UE capability signaling for uplink reference signal transmission

Abstract

There is provided techniques for performing an uplink reference signal transmission for downlink channel sounding. A method is performed by a UE being equipped with more antennas than receive chains. The method comprises sending a report to a network node serving the UE. The report comprises an indication of the UE supporting uplink reference signal based downlink channel sounding for reciprocity-based downlink transmission and indicates that the UE is equipped with more antennas than receive chains. The method comprises receiving uplink reference signal configuration from the network node. The uplink reference signal configuration is based on the report and comprises configuration of a set of uplink reference signal resources. According to the uplink reference signal configuration, at least one uplink reference signals is to be transmitted per antenna. The method comprises transmitting the uplink reference signals on the set of uplink reference signal resources in accordance with the uplink reference signal configuration.

Description

[0001] P111007W001 1

[0002] UE CAPABILITY SIGNALING FOR UPLINK REFERENCE SIGNAL TRANSMISSION

[0003] TECHNICAL FIELD

[0004] Embodiments presented herein relate to a method, a user equipment, a computer program, and a computer program product for performing an uplink reference signal transmission for downlink channel sounding.

[0005] Embodiments presented herein further relate to a method, a network node, a computer program, and a computer program product for configuring the user equipment for this uplink reference signal transmission.

[0006] BACKGROUND

[0007] Uplink reference signals, such as sounding reference signals (SRS) can be used for different purposes, such as for providing channel state information (CSI) in the uplink to the network. Fig. 1 is a schematic diagram illustrating a communication network 100 where embodiments presented herein can be applied. The communication network 100 comprises a network node 120 representing the network. The network node 120 could be any of a (radio) access network node, radio base station, base transceiver station, node B (NB), evolved node B (eNB), gNB, access point, access node, integrated access and backhaul (IAB) node, transmission and reception point, etc. The network node 120 is configured to serve, and thus provide network access, to pieces of user equipment (UE) 110a: 11 ON, such as any, or any combination of, portable wireless devices, mobile stations, mobile phones, handsets, wireless local loop phones, smartphones, laptop computers, tablet computers, wireless modem devices, wireless sensor device, network equipped vehicles, Internet of things devices, gaming equipment, etc.

[0008] The use of uplink reference signals comprises, e.g., deriving the appropriate transmission / reception beams, performing link adaptation (i.e., setting the transmission rank and the mobility and coding scheme), and selecting downlink (e.g., for downlink data transmissions) and uplink (e.g., for uplink data transmissions) multiple-input multiple-output (Ml MO) precoding schemes.

[0009] For the long-term evolution (LTE) and new radio (NR) air interfaces, SRS is configured via radio resource control (RRC) signaling, where part of the configuration can be updated (for reduced latency) through medium access control (MAC) control element (CE) signaling. The configuration comprises, for example, the SRS resource allocation (in terms of the time-and-frequency mapping and the sequence to use) as well as the time-domain behavior (i.e., aperiodic, semi-persistent, or periodic transmission of the SRS). For aperiodic SRS transmission, the RRC configuration does not activate an SRS transmission from the UE, but instead a dynamic activation trigger is transmitted from the network which instructs the UE to transmit the SRS once, at a predetermined time.

[0010] When configuring SRS transmissions, the network can configures, through a SRS-Config information element (IE), a set of SRS resources and a set of SRS resource sets, where each SRS resource set contains one or more SRS resources. Each SRS resource set can be configured with a certain usage (depending on what the SRS transmission shall be used for, as specified in the technical specification 3GPP TS 38.214, "NR; Physical layer P111007W001 2 procedures for data”, version 18.3.0), by setting the RRC parameter ‘usage’ to one of 'antennaSwitching', ‘codebook', ‘nonCodebook', and ‘beamManagement’, as will be further explained next.

[0011] An SRS resource set that is configured with usage 'antennaSwitching' is used for reciprocity-based DL precoding (i.e., used to sound the channel in the uplink so that the network can use reciprocity to set a suitable downlink precoders). The UE is expected to transmit one SRS port per UE antenna port.

[0012] An SRS resource set that is configured with usage ‘codebook' is used for codebook (CB)-based uplink transmission (i.e., used to sound the different UE antennas, or physical antenna ports, and help the network to determine a suitable uplink precoder, transmission rank, and modulation and coding scheme for uplink data transmission). There are up to two SRS resources in an SRS resource set with usage ‘codebook'. How SRS ports are mapped to physical antenna ports in the UE is, however, up to UE implementation and not known to the network.

[0013] An SRS resource set that is configured with usage ‘nonCodebook' is used for NCB-based uplink transmission (where NCB is short for non-codebook). Specifically, the UE transmits one SRS resource per candidate beam (where suitable candidate beams are determined by the UE based on, e.g., downlink reference signal measurements in the downlink, and in this case some degree of beam reciprocity needs to hold). Measurements on the SRS resources are then made by the network, and the network determines which uplink beam(s) that the UE should apply for uplink data transmission. The network then indicates to the UE which beams the UE should apply via indicating a subset of said SRS resources. One uplink layer will be transmitted per indicated SRS resource. How the UE maps SRS ports to physical antenna ports in the UE is up to UE implementation and not known to the network.

[0014] An SRS resource set that is configured with usage 'beamManagement' is used (mainly for frequency bands above 6 GHz (i.e., for FR2)) to evaluate different UE beams for analog beamforming arrays. The UE transmits one SRS resource per analog beam, and the network will perform a measurement of reference signal received power per transmitted SRS resource and, in this way, determine a suitable UE beam that is reported back to the UE.

[0015] It is desirable for the network to sound all UE antennas (where sounding a given antenna means that the UE is transmitting an uplink reference signal from that given antenna; this enables the network to estimate the channel between the given antenna and the antennas at the network). However, it can be costly to equip UEs with many transmit chains. So-called SRS antenna switching has therefore been introduced for several different UE architectures for which the number of receive chains is larger than the number of transmit chains. If a UE supports antenna switching, the UE will report so by means of UE capability signaling.

[0016] Examples of SRS antenna-switching capabilities that can be reported from a UE to the network are listed in the technical specification 3GPP TS 38.306, "NR; User Equipment (UE) radio access capabilities”, version 18.2.0. As P111007W001 3 an illustrative example, if a UE reports t1r2 in the UE-capability signaling, this means that the UE is equipped with two receive antennas (i.e., two receive chains) but only has the possibility of transmitting from one of those antennas at a time (i.e., one transmission chain) with support for antenna switching. The UE is thus capable of simultaneous reception using two receive chains. In this case, two single-port SRS resources can be configured to the UE such that both receive ports can be sounded using a single transmit port with an antenna switch in between.

[0017] Additional UE capabilities indicate support for the UE to be configured with SRS resource set(s) with usage ‘antennaSwitching’ but where only a subset of all UE antennas is sounded. As an illustrative example, the UE capability t1r1-t1r2 means that the network can configure one single-port SRS resource (i.e., the same as no antenna-switching capability) or two single-port SRS resources (i.e., the same as for the capability “t1i2" described above) with usage ‘antennaSwitching’ per SRS resource set. In this case, if the UE is configured with a single SRS resource (i.e., no antenna switching) the UE will only sound only one of its two antennas, which will save power consumption in the UE at the cost of reduced channel knowledge at the network (since the network can only estimate the channel between itself and the UE based on one of the two antennas in the UE).

