Transmission of SRS ports from a user equipment

Abstract

There is provided techniques for transmission of SRS ports. A method is performed by a user equipment. The user equipment comprises at least N2 transmit and / or receive chains. The method comprises obtaining an indication to transmit N2 SRS ports in an SRS transmission occasion, where N2>1. The method comprises transmitting the N2 SRS ports on N1 antenna ports out of at least N2 antenna ports, where N1<N2, in the SRS transmission occasion towards a network node (110) whilst turning off the N2 – N1 transmit and / or receive chains connected to antenna ports not associated with any SRS port.

Description

[0001] TRANSMISSION OF SRS PORTS FROM A USER EQUIPMENT

[0002] TECHNICAL FIELD

[0003] Embodiments presented herein relate to a method, a user equipment, a computer program, and a computer program product for transmission of sounding reference signal ports.

[0004] BACKGROUND

[0005] In general terms, uplink reference signals are signals that are sent by the user equipment (UE) and used for providing channel state information to the network. The network generally uses the uplink reference signals for a particular resource usage. The network might, for example, use uplink reference signals for deriving the appropriate transmission and / or reception beams, for performing link adaptation (i.e., setting the transmission rank and the modulation scheme), and for determining precoding matrices for downlink and / or uplink data channels.

[0006] One example of an uplink reference signal is sounding reference signals (SRSs). SRSs are configured via radio resource control (RRC) signaling, where parts of the configuration can be updated via medium access control (MAC) control element (CE) signaling. As an example, when the network configures SRS transmissions, the network configures, through the SRS-Config information element (IE), a list of SRS resources and a list of SRS resource sets. In this respect, SRS resource(s) will be transmitted as part of an SRS resource set, where each SRS resource set contains one or more SRS resources, and where all SRS resources in an SRS resource set share the same time-domain behavior.

[0007] The resource usage, which is configured by the higher-layer parameter usage determines constraints and assumptions on the resource properties, as further disclosed in 3GPP TS 38.214, version 18.0.0). SRS resource sets can be configured with one of four different usages: antennaSwitching, codebook, nonCodebook, or beamManagement.

[0008] An SRS resource set that is configured with usage antennaSwitching is used for reciprocity-based downlink 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 antenna port. In this respect, an SRS port can be regarded as an uplink reference signal that is used to sound a certain antenna port.

[0009] An SRS resource set that is configured with usage codebook is used for codebookbased uplink transmission (i. e. , used to sound the different UE antennas 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 (except when full-power mode 2 is configured). How SRS ports are associated with antenna ports is, however, up to UE implementation and not known to the network.

[0010] An SRS resource set that is configured with usage nonCodebook is used for non- codebook-based uplink transmission. Specifically, the UE transmits one SRS resource per candidate beam (where suitable candidate beams are determined by the UE, e.g., based on measurements on downlink reference signals). The network can then, by indicating a subset of these SRS resources, determine which uplink beam(s) the UE should apply for uplink data transmission. One uplink layer will be transmitted per indicated SRS resource. How the UE maps SRS ports to antenna ports is up to UE implementation and not known to the network.

[0011] An SRS resource set that is configured with usage beamManagement is used (mainly for frequency bands above 6 GHz (i.e., for frequency range 2, FR2)) to evaluate different UE analog beams (e.g., antenna panels). The UE transmits one SRS resource per analog beam, and the network perform a power measurement per transmitted SRS resource and, in this way, determine a suitable UE beam that is reported back to the UE.

[0012] For reciprocity-based downlink precoding, one or more SRS resource sets configured with usage antennaSwitching is used to obtain channel state information in the uplink. It is desirable for the network to sound all UE antennas (where sounding an antenna implies that SRS is transmitted from that antenna) but costly to equip the UE with many transmit (Tx) chains; a UE typically has more receive (Rx) chains than Tx chains. Therefore, SRS antenna switching can be used for UE equipped with more Rx chains than Tx chains. If a UE support antenna switching, the UE will report this by means of capability signaling to the network. As noted above, the network configures the UE with a certain number of SRS resources and SRS ports per SRS resource, based on what the UE has indicated support for during capability signaling. On the other hand, in some scenarios the UE may prefer to save power by e.g., turning off one or more Tx chains and, hence, not transmitting an SRS resource from one or more Tx chains. However, the number of SRS resource sets, or SRS resources, or SRS ports is configured via radio resource control signaling (i.e., it is semi-static) and there is no way for the UE to indicate to the network to reduce the configured number of SRS resource sets, SRS resources, or SRS ports.

[0013] Hence, there is still a need for an improved handling of configured SRS resource sets, or SRS resources, or SRS ports at the UE.

[0014] SUMMARY

[0015] An object of embodiments herein is for the UE to handle configured SRS resource sets, or SRS resources, or SRS ports in a way that enables the UE to save power.

[0016] A particular object is to enable power savings in the UE whilst still transmitting the configured number of SRS ports.

