EP4639978A1

EP4639978A1 - Methods for calibrating a repeater in a mimo network having tdd-reciprocity based communications, and corresponding devices

Info

Publication number: EP4639978A1
Application number: EP22839521.6A
Authority: EP
Inventors: Joao VIEIRA; Pål FRENGER; Erik G. Larsson
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2022-12-22
Filing date: 2022-12-22
Publication date: 2025-10-29
Also published as: WO2024136705A1

Abstract

There is provided techniques for calibrating a repeater in a wireless network. A method is performed by a network node. The method comprises configuring the repeater to operate in a first mode in a first time period and to operate in a second mode in a second time period. The method comprises determining a compensation factor as a function of a first bidirectional channel response measurement obtained for a first bidirectional sounding performed by a first transceiver device and a second transceiver device in the first time period and a second bidirectional channel response measurement obtained for a second bidirectional sounding performed by the first transceiver device and the second transceiver device in the second time period. The method comprises calibrating the repeater by configuring the repeater with the compensation factor.

Description

METHOD FOR REPEATER CALIBRATION TECHNICAL FIELD Embodiments presented herein relate to a methods, a network node, a repeater, computer programs, and a computer program product for calibrating the repeater when operated in a wireless network. The project leading to this application has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101013425. BACKGROUND Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO systems, or just MIMO for short. Cellular massive MIMO, also known as co-located massive MIMO, operating in time- division duplexing (TDD) relies on channel reciprocity to obtain downlink (DL) channel estimates at the access points (APs) from uplink pilots transmitted by the user equipment (UEs). Channel reciprocity is key to harvest most performance gains, since training the channel in the UL requires a pilot overhead that is proportional to the number of antenna ports at the UE side of the link, which is much smaller compared to training the channel in the DL which requires a pilot overhead that is proportional to the number of antenna ports at the APs. Moreover, in such reciprocity-based operation, there is no need for feedback of the measured DL channels to the AP side, as is required in systems that perform explicit DL channel estimation such as non-reciprocal frequency-division duplex (FDD) systems. Throughout this disclosure, reciprocity-based massive MIMO with TDD operation is considered. One challenge of co-located massive MIMO is to efficiently serve UEs in unfavorable locations. One example of this is scenarios with outdoor-to-indoor communications, where indoor UEs are served by APs located outdoors. Another example of this is scenarios in rural environments, where only a small number of channel scatters (e.g., physical objects in the environment) exits, and where the path loss can be very high. Some of the issues resulting from these challenges will be disclosed next. One issue pertains to coverage degradation. In outdoor-to-indoor communications, coverage degradation is due to an additional path loss from signal propagation through walls and windows. In rural environments coverage degradation is due to electromagnetic shadowing from large objects (plus lack of alternative scatterers). The former is especially an issue in buildings that have thick walls containing sheet metal and/or concrete reinforcement bars, or windows with energy-saving coatings, or both. One issue pertains to that richness of the electromagnetic channel might be compromised. In outdoor-to-indoor communications, if the only significant propagation path from an AP located outdoors to a UE located indoors is through a window, the resulting channel will be effectively rank one, also known as a key-hole channel, irrespective of how many antennas the AP and the UE have, and irrespective of how much local scattering there is around the AP and the UE. The issue is aggravated in a multi-user scenario, where many UEs are served through such a keyhole channel. In the rural environment scenario, one issue is that there are only few reliable scatterers between the AP and the UE. Some attempts to mitigate these issues involve installation and use of active repeaters that amplify the signal between the AP and the UE. Other attempts to mitigate these issues involve the use of reflecting intelligent surfaces (RIS) that essentially function as a repeater but without amplification (though with some beam steering capability). A repeater might be regarded as a transceiver device that amplifies the incoming radio-frequency (RF) field and re-transmits it instantaneously. Different types of repeaters exist. Single-antenna repeaters use the same antenna for reception and transmission and require the use of a circulator or similar device to isolate the outgoing wave from the incoming. Single-antenna repeaters have limited amplification by the transmit/receive (Tx/Rx) isolation circuit (e.g., circulator). Further, single-antenna repeaters lack beamforming capabilities (since there is only a single-antenna, and its antenna pattern needs to be wide enough to simultaneously cover both the direction in which the AP is located and the direction in which potential UEs are located). The former results in energy-inefficient communications, in general. For example, if the repeater is installed in the outer part of a building to enable outdoor-to-indoor communications, then the energy that effectively enters the building is only a small fraction of the energy transmitted by the repeater (since some of the energy will be transmitted back towards the AP, and some of the energy will be reflected back from the building). Dual-antenna repeaters, in contrast, have two antenna ports (e.g., two antenna elements or two panels with two or more antenna elements each). Moreover, under the assumption that the dual-antenna repeater’s operation is aligned with the TDD frame structure, then there is no need to have RF circulators which put a cap in the maximum amplification that can be provided (as in the single-antenna repeater case). Here, the angular spread associated with at least one antenna port may be narrow (either by beamforming in the case of a panel, or narrow antenna pattern in the case of a single antenna) since there is no requirement that each antenna simultaneously covers the AP and UE (as in single-antenna repeaters). For example, the width of the antenna pattern antenna covering the direction in which the AP is located may be narrow, which effectively results in additional link margins. Moreover, in the context of outdoor-to-indoor communications, one antenna can be installed indoors and the other outdoors, thereby avoiding the wall effect and increasing the energy efficiency of the communications. Dual-antenna repeaters have a forward path and a reverse path. The forward path is comprised of the repeater’s circuity linking the first antenna (e.g., the outdoor antenna) to the second antenna (e.g., the indoor antenna), and the reverse path is comprised by the repeater’s circuity linking the second antenna to the first antenna. Noticeably, the responses, or complex-valued gains (i.e., amplitude and phase shift), of the forward and reverse paths are different since the RF circuitries are different. For example, in the indoor/outdoor scenario, the signal path from a UE located indoors to an AP located outdoors via the repeater might undergo a different phase shift (and amplification) compared to the signal that travels in the reverse direction, i.e., from the AP towards the UE. The difference between the forward and reverse path gains of a dual-antenna repeater is a drawback in the context of reciprocity-based communications, since it implies a lack of reciprocity between uplink and downlink. Therefore, although a dual-antenna repeater can enhance coverage, but due to its non-reciprocity, it does not work well with in systems that rely on reciprocity-based multiuser MIMO beamforming, as for example disclosed in “Channel estimation error and beamforming performance in repeater-enhanced massive MIMO systems,” by Yiming Ma et al, published in the 2015 IEEE 26th Annual International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC), DOI: 10.1109/PIMRC.2015.7343383. Specifically, consider a scenario with two significant propagation paths from an AP to a UE; one direct link and one link through a dual-antenna repeater. The channel impulse response of the direct link, not considering the repeater, can be estimated at the AP from uplink pilots and remains the same on downlink (by virtue of reciprocity of propagation). But owing to the repeater non-reciprocity, the response of the channel from the AP to the UE via the repeater is different from the response from the UE to the AP via the repeater. Consequently, a channel estimate obtained from uplink pilots cannot be used to ensure coherent DL receptions. The consequence is that if the AP performs beamforming based on channel estimates obtained from the uplink pilots, the direct path and the path via the repeater may end up out-of-phase at the (single-antenna) UE, causing destructive interference and therefore loss of beamforming gain. The situation can be even worse if multi-user communication is attempted (via one or more repeaters), since the most enhanced form of interference suppression requires (amplitude and) phase coherency between the beamformer and the channel propagation paths. SUMMARY An object of embodiments herein is to address the above issues when using a repeater to forward wireless signals between two APs. A particular object is to enable the repeater to behave as a reciprocal node (e.g., behave as a regular channel scatterer but with amplification) such that it could aid a MIMO network to perform TDD reciprocity-based communications. According to a first aspect there is presented a method for calibrating a repeater in a wireless network. The wireless network further comprises a first transceiver device and a second transceiver device configured to wirelessly communicate with each other without and via the repeater. The method is performed by a network node. The method comprises configuring the repeater to operate in a first mode in a first time period and to operate in a second mode in a second time period. The method comprises determining a compensation factor as a function of a first bidirectional channel response measurement obtained for a first bidirectional sounding performed by the first transceiver device and the second transceiver device in the first time period and a second bidirectional channel response measurement obtained for a second bidirectional sounding performed by the first transceiver device and the second transceiver device in the second time period. The method comprises calibrating the repeater by configuring the repeater with the compensation factor. According to a second aspect there is presented a network node for calibrating a repeater in a wireless network. The wireless network further comprises a first transceiver device and a second transceiver device configured to wirelessly communicate with each other without and via the repeater. The network node comprises processing circuitry. The processing circuitry is configured to cause the network node to configure the repeater to operate in a first mode in a first time period and to operate in a second mode in a second time period. The processing circuitry is configured to cause the network node to determine a compensation factor as a function of a first bidirectional channel response measurement obtained for a first bidirectional sounding performed by the first transceiver device and the second transceiver device in the first time period and a second bidirectional channel response measurement obtained for a second bidirectional sounding performed by the first transceiver device and the second transceiver device in the second time period. The