Compensation of frequency offset

Abstract

A network node transmits (1210) downlink reference signals from first and second transmission sources and receives (1220) one or more reports from a UE. The reports indicate channel state information for coherent joint transmission on a physical downlink channel from the transmission sources. The reports also indicate a frequency offset between the transmission sources. The reports are based on measurements of the downlink reference signals from the transmission sources. The network node performs (1230) joint data transmission on the physical downlink channel from the transmission sources using a precoder which is based on the indicated channel state information. The network node performs frequency offset compensation which comprises performing the joint data transmission, wherein the precoder used for the joint data transmission is further based on the indicated frequency offset, and / or compensating at least one of the downlink reference signals based on the indicated frequency offset.

Description

[0001] COMPENSATION OF FREQUENCY OFFSET

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to wireless communications and, in particular, to compensation of frequency offset in joint transmissions.

[0004] BACKGROUND

[0005] The Third Generation Partnership Project (3GPP) has developed and is developing standards for Fourth Generation (4G) (also referred to as Long Term Evolution (LTE)) and Fifth Generation (5G) (also referred to as New Radio (NR)) wireless communication systems. Such systems provide, among other features, broadband communication between network nodes, such as base stations, and mobile user equipments (UE), as well as communication between network nodes and between UEs. The 3GPP is also developing standards for Sixth Generation (6G) wireless communication networks.

[0006] Similar to LTE, 5G mobile systems use OFDM (Orthogonal Frequency Division Multiplexing) in the downlink (i.e., from a network node, gNB, eNB, or base station, to a user equipment or UE). In the uplink (i.e., from UE to gNB), both OFDM and discrete Fourier transform (DFT)-spread OFDM (DFT-S-OFDM) are supported. The basic NR physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1, where a resource block (RB) in a 14-symbol slot is shown. A RB corresponds to 12 contiguous subcarriers in the frequency domain. RBs are numbered in the frequency domain, starting with 0 from one end of the system bandwidth. Each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.

[0007] Different subcarrier spacings are supported in NR. The supported subcarrier spacings (also referred to as numerologies) are given by Δf = (15 × 2μ) kHz where μ is a non-negative integer and may be one of {0,1,2,3,4}. Δf = 15kHz (e.g., μ = 0) is the basic (or reference) subcarrier spacing that is also used in LTE. μ is also referred to as the numerology.

[0008] In the time domain, downlink and uplink transmissions in NR are organized into equally-sized subframes of 1ms each. A subframe is further divided into multiple slots of equal duration. The slot length is dependent on the subcarrier spacing or numerology and is given by 1 / 2μ ms. Each slot consists of 14 OFDM symbols for normal Cyclic Prefix (CP).

[0009] Data scheduling in NR may be on a per slot basis. Downlink (DL) transmissions may be dynamically scheduled, i.e., in each slot the network node transmits downlinkcontrol information (DCI) about which UE data is to be transmitted to and which resource blocks in the current downlink slot the data is transmitted on. The control information is carried on Physical Control Channel (PDCCH) and data is carried on Physical Downlink Shared Channel (PDSCH). A UE first detects and decodes the PDCCH and if a PDCCH is decoded successfully, it then decodes the corresponding PDSCH based on the decoded control information in the PDCCH.

[0010] Uplink (UL) data transmission may also be dynamically scheduled using the PDCCH. Similar to downlink, a UE first decodes uplink grants in the PDCCH and then transmits data over the Physical Uplink Shared Channel (PUSCH) based on the decoded control information in the uplink grant. This may be done according to a modulation order, coding rate, uplink resource allocation, and etc.

[0011] Tracking Reference Signal (TRS)

[0012] Similar to LTE, a channel state Information Reference Signal (CSI-RS) was introduced in NR for channel measurement in the DL. A CSI-RS is transmitted over an antenna port (either a physical or virtual antenna) on certain resource elements (REs) for a UE to measure the DL channel associated with the antenna port. CSI-RS for this purpose is also referred to as Non-Zero Power (NZP) CSI-RS. The supported number of antenna ports or CSI-RS ports in NR are {1, 2, 4, 8, 12, 16, 24, 32}.

[0013] A Tracking Reference Signal (TRS) is a special NZP CSI-RS with one port and is used for time and frequency tracking in the DL. FIG. 2 shows an example of a TRS resource configuration in a physical resource block (PRB) and 2 slots. A UE may be configured with one or more periodic TRSs, or one or more periodic TRSs and aperiodic TRSs in NR. For a periodic TRS, it has a periodicity and a slot offset. The periodicity may be one of 2μXpslots where Xp=10, 20, 40, or 80. A TRS occupies multiple RBs. When a NZP CSI-RS resource set contains “trs- info”, then the NZP CSI-RS resource set is for TRS.

[0014] CSI framework in NR

[0015] In NR, a UE may be configured with one or multiple Channel State Information (CSI) report configurations for DL CSI feedback by the UE. A CSI report may contain one or more of:

[0016] Channel rank indicator (RI);

[0017] Antenna precoding matrix indicator (PMI);

[0018] Channel quality indicator (CQI);• DL reference signal received power (RSRP) or signal to interference and noise ratio (SINR); and / or

[0019] • CSI reference signal (CSI-RS) resource indicator (CRI).

[0020] Each CSI report configuration is associated with a bandwidth part (BWP) and contains all necessary information required for a CSI report, including:

[0021] • a CSI resource configuration for channel measurement;

[0022] • reporting type, i.e., aperiodic CSI (on PUSCH), periodic CSI (on PUCCH) or semi-persistent CSI (on PUCCH, and DCI activated on PUSCH); and

[0023] • report quantity specifying what to be reported, such as RI, PMI, CQI, RSRP, etc.

[0024] A UE may be configured with one or multiple CSI resource configurations for channel measurement. Each CSI resource configuration for channel measurement may contain one or more NZP CSI-RS resource sets. For each NZP CSI-RS resource set, it may further contain one or more NZP CSI-RS resources. A NZP CSI-RS resource may be periodic, semi-persistent, or aperiodic.

[0025] Periodic CSI starts after it has been configured by RRC and is reported on PUCCH The associated NZP CSI-RS resource(s) are also periodic.

[0026] For aperiodic CSI, it is reported on PUSCH and is activated by a CSI request bit field in downlink control information (DCI). The associated NZP CSI-RS resource(s) may be either periodic, semi-persistent, or aperiodic. The linkage between a code point of the CSI request field and a CSI report configuration is via an aperiodic CSI trigger state. A UE is configured by higher layer a list of aperiodic CSI trigger states, where each of the trigger states contains an associated CSI report configuration. The CSI request field is used to indicate one of the aperiodic CSI trigger states and thus, one CSI report configuration. If there are more than one NZP CSI-RS resource set and / or if more than one CSI interference measurement (CSI-IM) resource set are associated with a CSI report configuration, only one NZP CSI-RS resource set is selected in the aperiodic CSI trigger state. Thus, each aperiodic CSI report is based on a single NZP CSI-RS resource set. CQI and PMI may be reported per subband or wideband. In case of wideband CQI or PMI, the CQI or PMI is for the whole bandwidth configured for CSI report. In case of subband CQI or PMI, the CQI or PMI is reported for each subband. The subband size in NR may be from 4 RBs to 32 RBs, depending on the size of the BWP.Coherent joint transmission from multiple TRPs

[0027] In 3 GPP NR (new radio) technical release 18 (3 GPP Rel-18), channel state information (CSI) feedback for coherent joint transmission (CJT) of PDSCH over multiple transmission and reception points (TRPs) was introduced. In CJT, each multiple inputmultiple output (MIMO) layer of PDSCH is transmitted from multiple TRPs to a UE in a same time and frequency resource. Before the transmission, each MIMO layer is phase adjusted at each TRP such that they are phase aligned when reaching the UE and thus, are coherently combined to enhance signal quality.

[0028] An example is shown in FIG. 3, where a PDSCH with r layers, i.e., 5 =

[0029] [s1(s2,..., sr]T, is transmitted from two TRPs (TRP#1 denoted by 31 and TRP#2 denoted by 32) after being precoded by a precoding matrix

[0030]

[0031] at TRP#1 and a precoding matrix W2at TRP#2. Each element of the precoding matrices is a complex coefficient. The precoding helps to achieve coherent (or constructive) combining of signals from the two TRPs at the UE for each layer.

[0032] The precoders

[0033]

[0034] and W2may be reported by the UE as part of a CSI report for CJT, which was introduced in 3GPP Rel-18. A CSI report for CJT typically comprises a rank (i.e., number of layers) indicator (RI), a channel quality indicator (CQI), and a precoding matrix indicator (PMI). Information about the matrices

[0035]

[0036] and W2depicted in FIG. 3 would be indicated as part of the PMI.

[0037] 3 GPP Rel-19 CJT enhancement

[0038] A number of challenges were identified in practical CJT deployment. For instance, the cooperating TRPs may not be perfectly synchronized. Although the same nominal transmit frequency may be considered for the cooperating TRPs, due to local oscillator stability, there may be some actual transmit frequency offset(s) between different TRPs. In 3GPP Rel-19, it was considered that the UE may measure these frequency offset(s) and report them to the network node. According to 3GPP Rel-19 specifications, the frequency offset(s) report is triggered either jointly or separately from the CSI report for CJT discussed above. The frequency offsets (FOs) reported by the UE are referred to as CSI calibration report on frequency offset in this disclosure.

[0039] It would be desirable to provide ways for how a network node can use the reported frequency offset.

