Enhanced new radio (NR) type II channel state information (CSI) feedback using angle and delay reciprocity

Abstract

Method (800) performed by a base station (e.g., gNB). The method includes selecting (s802) a set of frequency domain (FD) basis vectors. The method also includes transmitting (s804) information identifying the selected FD basis vectors to a UE.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to channel state information (CSI) feedback. BACKGROUND

[0002] 1.1.1 Codebook-based precoding

[0003] Multi-antenna technology can significantly increase the data rates and reliability of a wireless communication system. The performance is particularly improved if both the transmitter and the receiver are equipped with multiple antennas, which form a multiple-input multiple-output (MIMO) communication channel. Such systems and / or related technology are often referred to as MIMO.

[0004] New radio (NR) standards are currently evolving with enhanced MIMO support. A core component in NR is to support MIMO antenna deployments and MIMO related technology, such as e.g. spatial multiplexing. The goal of spatial multiplexing mode is high data rates with favorable channel conditions. Figure 1 An illustration of spatial multiplexing operation is provided in

[0005] As Figure 1 illustrated, the information carried by the symbol vector s is multiplied with an N T x r precoder matrix W, which is used to allocate transmit energy in a subspace of an N T ( corresponding to N T antenna ports) dimensional vector space. The precoder matrix is typically selected from a codebook of possible precoder matrices, and is typically indicated by a precoder matrix indicator (PMI) that specifies a unique precoder matrix for a given number of symbol streams in that codebook. The r symbols in s each correspond to one layer, and r is referred to as the transmission rank. In this way, spatial multiplexing can be achieved, since multiple symbols can be transmitted simultaneously on the same time / frequency resource element (TFRE). The number of symbols r is typically adapted to suit the current channel properties.

[0006] NR uses orthogonal frequency division multiplexing (OFDM) in the downlink (and DFT-precoded OFDM in the uplink for rank-1 transmission), and thus for a certain TFRE on a subcarrier n (or alternatively data TFRE number n), the received N R x 1 vector y n is thus modeled as:

[0007] y n = H n Ws n + e n ,

[0008] where e nis the noise / interference vector obtained as an implementation of the random process. The precoder W can be a wideband precoder, which is constant over frequency, or frequency-selective.

[0009] The precoder matrix W is typically chosen to match the properties of H R xN T the MIMO channel matrix H n , resulting in so-called channel-dependent precoding. This is also often referred to as closed-loop precoding, and is essentially to concentrate the transmit energy into a subspace that is good in the sense of delivering most of the transmit energy to the UE.

[0010] In closed-loop precoding for NR downlink, the UE sends a recommendation of a suitable precoder to use to its serving base station (aka gNB) based on channel measurements in the forward link (downlink). The gNB configures the UE to provide feedback according to a CSI-ReportConfig (CSI-ReportConfig), and can send CSI-RS and configure the UE to use measurements of the CSI-RS to feedback a recommended precoder matrix that the UE selects from a codebook. A single precoder that should cover a large bandwidth (wideband precoding) can be fed back. It can also be beneficial to match the frequency variations of the channel and instead feed back a frequency-selective precoding report, e.g. a few precoders, one per subband. This is an example of a more general case of channel state information (CSI) feedback, which also includes feeding back other information than the recommended precoder to help the gNB in subsequent transmissions to the UE. Such other information can include a channel quality indicator (CQI) as well as a transmission rank indicator (RI). In NR, the CSI feedback can be wideband, where one CSI is reported for the entire channel bandwidth, or frequency-selective, where one CSI is reported for each subband, each subband being defined as a number of contiguous resource blocks ranging between 4-32 PRBS depending on the bandwidth part (BWP) size.

[0011] Given the CSI feedback from the UE, the gNB determines the transmission parameters it wants to use for transmissions to the UE, including the precoding matrix, the transmission rank, and the modulation and coding scheme (MCS). These transmission parameters can be different from the recommendations given by the UE. The transmission rank, and thus the number of spatial multiplexing layers, is reflected in the number of columns of the precoder W. It is important to select a transmission rank that matches the channel properties for efficient performance.

[0012] 1.1.2 2D antenna array

[0013] Embodiments presented in this disclosure can be used with two-dimensional antenna arrays, and some of the presented embodiments use such antennas. Such an antenna array can (partly) be characterized by a number of antenna columns N h, the number of antenna rows N corresponding to the vertical dimension v and the number of dimensions N corresponding to different polarizations p are described. Thus, the total number of antennas is N = N h N v N p It should be noted that the concept of antennas is non-restrictive in the sense that it can refer to any virtualization (e.g., linear mapping) of physical antenna elements. For example, pairs of physical sub-elements can be fed with the same signal and thus share the same virtualized antenna port.

[0014] An example of a 4x4 array with dual-polarized antenna elements is illustrated in Figure 2 . Figure 2 A two-dimensional antenna array with dual-polarized antenna elements with horizontal and vertical antenna elements is shown.

[0015] Precoding can be interpreted as multiplying the signal with different beamforming weights for each antenna before transmission. A typical approach is to tailor the precoder according to the antenna form factor, i.e., to consider N h , N v , and N p when designing the precoder codebook.

[0016] 1.1.3 Channel State Information Reference Signal (CSI-RS)

[0017] For CSI measurement and feedback, a CSI-RS is defined. The CSI-RS is transmitted on each antenna port and is used by the UE to measure the downlink channel between each transmit antenna port and each of its receive antenna ports. The transmit antenna port is also referred to as a CSI-RS port. The number of antenna ports supported in NR is {1, 2, 4, 8, 12, 16, 24, 32}. By measuring the received CSI-RS, the UE can estimate the channel through which the CSI-RS is passing, including the radio propagation channel and the antenna gain. The CSI-RS used for the above purpose is also referred to as non-zero-power (NZP) CSI-RS.

