Calibration of repeaters in a wireless network

Abstract

There is provided techniques for calibrating repeaters in a wireless network, the method is performed by a network node. The method comprises configuring the repeaters with patterns, wherein each of the patterns defines a sequence of non-zero complex-valued gains one of the repeaters is to apply over time during bidirectional sounding. The method comprises obtaining bidirectional channel response measurements for each of the patterns from bidirectional sounding as performed by a first transceiver device and a second transceiver device configured to wirelessly communicate with each other without and via the repeaters in the wireless network. The method comprises determining compensation factors, one per repeater, as a function of linear combinations of the bidirectional channel response measurements. There is one linear combination of bidirectional channel response measurements per pattern. The method comprises calibrating the repeaters by configuring the repeaters with the compensation factors.

Description

[0001]CALIBRATION OF REPEATERS 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. TECHNICAL FIELD Embodiments presented herein relate to a method, a network node, a computer program, and a computer program product for calibrating repeaters in a wireless network. Embodiments presented herein further relate to a method, a system of repeaters, a computer program, and a computer program product for the repeaters to be configured. 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 environments 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, 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 repeaters to behave as a reciprocal node (e.g., behave as a regular channel scatterer but with amplification) such that the repeaters could aid a MIMO network to perform TDD reciprocity-based communications. A particular object is to enable calibration of the repeaters without interrupting the operational connection of UEs with poor direct channel to the serving AP during the repeater calibration process. According to a first aspect there is presented a network node for calibrating ^ repeaters in a wireless network. The network node is configured to configure the repeaters with at least ^ patterns, wherein each of the ^ patterns defines a sequence of non-zero complex-valued gains one of the repeaters is to apply over time during bidirectional sounding. The network node is configured to obtain bidirectional channel response measurements for each of the at least ^ patterns from bidirectional sounding as performed by a first transceiver device and a second transceiver device configured to wirelessly communicate with each other without and via the repeaters in the wireless network. The network node is configured to determine compensation factors, one per repeater, as a function of linear combinations of the bidirectional channel response measurements. There is one linear combination of bidirectional channel response measurements per pattern. The network node is configured to calibrate the repeaters by configuring the repeaters with the compensation factors. According to a second aspect there is presented a method for calibrating ^ repeaters in a wireless network. The method is performed by a network node. The method comprises configuring the repeaters with at least ^ patterns, wherein each of the ^ patterns defines a sequence of non-zero complex-valued gains one of the repeaters is to apply over time during bidirectional sounding. The method comprises obtaining bidirectional channel response measurements for each of the at least ^ patterns from bidirectional sounding as performed by a first transceiver device and a second transceiver device configured to wirelessly communicate with each other without and via the repeaters in the wireless network. The method comprises determining compensation factors, one per repeater, as a function of linear combinations of the bidirectional channel response measurements. There is one linear combination of bidirectional channel response measurements per pattern. The method comprises calibrating the repeaters by configuring the repeaters with the compensation factors. According to a third aspect there is presented a computer program for calibrating ^ repeaters 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 repeaters with at least ^ patterns, wherein each of the ^ patterns defines a sequence of non- zero complex-valued gains one of the repeaters is to apply over time during bidirectional sounding. One action comprises the network node to obtain bidirectional channel response measurements for each of the at least ^ patterns from bidirectional sounding as performed by a first transceiver device and a second transceiver device configured to wirelessly communicate with each other without and via the repeaters in the wireless network. One action comprises the network node to determine compensation factors, one per repeater, as a function of linear combinations of the bidirectional channel response measurements. There is one linear combination of bidirectional channel response measurements per pattern. One action comprises the network node to calibrate the repeaters by configuring the repeaters with the compensation factors. According to a fourth aspect there is presented a system comprising ^ repeaters to be calibrated in a wireless network. The repeaters are configured to receive configuration from a network node with at least ^ patterns according to which the repeaters are to change their complex-valued gains over time during bidirectional sounding. The repeaters are configured to operate according to the configuration whilst, for each of the at least ^ patterns by the complex-valued gains of the repeaters being changed over time in accordance with the at least ^ patterns, taking part in the bidirectional sounding between a first transceiver device and a second transceiver device configured to wirelessly communicate with each other without and via the repeaters in the wireless network. The