Patent Description:
Certain aspects of the present disclosure generally relate to wireless communications and, more particularly,to demodulation reference signal (DMRS) enhancement for higher order multi-user multiple-input multiple-output (MU-MIMO) communications.

Wireless communication networks are widely deployed to provide various communication services such as voice, video, packet data, messaging, broadcast, etc. These wireless networks may be multiple-access networks capable of supporting multiple users by sharing the available network resources(e.g., bandwidth and transmit power). Examples of such multiple-access networks include Code Division Multiple Access (CDMA) networks, Time Division Multiple Access (TDMA) networks, Frequency Division Multiple Access (FDMA) networks, Orthogonal FDMA (OFDMA) networks, Single-Carrier FDMA (SC-FDMA) networks, <NUM>rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) networks, and Long Term Evolution Advanced (LTE-A) networks.

A wireless communication network may include a number of base stations that can support communication with a number of user equipments (UEs). A UE may communicate with a base station via the downlink and uplink. This communication link may be established via a single-input single-output, multiple-input single-output or a multiple-input multiple-output (MIMO) system.

A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS ≤ min{NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.

A MIMO system supports time division duplex (TDD) and frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows the estimation of the forward link channel from the reverse link channel. This enables the base station to extract transmit beamforming gain on the forward link when multiple antennas are available at the base station.

"<NPL> relates to DM-RS enhancement for EBF/FD-MIMO.

"<NPL> relates to DMRS design enhancement and evaluation for higher order MU-MIMO.

"<NPL> relates to proposals on the use of DMRS ports/scrambling sequences for MU-MIMO, and alternatives for further study.

In accordance with the present invention, there is provided a method for wireless communications by a base station as set out in claim <NUM>; an apparatus for wireless communications by an eNodeB as set out in claim <NUM>; a method for wireless communications by a user equipment as set out in claim <NUM> and an apparatus for wireless communications by a user equipment as set out in claim <NUM>. Other aspects of the invention can be found in the dependent claims.

Numerous other aspects are provided including methods, apparatus, systems, computer program products, and processing systems.

Although particular aspects are described herein, many variations and permutations of these aspects fall within the scope of the disclosure. Although some benefits and advantages of the preferred aspects are mentioned, the scope of the disclosure is not intended to be limited to particular benefits, uses, or objectives. Rather, aspects of the disclosure are intended to be broadly applicable to different wireless technologies, system configurations, networks, and transmission protocols, some of which are illustrated by way of example in the figures and in the following description of the preferred aspects. The detailed description and drawings are merely illustrative of the disclosure rather than limiting, the scope of the disclosure being defined by the appended claims and equivalents thereof.

The techniques described herein may be used for various wireless communication networks such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA and other networks. cdma2000 covers IS--<NUM>, IS-<NUM> and IS-<NUM> standards. An OFDMA network may implement a radio technology such as Evolved UTRA (E-UTRA), Ultra Mobile Broadband (UMB), IEEE <NUM> (Wi-Fi), IEEE <NUM> (WiMAX), IEEE <NUM>, Flash-OFDM®, etc. UTRA and E-UTRA are part of Universal Mobile Telecommunication System (UMTS). 3GPP Long Term Evolution (LTE) and LTE-Advanced (LTE-A) are new releases of UMTS that use E-UTRA. For clarity, certain aspects of the techniques are described below for LTE, and LTE terminology is used in much of the description below.

Single carrier frequency division multiple access (SC-FDMA) is a transmission technique that utilizes single carrier modulation at a transmitter side and frequency domain equalization at a receiver side. The SC-FDMA has similar performance and essentially the same overall complexity as those of OFDMA system. However, SC-FDMA signal has lower peak-to-average power ratio (PAPR) because of its inherent single carrier structure. The SC-FDMA has drawn great attention, especially in the uplink communications where lower PAPR greatly benefits the mobile terminal in terms of transmit power efficiency. It is currently a working assumption for uplink multiple access scheme in the 3GPP LTE, LTE-A, and the Evolved UTRA.

<FIG> shows a wireless communication network <NUM>, which may be an LTE network or some other wireless network, in which aspects of the present disclosure may be practiced. Wireless network <NUM> may include a number of evolved Node Bs (eNBs) <NUM> and other network entities. An eNB is an entity that communicates with user equipments (UEs) and may also be referred to as a base station, a Node B, an access point (AP), etc. Each eNB may provide communication coverage for a particular geographic area. In 3GPP, the term "cell" can refer to a coverage area of an eNB and/or an eNB subsystem serving this coverage area, depending on the context in which the term is used.

An eNB may provide communication coverage for a macro cell, a pico cell, a femto cell, and/or other types of cell. An eNB for a pico cell may be referred to as a picoeNB. An eNB for a femto cell may be referred to as a femtoeNB or a home eNB (HeNB). In the example shown in <FIG>, an eNB 110a may be a macro eNB for a macro cell 102a, an eNB 110b may be a picoeNB for a pico cell 102b, and an eNB 110c may be a femtoeNB for a femto cell 102c. An eNB may support one or multiple (e.g., three) cells. The terms "eNB", "base station," and "cell" may be used interchangeably herein.

A relay station is an entity that can receive a transmission of data from an upstream station (e.g., an eNB or a UE) and send a transmission of the data to a downstream station (e.g., a UE or an eNB). In the example shown in <FIG>, a relay station 110d may communicate with macro eNB 110a and a UE 120d in order to facilitate communication between eNB 110a and UE 120d. A relay station may also be referred to as a relay eNB, a relay base station, a relay, etc..