[0018] For SRS resources with usage 'antennaSwitching' for a UE with fewer number of transmit chains than number of receive chains, a guard period has to be configured between SRS resources to account for the switching transient time. For a subcarrier spacing below 120 kHz the guard period is 1 orthogonal frequency-division multiplexing (OFDM) symbol, while for a sub-carrier spacing of 120 kHz it is 2 OFDM symbols. This means that a UE is expected to be able to switch antenna within one or two OFDM symbols, depending on sub-carrier spacing.

[0019] For communication in frequency bands between 7-24 GHz (i.e., between FR1 and FR2), it is expected that the UE will be equipped with directional antennas, each directional antenna pointing to a different direction to attain omni-directional coverage and to compensate for the comparatively larger path loss at these frequencies. More antennas in the UE, compared to the number of antennas at sub-6GHz UEs, are feasible due to the smaller size of antennas configured for operation at these frequencies. In addition, having two or more antennas overlapping in coverage in a particular direction (e.g. as in a dual-polarized antenna) can be beneficial to enable higher-order spatial multiplexing more easily and hence increase user throughput (compared to only having a single-polarized antenna, each pointing towards a distinct direction).

[0020] However, to limit production costs, as well as energy consumption during operation, UEs might be implemented with a smaller number of receive chains compared to the number of UE antennas. Reference is here made to Fig. 2. Fig. 2 is a schematic block diagram of a UE 200 according to an example, focusing on transmission and reception components of the UE 200. In this example, the UE 200 is equipped with two receive chains 210a, 210b, one transmit chain 220, and four (directional) antennas 230a:230d (giving rise to antenna lobes 270a:270d). A low-noise amplifier (LNA) 250a, 250b is provided in each receive chain and a power amplifier (PA) 260 is provided in the transmit chain. In this example, the antennas are shared by the two receive chains and the P111007W001 4 single transmit chain in a way defined by a switching network 240. Only one of the chains 210a, 210b, 220 can be connected to each of the antennas 230a:230d per time instant. That is, according to the illustrative example, if receive chain 210a is connected to antenna 230b, then receive chain 210b is either connected to antenna 230c or turned off, etc.

[0021] The SRS schemes for "antenna switching” capability signaling and configuration have been conceived for UEs where the number of receive chains is equal to the number of UE antennas. However, when the number of UE antennas is larger than the number of receive chains, these schemes do not enable all UE antennas to be sounded. This is because the maximum number of SRS ports cannot exceed the number of receive chains. As a result, only partial channel knowledge can be obtained for downlink reciprocity based beamforming. In turn this reduce the downlink performance in the network.

[0022] For operation in FR2, the UEs may be equipped with antenna panels, where the number of transmit and / or receive chains typically is fewer than the number of antenna panels. As an illustrative example, a UE might be equipped with two transmit chains and two receive chains, and these transmit and receive chains can then be switched between the different antenna panels, where typically each antenna panel supports two transmit and receive chains). However, there does not exist any SRS transmission scheme which covers the SRS transmission for sounding all the antenna panels of UE. It is here noted that, for FR2, an SRS transmission scheme pertaining to SRS with usage 'beamManagement' has been introduced to handle UEs with antenna panels and analog beamforming, but this SRS transmission scheme does not indicate that the UE should sound different antenna panels. SRS with usage 'beamManagement' differs from SRS with usage ‘antenna switching', where the latter indicates to the UE to transmit SRS from different UE physical antenna ports, or antenna elements.

[0023] Hence, there is a need for techniques enabling sounding of all antennas at a UE equipped with more antennas than receive chains.

[0024] SUMMARY

[0025] An object of embodiments herein is to enable sounding of all antennas at a UE equipped with more antennas than receive chains.

[0026] A particular object is to provide an SRS transmission scheme for SRS with usage 'beamManagement' that enable sounding of all antennas at a UE equipped with more antennas than receive chains.

[0027] According to a first aspect there is presented a method for performing an uplink reference signal transmission for downlink channel sounding. The method is performed by a UE. The UE is equipped with more antennas than receive chains. The method comprises sending a report to a network node serving the UE. The report comprises an indication of the UE supporting uplink reference signal based downlink channel sounding for reciprocity-based downlink transmission and indicates that the UE is equipped with more antennas than receive chains. The P111007W001 5 method comprises receiving uplink reference signal configuration from the network node. The uplink reference signal configuration is based on the report and comprises configuration of a set of uplink reference signal resources. According to the uplink reference signal configuration, at least one uplink reference signals is to be transmitted per antenna. The method comprises transmitting the uplink reference signals on the set of uplink reference signal resources in accordance with the uplink reference signal configuration.

[0028] According to a second aspect there is presented a UE for performing an uplink reference signal transmission for downlink channel sounding. The UE is equipped with more antennas than receive chains and comprises processing circuitry. The processing circuitry is configured to cause the UE to send a report to a network node serving the UE. The report comprises an indication of the UE supporting uplink reference signal based downlink channel sounding for reciprocity-based downlink transmission and indicates that the UE is equipped with more antennas than receive chains. The processing circuitry is configured to cause the UE to receive uplink reference signal configuration from the network node. The uplink reference signal configuration is based on the report and comprises configuration of a set of uplink reference signal resources. According to the uplink reference signal configuration, at least one uplink reference signals is to be transmitted per antenna. The processing circuitry is configured to cause the UE to transmit the uplink reference signals on the set of uplink reference signal resources in accordance with the uplink reference signal configuration.

[0029] According to a third aspect there is presented a computer program for performing an uplink reference signal transmission for downlink channel sounding. The computer program comprises computer code which, when run on processing circuitry of a UE equipped with more antennas than receive chains, causes the UE to perform actions. One action comprises the UE to send a report to a network node serving the UE. The report comprises an indication of the UE supporting uplink reference signal based downlink channel sounding for reciprocity-based downlink transmission and indicates that the UE is equipped with more antennas than receive chains. One action comprises the UE to receive uplink reference signal configuration from the network node. The uplink reference signal configuration is based on the report and comprises configuration of a set of uplink reference signal resources. According to the uplink reference signal configuration, at least one uplink reference signals is to be transmitted per antenna. One action comprises the UE to transmit the uplink reference signals on the set of uplink reference signal resources in accordance with the uplink reference signal configuration.

[0030] According to a fourth aspect there is presented a method for configuring a UE for an uplink reference signal transmission used for downlink channel sounding. The UE is equipped with more antennas than receive chains. The method is performed by a network node. The method comprises receiving a report to from the UE. The report comprises an indication of the UE supporting uplink reference signal based downlink channel sounding for reciprocity-based downlink transmission and indicates that the UE is equipped with more antennas than receive chains. The method comprises sending) uplink reference signal configuration to the UE. The uplink reference signal configuration is based on the report and comprises configuration of a set of uplink reference signal resources. According to the uplink reference signal configuration, at least one uplink reference signals is to be P111007W001 6 transmitted per antenna. The method comprises receiving the uplink reference signals as transmitted by the UE on the set of uplink reference signal resources.