[0040] SUMMARYA first aspect provides embodiments of a method implemented in a network node that is configured to communicate with a user equipment (UE). The method comprises transmitting downlink reference signals from first and second transmission sources, and receiving one or more reports from the UE. The one more reports indicate channel state information for coherent joint transmission on a physical downlink channel from the first and second transmission sources. The one or more reports also indicate a frequency offset between the first and second transmission sources. The one or more reports are based on measurements of the downlink reference signals from the first and second transmission sources. The method comprises performing j oint data transmission on the physical downlink channel from the first and second transmission sources using a precoder which is based on the indicated channel state information. The network node performs frequency offset compensation which comprises:

[0041] • performing the j oint data transmission, wherein the precoder used for the j oint data transmission is further based on the indicated frequency offset; and / or

[0042] • pre-compensating at least one of the downlink reference signals based on the indicated frequency offset, wherein the frequency offset is indicated by a first received report which is based on measurements on other downlink reference signals from the first and second transmission sources than the at least one precompensated downlink reference signal, and wherein the channel state information is indicated by a second received report which is based on measurement on downlink reference signals from the first and second transmission sources including the at least one pre-compensated downlink reference signal.

[0043] A second aspect provides embodiments of a corresponding network node.

[0044] BRIEF DESCRIPTION OF THE DRAWINGS

[0045] A more complete understanding of the present embodiments, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:

[0046] FIG. 1 shows NR physical resources;

[0047] FIG. 2 shows an example of resource element (RE) allocation for a TRS in NR; FIG. 3 shows an example of CJT PDSCH transmission over two TRPs;FIG. 4 is a schematic diagram of an example network architecture illustrating a communication system according to principles disclosed herein;

[0048] FIG. 5 is a block diagram of a network node in communication with a user equipment over a wireless connection according to some embodiments of the present disclosure;

[0049] FIG. 6 is a flowchart of an example process in a network node for compensation of frequency offset (FO) in joint transmissions such as feedback based coherent joint transmission (CJT) according to some embodiments of the present disclosure;

[0050] FIG. 7 is a flowchart of an example process for compensation of FO in feedback based CJT according to principles disclosed herein;

[0051] FIG. 8 shows an example of FO induced phase difference;

[0052] FIG. 9 shows an example of performance of network node compensation of FO; FIG. 10 is a flowchart of another example process for compensation of FO in feedback based CJT;

[0053] FIG. 11 shows an example of FO induced phase difference, where the network node receives a FO report before CSI-RS transmission and applies compensation in the CSI-RS transmission; and

[0054] FIG. 12 is a flow chart of a method implemented in a network node according to some embodiments of the present disclosure.

[0055] DETAILED DESCRIPTION

[0056] Before describing in detail exemplary embodiments, it is noted that the embodiments reside primarily in combinations of apparatus components and processing steps related to methods of compensation of frequency offset (FO) in joint transmissions such as feedback based coherent joint transmission (CJT). Accordingly, components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.

[0057] As used herein, relational terms, such as “first” and “second,” “top” and “bottom,” and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the conceptsdescribed herein. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes” and / or “including” when used herein, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0058] In embodiments described herein, the joining term, “in communication with” and the like, may be used to indicate electrical or data communication, which may be accomplished by physical contact, induction, electromagnetic radiation, radio signaling, infrared signaling or optical signaling, for example. One having ordinary skill in the art will appreciate that multiple components may interoperate and modifications and variations are possible of achieving the electrical and data communication.

[0059] In some embodiments described herein, the term “coupled”, “connected,” and the like, may be used herein to indicate a connection, although not necessarily directly, and may include wired and / or wireless connections.

[0060] Reference made herein to the “network” (NW) performing a function, step, etc., can be understood to mean that a network node and / or combination of network nodes are configured to perform the described function, step, etc. The term “network node” used herein can be any kind of network node comprised in a radio network which may further comprise any of base station (BS), radio base station, base transceiver station (BTS), base station controller (BSC), radio network controller (RNC), g Node B (gNB), evolved Node B (eNB or eNodeB), Node B, multi-standard radio (MSR) radio node such as MSR BS, multi-cell / multicast coordination entity (MCE), relay node, donor node controlling relay, radio access point (AP), transmission points, transmission nodes, Remote Radio Unit (RRU) Remote Radio Head (RRH), a core network node (e.g., mobile management entity (MME), self-organizing network (SON) node, a coordinating node, positioning node, MDT node, etc.), an external node (e.g., 3rd party node, a node external to the current network), nodes in distributed antenna system (DAS), a spectrum access system (SAS) node, an element management system (EMS), etc. The network node may also comprise test equipment. The term “radio node” used herein may be used to also denote a user equipment (UE) such as a wireless device (WD) or a radio network node.

[0061] In some embodiments, the non-limiting terms wireless device (WD) or a user equipment (UE) are used interchangeably. The UE herein can be any type of userequipment capable of communicating with a network node or another UE over radio signals, such as a wireless device (WD). The UE may also be a radio communication device, target device, device to device (D2D) UE, machine type UE or UE capable of machine to machine communication (M2M), low-cost and / or low-complexity UE, a sensor equipped with UE, Tablet, mobile terminals, smart phone, laptop embedded equipped (LEE), laptop mounted equipment (LME), USB dongles, Customer Premises Equipment (CPE), an Internet of Things (loT) device, or a Narrowband loT (NB-IOT) device etc.

[0062] Also, in some embodiments the generic term “radio network node” is used. It can be any kind of a radio network node which may comprise any of base station, radio base station, base transceiver station, base station controller, network controller, RNC, evolved Node B (eNB), Node B, gNB, Multi-cell / multicast Coordination Entity (MCE), relay node, access point, radio access point, Remote Radio Unit (RRU) Remote Radio Head (RRH).

[0063] Note that although terminology from one particular wireless system, such as, for example, 3GPP LTE and / or New Radio (NR) and / or 6G, may be used in this disclosure, this should not be seen as limiting the scope of the disclosure to only the aforementioned system. It is contemplated that other 3GPP systems may make use of the concepts and arrangements disclosed herein. For example, a disclosure relating to NR may also be implementable in a 6G system and / or an LTE system, a disclosure relating to 6G may also be implementable in a NR and / or LTE system, and a disclosure relating to LTE may also be implementable in a NR and / or 6G system. Other wireless systems, including without limitation Wide Band Code Division Multiple Access (WCDMA), Worldwide Interoperability for Microwave Access (WiMax), Ultra Mobile Broadband (UMB) and Global System for Mobile Communications (GSM), may also benefit from exploiting the ideas covered within this disclosure.

[0064] Note further, that functions described herein as being performed by a user equipment or a network node may be distributed over a plurality of user equipments and / or network nodes. In other words, it is contemplated that the functions of the network node and user equipment described herein are not limited to performance by a single physical device and, in fact, can be distributed among several physical devices.

[0065] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in thecontext of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0066] The principles disclosed herein are not limited to NR nor to any specific release. The principles disclosed herein may also apply to 6G where different terminology may be used.

[0067] Although the term TRP is used in this disclosure, the term TRP may not be captured in 3GPP specifications. Instead, a TRP may be represented by any one of ‘NZP CSI-RS resource set’, ‘NZP CSI-RS resource’, ‘TRS resource set’, and / or ‘TRS resource’, or in general downlink reference signal (DL-RS). The terminologies ‘NZP CSI-RS’ and ‘CSI-RS’ may be used interchangeably in the disclosure.

[0068] In 3GPP 6G, other terms than NZP CSI-RS may be used. For example, a new downlink reference signal or downlink synchronization signal may be introduced which then may be used instead of NZP CSI-RS. The 3GPP 6G downlink reference signals and / or downlink synchronization signals may be aperiodically, semi-persistently or periodically transmitted from the NW (e.g., network node) to the UE.

[0069] Some embodiments are described with reference to NZP CSI-RS resource sets, but the below embodiments are non-limiting and equally applicable when NZP CSI-RS resource sets are replaced by NZP CSI-RS resource(s), TRS(s), TRS resource set(s), and / or DL-RS(s).

[0070] Some embodiments are described with reference to CSI / PMI reporting for CJT based on Enhanced Type II (eType-II) codebook for CJT (as defined in Clause 5.2.2.2.8 of 3GPP Technical Standard (TS) 38.214 V18.4.0), but the below embodiments are nonlimiting and equally applicable to general channel state feedback.

[0071] Some embodiments are directed to compensation of frequency offset (FO) in feedback based coherent joint transmission (CJT).

[0072] Returning to the drawing figures, in which like elements are referred to by like reference numerals, there is shown in FIG. 4 a schematic diagram of a communication system 10, according to an embodiment, such as a 3 GPP-type cellular network that may support standards such as LTE and / or NR (5G) and / or 6G, which comprises an access network 12, such as a radio access network, and a core network 14. The access network 12 comprises a plurality of network nodes 16a, 16b, 16c (referred to collectively as network nodes 16), such as NBs, eNBs, gNBs or other types of wireless access points, each defining a corresponding coverage area 18a, 18b, 18c (referred to collectively as coverage areas 18). Each network node 16a, 16b, 16c is connectable to the core network 14 over awired or wireless connection 20. A first user equipment (UE) 22a located in coverage area 18a is configured to wirelessly connect to, or be paged by, the corresponding network node 16a. A second UE 22b in coverage area 18b is wirelessly connectable to the corresponding network node 16b. While a plurality of UEs 22a, 22b (collectively referred to as user equipments 22) are illustrated in this example, the disclosed embodiments are equally applicable to a situation where a sole UE is in the coverage area or where a sole UE is connecting to the corresponding network node 16. Note that although only two UEs 22 and three network nodes 16 are shown for convenience, the communication system may include many more UEs 22 and network nodes 16.

[0073] Also, it is contemplated that a UE 22 can be in simultaneous communication and / or configured to separately communicate with more than one network node 16 and more than one type of network node 16. For example, a UE 22 can have dual connectivity with a network node 16 that supports LTE and the same or a different network node 16 that supports NR. As an example, UE 22 can be in communication with an eNB for LTE / E-UTRAN and a gNB for NR / NG-RAN.