Wireless network <NUM> may be a heterogeneous network that includes eNBs of different types, e.g., macro eNBs, picoeNBs, femtoeNBs, relay eNBs, etc. These different types of eNBs may have different transmit power levels, different coverage areas, and different impact on interference in wireless network <NUM>. For example, macro eNBs may have a high transmit power level (e.g., <NUM> to <NUM> W) whereas picoeNBs, femtoeNBs, and relay eNBs may have lower transmit power levels (e.g., <NUM> to <NUM> W).

A network controller <NUM> may couple to a set of eNBs and may provide coordination and control for these eNBs. Network controller <NUM> may communicate with the eNBs via a backhaul. The eNBs may also communicate with one another, e.g., directly or indirectly via a wireless or wireline backhaul.

A UE may also be referred to as an access terminal, a terminal, a mobile station (MS), a subscriber unit, a station (STA), etc. A UE may be a cellular phone, a personal digital assistant (PDA), a wireless modem, a wireless communication device, a handheld device, a laptop computer, a cordless phone, a wireless local loop (WLL) station, a tablet, a smart phone, a netbook, a smartbook, etc..

<FIG> is a block diagram of a design of base station/eNB <NUM> and UE <NUM>, which may be one of the base stations/eNBs and one of the UEs in <FIG>.

At base station <NUM>, a transmit processor <NUM> may receive data from a data source <NUM> for one or more UEs, select one or more modulation and coding schemes (MCSs) for each UE based on channel quality indicators (CQIs) received from the UE, process (e.g., encode and modulate) the data for each UE based on the MCS(s) selected for the UE, and provide data symbols for all UEs. Transmit processor <NUM> may also process system information (e.g., for semi-static resource partitioning information (SRPI), etc.) and control information (e.g., CQI requests, grants, upper layer signaling, etc.) and provide overhead symbols and control symbols. Processor <NUM> may also generate reference symbols for reference signals (e.g., the common reference signal (CRS)) and synchronization signals (e.g., the primary synchronization signal (PSS) and secondary synchronization signal (SSS)).

Each demodulator <NUM> may condition (e.g., filter, amplify, downconvert, and digitize) its received signal to obtain input samples. A channel processor may determine reference signal received power (RSRP), received signal strength indicator (RSSI), reference signal received quality (RSRQ), CQI, etc..

On the uplink, at UE <NUM>, a transmit processor <NUM> may receive and process data from a data source <NUM> and control information (e.g., for reports comprising RSRP, RSSI, RSRQ, CQI, etc.) from controller/processor <NUM>. Processor <NUM> may also generate reference symbols for one or more reference signals. The symbols from transmit processor <NUM> may be precoded by a TX MIMO processor <NUM> if applicable, further processed by modulators 254a through 254r (e.g., for SC-FDM, OFDM, etc.), and transmitted to base station <NUM>. Processor <NUM> may provide the decoded data to a data sink <NUM> and the decoded control information to controller/processor <NUM>.

Controllers/processors <NUM> and <NUM> may direct the operation at base station <NUM> and UE <NUM>, respectively. Processor <NUM> and/or other processors and modules at base station <NUM>, and/or processor <NUM> and/or other processors and modules at UE <NUM>, may perform or direct processes for the techniques described herein. Memories <NUM> and <NUM> may store data and program codes for base station <NUM> and UE <NUM>, respectively.

When transmitting data to the UE <NUM>, the base station <NUM> may be configured to determine a bundling size based at least in part on a data allocation size and precode data in bundled contiguous resource blocks of the determined bundling size, wherein resource blocks in each bundle may be precoded with a common precoding matrix. That is, reference signals (RSs) such as UE-RS and/or data in the resource blocks may be precoded using the same precoder. The power level used for the UE-RS in each resource block (RB) of the bundled RBs may also be the same.

The UE <NUM> may be configured to perform complementary processing to decode data transmitted from the base station <NUM>. For example, the UE <NUM> may be configured to determine a bundling size based on a data allocation size of received data transmitted from a base station in bundles of contiguous RBs, wherein at least one reference signal in resource blocks in each bundle are precoded with a common precoding matrix, estimate at least one precoded channel based on the determined bundling size and one or more RSs transmitted from the base station, and decode the received bundles using the estimated precoded channel.

<FIG> shows an exemplary frame structure <NUM> for FDD in LTE. Each radio frame may have a predetermined duration (e.g., <NUM> milliseconds (ms)) and may be partitioned into <NUM> subframes with indices of <NUM> through <NUM>. Each subframe may include two slots. Each radio frame may thus include <NUM> slots with indices of <NUM> through <NUM>. Each slot may include L symbol periods, e.g., seven symbol periods for a normal cyclic prefix (as shown in <FIG>) or six symbol periods for an extended cyclic prefix. The <NUM>L symbol periods in each subframe may be assigned indices of <NUM> through <NUM>L-<NUM>.

In LTE, an eNB may transmit a primary synchronization signal (PSS) and a secondary synchronization signal (SSS) on the downlink in the center <NUM> of the system bandwidth for each cell supported by the eNB. The PSS and SSS may be transmitted in symbol periods <NUM> and <NUM>, respectively, in subframes <NUM> and <NUM> of each radio frame with the normal cyclic prefix, as shown in <FIG>. The eNB may transmit a cell-specific reference signal (CRS) across the system bandwidth for each cell supported by the eNB. The CRS may be transmitted in certain symbol periods of each subframe and may be used by the UEs to perform channel estimation, channel quality measurement, and/or other functions. The eNB may also transmit a physical broadcast channel (PBCH) in symbol periods <NUM> to <NUM> in slot <NUM> of certain radio frames. The PBCH may carry some system information. The eNB may transmit other system information such as system information blocks (SIBs) on a physical downlink shared channel (PDSCH) in certain subframes. The eNB may transmit control information/data on a physical downlink control channel (PDCCH) in the first B symbol periods of a subframe, where B may be configurable for each subframe. The eNB may transmit traffic data and/or other data on the PDSCH in the remaining symbol periods of each subframe.

