WO2018143995A1

WO2018143995A1 - Codebook selection among codebooks with different spatial granularity for wireless networks

Info

Publication number: WO2018143995A1
Application number: PCT/US2017/016297
Authority: WO
Inventors: Eugene Visotsky; Xiaomao Mao
Original assignee: Nokia Technologies Oy
Priority date: 2017-02-02
Filing date: 2017-02-02
Publication date: 2018-08-09

Abstract

A technique includes determining, by the user device, a recommended codebook of a plurality of codebooks with different spatial granularities, sending, by the user device to the base station, a first codebook index (CBI) that indicates the recommended codebook for use in precoding, and receiving, by the user device from the base station, information regarding a selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected by the base station for performing precoding for downlink transmissions to the user device.

Description

Codebook Selection Among Codebooks With Different Spatial Granularity For Wireless

Networks

TECHNICAL FIELD

[0001] This description relates to communications.

BACKGROUND

[0002] A communication system may be a facility that enables communication between two or more nodes or devices, such as fixed or mobile communication devices. Signals can be carried on wired or wireless carriers.

[0003] An example of a cellular communication system is an architecture that is being standardized by the 3^rd Generation Partnership Project (3GPP). A recent development in this field is often referred to as the long-term evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) radio-access technology. E- UTRA (evolved UMTS Terrestrial Radio Access) is the air interface of 3GPP's Long Term Evolution (LTE) upgrade path for mobile networks. In LTE, base stations or access points (APs), which are referred to as enhanced Node AP (eNBs), provide wireless access within a coverage area or cell. In LTE, mobile devices, or mobile stations are referred to as user equipments (UE). LTE has included a number of improvements or developments.

[0004] A global bandwidth shortage facing wireless carriers has motivated the consideration of the underutilized millimeter wave (mmWave) frequency spectrum for future broadband cellular communication networks, for example. mmWave (or extremely high frequency) may, for example, include the frequency range between 30 and 300 gigahertz (GHz). Radio waves in this band may, for example, have wavelengths from ten to one millimeters, giving it the name millimeter band or millimeter wave. The amount of wireless data will likely significantly increase in the coming years. Various techniques have been used in attempt to address this challenge including obtaining more spectrum, having smaller cell sizes, and using improved technologies enabling more bits/s/Hz. One element that may be used to obtain more spectrum is to move to higher frequencies, above 6 GHz. For fifth generation wireless systems (5G), an access architecture for deployment of cellular radio equipment employing mmWave radio spectrum has been proposed. Other example spectrums may also be used, such as cmWave radio spectrum (3-30 GHz).

[0005] MIMO (multiple input, multiple output) is an antenna technology for wireless communications in which multiple antennas are used at both the source

(transmitter) and the destination (receiver) in order to reduce errors and/or improve data speed. Beamforming or spatial filtering is a signal processing technique used in arrays for directional signal transmission or reception. This may be achieved by combining elements in a phased array in such a way that signals at particular angles experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity. For example, a complex antenna weight (including amplitude and phase) may be applied to each antenna to perform beamforming. A direction and width of a beam may be controlled based on the amplitude and phase of a set of antenna weights applied to set of antennas.

[0006] Also, a user device (or a user equipment/UE) may determine and report channel state information (CSI) to a base station (BS). The BS may use the CSI for scheduling or transmission of data to the UE. The CSI provided by a UE to a BS may include one or more of a channel quality indicator (CQI), which may be or may include a quantized signal-to-interference plus noise ratio (SINR) and which may represent a highest recommended modulation and coding scheme for downlink transmission, a rank indication (RI), which may be a recommended transmission rank or number of layers that should be used for downlink transmission, and a precoding matrix index (PMI) that indicates a recommended precoder or precoding matrix to be used for precoding for downlink transmission. Precoding is a generalization of beamforming to support multi- stream (or multi-layer) transmission in multi-antenna wireless communications. In conventional single-stream beamforming, the same signal is emitted from each of the transmit antennas with appropriate weighting (phase and gain) such that the signal power is maximized at the receiver output.

SUMMARY

[0007] According to an example implementation, a method may include determining, by the user device, a recommended codebook of a plurality of codebooks with different spatial granularities, sending, by the user device to the base station, a first codebook index (CBI) that indicates the recommended codebook for use in precoding, and receiving, by the user device from the base station, information regarding a selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected by the base station for performing precoding for downlink transmissions to the user device.

[0008] According to an example implementation, an apparatus includes at least one processor and at least one memory including computer instructions, when executed by the at least one processor, cause the apparatus to: determine, by the user device, a recommended codebook of a plurality of codebooks with different spatial granularities, send, by the user device to the base station, a first codebook index (CBI) that indicates the recommended codebook for use in precoding, and receive, by the user device from the base station, information regarding a selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected by the base station for performing precoding for downlink transmissions to the user device.

[0009] According to an example implementation, an apparatus includes means for determining, by the user device, a recommended codebook of a plurality of codebooks with different spatial granularities, means for sending, by the user device to the base station, a first codebook index (CBI) that indicates the recommended codebook for use in precoding, and means for receiving, by the user device from the base station, information regarding a selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected by the base station for performing precoding for downlink transmissions to the user device.

[0010] According to an example implementation, a computer program product includes a computer-readable storage medium and storing executable code that, when executed by at least one data processing apparatus, is configured to cause the at least one data processing apparatus to perform a method including: determining, by the user device, a recommended codebook of a plurality of codebooks with different spatial granularities, sending, by the user device to the base station, a first codebook index (CBI) that indicates the recommended codebook for use in precoding, and receiving, by the user device from the base station, information regarding a selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected by the base station for performing precoding for downlink transmissions to the user device.

[0011] According to an example implementation, a method may include determining, by a user device in a wireless network, at least one channel variation characteristic of a channel between the user device and a base station, determining, by the user device based on the at least one channel variation characteristic, a recommended codebook of a plurality of codebooks with different spatial granularities, sending, by the user device to the base station, information identifying the at least one channel variation characteristic, and receiving, by the user device from the base station, information regarding a selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected by the base station for performing precoding for downlink transmissions to the user device.

[0012] According to an example implementation, an apparatus includes at least one processor and at least one memory including computer instructions, when executed by the at least one processor, cause the apparatus to: determine, by a user device in a wireless network, at least one channel variation characteristic of a channel between the user device and a base station, determine, by the user device based on the at least one channel variation characteristic, a recommended codebook of a plurality of codebooks with different spatial granularities, send, by the user device to the base station, information identifying the at least one channel variation characteristic, and receive, by the user device from the base station, information regarding a selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected by the base station for performing precoding for downlink transmissions to the user device.

[0013] According to an example implementation, a computer program product includes a computer-readable storage medium and storing executable code that, when executed by at least one data processing apparatus, is configured to cause the at least one data processing apparatus to perform a method including: determining, by a user device in a wireless network, at least one channel variation characteristic of a channel between the user device and a base station, determining, by the user device based on the at least one channel variation characteristic, a recommended codebook of a plurality of codebooks with different spatial granularities, sending, by the user device to the base station, information identifying the at least one channel variation characteristic, and receiving, by the user device from the base station, information regarding a selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected by the base station for performing precoding for downlink transmissions to the user device.

[0014] According to an example implementation, a method may include receiving, by a base station in a wireless network from a user device, at least one of the following: at least one channel variation characteristic of a channel between the user device and a base station; and a first codebook index (CBI) indicating a recommended codebook of a plurality of codebooks with different spatial granularities, determining, by the base station, based on either the received at least one channel variation characteristic or the received first codebook index (CBI) indicating a recommended codebook of the plurality of codebooks with different spatial granularities, a selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected for use in precoding, and sending, by the base station to the user device, information regarding the selected codebook of the plurality of codebooks with different spatial granularities.