repeaters are configured to receive compensation factors from the network node, one per repeater. The repeaters are configured to apply the compensation factors during subsequent operation. According to a fifth aspect there is presented a method for ^ repeaters to be calibrated in a wireless network, the method being performed by a system comprising the ^ repeaters. The method comprises each of the repeaters receiving configuration from a network node with at least ^ patterns according to which the repeaters are to change their complex-valued gains over time during bidirectional sounding. The method comprises each of the repeaters operating according to the configuration whilst, for each of the at least ^ patterns by the complex-valued gains of the repeaters being changed over time in accordance with the at least ^ patterns, taking part in the bidirectional sounding between a first transceiver device and a second transceiver device configured to wirelessly communicate with each other without and via the repeaters in the wireless network. The method comprises each of the repeaters receiving compensation factors from the network node, one per repeater. The method comprises each of the repeaters applying the compensation factors during subsequent operation. According to a sixth aspect there is presented a computer program for^^ repeaters to be calibrated in a wireless network. The computer program comprises computer code which, when run on processing circuitry of the repeaters, causes the repeaters to perform actions. One action comprises the repeaters to receive configuration from a network node with at least ^ patterns according to which the repeaters are to change their complex-valued gains over time during bidirectional sounding. One action comprises the repeaters to operate according to the configuration whilst, for each of the at least ^ patterns by the complex-valued gains of the repeaters being changed over time in accordance with the at least ^ patterns, taking part in the bidirectional sounding between a first transceiver device and a second transceiver device configured to wirelessly communicate with each other without and via the repeaters in the wireless network. One action comprises the repeaters to receive compensation factors from the network node, one per repeater. One action comprises the repeaters to apply the compensation factors during subsequent operation. 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 seventh 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, since all repeaters can be active during all calibration measurements, these aspects provide improved measurement accuracy compared to state-of-the-art methods. Further, since all repeaters can be active during the entire calibration process, the disclosed aspects enable continuous and uninterrupted connection to UEs with poor direct channel to the serving AP during the repeater calibration process. 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 the repeaters to possibly run in a transparent way to the network (or UEs) during reciprocity-based TDD communications since the repeaters essentially behave as a regular channel scatterer (with amplification). Advantageously, in the context of simultaneously using 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 schematic diagram showing functional units of a network node according to an embodiment; Fig.7 is a schematic diagram showing functional modules of a network node according to an embodiment; Fig.8 is a schematic diagram showing functional units of a repeater according to an embodiment; Fig.9 is a schematic diagram showing functional modules of a repeater according to an embodiment; and Fig.10 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 thus relate to techniques for calibrating repeaters 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 system comrpising repeaters, a method performed by the repeaters, and a computer program product comprising code, for example in the form of a computer program, that when run on processing circuitry of the repeaters, causes the repeaters 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 ^ repeaters 300a:300N. The repeaters 300a:300N 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 300n according to some examples. The repeater 300a:300n is a dual-antenna repeater. The repeater 300n is equipped with antennas 350a, 350b and comprises different signal paths for the forward direction and the reverse direction. In this respect, the repeater 300n comprises forward path circuitry 360a and reverse path circuitry 360b. In this way, the repeater 300n is configured for communication in the forward direction and the reverse direction, with the forward path circuitry 360a for communicating in the forward direction and with the reverse path circuitry 360b for communicating in the reverse direction. 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 300n is here assumed to have a forward direction with (complex-valued) gain ^^and a reverse directionwith gain ^^. That is, the forward path circuitry 360a for repeater ^, where ^ ൌ1,… ,^, has a complex-valued gain ^^, and the reverse path circuitry 360b forrepeater ^ has a complex-valued gain ^^. These gains, ^^and ^^, 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 300n 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 repeaters 300a:300N). The calibration is based on over-the-air measurements between transceiver device A and transceiver device B, with active involvement of the repeaters 300a:300N. 