[0031] According to a fifth aspect there is presented a network node for configuring a UE for an uplink reference signal transmission used for downlink channel sounding. The UE is equipped with more antennas than receive chains. The network node comprises processing circuitry. The processing circuitry is configured to cause the network node to receive a report to from the UE. The report comprises an indication of the UE supporting uplink reference signal based downlink channel sounding for reciprocity-based downlink transmission and indicates that the UE is equipped with more antennas than receive chains. The processing circuitry is configured to cause the network node to send uplink reference signal configuration to the UE. The uplink reference signal configuration is based on the report and comprises configuration of a set of uplink reference signal resources. According to the uplink reference signal configuration, at least one uplink reference signals is to be transmitted per antenna. The processing circuitry is configured to cause the network node to receive the uplink reference signals as transmitted by the UE on the set of uplink reference signal resources.

[0032] According to a sixth aspect there is presented a computer program for configuring a UE, equipped with more antennas than receive chains, for an uplink reference signal transmission used for downlink channel sounding. 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 receive a report to from the UE. The report comprises an indication of the UE supporting uplink reference signal based downlink channel sounding for reciprocity-based downlink transmission and indicates that the UE is equipped with more antennas than receive chains. One action comprises the network node to send uplink reference signal configuration to the UE. The uplink reference signal configuration is based on the report and comprises configuration of a set of uplink reference signal resources. According to the uplink reference signal configuration, at least one uplink reference signals is to be transmitted per antenna. One action comprises the network node to receive the uplink reference signals as transmitted by the UE on the set of uplink reference signal resources.

[0033] 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.

[0034] Advantageously, these aspects enable sounding of all antennas at a UE equipped with more antennas than receive chains.

[0035] Advantageously, these aspects enable UEs equipped with more antennas than receives chains to sound all UE antennas for downlink reciprocity-based precoding. Advantageously, in turn, this will increase the channel knowledge at the network and hence improve the downlink performance. P111007W001 7

[0036] Advantageously, these aspects enable better downlink performance. This is because the network node can indicate to the UE a subset of UE antennas suitable to receive precoded downlink signals.

[0037] It is to be noted that any feature of the first, second, third, fourth, fifth, sixth seventh, eight, and ninth aspects may be applied to any other aspect, wherever appropriate. Likewise, any advantage of the first aspect may equally apply to the second, third, fourth, fifth, sixth, seventh, eight, and / or ninth aspect, respectively, and vice versa. 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.

[0038] 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.

[0039] BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The inventive concept is now described, by way of example, with reference to the accompanying drawings, in which:

[0041] Fig. 1 is a schematic diagram illustrating a communication network according to embodiments;

[0042] Fig. 2 is a block diagram of a UE according to embodiments;

[0043] Figs. 3 and 5 are flowcharts of methods according to embodiments;

[0044] Fig. 4 is a schematic illustration of reception components of a UE according to embodiments;

[0045] Fig. 6 is a signaling diagram of a method according to an embodiment;

[0046] Fig. 7 is a schematic diagram showing structural units of a UE according to an embodiment;

[0047] Fig. 8 is a schematic diagram showing structural units of a network node according to an embodiment; and

[0048] Fig. 9 shows one example of a computer program product comprising computer readable means according to an embodiment.

[0049] DETAILED DESCRIPTION

[0050] 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, P111007W001 8 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.

[0051] As disclosed above there is a need for techniques enabling sounding of all antennas at a UE equipped with more antennas than receive chains.

[0052] In this respect, the inventors of the present disclosure have realized that at least part of this problem occurs because there is no type of existing capability signaling that covers this situation. The consequence of that is that the network node is not aware of that the UE is equipped with more antennas than receive chains, and the network therefore cannot take this information into account. This is because according to the baseline, the understanding is that the number of receive chains is equal to the number of antennas. Therefore, even if the UE has more antennas than receive chains, this is not reflected in existing capability signaling.

[0053] The present disclosure introduces a capability report, where the UE can indicate support for uplink reference signal targeting downlink channel sounding, where the UE is equipped with more antennas than receive chains. The present disclosure further introduces signaling and methods for how to perform configuration and transmission of uplink reference signals in a useful way for UEs equipped with more antennas than receive chains. The present disclosure further introduces signaling and methods for the network to indicate to the UE a subset of UE antennas that are suitable to receive precoded downlink signals.

[0054] Reference is now made to Fig. 3 illustrating a method for performing an uplink reference signal transmission for downlink channel sounding as performed by the UE 110a: 11 ON, 200 according to an embodiment. The UE 110a: 11 ON, 200 is equipped with more antennas 230a:230d than receive chains 210a, 210b.

[0055] S102: The UE 110a: 110N, 200 sends a report to a network node 120 serving the UE 110a:110N, 200. The report comprises an indication of the UE 110a: 110N, 200 supporting uplink reference signal based downlink channel sounding for reciprocity-based downlink transmission. The report indicates that the UE 110a: 110N, 200 is equipped with more antennas 230a:230d than receive chains 210a, 210b.

[0056] S104: The UE 110a: 110N, 200 receives uplink reference signal configuration from the network node 120. The uplink reference signal configuration is based on the report. The uplink reference signal configuration comprises configuration of a set of uplink reference signal resources. According to the uplink reference signal configuration, at least one uplink reference signals is to be transmitted per antenna.

[0057] S108: The UE 110a: 110N, 200 transmits the uplink reference signals on the set of uplink reference signal resources in accordance with the uplink reference signal configuration.

[0058] Embodiments relating to further details of performing an uplink reference signal transmission for downlink channel sounding as performed by the UE 110a: 110N, 200 will now be disclosed with continued reference to Fig. 3. P111007W001 9

[0059] Further aspects of the report will be disclosed next.

[0060] In some aspects, the report is sent as UE capability signaling.

[0061] In some embodiments, the report comprises an indication of the number of transmit chains 220 in the UE 110a: 11 ON, 200, the number of receive chains 210a, 210b in the UE 110a: 11 ON, 200 and the number of antennas 230a:230d in the UE 110a: 110N, 200. This information would then correspond to a certain SRS configuration. For example, the UE might indicate support for K number of transmit chains, M number of receive chains, and N number of antennas. Here, K, M, L are integer values larger than 0 and M < N.

[0062] In some aspects, the report indicates supported SRS configurations related to relaxed communication (i.e. where one or more transmit chain and / or receive chain is turned off at the UE to save energy). That is, in some embodiments, at least some of the transmit chains 220 and / or receive chains 210a, 210b in the UE 110a: 110N, 200 can be turned off. The report might then comprise an indication of different combinations of transmit chains 220 and receive chains 210a, 210b usable by the UE 110a: 11 ON, 200 during transmission of the uplink reference signals. As an illustrative example, assume that the UE 110a: 110N, 200 is equipped with 2 transmit chains, 4 receive chains and 8 antennas. Then the UE 110a: 11 ON, 200 can indicate support for being configured with e.g. one or more of: {1 Tx, 4 Rx, 8 antennas}, {2 Tx, 4 Rx, 8 antennas}, {1 Tx, 3 Rx, 8 antennas}, {2 Tx, 3 Rx, 8 antennas}, {1 Tx, 2 Rx, 8 antennas}, {2 Tx, 2 Rx, 8 antennas}, {1 Tx, 1 Rx, 8 antennas}, where Tx is short for transmit (chain), and Rx is short for receive (chain).

