Patent Description:
The present disclosure relates to Channel State Information (CSI) feedback in a wireless communication system.

The next generation mobile wireless communication system (Fifth Generation (<NUM>)), or New Radio (NR), will support a diverse set of use cases and a diverse set of deployment scenarios. The later includes deployment at both low frequencies (e.g., below <NUM> Gigahertz (GHz)) and very high frequencies (e.g., up to tens of GHz).

As in Long Term Evolution (LTE), NR uses Cyclic Prefix Orthogonal Frequency Division Multiplexing (CP-OFDM) in the downlink from a network node (e.g., a NR base station (gNB), an enhanced or evolved Node B (eNB), or a base station) to a User Equipment (UE) and both CP-OFDM and Discrete Fourier Transform (DFT)-Spread Orthogonal Frequency Division Multiplexing (DFT-S-OFDM) in the uplink from a UE to a network node (e.g., gNB, eNB, or base station). In the time domain, NR downlink and uplink are organized into equally-sized subframes of <NUM> millisecond (ms) each. A subframe is further divided into multiple slots of equal duration. The slot length depends on subcarrier spacing. For a subcarrier spacing of Δf = <NUM> kilohertz (kHz), there is only one slot per subframe and each slot consists of fourteen Orthogonal Frequency Division Multiplexing (OFDM) symbols.

Data scheduling in NR can be on a slot basis, as in LTE. <FIG> illustrates an example NR time-domain structure with <NUM> subcarrier spacing with a <NUM> symbol slot, where the first two symbols contain Physical Downlink Control Channel (PDCCH) and the rest contain Physical Data Channel (PDCH), i.e., either Physical Downlink Shared Channel (PDSCH) or Physical Uplink Shared Channel (PUSCH).

Different subcarrier spacing values are supported in NR. The supported subcarrier spacing values (also referred to as different numerologies) are given by Δf = (<NUM> × <NUM>a) kHz where α is a non-negative integer. Δf = <NUM> is the basic subcarrier spacing that is also used in LTE. The slot durations at different subcarrier spacings are shown in Table <NUM>.

In the frequency domain, a system bandwidth is divided into Resource Blocks (RBs), each corresponds to twelve contiguous subcarriers. The RBs are numbered starting with <NUM> from one end of the system bandwidth. The basic NR physical time-frequency resource grid is illustrated in <FIG>, where only one RB within a <NUM> symbol slot is shown. One OFDM subcarrier during one OFDM symbol interval forms one Resource Element (RE).

Downlink transmissions are dynamically scheduled, i.e., in each slot the gNB transmits Downlink Control Information (DCI) over PDCCH about which UE data is to be transmitted to and which RBs in the current downlink slot the data is transmitted on. PDCCH is typically transmitted in the first one or two OFDM symbols in each slot in NR. The UE data is carried on PDSCH. A UE first detects and decodes PDCCH and, upon decoding the PDCCH successfully, it then decodes the corresponding PDSCH based on the decoded DCI in the PDCCH.

Uplink data transmissions are also dynamically scheduled using PDCCH. Similar to downlink, a UE first decodes uplink grants in PDCCH and then transmits data over PUSCH based the decoded control information in the uplink grant such as modulation order, coding rate, uplink resource allocation, etc..

Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance can be improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a Multiple Input Multiple Output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO.

A core component in NR is the support of MIMO antenna deployments and MIMO related techniques. Spatial multiplexing is one of the MIMO techniques used to achieve high data rates in favorable channel conditions. An illustration of the spatial multiplexing operation is provided in <FIG>.

As seen, the information carrying symbol vector s = [s<NUM>,s<NUM>,. , sr]T is multiplied by an NT x r precoder matrix W, which serves to distribute the transmit energy in a subspace of the NT (corresponding to NT antenna ports) dimensional vector space. The precoder matrix is typically selected from a codebook of possible precoder matrices, and typically indicated by means of a Precoder Matrix Indicator (PMI), which specifies a unique precoder matrix in the codebook for a given number of symbol streams. The rsymbols in the symbol vector s each correspond to a MIMO layer, and r is referred to as the transmission rank. In this way, spatial multiplexing is achieved since multiple symbols can be transmitted simultaneously over the same time and frequency RE. The number of symbols ris typically adapted to suit the current channel properties.

The received signal at a UE with NR receive antennas at a certain RE n is given by <MAT> where yn is a NR × <NUM> received signal vector, Hn is a NR × NT channel matrix at the RE, and en is a NR × <NUM> noise and interference vector received at the RE by the UE. The precoder W can be a wideband precoder, which is constant over frequency, or frequency selective, i.e. different over frequency.

The precoder matrix is often chosen to match the characteristics of the NR x NT MIMO channel matrix Hn, resulting in so-called channel dependent precoding. This is also commonly referred to as closed-loop precoding and essentially strives for focusing the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to the UE. In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that, after proper linear equalization at the UE, the inter-layer interference is reduced.

The transmission rank, and thus the number of spatially multiplexed layers, is reflected in the number of columns of the precoder. The transmission rank is also dependent on the Signal to Interference plus Noise Ratio (SINR) observed at the UE. Typically, a higher SINR is required for transmissions with higher ranks. For efficient performance, it is important that a transmission rank that matches the channel properties as well as the interference is selected. The precoding matrix, the transmission rank, and the channel quality are part of Channel State Information (CSI), which is typically measured by a UE and fed back to a network node or gNB.

When all the data layers are transmitted to one UE, it is referred to as SU-MIMO. On the other hand, when the data layers are transmitted to multiple UEs, it is referred to as MU-MIMO. MU-MIMO is possible when, for example, two UEs are in different areas of a cell such that they can be separated through different precoders (or beamforming) at the gNB, the two UEs may be served on the same time-frequency resources (i.e. Physical RBs (PRBs)) by using different precoders or beams.