[0018] The CSI-RS can be configured to be transmitted in one slot and certain resource elements (REs) in certain slots. Figure 3 An example of CSI-RS REs for 12 antenna ports is shown, where 1 RE per port per RB is shown.

[0019] In addition, interference measurement resources (IMRs) are defined in NR for the UE to measure interference. An IMR resource contains 4 REs, either 4 adjacent REs in frequency in the same OFDM symbol or 2 by 2 adjacent REs in time and frequency in a slot. By measuring the channel based on NZP CSI-RS and the interference based on IMR, the UE can estimate the effective channel and noise plus interference to determine the CSI, i.e. rank, precoding matrix and channel quality. In addition, a UE in NR can be configured to measure interference based on one or multiple NZP CSI-RS resources.

[0020] 1.1.4 CSI framework in NR

[0021] 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 8 CSI-RS resources. For each CSI reporting setting, the UE feeds back a CSI report.

[0022] Each CSI reporting setting contains at least the following information:

[0023] • CSI-RS resource sets for channel measurement

[0024] • IMR resource sets for interference measurement

[0025] • Optionally, CSI-RS resource sets for interference measurement

[0026] • Time domain behavior, i.e. periodic, semi-persistent or aperiodic reporting

[0027] • Frequency granularity, i.e. wideband or subband

[0028] • CSI parameters to be reported, e.g. RI, PMI, CQI and CSI-RS resource indicator (CRI) in case of multiple CSI-RS resources in a resource set

[0029] • Codebook type, i.e. Type I or Type II, and codebook subset restriction

[0030] • Measurement restriction

[0031] • Subband size. One of two possible subband sizes is indicated, the value range depends on the bandwidth of the BWP. One CQI / PMI is fed back per subband (if configured for subband reporting).

[0032] When the CSI-RS resource set in the CSI report setting contains multiple CSI-RS resources, the UE selects one of them and also reports a CSI-RS resource indicator (CRI) to indicate the selected CSI-RS resource to the gNB, along with the RI, PMI and CQI associated with the selected CSI-RS resource.

[0033] For aperiodic CSI reporting in NR, multiple CSI report settings can be configured and triggered simultaneously, each with a different set of CSI-RS resources for channel measurement and / or resource sets for interference measurement. In this case, multiple CSI reports are aggregated and transmitted from the UE to the gNB in a single PUSCH.

[0034] 1.1.5 DFT-based precoder

[0035] A common type of precoding is using a DFT precoder, where the precoder vector for precoding for single-layer transmission using a single-polarization uniform linear array (ULA) with N antennas is defined as

[0036]

[0037] where k = 0, 1,... QN-1 is the precoder index, + is an integer oversampling factor. The corresponding precoder vector for a two-dimensional uniform planar array (UPA) can be created by taking the Kronecker product of two precoder vectors as where e jφ is a co-phasing factor that can be selected, for example, from the QPSK alphabet

[0038] Precoder matrix W for multi-layer transmission 2D,DP The precoder matrix W for multi-layer transmission can be created by appending the columns of the DFT precoder vector as follows:

[0039] W 2D,DP = [w 2D,DP (k1,l1,φ1)w 2D,DP (k2,l2,φ2)... w 2D,DP (k R ,l R ,φ R )],

[0040] where R is the number of transmission layers, i.e., the transmission rank. In the common special case of a rank-2 DFT precoder, k1 = k2 = k and l1 = l2 = l, meaning ​​

[0041]

[0042] Such DFT-based precoders are used for e.g. NR Type I CSI feedback.

[0043] 1.1.6 MU-MIMO

[0044] With multi-user MIMO, two or more users in the same cell are co-scheduled on the same time-frequency resources. That is, two or more independent data streams are transmitted to different UEs simultaneously, and the spatial domain is used to separate the individual data streams. By transmitting multiple streams simultaneously, the capacity of the system can be increased. However, this comes at the cost of a reduced SINR per stream, since power has to be shared between the streams, and the streams interfere with each other.

[0045] 1.1.7 Multi-beam (linear combination) precoders

[0046] A central part of MU-MIMO is to obtain accurate CSI, so that null forming between the co-scheduled users can be performed. Therefore, support for codebooks that provide more detailed CSI than the traditional single-DFT-beam precoders was added in LTE Rel. 14-16. These codebooks, called advanced CSI (LTE), Type II codebook (NR Rel. 15) and enhanced Type II codebook (NR Rel. 16) can be described as a set of precoders, where each precoder is created from multiple DFT beams. A multi-beam precoder can be defined as a linear combination of several DFT precoder vectors, like

[0047]

[0048] where {c i} can be complex coefficients in general. Such multi-beam precoders can describe the channel of a UE more accurately and therefore can bring additional performance benefits compared to DFT precoders, especially for MU-MIMO which requires rich channel knowledge in order to perform null forming between the co-scheduled UEs.

[0049] 1.1.7.1 NR rel-15 Type II

[0050] For the NR Type II codebook in Rel-15, for a given dual-polarized antenna array with N1 and N2 elements in each dimension per polarization, the precoding vector for each layer and subband is represented in 3GPP TS 38.214 as:

[0051]

[0052] If the above equation is restructured and expressed a little simpler, one can write the precoder vector w SB for a layer l = 0, 1, polarization p = 0, 1, and subband k = 0,..., N l,p (k) is formed as

[0053]

[0054] where is the i-th selected 2D beam, for i = 0, 1, 2, 3 For p = 1, and, N SB is the number of subbands in the CSI reporting bandwidth. Thus, the beam coefficients vary in frequency c l,i (k) is based on 2N SB parameters and are determined, where the subband amplitude parameter is quantized using 0 or 1 bits, while the subband phase parameter is quantized using 2 or 3 bits (i.e., QPSK or 8PSK alphabet), depending on the codebook configuration.