The PSS, SSS, CRS, and PBCH in LTE are described in 3GPP TS <NUM>, entitled "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation," which is publicly available.

<FIG> shows two example subframe formats <NUM> and <NUM> for the downlink with a normal cyclic prefix. The available time frequency resources for the downlink may be partitioned into resource blocks. Each resource block may cover <NUM> subcarriers in one slot and may include a number of resource elements. Each resource element may cover one subcarrier in one symbol period and may be used to send one modulation symbol, which may be a real or complex value.

Subframe format <NUM> may be used for an eNB equipped with two antennas. A CRS may be transmitted from antennas <NUM> and <NUM> in symbol periods <NUM>, <NUM>, <NUM>, and <NUM>. A reference signal is a signal that is known a priori by a transmitter and a receiver and may also be referred to as pilot. A CRS is a reference signal that is specific for a cell, e.g., generated based on a cell identity (ID). In <FIG>, for a given resource element with label Ra, a modulation symbol may be transmitted on that resource element from antenna a, and no modulation symbols may be transmitted on that resource element from other antennas. Subframe format <NUM> may be used for an eNB equipped with four antennas. A CRS may be transmitted from antennas <NUM> and <NUM> in symbol periods <NUM>, <NUM>, <NUM>, and <NUM> and from antennas <NUM> and <NUM> in symbol periods <NUM> and <NUM>. For both subframe formats <NUM> and <NUM>, a CRS may be transmitted on evenly spaced subcarriers, which may be determined based on cell ID. Different eNBs may transmit their CRSs on the same or different subcarriers, depending on their cell IDs. For both subframe formats <NUM> and <NUM>, resource elements not used for the CRS may be used to transmit data (e.g., traffic data, control data, and/or other data).

An interlace structure may be used for each of the downlink and uplink for FDD in LTE. For example, Q interlaces with indices of <NUM> through Q-<NUM> may be defined, where Q may be equal to <NUM>, <NUM>, <NUM>, <NUM>, or some other value. Each interlace may include subframes that are spaced apart by Q frames. In particular, interlace q may include subframesq, q+Q, q+<NUM>Q, etc., where q ∈ {<NUM>,. ,Q - <NUM>}.

The wireless network may support hybrid automatic retransmission request (HARQ) for data transmission on the downlink and uplink. For HARQ, a transmitter (e.g., an eNB <NUM>) may send one or more transmissions of a packet until the packet is decoded correctly by a receiver (e.g., a UE <NUM>) or some other termination condition is encountered. For synchronous HARQ, all transmissions of the packet may be sent in subframes of a single interlace. For asynchronous HARQ, each transmission of the packet may be sent in any subframe.

A UE may be located within the coverage of multiple eNBs. One of these eNBs may be selected to serve the UE. The serving eNB may be selected based on various criteria such as received signal strength, received signal quality, path loss, etc. Received signal quality may be quantified by a signal-to-interference-plus-noise ratio (SINR), or a reference signal received quality (RSRQ), or some other metric. The UE may operate in a dominant interference scenario in which the UE may observe high interference from one or more interfering eNBs.

Three-dimensional (3D) MIMO is specified in the 3GPP Rel-<NUM> and has the potential of drastically enhancing Long Term Evolution (LTE) systems. 3D MIMO differs from conventional MIMO in that 3D MIMO supports the use of (<NUM>) a two-dimensional array with antenna ports on both horizontal and vertical axis and(<NUM>) a greaternumber of TXRU's (e.g., Transceiver Units) relative to conventional MIMO. A TXRU generally may control amplitude and phase independently of other TXRUs(e.g., via independent amplitude and phase control devices at each TXRU). Such capability together with the two-dimensional antenna array generally allows the transmitted signal to be steered bothonly in the horizontal direction, as in conventional multi-antenna systems, and simultaneously in both the horizontaland the vertical direction, providing more flexibility in shaping beam directions from eNB to UE.

Higher order multi-user multiple-input multiple-output (MU-MIMO)may enable wireless devices to fully exploit the degree of freedom in elevation dimension introduced by 3D-MIMO. To efficiently support higher order MU-MIMO, DMRS enhancement is needed to have orthogonalDMRS port multiplexing (e.g., using orthogonal DMRS port multiplexing) to reduce DMRS inter-layer interference and improve channel estimation performance.

There are two types of MU resource sharingthat can be used with 3D-MIMO techniques and a two-dimensional (2D) antenna array. UEs may be separated in the horizontal direction only, or UEs may be separated in both vertical and horizontaldirections. In such way, up to <NUM> UEs or <NUM>-layer transmission in total is possible for 3D-MIMO with <NUM> antenna ports. For the support of higher order MU-MIMO, the design of downlink (DL) control signaling and DMRS allocation is needed.

Aspects of the present disclosure presentseveral orthogonal DMRS patterns that can be used to support higher order MU-MIMO. Additionally, aspects of the present disclosure provide for transmission and reception of control signaling for a DMRS pattern and a port indication. The orthogonal DMRS patterns and signaling resented herein is backward compatible (e.g., with conventional devices that employ 2D-MIMO techniques for communications) and has minimumsignaling overhead.

