1111111111111111 IIIIII IIIII 111111111111111 11111 111111111111111 IIIII IIIII IIIIII IIII 11111111
`US 20140016549Al
`
`c19) United States
`c12) Patent Application Publication
`N ovlan et al.
`
`c10) Pub. No.: US 2014/0016549 Al
`Jan. 16, 2014
`(43) Pub. Date:
`
`(54) METHODS AND APPARATUS FOR
`CODEBOOK SUBSET RESTRICTION FOR
`TWO-DIMENSIONAL ADVANCED ANTENNA
`SYSTEMS
`
`(71) Applicant: Samsung Electronics Co., LTD,
`Suwon-si (KR)
`
`(72)
`
`Inventors: Thomas David Novlan, Dallas, TX
`(US); Krishna Sayana, San Jose, CA
`(US); Young-Han Nam, Richardson, TX
`(US); Jin-Kyu Han, Allen, TX (US)
`
`(21) Appl. No.: 13/939,934
`
`(22) Filed:
`
`Jul. 11, 2013
`
`Related U.S. Application Data
`
`(60) Provisional application No. 61/670,936, filed on Jul.
`12, 2012.
`
`Publication Classification
`
`(51)
`
`Int. Cl.
`H04B 7104
`(52) U.S. Cl.
`CPC .................................... H04B 710417 (2013.01)
`USPC .......................................................... 370/328
`
`(2006.01)
`
`(57)
`
`ABSTRACT
`
`A user equipment (UE) in a wireless network having two(cid:173)
`dimensional antenna systems performs a method of codebook
`sampling. The method includes receiving from an eNodeB
`( eNB) an indication of a restricted subset M of vertical pre(cid:173)
`coding matrices, wherein M is less than a total number of
`vertical precoding matrices N in a codebook, the codebook
`comprising a plurality of vertical precoding matrices and
`horizontal precoding matrices. The method also includes
`feeding back vertical precoding matrix indicators (V-PMI) to
`the eNB based on the restricted subset of vertical precoding
`matrices.
`
`Samsung Ex. 1005
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`

`

`Patent Application Publication
`
`Jan. 16, 2014 Sheet 1 of 6
`
`US 2014/0016549 Al
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`
`Samsung Ex. 1005
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`

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`205
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`

`Patent Application Publication
`
`Jan. 16, 2014 Sheet 3 of 6
`
`US 2014/0016549 Al
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`
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`

`Patent Application Publication
`
`Jan. 16, 2014 Sheet 4 of 6
`
`US 2014/0016549 Al
`
`MASSIVE MIMO
`
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`
`Samsung Ex. 1005
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`

`

`Patent Application Publication
`
`Jan. 16, 2014 Sheet 5 of 6
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`US 2014/0016549 Al
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`
`

`

`Patent Application Publication
`
`Jan. 16, 2014 Sheet 6 of 6
`
`US 2014/0016549 Al
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`Samsung Ex. 1005
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`