[0063] In some aspects, the UE 110a: 110N, 200 indicates support for a configuration containing a smaller number of antenna elements compared to the ones it possesses, e.g. when the transmit chain(s) 220 and / or receive chain(s) 210a, 210b that the UE 110a: 110N, 200 has turned off have a fixed association (e.g., due to hardware switching network, etc.) with a specific UE antennas 230a:230d. Hence, in some embodiments, less than all the transmit chains 220 and / or less than all the receive chains 210a, 210b are connectible to the antennas 230a:230d in the UE 110a: 11 ON, 200. The different combinations might then further comprise information of the number of antennas 230a:230d usable by the UE 110a: 110N, 200 for each of the different combinations. This may imply that when the UE 110a: 110N, 200 turns off one or more transmit chain(s) and / or receive chain(s), one or more of the antennas 230a:230d might be not accessible as well. Hence, for the aforementioned example, in this case one or more of the following supported configurations might be indicated by the UE 110a: 110N, 200: {1 Tx, 4 Rx, 6 antennas}, {2 Tx, 4 Rx, 6 antennas}, {1 Tx, 3 Rx, 6 antennas}, {2 Tx, 3 Rx, 6 antennas}, {1 Tx, 2 Rx, 6 antennas}, {2 Tx, 2 Rx, 6 antennas}, {1 Tx, 1 Rx, 6 antennas}, {1 Tx, 4 Rx, 4 antennas}, {2 Tx, 4 Rx, 4 antennas},

[0064] {1 Tx, 3 Rx, 4 antennas}, {2 Tx, 3 Rx, 4 antennas}, {1 Tx, 2 Rx, 4 antennas}, {2 Tx, 2 Rx, 4 antennas}, {1 Tx, 1 Rx, 4 antennas}, {1 Tx, 3 Rx, 3 antennas}, {2 Tx, 3 Rx, 3 antennas}, {1 Tx, 2 Rx, 3 antennas}, {2 Tx, 2 Rx, 3 antennas}, {1 Tx, 1 Rx, 3 antennas}, {1 Tx, 2 Rx, 2 antennas}, {2 Tx, 2 Rx, 2 antennas}, {1 Tx, 1 Rx, 2 antennas}. P111007W001 10

[0065] In some aspects, the UE 110a: 110N, 200 also indicates to the network node 120 the type of switching network 240, 420 between its receive chains 210a, 210b and antennas 230a:230d. That is, in some embodiments, the transmit chains 220 and / or the receive chains 210a, 210b are connectible to the antennas 230a: 230d via a switching network 240, 420. The report might then comprise an indication of which type the switching network 240, 420 is.

[0066] Three different examples of switching networks 420 are illustrated in Fig. 4. In more detail, in Fig. is illustrated three examples 400a, 400b, 400c of reception components of a UE. In each example 400a: 400c is illustrated reception components in terms of antennas 410, switching network 420a:420c, and receive chains 430.

[0067] Example 400a represents a balanced switching network 420a. Accordingly, there are half as many receive chains, and each receive chain is switchable between two respective antennas. That is, receive chain m can be connected to antenna 2m- 1 or antenna 2m, one at a time (i.e., receive chain 1 can be connected to antenna 1 or antenna 2, one at a time, etc.).

[0068] Example 400b represents a fully capable switching network 420b. Accordingly, any receive chain can be associated with any antenna. But two antenna chain cannot at the same time be connected to the same antenna.

[0069] Example 400c represents an un-balanced switching network 420c. Accordingly, some receive chains (receive chains 1 and 2) have fixed connections to some antennas, and some receive chains (receive chains 3 and 4) can be switched between different antennas.

[0070] In some aspects, and as will be further disclosed below, the network node 120 triggers the UE 110a: 110N, 200 to transmit the uplink reference signals. Hence, in some embodiments, the UE 110a: 110N, 200 is configured to perform (optional) step S106.

[0071] S106: The UE 110a: 110N, 200 receives a trigger from the network node 120 for the UE 110a: 110N, 200 to transmit the uplink reference signals. The uplink reference signals are then transmitted in response thereto (i.e., in response to the UE 110a: 110N, 200 having received the trigger).

[0072] In some aspects, and as will be further disclosed below, the network node 120 indicates which UE antennas 230a:230d that are to be used by the UE UE 110a: 11 ON, 200 for reception of downlink transmission. That is, in some aspects, the UE 110a: 110N, 200 receives an indication of which UE antennas 230a:230d (or which subset) that the downlink transmission is associated with. Hence, in some embodiments, the UE 110a: 110N, 200 is configured to perform (optional) step S110.

[0073] S110: The UE 110a: 11 ON, 200 receives an indication from the network node 120 of which of the antennas 230a:230d for the UE 110a: 110N, 200 to use for reception of the reciprocity-based downlink transmission and / or of which of the antennas 230a:230d that by the network node 120 are associated with the reciprocity-based downlink transmission. P111007W001 11

[0074] Different types of such indications will be disclosed in the context of the network node 120 but apply here in the context of the UE 110a: 110N, 200 as well. For example, downlink control information (DCI) triggering a downlink transmission might comprise an indication of which of the antennas 230a:230d the UE 110a: 11 ON, 200 should use for (or the network has associated with) a given downlink transmission (i.e., the antenna indication in a DCI is only applicable for the downlink transmission triggered by the same DCI).

[0075] In some aspects, and as will be further disclosed below, the network node 120 performs a downlink transmission to the UE 110a: 110N, 200. That is, in some aspects, the UE 110a: 110N, 200 receives the downlink transmission and uses the indicated antennas 230a:230d when doing so. Hence, in some embodiments, the UE 110a: 11 ON, 200 is configured to perform (optional) step S112.

[0076] S112: The UE 110a: 11 ON, 200 receives, whilst having receive chains 210a, 210b connected to the indicated antennas 230a:230d, the reciprocity-based downlink transmission from the network node 120.

[0077] Reference is now made to Fig. 5 illustrating a method for configuring a UE 110a: 110N, 200 for an uplink reference signal transmission used for downlink channel sounding as performed by the network node 120 according to an embodiment.

[0078] S202: The network node 120 receives a report to from the UE 110a: 110N, 200. The report comprises an indication of the UE 110a: 11 ON, 200 supporting uplink reference signal based downlink channel sounding for reciprocity-based downlink transmission. Further, the report and indicates that the UE 110a: 110N, 200 is equipped with more antennas 230a:230d than receive chains 210a, 210b.

[0079] S204: The network node 120 sends uplink reference signal configuration to the UE 110a: 11 ON, 200. The uplink reference signal configuration is based on the report. The uplink reference signal configuration comprises configuration of a set of uplink reference signal resources. According to the uplink reference signal configuration, at least one uplink reference signals is to be transmitted per antenna.

[0080] S208: The network node 120 receives the uplink reference signals as transmitted by the UE 110a: 110N, 200 on the set of uplink reference signal resources.