[0017] According to a first aspect there is presented a method for transmission of SRS ports. The method is performed by a UE. The UE comprises at least N2 Tx and / or Rx chains. The method comprises obtaining an indication to transmit N2 SRS ports in an SRS transmission occasion, where N2>1. The method comprises transmitting the N2 SRS ports on Ni antenna ports out of at least N2 antenna ports, where N1<N2, in the SRS transmission occasion towards a network node whilst turning off the N2 - Ni transmit and / or receive chains connected to antenna ports not associated with any SRS port.

[0018] According to a second aspect there is presented a UE for transmission of SRS ports. The UE comprises at least N2 Tx and / or Rx chains. The UE further comprises processing circuitry. The processing circuitry is configured to cause the UE to obtain an indication to transmit N2 SRS ports in an SRS transmission occasion, where N2>1. The processing circuitry is configured to cause the UE to transmit the N2 SRS ports on Ni antenna ports out of at least N2 antenna ports, where N1<N2, in the SRS transmission occasion towards a network node whilst turning off the N2 - Ni transmit and / or receive chains connected to antenna ports not associated with any SRS port.

[0019] According to a third aspect there is presented UE for transmission of SRS ports. The UE comprises at least N2 Tx and / or Rx chains. The UE further comprises an obtain module configured to obtain an indication to transmit N2 SRS ports in an SRS transmission occasion, where N2>1. The UE further comprises a transmit module configured to transmit the N2 SRS ports on Ni antenna ports out of at least N2 antenna ports, where N1<N2, in the SRS transmission occasion towards a network node whilst turning off the N2 - Ni transmit and / or receive chains connected to antenna ports not associated with any SRS port.

[0020] According to a fourth aspect there is presented a computer program for transmission of SRS ports. The computer program comprises computer code which, when run on processing circuitry of a UE comprising at least N2 Tx and / or Rx chains, causes the UE to perform actions. One action comprises the UE to obtain an indication to transmit N2 SRS ports in an SRS transmission occasion, where N2>1. One action comprises the UE to transmit the N2 SRS ports on Ni antenna ports out of at least N2 antenna ports, where N1<N2, in the SRS transmission occasion towards a network node whilst turning off the N2 - Ni transmit and / or receive chains connected to antenna ports not associated with any SRS port.

[0021] According to a fifth aspect there is presented a computer program product comprising a computer program according to the fourth 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.

[0022] Advantageously, these aspects enable the UE to handle configured SRS resource sets, or SRS resources, or SRS ports and still save power.

[0023] Advantageously, these aspects enable power savings in the UE whilst still allowing the UE to transmit the configured number of SRS ports. Advantageously, these aspects enable the UE to save power, e.g., when the UE needs to preserve battery (such as in a power saving mode) and / or when the amount of uplink data traffic is low.

[0024] 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.

[0025] 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.

[0026] BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The inventive concept is now described, by way of example, with reference to the accompanying drawings, in which:

[0028] Fig. 1 is a schematic diagram illustrating a communications network according to embodiments;

[0029] Fig. 2 is a flowchart of methods according to embodiments;

[0030] Figs. 3, 4, and 5 schematically illustrate associations of SRS ports with antenna ports according to embodiments;

[0031] Fig. 6 is a schematic diagram showing structural units of a UE according to an embodiment;

[0032] Fig. 7 is a schematic diagram showing functional modules of a UE according to an embodiment;

[0033] Fig. 8 shows one example of a computer program product comprising computer readable storage medium according to an embodiment. DETAILED DESCRIPTION

[0034] 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.

[0035] Fig. i is a schematic diagram illustrating, at the top, a communications network too where embodiments presented herein can be applied, and at the bottom, a UE 120. The communications network 100 comprises a network node 110, such as 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 (TRP), etc. The network node 110 is configured to provide coverage, and thus network access, to UE 120 via an uplink / downlink link 130. The UE 120 might be any of a portable wireless device, mobile station, mobile phone, handset, wireless local loop phone, smartphone, laptop computer, tablet computer, wireless sensor device, Internet of Things device, network equipped vehicle, network equipped gaming control, etc. as long as the UE 120 is capable of implementing the herein disclosed embodiments. In the bottom part of Fig. 1 is provided a block diagram of the UE 120.

[0036] The UE 120 is illustrated as having one antenna panel 122 as connectable to a baseband unit 124 via Tx and / or Rx chains 126. In this respect, the antenna panel 122 is illustrated as an antenna array with four columns and one row of single- or dualpolarized antenna elements 128, and where each of the antenna elements 128 is connectable to the baseband unit 124 via a respective Tx and / or Rx chain 126. This is only one example implementation of the UE 120, and the example implementation is provided to illustrate the herein disclosed embodiments. For example, for the herein disclosed embodiments it is sufficient that the UE 200 has at least N2 Tx and / or Rx chains 126, where N2>1. It is thus noted that the herein disclosed embodiments apply also to other implementation of the UE 120, for example a UE with a two- dimensional antenna panel 122, and / or a UE 120 with more than one antenna panel 122, as long as the UE 120 is capable of implementing the herein disclosed embodiments.

