Multiplexed transmission of channel state information reference signals

Abstract

There is provided techniques for multiplexed transmission of CSI-RSs. A method is performed by a network node. The method comprises scheduling CSI opportunities within a DRX on duration to at least two UEs by assigning CSI-RS resources to a time-frequency grid for the at least two UEs. The method comprises performing a beamformed CSI-RS transmission in the CSI opportunities within the DRX on duration in accordance with the assigned CSI-RS resources.

Description

[0001] MULTIPLEXED TRANSMISSION OF CHANNEL STATE INFORMATION REFERENCE SIGNALS

[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.

[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 is 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. A schematic illustration of such a CSI opportunity 300 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 (starting at time ti), such as a physical downlink control channel (PDCCH), that the UE then measures on the CSI-RSs (starting at time t2), and that the UE reports the measurements (i.e., transmits a CSI report) via signaling on an uplink data channel (starting at time t3), such as a physical uplink shared cannel (PUSCH). 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 are 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 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 frequency DomainAllo cation) 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, Fig. 4 shows 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, 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] To support high capacity with many served UEs per network node, many CSI-RS resources should be multiplexed on the same symbol (for example, in order to support downlink-based beam management). This means that UEs sharing a CSI opportunity being part of a DRX on duration, such that all these UEs, when data inactive, can monitor the downlink control channel to be scheduled with a CSI report, will be configured CSI-RS resources using different frequency resources. However, 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.

[0018] Hence, there is still a need for an improved transmission of CSI-RSs.

[0019] SUMMARY

[0020] 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.

[0021] A particular object is to provide transmission of CSI-RSs that enables the number of connected, and thus served, UEs to be increased.

[0022] A particular object is to utilize the CSI opportunities within a DRX on duration in a resource-efficient manner.

[0023] According to a first aspect there is presented a method for multiplexed transmission of CSI-RSs. The method is performed by a network node. The method comprises scheduling CSI opportunities within a DRX on duration to at least two UEs by assigning CSI-RS resources to a time-frequency grid for the at least two UEs. The time-frequency grid is divided into frequency resources in frequency domain and into time resources in time domain. The CSI-RS resources for each of the UEs is contained within one respective CSI opportunity in the time domain. The same frequency resource is assigned to at least two different UEs in different CSI opportunities within the DRX on duration. The method comprises performing a beamformed CSI-RS transmission in the CSI opportunities within the DRX on duration in accordance with the assigned CSI-RS resources. The CSI-RS resources in the scheduled CSI opportunity for each of the at least two UEs are transmitted in a respective grid-of- beams. Each grid-of-beams is composed of a respective set of beams.

[0024] According to a second aspect there is presented a network node for multiplexed transmission of CSI-RSs. The network node comprises processing circuitry. The processing circuitry is configured to cause the network node to schedule CSI opportunities within a DRX on duration to at least two UEs by assigning CSI-RS resources to a time-frequency grid for the at least two UEs. The time-frequency grid is divided into frequency resources in frequency domain and into time resources in time domain. The CSI-RS resources for each of the UEs is contained within one respective CSI opportunity in the time domain. The same frequency resource is assigned to at least two different UEs in different CSI opportunities within the DRX on duration. The processing circuitry is configured to cause the network node to perform a beamformed CSI-RS transmission in the CSI opportunities within the DRX on duration in accordance with the assigned CSI-RS resources. The CSI-RS resources in the scheduled CSI opportunity for each of the at least two UEs are transmitted in a respective grid-of-beams. Each grid-of-beams is composed of a respective set of beams.