[0081] Embodiments relating to further details of configuring a UE 110a: 110N, 200 for an uplink reference signal transmission used for downlink channel sounding as performed by the network node 120 will now be disclosed with continued reference to Fig. 5.

[0082] Further aspects of the uplink reference signal configuration will be disclosed next.

[0083] In general terms, the network node 120 configures the UE 110a: 110N, 200 with uplink reference signal resources according to the reported UE capability signaling and that enables sounding of all UE antennas 230a:230d. P111007W001 12

[0084] Hence, in some embodiments, the uplink reference signal configuration specifies uplink reference signal resources to be transmitted from all the antennas 230a:230d the antennas 230a:230d of the UE 110a: 110N, 200.

[0085] In some examples, the uplink reference signals are SRSs and hence the uplink reference signal resources are SRS resources.

[0086] Below follows some non-limiting examples of SRS configuration based on the UE capability signaling.

[0087] In one example the UE indicates, during UE capability signaling, the support {2 Tx, 3 Rx, 6 antennas}. In this case the network node might configure the UE with one SRS resource set, where the SRS resource set consist of three 2-port SRS resources. In addition, there might be a gap period of one, or two, time domain (OFDM) symbols between the different SRS resources to allow for the UE to switch the transmit and receive chains between the antennas. In another example, for aperiodic SRS, the network node might configure the UE with 2 SRS resource set, where a first SRS resource set consists of two 2-port SRS resources and a second SRS resource set consists of a single 2-port SRS resources.

[0088] In another example, the UE indicates, during UE capability signaling, the support {3 Tx, 3 Rx, 6 antennas}. In this case the network node might configure the UE with one SRS resource set, where the SRS resource set consists of two 3-port SRS resources. In addition, there might be a gap period of one, or two, time domain (OFDM) symbols between the different SRS resources to allow for the UE to switch the transmit and receive chains between the antennas.

[0089] In yet another example, the UE indicates, during UE capability signaling, the support {1 Tx, 2 Rx, 4 antennas}. In this case, the network might configure the UE with one SRS resource set, where the SRS resource set consists of four single-port SRS resources. In addition, there might be a gap period of one, or two, time domain (OFDM) symbols between the different SRS resources to allow for the UE to switch the receive chains between the antennas.

[0090] In some aspects, as disclosed above, the network node 120 triggers the UE 110a: 110N, 200 to transmit the uplink reference signals. This trigger may depend on the time-domain behavior (i.e., aperiodic, semi-persistent, or periodic transmission) of the uplink reference signals. Hence, in some embodiments, the network node 120 is configured to perform (optional) step S206.

[0091] S206: The network node 120 transmits a trigger to the UE 110a: 11 ON, 200 for the UE 110a: 11 ON, 200 to transmit the uplink reference signals.

[0092] In some aspects, the network node 120 receives the uplink reference signals and estimates a downlink channel between the network node 120 and UE 110a: 110N, 200. Based on the estimated channel and considering the report received from the UE 110a: 110N, 200, the network node 120 then determines a precoder for coming P111007W001 13 downlink transmission to the UE 110a:110N, 200. Hence, in some embodiments, the network node 120 is configured to perform (optional) steps S210a, 210b and S212.

[0093] S210: The network node 120 estimates a downlink channel from the network node 120 to the UE 110a: 110N, 200 based on the received uplink reference signals.

[0094] S212: The network node 120 selects a precoder for the network node 120 to use for reciprocity-based downlink transmission towards the UE 110a: 110N, 200. The precoder is selected based on the estimated downlink channel and according to that the UE 110a: 110N, 200 is equipped with more antennas 230a:230d than receive chains 210a, 210b.

[0095] In some aspects, as disclosed above, the network node 120 indicates to the UE 110a: 110N, 200 which UE antennas 230a:230d the UE 110a: 110N, 200 is to use for reception of the downlink transmission. Hence, in some embodiments, the network node 120 is configured to perform (optional) step S214.

[0096] S214: The network node 120 sends an indication to the UE 110a: 110N, 200 of which of the antennas 230a:230d for the UE 110a: 11 ON, 200 to use for reception of the reciprocity-based downlink transmission and / or of which of the antennas 230a:230d that by the network node 120 are associated with the reciprocity-based downlink transmission.

[0097] There could be different ways for the network node 120 to send the indication to the UE 110a: 110N, 200 in step S214.

[0098] In some aspects, a field is used in a DCI triggering a downlink data transmission, where the field indicates one or more UE antennas. This could be the case where the downlink transmission is an aperiodic transmission. For a periodic downlink transmission, the indication can be sent in a field of RRC signaling. In some aspects, the network node 120 indicates the UE antennas by an index related to a previous transmission of uplink reference signals from the UE 110a: 110N, 200. In some aspects, each uplink reference signal is transmitted from one antenna and each uplink reference signal is associated with an index, and the field indicates the UE antennas by indicating the associated uplink reference signal indexes. Hence, in some embodiments, the indication is sent as any of: a field is used in a DCI message triggering a downlink data transmission, an index relating to the received uplink reference signals, an index relating to indexes of the received uplink reference signals.

[0099] In some aspects, a bitfield of length M (where M is equal to the number of UE antennas, or the number of UE antennas that are currently active, considering that some UE antennas might be de-activated due to turned off or un-used transmit and / or receive chains) is used to indicate which UE antennas that should be used by the UE for coming DL transmission. That is, in some embodiments, the indication is sent as a bitfield of length M, where M is either the total number of antennas 230a:230d or number of active antennas 230a:230d (i.e., the number of the antennas 230a:230d that are active) at the UE 110a: 11 ON, 200. Here, each bit of the bitfield is associated to one P111007W001 14 of the UE antennas (or uplink reference signals), and setting a bit to "1” means that the corresponding UE antenna should be used, while setting a bit to "0” indicates that the corresponding UE antennas should not be used. That is, a certain bit set in the bitfield indicates that the antenna corresponding to this certain bit is to be used for reception of the reciprocity-based downlink transmission.

[0100] In some aspects, instead of letting each bit of a bitfield indicate if a UE antenna should be used or not, different codepoints of a bitfield (e.g., in DCI) are used to indicate different combinations of UE antennas that should be used. Hence, in some embodiments, the indication is sent as a bitfield, where each bit of the bitfield indicates which combination of antennas 230a:230d to be used for reception of the reciprocity-based downlink transmission. For example, for a UE with 8 antennas in total, a first codepoint of a bitfield, say codepoint "0”, indicates that UE antennas 1, 2, 3, 4 should be used whilst UE antennas 5, 6, 7, 8 should not be used, and codepoint "1” of the bitfield indicates that UE antennas 1, 2, 3, 5 should be used, whilst UE antennas 4, 6, 7, 8 should not be used, etc. This reduces the number of bits required in the DCI compared to using one bit per UE antenna.

[0101] In general terms, if the UE 110a: 11 ON, 200 is configured for receive chain to UE antenna switching, this limits which UE antennas that can be connected to which receive chain(s). One such example is example 400c (see, Fig. 4). In this respect, depending on the implementation of the switching network 240 in the UE for receive chain to UE antenna switching, only a subset of all possible UE antennas combination for downlink reception may be possible. Therefore, in some aspects, the field in DCI is adapted to only indicate the candidate UE antenna combinations allowed by the receive chain to UE antenna switching.