processing circuitry is configured to cause the network node to calibrate the repeater by configuring the repeater with the compensation factor. According to a third aspect there is presented a network node for calibrating a repeater in a wireless network. The wireless network further comprises a first transceiver device and a second transceiver device configured to wirelessly communicate with each other without and via the repeater. The network node comprises a configure module configured to configure the repeater to operate in a first mode in a first time period and to operate in a second mode in a second time period. The network node comprises a determine module configured to determine a compensation factor as a function of a first bidirectional channel response measurement obtained for a first bidirectional sounding performed by the first transceiver device and the second transceiver device in the first time period and a second bidirectional channel response measurement obtained for a second bidirectional sounding performed by the first transceiver device and the second transceiver device in the second time period. The network node comprises a calibrate module configured to calibrate the repeater by configuring the repeater with the compensation factor. According to a fourth aspect there is presented a computer program for calibrating a repeater in a wireless network. The computer program comprises computer code which, when run on processing circuitry of a network node, causes the network node to perform actions. One action comprises the network node to configure the repeater to operate in a first mode in a first time period and to operate in a second mode in a second time period. One action comprises the network node to determine a compensation factor as a function of a first bidirectional channel response measurement obtained for a first bidirectional sounding performed by the first transceiver device and the second transceiver device in the first time period and a second bidirectional channel response measurement obtained for a second bidirectional sounding performed by the first transceiver device and the second transceiver device in the second time period. One action comprises the network node to calibrate the repeater by configuring the repeater with the compensation factor. According to a fifth aspect there is presented a method for a repeater to be calibrated. The repeater is configured for communication in a forward direction and a reverse direction. The repeater comprises forward path circuitry for communicating in the forward direction and reverse path circuitry for communicating in the reverse direction. The method is performed by the repeater. The method comprises receiving configuration from a network node to operate in a first mode in a first time period and to operate in a second mode in a second time period. The method comprises operating according to the configuration in the first time period and in the second time period. The method comprises receiving a compensation factor from the network node. The compensation factor is to be applied to either the forward path circuitry or the reverse path circuitry after the second time period. The method comprises applying the compensation factor after the second time period. According to a sixth aspect there is presented a repeater to be calibrated. The repeater is configured for communication in a forward direction and a reverse direction. The repeater comprises forward path circuitry for communicating in the forward direction and reverse path circuitry for communicating in the reverse direction. The repeater comprises processing circuitry. The processing circuitry is configured to cause the repeater to receive configuration from a network node to operate in a first mode in a first time period and to operate in a second mode in a second time period. The processing circuitry is configured to cause the repeater to operate according to the configuration in the first time period and in the second time period. The processing circuitry is configured to cause the repeater to receive a compensation factor from the network node. The compensation factor is to be applied to either the forward path circuitry or the reverse path circuitry after the second time period. The processing circuitry is configured to cause the repeater to apply the compensation factor after the second time period. According to a seventh aspect there is presented a repeater to be calibrated. The repeater is configured for communication in a forward direction and a reverse direction. The repeater comprises forward path circuitry for communicating in the forward direction and reverse path circuitry for communicating in the reverse direction. The repeater comprises a receive module configured to receive configuration from a network node to operate in a first mode in a first time period and to operate in a second mode in a second time period. The repeater comprises an operate module configured to operate according to the configuration in the first time period and in the second time period. The repeater comprises a receive module configured to receive a compensation factor from the network node. The compensation factor is to be applied to either the forward path circuitry or the reverse path circuitry after the second time period. The repeater comprises an apply module configured to apply the compensation factor after the second time period. According to an eighth aspect there is presented a computer program for a repeater to be calibrated. The computer program comprises computer code which, when run on processing circuitry of a repeater, causes the repeater to perform actions. One action comprises the repeater to receive configuration from a network node to operate in a first mode in a first time period and to operate in a second mode in a second time period. One action comprises the repeater to operate according to the configuration in the first time period and in the second time period. One action comprises the repeater to receive a compensation factor from the network node. The compensation factor is to be applied to either the forward path circuitry or the reverse path circuitry after the second time period. One action comprises the repeater to apply the compensation factor after the second time period. According to a ninth aspect there is presented a computer program product comprising a computer program according to at least one of the fourth aspect and the eighth aspect and a computer readable storage medium on which the computer program is stored. The computer readable storage medium could be a non-transitory computer readable storage medium. Advantageously, these aspects resolve the above when using a repeater to forward wireless signals between two APs. Advantageously, these aspects enable the repeater to behave as a reciprocal node (e.g., behave as a regular channel scatterer but with amplification) such that it could aid a MIMO network to perform TDD reciprocity-based communications. Advantageously, these aspects enable efficient use of repeaters in cellular networks that use reciprocity-based (i.e., TDD based) massive MIMO beamforming. Advantageously, in the context of a single dual-antenna repeater serving one UE, the herein disclosed aspects provide improved coverage due to array gain (since signals add coherently from direct-link paths and repeated paths) as well as an improved ability to multiplex data streams. Advantageously, these aspects enable a reduction in inter-user interference when a single dual-antenna repeater is used in multi-user MIMO communications. Advantageously, these aspects enable the repeater to possibly run in a transparent way to the network (or UEs) during reciprocity-based TDD communications since the repeater essentially behaves as a regular channel scatterer (with amplification). Advantageously, in the context of simultaneously using more two or more repeaters, such repeaters do not interference within themselves even if they are in the coverage area of each other (as has traditionally been the case when repeater deployments were densified too much). Expressed differently, each such repeater behaves like a regular (reciprocal) channel scatterers (but with amplification) and a UE can thus receive signals from more than one repeater without experiencing interference. Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings. Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, module, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, module, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. BRIEF DESCRIPTION OF THE DRAWINGS The inventive concept is now described, by way of example, with reference to the accompanying drawings, in which: Figs.1 and 2 are schematic diagrams illustrating a wireless network with transceiver devices and a repeater according to embodiments; Fig.3 is a schematic illustration of a repeater according to embodiments; Figs.4 and 5 are flowcharts of methods according to embodiments; Fig.6 is a signaling diagram of a method according to embodiments; Fig.7 show simulation results according to embodiments; Fig.8 is a schematic diagram showing functional units of a network node according to an embodiment; Fig.9 is a schematic diagram showing functional modules of a network node according to an embodiment; Fig.10 is a schematic diagram showing functional units of a repeater according to an embodiment; Fig.11 is a schematic diagram showing functional modules of a repeater according to an embodiment; and Fig.12 shows one example of a computer program product comprising computer readable means according to an embodiment. DETAILED DESCRIPTION The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the inventive concept are shown. This inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments 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 inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description. Any step or feature illustrated by dashed lines should be regarded as optional. The embodiments disclosed herein relate to techniques for calibrating a repeater when operated in a wireless network. In order to obtain such techniques, there is provided a network node, a method performed by the network node, a computer program product comprising code, for example in the form of a computer program, that when run on processing circuitry of the network node, causes the network node to perform the method. In order to obtain such techniques, there is further provided a repeater, a method performed by the repeater, and a computer program product comprising code, for example in the form of a computer program, that when run on processing circuitry of the repeater, causes the repeater to perform the method. Fig.1 is a schematic diagram illustrating a wireless network 100a where embodiments presented herein can be applied. The wireless network 100a comprises a first transceiver device 110a, a second transceiver device 110b, and a repeater 300. The repeater 300 is controllable, or at least configurable, by a network node (not shown). It is further assumed that the transceiver devices 110a, 110b also are configured for communication with the network node. In some examples also the transceiver devices 110a, 110b are controllable, or at least configurable, by the network node. The first transceiver device 110a is assumed to be equipped with ^^^^ _^^^^ antennas and the second transceiver device 110a is assumed to be equipped with ^^^^ _^^^^ antennas. however, nothing prevents one or both of the first transceiver device 110a and the second transceiver device 110b have only a single antenna (such that ^^^^ _^^^^ = 1 and/or ^^^^ _^^^^ = 1). Also, it can be the case that, for the calibration methods at hand, only one or a subset of the antennas of the antenna arrays at the first transceiver device 110a and the second transceiver device 110b are used. In this case, ^^^^ _^^^^ and ^^^^ _^^^^ represent the number of antennas that are effectively used during the calibration. In some examples, the first transceiver device 110a is any of a radio access network node 200, radio base station, base transceiver station, node B (NB), evolved node B (eNB), gNB, access point, access node, transmission and reception point, integrated access and backhaul node. In some examples, the second transceiver device 110b is any of a portable wireless device, mobile station, mobile phone, handset, wireless local loop phone, UE, smartphone, laptop computer, tablet computer, wireless modem, wireless sensor device, network equipped vehicle. In some examples, the second transceiver device 110b is of the same type as the first transceiver device 110a. In some examples, the first transceiver device 110a and the second transceiver device 110b are physically separated. An example of this is illustrated in the wireless network 100b of Fig.2(a). This could be the case when the second transceiver device 110b is of different type than the first transceiver device 110a. In some examples, the first transceiver device 110a and the second transceiver device 110b are collocated. An example of this is illustrated in the wireless network 100c of Fig.2(b). This could be the case when the first transceiver device 110a and the second transceiver device 110b are two different antenna panels of one and the same access point, or the like. Some specific illustrative examples of this will be disclosed next. In a first example the first transceiver device 110a and the second transceiver device 110b are two access points of a D-MIMO network, deployed at two different locations. That is, in some embodiments, the first transceiver device 110a is a first access point in the network 100a, 100b, 100c, and the second transceiver device 110b is a second access point in the network 100a, 100b, 100c. This could be the case in Fig.2(a). In a second example the first transceiver device 110a is an AP (or a gNB) of a D- MIMO network and the second transceiver device 110b is a UE served by the AP in the D-MIMO network. That is, in some embodiments, the first transceiver device 110a is an access point in the network 100a, 100b, 100c, and the second transceiver device 110b is a UE served by the access point. This could be the case in Fig.2(a). In a third example the first transceiver device 110a and the second transceiver device 110b are two subarrays of a single AP, or of a co-located gNB, equipped with ^^^^ _^^^^ + ^^^^ _^^^^ antennas in total. That is, in some embodiments, the first transceiver device 110a is a first antenna subarray of an access point in the network 100a, 100b, 100c, and the second transceiver device 110b is a second subarray of the access point. This could be the case in Fig.2(b). Fig.3 schematically illustrates a repeater 300 according to some examples. The repeater 300 is equipped with antennas 350a, 350b and comprises different signal paths for the forward direction and the reverse direction. In this respect, the repeater 300 comprises forward path circuitry 360a and reverse path circuitry 360b. A switch 370 is configured to selectively between engaging the forward path circuitry 360a and engaging the reverse path circuitry 360b. Other repeater implementations, e.g. without a physical switch 370 are also possible. For example, a filter may be used to isolate signals in the forward path circuitry 360a and prevent them from entering the reverse path circuitry 360b (and vice versa). The repeater 300 is here assumed to have a forward direction with (complex-valued) gain α and a reverse direction with gain β. These gains, α and β, are in general different due to differences in the RF components in the forward path circuitry 360a and the reverse path circuitry 360b. This makes the repeater 300 non-reciprocal, since the complex-valued gain applied to the repeated signal depends on the current link's direction (i.e., from the first transceiver device 110a towards the second transceiver device 110b, or vice versa). In general terms, the calibration involves the two transceiver devices 110a, 110b, (subsequently also referred to as transceiver device A and transceiver device B, respectively), that are utilized for over-the-air (OTA) calibration of the repeater 300). Two bidirectional measurements are performed between the transceiver devices, where the forward and reverse path circuitries of the repeater 300 switch states in between the first and second bidirectional measurements. As one illustrative example, the repeater 300 can be activated during the first bidirectional measurements but be shut off during the second bidirectional measurement. A network node 200 processes the four unidirectional measurements (i.e., the two bidirectional measurements) and estimates a calibration factor to be applied to one of the repeater 300s paths. After signaling this calibration factor to the repeater 300, the repeater 300 adjusts the gains of one of its paths, and it effectively becomes a reciprocal node as intended. With reference back to Fig.1, consider first the (narrowband time-invariant) channel (e.g., the channel at an OFDM sub-carrier at a given OFDM symbol) with the repeater 300 turned off. A standard model for propagation over a reciprocal channel but with non-reciprocal hardware components at transceiver device A and transceiver device B is used. The propagation channel from transceiver device A to transceiver device B (not including Tx or Rx hardware effects), when the repeater 300 is off, is represented by the MIMO channel matrix ^^^^ ( ^^^^ _^^^^ × ^^^^ _^^^^). This channel could include both line-of-sight and multipath components. To account for a non-reciprocity of the hardware at transceiver device A and transceiver device B, the diagonal matrices ^^^^ _^^^^, ^^^^ _^^^^, ^^^^ _^^^^ and ^^^^ _^^^^ that contain the coefficients of the transmitter and receiver RF chains are defined. Specifically, ^^^^ _^^^^ and ^^^^ _^^^^ are diagonal matrices with transmitter gains on their diagonals; ^^^^ _^^^^ and ^^^^ _^^^^ are diagonal matrices with the receiver gains on their diagonals. Owing to reciprocity of the propagation, this means that, when the repeater 300 is turned off, the channel from transceiver device A to transceiver device B is ^^^^ _^^^^ ^^^^ ^^^^ _^^^^, and the channel from transceiver device B to transceiver device A is ^^^^ _^^^^ ^^^^ ^{^^^^} ^^^^ _^^^^, where (. ) ^{^^^^} denotes the matrix transpose operator. In general terms, such a bidirectional measurement should be executed within the coherence time/frequency of the channel ^^^^ for the reciprocity to hold. With continued reference back to Fig.1, consider now that the repeater 300 is active. The propagation channel from transceiver device A to the repeater 300 is denoted by the vector ^^^^ ^{^^^^} (1 × ^^^^ _^^^^) and the reverse channel, from the repeater 300 to transceiver device A, is denoted by the vector ^^^^ since reciprocity is assumed to hold for this channel. By convention, throughout, all vectors are column vectors. The channel from transceiver device B to the repeater 300 is denoted by the vector ^^^^ ^{^^^^} (1 × ^^^^ _^^^^) and the reverse channel is denoted by ^^^^. As above, the repeater 300 is assumed to have a forward direction with (complex- valued) gain α and a reverse direction with gain β. Excluding Tx and Rx hardware effects, the channel from transceiver device A to transceiver device B via the repeater 300 is ^^^^ ^^^^ ^^^^ ^{^^^^} and the channel from transceiver device B to transceiver device A via the repeater 300 is ^^^^ ^^^^ ^^^^ ^{^^^^}. During repeater 300 calibration, the repeater 300 is configured to know the time instance(s) when transceiver device A transmits and transceiver device B receives, and vice versa, so that the repeater 300 can activate either the forward direction or the reverse direction accordingly. Thus, there is no channel component from transceiver device A to the right-hand side antenna of the repeater 300 in Fig.1, and no channel component from transceiver device B to the left-hand side antenna of the repeater 300 in Fig.1. Yet, another reason for not having such channels is the attenuation wall effect previously described, if the repeater 300 is a dual-antenna repeater 300 used for outdoor-to-indoor communications and one antenna sits outdoors and the other indoors. Further, it is assumed that the repeater 300 is configured to make a (relative) change to the amplitude and phase α in the forward path (or β in the reverse path) in a controlled manner even though the repeater 300 does not know the value of α or β (or β/α). Expressed differently, if at some time instance the unknown amplitude and phase response of α equals | ^^^^| ^^^^ ^{^^^^ ^^^^}, the repeater 300 can at the next instance effectively adapt such response to� ^^^^ ^^^^ _^^^^� ^^^^ ^{^^^^ ^^^^+ ^^^^ ^^^^} by increasing its amplitude gain by a factor of and adding ^^^^ _^^^^ degrees to the current unknown phase. Such functionality can be achieved in different ways depending on the repeater 300’s implementation. For example, there could be an RF phase-shifting circuitry in the forward path that can be tuned such that it effectively changes the current phase (and amplitude) of the forward path. In another example, this phase shifting tuning functionality could be achieved in the baseband domain, if the repeater 300 is implemented by down- converting the signal to the digital domain, by multiplying the signal to be transmitted in the forward path by a complex scalar before upconverting the signal to the analog domain (i.e. as in a regenerative repeater 300). The same applies with respect to the gain β in the reverse path. The adjustment on the repeater 300’s forward and/or reverse path responses is based on the result of the calibration as will be described next. As will also be further disclosed below, such adjustments are instructed to the repeater 300 by the network node 200 which performs the calibration measurements and respective post- processing computations. As will also be further disclosed below, the repeater 300 is therefore configured to receive such adjustment instructions from the network node 200, either via a dedicated (e.g., broadcast) control channel, or a cabled interface, etc. When the repeater 300 is on, and instructed by the network node 200 to not change its amplitude or phase in any particular way, the channel from transceiver device A to transceiver device B becomes ^^^^ _^^^^( ^^^^ + ) ^^^^ _^^^^ and the channel from transceiver device B to transceiver device A becomes ^^^^ _^^^^( ^^^^ ^{^^^^} + ^^^^ ^^^^ ^^^^ ^{^^^^}) ^^^^ _^^^^. If the repeater 300 is on and instructed to change its phase by 180 degrees, the channel from transceiver device A to transceiver device B becomes ^^^^ _^^^^( ^^^^ − ^^^^ ^^^^ ^^^^ ^{^^^^} ) ^^^^ _^^^^ and the channel from transceiver device B to transceiver device A becomes ^^^^ _^^^^( ^^^^ ^{^^^^} − ^^^^ ^^^^ ^^^^ ^{^^^^}) ^^^^ _^^^^. Reference is now made to Fig.4 illustrating a method for calibrating a repeater 300 when operated in a wireless network 100a, 100b, 100c as performed by the network node 200 according to an embodiment. The wireless network 100a, 100b, 100c further comprises a first transceiver device 110a and a second transceiver device 110b configured to wirelessly communicate with each other without and via the repeater 300. It is here noted that when the first transceiver device 110a and the second transceiver device 110b wirelessly communicate with each other without the repeater 300, this does not necessarily correspond to there being a line of sight (LoS) between the first transceiver device 110a and the second transceiver device 110b, it just means that the repeater 300 is not involved in the communication between the first transceiver device 110a and the second transceiver device 110b. S102: The network node 200 configures the repeater 300 to operate in a first mode in a first time period and to operate in a second mode in a second time period. S106: The network node 200 determines a compensation factor as a function of a first bidirectional channel response measurement obtained for a first bidirectional sounding performed by the first transceiver device 110a and the second transceiver device 110b in the first time period and a second bidirectional channel response measurement obtained for a second bidirectional sounding performed by the first