NR data transmission over multiple MIMO layers is shown in <FIG>. Depending on the total number of MIMO layers or the rank, either one codeword or two codewords is used. One codeword is used when the total number of layers is equal or less than four, two codewords are used when the number of layers is more than four. Each codeword contains the encoded data bits of a Transport Block (TB). After bit level scrambling, the scrambled bits are mapped to complex-valued modulation symbols <MAT> for codeword q. The complex-valued modulation symbols are then mapped onto the layers x(i) = [x(<NUM>)(i). x(v-<NUM>)(i)]T, <MAT>, according to Table <NUM>. <NUM>-<NUM> (which is copied below) of Third Generation Partnership Project (3GPP) Technical Specification (TS) <NUM> v15. <NUM>, where v is the number of layers and <MAT> is the number of modulation symbols per layer.

For demodulation purposes, a Demodulation Reference Signal (DMRS), also referred to as a DMRS port, is transmitted along each data layer. The block of vectors [x(<NUM>)(i). x(v-<NUM>)(i)]T, <MAT> is mapped to DMRS antenna ports according to <MAT> where <MAT>. The set of DMRS antenna ports {[p<NUM>,. , pv-<NUM>} and port to layer mapping are dynamically indicated in DCI according to Tables <NUM>. <NUM>-<NUM>/<NUM>/<NUM>/<NUM> in 3GPP TS <NUM> v15.

For CSI feedback, NR has adopted an implicit CSI mechanism where a UE feeds back the downlink CSI which typically includes a transmission Rank Indicator (RI), a PMI, and Channel Quality Indicator (CQI) for each codeword. The CQI/RI/PMI report can be either wideband or subband based on configuration.

The RI corresponds to a recommended number of layers that are to be spatially multiplexed and thus transmitted in parallel over the effective channel. The PMI identifies a recommended precoding matrix to use. The CQI represents a recommended modulation level (i.e., Quadrature Phase Shift Keying (QPSK), <NUM> Quadrature Amplitude Modulation (QAM), etc.) and coding rate for each codeword or TB. NR supports transmission of one or two codewords to a UE in a slot. There is thus a relation between a CQI and an SINR of the spatial layers over which the codewords are transmitted.

For CSI measurement and feedback, CSI-RSs are defined. A CSI-RS is transmitted on each transmit antenna (or antenna port) and is used by a UE to measure the downlink channel between each of the transmit antenna ports and each of its receive antenna ports. The antenna ports are also referred to as CSI-RS ports. The supported number of antenna ports in NR are {<NUM>,<NUM>,<NUM>,<NUM>,<NUM>,<NUM>,<NUM>,<NUM>}. By measuring the received CSI-RS, a UE can estimate the channel that the CSI-RS is traversing, including the radio propagation channel and antenna gains. The CSI-RS for the above purpose is also referred to as Non-Zero Power (NZP) CSI-RS.

CSI-RS can be configured to be transmitted in certain REs in a slot and certain slots. <FIG> shows an example of CSI-RS REs for twelve (<NUM>) antenna ports, where one RE per RB per port is shown.

In addition, Interference Measurement Resource (IMR) is also defined in NR for a UE to measure interference. An IMR contains four REs, i.e., either four adjacent REs in frequency in the same OFDM symbol or a two-by-two grid of adjacent REs in both time and frequency in a slot. By measuring both the channel based on NZP CSI-RS and the interference based on an IMR, a UE can estimate the effective channel and noise plus interference to determine the CSI, i.e. rank, precoding matrix, and the channel quality.

Furthermore, a UE in NR may be configured to measure interference based on one or multiple NZP CSI-RS resources.

In NR, a UE can be configured with multiple CSI reporting settings and multiple CSI-RS resource settings. Each resource setting can contain multiple resource sets, and each resource set can contain up to eight CSI-RS resources. For each CSI reporting setting, a UE feeds back a CSI report.

In each CSI reporting setting, it contains at least the following information:.

When the CSI-RS resource set in a CSI reporting setting contains multiple CSI-RS resources, one of the CSI-RS resources is selected by a UE and a CRI is also reported by the UE to indicate to the gNB about the selected CSI-RS resource in the resource set, together with RI, PMI, and CQI associated with the selected CSI-RS resource.

For aperiodic CSI reporting in NR, more than one CSI reporting setting, each with a different CSI-RS resource set for channel measurement and/or resource set for interference measurement, can be configured and triggered at the same time. In this case, multiple CSI reports are aggregated and sent from the UE to the gNB in a single PUSCH.

In NR, DPS is supported in which data for a UE can be sent over different Transmission Points (TRPs) in different slots. In this case, the gNB may request the UE to measure and feedback downlink CSI for each of the TRPs, and the gNB then decides the TRP for data transmission to the UE. Since different TRPs may have different configurations for CSI-RS and other downlink reference signals such as Synchronization Signal Block (SSB), the available REs in a slot may also be different. Furthermore, since the different TRPs may be physically in different locations, the propagation channels to the UE can also be different. To facilitate receiving PDSCH data from different TRPs, a parameter called Transmission Configuration Indicator (TCI) state is signaled to a UE in the corresponding DCI carried on PDCCH. A UE may be configured with multiple TCI states, and one of the configured TCI states is selected and indicated in the DCI. A TCI state contains Quasi Co-location (QCL) information between the DMRS for PDSCH and one or two downlink reference signals such as CSI-RS or SSB. The supported QCL information types in NR are:.

The QCL information is used by a UE to apply large scale channel properties associated with the downlink reference signals (CSI-RS or SSB) to DMRS based channel estimation for PDSCH reception.

