TECHNICAL FIELD

[0001] Certain embodiments of the present disclosure generally relate to channel estimation in wireless communication systems and, more particularly, to channel estimation based on user equipment reference signals (UE S).

BACKGROUND

[0002] Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, 3 GPP Long Term Evolution (LTE) systems, and orthogonal frequency division multiple access (OFDM A) systems.

[0003] Generally, a wireless multiple-access communication system can simultaneously support communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-in-single-out, multiple-in-single-out or a multiple-in- multiple-out (MIMO) system.

[0004] Wireless devices comprise user equipments (UEs) and remote devices. A UE is a device that operates under direct control by humans. Some examples of UEs include cellular phones, smart phones, personal digital assistants (PDAs), wireless modems, handheld devices, laptop computers, netbooks, etc. A remote device is a device that operates without being directly controlled by humans. Some examples of remote devices include sensors, meters, location tags, etc. A remote device may communicate with a base station, another remote device, or some other entity. Machine type communication (MTC) refers to communication involving at least one remote device on at least one end of the communication. [0005] UE reference signals (UE S) can be used for a variety of purposes, including channel estimation and beamforming. In performing channel estimation with a UERS, robust minimum mean square error (MMSE) algorithms can be used to decode the UERS. However, robust MMSE may involve a high amount of complexity in implementation.

SUMMARY

[0006] Certain aspects of the present disclosure provide a method for performing channel estimation. The method generally includes receiving a reference signal from an evolved Node B (eNB), selecting one or more parameters for channel estimation from the reference signal, wherein selecting one or more parameters comprises applying an orthogonal analysis matrix to the reference signal, and taking one or more actions to perform channel estimation based on the selected parameters.

[0007] Certain aspects of the present disclosure provide a method of parameter selection in UERS-based channel estimation. The method generally includes receiving at least one UE-specific reference signal from an eNB, selecting one or more parameters for channel estimation based on a signal-to-noise (SNR) estimation of the at least one UE-specific reference signal, and taking one or more actions to perform channel estimation based on the selected one or more parameters.

[0008] Certain aspects of the present disclosure provide a method for detecting whether a receiver is operating in single user multiple input, multiple output (SU- MIMO) or multiple user multiple input, multiple output (MU-MIMO) mode. The method generally includes selecting at least one pilot subcarrier N _p assuming MU- MIMO mode, generating an analysis matrix A _p assuming MU-MIMO mode, selecting parameters for a signal layer and interferer layer based on the selected at least one pilot subcarrier N _p , and detecting whether a receiver is operating in SU-MIMO mode or MU- MIMO mode based on the selected parameters.

[0009] Certain aspects of the present disclosure provide for a method of performing noise estimation. The method generally includes receiving at least one UE-specific reference signal, and performing noise estimation based on one or more of the at least one UERS.

[0010] 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

[0011] The features, nature, and advantages of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:

[0012] Figure 1 illustrates a multiple access wireless communication system, according to aspects of the present disclosure.

[0013] Figure 2 is a block diagram of a communication system, according to aspects of the present disclosure.

[0014] Figure 3 illustrates an example frame structure, according to aspects of the present disclosure.

[0015] Figure 4 illustrates an example sub frame resource element mapping, according to aspects of the present disclosure.

[0016] Figure 5 illustrates example operations that may be performed by a user equipment to select parameters for channel estimation and perform channel estimation based on the selected parameters in accordance with an aspect of the present disclosure.

[0017] Figure 6 illustrates an example of throughput performance for various methods of channel estimation.

[0018] Figures 7A-B illustrate an example of throughput performance in channel estimation utilizing comparisons of a parameter to a threshold and a cumulative distribution function showing the position of parameters in relation to a noise floor.

[0019] Figure 8 illustrates example operations that may be performed by a user equipment to select parameters based on one or more UE-specific reference signals in accordance with an aspect of the present disclosure.

[0020] Figure 9 illustrates an example comparison of throughput performance between CRS-based channel estimation and UERS-based channel estimation.

[0021] Figure 10 illustrates example operations that may be performed by a receiver for selecting parameters and detecting whether a receiver is operating in SU-MIMO or MU-MIMO mode in accordance with an aspect of the present disclosure.

[0022] Figures 11A-B illustrate examples of throughput performance in MIMO mode detection using comparison of parameters with a threshold.

[0023] Figure 12 illustrates example situations of UERS and CRS interference structures.

[0024] Figure 13 illustrates example operations that may be performed by a user equipment for performing UERS-based noise estimation in accordance with an aspect of the present disclosure.