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. If the repeater gains are large enough in relation to the inter-repeater path gains (ห^^,^ห ; ^,^ ൌ 1,… ,^^, then resonant (loopback) phenomena could occur. But if the inter-repeater path gains are sufficiently relatively small, these effects can be ignored. In situations where dual antenna repeaters are most likely to be deployed, such as to provide coverage for indoor UEs to outdoor base stations, such cross- repeater effects can be neglected. When there are multiple repeaters, ^^,^ଶ^… ,^ே, then new considerations arise. According to a first example, the direct link from transceiver device A to transceiver device B is sufficiently strong to be useful to aid in the calibration. According to a second example, the direct link from transceiver device A to transceiver device B is too weak to be useful to aid in the calibration. In some examples, the network node 200 therefore first estimates the SNR of the direct link from transceiver device A to transceiver device B (for example, by turning off all the repeaters 300a:300N). If this SNR exceeds a pre-determined threshold, then a procedure utilizing the direct link from transceiver device A to transceiver device B can be used. Else, a procedure not utilizing the direct link from transceiver device A to transceiver device B can be used. An example procedure for obtaining calibration with the repeaters 300a:300N selectively switched on and off will be disclosed next. For illustrative purposes and without loss of generality, a system comprising two repeaters, ^^,^ଶ(i.e., with ^ ൌ 2) is considered. However, the same methodology can be used systems comprising three or more repeaters 300a:300N. According to this procedure, there are six single- direction measurements (i.e., three bidirectional measurements) of the form given below, neglecting noise for simplicity of the exposition: ^^^ ൌ ^^G^^^ ^^^ ^^ ^^Here, superscripts 0, 1 and 2 co espo o: epeater ^^and repeater ^ଶ^switched off; 1 = repeater ^^switched on and repeater ^ଶswitched off; and 2 = repeater ^^swictehd on and repeater ^ଶoff. As also seen in Fig.1, ^ is the direct channel from transceiver device A to transceiver device node B, ^^^^is the channel from transceiver device A to transceiver device B via repeater ^, and ^^and ^^are the responses depending on depending on which repeater antenna is the one and which antenna is the receiving one. Repeater non-reciprocity steams from the fact that ^^and ^^are not equal to each other. The diagonal matrices ^^and ^^represent the gains at transceiver device A and transceiver device B, respectively. The diagonal matrices^^and^^represent the receiver gains at transceiver device A and transceiver device B, respectively. The following reparameterization is introduced: ^^^ ൌ ^^^^^^^^^^ ൌ ^^^ ^^^^^^ ൌ ^^^^^^^^^^ൌ ^^^^^^^^^^ ^^,^which allows the (noise-free) be rewritten as follows: ^^^ ൌ ^^ Firstly, ^,^^ and ^, up to a multiplicative ambiguity between ^ and ^ can be estimated from^^^ and ^^^. Further, ^^and γ^can be stimated from^^^ and ^^^. Further, ^ଶγଶcan be estimated from from^ଶ^ and ^^ଶ. A joint least-squares (maximum-likelihood) criterion can also be formulated; minimizing that criterion jointly with respect to all unknowns could yield better estimates. With that, the network node 200 obtains in terms of γ^and γଶ, estimates of the reciprocity imbalance ratios. For example, if repeater ^^adapts one of its RF chain gains using γ^, then this repeater essentially operates as a reciprocal node, as originally desired. As will be disclosed next with reference to Fig.4, instead of selectively switching the repeaters 300a:300N on and off, the repeaters 300a:300N can be instructed to change their complex-valued gains according to some appropriate, pre-determined pattern. Reference is therefore next made to Fig. 4 illustrating a method for calibrating ^ repeaters 300a:300N in a wireless network 100a:100c as performed by the network node 200 according to an embodiment. Consider a system with ^ repeaters, ^^,^ଶ^… ,^ே, with corresponding reciprocity imbalance ratios γ^, ..., γே. The repeaters are instructed to over time change their complex-valued gains (phase and / or amplitude) according to a pattern, as in S102. S102: The network node 200 configures the repeaters 300a:300N with at least ^ patterns. Each of the ^ patterns defines a sequence of non-zero complex-valued gains one of the repeaters 300a:300N is to apply over time during bidirectional sounding. Further examples of patterns will be disclosed below. For each pattern, bidirectional measurements are taken, as in S104. S104: The network node 200 obtains bidirectional channel response measurements for each of the at least ^ patterns. The bidirectional channel response measurements are obtained from bidirectional sounding as performed by the first transceiver device 110a and the second transceiver device 110b configured to wirelessly communicate with each other without and via the repeaters 300a:300N in the wireless network 100a:100c. By forming linear combinations of these bidirectional measurements (i.e., by performing linear post-processing), the individual reciprocity imbalance ratios ^^, ..., ^ேcan be estimated and calibration factors can be determined, as in S106 and S108. S106: The network node 200 determines compensation factors, one per repeater 300a:300N, as a function of linear combinations of the bidirectional channel response measurements. There is one linear combination of bidirectional channel response measurements per pattern. S108: The network node 200 calibrates the repeaters 300a:300N by configuring the repeaters 300a:300N with the compensation factors. Embodiments relating to further details of calibrating ^ repeaters 300a:300N in a wireless network 100a:100c as performed by the network node 200 will now be disclosed with continued reference to Fig.4. As disclosed above, each of the the ^ patterns defines a sequence of non-zero complex-valued gains one of the repeaters 300a:300N is to apply over time during bidirectional sounding. There could be different such patterns. In some embodiments, each of the patterns is taken as a respective row from one and the same orthonormal matrix. In some examples, the matrix is a square matrix. That is, the orthonormal matrix has ^ columns and ^ rows. In some non-limiting examples, the orthonormal matrix is any of: a unimodular matrix, a Hadamard matrix, a Discrete Fourier Transform (DFT) matrix. For the example, where the patterns are described by a Hadamard matrix, it is assumed that ^ is a power of 2. However, other