`

`US 2014/0016549 Al
`
`Jan. 16, 2014
`
`METHODS AND APPARATUS FOR
`CODEBOOK SUBSET RESTRICTION FOR
`TWO-DIMENSIONAL ADVANCED ANTENNA
`SYSTEMS
`
`CROSS-REFERENCE TO RELATED
`APPLICATION(S) AND CLAIM OF PRIORITY
`
`[0001] The present application claims priority to U.S. Pro(cid:173)
`visional Patent Application Ser. No. 61/670,936, filed Jul. 12,
`2012, entitled "CODEBOOK SUBSET RESTRICTION
`FOR 2-DIMENSIONAL ADVANCED ANTENNA SYS(cid:173)
`TEMS". The content of the above-identified patent docu(cid:173)
`ments is incorporated herein by reference.
`
`TECHNICAL FIELD
`
`[0002] The present application relates generally to wireless
`communication and, more specifically, to methods and appa(cid:173)
`ratus for codebook subset restriction for two-dimensional
`advanced antenna systems.
`
`BACKGROUND
`
`[0003] Wireless communication has been one of the most
`successful innovations in modem history. Recently, the num(cid:173)
`ber of subscribers to wireless communication services
`exceeded 5 billion and continues to grow quickly. The
`demand of wireless data traffic is rapidly increasing due to
`growing popularity among consumers and businesses of
`smart phones and other mobile data devices, such as tablets,
`"note pad" computers, net books, and eBook readers. In order
`to meet the high growth in mobile data traffic, improvements
`in radio interface efficiency and communication technology
`is of paramount importance. One such improvement is the
`growing use of two-dimensional advanced antenna systems
`in multi-user (MU) multiple-input multiple-output (MIMO)
`communication systems.
`
`SUMMARY
`
`[0004] A method of codebook sampling for use in a user
`equipment (UE) in a wireless network having two-dimen(cid:173)
`sional antenna systems is provided. The method includes
`receiving from an eNodeB ( eNB) an indication of a restricted
`subset M of vertical precoding matrices, wherein M is less
`than a total number of vertical precoding matrices N in a
`codebook, the codebook comprising a plurality of vertical
`precoding matrices and horizontal precoding matrices. The
`method also includes feeding back vertical precoding matrix
`indicators (V-PMI) to the eNB based on the restricted subset
`of vertical precoding matrices.
`[0005] A user equipment (UE) configured for communica(cid:173)
`tion with an eNodeB (eNB) in a wireless network having
`two-dimensional antenna systems is provided. The UE
`includes at least one antenna and a processor coupled to the at
`least one antenna. The processor is configured to receive from
`the eNB an indication of a restricted subset M of vertical
`precoding matrices, wherein Mis less than a total number of
`vertical precoding matrices N in a codebook, the codebook
`comprising a plurality of vertical precoding matrices and
`horizontal precoding matrices. The processor is also config(cid:173)
`ured to determine vertical precoding matrix indicators
`(V-PMI) to feed back to the eNB based on the restricted subset
`of vertical precoding matrices.
`[0006] An eNodeB (eNB) configured for communication
`with a plurality of user equipments (UEs) in a wireless net-
`
`work having two-dimensional antenna systems is provided.
`The eNB includes at least one antenna and a processor
`coupled to the at least one antenna. The processor is config(cid:173)
`ured to transmit to a UE an indication of a restricted subset M
`of vertical precoding matrices, wherein Mis less than a total
`number of vertical precoding matrices N in a codebook, the
`codebook comprising a plurality of vertical precoding matri(cid:173)
`ces and horizontal precoding matrices. The processor is also
`configured to receive feedback from the UE, the feedback
`comprising a plurality of vertical precoding matrix indicators
`(V-PMI) based on the restricted subset of vertical precoding
`matrices.
`[0007] Before undertaking the DETAILED DESCRIP(cid:173)
`TION below, it may be advantageous to set forth definitions
`of certain words and phrases used throughout this patent
`document: the terms "include" and "comprise," as well as
`derivatives thereof, mean inclusion without limitation; the
`term "or," is inclusive, meaning and/or; the phrases "associ(cid:173)
`ated with" and "associated therewith," as well as derivatives
`thereof, may mean to include, be included within, intercon(cid:173)
`nect with, contain, be contained within, connect to or with,
`couple to or with, be communicable with, cooperate with,
`interleave, juxtapose, be proximate to, be bound to or with,
`have, have a property of, or the like; and the term "controller"
`means any device, system or part thereof that controls at least
`one operation, such a device may be implemented in hard(cid:173)
`ware, firmware or software, or some combination of at least