NC-JT refers to MIMO data transmission over multiple TRPs in which different MIMO layers are transmitted over different TRPs. An example is shown in <FIG>, where a PDSCH is sent to a UE over two TRPs, each carrying one codeword. When the UE has four receive antennas while each of the TRPs has only two transmit antennas, the UE can support up to four MIMO layers but there is a maximum of two MIMO layers from each TRP. In this case, by transmitting data over two TRPs to the UE, the peak data rate to the UE can be increased, as up to four aggregated layers from the two TRPs can be used. This is beneficial when the traffic load, and thus the resource utilization, is low in each TRP. The scheme can also be beneficial in the case where the UE is in Line of Sight (LOS) of both the TRPs and the rank per TRP is limited even when there are more transmit antennas available at each TRP.

This type of NC-JT is supported in LTE with two TRPs, each having up to eight antenna ports. For CSI feedback purpose, a UE is configured with a CSI process with two NZP CSI-RS resources, one for each TRP, and one interference measurement resource. The UE calculates CSI per NZP CSI-RS resource and reports a pair of RIs, (RI1, RI2), a pair of PMIs, (PMI1, PMI2), and a pair of CQIs, (CQI1, CQI2), by considering the mutual interference between the two codewords from the two TRPs. With two codewords, different Modulation and Coding Schemes (MCSs) can be used for the two TRPs. An advanced receiver with Codeword Level Interference Cancellation (CWIC) can be used at the UE. Furthermore, when one codeword is in error, only that codeword needs to be re-transmitted. It also implies that maximum TRPs can be supported as only two codewords are supported in LTE and NR.

An alternative approach is to use a single codeword over multiple TRPs. An example is shown in <FIG>, where different layers are transmitted from three TRPs. This allows data transmission over more than two TRPs.

In one scenario, a gNB may configure a UE with two CSI reporting settings, one for DPS and the other for NC-JT. The UE then feeds back two CSI reports, one for DPS and the other for NC-JT. The gNB can decide whether to use DPS or NC-JT based on other information available at the gNB. In another scenario as discussed in <CIT>, a gNB may configure a UE with N><NUM> NZP CSI-RS resources, each associated with one TRP, in a resource setting as part of a CSI reporting setting for channel measurement, and the UE is allowed to select M = (<NUM>, <NUM>,. ,N) preferred NZP CSI-RS resources. In this case, the UE would feedback a single CSI report consisting of at least an indicator of "Number of Selected Resources", i.e. NSRI, which indicates how many resources are selected along with a set of the selected CRIs, i.e. {CRT<NUM>,. NSRI=<NUM> means that only a single TRP is selected, i.e., DPS transmission is preferred, while NRSI><NUM> indicates that NC-JT transmission is preferred.

In addition to using multi-TRP transmission for improved data throughput such as DPS and NC-JT discussed above (which is referred to herein as spatial multiplexing based multi-TRP transmission), another application of multi-TRP transmission is to provide increased reliability of data transmission, which is important in some mission critical applications such as auto driving or industrial control. In this case, a same data packet may be transmitted over multiple TRPs as shown <FIG>.

Either the same resource or different resources may be used in different TRPs. Soft combining may be performed at the UE. When the same resource is used, then different MIMO layers would be used to carry the same data from different TRPs and a MIMO receiver is needed at the UE to separate the layers. In one scenario, the same codeword with different redundancy versions may be transmitted from different TRPs and soft combining is performed at the UE. In another scenario, the same PDSCH may simply be transmitted over multiple TRPs, which is transparent to the UE. This type of multi-TRP transmission is referred to herein as diversity based multi-TRP transmission.

There currently exist certain challenge(s). In particular, the conventional CSI feedback mechanism (e.g., the CSI feedback mechanisms for LTE) are less than ideal when considering both DPS based multi-TRP transmission and NC-JT and when considering UE selection of M out of N TRPs.

Document "<NPL>, discloses an evaluation of CSI feedback for non-coherent joint transmission. The following observations were made. Observation <NUM>: Both NC-JT and DPS hypotheses need to be supported for multi-TRP CSI feedback. Observation <NUM>: To indicate QCL with corresponding DMRS, each TRP must transmit CSI-RS in separate CSI-RS resources. Observation <NUM>: TRPs participating in NC-JT can be equipped with different antennas and require different codebooks. Observation <NUM>: It is beneficial for the UE to dynamically select how many TRPs shall participate in the NC-JT. Observation <NUM>: As each TRP can transmit beamformed CSI-RS in different CSI-RS resources, CSI-RS resource selection within a TRP must be differentiated from selection of multiple CSI-RS resource corresponding to different TRPs for NC-JT hypothesis. Based on the discussion in the document, extensions of the NR CSI feedback framework to support NC-JT were proposed. Proposal <NUM>: To support multi-TRP CSI feedback, a CSI Report Setting can be linked with more than one Resource Setting for channel measurement: the CSI Report Setting associates each Resource Setting with a precoder codebook; a CSI Report Setting is further configured with a set of hypotheses for channel measurement, wherein each hypothesis selects a subset of the linked Resource Settings for channel measurement and where the UE selects one hypothesis from the set as part of the CSI report; and for the selected Resource Settings, the UE determines PMI, RI and if applicable CRI for each Resource Setting jointly, assuming the layers from CSI-RS resources in different Resource Settings mutually interfere. Proposal <NUM>: For CSI feedback with NC-JT hypothesis, the CSI Report Setting contains information on if single or multiple PDSCH shall be assumed for CQI calculation. Proposal <NUM>: CSI feedback for NC-JT is contained in a single report, and not split up in per-TRP CSI reports on separate PUCCH/PUSCH transmissions.