[0025] Figures 14A-B illustrate examples of resource structures showing channel estimates and UE-specific reference signals for different MIMO modes.

[0026] Figure 15 illustrates a block diagram of an example reference design of a UERS receiver.

[0027] Figure 16 illustrates a block diagram of a UERS receiver configured to perform noise estimation based on one or more elements in a parameter matrix in accordance with an aspect of the present disclosure.

[0028] Figure 17 illustrates a block diagram of a UERS receiver configured to perform noise estimation based on one or more elements in a parameter matrix and further on a post-whitening SINR in accordance with an aspect of the present disclosure.

DETAILED DESCRIPTION

[0029] According to certain aspects provided herein, a low complexity method of channel estimation based on a parametric model with an orthogonal basis may be provided. Certain aspects of the present disclosure provide a method for selecting parameters for UERS-based channel estimation. Certain aspects of the present disclosure provide a method of selecting parameters and determining if a receiver is operating in single user multiple in, multiple out (SU-MIMO) or multiple user, multiple in, multiple out (MU-MIMO) mode. Aspects of the present disclosure provide for methods of performing noise estimation based on received UERSs. [0030] 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 the 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.

[0031] The techniques described herein may be used for various wireless communication networks such as 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, etc. The terms "networks" and "systems" are often used interchangeably. A CDMA network may implement a radio technology such as Universal Terrestrial Radio Access (UTRA), cdma2000, etc. UTRA includes Wideband-CDMA (W-CDMA) and Low Chip Rate (LCR). cdma2000 covers IS-2000, IS-95 and IS-856 standards. A TDMA network may implement a radio technology such as Global System for Mobile Communications (GSM). An OFDMA network may implement a radio technology such as Evolved UTRA (E-UTRA), IEEE 802.11, IEEE 802.16, IEEE 802.20, Flash-OFDM®, etc. UTRA, E-UTRA, and GSM are part of Universal Mobile Telecommunication System (UMTS). Long Term Evolution (LTE) is an upcoming release of UMTS that uses E-UTRA. UTRA, E-UTRA, GSM, UMTS and LTE are described in documents from an organization named "3rd Generation Partnership Project" (3 GPP). cdma2000 is described in documents from an organization named "3rd Generation Partnership Project 2" (3GPP2). These various radio technologies and standards are known in the art. For clarity, certain aspects of the techniques are described below for LTE, and LTE terminology is used in much of the description below.

[0032] Single carrier frequency division multiple access (SC-FDMA), which utilizes single carrier modulation and frequency domain equalization is a technique. SC-FDMA has similar performance and essentially the same overall complexity as those of OFDMA system. SC-FDMA signal has lower peak-to-average power ratio (PAP ) because of its inherent single carrier structure. 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 3 GPP Long Term Evolution (LTE), or Evolved UTRA.

[0033] Figure 1 shows a wireless communication network 100, which may be an LTE network. The wireless network 100 may include a number of evolved Node Bs (eNBs) 110 and other network entities. An eNB may be a station that communicates with the UEs and may also be referred to as a base station, an access point, etc. A Node B is another example of a station that communicates with the UEs.

[0034] Each eNB 110 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.

[0035] An eNB may provide communication coverage for a macro cell, a pico cell, a femto cell, and/or other types of cell. A macro cell may cover a relatively large geographic area (e.g., several kilometers in radius) and may allow unrestricted access by UEs with service subscription. A pico cell may cover a relatively small geographic area and may allow unrestricted access by UEs with service subscription. A femto cell may cover a relatively small geographic area (e.g., a home) and may allow restricted access by UEs having association with the femto cell (e.g., UEs in a Closed Subscriber Group (CSG), UEs for users in the home, etc.). An eNB for a macro cell may be referred to as a macro eNB. An eNB for a pico cell may be referred to as a pico eNB. An eNB for a femto cell may be referred to as a femto eNB or a home eNB. In the example shown in Figure 1, the eNBs 110a, 110b and 110c may be macro eNBs for the macro cells 102a, 102b and 102c, respectively. The eNB HOx may be a pico eNB for a pico cell 102x. The eNBs HOy and HOz may be femto eNBs for the femto cells 102y and 102z, respectively. An eNB may support one or multiple (e.g., three) cells.

[0036] The wireless network 100 may also include relay stations. A relay station is a station that receives a transmission of data and/or other information from an upstream station (e.g., an eNB or a UE) and sends a transmission of the data and/or other information to a downstream station (e.g., a UE or an eNB). A relay station may also be a UE that relays transmissions for other UEs. In the example shown in Figure 1, a relay station 11 Or may communicate with the eNB 110a and a UE 120r in order to facilitate communication between the eNB 110a and the UE 120r. A relay station may also be referred to as a relay eNB, a relay, etc.

