EP4335038A1

EP4335038A1 - Phase-alignment in a distributed mimo communication system

Info

Publication number: EP4335038A1
Application number: EP21724249.4A
Authority: EP
Inventors: Pål FRENGER; Joao VIEIRA; Erik G. Larsson; Unnikrishnan KUNNATH GANESAN; Sarvendranath RIMALAPUDI
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2021-05-05
Filing date: 2021-05-05
Publication date: 2024-03-13
Also published as: WO2022233406A1

Abstract

There is provided mechanisms for phase -alignment between antenna panels at different APs in a distributed MIMO communication system. Each of the antenna panels comprises multiple antenna elements calibrated for uplink-downlink reciprocity. A method is performed by a first AP of the APs. The method comprises determining a reference phase rotation of a second reference signal received from a second AP of the APs. The method comprises transmitting a third reference signal towards the second AP. The third reference signal is phase adjusted with a phase adjustment factor determined from the reference phase rotation.

Description

PHASE- ALIGNMENT IN A DISTRIBUTED MIMO COMMUNICATION SYSTEM

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.

TECHNICAL FIELD

Embodiments presented herein relate to access points, systems, methods, computer programs, and a computer program product for phase-alignment between antenna panels at different access points in a distributed multiple -input multiple-output communication system.

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.

Distributed MIMO (D-MIMO, also referred to as cell-free massive MIMO, RadioStripes, RadioWeaves, and ubiquitous MIMO) is a candidate for the physical layer of the 6^th generation (6G) telecommunication system. D-MIMO is based on geographically distributing the antennas of the network and configure them to operate phase-coherently together.

There are many different variants of D-MIMO just as there are many different variants of traditional centralized MIMO. Beamforming may e.g. be analog, digital, or hybrid. MIMO techniques can provide diversity, directivity, spatial multiplexing, etc. D-MIMO techniques suitable for uplink communication can differ significantly from D-MIMO techniques suitable for downlink communication.

In a typical architecture multiple access points (APs), where each AP has one or more antenna panel, are interconnected and configured such that two or more APs can cooperate in coherent decoding of data from a given user equipment (UE) served by the network, and such that two or more APs can cooperate in coherent transmission of data to a UE. Each panel might comprise multiple antenna elements that are configured to operate phase-coherently together. One way of operation is time-division duplexing (TDD), relying on reciprocity of the physical radio propagation channel between the APs and the UE, whereby uplink pilot signals transmitted by served UEs are used by the APs to simultaneously obtain both uplink and downlink channel responses. This type of TDD operation is commonly referred to as reciprocity- based operation.

For reciprocity -based coherent D-MIMO transmission to work, all antennas at the network side must be appropriately synchronized and calibrated. Specifically, the following three tasks need be accomplished. Firstly, each antenna element in every panel has to be calibrated for uplink-downlink reciprocity. This in order to compensate for phase (and amplitude) mismatches between the receive and transmit branches of the hardware. This can be achieved, for example, by performing pairwise measurements between the antenna elements located in the same panel, and without interaction among different ones of the panels.

Secondly, the APs need to agree on a common frequency reference to drive their mixer. This can be achieved by communication between the APs using an over-the-air protocols. For example, a master transmitter can broadcast a frequency correction burst. Cable-based (e.g. fiber or ethemet) technologies for this communication are also possible.

Thirdly, the APs need to agree on a common global phasor in order to be aligned in phase. Since a small time-shift of a narrowband signal is equivalent to a phase-shift, synchronizing to the global phasor can be viewed as performing a fine time synchronization. This is required for joint coherent beamforming in the downlink to work properly when multiple APs co-operate. One solution is to connect all APs by means of a cable or fiber with precise calibration of all electronics to achieve this phase alignment. However, protocols that rely on measurements over-the-air are also possible. On example is to use pairwise bi directional measurements between the APs to obtain a common phase reference, or time-base.

It is also possible to involve the UEs in the synchronization and calibration tasks, but this is commonly considered undesirable for several reasons. For example, there might not even be a UE available, or the physical radio propagation channel between the APs and a UE may weak and/or fading. Relying on feedback from the UEs also places the burden of network synchronization on the UEs, which could result in a higher need for processing at the UE side, thus possibly reduced the battery life at the UE side. Involving the UEs introduces additional delay and overhead, resulting in reduced synchronization accuracy and capacity, since the measurements made by the UEs need to be communicated to the network before the measurements can be used for synchronization purposes of the APs.

In this respect, synchronization can refer to time, phase, or frequency. Further, there can be different levels of synchronization (such as either absolute synchronization, or relative synchronization) with different required accuracy for different purposes. One aspect of synchronization concerns the aligning the phases of signals transmitted from different distributed APs. One purpose of this phase alignment is that signals transmitted from different APs should add up coherently at the location of the receiving UE. The coherent phase alignment does not have to be perfect to be beneficial. The smaller phase alignment errors that can be achieve, the better the signal-to-interference-plus-noise ratio (SINR) will be obtained at the receiver of the UEs. Considering the case with only two distributed APs, the signals at a UE will add up to a larger resulting signal if the phase difference between the two components is less than +120 degrees (i.e. two vectors of the same length add up to a longer vector if the angle between them is less than 120 degrees). Techniques exist for reciprocity calibration of a single antenna panel (i.e., for antenna elements within an antenna panel). However, there is still a need for techniques for phase alignment (timing) and frequency synchronization between antenna panels of different APs in a D-MIMO system. This cannot be performed locally at each AP but hence it requires interaction between different APs. This imposes another challenge since the antenna panels of the APs might be installed in such a way that there is no strong (line-of-sight) channel between antenna panels of different APs, and/or that the APs are not aware of the locations of antenna panels at other APs (or not even the location of other APs). Further, there might be strong interference, for example when the D-MIMO system is deployed for operation in an unlicensed frequency band. This can result in a loss of signal-to-noise ratio (SNR) and signal-to-interference ratio (SIR) and eventually to a loss of reliability.

SUMMARY

An object of embodiments herein is to overcome the above noted issues and enable phase alignment (timing) and frequency synchronization between antenna panels of different APs in a D-MIMO system.