[0015] According to an example implementation, an apparatus includes at least one processor and at least one memory including computer instructions, when executed by the at least one processor, cause the apparatus to: receive, by a base station in a wireless network from a user device, at least one of the following: at least one channel variation characteristic of a channel between the user device and a base station; and a first codebook index (CBI) indicating a recommended codebook of a plurality of codebooks with different spatial granularities, determine, by the base station, based on either the received at least one channel variation characteristic or the received first codebook index (CBI) indicating a recommended codebook of the plurality of codebooks with different spatial granularities, a selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected for use in precoding, and send, by the base station to the user device, information regarding the selected codebook of the plurality of codebooks with different spatial granularities.

[0016] According to an example implementation, a computer program product includes a computer-readable storage medium and storing executable code that, when executed by at least one data processing apparatus, is configured to cause the at least one data processing apparatus to perform a method including: receiving, by a base station in a wireless network from a user device, at least one of the following: at least one channel variation characteristic of a channel between the user device and a base station; and a first codebook index (CBI) indicating a recommended codebook of a plurality of codebooks with different spatial granularities, determining, by the base station, based on either the received at least one channel variation characteristic or the received first codebook index (CBI) indicating a recommended codebook of the plurality of codebooks with different spatial granularities, a selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected for use in precoding, and sending, by the base station to the user device, information regarding the selected codebook of the plurality of codebooks with different spatial granularities.

[0017] The details of one or more examples of implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 is a block diagram of a wireless network according to an example implementation. [0019] FIGs. 2, 3 A and 3B are diagrams illustrating a plurality of codebooks having different spatial granularity according to example implementations.

[0020] FIG. 4 is a flow chart illustrating operation of a user device according to an example implementation.

[0021] FIG. 5 is a flow chart illustrating operation of a user device according to another example implementation.

[0022] FIG. 6 is a flow chart illustrating operation of a base station according to an example implementation.

[0023] FIG. 7 is a block diagram of a node or wireless station (e.g., base station/access point or mobile station/user device) according to an example

implementation.

DETAILED DESCRIPTION

[0024] FIG. 1 is a block diagram of a wireless network 130 according to an example implementation. In the wireless network 130 of FIG. 1, user devices 131, 132, 133 and 135, which may also be referred to as mobile stations (MSs) or user equipment (UEs), may be connected (and in communication) with a base station (BS) 134, which may also be referred to as an access point (AP), an enhanced Node B (eNB), a gNB (which may be a 5G base station) or a network node. At least part of the functionalities of an access point (AP), base station (BS) or (e)Node B (eNB) may be also be carried out by any node, server or host which may be operably coupled to a transceiver, such as a remote radio head. BS (or AP) 134 provides wireless coverage within a cell 136, including to user devices 131, 132, 133 and 135. Although only four user devices are shown as being connected or attached to BS 134, any number of user devices may be provided. BS 134 is also connected to a core network 150 via a SI interface 151. This is merely one simple example of a wireless network, and others may be used.

[0025] A user device (user terminal, user equipment (UE)) may refer to a portable computing device that includes wireless mobile communication devices operating with or without a subscriber identification module (SIM), including, but not limited to, the following types of devices: a mobile station (MS), a mobile phone, a cell phone, a smartphone, a personal digital assistant (PDA), a handset, a device using a wireless modem (alarm or measurement device, etc.), a laptop and/or touch screen computer, a tablet, a phablet, a game console, a notebook, and a multimedia device, as examples. It should be appreciated that a user device may also be a nearly exclusive uplink only device, of which an example is a camera or video camera loading images or video clips to a network.

[0026] In LTE (as an example), core network 150 may be referred to as Evolved Packet Core (EPC), which may include a mobility management entity (MME) which may handle or assist with mobility /handover of user devices between BSs, one or more gateways that may forward data and control signals between the BSs and packet data networks or the Internet, and other control functions or blocks.

[0027] The various example implementations may be applied to a wide variety of wireless technologies or wireless networks, such as LTE, LTE-A, 5G, cmWave, and/or mmWave band networks, or any other wireless network. LTE, 5G, cmWave and mmWave band networks are provided only as illustrative examples, and the various example implementations may be applied to any wireless technology/wireless network.

[0028] Various example implementations may relate, for example, to 5G radio access systems (or other systems) with support for Massive MIMO (multiple input, multiple output) and optimized for operating in high carrier frequencies such as cmWave frequencies (e.g. from 3 GHz onwards) or mmWave frequencies, as examples, according to an illustrative example implementation. Those illustrative systems are typically characterized by the need for high antenna gain to compensate for increased pathloss and by the need for high capacity and high spectral efficiency to respond to ever increasing wireless traffic. According to an example implementation, the increased attenuation at higher carrier frequencies may, for example, be compensated by introducing massive (multi-element) antenna arrays and correspondingly antenna gain via beamforming at the access point (AP) / base station (BS) and/or user device. The spectral efficiency may typically improve with the number spatial streams the system can support and thus with the number of antenna ports at the BS. According to an example implementation, spatial multiplexing may include a transmission technique in MIMO wireless communication to transmit independent and separately encoded data signals, so-called streams, from each of the multiple transmit antennas.

[0029] For example, for massive multiple input multiple output (M-MIMO) system, a large number of antenna elements may typically be used at a transmitter and/or receiver (e.g., at a base station/access point or other network node). M-MIMO may typically have more spatial links/layers and provides more spatial degrees of freedom. In an illustrative example, with well designed antenna weights, a MIMO or M-MFMO transmitter can generate relatively narrow beams with good spatial separation. Thus, such a transmitter can achieve greater beamforming gain, reduce the spatial interference range and obtain greater multiple user spatial multiplexing gain. A MIMO or M-MFMO system may typically have better performance in terms of data rate and link reliability compared with other systems.

[0030] For example, as shown in FIG. 1, to cover a cell, multiple beams are typically used, such as beam 1, beam 2, beam 3 up to N beams, for example. However, in many cases, only a subset of beams can be active at the same time, e.g., to reduce cost and complexity. Thus, beam sweeping may be used to transmit signals over each of a plurality of beams or sets of beams over multiple time periods. Beam sweeping may include activating each beam or a set of beams over multiple time periods. A

transmission between a user device and a BS may be communicated via a beam pair, which may include a beam applied by a BS and a beam applied by a user device. For example, for a transmission, a beam pair may be used including a transmit beam applied by a BS and a receive beam applied by a user device.

[0031] A user device (or a user equipment/UE) may determine and report channel state information (CSI) to a base station (BS). The BS may use the CSI for scheduling or transmission of data to the UE. The CSI provided by a UE to a BS may include one or more of a channel quality indicator (CQI), which may be or may include a quantized signal-to-interference plus noise ratio (STNR) and which may represent a highest recommended modulation and coding scheme for downlink transmission, a rank indication (RI), which may be a recommended transmission rank or number of layers that should be used for downlink transmission, and a precoding matrix index (PMI) that indicates a recommended precoder or precoding matrix to be used for precoding for downlink transmission. Precoding is a generalization of beamforming to support multi- stream (or multi-layer) transmission in multi-antenna wireless communications.

[0032] MIMO system performance may, for example, depend on the accuracy and the efficiency of CSI feedback, in which PMI (precoding matrix index) feedback is an important feedback. To facilitate the CSI feedback, CSI feedback with different granularity in time and frequency domain can be configured in MIMO transmission. For time domain, periodic CSI feedback with different periodicity can be configured, as well as aperiodic/multi-shot CSI feedback. For frequency domain, subband or wideband CSI reporting can be configured. A PMI (precoding matrix index, or a precoding matrix indicator) may identify a precoding matrix (also referred to as a precoding vector or a precoder), which may have a plurality of complex value antenna weights (e.g., each antenna weight including an amplitude and phase). The antenna weights (or elements) of the precoding matrix may be applied to provide a beam for transmission (e.g., for providing precoding, which may include beamforming). The amplitudes and phases of the antenna weights of a precoding matrix may determine a direction and beam width of a beam.