[0025] According to a third aspect there is presented a network node for multiplexed transmission of CSI-RSs. The network node comprises a schedule module configured to schedule CSI opportunities within a DRX on duration to at least two UEs by assigning CSI-RS resources to a time-frequency grid for the at least two UEs. The time-frequency grid is divided into frequency resources in frequency domain and into time resources in time domain. The CSI-RS resources for each of the UEs is contained within one respective CSI opportunity in the time domain. The same frequency resource is assigned to at least two different UEs in different CSI opportunities within the DRX on duration. The network node comprises a transmit module configured to perform a beamformed CSI-RS transmission in the CSI opportunities within the DRX on duration in accordance with the assigned CSI-RS resources. The CSI-RS resources in the scheduled CSI opportunity for each of the at least two UEs are transmitted in a respective grid-of-beams. Each grid-of-beams is composed of a respective set of beams.

[0026] According to a fourth aspect there is presented a computer program for multiplexed transmission of CSI-RSs. 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 schedule CSI opportunities within a DRX on duration to at least two UEs by assigning CSI-RS resources to a time-frequency grid for the at least two UEs. The time-frequency grid is divided into frequency resources in frequency domain and into time resources in time domain. The CSI-RS resources for each of the UEs is contained within one respective CSI opportunity in the time domain. The same frequency resource is assigned to at least two different UEs in different CSI opportunities within the DRX on duration. One action comprises the network node to perform a beamformed CSI-RS transmission in the CSI opportunities within the DRX on duration in accordance with the assigned CSI-RS resources. The CSI-RS resources in the scheduled CSI opportunity for each of the at least two UEs are transmitted in a respective grid-of- beams. Each grid-of-beams is composed of a respective set of beams.

[0027] 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.

[0028] Advantageously, these aspects enable transmission of CSI-RSs without suffering from the above issues.

[0029] Advantageously, these aspects provide increased capacity for higher number of connected, and thus served, UEs.

[0030] Advantageously, these aspects enable the CSI opportunities within a DRX on duration to be used in a resource-efficient manner.

[0031] 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.

[0032] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a / an / the element, apparatus, component, means, module, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, module, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The inventive concept is now described, by way of example, with reference to the accompanying drawings, in which:

[0034] Fig. 1 is a schematic diagram illustrating a communications network according to embodiments;

[0035] Fig. 2 schematically illustrates beam management procedures according to an example;

[0036] Fig. 3 schematically illustrates a CSI opportunity according to an example;

[0037] Fig. 4 schematically illustrates a time-frequency grid according to an example;

[0038] Figs. 5 and 6 schematically illustrate DRX on durations with CSI opportunities according to examples;

[0039] Fig. 7 is a flowchart of methods according to embodiments;

[0040] Figs. 8 and 9 schematically illustrate DRX on durations with CSI opportunities according to embodiments;

[0041] Fig. 10 is a schematic diagram showing structural units of a network node according to an embodiment;

[0042] Fig. 11 is a schematic diagram showing functional modules of a network node according to an embodiment; and

[0043] Fig. 12 shows one example of a computer program product comprising computer readable storage medium according to an embodiment.

[0044] DETAILED DESCRIPTION

[0045] 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.

[0046] As noted above, there is still a need for improved transmission of CSI-RSs.

[0047] In further detail, when multiplexing many UEs in one CSI opportunity it is uncertain in what subarea induced by analog beamforming the UEs will actually be located (for this specific CSI opportunity). This needs to be considered when a UE is data inactive and only monitors the downlink control channel at the DRX offset. A UE might move across several of these subareas during its connection lifetime. Further, a UE with CSI-RS resources allocated to a specific subset of subcarriers / needs to be reachable during as many CSI opportunities as there are subareas during the DRX on duration time. This is schematically illustrated at reference numeral 500 in Fig. 5 which shows two DRX on durations (denoted “On duration 1” and “On duration 2”) with three CSI opportunities (denoted “Co”, “Ci”, and “C2”) in each DRX on duration, and where a UE (denoted “UEi”) is moved between the subarea induced by analog beam At to the subarea induced by analog beam A2 between the two DRX on durations. The DRX on durations are thus periods of time where the UEs wake up and stay awake (e.g., to listen to a downlink control channel) before going to sleep again. The DRX on duration is therefore sometimes referred to as the DRX ON period, whereas the time during which UE is asleep and does not listen to the downlink control channel is called DRX sleep state or DRX OFF period. The uplink is usually unaffected by DRX as discontinuous reception generally is applicable only for the downlink. Whenever data is available in the uplink buffer, the UE can send a Scheduling Request (SR), irrespective of active or sleep state i.e., the UE can interrupt the sleep state when there is a need to transmit uplink data.