According to a first aspect there is presented a method for phase-alignment between antenna panels at different APs in a distributed MIMO communication system. Each of the antenna panels comprises multiple antenna elements calibrated for uplink-downlink reciprocity. The method is performed by a first AP of the APs. The method comprises determining a reference phase rotation a of a second reference signal received from a second AP of the APs. The method comprises transmitting a third reference signal towards the second AP. The third reference signal is phase adjusted with a phase adjustment factor g determined from the reference phase rotation a.

According to a second aspect there is presented a first AP for phase-alignment between antenna panels at different APs in a distributed MIMO communication system. Each of the antenna panels comprises multiple antenna elements calibrated for uplink-downlink reciprocity. The first AP comprises processing circuitry (210). The processing circuitry is configured to cause the first AP to determine a reference phase rotation a of a second reference signal received from a second AP of the APs. The processing circuitry is configured to cause the first AP to transmit a third reference signal towards the second AP. The third reference signal is phase adjusted with a phase adjustment factor g determined from the reference phase rotation a.

According to a third aspect there is presented a computer program for phase -alignment between antenna panels at different APs, the computer program comprising computer program code which, when run on processing circuitry of a first AP, causes the first AP to perform a method according to the first aspect.

According to a fourth aspect there is presented a method for phase-alignment between antenna panels at different APs in a distributed MIMO communication system. Each of the antenna panels comprises multiple antenna elements calibrated for uplink-downlink reciprocity. The method is performed by a second AP of the APs. The method comprises transmitting a second reference signal towards a first AP of the APs. The method comprises determining a phase alignment factor d from a third reference signal received from the first AP. The third reference signal has a phase rotation b compared to the second reference signal. The phase alignment factor d is determined from the phase rotation b.

According to a fifth aspect there is presented a second AP for phase-alignment between antenna panels at different APs in a distributed MIMO communication system. Each of the antenna panels comprises multiple antenna elements calibrated for uplink-downlink reciprocity. The second AP comprises processing circuitry (310). The processing circuitry is configured to cause the second AP to transmit a second reference signal towards a first AP of the APs. The processing circuitry is configured to cause the second AP to determine a phase alignment factor d from a third reference signal received from the first AP. The third reference signal has a phase rotation b compared to the second reference signal. The phase alignment factor d is determined from the phase rotation b.

According to a sixth aspect there is presented a computer program for phase-alignment between antenna panels at different APs, the computer program comprising computer program code which, when run on processing circuitry of a second AP, causes the second AP to perform a method according to the fourth aspect.

According to a seventh aspect there is presented a computer program product comprising a computer program according to at least one of the third aspect and the sixth 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.

According to an eight aspect there is presented a distributed MIMO communication system comprising a first AP according to the second aspect and at least one a second AP according to the fifth.

Advantageously, these aspects enable phase alignment (timing) and frequency synchronization between antenna panels of different APs in a D-MIMO system.

Advantageously, these aspects enable a fast way for phase alignment (timing) and frequency synchronization since the phase alignment process can be considered as consisting of one uplink/downlink handshake per AP-pair (where the uplink/downlink handshake is defined by transmission and reception of the second reference signal and transmission and reception of the third reference signal). Such uplink/downlink handshake can be performed simultaneously for a plurality of AP -pairs. Due to the quickness of the method, it guarantees that a bi-direction channel between two APs is phase reciprocal, which is a necessary condition for the alignment.

Advantageously, these aspects enable improved SNR and SIR in the detection of the phase alignment signals (i.e. the second and third reference signals), by virtue of beamforming gain achieved in the transmission of these signal, and the spatial processing gain when receiving these signal with multiple antennas. Advantageously, these aspects enable the overhead for reference signals to be lowered in a typical deployment scenario (compared to full or pruned beam-sweep of all AP pairs). Even if time-repetition is required, the amount of repetition required cannot be larger than the amount of spatial repetition in a beam-sweeping procedure. At worst the required overhead is the same.

Advantageously, these aspects enable that pre-compensation of phase alignment can be done locally at each AP.

Advantageously, these aspects enable both phase errors and frequency errors to be corrected for.

Advantageously, these aspects enable the overhead caused by the transmission of the reference signals to be adapted. The periodicity of the phase alignment handshake (second and third reference signals) can be determined based on statistics of phase alignment compensation factor updates. The periodicity of the beam-forming determination signal (first reference signal) can be adapted e.g. based on variations of observed SINR on the received phase alignment handshake signals.

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:

Fig. 1 is a schematic diagram illustrating a communication system according to embodiments;

Figs. 2, 3, and 4 are flowcharts of methods according to embodiments;

Figs. 5 and 6 are sequence diagrams of methods according to embodiments;

Fig. 7 is a schematic illustration of transmission of second reference signals according to embodiments;

Fig. 8 is a schematic illustration of reception of second reference signals and transmission of third reference signals according to embodiments; Fig. 9 is a schematic illustration of transmission of a first reference signal, a second reference signal, and a third reference signal according to embodiments;

Fig. 10 is a schematic illustration of phase alignment error as a function of time according to embodiments;

Fig. 11 is a schematic diagram showing functional units of an AP 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.

Fig. 1 is a schematic diagram illustrating a communication system 100 where embodiments presented herein can be applied. In some aspects the communication system 100 is a distributed MIMO communication system 100. The communication system 100 comprises APs, five of which are identified at reference numerals 200a, 200b, 200c, 200d, 200e. In this respect, the herein disclosed embodiments are not limited to any particular number of APs 200a:200e as long as there are at least two APs 200a:200e. Each AP 200a:200e could be a (radio) access network node, radio base station, base transceiver station, node B (NB), evolved node B (eNB), gNB, integrated access and backhaul (IAB) node, or the like. The APs 200a:200e operatively connected over interfaces 120 to a centralized node 300, which could represent a core network. The APs 200a:200e are configured to provide network access to UEs, one of which is illustrated at reference numeral 400. Each such UE 400 could be any of a portable wireless device, mobile station, mobile phone, handset, wireless local loop phone, smartphone, laptop computer, tablet computer, wireless modem, wireless sensor device, Internet of Things (IoT) device, network equipped vehicle, or the like.

