EP4670297A1 - METHOD AND DEVICES FOR ENCHANTING RECIPROCITY CALIBRATION IN A WIRELESS COMMUNICATION NETWORK - Google Patents

Abstract

Methods (1000, 1100, 1200), a wireless device (121), and an access point (AP) for enabling reciprocity calibration in a wireless communications network (100) comprising a wireless device (121) and a plurality of distributed APs, wherein the plurality of distributed APs comprises a primary AP (111) and a secondary AP (112). An example method (1000) comprises: transmitting (1020), by the wireless device (121), a primary uplink, UL, reference signal (s_UL1) to the primary AP (111);transmitting (1030), by the primary AP (111), a primary downlink, DL, reference signal (s_DL1) to the wireless device (121), where the primary DL reference signal (s_DL1) is based on the primary UL reference signal (s_UL1) as received by the primary AP (111);transmitting (1040), by the secondary AP (112), a secondary DL reference signal (s_DL2) to the wireless device (121); transmitting (1050), by the wireless device (121), a secondary UL reference signal (s_UL2) to the secondary AP (112), where the secondary UL reference signal (s_UL2) is based on the primary and the secondary DL reference signals (s_DL1, s_DL2) as received by the wireless device (121); and determining (1070), based on the secondary UL reference signal (s_UL2) as received by the secondary AP (112), a first calibration coefficient indicative of a channel (y_AP2,AP1) from the primary AP (111) to the secondary AP (112) and of a channel (y_AP1,AP2) from the secondary AP (112) to the primary AP (111).