Systems and methods for Channel State Information (CSI) feedback for multiple Transmission Points (TRPs) in a cellular communications network are disclosed. According to the present disclosure, methods, a wireless device and a base station according to the independent claims are provided. Developments are set forth in the dependent claims.

According to a first aspect of the present disclosure, there is provided a method performed by a wireless device for Channel State Information, CSI, feedback. The method comprises receiving, from a base station, a configuration comprising: N Non-Zero Power, NZP, CSI Reference Signal, CSI-RS, resources, each of the N NZP CSI-RS resources associated with a different one of N Transmission Points, TRPs; or N sets of NZP CSI-RS resources, each of the N sets associated with a different one of the N TRPs; receiving a request for CSI feedback based on the configuration; selecting a preferred subset of the NZP CSI-RS resources comprised in the N NZP CSI-RS resources or the N sets of NZP CSI-RS resources, wherein, when the configuration comprises N NZP CSI-RS resources, the preferred subset of the NZP CSI-RS resources is a subset of the N NZP CSI-RS resources, and when the configuration comprises N sets of NZP CSI-RS resources, the preferred subset of the NZP CSI-RS resources is one or more of the N sets of NZP CSI-RS resources, wherein the preferred subset of the NZP CSI-RS resources has size M, where M<N; wherein selecting the preferred subset of the NZP CSI-RS resources corresponds to selecting M preferred TRPs out of the N TRPs, and wherein M is signaled by the base station to the wireless device; and reporting (<NUM>), to the<NUM> base station, CSI based on the selected preferred subset of the NZP CSI-RS resources, wherein the CSI comprises an identifier of each of the NZP CSI-RS resources in the preferred subset. <NUM> Adaptation to the foregoing amendments.

According to a second aspect of the present disclosure, there is provided a method performed by a base station for Channel State Information, CSI, feedback for Physical Downlink Shared Channel, PDSCH, transmission over multiple Transmission Points, TRPs, in a wireless network. The method comprises providing, to a wireless device, a configuration comprising: N Non-Zero Power, NZP, CSI Reference Signal, CSI-RS, resources, each associated with a different one of N TRPs; or N sets of NZP CSI-RS resources, each associated with a different one of the N TRPs; requesting a CSI feedback from the wireless device based on the configuration; and receiving, from the wireless device (<NUM>), CSI based on the selected preferred subset of the NZP CSI-RS resources comprised in the N NZP CSI-RS resources or the N sets of NZP CSI-RS resources, wherein, when the configuration comprises N NZP CSI-RS resources, the preferred subset of the NZP CSI-RS resources is a subset of the N NZP CSI-RS resources, and when the configuration comprises N sets of NZP CSI-RS resources, the preferred subset of the NZP CSI-RS resources is one or more of the N sets of NZP CSI-RS resources, wherein the preferred subset of the NZP CSI-RS resources has size M, where M<N; wherein selecting the preferred subset of the NZP CSI-RS resources corresponds to selecting M preferred TRPs out of the N TRPs, wherein M is signaled by the base station to the wireless device, and wherein the CSI comprises an identifier of each of the NZP CSI-RS resources in the preferred subset.

According to a third aspect of the present disclosure, there is provided a wireless device for Channel State Information, CSI, feedback, the wireless device comprising one or more transmitters one or more receivers and processing circuitry associated with the one or more transmitters and the one or more receivers. The processing circuitry is configured to cause the wireless device to perform all steps of a method according to the first aspect.

According to a fourth aspect of the present disclosure, there is provided a base station for Channel State Information, CSI, feedback, the base station comprising processing circuitry configured to cause the base station to perform all steps of a method according to the second aspect.

Whenever in the following disclosure any of the above-stated aspects (independent claims) is disclosed as "optional" (e.g. due to usage of conjunctive terms, such as "can", "may", "should" etc.), it is nevertheless to be read as "mandatory".

Hereinabove and in the following, "examples" pertain to principles underlying the claimed subject-matter and/or being useful for understanding the claimed subject-matter, while "embodiments" pertain to the claimed subject-matter within the claim scope.

Whenever in the following disclosure the term "embodiment" occurs, reference is to be made to the figure description above to clarify whether an embodiment or an example is meant.

Radio Access Node or Transmission Point (TRP): As used herein, a "radio access node" or "radio network node" or TRP is any node in a Radio Access Network (RAN) of a cellular communications network that operates to wirelessly transmit and/or receive signals.

There currently exist certain challenge(s). First, only Dynamic Point Selection (DPS) based multi-TRP transmission is currently supported in NR. For Non-Coherent Joint Transmission (NC-JT), although LTE like Channel State Information (CSI) feedback could be similarly used in NR, there are some limitations, e.g., the NC-JT specified in LTE is limited to two TRPs, each with up to eight antenna ports, and TRP selection is done by the eNB, i.e. no dynamic UE selection of two TRPs from more than two TRP candidates. Second, when the UE is allowed to select M out of N TRPs or select one from multiple transmission hypotheses, an issue is that the CSI payload can be different for different values of M or different hypotheses. It would be difficult for the gNB to schedule the right amount of Physical Uplink Shared Channel (PUSCH) resources to carry this CSI payload. In addition, if the PUSCH resource is allocated based on the worst case CSI payload while the actual CSI payload may be smaller, there is an issue on how to determine and decode the actual CSI payload correctly at the gNB.

Certain aspects of the present disclosure and their embodiments may provide solutions to the aforementioned or other challenges. In some embodiments, the solution allows a UE to select M out N TRPs for data transmission, where both M and N are signaled by the base station (e.g., gNB in the case of NR) to the wireless device (e.g., UE), so the CSI payload is deterministic and is known to the base station (e.g., gNB) so the correct amount of PUSCH resources can be scheduled for carrying the CSI.