[0037] At least some of the herein disclosed embodiments pertain to different ways for the UE to turn off Tx and / or Rx chains 126 by proper association of SRS ports, or SRS resources, to antenna ports and / or proper muting of one or more SRS ports, or SRS resources.

[0038] The embodiments disclosed herein in particular relate to techniques for transmission of SRS ports. In order to obtain such techniques, there is provided a UE 120, 600, 700, a method performed by the UE 120, 600, 700, a computer program product comprising code, for example in the form of a computer program, that when run on a UE 120, 600, 700, causes the UE 120, 600, 700 to perform the method.

[0039] Fig. 2 is a flowchart illustrating embodiments of methods for transmission of SRS ports. The methods are performed by the UE 120, 600, 700. The UE 120, 600, 700 comprises at least N2 Tx and / or Rx chains 126. The methods are advantageously provided as computer programs 820.

[0040] S102: The UE 120, 600, 700 obtains an indication to transmit N2 SRS ports in an SRS transmission occasion, where N2>1.

[0041] S110: The UE 120, 600, 700 transmits the N2 SRS ports on Ni antenna ports out of at least N2 antenna ports, where N1<N2, in the SRS transmission occasion towards a network node 110 whilst turning off the N2 - Ni transmit and / or receive chains connected to antenna ports not associated with any SRS port.

[0042] In this respect, a transmit / receive antenna port can be regarded as an antenna port connected to a transmit / Rx chain. Therefore, turning off a transmit / receive port is equivalent to turning off the corresponding transmit / Rx chain. For this purpose, antenna ports are can be regarded as either physical ports or logical ports with one antenna port per each Tx and / or Rx chain 126. Further in this respect, SRS ports can be considered as logic ports.

[0043] Embodiments relating to further details of transmission of SRS ports as performed by the UE 120, 600, 700 will now be disclosed with continued reference to Fig. 2. There could be different ways for the UE to determine on which antenna ports the N2 SRS ports are to be transmitted in step Sno. In this respect, in some embodiments, the UE 120, 6oo, 700 is configured to perform (optional) step S108:

[0044] S108: The UE 120, 600, 700 associates the N2 SRS ports to the Ni antenna ports. The Ni antenna ports to which the N2 SRS ports are in turn associated with Tx and / or Rx chains 126.

[0045] Typically, there is one antenna port per Tx and / or Rx chain 126. An exception is for SRS antenna switching, where one or more Tx chains are switched between different antenna ports associated with different Rx chains. In this respect, for SRS antenna switching, one Rx chain can be associated with one antenna port, but one Tx chain can be associated with multiple antenna ports.

[0046] As disclosed above, the network generally uses uplink reference signals (such as SRSs) for a particular resource usage. Therefore, in some embodiments, the indication as received in step S102 for the UE 120, 600, 700 to transmit N2 SRS ports comprises information of a usage of the N2 SRS ports. Which of the Tx and / or Rx chains 126 to turn off can then further be based on the usage of the N2 SRS ports. In some examples, the indication is received as configuration from the network node 110.

[0047] In one example, a UE 120, 600, 700 with N2 Tx chains (e.g., associated with N2 antenna connectors) that is configured to transmit N2 SRS ports maps N2 SRS ports to Ni antenna port(s) associated with different Tx and / or Rx chain 126(S), where Ni < N2, and where the UE 120, 600, 700, to save power, turns off one or more Tx and / or Rx chain 126(S) associated with an antenna port not used for SRS transmission.

[0048] In one example, the UE 120, 600, 700 transmits one or more SRS ports with zero power, resulting in that no SRS port is transmitted over an antenna port associated with a Tx and / or Rx chain 126. That is, in some embodiments, according to the associating in step S108, output power of at least one of the SRS ports is zero.

[0049] In one example, the UE 120, 600, 700 maps multiple SRS ports to one and the same antenna port associated with a Tx and / or Rx chain 126, resulting in that no SRS port is transmitted over at least one antenna port associated with another Tx and / or Rx chain 126. That is, in some embodiments, according to the association? in step S108, at least two of the SRS ports are associated with one same antenna port.

[0050] In one example, the UE 120, 600, 700 measures on a downlink reference signal, control signal, or data signal, on different antennas and determines a downlink and / or uplink performance metric for each UE antenna based on the measurements on the downlink signal. Hence, in some embodiments, the UE 120, 600, 700 is configured to perform (optional) step S104:

[0051] S104: The UE 120, 600, 700 receives a downlink signal from the network node 110 at the N2 antenna ports.

[0052] Which of the SRS ports (or antenna ports) to be, or not to be, associated with any of the Ni antenna ports can then be determined based on a measurement as performed by the UE 120, 600, 700 on the downlink signal.

[0053] In some embodiments, the measurement is of received power, signal to noise ratio (SNR), or signal to interference plus noise ratio (SINR), and the N2 SRS ports are not associated with the Ni antenna ports at which the downlink signal was received with lowest received power, SNR, or SINR.