[0033] According to an example implementation, an implicit CSI (including PMI) feedback mechanism may be used for MIMO transmission. LTE-Advanced has defined two major types of feedback, i.e. implicit and explicit feedback. Implicit feedback makes some assumptions on the transmit precoding and receiver processing at the time of CSI and CQI feedback. The CSI may be expressed in terms of a recommended precoding matrix, indicated by a PMI. Explicit feedback refers to the feedback of channel information without making any assumption on the transmit and receiver processing. According to an illustrative example, a, UE first reports a PMI to eNB/gNB (where gNB may be a 5G BS, for example) based on one predefined codebook. Then, the eNB/gNB, taking into consideration the UE reported PMI, determines the precoding matrix (or precoding vector) and eNB/BS may apply the precoding to both DL (downlink) data and/or some reference signals, such as demodulation reference signals (DMRS). At the receiver, UE does not necessarily need to resolve the actual selected precoder but may, for example, simply estimate the channel based on received DMRS and then uses the estimated channel information to demodulate the transmitted data.

[0034] Also, according to an example implementation, a dual or two-step codebook for cross-polarized antenna array may be used or provided. And, implicit CSI feedback based on two-step codebook may be used, e.g., where a UE will report two PMIs (precoding matrix indexes) and a precoder will be constructed by combing PMI1 (a precoding matrix index from Wl codebook) and PMI2 (a precoding matrix index from W2 codebook). For example, the Wl codebook elements (precoding matrices) may indicate a group of optional beams or steering beams (representing the steering directions of the precoders), and W2 codebook elements (precoding matrices) indicating the best beam or steering beam selection (representing the best steering direction for the UE) and the co-phasing between two polarizations (azimuth and elevation). Co-phasing may involve transmitting the same signal on multiple antennas. This is typically done to shape the radiation pattern in order to direct the energy in a specific direction. Also, according to an example implementation, a two-dimensional (in both azimuth and elevation dimensions) codebook (2D codebook), e.g., including a PMI1 (PMI1 indicating a precoding matrix from Wl codebook) may be extended to include two precoder indexes, PMI11 (precoding matrix index from Wl 1 codebook in elevation dimension) and PMI12 (precoding matrix index from W12 codebook in azimuth dimension), which represent the best beams/steering beams for the elevation or the azimuth dimension, for example. Thus, for a 2D codebook, CSI (including PMI) feedback from the UE to the BS, and a precoder selection (from the BS to the UE) may include a PMI11 and PMI 12 (indicating the best steering beams for azimuth and elevation dimensions).

[0035] In general, there may be a tradeoff between a performance of a codebook (e.g., a larger size of a codebook, having more precoding matrices, allows for better performance) and CSI (including PMI) feedback or reporting overhead. Typically, a codebook, such as a Wl codebook and/or a W2 codebook, are fixed (including being fixed in size, and a fixed set of precoders/precoding matrices). CSI feedback with different granularity in time domain and frequency domain may also, for example, be configured in MIMO transmission, which may improve flexibility and achieve a desired tradeoff between performance and reporting overhead in time and frequency domain.

[0036] Also, according to an example implementation, a plurality of codebooks with different spatial granularity, which means that the plurality of codebooks have different beam width and a different number of precoders/precoding matrices. Thus, each of a plurality of the codebooks with different spatial granularity would have a different beam width. And, according to an example implementation, a uniform beam width may (for example) be applied or used by each of the codebooks (same beam width for each precoder within a codebook, and different beam widths among different codebooks) Thus, a set of codebooks may be provided in which each of the codebooks may have a different spatial granularity (e.g., including a different beam width and a different number of precoders or precoding matrices). In an example implementation, these plurality of codebooks (having different spatial granularity) may also be referred to as a set of multilevel spatial codebooks that are spatially diverse or have different spatial granularity. According to an example implementation, CSI feedback may be provided with different spatial granularity (e.g., with different beam width, based on a selected one of these codebooks), depending on one or more measured conditions or characteristics (such as based on a channel variation characteristic).

[0037] For example, a codebook may include a plurality of precoding matrices or precoder vectors (or simply precoders), with each precoding matrix including a set of complex antenna weights, each antenna weight including an amplitude and phase. Each antenna weight may be applied to an antenna within a set of antennas, where the amplitude and phase of the antenna weights of the precoding matrix may determine the beam characteristics, such as beam direction, beam width, etc. For example, each precoding matrix within a codebook may provide a different beam direction, and, for example, all precoding matrices within a codebook may include the same beam width. Therefore, according to an example implementation, a set or plurality of codebooks with different spatial granularities may be provided (and known by both the UE/user device and the BS/e B), where each codebook of the plurality of codebooks with different spatial granularities may use a different beam width for its precoders/precoding matrices, and may have a different number of precoders/precoding matrices. For example, a first codebook may include a first number of precoding matrices, and a first beam width; while a second codebook, of the plurality of codebooks with different spatial

granularities, may include a second number (greater than the first number) of precoding matrices and a second beam width (narrower than the first beam width). Thus, a codebook that has a larger number of precoders/precoding matrices may be associated with a finer spatial granularity (e.g., smaller/narrower beam width) for the codebook. Whereas, a codebook that has a smaller number of precoders/precoding matrices may be associated with a coarser spatial granularity (e.g., larger/wider beam width) for the codebook. Also, a codebook that uses or provides a finer spatial granularity (having more precoders/precoding matrices, and with a smaller/narrower beam width) may provide higher performance, but at the cost of increased (or greater) PMI signaling overhead, because more bits are required to signal a recommended or selected precoding matrix out of all possible precoding matrices within such codebook, as compared to a smaller codebook which may have fewer precoders and a larger beam width (thus requiring fewer bits to indicate a recommended or selected precoder/precoding matrix within such codebook).

[0038] For example, a plurality of multi -level spatial Wl codebooks may be provided, where the precoders/precoding matrices of the codebook have a beamwidth that is different for each of the Wl codebooks, e.g., spatially diverse codebooks. Also, a plurality of codebooks with different spatial granularity may also (or alternatively) be applied to (or provided for) W2 codebooks, for example.

[0039] Therefore, according to an example implementation, a plurality of codebooks may be provided with different spatial granularity, such as having different beam width. Different beam width may, for example, include precoders/precoding matrices of each codebook have or cause a different beam width. In an example implementation, one of these codebooks that have different spatial granularity, may be recommended by a UE to the BS, and/or may be selected by a BS for use in precoding. As a result, precoding may be performed with different spatial CSI feedback mechanism, which can achieve CSI feedback with different granularity in spatial domain. In an example implementation, the codebooks having different spatial granularity may include codebooks that include or provide 2D steering beams with different beam widths (e.g., Wl precoders with different spatial granularity among the different codebooks). Based on a measured condition, such as a channel variation, one of the codebooks may be selected and a codebook index (CBI), indicating the selected codebook, may be communicated between e B/gNB and UE. After that, legacy CSI feedback mechanisms can be applied for the other CSI report, such as PMI, RI, CQI, e.g., to allow one of the

precoders/precoding matrices within the selected codebook to be selected for precoding, for example.