[0102] In example 400c (see, Fig. 4), there are only 16 different possible UE antenna combinations to select from: {1, 3,

[0103] 5, 7}, {1, 3, 5, 8}, {1, 3, 6, 7}, {1, 3, 6, 8}, {1, 4, 5, 7}, {1, 4, 5, 8}, {1, 4, 6, 7}, {1, 4, 6, 8}, {2, 3, 5, 7}, {2, 3, 5, 8}, {2, 3, 6, 7}, {2, 3, 6, 8}, {2, 4, 5, 7}, {2, 4, 5, 8}, {2, 4, 6, 7} and {2, 4, 6, 8}. Hence, since only 16 different possible UE antenna combinations are possible, only 4 bits are needed in the field in DCI to indicate which UE antennas the UE should use for coming downlink transmission. For example, the bit combination {0, 0, 0, 0} could be used to signal that antennas {1, 3, 5, 7} should be used, the bit combination {0, 0, 1, 0} could be used to signal that antennas {1, 3, 6, 7} should be used, the bit combination {0, 1, 1, 0} could be used to signal that antennas {1, 4,

[0104] 6, 7} should be used, etc.

[0105] In some aspects, the network node 120 perform the downlink transmission using the determined precoder. Hence, in some embodiments, the network node 120 is configured to perform (optional) step S216.

[0106] S216: The network node 120 performs, whilst using the precoder, the reciprocity-based downlink transmission towards the UE 110a: 110N, 200. P111007W001 15

[0107] One particular embodiment for configuring a UE for uplink reference signal transmission for downlink channel sounding based on at least some of the above disclosed embodiments will now be disclosed in detail with reference to the signaling diagram of Fig. 6. In this embodiment, the uplink reference signals are SRSs.

[0108] S301 : The UE 620 indicates, e.g. during UE capability signaling, support for SRS based channel sounding for reciprocity-based downlink transmission, where the UE 110a: 110N, 200 is equipped with more antennas than receive chains.

[0109] S302: The network node (NN) 610 configures the UE 620 with SRS resources according to the UE capability signaling.

[0110] S303: The network node 610 triggers the UE 620 to transmit SRSs on the configured SRS resources.

[0111] S304: The UE 620 performs the SRS transmission in accordance with the configuration received in step S302 and the trigger received in step S303.

[0112] S305: The network node 610 receives the SRSs and estimates a downlink channel between the network node 610 and UE 620. Based on the estimated channel the network node 610 determines a precoder for coming downlink transmission to the UE 620. Since the UE 620 only can select a subset of UE antennas to use at each time instance (since it has a smaller number of receive chains compared to number of antennas), the network node 610 takes this into account when determining the precoder.

[0113] S306: The network node 610, after having determined the precoder, indicates to the UE 620 which UE antennas or subset of UE antennas the downlink precoder is associated with. This can be indicated to the UE 620 in accordance with what has been disclosed above.

[0114] S307: The network node 610 perform the downlink transmission using the determined precoder.

[0115] S308: The UE 620 receives the downlink transmission using the indicated UE antennas or subset of UE antennas.

[0116] In general terms, there should be an uplink reference signal to UE antenna mapping that is agreed between the UE and the network node, such that the network node knows which uplink reference signals are associated with which UE antennas or which group of UE antennas. This mapping can, for example, be included in written specifications and / or be configured by the network node via RRC signaling, where, for example, different UE antennas or groups of UE antennas can be associated with different groups of uplink reference signals. The groups of uplink reference signals can, e.g., be based on uplink reference signal indexes, using for example one or more of SRS resource set index, SRS resource index, SRS port index etc. P111007W001 16

[0117] Fig. 7 schematically illustrates, in terms of a number of structural units, the components of a UE 700 according to an embodiment. Processing circuitry 710 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 910a (as in Fig. 9), e.g. in the form of a storage medium 730. The processing circuitry 710 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA).

[0118] Particularly, the processing circuitry 710 is configured to cause the UE 700 to perform a set of operations, or steps, as disclosed above. For example, the storage medium 730 may store the set of operations, and the processing circuitry 710 may be configured to retrieve the set of operations from the storage medium 730 to cause the UE 700 to perform the set of operations. The set of operations may be provided as a set of executable instructions. Thus the processing circuitry 710 is thereby arranged to execute methods as herein disclosed.

[0119] The storage medium 730 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.

[0120] The UE 700 may further comprise a communications (comm.) interface 720 at least for communications with the network node, as in Fig. 1. As such the communications interface 720 may comprise one or more transmitters and receivers, comprising analogue and digital components.

[0121] The processing circuitry 710 controls the general operation of the UE 700 e.g. by sending data and control signals to the communications interface 720 and the storage medium 730, by receiving data and reports from the communications interface 720, and by retrieving data and instructions from the storage medium 730. Other components, as well as the related functionality, of the UE 700 are omitted in order not to obscure the concepts presented herein.

[0122] Fig. 8 schematically illustrates, in terms of a number of structural units, the components of a network node 800 according to an embodiment. Processing circuitry 810 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 910b (as in Fig. 9), e.g. in the form of a storage medium 830. The processing circuitry 810 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA).

[0123] Particularly, the processing circuitry 810 is configured to cause the network node 800 to perform a set of operations, or steps, as disclosed above. For example, the storage medium 830 may store the set of operations, and the processing circuitry 810 may be configured to retrieve the set of operations from the storage medium 830 to cause the network node 800 to perform the set of operations. The set of operations may be provided as a set of executable instructions. Thus the processing circuitry 810 is thereby arranged to execute methods as herein disclosed. P111007W001 17

[0124] The storage medium 830 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.

[0125] The network node 800 may further comprise a communications interface 820 for communications with its served UEs, as in Fig. 1, as well as with other entities, functions, nodes, and devices, in the network. As such the communications interface 820 may comprise one or more transmitters and receivers, comprising analogue and digital components.

[0126] The processing circuitry 810 controls the general operation of the network node 800 e.g. by sending data and control signals to the communications interface 820 and the storage medium 830, by receiving data and reports from the communications interface 820, and by retrieving data and instructions from the storage medium 830. Other components, as well as the related functionality, of the network node 800 are omitted in order not to obscure the concepts presented herein.

[0127] The network node 120, 610, 800 may be provided as a standalone device or as a part of at least one further device. For example, the network node 120, 610, 800 may be provided in a node of the radio access network or in a node of the core network. Alternatively, functionality of the network node 120, 610, 800 may be distributed between at least two devices, or nodes. These at least two nodes, or devices, may either be part of the same network part (such as the radio access network or the core network) or may be spread between at least two such network parts. In general terms, instructions that are required to be performed in real time may be performed in a device, or node, operatively closer to the cell than instructions that are not required to be performed in real time. Thus, a first portion of the instructions performed by the network node 120, 610, 800 may be executed in a first device, and a second portion of the instructions performed by the network node 120, 610, 800 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 120, 610, 800 may be executed. Hence, the methods according to the herein disclosed embodiments are suitable to be performed by a network node 120, 610, 800 residing in a cloud computational environment. Therefore, although a single processing circuitry 810 is illustrated in Fig. 8 the processing circuitry 810 may be distributed among a plurality of devices, or nodes. The same applies to the computer program 920b of Fig. 9.