extensions are possible if the patterns are described by a DFT matrix. As a non-limiting example, a procedure will be disclosed that only requires the repeaters to change the phase of their amplification gains by 180 degrees. However, there are several possibilities that yield equivalent performance. Consider a setup with ^ repeaters, , ^^,^ଶ^… ,^ே, with corresponding rank-one channels ^^, ..., ^ேand imbalance ratios ^^, ..., ^ே. The channels ^^, ..., ^ேcan be considered to be rank-one because repeaters with single-transmit and / or single- receive antennas effectively work as a keyhole channel. In some examples, the repeaters 300a:300N are configured to first not rotate their phases, i.e., keep their amplitudes equal to the nominal complex-valued gains. Therefore, in some embodiments, the network node 200 further is configured to perform (optional) step S102a as part of step S102. S102a: The network node 200 configures the repeaters 300a:300N to keep their phases unrotated during a first bidirectional sounding performed between the first transceiver device 110a and the second transceiver device 110b. This yields the (noise-free) bidirectional measurement ^^^ൌ ^ ^ ^^^⋯^ ^ே^ ^^^ൌ ^^^ ^ γ^^^ ^ ⋯^ γே^ே^்^Hence, in some embodiments, the compensation factors are based on channel properties ^ of a wireless direct link between the first transceiver device 110a and the second transceiver device 110b. As noted above, since ^ ൌ^^^^^^^, where ^ ൌ ^^^ ^^^, and ^ ൌ ^^^^^^, in some embodiments, the compensation factors are based transmitter gains^^,^^and receiver gains ^^,^^at the first transceiver device 110a and the second transceiver device 110b as estimated from the channel properties ^ of the wireless direct link. In some examples, the repeaters 300a:300N are configured to then rotate the phase of their complex-valued gains by 180 degrees. Therefore, in some embodiments, the network node 200 further is configured to perform (optional) step S102b as part of step S102. S102b: The network node 200 configures the repeaters 300a:300N to rotate their phases 180 degrees during a second bidirectional sounding performed between the first transceiver device 110a and the second transceiver device 110b. This yields the bidirectional measurement ^^^ ൌ ^ െ ^^െ⋯െ ^ே^ From these two can be recovered (up to a multiplicative ambiguity between ^ and ^). Hence, in some embodiments, the channel properties ^ of the wireless direct link are estimated as a function of the first bidirectional sounding and the second bidirectional sounding. For example ^ can be estimated by averaging^^^and^^^, and ^^^ can be estimated by averaging ^^^^^்and ^^^^^். Consequently, any terms of the form ^ or ^^^ (equivalently, ^^^^^்ൌ ^^்^) in subsequent measurement can be eliminated. Henceforth, it is assumed that such elimination has been performed. Also, it can be assumed that the diagonal matrices ^ and ^ are known (up to a multiplicative ambiguity) since they can be determined based on ^^^, under knowledge of ^. In some examples, the repeaters 300a:300N are configured to then rotate their phases according to the patterns, which hereinafter for illustrative pruposes will be exemplified by a Hadamard matrix. For each pattern, bidirectional measurements are taken. For ^ ൌ 4 this yields the following measurements, after subtracting the already-obtained estimate of ^^^, and after transposition (not Hermitian transposition): ^^^:^^^ ^ଶ^ ^ଷ^ ^ସ^ ^^^:^^γ^^^^ γଶ^ଶ^ γଷ^ଷ^ γସ^ସ^^^ ^^^:^^െ ^ଶ^ ^ଷെ ^ସ^ ^^^:^^γ^^^െ γଶ^ଶ^ γଷ^ଷെ γସ^ସ^^^ ^^^:^^ ^ ^ଶ െ ^ଷ െ ^ସ^^^^:^^γ^^^ ^ γଶ^ଶ െ γଷ^ଷ െ γସ^ସ^^^^^^:^^െ ^ଶെ ^ଷ^ ^ସ^ ^^^:^^γ^^^െ γଶ^ଶെ γଷ^ଷ^ γସ^ସ^^ Hence, the measurements ^^^ to ^^^ correspond to four bidirectional measurements, i.e., eight single-direction measurements in total. In this way, the coefficients of the linear combinations can be given by matrix entries per row of the orthonormal matrix. Measurements ^^^ and ^^^ were already obtained as described above when estimating ^ and ^^^. In the present example with^^ ൌ 4 repeaters, the corresponding ^ ൈ ^ Hadamard matrix is given by1^The first row of the ^ ൈ ^ Hadamar ma r x can e used to describe the first two measurements ^^^ and ^^^; the second row of the ^ ൈ ^ Hadamard matrix can be used to describe the next two measurements ^^^ and ^^^, and so on. That is, each row can be used to describe one bidirectional measurement. Since ^ and ^ are known, by forming linear combinations of these measurements, ^^and γ^^^^^ can be recovered, and from them, ^^, … , ^ସcan be recovered. Specifically, the coefficients in these linear combinations can (again) be given by the ^:th row of the ^ ൈ ^ Hadamard matrix. For example, the factors^^^^ ^ ^^^ ^ ^^^ ^ ^^^ and ^^^ ^^^^^^^^^^^^^yield, up to an irrelevant scaling factor, ^^ and γ^^^^^. Since ^ and ^are known, γ^can be recovered. As another example (based on the third row of the Hadamard matrix above), the factors ^^^ ^ ^^^ െ ^^^ െ ^^^ and ^^^ ^ ^^^ െ ^^^ െ ^^^ yield, up to an irrelevant scaling, ^ଷand γଷ^^ଷ^, from which γଷcan be determined (again, since ^ and ^ are known). In some aspects, the total number of single-direction measurements required for calibrating a system with ^ repeaters in this example is thus 2^ 2^. Here, 2^ single- direction measurements (i.e. ^ bidirectional measurements) are needed according to equations ^^^ to ^^^above. Two additional measurements (i.e. one additional bidirectional measurement) is to determine ^,^^ and ^. Aspects where the direct link channel ^ is too weak to aid in the calibration and does not affect the measurements significantly will be disclosed next. In this case, the procedure can start directly with measurements ^^^ to ^^^. This implies that the total number of single-direction measurements required for calibrating a system with ^ repeaters in this example is thus 2^. In som aspects, this assumes that the phase misalignment matrices ^ and ^ already are known, or can be estimated in some other way than disclosed above. Particularly, in some embodiments, the channel properties of the wireless direct link are