Description

METHODS AND APPARATUSES FOR ENABLING RECIPROCITY CALIBRATION IN A WIRELESS COMMUNICATIONS NETWORK TECHNICAL FIELD The present disclosure relates generally to the field of wireless communication. More particularly, it relates to methods, a wireless communications network, a wireless device, and an access point (AP) for enabling reciprocity calibration in a wireless communications network comprising a wireless device and a plurality of distributed APs. BACKGROUND In wireless communications, multiple-input and multiple-output (MIMO) is a technology utilizing multiple transmission antennas and receiving antennas to increase capacity. Distributed MIMO (D-MIMO), also known as cell-free MIMO, Radio Stripes etc., performing coherent joint transmissions (CJTs) is a strong candidate technology for sixth generation (6G) wireless communications. D-MIMO relies on phase-coherent operation of large numbers of antennas that are distributed over an area. It is foreseen that practical D-MIMO systems will be built of multiple panels where each panel typically comprises multiple antennas elements. Such panel may called be transmission/reception access points (APs) or points (TRPs). All APs are interconnected with one another and with a data processing unit. Channel state information (CSI) required for CJTs may be derived from uplink (UL) channel soundings, where reciprocity is assumed for the propagation channels (e.g. between an AP and a user equipment, UE). This type of CJT operation mode is typically referred to as reciprocity-based operation. A benefit of this type of CJT operation is that smaller training overheads are needed to learn the downlink (DL) CSI compared to performing a full DL beam/antenna sweep (plus feedback of the measured DL signals/channels to the APs). Reciprocity-based operation is especially suitable for time-division duplex (TDD) systems since full (i.e. amplitude and phase) channel reciprocity may be exploited if the UL training and DL data transmissions are performed within a time smaller than the coherent time of the channel (e.g. within an Orthogonal Frequency Division Multiplexing, OFDM, slot). However, even though the propagation channel between an AP and a UE is reciprocal, the presence of the analog front-end circuitry in the radio transceivers of the APs and UEs complicates the situation and makes the baseband-to-baseband channel non-reciprocal. Hence, in order to make use of the reciprocity assumption and rely on the uplink reference signals to compute DL precoding coefficients, the non-reciprocal transceiver responses need to be calibrated. One calibration approach that is suitable to restore reciprocity of a wireless link and enable reciprocity-based operation, is to conduct the entire calibration procedure solely at the data processing unit. In such approach, the calibration coefficients can be obtained via over-the-air (OTA) measurements between APs. One example of such approach is disclosed in J. Vieira and E. G. Larsson, "Reciprocity calibration of Distributed Massive MIMO Access Points for Coherent Operation," 2021 IEEE 32nd Annual International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC), 2021, pp.783-787. Another example of such approach is disclosed in R. Rogalin et al., "Scalable Synchronization and Reciprocity Calibration for Distributed Multiuser MIMO," in IEEE Transactions on Wireless Communications, vol.13, no.4, pp.1815-1831, April 2014. However, there may exist D-MIMO deployments where there exists no suitable link between APs (e.g. at some point in time the path loss may be too large, e.g. due to a structure obstructing the link) in order to perform measurements which are reliable enough for calibration. An alternative approach to inter-AP calibration involves one or more of the UEs in the calibration procedure as intermediary nodes. Said differently, if the direct link between two APs, say AP1 and AP2, is not reliable enough for calibration measurements, a combined link AP1-UE-AP2 may be suitable for inter-AP calibration. This can be the case if the UE is located at the boundaries of the respective local cells created by two APs. However, there is a need for improved ways of enabling inter-AP calibration using an UE as an intermediate node. SUMMARY It is an object of the present disclosure to mitigate, alleviate or eliminate one or more of the above-identified deficiencies and disadvantages in the prior art and solve at least the above- mentioned problem. In particular, an object is to provide improved ways of enabling reciprocity calibration in a wireless communications network. This object is obtained at least in part by a method for enabling reciprocity calibration in a wireless communications network comprising a wireless device and a plurality of distributed access points (APs). The plurality of distributed APs comprises a primary AP and a secondary AP. The method comprises: transmitting, by the wireless device, a primary uplink (UL) reference signal to the primary AP; transmitting, by the primary AP, a primary downlink (DL) reference signal to the wireless device, where the primary DL reference signal is based on the primary UL reference signal as received by the primary AP; transmitting, by the secondary AP, a secondary DL reference signal to the wireless device; and transmitting, by the wireless device, a secondary UL reference signal to the secondary AP, where the secondary UL reference signal is based on the primary and the secondary DL reference signals as received by the wireless device. The method further comprises determining, based on the secondary UL reference signal as received by the secondary AP, a first calibration coefficient indicative of a channel from the primary AP to the secondary AP and of a channel from the secondary AP to the primary AP. Since the secondary UL reference signal is based on the primary and the secondary DL reference signals, where the primary DL reference signal is in turn based on the primary UL reference signal, information (in the form of the first calibration coefficient) indicative of the channel from the primary AP to the secondary AP and of the channel from the secondary AP to the primary AP is possible to be estimated from the secondary UL reference signal as received by the secondary AP. The first calibration coefficient, can subsequently be used to calibrate the secondary AP relative to the primary AP in a reciprocity calibration in the wireless communications network. Thus, the disclosed method enables a reciprocity calibration, which in turn enables the plurality of distributed APs to operate using time-division duplex (TDD) reciprocity-based coherent joint transmissions (CJTs). In particular, the disclosed method does not require explicit feedback reporting from the wireless device to obtain the calibration coefficient. As a consequence, the first calibration coefficient may be obtained with less computational overhead and latency compared to other methods that rely on explicit feedback reporting from the wireless device. According to some aspects, the primary DL reference signal comprises a pilot sequence scaled by a first factor derived from the primary UL reference signal as received by the primary AP. Here, the first factor may be based on a magnitude and/or phase of a channel from the wireless device to the primary AP, and wherein the channel from the wireless device to the primary AP is based on the primary UL reference signal as received by primary AP. Furthermore, the secondary UL reference signal may comprise a pilot sequence scaled by a second factor derived from the primary DL reference signal as received by the wireless device and by a third factor derived from the secondary DL reference signal as received by the wireless device. Here, the second factor may be based on a magnitude and/or phase of a first ratio of a channel from the primary AP to the wireless device over the channel from the wireless device to the primary AP, and the third factor may be based on a magnitude and/or phase of a channel from the secondary AP to the wireless device. The first ratio may be based on the primary DL reference signal as received by the wireless device, and the channel from the secondary AP to the wireless device may be based on the secondary DL reference signal as received by the wireless device. In this way, it is possible to extract information indicative of the channels from the primary AP to the wireless device, from the wireless device to the primary AP, from secondary AP to the wireless device, and from the wireless device to the secondary AP from the secondary UL reference signal as received by the secondary AP. Again, the information may be obtained without any explicit feedback reporting by the wireless device. The extracted information may be used to factor out propagation channel information between the first AP and the second AP, such that the first calibration coefficient only comprises information of respective transmitters and receivers of the first and the second APs, which is desired if the first calibration coefficient is used to calibrate the secondary AP relative to the primary AP in a reciprocity calibration in the wireless communications network. There is also disclosed herein a wireless communications network for enabling reciprocity calibration in a wireless communications network. The wireless communications network is associated with the above-discussed advantages. The wireless communications network comprises a wireless device and a plurality of distributed access points (APs), where the plurality of distributed APs comprises a primary AP and a secondary AP. The wireless communications network is configured to: transmit, by the wireless device, a primary uplink (UL) reference signal to the primary AP; transmit, by the primary AP, a primary downlink (DL) reference signal to the wireless device, where the primary DL reference signal is based on the primary UL reference signal as received by the primary AP; transmit, by the secondary AP, a secondary DL reference signal to the wireless device; and transmit, by the wireless device, a secondary UL reference signal to the secondary A, where the secondary UL reference signal is based on the primary and the secondary DL reference signals as received by the wireless device. The wireless communications network is configured to determine, based on the secondary UL reference signal as received by the secondary AP, a first calibration coefficient indicative of a channel from the primary AP to the secondary AP and of a channel from the secondary AP to the primary AP. There is also disclosed herein a method performed by a wireless device for enabling reciprocity calibration in a wireless communications network. The method is associated with the above- discussed advantages. Here, the wireless communications network comprises a plurality of distributed access points (APs), where the plurality of distributed APs comprises a primary AP and a secondary AP. The method comprises receiving a primary downlink (DL) reference signal transmitted by the primary AP, receiving a secondary DL reference signal transmitted by the secondary AP, and transmitting a secondary uplink (UL) reference signal to the secondary AP. The secondary UL reference signal is based on the primary and the secondary DL reference signals as received by the wireless device. There is also disclosed herein a wireless device for enabling reciprocity calibration in a wireless communications network. The wireless device is associated with the above-discussed advantages. Here, the wireless communications network comprising a plurality of distributed access points (APs), where the plurality of distributed APs comprises a primary AP and a secondary AP. The wireless device is configured to receive a primary downlink (DL) reference signal transmitted by the primary AP, receive a secondary DL reference signal transmitted by the secondary AP, and transmit a secondary uplink (UL) reference signal to the secondary AP. The secondary UL reference signal is based on the primary and the secondary DL reference signals as received by the wireless device. There is also disclosed herein a method performed by a primary access point (AP) for enabling reciprocity calibration in a wireless communications network. The method is associated with the above-discussed advantages. Here, the wireless communications network comprises a plurality of distributed APs and a wireless device. The method comprises receiving a primary uplink (UL) reference signal transmitted by the wireless device, and transmitting a primary downlink (DL) reference signal to the wireless device. The primary DL reference signal is based on the primary UL reference signal as received by the primary AP. There is also disclosed herein a primary access point (AP) for enabling reciprocity calibration in a wireless communications network. The primary AP is associated with the above-discussed advantages. Here, the wireless communications network comprises a plurality of distributed APs and a wireless device. The primary AP is configured to receive a primary uplink (UL) reference signal transmitted by the wireless device, and transmit a primary downlink (DL) reference signal to the wireless device. The primary DL reference signal is based on the primary UL reference signal as received by the primary AP. There is also disclosed herein a computer program product comprising instructions which, when executed on at least one processing circuitry, cause the at least one processing circuitry to carry out the method according to the discussion above. The computer program is associated with the above-discussed advantages. There is also disclosed herein a computer program carrier carrying a computer program product according to the discussion above, wherein the computer program carrier is one of an electronic signal, optical signal, radio signal, or computer-readable storage medium. The computer program carrier is associated with the above-discussed advantages. BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, below follows a more detailed description of embodiments of the present disclosure cited as examples. In the drawings: Figure 1 is a schematic illustration of a wireless communications network; Figures 2 and 3 show different aspects of an example distributed multiple-input-multiple output system with a single antenna port per access point and a single antenna port per wireless device; Figure 4 illustrates a direct calibration between a first access point and a second access point; Figure 5 illustrates an indirect calibration between a first access point and a second access point via a wireless device; Figure 6 shows an example of resource element allocation for a 12-port Channel State Information Reference Signal (CSI-RS) in New Radio (NR); Figures 7A-7D illustrate respective steps in a procedure according to embodiments disclosed herein; Figures 8A-8D illustrate respective steps in a procedure according to embodiments disclosed herein; Figure 9 illustrates another procedure according to embodiments disclosed herein; Figures 10-12 are flow charts illustrating methods according to embodiments disclosed herein; Figure 13 schematically illustrates a wireless device according to embodiments disclosed herein; Figure 14 schematically illustrates an access point according to embodiments disclosed herein; Figure 15 schematically illustrates a node according to embodiments disclosed herein; Figure 16 shows an example of a communication system 1600 in accordance with some embodiments; Figure 17 is a block diagram of a host 1700, which may be an embodiment of the host 1616 of Figure 16, in accordance with various aspects described herein; and Figure 18 shows a communication diagram of a host 1802 communicating via a network node 1804 with a UE 1806 over a partially wireless connection in accordance with some embodiments. DETAILED DESCRIPTION The present disclosure is described below with reference to the accompanying drawings, in which certain aspects of the present disclosure are shown. The present disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments and aspects set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art. Like numbers refer to like elements throughout the description. It is to be understood that the present disclosure is not limited to the embodiments described herein and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims. Figure 1 depicts a wireless communications network 100 in which embodiments herein may operate. In some embodiments, the wireless communications network 100 may be a radio communications network, such as, 6G, NR or NR+ telecommunications network. However, the wireless communications network 100 may also employ technology of any one of 3/4/5G, LTE, LTE-Advanced, WCDMA, GSM/EDGE, WiMax, UMB, GSM, or any other similar network or system. The wireless communications network 100 may also employ technology transmitting on millimeter-waves (mmW), such as, e.g. an Ultra Dense Network, UDN. In some embodiments, the wireless communications network 100 may also employ transmissions supporting WiFi transmissions, e.g. the wireless communications standard IEEE 802.11ad or similar, or other non-cellular wireless transmissions. In particular, the wireless communications network 100 in Figure 1 comprises distributed access points (APs) 111;...;119. The APs are geographically spread out over an area in a planned or random fashion. The wireless communications network 100 in Figure 1 may be called a distributed multiple-input and multiple-output (D-MIMO) network or a D-MIMO system. The APs are arranged in communication with a data processing unit 140 through e.g. high- capacity backhaul links (such as fiber optic cables). The data processing unit 140 may e.g. be a remote standalone server, a cloud-implemented server, a distributed server, dedicated data processing resources in a server farm, or similar. The data processing unit 140 may be configured to operate the APs to perform TDD reciprocity-based CJTs. The APs 111;...;119 may serve wireless devices 121;...;124 in at least coverage area 115, which may also be called a cell. An AP may correspond to any type of network node or radio network node capable of communicating with a wireless device and/or with another network node, such as, a base station (BS), a radio base station, gNB, eNB, eNodeB, a Home NodeB, a Home eNodeB, a femto Base Station (BS), or a pico BS in the wireless communications network 100. As is also shown in Figure 1, wireless devices 121;...;124 are located within the service area 115. Each wireless device is configured to communicate within the wireless communications network 100 via one or more of the APs over radio links served by the one or more APs. The wireless devices may transmit data over an air or radio interface to one or more APs in uplink (UL) transmissions and the APs may transmit data over an air or radio interface to one or more wireless devices 121 in downlink (DL) transmissions. The wireless devices 121;...;124 may refer to any type of wireless devices or user equipment (UE) communicating with a network node and/or with another wireless device in a cellular, mobile or radio communication network or system. Examples of such wireless devices are mobile phones, cellular phones, Personal Digital Assistants (PDAs), smart phones, tablets, sensors equipped with a UE, Laptop Mounted Equipment (LME) (e.g. USB), Laptop Embedded Equipment (LEE), Machine Type Communication (MTC) devices, or Machine to Machine (M2M) device, Customer Premises Equipment (CPE), target device, device-to-device (D2D) wireless device, wireless device capable of machine to machine (M2M) communication. As mentioned, the base-band-to baseband channels between different APs and between APs and wireless devices are non-reciprocal due to the analog front-end circuitry in the radio transceivers of the APs and UEs. Thus, in order to enable reciprocity-based operation of the wireless communications network 100, the non-reciprocal transceiver responses need to be calibrated. The methods and apparatuses disclosed herein are suitably for any wireless devices. However, UEs are used as an example below. Below, a mathematical form for reciprocity calibration coefficients that ensure channel reciprocity, when applied at the APs, is described. For illustrative but non-limiting purposes, assume a narrowband MIMO link with M antenna ports at one end, and K antenna ports on the other end. We call “side A” the end of the link with M antenna ports. We call “side B” the end of the link with K antenna ports. An example system with a single antenna port per AP (e.g. single polarization) is depicted in Figure 2. More particularly, Figure 2 shows an UL system model. Side A can be the APs’ side of a distributed MIMO link, where each AP is single- antenna/single-transceiver and each AP is geographically distributed. We keep this formulation of one transceiver per distributed AP for simplicity, and remark that the methods and apparatuses disclosed herein also hold for the case of several transceivers per AP. Side B can be, e.g., ^ single-antenna UEs, a ^-antenna UE, or a mix of the previous two situations. In a massive MIMO regime, we typically have ^ ≫ ^. For illustrative but non-limiting purposes, we refer to side B as ^ single-antenna UEs, and to side A as the APs’ side of the distributed massive MIMO link. Assuming a noiseless channel for the moment, the ^ × ^ UL narrowband radio channel ^_^^, representing e.g. an OFDM subcarrier or PRB, is modelled as ^_^^ = ^_^^^ ^_^^ , (1) where ^ is a matrix comprising all channels effects occurring between the transmitter and receiver chains. For example, in fully-digital beamforming systems, the channel matrix ^ typically denotes the propagation channel. The matrix ^_^^ = ^^^^{^^^ ^^ ^ , … , ^_^ } is a diagonal matrix where each diagonal entry models the complex gain of each UE’s transmitter chain, and ^_^^ = diag{^^^ ^ , … , ^^^ ^ } is a diagonal matrix where each diagonal entry models the complex gain of each AP’s receiver chain. Consequently, ^_^^ may also be called a baseband-to-baseband UL channel. Figure 3 shows the same system as Figure 2, but during DL instead of UL. Within the same time/frequency coherence interval, the associated baseband-to-baseband DL channel is given by ^_^^ = ^_^^^^{^} ^_^^, (2) where ⁽. ^{)^} denotes the transpose operator, and ^_^^ = ^^^^{^^^ ^^ ^ , … , ^_^ } and ^_^^ = diag{^^^ ^ , … , ^^^ ^ } model the associated transmitter and receiver gains of each UE and each AP, respectively. Note that the matrix ^ is assumed to be reciprocal. However, the baseband-to-baseband UL channel and the baseband-to-baseband DL channel are not reciprocal, i.e. ^_^^ ≠ ^^ ^_^ . This is because the gains of the transceiver circuitries are not reciprocal (e.g., ^_^^ ≠ ^_^^). Due to this non-reciprocity aspect, it is not immediately obvious how coherent DL transmissions can be performed based on channel estimates obtained from UL pilot signals. To indicate how to address this challenge with the non-reciprocal transceiver terms, let’s assume for now that the AP side of the link has knowledge of the following matrix ^ = ^^^^{^_^, … , ^_^} = ^_^^ ⁽^_^^ ^{)^^}, (3) up to a non-zero complex-valued unknown scaling term ^. Via UL pilot signals, a data processing unit 140 of the D-MIMO system can estimate ^_^^. If the APs want to jointly perform, e.g. zero forcing (ZF) transmissions towards the UEs, they may do so by first the computing the Moore–Penrose inverse of ^^ ^_^ , namely ^ = ^ ^∗ ^ ^_^ ⁽ _^^^ ^ ^∗ ^_^ ^{)^^} , where ( )* denotes element-wise complex conjugation. However, since the matrix ^ was computed via UL signals, it cannot be directly used as a DL pre-coder since it is not matched to the (non-reciprocal) DL channel ^_^^. To solve this, each AP multiplies its pre-coded DL signals with its associated entry of ⁽^^^{)^^}. More specifically, the pre-coded signal at AP transceiver ^ is multiplied with 1/ (^^_^), with 1 ≤ ^ ≤ ^. With that, the effective DL channel is written as = ^^{^^} ^_^^ (^_^^)^{^^} , which is a diagonal channel matrix with unknown diagonal entries. The operator (. )^{^} denotes the Moore-Penrose inverse, and ^|. ^{|^} denotes element-wise squared absolute value. The unknown diagonal entries of ^^{^} _^^ can be estimated in the DL using only one DL reference signal, which is beamformed in the DL towards all UEs, using the calibrated channels. Thus, ^ UL pilot signals (one per UE) plus one DL reference signal are sufficient to conduct all training needed for this type of calibrated reciprocity-based transmissions. This results in much less training overhead compared to explicit DL channel estimation. In conclusion, knowledge of the matrix ^ allows CJTs, e.g. ZF DL transmissions, with no (or very little) inter-user interference over what is effectively a calibrated UL/DL channel setup. The matrix ^ can thus be seen as a calibration matrix, and thus we are interested to estimate its diagonal entries, i.e. the reciprocity calibration coefficients. Below, inter-AP calibration is discussed. As described above, a scaled version of the calibration matrix ^ = ^^^^{^_^, … , = _^^^ ⁽ _^^^ ^{)^^}, is required for reciprocity calibration. One example of the scaling factor ^ is ^ = 1/^_^ which provides the following calibration matrix 1/^_^^ = ^^^^{1, ^_^/^_^ … , ^_^/^_^} (4) With that, we see that AP1 does not need to apply any calibration compensation, but the remaining APs do. This can also be seen as calibrating all APs other than AP1 relative to AP1. AP1 can therefore be called a primary AP. Any AP other than AP1 can be called a secondary AP. The most straightforward way to estimate the diagonal elements of matrix in Equation (4) consists of performing bi-directional measurements between one reference AP (i.e. the primary AP), say AP1, and the other APs (i.e. any secondary AP, say AP2). For example, if we would like to calibrate AP2 with respect to AP1, then we perform a bi-directional measurement between them. A noise free version of this bi-directional measurement is written as ^_^^^,^^^ = ^_^^^ ℎ_^^^,^^^^_^^^ and ^_^^^,^^^ = ^_^^^ ℎ_^^^,^^^^_^^^. Figure 4 illustrates this case. More specifically, Figure 4 shows a direct calibration between AP1 and AP2, where AP1 transmits a first reference signal (ref. sig. 1) and where AP2 transmits a second reference signal (ref. sig.2). The baseband-to-baseband channel from AP1 to AP2 (^_^^^,^^^) is estimated from ref. sig. 1 as received by AP2, and the baseband-to- baseband channel from AP2 to AP1 (^_^^^,^^^) is estimated from ref. sig.2 as received by AP1. Here, ^_^^^ and ^_^^^ are the complex gains of the receiver chains of AP1 and AP2, respectively, and ^_^^^ and ^_^^^ are the complex gains of the transmitter chains of AP1 and AP2, respectively. Assuming that the two measurements, which comprise a bi-directional measurement, are performed within a time/bandwidth smaller than the coherent time/bandwidth of the channel, the propagation channel is reciprocal, i.e. ℎ_^^^,^^^ = ℎ_^^^,^^^. This is a necessary condition for the calibration shown below. This implies that the instantaneous phase of the propagation channels ℎ_^^^,^^^ and ℎ_^^^,^^^ is the same during both measurements. With that, the calibration coefficients for AP1 can be set to 1, and the calibration coefficient of AP2 can be computed by dividing the two measurements as which is consistent with the second diagonal entry of the example calibration matrix in Equation (4), and thus achieve the desired calibration (i.e. AP1 and AP2 can perform CJTs). Note that, even though the channels ℎ_^^^,^^^ and ℎ_^^^,^^^ cancel out in Equation (5), they require to yield enough channel gain/energy/power for the calibration to meet a certain level of accuracy. When this is not the case in D-MIMO deployments, one may instead perform calibration with the aid of a UE as it will be