[0074] A network node 16 (e.g. eNB or gNB) may be configured to include an FO compensation unit 24 which may be configured to perform FO compensation for joint data transmission on a physical downlink channel from at least two transmission sources such as TRPs. The FO compensation may be based at least in part on a received calibration report.

[0075] Example implementations, in accordance with an embodiment, of the UE 22 and network node 16 discussed in the preceding paragraphs will now be described with reference to FIG. 5.

[0076] The communication system 10 includes a network node 16 including hardware (HW) 28 enabling it to communicate with the UE 22. The hardware 28 may include a radio interface 30 for setting up and maintaining at least a wireless connection 32 with a UE 22 located in a coverage area 18 served by the network node 16. The radio interface 30 may be formed as or may include, for example, one or more RF transmitters, one or more RF receivers, and / or one or more RF transceivers. The radio interface 30 includes an array of antennas 34 to radiate and receive signal(s) carrying electromagnetic waves.

[0077] In the embodiment shown, the hardware 28 of the network node 16 further includes processing circuitry 36. The processing circuitry 36 may include a processor 38 and a memory 40. In particular, in addition to or instead of a processor, such as a central processing unit, and memory, the processing circuitry 36 may comprise integratedcircuitry for processing and / or control, e.g., one or more processors and / or processor cores and / or FPGAs (Field Programmable Gate Array) and / or ASICs (Application Specific Integrated Circuitry) adapted to execute instructions. The processor 38 may be configured to access (e.g., write to and / or read from) the memory 40, which may comprise any kind of volatile and / or nonvolatile memory, e.g., cache and / or buffer memory and / or RAM (Random Access Memory) and / or ROM (Read-Only Memory) and / or optical memory and / or EPROM (Erasable Programmable Read-Only Memory).

[0078] Thus, the network node 16 further has software (SW) 42 stored internally in, for example, memory 40, or stored in an external memory (e.g., database, storage array, network storage device, etc.) accessible by the network node 16 via an external connection. The software 42 may be executable by the processing circuitry 36. The processing circuitry 36 may be configured to control any of the methods and / or processes described herein and / or to cause such methods, and / or processes to be performed, e.g., by network node 16. Processor 38 corresponds to one or more processors 38 for performing network node 16 functions described herein. The memory 40 is configured to store data, programmatic software code and / or other information described herein. In some embodiments, the software 42 may include instructions that, when executed by the processor 38 and / or processing circuitry 36, causes the processor 38 and / or processing circuitry 36 to perform the processes described herein with respect to network node 16. For example, processing circuitry 36 of the network node 16 may include an FO compensation unit 24 which may be configured to perform FO compensation for joint data transmission on a physical downlink channel from at least two transmission sources. The FO compensation may be based at least in part on a received calibration report. The calibration report may be received from the UE 22 and may indicate the FO to the network node 16.

[0079] The communication system 10 further includes the UE 22 already referred to. The UE 22 may have hardware 44 that may include a radio interface 46 configured to set up and maintain a wireless connection 32 with a network node 16 serving a coverage area 18 in which the UE 22 is currently located. The radio interface 46 may be formed as or may include, for example, one or more RF transmitters, one or more RF receivers, and / or one or more RF transceivers. The radio interface 46 includes an array of antennas 48 to radiate and receive signal(s) carrying electromagnetic waves.

[0080] The hardware 44 of the UE 22 further includes processing circuitry 50. The processing circuitry 50 may include a processor 52 and memory 54. In particular, inaddition to or instead of a processor, such as a central processing unit, and memory, the processing circuitry 50 may comprise integrated circuitry for processing and / or control, e.g., one or more processors and / or processor cores and / or FPGAs (Field Programmable Gate Array) and / or ASICs (Application Specific Integrated Circuitry) adapted to execute instructions. The processor 52 may be configured to access (e.g., write to and / or read from) memory 54, which may comprise any kind of volatile and / or nonvolatile memory, e.g., cache and / or buffer memory and / or RAM (Random Access Memory) and / or ROM (Read-Only Memory) and / or optical memory and / or EPROM (Erasable Programmable Read-Only Memory).

[0081] Thus, the UE 22 may further comprise software 56, which is stored in, for example, memory 54 at the UE 22, or stored in external memory (e.g., database, storage array, network storage device, etc.) accessible by the UE 22. The software 56 may be executable by the processing circuitry 50. The software 56 may include a client application 58. The client application 58 may be operable to provide a service to a human or non-human user via the UE 22.

[0082] The processing circuitry 50 may be configured to control any of the methods and / or processes described herein and / or to cause such methods, and / or processes to be performed, e.g., by UE 22. The processor 52 corresponds to one or more processors 52 for performing UE 22 functions described herein. The UE 22 includes memory 54 that is configured to store data, programmatic software code and / or other information described herein. In some embodiments, the software 56 and / or the client application 58 may include instructions that, when executed by the processor 52 and / or processing circuitry 50, causes the processor 52 and / or processing circuitry 50 to perform the processes described herein with respect to UE 22.

[0083] In some embodiments, the inner workings of the network node 16 and UE 22 may be as shown in FIG. 5 and independently, the surrounding network topology may be that of FIG. 4.

[0084] The wireless connection 32 between the UE 22 and the network node 16 is in accordance with the teachings of the embodiments described throughout this disclosure. More precisely, the teachings of some of these embodiments may improve the data rate, latency, and / or power consumption and thereby provide benefits such as reduced user waiting time, relaxed restriction on file size, better responsiveness, extended battery lifetime, etc. In some embodiments, a measurement procedure may be provided for thepurpose of monitoring data rate, latency and other factors on which the one or more embodiments improve.

[0085] Although FIGS. 4 and 5 show various “units” such as FO compensation unit 24 as being within a respective processor, it is contemplated that these units may be implemented such that a portion of the unit is stored in a corresponding memory within the processing circuitry. In other words, the units may be implemented in hardware or in a combination of hardware and software within the processing circuitry.

[0086] As described above in the background section, cooperating TRPs may not be perfectly synchronized. Although the same nominal transmit frequency may be considered for the cooperating TRPs, due to local oscillator stability, there may be some actual transmit frequency offset between different TRPs. A UE may measure this frequency offset and report it to the network node in a report which may be referred to as a CSI calibration report on frequency offset.

[0087] When CSI-RSs from different TRPs are transmitted in different OFDM symbols or different slots, and in case of frequency offsets between the TRPs, the phase difference between the TRPs due to the frequency offsets at the time of channel measurements for computing a CSI report for CJT may differ from the phase difference between the TRPs during a subsequent PDSCH transmission. Put another way, the phase difference, Δφ, between the two TRPs at the time the two CSI-RSs are transmitted may be different from the phase difference between the two TRPs at the PDSCH transmission time. If a precoder across the two TRPs is computed based on the channel measurements on the CSI-RSs and is used for PDSCH transmission over the two TRPs, there may be a phase mismatch between the actual and assumed phase offset between the two TRPs. This phase mismatch means that the PDSCH from the two TRPs may not be coherently combined at the UE.

[0088] Note that although the problem is described in the context of coherent joint transmission among multiple TRPs with frequency offset, the same problem may occur in a more generic case when multiple references signals, transmitted at different time instances (e.g., symbol and / or slot), are used in combination for DL CSI acquisition, based on which the PDSCH will be transmitted.

[0089] Some embodiments advantageously provide methods and network nodes for compensation of frequency offset (FO) in joint transmissions such as feedback based coherent joint transmission (CJT).In some embodiments, the network node may perform compensation for the phase difference caused by frequency offset (FO) for cases where DL-RSs (e.g., CSI-RSs) are transmitted in different time symbols and / or slots. This may allow the UE to report CSI for CJT from different TRPs as in 3GPP Rel-18. The compensation performed by the network node may be implemented in software, hardware or both.

[0090] The network node may perform compensation for the phase difference introduced by FO using information such as:

[0091] • UE reported FO between the TRPs;

[0092] • UE reported CSI for CJT reports such as, for example:

[0093] o Enhanced Type II (eType-II) codebook for CJT as defined in Clause 5.2.2.2.8 of 3GPP Technical Standard (TS) 38.214 V18.4.0; and / or

[0094] o Further Enhanced Type II port selection (feType-II PS) codebook for CJT as defined in Clause 5.2.2.2.9 of 3GPP TS 38.214 V18.4.0; and / or

[0095] • DL-RS (CSI-RS, TRS, etc.) time domain configurations.

[0096] In a first set of embodiments, the network node may send CSI-RSs from multiple TRPs independent from the UE reported CSI calibration report on frequency offset. In this case, the network node may compensate frequency offset in the CJT precoder, i.e., the precoder used by the network node when transmitting the PDSCH from the TRPs.

[0097] In a second set of embodiments, the network node may send CSI-RSs from multiple TRPs based on the UE reported CSI calibration report on frequency offset. In this case, the network node may compensate UE reported frequency offset at least partly in CSI-RSs from the multiple TRPs.

[0098] An advantage of some embodiments is that the network may perform FO compensation and derive correct PDSCH precoder(s) based on the received UE report on CSI / PMI for CJT and the UE reported FO, together with the knowledge of DL-RS (CSI-RS, TRS etc.) configurations.

[0099] Some proposed methods are related to a topic of interest in 3GPP Rel-19 of NR, and may for example be applied for D-MIMO, which is expected to be a component in 3GPP 6G.