transceiver device 110a and the second transceiver device 110b in the second time period. S108: The network node 200 calibrates the repeater 300 by configuring the repeater 300 with the compensation factor. Embodiments relating to further details of calibrating a repeater 300 when operated in a wireless network 100a, 100b, 100c as performed by the network node 200 will now be disclosed. In some aspects the network node 200 explicitly configures the first transceiver device 110a and the second transceiver device 110b to perform bidirectional sounding. That is, in some embodiments, the network node 200 is configured to perform (optional) step S104. S104: The network node 200 configures the first transceiver device 110a and the second transceiver device 110b to perform the first bidirectional sounding in the first time period and the second bidirectional sounding in the second time period. The first transceiver device 110a and the second transceiver device 110b are configured to perform the first bidirectional sounding by wirelessly exchanging reference signals for calibration with each other in each of the first time period and the second time period. As disclosed above, the repeater 300 is by the network node 200 configured to operate in a first mode in the first time period and to operate in a second mode in the second time period. There could be different such first modes and second modes. In a first example, the first mode corresponds to the repeater 300 being switched on, and wherein the second mode corresponds to the repeater 300 being switched off. In a second example, the first mode corresponds to the repeater 300 operating with a first power level and wherein the second mode corresponds to the repeater 300 operating with a second power level lower than the first power level. In a third example, the first mode corresponds to the repeater 300 applying a phase shift of Δ degrees, and wherein the second mode corresponds to the repeater 300 applying a phase shift of Δ + δ degrees, where δ = ±90 degrees or δ = ±180 degrees. For ease of disclosure but without loss of generality it will hereinafter be assumed that the first mode corresponds to the repeater 300 being switched on, and wherein the second mode corresponds to the repeater 300 being switched off. The calibration is based on OTA measurements between transceiver device A and transceiver device B, some measurements with active involvement of the repeater 300. More specifically, two bidirectional measurements between transceiver device A and transceiver device B are taken, for example by transmitting pre-determined reference signals (RSs) from transceiver device A that are measured at transceiver device B, and vice versa. That is, in some embodiments, each of the first transceiver device 110a and the second transceiver device 110b is instructed to wirelessly transmit a first respective reference signal in the first time period, and each of the first transceiver device 110a and the second transceiver device 110b is instructed to wirelessly transmit a second respective reference signal in the second time period. Further, in some embodiments, the first bidirectional channel response measurement pertains to measurements on the first reference signals transmitted in the first time period, and wherein the second bidirectional channel response measurement pertains to measurements on the second reference signals transmitted in the second time period. Specifically, these bidirectional measurements collectively comprise four unidirectional measurements: (1) from transceiver device A to transceiver device B with the repeater 300 off, (2) from transceiver device A to transceiver device B with the repeater 300 on, (3) from transceiver device B to transceiver device A with the repeater 300 off, (4) and from transceiver device B to transceiver device A with the repeater 300 on. Therefore, in some embodiments, the repeater 300 is, by the network node 200, configured to, in the first time period, switch states of the forward path circuitry 360a and the reverse path circuitry 360b in-between transmission of the first reference signal from the first transceiver device 110a and the first reference signal from the second transceiver device 110b. Further, in some embodiments, the repeater 300 is, by the network node 200, configured to, in the second time period, switch states of the forward path circuitry 360a and the reverse path circuitry 360b in-between transmission of the second reference signal from the first transceiver device 110a and the second reference signal from the second transceiver device 110b. These measurements can be written as ^^^^ _^ ⁰ ^_^^ = ^^^^ _^^^^ ^^^^ ^^^^ _^^^^ + ^^^^ _^ ⁰ ^_^^ (1) ^^^^⁰ ^_{^^^ = ^^^^ ^^^^ ^^^^} ^{^^^^} _{^^^^ ^^^^ + ^^^^} ⁰ ^_^^^ ⁽ ₂ ⁾ ^^^^¹ ^_^^^ = ^^^^ _^^^^( ^^^^ ^{^^^^} + ^^^^ ^^^^ ^^^^ ^{^^^^}) ^^^^ _^^^^ + ^^^^¹ ^_^^^ (4) where the subscripts indicate the identity of the transmitting array (transceiver device A or transceiver device B), and the superscripts indicate the repeater 300 status (0=off, 1=on). The additive terms ^^^^ _^ ^{^^} ^^{^} ^^{^} ^ contain measurements noise. In some aspects, the bidirectional measurements, i.e. one when the repeater 300 is off and the other when the repeater 300 is on, are all be executed within the coherence time/frequency of the channel ^^^^ and of the channel ⁽ ^^^^ − ^^^^ ^^^^ ^^^^ ^{^^^^ )} for reciprocity to hold. Therefore, in some aspects, the two bidirectional measurements ^{occur within the minimum of the coherence time of ^^^^ and the coherence time of} _{( ^^^^ − ^^^^ ^^^^ ^^^^ ^^^^ ). That is, in some embodiments, the first time period and the second} time period extend over a time period being shorter than minimum coherence time of: a direct wireless between the first transceiver device 110a and the second transceiver device 110b, an indirect wireless between the first transceiver device 110a and the second transceiver device 110b via the repeater 300. In general terms, all parameters in ^^^^ _^^^^, ^^^^ _^^^^, ^^^^ _^^^^, ^^^^ _^^^^, ^^^^, ^^^^, ^^^^, ^^^^ and ^^^^ might be assumed to be a priori unknown. One objective is to estimate ^^^^ or ^^^^ from the bidirectional measurements of the channel between transceiver device A and transceiver device B. The problem of estimating both α and β is non-identifiable. However, the ratio ^^^^/ ^^^^ (or ^^^^/ ^^^^) can be estimated and this is all what is required to calibrate the repeater 300 in order for the repeater 300 to operate as a reciprocal node in reciprocity-based communications. That is, in some embodiments, determining the compensation factor involves estimating the ratio between α and β from the first bidirectional channel response measurement and the second bidirectional channel response measurement. That is, once the ratio ^^^^/ ^^^^ is estimated, the repeater 300 can be instructed to adjust the (complex-valued) gain in one (of its two) path(s) such that after the adjustment, the effective gain in each path is equal. For example, if the repeater 300 adjusts the gain of is forward path by ^^^^/ ^^^^, then after the adjustment the effective again of said forward path is ^^^^ ∙ ^^^^/ ^^^^ = ^^^^, which is equal to the gain of the reverse path. All unknowns, other than ^^^^ and ^^^^, can thereby be treated as nuisance parameters (i.e. parameters that are present in the model but are of no particular interest). It is here noted that all the herein disclosed estimators automatically eliminate the nuisance parameters ^^^^, ^^^^, ^^^^, ^^^^ and ^^^^ ^^^^ ^{^^^^} although these parameters are intrinsically present in the problem and their presence eventually affects the achievable accuracy for the estimation of ^^^^; adding nuisance parameters to an estimation problem can only increase the Cramér-Rao Bound, which is a lower bound on the variance of unbiased estimators. Some estimators to find ^^^^/ ^^^^ based on the measurements in Equations (1)-(4) will be disclosed next. Estimation approach 1 This approach ignores the contribution, and respective probability distribution function, of the additive noise during the measurements. In the noise-free case: ^^^^ _^ ¹ ^_{^^ ./ ^^^^} ⁰ ^_{^^^ − ^^^^ ^^^^ = ^^^^} ⁽ _{^^^^ ^^^^} ^{^^^^)} _{./ ^^^^} denotes element-wise division and ^^^^ _^^^^ is a matrix with ones in all of its ^{entries. Similarly,} (_^^^^1 _{^^^^ ./ ^^^^0} _{^^^^ − ^^^^ ^^^^) ^^^^ = ^^^^( ^^^^ ^^^^ ^^^^)./ ^^^^} ^{Dividing the two expressions above it follow that} (_^^^^1 _{^^^^ ./ ^^^^0} _{^^^^ − ^^^^ ^^^^) ^^^^ ./ ( ^^^^1 0} _{^^^^ ./ ^^^^ ^^^^ − ^^^^ ^^^^) = ^^^^ ^^^^ ^^^^} from which the ratio of interest ^^^^ = ^^^^/ ^^^^ can be easily obtained by averaging the elements. Specifically, the estimate is, in closed form: where ^^^^ _^^^^ and ^^^^ _^^^^ are column vectors with ones in all their entries, respectively. This corresponds to an embodiment where estimating the ratio between α and β involves averaging with equal weights of all component of the bidirectional channel response measurements. Estimation approach 2 Estimation approach 1 is statistically consistent (i.e., correct if there is no noise). But if measurement noise is considered in the above equations, then in the division by ^^^^⁰ ^_^^^ for example, if some element of ^^^^ is small, then the noise will be enhanced. This enhanced noise might propagate into the averaging in the last step of the algorithm. ^{One possible remedy is to take the average of only a subset of the components of} ._{/ ^^^^0 ^^^^ 1 0} _{^^^^ − ^^^^ ^^^^) ./ ( ^^^^ ^^^^ ./ ^^^^ ^^^^ − ^^^^ ^^^^), for example excluding the components with the} smallest magnitude. This modification improves performance in many scenarios. This corresponds to an embodiment where estimating the ratio between α and β involves averaging with unequal weights of at least some components of the bidirectional channel response measurements. Estimation approach 3 According to an embodiment the ratio between α and β is estimated using a maximum-likelihood / least-squares estimator. For a maximum-likelihood / least- squares estimation to be made, the following re-parameterizing is made. One purpose of this is to guarantee identifiability, that is, that the solution (i.e., the estimate ^^�^^) is unique in a noise-free case. Define ^^^^ = ^^^^ _^^^^ ^^^^ ^^^^ _^^^^ ^^^^ = ^^^^ ^^^^ _^^^^ ^^^^ ^^^^ ^{^^^^} ^^^^ _^^^^ ^^^^ = ^^^^ _^^^^ ^^^^⁻ ^_^^^ ¹ ^^^^ = ^^^^⁻ ^_^^^ ¹ ^^^^ _^^^^ With this re-parameterization the measurements in Equations (1)-(4) can be expressed as ^^^^ _^ ⁰ ^_^^ = ^^^^ + ^^^^ _^ ⁰ ^_^^ ^^^^⁰ ^_^^^ = ^^^^ ^^^^ ^{^^^^} ^^^^ + ^^^^⁰ ^_^^^ ^^^^¹ ^_^^^ = ^^^^( ^^^^ + ^^^^ ^^^^) ^{^^^^} ^^^^ + ^^^^¹ ^_^^^ where the unknowns now are ^^^^, ^^^^, ^^^^, ^^^^ and γ. This re-parameterization guarantees identifiability of all variables, up to an irrelevant scaling factor. Multiplying ^^^^ with a constant and dividing ^^^^ with the same constant does not change the measurements. Eventually, to calibrate the repeater 300, the sought parameter is the ratio ^^^^. Based on the observations, the following least-squares criterion can be formulated for the estimation: If an assumption is made that all noise components are mutually independent, zero- mean Gaussian with the same variance, then minimizing the above least-squares criterion gives the maximum-likelihood estimate. However, the criterion is technically sound to use even for other noise distributions or in case the noise distribution in unknown. This criterion can be minimized with respect to ^^^^, ^^^^, ^^^^, ^^^^ and γ, subject to the constraints that ^^^^ and ^^^^ are diagonal, and that ^^^^ has rank one. The scaling factor ambiguity can now be seen again: multiplication of ^^^^ by an arbitrary constant and division of ^^^^ by the same constant yields no change in the objective function. But other than that ambiguity, which is eventually unimportant, the problem is identifiable and well-posed. An approximate solution to the least-squares minimization can be found as described next. First estimate ^^^^ by minimizing the first term of the criterion: Next find ^^^^ and ^^^^ by minimizing the second term with respect to ^^^^ and ^^^^, inserting ^^�^^^ for ^^^^. This can be achieved by cyclically minimizing the following with respect to ^^^^ and ^^^^, until convergence (to a local optimum). _{Initialize ^^̂^^ = ^^} ^� _{^^ = ^^^^.