described next. In case the channel between AP1 and AP2 does not yield high path gains, but the channel between AP1 and a first UE (UE1) and the channel between AP2 and UE1 yield large enough path gains, then calibration between AP1 and AP2 can be done through bi-directional measurements between AP1 and UE1 and between UE1 and AP2. This approach may be termed indirect inter-AP calibration in the sense that an intermediate node (i.e. UE1) is being used to aid the calibration of two APs. A noise free version of these bi-directional measurements is written as ^_^^^,^^^ = ^_^^^ ℎ_^^^,^^^^_^^^ and ^_^^^,^^^ = ^_^^^ ℎ_^^^,^^^^_^^^. ^_^^^,^^^ = ^_^^^ ℎ_^^^,^^^^_^^^ and ^_^^^,^^^ = ^_^^^ ℎ_^^^,^^^^_^^^. Figure 5 illustrates an example indirect inter-AP calibration. In Figure 5, four reference signals are transmitted in any order (or simultaneously). Here, UE1 transmits a first reference a first reference signal (ref. sig. 1), AP1 transmits a second reference signal (ref. sig. 2), AP2 transmits a third reference signal (ref. sig.3), and UE1 transmits a fourth reference signal (ref. sig.4). The baseband-to-baseband channel from UE1 to AP2 (^_^^^,^^^) is estimated from ref. sig.1 as received by AP1, the baseband-to-baseband channel from AP1 to UE1 (^_^^^,^^^) is estimated from ref. sig.2 as received by UE1, the baseband-to-baseband channel from AP2 to UE1 (^_^^^,^^^) is estimated from ref. sig. 3 as received by UE1, and the baseband-to- baseband channel from UE1 to AP2 (^_^^^,^^^) is estimated from ref. sig.4 as received by AP2. Here, ^_^^^, ^_^^^ and ^_^^^ are the complex gains of the receiver chains of AP1, AP2, and UE1, respectively, and ^_^^^, ^_^^^ and ^_^^^ are the complex gains of the transmitter chains of AP1, AP2, and UE1, respectively. Assuming that each bi-directional measurement is taken within a time/bandwidth smaller than the coherence time/bandwidth of the respective channels, the calibration coefficient for AP2 can be calculated as which is the same coefficient as the second diagonal entry of Equation (4) and thus achieves the desired calibration. Equation (6) shows how the 2 bi-directional channel measurements, i.e. 4 unidirectional channel measurements, can be co-processed in order to compute an AP calibration coefficient for CJTs with AP1 as a reference. For completeness, the four involved channel measurements are ^ The channel measurement from AP1 to UE1, which gives ^_^^^,^^^ ^ The channel measurement from AP2 to UE1, which gives ^_^^^,^^^ ^ The channel measurement from UE1 to AP1, which gives ^_^^^,^^^ ^ The channel measurement from UE1 to AP2, which gives ^_^^^,^^^. To sum up, based on the above example of indirect inter-AP calibration, AP1 does not need to apply calibration compensation and AP2 applies the calibration coefficient as in Equation (6). With that, both APs are capable of TDD reciprocity-based CJTs. Below, Sounding Reference Signal (SRS) in New Radio (NR) is discussed. SRS is supported in NR for UL channel sounding. Similar to Long-Term Evolution (LTE), configurable SRS bandwidth is supported in NR. SRS can be configurable with regard to density in frequency domain (e.g., comb levels) and/or in time domain (including multi-symbol SRS transmissions). A UE can be configured with one or more SRS resource sets, each SRS resource set can contain one or more SRS resources. Each SRS resource can contain ^_a ^S _p ^RS ∈ {1,2,4} SRS antenna ports in a time-frequency resource with ^ SRS s_ymb ∈ {1,2,4,8,10,12,14} consecutive OFDM symbols in a slot starting from OFDM symbol ^_^ and a number Physical Resource Blocks (PRBs) starting from subcarrier ^_^. An SRS resource can be periodic, semi-persistent, or aperiodic. In case of periodic or semi- persistent SRS, a UE transmits SRS periodically at certain configured SRS slots. In case of aperiodic SRS, a UE transmits SRS only when it is requested by the data processing unit 140. Below, Channel State Information Reference Signals (CSI-RS) in NR is discussed. For Channel State Information (CSI) measurement and feedback, CSI-RS are defined. A CSI- RS is transmitted on each antenna port and is used by a UE to measure DL channel between each of the transmit antenna ports and each of its receive antenna ports. The transmit antenna ports are also referred to as CSI-RS ports. The supported number of antenna ports in NR are {1,2,4,8,12,16,24,32}. By measuring the received CSI-RS, a UE can estimate the channel that the CSI-RS is traversing, including the radio propagation channel and antenna gains. The CSI- RS for the above purpose is also referred to as Non-Zero Power (NZP) CSI-RS. CSI-RS can be configured to be transmitted in certain Resource Elements (REs) in a slot and certain slots. Figure 6 shows an example of CSI-RS REs for 12 antenna ports, where 1 RE per RB per port is shown. Below, CSI framework in NR is discussed. In NR, a UE can be configured with multiple CSI reporting settings and multiple CSI-RS resource settings. Each resource setting can contain multiple resource sets, and each resource set can contain up to 8 CSI-RS resources. For each CSI reporting setting, a UE feeds back a CSI report. Each CSI reporting setting contains at least the following information: ^ A CSI-RS resource set for channel measurement ^ An Interference Measurement Resource (IMR) resource set for interference measurement ^ Optionally, a CSI-RS resource set for interference measurement ^ Time-domain behavior, i.e. periodic, semi-persistent, or aperiodic reporting ^ Frequency granularity, i.e. wideband or subband ^ CSI parameters to be reported such as Rank Indicator (RI), Precoder Matrix Indicator (PMI), Channel Quality indicator (CQI), and CSI-RS Resource Indicator (CRI) in case of multiple CSI-RS resources in a resource set ^ Codebook types, i.e. type I or II, and codebook subset restriction ^ Measurement restriction enabled or disabled ^ Subband size. One out of two possible subband sizes is indicated, the value range depends on the bandwidth of the DL bandwidth part (BWP). One CQI/PMI (if configured for subband reporting) is fed back per subband). When the CSI-RS resource set in a CSI report setting contains multiple CSI-RS resources, one of the CSI-RS resources is selected by a UE and a CRI is also reported by the UE to indicate to the data processing unit 140 about the selected CSI-RS resource in the resource set, together with RI, PMI and CQI associated with the selected CSI-RS resource. The APs may then transmit the different CSI-RS resources using different MIMO precoders or by using different beam directions. For aperiodic CSI reporting in NR, more than one CSI report settings, each with a different CSI-RS resource set for channel measurement and/or different resource set for interference measurement can be configured and triggered at the same time, i.e. with a single trigger command in the DL control channel from the AP to the UE. In this case, multiple CSI reports are measured, computed, aggregated and sent from the UE to the AP in a single PUSCH message. As part of developing the embodiment disclosed herein, it has been realized that there is need for improved ways of implementing indirect inter-AP calibration. In particular, there are some practical aspects that should be addressed. For example, the indirect inter-AP calibration described in connection to Figure 5 requires that the UE report back the DL measurements, namely ^_^^^,^^^ to the data processing unit 140 of the link so that the calibration coefficient in Equation (6) can be computed. Such UE feedback implies overhead. It would therefore be of interest to obtain a procedure that would enable a data processing unit 140 to essentially obtain the desired calibration coefficient for AP2 but without the need of feedback reporting by UE1. Another aspect that may be addressed is how to perform such an indirect inter-AP calibration procedure, that does not use explicit feedback reporting, based on reference signals (RSs) of current standards such as 3rd Generation Partnership Project (3GPP). The present disclosure therefore presents ways for performing UE assisted inter-AP calibration without explicit CSI feedback from the UE. In general, there is disclosed herein methods and apparatuses for enabling reciprocity calibration in a wireless communications network. In an example embodiment, a wireless communications network 100 comprise two APs, namely a primary AP 111 (AP1) and a secondary AP 112 (AP2), and a UE 121 (UE1). Instead of using CSI-feedback from UE1, the following procedure is performed ^ Step 1: UE1 transmits a primary UL reference signal (sUL1) to AP1. ^ Step 2: AP1 transmits a primary DL reference signal (sDL1) to UE1. Here, sDL1 is based on sUL1 as received by AP1 in Step 1. Here, “based on” may be also be called “pre- coded based on” or “configured based on”. ^ Step 3: AP2 transmits a secondary reference signal (s_DL2) to UE1. ^ Step 4: UE1 transmits a secondary UL reference signal (sUL2) to AP2. Here, sUL2 is based on sDL1 as received by UE1 in Step 2 and on sDL2 as received by UE1 in Step 3. Here, “based on” may be also be called “pre-coded based on” or “configured based on”. Based on sUL2 as received by AP2, the wireless communications network 100 has the required information to perform calibration between AP1 and AP2. The information based on sUL2 as received by AP2 can e.g. be communicated to the data processing unit 140, which subsequently may perform a reciprocity calibration in the wireless communications network 100 based on the communicated information. The disclosed calibration procedure allows for TDD reciprocity-based CJTs without the need of UE feedback reporting, and may fit 3GPP standards such as NR. The procedure is comprised by Steps 1-4 should be executed in the disclosed order (except that the order of Step 2 and Step 3 can be changed). Each bi-directional measurement is performed within the channel coherence time (e.g. one or more slots/sub-frames as defined in NR/6G) and channel coherence bandwidth (e.g. within a PRB). The signaling procedure is described in the context of a narrowband system, but it can be generalized straightforwardly to a multi-carrier system by being employed directly per sub- carrier or PRB of an OFDM system. Figures 7A-7D show an example of Steps 1-4. Below, Steps 1-4 are discussed in more detail. ^ Step 1. UE1 transmits sUL1 to AP1. Here, sUL1 may e.g. be an SRS or a new type of dedicated UL reference signal. AP1 receives and measures sUL1. With sUL1 received by AP1, AP1 is able to obtain an estimate of the UL channel from UE1 to AP1, namely yAP1,UE1. Step 1 is illustrated in Figure 7A. ^ Step 2. AP1 transmits s_DL1 to UE1. Here, s_DL1 may e.g. be a modified CSI-RS or a modified new type of dedicated DL reference signal. The modification is based on the received signal in Step 1. In this example, s_DL1 comprises an original reference signal (i.e., pilot) divided by the UL channel estimate y_AP1,UE1 obtained in Step 1. With s_DL1 received by UE1, UE1 may obtain an estimate of a first ratio between the DL from UE1 to AP1 and the UL channel from AP1 to UE1, namely y_UE1,AP1/y_AP1,UE1. Here, it is assumed that the original reference signal of s_DL1 is known by UE1. Step 2 is illustrated in Figure 7B. ^ Step 3. AP2 transmit sDL2 to UE1. Here, sDL2 may e.g. be a CSI-RS or a new type of dedicated DL reference signal. With sDL2 received by UE1, UE1 is able to obtain an estimate of the DL channel from AP2 to UE1, namely yUE1,AP2. Step 3 is illustrated in Figure 7C. Please note that in some embodiments, the DL transmission from AP1 (in Step 2) and the DL transmission from AP1 (in Step 3) may occur in the same transmission time interval. For example the DL transmissions may use orthogonal time and frequency resource elements and UE1 may be capable of receiving two different DL signals using different receiver combining weights, separately tuned for each of AP1 and AP2. ^ Step 4. UE1 transmits sUL2 to AP2. Here, sUL2 may be a modified SRS or a modified new type of dedicated UL reference signal. The modification is based on the received signal in Step 2 and on the received signal in Step 3. More specifically, in this example, sUL2 comprises an original reference signal multiplied by the first ratio obtained in Step 2 (i.e., yUE1,AP1/yAP1,UE1) and divided by the channel estimate obtained in Step 3 (i.e., yUE1,AP2). With sUL2 received by AP2, it is possible to obtain an estimate the following ratio ^ ^̂ = ^ _{^^^,^^^ ^^^,^^^} . (7) ^_^^^,^^^ ^_^^^,^^^ The original reference signal of sUL2 may be the same or a different one from the one mentioned in Step 2. It is assumed that the original reference signal of sUL2 is known by AP2. Step 4 is illustrated in Figure 7D. Note that the ratio estimate obtained in Equation (7) equals to the inverse of the desired calibration coefficient to be applied at AP2 as shown in Equation (6), namely With that, AP2 can use the inverse of the ratio estimate ^̂ as a calibration coefficient to be able to perform CJTs with AP1. Please note that although Figure 5 may look somewhat similar to Figures 7A-7D, the calibration method in Figure 5 requires explicit UE feedback while the proposed method in Figures 7A-7D does not. The procedure in Figure 5 requires that all four measurements are collected at once central place, i.e. the UE needs to explicitly feedback the two measurements to the wireless communications network. Such explicit feedback is not needed in the methods disclosed herein. In Steps 1-4, the procedure is described in the context of a single reference signal transmitted in each step of the procedure for both the APs and UE1. However, the procedure can easily be extended to multiple reference signals transmitted one or more of the steps – as will be described below. This could be useful for example if UE1 or any of AP1 and AP2 has two antennas, e.g. one antenna per polarization. To mitigate potential polarization miss-match between UE1 and any of AP1 and AP2, one reference signal can be transmitted per polarization. It will also be described below how Steps 1-4 can be modified when two or more secondary APs are to be calibrated relative to the primary AP. Note that both amplitude and phase of the reference signals (sUL1, sDL1, sDL2, sUL2) may be used for calibration are pre-processed, measured, and post-processed. In another embodiment, each (or at least one) of Steps 1-4 may be executed using only the phase of the received reference signal. This may be convenient, e.g., from a node implementation point-of-view. For example, at Step 2, s_DL1 may comprises an original reference signal shifted in phase by ∠yAP1,UE1 (where ∠yAP1,UE1 denotes the phase of yAP1,UE1). In that case, when sDL1 is received by UE1, the difference ∠yUE1,AP1−∠yAP1,UE1 may be estimated. Additionally or alternatively, the modification of the reference signal transmitted by UE1 at Step 4 may comprises an original reference signal shifted in phase by ∠y_UE1,AP1−∠y_UE1,AP2−∠y_AP1,UE1. Now we focus on the angle where UE1 has multiple antennas. In one embodiment, in case UE1 has two antennas, UE1 is configured with two SRS ports per SRS resource, as schematically illustrated in Figures 8A-8D. In particular, Figures 8A-8D show an example of the calibration procedure where UE1 has two antennas, namely a first antenna element 811 and a second antenna element 812, where one SRS port is transmitted from respective UE antenna, both during Step 1 and Step 4, which improves the calibration accuracy since the risk of polarization mismatch between UE1 and any of AP1 and AP2 is mitigated in case the two UE antennas have different polarizations. In this example, it is assumed that all UL reference signals are SRSs and that all DL reference signals are CSI-RSs. In one embodiment, each SRS resource (both for Step 1 and Step 4) consists of two SRS ports, where a first SRS port is transmitted from the first antenna element 811 of UE1 and a second SRS port is transmitted from the second antenna element 812 of UE1. In another embodiment, when UE1 has two antennas and each SRS resource consists of two SRS ports, the CSI-RS resource used during Step 2 consists of two CSI-RS ports. However, the CSI-RS resource transmitted in Step 3 from AP2 only consist of a single CSI- RS port. In this case, the first CSI-RS port of the CSI-RS resource used in Step 2 is multiplied with the estimated channel from the first SRS port of the SRS resource used in Step 1, and the second CSI-RS port of the CSI-RS resource used in Step 2 is multiplied with the estimated channel from the second SRS port of the SRS resource used in Step 1. And in Step 4, UE1 transmits a first SRS port of the SRS resource from the first antenna element 811 of UE1 (i.e. the same SRS port to UE antenna mapping in Step 4 and Step 1), and multiplies the first SRS port with the estimated channel from the first CSI-RS port of the CSI- RS resource received during Step 2, and divide the first SRS port with the estimated channel from the (single-port) CSI-RS resource received during Step 3. In a similar way, UE1 transmits a second SRS port of the SRS resource used in Step 4 from the second antenna element 812 of UE1, and multiplies the second SRS port with the estimated channel from the second CSI-RS port of the CSI-RS resource received during Step 2, and divide the second SRS port with the estimated channel from the (single-port) CSI-RS resource received during Step 3. In this way, the calibration can become more reliable, since both of the UEs antennas are used for calibration, which reduces the risk of polarization mismatch in case the two UE antennas have different polarizations. Below, potential NR standard upgrade that may be implemented to be able to execute the disclosed procedure with the NR standard are discussed. In one embodiment a “new UE capability” is introduced in a 3GPP standardization specification (like NR for 5G or similar for 6G) that indicates support for the calibration procedure disclosed herein, where the new UE capability can be signaled from the UE to the wireless communications network e.g. during UE capability signaling. The new UE capability can also contain one or more of the following information: ^ Maximum number of APs that the UE can perform simultaneous calibration for o For example, in case the UE reports a capability to perform simultaneous calibration for three APs, the UE can receive one or more reference signals from two different APs (other than the primary AP, i.e. AP1) in Step 3, and the UE can transmit one or more reference signals to two different APs (other than AP1) in Step 4. Note that the word AP might not be used in the specification, instead this might be indicated in the specification by for example indicating the maximum number of CSI-RS resources supported for the calibration procedure, or maximum number of Transmission Configuration Indicator (TCI) states supported for the CSI-RS resources configured for the calibration procedure. ^ Maximum number of supported reference signals transmitted by the UE in Step 1. o For example, if the UE is equipped with two UE antennas, the UE might transmit two reference signals to AP1, i.e., one per UE antenna. o In NR, the maximum number of supported reference signals can for example be the maximum total number of supported SRS ports across one or multiple SRS resources configured for Step 1. ^ Maximum number of supported reference signals that the UE can receive from AP1 in Step 2. o For example, if AP1 is equipped with antennas of two separate polarizations, AP1 might transmit two reference signals to the UE, one per polarization. o In NR, the maximum number of supported reference signals can for example be the maximum total number of supported CSI-RS ports across one or multiple CSI-RS resources configured for Step 2 of the calibration procedure. ^ Maximum number of supported reference signals that the UE can receive from respective APs (other than AP1) in Step 3. o In NR, the maximum number of supported reference signals for a given AP can for example be the maximum total number of supported CSI-RS ports across one or multiple CSI-RS resources associated with that AP (and configured for Step 3). ^ Maximum number of supported reference signals that the UE can transmit to respective AP (other than AP1) in Step 4. o In NR, the maximum number of supported reference signals for a given AP can for example be the maximum total number of supported SRS ports across one or multiple SRS resources associated with that AP (and configured for Step 4). ^ Minimum delay between receiving the reference signals associated with Step 2 and/or Step3 until the UE can perform the transmission of the RS for Step 4. ^ TX coherency capability for one or more antenna ports. o For example the maximum phase difference between an SRS port transmitted during Step 1 at a first antenna port, and a SRS port transmitted during Step 4 using the same first antenna port. o Maximum duration in time between Step 1 and Step 4 in order to maintain the maximum coherency capability. ^ RX coherency capability for one or more antenna ports. o For example, the maximum phase difference of the RX antenna port used to receive DL-RS from Step 2 and the DL-RS received during Step 3. o Maximum duration in time between DL-RS in Step 2 and DL-RS in Step 3 in order to maintain the maximum coherency capability. In one embodiment a new report quantity is introduced in a Report setting in a 3GPP specification (e.g. in NR that would correspond to a new value to the parameter “reportQuantity” in a Report setting defined by “CSI-ReportConfig information element (IE)” as specified in Technical Specification (TS) 38.331, Chapter 6.3.2, version 17.2.0). The new report quantity is indicating to the UE to perform the calibration procedure as described in this disclosure. In one embodiment, a Report setting associated with the calibration procedure is configured with N CSI-RS resources, where N is the number of APs the calibration procedure should be applied to, and where each CSI-RS resource is associated with one AP. In one embodiment, a Report setting associated with the calibration procedure is configured with N groups of CSI-RS resources, where N is the number of APs the calibration procedure should be applied to. In one embodiment, each group of CSI-RS resources is defined by a CSI-RS resource set (e.g. NZP-CSI-RS-ResourceSet IE as specified in TS 38.331, Chapter 6.3.2, version 17.2.0). In one embodiment, the Report setting associated with the calibration procedure indicates one or more UL reference signals to be transmitted by the UE during Step 1 and/or Step 4. In one embodiment the Report setting is configured with one SRS resource set IDs pointing towards one SRS resource set containing one or more SRS resources to be used for Step 1 and/or Step 4. In one embodiment, the SRS resource set indicated by the SRS resource set ID in the Report setting consists of N SRS resources, where each of the N SRS resources is associated with one out of N APs that are associated with the calibration procedure (for example a first SRS resource is transmitted in Step 1 towards a first AP, and the remaining N-1 SRS resources are transmitted in Step 4 towards the remaining N-1 APs). In one embodiment, the Report setting is configured with N SRS resource set IDs (pointing towards N SRS resource set) to support calibration over N APs, where a first SRS resource set is used for Step 1 towards a first AP, and the remaining N-1 SRS resource sets is used for the remaining N-1 APs for Step 4. In one embodiment, a new field in DCI is used to trigger the transmission of one or more UL reference signals associated with Step 1 and/or Step 4 of the calibration procedure (instead of using a filed in Report setting to trigger the UL reference signals). In one embodiment, an SRS resource set used for Step 1 and/or Step 4 of the calibration procedure is configured with a new usage, i.e. a new dedicate value of the parameter “usage” as defined per SRS-ResourceSet in SRS-Config IE as specified in 38.331, Chapter 6.3.2, version 17.2.0. The new usage indicates that the SRS should be transmitted according to Step 1 and/or Step 4. Below, the calibration procedure when more than one secondary AP should be calibrated with respect to the primary AP is discussed. In Step 4, the UE should use the channel estimates based on the received DL reference signal from Step 2 and Step 3 to modify the transmitted UL reference signal(s). In one embodiment, it is specified that the UE, when triggered with the calibration procedure, should divide the channel estimate of the received DL reference signal from Step 2 with the channel estimate of the received DL reference signal from Step 3, and then multiply this value with the transmitted UL reference signal. This is straightforward if only two APs are calibrated and if only a single reference signal is transmitted during Steps 1-4. However, in case for example more than two APs are to be calibrated, there should preferable be some kind of association between different DL reference signals and UL reference signals. One way to handle this is described for the case where we perform calibration over N APs, and where UE1 is configured with N single-port CSI-RS resources (one per AP) and N single-port SRS resources (one per AP). In one embodiment, the specification indicates that a first SRS resource should be transmitted towards a first AP (also known as a reference AP as it will be explained below) during Step 1. The first SRS resource can for example be the SRS resource in an SRS resource set with lowest SRS resource ID (in case a single SRS resource set is configured for the calibration procedure), or if multiple SRS resource sets are configured, the first SRS resource could be the SRS resource with lowest SRS resource ID in the SRS resource set with lowest SRS resource set ID. In a similar way for CSI-RS, the specification can indicate that a first CSI-RS resource should be transmitted from a first AP (i.e. the primary AP) during Step 2, where the first CSI-RS resource can for example be the CSI-RS resource in an CSI-RS resource set with lowest CSI-RS resource ID (in case a single CSI-RS resource set is configured for the calibration procedure), or if multiple CSI-RS resource sets are configured, the first CSI-RS resource could be the CSI-RS resource with lowest CS-RS resource ID in the CSI-RS resource set with lowest CSI-RS resource set ID. Note that in some embodiments the transmission of all DL CSI-RS in Step 3 (CSI-RS1, CSI- RS2, CSI-RS3, CSI-RS4) signals may occur in parallel, e.g. using orthogonal time and frequency resource elements within the same transmission time interval. This may require that the UE can receive and process several CSI-RS signals simultaneously from different APs. In some embodiments additional UE capability signaling is used to indicate such capabilities to the network. In further embodiments the network selects a UE capable of receiving and processing multiple CSI-RS signals from multiple APs to perform the disclosed calibration procedure. The benefit of transmitting and processing multiple CSI-RS signals in parallel is that the entire calibration procedure can be performed faster and the channel will change less during the calibration procedure time, thereby increasing the accuracy of the calibration. In Step 4, the UE should transmit N-1 SRS resources towards the remaining (non-reference) N-1 APs. Each SRS resource should be multiplied with the channel estimate from the CSI- RS received in Step 2. In addition, an SRS resource transmitted towards a certain AP should divide the SRS resource with the channel estimate from the CSI-RS resource transmitted from that AP. To enable this, in one embodiment, there is an association between the SRS resources and the CSI-RS resources used in the calibration procedure, so the UE knows which SRS resource that should be divided with which channel estimate. One example of this is