Signal Precoding

Abstract

A method (700) for precoding a signal. The method includes determining (s702) a spatial domain (SD) precoding vector and a frequency domain (FD) precoding vector for a downlink (DL) transmission of the signal based on uplink (UL) measurements. The method also includes applying (s704) the FD precoding vector to the signal on subcarriers for carrying the signal, thus producing a modified signal. The method further includes applying (s706) the SD precoding vector to the modified signal to produce a precoded signal, where the SD precoding vector is common for all the subcarriers in one symbol.

Description

[Technical field]

[0001] This disclosure relates to precoding of signals, such as, for example, channel state information reference signals (CSI-RS). [Background technology]

[0002] 1. Codebook-based Precoding

[0003] Multi-antenna techniques can significantly increase the data rate and reliability of wireless communication systems. Performance is particularly improved when both the transmitter and receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and / or related techniques are typically referred to as MIMO.

[0004] The 3rd Generation Partnership Project (3GPP) New Radio (NR) standard is currently evolving with enhanced MIMO support. The core components in NR are MIMO antenna deployment and support for MIMO-related techniques, such as spatial multiplexing. The spatial multiplexing mode aims for high data rates in favorable channel conditions. An illustration of spatial multiplexing operation is provided in FIG. 1A.

[0005] As can be seen in FIG. 1A, the information carrying symbol vector s is N T ×r matrix W (hereafter referred to as the “precoder matrix”), which is expressed as (N T (N corresponding to antenna ports) TIt serves to distribute the transmission energy among subspaces of a x-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), which specifies a unique precoder matrix in the codebook for a given number of symbol streams. Each of the r symbols in s corresponds to a layer, and r is called the transmission rank. In this way, spatial multiplexing is 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 therefore the received N R x1 vector y n But, therefore, y n =H n Ws n +e n It is modeled by, where e n is the noise / interference vector obtained as a realization of a random process. The precoder matrix W may be a wideband precoder, which is either constant over frequency or frequency selective.

[0007] The precoder matrix W is often R ×N T MIMO channel matrix H n , resulting in so-called channel-dependent precoding, which is also commonly referred to as closed-loop precoding, which essentially strives to concentrate the transmission energy in a subspace that is strong in the sense that it transfers most of the transmitted energy to the UE.

[0008] In closed-loop precoding for the NR downlink, the UE sends a recommendation to the NR base station (denoted as gNB) of a suitable precoder matrix to use based on channel measurements in the downlink. The gNB may configure the UE to provide feedback according to CSI-ReportConfig, transmit CSI-RS, and configure the UE to use measurements of the CSI-RS to feed back a recommended precoding matrix that the UE selects from a codebook. A single precoder matrix that is considered to cover a large bandwidth (wideband precoding) may be fed back. It may also be beneficial to feed back a frequency-selective precoding report instead, e.g., several precoder matrices, one per subband, that match the frequency variations of the channel. This is an example of a more general case of channel state information (CSI) feedback, which also encompasses feeding back other information besides the recommended precoder matrix to assist the gNode B in subsequent transmissions to the UE. Such other information may include a channel quality indicator (CQI) as well as a transmission rank indicator (RI). In NR, CSI feedback can be either wideband, where one CSI is reported for the entire channel bandwidth, or frequency-selective, where one CSI is reported for each subband, defined as a number of adjacent resource blocks spanning between 4 PRBs and 32 PRBs, depending on the size of the bandwidth part (BWP).

[0009] Given the CSI feedback from the UE, the gNB determines the transmission parameters it wants to use to transmit data to the UE, including the precoding matrix, transmission rank, and modulation and coding scheme (MCS). These transmission parameters may differ from the recommendation made by the UE. The transmission rank, and therefore the number of spatially multiplexed layers, is reflected in the number of columns of the precoder matrix W. For efficient performance, it is important that a transmission rank that matches the channel properties is selected.

[0010] 2. 2D Antenna Array

[0011] A two-dimensional antenna array has a number of antenna columns, N, corresponding to the horizontal dimension. h and the number of antenna rows corresponding to the vertical dimension, N v and the number of dimensions corresponding to different polarizations, N p Therefore, the total number of antennas is N = N h ×N v ×N p It should be pointed out that the antenna concept is non-limiting in the sense that it can refer to any virtualization (e.g., linear mapping) of a physical antenna element. For example, a pair of physical sub-elements may be fed with the same signal and therefore share the same virtualized antenna port. An example of a 4×4 array with dual polarized antenna elements is shown in FIG. 1B.

[0012] Precoding a signal can be interpreted as multiplying the signal with different precoding (aka "beamforming") weights for each antenna prior to transmission. A common approach is to tailor the precoder matrix to the antenna form factor, i.e., to use N precoder matrix codebooks when designing the codebook. h , N v , and N p The key is to take into account the following:

[0013] The codebook is designed with a particular antenna numbering in mind (or rather a port numbering scheme, where the mapping of antenna ports to physical antennas is up to each deployment). For a given P antenna ports, the precoding codebook is designed such that the P / 2 first antenna ports should map to a set of co-polarized antennas, and the P / 2 last antenna ports are mapped to another set of co-polarized antennas with orthogonal polarization to the first set. This is therefore targeted at dual-polarized antenna arrays. Figure 1C shows an example with 8 antenna ports.