[0037] The wireless network 100 may be a heterogeneous network that includes eNBs of different types, e.g., macro eNBs, pico eNBs, femto eNBs, relays, etc. These different types of eNBs may have different transmit power levels, different coverage areas, and different impact on interference in the wireless network 100. For example, macro eNBs may have a high transmit power level (e.g., 20 Watts) whereas pico eNBs, femto eNBs and relays may have a lower transmit power level (e.g., 1 Watt).

[0038] The wireless network 100 may support synchronous or asynchronous operation. For synchronous operation, the eNBs may have similar frame timing, and transmissions from different eNBs may be approximately aligned in time. For asynchronous operation, the eNBs may have different frame timing, and transmissions from different eNBs may not be aligned in time. The techniques described herein may be used for both synchronous and asynchronous operation.

[0039] A network controller 130 may couple to a set of eNBs and provide coordination and control for these eNBs. The network controller 130 may communicate with the eNBs 110 via a backhaul. The eNBs 110 may also communicate with one another, e.g., directly or indirectly via wireless or wireline backhaul.

[0040] The UEs 120 may be dispersed throughout the wireless network 100, and each UE may be stationary or mobile. A UE may also be referred to as a terminal, a mobile station, a subscriber unit, a station, 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, etc. A UE may be able to communicate with macro eNBs, pico eNBs, femto eNBs, relays, etc. In Figure 1, a solid line with double arrows indicates desired transmissions between a UE and a serving eNB, which is an eNB designated to serve the UE on the downlink and/or uplink. A dashed line with double arrows indicates interfering transmissions between a UE and an eNB. [0041] LTE utilizes orthogonal frequency division multiplexing (OFDM) on the downlink and single-carrier frequency division multiplexing (SC-FDM) on the uplink. OFDM and SC-FDM partition the system bandwidth into multiple (K) orthogonal subcarriers, which are also commonly referred to as tones, bins, etc. Each subcarrier may be modulated with data. In general, modulation symbols are sent in the frequency domain with OFDM and in the time domain with SC-FDM. The spacing between adjacent subcarriers may be fixed, and the total number of subcarriers (K) may be dependent on the system bandwidth. For example, the spacing of the subcarriers may be 15 kHz and the minimum resource allocation (called a 'resource block') may be 12 subcarriers (or 180 kHz). Consequently, the nominal FFT size may be equal to 128, 256, 512, 1024 or 2048 for system bandwidth of 1.25, 2.5, 5, 10 or 20 megahertz (MHz), respectively. The system bandwidth may also be partitioned into subbands. For example, a subband may cover 1.08 MHz (i.e., 6 resource blocks), and there may be 1, 2, 4, 8 or 16 subbands for system bandwidth of 1.25, 2.5, 5, 10 or 20 MHz, respectively.

[0042] The wireless network 100 may also include UEs 120 capable of communicating with a core network via one or more radio access networks (RANs) that implement one or more radio access technologies (RATs). For example, according to certain aspects provided herein, the wireless network 100 may include co-located access points (APs) and/or base stations that provide communication through a first RAN implementing a first RAT and a second RAN implementing a second RAT. According to certain aspects, the first RAN may be a wide area wireless access network (WW AN) and the second RAN may be a wireless local area network (WLAN). Examples of WW AN may include, but not be limited to, for example, radio access technologies (RATs) such as LTE, UMTS, cdma2000, GSM, and the like. Examples of WLAN may include, but not be limited to, for example, RATs such as Wi-Fi or IEEE 802.11 based technologies, and the like.

[0043] According to certain aspects provided herein, the wireless network 100 may include co-located Wi-Fi access points (APs) and femto eNBs that provide communication through Wi-Fi and cellular radio links. As used herein, the term "co- located" generally means "in close proximity to," and applies to Wi-Fi APs or femto eNBs within the same device enclosure or within separate devices that are in close proximity to each other. According to certain aspects of the present disclosure, as used herein, the term "femtoAP" may refer to a co-located Wi-Fi AP and femto eNB.

[0044] Figure 2 is a block diagram of an embodiment of a transmitter system 210 (also known as an access point (AP)) and a receiver system 250 (also known as an user equipment (UE)) in a MIMO system 200. At the transmitter system 210, traffic data for a number of data streams is provided from a data source 212 to a transmit (TX) data processor 214.