obtained from the first transceiver device 110a and the second transceiver device 110b. Aspects of how often, i.e., at which time nterval, the calibration will be performed will be disclosed next. In general terms, to determine a time interval for calibrating each repeater, the network node 200 might initially perform unnecessary frequent calibrations (e.g. every 1-10 ms) where the phase calibration drift for each repeater 300a:300N is determined. The time interval can then be gradually increased (e.g. every 1 second, every 10 seconds, every minute, and so forth) until a suitable time interval is determined. Therefore, in some aspects, the phase calibration error drift is tracked for each repeater 300a:300N by the network node 200 estimating the change in the phase correction factor between consecutive calibration occasions. In particular, in some embodiments, the network node 200 further is configured to perform (optional) step S110. S110: The network node 200 tracks a drift in calibration error of the repeaters 300a:300N by estimating a change in compensation factor between consecutive occasions in which the repeaters 300a:300N are calibrated. In some examples, different repeaters 300a:300N are recalibrated with different time intervals based on an estimated calibration drift. In some aspects, the ^ repeaters 300a:300N that are jointly calibrated is therefore a subset of a larger number ^ of repeaters. In further, detail, the network node 200 might divide the repeaters 300a:300N into different calibration sub-sets and perform joint calibration, using the previously disclosed methodologies, only for the repeaters 300a:300N of the same subset. The set of ^ repeaters 300a:300N might be selected by the network node 200 based on examining the phase drift of each repeater 300a:300N. Therefore, in some embodiments, the network node 200 further is configured to perform (optional) step S112. S112: The network node 200 selects the ^ repeaters 300a:300N from a set of ^ ^ ^ repeaters based on the drift in calibration error of the ^ repeaters 300a:300N. In this way, to reduce the number of measurements required for maintaining calibration of a set of N repeaters 300a:300N, periodic recalibration might be performed for only the subset of the repeaters for which the expected phase drift exceeds a threshold value. Hence, in some embodiments, the ^ repeaters 300a:300N are selected by their drift in calibration error being larger than a threshold value. This can further reduce the number of calibration measurements required, since repeaters 300a:300N with more stable phase calibration are allowed to be recalibrated more seldom. 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 repeaters 300a:300N are by the network node 200 configured to apply the compensation factors to either the forward path circuitry 360a or the reverse path circuitry 360b. Further, as disclosed above, the forward path circuitry 360a for repeater ^ has a complex-valued gain ^^, and the reverse path circuitry 360b has a complex-valued gain ^^. where the compensation factor is determined as a function of a ratio between ^ and ^. Therefore, in some embodiments, the compensation factor for repeater ^ is a function of a ratio between ^^and ^^. Reference is now made to Fig. 5 illustrating a method for ^ repeaters 300a:300N to be calibrated in a wireless network 100a:100c according to an embodiment. S202: The repeaters 300a:300N receive configuration from the network node 200 with at least ^ patterns according to which the repeaters 300a:300N are to change their complex-valued gains over time during bidirectional sounding. S204: The repeaters 300a:300N operate according to the configuration whilst, for each of the at least ^ patterns by the complex-valued gains of the repeaters 300a:300N being changed over time in accordance with the at least ^ patterns, taking part in the bidirectional sounding between a first transceiver device 110a and a second transceiver device 110b configured to wirelessly communicate with each other without and via the repeaters 300a:300N in the wireless network 100a:100c. S206: The repeaters 300a:300N receive compensation factors from the network node 200, one per repeater 300a:300N. S208: The repeaters 300a:300N apply the compensation factors during subsequent operation. Embodiments relating to further details of ^ repeaters 300a:300N to be calibrated in a wireless network 100a:100c will now be disclosed with continued reference to Fig. 5. As disclosed above, each of the the ^ patterns defines a sequence of non-zero complex-valued gains one of the repeaters 300a:300N is to apply over time during bidirectional sounding. As further disclosed above, there could be different such patterns. In some embodiments, each of the patterns is taken as a respective row from one and the same orthonormal matrix. In some examples, the matrix is a square matrix. That is, the orthonormal matrix has ^ columns and ^ rows. In some non- limiting examples, the orthonormal matrix is any of: a unimodular matrix, a Hadamard matrix, a DFT matrix. As disclosed above, in some examples, the repeaters 300a:300N are configured to first not rotate their phases, i.e., keep their amplitudes equal to the nominal complex- valued gains. Therefore, in, in some embodiments, each of the repeaters 300a:300N s configured to perform (optional) step S202a. S202a: The repeaters 300a:300N receive configuration from the network node 200 to keep their phases unrotated during a first bidirectional sounding performed between the first transceiver device 110a and the second transceiver device 110b. As further disclosed above, in some examples, the repeaters 300a:300N are configured to then rotate the phase of their complex-valued gains by 180 degrees. Therefore, in, in some embodiments, each of the repeaters 300a:300N is configured to perform (optional) step S202b. S202b: The repeaters 300a:300N receive configuration from the network node 200 to rotate their phases 180 degrees during a second bidirectional sounding performed between the first transceiver device 110a and the second transceiver device 110b. As disclosed above, in some embodiments, the repeaters 300a:300N are by the network node 200 configured to apply the compensation factors to either the forward path circuitry 360a or the reverse path circuitry 360b. As further disclosed above, in some embodiments, the compensation factor for repeater ^ is a function of a ratio between ^^and ^^. There could be different ways in which t e configuration in step S202 (including optional steps S202a and S202b) is provided from the network node 200 to the repeaters 300a:300N. 