In some other embodiments, the UE is allowed to select a variable number of TRPs, M<=N, out of N TRPs. Further, in some embodiments, the number of selected TRPs is encoded separately so that when a PUSCH carrying the CSI is received by the base station (e.g., gNB), the base station (e.g., gNB) can decode the CSI correctly.

Certain embodiments may provide one or more of the following technical advantage(s). The solutions allow a UE to select the best TRPs for data transmission over multiple TRPs and feed back the corresponding CSI, while allowing the gNB to decode the CSI feedback correctly without any ambiguity.

<FIG> illustrates one example of a cellular communications network <NUM> according to some embodiments of the present disclosure. In the embodiments described herein, the cellular communications network <NUM> is a <NUM> NR network; however, the embodiments described herein are not limited thereto. In this example, the cellular communications network <NUM> includes base stations <NUM>-<NUM> and <NUM>-<NUM>, which in <NUM> NR are referred to as gNBs, controlling corresponding macro cells <NUM>-<NUM> and <NUM>-<NUM>. The base stations <NUM>-<NUM> and <NUM>-<NUM> are generally referred to herein collectively as base stations <NUM> and individually as base station <NUM>. Likewise, the macro cells <NUM>-<NUM> and <NUM>-<NUM> are generally referred to herein collectively as macro cells <NUM> and individually as macro cell <NUM>. The cellular communications network <NUM> may also include a number of low power nodes <NUM>-<NUM> through <NUM>-<NUM> controlling corresponding small cells <NUM>-<NUM> through <NUM>-<NUM>. The low power nodes <NUM>-<NUM> through <NUM>-<NUM> can be small base stations (such as pico or femto base stations) or Remote Radio Heads (RRHs), or the like. Notably, while not illustrated, one or more of the small cells <NUM>-<NUM> through <NUM>-<NUM> may alternatively be provided by the base stations <NUM>. The low power nodes <NUM>-<NUM> through <NUM>-<NUM> are generally referred to herein collectively as low power nodes <NUM> and individually as low power node <NUM>. Likewise, the small cells <NUM>-<NUM> through <NUM>-<NUM> are generally referred to herein collectively as small cells <NUM> and individually as small cell <NUM>. The base stations <NUM> (and optionally the low power nodes <NUM>) are connected to a core network <NUM>.

In some embodiments, a UE (e.g., wireless device <NUM>) selects M out of N TRPs with M being signaled to the UE.

More specifically, in some embodiments, a UE is either configured with N Non-Zero Power (NZP) CSI Reference Signal (CSI-RS) resource settings in a CSI reporting setting for channel measurement, each associated with one of N TRPs, or configured with N CSI-RS resources within an NZP CSI-RS resource setting in a CSI reporting setting for channel measurement, each associated with one of N TRPs. The UE may also be signaled with the same number of CSI Interference Measurement (CSI-IM) (and/or NZP CSI-RS) resources for interference measurement, each associated with one of the CSI-RS resource settings or the CSI-RS resources for channel measurement, respectively. In addition, the UE is signaled to select M (<=N) preferred TRPs, where M is also signaled to the UE, either explicitly or implicitly. With M being signaled to the UE (for instance via Radio Resource Control (RRC) signaling), the CSI payload size is deterministic and is known to both the gNB and the UE.

In the CSI report, the UE feeds back the M selected TRPs in the form of the identifiers (IDs) of the selected NZP CSI-RS resource settings or NZP CSI-RS resources. These IDs may typically be local IDs, i.e. it refers to a position in a list, for example comprised in CSI-ReportConfig, rather than a global ID, in order to minimize the feedback payload. For instance, they may be conveyed as a number of CSI-RS Resource Indicator (CRI) parameters. Alternatively, a size-N bitmap may be used, where each bit is associated with NZP CSI-RS resource settings or NZP CSI-RS resource (and thus indirectly one TRP) and a value of '<NUM>' indicates the corresponding NZP CSI-RS resource settings or NZP CSI-RS resource is selected while a value of '<NUM>' indicates the NZP CSI-RS resource settings or NZP CSI-RS resource is not selected. An alternative approach is to jointly encode the selected NZP CSI-RS resource settings or NZP CSI-RS resources to reduce the feedback overhead. For instance, the number of combinations for choosing M out of N TRPs is <MAT> and hence the number of bits needed can be reduced to <MAT> bits.

For each selected TRP, the estimated rank and precoding matrix are also reported. When separate codewords are used in each TRP, a Channel Quality Indicator (CQI) is also reported for each TRP, with inter-TRP interference being considered. In case a single codeword is used across the TRPs, then a single CQI is reported conditioned on the reported ranks and precoding matrices.

In some scenarios, the gNB may want to restrict the rank for each TRP and signal the rank restriction per each of the N CSI-RS resources associated with the CSI reporting setting. For example, the gNB may want to transmit a single layer from each TRP. In this case, the UE would measure and report CSI assuming the restricted rank in each of the N CSI-RS resources corresponding to the N TRPs.

In a related embodiment, the UE may be allowed to also search for NC-JT involving fewer TRPs than the signaled value M. If the best solution involves KTRPs, with K < M, it could still be worthwhile to feed back a CSI report with M sets of Precoding Matrix Indicator (PMI) / Rank Indicator (RI) / CQI. Apart from the advantage of maintaining a fixed CSI payload, the available M - K sets of PMI/RI/CQI can be used to provide CSI for alternative kinds of transmission, e.g., DPS. Indeed, for M = <NUM>, e.g., the CSI report could either convey CSI for <NUM>-TRP NC-JT, or CSI for DPS between two TRPs. In this embodiment, the CSI report would need to be complemented to specify <NUM>) K; <NUM>) the subset of Kout of MTRPs for the NC-JT CSI; as well as <NUM>) the subset of M - Kout of MTRPs for the DPS CSI. Alternatively, it could be specified that the M - K sets of PMI/RI/CQI instead refer to CSI for a second NC-JT involving M - KTRPs.