`two of the same. It should be noted that the functionality
`associated with any particular controller may be centralized
`or distributed, whether locally or remotely. Definitions for
`certain words and phrases are provided throughout this patent
`document, those of ordinary skill in the art should understand
`that in many, if not most instances, such definitions apply to
`prior, as well as future uses of such defined words and
`phrases.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0008] For a more complete understanding of the present
`disclosure and its advantages, reference is now made to the
`following description taken in conjunction with the accom(cid:173)
`panying drawings, in which like reference numerals represent
`like parts:
`[0009] FIG. 1 illustrates a wireless network according to an
`embodiment of this disclosure;
`[0010] FIG. 2 illustrates a high-level diagram of a wireless
`transmit path according to an embodiment of this disclosure;
`[0011] FIG. 3 illustrates a high-level diagram of a wireless
`receive path according to an embodiment of this disclosure;
`[0012] FIG. 4 illustrates a transmission point according to
`embodiments of this disclosure;
`[0013] FIG. 5 illustrates azimuth and elevation angles from
`a transmission point to a user equipment, according to
`embodiments of this disclosure;
`[0014] FIG. 6 illustrates an example operation of a multi(cid:173)
`user MIMO system with a two-dimensional (2D) array,
`according to embodiments of this disclosure;
`[0015] FIG. 7 illustrates an example deployment of a 2D
`antenna array according to embodiments of this disclosure;
`[0016] FIG. 8 illustrates a configuration of codebook subset
`restriction feedback using general subset restriction, accord(cid:173)
`ing to embodiments of this disclosure;
`[0017] FIG. 9 illustrates fine and coarse PMI (precoding
`matrix indicator) sub sampling of a codebook for use in subset
`restriction, according to embodiments of this disclosure; and
`
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`US 2014/0016549 Al
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`Jan. 16, 2014
`
`2
`
`[0018] FIGS. 10 and 11 illustrate examples of sliding win(cid:173)
`dow based subset restriction, according to embodiments of
`this disclosure.
`
`DETAILED DESCRIPTION
`
`[0019] FIGS. 1 through 11, discussed below, and the vari(cid:173)
`ous embodiments used to describe the principles of the
`present disclosure in this patent document are by way of
`illustration only and should not be construed in any way to
`limit the scope of the disclosure. Those skilled in the art will
`understand that the principles of the present disclosure may
`be implemented in any suitably arranged wireless communi(cid:173)
`cation system.
`[0020] The following documents and standards descrip(cid:173)
`tions are hereby incorporated into this disclosure as if fully set
`forth herein: (i) 3GPP Technical Specification No. 36.211,
`version 10.1.0, "E-UTRA, Physical channels and modula(cid:173)
`tion" (hereinafter "REFI"); (ii) 3GPP Technical Specification
`No. 36.212, version 10.1.0, "E-UTRA, Multiplexing and
`Channel coding" (hereinafter "REF2"); and (iii) 3GPP Tech(cid:173)
`nical Specification No. 36.213, version 10.1.0, "E-UTRA,
`Physical Layer Procedures" (hereinafter "REF3").
`[0021] FIG. 1 illustrates a wireless network 100 according
`to one embodiment of this disclosure. The embodiment of
`wireless network 100 illustrated in FIG. 1 is for illustration
`only. Other embodiments of wireless network 100 could be
`used without departing from the scope of this disclosure.
`[0022] The wireless network 100 includes eNodeB (eNB)
`101, eNB 102, and eNB 103. The eNB 101 communicates
`with eNB 102 and eNB 103. The eNB 101 also communicates
`with Internet protocol (IP) network 130, such as the Internet,
`a proprietary IP network, or other data network.
`[0023] Depending on the network type, other well-known
`terms may be used instead of "eNodeB," such as "base sta(cid:173)
`tion" or "access point". For the sake of convenience, the term
`"eNodeB" shall be used herein to refer to the network infra(cid:173)
`structure components that provide wireless access to remote
`terminals.
`[0024] The eNB 102 provides wireless broadband access to
`network 130 to a first plurality of user equipments (UEs)
`within coverage area 120 of eNB 102. The first plurality of
`UEs includes UE 111, which may be located in a small
`business; UE 112, which may be located in an enterprise; UE
`113, which may be located in a WiFi hotspot; UE 114, which
`may be located in a first residence; UE 115, which may be
`located in a second residence; and UE 116, which may be a
`mobile device, such as a cell phone, a wireless laptop, a
`wireless PDA, or the like. UEs 111-116 may be any wireless