[0100] FIG. 6 is a flowchart of an example process in a network node 16 for compensation of frequency offset (FO) in joint transmissions such as feedback based coherent joint transmission (CJT). One or more blocks described herein may be performed by one or more elements of network node 16 such as by one or more of processing circuitry 36 (including the FO compensation unit 24), processor 38, and / or radio interface 30. Networknode 16 such as via processing circuitry 36 and / or processor 38 and / or radio interface 30 is configured to transmit downlink reference signals from a plurality of transmission sources (Block S10). The process also includes receiving from the UE 22 a calibration report on one or more frequency offsets (FOs), the calibration report being based at least in part on measurements by the UE 22 of the downlink reference signals (Block S12). The process further includes performing FO compensation for joint data transmission on a physical downlink channel from at least two of the plurality of transmission sources, the FO compensation being based at least in part on the received calibration report (Block S14).

[0101] In some embodiments, at least two downlink reference signals of the downlink reference signals are configured with a different time resource. In some embodiments, performing the FO compensation includes using a coherent joint transmission (CJT) precoder for performing the joint data transmission, the CJT precoder being based at least in part on measurements by the UE 22 of the downlink reference signals on different time resources. In some embodiments, the CJT precoder is compensated with a first phase compensation component based at least in part on an FO and a time difference between downlink reference signal transmission and physical downlink channel transmission. In some embodiments, the CJT precoder is compensated with a second phase compensation component based at least in part on an FO and a time of a first downlink reference signal transmission from a first transmission source relative to a second downlink reference signal transmitted from a second transmission source. In some embodiments, the FO compensation of the CJT precoder is based at least in part on a phase drift between measurements on the downlink reference signals at different measurement times. In some embodiments, the phase drift is based at least in part on a reported FO. In some embodiments, the phase drift is based at least in part on a time difference between two different times of the measurement times. In some embodiments, the two different times of the measurement times are determined based at least in part on a slot or symbol of the joint data transmission on the physical downlink channel or a downlink reference signal transmission. In some embodiments, at least two transmission sources of the plurality of transmission source are transmission-reception points (TRPs) operating at different frequencies. In some embodiments, each downlink reference signal includes a channel state information reference signal (CSI-RS). In some embodiments, the physical downlink channel includes a physical downlink shared channel (PDSCH). In some embodiments, the network node, radio interface and / or processing circuitry are configured to configure the UE for FO reporting.Having described the general process flow of arrangements of the disclosure and having provided examples of hardware and software arrangements for implementing the processes and functions of the disclosure, the sections below provide details and examples of arrangements for compensation of frequency offset (FO) in joint transmissions such as feedback based coherent joint transmission (CJT).

[0102] Let Hi(i = 1,..., N) denote the measured channel associated to TRP #i based on a reference signal (e.g., NZP CSI-RS) transmitted from TRP #i. Then, the channel H = [H1H2

[0103]

[0104] corresponds to the TV TRPs and associated reference signals measured by the UE 22. Information regarding a joint precoder, W =

[0105]

[0106] [iv^... is contained in the CSI / PMI for CJT, wherein widenotes a precoder matrix corresponding to TRP#i. wicontains co-phasing factor(s) for compensating any phase difference between TRP#i and a reference TRP so that PDSCH transmitted from the multiple TRPs may be coherently combined at the UE 22.

[0107] When frequency differences / offsets are present among the multiple TRPs and if the downlink reference signals (DL-RSs) from the multiple TRPs are transmitted at different time instances such as different OFDM symbols or slots, the measured phase differences between the TRPs would be different from the actual phase differences between the TRPs at PDSCH transmission time. This phase mismatch is a problem for coherent joint transmission as it may result in the PDSCH transmitted from the multiple TRPs being combined non- coherently at the UE 22.

[0108] Methods and arrangements are disclosed herein that enable compensation of the phase mismatch for coherent joint transmission by the network node 16 based on CSI / PMI feedback for CJT (e.g., based on Enhanced Type II codebook for CJT). Two frequency compensation methods are disclosed: (1) embodiments related to the scenario where the network node 16 compensates frequency offset on the CJT precoder associated to each TRP (e.g., using the CSI / PMI feedback for CJT from the UE 22); and (2) embodiments related to the scenario where the network node 16 compensates frequency offset on the transmitted CSI-RS associated to each TRP.

[0109] Embodiments related to NW compensating FO in the CJT precoder

[0110] One example embodiment is described with reference to FIG. 7.

[0111] In STEP 7-1, the network node 16 provides the UE 22 with higher layer (e.g., radio resource control (RRC)) configurations on:

[0112] • DL RSs resources (e.g., TRS and / or CSI-RS);• a CSI calibration report on frequency offset(s) (e.g., a FO report introduced in 3GPP Rel-19); and / or

[0113] • CSI / PMI reporting for CJT (e.g., using eType-II codebook for CJT). In some embodiments, the network node 16 provides the UE 22 with one CSI reporting configuration for reporting FO between TRPs, e.g., the 3GPP Rel-19 FO report, and another CSI reporting configuration for reporting the CSI / PMI reporting for CJT based on eType-II codebook for CJT.

[0114] The CSI / PMI report for CJT may be one of the following:

[0115] • Enhanced Type II (eType-II) codebook for CJT as defined in Clause 5.2.2.2.8 of 3GPP Technical Specification (TS) 38.214 V18.4.0; or

[0116] • Further enhanced Type II port selection (feType-II PS) codebook for CJT as defined in Clause 5.2.2.2.9 of 3GPP TS 38.214 V18.4.0.

[0117] In STEP 7-2, the network node 16 transmits DL RSs (e.g., TRS and CSI-RS) for the UE 22 to compute and report the CSI calibration report on frequency offset(s) and CSI / PMI report for CJT.

[0118] In STEP 7-3, the network node 16 receives the CSI calibration report on frequency offset(s) and the CSI / PMI report for CJT from the UE 22.

[0119] In some embodiments, the CSI calibration report on frequency offsets (FOs) and the CSI / PMI report for CJT are separately triggered. This means the network node 16 requests the UE 22 to report the CSI calibration report on FOs using a first DCI and requests the UE 22 to report the CSI / PMI for CJT using a second DCI. In some embodiments, the CSI calibration report on FOs and the CSI / PMI report for CJT are received by the network node 16 from the UE 22 in different slots.

[0120] In some embodiments, the CSI calibration report on FOs and CSI / PMI report for CJT are jointly triggered. This means the network node 16 requests the UE 22 to report the CSI calibration report on FOs and the CSI / PMI for CJT using a single DCI. In some embodiments, the CSI calibration report on FOs and the CSI / PMI report for CJT are received by the network node 16 from the UE 22 in the same slot.

[0121] In STEP 7-4, the network node 16 performs frequency offset compensation and CJT on PDSCH based on one or more of:

[0122] • received CSI calibration report (e.g., a CJT calibration report on frequency offset, also referred to as a CJT-C FO report);

[0123] • received CSI / PMI report for CJT (e.g., eType-II CJT PMI report);

[0124] • knowledge of DL-RS time / frequency resource configuration;knowledge of the reference resource used to measure CSI / PMI report for CJT; and / or

[0125] • knowledge of downlink numerology or subcarrier spacing.

[0126] How the frequency offset compensation may be performed will be exemplified with reference to FIG. 8. FIG. 8 is an example illustrating frequency offset induced phase difference A<p. As shown in FIG. 8, a CSI-RS#260 from TRP#2 may be configured with the same time resource ti as a CSI-RS#1 62 from TRP#1. Alternatively, as shown in FIG.

[0127] 8, a CSI-RS#264 from TRP#2 at ti ’ may be configured with a different time resource than the CSI-RS#! 62 from TRP#1.

[0128] As illustrated in FIG. 8, consider a reference TRP#1 and a second TRP#2 in CJT operation. Due to the separate local oscillators of TRP#1 and TRP#2, the phase drift given by the carrier frequencies and f2may be written as:

[0129] φ₁(t) = 2πf₁t + φ₀,₁

[0130] φ₂(t) = 2πf₂t + φ₀,₂,

[0131] A

[0132]

[0133] <p(t) = <p2(t)

[0134] where φ₀,₁ and φ₀,₂ are the initial phases. Note that Δφ(t) is time varying because of the frequency offset between the two TRPs. In this example, the phase difference at the channel measurement time for computing CSI / PMI for CJT at the UE 22 is Δφ(t₁, t₁') = φ₂(t'₁) −

[0135]

[0136] , while the phase difference at the PDSCH transmission time is Δφ(t₃) = φ₂(t₃) − φ₁(t₃). Note that Δφ(t₁, t'₁) is implicitly contained in the channel measurements and will be compensated for when the corresponding CSI / PMI for CJT is used for joint transmission of PDSCH. In order to perform joint PDSCH transmission at t3, the network node 16 may compensate the phase difference between the time instances the channels corresponding to CSI-RS #1 and #2 are measured and the time instance the PDSCH is transmitted due to the frequency offset, based on received CSI calibration report on FO and CSI / PMI report for CJT (at t2).

[0137] In some embodiments, when CSI-RS#1 62 from TRP#1 and CSI-RS#2 60 from TRP#2 are transmitted at the same time resource t₁, the network node 16 may derive the actual CJT precoder, W̃ = [w̃₁ w̃₂]T, at t3 with:

[0138] • UE reported PMI for CJT W = [w₁ w₂]Tbased on (H₁(t₁), H₂(t₁)); and • the phase drift between t₁ and t₃ given by Δφ' =

[0139]

[0140] = t₃ - t₁,

[0141]

[0142] where w̃₁ = w₁ and w̃₂ = w₂ exp(−jΔφ') = w₂ exp(−j2πΔf(t₃ − t₁)).In some embodiments, when CSI-RS#1 62 from TRP#1 and CSI-RS#2 64 from TRP#2 are transmitted at different time instances t₁ and t₁', respectively, the network node 16 may derive the CJT precoder, W̃ = [w̃₁ w̃₂]T, at t₃ with:

[0143] • UE reported PMI for CJT W =

[0144]

[0145] [w̃₁ w̃₂]Tbased on (H₁(t₁), H₂(t₁')); • the phase drift between t₁ and t₃ given by Δφ' = Δφ(t₃)-Δφ(t₁); and • the phase difference between tx

[0146]

[0147] and t₁': φ₂(t₁') − φ₂(t₁), which may be approximated with

[0148]

[0149] 2πΔf(t₁' − t₁), where Δf is the UE reported frequency offset; and where: w̃₁ = w₁ and w̃₂ = w₂ exp(−jΔφ' + j(φ₂(t₁') − φ₂(t₁))) ≈ w₂ exp(−jΔφ' + j

[0150]

[0151] 2πΔf(t₁' − t₁)) = w₂ exp(−j2πΔf(t₃ − t₁')).