[0040] For example, a codebook of a plurality of codebooks that have different spatial granularity (e.g., different beam width) may be recommended by a UE or selected by a BS based on one or more measured conditions, such as based on at least one channel variation characteristic. A channel variation characteristic may include a characteristic or parameter that may indicate or provide an estimate of a variation of a channel, such as providing an estimate of an amount of channel variation or a rate of channel variation, as example. By way of example implementation, a channel variation characteristic may include, for example, a speed or velocity of a UE/user device, an angular spread (at either the UE side or the BS side) of the channel between the UE and BS, or other channel variation characteristic. For example, a speed of a UE/user device may indicate (or may be used to estimate) an amount of channel variation or a channel variation rate. For example, a UE traveling relatively fast (higher speed, e.g., 60 mph) suggests that the channel (e.g., amplitude and phase changes of a signal due to the channel) between the UE and BS is varying or changing quite rapidly (e.g., the channel characteristics may be changing relatively quickly). Whereas, a channel between a stationary (e.g., a zero speed or low speed) UE and a BS may be expected to vary or change much more slowly than for a UE that is moving fast. Thus, the speed or velocity (where velocity may include speed and direction of travel) of a UE may, for example, be used as a channel variation characteristic since UE speed may be indicative of or may be used to estimate a rate of change or variation rate of the UE-BS channel, for example.

[0041] As noted, an angular spread (at either the UE or BS, e.g., as measured by the UE) may also be an example of a channel variation characteristic. Angular spread of a channel may include, for example, a distribution (or range) of the angle of arriving energy or signals. Thus, a large angular spread may indicate that energy/signals are arriving from many different directions or angles, whereas a small (or smaller) angular spread of a channel may indicate or suggest that energy/signals are arriving from a narrow/narrower range of directions or angles, for example. An angular spread may be measured by a UE or by a BS. A UE, for example, may determine or measure an angular spread of a channel at a UE side or a BS side. An angular spread may be an example of a channel variation characteristic. According to an example implementation, a larger angular spread for a channel may indicate a greater channel variation, while a smaller angular spread may indicate a smaller channel variation.

[0042] According to an example implementation, a codebook, of a plurality of codebooks having different spatial granularity (e.g., different beam width) may be selected for use in precoding based on a channel variation characteristic. In this manner, a codebook having a spatial granularity may be selected for precoding that may be suited or tailored for the measured channel variation characteristic. For example, if only one codebook is used (no codebooks with different spatial granularity), then, for example, in the case of a fast moving UE, a PMI (precoding matrix indicator to indicate a selected precoding matrix/precoder within the one codebook) may be selected and fed back (or signaled) to the BS, and then used by the BS for the precoding. However, if the UE is traveling fast, a problem may arise where the UE-BS channel is changing faster than the UE can feed back CSI/PMI and receive precoded data based on such PMI. This may cause the precoding for the channel to be incorrect or inaccurate due to the channel varying or changing at a rate that is faster than the CSI feedback mechanism, at least in some cases.

[0043] Therefore, according to an example implementation, multiple codebooks with different spatial granularity may be provided or used. In an illustrative example implementation, a first codebook, for example, having a coarse spatial granularity (e.g., a codebook having fewer precoders/precoding matrices, and larger beam width) may be selected for precoding in the case of a high channel variation characteristic (e.g., in the case of a fast moving UE, or where a channel variation characteristic is greater than a threshold), e.g., so that a precoding matrix within such codebook may be selected and used that has a larger beam width to accommodate the faster changing channel for the UE. Thus, with a larger beam width, the precoding may still be correct or accurate even if then channel changes or varies, due to the wider or larger beam width used by the codebook. Also, for example, a second codebook having a fine (or finer) spatial granularity (e.g., a codebook having more precoders/precoding matrices, and using smaller beam width) may be selected for precoding in the case of a slow (or slower) channel variation characteristic (e.g., in the case of a slow/slower moving UE, or where channel variation characteristic is less than a threshold), so that a higher performance may be achieved via narrower beam width when the channel variation is less or below a threshold. For example, thresholds may be used to select a codebook of the plurality of codebooks. In this manner, a codebook, from a plurality of codebooks having different spatial granularity, may be selected and used, e.g., based on a measured condition or characteristic, such as a channel variation characteristic.

[0044] FIGs. 2 and 3 are diagrams illustrating a plurality of codebooks having different spatial granularity according to an example implementation. FIG. 2 illustrates transmit beams for different precoders for different codebooks, where different codebooks may use or provide different beam width. FIG. 3 A illustrates the beams from a top view or birds eye view. Referring to FIG. 2, a BS 134 may transmit data or signals to a UE/user device using precoding based on a precoder/precoding matrix of one of a plurality of codebooks. For example, a codebook 1 may have a coarse spatial granularity, with a larger beam width, as shown by the 8 larger ovals or circles, including, for example, a beam 210 (associated with PMIl of codebook 1) and beam 212 (associated with PMI2 of codebook 1). Similarly, a codebook 2 may have a fine (or finer) spatial granularity, as shown by the smaller ovals/beams in FIGs. 2 and 3. Thus, beam 220 (associated with PMIl of codebook 2) and beam 222 (associated with PMI2 of codebook 2) are examples of beams for codebook 2 (having a finer spatial granularity). Thus, the spatial coverage of the beams of codebook 1 (having a larger or coarser spatial granularity) are shown by the larger ovals/circles (e.g., 210, 212), while the spatial coverage of the beams of codebook 2 (having a finer spatial granularity) are shown by the smaller ovals/circles (e.g., 220, 222). Thus, due to the overlapping nature of the beams for these two codebooks, these two codebooks may be referred to as nested codebooks.

[0045] It can be seen in FIGs. 2 and 3 A that there are 8 beams (the larger ovals) for codebook 2 (coarser spatial granularity), and 32 beams (the smaller ovals) for codebook 1 (finer spatial granularity). Thus, in this illustrative example, there are 4 beams of codebook 2 that cover the same area as one beam of codebook 1. And, while the beams of codebooks 1 and 2 cover the same area, they cover this area with different spatial granularity, with beams of codebook 1 covering the area with 8 beams, and the beams of codebook 2 covering the same area with 32 beams. While only two codebooks, with different spatial granularity, are shown, any number of codebooks (with different spatial granularity) may be used. Codebook 1 may have a first uniform beam width for its precoders, and codebook 2 may have or use a second uniform beam width for its precoders/precoding matrices.

[0046] By adjusting the amplitude and phase of the antenna weights of a precoding matrix, the beam width and beam direction of the corresponding beams may be varied, so that different codebooks, having different spatial granularity, may be provided.

[0047] Some additional details will now be provided regarding an illustrative example implementation.

[0048] By adjusting a complex value of each element (each antenna weight) of a precoder, an arbitrary steering direction and width for the corresponding steering beam may be achieved. Thus, this may allow a selectable or arbitrary spatial granularity of the corresponding codebook. Below, a generic technique is described to generate L layers of codebooks that have different spatial granularity by antenna port virtualization and DFT (Discrete Fourier Transform) beams. Some of the precoder elements (antenna weights) in azimuth and elevation dimensions may be punctured or made the same value as the neighbor element (virtualization of a pair of neighbor antenna elements (or antenna weights) into one), as a way to generate a smaller codebooks (e.g., having fewer precoders) or codebooks having fewer precoders and thus lower spatial granularity. Thus, a larger codebook will have more precoders and use a smaller/finer beam width (finer spatial granularity), while a smaller codebook will have fewer precoders and use a larger/wider beam width (coarser spatial granularity).

[0049] Assume the antenna port number in elevation N_x and azimuth domain N₂ are both equal to N = 2^L. According to an example 2D two-step codebook design, Wl precoders can be constructed as below,

[0050] where, i and i₂ are the indexes for elevation and azimuth domain selecting the 2D precoders in B, and B is a vector consisting of 2D precoders I7_{i m}, which is built by taking a Kronecker product of DFT precoders from elevation and azimuth domain.