In some embodiments, the UE selects M out of N TRPs, where M is determined by the UE.

More specifically, in some embodiments, a UE is either configured with N NZP CSI-RS resource settings in a CSI reporting setting for channel measurement, each associated with one of N TRPs, or configured with N CSI-RS resources within an NZP CSI-RS resource setting in a CSI reporting setting for channel measurement, each associated with one of N TRPs. The UE may also be signaled with the same number of CSI-IM (and or NZP CSI-RS) resources for interference measurement, each associated with one of the CSI-RS resource settings or the CSI-RS resources for channel measurement, respectively. The UE is allowed to select M (<=N) preferred TRPs out of the N TRPs, where M is determined by the UE. In this case, the CSI payload size may change with each CSI reporting occasion, depending on the value of M, and is thus unknown to the gNB.

In one embodiment, the CSI report is divided into two parts. The first part has a fixed payload size and is known to the gNB based on the CSI configuration. The number of selected TRPs, M, is included in the first CSI part and can be decoded by the gNB without multiple blinding decoding attempts using multiple hypotheses of different payload, since the payload size is fixed and known. After decoding the first part correctly, the payload size of the second part can be determined and thus the second part can also be decoded.

In case the scheduled PUSCH resource is not enough to carry the full CSI report, it is assumed that the first part of the CSI report still can be carried and decoded. The second part of the CSI report may be truncated with certain CSI parameters being dropped. Dropping rules can be defined so that both the gNB and the UE know what has been dropped.

Alternatively, the number of selected TRPs may be reduced, for example from three to one, so that the second part of the CSI can fit in the scheduled PUSCH resource.

When it comes to transmission from multiple-TRPs, it is possible to receive either diversity based transmission or spatial multiplexing based transmission. In some scenarios, a UE may have enhanced Mobile Broadband (eMBB) and Ultra-Reliable and Low Latency Communication (URLLC) type of traffic serviced simultaneously. Hence, the UE can be indicated dynamically whether the Physical Downlink Shared Channel (PDSCH) being transmitted from the selected multiple TRPs should be treated either (<NUM>) by combining layers from the selected multiple TRPs (i.e., diversity based transmission) or (<NUM>) by performing layer independent decoding from the selected multiple TRPs (i.e., spatial multiplexing based transmission).

Hence, for CSI feedback, the UE's CSI report should reflect the possibility to dynamically switch between diversity based transmission and spatial multiplexing based transmission from the selected multiple TRPs. This can be satisfied by having different reporting quantities such as:.

One way of realizing this is for the UE to feed back a Hypothesis Index (HI) where different HIs define a particular type of transmission. The gNB can predefine the particular type of transmission for each HI. An example is shown in Table <NUM> below. In this table, NZP CSI-RS IDs A and B are associated with two different TRPs A and B. If the UE feedback HI=<NUM> or HI=<NUM>, then this corresponds to receiving DPS based transmission from TRPs A and B respectively. If the UE feeds back HI=<NUM>, then this is for receiving NC-JT from TRPs A and B, and the CSI is calculated assuming the layers transmitted from TRPs A and B are independent. If HI=<NUM> is fed back by the UE, then this is for receiving diversity based transmission and the CSI is calculated assuming combining of layers from TRPs A and B.

In other embodiments, the CSI report configuration includes an instruction if the UE shall report CSI according to the assumption on diversity-based transmission or spatial multiplexing-based transmission. The UE may in some such embodiments be configured with two (or more) CSI-ReportConfigs to report two sets of CSI, one for each assumption.

In other embodiments, the observation that PMI/RI report per TRP may be the same regardless of if the hypothesis is diversity-based or spatial multiplexing (SM) based transmission is utilized and the UE feeds back two sets of CQI reports, one for each hypothesis, but only a single PMI/RI report for each TRP.

<FIG> illustrates the operation of a base station (e.g., gNB) and a UE in accordance with at least some aspects of the embodiments described above. The base station may be, e.g., a base station <NUM> or <NUM> and the UE may be, e.g., a UE <NUM>. As illustrated, the base station sends, to the UE, a configuration of either: (a) N CSI-RS resource settings containing N NZP CSI-RS resources each associated with a different one of N TRPs or (b) N sets of NZP CSI-RS resources each associated with a different one of the N TRPs (step <NUM>). The base station sends, to the UE, a request for CSI feedback (step <NUM>). The UE selects a preferred subset of the NZP CSI-RS resources (step <NUM>). In some embodiments, if the configuration is of N CSI-RS resource settings containing N NZP CSI-RS resources, the preferred subset of the NZP CSI-RS resources is a subset of those N CSI-RS resources. In some embodiments, if the configuration is of N sets of NZP CSI-RS resources, then the preferred subset of the NZP CSI-RS resources is one or more of the N sets of NZP CSI-RS resources. The selected preferred subset of NZP CSI-RS resources then corresponds to a preferred subset of the N TRPs. In some embodiments, N≥<NUM>, and the preferred subset of the NZP CS-RS resources is M of the NZP CSI-RS resources where M<N. In some embodiments, M≥<NUM>. In some other embodiments, M≥<NUM>. As described above, the number M is configured for the UE, e.g., by the base station. In some other embodiments, the number M is determined by the UE. The UE then reports CSI to the base station based on the selected preferred subset of the NZP CSI-RS resources (step <NUM>).