`communication device, such as, but not limited to, a mobile
`phone, mobile PDA and any mobile station (MS).
`[0025] For the sake of convenience, the term "user equip(cid:173)
`ment" or "UE" is used herein to designate any remote wire(cid:173)
`less equipment that wirelessly accesses an eNB, whether the
`UE is a mobile device (e.g., cell phone) or is normally con(cid:173)
`sidered a stationary device ( e.g., desktop personal computer,
`vending machine, etc.). In other systems, other well-known
`terms may be used instead of "user equipment", such as
`"mobile station" (MS), "subscriber station" (SS), "remote
`terminal" (RT), "wireless terminal" (WT), and the like.
`[0026] The eNB 103 provides wireless broadband access to
`a second plurality of UEs within coverage area 125 of eNB
`103. The second plurality ofUEs includes UE 115 and UE
`116. In some embodiment, eNBs 101-103 may communicate
`with each other and with UEs 111-116 using LTE or LTE-A
`
`techniques. In some embodiments, one or more of base sta(cid:173)
`tions 101-103 may communicate with each other and with
`UEs 111-116 using 5G, LTE-A, or WiMAX techniques
`including techniques for: codebook subset restriction as
`described in embodiments of the present disclosure
`[0027] Dotted lines show the approximate extents of cov(cid:173)
`erage areas 120 and 125, which are shown as approximately
`circular for the purposes of illustration and explanation only.
`It should be clearly understood that the coverage areas asso(cid:173)
`ciated with base stations, for example, coverage areas 120 and
`125, may have other shapes, including irregular shapes,
`depending upon the configuration of the base stations and
`variations in the radio environment associated with natural
`and man-made obstructions.
`[0028] Although FIG. 1 depicts one example of a wireless
`network 100, various changes may be made to FIG. 1. For
`example, another type of data network, such as a wired net(cid:173)
`work, may be substituted for wireless network 100. In a wired
`network, network terminals may replace eNBs 101-103 and
`UEs 111-116. Wired connections may replace the wireless
`connections depicted in FIG. 1.
`[0029] FIG. 2 is a high-level diagram of a wireless transmit
`path. FIG. 3 is a high-level diagram ofa wireless receive path.
`In FIGS. 2 and 3, the transmit path 200 may be implemented,
`e.g., in eNB 102 and the receive path 300 may be imple(cid:173)
`mented, e.g., in a UE, such as UE 116 of FIG. 1. It will be
`understood, however, that the receive path 300 could be
`implemented in an eNB (e.g. eNB 102 of FIG. 1) and the
`transmit path 200 could be implemented in a UE. In certain
`embodiments, transmit path 200 and receive path 300 are
`configured to perform methods for codebook subset restric(cid:173)
`tion reporting as described in embodiments of the present
`disclosure.
`[0030] Transmit path 200 comprises channel coding and
`modulation block 205, serial-to-parallel (S-to-P) block 210,
`Size N Inverse Fast Fourier Transform (IFFT) block 215,
`parallel-to-serial (P-to-S) block 220, add cyclic prefix block
`225, up-converter (UC) 230. Receive path 300 comprises
`down-converter (DC) 255, remove cyclic prefix block 260,
`serial-to-parallel (S-to-P) block 265, Size N Fast Fourier
`Transform (FFT) block 270, parallel-to-serial (P-to-S) block
`275, channel decoding and demodulation block 280.
`[0031] At least some of the components in FIGS. 2 and 3
`may be implemented in software while other components
`may be implemented by configurable hardware (e.g., a pro(cid:173)
`cessor) or a mixture of software and configurable hardware.
`In particular, it is noted that the FFT blocks and the IFFT
`blocks described in this disclosure document may be imple(cid:173)
`mented as configurable software algorithms, where the value
`of Size N may be modified according to the implementation.
`[0032] Furthermore, although this disclosure is directed to
`an embodiment that implements the Fast Fourier Transform
`and the Inverse Fast Fourier Transform, this is by way of
`illustration only and should not be construed to limit the scope
`of the disclosure. It will be appreciated that in an alternate
`embodiment of the disclosure, the Fast Fourier Transform
`functions and the Inverse Fast Fourier Transform functions
`may easily be replaced by Discrete Fourier Transform (DFT)
`functions and Inverse Discrete Fourier Transform (IDFT)
`functions, respectively. It will be appreciated that for DFT
`and IDFT functions, the value of the N variable may be any
`integer number (i.e., 1, 2, 3, 4, etc.), while for FFT and IFFT
`functions, the value of the N variable may be any integer
`number that is a power of two (i.e., 1, 2, 4, 8, 16, etc.).
`
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`