Vi,_m = h ® U_ri (3)

[0051] Here, O_x and 0₂ are the oversampling factors, by which the spatial resolution of the DFT precoders can be adjusted. It can be seen that there are in total N_x and N₂ DFT precoders in elevation and azimuth domain, which leads to in total N_x x N₂ 2D DFT precoders for Wl .

[0052] According to an example implementation, it is proposed here to virtualize (e.g., make elements the same value, so as to create a smaller codebook) every two neighboring antenna ports (or antenna weights or elements) for both elevation and azimuth domain and apply the same complex value to them to form a virtualized antenna port.

[0053] For example, the same complex value may be used or applied to the first and second antenna weights. Similarly one antenna weight may be used for the third and fourth antenna weight, etc., which will create a smaller codebook.

[0054] In this way, a new Wl codebook may be constructed with coarser spatial granularity and ^WlXW2 2D DFT precoders. Specifically,

. 2πΙ*2 .2πΙ*2 2πΙ*(Ν_ί-2) .2πΙ*(Ν_ί-2)

t, = 1 1 e °i^Ni e °i^Ni ... e °i^Ni j-

(6)

.2πτη*2 .2πτη*2 2ππι*(Ν₂-2) .2ππι*(Ν₂-2)

J-

(7)

[0055] Similarly, a next level (next smaller) codebook may be built by

virtualizing four neighbor antenna ports and arrive a new Wl codebook with even coarser spatial granularity and ^Nl ^² 2D DFT precoders. Repeating this procedure, the process will eventually create a Wl codebook with only one 2D DFT precoder.

[0056] Note spatial codebooks with different spatial granularity may also be built between elevation and azimuth domain, or even non-uniformly as to better cover a hexagonal cell or the handover area. One typical scenario will be urban macro scenario, where vertical variation of the UE channel will be comparatively smaller, a series of codebooks as shown in FIG. 3B is better to fit into this scenario. FIG. 3B is a diagram illustrating beams for multiple codebooks, with at least one of the codebooks (e.g., codebook 2) including different (or uneven) spatial granularity in azimuth and elevation dimensions. As shown in FIG. 3B, beams 310 for codebook 1 have a wide or coarse spatial granularity, while beams 320 of codebook 2 have a finer or smaller spatial granularity.

[0057] According to an example implementation, with the plurality of codebooks (having different spatial granularity) constructed, a best preferred codebook which should be used in the later CSI reporting (including PMI, CQI, RI feedback) may be selected based on eNB/gNB channel variation characteristic (for example, or other characteristic or condition), which may be measured by eNB/gNB angular spread and/or UE speed. The UE may measure the BS angular spread (angular spread at BS side). According to an example implementation, a threshold based codebook selection may use a metric function on both eNB/gNB angular spread and UE speed. Specifically the metric function may, for example, be defined as:

[0058] Where, v_UE represents the UE speed estimation, eNB/gNB represents the angular spread at eNB/gNB, and γ is the calculated metric. Using the codebooks with different spatial granularity, the threshold based method may be formulated as: if γ > T, use larger beams of codebook 1, FIG. 2 — 3

if γ≤T, use smaller beams of codebook 2, FIGs. 2 — 3 ^

[0059] With multiple thresholds for angular spread and UE speed, a decision matrix may be defined to select one codebook from a series or plurality of L codebooks that have different spatial granularity.

[0060] The codebook selection from the plurality of codebooks with different spatial granularity may be made either at eNB/gNB or UE side. UE can measure the angular spread (at UE side and/or BS side) and the UE speed, then report the

measurements to eNB/gNB. The BS (e.g., eNB/gNB) based on the reported

measurement, may select the best codebook, and then signal to UE the codebook index (CBI) of the selected codebook. On the other hand, UE can select best codebook (e.g., select a recommended codebook) based on its measurements and only report the codebook index (CBI) of the recommended codebook to eNB/gNB. For example, the report overhead may be less for the UE reporting of CBI for recommended codebook (instead of reporting its measured channel variation characteristic(s). Thus, one example implementation may use a UE codebook index report procedure (e.g., UE to report to BS/eNB the CBI of recommended codebook, and then UE to receive a confirmation that recommended codebook will be used or UE receive a CBI of a selected (e.g., different) codebook) to facilitate the codebook selection.

[0061] An example, flow of operations may include one or more, or even all, of the following operations, by way of illustrative example.

[0062] Step 1 : e B/g B/BS signals to UE (to cause a measurement of channel variation characteristics and to cause a reporting by UE of codebook index (CBI) report (including CBI of recommended codebook).

[0063] Step 2: UE measures its speed v_UE and angular spread p_eNB-

[0064] Step 3 : UE calculates γ based on the metric function / and the measurement v_UE and p_eNB -, and compares the metric y to a threshold. A recommended codebook, of the plurality of codebooks with different spatial granularity, is determined by UE based on the comparison of the metric to the threshold, for example.

[0065] Step 4: UE reports codebook index (CBI) of recommended codebook based on the comparison results in Eq. (9).

[0066] Step 5: eNB/gNB/BS selects a codebook, of the plurality of codebooks having different spatial granularity, for later CSI reporting considering UE reported codebook index (CBI) for recommended codebook, and BS/eNB signals to UE the final (selected) CBI that indicates the codebook to be used for precoding.

[0067] According to the channel condition, the codebook index report procedure can be triggered from time to time to realize dynamic codebook selection, so the following CSI estimation (including PMI, CQI, RI) can be done based on the best codebook fit to the channel.

[0068] One of the typical use cases may include, for example, a high speed UE scenario. When UE speed is high, the latency of CSI feedback sometimes makes the CSI report useless (inaccurate based on rapidly changing channel). When the Wl precoders mirror to very narrow 2D steering beams, the reported PMIl (Wl codebook) may thus be outdated in a very short time and may not properly track a steering angle of the fast moving UE. In such a case, adjusting the spatial domain granularity of CSI report by reconfiguring to a coarser spatial granularity codebook can mitigate the problem. In such a case, the selected Wl precoder will have wider coverage (due to coarser spatial granularity or wider beam width), thus sustain its effectiveness (accuracy of precoding) for a longer period of time. When UE speed is low, extreme case would be fixed wireless access scenario where UEs are actually static. In this case, a UE steering angle is stable and can be easily tracked. In such case, it is better to serve the UE with a finer spatial granularity codebook as narrow beams (Wl precoder) can achieve high beamforming gain. Thus, according to an example implementation, a codebook may be selected, from a plurality of codebooks with different spatial granularity, that best fits the application scenario, and therefore, better MTMO system performance can be achieved.

[0069] According to an example implementation, for the multi-level spatial codebooks (codebooks having different spatial granularity), coarser granularity codebook may include fewer (or less) Wl precoders than the codebooks having a finer spatial granularity. Therefore, a CSI report based on coarser spatial granularity codebook consumes less overhead in uplink (fewer bits required to identify a precoder of such smaller codebook). Selecting coarser granularity codebook for high speed scenario can also save overhead besides bring better system performance. In general, selecting a codebook, from the plurality of codebooks having different spatial granularity, the best codebook fit to application scenario may avoid unnecessary CSI overhead and may achieve improved performance, and may also achieve the best tradeoff between overhead and system performance.

[0070] Example 1 : FIG. 4 is a flow chart illustrating operation of a user device according to an example implementation. Operation 410 includes determining, by the user device, a recommended codebook of a plurality of codebooks with different spatial granularities. Operation 420 includes sending, by the user device to the base station, a first codebook index (CBI) that indicates the recommended codebook for use in precoding. And operation 430 includes receiving, by the user device from the base station, information regarding a selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected by the base station for performing precoding for downlink transmissions to the user device.