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, each of the repeaters 300a:300N 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 repeaters 300a:300N implementation to support this. The repeaters 300a:300N 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 repeaters 300a:300N 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 repeaters 300a:300N. 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. Fig. 6 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 1010a (as in Fig.10), 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, such as the transceiver devices and the repeaters. 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. 7 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.7 comprises a number of functional modules; a configure module 210a configured to perform step S102, an obtain module 210b configured to perform step S104, a determine module 210e configured to perform step S106, and a calibrate module 210f configured to perform step S108. The network node 200 of Fig.7 may further comprise a number of optional functional modules, such as any of a configure module 210c configured to perform step S102a, a configure module 210d configured to perform step S102b, a track module 210g configured to perform step S110, and a select module 210h configured to perform step S112. In general terms, each functional module 210a:210h may be implemented in hardware or in software. Preferably, one or more or all functional modules 210a:210h may be implemented by the processing c cuitry 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:210h 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 a (radio) access network or in a node of a core network. For example, the network node 200 might be part of, integrated with, or 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. 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.6 the processing circuitry 210 may be distributed among a plurality of devices, or nodes. The same applies to the functional modules 210a:210h of Fig.7 and the computer program 1020a of Fig.10. Some (radio) access network architectures define network nodes (or APs, 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 is 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. 8 schematically illustrates, in terms of a number of functional units, the components of a repeater 300a:300N 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 1010b (as in Fig.10), 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 300a:300N 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 300a:300N 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 300a:300N may further comprise a communications interface 320 for communications with other entities, functions, nodes, and devices, such as the transceiver devices and the network node. 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 300a:300N 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 300a:300N 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 repeater 300a:300N according to an embodiment. The repeater 300a:300N of Fig.9 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 300a:300N of Fig. 9 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 300a:300N as disclosed herein. Fig. 10 shows one example of a computer program product 1010a, 1010b comprising computer readable means 1030. On this computer readable means 1030, a computer program 1020a can be stored, which computer program 1020a 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 1020a and / or computer program product 1010a may thus provide means for performing any steps of the network node 200 as herein disclosed. On this computer readable means 1030, a computer program 1020b can be stored, which computer program 1020b 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 descr bed herein. The computer program 1020b and / or computer program product 1010b may thus provide means for performing any steps of the repeater 300a:300N as herein disclosed. In the example of Fig.10, the computer program product 1010a, 1010b 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 1010a, 1010b 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 1020a, 1020b is here schematically shown as a track on the depicted optical disk, the computer program 1020a, 1020b can be stored in any way which is suitable for the computer program product 1010a, 1010b. 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 network node (200) for calibrating ^ repeaters (300a:300N) in a wireless network (100a:100c), wherein the network node (200) is configured to: configure the repeaters (300a:300N) with at least ^ patterns, wherein each of the ^ patterns defines a sequence of non-zero complex-valued gains one of the repeaters (300a:300N) is to apply over time during bidirectional sounding; obtain bidirectional channel response measurements for each of the at least ^ patterns from bidirectional sounding as performed by a first transceiver device (110a) and a second transceiver device (110b) configured to wirelessly communicate with each other without and via the repeaters (300a:300N) in the wireless network (100a:100c); determine compensation factors, one per repeater (300a:300N), as a function of linear combinations of the bidirectional channel response measurements, wherein there is one linear combination of bidirectional channel response measurements per pattern; and calibrate the repeaters (300a:300N) by configuring the repeaters (300a:300N) with the compensation factors.