illustrated in Figure 9, where a first SRS resource is associated with a first CSI-RS resource, a second SRS resource is associated with a second CSI-RS resource and so on. In particular, Figure 9 shows an example comprising multiple APs 910 in addition to AP1 and AP2 in the example shown in Figures 7A-7D. A third AP 113 (AP3) of the multiple APs is used to demonstrate the additional UL and DL reference signals in the calibration procedure. In one embodiment, the order of the CSI-RS resources and SRS resources is determined based on the SRS resource ID and/or SRS resource set ID as well as CSI-RS resource ID and/or CSI-RS resource set ID (in a similar way as described above). In one embodiment the order is explicitly configured per SRS and CSI-RS resource in a new field/parameter. In Figure 9, Steps 1 and 2 are executed once and for the first AP (i.e. the primary AP, AP1, which can be seen as the reference AP), and Steps 3 and 4 are executed N-1 times, specifically, once for each of the remaining (non-reference) APs. In this way, AP1 can be seen as a reference node for calibration since it implicitly aligns the calibrations between all other APs. In particular, in Figure 9, the calibration can be described as comprising Steps 1-2 according to the discussion above, and with an iteration of Steps 3-4, which is described as Steps 3’, 3’’, 4’, and 4’’. The iteration can be described as ^ Step 3’: AP2 transmits a first secondary reference signal (s_DL2’) to UE1. ^ Step 4’: UE1 transmits a first secondary UL reference signal (sUL2’) to AP2. Here, sUL2’ is based on sDL1 as received by UE1 in Step 2 and on sDL2‘ as received by UE1 in Step 3’. ^ Step 3’’: AP3 transmits a second secondary reference signal (sDL2’’) to UE1. ^ Step 4’’: UE1 transmits a second secondary UL reference signal (sUL2’’) to AP3. Here, s_UL2’’ is based on s_DL1 as received by UE1 in Step 2 and on s_DL2’’ as received by UE1 in Step 3’’. With reference to Figure 10, there is disclosed herein a method 1000 for enabling reciprocity calibration in a wireless communications network 100 comprising a wireless device 121 and a plurality of distributed access points (APs), where the plurality of distributed APs comprises a primary AP 111 and a secondary AP 112. The method 1000 may comprise a number of actions, which are discussed below. Action 1010. Optionally, the method comprises reporting, by the wireless device 121, a capability report of the wireless device 121 to any AP of the plurality of distributed APs. The capability report generally may comprise information indicating the wireless device is capable of supporting the calibration procedure according to the method 1000. In particular, the capability report may be indicative of the wireless device 121 being capable of transmitting the secondary UL reference signal sUL2 to the secondary AP 112. The capability report may further, or alternatively, comprise any of the “new UE capability” as discussed above. In particular, the capability report may be a quantity value in a report defined by a Channel State Information (CSI) Information Element (IE). The wireless device may e.g. report the capability report to the wireless communications network 100, e.g. to a node 140, such as a data processing unit 140 coordinating the plurality of distributed APs, by transmitting the capability report to one of the APs of plurality of distributed APs. In some embodiments, the wireless device 121 is capable of transmitting a first number of secondary uplink (UL) reference signals to respective secondary APs comprised in the plurality of distributed APs, where each secondary UL reference signal of the first number of secondary UL reference signals is based on a primary DL reference signal s_DL1 (transmitted from the primary AP 111) and an associated secondary DL reference signal (transmitted by an associated secondary AP) as received by the wireless device 121. The associated secondary DL reference signal is associated with the respective secondary AP that the respective secondary UL reference signal is transmitted to. In that case, the capability report may comprises the first number. This enables the wireless device to participate in calibrating the first number of secondary APs relative to the primary AP. In some embodiments, the primary and the secondary UL reference signals comprise a respective (Sounding Reference Signal) SRS, and primary and the secondary DL reference signals comprise a respective Channels State Information-Reference Signal (CSI-RS). In that case, the first number may be associated with a corresponding number of SRS resources for each secondary UL reference signal of the first number of secondary UL reference signals and with a corresponding number of CSI-RS resources for each corresponding secondary DL reference signal. Action 1020. The method 1000 comprises transmitting, by the wireless device 121, a primary UL reference signal s_UL1 to the primary AP 111. Here, s_UL1 may e.g. comprise an SRS, be an SRS, or be a new type of dedicated UL reference signal. The primary AP 111 is aware of reference information contained in s_UL1, such as a pilot sequence. Thus, the primary AP may estimate the channel y_AP1,UE1 from the wireless device 121 to the primary AP 111 from s_UL1 as received by the primary AP. This channel y_AP1,UE1 may be a baseband-to-baseband channel. However, it is also possible that the estimated channel does not comprise the full transmitter chain of the wireless device 121 and/or the full receiver chain of the primary AP 111. Action 1030. The method comprises transmitting, by the primary AP 111, a primary DL reference signal sDL1 to the wireless device 121. The primary DL reference signal sDL1 is based on the primary UL reference signal sUL1 as received by the primary AP 111. Here, sDL1 may e.g. comprise a CSI-RS, be a modified CSI-RS, or be a new type of dedicated DL reference signal. Here, “sDL1 being based on” may mean that a pilot sequence of sDL1, i.e., an original reference sequence, is modified based on information of sUL1 as received by the primary AP 111. In other words, sDL1 may be pre-coded based on the primary UL reference signal sUL1 as received by the primary AP 111. In particular, the primary DL reference signal sDL1 may comprise a pilot sequence scaled by a first factor f1 derived from the primary UL reference signal sUL1 as received by the primary AP 111. Furthermore, the first factor f1 may be based on a magnitude and/or phase of the channel yAP1,UE1 from the wireless device 121 to the primary AP 111, where the channel yAP1,UE1 from the wireless device 121 to the primary AP 111 is based on the primary UL reference signal sUL1 as received by primary AP 111. Preferably, the pilot sequence of sDL1 is modified by dividing the pilot sequence by f1. It is assumed wireless device 121 is aware of the unmodified pilot sequence contained in sDL1. Thus, the wireless device 121 may estimate a first ratio yUE1,AP1/yAP1,UE1 from sDL1 as received by the primary AP. Here, yUE1,AP1 may be a baseband-to-baseband channel. However, it is also possible that y_UE1,AP1 does not comprise the full transmitter chain of the primary AP 111 and/or the full receiver chain of the wireless device 121. If f₁ is the phase and magnitude of y_AP1,UE1, the wireless device 121 may estimate phase and magnitude of the ratio y_UE1,AP1/y_AP1,UE1. If f₁ is the phase of y_AP1,UE1, the wireless device 121 may estimate phase of the ratio y_UE1,AP1/y_AP1,UE1. Action 1040. The method comprises transmitting, by the secondary AP 112, a secondary DL reference signal s_DL2 to the wireless device 121. Here, s_DL2 may e.g. comprise a CSI-RS, be a CSI-RS, or be a new type of dedicated DL reference signal. The wireless device 121 is aware of reference information contained in s_DL2 (such as a pilot sequence). Thus, the wireless device 121 may estimate the channel y_UE1,AP2 from the secondary AP 112 to the wireless device 121 from s_DL2 as received by the wireless device 121. This channel y_UE1,AP2 may be a baseband-to-baseband channel. However, it is also possible that the estimated channel does not comprise the full transmitter chain of the secondary AP 112 and/or the full receiver chain of the wireless device 121. Action 1050. The method comprises transmitting, by the wireless device 121, a secondary UL reference signal sUL2 to the secondary AP 112. The secondary UL reference signal sUL2 is based on the primary and the secondary DL reference signals sDL1, sDL2 as received by the wireless device 121. Here, sUL2 may e.g. comprise an SRS, be a modified SRS, or be a new type of dedicated DL reference signal. Here, “sUL2 being based on” may mean that a pilot sequence of sUL2, i.e., an original reference sequence, is modified based on information of sDL1, and sDL2 as received by the wireless device 121. In other words, sUL2 may be pre-coded based on the primary and the secondary DL reference signals sDL1, sDL2 as received by the wireless device 121. In particular, sUL2 may comprises a pilot sequence scaled by a second factor f2 derived from sDL1 as received by the wireless device 121 and by a third factor f3 derived from sDL2 as received by the wireless device 121. Furthermore, the second factor f2 may be based on a magnitude and/or phase of the first ratio yUE1,AP1/yAP1,UE1, which is discussed in connection to Action 1030. The third factor f3 may be based on a magnitude and/or phase of a channel yUE1,AP2 from the secondary AP 112 to the wireless device 121, which is discussed in connection to Action 1040. Thus, the first ratio may be based on sDL1 as received by the wireless device 121, and yUE1,AP2 may be based on sDL2 as received by the wireless device 121. Preferably, the pilot sequence of sUL2 is modified by multiplying the pilot sequence by f2 and by dividing the pilot sequence by f3. It is assumed that the secondary AP 112 is aware of the unmodified pilot sequence contained in sUL2. Thus, the secondary AP 112 may estimate a ratio (yAP2,UE1yUE1,AP1)/(yUE1,AP2yAP1,UE1) from sUL2 as received by the secondary AP. Here, yAP2,UE1 may be a baseband-to-baseband channel. However, it is also possible that y_AP2,UE1 does not comprise the full transmitter chain of the wireless device 121 and/or the full receiver chain of secondary AP 112. If f₂ is the phase and magnitude of the first ratio and f₃ is the phase and magnitude of y_UE1,AP2, the secondary AP may estimate phase and magnitude of the ratio (y_AP2,UE1 y_UE1,AP1)/(y_UE1,AP2y_AP1,UE1). If f₂ is the phase of the first ratio and f₃ is the phase of y_UE1,AP2, the secondary AP may estimate phase of the ratio (y_AP2,UE1 y_UE1,AP1)/(y_UE1,AP2y_AP1,UE1). It should be noted that Action 1030 and Action 1040 may be performed in any order or simultaneously. As mentioned, the reference signals s_DL1, s_DL2, s_DL1, and s_DL2 may comprise respective pilot signal sequences. The pilot signal sequence may be the same or different for each reference signal. It is assumed that the node (wireless device or AP) receiving the reference signal is aware of the pilot signal sequence of the received reference signal. As mentioned, in the actions of method 1000, the channel y_UE1,AP1 from the primary AP 111 to the wireless device 121, the channel y_AP1,UE1 from the wireless device 121 to the primary AP 111, the channel yUE1,AP2 from the secondary AP 112 to the wireless device 121, and the channel yAP2,UE1 from the wireless device 121 to the secondary AP 112 may be respective baseband-to-baseband channels. Each bi-directional measurement, i.e., each pair of an UL reference signal and a DL signal between two nodes, is preferably performed within the channel coherence time (e.g. one or more slots/sub-frames as defined in NR/6G) and channel coherence bandwidth (e.g. within a PRB) of the propagation channel between the two nodes. In particular, sUL1 and sDL1 may be transmitted within a channel coherence time and a channel coherence bandwidth of a propagation channel between the wireless device 121 and the primary AP 111. Similarly, sUL2 and sDL2 may transmitted within a channel coherence time and a channel coherence bandwidth of a propagation channel between the wireless device 121 and the secondary AP 112. In a standard specification, there might be a maximum time interval specified which indicates the maximum time interval difference between DL and UL reference signals of a propagation channel, where it is assumed that the propagation channel is coherent during that maximum time interval. Thus, the primary and the secondary UL reference signals sUL1, sUL2, and the primary and the secondary DL reference signals sDL1, sDL2 may be transmitted within a specified time interval. This specified time interval may e.g. be the maximum time interval in the standard. As mentioned, the primary and the secondary UL reference signals sUL1, sUL2 may comprise a respective SRS. Furthermore, the primary and the secondary DL reference signals sDL1, sDL2 may comprise a respective CSI-RS. Action 1070. The method comprises determining, based on the secondary UL reference signal s_UL2 as received by the secondary AP 112, a first calibration coefficient indicative of a channel y_AP2,AP1 from the primary AP 111 to the secondary AP 112 and of a channel y_AP1,AP2 from the secondary AP 112 to the primary AP 111. Since s_UL2 is based on s_DL1 and s_DL2 as received by the wireless device 121 (were s_DL1 is based on s_UL1 as received by the primary AP), information of y_AP2,AP1 and y_AP1,AP2 may be estimated from s_UL2 as received by the secondary AP. Thus, the first calibration coefficient may indicative of the ratio y_AP2,AP1/y_AP1,AP2. In other words, the the first calibration coefficient may be indicative of a function of y_AP2,AP1 and y_AP1,AP2. The channel y_AP2,AP1 from the primary AP 111 to the secondary AP 112 and the channel y_AP1,AP2 from the secondary AP 112 to the primary AP 111 may be respective baseband-to-baseband channels. However, it is also possible that y_AP2,AP1 does not comprise the full transmitter chain of the primary AP 111 and/or the full receiver chain of secondary AP 112, and that y_AP1,AP2 does not comprise the full transmitter chain of the secondary AP 112 and/or the full receiver chain of primary AP 111. In particular, if sUL2 comprises a pilot sequence multiplied by yUE1,AP1/yAP1,UE1 and divided by yUE1,AP2, the ratio (yAP2,UE1 yUE1,AP1)/(yUE1,AP2yAP1,UE1) may be estimated from sUL2 as received by the secondary AP 112, as is discussed above in connection to Action 1060. This ratio (yAP2,UE1 yUE1,AP1)/(yUE1,AP2yAP1,UE1) is the first calibration coefficient, which is equal to a ratio yAP1,AP2/yAP2,AP1. Thus, the ratio c2/c1 may be estimated from sUL2 as received by the secondary AP 112. As is discussed in connection to Equation (7), the ratio c2/c1 may be used for reciprocity calibration that enables the plurality of distributed APs to operating using TDD reciprocity-based CJTs. Action 1071. The method may comprise determining the first calibration coefficient by the secondary AP 112. Actions 1060 and 1072. More generally, information may be estimated form sUL2 as received by the secondary AP 112 using a node 140 comprised in the wireless communications network 100. This node may e.g. be a data processing unit 140 according to the discussions above. In this case, the method may comprise communicating 1060, by the secondary AP 112, information based on the secondary UL reference signal sUL2 as received by the secondary AP 112 to the node 140, and determining 1072, by the node 140, the first calibration coefficient based on the communicated information. Action 1080. With the first calibration coefficient determined, the method may comprise performing, by the node 140, a reciprocity calibration in the wireless communications network 100 based on the determined first calibration coefficient. In some embodiments, a resource block of the primary UL reference signal s_UL1 is associated with a resource block of the primary DL reference signal s_DL1, and a resource block of the secondary UL reference signal s_UL2 is associated with a resource block of the secondary DL reference signal s_DL2. This a way to differentiate between different DL reference signals. In some embodiments, a respective resource block of the primary and secondary UL reference signals s_UL1, s_UL2 is based on an UL resource identity, ID, and/or UL resource set ID, and wherein a respective resource block of the primary and secondary DL reference signals s_DL1, s_DL2 is based on a DL resource ID and/or DL resource set ID. This another way to differentiate between different DL reference signals. Actions 1021, 1031, 1041, and 1051. As mentioned, the wireless device 121 may be provided with multiple antennas. In particular, the wireless device 121, the primary AP 111, and the secondary AP 112 may each comprise respective first antenna elements and respective second antenna elements. The first antenna element and the second antenna element of any of wireless device 121, the primary AP 111, and the secondary AP 112, may be respective distributed physical structures, or share the same physical structure (e.g. a dually polarized patch antenna). In this case, the method may comprise Actions 1021, 1031, 1041, and 1051. Action 1021. The method may comprise transmitting the primary UL reference signal sUL1 as a first primary UL component sUL1,P1 and a second primary UL component sUL1,P2. The first primary UL component sUL1,P1 is transmitted by the first antenna element 811 of the wireless device 121, and the second primary UL component sUL1,P2 is transmitted by the second antenna element 812 of the wireless device 121. Action 1031. The method may further comprise transmitting the primary DL reference signal sDL1 as a first primary DL component sDL1,P1 and a second primary DL component sDL1,P2. The first primary DL component sDL1,P1 is transmitted by the first antenna element of the primary AP 111, and the second primary DL component sDL1,P2 is transmitted by the second antenna element of the primary AP 111. The first primary DL component sDL1,P1 is based on the first primary UL component sUL1,P1 as received by the first antenna element of the primary AP 111 and the second primary DL component sDL1,P2 is based on the second primary UL component sUL1,P2 as received by the second antenna element of the primary AP 111. Action 1041. The method may further comprise transmitting the secondary DL reference signal sDL2 as a first secondary DL component sDL2,P1 and a second secondary UL component sDL2,P2. The first secondary UL component sDL2,P1 is transmitted by the first antenna element of the secondary AP 112, and the second secondary UL component sDL2,P2 is transmitted by the second antenna element of the secondary AP 112. Action 1051. The method may further comprise transmitting the secondary UL reference signal s_UL2 as a first secondary UL component s_UL2,P1 and a second secondary UL component s_UL2,P2. The first secondary UL component s_UL2,P1 is transmitted by the first antenna element 811 of the wireless device 121, and the second secondary UL component s_UL2,P2 is transmitted by the second antenna element 812 of the wireless device 121. The first secondary UL component s_UL2,P1 is based on the first primary DL component s_DL1,P1 and the first secondary DL component s_DL2,P1 as received by the first antenna element 811 of the wireless device 121, and the second secondary UL component s_UL2,P2 is based on the second primary DL component s_DL1,P2 and the second secondary DL component s_DL2,P2 as received by the second antenna element 812 of the wireless device 121. Thereafter, the first calibration coefficient may be estimated from s_UL2 as received by the secondary AP 112 according to the discussions above (where s_UL2 now comprises the two components s_UL2,P1 and s_UL2,P2). Implementing Actions 1021, 1031, 1041, and 1051 provides more reliability in estimating the first calibration coefficient since two different antennas for each of the wireless device 121, the primary AP 111, and the secondary AP 112 are utilized. This may e.g. reduce the risk of polarization mismatch, particularly in case the antenna element 811 of the wireless device 121, and the second antenna element 812 of the wireless device 121 have different polarizations. In an alternative embodiment, the secondary AP 112 does not utilize two respective antenna elements during its UL transmission. In other words, the wireless device 121 and the primary AP 111 each comprises respective first antenna elements and respective second antenna elements. In that case, the method may perform Actions 1021, 1031, and 1052 (where 1021 and 1031 remain the same as above. After performing actions 1021, 1031, the method performs Action 1052, which is discussed below. Action 1052. The method may comprise transmitting the secondary UL reference signal sUL2 as a first secondary UL component sUL2,P1 and a second secondary UL component sUL2,P2. The first secondary UL component sUL2,P1 is transmitted by the first antenna element 811 of the wireless device 121, and the second secondary UL component sUL2,P2 is transmitted by the second antenna element 812 of the wireless device 121. The first secondary UL component sUL2,P1 is based on the first primary DL component sDL1,P1 as received by the first antenna element 811 of the wireless device 121 and on the secondary DL reference signal sDL2 as received by the first and/or second antenna elements of the wireless device 121. The second secondary UL component sUL2,P2 is based on the second primary DL component sDL1,P2 as received by the second antenna element 812 of the wireless device 121 and on the secondary DL reference signal sDL2 as received by the first and/or second antenna elements of the wireless device 121. Thereafter, the first calibration coefficient may be estimated from sUL2 as received by the secondary AP 112 according to the discussions above (where s_UL2 now comprises the two components s_UL2,P1 and s_UL2,P2). Implementing Actions 1021, 1031, and 1052 requires less computational overhead compared to Actions 1021, 1031, 1041, and 1051, but still provides some improvements in reliability in the estimation of the first calibration coefficient. With reference to Figure 11, there is disclosed herein a method 1100 performed by a wireless device 121 for enabling reciprocity calibration in a wireless communications network 100. The wireless communications network 100 comprises a plurality of distributed access points (APs), where the plurality of distributed APs comprises a primary AP 111 and a secondary AP 112. The method 1100 may comprise a number of actions, which are discussed below. Action 1130. The method comprises receiving a primary downlink (DL) reference signal s_DL1 transmitted by the primary AP 111. Action 1130. The method comprises receiving a secondary DL reference signal s_DL2 transmitted by the secondary AP 112. Action 1130. The method comprises transmitting a secondary uplink (UL) reference signal sUL2 to the secondary AP 112. The secondary UL reference signal sUL2 is based on the primary and the secondary DL reference signals sDL1, sDL2 as received by the wireless device 121. Since sUL2 is based on sDL1 and sDL2 as received by the wireless device 121, information of yAP2,AP1 and yAP1,AP2 may be estimated from sUL2 as received by the secondary AP 112. Consequently, the method 1100 enables a calibration procedure according to method 1000. Thus, the method 1100 enables a reciprocity calibration that enables the plurality of distributed APs to operate using TDD reciprocity-based CJTs. The reference signals transmitted and received by the wireless device 121 in method 1100 may be configured in the same ways as discussed in connection to method 1000. In some embodiments, the secondary UL reference signal sDL2 comprises a pilot sequence scaled by a second factor f2 derived from the primary DL reference signal sDL1 as received by the wireless device 121 and by a third factor f3 derived from the secondary DL reference signal sDL2 as received by the wireless device 121. In some embodiments, the primary UL reference signal sUL1 is received and the primary DL reference signal sDL1 is transmitted within a channel coherence time and a channel coherence bandwidth of a propagation channel between the wireless device 121 and the primary AP 111. In some embodiments, the secondary UL reference signal sUL1 and the primary and the secondary DL reference signals sDL1, sDL2 are transmitted and received, respectively, within a specified time interval. In some embodiments, the secondary UL reference signal s_UL2 comprises a Sounding Reference Signal, SRS. In some embodiments, the primary and the secondary DL reference signals s_DL1, s_DL2 comprise a respective Channels State Information-Reference Signal (CSI-RS). In some embodiments, a resource block of the secondary UL reference signal s_UL2 is associated with a resource block of the secondary DL reference signal s_DL2. In some embodiments, the method 1100 comprises reporting 1010 a capability report of the wireless device 121 to any AP of the plurality of distributed APs, where the capability report is indicative of the wireless device 121 being capable of transmitting the secondary UL reference signal s_UL2 to the secondary AP 112. In some embodiments, the wireless device 121 is capable of transmitting a first number of secondary UL reference signals to respective secondary APs comprised in the plurality of distributed APs. Each secondary UL reference signal of the first number of secondary UL reference signals is based on the primary DL reference signal sDL1 and an associated secondary DL reference signal as received by the wireless device 121. The associated secondary DL reference signal is associated with the respective secondary AP that the respective UL reference signal is transmitted to, and wherein the capability report comprises the first number. In some embodiments, the first number is associated with a corresponding number of SRS resources for each secondary UL reference signal of the first number of secondary UL reference signals and with a corresponding number of CSI-RS resources for each corresponding secondary DL reference signal. In some embodiments, the capability report is a quantity value in a report defined by a Channel State Information (CSI) Information Element (IE). With reference to Figure 12, there is disclosed herein a method 1200 performed by a primary access point (AP) 110 for enabling reciprocity calibration in a wireless communications network 100. The wireless communications network 100 comprising a plurality of distributed APs and a wireless device 121. The method 1200 may comprise a number of actions, which are discussed below. Action 1220. The method comprises receiving a primary uplink (UL) reference signal sUL1 transmitted by the wireless device 121. Action 1220. The method comprises transmitting a primary downlink (DL) reference signal s_DL1 to the wireless device 121. The primary DL reference signal s_DL1 is based on the primary UL reference signal s_UL1 as received by the primary AP 111. Since s_DL1 is based on s_UL1 as received by the primary AP 11, information of information of y_UE1,AP1 and y_AP1,UE1 may be estimated from s_DL1 is as received by the wireless device 121. Consequently, the method 1200 enables a calibration procedure according to method 1000. Thus, the method 1200 enables a reciprocity calibration that enables the plurality of distributed APs to operate using TDD reciprocity-based CJTs. The reference signals transmitted and received by the primary AP 111 in method 1200 may be configured in the same ways as discussed in connection to method 1000. In some embodiments, the primary DL reference signal s_DL1 comprises a pilot sequence scaled by a first factor f₁ derived from the primary UL reference signal s_UL1 as received by the primary AP 111. In some embodiments, the first factor f1 is based on a