[0128] Some (radio) access network architectures define network nodes (or gNBs) comprising multiple component parts or nodes: a central unit (CU), one or more distributed units (DUs), and one or more radio units (RUs). The protocol layer stack of the network node 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, P111007W001 18

[0129] CU, or DU is referred to as a fronthaul network or interface. Messages or packets may be transmitted from the network node 700 in the downlink (i.e., from the CU to the RU) or received by the network node 700 in the uplink (i.e., from the RU to the CU).

[0130] Fig. 9 shows one example of a computer program product 910a, 910b comprising computer readable means 930. On this computer readable means 930, a computer program 920a can be stored, which computer program 920a can cause the processing circuitry 710 and thereto operatively coupled entities and devices, such as the communications interface 720 and the storage medium 730, to execute methods according to embodiments described herein. The computer program 920a and / or computer program product 910a may thus provide means for performing any steps of the UE 110a: 11 ON, 200, 620, 700 as herein disclosed. On this computer readable means 930, a computer program 920b can be stored, which computer program 920b can cause the processing circuitry 810 and thereto operatively coupled entities and devices, such as the communications interface 820 and the storage medium 830, to execute methods according to embodiments described herein. The computer program 920b and / or computer program product 910b may thus provide means for performing any steps of the network node 120, 610, 800 as herein disclosed.

[0131] In the example of Fig. 9, the computer program product 910a, 910b 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 910a, 910b 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 920a, 920b is here schematically shown as a track on the depicted optical disk, the computer program 920a, 920b can be stored in any way which is suitable for the computer program product 910a, 910b.

[0132] 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

P111007W001 19CLAIMS1. A method for performing an uplink reference signal transmission for downlink channel sounding, wherein the method is performed by a user equipment, UE (110a:110N, 200, 620, 700), wherein the UE (110a:110N, 200, 620, 700) is equipped with more antennas (230a:230d, 410) than receive chains (210a, 210b, 430), and wherein the method comprises: sending (S102) a report to a network node (120, 610, 800) serving the UE (110a: 11 ON, 200, 620, 700), wherein the report comprises an indication of the UE (110a: 110N, 200, 620, 700) supporting uplink reference signal based downlink channel sounding for reciprocity-based downlink transmission and indicating that the UE (110a: 11 ON, 200, 620, 700) is equipped with more antennas (230a:230d, 410) than receive chains (210a, 210b, 430); receiving (S104) uplink reference signal configuration from the network node (120, 610, 800), wherein the uplink reference signal configuration is based on the report and comprises configuration of a set of uplink reference signal resources, and wherein, according to the uplink reference signal configuration, at least one uplink reference signals is to be transmitted per antenna; and transmitting (S108) the uplink reference signals on the set of uplink reference signal resources in accordance with the uplink reference signal configuration.

2. The method according to claim 1 , wherein the report comprises an indication of number of transmit chains (220) in the UE (110a: 11 ON, 200, 620, 700), number of receive chains (210a, 210b, 430) in the UE (110a: 11 ON, 200, 620, 700) and number of antennas (230a:230d, 410) in the UE (110a:110N, 200, 620, 700).

3. The method according to claim 1 or 2, wherein at least some of the transmit chains (220) and / or receive chains (210a, 210b, 430) in the UE (110a: 110N, 200, 620, 700) can be turned off, and wherein the report comprises an indication of different combinations of transmit chains (220) and receive chains (210a, 210b, 430) usable by the UE (110a: 110N, 200, 620, 700) during transmission of the uplink reference signals.

4. The method according to claim 3, wherein less than all the transmit chains (220) and / or less than all the receive chains (210a, 210b, 430) are connectible to the antennas (230a:230d, 410) in the UE (110a: 110N, 200, 620, 700), and wherein the different combinations further comprises information of number of antennas (230a:230d, 410) usable by the UE (110a: 11 ON, 200, 620, 700) for each of the different combinations.

5. The method according to claim 3 or 4, wherein the transmit chains (220) and / or the receive chains (210a, 210b, 430) are connectible to the antennas (230a:230d, 410) via a switching network (240, 420), and wherein the report comprises an indication of which type the switching network (240, 420) is.

6. The method according to any preceding claim, wherein the method further comprises:P111007W001 20 receiving (S106) a trigger from the network node (120, 610, 800) for the UE (110a: 11 ON, 200, 620, 700) to transmit the uplink reference signals, and wherein the uplink reference signals are transmitted in response thereto.

7. The method according to any preceding claim, wherein the method further comprises: receiving (S110) an indication from the network node (120, 610, 800) of which of the antennas (230a:230d, 410) for the UE (110a: 11 ON, 200, 620, 700) to use for reception of the reciprocity-based downlink transmission and / or of which of the antennas (230a:230d, 410) that by the network node (120, 610, 800) are associated with the reciprocity-based downlink transmission.

8. The method according to claim 7, wherein the method further comprises: receiving (S112), whilst having receive chains (210a, 210b, 430) connected to the indicated antennas (230a:230d, 410), the reciprocity-based downlink transmission from the network node (120, 610, 800).

9. A method for configuring a user equipment, UE (110a: 11 ON, 200, 620, 700) for an uplink reference signal transmission used for downlink channel sounding, wherein the UE (110a: 11 ON, 200, 620, 700) is equipped with more antennas (230a:230d, 410) than receive chains (210a, 210b, 430), wherein the method is performed by a network node (120, 610, 800), and wherein the method comprises: receiving (S202) a report to from the UE (110a: 110N, 200, 620, 700), wherein the report comprises an indication of the UE (110a: 11 ON, 200, 620, 700) supporting uplink reference signal based downlink channel sounding for reciprocity-based downlink transmission and indicating that the UE (110a: 110N, 200, 620, 700) is equipped with more antennas (230a:230d, 410) than receive chains (210a, 210b, 430); sending (S204) uplink reference signal configuration to the UE (110a:110N, 200, 620, 700), wherein the uplink reference signal configuration is based on the report and comprises configuration of a set of uplink reference signal resources, and wherein, according to the uplink reference signal configuration, at least one uplink reference signals is to be transmitted per antenna; and receiving (S208) the uplink reference signals as transmitted by the UE (110a: 110N, 200, 620, 700) on the set of uplink reference signal resources.

10. The method according to claim 9, wherein the uplink reference signal configuration specifies uplink reference signal resources to be transmitted from all the antennas (230a:230d, 410) the antennas (230a:230d, 410) of the UE (110a: 11 ON, 200, 620, 700).11 . The method according to claim 9 or 10, wherein the method further comprises:P111007W001 21 transmitting (S206) a trigger to the UE (110a: 110N, 200, 620, 700) for the UE (110a: 110N, 200, 620, 700) to transmit the uplink reference signals.

