Multiplexed transmission of CSI-rs

Abstract

There is provided techniques for multiplexed transmission of CSI-RSs in a CSI opportunity. A method is performed by a network node. The method comprises distributing UEs served by the network node into different groups of UEs. There are shared CSI-RS resources per each group of UEs. The method comprises scheduling the UEs from at least two of the groups of UEs, and no more than one UE from each of the groups of UEs, to report CSI in the CSI opportunity. The method comprises performing a beamformed CSI-RS transmission in the CSI opportunity in accordance with the assigned CSI-RS resources. The CSI-RS resources in each time resource are transmitted in one beam per each grid-of-beam.

Description

[0001] MULTIPLEXED TRANSMISSION OF CSI-RS

[0002] TECHNICAL FIELD

[0003] Embodiments presented herein relate to a method, a network node, a computer program, and a computer program product for multiplexed transmission of channel state information reference signals in a channel state information reference signal opportunity.

[0004] BACKGROUND

[0005] In general terms, in fifth generation (5G) new radio (NR) millimeter wave (mmW) frequency telecommunication systems, maintaining decent coverage for user equipment (UEs) can be challenging unless there are beams directed towards the UEs. Beamforming enables a dynamic creation of directed beams, by changing phase and amplitude of individual antenna elements, towards a particular UE. One purpose of this is to enhance the link budget for the UEs. To account for the dynamic change of the radio channel conditions and UE mobility, it might be necessary to regularly measure and evaluate different beam directions and pick the one best serving the UE for communication with a network node.

[0006] Fig. 1 is a schematic diagram illustrating a communication network 100 where embodiments presented herein can be applied. The communication network 100 could be a third generation (3G) telecommunications network, a fourth generation (4G) telecommunications network, a fifth (5G) telecommunications network, a sixth (6G) telecommunications network, and support any 3rd Generation Partnership Project (3GPP) telecommunications standard. The communication network 100 comprises a transmission and reception point 140 configured to provide network access to user equipment 160 in an (radio) access network 110 in beams, as represented by beam 150. The access network 110 is operatively connected to a core network 120. The core network 120 is in turn operatively connected to a service network 130, such as the Internet. The user equipment 160 is thereby, via the transmission and reception point 140, enabled to access services of, and exchange data with, the service network 130. Operation of the transmission and reception point 140 is controlled by a network node 200. The network node 200 might be part of, collocated with, or integrated with the transmission and reception point 140. Examples of network nodes 200 are (radio) access network nodes, radio base stations, base transceiver stations, Node Bs (NBs), evolved Node Bs (eNBs), gNBs, access points, access nodes, and integrated access and backhaul nodes. Examples of user equipment 160 are wireless devices, mobile stations, mobile phones, handsets, wireless local loop phones, smartphones, laptop computers, tablet computers, network equipped sensors, network equipped vehicles, and so-called Internet of Things devices.

[0007] 3GPP has in 3GPP TR 38.802 “Study on new radio access technology Physical layer aspects”, version 14.2.0, section 6.1.6, defined layer 1 and layer 2 beam management procedures. Procedure 1 (Pi) represents an initial access procedure, Procedure 2 (P2) is used for beam refinement and tracking at the network node, and Procedure 3 (P3) is used for UE beam refinement.

[0008] The procedures Pi, P2, and P3 are schematically illustrated in Fig. 2 and can be summarized as follows.

[0009] For the Pi procedure the network node, via its TRP 140, broadcasts Synchronization Signal Blocks (SSBs) in the downlink in a set of wide beams 150a such that the UE 160, using a beam 170a for reception of the SSBs, is able to measure and send a random-access preamble for the beam corresponding to the strongest SSB. SSBs are transmitted over a set of wide beams 150 to cover several UEs 160 at the same time. After the initial access is completed, the network node 200 triggers a P2 procedure to identify the best narrow beam within the strongest wide beam that was identified during the Pi procedure. Typically, the P2 procedure involves the network node to, via its TRP 140, transmit channel state information reference signals (CSI-RSs) in the downlink in a set of narrow beams 150b such that the UE 160, using a beam 170b for reception of the CSI-RSs, is able to measure and report (such as in a CSI report) at least the narrow beam for which the reference signal received power (RSRP) was highest. In case the UE 160 is capable of beamforming, a P3 procedure can be performed where the network, via its TRP 140, transmits CSI-RSs in the downlink in the beam 150c identified in the P2 procedure as the best for serving the UE 160 such that the UE 160, using a set of beams 170c for reception of the CSI-RSs, is able to identify which of the narrow beams is the best for subsequent communication with the network node. After the completion of the P2 procedure (possibly followed by the optional P3 procedure), data transmission and reception are at the network node performed on narrow beams, as determined during the P2 procedure. However, to account for the dynamic changes in the radio channel and mobility of UEs, the network node needs to track each UE by transmitting further CSI-RSs (per candidate narrow beams) towards each UE and process the response from UEs to determine the best new narrow beam direction.

[0010] Time-domain beamforming might be used to build affordable beamforming systems for high-band deployments. This means that beamforming weights are applied per antenna element at the TRP after the point of Inverse Fast Fourier Transform (IFFT) as seen from the downlink perspective. To have even more cost-efficient deployments only a few beam directions may be allowed, to reduce the amount of data the beamforming machinery needs to manage.

[0011] The P2 procedure, based on aperiodic CSI-RS transmissions for refinement and tracking, typically requires a CSI-RS transmission per candidate narrow beam and UE. Ideally, the candidate beams selected by the network node should cover (and exhaust) the region within the wide beam area of the UE. The CSI-RS transmissions may then consume several downlink slots even for a single request for a CSI report. A corresponding uplink CSI-report (per UE) is also required, requiring resources in one slot.

[0012] For the CSI report initiation (performed by the network node) and transmission of the CSI report (performed by the UE), resources on a downlink control channel and on an uplink data or control channel are needed. This is in addition to the downlink resources related to the actual CSI-RS (for each candidate narrow beam). All resources needed for a CSI report will hereinafter be referred to as a CSI opportunity. The CSI-RS resources within the CSI opportunity are referred to as a CSI-RS opportunity. A schematic illustration of such a CSI opportunity is shown in Fig. 3. In Fig. 3 is illustrated that a UE is configured for CSI report initiation via signaling on a downlink control channel, such as a physical downlink control channel (PDCCH), that the UE then measures on the CSI-RSs, and that the UE the reports the measurements (i.e., transmits a CSI report) via signaling on an uplink channel, such as a physical uplink shared cannel (PUSCH) or a physical uplink control channel (PUCCH). The configuration for CSI report initiation and the transmission of the CSI report each requires at least one symbol of resources, thus resulting in overhead signaling. This overhead signaling scales proportional to the number of UEs served by the network node. As a result, throughput as well as the overall system performance, is reduced.