[0055] 1.1.7.2 NR rel-16 Type II

[0056] For NR Rel-16 Type II, an overhead reduction mechanism has been specified. The basic principle is that it has been observed that there is a strong correlation between different values of c l,i for different subbands, and one can exploit this correlation to perform an efficient compression to reduce the number of bits needed to represent the information. This will thus reduce the amount of information that needs to be signaled from the UE to the gNB, which is relevant in multiple aspects.

[0057] Thus, in the NR Rel-16 Type II codebook, a set of frequency domain (FD) DFT vectors over the set of subbands is introduced. The agreed codebook design for the NR Rel-16 Type II codebook can be described as follows:

[0058]

[0059]

[0060]

[0061]

[0062]

[0063] Figure 4 The agreed codebook structure using SD and FD compression is shown.

[0064] 1.1.8 NR Rel-16 Enhanced Type-II Port Selection Codebook

[0065] Based on the above Enhanced Type-II codebook, an Enhanced Type-II Port Selection codebook is also introduced in NR Rel-16, where it is assumed that each antenna port is actually associated with a 2D spatial beam, thus the beam selection in the Enhanced Type-II codebook becomes port selection. The Type-II PS CB is intended for beamformed CSI-RS. The UE measures the beamformed CSI-RS ports and recommends which ports to use, the selected ports are indicated and reported in a P CSI-RS x 2L matrix W1, where P CSI-RS is the number of beamformed CSI-RS ports. The linear combination coefficient (LCC) matrix and the FD basis W f are reported in the same way as the regular Enhanced Type-II codebook. SUMMARY

[0066] There are certain challenges at present. In FDD operation, UL and DL transmissions are conducted on different frequencies, thus the propagation channels in UL and DL are not reciprocal. Nevertheless, some physical channel parameters, such as delay and angle of different clusters, which depend on the spatial properties of the channel but not on the carrier frequency, are reciprocal between UL and DL. This reciprocity can potentially be exploited in codebook-based DL transmission for FDD in order to, for example, reduce the feedback overhead in UL when using the NR Type-II port selection codebook. However, how to apply UL channel measurements to assist II-type based DL CSI feedback in FDD systems is an issue.

[0067] More specifically, if the Rel.16 Enhanced Type-II port selection codebook is used for angle and / or delay reciprocity based FDD operation, the frequency domain (FD) basis W f still needs to be determined by the UE. Therefore, in the CSI report, the feedback overhead for indicating which FD bases to select can be large, especially when the number of PMI subbands N3 is large. Moreover, the computational complexity at the UE for evaluating and selecting the best FD basis also increases with N3.

[0068] With the delay reciprocity between UL and DL, the gNB can pre-determine a subset of FD basis vectors based on the estimated delay information to the selected clusters in UL. Then, the gNB can signal to the UE about this pre-determined subset of FD basis vectors , then the UE can evaluate and select the FD basis vectors in the pre-determined subset of FD basis vectors.

[0069] This has the advantage of reducing the CSI feedback overhead for indicating which FD basis vectors to use. It also has the advantage of reducing the computational complexity of the UE selecting the best FD basis vectors, thereby reducing the use of computational resources and prolonging battery life.

[0070] Thus, in one aspect, a method performed by a base station (e.g., gNB) is provided. The method includes the base station selecting a set of frequency domain (FD) basis vectors. The method also includes the base station transmitting, to a UE, information identifying the selected FD basis vectors. In some embodiments, the selected FD basis vectors are selected based on measurements of reference signals (RSs) transmitted by the UE. In some embodiments, the RSs are sounding reference signals (SRSs). In another aspect, a base station configured to perform the method is provided. In another aspect, a computer program stored in the memory of the base station, which, when run on processing circuitry of the base station, causes the base station to perform the method is provided.

[0071] In another aspect, a method performed by a UE is provided. The method includes the UE transmitting, to a base station, reference signals (e.g., SRSs). The method also includes the UE receiving, from the base station, information identifying a set of FD basis vectors selected by the base station. The method also includes the UE receiving, from the base station, a plurality of downlink (DL) reference signals (RSs) (e.g., CSI-RSs), and a request for channel state information (CSI) feedback based on the DL RSs and the set of FD basis vectors. The method also includes the UE selecting a subset of the set of FD basis vectors and estimating CSI based on the DL RSs and the subset of the set of FD basis vectors. The method also includes the UE transmitting, to the base station, a CSI report message including a set of coefficients, each coefficient associated with at least one of the DL reference signals and at least one of the selected FD basis vectors. In another aspect, a UE configured to perform the method is provided. In another aspect, a computer program stored in the memory of the UE, which, when run on processing circuitry of the UE, causes the UE to perform the method is provided. BRIEF DESCRIPTION OF DRAWINGS

[0072] Figure 1 Spatial multiplexing operation is shown.

[0073] Figure 2 An example of a 4x4 array with dual-polarized antenna elements is shown.

[0074] Figure 3 An example of CSI-RS REs for 12 antenna ports is shown, where 1 RE per port per RB is shown.

[0075] Figure 4 An agreed codebook structure using SD and FD compression is shown.

[0076] Figure 5 A process for reciprocity-based FDD transmission scheme is shown.

[0077] Figure 6 Angular delay power spectrum of the channel before and after spatial precoding is shown.

[0078] Figure 7 Angular delay power spectrum of the channel before and after spatial precoding and delay pre-compensation is shown.

[0079] Figure 8 is a flowchart showing processing according to an embodiment.

[0080] Figure 9 is a flowchart showing processing according to an embodiment.

[0081] Figure 10 is a block diagram of a base station according to an embodiment.