The APs 200a:200e are configured for wireless communication with each other (as well as with the UE 400). In some examples, the APs 200a:200e use beamforming for this communication, as represented by beams 110a, 110b, 110c. Beam 110a is an example of a wide beam, or omni-directional beam, whereas beams 110b, 110c are examples of narrow beams, or so-called pencil beams. Each AP 200a:”00b comprises at least one antenna panel. For ease of description but without loss of generality it will hereinafter be assumed that each AP 200a:200e comprises one antenna panel. Each antenna panel is composed of a plurality of individual antenna elements.

As disclosed above, there is still a need for techniques for phase alignment (timing) and frequency synchronization between antenna panels of different APs in a D-MIMO system.

The embodiments disclosed herein therefore relate to mechanisms for phase-alignment between antenna panels at different APs 200a:200e in a distributed MIMO communication system 100. In order to obtain such mechanisms there is provided APs 200a:200e, methods performed by the APs 200a:200e, computer program products comprising code, for example in the form of computer programs, that when run on processing circuitry of the APs 200a:200e, causes the APs 200a:200e to perform the methods.

One of the APs is selected as reference AP. Without loss of generality, AP 200a will hereinafter be referred to as a first AP, or reference AP (denoted AP_R), whereas AP 200b will be referred to as a second AP, or non-calibrated panel AP (denoted AP_npa). HOW to select the reference AP will be disclosed below.

The antenna panel at the first AP 200a is assumed to haveM_R antenna elements. The antenna panel at the second AP 200b is assumed to have M_NPA antenna elements. It is assumed that the individual antenna panels are reciprocity -calibrated (but not phase aligned), such that, in mathematical representation, the physical radio propagation channel from the first AP 200a to the second AP 200b is equal to the (transpose of) the physical radio propagation channel from the second AP 200b to the first AP 200a. This can be achieved using known techniques for intra-panel reciprocity calibration.

A mechanism is disclosed for improved over-the-air phase-alignment (timing) and frequency synchronization based on pairwise transmission between APs 200a:200e.

Reference is now made to Fig. 2 illustrating a method for phase-alignment between antenna panels at different APs 200a:200e in a distributed MIMO communication system 100 as performed by a first AP 200a of the APs 200a:200e according to an embodiment. Each of the antenna panels comprises multiple antenna elements calibrated for uplink-downlink reciprocity, but the antenna panels are not mutually phase aligned.

S104: The first AP 200a determines a reference phase rotation a of a second reference signal received from a second AP 200b of the APs 200a:200e.

S106: The first AP 200a transmits a third reference signal towards the second AP 200b. The third reference signal is phase adjusted with a phase adjustment factor g determined from the reference phase rotation a.

Advantageously, this method enables phase alignment (timing) and frequency synchronization between antenna panels of different APs in a D-MIMO system. Advantageously, this method enables a fast way for phase alignment (timing) and frequency synchronization since the phase alignment process can be considered as consisting of one uplink/downlink handshake per AP-pair (where the uplink/downlink handshake is defined by transmission and reception of the second reference signal and transmission and reception of the third reference signal). Such uplink/downlink handshake can be performed simultaneously for a plurality of AP -pairs. Due to the quickness of the method, it guarantees that a bi-direction channel between two APs is phase reciprocal, which is a necessary condition for the alignment.

Advantageously, this method enables improved SNR and SIR in the detection of the phase alignment signals (i.e. the second and third reference signals), by virtue of beamforming gain achieved in the transmission of these signal, and the spatial processing gain when receiving these signal with multiple antennas.

Advantageously, this method enables the overhead for reference signals to be lowered in a typical deployment scenario (compared to full or pruned beam-sweep of all AP pairs). Even if time-repetition is required, the amount of repetition required cannot be larger than the amount of spatial repetition in a beam-sweeping procedure. At worst the required overhead is the same.

Advantageously, this method enables pre-compensation of phase alignment can be done locally at each AP.

Advantageously, this method enables both phase errors and frequency errors to be corrected for.

Advantageously, these aspects enable the overhead caused by the transmission of the reference signals to be adapted. The periodicity of the phase alignment handshake (second and third reference signals) can be determined based on statistics of phase alignment compensation factor updates.

Embodiments relating to further details of phase-alignment between antenna panels at different APs 200a:200e as performed by the first AP 200a will now be disclosed.

There could be different ways in which the phase adjustment factor g is determined. In some aspects, the phase adjustment factor g is equal to the conjugate of the reference phase rotation a. That is, in some embodiments, the phase adjustment factor g used when transmitting the third reference signal is determined as a conjugate of the reference phase rotation a. As will be further disclosed below, this phase adjustment factor g is directly used by the second AP 200b when transmitting a data signals towards the UE 400. In some embodiments, the phase adjustment factor g used when transmitting the third reference signal is determined as g = —a.

As will be further disclosed below with reference to Fig. 6, there could be a difference, denoted D between the local clock phase at the first AP 200a and the second AP 200b. The reference phase rotation a is then dependent on this difference D. Further, properties of the radio propagation channel between the first AP 200a and the second AP 200b might impose a channel phase rotation factor t on signals transmitted between the APs 200a:200e. Therefore, in some embodiments, the reference phase rotation a is equal to a = A + t, where D is a difference in local clock phase between the first AP 200a and the second AP 200b, and where t is a channel phase rotation factor of a radio propagation channel between the first AP 200a and the second AP 200b.

As disclosed above, the APs 200a:200e might use beamforming for communication between each other. Hence, in some embodiments, the second reference signal and the third reference signal are beamformed.

Before receiving the second reference signal, the first AP 200a itself might transmit a first reference signal. One purpose of this reference signal is to enable the second AP 200b:200e to estimate a direction towards the first AP 200a. In turn, this could allow the second AP 200b:200e to beamform the transmission of the second reference signal. Hence, in some embodiments, the first AP 200a is configured to perform (optional) step SI 02:

S102: The first AP 200a transmits a first reference signal towards the second AP 200b. The first reference signal is transmitted at least once prior to the first AP 200a receiving the second reference signal from the second AP 200b.

The periodicity of the beam-forming determination signal (first reference signal) can be adapted e.g. based on variations of observed SINR on the received phase alignment handshake signals.