} Compute the minimum with respect to ^^^^ _{^^^^ ^^^^} (for all ^^^^): Compute the minimum with respect to ^^^^ _{^^^^ ^^^^} (for all ^^^^), Then iterate between the computation of ^^^^ and ^^^^, until convergence. Next, ^^^^ can be estimated from the third term, inserting the already-obtained estimate of H: ^_{^̂^^ = ^^^^� ^^^^ ^} ¹ ^_{^^ − ^} ^� _^^^� where ^^^^[. ] denotes the best rank-one approximation to a matrix in the least-squares sense, which in turn is given by the first term in the singular value decomposition of Finally, γ can be estimated by minimizing the last term of the criterion, inserting the estimates of ^^^^, ^^^^ and ^^^^: One advantages of this least-squares approach are that it is statistically sound, easy to implement, and requires no tuning of any user parameters. The above least-squares criterion can be further minimized with respect to all unknowns, for example, using cyclic optimization. That is, after having obtained the estimates in Estimation approach 3, ^^^^ can be re-estimated by minimizing the complete objective function (including all its four terms) with respect to ^^^^, treating ^^^^, ^^^^, ^^^^ and γ as known; then, treating ^^^^, ^^^^ and γ as known minimize the objective with respect to ^^^^ and ^^^^; then, treating ^^^^, ^^^^, ^^^^ and γ as known minimize it with respect to ^^^^; then, treating ^^^^, ^^^^, ^^^^ and ^^^^ as known, minimize the objective with respect to γ; then, start over, and iterate this process until some pre-determined termination criterion is satisfied. Yet another alternative to minimize the objective function is to use a gradient search. In some aspects, instead of performing one bidirectional measurement with the repeater 300 turned off and one with the repeater 300 on, two bidirectional measurements can be taken with repeater 300 on, but configured to rotate the phase by two predetermined values. For example, these values can be 0 and 180 degrees, which results in the following two bidirectional measurements (ignoring noise here for simplicity): ^^^^ _^ ⁰ ^_{^^ = ^^^^ ^^^^} ⁽ _{^^^^ + ^^^^ ^^^^ ^^^^} ^{^^^^ )} _{^^^^ ^^^^} ^^^^¹ ^_{^^^ = ^^^^ ^^^^} ⁽ _^^^^ ^{^^^^} _{− ^^^^ ^^^^ ^^^^} ^{^^^^)} _{^^^^ ^^^^} From these equations, it is clear that ^^^^ _^ ⁰ ^_^^ = ^^^^ _^ ¹ ^_^^ and ^^^^⁰ ^_^^^ = ^^^^¹ ^_^^^ and that by adding ^^^^ _^ ⁰ ^_^^ and ^^^^ _^ ¹ ^_^^ we obtain 2 ^^^^ _^ ⁰ ^_^^; similarly, ^^^^⁰ ^_^^^ + ^^^^¹ ^_^^^ = 2 ^^^^⁰ ^_^^^ . Hence, the algorithms proposed in above (such as Estimation approach 1, Estimation approach 2, and Estimation approach 3) _{can be directly applied to the four measurements} ⁽ _{^^^^ ^} ⁰ ^_{^^ + ^^^^ ^} ¹ ^_^^ ⁾ _/2, ⁽ _^^^^ ⁰ ^_{^^^ + ^^^^} ¹ ^_^^^ ⁾ _{/2, ^^^^ ^} ⁰ ^_{^^ and} ^^^^⁰ ^_^^^ . Alternatively, other processing schemes operating directly on ^^^^ _^ ⁰ ^_^^, ^^^^ _^ ¹ ^_^^, ^^^^⁰ ^_^^^ , ^^^^¹ ^_^^^ (instead of operating on ^^^^ _^ ⁰ ^_^^, ^^^^⁰ ^_^^^ , ^^^^⁰ ^_^^^ + ^^^^¹ ^_^^^ and ^^^^ _^ ⁰ ^_^^ + ^^^^ _^ ¹ ^_^^ which have correlated additive noise terms) can derived to estimate ^^^^. For example, a direct maximum-likelihood approach can be used. One advantage of this approach, in which the repeater 300 is tuned on during the entire round of calibration measurements, is that in coverage-limited scenarios, the repeater 300 gain is not lost during the time when the calibration procedure is performed. Expressed differently, in scenarios where the repeater 300 provides the sole propagation path between node transceiver device A and transceiver device B (i.e., ^^^^ ≈ ^^^^), then the calibration accuracy benefits by having the repeater 300 on during the entire calibration procedure. As disclosed above, the network node 200 calibrates the repeater 300 by configuring the repeater 300 with the compensation factor. In further detail, in some embodiments, the repeater 300 is, by the network node 200, configured to apply the compensation factor to either the forward path circuitry 360a or the reverse path circuitry 360b. As disclosed above, the forward path circuitry 360a has a complex- valued gain ^^^^. Therefore, in some embodiments, the reverse path circuitry 360b has a complex-valued gain ^^^^, where the compensation factor is determined as a function of a ratio between ^^^^ and ^^^^. The calibration methods disclosed above allow reciprocity calibration of one non- reciprocal repeater 300. It is noted that disclosed methods extend readily to the case of calibrating a network with multiple repeater 300s. In some aspects, when calibrating one non-reciprocal repeater 300, the only source of channel non-reciprocity present in the first and second (bidirectional) measurements must come from such a non-calibrated repeater 300 itself, which follows from Equations (1)-(4). Otherwise, the calibration adjustment that will be computed for such a repeater 300 will be influence by external sources of channel non-reciprocity (and might therefore be incorrect). This implies that, when calibrating one such repeater 300, no other network non-calibrated repeater 300s should be active during the bidirectional measurements. On the other hand, already calibrated repeater 300s may, or may not, participate in the measurement process. That is, in some embodiments, the network 100a, 100b, 100c further comprises at least one further repeater 300, and actions for configuring of the repeater 300, configuring of the first transceiver device 110a and the second transceiver device 110b, determining the compensation factor, and calibrating the repeater 300 are repeated in turn for each repeater 300, where each repeater 300 is switched off until having been calibrated. Reference is now made to Fig.5 illustrating a method for a repeater 300 to be calibrated when operated in a wireless network 100a, 100b, 100c according to an embodiment. The repeater 300 is configured for communication in a forward direction and a reverse direction. The repeater 300 comprises forward path circuitry 360a for communicating in the forward direction and reverse path circuitry 360b for communicating in the reverse direction S202: The repeater 300 receives configuration from a network node 200 to operate in a first mode in a first time period and to operate in a second mode in a second time period. S204: The repeater 300 operates according to the configuration in the first time period and in the second time period. S206: The repeater 300 receives a compensation factor from the network node 200. The compensation factor is to be applied to either the forward path circuitry 360a or the reverse path circuitry 360b after the second time period. S208: The repeater 300 applies the compensation factor after the second time period. Embodiments relating to further details for the repeater 300 to be calibrated will now be disclosed. There could be different ways in which the configuration is provided from the network node 200 to the repeater 300. Alternatives differ in the required complexity of the receiver, the spectral efficiency, the flexibility of the communication protocol, etc. For example, the configuration could be provided in one or more control messages. Many alternatives exist. In a first example, the repeater 300 is capable of demodulating and decoding common downlink control and data channels. A sub-set of the hardware used in common UEs might be part of the repeater 300 implementation to support this. The repeater 300 might then be assigned a Radio Network Temporary Identifier (RNTI) by the serving cell upon registration. This will enable the network to schedule control messages to the repeater 300 using the common physical downlink control channel (PDCCH) and physical downlink shared channel (PDSCH). That is, the configuration might be sent either in a PDCCH message or in a PDSCH message. In a second example, the configuration is transmitted on a broadcast channel. In a third example, the configuration is sent over separate and/or proprietary communication channel from the network node 200 to the repeater 300. This communication channel might be designed to enable simple demodulation and detection, e.g. by using a matched filter searching for the presence of a set of pre- defined signals. As disclosed above, in some embodiments, the repeater 300 is, by the network node 200, configured to, in the first time period, switch states of the forward path circuitry 360a and the reverse path circuitry 360b. As further disclosed above, in some embodiments, the repeater 300 is, by the network node 200, configured to, in the second time period, switch states of the forward path circuitry 360a and the reverse path circuitry 360b. As disclosed above, the repeater 300 is by the network node 200 configured to operate in a first mode in the first time period and to operate in a second mode in the second time period. As also disclosed above, there could be different such first modes and second modes. In a first example, the first mode corresponds to the repeater 300 being switched on, and wherein the second mode corresponds to the repeater 300 being switched off. In a second example, the first mode corresponds to the repeater 300 operating with a first power level and wherein the second mode corresponds to the repeater 300 operating with a second power level lower than the first power level. In a third example, the first mode corresponds to the repeater 300 applying a phase shift of Δ degrees, and wherein the second mode corresponds to the repeater 300 applying a phase shift of Δ + δdegrees, where δ = ±90 degrees or δ = ±180 degrees. As disclosed above, the network node 200 calibrates the repeater 300 by configuring the repeater 300 with the compensation factor. In further detail, in some embodiments, the repeater 300 is, by the network node 200, configured to apply the compensation factor to either the forward path circuitry 360a or the reverse path circuitry 360b. As disclosed above, the forward path circuitry 360a has a complex- valued gain ^^^^. Therefore, in some embodiments, the reverse path circuitry 360b has a complex-valued gain ^^^^, where the compensation factor is determined as a function of a ratio between ^^^^ and ^^^^. One particular embodiment for calibrating a repeater 300 based on at least some of the above disclosed embodiments will now be disclosed in detail with reference to the signaling diagram of Fig.6. It is here assumed, for illustrative purposes, that the network node 200 is collocated with the first transceiver device. S301: The repeater 300 to be calibrated operates in a first mode and applies a first configuration to the circuity of its forward and reverse paths. In some examples, according to this first configuration the repeater 300 is on but does not apply any particular gain adjustments to the forward or reverse paths. S302a, S302b: A first bidirectional measurement process is performed between the first transceiver device and the second transceiver device. The first transceiver device transmits reference signal RS #1 and the second transceiver device transmits reference signal RS #2. S303: The repeater 300 applies a second configuration to the circuity of its forward and reverse paths. In some examples, according to this second configuration the repeater 300 is turned off. S304a, S304b: A second bidirectional measurement process is performed between the first transceiver device and the second transceiver device. The first transceiver device transmits reference signal RS #3 and the second transceiver device transmits reference signal RS #4. S305: The measurements of the first bidirectional measurement process and the second bidirectional measurement process are forwarded to the network node 200. S306: The network node 200 estimates the calibration factor ^^�^^ according to any of the above disclosed estimation techniques. S307: The network node 200 configures the repeater 300 with the calibration factor. S308: The repeater 300 applies the calibration factor to adjust the gain of its forward path, reverse path, or both. Simulation results will now be provided with reference to