[0082] Figure 11 is a block diagram of a UE according to an embodiment. DETAILED DESCRIPTION

[0083] Enhanced Type-II port selection codebook for FDD systems

[0084] Figure 5 A process for reciprocity-based FDD transmission scheme is shown, assuming the use of the NR Rel. 16 enhanced Type-II port selection codebook.

[0085] In step 1, the UE 102 transmits a sounding reference signal (SRS) in the UL, allowing the gNB 104 to estimate the angle and delay of different clusters associated with different propagation paths.

[0086] In step 2, the gNB 104 selects the dominant clusters from the estimated angle-delay power spectrum profile and, for each of the selected clusters, the gNB pre- codes one CSI-RS port according to the obtained angle and / or delay estimates. The gNB also selects a set of frequency domain (FD) basis vectors and transmits information identifying the selected FD basis vectors to the UE. For example, the gNB can transmit a message to the UE including an index of each FD basis vector included in the selected set of FD basis vectors, where the index of the FD basis vector identifies (e.g., points to) the FD basis vector. The selected set of FD basis vectors is selected based on the estimated angle-delay power spectrum profile.

[0087] In step 3, the UE measures the received CSI-RS ports and then determines Type II CSI, including RI, PMI for each layer, and CQI. The PMI indicates a precoding matrix that includes the best phase and amplitude for co-phasing the corresponding beams. The phase and amplitude for each beam are quantized and fed back to the gNB.

[0088] In step 4, the gNB computes the DL precoding matrix for each layer based on the selected beams and the corresponding amplitude and phase feedback, and performs Physical Downlink Shared Channel (PDSCH) transmission directly based on the precoding matrix (e.g., Single-User MIMO (SU-MIMO)) or based on a precoder derived from the precoding matrix (e.g., Zero-Forcing precoder in case of Multi-User MIMO (MU-MIMO)) including CSI reports from co-scheduled UEs.

[0089] In one embodiment, the gNB can help the UE to select the best M FD basis vectors according to the estimated cluster angles and delays to reduce the feedback overhead for CSI reporting and the computational complexity of selecting the best M FD basis vectors at the UE. The gNB can determine the angles and delays to different clusters by analyzing the angle-delay power spectrum of the channel.

[0090] For example, Figure 6 The 8x10 grid in the left side shows the angle-delay power spectrum of the UL channel with 8 angle bins and 10 delay taps, where each shaded square represents the power level of a given cluster at a particular angle and delay. Based on the angle reciprocity, in this example, the gNB selects the 2 strongest clusters and precodes one CSI-RS port for each polarization of each cluster (i.e., a total of 4 CSI-RS ports).

[0091] In the right side of Figure 6 , there are only 4 taps in the delay domain of the two beamformed channels, while there are 10 in the original channel. Therefore, the remaining 6 can be converted to FD bases with 4 vectors, The 4 delay taps of , can be conveyed to the UE by the gNB, so that the UE only needs to select the best frequency basis vector from 4, instead of 10, FD basis vector candidates. Therefore, in this example, the overhead of indicating which FD bases can be selected can be reduced, and the computational complexity of selecting the best FD basis at the UE can be reduced.

[0092] In one embodiment, the gNB pre-compensates the delay of each beamforming channel so that the strongest paths in all beamforming channels arrive at the UE simultaneously. For example, delay pre-compensation can be accomplished by applying a linear phase slope over frequency for each beam with beamforming weights, where the slope is given by the delay of the strongest tap in each beam.

[0093] like Figure 7 As shown, after pre-compensating for the delay of the beamforming channel, the number of delay taps is reduced to 3, compared to 10 delay taps in the original channel. Furthermore, since the zeroth delay component (corresponding to the zeroth FD basis vector, i.e., the DC basis) always exists, gNB only needs to use the remaining 2 FD basis vectors... The signal is sent to the UE. Therefore, the UE only needs to select the optimal frequency basis vector from 2 instead of 4 FD basis vector candidates. Thus, in this example, not only is the overhead of indicating which FD components have been selected reduced, but also the overhead of reporting the corresponding LC coefficients is reduced. Furthermore, the computational complexity at the UE for selecting the optimal FD basis can be reduced.

[0094] In one embodiment, gNB predetermines and signals M FD basis vectors to UE. The layer common set. For each transport layer, the UE can select a subset of the corresponding best FD basis vectors from the layer common FD basis vector set and report it to the gNB (as used herein, set A is a subset of set B as long as every element in set A is also in set B—therefore, set A does not have to be smaller than set B (i.e., A may be equal to B), but set A cannot be larger than set B). Note that for this embodiment, the UE does not need to perform a two-step FD basis selection for a large number of PMI subbands (i.e., N3>19) as in the Rel-16 Type II Enhanced Codebook. This is because the gNB has already predetermined and signaled M FD basis vectors. The layer common set is such that the UE does not need to report an indication of which FD base subset the UE has selected. For example, in this embodiment, it is not necessary to specify index i. 1,5 (For N3>19, it is reported as part of the PMI in the NR Rel-16 Enhanced Type II CSI report.) Therefore, in this embodiment, CSI reporting overhead is reduced. Furthermore, since the UE does not need to perform window-based intermediate subset selection in this embodiment, complexity is also saved.

[0095] In one embodiment, the gNB pre-determines and signals to the UE multiple FD basis vector subsets. For each transmission layer, the UE selects the corresponding best subset and the corresponding best FD basis vector from the selected best subset and reports to the gNB. In this embodiment, the complexity is also saved since the UE does not need to perform window-based intermediate subset selection as the UE does in Rel-16 NR enhanced Type II CSI feedback.

[0096] In some embodiments, the UE selects the same best FD basis vector subset for all layers (i.e., the selected best FD basis vector subset is layer-common). In this case, the UE will report one index per PMI to the gNB to indicate the selected best FD basis vector subset.