magnitude and/or phase of a channel yAP1,UE1 from the wireless device 121 to the primary AP 111, and the channel yAP1,UE1 from the wireless device 121 to the primary AP 111 is based on the primary UL reference signal sUL1 as received by primary AP 111. In some embodiments, channel yAP1,UE1 from the wireless device 121 to the primary AP 111 is a baseband-to-baseband channel. In some embodiments, the primary reference signal sUL1 and the primary DL reference signal sDL1 are received and transmitted, respectively, within a channel coherence time and a channel coherence bandwidth of a propagation channel between the wireless device 121 and the primary AP 111. In some embodiments, the primary UL reference signal sUL1 and the primary DL reference signal sDL1 are received and transmitted, respectively, within a specified time interval. In some embodiments, the primary UL reference signal sUL1 comprise a Sounding Reference Signal, SRS. In some embodiments, the primary DL reference signal sDL1 comprise a Channels State Information-Reference Signal (CSI-RS). In some embodiments, a resource block of the primary UL reference signal sUL1 is associated with a resource block of the primary DL reference signal sDL1. There is also disclosed herein a wireless communications network 100 for enabling reciprocity calibration in a wireless communications network 100. The wireless communications network 100 comprises a wireless device 121 and a plurality of distributed access points (APs), where the plurality of distributed APs comprises a primary AP 111 and a secondary AP 112. The wireless communications network 100 is configured to transmit, by the wireless device 121, a primary uplink (UL) reference signal s_UL1 to the primary AP 111. The wireless communications network 100 is further configured to transmit, by the primary AP 111, a primary downlink (DL) reference signal s_DL1 to the wireless device 121, where the primary DL reference signal s_DL1 is based on the primary UL reference signal (s_UL1) as received by the primary AP 111. The wireless communications network 100 is further configured to transmit, by the secondary AP 112, a secondary DL reference signal s_DL2 to the wireless device 121. The wireless communications network 100 is further configured to transmit, by the wireless device 121, a secondary UL reference signal s_UL2 to the secondary AP 112, where the secondary UL reference signal sUL2 is based on the primary and the secondary DL reference signals sDL1, sDL2 as received by the wireless device 121. The wireless communications network 100 is further configured to determine, based on the secondary UL reference signal sUL2 as received by the secondary AP 112, a first calibration coefficient indicative of a channel yAP2,AP1 from the primary AP 111 to the secondary AP 112 and of a channel yAP1,AP2 from the secondary AP 112 to the primary AP 111. Since sUL2 is based on sDL1 and sDL2 as received by the wireless device 121 (were sDL1 is based on sUL1 as received by the primary AP), information of yAP2,AP1 and yAP1,AP2 may be estimated from sUL2 as received by the secondary AP. In particular, if sUL2 comprises a pilot sequence multiplied by yUE1,AP1/yAP1,UE1 and divided by yUE1,AP2, the ratio (yAP2,UE1 yUE1,AP1)/(yUE1,AP2yAP1,UE1) may be estimated from sUL2 as received by the secondary AP 112. This ratio (yAP2,UE1 yUE1,AP1)/(yUE1,AP2yAP1,UE1) is the first calibration coefficient, which is equal to a ratio yAP1,AP2/yAP2,AP1. Thus, the ratio c2/c1 may be estimated from sUL2 as received by the secondary AP 112. As is discussed in connection to Equation (7), the ratio c2/c1 may be used for reciprocity calibration that enables the plurality of distributed APs to operating using TDD reciprocity-based CJTs. In some embodiments, the channel yAP2,AP1 from the primary AP 111 to the secondary AP 112 and the channel yAP1,AP2 from the secondary AP 112 to the primary AP 111 are respective baseband-to-baseband channels. In some embodiments, the primary DL reference signal sDL1 comprises a pilot sequence scaled by a first factor f₁ derived from the primary UL reference signal s_UL1 as received by the primary AP 111. In some embodiments, the first factor f₁ is based on a magnitude and/or phase of a channel y_AP1,UE1 from the wireless device 121 to the primary AP 111, and the channel y_AP1,UE1 from the wireless device 121 to the primary AP 111 is based on the primary UL reference signal s_UL1 as received by primary AP 111. In some embodiments, the secondary UL reference signal s_UL2 comprises a pilot sequence scaled by a second factor f₂ derived from the primary DL reference signal s_DL1 as received by the wireless device 121 and by a third factor f₃ derived from the secondary DL reference signal s_DL2 as received by the wireless device 121. In some embodiments, the second factor f₂ is based on a magnitude and/or phase of a first ratio y_UE1,AP1/y_AP1,UE1 of a channel y_UE1,AP1 from the primary AP 111 to the wireless device 121 over an channel y_AP1,UE1 from the wireless device 121 to the primary AP 111. The third factor f₃ is based on a magnitude and/or phase of a channel y _UE1,AP2 from the secondary AP 112 to the wireless device 121. The first ratio is based on the primary DL reference signal sDL1 as received by the wireless device 121. The channel yUE1,AP2 from the secondary AP 112 to the wireless device 121 is based on the secondary DL reference signal sDL2 as received by the wireless device 121. In some embodiments, the channel yUE1,AP1 from the primary AP 111 to the wireless device 121, the channel yAP1,UE1 from the wireless device 121 to the primary AP 111, and the channel yUE1,AP2 from the secondary AP 112 to the wireless device 121 are respective baseband-to- baseband channels. In some embodiments, the secondary AP 112 is configured to determine the first calibration coefficient. In some embodiments, the wireless communications network 100 comprises a node 140. The secondary AP 112 is configured to communicate information based on the secondary UL reference signal sUL2 as received by the secondary AP 112 to the node 140. The node 140 is configured to determine the first calibration coefficient based on the communicated information. In some embodiments, the node 140 is configured to perform a reciprocity calibration in the wireless communications network 100 based on the determined first calibration coefficient. In some embodiments, the primary UL reference signal sUL1 and the primary DL reference signal sDL1 are transmitted within a channel coherence time and a channel coherence bandwidth of a propagation channel between the wireless device 121 and the primary AP 111. The secondary UL reference signal s_UL2 and the secondary DL reference signal s_DL2 are transmitted within a channel coherence time and a channel coherence bandwidth of a propagation channel between the wireless device 121 and the secondary AP 112. In some embodiments, the wireless device 121 and the primary and the secondary APs 111, 112 are configured to transmit the primary and the secondary UL reference signals s_UL1, s_UL2 and the primary and the secondary DL reference signals s_DL1, s_DL2 within a specified time interval. In some embodiments, the primary and the secondary UL reference signals (s_UL1, s_UL2) comprise a respective Sounding Reference Signal (SRS). In some embodiments, the primary and the secondary DL reference signals s_DL1, s_DL2 comprise a respective Channels State Information-Reference Signal, CSI-RS. In some embodiments, a resource block of the primary UL reference signal s_UL1 is associated with a resource block of the primary DL reference signal s_DL1, and a resource block of the secondary UL reference signal sUL2 is associated with a resource block of the secondary DL reference signal sDL2. In some embodiments, a respective resource block of the primary and secondary UL reference signals sUL1, sUL2 is based on an UL resource identity (ID) and/or UL resource set ID, and wherein a respective resource block of the primary and secondary DL reference signals sDL1, sDL2 is based on a DL resource ID and/or DL resource set ID. In some embodiments, the wireless device 121 is configured to report a capability report of the wireless device 121 to any AP of the plurality of distributed APs. The capability report is indicative of the wireless device 121 being capable of transmitting the secondary UL reference signal sUL2 to the secondary AP 112. In some embodiments, the wireless device 121 is capable of transmitting a first number of secondary UL reference signals to respective secondary APs comprised in the plurality of distributed APs. Each secondary UL reference signal of the first number of secondary UL reference signals is based on the primary DL reference signal sDL1 and an associated secondary DL reference signal as received by the wireless device 121. The associated secondary DL reference signal is associated with the respective secondary AP that the respective UL reference signal is transmitted to, and wherein the capability report comprises the first number. In some embodiments, the first number is associated with a corresponding number of SRS resources for each secondary UL reference signal of the first number of secondary UL reference signals and with a corresponding number of CSI-RS resources for each corresponding secondary DL reference signal. In some embodiments, the capability report is a quantity value in a report defined by a Channel State Information (CSI) Information Element (IE). With reference to Figure 13, there is disclosed herein a wireless device 121 for enabling reciprocity calibration in a wireless communications network 100. The wireless communications network 100 comprises a plurality of distributed access points (APs), where the plurality of distributed APs comprises a primary AP 111 and a secondary AP 112. Figure 13 shows a schematic block diagram of embodiments of the wireless device 121. The embodiments of the wireless device 121 may be considered as independent embodiments or may be considered in any combination with each other. It should also be noted that, although not shown in Figure 13, the wireless device 121 may comprise known conventional features for such device, such as a power source like a battery or main connection. The conventional features may also be, e.g., an antenna arrangement. The wireless device 121 may comprise processing circuitry 1310 and a memory 1320. The processing circuitry 1310 may, in turn, comprise a receiving module 1311 and a transmitting module 1312. The receiving module 1311 and the transmitting module 1312 may comprise radio frequency circuitry and baseband processing circuitry capable of transmitting and receiving a radio signal in the wireless communications network 100. The receiving module 1311 and the transmitting module 1312 may also form part of a single transceiver. It should also be noted that some or all of the functionality described in the embodiments above as being performed by the wireless device 121 may be provided by the processing circuitry 1310 executing instructions stored on a computer-readable medium, such as, e.g. the memory 1320 shown in Figure 13. Alternative embodiments of the wireless device 121 may comprise additional components, such as, a determining module 1313 responsible for providing functionality to support the embodiments of the wireless device 121 described herein. The wireless device 121 or processing circuitry 1310 is configured to receive a primary downlink (DL) reference signal sDL1 transmitted by the primary AP 111, receive a secondary DL reference signal sDL2 transmitted by the secondary AP 112, and transmit a secondary uplink (UL) reference signal sUL2 to the secondary AP 112. The secondary UL reference signal sUL2 is based on the primary and the secondary DL reference signals sDL1, sDL2 as received by the wireless device 121. In some embodiments, the secondary UL reference signal sDL2 comprises a pilot sequence scaled by a second factor f₂ derived from the primary DL reference signal s_DL1 as received by the wireless device 121 and by a third factor f₃ derived from the secondary DL reference signal s_DL2 as received by the wireless device 121. In some embodiments, the wireless device 121 or processing circuitry 1310 is configured to transmit the secondary UL reference signal s_UL2 and receive the primary and the secondary DL reference signals s_DL1, s_DL2 within a channel coherence time and a channel coherence bandwidth of a propagation channel between the wireless device 121 and the primary AP 111. In some embodiments, the wireless device 121 or processing circuitry 1310 is configured to transmit the secondary UL reference signal s_UL2 and receive the primary and the secondary DL reference signals s_DL1, s_DL2 within a specified time interval. In some embodiments the secondary UL reference signal s_UL2 comprise a Sounding Reference Signal (SRS). In some embodiments the primary and the secondary DL reference signals s_DL1, s_DL2 comprise a respective Channels State Information-Reference Signal (CSI-RS). In some embodiments a resource block of the secondary UL reference signal s_UL2 is associated with a resource block of the secondary DL reference signal sDL2. In some embodiments the wireless device 121 or processing circuitry 1310 is configured to report a capability report of the wireless device 121 to any AP of the plurality of distributed APs. The capability report is indicative of the wireless device 121 being capable of transmitting the secondary UL reference signal sUL2 to the secondary AP 112. In some embodiments, the wireless device 121 is capable of transmitting a first number of secondary UL reference signals to respective secondary APs comprised in the plurality of distributed APs, where each secondary UL reference signal of the first number of secondary UL reference signals is based on the primary DL reference signal sDL1 and an associated secondary DL reference signal as received by the wireless device 121. The associated secondary DL reference signal is associated with the respective secondary AP that the respective UL reference signal is transmitted to, and wherein the capability report comprises the first number. In some embodiments, the first number is associated with a corresponding number of SRS resources for each secondary UL reference signal of the first number of secondary UL reference signals and with a corresponding number of CSI-RS resources for each corresponding secondary DL reference signal. In some embodiments the capability report is a quantity value in a report defined by a Channel State Information (CSI) Information Element (IE). With reference to Figure 14, there is disclosed herein a primary access point (AP) 111 for enabling reciprocity calibration in a wireless communications network 100. The wireless communications network 100 comprises a plurality of distributed APs and a wireless device 121. Figure 14 shows a schematic block diagram of embodiments of the primary AP 111. The embodiments of the primary AP 111may be considered as independent embodiments or may be considered in any combination with each other. It should also be noted that, although not shown in Figure 14, the primary AP 111 may comprise known conventional features for such device, such as a power source like a battery or main connection. The conventional features may also be, e.g., an antenna arrangement. The primary AP 111 may comprise processing circuitry 1410 and a memory 1420. The processing circuitry 1410 may, in turn, comprise a receiving module 1411 and a transmitting module 1412. The receiving module 1411 and the transmitting module 1412 may comprise radio frequency circuitry and baseband processing circuitry capable of transmitting and receiving a radio signal in the wireless communications network 100. The receiving module 1411 and the transmitting module 1412 may also form part of a single transceiver. It should also be noted that some or all of the functionality described in the embodiments above as being performed by the primary AP 111 may be provided by the processing circuitry 1410 executing instructions stored on a computer-readable medium, such as, e.g. the memory 1420 shown in Figure 14. Alternative embodiments of the primary AP 111may comprise additional components, such as, a determining module 1413 responsible for providing functionality to support the embodiments of the primary AP 111 described herein. The primary AP 111 or processing circuitry 1410 is configured to receive a primary uplink (UL) reference signal sUL1 transmitted by the wireless device 121, and transmit a primary downlink (DL) reference signal sDL1 to the wireless device 121, where the primary DL reference signal sDL1 is based on the primary UL reference signal sUL1 as received by the primary AP 111. In some embodiments, the primary DL reference signal sDL1 comprises a pilot sequence scaled by a first factor f1 derived from the primary UL reference signal sUL1 as received by the primary AP 111. In some embodiments, the first factor f1 is based on a magnitude and/or phase of a channel yAP1,UE1 from the wireless device 121 to the primary AP 111, and wherein the channel yAP1,UE1 from the wireless device 121 to the primary AP 111 is based on the primary UL reference signal sUL1 as received by primary AP 111. In some embodiments, the channel yAP1,UE1 from the wireless device 121 to the primary AP 111 is a baseband-to-baseband channel. In some embodiments, the primary AP 111 or processing circuitry 1410 is configured to receive and transmit the primary reference signal s_UL1 and the primary DL reference signal s_DL1, respectively, within a channel coherence time and a channel coherence bandwidth of a propagation channel between the wireless device 121 and the primary AP 111. In some embodiments, the primary AP 111 or processing circuitry 1410 is configured to receive and transmit the primary UL reference signal s_UL1 and the primary DL reference signal s_DL1, respectively, within a specified time interval. In some embodiments, the primary UL reference signal s_UL1 comprise a Sounding Reference Signal (SRS). In some embodiments, the primary DL reference signal s_DL1 comprise a Channels State Information-Reference Signal (CSI-RS). In some embodiments, a resource block of the primary UL reference signal s_UL1 is associated with a resource block of the primary DL reference signal s_DL1. Figure 15 shows a schematic block diagram of embodiments of a node 140. The node 140 may be a data processing unit 140 coordinating the plurality of distributed APs. The embodiments of the node 140 may be considered as independent embodiments or may be considered in any combination with each other. It should also be noted that, although not shown in Figure 15, the node 140 may comprise known conventional features for such device, such as a power source like a battery or main connection. The node 140 may comprise processing circuitry 1510 and a memory 1520. The processing circuitry 1510 may, in turn, comprise a receiving module 1511 and a transmitting module 1512. The receiving module 1511 and the transmitting module 1512 may comprise radio frequency circuitry and baseband processing circuitry capable of transmitting and receiving a radio signal in the wireless communications network 100. The receiving module 1511 and the transmitting module 1512 may also form part of a single transceiver. It should also be noted that some or all of the functionality described in the embodiments above as being performed by the node 140 may be provided by the processing circuitry 1510 executing instructions stored on a computer-readable medium, such as, e.g. the memory 1520 shown in Figure 15. Alternative embodiments of the node 140 may comprise additional components, such as, a determining module 1513 responsible for providing functionality to support the embodiments of the node 140 described herein. The methods disclosed herein may be implemented through one or more processors – such as the processing circuitry 1310 in the wireless device 121 depicted in Figure 13, the processing circuitry 1410 in the primary AP 111 depicted in Figure 14, or the processing circuitry 1510 in the node 140 depicted in Figure 15 – together with computer program code for performing the functions and actions of the embodiments herein. The program code may also be provided as a computer program product, for instance in the form of a data carrier carrying computer program code or code means for performing the embodiments herein when being loaded into the processing circuitry 1310 in the wireless device 121, into the processing circuitry 1410 in the primary AP 111, and/or into the processing circuitry 1510 in the node 140. The computer program code may e.g. be provided as pure program code in the wireless device 121, the primary AP 111, and/or the node 140, or on a server and downloaded to the wireless device 121, the primary AP 111, and/or the node 140. Thus, it should be noted that the modules of the wireless device 121, the primary AP 111, and/or the node 140 may in some embodiments be implemented as computer programs stored in memory, e.g. in the memory modules 1320, 1420, and/or 1520, for execution by respective processors or processing modules, e.g. the processing circuitry 1310, 1410, and/or 1510. Those skilled in the art will also appreciate that the processing circuitry 1310, 1410, and/or 1510 and the memory 1320, 1420, and/or 1520 described above may refer to a combination of analog and digital circuits, and/or one or more processors configured with software and/or firmware, e.g. stored in a memory, that when executed by the one or more processors such as the processing circuitry 1310, 1410, and/or 1510 perform as described above. One or more of these processors, as well as the other digital hardware, may be included in a single application- specific integrated circuit (ASIC), or several processors and various digital hardware may be distributed among several separate components, whether individually packaged or assembled into a system-on-a-chip (SoC). Figure 16 shows an example of a communication system 1600 in accordance with some embodiments. In the example, the communication system 1600 includes a telecommunication network 1602 that includes an access network 1604, such as a radio access network (RAN), and a core network 1606, which includes one or more core network nodes 1608. The access network 1604 includes one or more access network nodes, such as network nodes 1610a and 1610b (one or more of which may be generally referred to as network nodes 1610), or any other similar 3^rd Generation Partnership Project (3GPP) access node or non-3GPP access point. The network nodes 1610 facilitate direct or indirect connection of user equipment (UE), such as by connecting UEs 1612a, 1612b, 1612c, and 1612d (one or more of which may be generally referred to as UEs 1612) to the core network 1606 over one or more wireless connections. Example wireless communications over a wireless connection include transmitting and/or receiving wireless signals using electromagnetic waves, radio waves, infrared waves, and/or other types of signals suitable for conveying information without the use of wires, cables, or other material conductors. Moreover, in different embodiments, the communication system 1600 may include any number of wired or wireless networks, network nodes, UEs, and/or any other components or systems that may facilitate or participate in the communication of data and/or signals whether via wired or wireless connections. The communication system 1600 may include and/or interface with any type of communication, telecommunication, data, cellular, radio network, and/or other similar type of system. The UEs 1612 may be any of a wide variety of communication devices, including wireless devices arranged, configured, and/or operable to communicate wirelessly with the network nodes 1610 and other communication devices. Similarly, the network nodes 1610 are arranged, capable, configured, and/or operable to communicate directly or indirectly with the UEs 1612 and/or with other network nodes or equipment in the telecommunication network 1602 to enable and/or provide network access, such as wireless network access, and/or to perform other functions, such as administration in the telecommunication network 1602. In the depicted example, the core network 1606 connects the network nodes 1610 to one or more hosts, such as host 1616. These connections may be direct or indirect via one or more intermediary networks or devices. In other examples, network nodes may be directly coupled to hosts. The core network 1606 includes one more core network nodes (e.g., core network node 1608) that are structured with hardware and software components. Features of these components may be substantially similar to those described with respect to the UEs, network nodes, and/or hosts, such that the descriptions thereof are generally applicable to the corresponding components of the core network node 1608. Example core network nodes include functions of one or more of a Mobile Switching Center (MSC), Mobility Management Entity (MME), Home Subscriber Server (HSS), Access and Mobility Management Function (AMF), Session Management Function (SMF), Authentication Server Function (AUSF), Subscription Identifier De-concealing function (SIDF), Unified Data Management (UDM), Security Edge Protection Proxy (SEPP), Network Exposure Function (NEF), and/or a User Plane Function (UPF). The host 1616 may be under the ownership or control of a service provider other than an operator or provider of the access network 1604 and/or the telecommunication network 1602, and may be operated by the service provider or on behalf of the service provider. The host 1616 may host a variety of applications to provide one or more service. Examples of such applications include live and pre-recorded audio/video content, data collection services such as retrieving and compiling data on various ambient conditions detected by a plurality of UEs, analytics functionality, social media, functions for controlling or otherwise interacting with remote devices, functions for an alarm and surveillance center, or any other such function performed by a server. As a whole, the communication system 1600 of Figure 16 enables connectivity between the UEs, network nodes, and hosts. In that sense, the communication system may be configured to operate according to predefined rules or procedures, such as specific standards that include, but are not limited to: Global System for Mobile Communications (GSM); Universal Mobile Telecommunications System (UMTS); Long Term Evolution (LTE), and/or other suitable 2G, 3G, 4G, 5G standards, or any applicable future generation standard (e.g., 6G); wireless local area network (WLAN) standards, such as the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards (WiFi); and/or any other appropriate wireless communication standard, such as the Worldwide Interoperability for Microwave Access (WiMax), Bluetooth, Z- Wave, Near Field Communication (NFC) ZigBee, LiFi, and/or any low-power wide-area network (LPWAN) standards such as LoRa and Sigfox. In some examples, the telecommunication network 1602 is a cellular network that implements 3GPP standardized features. Accordingly, the telecommunications network 1602 may support network slicing to provide different logical networks to different devices that are connected to the telecommunication network 1602. For example, the telecommunications network 1602 may provide Ultra Reliable Low Latency Communication (URLLC) services to some UEs, while providing Enhanced Mobile Broadband (eMBB) services to other UEs, and/or Massive Machine Type Communication (mMTC)/Massive IoT services to yet further UEs. In some examples, the UEs 1612 are configured to transmit and/or receive information without direct human interaction. For instance, a UE may be designed to transmit