`US 2014/0016549 Al
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`Jan. 16, 2014
`
`3
`
`In transmit path 200, channel coding and modula(cid:173)
`[0033]
`tion block 205 receives a set of information bits, applies
`coding (e.g., LDPC coding) and modulates (e.g., Quadrature
`Phase Shift Keying (QPSK) or Quadrature Amplitude Modu(cid:173)
`lation (QAM)) the input bits to produce a sequence of fre(cid:173)
`quency-domain modulation symbols. Serial-to-parallel block
`210 converts (i.e., de-multiplexes) the serial modulated sym(cid:173)
`bols to parallel data to produce N parallel symbol streams
`where N is the IFFT/FFT size used in eNB 102 and UE 116.
`Size N IFFT block 215 then performs an IFFT operation on
`the N parallel symbol streams to produce time-domain output
`signals. Parallel-to-serial block 220 converts (i.e., multi(cid:173)
`plexes) the parallel time-domain output symbols from Size N
`IFFT block 215 to produce a serial time-domain signal. Add
`cyclic prefix block 225 then inserts a cyclic prefix to the
`time-domain signal. Finally, up-converter 230 modulates
`(i.e., up-converts) the output of add cyclic prefix block 225 to
`RF frequency for transmission via a wireless channel. The
`signal may also be filtered at baseband before conversion to
`RF frequency.
`[0034] The transmitted RF signal arrives at UE 116 after
`passing through the wireless channel and reverse operations
`to those at eNB 102 are performed. Down-converter 255
`down-converts the received signal to baseband frequency and
`remove cyclic prefix block 260 removes the cyclic prefix to
`produce the serial time-domain baseband signal. Serial-to(cid:173)
`parallel block 265 converts the time-domain baseband signal
`to parallel time domain signals. Size N FFT block 270 then
`performs an FFT algorithm to produce N parallel frequency(cid:173)
`domain signals. Parallel-to-serial block 275 converts the par(cid:173)
`allel frequency-domain signals to a sequence of modulated
`data symbols. Channel decoding and demodulation block 280
`demodulates and then decodes the modulated symbols to
`recover the original input data stream.
`[0035] Each of eNBs 101-103 may implement a transmit
`path that is analogous to transmitting in the downlink to UEs
`111-116 and may implement a receive path that is analogous
`to receiving in the uplink from UEs 111-116. Similarly, each
`one ofUEs 111-116 may implement a transmit path corre(cid:173)
`sponding to the architecture for transmitting in the uplink to
`eNBs 101-103 and may implement a receive path correspond(cid:173)
`ing to the architecture for receiving in the downlink from
`eNBs 101-103.
`[0036] FIG. 4 illustrates a transmission point according to
`one embodiment of this disclosure. The embodiment of trans(cid:173)
`mission point 400 illustrated in FIG. 4 is for illustration only.
`Other embodiments of transmission point 400 could be used
`without departing from the scope of this disclosure.
`[0037] Transmission point (TP) 400 is equipped with a
`two-dimensional (2D) active antenna array comprising a plu(cid:173)
`rality of antenna elements 402, and is configured for multi(cid:173)
`user multiple-input multiple-output (MU-MIMO) transmis(cid:173)
`sions. In some embodiments, TP 400 may also be configured
`for full dimension (FD) MIMO transmissions. As used
`herein, the term "transmission point" refers to a network node
`that can transmit downlink signals and receive uplink signals
`in a cellular network. Examples of TPs may include base
`stations, NodeBs, enhanced NodeBs (eNBs), remote radio
`heads (RRHs), and the like. As particular examples, TP 400
`may represent one or more of eNBs 101-103 of FIG. 1. An