[0071] Example 2: According to an example implementation of example 1, the determining a recommended codebook includes: determining, by a user device in a wireless network, at least one channel variation characteristic of a channel between the user device and a base station; and determining, by the user device based on the at least one channel variation characteristic, a recommended codebook of a plurality of codebooks with different spatial granularities.

[0072] Example 3 : According to an example implementation of any of examples 1-2, the receiving information regarding a selected codebook includes at least one of: receiving a confirmation that the recommended codebook has been selected for performing precoding; and receiving a second codebook index (CBI), different from the first codebook index (CBI), that indicates the selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected by the base station for precoding.

[0073] Example 4: According to an example implementation of any of examples 1-3, and further including receiving, by the user device from the base station, data that has been precoded based on the selected codebook of the plurality of codebooks with different spatial granularities.

[0074] Example 5. According to an example implementation of any of examples 1-4, and further including determining, by the user device with respect to the channel between the base station and the user device, a channel state information (CSI) including a rank indication (RI), a channel quality indication (CQI) and a precoding matrix index (PMI), the precoding matrix index (PMI) indicating a recommended precoding matrix within the selected codebook; and sending, by the user device to the base station, at least the precoding matrix index (PMI) indicating a recommended precoding matrix within the selected codebook.

[0075] Example 6. According to an example implementation of any of examples 1-5, wherein each of the plurality of codebooks with different spatial granularities uses a different beam width.

[0076] Example 7. According to an example implementation of any of examples 1-6, wherein the plurality of codebooks with different spatial granularities include at least a first codebook that uses a first beam width for a first number of precoding matrices, and a second codebook that uses a second beam width for a second number of precoding matrices, wherein the first beam width is different than the second beam width and the first number is different than the second number.

[0077] Example 8. According to an example implementation of any of examples 1-7, wherein the determining, by the user device, at least one channel variation characteristic of a channel between the user device and a base station comprises determining at least one of the following: an estimated speed or velocity of the user device; and an angular spread of the channel at base station side.

[0078] Example 9. According to an example implementation of any of examples 1-8, wherein the determining, by the user device, at least one channel variation characteristic of a channel between the user device and a base station comprises:

determining, by the user device, both an estimated speed or velocity of the user device and an angular spread of the channel; and, wherein the determining, by the user device, a recommended codebook of a plurality of codebooks with different spatial granularities comprises: determining, by the user device, a recommended codebook of the plurality of codebooks with different spatial granularities based on the angular spread and the estimated speed of the user device.

[0079] Example 10. According to an example implementation, an apparatus including at least one processor and at least one memory including computer instructions, when executed by the at least one processor, cause the apparatus to perform a method of any of examples 1-9.

[0080] Example 11. According to an example implementation, an apparatus includes means for performing the method of any of examples 1-9.

[0081] Example 12. According to an example implementation, an apparatus includes a computer program product including a non-transitory computer-readable storage medium and storing executable code that, when executed by at least one data processing apparatus, is configured to cause the at least one data processing apparatus to perform a method of any of examples 1-9.

[0082] Example 13. An apparatus including at least one processor and at least one memory including computer instructions, when executed by the at least one processor, cause the apparatus to: determine, by the user device, a recommended codebook of a plurality of codebooks with different spatial granularities; send, by the user device to the base station, a first codebook index (CBI) that indicates the recommended codebook for use in precoding; and receive, by the user device from the base station, information regarding a selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected by the base station for performing precoding for downlink transmissions to the user device.

[0083] Example 14: FIG. 5 is a flow chart illustrating operation of a user device according to an example implementation. Operation 510 includes determining, by a user device in a wireless network, at least one channel variation characteristic of a channel between the user device and a base station. Operation 520 includes determining, by the user device based on the at least one channel variation characteristic, a recommended codebook of a plurality of codebooks with different spatial granularities. Operation 530 includes sending, by the user device to the base station, information identifying the at least one channel variation characteristic. And, operation 540 includes receiving, by the user device from the base station, information regarding a selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected by the base station for performing precoding for downlink transmissions to the user device.

[0084] Example 15. According to an example implementation of example 14, the receiving includes receiving a codebook index (CBI) that indicates the selected codebook, of the plurality of codebooks with different spatial granularities, for use in precoding.

[0085] Example 16. According to an example implementation of any of examples 14-15, and further including receiving, by the user device from the base station, data that has been precoded based on the selected codebook of the plurality of codebooks with different spatial granularities.

[0086] Example 17. According to an example implementation of any of examples 14-16, and further including sending, by the user device to the base station, a precoding matrix index (PMI) indicating a recommended precoding matrix within the selected codebook, of the plurality of codebooks with different spatial granularities, for use in precoding.

[0087] Example 18. According to an example implementation of any of examples 14-17, wherein each of the plurality of codebooks with different spatial granularities uses a different beam width. [0088] Example 19. According to an example implementation of any of examples 14-18, wherein the plurality of codebooks with different spatial granularities comprise at least a first codebook that uses a first beam width for a first number of precoding matrices, and a second codebook that uses a second beam width for a second number of precoding matrices, wherein the first beam width is different than the second beam width and the first number is different than the second number.

[0089] Example 20. According to an example implementation of any of examples 14-19, wherein the determining, by the user device, at least one channel variation characteristic of a channel between the user device and a base station includes determining at least one of the following: an estimated speed or velocity of the user device; and an angular spread of the channel at a base station side.

[0090] Example 21. According to an example implementation, an apparatus includes at least one processor and at least one memory including computer instructions, when executed by the at least one processor, cause the apparatus to perform a method of any of examples 14-20.

[0091] Example 22. According to an example implementation, an apparatus includes means for performing the method of any of examples 14-20.

[0092] Example 23. According to an example implementation, an apparatus includes a computer program product including a non-transitory computer-readable storage medium and storing executable code that, when executed by at least one data processing apparatus, is configured to cause the at least one data processing apparatus to perform a method of any of examples 14-20.

[0093] Example 24: FIG. 6 is a flow chart illustrating operation of a base station according to an example implementation. Operation 610 includes receiving, by a base station in a wireless network from a user device, at least one of the following: at least one channel variation characteristic of a channel between the user device and a base station; and a first codebook index (CBI) indicating a recommended codebook of a plurality of codebooks with different spatial granularities. Operation 620 includes determining, by the base station, based on either the received at least one channel variation characteristic or the received first codebook index (CBI) indicating a recommended codebook of the plurality of codebooks with different spatial granularities, a selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected for use in precoding. Operation 630 includes sending, by the base station to the user device, information regarding the selected codebook of the plurality of codebooks with different spatial granularities.

[0094] Example 25. According to an example implementation of example 24, wherein the sending includes at least one of the following: sending a confirmation that confirms that the recommended codebook will be used for precoding; and sending a second codebook index (CBI), which is different from the first codebook index, that indicates the selected codebook to be used for precoding.

[0095] Example 26. According to an example implementation of any of examples 24-25, and further including: sending, by the base station to user device, data that has been precoded based on the selected codebook.

[0096] Example 27. According to an example implementation of any of examples 24-26, and further including: determining, by the base station, a selected precoding matrix within the selected codebook of the plurality of codebooks with different spatial granularities; and sending, by the base station to the user device, information regarding the selected precoding matrix within the selected codebook that has been selected by the base station for performing precoding.

[0097] Example 28. According to an example implementation of any of examples 24-27, wherein each of the plurality of codebooks with different spatial granularities uses a different beam width.