[0048] In Fig. 5 it is not shown that determination of the subarea for the UE would involve many other UEs allocated to other non-overlapping subsets of subcarriers. The impact from such other UEs is shown in Fig. 6. In principle, Fig. 6 shows at reference numeral 600 the second DRX on duration in Fig. 5 but also including three other UEs (denoted “UE2”, “UE3”, and “UE4”), where each of these UEs belong to a respective group (denoted “Group 1”, “Group 2”, “Group 3”, and “Group 4”) of one or more UEs. In general term, and as illustrated in Fig. 6, for a DRX on duration, there could be a long list of UEs to be assigned CSI resources, specifically CSI-RS resources. The CSI- RS resources expected to occur in one DRX on duration are non-overlapping in frequency, as indicated in Fig. 6. In fact, the different groups of UEs could be served on different carriers. A scheduler assigns CSI opportunities (and appropriate subarea) to the UEs in order. For example, in Fig. 6, UE4 is put in CSI opportunity Co (and assigned subarea Ao). UE2 is also put in Co (and assigned subarea Ao). UE3 is located in Al and therefore put in another CSI opportunity, namely CSI opportunity Ci (and assigned subarea Al). Similarly, UE1 is assigned CSI opportunity C2 (and assigned subarea A2). All remaining UEs in the list (not shown in Fig. 6) can now be assigned any CSI opportunity Co (and assigned subarea Ao), Ci (and assigned subarea Al), etc., according to where these UEs are located.

[0049] In any case, there are as many CSI opportunities as there are subareas to cater for the worst-case scenario of the DRX on duration representing UEs in all different deployed subareas. It is not important in what order the UEs occur in this DRX on duration; they will all be served. This essentially means that the system is overallocating time-domain resources to allow for mobility of the UEs. To avoid collisions when UEs move into same subarea, no other UEs would be assigned to the same DRX offset (with same subset of subcarriers / for CSI-RS resources).

[0050] Scheduling a CSI report using the CSI-RS resources is implemented in 3GPP supported telecommunication systems based on so-called trigger states. The trigger states are configured to represent the CSI-RS resources. One such trigger state is indicated in the downlink control information (DCI) at the time of scheduling the CSI report. Since the number of trigger states are limited to 128 (by the parameter maxNrOfCSI-AperiodicTriggers, see Section 6.4 in aforementioned 3GPP TS 38.331), it is in many cases not possible to offer alternative CSI-RS resources (mapped to different subset of subcarriers) for a specific UE.

[0051] At least some of the herein disclosed embodiments aim at reducing the overallocation of CSI opportunities by considering how mobile the UEs are.

[0052] The embodiments disclosed herein in particular relate to techniques for multiplexed transmission of CSI-RSs. 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.

[0053] Fig. 7 is a flowchart illustrating embodiments of methods for multiplexed transmission of CSI-RSs. The methods are performed by the network node 200. The methods are advantageously provided as computer programs 1220.

[0054] S106: The network node 200 schedules CSI opportunities 820a, 920a within a DRX on duration 810a: 910c to at least two UEs 160 by assigning CSI-RS resources to a time-frequency grid for the at least two UEs 160.

[0055] The time-frequency grid is divided into frequency resources in frequency domain and into time resources in time domain.

[0056] The CSI-RS resources for each of the UEs 160 is contained within one respective CSI opportunity in the time domain.

[0057] The same frequency resource is assigned to at least two different UEs 160 in different CSI opportunities 820a, 920a within the DRX on duration 8ioa:9ioc.

