WO2014198068A1

WO2014198068A1 - Methods and apparatus for linear precoding in full-dimensional mimo systems

Info

Publication number: WO2014198068A1
Application number: PCT/CN2013/077277
Authority: WO
Inventors: Yu Zhang; Wei Chao; Peng Cheng; Neng Wang; Jilei Hou
Original assignee: Qualcomm Incorporated
Priority date: 2013-06-14
Filing date: 2013-06-14
Publication date: 2014-12-18

Abstract

Certain aspects of the present disclosure provide methods and apparatus for linear precoding in full-dimensional MIMO (FD-MIMO) systems. According to aspects, a BS may generate a port precoding matrix which compress a larger number of antenna elements to a smaller number of antenna ports, transmit UE-specific port reference signals to a user equipment (UE) using the port precoding matrix, receive feedback regarding channel state information (CSI) measured by the UE based on the UE- specific port reference signals, map multiple data layers to UE-specific antenna ports based on the CSI, map each of the UE-specific antenna ports to physical antenna elements, and transmit data to the UE, based on the mapping of the multiple data layers and the mapping of antenna ports to physical antenna elements.

Description

METHODS AND APPARATUS FOR LINEAR PRECODING IN FULL- DIMENSIONAL MIMO SYSTEMS

BACKGROUND

Field

[0001] The present disclosure relates generally to wireless communication, and more particularly, processing in FD-MIMO systems

Background

[0002] Wireless communication systems are widely deployed to provide various telecommunication services such as telephony, video, data, messaging, and broadcasts. Typical wireless communication systems may employ multiple-access technologies capable of supporting communication with multiple users by sharing available system resources (e.g., bandwidth, transmit power). Examples of such multiple-access technologies include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, orthogonal frequency division multiple access (OFDM A) systems, single-carrier frequency divisional multiple access (SC-FDMA) systems, and time division synchronous code division multiple access (TD-SCDMA) systems.

[0003] These multiple access technologies have been adopted in various telecommunication standards to provide a common protocol that enables different wireless devices to communicate on a municipal, national, regional, and even global level. An example of an emerging telecommunication standard is Long Term Evolution (LTE). LTE is a set of enhancements to the Universal Mobile Telecommunications System (UMTS) mobile standard promulgated by Third Generation Partnership Project (3GPP). It is designed to better support mobile broadband Internet access by improving spectral efficiency, lower costs, improve services, make use of new spectrum, and better integrate with other open standards using OFDMA on the downlink (DL), SC-FDMA on the uplink (UL), and multiple-input multiple-output (MIMO) antenna technology. However, as the demand for mobile broadband access continues to increase, there exists a need for further improvements in LTE technology. Preferably, these improvements should be applicable to other multi-access technologies and the telecommunication standards that employ these technologies. SUMMARY

[0004] The following presents a simplified summary of one or more aspects in order to provide a basic understanding of such aspects. This summary is not an extensive overview of all contemplated aspects, and is intended to neither identify key or critical elements of all aspects nor delineate the scope of any or all aspects. Its sole purpose is to present some concepts of one or more aspects in a simplified form as a prelude to the more detailed description that is presented later.

[0005] Aspects of the present disclosure provide a method for wireless communications by a base station (BS). The method generally includes generating a port precoding matrix which compress a larger number of antenna elements to a smaller number of antenna ports, transmitting UE-specific port reference signals to a user equipment (UE) using the port precoding matrix, receiving feedback regarding channel state information (CSI) measured by the UE based on the UE- specific port reference signals, mapping multiple data layers to UE- specific antenna ports based on the CSI, mapping each of the UE-specific antenna ports to physical antenna elements, and transmitting data to the UE, based on the mapping of the multiple data layers and the mapping of antenna ports to physical antenna elements.

[0006] Aspects of the present disclosure provide a method for wireless communication by a user equipment (UE). The method generally includes receiving UE-specific port reference signals transmitted by a base station (BS) using a long-term port precoding matrix which compress a large number of antenna elements to a small number of antenna ports, measuring and quantizing short-term CSI based on the UE-specific port reference signals, and transmitting feedback regarding the (quantized) short-term CSI to the BS.

[0007] Aspects generally include methods, apparatus, systems, computer program products, and processing systems, as substantially described herein with reference to and as illustrated by the accompanying drawings and APPENDIX.

[0008] To the accomplishment of the foregoing and related ends, the one or more aspects comprise the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative features of the one or more aspects. These features are indicative, however, of but a few of the various ways in which the principles of various aspects may be employed, and this description is intended to include all such aspects and their equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 is a diagram illustrating an example of a network architecture. [0010] FIG. 2 is a diagram illustrating an example of an access network.

[0011] FIG. 3 is a diagram illustrating an example of a frame structure for use in an access network.

[0012] FIG. 4 shows an exemplary format for the UL in LTE.

[0013] FIG. 5 is a diagram illustrating an example of a radio protocol architecture for the user and control planes.

[0014] FIG. 6 is a diagram illustrating an example of an evolved Node B and user equipment in an access network.

[0015] FIG. 7 illustrates an example of a traditional MIMO with one-dimensional array.

[0016] FIG. 8 illustrates an example FD-MIMO with two-dimensional array, according to aspects of the present disclosure.