Fig.7. In Fig.7, simulation results are shown for an example where transceiver device A is equipped with ^^^^ _^^^^ = 4 antennas, and transceiver device B is equipped with ^^^^ _^^^^ = 3 antennas. For simplicity, the column vectors ^^^^ and ^^^^ are taken as random vector entries of discrete Fourier transform (DFT) matrices of size ^^^^ _^^^^ and ^^^^ _^^^^, respectively. The entries of the noise matrices ^^^^ _^ ⁰ ^_^^, ^^^^⁰ ^_^^^ , ^^^^ _^ ¹ ^_^^, and ^^^^¹ ^_^^^ are modelled as independent and identically distributed (i.i.d.) complex-valued circularly symmetric zero-mean Gaussian random variables with variance ^^^^₀. The entries of the propagation channel ^^^^ are modelled as i.i.d. complex-valued circularly symmetric unit-variance zero-mean Gaussian random variables. The signal to noise ratio (SNR) is defined per-antenna pair as ^^^^₀ ⁻¹. The diagonal entries of ^^^^ _^^^^, ^^^^ _^^^^, ^^^^ _^^^^ and ^^^^ _^^^^ are modelled as i.i.d. unit-norm phasors with phases drawn uniformly from the interval [0, 2 ^^^^]. In Fig.7 is shown a performance comparison of four different proposed estimators as disclosed herein. The minimum- squared error (MSE) performance, namely ^^^^{| ^^�^^ − ^^^^|²} , of the estimators converges to zero when the SNR is increased. This suggests that the disclosed estimators are fundamentally suitable to estimate the ratio ^^^^. Fig.8 schematically illustrates, in terms of a number of functional units, the components of a network node 200 according to an embodiment. Processing circuitry 210 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), etc., capable of executing software instructions stored in a computer program product 1210a (as in Fig.12), e.g. in the form of a storage medium 230. The processing circuitry 210 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA). Particularly, the processing circuitry 210 is configured to cause the network node 200 to perform a set of operations, or steps, as disclosed above. For example, the storage medium 230 may store the set of operations, and the processing circuitry 210 may be configured to retrieve the set of operations from the storage medium 230 to cause the network node 200 to perform the set of operations. The set of operations may be provided as a set of executable instructions. Thus the processing circuitry 210 is thereby arranged to execute methods as herein disclosed. The storage medium 230 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The network node 200 may further comprise a communications (comm.) interface 220 for communications with other entities, functions, nodes, and devices in the wireless network 100a, 100b, 100c, such as the transceiver devices 110a, 110b and the repeater 300. As such the communications interface 220 may comprise one or more transmitters and receivers, comprising analogue and digital components. The processing circuitry 210 controls the general operation of the network node 200 e.g. by sending data and control signals to the communications interface 220 and the storage medium 230, by receiving data and reports from the communications interface 220, and by retrieving data and instructions from the storage medium 230. Other components, as well as the related functionality, of the network node 200 are omitted in order not to obscure the concepts presented herein. Fig.9 schematically illustrates, in terms of a number of functional modules, the components of a network node 200 according to an embodiment. The network node 200 of Fig.9 comprises a number of functional modules; a configure module 210a configured to perform step S102, a determine module 210c configured to perform step S106, and a calibrate module 210d configured to perform step S110. The network node 200 of Fig.9 may further comprise a number of optional functional modules, such a configure module 210b configured to perform step S104. In general terms, each functional module 210a:210d may be implemented in hardware or in software. Preferably, one or more or all functional modules 210a:210d may be implemented by the processing circuitry 210, possibly in cooperation with the communications interface 220 and/or the storage medium 230. The processing circuitry 210 may thus be arranged to from the storage medium 230 fetch instructions as provided by a functional module 210a:210d and to execute these instructions, thereby performing any steps of the network node 200 as disclosed herein. The network node 200 may be provided as a standalone device or as a part of at least one further device. For example, the network node 200 may be provided in a node of the radio access network or in a node of the core network. For example, the network node 200 might be collocated with one of the transceiver devices 110a, 110b. Alternatively, functionality of the network node 200 may be distributed between at least two devices, or nodes. These at least two nodes, or devices, may either be part of the same network part (such as the radio access network or the core network) or may be spread between at least two such network parts. In general terms, instructions that are required to be performed in real time may be performed in a device, or node, operatively closer to the cell than instructions that are not required to be performed in real time. Thus, a first portion of the instructions performed by the network node 200 may be executed in a first device, and a second portion of the instructions performed by the network node 200 may be executed in a second device; the herein disclosed embodiments are not limited to any particular number of devices on which the instructions performed by the network node 200 may be executed. Hence, the methods according to the herein disclosed embodiments are suitable to be performed by a network node 200 residing in a cloud computational environment. Therefore, although a single processing circuitry 210 is illustrated in Fig.8 the processing circuitry 210 may be distributed among a plurality of devices, or nodes. The same applies to the functional modules 210a:210d of Fig.9 and the computer program 1220a of Fig.12. Some (radio) access network architectures define network nodes (or gNBs) comprising multiple component parts or nodes: a central unit (CU), one or more distributed units (DUs), and one or more radio units (RUs). The protocol layer stack of the network node 200 can be divided between the CU, the DUs and the RUs, with one or more lower layers of the stack implemented in the RUs, and one or more higher layers of the stack implemented in the CU and/or DUs. The CU is coupled to the DUs via a fronthaul higher layer split (HLS) network; the CU/DUs are connected to the RUs via a fronthaul lower-layer split (LLS) network. The DU may be combined with the CU in some embodiments, where a combined DU/CU may be referred to as a CU or simply a baseband unit. A communication link for communication of user data messages or packets between the RU and the baseband unit, CU, or DU is referred to as a fronthaul network or interface. Messages or packets may be transmitted from the network node 200 in the downlink (i.e., from the CU to the RU) or received by the network node 200 in the uplink (i.e., from the RU to the CU). Fig.10 schematically illustrates, in terms of a number of functional units, the components of a repeater 300 according to an embodiment. Processing circuitry 310 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), etc., capable of executing software instructions stored in a computer program product 1210b (as in Fig.12), e.g. in the form of a storage medium 330. The processing circuitry 310 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA). Particularly, the processing circuitry 310 is configured to cause the repeater 300 to perform a set of operations, or steps, as disclosed above. For example, the storage medium 330 may store the set of operations, and the processing circuitry 310 may be configured to retrieve the set of operations from the storage medium 330 to cause the repeater 300 to perform the set of operations. The set of operations may be provided as a set of executable instructions. Thus the processing circuitry 310 is thereby arranged to execute methods as herein disclosed. The storage medium 330 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The repeater 300 may further comprise a communications interface 320 for communications with other entities, functions, nodes, and devices in the wireless network 100a, 100b, 100c, such as the network node 200. As such the communications interface 320 may comprise one or more transmitters and receivers, comprising analogue and digital components. The processing circuitry 310 controls the general operation of the repeater 300 e.g. by sending data and control signals to the communications interface 320 and the storage medium 330, by receiving data and reports from the communications interface 320, and by retrieving data and instructions from the storage medium 330. Other components, as well as the related functionality, of the repeater 300 are omitted in order not to obscure the concepts presented herein. Fig.11 schematically illustrates, in terms of a number of functional modules, the components of a repeater 300 according to an embodiment. The repeater 300 of Fig. 11 comprises a number of functional modules; a receive module 310a configured to perform step S202, an operate module 310b configured to perform step S204, a receive module 310c configured to perform step S206, and an apply module 310d configured to perform step S208. The repeater 300 of Fig.11 may further comprise a number of optional functional modules, as represented by functional module 310e. In general terms, each functional module 310a:310e may be implemented in hardware or in software. Preferably, one or more or all functional modules 310a:310e may be implemented by the processing circuitry 310, possibly in cooperation with the communications interface 320 and/or the storage medium 330. The processing circuitry 310 may thus be arranged to from the storage medium 330 fetch instructions as provided by a functional module 310a:310e and to execute these instructions, thereby performing any steps of the repeater 300 as disclosed herein. Fig.12 shows one example of a computer program product 1210a, 1210b comprising computer readable means 1230. On this computer readable means 1230, a computer program 1220a can be stored, which computer program 1220a can cause the processing circuitry 210 and thereto operatively coupled entities and devices, such as the communications interface 220 and the storage medium 230, to execute methods according to embodiments described herein. The computer program 1220a and/or computer program product 1210a may thus provide means for performing any steps of the network node 200 as herein disclosed. On this computer readable means 1230, a computer program 1220b can be stored, which computer program 1220b can cause the processing circuitry 310 and thereto operatively coupled entities and devices, such as the communications interface 320 and the storage medium 330, to execute methods according to embodiments described herein. The computer program 1220b and/or computer program product 1210b may thus provide means for performing any steps of the repeater 300 as herein disclosed. In the example of Fig.12, the computer program product 1210a, 1210b is illustrated as an optical disc, such as a CD (compact disc) or a DVD (digital versatile disc) or a Blu-Ray disc. The computer program product 1210a, 1210b could also be embodied as a memory, such as a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM), or an electrically erasable programmable read-only memory (EEPROM) and more particularly as a non-volatile storage medium of a device in an external memory such as a USB (Universal Serial Bus) memory or a Flash memory, such as a compact Flash memory. Thus, while the computer program 1220a, 1220b is here schematically shown as a track on the depicted optical disk, the computer program 1220a, 1220b can be stored in any way which is suitable for the computer program product 1210a, 1210b. The inventive concept has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended patent claims.