[0013] Some wireless communication systems comprise options of discontinuous reception (DRX). In short, a UE configured for DRX operation monitors the downlink control channel (such as the PDCCH) only when in an active state. The UE is otherwise in a sleep state where the reception hardware is turned off to reduce UE battery power consumption. The DRX operation for the UE can be configured with different timers (as defined in 3GPP TS 38.321, “NR; Medium Access Control (MAC) protocol specification”, version 17.6.0) to specify how long the UE is to be in the active state (for monitoring downlink control channel). UEs with no data to transmit or receive can be referred to as data inactive UEs. Data inactive UEs can be configured to be only active at certain periodic instants (as specified by the parameter drx-SlotOffset in Section 6.3.2, DRX-Config IE, of 3GPP TS 38.331, “NR; Radio Resource Control (RRC); Protocol specification”, version 17.6.0) for a certain time span (as specified by the parameter drx-onDurationTimer). UEs which are transmitting or receiving data are referred to as data active UEs. Active UEs stay in the active state as long as they continue to transmit or receive information.

[0014] Downlink CSI-RS based beam management for data inactive UEs requires that the network node is allowed to transmit downlink control information on a downlink control channel that schedules a CSI report during the time span defined by drx- onDurationTimer. In other words, there needs to be a CSI opportunity within that time span.

[0015] In short, the resources for CSI-RS can be defined (configured by RRC signaling) by selecting a range based on a frequency band (as defined by the parameter freqBand) and then a subset of subcarriers within that range based on the pattern of subcarriers for CSI-RS (as defined by the parameter frequencyDomainAUocation) and a density parameter defining how dense the CSI-RSs are to be transmitted in the frequency domain. This can be regarded as each UE being assigned its own subset of subcarriers (denoted f) of all the subcarriers part of the bandwidth. This is valid also when multiple carriers are deployed. Then the range is not only defined from freqBand but also from the carrier (on which the CSI-RS shall be transmitted).

[0016] In Fig. 4 is illustrated a traditional approach for transmission of CSI-RSs in narrow beams. In particular, in Fig. 4 is shown an example of CSI-RS resource mapping for beam refinement and tracking for one single TCI state according to the “row 1”- configuration specified in 3GPP TS 38.211 “NR; Physical channels and modulation”, version 17.4.0. It is illustrated how the CSI-RS resources are scheduled in a timefrequency grid (i.e., in the time-frequency domain) 400 to the UEs for one single TCI state (i.e., for one SSB, or one wide beam). One CSI-RS resource (represented by (many) subcarriers - in this figure represented by 3 subcarriers in one physical resource block, PRB) is transmitted in each symbol and represents one narrow beam for the UEs to measure on. That is, in symbol So the CSI-RS resource denoted “A” is transmitted in a first narrow beam, in symbol Si the CSI-RS resource denoted “B” is transmitted in a second narrow beam, in symbol S2 the CSI-RS resource denoted “C” is transmitted in a third narrow beam, and in symbol S3 the CSI-RS resource denoted “D” is transmitted in a fourth narrow beam. Since there are four narrow beams in this example, four symbols are needed in the time domain for the CSI-RS to be transmitted in each narrow beam. After the UEs have measured on the CSI-RS as transmitted in all the narrow beams, the UEs report certain details of the measurements, for instance which narrow beam is the strongest one (i.e., the beam for which the RSRP was the highest). This information can then be used by the network node for efficient scheduling of the UEs. The CSI-RS resources shown in Fig. 4 as used for beam management are one-port signals (see, row 1 in table 7.4.1.5.3-1 of aforementioned 3GPP TS 38.211). As illustrated in Fig. 4, the one-port CSI-RS is transmitted on every fourth subcarrier, with a subcarrier offset indicated by the parameter frequency DomainAllocation as defined in aforementioned 3GPP TS 38.211.

[0017] In analog beamforming, only one single beam-pair (pointing to a single UE, using both polarizations) is active at a time. This effectively means that only a single UE can be allowed to use the downlink or uplink air interface resources at a time. In the context of a CSI opportunity, it only allows for one CSI-RS at a time. That is, for analog beamforming it is only possible to allocate one UE per CSI opportunity. Hence, there is still a need for an improved transmission of CSI-RSs.

[0018] SUMMARY

[0019] An object of embodiments herein is to provide transmission of CSI-RSs that does not suffer from the above issues, or at least where the above issues are mitigated or reduced.

[0020] A particular object is to provide transmission of CSI-RSs where the overhead signaling at least per CSI opportunity does not depend on the number of UEs served by the network node.

[0021] A particular object is to enable concurrent transmission of CSI-RSs for many UEs.

[0022] According to a first aspect there is presented a method for multiplexed transmission of CSI-RSs in a CSI opportunity. The method is performed by a network node. The method comprises distributing UEs served by the network node into different groups of UEs. There are shared CSI-RS resources per each group of UEs. The method comprises scheduling the UEs from at least two of the groups of UEs, and no more than one UE from each of the groups of UEs, to report CSI in the CSI opportunity. The CSI-RS resources are assigned to a time-frequency grid divided into frequency resources in frequency domain and into time resources in time domain, wherein for each of the scheduled UEs, the CSI-RS resources are transmitted in a respective grid- of-beams. Each grid-of-beams is composed of a respective set of beams. The CSI-RS resources for all the UEs scheduled in the CSI opportunity are multiplexed in the frequency domain. The CSI-RS resources for each of the scheduled UEs span the CSI opportunity in the time domain. The method comprises performing a beamformed CSI-RS transmission in the CSI opportunity in accordance with the assigned CSI-RS resources. The CSI-RS resources in each time resource are transmitted in one beam per each grid-of-beam.