information to the access network 1604 on a predetermined schedule, when triggered by an internal or external event, or in response to requests from the access network 1604. Additionally, a UE may be configured for operating in single- or multi-RAT or multi-standard mode. For example, a UE may operate with any one or combination of Wi-Fi, NR (New Radio) and LTE, i.e. being configured for multi-radio dual connectivity (MR-DC), such as E-UTRAN (Evolved-UMTS Terrestrial Radio Access Network) New Radio – Dual Connectivity (EN-DC). In the example, the hub 1614 communicates with the access network 1604 to facilitate indirect communication between one or more UEs (e.g., UE 1612c and/or 1612d) and network nodes (e.g., network node 1610b). In some examples, the hub 1614 may be a controller, router, content source and analytics, or any of the other communication devices described herein regarding UEs. For example, the hub 1614 may be a broadband router enabling access to the core network 1606 for the UEs. As another example, the hub 1614 may be a controller that sends commands or instructions to one or more actuators in the UEs. Commands or instructions may be received from the UEs, network nodes 1610, or by executable code, script, process, or other instructions in the hub 1614. As another example, the hub 1614 may be a data collector that acts as temporary storage for UE data and, in some embodiments, may perform analysis or other processing of the data. As another example, the hub 1614 may be a content source. For example, for a UE that is a VR headset, display, loudspeaker or other media delivery device, the hub 1614 may retrieve VR assets, video, audio, or other media or data related to sensory information via a network node, which the hub 1614 then provides to the UE either directly, after performing local processing, and/or after adding additional local content. In still another example, the hub 1614 acts as a proxy server or orchestrator for the UEs, in particular in if one or more of the UEs are low energy IoT devices. The hub 1614 may have a constant/persistent or intermittent connection to the network node 1610b. The hub 1614 may also allow for a different communication scheme and/or schedule between the hub 1614 and UEs (e.g., UE 1612c and/or 1612d), and between the hub 1614 and the core network 1606. In other examples, the hub 1614 is connected to the core network 1606 and/or one or more UEs via a wired connection. Moreover, the hub 1614 may be configured to connect to an M2M service provider over the access network 1604 and/or to another UE over a direct connection. In some scenarios, UEs may establish a wireless connection with the network nodes 1610 while still connected via the hub 1614 via a wired or wireless connection. In some embodiments, the hub 1614 may be a dedicated hub – that is, a hub whose primary function is to route communications to/from the UEs from/to the network node 1610b. In other embodiments, the hub 1614 may be a non-dedicated hub – that is, a device which is capable of operating to route communications between the UEs and network node 1610b, but which is additionally capable of operating as a communication start and/or end point for certain data channels. Figure 17 is a block diagram of a host 1700, which may be an embodiment of the host 1616 of Figure 16, in accordance with various aspects described herein. As used herein, the host 1700 may be or comprise various combinations hardware and/or software, including a standalone server, a blade server, a cloud-implemented server, a distributed server, a virtual machine, container, or processing resources in a server farm. The host 1700 may provide one or more services to one or more UEs. The host 1700 includes processing circuitry 1702 that is operatively coupled via a bus 1704 to an input/output interface 1706, a network interface 1708, a power source 1710, and a memory 1712. Other components may be included in other embodiments. Features of these components may be substantially similar to those described with respect to the devices of previous figures, such as Figures QQ2 and QQ3, such that the descriptions thereof are generally applicable to the corresponding components of host 1700. The memory 1712 may include one or more computer programs including one or more host application programs 1714 and data 1716, which may include user data, e.g., data generated by a UE for the host 1700 or data generated by the host 1700 for a UE. Embodiments of the host 1700 may utilize only a subset or all of the components shown. The host application programs 1714 may be implemented in a container-based architecture and may provide support for video codecs (e.g., Versatile Video Coding (VVC), High Efficiency Video Coding (HEVC), Advanced Video Coding (AVC), MPEG, VP9) and audio codecs (e.g., FLAC, Advanced Audio Coding (AAC), MPEG, G.711), including transcoding for multiple different classes, types, or implementations of UEs (e.g., handsets, desktop computers, wearable display systems, heads-up display systems). The host application programs 1714 may also provide for user authentication and licensing checks and may periodically report health, routes, and content availability to a central node, such as a device in or on the edge of a core network. Accordingly, the host 1700 may select and/or indicate a different host for over-the-top services for a UE. The host application programs 1714 may support various protocols, such as the HTTP Live Streaming (HLS) protocol, Real-Time Messaging Protocol (RTMP), Real-Time Streaming Protocol (RTSP), Dynamic Adaptive Streaming over HTTP (MPEG-DASH), etc. Figure 18 shows a communication diagram of a host 1802 communicating via a network node 1804 with a UE 1806 over a partially wireless connection in accordance with some embodiments. Example implementations, in accordance with various embodiments, of the UE (such as a UE 1612a of Figure 16 and/or UE QQ200 of Figure QQ2), network node (such as network node 1610a of Figure 16 and/or network node QQ300 of Figure QQ3), and host (such as host 1616 of Figure 16 and/or host 1700 of Figure 17) discussed in the preceding paragraphs will now be described with reference to Figure 18. Like host 1700, embodiments of host 1802 include hardware, such as a communication interface, processing circuitry, and memory. The host 1802 also includes software, which is stored in or accessible by the host 1802 and executable by the processing circuitry. The software includes a host application that may be operable to provide a service to a remote user, such as the UE 1806 connecting via an over-the-top (OTT) connection 1850 extending between the UE 1806 and host 1802. In providing the service to the remote user, a host application may provide user data which is transmitted using the OTT connection 1850. The network node 1804 includes hardware enabling it to communicate with the host 1802 and UE 1806. The connection 1860 may be direct or pass through a core network (like core network 1606 of Figure 16) and/or one or more other intermediate networks, such as one or more public, private, or hosted networks. For example, an intermediate network may be a backbone network or the Internet. The UE 1806 includes hardware and software, which is stored in or accessible by UE 1806 and executable by the UE’s processing circuitry. The software includes a client application, such as a web browser or operator-specific “app” that may be operable to provide a service to a human or non-human user via UE 1806 with the support of the host 1802. In the host 1802, an executing host application may communicate with the executing client application via the OTT connection 1850 terminating at the UE 1806 and host 1802. In providing the service to the user, the UE's client application may receive request data from the host's host application and provide user data in response to the request data. The OTT connection 1850 may transfer both the request data and the user data. The UE's client application may interact with the user to generate the user data that it provides to the host application through the OTT connection 1850. The OTT connection 1850 may extend via a connection 1860 between the host 1802 and the network node 1804 and via a wireless connection 1870 between the network node 1804 and the UE 1806 to provide the connection between the host 1802 and the UE 1806. The connection 1860 and wireless connection 1870, over which the OTT connection 1850 may be provided, have been drawn abstractly to illustrate the communication between the host 1802 and the UE 1806 via the network node 1804, without explicit reference to any intermediary devices and the precise routing of messages via these devices. As an example of transmitting data via the OTT connection 1850, in step 1808, the host 1802 provides user data, which may be performed by executing a host application. In some embodiments, the user data is associated with a particular human user interacting with the UE 1806. In other embodiments, the user data is associated with a UE 1806 that shares data with the host 1802 without explicit human interaction. In step 1810, the host 1802 initiates a transmission carrying the user data towards the UE 1806. The host 1802 may initiate the transmission responsive to a request transmitted by the UE 1806. The request may be caused by human interaction with the UE 1806 or by operation of the client application executing on the UE 1806. The transmission may pass via the network node 1804, in accordance with the teachings of the embodiments described throughout this disclosure. Accordingly, in step 1812, the network node 1804 transmits to the UE 1806 the user data that was carried in the transmission that the host 1802 initiated, in accordance with the teachings of the embodiments described throughout this disclosure. In step 1814, the UE 1806 receives the user data carried in the transmission, which may be performed by a client application executed on the UE 1806 associated with the host application executed by the host 1802. In some examples, the UE 1806 executes a client application which provides user data to the host 1802. The user data may be provided in reaction or response to the data received from the host 1802. Accordingly, in step 1816, the UE 1806 may provide user data, which may be performed by executing the client application. In providing the user data, the client application may further consider user input received from the user via an input/output interface of the UE 1806. Regardless of the specific manner in which the user data was provided, the UE 1806 initiates, in step 1818, transmission of the user data towards the host 1802 via the network node 1804. In step 1820, in accordance with the teachings of the embodiments described throughout this disclosure, the network node 1804 receives user data from the UE 1806 and initiates transmission of the received user data towards the host 1802. In step 1822, the host 1802 receives the user data carried in the transmission initiated by the UE 1806. One or more of the various embodiments improve the performance of OTT services provided to the UE 1806 using the OTT connection 1850, in which the wireless connection 1870 forms the last segment. More precisely, the teachings of these embodiments may improve the latency and thereby provide benefits such as reduced user waiting time. In an example scenario, factory status information may be collected and analyzed by the host 1802. As another example, the host 1802 may process audio and video data which may have been retrieved from a UE for use in creating maps. As another example, the host 1802 may collect and analyze real-time data to assist in controlling vehicle congestion (e.g., controlling traffic lights). As another example, the host 1802 may store surveillance video uploaded by a UE. As another example, the host 1802 may store or control access to media content such as video, audio, VR or AR which it can broadcast, multicast or unicast to UEs. As other examples, the host 1802 may be used for energy pricing, remote control of non-time critical electrical load to balance power generation needs, location services, presentation services (such as compiling diagrams etc. from data collected from remote devices), or any other function of collecting, retrieving, storing, analyzing and/or transmitting data. In some examples, a measurement procedure may be provided for the purpose of monitoring data rate, latency and other factors on which the one or more embodiments improve. There may further be an optional network functionality for reconfiguring the OTT connection 1850 between the host 1802 and UE 1806, in response to variations in the measurement results. The measurement procedure and/or the network functionality for reconfiguring the OTT connection may be implemented in software and hardware of the host 1802 and/or UE 1806. In some embodiments, sensors (not shown) may be deployed in or in association with other devices through which the OTT connection 1850 passes; the sensors may participate in the measurement procedure by supplying values of the monitored quantities exemplified above, or supplying values of other physical quantities from which software may compute or estimate the monitored quantities. The reconfiguring of the OTT connection 1850 may include message format, retransmission settings, preferred routing etc.; the reconfiguring need not directly alter the operation of the network node 1804. Such procedures and functionalities may be known and practiced in the art. In certain embodiments, measurements may involve proprietary UE signaling that facilitates measurements of throughput, propagation times, latency and the like, by the host 1802. The measurements may be implemented in that software causes messages to be transmitted, in particular empty or ‘dummy’ messages, using the OTT connection 1850 while monitoring propagation times, errors, etc. Additional aspects. According to a first additional aspect of the embodiments described herein, it is also presented a host configured to operate in a communication system to provide an over-the-top (OTT) service. The host comprises processing circuitry configured to provide user data; and a network interface configured to initiate transmission of the user data to a network node in a cellular network for transmission to a user equipment (UE), the network node having a communication interface and processing circuitry, the processing circuitry of the network node configured to perform the following operations to operations to transmit the user data from the host to the UE: receiving a primary uplink (UL) reference signal s_UL1 transmitted by the UE; and transmitting a primary downlink (DL) reference signal s_DL1 to the UE, where primary DL reference signal sDL1 is based on the primary UL reference signal sUL1 as received by the network node. The processing circuitry of the host may be configured to execute a host application that provides the user data; and the UE may comprise processing circuitry configured to execute a client application associated with the host application to receive the transmission of user data from the host. According to a second additional aspect of the embodiments described herein, it is also presented a method implemented in a host configured to operate in a communication system that further includes a network node and a user equipment (UE), the method comprising: providing user data for the UE; and initiating a transmission carrying the user data to the UE via a cellular network comprising the network node, wherein the network node performs the following operations to transmit the user data from the host to the UE: receiving a primary uplink (UL) reference signal sUL1 transmitted by the UE; and transmitting a primary downlink (DL) reference signal sDL1 to the UE, where primary DL reference signal sDL1 is based on the primary UL reference signal sUL1 as received by the network node. The method may further comprise, at the network node, transmitting the user data provided by the host for the UE. The user data may be provided at the host by executing a host application that interacts with a client application executing on the UE, the client application being associated with the host application. According to a third additional aspect of the embodiments described herein, it is also presented a communication system configured to provide an over-the-top service, the communication system comprising: a host comprising: processing circuitry configured to provide user data for a user equipment (UE), the user data being associated with the over-the-top service; and a network interface configured to initiate transmission of the user data toward a cellular network node for transmission to the UE, the network node having a communication interface and processing circuitry, the processing circuitry of the network node configured to perform the following operations to transmit the user data from the host to the UE: receiving a primary uplink (UL) reference signal s_UL1 transmitted by the UE; and transmitting a primary downlink (DL) reference signal s_DL1 to the UE, where primary DL reference signal s_DL1 is based on the primary UL reference signal s_UL1 as received by the network node. The communication system may further comprise the network node; and/or the user equipment. The processing circuitry of the host may be configured to execute a host application, thereby providing the user data; and the host application may be configured to interact with a client application executing on the UE, the client application being associated with the host application. According to a fourth additional aspect of the embodiments described herein, it is also presented a host configured to operate in a communication system to provide an over-the-top (OTT) service, the host comprising: processing circuitry configured to initiate receipt of user data; and a network interface configured to receive the user data from a network node in a cellular network, the network node having a communication interface and processing circuitry, the processing circuitry of the network node configured to perform the following operations to receive the user data from the UE for the host: receiving a primary uplink (UL) reference signal sUL1 transmitted by the UE; and transmitting a primary downlink (DL) reference signal sDL1 to the UE, where primary DL reference signal sDL1 is based on the primary UL reference signal sUL1 as received by the network node. The processing circuitry of the host may be configured to execute a host application, thereby providing the user data; and the host application may be configured to interact with a client application executing on the UE, the client application being associated with the host application. The initiating receipt of the user data may comprise requesting the user data. According to a fifth additional aspect of the embodiments described herein, it is also presented a method implemented by a host configured to operate in a communication system that further includes a network node and a user equipment (UE), the method comprising: at the host, initiating receipt of user data from the UE, the user data originating from a transmission which the network node has received from the UE, wherein the network node performs the following operations to receive the user data from the UE for the host: receiving a primary uplink (UL) reference signal sUL1 transmitted by the UE; and transmitting a primary downlink (DL) reference signal sDL1 to the UE, where primary DL reference signal sDL1 is based on the primary UL reference signal sUL1 as received by the network node. The method may further comprise at the network node, transmitting received user data to the host. According to a sixth additional aspect of the embodiments described herein, it is also presented a host configured to operate in a communication system to provide an over-the-top (OTT) service, the host comprising: processing circuitry configured to provide user data; and a network interface configured to initiate transmission of the user data to a cellular network for transmission to a user equipment (UE), wherein the UE comprises a communication interface and processing circuitry, the communication interface and processing circuitry of the UE being configured to perform the following operations to receive the user data from the host: receiving a primary downlink (DL) reference signal s_DL1 transmitted by a primary AP 111; receiving a secondary DL reference signal s_DL2 transmitted by a secondary AP 112; and transmitting a secondary uplink (UL) reference signal s_UL2 to the secondary AP 112, where the secondary UL reference signal s_UL2 is based on the primary and the secondary DL reference signals s_DL1, s_DL2 as received by the UE. The cellular network may further include a network node configured to communicate with the UE to transmit the user data to the UE from the host. The processing circuitry of the host may be configured to execute a host application, thereby providing the user data; and the host application may be configured to interact with a client application executing on the UE, the client application being associated with the host application. According to a seventh additional aspect of the embodiments described herein, it is also presented a method implemented by a host operating in a communication system that further includes a network node and a user equipment (UE), the method comprising: providing user data for the UE; and initiating a transmission carrying the user data to the UE via a cellular network comprising the network node, wherein the UE performs the following operations to receive the user data from the host: receiving a primary downlink (DL) reference signal sDL1 transmitted by a primary AP 111; receiving a secondary DL reference signal sDL2 transmitted by a secondary AP 112; and transmitting a secondary uplink (UL) reference signal sUL2 to the secondary AP 112, where the secondary UL reference signal sUL2 is based on the primary and the secondary DL reference signals sDL1, sDL2 as received by the UE. The method may further comprise, at the host, executing a host application associated with a client application executing on the UE to receive the user data from the UE. The method may further comprise: at the host, transmitting input data to the client application executing on the UE, the input data being provided by executing the host application, wherein the user data is provided by the client application in response to the input data from the host application. According to an eight additional aspect of the embodiments described herein, it is also presented a host configured to operate in a communication system to provide an over-the-top (OTT) service, the host comprising: processing circuitry configured to utilize user data; and a network interface configured to receipt of transmission of the user data to a cellular network for transmission to a user equipment (UE), wherein the UE comprises a communication interface and processing circuitry, the communication interface and processing circuitry of the UE being configured to perform the following operations to transmit the user data to the hos: receiving a primary downlink (DL) reference signal s_DL1 transmitted by a primary AP 111; receiving a secondary DL reference signal s_DL2 transmitted by a secondary AP 112; and transmitting a secondary uplink (UL) reference signal s_UL2 to the secondary AP 112, where the secondary UL reference signal s_UL2 is based on the primary and the secondary DL reference signals s_DL1, s_DL2 as received by the UE. The cellular network may further include a network node configured to communicate with the UE to transmit the user data from the UE to the host. The processing circuitry of the host may be configured to execute a host application, thereby providing the user data; and the host application may be configured to interact with a client application executing on the UE, the client application being associated with the host application. According to a ninth additional aspect of the embodiments described herein, it is also presented a method implemented by a host configured to operate in a communication system that further includes a network node and a user equipment (UE), the method comprising: at the host, receiving user data transmitted to the host via the network node by the UE, wherein the UE performs the following operations to transmit the user data to the host: receiving a primary downlink (DL) reference signal sDL1 transmitted by a primary AP 111; receiving a secondary DL reference signal sDL2 transmitted by a secondary AP 112; and transmitting a secondary uplink (UL) reference signal sUL2 to the secondary AP 112, where the secondary UL reference signal sUL2 is based on the primary and the secondary DL reference signals sDL1, sDL2 as received by the UE. The method may further comprise, at the host, executing a host application associated with a client application executing on the UE to receive the user data from the UE. The method may further comprise: at the host, transmitting input data to the client application executing on the UE, the input data being provided by executing the host application, wherein the user data is provided by the client application in response to the input data from the host application. The description of the example embodiments provided herein have been presented for purposes of illustration. The description is not intended to be exhaustive or to limit example embodiments to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of various alternatives to the provided embodiments. The examples discussed herein were chosen and described in order to explain the principles and the nature of various example embodiments and its practical application to enable one skilled in the art to utilize the example embodiments in various manners and with various modifications as are suited to the particular use contemplated. The features of the embodiments described herein may be combined in all possible combinations of methods, apparatus, modules, systems, and computer program products. It should be appreciated that the example embodiments presented herein may be practiced in any combination with each other. It should be noted that the word “comprising” does not necessarily exclude the presence of other elements or steps than those listed and the words “a” or “an” preceding an element do not exclude the presence of a plurality of such elements. It should further be noted that any reference signs do not limit the scope of the claims, that the example embodiments may be implemented at least in part by means of both hardware and software, and that several “means”, “units” or “devices” may be represented by the same item of hardware. It should also be noted that the various example embodiments described herein are described in the general context of method steps or processes, which may be implemented in one aspect by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers in networked environments. A computer-readable medium may include removable and non- removable storage devices including, but not limited to, Read Only Memory (ROM), Random Access Memory (RAM), compact discs (CDs), digital versatile discs (DVD), etc. Generally, program modules may include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Computer-executable instructions, associated data structures, and program modules represent examples of program code for executing steps of the methods disclosed herein. The particular sequence of such executable instructions or associated data structures represents examples of corresponding acts for implementing the functions described in such steps or processes. The embodiments herein are not limited to the above-described preferred embodiments. Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be construed as limiting.