`entity controlling at least one TP is called the controller,
`network, or eNB. As shown in FIG. 4, TP 400 includes a
`controller 404. Each active antenna array may have a separate
`
`base band, which can dynamically control the antenna
`weights in a frequency selective manner.
`[0038] TP 400 includes N (N=NHxNv) 2D active antenna
`elements 402, and the N antenna elements 402 are placed in a
`2D grid ofNHxN v· The horizontal spacing between any two
`adjacent antenna elements 402 is denoted by dH, and the
`vertical spacing between any two adjacent antenna elements
`402 is denoted by dv.
`[0039] FIG. 5 illustrates azimuth and elevation angles from
`transmission point 400 to a user equipment, according to one
`embodiment of this disclosure. The embodiment of TP 400
`illustrated in FIG. 5 is for illustration only. Other embodi(cid:173)
`ments could be used without departing from the scope of this
`disclosure.
`[0040] FIG. 5 illustrates the azimuth and elevation angles to
`a UE k from the 2D antenna array of antenna elements of TP
`400. As shown in FIG. 5, the antenna elements 402 ofTP 400
`are arranged in a rectangle on a XZ plane in an orthogonal
`XYZ coordinate system. The origin of the XYZ coordinate
`system is placed at the center of the rectangle. The azimuth
`(horizontal) angle <Pk for UE k is defined as the angle between
`the Y axis and the projection vector of a straight line between
`TP 400 and UE k to the XY plane. The elevation (vertical)
`angle 8k is defined as the angle between the Y axis and the
`projection vector of the straight line between TP 400 and UE
`k to the YZ plane.
`[0041]
`In cellular networks, the network utilizes channel
`state information ( CSI) from UEs to schedule time-frequency
`resources, and to select precoders and modulation and coding
`schemes (MCS) for each individual UE. To facilitate the
`estimation of CSI at the UE, the network can configure and
`transmit CSI reference signals (CSI-RS). At the same time,
`each UE can be configured to feed back estimated precoding
`matrix indicators (PMI), channel quality information (CQI),
`and rank information (RI), by receiving and processing the
`CSI-RS. In many cases, the CSI feedback from the UE is
`primarily associated with horizontal CSI associated with the
`azimuth angles. For example, PMI/CQI feedback for down(cid:173)
`link beamforming in LTE informs the eNB the horizontal
`direction ( or the azimuth angle) in which the UE receives the
`strongest signal, and the associated channel strength. When
`active antenna array elements are introduced in the vertical
`domain as well, the use of vertical CSI feedback emerges.
`[0042] The codebook used for feedback can be designed
`based on a 64-antenna MIMO system. However, it is advan(cid:173)
`tageous to simplify codebook design to facilitate reasonable
`codebook size and acceptable computational complexity at
`the UE receivers. Some observations can be made regarding
`the channel behavior corresponding to a 2D active antenna
`array. The overall transmit covariance matrix corresponding
`to all of the 64 antennas in an 8x8 array may be separated into
`two components using Kronecker decompositions as an
`approximation,
`
`(1)
`
`[0043]
`It can be shown that the precoder can be approxi(cid:173)
`mated into horizontal and vertical components,
`
`(2)
`
`where n is the rank of transmission.
`[0044]
`In LTE Release 10, the UE feeds back a CQI in
`addition to the PMI and RI, which corresponds to a MCS level
`that can be supported reliably by the UE, with a certain target
`error probability. The feedback designs in LTE Release 10 are
`optimized for single user MIMO. PMI and CQI determined
`
`Samsung Ex. 1005
`
`