2. The network node (200) according to claim 1, wherein each of the patterns is taken as a respective row from one and the same orthonormal matrix.

3. The network node (200) according to claim 2, wherein the orthonormal matrix has ^ columns and ^ rows.

4. The network node (200) according to claim 1 or 2, wherein the orthonormal matrix is any of: a unimodular matrix, a Hadamard matrix, a DFT matrix.

5. The network node (200) according to any of claims 2 to 4, wherein coefficients of the linear combinations are given by matrix entries per row of the orthonormal matrix.

6. The network node (200) according o any preceding claim, wherein the compensation factors are based on channel properties ^ of a wireless direct link between the first transceiver device (110a) and the second transceiver device (110b).

7. The network node (200) according to claim 6, wherein the compensation factors are based on transmitter gains^^,^^and receiver gains ^^,^^at the first transceiver device (110a) and the second transceiver device (110b) as estimated from the channel properties ^ of said wireless direct link.

8. The network node (200) according to claim 7, wherein the network node (200) further is configured to: configure the repeaters (300a:300N) to keep their phases unrotated during a first bidirectional sounding performed between the first transceiver device (110a) and the second transceiver device (110b).

9. The network node (200) according to claim 7 or 8, wherein the network node (200) further is configured to: configure the repeaters (300a:300N) to rotate their phases 180 degrees during a second bidirectional sounding performed between the first transceiver device (110a) and the second transceiver device (110b).

10. The network node (200) according to claims 8 and 9, wherein the channel properties ^ of said wireless direct link are estimated as a function of the first bidirectional sounding and the second bidirectional sounding.

11. The network node (200) according to claim 6, wherein the channel properties of said wireless direct link are obtained from the first transceiver device (110a) and the second transceiver device (110b).

12. The network node (200) according to any preceding claim, wherein the network node (200) further is configured to: track a drift in calibration error of the repeaters (300a:300N) by estimating a change in compensation factor between consecutive occasions in which the repeaters (300a:300N) are calibrated.

13. The network node (200) according o claim 12, wherein the network node (200) further is configured to: select the ^ repeaters (300a:300N) from a set of ^ ^ ^ repeaters based on the drift in calibration error of the ^ repeaters (300a:300N).

14. The network node (200) according to claim 13, wherein the ^ repeaters (300a:300N) are selected by their drift in calibration error being larger than a threshold value.

15. The network node (200) according to any preceding claim, wherein each of the repeaters (300a:300N) is configured for communication in a forward direction and a reverse direction, wherein each of the repeaters (300a:300N) comprises forward path circuitry (360a) for communicating in the forward direction and reverse path circuitry (360b) for communicating in the reverse direction, and wherein the repeaters (300a:300N) are by the network node (200) configured to apply the compensation factors to either the forward path circuitry (360a) or the reverse path circuitry (360b).

16. The network node (200) according to claim 15, wherein the forward path circuitry (360a) for repeater ^, where ^ ൌ 1,… ,^, has a complex-valued gain ^^, wherein the reverse path circuitry (360b) for repeater ^ has a complex-valued gain ^^, and wherein the compensation factor for repeater ^ is a function of a ratio between ^^and ^^.

17. A system comprising ^ repeaters (300a:300N) to be calibrated in a wireless network (100a:100c), wherein the repeaters (300a:300N) are configured to: receive configuration from a network node (200) with at least ^ patterns according to which the repeaters (300a:300N) are to change their complex-valued gains over time during bidirectional sounding; operate according to the configuration whilst, for each of the at least ^ patterns by the complex-valued gains of the repeaters (300a:300N) being changed over time in accordance with the at least ^ patterns, taking part in the bidirectional sounding between a first transceiver device (110a) and a second transceiver device (110b)configured to wirelessly communicate w each other without and via the repeaters (300a:300N) in the wireless network (100a:100c); receive compensation factors from the network node (200), one per repeater (300a:300N); and apply the compensation factors during subsequent operation.

18. The system according to claim 17, wherein each of the repeaters (300a:300N) is a dual-antenna repeater (300a:300N).

19. The system according to claim 17 or 18, wherein each of the repeaters (300a:300N) is configured for communication in a forward direction and a reverse direction, wherein each of the repeaters (300a:300N) comprises forward path circuitry (360a) for communicating in the forward direction and reverse path circuitry (360b) for communicating in the reverse direction, and wherein the compensation factors are to be applied to either the forward path circuitry (360a) or the reverse path circuitry (360b).