Claims

CLAIMS 1. A method for calibrating a repeater (300) in a wireless network (100a, 100b, 100c), the wireless network (100a, 100b, 100c) further comprising a first transceiver device (110a) and a second transceiver device (110b) configured to wirelessly communicate with each other without and via the repeater (300), wherein the method is performed by a network node (200), and wherein the method comprises: configuring (S102) the repeater (300) to operate in a first mode in a first time period and to operate in a second mode in a second time period; determining (S106) a compensation factor as a function of a first bidirectional channel response measurement obtained for a first bidirectional sounding performed by the first transceiver device (110a) and the second transceiver device (110b) in the first time period and a second bidirectional channel response measurement obtained for a second bidirectional sounding performed by the first transceiver device (110a) and the second transceiver device (110b) in the second time period; and calibrating (S108) the repeater (300) by configuring the repeater (300) with the compensation factor.

2. The method according to claim 1, further comprising: configuring (S104) the first transceiver device (110a) and the second transceiver device (110b) to perform the first bidirectional sounding in the first time period and the second bidirectional sounding in the second time period by wirelessly exchanging reference signals for calibration with each other in each of the first time period and the second time period.

3. The method according to claim 1 or 2, wherein each of the first transceiver device (110a) and the second transceiver device (110b) is instructed to wirelessly transmit a first respective reference signal in the first time period, and each of the first transceiver device (110a) and the second transceiver device (110b) is instructed to wirelessly transmit a second respective reference signal in the second time period.

4. The method according to claim 3, wherein the first bidirectional channel response measurement pertains to measurements on the first reference signals transmitted in the first time period, and wherein the second bidirectional channel response measurement pertains to measurements on the second reference signals transmitted in the second time period.

5. The method according to any preceding claim, wherein the repeater (300) comprises forward path circuitry (360a) for communicating in a forward direction and reverse path circuitry (360b) for communicating in a reverse direction.

6. The method according to claim 5, wherein the repeater (300) is, by the network node (200), configured to, in the first time period, switch states of the forward path circuitry (360a) and the reverse path circuitry (360b) in-between transmission of the first reference signal from the first transceiver device (110a) and the first reference signal from the second transceiver device (110b).

7. The method according to claim 5 or 6, wherein the repeater (300) is, by the network node (200), configured to, in the second time period, switch states of the forward path circuitry (360a) and the reverse path circuitry (360b) in-between transmission of the second reference signal from the first transceiver device (110a) and the second reference signal from the second transceiver device (110b).

8. The method according to any of claims 1 to 7, wherein: the first mode corresponds to the repeater (300) being switched on, and wherein the second mode corresponds to the repeater (300) being switched off, or the first mode corresponds to the repeater (300) operating with a first power level and wherein the second mode corresponds to the repeater (300) operating with a second power level lower than the first power level, and/or the first mode corresponds to the repeater (300) applying a phase shift of Δ degrees, and wherein the second mode corresponds to the repeater (300) applying a phase shift of Δ + δdegrees, where δ = ±90 degrees or δ = ±180 degrees.

9. The method according to any preceding claim, wherein the first time period and the second time period extend over a time period being shorter than minimum coherence time of: a direct wireless between the first transceiver device (110a) and the second transceiver device (110b), an indirect wireless between the first transceiver device (110a) and the second transceiver device (110b) via the repeater (300).

10. The method according to claim 5, wherein the repeater (300) is, by the network node (200), configured to apply the compensation factor to either the forward path circuitry (360a) or the reverse path circuitry (360b).