[0014] 3. Channel State Information Reference Signal (CSI-RS)

[0015] In NR, by measuring with respect to CSI-RS, which is a reference symbol sequence introduced with the intention of estimating channel state information, the UE can estimate the effective channel that the CSI-RS is traversing, including the radio propagation channel and antenna gain. More mathematically precise, this implies that if a known CSI-RS signal x is transmitted, the UE can estimate the coupling between the transmitted signal and the received signal (i.e., the effective channel). Thus, if no virtualization is implemented in the transmission, the received signal y can be expressed as y=Hx+e, and the UE can estimate the effective channel H. Up to 32 CSI-RS ports can be configured for an NR UE. That is, the UE can estimate the channel from up to 32 transmit antenna ports.

[0016] An antenna port is equivalent to a reference signal resource that a UE shall use to measure the channel. Thus, a gNB with two antennas may define two CSI-RS ports, where each port is a set of resource elements in a time-frequency grid within a subframe or slot. The base station transmits each of these two reference signals from each of the two antennas so that the UE can measure the two radio channels and report channel state information to the base station based on these measurements. In NR, CSI-RS resources with 1, 2, 4, 8, 12, 16, 24 and 32 ports are supported.

[0017] The sequence used for CSI-RS is r(m), The pseudo-random sequence c(i) is specified in 3GPP TS 38.211, clause 5.2.1. The pseudo-random sequence generator generates at the beginning of each OFDM symbol: It is assumed that the image is initialized to TIFF0007602651000002.tif8170, where: TIFF0007602651000003.tif7170 is the slot number in the radio frame, l is the OFDM symbol number in the slot, and n ID is equal to the upper layer parameter scramblingID or sequenceGenerationConfig.

[0018] 3.1 CSI-RS Port Mapping

[0019] There are 18 different CSI-RS resource configurations in NR, where each has a specific number of ports X. See Table 1 below, taken from 3GPP Technical Specification (TS) 38.211. When CDM is applied, the index k i Let k denote the first subcarrier in the PRB used to map the CSI-RS sequence to resource elements, where k is the second subcarrier. i +1. This set of two subcarriers (k i, k i +1) is associated with CDM group j, where the CDM group covers one, two or four OFDM symbols. i’ or i’ +1 indicates the first OFDM symbol in the slot associated with the CDM group. i and l i’ is signaled from the gNB to the UE by RRC signaling when configuring CSI-RS resources.

[0020] When CDM is applied, the size of the CDM group (L) is either 2, 4 or 8, and the total number of CDM groups is given by the setting (k i ,l i’ ), (k i ,l i’+1) pairs. Thus, a CDM group can refer to a set of two, four or eight antenna ports, where a set of two antenna ports occurs when only CDM in the frequency domain (FD) on two adjacent subcarriers is considered (FD-CDM2).

[0021] According to TS38.211 section 7.4.1.5.3, CSI-RS ports are numbered first within a CDM group and then across CDM groups. The UE shall assume that CSI-RS is transmitted using antenna port p numbered according to: p=3000+s+jL, j=0, 1, ..., N / L-1, s=0, 1, ,..., L-1, where s is the sequence index, L ∈ {1, 2, 4, 8} is the CDM group size, and N is the number of CSI-RS ports. The CDM group index j given in Table 7.4.1.5.3-1 in 38.211 corresponds to the time / frequency location for a given row of the table. TIFF0007602651000004.tif7170. This table is reproduced in Table 1 for convenience. For example, the CSI-RS resource configuration given by row 4 in Table 1 has two CDM groups (j=0, 1) of size L=2, where ports 3000 and 3001 map to the CDM group indicated by k0, and ports 3002 and 3003 map to the CDM group indicated by k0+2. TIFF0007602651000005.tif255170TIFF0007602651000006.tif18170

[0022] 4. CSI Framework in NR

[0023] In NR, a UE may be configured with multiple CSI reporting settings and multiple CSI-RS resource settings. Each resource setting may include multiple resource sets, and each resource set may include up to eight CSI-RS resources. For each CSI reporting setting, the UE feeds back a CSI report. Each CSI reporting setting may include at least the following information: CSI-RS resource set for channel measurements; IMR resource set for interference measurements; CSI-RS resource set for interference measurements; time domain behavior, i.e., periodic, semi-persistent, or aperiodic reporting; frequency granularity, i.e., wideband or subband; CSI parameters to be reported, such as RI, PMI, CQI, and CSI-RS Resource Indicator (CRI), in case of multiple CSI-RS resources in a resource set; codebook type, i.e., Type I or Type II, and codebook subset restriction; measurement restriction; and subband size (one out 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)).