In some examples, the first reference signal is transmitted in a wider beam than the third reference signal. In some examples, the first reference signal is transmitted in a wide beam, or omni-directional beam 110a. One efficient method to generate a wide beam from an antenna panel is that of dual-polarized array size invariant pre-coding. In other examples, the first reference signal is transmitted in a narrow beam, but this might require the first AP 200a to transmit the first signal in several narrow beams, for example by transmitting the first reference signal by performing a full beam-sweep.

In some examples, the first reference signal and the third reference signal are by the first AP 200a beamformed using a phase-neutral beamformer. In some examples, this implies that one of the antenna precoder coefficient, such as the first antenna precoder coefficient, is 1. In some examples, this implies that using a different phase rotation of the beamformers when transmitting and receiving (the first and second) reference signals and when transmitting data to a UE should be avoided. One way to accomplish this is to use a fixed value (e.g. 1) for one component in all beamformers. This would avoid introducing phase ambiguities by the transmit and receive beamformers. Each and every APs 210a:210e should thus use the same beamforming phase reference when calibrating and when transmitting data to the UEs. To avoid introducing any systematic beamforming error, a good approach is to rotate each transmit and receive beamformer used during phase alignment to a neutral phase e.g. by requiring that the first coefficient (y- in the beamforming vector (v) to be equal to 1. This does not change the direction of the transmit or receive beam.

In some aspects, the first reference signal is transmitted less frequently in time than the third reference signal. Hence, there might be at least two occurrences of the third reference signal between each occurrence of the first reference signal. Assume therefore that at least two occurrences of the first reference signal might be transmitted. Then, in some embodiments, at least two occurrences of the second reference signal are received and at least two occurrences of the third reference signal are transmitted between any two adjacent occurrences of the first reference signal. Further details relating thereto will be disclosed below with reference to Fig. 9.

For example, as in the example of Fig. 1, there might be several second APs 200b:200e. All these second APs 200b:200e might be simultaneously phase-aligned with the first AP 200a. Thus, in some embodiments, one separate second reference signal is received from each of at least two second APs 200b:200e, and one separate value of the reference phase rotation is determined for each of the at least two second APs 200b:200e.

In some aspects, the second APs 200b:200e transmit second reference signals that are orthogonal with respect to each other. This enable the second reference signals of all the second APs 200b:200e to be simultaneously transmitted (and simultaneously received). Hence, in some embodiments, the second reference signal received from one of the at least two second APs 200b is orthogonal to the second reference signal received from any other of the at least two second APs 200c:200e. Alternatively, each second AP 200b:200e might transmit its second reference signal in a very narrow beam. This could also enable the second reference signals of all the second APs 200b:200e to be simultaneously transmitted (and simultaneously received). Further details relating thereto will be disclosed below with reference to Fig. 7.

When there are several second APs 200b:200e, the first AP 200a might transmit one third reference signal to each of the second APs 200b:200e. In particular, in some embodiments, one separate third reference signal is transmitted towards each of the at least two second APs 200b:200e. The third reference signal for a given one of the at least two second APs 200b:200e is phase adjusted with the phase adjustment factor determined from the reference phase rotation of the second reference signal received from this given one of the at least two second APs 200b:200e. In other words, each third reference signal is phase adjusted with the phase adjustment factor determined for the intended receiver.

In some aspects, when there are several second APs 200b:200e, the first AP 200a transmits third reference signals that are orthogonal with respect to each other. That is, in some embodiments, the third reference signal transmitted towards one of the at least two second APs 200b is orthogonal to the third reference signal transmitted towards any other of the at least two second APs 200c:200e. Using third reference signals that are orthogonal with respect to each other enables all third reference signals to be simultaneously transmitted. Hence, in some embodiments, the third reference signals for all of the at least two second APs 200b:200e are simultaneously transmitted towards the at least two second APs 200b:200e. Alternatively, the first AP 200a might transmit the third reference signals in very narrow beams. This could also enable the third reference signals to be simultaneously transmitted, without causing mutual interference at the second APs 200b:200e. Further details relating thereto will be disclosed below with reference to Fig. 8.

As disclosed above, the first AP 200a is selected as reference AP. There could be different criteria for selecting the first AP 200a as reference AP. One criterion depends on the geographical location where the APs 200a:”00e are deployed. Assume therefore that each of the APs 200a:200e has a deployment location. In some embodiments, the first AP 200a is selected from all the APs 200a:200e to transmit the third reference signal based on its deployment location in relation to the deployment locations of the other APs 200b:200e. In one example, the AP being closest or the geographical center of all the APs (or a subset of the APs) is selected as reference AP. In one example, the AP being on an edge between two subsets, or clusters, of APs is selected as reference AP. The selection could be made by the centralized node 300. Hence, in some embodiments, the deployment location of the first AP 200a is either more centralized than the deployment location of the second AP 200b or is on an edge between a first sub-set of the APs 200a:200e and a second sub-set of the APs 200a:200e where the second AP 200b belongs to either the first sub-set or the second sub-set. In further detail, a decent quality AP-to-AP link (such as either line-of- sight (LOS) or without any significant reflection/refraction) is required for each pair or APs. This could be achieved by selecting a centrally located AP as the first AP 200a. In some examples, the network is divided into sub-sections and the herein disclosed methods are performed separately for each such subsection of the network. In some examples, once the herein disclosed methods have been performed for all APs in a cluster, or sub-section of the network, one of the APs at the edge of this cluster, or sub-section of the network is selected as the next reference AP.

Reference is now made to Fig. 3 illustrating a method for phase-alignment between antenna panels at different APs 200a:200e in a distributed MIMO communication system 100 as performed by a second AP 200b:200e of the APs 200a: 200e according to an embodiment. Each of the antenna panels comprises multiple antenna elements calibrated for uplink-downlink reciprocity, but the antenna panels are not mutually phase aligned.

S206: The second AP 200b:200e transmits a second reference signal towards a first AP 200a of the APs 200a:200e.

S208: The second AP 200b:200e determines a phase alignment factor d from a third reference signal received from the first AP 200a. The third reference signal has a phase rotation b compared to the second reference signal. The phase alignment factor d is determined from the phase rotation b. Embodiments relating to further details of phase-alignment between antenna panels at different APs 200a:200e as performed by the second AP 200b:200e will now be disclosed.