20. The system according to claim 19, wherein the forward path circuitry (360a) for repeater ^, where ^ ൌ 1,… ,^, has a complex-valued gain ^^, wherein the reverse path circuitry (360b) for repeater ^ has a complex-valued gain ^^, and wherein the compensation factor for repeater ^ is a function of a ratio between ^^and ^^.

21. The system according to any of claims 17 to 20, wherein each of the patterns is taken as a respective row from one and the same orthonormal matrix.

22. The system according to claim 21, wherein the orthonormal matrix has ^ columns and ^ rows.

23. The system according to claim 21 or 22, wherein the orthonormal matrix is any of: a unimodular matrix, a Hadamard matrix, a DFT matrix.

24. The system according to any of claims 17 to 23, wherein the repeaters (300a:300N) are configured to:receive configuration from the network node (200) to keep their phases unrotated during a first bidirectional sounding performed between the first transceiver device (110a) and the second transceiver device (110b).

25. The system according to claim 24, wherein the repeaters (300a:300N) are configured to: receive configuration from the network node (200) to rotate their phases 180 degrees during a second bidirectional sounding performed between the first transceiver device (110a) and the second transceiver device (110b).

26. A method for calibrating ^ repeaters (300a:300N) in a wireless network (100a:100c), the method being performed by a network node (200), the method comprising: configuring (S102) the repeaters (300a:300N) with at least ^ patterns, wherein each of the ^ patterns defines a sequence of non-zero complex-valued gains one of the repeaters (300a:300N) is to apply over time during bidirectional sounding; obtaining (S104) bidirectional channel response measurements for each of the at least ^ patterns from bidirectional sounding as performed by a first transceiver device (110a) and a second transceiver device (110b) configured to wirelessly communicate with each other without and via the repeaters (300a:300N) in the wireless network (100a:100c); determining (S106) compensation factors, one per repeater (300a:300N), as a function of linear combinations of the bidirectional channel response measurements, wherein there is one linear combination of bidirectional channel response measurements per pattern; and calibrating (S108) the repeaters (300a:300N) by configuring the repeaters (300a:300N) with the compensation factors.

27. A method for ^ repeaters (300a:300N) to be calibrated in a wireless network (100a:100c), the method being performed by a system comprising the ^ repeaters (300a:300N), the method comprising each of the repeaters (300a:300N):receiving (S202) configuration from a network node (200) with at least ^ patterns according to which the repeaters (300a:300N) are to change their complex- valued gains over time during bidirectional sounding; operating (S204) according to the configuration whilst, for each of the at least ^ patterns by the complex-valued gains of the repeaters (300a:300N) being changed over time in accordance with the at least ^ patterns, taking part in the bidirectional sounding between a first transceiver device (110a) and a second transceiver device (110b) configured to wirelessly communicate with each other without and via the repeaters (300a:300N) in the wireless network (100a:100c); receiving (S206) compensation factors from the network node (200), one per repeater (300a:300N); and applying (S208) the compensation factors during subsequent operation.

28. A computer program (1020a) for calibrating ^ repeaters (300a:300N) in a wireless network (100a:100c), 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 repeaters (300a:300N) with at least ^ patterns, wherein each of the ^ patterns defines a sequence of non-zero complex-valued gains one of the repeaters (300a:300N) is to apply over time during bidirectional sounding; obtain (S104) bidirectional channel response measurements for each of the at least ^ patterns from bidirectional sounding as performed by a first transceiver device (110a) and a second transceiver device (110b) configured to wirelessly communicate with each other without and via the repeaters (300a:300N) in the wireless network (100a:100c); determine (S106) compensation factors, one per repeater (300a:300N), as a function of linear combinations of the bidirectional channel response measurements, wherein there is one linear combination of bidirectional channel response measurements per pattern; andcalibrate (S108) the repeaters (300a:300N) by configuring the repeaters (300a:300N) with the compensation factors.

29. A computer program (1020b) for ^ repeaters (300a:300N) to be calibrated in a wireless network (100a:100c), the computer program comprising computer code which, when run on processing circuitry (310) of a system comprising the ^ repeaters (300a:300N), causes the the ^ repeaters (300a:300N) to: receive (S202) configuration from a network node (200) with at least ^ patterns according to which the repeaters (300a:300N) are to change their complex-valued gains over time during bidirectional sounding; operate (S204) according to the configuration whilst, for each of the at least ^ patterns by the complex-valued gains of the repeaters (300a:300N) being changed over time in accordance with the at least ^ patterns, taking part in the bidirectional sounding between a first transceiver device (110a) and a second transceiver device (110b) configured to wirelessly communicate with each other without and via the repeaters (300a:300N) in the wireless network (100a:100c); receive (S206) compensation factors from the network node (200), one per repeater (300a:300N); and apply (S208) the compensation factors during subsequent operation.

30. A computer program product (1010a, 1010b) comprising a computer program (1020a, 1020b) according to at least one of claims 28 and 29, and a computer readable storage medium (1030) on which the computer program is stored.

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