[0058] S108: The network node 200 performs a beamformed CSI-RS transmission in the CSI opportunities within the DRX on duration 8103:910c in accordance with the assigned CSI-RS resources.

[0059] The CSI-RS resources in the scheduled CSI opportunity for each of the at least two UEs 160 are transmitted in a respective grid-of-beams. Each grid-of-beams is composed of a respective set of beams. In some examples, each grid-of-beams is created by digital beamforming, assuming one of the subareas induced by analog beamforming.

[0060] In some embodiments, the beamformed CSI-RS transmission is performed as part of a beam management process for the UEs 160.

[0061] In some examples, the CSI-RS resources are transmitted using millimeter wave communication. Embodiments relating to further details of multiplexed transmission of CSI-RSs as performed by the network node 200 will now be disclosed with continued reference to Fig. 7.

[0062] In some embodiments, each of the UEs 160 is assumed to be stationary (i.e., not moving, or at least moving very little compared to other UEs). The network node 200 might therefore verify that the UEs 160 are stationary. Hence, in some embodiments, the network node 200 is configured to perform (optional) step S102.

[0063] S102: The network node 200 obtains information that the UEs 160 are stationary before assigning the CSI-RS resources.

[0064] There could be different ways in which the network node 200 obtains this information. For example, the network node 200 might be provided with information on whether a UE is a Fixed Wireless Access (FWA) device, based on slice identity Single Network Slice Selection Assistance Information (S-NSSAI) and / or on a specific set of Quality of Service (QoS) parameters at protocol data unit (PDU) session setup from the core network. Additionally, or alternatively, the network node 200 might derive FWA properties based on the International Mobile Equipment Identity (IMEI) of the UE, if available.

[0065] In some aspects, the network node 200 receives, or otherwise obtains, information of the location of the UEs 160 in the subareas. Hence, in some embodiments, the network node 200 is configured to perform (optional) step S104.

[0066] S104: The network node 200 obtains information about which of the UEs 160 is located in which of the subareas before assigning the CSI-RS resources.

[0067] There could be different ways in which the network node 200 obtains this information. For example, the network node 200 might obtain this information directly from the UEs (for examples as coordinates), by means of performing an explicit positioning procedure with the UEs (for example by transmitting and / or receiving positioning reference signals), or by means of measurements, such as signal strength measurements, on downlink and / or uplink reference signals.

[0068] Knowing the UEs are not moving during their connection, together with information about their locations, (such as in any of A0 / A1 / A2, see Fig. 6), the network node 200 can for one subset / of subcarriers (with one specific subset of subcarriers all contained within one carrier) use all the CSI opportunities within one DRX on duration. An example of this is illustrated at reference numeral 8oo in Fig. 8. In Fig.

[0069] 8 is schematically illustrated transmission of CSI-RSs in narrow beams in accordance with at least some of the herein disclosed embodiments. In particular, Fig. 8 shows two DRX on durations 8ioa, 8iob (denoted “On duration 1” and “On duration 2”) with three CSI opportunities 820a, 820b (denoted “Co”, “Ci”, and “C2”) in each DRX on duration for three (stationary) UEs (denoted “UEo”, “UE1”, and “UE2”). The CSI opportunities 820a, 820b are all for one and the same subset / of subcarriers. The UEs are located in subareas Ao, Al, A2 induced by analog beams Ao, Al, A2. These UEs should be located in different subareas. Hence, in some embodiments, each of the UEs 160 is located in a respective subarea. That is, in the example of Fig. 8, there is one UE per subarea. Otherwise, the UEs in some subareas cannot be served in each DRX on duration. In some embodiments, each subarea is served by a respective one of the grid-of-beams (with only one beam of each grid-of-beams shown in Fig. 8). The subarea induced by the analog beamforming applies to all carriers, and thus all subcarriers. That is, in some embodiments, each of the subareas is induced by analog beamforming and / or each of the grid-of-beams is created by digital beamforming with one of the subareas induced by analog beamforming. The amount CSI opportunities (e.g., how many UEs share the CSI opportunities in one and the same DRX on duration) can be made per subset of subcarriers.