[0023] According to a second aspect there is presented a network node for multiplexed transmission of CSI-RSs in a CSI opportunity. The network node comprises processing circuitry. The processing circuitry is configured to cause the network node to distribute UEs served by the network node into different groups of UEs. There are shared CSI-RS resources per each group of UEs. The processing circuitry is configured to cause the network node to schedule the UEs from at least two of the groups of UEs, and no more than one UE from each of the groups of UEs, to report CSI in the CSI opportunity. The CSI-RS resources are assigned to a time-frequency grid divided into frequency resources in frequency domain and into time resources in time domain, wherein for each of the scheduled UEs, the CSI-RS resources are transmitted in a respective grid-of-beams. Each grid-of-beams is composed of a respective set of beams. The CSI-RS resources for all the UEs scheduled in the CSI opportunity are multiplexed in the frequency domain. The CSI-RS resources for each of the scheduled UEs span the CSI opportunity in the time domain. The processing circuitry is configured to cause the network node to perform a beamformed CSI-RS transmission in the CSI opportunity in accordance with the assigned CSI-RS resources. The CSI-RS resources in each time resource are transmitted in one beam per each grid-of-beams.

[0024] According to a third aspect there is presented a network node for multiplexed transmission of CSI-RSs in a CSI opportunity. The network node comprises a distribute module configured to distribute UEs served by the network node into different groups of UEs. There are shared CSI-RS resources per each group of UEs. The network node comprises a schedule module configured to schedule the UEs from at least two of the groups of UEs, and no more than one UE from each of the groups of UEs, to report CSI in the CSI opportunity. The CSI-RS resources are assigned to a time-frequency grid divided into frequency resources in frequency domain and into time resources in time domain, wherein for each of the scheduled UEs, the CSI-RS resources are transmitted in a respective grid-of-beams. Each grid-of-beams is composed of a respective set of beams. The CSI-RS resources for all the UEs scheduled in the CSI opportunity are multiplexed in the frequency domain. The CSI- RS resources for each of the scheduled UEs span the CSI opportunity in the time domain. The network node comprises a transmit module configured to perform a beamformed CSI-RS transmission in the CSI opportunity in accordance with the assigned CSI-RS resources. The CSI-RS resources in each time resource are transmitted in one beam per each grid-of-beams.

[0025] According to a fourth aspect there is presented a computer program for multiplexed transmission of CSI-RSs in a CSI opportunity. 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 distribute UEs served by the network node into different groups of UEs. There are shared CSI-RS resources per each group of UEs. One action comprises the network node to schedule the UEs from at least two of the groups of UEs, and no more than one UE from each of the groups of UEs, to report CSI in the CSI opportunity. The CSI-RS resources are assigned to a time-frequency grid divided into frequency resources in frequency domain and into time resources in time domain, wherein for each of the scheduled UEs, the CSI-RS resources are transmitted in a respective grid- of-beams. Each grid-of-beams is composed of a respective set of beams. The CSI-RS resources for all the UEs scheduled in the CSI opportunity are multiplexed in the frequency domain. The CSI-RS resources for each of the scheduled UEs span the CSI opportunity in the time domain. One action comprises the network node to perform a beamformed CSI-RS transmission in the CSI opportunity in accordance with the assigned CSI-RS resources. The CSI-RS resources in each time resource are transmitted in one beam per each grid-of-beams.

[0026] 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.

[0027] Advantageously, these aspects enable transmission of CSI-RSs without suffering from the above issues.

[0028] Advantageously, by using frequency multiplexing of CSI-RSs, these aspects enable an increased number of connected UE per CSI opportunity.

[0029] Advantageously, these aspects can avoid over-allocating the frequency-multiplexed CSI-RS resources by estimating the fraction of active UEs in case they require shorter beam management periodicity.

[0030] Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings. Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a / an / the element, apparatus, component, means, module, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, module, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

[0031] BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The inventive concept is now described, by way of example, with reference to the accompanying drawings, in which:

[0033] Fig. 1 is a schematic diagram illustrating a communications network according to embodiments;

[0034] Fig. 2 schematically illustrates beam management procedures according to an example;

[0035] Fig. 3 schematically illustrates an CSI opportunity according to an example;

[0036] Fig. 4 schematically illustrates a time-frequency grid according to an example;

[0037] Fig. 5 is a flowchart of methods according to embodiments;

[0038] Figs. 6, 7, 8, and 9 schematically illustrate time-frequency grids according to embodiments;

[0039] Fig. 10 is a schematic diagram showing structural units of a network node according to an embodiment;

[0040] Fig. 11 is a schematic diagram showing functional modules of a network node according to an embodiment; and

[0041] Fig. 12 shows one example of a computer program product comprising computer readable storage medium according to an embodiment. DETAILED DESCRIPTION

[0042] 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.

[0043] As noted above, there is still a need for improved transmission of CSI-RSs.

[0044] The embodiments disclosed herein therefore relate to techniques for multiplexed transmission of CSI-RSs in a CSI opportunity. In order to obtain such techniques there is provided a network node 200, a method performed by the network node 200, a computer program product comprising code, for example in the form of a computer program, that when run on a network node 200, causes the network node 200 to perform the method.

[0045] In legacy, with increasing load (means increasing number of connected UEs), the system behavior can be modified to make it possible to admit additional UEs into the system. As an example, the periodicity for the P2 procedure can be increased if the number of connected UEs exceed a certain threshold value.

[0046] At least some of the herein disclosed embodiments are based on evaluating the load (or number of connected UEs) per subgroups of UEs, where each subgroup contains all UEs configured with identical CSI-RS resources in the frequency domain used for measurement events related to beam management functionality (i.e., all UEs with the same CSI-RS frequency offset). This takes advantage of the fact that only UEs with identical CSI-RS resource configurations are competing for the same CSI opportunity resources and can cause potential collisions in the scheduler.

[0047] At least some of the herein disclosed embodiments are based on multiplexing CSI-RS resources, for example for the purpose of downlink-based beam management. This is enabled by digital beamforming. Here, the digital beamforming is a component of hybrid beamforming. At least some of the herein disclosed embodiments are based on acknowledging that there is only a limited number of CSI opportunities offering beam management per time unit. At least some of the herein disclosed embodiments are based on matching the CSI opportunities to the required beam management periodicity (e.g., how often in time the above disclosed P2 procedure needs to be performed). At least some of the herein disclosed embodiments are based on maintaining a record of allocated UEs per subset of subcarriers used for CSI-RS to fulfill beam management periodicity (for the UEs allocated CSI-RS on those specific subcarriers). At least some of the herein disclosed embodiments are based on that data active UEs might require more CSI opportunities than data inactive UEs because of smaller beam management periodicity for the data active UEs.