There could be different ways in which the phase alignment factor d is determined. In some aspects, the phase alignment factor d is equal to half of the negative value of the phase rotation b. That is, in some embodiments, the phase alignment factor d is determined as d = — b/2.

As will be further disclosed below with reference to Fig. 6, there could be a difference, denoted D between the local clock phase at the first AP 200a and the second AP 200b. The phase rotation b is determined is then dependent on this difference D. Further, properties of the radio propagation channel between the first AP 200a and the second AP 200b might impose a channel phase rotation factor t on signals transmitted between the APs 200a:200e. Therefore, in some embodiments, the phase rotation b is determined as b = 2D, where 2D is a difference in local clock phase at the second AP 200b between when the second reference signal was transmitted and the third reference signal was received.

As disclosed above, the APs 200a:200e might use beamforming for communication between each other. Hence, in some embodiments, the second reference signal and the third reference signal are beamformed. In some embodiments, the same beamformer is used when transmitting the second reference signal as when receiving the third reference signal.

As disclosed above, before receiving the second reference signal, the first AP 200a itself might transmit a first reference signal. One purpose of this reference signal is to enable the second AP 200b:200e to estimate a direction towards the first AP 200a. Hence, in some embodiments, the second AP 200b:200e is configured to perform (optional) step S202 and step S204:

S202: The second AP 200b:200e receives a first reference signal from the first AP 200a. The first reference signal is received by the second AP 200b:200e prior to the second AP 200b:200e transmitting the second reference signal towards the first AP 200a.

S204: The second AP 200b:200e estimates a dominant spatial direction from which the first reference signal emanates.

The second reference signal could then be beamformed to be transmitted in the dominant spatial direction in which the first reference signal was received. That is, in some embodiments, the second reference signal is beamformed in the thus estimated dominant spatial direction.

In some aspects, the first reference signal is transmitted less frequently in time than the third reference signal. Hence, there might be at least two occurrences of the third reference signal between each occurrence of the first reference signal. Assume therefore that at least two occurrences of the first reference signal might be transmitted. Then, in some embodiments, at least two occurrences of the second reference signal are transmited and at least two occurrences of the third reference signal are received between any two adjacent occurrences of the first reference signal.

In some examples, the second reference signal is by the second AP 200b beamformed using a phase- neutral beamformer. In some examples, this implies that the first antenna precoder coefficient is 1. In some examples, this implies that using a different phase rotation of the beamformers when receiving and transmiting (the first and second) reference signals and when transmiting data to a UE should be avoided. One way to accomplish this is to use a fixed value (e.g. 1) for one component in all beamformers.

As disclosed above, in some aspects, the second APs 200b:200e transmit second reference signals that are orthogonal with respect to each other. That is, in some embodiments, the second reference signal transmited from the second AP 200b is orthogonal to any other second reference signal transmited from any other second AP 200c:200e.

The phase alignment factor d can by the second AP 200b:200e be applied for data transmission towards UE 400 served by the second AP 200b:200e. Particularly, in some embodiments, the second AP 200b:200e is configured to perform (optional) step S210:

S210: The second AP 200b:200e transmits a data signal towards UEs 400 served by the second AP 200b:200e. The data signal when transmited is phase rotated by the phase alignment factor d.

In some aspects, also the first AP 200a transmits a data signal towards the same UEs 400 (which are thus also served by the first AP 200b). This data signal when transmited does not need to be phase rotated.

Reference is now made to the flowchart of Fig. 4 and the sequence diagram of Fig. 5 which both illustrate a method for phase-alignment between antenna panels at different APs 200a:200e in a distributed MIMO communication system 100 based on at least some of the above disclosed embodiments. For simplicity, only two APs 200a, 200b are involved in the method but the skilled person would understand how the method could be applied also for more than two APs 200a, 200b.

S300: The centralized node 300 provides control and coordination information to the APs 200a, 200b as well as to the UE 400 (via one or more of the APs 200a, 200b).

S301 : The first AP 200a transmits a first reference signal. The first reference signal might be transmited in an omni-directional beam 110a at the first AP 200a.

As disclosed above, in some examples, the first reference signal is transmited in a wide beam, or omni directional beam 110a. Further, since the transmission of the first reference signal cannot benefit from any transmit beamforming gain, the first reference signal may be repeatedly transmited several times to accumulate enough energy at each of the receivers in the second APs 200b:200e. As further disclosed above, in other examples, the first reference signal is transmitted in a narrow beam, but this might require the first AP 200a to transmit the first signal in several narrow beams, for example by transmitting the first reference signal by performing a full beam-sweep. As further disclosed above, in some aspects, the first reference signal is transmitted less frequently in time than the third reference signal.

In some examples, the first AP 200a transmits an M_s X P-dimensional reference signal (pilot) F =

[f₁ ... , f_R] using its M_R antennas, where P is the number of channel uses spent (i.e., the length of the pilot). In some examples, the pilot waveforms (rows of F) are orthogonal. In some examples, the pilot waveforms are adapted based on knowledge of the propagation environment and specifically based on information about the location of the first AP 200a relative to the second APs 200b:200e.

S302: The second AP 200b receives the first reference signal.

S303: The second AP 200b estimates the dominant spatial direction from which the first reference signal emanates. The second AP 200b thereby estimates the dominant spatial direction towards the first AP 200a. The second AP 200b might further determine a receive beamformer and a transmit beamformer based on the dominant spatial direction towards the first AP 200a.

In further detail, the antenna panel at second AP 200b at time index n receives the time-domain signal y_{p n} according to the following: where p represents transmission power, G is the channel matrix and w_n is noise. Here m and e denote frequency and phase (small time) offsets, respectively, which are a priori unknown to all APs 200a:200e.

The second AP 200b determines a beamforming vector, v (of dimension M_p), that matches the dominant spatial direction of G. In some examples, the second AP 200b selects v = t/^*(: ,1), which is the complex conjugate of the first column of the matrix U obtained by performing a singular value decomposition (SVD) of Y_p as follows: Y_p = UåV^H . One interpretation of this is that the second AP 200b should beamform into the strongest spatial direction of the channel. When obtaining v. no knowledge of G, m or e is used. In some examples, v is obtained through parametric estimation of G, treating m and e as explicit nuisance parameters. In some examples, second-order statistics of w_n are jointly estimated with the dominant singular vector of G in order to suppress interference through spatial filtering.