<FIG> illustrates an example of quasi-orthogonal demodulation reference signal (DMRS) structure <NUM>, in accordance with certain aspects of the present disclosure. In the current specification, two orthogonal DMRS ports (e.g., ports <NUM> and <NUM> illustrated in <FIG>) and two scrambling sequences (e.g., Id <NUM> and Id <NUM>) are defined for MU-MIMO.

In the current specification, the quasi-orthogonal DMRS structure <NUM> supports no more than <NUM>-layer transmission in total for MU-MIMO transmission, and no more than two layers per UE with two orthogonal DM-RS ports. As illustrated in <FIG>, there is one code division multiplexing (CDM) group with orthogonal DMRS ports, and the CDM group may be multiplexed using a length-<NUM> orthogonal cover code (OCC). In some cases, CDM groups may use different scrambling sequences (e.g., a first CDM group may use the scrambling sequence Id <NUM>, and a second CDM group may use the scrambling sequence Id <NUM>).

In some cases,virtual cell ID (VCID) may be used for DMRS sequence initialization to support more than <NUM>-layer MU-MIMO. For example, VCID can be configured by radio resource control (RRC) signaling on per-UE basis. However, in such case, channel estimation performance may be degraded by interference between different DMRS layers because the orthogonality provided by different scrambling codes (or sequences) is weak. In general, orthogonal DMRS design is preferred overnon-orthogonal DMRS design especially for higher order MU-MIMO. Therefore, higher-rank orthogonal DMRS patterns may be needed for <NUM>-layer and <NUM>-layer MU-MIMO.

<FIG> illustrates example DMRS patterns for <NUM>-layer MU-MIMO communications with a normal cyclic prefix (CP), in accordance with certain aspects of the present disclosure. For the support of <NUM>-layer orthogonal MU-MIMO, the two options for DMRS pattern design (e.g., patterns <NUM> and <NUM>) illustrated in <FIG> may be used.

As illustrated in <FIG>, for the DMRS pattern <NUM>, CDM and Time Division Multiplexing (TDM) is combined with a length-<NUM> OCC. CDM group 604may be allocated for layers {<NUM>,<NUM>} or DMRSports {<NUM>,<NUM>}, and CDM group <NUM> may be allocated for layers {<NUM>,<NUM>} or DMRSports {<NUM>,<NUM>}. TDM may be applied between the CDM group <NUM> and the CDM group <NUM>.

For the DMRS pattern <NUM>, there may be <NUM> resource elements (REs) per layer using alength-<NUM> OCC. As illustrated in <FIG>, for the DMRS pattern <NUM>, CDMgroup <NUM> may span over <NUM> REs in time which are not contiguous. At high Doppler, the orthogonality may be lost, leading to performance loss. For the DMRS pattern <NUM>, the CDMgroup <NUM> may be allocated for layers {<NUM>,<NUM>,<NUM>,<NUM>} or DMRS ports {<NUM>, <NUM>, <NUM>, <NUM>} via length-<NUM> OCC.

DMRSpattern 608allows for the use of auniform DMRS to PDSCH (Physical Downlink Shared Channel) power ratio for each layer. Compared to the DMRSpattern <NUM>, 3dB DMRS power boosting may need to be used for the DMRS pattern <NUM> due to TDM of two CDM groups.

<FIG> illustrates examples DMRS patterns for <NUM>-layer MU-MIMO communications with an extended cyclic prefix (CP), in accordance with certain aspects of the present disclosure. As illustrated in <FIG>, for the DMRS pattern <NUM>, CDM and TDM may be combined with length-<NUM> OCC. For the DMRS pattern <NUM>, CDM group <NUM> may beallocated for layers <NUM> and <NUM>, and CDM group <NUM> may be allocated for layers <NUM> and <NUM>. TDM may be applied between the CDM group <NUM> and the CDM group <NUM>. For the DMRS pattern <NUM>,there may be <NUM> RE per layer with length-<NUM> OCC. For the DMRS pattern <NUM>, a single CDM group <NUM> may be allocated for layers {<NUM>,<NUM>,<NUM>,<NUM>} via length-4OCC. In some aspects, orthogonal TDM and FDM may be applied between CDM group <NUM> and CDM group <NUM>.

<FIG> illustrates an example DMRS pattern <NUM> for <NUM>-layer MU-MIMO with a normal CP, in accordance with certain aspects of the present disclosure. As illustrated in <FIG>, if eight UEs or total <NUM>-layer MU-MIMO is supported, the orthogonal DMRS pattern can be extended by combining CDM and Frequency Division Multiplexing (FDM) with length-<NUM> OCC. In some aspects,<NUM>-layers may be multiplexed usinga length-<NUM> OCC, and <NUM> CDM groups may be further multiplexed in the frequency domain. From a UE perspective, the DMRS pattern may use <NUM> DMRSREs for demodulation of PDSCH, but the DMRS location may be determined by the CDM group. As illustrated in <FIG>, CDM group <NUM> may be allocated for layers {<NUM>,<NUM>,<NUM>,<NUM>} or DMRS ports {<NUM>,<NUM>,<NUM>,<NUM>}, and CDM group <NUM> may be allocated for layers {<NUM>,<NUM>,<NUM>,<NUM>} or DMRS ports {<NUM>,<NUM>,<NUM>,<NUM>}. Orthogonal TDM and/or FDM may be applied between the CDM group <NUM> and the CDM group <NUM>.