[0017] FIG. 9 illustrates example components used in accordance with methods described herein.

[0018] FIG. 10 illustrates an example components used in accordance with methods described herein.

[0019] FIG. 11 illustrates an example sub-array partitions, according to aspects of the present disclosure.

[0020] FIGs. 12-18 illustrate various sub-array partitions, according to aspects of the present disclosure.

[0021] FIG. 19 illustrates example operations performed, for example, by a base station (BS), in accordance with aspects of the present disclosure.

[0022] FIG. 20 illustrates example operations performed, for example, by a user equipment (UE) in accordance with aspects of the present disclosure.

[0023] The APPENDIX provides details and descriptions of various aspects in accordance with aspects of the present disclosure.

DETAILED DESCRIPTION

[0024] The detailed description set forth below in connection with the appended drawings is intended as a description of various configurations and is not intended to represent the only configurations in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. In some instances, well known structures and components are shown in block diagram form in order to avoid obscuring such concepts.

[0025] Several aspects of telecommunication systems will now be presented with reference to various apparatus and methods. These apparatus and methods will be described in the following detailed description and illustrated in the accompanying drawings by various blocks, modules, components, circuits, steps, processes, algorithms, etc. (collectively referred to as "elements"). These elements may be implemented using electronic hardware, computer software, or any combination thereof. Whether such elements are implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system.

[0026] By way of example, an element, or any portion of an element, or any combination of elements may be implemented with a "processing system" that includes one or more processors. Examples of processors include microprocessors, microcontrollers, digital signal processors (DSPs), field programmable gate arrays (FPGAs), programmable logic devices (PLDs), state machines, gated logic, discrete hardware circuits, and other suitable hardware configured to perform the various functionality described throughout this disclosure. One or more processors in the processing system may execute software. Software shall be construed broadly to mean instructions, instruction sets, code, code segments, program code, programs, subprograms, software modules, applications, software applications, software packages, routines, subroutines, objects, executables, threads of execution, procedures, functions, etc., whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise.

[0027] Accordingly, in one or more exemplary embodiments, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or encoded as one or more instructions or code on a computer-readable medium. Computer-readable media includes computer storage media. Storage media may be any available media that can be accessed by a computer. By way of example, and not limitation, such computer- readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media.

[0028] FIG. 1 is a diagram illustrating an LTE network architecture 100. The LTE network architecture 100 may be referred to as an Evolved Packet System (EPS) 100. The EPS 100 may include one or more user equipment (UE) 102, an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) 104, an Evolved Packet Core (EPC) 110, a Home Subscriber Server (HSS) 120, and an Operator's IP Services 122. The EPS can interconnect with other access networks, but for simplicity those entities/interfaces are not shown. As shown, the EPS provides packet- switched services, however, as those skilled in the art will readily appreciate, the various concepts presented throughout this disclosure may be extended to networks providing circuit-switched services.

[0029] The E-UTRAN includes the evolved Node B (eNB) 106 and other eNBs 108. The eNB 106 provides user and control plane protocol terminations toward the UE 102. The eNB 106 may be connected to the other eNBs 108 via an X2 interface (e.g., backhaul). The eNB 106 may also be referred to as a base station, a base transceiver station, a radio base station, a radio transceiver, a transceiver function, a basic service set (BSS), an extended service set (ESS), or some other suitable terminology. The eNB 106 provides an access point to the EPC 110 for a UE 102. Examples of UEs 102 include a cellular phone, a smart phone, a session initiation protocol (SIP) phone, a laptop, a personal digital assistant (PDA), a satellite radio, a global positioning system, a multimedia device, a video device, a digital audio player (e.g., MP3 player), a camera, a game console, or any other similar functioning device. The UE 102 may also be referred to by those skilled in the art as a mobile station, a subscriber station, a mobile unit, a subscriber unit, a wireless unit, a remote unit, a mobile device, a wireless device, a wireless communications device, a remote device, a mobile subscriber station, an access terminal, a mobile terminal, a wireless terminal, a remote terminal, a handset, a user agent, a mobile client, a client, or some other suitable terminology. [0030] The eNB 106 is connected by an SI interface to the EPC 110. The EPC 110 includes a Mobility Management Entity (MME) 112, other MMEs 114, a Serving Gateway 116, and a Packet Data Network (PDN) Gateway 118. The MME 112 is the control node that processes the signaling between the UE 102 and the EPC 110. Generally, the MME 112 provides bearer and connection management. All user IP packets are transferred through the Serving Gateway 116, which itself is connected to the PDN Gateway 118. The PDN Gateway 118 provides UE IP address allocation as well as other functions. The PDN Gateway 118 is connected to the Operator's IP Services 122. The Operator's IP Services 122 may include the Internet, the Intranet, an IP Multimedia Subsystem (IMS), and a PS Streaming Service (PSS).