[0098] Example 29. According to an example implementation of any of examples 24-28, wherein the plurality of codebooks with different spatial granularities include at least a first spatial codebook that uses a first beam width for precoding matrices, and a second spatial codebook that uses a second beam width for precoding matrices, wherein the first beam width is different than the second beam width.

[0099] Example 30. According to an example implementation of any of examples 24-29, wherein the at least one channel variation characteristic of a channel between the user device and a base station includes at least one of the following: an estimated speed or velocity of the user device; and an angular spread of the channel.

[00100] Example 31. According to an example implementation, an apparatus includes at least one processor and at least one memory including computer instructions, when executed by the at least one processor, cause the apparatus to perform a method of any of examples 24-30.

[00101] Example 32. According to an example implementation, an apparatus includes means for performing the method of any of examples 24-30.

[00102] Example 33. According to an example implementation, an apparatus includes a computer program product including a non-transitory computer-readable storage medium and storing executable code that, when executed by at least one data processing apparatus, is configured to cause the at least one data processing apparatus to perform a method of any of examples 24-30.

[00103] FIG. 7 is a block diagram of a wireless station (e.g., AP, BS, eNB, UE or user device) 1000 according to an example implementation. The wireless station 1000 may include, for example, one or two RF (radio frequency) or wireless transceivers 1002A, 1002B, where each wireless transceiver includes a transmitter to transmit signals and a receiver to receive signals. The wireless station also includes a processor or control unit/entity (controller) 1004 to execute instructions or software and control transmission and receptions of signals, and a memory 1006 to store data and/or instructions.

[00104] Processor 1004 may also make decisions or determinations, generate frames, packets or messages for transmission, decode received frames or messages for further processing, and other tasks or functions described herein. Processor 1004, which may be a baseband processor, for example, may generate messages, packets, frames or other signals for transmission via wireless transceiver 1002 (1002A or 1002B). Processor 1004 may control transmission of signals or messages over a wireless network, and may control the reception of signals or messages, etc., via a wireless network (e.g., after being down-converted by wireless transceiver 1002, for example). Processor 1004 may be programmable and capable of executing software or other instructions stored in memory or on other computer media to perform the various tasks and functions described above, such as one or more of the tasks or methods described above. Processor 1004 may be (or may include), for example, hardware, programmable logic, a programmable processor that executes software or firmware, and/or any combination of these. Using other terminology, processor 1004 and transceiver 1002 together may be considered as a wireless transmitter/receiver system, for example.

[00105] In addition, referring to FIG. 7, a controller (or processor) 1008 may execute software and instructions, and may provide overall control for the station 1000, and may provide control for other systems not shown in FIG. 7, such as controlling input/output devices (e.g., display, keypad), and/or may execute software for one or more applications that may be provided on wireless station 1000, such as, for example, an email program, audio/video applications, a word processor, a Voice over IP application, or other application or software.

[00106] In addition, a storage medium may be provided that includes stored instructions, which when executed by a controller or processor may result in the processor 1004, or other controller or processor, performing one or more of the functions or tasks described above.

[00107] According to another example implementation, RF or wireless transceiver(s) 1002A/1002B may receive signals or data and/or transmit or send signals or data. Processor 1004 (and possibly transceivers 1002A/1002B) may control the RF or wireless transceiver 1002A or 1002B to receive, send, broadcast or transmit signals or data.

[00108] The embodiments are not, however, restricted to the system that is given as an example, but a person skilled in the art may apply the solution to other

communication systems. Another example of a suitable communications system is the 5G concept. It is assumed that network architecture in 5G will be quite similar to that of the LTE-advanced. 5G is likely to use multiple input - multiple output (MTMO) antennas, many more base stations or nodes than the LTE (a so-called small cell concept), including macro sites operating in co-operation with smaller stations and perhaps also employing a variety of radio technologies for better coverage and enhanced data rates.

[00109] It should be appreciated that future networks will most probably utilise network functions virtualization (NFV) which is a network architecture concept that proposes virtualizing network node functions into "building blocks" or entities that may be operationally connected or linked together to provide services. A virtualized network function (VNF) may comprise one or more virtual machines running computer program codes using standard or general type servers instead of customized hardware. Cloud computing or data storage may also be utilized. In radio communications this may mean node operations may be carried out, at least partly, in a server, host or node operationally coupled to a remote radio head. It is also possible that node operations will be distributed among a plurality of servers, nodes or hosts. It should also be understood that the distribution of labour between core network operations and base station operations may differ from that of the LTE or even be non-existent.

[00110] Implementations of the various techniques described herein may be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. Implementations may implemented as a computer program product, i.e., a computer program tangibly embodied in an information carrier, e.g., in a machine-readable storage device or in a propagated signal, for execution by, or to control the operation of, a data processing apparatus, e.g., a programmable processor, a computer, or multiple computers. Implementations may also be provided on a computer readable medium or computer readable storage medium, which may be a non-transitory medium. Implementations of the various techniques may also include implementations provided via transitory signals or media, and/or programs and/or software

implementations that are downloadable via the Internet or other network(s), either wired networks and/or wireless networks. In addition, implementations may be provided via machine type communications (MTC), and also via an Internet of Things (IOT).

[00111] The computer program may be in source code form, object code form, or in some intermediate form, and it may be stored in some sort of carrier, distribution medium, or computer readable medium, which may be any entity or device capable of carrying the program. Such carriers include a record medium, computer memory, readonly memory, photoelectrical and/or electrical carrier signal, telecommunications signal, and software distribution package, for example. Depending on the processing power needed, the computer program may be executed in a single electronic digital computer or it may be distributed amongst a number of computers.

[00112] Furthermore, implementations of the various techniques described herein may use a cyber-physical system (CPS) (a system of collaborating computational elements controlling physical entities). CPS may enable the implementation and exploitation of massive amounts of interconnected ICT devices (sensors, actuators, processors microcontrollers,...) embedded in physical objects at different locations. Mobile cyber physical systems, in which the physical system in question has inherent mobility, are a subcategory of cyber-physical systems. Examples of mobile physical systems include mobile robotics and electronics transported by humans or animals. The rise in popularity of smartphones has increased interest in the area of mobile cyber- physical systems. Therefore, various implementations of techniques described herein may be provided via one or more of these technologies.

[00113] A computer program, such as the computer program(s) described above, can be written in any form of programming language, including compiled or interpreted languages, and can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit or part of it suitable for use in a computing environment. A computer program can be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network.

[00114] Method steps may be performed by one or more programmable processors executing a computer program or computer program portions to perform functions by operating on input data and generating output. Method steps also may be performed by, and an apparatus may be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit).

[00115] Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer, chip or chipset. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. Elements of a computer may include at least one processor for executing instructions and one or more memory devices for storing instructions and data. Generally, a computer also may include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto-optical disks, or optical disks. Information carriers suitable for embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory may be supplemented by, or incorporated in, special purpose logic circuitry.

[00116] To provide for interaction with a user, implementations may be implemented on a computer having a display device, e.g., a cathode ray tube (CRT) or liquid crystal display (LCD) monitor, for displaying information to the user and a user interface, such as a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer. Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speech, or tactile input.

[00117] Implementations may be implemented in a computing system that includes a back-end component, e.g., as a data server, or that includes a middleware component, e.g., an application server, or that includes a front-end component, e.g., a client computer having a graphical user interface or a Web browser through which a user can interact with an implementation, or any combination of such back-end, middleware, or front-end components. Components may be interconnected by any form or medium of digital data communication, e.g., a communication network. Examples of communication networks include a local area network (LAN) and a wide area network (WAN), e.g., the Internet.

[00118] While certain features of the described implementations have been illustrated as described herein, many modifications, substitutions, changes and

equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the various embodiments.