[0152] The frequencies of the oscillators at TRP#1 and TRP#2 (i.e. fi and f2) may not be fixed, and may drift all the time. Neither the network nor the UE may know the exact values of these frequencies. The UE may estimate the frequency offset between its own oscillator and an oscillator at the network (for example at TRP#1) based on downlink reference signals (such as CSI-RS or periodic tracking reference signals (TRS)) and may correct for this offset. Assuming TRP#1 is the reference TRP for frequency offsets report, the UE could frequency lock to TRP#1 (i.e., fi) and the phase change over time due to fi can (at least theoretically) be fully tracked and removed at the UE side, i.e., the line associated to fi in FIG. 8 becomes a horizontal line, and then f2 is effectively the same as f2-fi.

[0153] Note that in the above embodiments, to compute phase drift between t₁ and t₃, the time difference (t₃ − t₁) may be computed. This time difference may be determined by the network node 16 by:

[0154] • Knowing the slot / symbol in which PDSCH is jointly transmitted at t3; • Knowing the slot / symbol in which CSI-RS #1 is transmitted that corresponds to the reference resource of the UE reported PMI for CJT (i.e., U); here, the reference resource refers to the transmission occasion of CSI-RS #1 that was used to measure and compute the UE reported PMI for CJT; reference resource definition for different CSI reports are given in Clause 5.2.2.5 of 3GPP TS 38.214 V18.4.0; and / or • Knowing the downlink numerology or subcarrier spacing allows the network to determine the time difference (t3— U) in units of seconds / milli-seconds from the slot-level or symbol-level time differences; for instance, for a 15 kHz numerology, the duration of a slot is 1 millisecond, for a 30 kHz numerology, the duration of a slot is 0.5 millisecond. If the network knows the time difference between transmission of CSI-RS #1 and transmission of PDSCH in units of slots, then thenetwork may determine the time difference (t3— ti) by multiplying this time difference in slots by the time duration of a slot in seconds / milliseconds which is given by the downlink numerology or subcarrier spacing.

[0155] The time difference (t₁' − t₁) may be computed in a similar way as the time difference (t₃ − t₁). The time difference (t₁' −

[0156]

[0157] may be determined by the network node 16 by:

[0158] • Knowing the slot / symbol in which CSI-RS #1 and CSI-RS #2 are transmitted that corresponds to the reference resource of the UE reported PMI for CJT (i.e., t₁ and t₁'); here, the reference resource refers to the transmission occasions of CSI-RS #1 and CSI-RS #2 that were used to measure and compute the UE reported PMI for CJT; and / or

[0159] • Knowing the downlink numerology or subcarrier spacing allows the network to determine the time difference (t — U) in units of seconds / milli-seconds from the slot-level or symbol-level time differences; for instance, for a 15 kHz numerology, the duration of a slot is 1 millisecond, for a 30 kHz numerology, the duration of a slot is 0.5 millisecond. If the network knows the time difference between transmissions of CSI-RS #1 and CSI-RS #2 in units of slots, then the network may determine the time difference (t₁' − t₁) by multiplying this time difference in slots by the time duration of a slot in seconds / milliseconds which is given by the downlink numerology or subcarrier spacing.

[0160] Note that the PDSCH may be scheduled over multiple OFDM symbols. In this case, t3above represents the time of each of the multiple symbols, and different t3values would be used for computing the compensation, i.e., exp(−jΔφ' + j2πΔf(t₁' − t₁)), for different OFDM symbols over which the PDSCH is to be transmitted. The compensation may be performed per PDSCH OFDM symbol.

[0161] In some scenarios, each of the CSI-RS resources may occupy multiple OFDM symbols. In this case, t₁ above may be any time instance within the multiple OFDM symbols of CSI-RS resource #1 associated to TRP#1 and t₁' above may be any time instance within the multiple OFDM symbols of CSI-RS resource #2 associated to TRP#2. For example, t₁ may be the time of the first symbol, last symbol, or a symbol between the first and the last symbols.

[0162] In some embodiments, the network node 16 may compensate the CJT CSI precoder for each TRP (which is not the reference TRP) with two phase compensation components:• A first phase compensation component based on the received frequency offset (e.g., 3GPP Rel-19 CJT-C FO report) and a time difference between CSI-RS reception and PDSCH transmission. In some embodiments, the time difference is between reception of the CSI-RS from the reference (first) TRP and PDSCH transmission. In some embodiments, the time difference is between reception of the CSI-RS from the second TRP and PDSCH transmission; and

[0163] • A second phase compensation component based on the received frequency offset (e.g., 3GPP Rel-19 CJT-C FO) and a time difference in CSI-RS transmission with respect to the reference TRP.

[0164] In some embodiments, the network node 16 derives the second phase compensation component to handle CSI-RSs from different TRPs transmitted with symbol-level offset, and the actions performed may include one or more of the following:

[0165] • Compensating the PMI for CJT phase per TRP#k (ivfc) using the frequency offset FOkfor a reference time tkbased on the transmission time of the occupied symbols for the CSI-RS resource associated with TRP#k, where tkis a symbol time between the first and the last OFDM symbols of the CSI-RS resource;

[0166] • The transmission time of the first, or medium, or mean, or last symbol of CSI-RS resource associated with TRP#k may be used as the reference time tkas long as the same is used for all TRPs;

[0167] • The compensated PMI for TRP#k at transmission time t is: wkcomp(t) =

[0168] ke-2jπFO(t-t); and / or

[0169]

[0170] • The frequency offset for the reference TRP is 0, and no CJT PMI phase compensation is needed for the reference TRP.

[0171] In some embodiments, the network node 16 derives the second phase compensation component to handle CSI-RSs from different TRPs transmitted with slotlevel offset, and the actions performed may include one or more of:

[0172] • Compensating the CJT PMI phase per TRP# k (wk) using the frequency offset FOkfor the common reference time tref based on the reference CSI- RS resource;

[0173] • The compensated PMI for TRP#k at transmission time t is:kcomp(t) =ke-2jπFO(t-t); and / or• The frequency offset for the reference TRP is 0, and no CJT PMI phase compensation is needed for the reference TRP.

[0174] In some embodiments, the network node 16 may derive one combined phase compensation component including both the first and second phase compensation components for all PDSCH symbols in the CJT PMI period.

[0175] In some embodiments, the network node 16 may derive a combined phase compensation component including both the first and second phase compensation components to each PDSCH symbol in the CJT PMI period.

[0176] In some embodiments, the network node 16 may derive and apply a slot-level combined phase compensation component (including the first and second phase compensation components) to each PDSCH slot in the PMI period.

[0177] In some embodiments, the network node 16 may apply the first and second phase compensation components with different time units. In one example, the network node 16 may apply the first phase compensation component in symbol level and the second phase compensation component in slot level. In an alternative example, the network node 16 may apply the first phase compensation component in slot level and the second phase compensation component in symbol level.

[0178] FIG. 9 depicts the performance of the network node 16 FO compensation versus without FO compensation when CSI-RS#1 and CSI-RS #2 are transmitted in two adjacent slots with a frequency offset of 195 parts per billion (ppb) between two TRPs at a nominal carrier frequency of 7GHz. FIG. 9 also shows the gain by the network node 16 compensating FO including both the compensation component for the time difference between CSI-RS transmission and PDSCH transmission and the compensation component for slot-level CSI-RS transmission time difference between TRP#1 and TRP#2, compared to excluding the compensation component for CSI-RS transmission time difference between TRP# 1 and TRP#2.

[0179] Embodiments related to NW compensating FO in CSI-RS

[0180] In some embodiments, the network node 16 compensates frequency offset between multiple TRPs in CSI-RS transmission, as depicted in FIG. 10. This approach may be useful if the UE 22 is configured to perform CJT with, for example, the eType-II CJT port-selection PMI reporting. The CJT port-selection PMI reporting may require UE specific CSI-RS, which may also be needed when the network node 16 pre-compensates FO offset in CSI-RS.

[0181] Some embodiments may include one or more of the following steps.In STEP 10-1, the network node 16 provides the UE 22 with higher layer (e.g., RRC) resource and reporting configurations on:

[0182] • DL RSs including TRS and CSI-RS;

[0183] • CSI calibration report on frequency offset FO (e.g., 3GPP Rel-19 CJT-C FO report); and / or

[0184] • CJT CSI report (e.g., the eType-II CJT port-selection PMI report).

[0185] In STEP 10-2, the network node 16 transmits TRSs from different TRPs for the UE 22 to compute and report frequency offsets between TRPs.

[0186] In some embodiments, the TRSs from different TRPs are configured with orthogonal time / frequency resources. In some embodiments, the TRSs are configured with same the time resources but with different frequency resources; alternatively, the TRSs from different TRPs are configured with different time and with different frequency resources.

[0187] In STEP 10-3, the network node 16 receives the CSI calibration report on frequency offsets (e.g., 3GPP Rel-19 CJT-C FO report) from the UE 22.

[0188] In some embodiments, the CSI calibration report on frequency offsets and the CJT CSI report are separately triggered.