[0048] Fig. 5 is a flowchart illustrating embodiments of methods for multiplexed transmission of CSI-RSs in a CSI opportunity. The methods are performed by the network node 200. The methods are advantageously provided as computer programs 1220.

[0049] S102: The network node 200 distributes UEs 160 served by the network node 200 into different groups of UEs 160. There are shared CSI-RS resources per each group of UEs 160. That is, whilst there are separate CSI-RS resources for each group of UEs 160, the CSI-RS resources for each given group of UEs 160 are shared by that given group of UEs 160.

[0050] S104: The network node 200 schedules the UEs 160 from at least two of the groups of UEs 160, and no more than one UE 160 from each of the groups of UEs 160, to report CSI in the CSI opportunity.

[0051] The CSI-RS resources are assigned to a time-frequency grid divided into frequency resources in frequency domain and into time resources in time domain.

[0052] For each of the scheduled UEs, the CSI-RS resources are transmitted in a respective grid-of-beams. Each grid-of-beams is composed of a respective set of beams. In this respect, there can be one individual grid-of-beams for each of the UEs 160 in each group of UEs 160. The CSI-RS resources for all the UEs 160 scheduled in the CSI opportunity are multiplexed in the frequency domain. The CSI-RS resources for each of the scheduled UEs 160 span the CSI opportunity in the time domain.

[0053] The CSI-RS opportunities are thereby multiplexed in the time domain for all groups of UEs 160 such that one UE from each group is scheduled in one CSI opportunity.

[0054] Sio6: The network node 200 performs a beamformed CSI-RS transmission in the CSI opportunity in accordance with the assigned CSI-RS resources. The CSI-RS resources in each time resource are transmitted in one beam per each grid-of-beams. In some examples, the CSI-RS are transmitted using millimeter wave (mmW) communication.

[0055] Embodiments relating to further details of multiplexed transmission of CSI-RSs in a CSI opportunity as performed by the network node 200 will now be disclosed.

[0056] In some aspects, the CSI-RS resources are specific per group of UEs 160. That is, in some embodiments, in the CSI opportunity, one respective CSI-RS resource is assigned to one UE 160 in each of the at least two groups of UEs 160.

[0057] In some aspects, there are different CSI-RS opportunities for different UEs 160 in one and the same group of UEs 160. That is, in some embodiments, the UEs 160 in groups of UEs 160 with more than one UE 160 are scheduled in different CSI-RS opportunities.

[0058] In some aspects, the UEs are divided evenly over the subgroups. That is, in some embodiments, the UEs 160 are evenly distributed over the at least two groups of UEs 160. In this respect, the radio resource control (RRC) configuration a particular UE gets would consider the current distribution of UEs over the different subgroups.

[0059] In some aspects, only a certain a maximum number of UEs are allowed to be admitted to a particular subgroup. That is, in some embodiments, each of the groups of UEs 160 comprises at most a predetermined maximum amount of UEs 160. This maximum number can be set as a tradeoff between the number of UEs the network node should support and delays in the scheduler as caused by potential collisions, as occurs when several of the UEs of the same group compete for a CSI opportunity and just one of them can be scheduled in that CSI opportunity. In some embodiments, the beamformed CSI-RS transmission is performed as part of a beam management process for UEs 160 served in each grid-of-beams. Further, in some embodiments, the CSI-RS resources for the at least two groups of UEs 160 are assigned for at least two CSI-RS opportunities, and the CSI-RS opportunities are periodically scheduled in accordance with the periodicity of the beam management process.

[0060] In some aspects, the beam management periodicity for a given subgroup of UEs can be dynamically increased if the number of UEs within that given subgroup exceeds a certain number. That is, in some embodiments, the periodicity of the beam management process for each group of UEs (160) is dependent on number of UEs 160 per each group of UEs 160. This represents a tradeoff between how many UEs the network node can support and the beam management performance.

[0061] In some aspects, the number of unique subgroups can be increased with increasing network load. That is, in some embodiments, the number of groups of UEs 160 increases with increasing traffic load. This can be achieved either by changing the CSI-RS configuration to a sparser configuration type (for example only allocating one subcarrier per PRB instead of three subcarriers per PRB) or by only allocating a subrange of the complete bandwidth to certain subgroups of UEs and other part(s) of the BW to other subgroups of UEs. These two alternatives represent a tradeoff between increasing the number of UEs that can be served by the network node on the one hand and a performance degradation of the beam quality evaluation via measurements on the other hand (due to the sparser CSI-RS resource allocation).

[0062] The introduction of different subgroups also allows for the possibility to treat distinct types of UEs differently. In particular, in some embodiments, each group of UEs 160 includes only UEs 160 with data to transmit and / or receive or only UEs 160 with no data to transmit and / or receive. In some examples, time-critical UEs or high-priority UEs are mapped into one or more dedicated subgroups, where the beam management periodicity is not increased for any such subgroup. The beam management periodicity for other subgroups containing UEs of lower priority can then be increased.

[0063] In some examples, there are different maximum number of UEs that can be contained in different subgroups. One purpose of this could be to reduce the risk of collisions and / or the likelihood of delays in the scheduler for certain subgroups. This might enhance the performance for the high-priority users belonging to those subgroups. The sorting of UEs depending on type into subgroups also makes the estimate of activity levels more accurate, assuming different types of UEs can be expected to have different activity levels. Activity level generally can be estimated at one point in time by counting UEs that are data active (divided by total number of UEs).

[0064] Reference is next made to illustrations of how CSI-RS resources can be configured and multiplexed among the UEs according to the herein disclosed embodiments. In these examples, the frequency resources are subcarriers, such that one CSI-RS is transmitted per subset of subcarriers (i.e., where each frequency resource corresponds to one subcarrier).

[0065] Fig. 6 and Fig. 7 show examples of how the CSI-RS resources are scheduled in timefrequency grids (i.e., in the time-frequency domain) 600, 700 to the UEs when multiplexing is used (as opposed to Fig. 4 where there is no multiplexing). At least two CSI-RS resources (each represented by three subcarriers in one PRB) is transmitted in each symbol and represents one narrow beam for the UEs to measure on. That is, in Fig. 6, in symbol So there is a simultaneous transmission of the CSI- RS resource denoted “A” in the narrow beam “A” and the CSI-RS resource denoted “E” in the narrow beam “E”, etc. Likewise, in Fig. 7, in symbol So there is a simultaneous transmission of the CSI-RS resource denoted “A” in the narrow beam “A”, the CSI-RS resource denoted “E” in the narrow beam “E”, the CSI-RS resource denoted “I” in the narrow beam “I”, and the CSI-RS resource denoted “M” in the narrow beam “M”, etc.