[0054] Based on a performance metric, the UE 120, 600, 700 might not transmit SRS over antenna(s), and corresponding Tx and / or Rx chain 126(S), with lowest performance metric. That is, in some embodiments, which of the Tx and / or Rx chains 126 to turn off further is based on a performance metric computed per antenna port of each of the Tx and / or Rx chains 126. There can be different examples of such performance metrics. In one example, the performance metric is the estimated downlink link budget, taking downlink reference signal received power into account. Hence, in some embodiments, the performance metric for determining whether or not to turn off any Tx chain or Rx chain pertains to a downlink link budget. In one example, the performance metric is estimated uplink link budget. Hence, in some embodiments, the performance metric for determining whether or not to turn off any Tx chain pertains to an uplink link budget. In one example, the performance metric takes into account downlink reference signal received power and output power for the corresponding Tx chain into account (including potential Power Management Maximum Power Reduction (P-MPR) due to vicinity of human tissue). Hence, in some embodiments, the uplink link budget is dependent on an output power capability of each Tx chain.

[0055] In one example, the downlink signal that the UE 120, 600, 700 measures on is a synchronization signal block (SSB) associated with an applied Transmission Configuration Indicator (TCI) state. In one example, the downlink signal that the UE 120, 600, 700 measures on is a Tracking Reference Signal (TRS) associated with an applied TCI state.

[0056] In general terms, reducing the number of SRS ports from N2 to Ni implies that the maximum rank supported by the UE (or, rather, that can be decoded at the network node 110) is upper-bounded by Ni, even if the configured maximum rank is higher than Ni. Ideally, the scheduler in the network detects that the maximum rank supported by the UE 120, 600, 700 is limited by Ni. Hence, the network would indicate a precoding matrix and transmission rank for the uplink (i.e., to be used by the UE 120, 600, 700 during uplink data transmissions) which maps at most Ni uplink data transmission layers to the N2 configured SRS ports. Furthermore, the network node 110 would transmit in the downlink at most Ni downlink data transmission layers to the UE.

[0057] A UE can indicate to the network a preferred rank Li, which may be lower than the configured max rank L2, via UE assistance information signaling. For example, for the case Li = Ni and non-codebook-based precoding, if the network follows the preference as signaled by the preference, it follows that the UE use only Ni Tx chains for uplink data transmissions. However, N2 Tx chains are still used for SRS transmission, which implies that the corresponding Tx and / or Rx chains 126 cannot be turned off.

[0058] In one example, the UE 120, 600, 700 maps N2 SRS ports to Ni antenna port(s) associated with different Tx and / or Rx chain 126(S) only if the UE 120, 600, 700 has indicated to the network that the preferred uplink and / or downlink rank is Li = Ni. Hence, in some embodiments, the UE 120, 600, 700 is configured to perform (optional) step S106: Sio6: The UE 120, 600, 700 signals a preference for Ni number of uplink multipleinput multiple-output (MIMO) layers to the network node 110.

[0059] The associating in step S108 of the N2 SRS ports to the Ni antenna ports can then be performed in response thereto (i.e., in response to the UE 120, 600, 700 having signaled the preference for the Ni number of uplink MIMO layers).

[0060] In this respect, the UE 120, 600, 700 does not need to signal to the network the number of Tx and / or Rx chains 126 that is associated with the SRS transmission (and, consequently, with the uplink data transmission and / or downlink data reception).

[0061] In one example, the UE 120, 600, 700 transmits N2 SRS ports to Ni antenna port(s) associated with different Tx and / or Rx chain 126(S) only if the UE 120, 600, 700 is in a power-saving mode. That is, in some embodiments, the associating in step S108 of the N2 SRS ports to the Ni antenna ports is triggered by the UE having entered a power saving mode. In another example, the UE transmits N2 SRS ports to Ni antenna port(s) associated with different Tx and / or Rx chain 126(S) if the UE battery level is below some threshold. That is, in some embodiments, the associating in step S108 of the N2 SRS ports to the Ni antenna ports is triggered by a power level in the UE 120, 600, 700 being below a threshold value.

[0062] In some examples, the UE 120, 600, 700 determines to turn off one or more of the Tx and / or Rx chains 126 based on the capability of the UE to turn on / off the Tx and / or Rx chains 126, the on / off switching delays in the Tx and / or Rx chains 126, the amount of uplink data traffic, the buffered data, the possible combinations of the Tx and Rx chains, etc. That is, in some embodiments, which of the Tx and / or Rx chains 126 to turn off further is based on any of: a capability of the UE to turn off and turn on the Tx and / or Rx chains 126, switching delay for turning off and turning on the Tx and / or Rx chains 126, switching between Tx and / or Rx chains 126.