[0024] When a CSI-RS resource set in a CSI reporting setting includes multiple CSI-RS resources, one of the CSI-RS resources is selected by the UE, and a CSI-RS Resource Indicator (CRI) is reported by the UE to indicate to the gNB regarding the selected CSI-RS resource in the resource set, along with the RI, PMI, and CQI associated with the selected CSI-RS resource.

[0025] In aperiodic CSI reporting in NR, two or more CSI reporting settings, each with a different CSI-RS resource set for channel measurements and / or a different resource set for interference measurements, may be configured and triggered simultaneously, in which case multiple CSI reports are aggregated and sent from the UE to the gNB in ​​a single physical uplink shared channel (PUSCH) transmission.

[0026] 5. NR Release 16 (Rel-16) Extended Type II Port Selection Codebook

[0027] An enhanced Type II (eType II) port selection (PS) codebook was introduced in Rel-16, which is intended to be used for beamformed CSI-RS, where each CSI-RS port covers a small portion of the cell coverage area with high gain (compared to non-beamformed CSI-RS). Depending on the gNB implementation, it is usually assumed that each CSI-RS port is transmitted in a 2D spatial beam with a main lobe with azimuth pointing angle and elevation pointing angle. The actual precoder matrix used for CSI-RS is transparent to the UE. Based on measurements, the UE selects and recommends the best CSI-RS port to the gNB to use for DL ​​transmission. The eType II PS codebook can be used by the UE to feedback the selected CSI-RS ports and the method for combining them.

[0028] 5.1 eType II PS Codebook Structure, Configuration and Reporting

[0029] For a given transmission layer l, l ∈ {1,...,v}, where v is the rank indicator (RI), the precoder matrix for all FD-units is of size P CSI-RS ×N3 matrix W lwhere P_CSI-RS is the number of CSI-RS ports, N_3=N_SB×R is the number of PMI subbands, where the value R={1,2} (PMI subband size indicator) is RRC configured, N_SB is the number of CQI bands, also RRC configured, and the RI value v is set according to the configured higher layer parameter typeII-RI-Restriction-r16. The UE shall not report v>4.

[0030] Precoder matrix W l teeth, TIFF0007602651000007.tif6170 can be factorized as W l teeth, TIFF0007602651000008.tif5170, normalized such that l=1,...,v. W1 can be written as CSI-RS × 2L port selection matrix. TIFF0007602651000009.tif10170, where W PS is a vector of size 0 and 1. TIFF0007602651000010.tif10170 is the port selection matrix. The selected ports are indicated by 1's that are common for both polarizations. L is the number of selected CSI-RS ports per polarization. Supported L values ​​can be found in Table 2. W1 is common to all layers.

[0031] W f,l is the size N3×M for layer l. v is the frequency domain (FD) compression matrix, where: TIFF0007602651000011.tif9170 is the number of selected FD precoding vectors, which is determined by the rank indicator v and the RRC configured parameter p v Depends on (p v Supported values ​​of can be found in Table 1), TIFF0007602651000012.tif6170, where: TIFF0007602651000013.tif8170 is a set of N3 orthogonal DFT basis vectors of size N3×1. Selected from TIFF0007602651000014.tif6170, M v is the size N3 × 1 FD precoding vector, W f,l is layer specific.

[0032] TIFF0007602651000015.tif6170 is the selected M for the selected 2L CSI-RS ports. v 2LM for linearly combining FD precoding vectors v Contains coefficients, size 2L×M v This is the linear combination coefficient matrix. For layer l, Only a subset of 5170 coefficients in TIFF0007602651000016.tif are non-zero and are reported. TIFF0007602651000017.tif5170 unreported coefficients are assumed to be 0. The amplitudes and phases of the coefficients in TIFF0007602651000018.tif6170 shall be quantized for reporting. TIFF0007602651000019.tif6170 is layer specific. TIFF0007602651000020.tif55170 Summary of the Invention

[0033] Currently, there are several challenges. For example, a gNB may need to use many different beamformers (e.g., different WDM l ) is an implementation complexity issue in gNB. From a complexity point of view, it is necessary to have a single beam (single W) on one or at most two ports, one per polarization, transmitted in a wideband manner. l), i.e., in one OFDM symbol, over the entire transmission bandwidth and for both polarizations, the same precoder matrix W l (also known as a "beamformer") is used. The problem is that the current structure of the mapping between CSI-RS ports and resource elements and OFDM symbols in NR causes high implementation complexity.

[0034] The present disclosure provides a means for reducing implementation complexity. In one aspect, a method for precoding a signal (e.g., CSI-RS, PDSCH signal, etc.) is provided. The method is implemented by a base station (gNB). The method includes determining a spatial domain (SD) precoding vector and a frequency domain (FD) precoding vector for a downlink (DL) transmission of the signal based on an uplink measurement. The method also includes applying the FD precoding vector to the signal on a subcarrier for carrying the signal, thus producing a modified signal. The method further includes applying an SD precoding vector to the modified signal to produce a precoded signal (Y), where the SD precoding vector is common for all the subcarriers in one symbol (e.g., OFDM symbol).