[0097] In some embodiments, the UE can select different best FD basis vector subsets for different layers (i.e., the selected best FD basis vector subset is layer-specific). In this case, the UE will report one index per layer per PMI to the gNB to indicate the selected best FD basis vector subset.

[0098] In one embodiment, the gNB signals to the UE layer-specific FD basis subsets. For each transmission layer, the UE will either use all the signaled FD basis vectors for the corresponding layer, or select the best FD basis vector from the corresponding layer-specific FD basis subset and report to the gNB. If the UE uses all the signaled FD basis vectors for the corresponding layer, the index i 1,5 (which indicates the selected intermediate subset of FD basis vectors to the gNB in rel-16 Type II CSI reporting) and the index i 1,6,l (which indicates the selected FD basis vector subset to the gNB in rel-16 Type II CSI reporting) do not need to be reported by the UE to the gNB as part of the PMI report. This amounts to significant saving of CSI report overhead compared to Rel-16 Type II enhanced CSI reporting.

[0099] In some variants of the above embodiments, N3= N SB x R and where, it is assumed that R is the RRC-configured PMI subband size indicator and p is a higher layer parameter that depends on the rank. For example, for the embodiment where the entire set of FD basis vectors signaled by the gNB is used by the UE, the signaling by the gNB is expected to have M FD basis vectors. Note that in this case, N and M are semi-static since they are determined by the RRC-configured parameters R, p and N SBdetermined. In an alternative embodiment, the gNB can signal the M' FD basis vectors more dynamically depending on the angle-delay power spectrum it measures on the uplink. In this case, the number of FD basis vectors can also be explicitly indicated by the UE to the gNB. In some embodiments, the corresponding N (i.e., PMI subband size) can also be indicated by the gNB to the UE. In general, the number of beams selected by the gNB can be indicated to the UE by the number of CSI-RS ports in the CSI request. One way to achieve this is to configure multiple CSI-RS resources for aperiodic CSI feedback reporting, where different resources have different number of CSI-RS antenna ports X. The aperiodic CSI trigger point in the DCI is the same as in Rel. 15, which is used to select which CSI-RS should be used for the CSI reporting, i.e., the gNB is selecting the X-port CSI-RS resource for the feedback. The UE should determine the number of FD basis vectors for the CSI feedback based on the value X of the indicated CSI-RS resource.

[0100] The amount of delay spread across the selected beams can be converted to the channel coherence bandwidth in the frequency domain, which can be used to determine the number of frequency units or subbands (i.e., N) needed in Type-II CSI feedback. The determined number of frequency units can be signaled to the UE to accommodate the UE channel delay spread.

[0101] In addition, the power delay profile can also be used to determine the FD basis vectors in the frequency domain to feedback Type-II CSI by the UE. In one embodiment, M adjacent vectors can be selected from the FD basis, starting from the basis vector .

[0102] Figure 8 is a flowchart illustrating a process 800 according to an embodiment. The process 800 can begin at step s802. Step s802 includes selecting a set of frequency domain (FD) basis vectors. Step s804 includes sending information identifying the selected FD basis vectors to a UE (e.g., UE 102).

[0103] In some embodiments, the selected FD basis vectors are selected based on measurements of reference signals (RSs) transmitted by the UE. In some embodiments, the RSs are sounding reference signals (SRSs).

[0104] In some embodiments, the process 800 further includes the base station estimating an angle of arrival and an associated power delay profile of the reference signal based on the reference signal transmitted by the UE, wherein the base station selects the set of FD basis vectors based on the estimated angle of arrival and the associated power delay profile of the reference signal. In some embodiments, the process 800 further includes the base station transmitting a plurality of downlink (DL) reference signals based on i) the estimated power delay profile and / or ii) the estimated angle of arrival.

[0105] In some embodiments, the process further includes transmitting a plurality of downlink (DL) reference signals, each DL reference signal being associated with one of the angles of arrival and one or more delays associated with the power delay profile, and requesting the UE to measure the DL reference signals and feed back a channel state information (CSI) report based on the DL reference signals and the set of FD basis vectors. In some embodiments, the DL reference signals are channel state information reference signals (CSI-RS).

[0106] In some embodiments, the process further includes, after transmitting the information identifying the set of FD basis vectors and the DL reference signals to the UE, the base station receiving a CSI report message transmitted by the UE, wherein the message includes a set of coefficients, each coefficient being associated with one of the DL reference signals and one FD basis vector from a subset of the set of FD basis vectors selected by the UE.

[0107] In some embodiments, the set of FD basis vectors includes one or more FD basis vectors. In some embodiments, the set of FD basis vectors includes adjacent DFT vectors. In some embodiments, the set of adjacent DFT vectors is indicated by an index of a first DFT vector in the set and a total number of DFT vectors in the set.

[0108] In some embodiments, when the set of FD basis vectors includes one FD basis vector, the one FD basis vector can be predetermined to be a DFT vector associated with a zero Hertz frequency.

[0109] In some embodiments, the length of the DFT vectors is derived from a configured parameter.

[0110] In some embodiments, the set of FD basis vectors is common for all transmission layers. In other embodiments, the set of FD basis vectors can be different for different transmission layers.

[0111] In some embodiments, the transmission can be made through one of radio resource control (RRC) signaling or dynamic signaling in a physical downlink control channel (PDCCH) or a medium access control element (MAC CE).

[0112] Figure 9is a flowchart illustrating a process 900 performed by a UE (e.g., UE 102) according to an embodiment. The process 900 can begin at step s902. Step s902 includes the UE transmitting a reference signal (e.g., SRS) to a base station (e.g., gNB 104). Step s904 includes receiving, from the base station, information identifying a set of FD basis vectors selected by the base station. Step s906 includes receiving, from the base station, a plurality of downlink (DL) reference signals (RSs) (e.g., CSI-RSs) and a request for channel state information (CSI) feedback based on the DL RSs and the set of FD basis vectors. Step s908 includes selecting a subset of the set of FD basis vectors and estimating CSI based on the DL RSs and the subset of the set of FD basis vectors. Step s910 includes transmitting (s910), to the base station, a CSI report message including a set of coefficients, each coefficient associated with at least one of the DL reference signals and at least one of the selected FD basis vectors.