As described above, in some embodiments, the size (i.e., value of M) of the preferred subset is signaled to the UE, either semi-statically or dynamically. In some other embodiments, the size of the preferred subset is determined by the UE.

In some embodiments, the CSI reported to the base station contains identities of the NZP CSI-RS resources in the selected preferred subset. The identities can be indices of the NZP CSI-RS resources within the set of NZP CSI-RS resources configured in the CSI reporting setting. In some embodiments, in order to contain the identities of the NZP CSI-RS resources in the selected preferred subset, the CSI includes a bitmap that provides the identities of the NZP CSI-RS resources of the selected preferred subset, where each bit in the bitmap is associated with a configured NZP CSI-RS resource in the CSI reporting setting and where a bit value of '<NUM>' indicates that the corresponding NZP CSI-RS resource is selected and a value of '<NUM>' otherwise (or vice versa). The number of bits with '<NUM>' is equal to the size of the preferred subset.

In some embodiments, the maximum rank may be restricted for each of the multiple NZP CSI-RS resources and the restriction may be signaled to the UE.

In some embodiments, the preferred subset of the NZP CSI-RS resources corresponds to a preferred subset of the N TRPs for a PDSCH transmission to the UE, where the PDSCH transmission can be NC-JT, DPS, or diversity combining.

In some embodiments, the CSI report contains a first part and a second part, where the size of the selected preferred subset is encoded in the first part. In some embodiments, the first part has a predefined payload size and is encoded first before encoding the second part at the UE. Further, in some embodiments, some part of the CSI in the second part may be dropped without reporting if the scheduled resource is not enough to carry the full CSI content. In some embodiments, the first part is decoded first at the base station.

In some embodiments, the base station further configures the UE with a list of transmission hypothesis over the configured NZP CSI-RS resources. In some embodiments, the list of hypothesis comprises NC-JT, DPS, or diversity combining over one or more of the NZP CSI-RS resources. In some embodiments, selecting the preferred subset comprises selecting one hypothesis out of the list of the hypothesis. In some embodiments, reporting the CSI further comprises reporting the selected hypothesis.

In some embodiments, the base station provides, to the UE, two or more CSI reporting configurations, each for one transmission mode, whether NC-JT, DPS, or diversity combining, and the UE is instructed to feedback two or more CSI reports.

In some embodiments, the CSI report comprises a common CSI containing RIs and PMIs for each of the multiple NZP CSI-RS resources and two or more CQIs, each for a different transmission mode.

In some embodiments, the sets of NZP CSI-RS resources are in either a single CSI-RS resource setting or multiple CSI-RS resource settings.

In some embodiments, base station sends the request for CSI feedback either semi-statically over, e.g., RRC signaling or dynamically over, e.g., Physical Downlink Control Channel (PDCCH).

In some embodiments, selecting the preferred subset at the UE comprises comparing data throughputs and selecting the TRPs that can provide the maximum throughput.

<FIG> illustrates the operation of a base station (e.g., gNB) and a UE in accordance with at least some aspects of the embodiments described above. Note that optional steps are indicated with dashed lines. This embodiment is substantially the same as that of <FIG> other than "N" being replaced by "one or more" and the steps being optional to indicate that not all steps are required in all embodiments or implementations. As illustrated, the base station sends, to the UE, a configuration of either: (a) one or more CSI-RS resource settings containing one or more NZP CSI-RS resources each associated with a different one of one or more TRPs or (b) one or more sets of NZP CSI-RS resources each associated with a different one of the one or more TRPs (step <NUM>). The base station sends, to the UE, a request for CSI feedback (step <NUM>). The UE selects a preferred subset of the NZP CSI-RS resources (step <NUM>). The selected preferred subset of NZP CSI-RS resources then corresponds to a preferred subset of the one or more TRPs. The UE then reports CSI to the base station based on the selected preferred subset of the NZP CSI-RS resources (step <NUM>).

Note that the remaining details provided above with respect to <FIG> are equally applicable here to <FIG>.

<FIG> is a schematic block diagram of a radio access node <NUM> according to some embodiments of the present disclosure. The radio access node <NUM> may be, for example, a base station <NUM> or <NUM>. As illustrated, the radio access node <NUM> includes a control system <NUM> that includes one or more processors <NUM> (e.g., Central Processing Units (CPUs), Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs), and/or the like), memory <NUM>, and a network interface <NUM>. The one or more processors <NUM> are also referred to herein as processing circuitry. In addition, the radio access node <NUM> includes one or more radio units <NUM> that each includes one or more transmitters <NUM> and one or more receivers <NUM> coupled to one or more antennas <NUM>. The radio units <NUM> may be referred to or be part of radio interface circuitry. In some embodiments, the radio unit(s) <NUM> is external to the control system <NUM> and connected to the control system <NUM> via, e.g., a wired connection (e.g., an optical cable). However, in some other embodiments, the radio unit(s) <NUM> and potentially the antenna(s) <NUM> are integrated together with the control system <NUM>. The one or more processors <NUM> operate to provide one or more functions of a radio access node <NUM> as described herein. In some embodiments, the function(s) are implemented in software that is stored, e.g., in the memory <NUM> and executed by the one or more processors <NUM>.

As used herein, a "virtualized" radio access node is an implementation of the radio access node <NUM> in which at least a portion of the functionality of the radio access node <NUM> is implemented as a virtual component(s) (e.g., via a virtual machine(s) executing on a physical processing node(s) in a network(s)). As illustrated, in this example, the radio access node <NUM> includes the control system <NUM> that includes the one or more processors <NUM> (e.g., CPUs, ASICs, FPGAs, and/or the like), the memory <NUM>, and the network interface <NUM> and the one or more radio units <NUM> that each includes the one or more transmitters <NUM> and the one or more receivers <NUM> coupled to the one or more antennas <NUM>, as described above. The control system <NUM> is connected to the radio unit(s) <NUM> via, for example, an optical cable or the like. The control system <NUM> is connected to one or more processing nodes <NUM> coupled to or included as part of a network(s) <NUM> via the network interface <NUM>. Each processing node <NUM> includes one or more processors <NUM> (e.g., CPUs, ASICs, FPGAs, and/or the like), memory <NUM>, and a network interface <NUM>.