Claims

CLAIMS 1. A method (1000) for enabling reciprocity calibration in a wireless communications network (100) comprising a wireless device (121) and a plurality of distributed access points, APs, wherein the plurality of distributed APs comprises a primary AP (111) and a secondary AP (112), the method (1000) comprising: transmitting (1020), by the wireless device (121), a primary uplink, UL, reference signal (s_UL1) to the primary AP (111); transmitting (1030), by the primary AP (111), a primary downlink, DL, reference signal (s_DL1) to the wireless device (121), where the primary DL reference signal (s_DL1) is based on the primary UL reference signal (s_UL1) as received by the primary AP (111); transmitting (1040), by the secondary AP (112), a secondary DL reference signal (sDL2) to the wireless device (121); transmitting (1050), by the wireless device (121), a secondary UL reference signal (sUL2) to the secondary AP (112), where the secondary UL reference signal (sUL2) is based on the primary and the secondary DL reference signals (sDL1, sDL2) as received by the wireless device (121); and determining (1070), based on the secondary UL reference signal (sUL2) as received by the secondary AP (112), a first calibration coefficient indicative of a channel (yAP2,AP1) from the primary AP (111) to the secondary AP (112) and of a channel (yAP1,AP2) from the secondary AP (112) to the primary AP (111).

2. The method (1000) according to claim 1, wherein the channel (yAP2,AP1) from the primary AP (111) to the secondary AP (112) and the channel (yAP1,AP2) from the secondary AP (112) to the primary AP (111) are respective baseband-to-baseband channels.

3. The method (1000) according to any previous claim, wherein the primary DL reference signal (sDL1) comprises a pilot sequence scaled by a first factor (f1) derived from the primary UL reference signal (sUL1) as received by the primary AP (111).

4. The method (1000) according to claim 3, wherein the first factor (f1) is based on a magnitude and/or phase of a channel (yAP1,UE1) from the wireless device (121) to the primary AP (111), and wherein the channel (yAP1,UE1) from the wireless device (121) to the primary AP (111) is based on the primary UL reference signal (sUL1) as received by primary AP (111).

5. The method (1000) according to any previous claim, wherein the secondary UL reference signal (s_UL2) comprises a pilot sequence scaled by a second factor (f₂) derived from the primary DL reference signal (s_DL1) as received by the wireless device (121) and by a third factor (f₃) derived from the secondary DL reference signal (s_DL2) as received by the wireless device (121).