S304: The second AP 200b beamforms, using a transmit precoder that is based on the transmit beamformer, a second reference signal that is transmitted in a beam 110b into the estimated dominant spatial direction towards the first AP 200a.

S305: The first AP 200a receives the second reference signal. The first AP 200a determines a reference phase rotation a of the second reference signal.

S306: The first AP 200a determines a receive beamformer and a transmit beamformer. S307: The first AP 200a beamforms, using a transmit precoder that is based on the transmit beamformer, a third reference signal that is transmitted in a beam 110c towards the second AP 200b. The third reference signal is phase adjusted with a phase adjustment factor g = —a.

S308: The second AP 200b receives the third reference signal using the receive beamformer. The third reference signal has a phase rotation b compared to the second reference signal.

S309: The second AP 200b determines a phase alignment factor d = — b/2 from the third reference signal received from the first AP 200a.

S310: The UE 400 transmits uplink reference signals that are received by the first AP 200a and the second AP 200b. S311 : The second AP 200b applies the determined phase alignment factor d.

S312: The first AP 200a transmits data and/or control signals towards the UE 400.

S313: The second AP 200b transmits data and/or control signals towards the UE 400 whilst applying the determined phase alignment factor d.

It is noted that whatever phase ambiguity the reference first AP 200a has is irrelevant. The goal of the disclosed method is to enable calibration of the phase ambiguities of all APs (in the embodiment of Fig. 4 represented by the first AP 200a and the second AP 200b) to enable phase coherent downlink transmissions to UEs 400. When the APs 200a:200e transmit data signals to the same UE 400, the received data signals from the different APs 200a:200e should arrive phase aligned at the UE 400. This is accomplished by the disclosed embodiments. Any phase ambiguity of the first AP 200a acting as reference AP is common to all APs 200a: 200e after phase alignment and can be equalized via one downlink reference signal (which is simultaneously transmitted by all APs 200a:200e).

As disclosed above, there could be a difference, denoted D between the local clock phase at the first AP 200a and the second AP 200b. This is illustrated in Fig. 6. Fig. 6 schematically illustrates a sequence diagram of transmission and reception of the second reference signal and transmission and reception of the third reference signal, together with the local clock phases at the first AP 200a and the second AP 200b. As can be seen in Fig. 6, since the transmission timing of the second reference signal and the transmission timing of the third reference signal are determined by the local clocks, the observed phase of the third reference signal at the second AP 200b is twice as large as the actual phase alignment error between the second AP 200b and the first AP 200a. A phase alignment error is equivalent to a clock error and the transmission of a reference signal starts when the clock phase at the transmitting end is zero. This has the effect that the phase alignment error is applied twice to the observed phase of the reference signal at the second AP 200b when receiving the third reference signal. In comparison to Fig. 5, in Fig. 6 the reference phase 0 is defined from the perspective the second AP 200b whereas in Fig. 5 the reference i6 phase 0 is defined from the perspective of the first AP 200a. This implies that the observed phase of the third reference signal at the second AP 200b equals — 2D in Fig. 6, which thus is equal to b in Fig. 5. By applying a phase compensating factor of d = — b/2 = — (— 2D)/2 = D at the second AP 200b, after phase compensation both the first AP 200a and the second AP 200b have the same phase of D (which could take an arbitrary value).

Reference is now made to Fig. 7 which schematically illustrates an example of transmission of second reference signals from four second APs 200b:200e towards the first AP 200a. Here, all four second APs 200b:200e are to be simultaneously phase aligned with the first AP 200a. The second reference signals are denoted fi, f₂, F₃, and f₄, and can, for example in the case of orthogonal frequency -division multiplexing (OFDM) transmission, be placed on orthogonal resource elements in a time -frequency resource grid. Allocation of resource elements for transmission of the second reference signal in the time- frequency resource grid from each of the second APs 200b:200e is illustrated at 710b, 710c, 710d, and 710d. Beams used by each of the second APs 200b:200e for transmission of the second reference signals are illustrated at 1 lOb-b, 1 lOb-c, 1 lOb-d, and 1 lOb-e. In case the first AP 200a uses a fully digital receive beamformer, the first AP 200a can apply different receive beamforming coefficient to different resource elements in the time and frequency grid when receiving the second reference signals. The time-frequency resource grid of resource elements of the second reference signals as received by the first AP 200a from all four second APs 200b:200e is illustrated at 720. Beams used by the first AP 200a for reception of the second reference signals are illustrated at llOc-b, llOc-c, llOc-d, and llOc-e.

Reference is now made to Fig. 8 which schematically illustrates an example of reception of second reference signals by the first AP 200a from four second APs 200b:200e and transmission of third reference signals from the first AP 200a towards four second APs 200b:200e. As in the example of Fig. 7, all four second APs 200b:200e are to be simultaneously phase aligned with the first AP 200a. The time- frequency resource grid of resource elements of the second reference signals received by the first AP 200a from all four second APs 200b:200e is illustrated at 810. At 820 is illustrated that the first AP 200a de-multiplexes the second reference signals from the separate second APs 200b:200e. At 830b, 830c, 830d, 830e is illustrated the time-frequency resource grids for the thus separated second reference signals from the four second APs 200b:200e. At 840b, 840c, 840d, 840e is illustrated that the first AP 200a determines one reference phase rotation a_k for each of the four second APs 200b:200e. At 850b, 850c, 850d, 850e is illustrated that the first AP 200a applies a respective precoder with one phase adjustment factor y_k for each of the reference phase rotations, that is y_k is determined from a_k for k = 1,2, 3,4. At 860 is illustrated that the first AP 200a transmits the third reference signal towards the four second APs 200b:200e.