[0113] Figure 10 is a block diagram of a base station 104 according to some embodiments. As Figure 10As shown, the base station 104 can include processing circuitry (PC) 1002, which can include one or more processors (P) 1055 (e.g., one or more general purpose microprocessors and / or one or more other processors, such as an application specific integrated circuit (ASIC), field programmable gate array (FPGA), and / or the like), which can be co-located in a single housing or in a single data center, or which can be geographically distributed (i.e., the base station 104 can be a distributed computing device); a network interface 1068 including a transmitter (Tx) 1065 and a receiver (Rx) 1067 for enabling the base station 104 to send data to and receive data from other nodes connected to a network 110 (e.g., an Internet Protocol (IP) network) to which the network interface 1068 is connected; communication circuitry 1048 coupled to an antenna arrangement 1049, which includes one or more antennas and which includes a transmitter (Tx) 1045 and a receiver (Rx) 1047 for enabling the base station 104 to send data and receive data (e.g., wirelessly send / receive data); and a local storage unit (a.k.a., “data storage system”) 1008, which can include one or more non-volatile storage devices and / or one or more volatile storage devices. In embodiments in which the PC 1002 includes a programmable processor, a computer program product (CPP) 1041 can be provided. The CPP 1041 includes a computer readable medium (CRM) 1042 storing a computer program (CP) 1043, which includes computer readable instructions (CRI) 1044. The CRM 1042 can be a non-transitory computer readable medium, such as magnetic (e.g., a hard disk), optical, storage device (e.g., random access memory, flash memory), and / or the like. In some embodiments, the CRI 1044 of the computer program 1043 is configured such that, when executed by the PC 1002, the CRI causes the base station 104 to perform steps described herein (e.g., steps described herein with reference to flowcharts). In other embodiments, the base station 104 can be configured to perform steps described herein without the need for code. That is, for example, the PC 1002 can consist merely of one or more ASICs. Thus, features of embodiments described herein can be implemented in hardware and / or software.

[0114] Figure 11 is a block diagram of a UE 102 according to some embodiments. As Figure 11As shown, the UE 102 can include: processing circuitry (PC) 1102, which can include one or more processors (P) 1155 (e.g., one or more general purpose microprocessors and / or one or more other processors, such as an application specific integrated circuit (ASIC), field programmable gate array (FPGA), etc.); communication circuitry 1148 coupled to an antenna arrangement 1149 comprising one or more antennas and comprising a transmitter (Tx) 1145 and a receiver (Rx) 1147 for enabling the UE 102 to send and receive data (e.g., wirelessly send / receive data); and a local storage unit (also referred to as a “data storage system”) 1108, which can include one or more non-volatile storage devices and / or one or more volatile storage devices. In embodiments in which the PC 1102 includes a programmable processor, a computer program product (CPP) 1141 can be provided. The CPP 1141 includes a computer readable medium (CRM) 1142 storing a computer program (CP) 1143 comprising computer readable instructions (CRI) 1144. The CRM 1142 can be a non-transitory computer readable medium, such as magnetic (e.g., a hard disk), optical, physical storage device (e.g., random access memory, flash memory), etc. In some embodiments, the CRI 1144 of the computer program 1143 is configured such that, when executed by the PC 1102, the CRI causes the UE 102 to perform steps described herein (e.g., steps described herein with reference to flowcharts). In other embodiments, the UE 102 can be configured to perform steps described herein without the need for code. That is, for example, the PC 1102 can consist merely of one or more ASICs. Thus, features of embodiments described herein can be implemented in hardware and / or software.

[0115] Summary of various embodiments:

[0116] A1. A method (800) performed by a base station (e.g., gNB 104), the method comprising: selecting (s802) a set of frequency domain (FD) basis vectors; and transmitting (s804), to a UE (e.g., UE 102), information identifying the selected FD basis vectors.

[0117] A2. The method of embodiment A1, wherein the selected FD basis vectors are selected based on measurements of reference signals (RSs) transmitted by the UE.

[0118] A3. The method of embodiment A2, wherein the RSs are sounding reference signals (SRSs).

[0119] A4. The method of embodiments A1, A2, or A3, further comprising the base station estimating an angle of arrival and an associated power delay profile of a reference signal transmitted by the UE, wherein the base station selects the set of FD basis vectors based on the estimated angle of arrival and the associated power delay profile of the reference signal.

[0120] A5. The method of embodiment A4, further comprising the base station transmitting a plurality of downlink (DL) reference signals based on i) the estimated power delay profile and / or ii) the estimated angle of arrival.

[0121] A6. The method of any of embodiments A1-A5, wherein the method further comprises transmitting a plurality of downlink (DL) reference signals and requesting the UE to measure the DL reference signals and to feed back a channel state information (CSI) report based on the DL reference signals and the set of FD basis vectors.

[0122] A6a. The method of embodiment A6, wherein each of the plurality of DL reference signals is associated with one of the angles of arrival and one or more of the delays associated with the power delay profile.

[0123] A7. The method of embodiment A6 or A6a, wherein the DL reference signals are channel state information reference signals (CSI-RS).

[0124] A8. The method of any of embodiments A1-A7, further comprising, after transmitting information identifying the set of FD basis vectors and the DL reference signals to the UE, the base station receiving a CSI report message transmitted by the UE, wherein the message includes a set of coefficients, each coefficient being associated with one of the DL reference signals and one FD basis vector from a subset of the set of FD basis vectors selected by the UE.