[0189] In STEP 10-4, the network node 16 transmits frequency offset pre-compensated CSI-RSs based on the received CSI calibration report from the UE 22. The precompensation is done when CSI-RS associated to different TRPs are allocated in different time domain resources such as in different OFDM symbols or slots.

[0190] In some embodiments, the CSI-RSs from different TRPs are configured with orthogonal time / frequency resources. In one example, the CSI-RSs are configured with same time resources but different frequency resources; alternatively, the CSI-RSs from different TRPs are configured with different time and frequency resources.

[0191] The network node 16 may phase rotate CSI-RS to pre-compensate the frequency offset. An example is shown in FIG. 11, where the frequency offset between the reference TRP (TRP#1) and TRP#2 is reported by the UE 22 before CSI-RS transmissions from the two TRPs. The CSI-RS associated to the two TRPs are transmitted at time U and t, respectively, as illustrated in FIG. 11 by the transmissions 62 and 64, respectively (in analogy with FIG. 9, FIG. 11 also shows another scenario where a CSI-RS transmission 60 from TRP#2 is made at the same time Uas the CSI-RS transmission 62 from TRP#1).• In some embodiments, the network node 16 compensates the phase rotation,

[0192]

[0193] t'i) = (t \)—< Pi(G for CSI-RS #2 from TRP#2, and a phase derotation of the same amount is applied to CSI-RS#2 before transmission, i.e., by multiplying e

[0194]

[0195] xp(— j(^>2(t,1) — )) ~ exp(— j27iA / (t1 / — ) to CSI-RS#2.

[0196] • In an alternative embodiment, the network node 16 may in addition to

[0197]

[0198] t'i) = < P2(t'i) — also phase de-rotate CSI-RS #2 from TRP#2 by <p(t3) - A^(t = (<p2(t3) - < Pi(t3)) - (<?2 (ti) - < Pi(ti)), i e.,

[0199] exp(-j27iA (t3- ti) ).

[0200] In STEP 10-5, the network node 16 receives a CJT CSI report (e.g., the eType-II CJT port-selection PMI report).

[0201] In STEP 10-6, the network node 16 may perform frequency offset compensation and CJT on PDSCH based on received:

[0202] • CSI calibration report (e.g., CJT-C FO report);

[0203] • CJT CSI report (e.g., eType-II CJT port-selection PMI report); and / or • DL-RS transmit time from the reference TRP.

[0204] In some embodiments, the network node 16 performs CJT on PDSCH based on a FO from a CJT-C FO report and a PMI from a CJT CSI report. Using the example in FIG. 11 and let = [1T2T]Tbe the CJT precoder reported by the UE 22, the actual CJT precoder, = [1T2T]T, for PDSCH transmission at ts is computed by the network node 16, where w̃1= w1and w̃2= w2exp(−j2πΔf(t3− t1)).

[0205] exp(−j2πΔf(t3− t1)) is the FO pre-compensation factor. However, in some embodiments, the phase rotation related to exp(−j2πΔf(t3− t1)) may be precompensated in CSI-RS#2 as in the alternative embodiment described above in Step 10-4, whereby there may be no need for the compensation factor exp(−j2πΔf(t3− t1)) in the precoder.

[0206] In some embodiments, the FO pre-compensation factor may be applied to the PDSCH data at TRP#2 before applying the precoder = [1T2T]T.

[0207] Examples

[0208] Some embodiments may include one or more of the following:

[0209] Example 1. A method in a network node 16 to compensate frequency offset (FO) between multiple TRPs in CSI feedback based CJT operation, the method comprising:a. configuring a UE 22 with higher layer configurations for frequency offset reporting between the multiple TRPs, and for CSI / PMI reporting for CJT;

[0210] b. receiving a report of CSI / PMI for CJT CSI (e.g., eType-II CSI / PMI report) and a CSI calibration report on frequency offset (e.g., CSI CJT-C FO report);

[0211] c. performing FO compensation and CJT on PDSCH based on DL-RS configurations and received information in step b.

[0212] Example 2. Example 1 and where the CSI / PMI reporting for CJT may be based on one of

[0213] a. enhanced Type II (eType-II) codebook for CJT b. or, further enhanced Type II port selection (feType-II PS) codebook for CJT.

[0214] Example 3. Example 1 and where the DL-RSs (e.g., TRSs, CSI-RSs) may be configured with different time resources, where time difference between the DL-RSs may be in symbol-level or slot-level.

[0215] Example 4. Example 1 and where the network node 16 actions to compensate frequency offset between the TRPs in deriving CJT precoder will be based on one or more of

[0216] a. time resource difference in symbol-level or slot-level b. CSI / PMI reporting for CJT

[0217] c. knowledge of the reference resource used to measure CSI / PMI report for CJT

[0218] d. knowledge of downlink numerology or subcarrier spacing

[0219] Example 5. Example 1 and where based on UE 22 reported frequency offset, the network node 16 may derive a first phase compensation component to handle phase rotation between CSI-RS reception timing and PDSCH transmission timing, and a second phase compensation component to handle phase rotation between CSI-RS transmissions.

[0220] Overview regarding network compensation for frequency offset

[0221] As described above with reference to Figures 6-11, there are different ways for network nodes to compensate for frequency offsets. These ideas will be summarized below with reference to Figure 12.Figure 12 is a flow chart of a method 1200 implemented in a network node, such as the network node 16. The network node is configured to communicate with a user equipment (UE), such as the UE 22.

[0222] The method 1200 comprises transmitting 1210 downlink reference signals from first and second transmission sources. An example of the transmission sources is provided by the TRPs 31 and 32 in Figure 3. Examples of the transmission 1210 of downlink reference signals are provided by S10 in Figure 6, by STEP 7-2 in Figure 7, and by STEP 10-2 and STEP 10-4 in Figure 10.

[0223] The method 1200 comprises receiving 1220 one or more reports from the UE. The one more reports indicate channel state information for coherent joint transmission on a physical downlink channel from the first and second transmission sources. The one or more reports also indicate a frequency offset between the first and second transmission sources. The one or more reports are based on measurements of the downlink reference signals from the first and second transmission sources. Examples of the reception 1220 are provided by S12 in Figure 6, by STEP 7-3 in Figure 7, and by STEP 10-3 and STEP 10-5 in Figure 10.

[0224] The method 1200 comprises performing 1230 joint data transmission on the physical downlink channel from the first and second transmission sources using a precoder which is based on the indicated channel state information. Examples of the joint data transmission are provided by the joint data transmission referred to in S 14 in Figure 6, the joint PDSCH transmission at t3 in Figure 8 and the joint PDSCH transmission at t3 in Figure 11.

[0225] In the method 1200, the network node performs frequency offset compensation. In some embodiments, performing the frequency offset compensation comprises performing the joint data transmission, wherein the precoder used for the joint data transmission is further based on the indicated frequency offset. An example of using such a precoder is described above with reference to STEP 7-4 in Figure 7.

[0226] In some embodiments, performing the frequency offset compensation comprises pre-compensating at least one of the downlink reference signals based on the indicated frequency offset. In such embodiments, the frequency offset is indicated by a first received report which is based on measurements on other downlink reference signals from the first and second transmission sources than the at least one pre-compensated downlink reference signal. In such embodiments, the channel state information is indicated by a second received report which is based on measurements on downlinkreference signals from the first and second transmission sources including the at least one pre-compensated downlink reference signal. An example of using such precompensated downlink reference signals is provided in STEP 10-4 in Figure 10.

[0227] Examples of the first and second reports are provided in STEP 10-3 and STEP 10-5 in Figure 10.

[0228] In some embodiments, the precoder used for the joint data transmission is based on the indicated channel state information as well as the indicated frequency offset. In some embodiments, the precoder is compensated with a first phase compensation component based on the indicated frequency offset and a time difference between transmission of downlink reference signals on which the indicated channel state information is based and the joint data transmission. In some embodiments, the first phase compensation component comprises a factor exp(−j2πΔf(t3− t1)), where Δf is the indicated frequency offset and (t3− t1) is the time difference between transmission of downlink reference signals on which the indicated channel state information is based and the joint data transmission.

[0229] In some embodiments, the precoder is compensated with a second phase compensation component based on the indicated frequency offset and a time difference between transmission of a first downlink reference signal from the first transmission source and transmission of a second downlink reference signal from the second transmission source, wherein the indicated channel state information is based on measurements on the first and second downlink reference signals. In some embodiments, the second phase compensation component comprises a factor exp(j2πΔf(t1′ − t1)), where Δf is the indicated frequency offset and (t1′ − t1) is the time difference between transmission of the first downlink reference signal from the first transmission source and transmission of the second downlink reference signal from the second transmission source.

[0230] In some embodiments, the network node pre-compensates at least one of the downlink reference signals based on the indicated frequency offset, wherein a downlink reference signal is pre-compensated with a first phase compensation component based on the indicated frequency offset and a time difference between transmission of downlink reference signals on which the indicated channel state information is based and the joint data transmission. In some embodiments, the first phase compensation component comprises a factor exp(−j2πΔf(t3− t1)), where Δf is the indicated frequency offset and (t3− t1) is the time difference between transmission of downlink reference signals on which the indicated channel state information is based and the joint data transmission.In some embodiments, the network node pre-compensates at least one of the downlink reference signals based on the indicated frequency offset, wherein a downlink reference signal is pre-compensated with a second phase compensation component based on the indicated frequency offset and a time difference between transmission of a first downlink reference signal from the first transmission source and transmission of a second downlink reference signal from the second transmission source, wherein the indicated channel state information is based on measurements on the first and second downlink reference signals. In some embodiments, the second phase compensation component comprises a factor exp(−j2πΔf(t1′ − t1)), where Δf is the indicated frequency offset and (t1′ − t1) is the time difference between transmission of the first downlink reference signal from the first transmission source and transmission of the second downlink reference signal from the second transmission source.