As disclosed above, the first reference signal might be transmitted less frequently in time than the third reference signal. This is illustrated in Fig. 9. Fig. 9 at 900 schematically illustrates transmission of a first reference signal, a second reference signal, and a third reference signal along a timeline. As can be seen, one occurrence of the first reference signal 910 is followed by a first occurrence of the second reference signal 920a and a first occurrence of the third reference signal 930a. After some time delay, where first downlink or uplink data 940a is transmitted, follows a second occurrence of the second reference signal 920b and a second occurrence of the third reference signal 930b. After some time delay, where second downlink or uplink data 940b is transmitted, follows a third occurrence of the second reference signal 920c and a third occurrence of the third reference signal 930c.

In some examples the phase alignment (being represented by the transmission of the second reference signal and the third reference signal) occurs periodically. In other examples the phase alignment (being represented by the transmission of the second reference signal and the third reference signal) is opportunistically scheduled by the network node 200, or the centralized node 300, at instances when there is no or little user data to transmit. The time delay between two adjacent occurrences of the second reference signal (and the third reference signal) can be adapted based on observed phase errors, drifts. According to one example, in case the standard deviation of phase alignment errors is smaller than a threshold, the rate of occurrence of the transmission of the second reference signal and the third reference signal is decreased; otherwise the rate of occurrence is increased.

In further detail, if any of the second APs 200b:200e use a phase alignment factor d = — b/2 already when transmitting the second reference signal then any such second AP 200b:200e should receive a third reference signal from the first AP 200a that is phase adjusted with a phase adjustment factor g that is very close to zero (i.e. if no phase drift exists between he APs). However, in practice the phase adjustment factor g will never, or at least very seldom, be exactly zero due to measurement noise and/or phase drifts. By tracking how the phase adjustment factor g evolves over time, errors, drifts, in frequency calibration (i.e. frequency offsets) can be detected and compensated for. This is illustrated in Fig. 10. Fig. 10 schematically depicts the phase alignment error as a function of time and which at (a) illustrates an example of a first phase alignment error 1010a evolving over time where the mean phase alignment error is zero, indicating no systematic frequency error, and which at (b) illustrates an example of a second phase alignment error 1010b evolving over time where the mean phase alignment error is above zero, indicating a systematic frequency error. If there is a constant drift, then the frequency of the local oscillator should be corrected accordingly.

The standard deviation of all compensation factor updates from all the second APs 200b:200e could be used as a metric when determining if the phase alignment period is too large or too small. This may e.g. be reported by the first AP 200a to the centralized node 300, either periodically or event triggered).

Fig. 11 schematically illustrates, in terms of a number of functional units, the components of an AP 200a:200e 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 i8 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 AP 200a:200e 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 AP 200a:200e 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 AP 200a:200e may further comprise a communications interface 220 for communications with other functions, nodes, entities, and devices, in the distributed MIMO communication system 100. 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 AP 200a:200e 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 AP 200a:200e are omitted in order not to obscure the concepts presented herein.

The AP 200a:200e may be provided as a standalone device or as a part of at least one further device. For example, the AP 200a:200e may be provided in a node of the radio access network or in a node of the core network. Alternatively, functionality of the AP 200a:200e 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 AP 200a:200e may be executed in a first device, and a second portion of the instructions performed by the AP 200a:200e 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 AP 200a:200e may be executed. Hence, the methods according to the herein disclosed embodiments are suitable to be performed by an AP 200a:200e residing in a cloud computational environment. Therefore, although a single processing circuitry 210 is illustrated in Fig. 11 the processing circuitry 210 may be distributed among a plurality of devices, or nodes. The same applies to the computer programs 1220a, 1220b of Fig. 12.

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 first AP 200a 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 second AP 200b:200e 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

1. A first access point, AP (200a), for phase-alignment between antenna panels at different APs (200a:200e) in a distributed MIMO communication system (100), wherein each of the antenna panels comprises multiple antenna elements calibrated for uplink-downlink reciprocity, the first AP (200a) comprising processing circuitry (210), the processing circuitry being configured to cause the first AP (200a) to: determine a reference phase rotation a of a second reference signal received from a second AP (200b) of the APs (200a:200e); and transmit a third reference signal towards the second AP (200b), wherein the third reference signal is phase adjusted with a phase adjustment factor g determined from the reference phase rotation a.

2. The first AP (200a) according to claim 1, wherein the phase adjustment factor g used when transmitting said third reference signal is determined as a conjugate of the reference phase rotation a.

3. The first AP (200a) according to any preceding claim, wherein the phase adjustment factor used when transmitting said third reference signal is determined as g = —a.

4. The first AP (200a) according to any preceding claim, wherein the reference phase rotation a is equal to a = A + t, where D is a difference in local clock phase between the first AP (200a) and the second AP (200b), and where t is a channel phase rotation factor of a radio propagation channel between the first AP (200a) and the second AP (200b).

5. The first AP (200a) according to any preceding claim, wherein the second reference signal and the third reference signal are beamformed.

6. The first AP (200a) according to any preceding claim, wherein the processing circuitry further is configured to cause the first AP (200a) to: transmit a first reference signal towards the second AP (200b), wherein the first reference signal is transmitted at least once prior to receiving the second reference signal from the second AP (200b).

7. The first AP (200a) according to claim 6, wherein at least two occurrences of the first reference signal are transmitted, and wherein at least two occurrences of the second reference signal are received and at least two occurrences of the third reference signal are transmitted between any two adjacent occurrences of the first reference signal.

8. The first AP (200a) according to any preceding claim, wherein one separate second reference signal is received from each of at least two second APs (200b:200e), and wherein one separate value of the reference phase rotation is determined for each of the at least two second APs (200b:200e).

9. The first AP (200a) according to claim 8, wherein the second reference signal received from one of the at least two second APs (200b) is orthogonal to the second reference signal received from any other of the at least two second APs (200c:200e).

10. The first AP (200a) according to claim 8 or 9, wherein one separate third reference signal is transmitted towards each of the at least two second APs (200b:200e), and wherein the third reference signal for a given one of the at least two second APs (200b:200e) is phase adjusted with the phase adjustment factor determined from the reference phase rotation of the second reference signal received from said given one of the at least two second APs (200b:200e).

11. The first AP (200a) according to claim 10, wherein the third reference signal transmitted towards one of the at least two second APs (200b) is orthogonal to the third reference signal transmitted towards any other of the at least two second APs (200c:200e).