[0035] In another aspect, another method implemented by a base station is provided, the method including grouping ports that belong to a common beam but have different delay pre-compensation, the method further including transmitting the grouped ports in one OFDM symbol.

[0036] In another aspect, a further method performed by a base station is provided. The method comprises setting, for a UE, N antenna ports in a CSI-RS resource, where the CSI-RS resource is spread over L OFDM symbols, and each OFDM symbol transmits P CSI-RS ports, P < N, for the UE. The method further comprises, for each of the L OFDM symbols, using a single wideband precoding vector for all P CSI-RS ports, where each of the P CSI-RS ports is associated with a delay introduced by modifying a CSI-RS sequence.

[0037] In another aspect, a computer program is provided that, when executed by a processing circuit of a base station, causes the base station to perform the methods disclosed herein. In another aspect, a carrier containing the computer program is provided, the carrier being one of an electronic signal, an optical signal, a wireless signal, and a computer-readable storage medium.

[0038] In another aspect, a base station is provided that is adapted to perform the methods of any of the embodiments disclosed herein. In some embodiments, the base station comprises a processing circuit and a memory, the memory containing instructions executable by the processing circuit, whereby the base station is operable to perform the methods disclosed herein.

[0039] The advantage of the embodiment is that the implementation complexity is significantly reduced because the embodiment does not use a precoder matrix (i.e., a two-dimensional matrix) to generate delay compensation. Instead, a common beamforming weight vector (also known as a spatial domain (SD) precoding vector) can be applied over the entire bandwidth within an OFDM symbol, which reduces the implementation complexity of the beamformed signal. The freed processing resources can be used, for example, for precoding of PDDCH and / or PDSCH to increase performance.

[0040] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various embodiments. [Brief description of the drawings]

[0041] [Figure 1A] FIG. 1 illustrates spatial multiplexing. [Figure 1B] FIG. 1 shows an antenna array with dual polarized antenna elements. [Figure 1C] FIG. 1 is a diagram showing an example of port numbering for eight antenna ports. [Diagram 2] FIG. 10 compares the angle / delay power spectrum of a UL channel and the corresponding DL channel. [Diagram 3] FIG. 13 is a diagram showing an example in which two CSI-RS ports are precoded. [Figure 4] FIG. 1 illustrates an example procedure for reciprocity-based FDD transmission. [Diagram 5] FIG. 13 illustrates an example of how the phase of delay pre-compensation may vary across subcarriers when delay pre-compensation is implemented for the case where eight CSI-RS ports are mapped to the same symbol. [Figure 6] FIG. 2 illustrates a precoding process according to one embodiment. [Figure 7A] 1 is a flowchart illustrating a process according to one embodiment. [Figure 7B] 1 is a flowchart illustrating a process according to one embodiment. [Figure 7C] 1 is a flowchart illustrating a process according to one embodiment. [Figure 8] FIG. 2 is a block diagram of a base station according to one embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] In frequency division duplexing (FDD), perfect reciprocity does not hold between UL and DL channels because small-scale fading is different at two different frequencies. However, some large-scale channel parameters, such as angle and delay to cluster, may still be reciprocal because the physical propagation environment is the same at both frequencies. Figure 2 compares the angle / delay power spectrum of UL channel and the corresponding DL channel, which are plotted according to the 3GPP38.901 channel model at 2 GHz with 200 MHz duplex distance. It is observed that the power spectrum in UL and the power spectrum in DL are very similar. Also, the dominant cluster seen in UL appears at similar angle and delay in DL. However, there are still some differences caused by independent small-scale fading realization.

[0043] In light of the above, delay reciprocity may be utilized in CSI-RS precoding in addition to angle reciprocity to reduce the frequency selectivity of the precoded channel seen at the UE side. More specifically, each CSI-RS port may be precoded using a pair of vectors consisting of a spatial domain (SD) precoding vector and a frequency domain (FD) precoding vector, where the SD precoding vector is used to beamform towards a selected cluster and the FD precoding vector precompensates the delay for that cluster. Figure 3 shows an example in which two CSI-RS ports are precoded. After delay precompensation, the dominant tap in the two clusters is aligned to the first tap. The UE can then compress the FD channel with fewer FD precoding vectors. Also, multiple delays at the same angle may be precompensated by precoding one CSI-RS port per delay. The UE can then filter out the 0th delay component for each port to obtain the amplitude and phase for each delay.

[0044] Angle / delay reciprocity for FDD is currently being studied in 3GPP Rel-17 for extending the Type II port selection codebook. One exemplary procedure for a reciprocity-based FDD transmission scheme is shown in FIG.