[0125] A9. The method of any of embodiments A1-A8, wherein the set of FD basis vectors includes one or more FD basis vectors.

[0126] A10. The method of any of embodiments A1-A9, wherein the set of FD basis vectors includes adjacent DFT vectors.

[0127] A11. The method of embodiment A10, wherein the set of adjacent DFT vectors is indicated by an index of a first DFT vector in the set and a total number of DFT vectors in the set.

[0128] A12. The method of any of embodiments A1-A11, wherein, when the set of FD basis vectors includes one FD basis vector, the one FD basis vector can be predetermined to be a DFT vector associated with a zero hertz frequency.

[0129] A13. The method of any of embodiments A1-A12, wherein a length of the DFT vector is derived from a configured parameter.

[0130] A14. The method of any of embodiments A1-A13, wherein the FD basis vector set is common for all transmission layers.

[0131] A15. The method of any of embodiments A1-A13, wherein the FD basis vector set can be different for different transmission layers.

[0132] A16. The method of any of embodiments A1-A14, wherein the transmission can be by one or more of: radio resource control (RRC) signaling, dynamic signaling in a physical downlink control channel (PDCCH), downlink control information (DCI), or a medium access control element (MAC CE).

[0133] B1. A method (900) performed by a UE, the method comprising: transmitting (s902), to a base station, a reference signal; receiving (s904), from the base station, information identifying a set of FD basis vectors selected by the base station; receiving (s906), from the base station, a plurality of downlink (DL) reference signals (RSs), and a request for channel state information (CSI) feedback based on the DL RSs and the set of FD basis vectors; selecting (s908) a subset of the set of FD basis vectors, and estimating CSI based on the DL RSs and the selected subset of FD basis vectors; and transmitting (s910), to the base station, a CSI report message including a set of coefficients, each coefficient associated with one of the DL RSs and one of the selected subset of FD basis vectors.

[0134] B2. The method of embodiment B1, wherein the DL RSs are channel state information RSs, CSI-RSs.

[0135] B3. The method of embodiment B1, wherein the reference signal transmitted by the UE is a sounding reference signal (SRS).

[0136] B4. The method of embodiment B1, B2, or B3, wherein the selected subset is the same as the set of FD basis vectors.

[0137] B5. The method of any of embodiments B1-B4, wherein the DL RSs are channel state information RSs, CSI-RSs.

[0138] B6. The method of any of embodiments B1-B5, wherein the reference signal transmitted by the UE is a sounding reference signal (SRS).

[0139] B7. The method of any of embodiments B1-B6, wherein the set of FD basis vectors comprises one or more FD basis vectors.

[0140] B8. The method of any of embodiments B1-B7, wherein the set of FD basis vectors comprises adjacent DFT vectors.

[0141] B9. The method of embodiment B8, wherein the set of adjacent DFT vectors is indicated by an index of a first DFT vector in the set and a total number of DFT vectors in the set.

[0142] B10. The method of any of embodiments B1-B9, wherein, when the set of FD basis vectors comprises one FD basis vector, the one FD basis vector can be predetermined to be a DFT vector associated with a zero Hertz frequency.

[0143] B11. The method of any of embodiments B1-B10, wherein a length of the DFT vectors is derived from a configured parameter.

[0144] B12. The method of any of embodiments B1-B11, wherein the set of FD basis vectors is common for all transmission layers.

[0145] B13. The method of any of embodiments B1-B12, wherein the set of FD basis vectors can be different for different transmission layers.

[0146] B14. The method of any of embodiments B1-B13, wherein receiving information identifying the set of FD basis vectors from a base station can be performed by one or more of: radio resource control (RRC) signaling, dynamic signaling in a physical downlink control channel (PDCCH), a medium access control element (MAC CE), or downlink control information (DCI).

[0147] C1. A computer program (1043) comprising instructions (1044) which, when executed by a processing circuitry (1002), causes the processing circuitry (1002) to perform the method of any of embodiments A1-A16.

[0148] C2. A computer program (1143) comprising instructions (1144) which, when executed by a processing circuitry (1102), causes the processing circuitry (1102) to perform the method of any of embodiments B1-B3.

[0149] C3. A carrier containing the computer program of embodiment C1 or C2, wherein the carrier is one of an electronic signal, optical signal, radio signal, and computer readable storage medium (1042, 1142).

[0150] D1. A base station (104) adapted to perform the method of any of embodiments Al-A16.

[0151] D2. A base station (104) comprising: processing circuitry (1002); and a memory (1042) containing instructions (1044) executable by said processing circuitry, whereby said apparatus is operative to perform the method of any of embodiments Al-A6.

[0152] E1. A UE (102) adapted to perform the method of any of embodiments B1-B3.

[0153] E2. A UE (102) comprising: processing circuitry (1102); and a memory (1142) containing instructions (1144) executable by said processing circuitry, whereby said apparatus is operative to perform the method of any of embodiments B1-B3.

[0154] While various embodiments have been described herein, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present disclosure should not be limited by any of the above described exemplary embodiments, but should be defined in accordance with the following claims and their equivalents.

[0155] Furthermore, while the procedures described above and illustrated in the drawings show the steps as a discrete process, this is merely for illustration. Thus, it is contemplated that some steps can be added, some steps can be omitted, the order of the steps can be re-arranged, and some steps can be performed in parallel.

Claims

1. A method (800) performed by a base station (104), the method comprising: Select the (s802) frequency domain FD basis vector set; as well as Send (s804) information identifying the selected FD basis vector to the user equipment (UE) (102); The selected FD basis vector is chosen based on the measurement of the reference signal RS transmitted by the UE; The method further includes: sending multiple downlink DL reference signals, and requesting the UE to measure the DL reference signals and feed back a channel state information (CSI) report based on the DL reference signals and the FD basis vector set; The method further includes: after sending the information identifying the FD basis vector set and the DL reference signal to the UE, the base station receives a CSI report message sent by the UE, wherein the message includes a set of coefficients, each coefficient being associated with a DL reference signal in the DL reference signal and an FD basis vector from a subset of the FD basis vector set selected by the UE.