In some aspects, a first CDM group and a second CDM group may be nonorthogonally multiplexed on the same resource elements in the time and frequency domains The first CDM group may use a first scrambling sequence, and a second CDM group may use a second scrambling sequence.

DMRS patterns (e.g., legacy <NUM>-layer with length-<NUM> OCC or enhanced <NUM>-layer or <NUM>-layer orthogonal pattern withlength-<NUM> OCC) can be semi-statically configured by RRC or via dynamic L1 signaling on the PDCCH for each UE. For example, two bit signaling for DMRS pattern indication may be utilized. The pattern "<NUM>" may indicate a legacy quasi-orthogonal DMRS withlength-<NUM> OCC; the pattern "<NUM>" may indicate one enhanced <NUM>-layerDMRS pattern of either CDM+TDM via length-<NUM> OCC or CDM only via length-<NUM> OCC; and the patterns "<NUM>" and "<NUM>" may indicate an enhanced <NUM>-layerDMRS pattern of CDM group <NUM> and <NUM> (e.g., the CDM groups <NUM> and <NUM> in <FIG>).

The DMRS overhead for each configured DMRS pattern may be different, which may result in different rate matching pattern for PDSCH resource mapping. For example, if <NUM>-layer DMRS pattern is indicated, then the UE may use <NUM> DMRSREs for PDSCH rate matching instead of <NUM> REs. The dynamic configuration of the DMRS pattern may allow the network to dynamically switch between different DMRS patterns on a per-UE basis based on mobility (e.g., the speed of the UE) and the capability of the UE to support higher order MU-MIMO.

In some cases, the enhanced DMRS patterns described herein may be applicable only for PDCCH/EPDCCH (Enhanced PDCCH) located in a UE specific search space. For example, for (E)PDCCH in a common search space, the legacy <NUM>-layer DMRS pattern via length-<NUM> OCC may be used even if UE is configured with an enhanced DMRS pattern by RRC signaling. Using a legacy DMRS pattern in a common search space provides for backward compatibility with legacy UEs that may not support enhanced DMRS patterns and allows UEs that support enhanced DMRS patterns to coexist with legacy UEs.

<FIG> illustrates examples of DL control signaling for DMRS pattern and port indication, in accordance with certain aspects of the present disclosure. For DMRS port indication, the existing <NUM>-bits field in Downlink Control Information (DCI) format can be reused, but the content may be determined by the configured DMRS pattern. For example, UE may use the table <NUM> to determine DMRS port and number of layers indication if the legacy DMRS pattern is configured. The table <NUM> may be used for the enhanced <NUM>-layer or the CDM group <NUM> (e.g., the CDM group <NUM> in <FIG>) of the <NUM>-layer DMRS pattern.

The difference between tables <NUM> and <NUM> is that a quasi-orthogonal DMRS (e.g., scrambling ID=<NUM>/<NUM>) may be used in the table <NUM>, while orthogonal DMRS ports may be assumed in the table <NUM>. As illustrated in <FIG>, the table <NUM> provides antenna port(s), scrambling identity and number of layers indication for legacy MU DMRS pattern; the table <NUM> provides antenna port(s) and number of layers indication for enhanced MU DMRS pattern.

According to the present disclosure, joint indication of the DMRS pattern and antenna port indices may be transmitted to each UE via L1 control signaling. One bit is used for PDSCH rate matching indication (e.g., to indicate the use of <NUM> or <NUM> DMRS REs), and another <NUM> bits is used for DMRS pattern and port indication.

Examples of joint coding of DMRS pattern and port indication are illustrated in the table <NUM> from <FIG> and by the table <NUM> from <FIG>. <FIG> illustrates examples of joint DMRS pattern and port indication with one codeword enabled, in accordance with certain aspects of the present disclosure. <FIG> illustrates examples of joint DMRS pattern and port indication with two codewords enabled, in accordance with certain aspects of the present disclosure. For example, for joint DMRS and port indication using one codeword, there are <NUM> MU states corresponding to <NUM> orthogonal DMRS ports of rank <NUM>, as illustrated in the table <NUM> in <FIG>. For joint DMRS and port indication using two codewords, there are <NUM> MU states corresponding to four rank-<NUM> cases, one rank-<NUM> case and one rank-<NUM> case, as illustrated in the table <NUM> in <FIG>. In some aspects, based on the port indication, UE may know the assigned DMRS ports, total DRMS resources, and maximum number of orthogonal DRMS ports for MU.

For OCC (orthogonal cover code) mapping for DMRS ports {<NUM>,<NUM>,<NUM>,<NUM>}, the current method can be reused for backward compatibility. For example, anOCC design may be based on length-<NUM> Walsh code (<NUM><NUM>) for ports <NUM> and <NUM> and the length-<NUM> Walsh code (<NUM> -<NUM>) for ports <NUM> and <NUM>,or length-<NUM> Walsh codes of (<NUM>, <NUM>, <NUM>, <NUM>) and (<NUM> -<NUM><NUM> -<NUM>). In some aspects, 2D orthogonal mapping may be achieved by reversing the mapping direction every subcarrier.

In some aspects, for DMRS ports {<NUM>, <NUM>, <NUM>, <NUM>}, it may be possible to reuse the OCC and mapping pattern for ranks <NUM>-<NUM> of SU-MIMO. For example, length-<NUM> Walsh codes {<NUM>, <NUM>, -<NUM>, -<NUM>} and {<NUM>, -<NUM>, -<NUM>, <NUM>} can be used for ports <NUM> and <NUM>, and length-<NUM> Walshcodes{-<NUM>, -<NUM>, <NUM>, <NUM>} and {-<NUM>, <NUM>, <NUM>, -<NUM>} can be used for ports <NUM> and <NUM>. However, if only DMRS ports <NUM> and <NUM> are configured using sequences {<NUM>, <NUM>, <NUM>, <NUM>} and {<NUM>, <NUM>, -<NUM>, - <NUM>}, then, at high Doppler, there is a strong inter-layer interference due to the loss of the orthogonality since the CDM is over <NUM> REs in time that are not contiguous.