Claims

WHAT IS CLAIMED IS:

1. A method comprising:

determining, by the user device, a recommended codebook of a plurality of codebooks with different spatial granularities;

sending, by the user device to the base station, a first codebook index (CBI) that indicates the recommended codebook for use in precoding; and

receiving, by the user device from the base station, information regarding a selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected by the base station for performing precoding for downlink

transmissions to the user device.

2. The method of claim 1 wherein the determining a recommended codebook comprises:

determining, by a user device in a wireless network, at least one channel variation characteristic of a channel between the user device and a base station; and

determining, by the user device based on the at least one channel variation characteristic, a recommended codebook of a plurality of codebooks with different spatial granularities.

3. The method of any of claims 1-2 wherein the receiving information regarding a selected codebook comprises at least one of:

receiving a confirmation that the recommended codebook has been selected for performing precoding; and

receiving a second codebook index (CBI), different from the first codebook index (CBI), that indicates the selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected by the base station for precoding.

4. The method of any of claims 1-3 and further comprising: receiving, by the user device from the base station, data that has been precoded based on the selected codebook of the plurality of codebooks with different spatial granularities.

5. The method of any of claims 1-4 and further comprising:

determining, by the user device with respect to the channel between the base station and the user device, a channel state information (CSI) including a rank indication (RI), a channel quality indication (CQI) and a precoding matrix index (PMI), the precoding matrix index (PMI) indicating a recommended precoding matrix within the selected codebook; and

sending, by the user device to the base station, at least the precoding matrix index (PMI) indicating a recommended precoding matrix within the selected codebook.

6. The method of any of claims 1-5 wherein each of the plurality of codebooks with different spatial granularities uses a different beam width.

7. The method of any of claims 1-6 wherein the plurality of codebooks with different spatial granularities comprise at least a first codebook that uses a first beam width for a first number of precoding matrices, and a second codebook that uses a second beam width for a second number of precoding matrices, wherein the first beam width is different than the second beam width and the first number is different than the second number.

8. The method of any of claims 2-7 wherein the determining, by the user device, at least one channel variation characteristic of a channel between the user device and a base station comprises determining at least one of the following:

an estimated speed or velocity of the user device; and

an angular spread of the channel at base station side.

9. The method of any of claims 2-8 wherein the determining, by the user device, at least one channel variation characteristic of a channel between the user device and a base station comprises:

determining, by the user device, both an estimated speed or velocity of the user device and an angular spread of the channel;

wherein the determining, by the user device, a recommended codebook of a plurality of codebooks with different spatial granularities comprises:

determining, by the user device, a recommended codebook of the plurality of codebooks with different spatial granularities based on the angular spread and the estimated speed of the user device.

10. An apparatus comprising at least one processor and at least one memory including computer instructions, when executed by the at least one processor, cause the apparatus to perform a method of any of claims 1-9.

11. An apparatus comprising means for performing the method of any of claims 1-9.

12. An apparatus comprising a computer program product including a non- transitory computer-readable storage medium and storing executable code that, when executed by at least one data processing apparatus, is configured to cause the at least one data processing apparatus to perform a method of any of claims 1- 9.

13. An apparatus comprising at least one processor and at least one memory including computer instructions, when executed by the at least one processor, cause the apparatus to:

determine, by the user device, a recommended codebook of a plurality of codebooks with different spatial granularities; send, by the user device to the base station, a first codebook index (CBI) that indicates the recommended codebook for use in precoding; and

receive, by the user device from the base station, information regarding a selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected by the base station for performing precoding for downlink transmissions to the user device.

14. A method comprising:

determining, by a user device in a wireless network, at least one channel variation characteristic of a channel between the user device and a base station;

determining, by the user device based on the at least one channel variation characteristic, a recommended codebook of a plurality of codebooks with different spatial granularities;

sending, by the user device to the base station, information identifying the at least one channel variation characteristic; and

transmissions to the user device.

15. The method of claim 14 wherein the receiving comprises:

receiving a codebook index (CBI) that indicates the selected codebook, of the plurality of codebooks with different spatial granularities, for use in precoding.

16. The method of any of claims 14-15 and further comprising:

receiving, by the user device from the base station, data that has been precoded based on the selected codebook of the plurality of codebooks with different spatial granularities.

17. The method of any of claims 14-16 and further comprising:

sending, by the user device to the base station, a precoding matrix index (PMI) indicating a recommended precoding matrix within the selected codebook, of the plurality of codebooks with different spatial granularities, for use in precoding.

18. The method of any of claims 14-17 wherein each of the plurality of codebooks with different spatial granularities uses a different beam width.

19. The method of any of claims 14-18 wherein the plurality of codebooks with different spatial granularities comprise at least a first codebook that uses a first beam width for a first number of precoding matrices, and a second codebook that uses a second beam width for a second number of precoding matrices, wherein the first beam width is different than the second beam width and the first number is different than the second number.

20. The method of any of claims 14-19 wherein the determining, by the user device, at least one channel variation characteristic of a channel between the user device and a base station comprises determining at least one of the following: an estimated speed or velocity of the user device; and

an angular spread of the channel at a base station side.

21. An apparatus comprising at least one processor and at least one memory including computer instructions, when executed by the at least one processor, cause the apparatus to perform a method of any of claims 14-20.

22. An apparatus comprising means for performing the method of any of claims 14-20.

23. An apparatus comprising a computer program product including a non- transitory computer-readable storage medium and storing executable code that, when executed by at least one data processing apparatus, is configured to cause the at least one data processing apparatus to perform a method of any of claims 14-20.

24. A method comprising:

receiving, by a base station in a wireless network from a user device, at least one of the following:

at least one channel variation characteristic of a channel between the user device and a base station; and

a first codebook index (CBI) indicating a recommended codebook of a plurality of codebooks with different spatial granularities;

determining, by the base station, based on either the received at least one channel variation characteristic or the received first codebook index (CBI) indicating a recommended codebook of the plurality of codebooks with different spatial granularities, a selected codebook, of the plurality of codebooks with different spatial granularities, that has been selected for use in precoding; and

sending, by the base station to the user device, information regarding the selected codebook of the plurality of codebooks with different spatial granularities.

25. The method of claim 24 wherein the sending comprises at least one of the following:

sending a confirmation that confirms that the recommended codebook will be used for precoding; and

sending a second codebook index (CBI), which is different from the first codebook index, that indicates the selected codebook to be used for precoding.

26. The method of any of claims 24-25 and further comprising:

sending, by the base station to user device, data that has been precoded based on the selected codebook.

27. The method of any of claims 24-26 and further comprising:

determining, by the base station, a selected precoding matrix within the selected codebook of the plurality of codebooks with different spatial granularities; and sending, by the base station to the user device, information regarding the selected precoding matrix within the selected codebook that has been selected by the base station for performing precoding.

28. The method of any of claims 24-27 wherein each of the plurality of codebooks with different spatial granularities uses a different beam width.

29. The method of any of claims 24-28 wherein the plurality of codebooks with different spatial granularities comprise at least a first spatial codebook that uses a first beam width for precoding matrices, and a second spatial codebook that uses a second beam width for precoding matrices, wherein the first beam width is different than the second beam width.

30. The method of any of claims 24-29 wherein the at least one channel variation characteristic of a channel between the user device and a base station comprises at least one of the following:

an estimated speed or velocity of the user device; and

an angular spread of the channel.

31. An apparatus comprising at least one processor and at least one memory including computer instructions, when executed by the at least one processor, cause the apparatus to perform a method of any of claims 24-30.

32. An apparatus comprising means for performing the method of any of claims 24-30.

33. An apparatus comprising a computer program product including a non- transitory computer-readable storage medium and storing executable code that, when executed by at least one data processing apparatus, is configured to cause least one data processing apparatus to perform a method of any of claims