11. The method according to claim 5 or 10, wherein the forward path circuitry (360a) has a complex-valued gain α, wherein the reverse path circuitry (360b) has a complex-valued gain β, and wherein the compensation factor is determined as a function of a ratio between α and β.

12. The method according to claim 11, wherein determining the compensation factor involves estimating the ratio between α and β from the first bidirectional channel response measurement and the second bidirectional channel response measurement.

13. The method according to claim 12, wherein estimating the ratio between α and β involves averaging with equal weights of all components of the bidirectional channel response measurements.

14. The method according to claim 12, wherein estimating the ratio between α and β involves averaging with unequal weights of at least some components of the bidirectional channel response measurements.

15. The method according to claim 12, wherein the ratio between α and β is estimated using a maximum-likelihood / least-squares estimator.

16. The method according to any preceding claim, wherein the network (100a, 100b, 100c) further comprises at least one further repeater (300), and wherein actions for configuring of the repeater (300), configuring of the first transceiver device (110a) and the second transceiver device (110b), determining the compensation factor, and calibrating the repeater (300) are repeated in turn for each repeater (300), and wherein each repeater (300) is switched off until having been calibrated.

17. The method according to any preceding claim, wherein the repeater (300) is a dual-antenna repeater (300).

18. The method according to any preceding claim, wherein the first transceiver device (110a) is a first access point in the network (100a, 100b, 100c), and the second transceiver device (110b) is a second access point in the network (100a, 100b, 100c).

19. The method according to any of claims 1 to 17, wherein the first transceiver device (110a) is a first antenna subarray of an access point in the network (100a, 100b, 100c), and the second transceiver device (110b) is a second subarray of the access point.

20. The method according to any of claims 1 to 17, wherein the first transceiver device (110a) is an access point in the network (100a, 100b, 100c), and the second transceiver device (110b) is a user equipment served by the access point.

21. A method for a repeater (300) to be calibrated, the repeater (300) being configured for communication in a forward direction and a reverse direction, wherein the repeater (300) comprises forward path circuitry (360a) for communicating in the forward direction and reverse path circuitry (360b) for communicating in the reverse direction, wherein the method is performed by the repeater (300), and wherein the method comprises: receiving (S202) configuration from a network node (200) to operate in a first mode in a first time period and to operate in a second mode in a second time period; operating (S204) according to the configuration in the first time period and in the second time period; receiving (S206) a compensation factor from the network node (200), wherein the compensation factor is to be applied to either the forward path circuitry (360a) or the reverse path circuitry (360b) after the second time period; and applying (S208) the compensation factor after the second time period.

22. The method according to claim 21, wherein the repeater (300) is, by the network node (200), configured to, in the first time period, switch states of the forward path circuitry (360a) and the reverse path circuitry (360b).

23. The method according to claim 21 or 22, wherein the repeater (300) is, by the network node (200), configured to, in the second time period, switch states of the forward path circuitry (360a) and the reverse path circuitry (360b).

24. The method according to claim 21 or 22, wherein: the first mode corresponds to the repeater (300) being switched on, and wherein the second mode corresponds to the repeater (300) being switched off, or the first mode corresponds to the repeater (300) operating with a first power level and wherein the second mode corresponds to the repeater (300) operating with a second power level lower than the first power level, and/or the first mode corresponds to the repeater (300) applying a phase shift of Δ degrees, and wherein the second mode corresponds to the repeater (300) applying a phase shift of Δ + δdegrees, where δ = ±90 degrees or δ = ±180 degrees.

25. The method according to any of claims 21 to 24, wherein the repeater (300) is, by the network node (200), configured to apply the compensation factor to either the forward path circuitry (360a) or the reverse path circuitry (360b).

26. The method according to any of claims 21 to 25, wherein the forward path circuitry (360a) has a complex-valued gain α, wherein the reverse path circuitry (360b) has a complex-valued gain β, and wherein the compensation factor is a function of a ratio between α and β.

27. The method according to any of claims 21 to 26, wherein the repeater (300) is a dual-antenna repeater.

28. A network node (200) for calibrating a repeater (300) in a wireless network (100a, 100b, 100c), the wireless network (100a, 100b, 100c) further comprising a first transceiver device (110a) and a second transceiver device (110b) configured to wirelessly communicate with each other without and via the repeater (300), the network node (200) comprising processing circuitry (210), the processing circuitry being configured to cause the network node (200) to: configure the repeater (300) to operate in a first mode in a first time period and to operate in a second mode in a second time period; determine a compensation factor as a function of a first bidirectional channel response measurement obtained for a first bidirectional sounding performed by the first transceiver device (110a) and the second transceiver device (110b) in the first time period and a second bidirectional channel response measurement obtained for a second bidirectional sounding performed by the first transceiver device (110a) and the second transceiver device (110b) in the second time period; and calibrate the repeater (300) by configuring the repeater (300) with the compensation factor.

29. A network node (200) for calibrating a repeater (300) in a wireless network (100a, 100b, 100c), the wireless network (100a, 100b, 100c) further comprising a first transceiver device (110a) and a second transceiver device (110b) configured to wirelessly communicate with each other without and via the repeater (300), the network node (200) comprising: a configure module (210a) configured to configure the repeater (300) to operate in a first mode in a first time period and to operate in a second mode in a second time period; a determine module (210c) configured to determine a compensation factor as a function of a first bidirectional channel response measurement obtained for a first bidirectional sounding performed by the first transceiver device (110a) and the second transceiver device (110b) in the first time period and a second bidirectional channel response measurement obtained for a second bidirectional sounding performed by the first transceiver device (110a) and the second transceiver device (110b) in the second time period; and a calibrate module (210d) configured to calibrate the repeater (300) by configuring the repeater (300) with the compensation factor.

30. The network node (200) according to claim 28 or 29, further being configured to perform the method according to any of claims 2 to 20.

31. A repeater (300) to be calibrated, the repeater (300) being configured for communication in a forward direction and a reverse direction, wherein the repeater (300) comprises forward path circuitry (360a) for communicating in the forward direction and reverse path circuitry (360b) for communicating in the reverse direction, the repeater (300) comprising processing circuitry (310), the processing circuitry being configured to cause the repeater (300) to: receive configuration from a network node (200) to operate in a first mode in a first time period and to operate in a second mode in a second time period; operate according to the configuration in the first time period and in the second time period; receive a compensation factor from the network node (200), wherein the compensation factor is to be applied to either the forward path circuitry (360a) or the reverse path circuitry (360b) after the second time period; and apply the compensation factor after the second time period.

32. A repeater (300) to be calibrated, the repeater (300) being configured for communication in a forward direction and a reverse direction, wherein the repeater (300) comprises forward path circuitry (360a) for communicating in the forward direction and reverse path circuitry (360b) for communicating in the reverse direction, the repeater (300) comprising: a receive module (310a) configured to receive configuration from a network node (200) to operate in a first mode in a first time period and to operate in a second mode in a second time period; an operate module (310b) configured to operate according to the configuration in the first time period and in the second time period; a receive module (310c) configured to receive a compensation factor from the network node (200), wherein the compensation factor is to be applied to either the forward path circuitry (360a) or the reverse path circuitry (360b) after the second time period; and an apply module (310d) configured to apply the compensation factor after the second time period.

33. The repeater (300) according to claim 31 or 32, further being configured to perform the method according to any of claims 22 to 27.

34. A computer program (1220a) for calibrating a repeater (300) in a wireless network (100a, 100b, 100c), the wireless network (100a, 100b, 100c) further comprising a first transceiver device (110a) and a second transceiver device (110b) configured to wirelessly communicate with each other without and via the repeater (300), the computer program comprising computer code which, when run on processing circuitry (210) of a network node (200), causes the network node (200) to: configure (S102) the repeater (300) to operate in a first mode in a first time period and to operate in a second mode in a second time period; determine (S106) a compensation factor as a function of a first bidirectional channel response measurement obtained for a first bidirectional sounding performed by the first transceiver device (110a) and the second transceiver device (110b) in the first time period and a second bidirectional channel response measurement obtained for a second bidirectional sounding performed by the first transceiver device (110a) and the second transceiver device (110b) in the second time period; and calibrate (S108) the repeater (300) by configuring the repeater (300) with the compensation factor.

35. A computer program (1220b) for a repeater (300) to be calibrated, the repeater (300) being configured for communication in a forward direction and a reverse direction, wherein the repeater (300) comprises forward path circuitry (360a) for communicating in the forward direction and reverse path circuitry (360b) for communicating in the reverse direction, the computer program comprising computer code which, when run on processing circuitry (310) of the repeater (300), causes the repeater (300) to: receive (S202) configuration from a network node (200) to operate in a first mode in a first time period and to operate in a second mode in a second time period; operate (S204) according to the configuration in the first time period and in the second time period; receive (S206) a compensation factor from the network node (200), wherein the compensation factor is to be applied to either the forward path circuitry (360a) or the reverse path circuitry (360b) after the second time period; and apply (S208) the compensation factor after the second time period.

36. A computer program product (1210a, 1210b) comprising a computer program (1220a, 1220b) according to at least one of claims 34 and 35, and a computer readable storage medium (1230) on which the computer program is stored.