[0231] In some embodiments, the channel state information indicates a precoder for the joint data transmission, and the network node compensates the indicated precoder based on the indicated frequency offset. In some embodiments, the network node compensates the indicated precoder based on a phase drift, caused by the frequency offset, between a time of a measurement on a first downlink reference signal transmitted from the first transmission source and a time of a measurement on a second downlink reference signal transmitted from the second transmission source. In some embodiments, the network node compensates the indicated precoder based on a phase drift, caused by the frequency offset, between a time of a measurement on a downlink reference signal and the joint data transmission. In some embodiments, a time of a measurement on a downlink reference signal is determined based on a slot or symbol of a transmission of the downlink reference signal.

[0232] In some embodiments, the method 1200 further comprises configuring the UE with resources in which the downlink reference signals are to be transmitted, wherein the configuration indicates that a first downlink reference signal from the first transmission source is to be transmitted at a different time than a second downlink reference signal from the second transmission source, wherein the indicated channel state information is based on measurements on the first and second downlink reference signals. In some embodiments, the frequency offset compensation is based on the configured resources in which the downlink reference signals are to be transmitted.

[0233] In some embodiments, the downlink reference signals are channel state information reference signals (CSI-RS).In some embodiments, the physical downlink channel is a physical downlink shared channel (PDSCH).

[0234] In some embodiments, the method 1200 further comprises configuring the UE for frequency offset reporting.

[0235] Further description

[0236] As will be appreciated by one of skill in the art, the concepts described herein may be embodied as a method, data processing system, computer program product and / or computer storage media storing an executable computer program. Accordingly, the concepts described herein may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects all generally referred to herein as a “circuit” or “module.” Any process, step, action and / or functionality described herein may be performed by, and / or associated to, a corresponding module, which may be implemented in software and / or firmware and / or hardware. Furthermore, the disclosure may take the form of a computer program product on a tangible computer usable storage medium having computer program code embodied in the medium that can be executed by a computer. Any suitable tangible computer readable medium may be utilized including hard disks, CD-ROMs, electronic storage devices, optical storage devices, or magnetic storage devices.

[0237] Some embodiments are described herein with reference to flowchart illustrations and / or block diagrams of methods, systems and computer program products. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer (to thereby create a special purpose computer), special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions / acts specified in the flowchart and / or block diagram block or blocks.

[0238] These computer program instructions may also be stored in a computer readable memory or storage medium that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer readable memory produce an article of manufacture including instructionmeans which implement the function / act specified in the flowchart and / or block diagram block or blocks.

[0239] The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions / acts specified in the flowchart and / or block diagram block or blocks.

[0240] It is to be understood that the functions / acts noted in the blocks may occur out of the order noted in the operational illustrations. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality / acts involved. Although some of the diagrams include arrows on communication paths to show a primary direction of communication, it is to be understood that communication may occur in the opposite direction to the depicted arrows.

[0241] Computer program code for carrying out operations of the concepts described herein may be written in an object oriented programming language such as Python, Java® or C++. However, the computer program code for carrying out operations of the disclosure may also be written in conventional procedural programming languages, such as the " C" programming language. The program code may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer. In the latter scenario, the remote computer may be connected to the user's computer through a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider).

[0242] Many different embodiments have been disclosed herein, in connection with the above description and the drawings. It will be understood that it would be unduly repetitious and obfuscating to literally describe and illustrate every combination and subcombination of these embodiments. Accordingly, all embodiments can be combined in any way and / or combination, and the present specification, including the drawings, shall be construed to constitute a complete written description of all combinations and subcombinations of the embodiments described herein, and of the manner and process ofmaking and using them, and shall support claims to any such combination or subcombination.

[0243] It will be appreciated by persons skilled in the art that the embodiments described herein are not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings.

[0244] Example Embodiments:

[0245] Embodiment Al. A network node configured to communicate with a user equipment (UE), the network node configured to, and / or comprising a radio interface and / or comprising processing circuitry configured to:

[0246] transmit downlink reference signals from a plurality of transmission sources; receive from the UE a calibration report on one or more frequency offsets (FOs), the calibration report being based at least in part on measurements by the UE of the downlink reference signals; and

[0247] perform FO compensation for joint data transmission on a physical downlink channel from at least two of the plurality of transmission sources, the FO compensation being based at least in part on the received calibration report.

[0248] Embodiment A2. The network node of Embodiment Al, wherein at least two downlink reference signals of the downlink reference signals is configured with a different time resource.

[0249] Embodiment A3. The network node of any of Embodiment Al and A2, wherein performing the FO compensation comprises using a coherent joint transmission (CJT) precoder for performing the joint data transmission, the CJT precoder being based at least in part on measurements by the UE of the downlink reference signals on different time resources.

[0250] Embodiment A4. The network node of Embodiment A3, wherein the CJT precoder is compensated with a first phase compensation component based at least in part on an FO and a time difference between downlink reference signal transmission and physical downlink channel transmission.

[0251] Embodiment A5. The network node of any of Embodiment A3 and A4, wherein the CJT precoder is compensated with a second phase compensation component based at least in part on an FO and a time of a first downlink reference signal transmissionfrom a first transmission source relative to a second downlink reference signal transmitted from a second transmission source.

[0252] Embodiment A6. The network node of any of Embodiments A3-A5, wherein the FO compensation of the CJT precoder is based at least in part on a phase drift between measurements on the downlink reference signals at different measurement times.

[0253] Embodiment A7. The network node of Embodiment A6, wherein the phase drift is based at least in part on a reported FO.

[0254] Embodiment A8. The network node of any of Embodiments A6 and A7, wherein the phase drift is based at least in part on a time difference between two different times of the measurement times.

[0255] Embodiment A9. The network node of Embodiment A8, wherein the two different times of the measurement times are determined based at least in part on a slot or symbol of the joint data transmission on the physical downlink channel or a downlink reference signal transmission.

[0256] Embodiment A10. The network node of any of Embodiments A1-A9, wherein at least two transmission sources of the plurality of transmission sources is a transmissionreception point (TRP) operating at a different frequency.

[0257] Embodiment All. The network node of any of Embodiments Al -Al 0, wherein each downlink reference signal includes a channel state information reference signal (C SIRS).

[0258] Embodiment A12. The network node of any of Embodiments Al -Al 1, wherein the physical downlink channel includes a physical downlink shared channel (PDSCH).

[0259] Embodiment A13. The network node of any of Embodiments A1-A12, wherein the network node, radio interface and / or processing circuitry are configured to configure the UE for FO reporting.

[0260] Embodiment Bl. A method implemented in a network node that is configured to communicate with a user equipment (UE), the method comprising:

[0261] transmitting downlink reference signals from a plurality of transmission sources; receiving from the UE a calibration report on one or more frequency offsets (FOs), the calibration report being based at least in part on measurements by the UE of the downlink reference signals; and

[0262] performing FO compensation for joint data transmission on a physical downlink channel from at least two of the plurality of transmission sources, the FO compensation being based at least in part on the received calibration report.Embodiment B2. The method of Embodiment Bl, wherein at least two downlink reference signals of the downlink reference signals is configured with a different time resource.

[0263] Embodiment B3. The method of any of Embodiment Bl and B2, wherein performing the FO compensation comprises using a coherent joint transmission (CJT) precoder for performing the joint data transmission, the CJT being based at least in part on measurements by the UE of the downlink reference signals on different time resources.

[0264] Embodiment B4. The method of Embodiment B3, wherein the CJT precoder is compensated with a first phase compensation component based at least in part on an FO and a time difference between downlink reference signal transmission and physical downlink channel transmission.

[0265] Embodiment B5. The method of any of Embodiment B3 and B4, wherein the CJT precoder is compensated with a second phase compensation component based at least in part on an FO and a time of a first downlink reference signal transmission from a first transmission source relative to a second downlink reference signal transmitted from a second transmission source.

[0266] Embodiment B6. The method of any of Embodiments B3-B5, wherein the FO compensation of the CJT precoder is based at least in part on a phase drift between measurements on the downlink reference signals at different measurement times.

[0267] Embodiment B7. The method of Embodiment B6, wherein the phase drift is based at least in part on a reported FO.

[0268] Embodiment B8. The method of any of Embodiments B6 and B7, wherein the phase drift is based at least in part on a time difference between two different times of the measurement times.

[0269] Embodiment B9. The method of Embodiment B8, wherein the two different times of the measurement times are determined based at least in part on a slot or symbol of the joint data transmission on the physical downlink channel or a downlink reference signal transmission.

[0270] Embodiment BIO. The method of any of Embodiments B1-B9, wherein at least two transmission sources of the plurality of transmission source is a transmissionreception point (TRP) operating at a different frequency.

[0271] Embodiment B 11. The method of any of Embodiments B 1 -B 10, wherein each downlink reference signal includes a channel state information reference signal (CSI-RS).Embodiment B 12. The method of any of Embodiments B 1 -B 11, wherein the physical downlink channel includes a physical downlink shared channel (PDSCH).

[0272] Embodiment Bl 3. The method of any of Embodiments Bl -Bl 2, further comprising configuring the UE for FO reporting.