[0045] In an aspect, each data stream is transmitted over a respective transmit antenna. TX data processor 214 formats, codes, and interleaves the traffic data for each data stream based on a particular coding scheme selected for that data stream to provide coded data.

[0046] The coded data for each data stream may be multiplexed with pilot data using OFDM techniques. The pilot data is typically a known data pattern that is processed in a known manner and may be used at the receiver system to estimate the channel response. The multiplexed pilot and coded data for each data stream is then modulated (i.e., symbol mapped) based on a particular modulation scheme (e.g., BPSK, QSPK, M-PSK, or M-QAM) selected for that data stream to provide modulation symbols. The data rate, coding, and modulation for each data stream may be determined by instructions performed by processor 230.

[0047] The modulation symbols for all data streams are then provided to a TX MIMO processor 220, which may further process the modulation symbols (e.g., for OFDM). TX MIMO processor 220 then provides Ντ modulation symbol streams to Ντ transmitters (TMTR) 222a through 222t. In certain embodiments, TX MIMO processor 220 applies beamforming weights to the symbols of the data streams and to the antenna from which the symbol is being transmitted.

[0048] Each transmitter 222 receives and processes a respective symbol stream to provide one or more analog signals, and further conditions (e.g., amplifies, filters, and upconverts) the analog signals to provide a modulated signal suitable for transmission over the MIMO channel. Ντ modulated signals from transmitters 222a through 222t are then transmitted from N antennas 224a through 224t, respectively.

[0049] At receiver system 250, the transmitted modulated signals are received by N _R antennas 252a through 252r, and the received signal from each antenna 252 is provided to a respective receiver ( CV ) 254a through 254r. Each receiver 254 conditions (e.g., filters, amplifies, and downconverts) a respective received signal, digitizes the conditioned signal to provide samples, and further processes the samples to provide a corresponding "received" symbol stream.

[0050] An RX data processor 260 then receives and processes the N _R received symbol streams from N _R receivers 254 based on a particular receiver processing technique to provide Ντ "detected" symbol streams. The RX data processor 260 then demodulates, deinterleaves, and decodes each detected symbol stream to recover the traffic data for the data stream. The processing by RX data processor 260 is complementary to that performed by TX MIMO processor 220 and TX data processor 214 at transmitter system 210.

[0051] A processor 270 periodically determines which pre-coding matrix to use. Processor 270 formulates a reverse link message comprising a matrix index portion and a rank value portion.

[0052] The reverse link message may comprise various types of information regarding the communication link and/or the received data stream. The reverse link message is then processed by a TX data processor 238, which also receives traffic data for a number of data streams from a data source 236, modulated by a modulator 280, conditioned by transmitters 254a through 254r, and transmitted back to transmitter system 210.

[0053] At transmitter system 210, the modulated signals from receiver system 250 are received by antennas 224, conditioned by receivers 222, demodulated by a demodulator 240, and processed by a RX data processor 242 to extract the reserve link message transmitted by the receiver system 250. Processor 230 then determines which pre-coding matrix to use for determining the beamforming weights and then processes the extracted message.

[0054] According to certain aspects, the controllers/processors 230 and 270 may direct the operation at the transmitter system 210 and the receiver system 250, respectively. According to an aspect, the processor 230, TX data processor 214, and/or other processors and modules at the transmitter system 210 may perform or direct processes for the techniques described herein. According to another aspect, the processor 270, RX data processor 260, and/or other processors and modules at the receiver system 250 may perform or direct operations 500 in Figure 5, operations 800 in Figure 8, operations 1000 in Figure 10, operations 1300 in Figure 13, and/or other processes for the techniques described herein.

[0055] In an aspect, logical channels are classified into Control Channels and Traffic Channels. Logical Control Channels comprise Broadcast Control Channel (BCCH), which is a DL channel for broadcasting system control information. Paging Control Channel (PCCH) is a DL channel that transfers paging information. Multicast Control Channel (MCCH) is a point-to-multipoint DL channel used for transmitting Multimedia Broadcast and Multicast Service (MBMS) scheduling and control information for one or several MTCHs. Generally, after establishing an RRC connection, this channel is only used by UEs that receive MBMS (Note: old MCCH+MSCH). Dedicated Control Channel (DCCH) is a point-to-point bi-directional channel that transmits dedicated control information used by UEs having an RRC connection. In an aspect, Logical Traffic Channels comprise a Dedicated Traffic Channel (DTCH), which is a point-to-point bi-directional channel, dedicated to one UE, for the transfer of user information. Also, a Multicast Traffic Channel (MTCH) is a point-to-multipoint DL channel for transmitting traffic data.