[0063] In one example, higher priority is given to turning off Tx chains compared to Rx chains, as Tx chains might consume more power. That is, in some embodiments, the associating in step S108 of the N2 SRS ports to the Ni antenna ports is biased to prioritize turning off Tx chains. Further, whether to turn off any Tx and / or Rx chain 126 might be based on the total power consumption in the Tx and / or Rx chains 126. That is, in some embodiments, which of the Tx and / or Rx chains 126 to turn off further is based on a power consumption criterion of the Tx and / or Rx chains 126.

[0064] Examples related to codebook-based uplink precoding will be disclosed next.

[0065] For illustrative purposes, some examples are illustrated for a UE with four Tx chains and configured with an SRS resource with four SRS ports. However, the examples are applicable to any number of Tx chains and SRS ports at the UE.

[0066] Reference is here made to Fig. 3 in which is illustrated different examples of associations 300a, 300b of SRS ports 310a, 310b with antenna ports 330a, 330b. As disclosed above, a transmit / receive antenna port can be regarded as an antenna port connected to a transmit / Rx chain; this is applicable also to the examples illustrated in Fig. 4 and Fig. 5 as described below. In further detail, in Fig. 3 is illustrated two examples where four SRS ports are associated with three Tx chains (either by associating two SRS ports with one Tx chain (as in Fig. 3(a)), or to transmit one SRS port with zero output power (as in Fig. 3(b))), such that one Tx chain can be turned off (i.e., such that Tx chain 3 can be turned off in this example). In more detail, for the examples in both Fig. 3(a) and Fig. 3(b) it is assumed that the UE has detected that Tx chain 3 is associated with poor performance. According to the association 300a in Fig. 3(a) the UE associates SRS ports 1002 and 1003 with the same Tx chain (Tx chain 2 in this example), such that no SRS port is transmitted on Tx chain 3, and hence Tx chain 3 can be turned off. According to the association 300b in Fig. 3(b), the SRS port 1003 is transmitted with zero power and hence Tx chain 3 can be turned off.

[0067] In one example, for the example in Fig. 3(a), the UE transmits the four SRS ports with the same output power. One potential downside with this approach is that the UE might not be able to fully utilize the power amplifier power for the Tx ports that convey only one SRS port. For example, assume that the UE is of power class 3 (i.e., supporting maximum 23 dBm output power) and the maximum output power for each of the four power amplifiers is 17 dBm (i.e., one power amplifier per each of the four Tx chains). If the UE transmit two SRS ports on a TX chain with a maximum of 17 dBm output power, the maximum SRS output power per SRS port can be 14 dBm, which limits the maximum used SRS output power for all four SRS ports. However, as will be disclosed next, this issue could be mitigated in some cases by proper selection of the Tx chain used for transmission of two SRS ports. In one example, the UE transmits two SRS ports on the Tx chain that has the largest maximum output power. For example, for a UE with four Tx chains and maximum power amplifier output powers of 17, 17,17, and 20 dBm, respectively, if the UE maps two SRS ports to the Tx chain with 20 dBm maximum output power, the maximum output power per SRS port can be 17 dBm (which is the maximum allowed SRS output power supported for a UE with power class 3 configured with a 4 port SRS resource).

[0068] In one example, the UE transmits two SRS ports on the Tx chain that has at least maximum output power corresponding to half of the maximum allowed output power for the power class. This would mean that the UE does not have to reduce the output power per SRS port compared to a UE with 4 Tx chains.

[0069] In another example, the SRS Port 1003 is equally divided across the first three Tx chains as illustrated in Fig. 4. In Fig. 4 is illustrated an example of an association 400 of SRS ports 410 with antenna ports 430 via a mapper 420. In further detail, in Fig. 4 is illustrated an example where one SRS port (SRS Port 1003) is associated with all used Tx chains in order to maximize the available output power per SRS ports, assuming each SRS port is transmitted with the same output power to comply with SRS power scaling requirements. In case each power amplifier has a maximum output power of 17 dBm, this example maximizes the available output power per SRS port, assuming each SRS port is transmitted with the same output power.

[0070] Examples related to non-codebook-based uplink precoding will be disclosed next.

[0071] For illustrative purposes, some examples are illustrated for a UE with four Tx chains and configured with four single port SRS resources. However, the examples are applicable to any number of Tx chains and number of single-port SRS resources at the UE. In some examples, the at least two SRS ports that are mapped to one same Tx chain all have either an odd SRS port index or an even SRS port index. One reason for such mappings is that the network may schedule reference signals such that SRS ports with even and odd numbers are mapped to different comb offsets, respectively, and it may be preferred, depending on UE coherency, to keep SRS ports transmitted over one and the same Tx chain on the same comb offset (e.g., for transmission comb 8, the same cyclic shift is used by two SRS ports, but on different comb offsets). The UE might therefore associate multiple SRS ports with one antenna port (and thus one Tx chain).