[0031] FIG. 2 is a diagram illustrating an example of an access network 200 in an LTE network architecture. In this example, the access network 200 is divided into a number of cellular regions (cells) 202. One or more lower power class eNBs 208 may have cellular regions 210 that overlap with one or more of the cells 202. A lower power class eNB 208 may be referred to as a remote radio head (RRH). The lower power class eNB 208 may be a femto cell (e.g., home eNB (HeNB)), pico cell, or micro cell. The macro eNBs 204 are each assigned to a respective cell 202 and are configured to provide an access point to the EPC 110 for all the UEs 206 in the cells 202. There is no centralized controller in this example of an access network 200, but a centralized controller may be used in alternative configurations. The eNBs 204 are responsible for all radio related functions including radio bearer control, admission control, mobility control, scheduling, security, and connectivity to the serving gateway 116.

[0032] The modulation and multiple access scheme employed by the access network 200 may vary depending on the particular telecommunications standard being deployed. In LTE applications, OFDM is used on the DL and SC-FDMA is used on the UL to support both frequency division duplexing (FDD) and time division duplexing (TDD). As those skilled in the art will readily appreciate from the detailed description to follow, the various concepts presented herein are well suited for LTE applications. However, these concepts may be readily extended to other telecommunication standards employing other modulation and multiple access techniques. By way of example, these concepts may be extended to Evolution-Data Optimized (EV-DO) or Ultra Mobile Broadband (UMB). EV-DO and UMB are air interface standards promulgated by the 3rd Generation Partnership Project 2 (3GPP2) as part of the CDMA2000 family of standards and employs CDMA to provide broadband Internet access to mobile stations. These concepts may also be extended to Universal Terrestrial Radio Access (UTRA) employing Wideband-CDMA (W-CDMA) and other variants of CDMA, such as TD- SCDMA; Global System for Mobile Communications (GSM) employing TDM A; and Evolved UTRA (E-UTRA), Ultra Mobile Broadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, and Flash-OFDM employing OFDMA. UTRA, E-UTRA, UMTS, LTE and GSM are described in documents from the 3GPP organization. CDMA2000 and UMB are described in documents from the 3GPP2 organization. The actual wireless communication standard and the multiple access technology employed will depend on the specific application and the overall design constraints imposed on the system.

[0033] The eNBs 204 may have multiple antennas supporting MIMO technology. The use of MIMO technology enables the eNBs 204 to exploit the spatial domain to support spatial multiplexing, beamforming, and transmit diversity. Spatial multiplexing may be used to transmit different streams of data simultaneously on the same frequency. The data steams may be transmitted to a single UE 206 to increase the data rate or to multiple UEs 206 to increase the overall system capacity. This is achieved by spatially precoding each data stream (i.e., applying a scaling of an amplitude and a phase) and then transmitting each spatially precoded stream through multiple transmit antennas on the DL. The spatially precoded data streams arrive at the UE(s) 206 with different spatial signatures, which enables each of the UE(s) 206 to recover the one or more data streams destined for that UE 206. On the UL, each UE 206 transmits a spatially precoded data stream, which enables the eNB 204 to identify the source of each spatially precoded data stream.

[0034] Spatial multiplexing is generally used when channel conditions are good. When channel conditions are less favorable, beamforming may be used to focus the transmission energy in one or more directions. This may be achieved by spatially precoding the data for transmission through multiple antennas. To achieve good coverage at the edges of the cell, a single stream beamforming transmission may be used in combination with transmit diversity.

[0035] In the detailed description that follows, various aspects of an access network will be described with reference to a MIMO system supporting OFDM on the DL. OFDM is a spread- spectrum technique that modulates data over a number of subcarriers within an OFDM symbol. The subcarriers are spaced apart at precise frequencies. The spacing provides "orthogonality" that enables a receiver to recover the data from the subcarriers. In the time domain, a guard interval (e.g., cyclic prefix) may be added to each OFDM symbol to combat inter-OFDM-symbol interference. The UL may use SC- FDMA in the form of a DFT-spread OFDM signal to compensate for high peak-to- average power ratio (PAPR).

[0036] FIG. 3 is a diagram 300 illustrating an example of a DL frame structure in LTE. A frame (10 ms) may be divided into 10 equally sized sub-frames. Each sub- frame may include two consecutive time slots. A resource grid may be used to represent two time slots, each time slot including a resource block. The resource grid is divided into multiple resource elements. In LTE, a resource block contains 12 consecutive subcarriers in the frequency domain and, for a normal cyclic prefix in each OFDM symbol, 7 consecutive OFDM symbols in the time domain, or 84 resource elements. Some of the resource elements, as indicated as R 302, 304, include DL reference signals (DL-RS). The DL-RS include Cell-specific RS (CRS) (also sometimes called common RS) 302 and UE-specific RS (UE-RS) 304. UE-RS 304 are transmitted only on the resource blocks upon which the corresponding physical DL shared channel (PDSCH) is mapped. The number of bits carried by each resource element depends on the modulation scheme. Thus, the more resource blocks that a UE receives and the higher the modulation scheme, the higher the data rate for the UE.

[0037] FIG. 4 is a diagram 400 illustrating an example of an UL frame structure in LTE. The available resource blocks for the UL may be partitioned into a data section and a control section. The control section may be formed at the two edges of the system bandwidth and may have a configurable size. The resource blocks in the control section may be assigned to UEs for transmission of control information. The data section may include all resource blocks not included in the control section. The UL frame structure results in the data section including contiguous subcarriers, which may allow a single UE to be assigned all of the contiguous subcarriers in the data section.