[0045] In step 1, a user equipment (UE) 402 is configured by a gNB 404 to transmit a sounding reference signal (SRS) to enable the gNB to estimate angles and delays of different clusters associated with different propagation paths. As used herein, a UE is a device capable of communicating wirelessly with a base station (e.g., a gNB).

[0046] In step 2, the gNB selects a dominant cluster according to the estimated angle-delay power spectral profile, and a set of spatial domain and frequency domain (SD-FD) precoding vector pairs are calculated by the gNB for CSI-RS beamforming based on the estimated angle-delay power spectral profile. Each CSI-RS port may include one or more SD-FD precoding vector pairs. The gNB precodes all CSI-RS ports in the configured CSI-RS resources to the UE.

[0047] In step 3, the gNB configures the UE to measure CSI-RS, and the UE measures the received CSI-RS ports and then determines Type-II CSI, including RI, PMI for each layer, and CQI. The precoding matrix indicated by the PMI includes the selected SD-FD precoding vector pair / precoded CSI-RS port and the corresponding best phase and amplitude for cophasing the selected pair / port. The phase and amplitude for each pair / port are quantized and fed back to the gNB.

[0048] In step 4, the gNB implementation algorithm calculates the DL precoding matrix per layer based on the selected beam and the corresponding amplitude and phase feedback, and performs PDSCH transmission.

[0049] Such reciprocity-based transmission can potentially be exploited in codebook-based DL transmission for FDD, for example, to reduce feedback overhead in the UL when an NR Type II port selection codebook is used. Another potential benefit is the reduced complexity of CSI calculation at the UE.

[0050] In a radio propagation environment, there may be multiple channel clusters in similar directions but different delays. Thus, in the extended Type II port selection codebook considered for Rel. 17, a gNB may apply delay pre-compensation for multiple channel taps in the same beam. One way to achieve this is to allocate one CSI-RS port per channel tap (and per polarization) and apply delay pre-compensation for each such port. Thus, there may be multiple CSI-RS ports corresponding to the same beam but different taps.

[0051] Performing delay pre-compensation by operating in the frequency domain is equivalent to applying a linear phase ramp over frequency on the beamforming vector, where the ramp is proportional to the delay. This is TIFF0007602651000021.tif6170, where: TIFF0007602651000022.tif5170 is the delay precompensated beamforming vector, where f is the frequency, τ is the delay to be compensated, and b(Φ0) is the frequency independent (i.e. wideband) beamformer in direction Φ0. If there are P taps per beam, then the port is the tap p of the beamforming vector For each tap, a time may be assigned as associated with TIFF0007602651000023.tif7170, p=1,...,P, where τ p and Φ0 are the delay and angle, respectively, for the pth tap.

[0052] FIG. 5 shows the phase of delay pre-compensation 2πfτ when delay pre-compensation is implemented for the case where eight CSI-RS ports are mapped to the same symbol. p can vary over subcarriers. Here, it is assumed that there are four pairs of ports, where the ports in a pair correspond to the same tap but different polarizations, and different pairs correspond to taps with different delays. This CSI-RS configuration corresponds to a modified version of row 6 in Table 1, where the modification is made to make ports corresponding to the same beam and different polarizations belong to the same CDM group.

[0053] It can be seen in the figure that even if all ports in a symbol correspond to the same beam, multiple different phase tilts exist in each resource block. This means that the same beamforming vector cannot be used across the bandwidth, even though all ports correspond to beams in the same direction. Therefore, there is high implementation complexity when there are multiple delay pre-compensated CSI-RS ports in one symbol.

[0054] More generally, the precoded CSI-RS sequence r(m) in SD and FD on N antennas and M subcarriers can be expressed as: Y=WR (1) where Y is an N×M matrix containing the precoded CSI-RS signal samples on all antennas and subcarriers, W=bf H (2) is an N×M SD / FD precoding matrix, where b is an N×1 SD precoding vector and f is an M×1 FD precoding vector. For example, the SD and FD precoding vectors may be DFT vectors. Physically, this means that the CSI-RS ports are beamformed in a particular direction and shifted in delay to precompensate for the channel delay. The direction and delay are determined from measurements on the SRS or DMRS transmitted in the UL.

[0055] The SD and FD precoding vectors may also be more general than having a DFT structure, for example, based on the SVD of measurements on the UL channel matrix. R=diag(r), r=[r(1)...r(M)] T , For example, R may be an M×M diagonal matrix containing the CSI-RS sequences r(m), m=1 to M, along the diagonal. As another example, R may be an M×M diagonal matrix containing the PDSCH samples for each subcarrier.

[0056] Equation (1) is TIFF0007602651000024.tif7170, where: TIFF0007602651000025.tif5170 shows element-wise multiplication, TIFF0007602651000026.tif6170 is a signal r (e.g., a CSI-RS sequence) multiplied with the conjugate of f (an FD precoding vector). From equation (3), it can be concluded that the frequency-dependent precoder matrix W in equation (2) can be replaced by a wideband SD precoding vector b if the FD precoding vector f is applied to signal r instead of being included in the precoding matrix W. This is illustrated in FIG. 6, which shows applying f to signal r to generate a modified signal, and then applying the SD precoding vector (b) to the modified signal to produce Y, the precoded signal.