[0072] Some examples will be described with reference to Fig. 5. In Fig. 5 is illustrated examples of associations 500a, 500b of SRS ports 510a, 510b with antenna ports 530a, 530b via mappers 520. In further detail, in Fig. 5 is illustrated an example where four SRS resources are associated with three Tx chains (either by associating the four SRS resources with three Tx chains (as in Fig. 5(a)), or where one SRS resource is transmitted with zero output power (as in Fig. 5(b))), such that one Tx chain can be turned off (i.e., such that Tx chain 3 can be turned off in this example). In more detail, for the examples in both Fig. 5(a) and Fig. 5(b) it is assumed that the UE has detected that Tx chain 3 is associated with poor performance. According to the association 500a in Fig. 5(a) the UE associates the four SRS resources (SRS resource o, SRS resource 1, SRS resource 2 and SRS resource 3) with the first three Tx chains, such that no SRS resource is transmitted on Tx chain 3, and hence Tx chain 3 can be turned off. According to the association 500b in Fig. 5(b) the UE associates the three first SRS resources with the three first TX chains, and SRS resource 3 is transmitted with zero power. Hence Tx chain 3 can be turned off.

[0073] Examples related to reciprocity-based downlink precoding (i.e., antenna switching) will be disclosed next.

[0074] In general terms, for antenna switching, one or more Tx chains is switched between antenna ports, or connectors, associated with different Rx chains. Hence, transmitting an SRS port belonging to an SRS resource over an antenna port connected to an Rx chain can be considered equivalent to sounding the Rx chain for reciprocity-based downlink precoding.

[0075] In one example, an Rx chain associated with an antenna not used during an SRS transmission can be turned off, which could be useful for saving power during reciprocity-based downlink transmission. Indeed, if one or more antenna(s) is / are associated with poor link budget, the UE can turn off the associated one or more Rx chain(s) when receiving a downlink data transmission without significant downlink throughput losses. In some cases, the UE turns off one or more Tx chains. For example, assume that a UE has four Rx chains (denoted Rxi, Rx2, Rx3 and Rxq.) and two Tx chains (denoted Txi and Tx2). Assume further that the Tx chain Txi is used to sound the two Rx chains Rxi and Rx2, and that the Tx chain Tx2 is used to sound the two Rx chains Rx3 and Rx4. If the sounding of the Rx chains RX1 and RX2 is turned off, then also the Tx chain Txi can be turned off. Likewise, if the sounding of the Rx chains RX3 and RX4 is turned off, then also the Tx chain Tx2 can be turned off.

[0076] For downlink channel estimation based on SRS antenna switching, there might be SRS output power differences between different antenna ports. The SRS power differences are mainly caused by different insertion loss associated with sounding y different UE antennas (connected to different Rx chains) using x < y Tx chains by configuring an xTyR antenna switching. Since such antenna switching occur only for uplink transmissions (Rx chains do not require antenna switching and, hence, do not experience the same insertion losses), there is a mismatch due to SRS insertion loss imbalance between the reciprocity-based downlink channel estimate and the true downlink channel. Therefore, in one example, the UE does not transmit SRS over the antenna ports associated with largest SRS insertion losses. That is, in some embodiments, which Rx chain to turn off further is based on any SRS insertion loss associated with the Rx chains.

[0077] Fig. 6 schematically illustrates, in terms of a number of structural units, the components of a UE 600 according to an embodiment. Processing circuitry 610 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 810 (as in Fig. 8), e.g., in the form of a storage medium 630. The processing circuitry 610 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA).

[0078] Particularly, the processing circuitry 610 is configured to cause the UE 600 to perform a set of operations, or steps, as disclosed above. For example, the storage medium 630 may store the set of operations, and the processing circuitry 610 may be configured to retrieve the set of operations from the storage medium 630 to cause the UE 600 to perform the set of operations. The set of operations may be provided as a set of executable instructions. Thus, the processing circuitry 6io is thereby arranged to execute methods as herein disclosed. The storage medium 630 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 UE 600 may further comprise a communications (comm.) interface 620 at least configured for communications with other entities, functions, nodes, and devices, such as the network node 110 in Fig. 1. As such the communications interface 620 may comprise one or more transmitters and receivers, comprising analogue and digital components. The processing circuitry 610 controls the general operation of the UE 600 e.g. by sending data and control signals to the communications interface 620 and the storage medium 630, by receiving data and reports from the communications interface 620, and by retrieving data and instructions from the storage medium 630. Other components, as well as the related functionality, of the UE 600 are omitted in order not to obscure the concepts presented herein.