[0070] In general terms, less mobile UEs (i.e., UEs that stationary most of the time) change subareas during their connection time only with some low probability. How often UEs change subareas can be estimated for individual UEs or based on some average with respect to all UEs having been connected to the network node 200 during some time interval over which the averaging is performed. This implies that if several UEs are assigned the same DRX offset and the same subset / of subcarriers there will be collisions at some (low) frequency of occurrence. Collisions here means that two or more UEs end up in the same subarea, as illustrated at reference numeral 900 in Fig. 9. In Fig. 9 is schematically illustrated transmission of CSI-RSs in narrow beams in accordance with at least some of the herein disclosed embodiments. Fig. 9 shows three DRX on durations 910a, 910b, 910c (denoted “On duration 1”, “On duration 2”, and “On duration 3”) with three CSI opportunities 920a, 920b (denoted “Co”, “Ci”, and “C2”) in each DRX on duration for two UEs (denoted “UEo”, and “UE1”. The CSI opportunities 920a, 920b are all for one and the same subset / of subcarriers. The UEs are located in subareas Ao, Al, A2 induced by analog beams Ao, Al, A2. These UEs should be located in different subareas. However, as can be seen in the figure, from DRX on duration 1 to DRX on duration 2, UEo moves from the subarea induced by analog beam Ao to the subarea induced by analog beam Al, and from DRX on duration 2 to DRX on duration 3, UEi moves from the subarea induced by analog beam A2 to the subarea induced by analog beam Al. That is, in DRX on duration 3, both UEo and UEi are located in the subarea induced by analog beam Al, corresponding to CSI opportunity 1, thus causing a collision. As will be disclosed next, there are different ways in which the network node 200 can act to resolve such collisions.

[0071] According to a first example, the network node 200 can schedule CSI reports in a time-sharing manner, doubling the beam management periodicity for the sharing UEs. Hence, in some embodiments, in case two or more UEs 160 are located in a same subarea, each of these two or more UEs 160 is scheduled with a CSI opportunity in a respective DRX on duration 8ioa:9ioc. Each of the CSI opportunities within the DRX on duration 810a: 910c might be specified by a respective DRX offset.

[0072] According to a second example, the network node 200 by means of RRC reconfiguration changes the DRX offset for one of the colliding UEs to a DRX offset where the coverage area of the UE is unoccupied. Hence, in some embodiments, in case two or more UEs 160 are located in a same subarea, each of these two or more UEs 160 is configured with a respective DRX offset yielding at most one UE 160 per CSI opportunity within the DRX on duration 810a: 910c. In this way the CSI opportunity for the UE is moved from one DRX on duration to another.

[0073] According to a third example, the network node 200 changes the subcarriers of the CSI-RS resources for the colliding UE, to subcarriers not used by another UE in this subarea. Hence, in some embodiments, in case two or more UEs 160 are located in a same subarea, each of these two or more UEs 160 is configured with a respective subcarrier yielding at most one UE 160 per subcarrier within the DRX on duration 8ioa:9ioc. This is the same as reconfiguring the trigger states of the UE and can also be performed by the network node by means of RRC reconfiguration. According to a fourth example, the network node 200 moves the colliding UE to another primary carrier (primary cell) where there are more suitable CSI resources. For non-standalone NR deployment this can, for example, be accomplished by a secondary network node issuing a “SgNB Modification Required” message to a master network node, requesting a primary secondary cell (PSCell) change that results in RRC reconfiguration between the master network node and the UE. This is, for example, described in Section 10.3 of 3GPP TS 37.340, “NR; Multi-connectivity; Overall description; Stage-2”, version 18.0.0. In this document is also described how the secondary network node itself can send RRC reconfiguration to the UE to perform a PSCell change. In case the network node is provided in a New Radio (NR) standalone implementation, this could be implemented as intra-network node handover from one cell to another cell in which network coverage is provided by one and the same network node.