[0038] A UE may be assigned resource blocks 410a, 410b in the control section to transmit control information to an eNB. The UE may also be assigned resource blocks 420a, 420b in the data section to transmit data to the eNB. The UE may transmit control information in a physical UL control channel (PUCCH) on the assigned resource blocks in the control section. The UE may transmit only data or both data and control information in a physical UL shared channel (PUSCH) on the assigned resource blocks in the data section. A UL transmission may span both slots of a subframe and may hop across frequency.

[0039] A set of resource blocks may be used to perform initial system access and achieve UL synchronization in a physical random access channel (PRACH) 430. The PRACH 430 carries a random sequence and cannot carry any UL data/signaling. Each random access preamble occupies a bandwidth corresponding to six consecutive resource blocks. The starting frequency is specified by the network. That is, the transmission of the random access preamble is restricted to certain time and frequency resources. There is no frequency hopping for the PRACH. The PRACH attempt is carried in a single subframe (1 ms) or in a sequence of few contiguous subframes and a UE can make only a single PRACH attempt per frame (10 ms).

[0040] FIG. 5 is a diagram 500 illustrating an example of a radio protocol architecture for the user and control planes in LTE. The radio protocol architecture for the UE and the eNB is shown with three layers: Layer 1, Layer 2, and Layer 3. Layer 1 (LI layer) is the lowest layer and implements various physical layer signal processing functions. The LI layer will be referred to herein as the physical layer 506. Layer 2 (L2 layer) 508 is above the physical layer 506 and is responsible for the link between the UE and eNB over the physical layer 506.

[0041] In the user plane, the L2 layer 508 includes a media access control (MAC) sublayer 510, a radio link control (RLC) sublayer 512, and a packet data convergence protocol (PDCP) 514 sublayer, which are terminated at the eNB on the network side. Although not shown, the UE may have several upper layers above the L2 layer 508 including a network layer (e.g., IP layer) that is terminated at the PDN gateway 118 on the network side, and an application layer that is terminated at the other end of the connection (e.g., far end UE, server, etc.).

[0042] The PDCP sublayer 514 provides multiplexing between different radio bearers and logical channels. The PDCP sublayer 514 also provides header compression for upper layer data packets to reduce radio transmission overhead, security by ciphering the data packets, and handover support for UEs between eNBs. The RLC sublayer 512 provides segmentation and reassembly of upper layer data packets, retransmission of lost data packets, and reordering of data packets to compensate for out-of-order reception due to hybrid automatic repeat request (HARQ). The MAC sublayer 510 provides multiplexing between logical and transport channels. The MAC sublayer 510 is also responsible for allocating the various radio resources (e.g., resource blocks) in one cell among the UEs. The MAC sublayer 510 is also responsible for HARQ operations.

[0043] In the control plane, the radio protocol architecture for the UE and eNB is substantially the same for the physical layer 506 and the L2 layer 508 with the exception that there is no header compression function for the control plane. The control plane also includes a radio resource control (RRC) sublayer 516 in Layer 3 (L3 layer). The RRC sublayer 516 is responsible for obtaining radio resources (i.e., radio bearers) and for configuring the lower layers using RRC signaling between the eNB and the UE.

[0044] FIG. 6 is a block diagram of an eNB 610 in communication with a UE 650 in an access network. In the DL, upper layer packets from the core network are provided to a controller/processor 675. The controller/processor 675 implements the functionality of the L2 layer. In the DL, the controller/processor 675 provides header compression, ciphering, packet segmentation and reordering, multiplexing between logical and transport channels, and radio resource allocations to the UE 650 based on various priority metrics. The controller/processor 675 is also responsible for HARQ operations, retransmission of lost packets, and signaling to the UE 650.

[0045] The TX processor 616 implements various signal processing functions for the LI layer (i.e., physical layer). The signal processing functions include coding and interleaving to facilitate forward error correction (FEC) at the UE 650 and mapping to signal constellations based on various modulation schemes (e.g., binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), M-phase-shift keying (M-PSK), M-quadrature amplitude modulation (M-QAM)). The coded and modulated symbols are then split into parallel streams. Each stream is then mapped to an OFDM subcarrier, multiplexed with a reference signal (e.g., pilot) in the time and/or frequency domain, and then combined together using an Inverse Fast Fourier Transform (IFFT) to produce a physical channel carrying a time domain OFDM symbol stream. The OFDM stream is spatially precoded to produce multiple spatial streams. Channel estimates from a channel estimator 674 may be used to determine the coding and modulation scheme, as well as for spatial processing. The channel estimate may be derived from a reference signal and/or channel condition feedback transmitted by the UE 650. Each spatial stream is then provided to a different antenna 620 via a separate transmitter 618TX. Each transmitter 618TX modulates an RF carrier with a respective spatial stream for transmission.