2. The method according to claim 1, wherein, The RS is the detection reference signal SRS.

3. The method according to claim 1 or 2, further comprising: The base station estimates the angle of arrival and associated power delay profile of the reference signal based on the reference signal transmitted by the UE, wherein the base station selects the FD basis vector set based on the estimated angle of arrival and the associated power delay profile of the reference signal.

4. The method according to claim 3, further comprising: The base station transmits multiple downlink DL reference signals based on: i) the estimated power delay profile, and / or ii) the estimated angle of arrival.

5. The method according to claim 3, wherein, Each of the plurality of DL reference signals is associated with one of the angles of arrival and one or more delays associated with the power delay profile.

6. The method according to claim 1 or 5, wherein, The DL reference signal is the Channel State Information Reference Signal (CSI-RS).

7. The method according to any one of claims 1-2, wherein, The FD basis vector set includes one or more FD basis vectors.

8. The method according to any one of claims 1-2, wherein, The FD basis vector set includes adjacent DFT vectors.

9. The method according to claim 8, wherein, Adjacent DFT vector sets are indicated by the index of the first DFT vector in the FD basis vector set and the total number of DFT vectors in the FD basis vector set.

10. The method according to claim 9, wherein, The length of the DFT vector is derived from the configured parameters.

11. The method according to any one of claims 1-2, wherein, When the FD basis vector set includes an FD basis vector, the FD basis vector is predetermined as a DFT vector associated with a zero Hertz frequency.

12. The method according to any one of claims 1-2, wherein, The FD basis vector set is applicable to all transport layers.

13. The method according to any one of claims 1-2, wherein, The FD basis vector set is different for different transport layers.

14. The method according to any one of claims 1-2, wherein, Sending the information identifying the FD base vector set to the UE can be performed via one or more of the following: Radio Resource Control (RRC) signaling, Dynamic signaling in the Physical Downlink Control Channel (PDCCH) Downlink Control Information (DCI), or Media Access Control Element (MAC CE).

15. A computer program product (1041) comprising a computer program (1043) including instructions (1044) that, when executed by a processing circuit (1102) of a base station (104), cause the base station (104) to perform the method of any one of claims 1-14.

16. A computer-readable storage medium (1042) having a computer program (1043) stored thereon, the computer program (1043) including instructions (1044) that, when executed by a processing circuit (1102) of a base station (104), cause the base station (104) to perform the method of any one of claims 1-14.

17. A base station (104), the base station comprising: Processing circuit (1002); as well as A memory (1042) containing instructions (1044) executable by the processing circuitry, wherein the base station is configured to perform the method according to any one of claims 1-14.

18. A method (900) performed by a user equipment (UE) (102), the method comprising: Send (s902) reference signal to base station (104); Receive (s904) information from the base station identifying the FD basis vector set selected by the base station; Receives (s906) multiple downlink DL reference signals RS from the base station, as well as requests for channel state information (CSI) feedback based on the DL RS and the FD basis vector set; Select a subset of the FD basis vector set (s908), and estimate the CSI based on the DL RS and the selected subset of FD basis vectors; as well as Send (s910) a CSI report message to the base station, which includes a set of coefficients, each coefficient being associated with one of the DLRSs and one of the FD basis vectors included in a subset of the selected FD basis vectors.

19. The method according to claim 18, wherein, The selected subset is the same as the FD basis vector set.

20. The method according to claim 18 or 19, wherein, The DL RS is Channel State Information RS, i.e., CSI-RS.

21. The method according to claim 18 or 19, wherein, The reference signal transmitted by the UE is a sounding reference signal (SRS).

22. The method according to any one of claims 18-19, wherein, The FD basis vector set includes one or more FD basis vectors.

23. The method according to any one of claims 18-19, wherein, The FD basis vector set includes adjacent DFT vectors.

24. The method according to claim 23, wherein, Adjacent DFT vector sets are indicated by the index of the first DFT vector in the FD basis vector set and the total number of DFT vectors in the FD basis vector set.

25. The method according to claim 24, wherein, The length of the DFT vector is derived from the configured parameters.

26. The method according to any one of claims 18-19, wherein, When the FD basis vector set includes an FD basis vector, the FD basis vector is predetermined as a DFT vector associated with a zero Hertz frequency.

27. The method according to any one of claims 18-19, wherein, The FD basis vector set is applicable to all transport layers.

28. The method according to any one of claims 18-19, wherein, The FD basis vector set is different for different transport layers.

29. The method according to any one of claims 18-19, wherein, Receiving the information identifying the FD base vector set from the base station can be performed by one or more of the following: Radio Resource Control (RRC) signaling, Dynamic signaling in the Physical Downlink Control Channel (PDCCH) Media Access Control Element (MAC CE), or Downlink Control Information (DCI).

30. A computer program product (1141) comprising a computer program (1143) including instructions (1144) that, when executed by a processing circuit (1102) of a UE (102), cause the UE (102) to perform the method of any one of claims 18-29.

31. A computer-readable storage medium (1142) having a computer program (1143) stored thereon, the computer program including instructions (1144) which, when executed by a processing circuit (1102) of a UE (102), cause the UE (102) to perform the method of any one of claims 18-29.

32. A user equipment (UE) (102), the UE comprising: Processing circuit (1102); as well as A memory (1142) containing instructions (1144) executable by the processing circuitry, wherein the UE is configured to perform the method according to any one of claims 18-29.

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