Similar inter-layer interference may also be observed for DMRS ports {<NUM>, <NUM>}, {<NUM>, <NUM>}, or {<NUM>, <NUM>}. It should be noted that for DMRS ports {<NUM>, <NUM>} or {<NUM>, <NUM>}, there is no such problem since two sequences {<NUM>, <NUM>, <NUM>, <NUM>} and {<NUM>, -<NUM>, -<NUM>, <NUM>} or {<NUM>, -<NUM>, <NUM>, - <NUM>} and {<NUM>, <NUM>, -<NUM>, -<NUM>} are also orthogonal with CDM length-<NUM>.

To minimize the inter-layer interference of DMRS ports, one approach may includeadding the layer shift in the frequency domain so that the OCC for DMRS port {<NUM>, <NUM>, <NUM>, <NUM>} is switched on frequency domain between two spreading sequences, where the second spreading sequence is a cyclically shiftedversion of the first spreading sequence. For example, for DMRS port <NUM>, frequency switching may be performed between spreading sequence {<NUM>, <NUM>, -<NUM>, -<NUM>} and spreading sequence {<NUM>, -<NUM>, -<NUM>, <NUM>}.

<FIG> illustrates examples of DMRS to OCC mapping for DMRS ports {<NUM>, <NUM>, <NUM>, <NUM>}, in accordance with certain aspects of the present disclosure. In order to retain frequency orthogonality, the reserving mapping direction may also be used for the <NUM>nd and <NUM>th subcarrier, resulting in total four spreading sequences for each DMRS port. As illustrated in <FIG>,the <NUM>nd spreading sequence {d, c, b, a} may bea time reversal version of the first spreading sequence {a, b, c, d}; the <NUM>rd spreading sequence {b, c, d, a} may be a left cyclic shift version of the <NUM>st spreading sequence; and the <NUM>th spreading sequence {a, d, c, b} may be a time reversal version of the <NUM>rd spreading sequenceor a right cyclic shift version of the <NUM>nd spreading sequence.

<FIG> illustrates an example <NUM> of precoding resource block (PRB) bundling, in accordance with certain aspects of the present disclosure. PRB bundling may be used when Pre-coding Matrix Indicator/Rank Indicator (PMI/RI) feedback is configured so that UE can assume the same precoder is applied in multiple PRBs. The precoding RB groups (PRGs) may be fixed and comprise consecutive PRBs. The boundary may be cell specific independent of UE allocation. PRB bundling can improve the channel estimation performance by using joint channel estimation across the PRGs.

For MU-MIMO, there is one problem about RB bundling assumption for the co-scheduled UE, especially when more than two orthogonal ports are supported. A simple solution can be that UE assumes no bundling for co-scheduled UEs since there could be different UE pairing in the PRGs. This is a straightforward solution, as assuming no bundling for co-scheduled UEs may not impose restrictions on eNB scheduling. However, frequency orthogonality of OCC mapping may not be valid across the PRB since the precoder for interfering layers may change over RBs in one PRG. This may degrade the channel estimation performance for edge tones when frequency orthogonality (e.g., frequency OCC dispreading)is used for channel estimation (e.g., at high Doppler).

<FIG> illustrates an example of PRB bundling <NUM> with two RBs in one precoding RB group (PRG), in accordance with certain aspects of the present disclosure. As illustrated in <FIG>, one PRG with <NUM> PRBs is considered, with different UE pairings on RBs <NUM> and <NUM>. In this case, for channel estimation of edge tones across the two RBs, frequency OCC dispreading cannot be used since different precoders are generally used for DMRS port <NUM> between RB <NUM> and RB1, resulting in a loss of frequency orthogonality.

Aspects of the present disclosure provide certain assumptions for PRB bundling for other orthogonal ports assigned to the co-scheduled UEs. In an aspect of the present disclosure, UEs may assume the use of the same bundling boundary for the co-scheduled UEs. For example, UEs may assume that the bundling boundaries are fixed and dependent on system bandwidth, such as bundling boundaries of <NUM>/<NUM>/<NUM> RBs for <NUM>/<NUM>/<NUM>. Assuming the use of the same bundling boundary for co-scheduled UEs may add scheduling constraints since it is assumed the same UE pairing is applied to the bundled RBs of the PRG. In another aspect, UE may use a bundling boundary configured using higher level signaling, such as two RB boundaries for both the used DMRS ports and other DMRS ports of the co-scheduled UEs. This approach may achievea tradeoff between channel estimation performance and scheduling constraints and may provide more flexibility for eNB implementation.

<FIG> illustrates example operations <NUM> that may be performed at an evolved Node B (eNB) or a base station (BS), in accordance with certain aspects of the present disclosure. Operations <NUM> may be executed, for example, at the processor(s) <NUM>, <NUM>, and/or <NUM> of the eNB <NUM> from <FIG>. While operations <NUM> and other aspects of the present disclosure are described with reference to LTE-A systems, the techniques described herein are applicable to any other suitable MIMO system.