[0046] At the UE 650, each receiver 654RX receives a signal through its respective antenna 652. Each receiver 654RX recovers information modulated onto an RF carrier and provides the information to the receiver (RX) processor 656. The RX processor 656 implements various signal processing functions of the LI layer. The RX processor 656 performs spatial processing on the information to recover any spatial streams destined for the UE 650. If multiple spatial streams are destined for the UE 650, they may be combined by the RX processor 656 into a single OFDM symbol stream. The RX processor 656 then converts the OFDM symbol stream from the time-domain to the frequency domain using a Fast Fourier Transform (FFT). The frequency domain signal comprises a separate OFDM symbol stream for each subcarrier of the OFDM signal. The symbols on each subcarrier, and the reference signal, is recovered and demodulated by determining the most likely signal constellation points transmitted by the eNB 610. These soft decisions may be based on channel estimates computed by the channel estimator 658. The soft decisions are then decoded and deinterleaved to recover the data and control signals that were originally transmitted by the eNB 610 on the physical channel. The data and control signals are then provided to the controller/processor 659.

[0047] The controller/processor 659 implements the L2 layer. The controller/processor can be associated with a memory 660 that stores program codes and data. The memory 660 may be referred to as a computer-readable medium. In the UL, the control/processor 659 provides demultiplexing between transport and logical channels, packet reassembly, deciphering, header decompression, control signal processing to recover upper layer packets from the core network. The upper layer packets are then provided to a data sink 662, which represents all the protocol layers above the L2 layer. Various control signals may also be provided to the data sink 662 for L3 processing. The controller/processor 659 is also responsible for error detection using an acknowledgement (ACK) and/or negative acknowledgement (NACK) protocol to support HARQ operations.

[0048] In the UL, a data source 667 is used to provide upper layer packets to the controller/processor 659. The data source 667 represents all protocol layers above the L2 layer. Similar to the functionality described in connection with the DL transmission by the eNB 610, the controller/processor 659 implements the L2 layer for the user plane and the control plane by providing header compression, ciphering, packet segmentation and reordering, and multiplexing between logical and transport channels based on radio resource allocations by the eNB 610. The controller/processor 659 is also responsible for HARQ operations, retransmission of lost packets, and signaling to the eNB 610.

[0049] Channel estimates derived by a channel estimator 658 from a reference signal or feedback transmitted by the eNB 610 may be used by the TX processor 668 to select the appropriate coding and modulation schemes, and to facilitate spatial processing. The spatial streams generated by the TX processor 668 are provided to different antenna 652 via separate transmitters 654TX. Each transmitter 654TX modulates an RF carrier with a respective spatial stream for transmission.

[0050] The UL transmission is processed at the eNB 610 in a manner similar to that described in connection with the receiver function at the UE 650. Each receiver 618RX receives a signal through its respective antenna 620. Each receiver 618RX recovers information modulated onto an RF carrier and provides the information to a RX processor 670. The RX processor 670 may implement the LI layer.

[0051] The controller/processor 675 implements the L2 layer. The controller/processor 675 can be associated with a memory 676 that stores program codes and data. The memory 676 may be referred to as a computer-readable medium. In the UL, the control/processor 675 provides demultiplexing between transport and logical channels, packet reassembly, deciphering, header decompression, control signal processing to recover upper layer packets from the UE 650. Upper layer packets from the controller/processor 675 may be provided to the core network. The controller/processor 675 is also responsible for error detection using an ACK and/or NACK protocol to support HARQ operations.

LINEAR PRECODING IN FD-MIMO

[0052] Full-dimensional MIMO (FD-MIMO) technology may greatly improve system capacity by using a two-dimensional antenna array with up to 64 antenna ports at an eNB. Benefits of using up to 64 antenna ports at the eNB may include very small inter-cell interference as well as very high beamforming gain. The use of a two- dimensional antenna array allows UE- specific beamforming in both azimuth and elevation. Each antenna may be connected to its own RF transceiver. [0053] In traditional linear precoding, MIMO channel state information (CSI) about the full channel is needed by the eNB. In TDD systems, the CSI is mainly acquired at the eNB by exploiting the bi-directional channel reciprocity. In FDD systems, the CSI is usually measured and quantized at the UE side and is then fed back to the eNB via a dedicated uplink channel. In general, the size of the codebook used for CSI quantization increases as the number of transmit antennas increases. In FD-MIMO systems, each antenna is connected to its own RF transceiver. Hence, in order to provide sufficiently accurate CSI via UE feedback, may lead to extra overhead at the UE, for example, in terms of channel estimation and the codeword selection.

[0054] Accordingly, aspects described herein adopt two- stage precoding to reduce the overhead caused by CSI feedback in FD-MIMO systems. The structure of two- dimensional arrays and the channel reciprocity are exploited. UL channel estimation is used to acquire a long-term port precoding matrix which compresses the large number of antenna elements to a small number of antenna ports. The eNB uses the port precoding matrix to transmit UE- specific port reference signals.

[0055] The UE measures the short-term CSI on a small number of antenna ports instead of the large number of antenna elements. The UE then quantize the short-term CSI and feeds back it to the eNB. The eNB uses the quantized short-term CSI to map multiple data layers to UE specific antenna ports and followed by a second stage precoding which maps the each antenna port to antenna elements. In order to support the proposed two-stage precoding scheme, some related signaling are described in more detailed herein.

[0056] FIG. 7 illustrates an example of a traditional MIMO with one-dimensional array. As illustrated, UE-specific beamforming may be performed in azimuth only. A common elevation tilting may be applied.