12. The first AP (200a) according to any preceding claim, wherein the third reference signals for all of the at least two second APs (200b:200e) are simultaneously transmitted towards the at least two second APs (200b:200e).

13. The first AP (200a) according to any preceding claim, wherein each of the APs (200a:200e) has a deployment location, and wherein the first AP (200a) is selected from all the APs (200a:200e) to transmit the third reference signal based on its deployment location in relation to the deployment locations of the other APs (200b:200e).

14. The first AP (200a) according to claim 13, wherein the deployment location of the first AP (200a) is either more centralized than the deployment location of the second AP (200b) or is on an edge between a first sub-set of the APs (200a:200e) and a second sub-set of the APs (200a:200e) where the second AP (200b) belongs to either the first sub-set or the second sub-set.

15. A second access point, AP (200b:200e), for phase-alignment between antenna panels at different APs (200a:200e) in a distributed MIMO communication system (100), wherein each of the antenna panels comprises multiple antenna elements calibrated for uplink-downlink reciprocity, the second AP (200b:200e) comprising processing circuitry (310), the processing circuitry being configured to cause the second AP (200b:200e) to: transmit a second reference signal towards a first AP (200a) of the APs (200a:200e); and determine a phase alignment factor d from a third reference signal received from the first AP (200a), wherein the third reference signal has a phase rotation b compared to the second reference signal, and wherein the phase alignment factor d is determined from the phase rotation b.

16. The second AP (200b:200e) according to claim 15, wherein the phase alignment factor d is determined as d = — b/2.

17. The second AP (200b:200e) according to any of claim 15 to claim 16, wherein the phase rotation b is determined as b = 2D, where 2D is a difference in local clock phase at the second AP (200b) between when the second reference signal was transmitted and the third reference signal was received.

18. The second AP (200b:200e) according to any of claim 15 to claim 17, wherein the second reference signal and the third reference signal are beamformed.

19. The second AP (200b:200e) according to claim 18, where the same beamformer is used when transmitting the second reference signal as when receiving the third reference signal.

20. The second AP (200b:200e) according to any of claim 15 to claim 19, wherein the processing circuitry further is configured to cause the second AP (200b:200e) to: receive a first reference signal from the first AP (200a), wherein the first reference signal is received prior to transmitting the second reference signal towards the first AP (200a); and estimate a dominant spatial direction from which the first reference signal emanates.

21. The second AP (200b:200e) according to a combination of claim 20 with any of claim 18 or 19, wherein the second reference signal is beamformed in said dominant spatial direction.

22. The second AP (200b:200e) according to claim 20, wherein at least two occurrences of the first reference signal are received, and wherein at least two occurrences of the second reference signal are transmitted and at least two occurrences of the third reference signal are received between any two adjacent occurrences of the first reference signal.

23. The second AP (200b:200e) according to any of claim 15 to claim 22, wherein the second reference signal transmitted from the second AP (200b) is orthogonal to any other second reference signal transmitted from any other second AP (200c:200e).

24. The second AP (200b:200e) according to any of claim 15 to claim 23, wherein the processing circuitry further is configured to cause the second AP (200b:200e) to: transmit a data signal towards user equipment, UE (400), served by the second AP (200b), wherein the data signal when transmitted is phase rotated by the phase alignment factor d.

25. A distributed multiple -input multiple-output, MIMO, communication system (100), the distributed MIMO communication system (100) comprising a first AP (200a) according to any of claims 1 to 14 and at least one second AP (200b:200e) according to any of claims 15 to 24.

26. A method for phase-alignment between antenna panels at different access points, APs (200a:200e) in a distributed multiple-input multiple-output, MIMO, communication system (100), wherein each of the antenna panels comprises multiple antenna elements calibrated for uplink-downlink reciprocity, the method being performed by a first AP (200a) of the APs (200a:200e), the method comprising: determining (SI 04) a reference phase rotation a of a second reference signal received from a second AP (200b) of the APs (200a:200e); and transmitting (SI 06) a third reference signal towards the second AP (200b), wherein the third reference signal is phase adjusted with a phase adjustment factor g determined from the reference phase rotation a.

27. A method for phase-alignment between antenna panels at different access points, APs (200a:200e) in a distributed MIMO communication system (100), wherein each of the antenna panels comprises multiple antenna elements calibrated for uplink-downlink reciprocity, the method being performed by a second AP (200b) of the APs (200a:200e), the method comprising: transmitting (S206) a second reference signal towards a first AP (200a) of the APs (200a:200e); and determining (S208) a phase alignment factor d from a third reference signal received from the first AP (200a), wherein the third reference signal has a phase rotation b compared to the second reference signal, and wherein the phase alignment factor d is determined from the phase rotation b.

28. A computer program (1220a) for phase-alignment between antenna panels at different access points, APs (200a:200e) in a distributed MIMO communication system (100), wherein each of the antenna panels comprises multiple antenna elements calibrated for uplink-downlink reciprocity, the computer program comprising computer code which, when run on processing circuitry (210) of a first AP (200a) of the APs (200a:200e), causes the first AP (200a) to: determine (SI 04) a reference phase rotation a of a second reference signal received from a second AP (200b) of the APs (200a:200e); and transmit (SI 06) a third reference signal towards the second AP (200b), wherein the third reference signal is phase adjusted with a phase adjustment factor g determined from the reference phase rotation a.

29. A computer program (1220b) for phase-alignment between antenna panels at different access points, APs (200a:200e) in a distributed MIMO communication system (100), wherein each of the antenna panels comprises multiple antenna elements calibrated for uplink-downlink reciprocity, the computer program comprising computer code which, when run on processing circuitry (210) of a second AP (200b:200e) of the APs (200a:200e), causes the second AP (200b:200e) to: transmit (S206) a second reference signal towards a first AP (200a) of the APs (200a:200e); and determine (S208) a phase alignment factor d from a third reference signal received from the first AP (200a), wherein the third reference signal has a phase rotation b compared to the second reference signal, and wherein the phase alignment factor d is determined from the phase rotation b.

30. A computer program product (1210a, 1210b) comprising a computer program (1220a, 1220b) according to at least one of claims 28 and 29, and a computer readable storage medium (1230) on which the computer program is stored.