[0079] Fig. 7 schematically illustrates, in terms of a number of functional modules, the components of a UE 700 according to an embodiment. The UE 700 of Fig. 7 comprises a number of functional modules; an obtain module 710 configured to perform step S102, and a transmit module 750 configured to perform step S110. The UE 700 of Fig. 7 may further comprise a number of optional functional modules, such as any of a receive module 720 configured to perform step S104, a signal module 730 configured to perform step S106, and an associate module 740 configured to perform step S108. In general terms, each functional module 710:750 may in one embodiment be implemented only in hardware and in another embodiment with the help of software, i.e., the latter embodiment having computer program instructions stored on the storage medium 630 which when run on the processing circuitry makes the UE 600 perform the corresponding steps mentioned above in conjunction with Fig 7. It should also be mentioned that even though the modules correspond to parts of a computer program, they do not need to be separate modules therein, but the way in which they are implemented in software is dependent on the programming language used. Preferably, one or more or all functional modules 7io:75omay be implemented by the processing circuitry 610, possibly in cooperation with the communications interface 620 and / or the storage medium 630. The processing circuitry 610 may thus be configured to from the storage medium 630 fetch instructions as provided by a functional module 710:750 and to execute these instructions, thereby performing any steps as disclosed herein.

[0080] Fig. 8 shows one example of a computer program product 810 comprising computer readable storage medium 830. On this computer readable storage medium 830, a computer program 820 can be stored, which computer program 820 can cause the processing circuitry 610 and thereto operatively coupled entities and devices, such as the communications interface 620 and the storage medium 630, to execute methods according to embodiments described herein. The computer program 820 and / or computer program product 810 may thus provide means for performing any steps as herein disclosed.

[0081] In the example of Fig. 8, the computer program product 810 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 810 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 820 is here schematically shown as a track on the depicted optical disk, the computer program 820 can be stored in any way which is suitable for the computer program product 810.

[0082] 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

CLAIMS1. A method for transmission of sounding reference signal, SRS, ports, the method being performed by a user equipment (120, 600, 700), the user equipment (120, 600, 700) comprising at least N2 transmit and / or receive chains (126), the method comprising: obtaining (S102) an indication to transmit N2 SRS ports (310a, 310b, 410, 510a, 510b) in an SRS transmission occasion, where N2>1; and transmitting (S110) the N2 SRS ports (310a, 310b, 410, 510a, 510b) on Ni antenna ports (330a, 330b, 430, 530a, 530b) out of at least N2 antenna ports (330a, 330b, 430, 530a, 530b), where NI<N2, in the SRS transmission occasion towards a network node (110) whilst turning off the N2 - Ni transmit and / or receive chains (126) connected to antenna ports (330a, 330b, 430, 530a, 530b) not associated with any SRS port (310a, 310b, 410, 510a, 510b).

2. The method according to any claim 1, wherein the method further comprises: associating (S108) the N2 SRS ports (310a, 310b, 410, 510a, 510b) to the Ni antenna ports (330a, 330b, 430, 530a, 530b), wherein the Ni antenna ports (330a, 330b, 430, 530a, 530b) to which the N2 SRS ports (310a, 310b, 410, 510a, 510b) are associated in turn are associated with transmit and / or receive chains (126), with there being one antenna port (330a, 330b, 430, 530a, 530b) per transmit and / or receive chain (126).

3. The method according to any claim 2, wherein the method further comprises: receiving (S104) a downlink signal from the network node (110) at the N2 antenna ports (330a, 330b, 430, 530a, 530b), and wherein which of the SRS ports (310a, 310b, 410, 510a, 510b) to be or not to be associated with any of the Ni antenna ports (330a, 330b, 430, 530a, 530b) is determined based on a measurement as performed by the user equipment (120, 600, 700) on the downlink signal.

4. The method according to claim 3, wherein the measurement is of received power, signal to noise ratio, SNR, or signal to interference plus noise ratio, SINR, andwherein the N2 SRS ports (310a, 310b, 410, 510a, 510b) are not associated with the Ni antenna ports (330a, 330b, 430, 530a, 530b) at which the downlink signal was received with lowest received power, SNR, or SINR.

5. The method according to any preceding claim, wherein which of the transmit and / or receive chains (126) to turn off further is based on a performance metric computed per antenna port (330a, 330b, 430, 530a, 530b) of each of the transmit and / or receive chains (126).

6. The method according to claim 5, wherein the performance metric for determining whether or not to turn off any transmit chain or receive chain of the transmit and / or receive chains (126) pertains to a downlink link budget.

7. The method according to claim 5, wherein the performance metric for determining whether or not to turn off any transmit chain of the transmit and / or receive chains (126) pertains to an uplink link budget.

8. The method according to claim 7, wherein the uplink link budget is dependent on an output power capability of each transmit chain.

9. The method according to any preceding claim, wherein which receive chain of the transmit and / or receive chains (126) to turn off further is based on any SRS insertion loss associated with the receive chains of the transmit and / or receive chains (126).

10. The method according to any preceding claim, wherein said associating of the N2 SRS ports (310a, 310b, 410, 510a, 510b) to the Ni antenna ports (330a, 330b, 430, 530a, 530b) is biased to prioritize turning off transmit chains of the transmit and / or receive chains (126).

11. The method according to any preceding claim, wherein which of the transmit and / or receive chains (126) to turn off further is based on a power consumption criterion of the transmit and / or receive chains (126).