[0057] The above equation shows that for a given CSI-RS port, the CSI-RS precoder matrix (a.k.a. beamformer) can be made wideband by moving the delay pre-compensation (or, more generally, FD precoding) function from the beamformer to the CSI-RS sequence generation. Thus, a wideband beamforming vector (non-frequency dependent) b can be used over the entire bandwidth.

[0058] Using a wideband beamforming vector instead of a frequency-selective precoder matrix enables a transmitter implementation with significant reduction in complexity. Also, the calculations show that any FD precoding vector, e.g., an SVD precoding vector, can be transferred from the precoder matrix to the CSI-RS sequence generation as long as the SD and FD precoding vectors are separable according to (2), i.e., the same FD precoding vector is applied for all antennas.

[0059] Note that the modification of the CSI-RS sequence according to (4) is transparent to the UE, and since the purpose of delay pre-compensation is for the UE to experience an effective channel with the delay removed, the UE shall still base its channel estimation on the original sequence r(m). Hence, this is a patent for gNB implementations.

[0060] In one embodiment, a CSI-RS resource having N ports is configured from the network to the UE. The CSI-RS resource is spread over L OFDM symbols, and each OFDM symbol transmits P CSI-RS ports, where P < N. The UE is also configured to report CSI using a type II port selection codebook. The gNB implementation ensures that each symbol uses a single broadband precoding vector for all P CSI-RS ports, although in some cases there are different delays associated with the P ports in that symbol. These delays for the P ports are brought about by the CSI-RS sequence modification design as described in the present invention. Thus, the gNB port scheduler groups ports that belong to the same beam but have different delay precoding and transmits such groups within one OFDM symbol. The above techniques can be applied to other downlink signals, such as transmissions on the physical downlink shared channel (PDSCH), demodulation reference signals (DM-RS).

[0061] Figure 7A is a flowchart showing a process 700 for precoding a signal. Process 700 may be implemented by gNB 404 and may begin at step s702. Step s702 includes determining a spatial domain (SD) precoding vector and a frequency domain (FD) precoding vector for downlink (DL) transmission of the signal based on uplink measurements (e.g., based on measurements of SRS transmitted by the UE). Step s704 includes applying the FD precoding vector to the signal on the subcarriers for carrying the signal, thus creating a modified signal. Step s706 includes applying the SD precoding vector to the modified signal to create a precoded signal, where the SD precoding vector is common for all said subcarriers within one symbol (e.g., an OFDM symbol).

[0062] In some embodiments, the signal is a CSI-RS, the signal is a DM-RS, or the signal is a PDSCH signal (ie, a transmission on a PDSCH).

[0063] In some embodiments, the UL measurements are based on a reference signal (RS) transmitted by the UE, in some embodiments the RS is a sounding reference signal (SRS) or the RS is a demodulation reference signal (DMRS).

[0064] In some embodiments, the FD precoding vector is a DFT vector, thereby achieving delay pre-compensation by applying a phase ramp over frequency with respect to the CSI-RS sequence.

[0065] In some embodiments, applying the FD precoding vector to the signal comprises generating a conjugate of the FD precoding vector; and calculating TIFF0007602651000027.tif5170, where f * is the conjugate of the FD precoding vector, TIFF0007602651000028.tif5170 shows the element-wise multiplication, r is the signal, TIFF0007602651000029.tif4170 is the corrected signal.

[0066] In some embodiments, applying the SD precoding vector to the modified signal comprises: TIFF0007602651000030.tif5170, where TIFF0007602651000031.tif5170 is the transpose of the corrected signal, and b is the SD precoding vector.

[0067] FIG. 7B is a flowchart showing a process 710 for precoding a signal. The process 710 may be performed by the gNB 404 and may begin at step s712. Step s712 includes grouping ports that belong to a common beam but have different delay precorrections. Step s714 includes transmitting the grouped ports in one OFDM symbol.

[0068] FIG. 7C is a flowchart showing a process 720 for precoding a signal. The process 720 may be performed by the gNB 404 and may begin at step s722. Step s722 is to set N antenna ports for the UE in the CSI-RS resource, where the CSI-RS resource is distributed over L OFDM symbols, and each OFDM symbol transmits P CSI-RS ports, P < N, and includes setting N antenna ports for the UE. Step s724 is, for each of the L OFDM symbols, to use a single broadband precoding vector for all P CSI-RS ports, where each of the P CSI-RS ports is related to the delay introduced by modifying the CSI-RS sequence, and includes using a single broadband precoding vector.