[0066] Figs. 6 and 7 (as well as the CSI-RS resource multiplexing scheme 800a of Fig. 8) are examples of embodiments where each of the groups of UEs 160 is assigned to at least two respective subcarriers (such as three subcarriers) per PRB, and the at least two respective subcarriers for each group of UEs 160 are frequency offset with respect to each other.

[0067] In more detail, in Fig. 6 is illustrated an example of CSI-RS resource multiplexing for two UEs using the same time-domain resource. The JrequencyDomainAllocation parameter is set to rowi as per the aforementioned 3GPP TS 38.331. In Fig. 7 is illustrated an example of CSI-RS resource multiplexing for four UEs using the same time-domain resource. The frequency DomainAllocation parameter is set to rowi as per the aforementioned 3GPP TS 38.331. That is, a maximum of four UEs can be multiplexed in the same time-domain resource.

[0068] That is, in Fig. 6, two beam sweeps are performed simultaneously and in Fig. 7, four beams sweeps are performed simultaneously. It is here noted that these beam sweeps do not need to be performed in the same directions (although this is the case for both Fig. 6 and Fig. 7 to avoid unnecessary clutter). It is here also noted that one beam sweep is performed per UE in each subgroup, where the UEs denoted UEi and UE2 in Fig. 6 are in different subgroups and the UEs denoted UEi, UE2, UE3, UE4 in Fig. 7 are in different subgroups. There can be other UEs in each of these subgroups that also compete for the CSI-RS resources. For example, if a UE denoted UEx is in the same subgroup as UEi, then UEx could be scheduled in the next CSI opportunity (which thus would be identical to the illustrated CSI opportunity, but where UEi is replaced by UEx and the corresponding beam sweep is made in some other direction, depending on where UEx is located).

[0069] In Fig. 6 and Fig. 7, the frequency DomainAllocation parameter is set to a value rowi as per the aforementioned 3GPP TS 38.331. That is, a maximum of four UEs (as in Fig. 7) can be multiplexed per PRB. However, the capacity can be further extended by setting the frequency DomainAllocation to row2 for multiplexing up to 12 UEs at the same time. This will be illustrated next with reference to Fig. 8. In Fig. 8 are illustrated two examples of CSI-RS resource multiplexing schemes 800a, 800b for one symbol. Here, the CSI-RS resource multiplexing scheme 800a corresponds to one symbol of the scheduling of CSI-RS resources in the time-frequency grid 700 and where thus four UEs are multiplexed per PRB. In the CSI-RS resource multiplexing scheme 800b, on the other hand, there is just one single CSI-RS per UE and thus twelve UEs are multiplexed per PRB. That is, in some embodiments, there are at most 12 groups of UEs 160. Hence, frequency DomainAllocation = rowi for the CSI-RS resource multiplexing scheme 800a, and frequency DomainAllocation = row2 for the CSI-RS resource multiplexing scheme 800b. The CSI-RS resource multiplexing scheme 800b is an example of embodiments where each of the groups of UEs 160 is assigned to exactly one respective subcarrier per PRB. For the case frequency DomainAllocation = row2 the parameter density might be assigned the value 0.5 (as per the aforementioned 3GPP TS 38.331), indicating that only every second PRB shall be considered; it can also be indicated if odd or even PRBs apply. Assigning density to the value 1 means that all PRBs are considered (corresponding to the case covered in the previous paragraph). This means effectively that every 241th subcarrier is used when the density equals 0.5.

[0070] To further increase the level of multiplexing, the CSI-RS can be configured in multiple frequency chunks using the parameter freqBand (see, aforementioned 3GPP TS 38.331). Two UEs can then be configured with the same drx-SlotOffset and frequeny DomainAllocation, doubling the capacity and also reducing the potential for resource collision.

[0071] In some examples, for each DRX-SlotOffset the CSI-RS resources are split based on the parameter freqband to create more CSI-RS resources. This is illustrated in Fig. 9. Fig. 9 illustrates a CSI-RS resource multiplexing scheme 900 where two timefrequency grids occupy the same time-domain resource but span different chunks of the bandwidth. In Fig. 9, in symbol So, there is a simultaneous transmission of the CSI-RS resource denoted “Al” in the narrow beam “Al”, the CSI-RS resource denoted “El” in the narrow beam “El”, the CSI-RS resource denoted “11” in the narrow beam “11”, the CSI-RS resource denoted “Ml” in the narrow beam “Ml”, the CSI-RS resource denoted “A2” in the narrow beam “A2”, the CSI-RS resource denoted “E2” in the narrow beam “E2”, the CSI-RS resource denoted “I2” in the narrow beam “I2”, and the CSI-RS resource denoted “M2” in the narrow beam “M2”, etc. As illustrated in Fig. 9, one frequency chunk spans in frequency the interval represented by (o to NRB

[0072] — 1) and another frequency chunk spans in frequency the interval represented by is the number of PRBs within the system bandwidth. The resources in both frequency chunks in this example have the same frequency DomainAllocation. The CSI-RS resource multiplexing scheme 900 is an example of embodiments where the subcarriers are placed within a system bandwidth, where different groups of UEs 160 are assigned to subcarriers confined within different non-overlapping frequency chunks of the system bandwidth. The DRX-SlotOffsets can be calculated based on the formulas described in Section 5.7 of aforementioned 3GPP TS 38.321. Compared to Fig. 7, the CSI-RS capacity is doubled in Fig. 9 for the same multiplexing order (assuming four UEs being multiplexed in one frequency chunk). In addition, the same technique delivers lower CSI-RS resource collision probability for data active UE by assigning half of the UEs to one frequency chunk and the other half of UEs in another frequency chunk, given the same number of admitted UEs.

[0073] All the above examples setting up CSI-RS multiplexing allocates the UEs with CSI-RS on different subsets of subcarriers (in relation to all the subcarriers of the carrier bandwidth).