[0074] In some aspects, the frequency resources are subcarriers such that one CSI-RS resource is transmitted per subcarrier. That is, in some embodiment, each frequency resource corresponds to one subcarrier.

[0075] In some aspects, the time resources are symbols and the CSI-RS resources of a grid- of-beams are spread on symbols, for example on subsequent symbols. In some embodiments, there are as many time resources for the CSI-RS resources in each of the CSI opportunities as there are beams in each grid-of-beams. In some examples, 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.

[0076] Fig. 10 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). 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.

[0077] 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.

[0078] 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 schedule module 210c configured to perform step S106, and a transmit module 2iod configured to perform step S108. The network node 200 of Fig. 11 may further comprise a number of optional functional modules, such as any of a (first) obtain module 210a configured to perform step S102, and a (second) obtain module 210b configured to perform step S104. 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 2ioa:2iod maybe 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 2ioa:2iod and to execute these instructions, thereby performing any steps as disclosed herein.

[0079] 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. 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 2ioa:2iod of Fig. 11 and the computer program 1220 of Fig. 12.

[0080] 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).

[0081] 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.

[0082] 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.

[0083] 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, wherein the method is performed by a network node (200), and wherein the method comprises: scheduling (S106) CSI opportunities (820a, 920a) within a DRX on duration (810a: 910c) to at least two UEs (160) by assigning CSI-RS resources to a timefrequency grid for the at least two UEs (160), wherein the time-frequency grid is divided into frequency resources in frequency domain and into time resources in time domain, wherein the CSI-RS resources for each of the UEs (160) is contained within one respective CSI opportunity in the time domain, and wherein a same frequency resource is assigned to at least two different UEs (160) in different CSI opportunities (820a, 920a) within the DRX on duration (8ioa:9ioc); and performing (S108) a beamformed CSI-RS transmission in the CSI opportunities within the DRX on duration (810a: 910c) in accordance with the assigned CSI-RS resources, wherein the CSI-RS resources in the scheduled CSI opportunity for each of the at least two UEs (160) are transmitted in a respective grid-of-beams, wherein each grid-of-beams is composed of a respective set of beams.

2. The method according to claim 1, wherein each of the UEs (160) is stationary.

3. The method according to claim 2, wherein the method further comprises: obtaining (S102) information that the UEs (160) are stationary before assigning the CSI-RS resources.

4. The method according to any preceding claim, wherein each of the UEs (160) is located in a respective subarea.

5. The method according to claim 4, wherein the method further comprises:obtaining (S104) information about which of the UEs (160) is located in which of the subareas before assigning the CSI-RS resources.

6. The method according to any preceding claim, wherein each subarea is served by a respective one of the grid-of-beams.

7. The method according to any preceding claim, wherein each of the subareas is induced by analog beamforming.

8. The method according to any preceding claim, wherein each of the grid-of- beams is created by digital beamforming with one of the subareas induced by analog beamforming.

9. The method according to any preceding claim, wherein, in case two or more UEs (160) are located in a same subarea, each of these two or more UEs (160) is scheduled with a CSI opportunity in a respective DRX on duration (8ioa:9ioc).

10. The method according to claim 8, wherein each of the CSI opportunities within the DRX on duration (8ioa:9ioc) is specified by a respective DRX offset.

11. The method according to any preceding claim, wherein, in case two or more UEs (160) are located in a same subarea, each of these two or more UEs (160) is configured with a respective DRX offset yielding at most one UE (160) per CSI opportunity within the DRX on duration (8ioa:9ioc).

12. The method according to any preceding claim, wherein each frequency resource corresponds to one subcarrier.

13. The method according to claim 12, wherein, in case two or more UEs (160) are located in a same subarea, each of these two or more UEs (160) is configured with a respective subcarrier yielding at most one UE (160) per subcarrier within the DRX on duration (8ioa:9ioc).