[0069] 8 is a block diagram of a base station 404, according to some embodiments. As shown in FIG. 8, the base station 404 includes a processing circuit (PC) 802, which may include one or more processors (P) 855 (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.), which may be co-sited in a single housing or in a single data center or may be geographically distributed (i.e., the base station 404 may be a distributed computing device), and a network interface 868, which enables the base station 404 to communicate with the network 110 to which the network interface 868 is connected. The PC 802 may comprise a network interface 868 comprising a transmitter (Tx) 865 and a receiver (Rx) 867 for enabling the base station 404 to transmit data to and receive data from other nodes connected thereto (e.g., an Internet Protocol (IP) network); a communication circuit 848 coupled to an antenna arrangement 849 comprising one or more antennas and comprising a transmitter (Tx) 845 and a receiver (Rx) 847 for enabling the base station 404 to transmit data to and receive data from (e.g., transmit / receive data wirelessly); and a local storage unit (a.k.a. "data storage system") 808, which may include one or more non-volatile storage devices and / or one or more volatile storage devices. In an embodiment in which the PC 802 comprises a programmable processor, a computer program product (CPP) 841 may be provided. The CPP 841 comprises a computer readable medium (CRM) 842, which stores a computer program (CP) 843 comprising computer readable instructions (CRI) 844. The CRM 842 may be a non-transitory computer-readable medium, such as a magnetic medium (eg, a hard disk), an optical medium, a memory device (eg, a random access memory, a flash memory), or the like.In some embodiments, the CRI 844 of the computer program 843, when executed by the PC 802, is configured such that the CRI causes the base station 404 to perform the steps described herein (e.g., steps described herein with reference to flow charts). In other embodiments, the base station 404 may be configured to perform the steps described herein without the need for code; that is, for example, the PC 802 may simply consist of one or more ASICs. Thus, features of the embodiments described herein may be implemented in hardware and / or software.

[0070] Overview of Various Embodiments

[0071] A1. A method for precoding a signal (e.g., a CSI-RS, a PDSCH signal, etc.), the method including: determining a spatial domain (SD) precoding vector and a frequency domain (FD) precoding vector for a downlink (DL) transmission of the signal based on uplink measurements (e.g., based on measurements of an SRS transmitted by a UE); applying the FD precoding vector to the signal on subcarriers for carrying the signal, thus producing a modified signal; and applying the SD precoding vector to the modified signal to produce a precoded signal, wherein the SD precoding vector is common for all the subcarriers in one symbol (e.g., an OFDM symbol).

[0072] A2. The method of embodiment A1, wherein the signal is a CSI-RS, the signal is a DM-RS, or the signal is a PDSCH signal.

[0073] A3. The method of embodiment A1 or A2, wherein the UL measurements are based on a reference signal (RS) transmitted by the UE.

[0074] A4. The method of embodiment A3, wherein the RS is a sounding reference signal (SRS) or the RS is a demodulation reference signal (DMRS).

[0075] A5. The method of any one of embodiments A1 to A4, wherein the FD precoding vector is a DFT vector, thereby achieving delay pre-compensation by applying a phase ramp over frequency with respect to the CSI-RS sequence.

[0076] A6. Applying an FD precoding vector to a signal comprises generating the conjugate of the FD precoding vector; and calculating TIFF0007602651000032.tif5170, where f * is the conjugate of the FD precoding vector, TIFF0007602651000033.tif5170 shows the element-wise multiplication, r is the signal, TIFF0007602651000034.tif4170 is the corrected signal, The method according to any one of embodiments A1 to A5.

[0077] A7. Applying SD precoding vectors to the modified signal is TIFF0007602651000035.tif5170, where The method of any one of embodiments A1 to A6, wherein TIFF0007602651000036.tif5170 is the transpose of the corrected signal and b is the SD precoding vector.

[0078] A8. A method implemented by a base station, the method including: grouping ports that belong to a common beam but have different delay pre-compensation; and transmitting the grouped ports in one OFDM symbol.

[0079] A method performed by a base station, the method comprising: configuring N antenna ports for a user equipment (UE) (402) in a CSI-RS resource, wherein the CSI-RS resource is spread over L OFDM symbols, each OFDM symbol transmitting P CSI-RS ports, P < N; and for each of the L OFDM symbols, using a single broadband precoding vector for all P CSI-RS ports, each of the P CSI-RS ports being associated with a delay introduced by modifying a CSI-RS sequence.

[0080] A computer program (843) comprising instructions (844) which, when executed by a processing circuit (802) of a base station (404), cause the base station (404) to perform the method according to any one of the above embodiments.

[0081] A carrier comprising the computer program according to embodiment B1, the carrier being one of an electronic signal, an optical signal, a wireless signal, and a computer-readable storage medium (842).

[0082] A base station (404) adapted to perform the method according to any one of embodiments A1 to A9.

[0083] A base station (404) comprising a processing circuit (802) and a memory (842), the memory containing instructions (844) executable by the processing circuit, whereby the base station (404) is operable to perform the method according to any one of embodiments A1 to A9.