[0074] For illustrative purposes, but without loss of generality, assume that one subgroup of UEs is assigned subset fo. Another subgroup of UEs can be allocated subset fl, nonoverlapping with fo, and so on. One reason for having the UEs associated with one subset f of subcarriers for CSI-RS is that it might be costly to redefine the CSI-RS resources by means of RRC reconfiguration. Moreover, changing the CSI-RS resources does not increase the maximum allowed load of UEs since the other subsets are allocated to their own subgroup of UEs. Still, redefining CSI-RS by RRC reconfiguration distributes the load between the subgroups.

[0075] For the network node to multiplex N UEs on one CSI opportunity with regard to CSI- RS resources, N (nonoverlapping) subsets fi of subcarriers can be defined in accordance with any of the thus far disclosed embodiments. More subsets allow multiplexing more UEs; on the other hand, each subset contains fewer subcarriers which may jeopardize the quality of the CSI report. Construction of the subset fi based on the parameter freqBand (distributing the CSI-RS to different parts of the carrier bandwidth) may not represent a frequency-selective channel accurately. There is also an interference aspect with adjacent subcarriers from different CSI-RS that may corrupt the UE measurements.

[0076] As disclosed above, at least some of the herein disclosed embodiments are based on matching the CSI opportunities to the required beam management periodicity. In particular, in some embodiments, the CSI-RS resources for the at least two groups of UEs 160 are assigned for at least two CSI-RS opportunities, and the CSI-RS opportunities are periodically scheduled in accordance with a periodicity of the beam management process. First consider the case when the beam management periodicity is the same for data active UEs and data inactive UEs. The multiplexing of CSI-RS resources allocated for the users in the DRX on-duration (that will be used for data inactive UEs) would then be enough in case some (or even all) data inactive UEs would become data active. When a UE becomes data active there is one data inactive UE less; since they request the same amount of CSI opportunities per time unit (assuming the same beam management periodicity) the total CSI-RS load stays the same.

[0077] In some examples, the beam management periodicity is smaller for data active UEs compared to data inactive UEs. That is, in some embodiments, the periodicity of the beam management process is smaller for the UEs 160 with data to transmit and / or receive than for the UEs 160 with no data to transmit and / or receive. Then consideration needs to be made regarding that the data active UEs consume more CSI opportunities per time unit than the data inactive UEs. There may also be collisions in the sense that several data active UEs ready to perform beam management in one and the same specific CSI opportunity have conflicting (i.e., same) subcarriers for the CSI-RS resources. Then, one of these UEs needs to wait until later CSI opportunity. However, this is not an issue for data active UEs since they always monitor the downlink control signaling, for example as sent on PDCCH.

[0078] As disclosed above, at least some of the herein disclosed embodiments are based on that data active UEs might require more CSI opportunities than data inactive UEs because of smaller beam management periodicity for the data active UEs. In particular, in some embodiments, there are more CSI-RS opportunities scheduled for the groups of UEs 160 comprising UEs 160 with data to transmit and / or receive than for the groups of UEs 160 comprising UEs 160 with no data to transmit and / or receive.

[0079] Assume that x is the fraction of the connected UEs that are data active UEs. In the following is described how an estimate of x can be used to allocate no more connected UEs with a specific subset fi than can be provided CSI opportunities enough to maintain the wanted beam management periodicities for both data active UEs and data inactive UEs.

[0080] The allocation of UEs (at the point of admission) onto the CSI opportunities is considered per subset fi. A reasonable approach (for the purpose of load sharing) is to allocate a new UE to the subset fi of CSI-RS resources with fewest UEs. The underlying time-division duplex (TDD) pattern is assumed to allow for a number C of CSI opportunities per time unit (for a subset of subcarriers fi). These CSI opportunities can be used by both data active UEs and data inactive UEs. Denote by T the beam management periodicity for data inactive UEs. Further, a-T is the beam management periodicity for data active UEs. That is, a = 0.5 means that data active users are subject to half the beam management periodicity of data inactive UEs, and a = 2 means data active UEs are subject to twice the beam management periodicity of data inactive UEs. Further, Ni is the number of connected UEs allocated to fi. Only one UE can be allocated to a subset fi in one CSI opportunity, regardless if the UE is a data active UE or a data inactive UE. Then, to have enough of CSI opportunities for fi:

[0081] C> (i-x)Ni / T + x-Ni / (a-T).

[0082] In other words, Ni cannot be too large in order to fulfill expectations on the beam management periodicity. If Ni is too large, there will be collisions to a level that not even putting CSI reports in a queue would help.

[0083] Solving for the upper limit for number of connected UEs (i.e., for Nmaxi) would then result in:

[0084] Nmaxi = C-T / (i-x+(x / a)).

[0085] In some aspects there are CSI opportunities available for data active UE only. Refer to the number of CSI opportunities per time unit for data active UEs as Ca. Then, to have enough of CSI opportunities for fi:

[0086] C> (i-x)Ni / T + (x-Ni-Ca(a-T)) / (a-T).

[0087] The corresponding expression for Nmaxi would then be:

[0088] Nmaxi = (C + Ca) / (i-x + (x / a)).

[0089] Aspects of handling a new UE in a scenario when the number of UEs is considered to be large compared to Nmaxi will be disclosed next.

[0090] One variant is to let a new UE share CSI opportunities (in the same DRX offset, using the same subset fi) with another already admitted UE in the subset fi when the number of UEs allocated to the subset fi is comparable to Nmaxi (and in case no other subset fi is less loaded). That means for these two UEs, the beam management periodicity will be twice as long compared to the other UEs in the subset f. UEs on other subsets of subcarriers are unaffected since the comparison with an upper limit takes place per subset.

[0091] Another variant is to not admit the new UE. This could, for example, be the case when having different beam management periodicities for different UEs is considered a risk. This variant would not change the beam management periodicity as a function of load.

[0092] Another variant is to change to sparser CSI-RS allocation as load increases. Here sparser would mean allocating a CSI-RS resource to a UE with fewer subcarriers, enabling more UEs to be multiplexed at one point in time, for example going from the example in Fig. 8(b) to the example in Fig. 8(a).

[0093] Another variant is to use these CSI opportunity subsets for differentiating the network performances experienced by different UE types. An example is to allocate all time critical, or high-priority, UEs to a particular subset whilst simultaneously reducing the maximum number of UEs that the network node will admit to that subset (compared to other subsets). This could reduce the likelihood of collisions and delays in the scheduler for the time critical UEs. In turn, this could lead to better performance compared to other UE types belonging to other subsets.