6. The method (1000) according to claims 4 and 5, wherein the second factor (f₂) is based on a magnitude and/or phase of a first ratio (y_UE1,AP1/y_AP1,UE1) of a channel (y_UE1,AP1) from the primary AP (111) to the wireless device (121) over the channel (y_AP1,UE1) from the wireless device (121) to the primary AP (111), and wherein the third factor (f₃) is based on a magnitude and/or phase of a channel (y_UE1,AP2) from the secondary AP (112) to the wireless device (121), and wherein the first ratio is based on the primary DL reference signal (s_DL1) as received by the wireless device (121), and wherein the channel (y_UE1,AP2) from the secondary AP (112) to the wireless device (121) is based on the secondary DL reference signal (s_DL2) as received by the wireless device (121).

7. The method (1000) according to claims 4 and 6, wherein the channel (yUE1,AP1) from the primary AP (111) to the wireless device (121), the channel (yAP1,UE1) from the wireless device (121) to the primary AP (111), and the channel (yUE1,AP2) from the secondary AP (112) to the wireless device (121) are respective baseband-to-baseband channels.

8. The method (1000) according to any previous claim, wherein the method (1000) comprises determining (1071) the first calibration coefficient by the secondary AP (112).

9. The method (1000) according to any previous claim, wherein the wireless communications network (100) comprises a node (140), and wherein the method (1000) comprises communicating (1060), by the secondary AP (112), information based on the secondary UL reference signal (sUL2) as received by the secondary AP (112) to the node (140), and determining (1072), by the node (140), the first calibration coefficient based on the communicated information.

10. The method (1000) according to claim 9, comprising performing (1080), by the node (140), a reciprocity calibration in the wireless communications network (100) based on the determined first calibration coefficient.

11. The method (1000) according to any previous claim, wherein the primary UL reference signal (sUL1) and the primary DL reference signal (sDL1) are transmitted within a channel coherence time and a channel coherence bandwidth of a propagation channel between the wireless device (121) and the primary AP (111), and wherein the secondary UL reference signal (s_UL2) and the secondary DL reference signal (s_DL2) are transmitted within a channel coherence time and a channel coherence bandwidth of a propagation channel between the wireless device (121) and the secondary AP (112).

12. The method (1000) according to claim 11, wherein the primary and the secondary UL reference signals (s_UL1, s_UL2), and the primary and the secondary DL reference signals (s_DL1, s_DL2) are transmitted within a specified time interval.

13. The method (1000) according to any pervious claim, wherein the primary and the secondary UL reference signals (s_UL1, s_UL2) comprise a respective Sounding Reference Signal, SRS.

14. The method (1000) according to any pervious claim, wherein the primary and the secondary DL reference signals (s_DL1, s_DL2) comprise a respective Channels State Information- Reference Signal, CSI-RS.

15. The method (1000) according to any previous claim, wherein a resource block of the primary UL reference signal (sUL1) is associated with a resource block of the primary DL reference signal (sDL1), and wherein a resource block of the secondary UL reference signal (sUL2) is associated with a resource block of the secondary DL reference signal (sDL2).

16. The method (1000) according to any previous claim, wherein a respective resource block of the primary and secondary UL reference signals (sUL1, sUL2) is based on an UL resource identity, ID, and/or UL resource set ID, and wherein a respective resource block of the primary and secondary DL reference signals (sDL1, sDL2) is based on a DL resource ID and/or DL resource set ID.

17. The method (1000) according to any pervious claim, comprising reporting (1010), by the wireless device (121), a capability report of the wireless device (121) to any AP of the plurality of distributed APs, wherein the capability report is indicative of the wireless device (121) being capable of transmitting the secondary UL reference signal (sUL2) to the secondary AP (112).

18. The method (1000) according to claim 17, wherein the wireless device (121) is capable of transmitting a first number of secondary UL reference signals to respective secondary APs comprised in the plurality of distributed APs, wherein each secondary UL reference signal of the first number of secondary UL reference signals is based on the primary DL reference signal (sDL1) and an associated secondary DL reference signal as received by the wireless device (121), wherein the associated secondary DL reference signal is associated with the respective secondary AP that the respective secondary UL reference signal is transmitted to, and wherein the capability report comprises the first number.

19. The method (1000) according to claim 18 when dependent on claims 13 and 14, wherein the first number is associated with a corresponding number of SRS resources for each secondary UL reference signal of the first number of secondary UL reference signals and with a corresponding number of CSI-RS resources for each corresponding secondary DL reference signal.

20. The method (1000) according to any of claims 17-19, wherein the capability report is a quantity value in a report defined by a Channel State Information, CSI, Information Element, IE.

21. The method (1000) according to any previous claim, wherein the wireless device (121), the primary AP (111), and the secondary AP (112) each comprise respective first antenna elements and respective second antenna elements, and wherein the method (1000) comprises transmitting (1021) the primary UL reference signal (sUL1) as a first primary UL component (sUL1,P1) and a second primary UL component (sUL1,P2), wherein the first primary UL component (sUL1,P1) is transmitted by the first antenna element (811) of the wireless device (121), and the second primary UL component (sUL1,P2) is transmitted by the second antenna element (812) of the wireless device (121), transmitting (1031) the primary DL reference signal (sDL1) as a first primary DL component (sDL1,P1) and a second primary DL component (sDL1,P2), wherein the first primary DL component (sDL1,P1) is transmitted by the first antenna element of the primary AP (111), and the second primary DL component (sDL1,P2) is transmitted by the second antenna element of the primary AP (111), wherein the first primary DL component (sDL1,P1) is based on the first primary UL component (sUL1,P1) as received by the first antenna element of the primary AP (111) and the second primary DL component (sDL1,P2) is based on the second primary UL component (sUL1,P2) as received by the second antenna element of the primary AP (111), transmitting (1041) the secondary DL reference signal (sDL2) as a first secondary DL component (sDL2,P1) and a second secondary UL component (sDL2,P2), wherein the first secondary UL component (sDL2,P1) is transmitted by the first antenna element of the secondary AP (112), and the second secondary UL component (sDL2,P2) is transmitted by the second antenna element of the secondary AP (112), and transmitting (1051) the secondary UL reference signal (sUL2) as a first secondary UL component (s_UL2,P1) and a second secondary UL component (s_UL2,P2), wherein the first secondary UL component (s_UL2,P1) is transmitted by the first antenna element (811) of the wireless device (121), and the second secondary UL component (s_UL2,P2) is transmitted by the second antenna element (812) of the wireless device (121), wherein the first secondary UL component (s_UL2,P1) is based on the first primary DL component (s_DL1,P1) and the first secondary DL component (s_DL2,P1) as received by the first antenna element (811) of the wireless device (121), and wherein the second secondary UL component (s_UL2,P2) is based on the second primary DL component (s_DL1,P2) and the second secondary DL component (s_DL2,P2) as received by the second antenna element (812) of the wireless device (121).

22. The method (1000) according to any of claims 1-20, wherein the wireless device (121) and the primary AP (111) each comprises respective first antenna elements and respective second antenna elements, and wherein the method (1000) comprises transmitting (1021) the primary UL reference signal (s_UL1) as a first primary UL component (s_UL1,P1) and a second primary UL component (s_UL1,P2), wherein the first primary UL component (s_UL1,P1) is transmitted by the first antenna element (811) of the wireless device (121), and the second primary UL component (sUL1,P2) is transmitted by the second antenna element (812) of the wireless device (121), transmitting (1031) the primary DL reference signal (sDL1) as a first primary DL component (sDL1,P1) and a second primary DL component (sDL1,P2), wherein the first primary DL component (sDL1,P1) is transmitted by the first antenna element of the primary AP (111), and the second primary DL component (sDL1,P2) is transmitted by the second antenna element of the primary AP (111), wherein the first primary DL component (sDL1,P1) is based on the first primary UL component (sUL1,P1) as received by the first antenna element of the primary AP (111) and the second primary DL component (sDL1,P2) is based on the second primary UL component (sUL1,P2) as received by the second antenna element of the primary AP (111), transmitting (1052) the secondary UL reference signal (sUL2) as a first secondary UL component (sUL2,P1) and a second secondary UL component (sUL2,P2), wherein the first secondary UL component (sUL2,P1) is transmitted by the first antenna element (811) of the wireless device (121), and the second secondary UL component (sUL2,P2) is transmitted by the second antenna element (812) of the wireless device (121), wherein the first secondary UL component (sUL2,P1) is based on the first primary DL component (sDL1,P1) as received by the first antenna element (811) of the wireless device (121) and on the secondary DL reference signal (sDL2) as received by the first and/or second antenna elements of the wireless device (121), and wherein the second secondary UL component (sUL2,P2) is based on the second primary DL component (sDL1,P2) as received by the second antenna element (812) of the wireless device (121) and on the secondary DL reference signal (s_DL2) as received by the first and/or second antenna elements of the wireless device (121).

23. A wireless communications network (100) for enabling reciprocity calibration in a wireless communications network (100), the wireless communications network (100) comprising a wireless device (121) and a plurality of distributed access points, APs, wherein the plurality of distributed APs comprises a primary AP (111) and a secondary AP (112), wherein the wireless communications network (100) is configured to transmit, by the wireless device (121), a primary uplink, UL, reference signal (s_UL1) to the primary AP (111), transmit, by the primary AP (111), a primary downlink, DL, reference signal (s_DL1) to the wireless device (121), where the primary DL reference signal (s_DL1) is based on the primary UL reference signal (s_UL1) as received by the primary AP (111), transmit, by the secondary AP (112), a secondary DL reference signal (s_DL2) to the wireless device (121), transmit, by the wireless device (121), a secondary UL reference signal (s_UL2) to the secondary AP (112), where the secondary UL reference signal (sUL2) is based on the primary and the secondary DL reference signals (sDL1, sDL2) as received by the wireless device (121), and determine, based on the secondary UL reference signal (sUL2) as received by the secondary AP (112), a first calibration coefficient indicative of a channel (yAP2,AP1) from the primary AP (111) to the secondary AP (112) and of a channel (yAP1,AP2) from the secondary AP (112) to the primary AP (111).

24. A method (1100) performed by a wireless device (121) for enabling reciprocity calibration in a wireless communications network (100), the wireless communications network (100) comprising a plurality of distributed access points, APs, wherein the plurality of distributed APs comprises a primary AP (111) and a secondary AP (112), the method (1100) comprising: receiving (1130) a primary downlink, DL, reference signal (sDL1) transmitted by the primary AP (111); receiving (1140) a secondary DL reference signal (sDL2) transmitted by the secondary AP (112); and transmitting (1150) a secondary uplink, UL, reference signal (sUL2) to the secondary AP (112), where the secondary UL reference signal (sUL2) is based on the primary and the secondary DL reference signals (sDL1, sDL2) as received by the wireless device (121).

25. The method (1100) according to claim 24, wherein the secondary UL reference signal (sDL2) comprises a pilot sequence scaled by a second factor (f2) derived from the primary DL reference signal (s_DL1) as received by the wireless device (121) and by a third factor (f₃) derived from the secondary DL reference signal (s_DL2) as received by the wireless device (121).

26. The method (1100) according to any of claims 24-25, wherein the secondary UL reference signal (s_UL1) and the primary and the secondary DL reference signals (s_DL1, s_DL2) are transmitted and received, respectively, within a channel coherence time and a channel coherence bandwidth of a propagation channel between the wireless device (121) and the primary AP (111).

27. The method (1100) according to claim 26, wherein the secondary UL reference signal (s_UL1) and the primary and the secondary DL reference signals (s_DL1, s_DL2) are transmitted and received, respectively, within a specified time interval.

28. The method (1100) according to any of claims 24-27, wherein the secondary UL reference signal (s_UL2) comprises a Sounding Reference Signal, SRS.

29. The method (1100) according to any of claims 24-28, wherein the primary and the secondary DL reference signals (sDL1, sDL2) comprise a respective Channels State Information- Reference Signal, CSI-RS.

30. The method (1100) according to any of claims 24-29, and wherein a resource block of the secondary UL reference signal (sUL2) is associated with a resource block of the secondary DL reference signal (sDL2).

31. The method (1100) according to any of claims 24-30, comprising reporting (1010) a capability report of the wireless device (121) to any AP of the plurality of distributed APs, wherein the capability report is indicative of the wireless device (121) being capable of transmitting the secondary UL reference signal (sUL2) to the secondary AP (112).

32. The method (1000) according to claim 31, wherein the wireless device (121) is capable of transmitting a first number of secondary UL reference signals to respective secondary APs comprised in the plurality of distributed APs, wherein each secondary UL reference signal of the first number of secondary UL reference signals is based on the primary DL reference signal (sDL1) and an associated secondary DL reference signal as received by the wireless device (121), wherein the associated secondary DL reference signal is associated with the respective secondary AP that the respective UL reference signal is transmitted to, and wherein the capability report comprises the first number.

33. The method (1000) according to claim 32 when dependent on claims 47 and 48, wherein the first number is associated with a corresponding number of SRS resources for each secondary UL reference signal of the first number of secondary UL reference signals and with a corresponding number of CSI-RS resources for each corresponding secondary DL reference signal.

34. The method (1000) according to any of claims 31-33, wherein the capability report is a quantity value in a report defined by a Channel State Information, CSI, Information Element, IE.

35. A wireless device (121) for enabling reciprocity calibration in a wireless communications network (100), the wireless communications network (100) comprising a plurality of distributed access points, APs, wherein the plurality of distributed APs comprises a primary AP (111) and a secondary AP (112), wherein the wireless device (121) is configured to receive a primary downlink, DL, reference signal (s_DL1) transmitted by the primary AP (111), receive a secondary DL reference signal (s_DL2) transmitted by the secondary AP (112), and transmit a secondary uplink, UL, reference signal (sUL2) to the secondary AP (112), where the secondary UL reference signal (sUL2) is based on the primary and the secondary DL reference signals (sDL1, sDL2) as received by the wireless device (121).

36. A method (1200) performed by a primary access point, AP, (110) for enabling reciprocity calibration in a wireless communications network (100), the wireless communications network (100) comprising a plurality of distributed APs and a wireless device (121), the method (1200) comprising: receiving (1220) a primary uplink, UL, reference signal (sUL1) transmitted by the wireless device (121); and transmitting (1230) a primary downlink, DL, reference signal (sDL1) to the wireless device (121), where the primary DL reference signal (sDL1) is based on the primary UL reference signal (sUL1) as received by the primary AP (111).

37. The method (1200) according to claim 36, wherein the primary DL reference signal (sDL1) comprises a pilot sequence scaled by a first factor (f1) derived from the primary UL reference signal (sUL1) as received by the primary AP (111).

38. The method (1200) according to claim 37, wherein the first factor (f1) is based on a magnitude and/or phase of a channel (yAP1,UE1) from the wireless device (121) to the primary AP (111), and wherein the channel (yAP1,UE1) from the wireless device (121) to the primary AP (111) is based on the primary UL reference signal (sUL1) as received by primary AP (111).

39. The method (1200) according to claim 38, wherein the channel (yAP1,UE1) from the wireless device (121) to the primary AP (111) is a baseband-to-baseband channel.

40. The method (1200) according to any of claims 36-39, wherein the primary reference signal (s_UL1) and the primary DL reference signal (s_DL1) are received and transmitted, respectively, within a channel coherence time a channel coherence bandwidth of a propagation channel between the wireless device (121) and the primary AP (111).

41. The method (1200) according to claim 40, wherein the primary UL reference signal (s_UL1) and the primary DL reference signal (s_DL1) are received and transmitted, respectively, within a specified time interval.

42. The method (1200) according to any of claims 36-41, wherein the primary UL reference signal (s_UL1) comprise a Sounding Reference Signal, SRS.

43. The method (1200) according to any of claims 36-42, wherein the primary DL reference signal (s_DL1) comprise a Channels State Information-Reference Signal, CSI-RS.

44. The method (1200) according to any of claims 36-43, wherein a resource block of the primary UL reference signal (sUL1) is associated with a resource block of the primary DL reference signal (sDL1).

45. A primary access point, AP, (111) for enabling reciprocity calibration in a wireless communications network (100), the wireless communications network (100) comprising a plurality of distributed APs and a wireless device (121), wherein the primary AP (111) is configured to receive a primary uplink, UL, reference signal (sUL1) transmitted by the wireless device (121), and transmit a primary downlink, DL, reference signal (sDL1) to the wireless device (121), where the primary DL reference signal (sDL1) is based on the primary UL reference signal (sUL1) as received by the primary AP (111).

46. A computer program, comprising instructions which, when executed on at least one processing circuitry, cause the at least one processing circuitry to carry out the methods (1000, 1100, 1200) according to any of claims 1-22, 24-34, or 36-44.

47. A carrier containing the computer program according to claim 46, wherein the carrier is one of an electronic signal, optical signal, radio signal, or computer-readable storage medium.