14. The method according to any preceding claim, wherein the beamformed CSI-RS transmission is performed as part of a beam management process for the UEs (160).15- The method according to any preceding claim, wherein there are as many time resources for the CSI-RS resources in each of the CSI opportunities as there are beams in each grid-of-beams.

16. The method according to any preceding claim, wherein there are at least two time resources in each of the CSI opportunities.

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

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

19. A network node (200) for multiplexed transmission of channel state information reference signals, CSI-RSs, the network node (200) comprising processing circuitry (210), the processing circuitry being configured to cause the network node (200) to: schedule CSI opportunities (820a, 920a) within a DRX on duration (810a: 910c) to at least two UEs (160) by assigning CSI-RS resources to a time-frequency grid for the at least two UEs (160), wherein the time-frequency grid is divided into frequency resources in frequency domain and into time resources in time domain, wherein the CSI-RS resources for each of the UEs (160) is contained within one respective CSI opportunity in the time domain, and wherein a same frequency resource is assigned to at least two different UEs (160) in different CSI opportunities (820a, 920a) within the DRX on duration (8ioa:9ioc); and perform a beamformed CSI-RS transmission in the CSI opportunities within the DRX on duration (810a: 910c) in accordance with the assigned CSI-RS resources, wherein the CSI-RS resources in the scheduled CSI opportunity for each of the at least two UEs (160) are transmitted in a respective grid-of-beams, wherein each grid-of-beams is composed of a respective set of beams.

20. A network node (200) for multiplexed transmission of channel state information reference signals, CSI-RSs, the network node (200) comprising: a schedule module (210c) configured to schedule CSI opportunities (820a, 920a) within a DRX on duration (810a: 910c) to at least two UEs (160) by assigning CSI-RS resources to a time-frequency grid for the at least two UEs (160), wherein the time-frequency grid is divided into frequency resources in frequency domain and into time resources in time domain, wherein the CSI-RS resources for each of the UEs (160) is contained within one respective CSI opportunity in the time domain, and wherein a same frequency resource is assigned to at least two different UEs (160) in different CSI opportunities (820a, 920a) within the DRX on duration (8ioa:9ioc); and a transmit module (2iod) configured to perform a beamformed CSI-RS transmission in the CSI opportunities within the DRX on duration (810a: 910c) in accordance with the assigned CSI-RS resources, wherein the CSI-RS resources in the scheduled CSI opportunity for each of the at least two UEs (160) are transmitted in a respective grid-of-beams, wherein each grid-of-beams is composed of a respective set of beams.

21. The network node (200) according to claim 19 or 20, further being configured to perform the method according to any of claims 2 to 18.

22. A computer program (1220) for multiplexed transmission of channel state information reference signals, CSI-RSs, the computer program comprising computer code which, when run on processing circuitry (210) of a network node (200), causes the network node (200) to: schedule (S106) CSI opportunities (820a, 920a) within a DRX on duration (810a: 910c) to at least two UEs (160) by assigning CSI-RS resources to a timefrequency grid for the at least two UEs (160),wherein the time-frequency grid is divided into frequency resources in frequency domain and into time resources in time domain, wherein the CSI-RS resources for each of the UEs (160) is contained within one respective CSI opportunity in the time domain, and wherein a same frequency resource is assigned to at least two different UEs (160) in different CSI opportunities (820a, 920a) within the DRX on duration (8ioa:9ioc); and perform (S108) a beamformed CSI-RS transmission in the CSI opportunities within the DRX on duration (8ioa:9ioc) in accordance with the assigned CSI-RS resources, wherein the CSI-RS resources in the scheduled CSI opportunity for each of the at least two UEs (160) are transmitted in a respective grid-of-beams, wherein each grid-of-beams is composed of a respective set of beams.

23. A computer program product (1210) comprising a computer program (1220) according to claim 22, and a computer readable storage medium (1230) on which the computer program is stored.

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