[0084] Although various embodiments have been described herein, it should be understood that the embodiments have been presented by way of example only, and not by way of limitation. Thus, the breadth and scope of the present disclosure should not be limited by any of the exemplary embodiments described above. Moreover, unless otherwise indicated herein or clearly contradicted by context, any combination of the above-described elements in all possible variations thereof is encompassed by the present disclosure.

[0085] Additionally, while the processes described above and illustrated in the Figures have been shown as a sequence of steps, this has been done for purposes of illustration only, and it is therefore contemplated that some steps may be added, some steps may be omitted, the order of the steps may be rearranged, and some steps may be performed in parallel.

Claims

1. A method (700) for precoding a signal, the method comprising: determining (s702) a spatial domain (SD) precoding vector and a frequency domain (FD) precoding vector for a downlink (DL) transmission of the signal based on uplink (UL) measurements; applying (s704) the FD precoding vector to the signal on the subcarriers intended to carry the signal, thus producing a modified signal; applying the SD precoding vector to the modified signal to produce a precoded signal (s706), the SD precoding vector being common to all the subcarriers in one symbol; Including, Applying the FD precoding vector to the signal comprises: generating a conjugate of the FD precoding vector; where f* is the conjugate of the FD precoding vector; denotes element-wise multiplication, r is the signal, is the modified signal.

2. A method (700) for precoding a signal, the method comprising: determining (s702) a spatial domain (SD) precoding vector and a frequency domain (FD) precoding vector for a downlink (DL) transmission of the signal based on uplink (UL) measurements; applying (s704) the FD precoding vector to the signal on the subcarriers intended to carry the signal, thus producing a modified signal; applying the SD precoding vector to the modified signal to produce a precoded signal (s706), the SD precoding vector being common to all the subcarriers in one symbol; Including, Applying the SD precoding vector to the modified signal comprises: where: wherein b is the transpose of the modified signal and b is the SD precoding vector.

3. The signal is a channel state information reference signal (CSI-RS); the signal is a demodulation reference signal (DMRS); or the signal is a Physical Downlink Shared Channel (PDSCH) signal; The method according to claim 1 or 2.

4. The method according to any one of claims 1 to 3, wherein the UL measurements are based on a Reference Signal (RS) transmitted by a User Equipment (UE) (402).

5. The RS is a Sounding Reference Signal (SRS); or The RS is a demodulation reference signal (DMRS), The method according to claim 4.

6. The method of claim 1 , wherein the FD precoding vector is a Discrete Fourier Transform (DFT) vector.

7. A computer program (843) comprising instructions (844) which, when executed by a processing circuit (802) of a base station (404), cause said base station (404) to perform a method according to any one of claims 1 to 6.

8. A computer readable storage medium (842) containing the computer program of claim 7.

9. A base station (404) for precoding a signal, said base station (404) comprising a processing circuit (802), The processing circuit (802) determining (s702) a spatial domain (SD) precoding vector and a frequency domain (FD) precoding vector for a downlink (DL) transmission of the signal based on uplink (UL) measurements; applying (s704) the FD precoding vector to the signal on the subcarriers intended to carry the signal, thus producing a modified signal; applying the SD precoding vector to the modified signal to produce a precoded signal (s706), the SD precoding vector being common to all the subcarriers in one symbol; configured to: Applying the FD precoding vector to the signal comprises: generating a conjugate of the FD precoding vector; where f* is the conjugate of the FD precoding vector; denotes element-wise multiplication, r is the signal, is said modified signal.

10. A base station (404) for precoding a signal, the base station (404) comprising: a processing circuit (802); The processing circuit (802) determining (s702) a spatial domain (SD) precoding vector and a frequency domain (FD) precoding vector for a downlink (DL) transmission of the signal based on uplink (UL) measurements; applying (s704) the FD precoding vector to the signal on the subcarriers intended to carry the signal, thus producing a modified signal; applying the SD precoding vector to the modified signal to produce a precoded signal (s706), the SD precoding vector being common to all the subcarriers in one symbol; [0023] Applying the SD precoding vector to the modified signal comprises: where: a is the transpose of the modified signal, and b is the SD precoding vector.

11. The signal is a channel state information reference signal (CSI-RS); the signal is a demodulation reference signal (DMRS); or the signal is a Physical Downlink Shared Channel (PDSCH) signal; A base station as claimed in claim 9 or 10.

12. The base station according to any one of claims 9 to 11, wherein the UL measurements are based on a Reference Signal (RS) transmitted by a User Equipment (UE) (402).

13. The RS is a Sounding Reference Signal (SRS); or The RS is a demodulation reference signal (DMRS), The base station according to claim 12.

14. The base station according to claim 9 , wherein the FD precoding vector is a Discrete Fourier Transform (DFT) vector.

15. A base station (404), comprising: A processing circuit (802); Memory (842) Equipped with the memory includes instructions (844) executable by the processing circuit; A base station (404), wherein the processing circuit (802) is configured to perform the method according to any one of claims 1 to 6.

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