[0057] FIG. 8 illustrates an example FD-MIMO with two-dimensional array, according to aspects of the present disclosure. As illustrated, UE-specific beamforming may be performed in both azimuth and elevation.

[0058] In FD-MIMO systems, the number of transmit antennas at the eNB may be increased 8 to 10 folds as compared to legacy 8TX MIMO systems. These extra transmit antennas brings larger beamforming gain and sprays less interference to neighboring cells. [0059] Traditional one-shot beamforming/precoding methods rely on the availability of the channel state information (CSI) of the whole transmit dimension (e.g., the instantaneous/statistical knowledge of the channel from each eNB transmit antenna to one or more UE receive antennas are needed.) Such CSI is obtained either by UE PMI/RI feedback or by exploiting channel reciprocity.

[0060] The UE PMI/RI reporting is based on the pilot-aided estimation of the DL full channel. The pilot (or common reference signals) overhead and the complexity of DL channel estimation may be proportional to the number of eNB antennas. The complexity of PMI/RI selection may increase as the number of eNB antennas increases.

[0061] The channel reciprocity approach is limited by UE capability and the UL channel estimation error. For a low-end UE which cannot support sounding antenna switching, short-term CSI about the full channel is unavailable. The complexity of UL channel estimation and the complexity of calculating beamformer/precoder may be proportional to the number of eNB antennas.

[0062] In FD-MIMO systems, traditional one-shot beamforming/precoding is problematic due to the increased number of antennas. Thus, a challenge of supporting FD-MIMO is to design efficient beamforming/precoding algorithms and associated CSI acquisition schemes.

[0063] FIG. 9 illustrates example components used in accordance with methods described herein. Aspects of the present disclosure contain: a Hybrid CSI Acquisition module, to provide precoders for data/pilot precoding, a Data Precoding module, to precode data streams to antenna elements, and a Pilot Precoding module, to precode pilot sequence to antenna elements.

[0064] According to methods of the present disclsoure, both data and pilots are transmitted on a set of antenna ports. The number of antenna ports is much less than the number of antenna elements. Consequently, the overhead/computational complexity can be reduced significantly.

[0065] The Hybrid CSI Acquisition module of FIG. 9 generates two precoders, a port precoder and a layer precoder. The port precoder and layer precoder are illustrated in FIG. 10. [0066] The port precoder is used to map a small number of antenna ports to numerous antenna elements. It is obtained by exploiting (long-term) UL channel information.

[0067] The layer precoder is used to map data layers to antenna ports. It is obtained by UE feedback in terms of PMI/RI. The reported PMI/RI is selected from a predefined codebook based on the estimated DL channel on antenna ports. The estimation of DL channel on an antenna port is obtained by the associated the precoded pilot.

[0068] Data precoding (data stream precoding) of FIG. 9, is preformed in two consecutive stages, as detailed in FIG. 10. Stage 1 is Layer-to-port mapping where L data streams are first precoded by an L x P layer precoder. The layer precoder maps L data layers to P antenna ports. Stage 2 is Port-to-element mapping where P antenna ports are then precoded by a P x M port precoder. The port precoder maps P antenna ports to M antenna elements. Pilot sequences for estimating the channels on P antenna ports are precoded by the same port precoder.

[0069] Assumptions, notations, and terminology used herein will now be provided and may also be found in pages 5 and 6 of the attached APPENDIX.

* Ass y m i ns nd Notations

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- The layer precoder Is o d as f?*^L

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t p sS ¾s sa PfQ psrts.

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mttrf* *¾.€ «*** fs* * rifc

[0070] FIG. 11 illustrates an example sub-array partition, where M = 64 and P = 8. In this case, antenna elements may be cross-polarized. As well, elements are aligned in 8 rows by 8 columns.

[0071] FIGs. 12-18 and pages 8-14 of the APPENDIX illustrate various sub-array partitions according to aspects of the present disclosure.

[0072] FIG. 12 illustrates an example Type-1 sub-array, according to aspects of the present disclosure.

[0073] FIG. 13 illustrates an example Type-2 sub-array, according to aspects of the present disclosure.

[0074] FIG. 14 illustrates an example Type-4a sub-array, according to aspects of the present disclosure.

[0075] FIG. 15 illustrates an example Type-4b sub-array, according to aspects of the present disclosure.

[0076] FIG. 16 illustrates an example Type-8a sub-array, according to aspects of the present disclosure.

[0077] FIG. 17 illustrates an example Type-8b sub-array, according to aspects of the present disclosure. [0078] FIG. 18 illustrates an example Type-8c sub-array, according to aspects of the present disclosure.

[0079] In order to support the two-stage precoding scheme described herein, UE- specific parameters, including the type of sub-array partition and the CSI resource configuration may be semi-dynamically configured. The type of sub-array partition may include the structure of antenna port and the number of antenna ports.

[0080] Based on the above configuration, the UE may select one out of several predefined codebooks and use the selected codebook to report the PMI/RI for the layer precoder. UEs may be divided into multiple categories according to their capability of supporting multiple types of sub-array partition and the associated codebooks. According to aspects, low-end UEs may support limited types of sub-array partition.