Claims

CLAIMS1. A method (1200) implemented in anetwork node (16) that is configured to communicate with a user equipment, UE (22), the method comprising:transmitting (1210) downlink reference signals from first and second transmission sources (31, 32);receiving (1220) one or more reports from the UE, the one more reports indicating channel state information for coherent joint transmission on a physical downlink channel from the first and second transmission sources, the one or more reports also indicating a frequency offset between the first and second transmission sources, the one or more reports being based on measurements of the downlink reference signals from the first and second transmission sources; andperforming (1230) joint data transmission on the physical downlink channel from the first and second transmission sources using a precoder which is based on the indicated channel state information,wherein the network node performs frequency offset compensation which comprises: performing the joint data transmission, wherein the precoder used for the joint data transmission is further based on the indicated frequency offset; and / orpre-compensating at least one of the downlink reference signals based on the indicated frequency offset, wherein the frequency offset is indicated by a first received report which is based on measurements on other downlink reference signals from the first and second transmission sources than the at least one pre-compensated downlink reference signal, and wherein the channel state information is indicated by a second received report which is based on measurement on downlink reference signals from the first and second transmission sources including the at least one pre-compensated downlink reference signal.

2. The method of claim 1, wherein the precoder used for the joint data transmission is based on the indicated channel state information as well as the indicated frequency offset, wherein the precoder is compensated with a first phase compensation component based on the indicated frequency offset and a time difference between transmission of downlink reference signals on which the indicated channel state information is based and the joint data transmission.

3. The method of claim 2, wherein the first phase compensation component comprises a factor exp(— j27iA (t3— t^), where A is the indicated frequency offset and (t3— t is the time difference between transmission of downlink reference signals on which the indicated channel state information is based and the joint data transmission.

4. The method of any of claims 2-3, wherein the precoder is compensated with a second phase compensation component based on the indicated frequency offset and a time difference between transmission of a first downlink reference signal from the first transmission source and transmission of a second downlink reference signal from the second transmission source, wherein the indicated channel state information is based on measurements on the first and second downlink reference signals.

5. The method of claim 4, wherein the second phase compensation component comprises a factor exp( / 27iA (t1 / — t ), where A is the indicated frequency offset and (t^ — t is the time difference between transmission of the first downlink reference signal from the first transmission source and transmission of the second downlink reference signal from the second transmission source.

6. The method of claim 1, wherein the network node pre-compensates at least one of the downlink reference signals based on the indicated frequency offset, wherein a downlink reference signal is pre-compensated with a first phase compensation component based on the indicated frequency offset and a time difference between transmission of downlink reference signals on which the indicated channel state information is based and the joint data transmission.

7. The method of claim 6, wherein the first phase compensation component comprises a factor exp(— j27iA (t3— tx)), where A is the indicated frequency offset and (t3— t is the time difference between transmission of downlink reference signals on which the indicated channel state information is based and the joint data transmission.

8. The method of any of claims 2-3 or 6-7, wherein the network node pre-compensates at least one of the downlink reference signals based on the indicated frequency offset, wherein a downlink reference signal is pre-compensated with a second phase compensation component based on the indicated frequency offset and a time differencebetween transmission of a first downlink reference signal from the first transmission source and transmission of a second downlink reference signal from the second transmission source, wherein the indicated channel state information is based on measurements on the first and second downlink reference signals.

9. The method of claim 8, wherein the second phase compensation component comprises a factor exp(— j27iA (t1 / —), where A is the indicated frequency offset and (t^ — t is the time difference between transmission of the first downlink reference signal from the first transmission source and transmission of the second downlink reference signal from the second transmission source.

10. The method of any of the preceding claims, wherein the channel state information indicates a precoder for the joint data transmission, and wherein the network node compensates the indicated precoder based on the indicated frequency offset.

11. The method of claim 10, wherein the network node compensates the indicated precoder based on a phase drift, caused by the frequency offset, between a time of a measurement on a first downlink reference signal transmitted from the first transmission source and a time of a measurement on a second downlink reference signal transmitted from the second transmission source.

12. The method of any of claims 10-11, wherein the network node compensates the indicated precoder based on a phase drift, caused by the frequency offset, between a time of a measurement on a downlink reference signal and the joint data transmission.

13. The method of any of claims 11-12, wherein a time of a measurement on a downlink reference signal is determined based on a slot or symbol of a transmission of the downlink reference signal.

14. The method of any of the preceding claims, further comprising:configuring the UE with resources in which the downlink reference signals are to be transmitted, wherein the configuration indicates that a first downlink reference signal from the first transmission source is to be transmitted at a different time than a second downlink reference signal from the second transmission source, wherein the indicatedchannel state information is based on measurements on the first and second downlink reference signals.

15. The method of claim 14, wherein the frequency offset compensation is based on the configured resources in which the downlink reference signals are to be transmitted.

16. The method of any of the preceding claims, wherein:the downlink reference signals are channel state information reference signals, CSI-RS; and / orthe physical downlink channel is a physical downlink shared channel, PDSCH.

17. The method of any of the preceding claims, further comprising:configuring the UE for frequency offset reporting.

18. A network node (16) configured to communicate with a user equipment, UE (22), wherein the network node is configured to:transmit (1210) downlink reference signals from first and second transmission sources (31, 32);receive (1220) one or more reports from the UE, the one more reports indicating channel state information for coherent joint transmission on a physical downlink channel from the first and second transmission sources, the one or more reports also indicating a frequency offset between the first and second transmission sources, the one or more reports being based on measurements of the downlink reference signals from the first and second transmission sources; andperform (1230) joint data transmission on the physical downlink channel from the first and second transmission sources using a precoder which is based on the indicated channel state information,wherein the network node is configured to perform frequency offset compensation which comprises:performing the joint data transmission, wherein the precoder used for the joint data transmission is further based on the indicated frequency offset; and / orpre-compensating at least one of the downlink reference signals based on the indicated frequency offset, wherein the frequency offset is indicated by a first received report which is based on measurements on other downlink reference signals from the firstand second transmission sources than the at least one pre-compensated downlink reference signal, and wherein the channel state information is indicated by a second received report which is based on measurement on downlink reference signals from the first and second transmission sources including the at least one pre-compensated downlink reference signal.

19. The network node of claim 18, wherein the precoder used for the joint data transmission is based on the indicated channel state information as well as the indicated frequency offset, wherein the precoder is compensated with a first phase compensation component based on the indicated frequency offset and a time difference between transmission of downlink reference signals on which the indicated channel state information is based and the joint data transmission.

20. The network node of claim 19, wherein the first phase compensation component comprises a factor exp(— j27iA (t3— tx)), where A is the indicated frequency offset and (t3— tx) is the time difference between transmission of downlink reference signals on which the indicated channel state information is based and the joint data transmission.

21. The network node of any of claims 19-20, wherein the precoder is compensated with a second phase compensation component based on the indicated frequency offset and a time difference between transmission of a first downlink reference signal from the first transmission source and transmission of a second downlink reference signal from the second transmission source, wherein the indicated channel state information is based on measurements on the first and second downlink reference signals.

22. The network node of claim 21, wherein the second phase compensation component comprises a factor exp (j 2 nA (t — t ), where A is the indicated frequency offset and (t — tx) is the time difference between transmission of the first downlink reference signal from the first transmission source and transmission of the second downlink reference signal from the second transmission source.

23. The network node of claim 18, wherein the network node is configured to precompensate at least one of the downlink reference signals based on the indicated frequency offset, wherein a downlink reference signal is pre-compensated with a firstphase compensation component based on the indicated frequency offset and a time difference between transmission of downlink reference signals on which the indicated channel state information is based and the joint data transmission.

24. The network node of claim 23, wherein the first phase compensation component comprises a factor exp(−j2πΔf(t3− t1)), where Δf is the indicated frequency offset and (t3− t1) is the time difference between transmission of downlink reference signals on which the indicated channel state information is based and the joint data transmission.

25. The network node of any of claims 19-20 or 23-24, wherein the network node is configured to pre-compensate at least one of the downlink reference signals based on the indicated frequency offset, wherein a downlink reference signal is pre-compensated with a second phase compensation component based on the indicated frequency offset and a time difference between transmission of a first downlink reference signal from the first transmission source and transmission of a second downlink reference signal from the second transmission source, wherein the indicated channel state information is based on measurements on the first and second downlink reference signals.

26. The network node of claim 25, wherein the second phase compensation component comprises a factor exp(— j27iA (t1 / — t ), where A is the indicated frequency offset and (t^ — tx) is the time difference between transmission of the first downlink reference signal from the first transmission source and transmission of the second downlink reference signal from the second transmission source.

27. The network node of any of claims 18-26, wherein the channel state information indicates a precoder for the joint data transmission, and wherein the network node is configured to compensate the indicated precoder based on the indicated frequency offset.

28. The network node of claim 27, wherein the network node is configured to compensate the indicated precoder based on a phase drift, caused by the frequency offset, between a time of a measurement on a first downlink reference signal transmitted from the first transmission source and a time of a measurement on a second downlink reference signal transmitted from the second transmission source.

29. The network node of any of claims 27-28, wherein the network node is configured to compensate the indicated precoder based on a phase drift, caused by the frequency offset, between a time of a measurement on a downlink reference signal and the joint data transmission.

30. The network node of any of claims 28-29, wherein a time of a measurement on a downlink reference signal is determined based on a slot or symbol of a transmission of the downlink reference signal.

31. The network node of any of claims 18-30, further configured to:configure the UE with resources in which the downlink reference signals are to be transmitted, wherein the configuration indicates that a first downlink reference signal from the first transmission source is to be transmitted at a different time than a second downlink reference signal from the second transmission source, wherein the indicated channel state information is based on measurements on the first and second downlink reference signals.

32. The network node of claim 31, wherein the frequency offset compensation is based on the configured resources in which the downlink reference signals are to be transmitted.

33. The network node of any of claims 18-32, wherein:the downlink reference signals are channel state information reference signals, CSI-RS; and / orthe physical downlink channel is a physical downlink shared channel, PDSCH.

34. The network node of any of claims 18-33, further configured to:configure the UE for frequency offset reporting.

35. A memory (40) storing instructions which, when executed by processing circuitry (36) of a network node (16), cause the processing circuitry to perform the method of any of claims 1-17.

Images