Operations1500 may begin, at <NUM>, by determining a plurality of ports of a multi-dimensional array of transmit antennas and a number of spatial multiplexed layers for transmission to a plurality of user equipments (UEs). At <NUM>, an orthogonal demodulation reference signal (DMRS) pattern may be configured by multiplexing the layers or the ports in the DMRS pattern, using an orthogonal cover code (OCC) and one or more code division multiplexing (CDM) groups. At <NUM>, DMRS symbols may be transmittedbased on the configured DMRS pattern using the multiplexed layers and the ports.

In an aspect of the present disclosure, as discussed above (e.g., the DMRS pattern <NUM> in <FIG>), the OCC may comprise a length-<NUM> OCC. The one or more CDM groups may comprise a first CDM group allocated to a first pair of layers or a first pair of ports and a second CDM group allocated to a second pair of layers or a second pair of ports, and TDM may be applied between the first CDM group and the second CDM group. In another aspect (e.g., the DMRS pattern <NUM> in <FIG>), the OCC may comprise a length-<NUM> OCC. The one or more CDM groups may comprise a first CDM group allocated to a first pair of layers and a second CDM group allocated to a second pair of layers. TDM may be applied between the first CDM group and the second CDM group, and the first CDM group may be shifted in frequency relative to the second CDM group.

In yet another aspect, as discussed above (e.g., the DMRS pattern <NUM> in <FIG>), the OCC may comprise a length-<NUM> OCC, andthe one or more CDM groups may comprise a single CDM group allocated to four layers or four ports spanning four noncontiguous resource elements (REs) in the time domain. In yet another aspect (e.g., the DMRS pattern <NUM> in <FIG>), the OCC may comprise a length-<NUM> OCC. The one or more CDM groups may comprise a single CDM group allocated to four layers spanning four noncontiguous resource elements (REs) in time domain, and two of the four REs may be frequency shifted relative to other two of the four REs.

In yet another aspect, as discussed above (e.g., the DMRS pattern <NUM> in <FIG>),the OCC may comprise a length-<NUM> OCC. The one or more CDM groups may comprise a first CDM group allocated to a first set of four layers or four ports spanning four noncontiguous resource elements (REs) in time domain and a second CDM group allocated to a second set offour layers or four ports spanning four noncontiguous REs in time domain. , Frequency division multiplexing (FDM) may be applied between the first CDM group and the second CDM group.

In an aspect of the present disclosure, the BS may provide an indication about the configured DMRS pattern to the plurality of UEs using radio resource control (RRC) signaling. In another aspect, the BS may providean indication about the configured DMRS pattern to the plurality of UEs using dynamic L1 signaling on a Physical Downlink Control Channel (PDCCH) for each UE. In some cases, the indication about the configured DMRS pattern may include first data indicating a type of multiplexing used for a first and a second CDM group, and second data indicating a length of the OCC (e.g., whether the DMRS pattern uses a length-<NUM> OCC or a length-<NUM> OCC).

In some aspects, as discussed above, the configured DMRS pattern may be dynamically switched based, for example, on the speed of a user equipment (UE) of the plurality of UEs or a UE capability to support higher order multi-user multiple-input multiple-output (MU-MIMO) communications. In an aspect, the BS may communicate an indication about the ports of the multi-dimensional array of transmit antennas and the number of layers for transmitting the DMRS symbols using L1 control signaling on a Physical Downlink Control Channel (PDCCH).

As discussed above, the BS communicates a joint indication of the configured DMRS pattern and the ports of the multi-dimensional array of transmit antennas using L1 control signaling. The joint indication may comprise one bit for Physical Downlink Shared Channel (PDSCH) rate matching information and four bits indicating the configured DMRS pattern and the ports of the multi-dimensional array of transmit antennas.

In some aspects, as discussed above, the OCC may comprise a length-<NUM> Walsh code indicated by the sequence {a, b, c, d}, and switching may be performed in the frequency domain between the OCC and a second OCC. The second OCC may be is a cyclic shift version of the length-<NUM> Walsh code. For example, the second OCC may comprise a length-<NUM> Walsh code indicated by the sequence {b, c, d, a}.

In some aspects, as discussed above (e.g., the DMRS pattern illustrated in <FIG>), the OCC may be used for a first subcarrier of the DMRS pattern. A second OCC, which may be a time reversal version of the OCC, may be used for a second subcarrier of the DMRS pattern. A third OCC, which may be a left cyclic shift of the OCC, may be used for a third subcarrier of the DMRS pattern. Finally, a fourth OCC, which may be a right cyclic shift of the OCC, may be used for a fourth subcarrier of the DMRS pattern. For example, as illustrated in <FIG>, the OCC may comprise a length-<NUM> Walsh code indicated by the sequence {a, b, c, d}, the second OCC may comprise a length-<NUM> Walsh code indicated by the sequence {d, c, b, a}, the third OCC may comprise a length-<NUM> Walsh code indicated by the sequence {b, c, d, a}, and the fourth OCC may comprise a length-<NUM> Walsh code indicated by the sequence {a, d, c, b}.

The BS schedules some UEs of the plurality of UEs for a plurality of bundled resource blocks (RBs) used for transmitting the DMRS symbols. The same precoder is applied, at the BS, to the plurality of bundled RBs. The BS provides, to the co-scheduled UEs, indication about a size of the plurality of bundled RBs.

<FIG> illustrates example operations <NUM> that may be performed at a user equipment (UE), in accordance with certain aspects of the present disclosure. These operations <NUM> may be executed, for example, at the processor(s) <NUM>, <NUM>, and/or <NUM> of the UE <NUM> from <FIG>. While operations <NUM> and other aspects of the present disclosure are described with reference to LTE-A systems, the techniques described herein are applicable to any other suitable MIMO system.