12. The method according to any preceding claim, wherein which of the transmit and / or receive chains (126) to turn off further is based on any of: a capability of the user equipment (120, 600, 700) to turn off and turn on the transmit and / or receivechains (126), switching delay for turning off and turning on the transmit and / or receive chains (126), switching between transmit and / or receive chains (126).

13. The method according to claim 2, wherein, according to said associating, output power of at least one of the SRS ports (310a, 310b, 410, 510a, 510b) is zero.

14. The method according to claim 2 or 13, wherein, according to said associating, at least two of the SRS ports (310a, 310b, 410, 510a, 510b) are associated with one same antenna port (330a, 330b, 430, 530a, 530b).

15. The method according to any of claims 2, 13, or 14, wherein said associating of the N2 SRS ports (310a, 310b, 410, 510a, 510b) to the Ni antenna ports (330a, 330b, 430, 530a, 530b) is triggered by the user equipment (120, 600, 700) having entered a power saving mode.

16. The method according to any of claims 2, 13, 14, or 15, wherein said associating of the N2 SRS ports (310a, 310b, 410, 510a, 510b) to the Ni antenna ports (330a, 330b, 430, 530a, 530b) is triggered by a power level in the user equipment (120, 600, 700) being below a threshold value.

17. The method according to any of claims 2, 13, 14, 15, or 16, wherein the method further comprises: signaling (S106) a preference for Ni number of uplink multiple-input multipleoutput layers to the network node (110), and wherein said associating of the N2 SRS ports (310a, 310b, 410, 510a, 510b) to the Ni antenna ports (330a, 330b, 430, 530a, 530b) is performed in response thereto.

18. The method according to any preceding claim, wherein the indication to transmit N2 SRS ports (310a, 310b, 410, 510a, 510b) comprises information of a usage of the N2 SRS ports (310a, 310b, 410, 510a, 510b), and wherein which of the transmit and / or receive chains (126) to turn off further is based on said usage of the N2 SRS ports (310a, 310b, 410, 510a, 510b).

19. A user equipment (120, 600, 700) for transmission of sounding reference signal, SRS, ports, the user equipment (120, 600, 700) comprising at least N2 transmitand / or receive chains (126), the user equipment (120, 600, 700) further comprising processing circuitry (610), the processing circuitry being configured to cause the user equipment (120, 600, 700) to: obtain an indication to transmit N2 SRS ports (310a, 310b, 410, 510a, 510b) in an SRS transmission occasion, where N2>1; and transmit the N2 SRS ports (310a, 310b, 410, 510a, 510b) on Ni antenna ports (33oa, 330b, 430, 530a, 530b) out of at least N2 antenna ports (330a, 330b, 430, 530a, 530b), where N1<N2, in the SRS transmission occasion towards a network node (110) whilst turning off the N2 - Ni transmit and / or receive chains (126) connected to antenna ports (330a, 330b, 430, 530a, 530b) not associated with any SRS port (310a, 310b, 410, 510a, 510b).

19. A user equipment (120, 600, 700) for transmission of sounding reference signal, SRS, ports, the user equipment (120, 600, 700) comprising at least N2 transmit and / or receive chains (126), the user equipment (120, 600, 700) further comprising: an obtain module (710) configured to obtain an indication to transmit N2 SRS ports (310a, 310b, 410, 510a, 510b) in an SRS transmission occasion, where N2>1; and a transmit module (750) configured to transmit the N2 SRS ports (310a, 310b, 410, 510a, 510b) on Ni antenna ports (330a, 330b, 430, 530a, 530b) out of at least N2 antenna ports (330a, 330b, 430, 530a, 530b), where NI<N2, in the SRS transmission occasion towards a network node (110) whilst turning off the N2 - Ni transmit and / or receive chains (126) connected to antenna ports (330a, 330b, 430, 530a, 530b) not associated with any SRS port (310a, 310b, 410, 510a, 510b).

20. The user equipment (120, 600, 700) according to claim 18 or 19, further being configured to perform the method according to any of claims 2 to 17.

21. A computer program (820) for transmission of sounding reference signal, SRS, ports, the computer program comprising computer code which, when run on processing circuitry (610) of a user equipment (120, 600, 700) comprising at least N2 transmit and / or receive chains (126), causes the user equipment (120, 600, 700) to:obtain (S102) an indication to transmit N2 SRS ports (310a, 310b, 410, 510a, 510b) in an SRS transmission occasion, where N2>1; and transmit (S110) the N2 SRS ports (310a, 310b, 410, 510a, 510b) on Ni antenna ports (330a, 330b, 430, 530a, 530b) out of at least N2 antenna ports (330a, 330b, 430, 530a, 530b), where N1<N2, in the SRS transmission occasion towards a network node (110) whilst turning off the N2 - Ni transmit and / or receive chains (126) connected to antenna ports (330a, 330b, 430, 530a, 530b) not associated with any SRS port (310a, 310b, 410, 510a, 510b).

22. A computer program product (810) comprising a computer program (820) according to claim 21, and a computer readable storage medium (830) on which the computer program is stored.

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