12. The method according to any of claims 9 to 11, wherein the method further comprises: estimating (S21 Oa, 210b) a downlink channel from the network node (120, 610, 800) to the UE (110a: 11 ON, 200, 620, 700) based on the received uplink reference signals; and selecting (S212) a precoder for the network node (120, 610, 800) to use for reciprocity-based downlink transmission towards the UE (110a: 110N, 200, 620, 700), wherein the precoder is selected based on the estimated downlink channel and according to that the UE (110a: 11 ON, 200, 620, 700) is equipped with more antennas (230a:230d, 410) than receive chains (210a, 210b, 430).

13. The method according to claim 12, wherein the method further comprises: performing (S216), whilst using the precoder, the reciprocity-based downlink transmission towards the UE (110a: 11 ON, 200, 620, 700).

14. The method according to any of claims 9 to 13, wherein the method further comprises: sending (S214) an indication to the UE (110a:110N, 200, 620, 700) of which of the antennas (230a:230d, 410) for the UE (110a: 110N, 200, 620, 700) to use for reception of the reciprocity-based downlink transmission and / or of which of the antennas (230a:230d, 410) that by the network node (120, 610, 800) are associated with the reciprocity-based downlink transmission.

15. The method according to claim 14, wherein the indication is sent as any of: a field is used in a downlink control information, DCI, message triggering a downlink data transmission, an index relating to the received uplink reference signals, an index relating to indexes of the received uplink reference signals.

16. The method according to claim 14 or 15, wherein the indication is sent as a bitfield of length M, where M is either total number of antennas (230a:230d, 410) or number of active antennas (230a:230d, 410) at the UE(110a: 110N, 200, 620, 700), wherein each bit set in the bitfield indicates that the antenna corresponding to said bit is to be used for reception of the reciprocity-based downlink transmission.

17. The method according to claim 14 or 15, wherein the indication is sent as a bitfield, where each bit of the bitfield indicates which combination of antennas (230a:230d, 410) to be used for reception of the reciprocitybased downlink transmission.

18. A user equipment, UE (110a: 110N, 200, 620, 700), for performing an uplink reference signal transmission for downlink channel sounding, the UE (110a: 11 ON, 200, 620, 700) being equipped with more antennasP111007W001 22(230a:230d, 410) than receive chains (210a, 210b, 430) and comprising processing circuitry (710), the processing circuitry being configured to cause the UE (110a: 110N, 200, 620, 700) to: send a report to a network node (120, 610, 800) serving the UE (110a: 110N, 200, 620, 700), wherein the report comprises an indication of the UE (110a: 110N, 200, 620, 700) supporting uplink reference signal based downlink channel sounding for reciprocity-based downlink transmission and indicating that the UE (110a: 11 ON, 200, 620, 700) is equipped with more antennas (230a:230d, 410) than receive chains (210a, 210b, 430); receive uplink reference signal configuration from the network node (120, 610, 800), wherein the uplink reference signal configuration is based on the report and comprises configuration of a set of uplink reference signal resources, and wherein, according to the uplink reference signal configuration, at least one uplink reference signals is to be transmitted per antenna; and transmit the uplink reference signals on the set of uplink reference signal resources in accordance with the uplink reference signal configuration.

19. The UE (110a: 110N, 200, 620, 700) according to claim 18, further being configured to perform the method according to any of claims 2 to 8.

20. A network node (120, 610, 800) for configuring a user equipment, UE (110a: 110N, 200, 620, 700) for an uplink reference signal transmission used for downlink channel sounding, wherein the UE (110a: 110N, 200, 620, 700) is equipped with more antennas (230a:230d, 410) than receive chains (210a, 210b, 430), the network node (120, 610, 800) comprising processing circuitry (810), the processing circuitry being configured to cause the network node (120, 610, 800) to: receive a report to from the UE (110a: 110N, 200, 620, 700), wherein the report comprises an indication of the UE (110a:110N, 200, 620, 700) supporting uplink reference signal based downlink channel sounding for reciprocity-based downlink transmission and indicating that the UE (110a: 110N, 200, 620, 700) is equipped with more antennas (230a:230d, 410) than receive chains (210a, 210b, 430); send uplink reference signal configuration to the UE (110a: 11 ON, 200, 620, 700), wherein the uplink reference signal configuration is based on the report and comprises configuration of a set of uplink reference signal resources, and wherein, according to the uplink reference signal configuration, at least one uplink reference signals is to be transmitted per antenna; and receive the uplink reference signals as transmitted by the UE (110a: 110N, 200, 620, 700) on the set of uplink reference signal resources.

21. The network node (120, 610, 800) according to claim 20, further being configured to perform the method according to any of claims 10 to 17.P111007W001 2322. A computer program (920a) for performing an uplink reference signal transmission for downlink channel sounding, the computer program comprising computer code which, when run on processing circuitry (210a, 210b) of a user equipment, UE (110a: 110N, 200, 620, 700), being equipped with more antennas (230a:230d, 410) than receive chains (210a, 210b, 430), causes the UE (110a: 110N, 200, 620, 700) to: send (S102) a report to a network node (120, 610, 800) serving the UE (110a: 110N, 200, 620, 700), wherein the report comprises an indication of the UE (110a: 110N, 200, 620, 700) supporting uplink reference signal based downlink channel sounding for reciprocity-based downlink transmission and indicating that the UE (110a: 11 ON, 200, 620, 700) is equipped with more antennas (230a:230d, 410) than receive chains (210a, 210b, 430); receive (S104) uplink reference signal configuration from the network node (120, 610, 800), wherein the uplink reference signal configuration is based on the report and comprises configuration of a set of uplink reference signal resources, and wherein, according to the uplink reference signal configuration, at least one uplink reference signals is to be transmitted per antenna; and transmit (S108) the uplink reference signals on the set of uplink reference signal resources in accordance with the uplink reference signal configuration.

23. A computer program (920b) for configuring a user equipment, UE (110a:110N, 200, 620, 700) for an uplink reference signal transmission used for downlink channel sounding, wherein the UE (110a: 110N, 200, 620, 700) is equipped with more antennas (230a:230d, 410) than receive chains (210a, 210b, 430), the computer program comprising computer code which, when run on processing circuitry (810) of a network node (120, 610, 800), causes the network node (120, 610, 800) to: receive (S202) a report to from the UE (110a: 110N, 200, 620, 700), wherein the report comprises an indication of the UE (110a: 11 ON, 200, 620, 700) supporting uplink reference signal based downlink channel sounding for reciprocity-based downlink transmission and indicating that the UE (110a: 110N, 200, 620, 700) is equipped with more antennas (230a:230d, 410) than receive chains (210a, 210b, 430); send (S204) uplink reference signal configuration to the UE (110a:110N, 200, 620, 700), wherein the uplink reference signal configuration is based on the report and comprises configuration of a set of uplink reference signal resources, and wherein, according to the uplink reference signal configuration, at least one uplink reference signals is to be transmitted per antenna; and receive (S208) the uplink reference signals as transmitted by the UE (110a: 110N, 200, 620, 700) on the set of uplink reference signal resources.

24. A computer program product (910a, 910b) comprising a computer program (920a, 920b) according to at least one of claims 22 and 23, and a computer readable storage medium (930) on which the computer program is stored.

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