`

`US 2014/0016549 Al
`
`Jan. 16, 2014
`
`4
`
`by the VE assuming single user MIMO is referred to as single
`user PMI (SV-PMI) and single user CQI (SV-CQI), respec(cid:173)
`tively.
`[0045] FIG. 6 illustrates an example operation of a multi(cid:173)
`user MIMO system with a two-dimensional array, according
`to an embodiment of this disclosure. The embodiment of the
`multi-user MIMO system illustrated in FIG. 6 is for illustra(cid:173)
`tion only. Other embodiments could be used without depart(cid:173)
`ing from the scope of this disclosure.
`[0046] Multi-user MIMO corresponds to a transmission
`scheme where a transmitter can transmit data to two or more
`VEs at the same time/frequency resource, by relying on spa(cid:173)
`tial separation of the corresponding user's channels. With a
`smaller number of transmit antennas, the number of users that
`can be supported is limited. Since the number of transmit
`antennas supported in LTE Release 10 is limited to a maxi(cid:173)
`mum of eight antennas, many designs for multi-user MIMO
`support are optimized for a case of two-user MU-MIMO
`transmission with a single stream per each VE.
`[0047] However, with MU-MIMO, the MCS to be used by
`the scheduler for each user may need to be determined at the
`eNB. The MCS that can be supported reliably for each VE is
`dependent on co-channel PMI corresponding to the co-sched(cid:173)
`uled VE. For scheduling flexibility, a transmitter may pair a
`user with any other VE.
`[0048] Assuming a typical configuration of two receiver
`antennas at the VE, single user MIMO (SV-MIMO) up to
`rank 2 can be supported. Further, it is expected that MU(cid:173)
`MIMO is only scheduled by an eNB when the performance is
`better than the SV-MIMO. This means that scheduled users
`have good spatial separation. Single user CQI (SV-CQI) is an
`approximation ofMU-CQI for determination ofMCS at the
`eNB.
`[0049] However, for MIMO with a large number of trans(cid:173)
`mit antennas (e.g., number of transmit antennas is greater
`than number of receive antennas, or N y>N R), the spatial rank
`of SV-MIMO transmission is limited by the number of
`receive antennas. Hence, MU-MIMO is frequently used in
`such cases. Accordingly, methods are defined to determine
`MU-CQI at the VE. The eNB predictions ofMCS may not be
`accurate since the receiver implementation-specific algo(cid:173)
`rithms, like interference cancellation and suppression, also
`need to be accurately reflected in any MU-CQI calculation.
`[0050]
`In wireless communication standards such as LTE,
`efficient PMI selection is associated with reducing control
`information overhead and reducing complexity at the
`receiver. As a result, because certain PMI indices are infre(cid:173)
`quently selected or never selected, codebook subset restric(cid:173)
`tion has been specified in LTE to let a VE report PMI within
`the codebook subset configured by the serving eNB. This can
`be achieved, for example, by utilizing a bitmap, which is
`signaled via higher layer in a VE-specific manner. In one
`example, a bit value of zero in the bitmap indicates that the
`PMI and RI reporting is not allowed to correspond to the
`precoder associated with the bit. The number of bits in the
`codebook subset restriction bitmap is determined by the num(cid:173)
`ber of precoders allowed in both the configured VE-specific
`transmission mode and the number of antenna ports.
`[0051] The introduction of multi-user (MU) MIMO trans(cid:173)
`missions supporting large numbers of users due to transmis(cid:173)
`sion points equipped with two-dimensional (2D) antenna
`arrays with large numbers of elements further motivates the
`need for efficient PMI selection and feedback techniques. The
`amount of overhead required grows with both the number of
`
`users and the number of antenna elements, compared to pre(cid:173)
`vious LTE releases, which only support a maximum of eight
`transmit antenna elements.
`[0052] As described earlier, codebook design can take
`advantage of the spatial structure of the channel resulting
`from the 2D MIMO antenna array transmissions. Differen(cid:173)
`tiation between the horizontal and vertical dimensions can be
`used for flexible and efficient codebook design and also has
`implications on the PMI selection and feedback. Due to the
`variation in network user geographic distributions, users may
`experience different angles of elevation and azimuth relative
`to the transmission point. However the azimuth distribution
`and the elevation distribution in many networks are likely to
`be quite different and in fact may be uncorrelated. For
`example, since transmission points are often mounted several
`stories above ground, and many users (especially those out(cid:173)
`doors) are located on the ground plane, a typical elevation
`angle range may be within a 45 degree range for most users.
`However other users (e.g., in high-rise buildings or in hilly
`terrains) may experience much larger elevation angles with
`respect to the transmit point.
`[0053] FIG. 7 illustrates an example deployment of a 2D
`antenna array according to an embodiment of this disclosure.
`The embodiment of the antenna array 700 illustrated in FIG.
`7 is for illustration only. Other embodiments could be used
`without departing from the scope of this disclosure.
`[0054] As shown in FIG. 7, angle 8 p, which represents the
`elevation angle range experienced by the outdoor VEs, is
`much smaller than 8 0 which represents the elevation angle
`range experienced by all the VEs. Codebooks may be
`designed to sample the entire spatial domain (based on a
`sampled DFT for example). Thus, one method for improving
`the efficiency of vertical PMI selection and reducing CQI
`computation complexity is to restrict the VE to searching
`through the codebook only over those precoders that corre(cid:173)
`spond to relevant spatial domain.
`[0055] General Subset Restriction:
`[0056]
`In an embodiment of the current disclosure, subset
`restriction of Vertical PMI (V-PMI) is performed by first
`restricting the total number of unique V-PMI indications M
`that can be reported to be less than or equal to the total number
`of possible precoding matrices N.
`[0057] For example, an 8x8 antenna array at the eNB
`decomposes theprecoderinto Sxl vertical and Sxl horizontal
`vectors. IfN=16, the VE would need to determine which of
`the N vertical precoding matrices produces the best CQI (in
`combination with H-PMI) and then indicate this to the eNB
`(e.g., using a log2N sized bit field). However, ifM=4, the VE
`only measures CQI on the subset