[0094] In some aspects, the time resources are symbols and the CSI-RSs of a grid-of-beams are spread on symbols, for example on subsequent symbols. That is, in some embodiments, each time resource has a time duration of one symbol. The symbol might be an orthogonal frequency-division multiplexing (OFDM) symbol. In some embodiments there are at least two time resources in the CSI opportunity. In some aspects the CSI-RSs are, per symbol, transmitted in as many beams as there are grid- of-beams. Hence, in some embodiments, in each symbol, CSI-RSs are transmitted in at least as many beams as there are grid-of-beams.

[0095] Fig. io schematically illustrates, in terms of a number of structural units, the components of a network node 200 according to an embodiment. Processing circuitry 210 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), etc., capable of executing software instructions stored in a computer program product 1210 (as in Fig. 12), e.g. in the form of a storage medium 230. The processing circuitry 210 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA).

[0096] Particularly, the processing circuitry 210 is configured to cause the network node 200 to perform a set of operations, or steps, as disclosed above. For example, the storage medium 230 may store the set of operations, and the processing circuitry 210 may be configured to retrieve the set of operations from the storage medium 230 to cause the network node 200 to perform the set of operations. The set of operations may be provided as a set of executable instructions.

[0097] Thus the processing circuitry 210 is thereby arranged to execute methods as herein disclosed. The storage medium 230 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The network node 200 may further comprise a communications (comm.) interface 220 at least configured for communications with the UEs 160, as well as other entities, functions, nodes and devices, as in Fig. 1. As such the communications interface 220 may comprise one or more transmitters and receivers, comprising analogue and digital components. The processing circuitry 210 controls the general operation of the network node 200 e.g. by sending data and control signals to the communications interface 220 and the storage medium 230, by receiving data and reports from the communications interface 220, and by retrieving data and instructions from the storage medium 230. Other components, as well as the related functionality, of the network node 200 are omitted in order not to obscure the concepts presented herein.

[0098] Fig. 11 schematically illustrates, in terms of a number of functional modules, the components of a network node 200 according to an embodiment. The network node 200 of Fig. 11 comprises a number of functional modules; a distribute module 210a configured to perform step S102, a schedule module 210b configured to perform step S104, and a transmit module 210c configured to perform step S106. The network node 200 of Fig. 11 may further comprise a number of optional functional modules, as represented by functional module 2iod. In general terms, each functional module 2ioa:2iod 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 230 which when run on the processing circuitry makes the network node 200 perform the corresponding steps mentioned above in conjunction with Fig 11. 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 210a: 2iod may be implemented by the processing circuitry 210, possibly in cooperation with the communications interface 220 and / or the storage medium 230. The processing circuitry 210 may thus be configured to from the storage medium 230 fetch instructions as provided by a functional module 210a: 2iod and to execute these instructions, thereby performing any steps as disclosed herein.

[0099] The network node 200 may be provided as a standalone device or as a part of at least one further device. For example, the network node 200 may be provided in a node of the radio access network or in a node of the core network. Alternatively, functionality of the network node 200 may be distributed between at least two devices, or nodes. These at least two nodes, or devices, may either be part of the same network part (such as the radio access network or the core network) or may be spread between at least two such network parts. In general terms, instructions that are required to be performed in real time may be performed in a device, or node, operatively closer to the cell than instructions that are not required to be performed in real time. Thus, a first portion of the instructions performed by the network node 200 may be executed in a first device, and a second portion of the of the instructions performed by the network node 200 may be executed in a second device; the herein disclosed embodiments are not limited to any particular number of devices on which the instructions performed by the network node 200 may be executed. Hence, the methods according to the herein disclosed embodiments are suitable to be performed by a network node 200 residing in a cloud computational environment. Therefore, although a single processing circuitry 210 is illustrated in Fig. 10 the processing circuitry 210 may be distributed among a plurality of devices, or nodes. The same applies to the functional modules 210a: 2iod of Fig. 11 and the computer program 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, CU, or DU is referred to as a fronthaul network or interface. Messages or packets may be transmitted from the network node 200 in the downlink (i.e., from the CU to the RU) or received by the network node 200 in the uplink (i.e., from the RU to the CU).

[0100] Fig. 12 shows one example of a computer program product 1210 comprising computer readable storage medium 1230. On this computer readable storage medium 1230, a computer program 1220 can be stored, which computer program 1220 can cause the processing circuitry 210 and thereto operatively coupled entities and devices, such as the communications interface 220 and the storage medium 230, to execute methods according to embodiments described herein. The computer program 1220 and / or computer program product 1210 may thus provide means for performing any steps as herein disclosed.

[0101] In the example of Fig. 12, the computer program product 1210 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 1210 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 1220 is here schematically shown as a track on the depicted optical disk, the computer program 1220 can be stored in any way which is suitable for the computer program product 1210. 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 multiplexed transmission of channel state information reference signals, CSI-RSs, in a CSI opportunity, wherein the method is performed by a network node (200), and wherein the method comprises: distributing (S102) UEs (160) served by the network node (200) into different groups of UEs (160), wherein there are shared CSI-RS resources per each group of UEs (160); scheduling (S104) the UEs (160) from at least two of the groups of UEs (160), and no more than one UE (160) from each of the groups of UEs (160), to report CSI in the CSI opportunity, wherein the CSI-RS resources are assigned to a time-frequency grid (600:900) divided into frequency resources in frequency domain and into time resources in time domain, wherein for each of the scheduled UE (160), the CSI-RS resources are transmitted in a respective grid-of-beams , wherein each grid-of-beams is composed of a respective set of beams, wherein the CSI-RS resources for all the UEs (160) scheduled in the CSI opportunity are multiplexed in the frequency domain, and wherein the CSI-RS resources for each of the scheduled UE (160) span the CSI opportunity in the time domain; and performing (S106) a beamformed CSI-RS transmission in the CSI opportunity in accordance with the assigned CSI-RS resources, wherein the CSI-RS resources in each time resource are transmitted in one beam per each grid-of-beams.

2. The method according to claim 1, wherein, in the CSI opportunity, one respective CSI-RS resource is assigned to one UE (160) in each of the at least two groups of UEs (160).

3. The method according to claim 1 or 2, wherein the UEs (160) in groups of UEs (160) with more than one UE (160) are scheduled in different CSI-RS opportunities.4- The method according to any preceding claim, wherein the UEs (160) are evenly distributed over the at least two groups of UEs (160).