Operations <NUM> may begin at <NUM>, where a UE receives, from a base station (BS), a downlink (DL) control signaling indicative of an orthogonal demodulation reference signal (DMRS) pattern. At <NUM>, the UE may determine, based on the DL control signaling, ports of a multi-dimensional array of antennas and spatial multiplexed layers for DMRS symbols transmission, the ports and the layers being multiplexed in the orthogonal DMRS pattern using an orthogonal cover code (OCC) and one or more code division multiplexing (CDM) groups. At <NUM>, the UE may receive the DMRS symbols based on the determination.

In an aspect of the present disclosure, the UE may receive an indication about the DMRS pattern via radio resource control (RRC) signaling. In another aspect, the UE may receive an indication about the DMRS pattern via dynamic L1 signaling on a Physical Downlink Control Channel (PDCCH). In yet another aspect, the UE may receive an indication about the ports of the multi-dimensional array of antennas and the number of layers for transmitting the DMRS symbols using L1 control signaling on a Physical Downlink Control Channel (PDCCH).

The UE receives joint indication of the DMRS pattern and the ports of the multi-dimensional array of antennas using L1 control signaling. The joint indication may comprise one bit for Physical Downlink Shared Channel (PDSCH) rate matching information and four bits indicating the configured DMRS pattern and the ports of the multi-dimensional array of transmit antennas.

The UE processes, based on the same bundling boundary for co-scheduled user equipments (UEs), the DMRS symbols received within a plurality of bundled resource blocks (RBs). The same precoder has been used for each port within the plurality of bundled RBs. In another aspect, the UE may process, based on a higher layer configured bundling boundary, the DMRS symbols received in a plurality of bundled resource blocks (RBs). The same precoder has been used for each port within the plurality of bundled RBs.

For example, operations <NUM> and <NUM> illustrated in <FIG> and <FIG> correspond to means 1500A and 1600A illustrated in <FIG> and <FIG>.

The steps of a method or algorithm described in connection with the present disclosure may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in any form of storage medium that is known in the art. Some examples of storage media that may be used include random access memory (RAM), read only memory (ROM), flash memory, EPROM memory, EEPROM memory, registers, a hard disk, a removable disk, a CD-ROM and so forth. A storage medium may be coupled to a processor such that the processor can read information from, and write information to, the storage medium.

The functions described may be implemented in hardware, software, firmware, or any combination thereof. In the case of a user terminal, a user interface (e.g., keypad, display, mouse, joystick, etc.) may also be connected to the bus.

The processor may be responsible for managing the bus and general processing, including the execution of software stored on the machine-readable media. The processor may be implemented with one or more general-purpose and/or specialpurpose processors. Machine-readable media may include, by way of example, RAM (Random Access Memory), flash memory, ROM (Read Only Memory), PROM (Programmable Read-Only Memory), EPROM (Erasable Programmable Read-Only Memory), EEPROM (Electrically Erasable Programmable Read-Only Memory), registers, magnetic disks, optical disks, hard drives, or any other suitable storage medium, or any combination thereof. The computer-program product may comprise packaging materials.

In a hardware implementation, the machine-readable media may be part of the processing system separate from the processor. However, as those skilled in the art will readily appreciate, the machine-readable media, or any portion thereof, may be external to the processing system. By way of example, the machine-readable media may include a transmission line, a carrier wave modulated by data, and/or a computer product separate from the wireless node, all which may be accessed by the processor through the bus interface.

The processing system may be configured as a general-purpose processing system with one or more microprocessors providing the processor functionality and external memory providing at least a portion of the machine-readable media, all linked together with other supporting circuitry through an external bus architecture. Alternatively, the processing system may be implemented with an ASIC (Application Specific Integrated Circuit) with the processor, the bus interface, the user interface in the case of an access terminal), supporting circuitry, and at least a portion of the machine-readable media integrated into a single chip, or with one or more FPGAs (Field Programmable Gate Arrays), PLDs (Programmable Logic Devices), controllers, state machines, gated logic, discrete hardware components, or any other suitable circuitry, or any combination of circuits that can perform the various functionality described throughout this disclosure.

The machine-readable media may comprise a number of software modules. The software modules include instructions that, when executed by the processor, cause the processing system to perform various functions.

A storage medium may be any available medium 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.

Claim 1:
A method for wireless communications by a base station "BS", comprising:
determining (<NUM>) a plurality of ports of a multi-dimensional array of transmit antennas and a number of spatial multiplexed layers for transmission to a plurality of user equipments "UEs";
configuring (<NUM>) a demodulation reference signal "DMRS" pattern by multiplexing the layers or the ports in the DMRS pattern, using an orthogonal cover code "OCC" and one or more code division multiplexing "CDM" groups;
communicating joint indication of the configured DMRS pattern and the ports of the multi-dimensional array of transmit antennas using L1 control signaling, wherein:
the joint indication comprises one bit for Physical Downlink Shared Channel "PDSCH" rate matching indication and four bits indicating the configured DMRS pattern and the ports of the multi-dimensional array of transmit antennas; and
transmitting DMRS symbols based on the configured DMRS pattern using the multiplexed layers and the ports further comprising:
scheduling UEs of the plurality of UEs for a plurality of bundled resource blocks "RBs" used for transmitting the DMRS symbols;
applying the same precoder to the plurality of bundled RBs; and
providing, to the co-scheduled UEs, indication about a size of the plurality of bundled RBs.