In this example, functions <NUM> of the radio access node <NUM> described herein are implemented at the one or more processing nodes <NUM> or distributed across the control system <NUM> and the one or more processing nodes <NUM> in any desired manner. In some particular embodiments, some or all of the functions <NUM> of the radio access node <NUM> described herein are implemented as virtual components executed by one or more virtual machines implemented in a virtual environment(s) hosted by the processing node(s) <NUM>. As will be appreciated by one of ordinary skill in the art, additional signaling or communication between the processing node(s) <NUM> and the control system <NUM> is used in order to carry out at least some of the desired functions <NUM>. Notably, in some embodiments, the control system <NUM> may not be included, in which case the radio unit(s) <NUM> communicate directly with the processing node(s) <NUM> via an appropriate network interface(s).

<FIG> is a schematic block diagram of a UE <NUM> according to some embodiments of the present disclosure. As illustrated, the UE <NUM> includes one or more processors <NUM> (e.g., CPUs, ASICs, FPGAs, and/or the like), memory <NUM>, and one or more transceivers <NUM> each including one or more transmitters <NUM> and one or more receivers <NUM> coupled to one or more antennas <NUM>. The transceiver(s) <NUM> includes radio-front end circuitry connected to the antenna(s) <NUM> that is configured to condition signals communicated between the antenna(s) <NUM> and the processor(s) <NUM>, as will be appreciated by on of ordinary skill in the art. The processors <NUM> are also referred to herein as processing circuitry. The transceivers <NUM> are also referred to herein as radio circuitry. In some embodiments, the functionality of the UE <NUM> described above may be fully or partially implemented in software that is, e.g., stored in the memory <NUM> and executed by the processor(s) <NUM>. Note that the UE <NUM> may include additional components not illustrated in <FIG> such as, e.g., one or more user interface components (e.g., an input/output interface including a display, buttons, a touch screen, a microphone, a speaker(s), and/or the like and/or any other components for allowing input of information into the UE <NUM> and/or allowing output of information from the UE <NUM>), a power supply (e.g., a battery and associated power circuitry), etc..

With reference to <FIG>, in accordance with an embodiment, a communication system includes a telecommunication network <NUM>, such as a 3GPP-type cellular network, which comprises an access network <NUM>, such as a RAN, and a core network <NUM>. The access network <NUM> comprises a plurality of base stations 1706A, 1706B, 1706C, such as Node Bs, eNBs, gNBs, or other types of wireless Access Points (APs), each defining a corresponding coverage area 1708A, 1708B, 1708C. Each base station 1706A, 1706B, 1706C is connectable to the core network <NUM> over a wired or wireless connection <NUM>. A first UE <NUM> located in coverage area 1708C is configured to wirelessly connect to, or be paged by, the corresponding base station 1706C. A second UE <NUM> in coverage area 1708A is wirelessly connectable to the corresponding base station 1706A.

It is noted that the host computer <NUM>, the base station <NUM>, and the UE <NUM> illustrated in <FIG> may be similar or identical to the host computer <NUM>, one of the base stations 1706A, 1706B, 1706C, and one of the UEs <NUM>, <NUM> of <FIG>, respectively.

The wireless connection <NUM> between the UE <NUM> and the base station <NUM> is in accordance with the teachings of the embodiments described throughout this disclosure. One or more of the various embodiments improve the performance of OTT services provided to the UE <NUM> using the OTT connection <NUM>, in which the wireless connection <NUM> forms the last segment. More precisely, the teachings of these embodiments may improve, e.g., data rate, latency, and/or power consumption and thereby provide benefits such as, e.g., reduced user waiting time, relaxed restriction on file size, better responsiveness, and/or extended battery lifetime.

Claim 1:
A method performed by a wireless device (<NUM>) for Channel State Information, CSI, feedback, the method comprising:
• receiving (<NUM>), from a base station (<NUM>), a configuration comprising:
∘ N Non-Zero Power, NZP, CSI Reference Signal, CSI-RS, resources, each of the N NZP CSI-RS resources associated with a different one of N Transmission Points, TRPs; or N sets of NZP CSI-RS resources, each of the N sets associated with a different one of the N TRPs;
• receiving (<NUM>) a request for CSI feedback based on the configuration;
• selecting (<NUM>) a preferred subset of the NZP CSI-RS resources comprised in the N NZP CSI-RS resources or the N sets of NZP CSI-RS resources,
∘ wherein, when the configuration comprises N NZP CSI-RS resources, the preferred subset of the NZP CSI-RS resources is a subset of the N NZP CSI-RS resources, and when the configuration comprises N sets of NZP CSI-RS resources, the preferred subset of the NZP CSI-RS resources is one or more of the N sets of NZP CSI-RS resources,
∘ wherein the preferred subset of the NZP CSI-RS resources has size M, where M<N;
∘ wherein selecting the preferred subset of the NZP CSI-RS resources corresponds to selecting M preferred TRPs out of the N TRPs, and
∘ wherein M is signaled by the base station to the wireless device; and
• reporting (<NUM>), to the base station, CSI based on the selected preferred subset of the NZP CSI-RS resources, wherein the CSI comprises an identifier of each of the NZP CSI-RS resources in the preferred subset.