5. The method according to any preceding claim, wherein each group of UEs (160) includes only UEs (160) with data to transmit and / or receive or only UEs (160) with no data to transmit and / or receive.

6. The method according to any preceding claim, wherein number of groups of UEs (160) increases with increasing traffic load.

7. The method according to any preceding claim, wherein each of the groups of UEs (160) comprises at most a predetermined maximum amount of UEs (160).

8. The method according to any preceding claim, wherein the beamformed CSI-RS transmission is performed as part of a beam management process for UEs (160) served in each grid-of-beams.

9. The method according to claim 8, wherein the CSI-RS resources for the at least two groups of UEs (160) are assigned for at least two CSI-RS opportunities, and wherein the CSI-RS opportunities are periodically scheduled in accordance with a periodicity of the beam management process.

10. The method according to claim 9, wherein the periodicity of the beam management process is smaller for the UEs (160) with data to transmit and / or receive than for the UEs (160) with no data to transmit and / or receive.

11. The method according to claim 9 or 10, wherein there are more CSI-RS opportunities scheduled for the groups of UEs (160) comprising UEs (160) with data to transmit and / or receive than for the groups of UEs (160) comprising UEs (160) with no data to transmit and / or receive.

12. The method according to claim 9, 10, or 11, wherein the periodicity of the beam management process for each group of UEs (160) is dependent on number of UEs (160) per said each group of UEs (160).

13. The method according to any preceding claim, wherein there are at most 12 groups of UEs (160).14- The method according to any preceding claim, wherein each frequency resource corresponds to one subcarrier.

15. The method according to claim 14, wherein each of the groups of UEs (160) is assigned to at least two respective subcarriers per PRB, and wherein the at least two respective subcarriers for each group of UEs (160) are frequency offset with respect to each other.

16. The method according to claim 14, wherein each of the groups of UEs (160) is assigned to exactly one respective subcarrier per PRB.

17. The method according to claim 14, wherein the subcarriers are placed within a system bandwidth, wherein different groups of UEs (160) are assigned to subcarriers confined within different non-overlapping frequency chunks of the system bandwidth.

18. The method according to any preceding claim, wherein there are as many time resources in the CSI opportunity as there are beams in each grid-of-beams.

19. The method according to any preceding claim, wherein there are at least two time resources in the CSI opportunity.

20. The method according to any preceding claim, wherein each time resource has a time duration of one OFDM symbol.

21. The method according to any preceding claim, wherein the CSI-RS are transmitted using millimeter wave communication.

22. A network node (200) for multiplexed transmission of channel state information reference signals, CSI-RSs, in a CSI opportunity, the network node (200) comprising processing circuitry (210), the processing circuitry being configured to cause the network node (200) to: distribute UEs (160) served by the network node (200) into different groups of UEs (160), wherein there are shared CSI-RS resources per each group of UEs (160);schedule the UEs (160) from at least two of the groups of UEs (160), and no more than one UE (160) from each of the groups of UEs (160), to report CSI in the CSI opportunity, wherein the CSI-RS resources are assigned to a time-frequency grid (600:900) divided into frequency resources in frequency domain and into time resources in time domain, wherein for each of the scheduled UEs (160), the CSI-RS resources are transmitted in a respective grid-of-beams , wherein each grid-of-beams is composed of a respective set of beams, wherein the CSI-RS resources for all the UEs (160) scheduled in the CSI opportunity are multiplexed in the frequency domain, and wherein the CSI-RS resources for each of the scheduled UEs (160) span the CSI opportunity in the time domain; and perform a beamformed CSI-RS transmission in the CSI opportunity in accordance with the assigned CSI-RS resources, wherein the CSI-RS resources in each time resource are transmitted in one beam per each grid-of-beams.

23. A network node (200) for multiplexed transmission of channel state information reference signals, CSI-RSs, in a CSI opportunity, the network node (200) comprising: a distribute module (210a) configured to distribute UEs (160) served by the network node (200) into different groups of UEs (160), wherein there are shared CSI-RS resources per each group of UEs (160); a schedule module (210b) configured to schedule the UEs (160) from at least two of the groups of UEs (160), and no more than one UE (160) from each of the groups of UEs (160), to report CSI in the CSI opportunity, wherein the CSI-RS resources are assigned to a time-frequency grid (600:900) divided into frequency resources in frequency domain and into time resources in time domain,wherein for each of the scheduled UEs (160), the CSI-RS resources are transmitted in a respective grid-of-beams , wherein each grid-of-beams is composed of a respective set of beams, wherein the CSI-RS resources for all the UEs (160) scheduled in the CSI opportunity are multiplexed in the frequency domain, and wherein the CSI-RS resources for each of the scheduled UEs (160) span the CSI opportunity in the time domain; and a transmit module (210c) configured to perform a beamformed CSI-RS transmission in the CSI opportunity in accordance with the assigned CSI-RS resources, wherein the CSI-RS resources in each time resource are transmitted in one beam per each grid-of-beams.

24. The network node (200) according to claim 22 or 23, further being configured to perform the method according to any of claims 2 to 21.

25. A computer program (1220) for multiplexed transmission of channel state information reference signals, CSI-RSs, in a CSI opportunity, the computer program comprising computer code which, when run on processing circuitry (210) of a network node (200), causes the network node (200) to: distribute (S102) UEs (160) served by the network node (200) into different groups of UEs (160), wherein there are shared CSI-RS resources per each group of UEs (160); schedule (S104) the UEs (160) from at least two of the groups of UEs (160), and no more than one UE (160) from each of the groups of UEs (160), to report CSI in the CSI opportunity, wherein the CSI-RS resources are assigned to a time-frequency grid (600:900) divided into frequency resources in frequency domain and into time resources in time domain, wherein for each of the scheduled UEs (160), the CSI-RS resources are transmitted in a respective grid-of-beams , wherein each grid-of-beams is composed of a respective set of beams,wherein the CSI-RS resources for all the UEs (160) scheduled in the CSI opportunity are multiplexed in the frequency domain, and wherein the CSI-RS resources for each of the scheduled UEs (160) span the CSI opportunity in the time domain; and perform (S106) a beamformed CSI-RS transmission in the CSI opportunity in accordance with the assigned CSI-RS resources, wherein the CSI-RS resources in each time resource are transmitted in one beam per each grid-of-beams.

26. A computer program product (1210) comprising a computer program (1220) according to claim 25, and a computer readable storage medium (1230) on which the computer program is stored.

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