[0081] FIG. 19 illustrates example operations 1900 performed, for example, by a base station (BS) according to aspects of the present disclosure. At 1902 the BS may generate a port precoding matrix which compress a larger number of antenna elements to a smaller number of antenna ports. At 1904, the BS may transmit UE-specific port reference signals to a user equipment (UE) using the port precoding matrix. At 1906, the BS may receive feedback regarding channel state information (CSI) measured by the UE based on the UE- specific port reference signals. At 1908, the BS may map multiple data layers to UE-specific antenna ports based on the CSI. At 1910, the BS may map each of the UE-specific antenna ports to physical antenna elements. At 1912, the BS may transmit data to the UE, based on the mapping of the multiple data layers and the mapping of antenna ports to physical antenna elements.

[0082] According to aspects, the port precoding matrix is generated based on (UL) channel estimation and the physical antenna elements are arranged in a multidimensional array. As described herein the BS may further map pilot sequences to UE- specific antenna ports.

[0083] According to aspects, the feedback may comprise quantized feedback comprising at least one of a preferred matrix indicator (PMI) and a rank indication (RI). The quantized feedback may be selected from a predefined codebook.

[0084] The BS may also transmit, to the UE, information regarding a sub-array partition of the antenna elements. The information may comprise at least one of a type of sub-array partition, a structure of antenna ports, or a number of antenna ports. [0085] According to aspects, the UEs may be divided into multiple categories according to their capability of supporting multiple types of sub-array partition and the associated codebooks. Certain types of UEs may support less types of sub-array partition than other types of UEs.

[0086] FIG. 20 illustrates example operations 2000 performed, for example, by a user equipment (UE), according to aspects of the present disclosure. At 2002, the UE may receive UE- specific port reference signals transmitted by a base station using a long-term port precoding matrix which compress a large number of antenna elements to a small number of antenna ports. At 2004, the UE may measure and quantize short-term CSI based on the UE-specific port reference signals. At 2006, the UE may transmit feedback regarding the (quantized) short-term CSI to the BS.

[0087] According to aspects, the feedback may comprise quantized feedback comprising at least one of a preferred matrix indicator (PMI) and a rank indication (RI). The quantized feedback may be selected from a predefined codebook. The UE may select, based on the information, one of a plurality of predefined codebooks, and may use the selected codebook to report the feedback. .

[0088] According to aspects, the UE may receive information regarding a sub-array partition of the antenna elements. The information may comprise at least one of a type of sub-array partition, a structure of antenna ports, and a number of antenna ports.

[0089] Thus, aspects of the present disclosure provide techniques to improve system capacity of FD-MIMO technology by using a two-dimensional antenna array with up to 64 antenna ports at the eNB. The use of a two-dimensional antenna array allows UE- specific beamforming in both azimuth and elevation.

[0090] The APPENDIX provides details and descriptions of various aspects of the present disclosure.

[0091] It is understood that the specific order or hierarchy of steps in the processes disclosed is an illustration of exemplary approaches. Based upon design preferences, it is understood that the specific order or hierarchy of steps in the processes may be rearranged. The accompanying method claims present elements of the various steps in a sample order, and are not meant to be limited to the specific order or hierarchy presented. [0092] The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects. Thus, the claims are not intended to be limited to the aspects shown herein, but is to be accorded the full scope consistent with the language claims, wherein reference to an element in the singular is not intended to mean "one and only one" unless specifically so stated, but rather "one or more." Unless specifically stated otherwise, the term "some" refers to one or more. All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. §112, sixth paragraph, unless the element is expressly recited using the phrase "means for" or, in the case of a method claim, the element is recited using the phrase "step for."

WHAT IS CLAIMED IS:

Background

The FD-MIMO technology could greatly improve system

ca acity by using a two-dimensional antenna array with up

to 64 antenna ports at the e B

- Very small inter-cell interference Figure 1. Traditional MIMO w/ one- dimensional array. UE-specific beamformir

- Very high hearn forming gain can be done in azimuth only. A common elevation tilting is applied.

The FD- irVIO is a study item for 3GPP Rel^'12,

Each antenna is connected to its own RF transceiver. The

two-dimensional antenna array allows UE-specific

beamforming in both azimuth and elevation.

Figure 2. Full-dimensional MIMO w/ two- dimensional array. UE-specific beamformir can be done in both azimuth and elevation.

Problem Statement

In FD-MIMO systems, the number of transmit antennas at the eNB is increased 8 to 10 folds as compared to legacy 8TX Ml MO systems. These extra transmit antennas brings larger beamforming gain and spays less interference to neighboring cells.

Traditional one-shot beamforming/precoding method relies on the availability of the channel state information (CSI) of the whole transmit dimension, i.e, the instantaneous/statistical knowledge of the channel from each eNB transmit antenna to one or more UE receive antennas are needed. Such CSI is obtained either by exploiting channel reciprocity or by UE PMI/RI feedback.

- The UE PMI/RI reporting is based on the pilot-aided estimation of the DL full channel.

* The pilot (or common reference signals) overhead and the complexity of DL channel estimation is proportional to the number of eNB antennas.

* The complexity of PMI/ ! selection increases as the number of e B antennas increases.

- The channel reciprocity approach is limited by UE capability and the UL channel estimation error.

* For the low- end UE which cannot support sound ng antenna switching, short-term CSI about the full channel is unavailable.

* The complexity of UL channel estimation and the complexity of calculating

beamformer/precoder are proportional to the number of eNB antennas.

In FD-MIMO systems, the traditional one-shot beamforming/precoding is problematic due to the increased number of antennas.

The main challenges of supporting FD-MIMO is to design efficient beamforming/precoding algorithms and associated CSI acquisition schemes.

Proposed Method

The proposed method contains the following compone

- Hybrid CS1 acquisition

To provide precoders for data/pilot preceding

^{■■■■■} Data preceding

To precede data streams to antenna elements

- Pilot preceding

To precode pilot sequence to antenna elements

With the proposed method, both data and pilots are

transmitted on a set of antenna ports.

- The number of antenna ports is much less than the

number of antenna elements.

- Consequently, the overhead/computational complexity

can be reduced significantly.

The hybrid CSI acquisition unit generates two precoders,

- Port precoder

* Port precoder is used to map a small number of antenna ports to numerous antenna elements.

* It is obtained by exploiting (long-term) UL channel information.

- Layer precoder

^¾ Layer precoder is used to map data layers to antenna ports,

^¾ it is obtained by UE feedback in terms of P I/R1. The reported PMI/R1 is selected from a

predefined code book based on the estimated DL channel on ante na ports.

* The estimation of DL channel on an antenna port is obtained by the associated the preceded pilot.

Proposed Method

The data stream preceding is performed in two consecutive stages,

- Stage 1 : Layer-to- port mapping

L data streams are first precoded by an L x P layer precoder. The layer precoder maps L data layers to P antenna ports,

- Stage 2: Port-to-e!ement mapping

P antenna ports are then precoded by a P x M port precoder, The element precoder maps P antenna ports to M antenna elements.

The pilot sequences for estimating the channels on P antenna ports are precoded by the same port precoder.

st



LU

eigen ports

rts

2 eigen ports

2 eigen ports eigen ports

30

31

32

33

Signaling

^• In order to support the proposed two-stage precoding

scheme, the following U E-specific parameters may be semi- dynamically configured.

— Type of Sub-array partition

^• Structure of antenna port

* Number of antenna ports

— CSI resource configuration

^• Based on the above configuration, the U E shall select one

out of several predefined codebooks and use the selected codebook to report the PM I/RI for the layer precoder.

^• U Es can be divided into multiple categories according to

their capability of supporting multiple types of sub-array

partition and the associated codebooks,

— Low-end UEs may support limited types of sub-array partition.

Claims

1. A method for wireless communications by a base station (BS), comprising: generating a port precoding matrix which compress a larger number of antenna elements to a smaller number of antenna ports;

transmitting UE-specific port reference signals to a user equipment (UE) using the port precoding matrix;

receiving feedback regarding channel state information (CSI) measured by the UE based on the UE- specific port reference signals;

mapping multiple data layers to UE-specific antenna ports based on the CSI; mapping each of the UE-specific antenna ports to physical antenna elements; and

transmitting data to the UE, based on the mapping of the multiple data layers and the mapping of antenna ports to physical antenna elements.

2. The method of claim 1, wherein the port precoding matrix is generated based on (UL) channel estimation.

3. The method of claim 1, wherein the physical antenna elements are arranged in a multi-dimensional array.

4. The method of claim 1, further comprising mapping pilot sequences to UE- specific antenna ports.

5. The method of claim 1, wherein the feedback comprises quantized feedback comprising at least one of a preferred matrix indicator (PMI) and a rank indication (RI).

6. The method of claim 5, wherein the quantized feedback is selected from a predefined codebook.

7. The method of claim 1, further comprising transmitting, to the UE, information regarding a sub-array partition of the antenna elements.

8. The method of claim 7, wherein the information comprises at least one of a type of Sub-array partition, a structure of antenna ports, or a number of antenna ports.

9. The method of claim 1, wherein UEs are divided into multiple categories according to their capability of supporting multiple types of sub-array partition and the associated codebooks.

10. The method of claim 9, wherein certain types of UEs may support less types of sub-array partition than other types of UEs.

11. A method for wireless communications by a user equipment (UE), comprising: receiving UE-specific port reference signals transmitted by a base station using a long-term port precoding matrix which compress a large number of antenna elements to a small number of antenna ports;

measuring and quantizing short-term CSI based on the UE-specific port reference signals; and

transmitting feedback regarding the (quantized) short-term CSI to the BS.

12. The method of claim 11, wherein the feedback comprises quantized feedback comprising at least one of a preferred matrix indicator (PMI) and a rank indication (RJ).

13. The method of claim 12, wherein the quantized feedback is selected from a predefined codebook.

14. The method of claim 11, further comprising receiving information regarding a sub-array partition of the antenna elements.

15. The method of claim 14, wherein the information comprises at least one of a type of Sub-array partition, a structure of antenna ports, and a number of antenna ports.

16. The method of claim 14, further comprising:

selecting, based on the information, one out of a plurality of predefined codebooks; and

using the selected codebook to report the feedback.

17. A method, apparatus, system, computer program product, and processing system as substantially described herein with reference to and as illustrated by the accompanying drawings and APPENDIX.