[0056] In an aspect, Transport Channels are classified into DL and UL. DL Transport Channels comprise a Broadcast Channel (BCH), Downlink Shared Data Channel (DL-SDCH), and a Paging Channel (PCH), the PCH for support of UE power saving (DRX cycle is indicated by the network to the UE), broadcasted over entire cell and mapped to PHY resources which can be used for other control/traffic channels. The UL Transport Channels comprise a Random Access Channel (RACH), a Request Channel (REQCH), an Uplink Shared Data Channel (UL-SDCH), and a plurality of PHY channels. The PHY channels comprise a set of DL channels and UL channels.

[0057] In an aspect, a channel structure is provided that preserves low PAPR (at any given time, the channel is contiguous or uniformly spaced in frequency) properties of a single carrier waveform.

[0058] For the purposes of the present document, the following abbreviations apply: AM Acknowledged Mode

AMD Acknowledged Mode Data

A Q Automatic Repeat Request

BCCH Broadcast Control CHannel

BCH Broadcast CHannel

C- Control-

CCCH Common Control CHannel

CCH Control CHannel

CCTrCH Coded Composite Transport Channel

CP Cyclic Prefix

CRC Cyclic Redundancy Check

CTCH Common Traffic CHannel

DCCH Dedicated Control CHannel

DCH Dedicated CHannel

DL DownLink

DL-SCH DownLink Shared CHannel

DM-RS DeModulation-Reference Signal

DSCH Downlink Shared CHannel

DTCH Dedicated Traffic CHannel

FACH Forward link Access CHannel

FDD Frequency Division Duplex

LI Layer 1 (physical layer)

L2 Layer 2 (data link layer)

L3 Layer 3 (network layer)

LI Length Indicator

LSB Least Significant Bit

MAC Medium Access Control

MBMS Multimedia Broadcast Multicast Service

MCCH MBMS point-to-multipoint Control CHannel

MRW Move Receiving Window

MSB Most Significant Bit

MSCH MBMS point-to-multipoint Scheduling CHannel

MTCH MBMS point-to-multipoint Traffic CHannel

PCCH Paging Control CHannel PCH Paging CHannel

PDU Protocol Data Unit

PHY PHYsical layer

PhyCH Physical CHannels

RACH Random Access CHannel

RB Resource Block

RLC Radio Link Control

RRC Radio Resource Control

SAP Service Access Point

SDU Service Data Unit

SHCCH SHared channel Control CHannel

SN Sequence Number

SUFI SUper Field

TCH Traffic CHannel

TDD Time Division Duplex

TFI Transport Format Indicator

TM Transparent Mode

TMD Transparent Mode Data

TTI Transmission Time Interval

U- User-

UE User Equipment

UL UpLink

UM Unacknowledged Mode

UMD Unacknowledged Mode Data

UMTS Universal Mobile Telecommunications System

UTRA UMTS Terrestrial Radio Access

UTRAN UMTS Terrestrial Radio Access Network

MBSFN Multimedia Broadcast Single Frequency Network

MCE MBMS Coordinating Entity

MCH Multicast CHannel

MSCH MBMS Control CHannel

PDCCH Physical Downlink Control CHannel

PDSCH Physical Downlink Shared CHannel

PRB Physical Resource Block VRB Virtual Resource Block

In addition, Rel-8 refers to Release 8 of the LTE standard.

[0059] Figure 3 shows an exemplary frame structure 300 for FDD in LTE. The transmission timeline for each of the downlink and uplink may be partitioned into units of radio frames. Each radio frame may have a predetermined duration (e.g., 10 milliseconds (ms)) and may be partitioned into 10 sub frames with indices of 0 through 9. Each subframe may include two slots. Each radio frame may thus include 20 slots with indices of 0 through 19. Each slot may include L symbol periods, e.g., seven symbol periods for a normal cyclic prefix (as shown in Figure 2) or six symbol periods for an extended cyclic prefix. The 2L symbol periods in each subframe may be assigned indices of 0 through 2L-1.

[0060] In LTE, an eNB may transmit a primary synchronization signal (PSS) and a secondary synchronization signal (SSS) on the downlink in the center 1.08 MHz of the system bandwidth for each cell supported by the eNB. The PSS and SSS may be transmitted in symbol periods 6 and 5, respectively, in sub frames 0 and 5 of each radio frame with the normal cyclic prefix, as shown in Figure 3. The PSS and SSS may be used by UEs for cell search and acquisition. During cell search and acquisition the terminal detects the cell frame timing and the physical-layer identity of the cell from which the terminal learns the start of the references-signal sequence (given by the frame timing) and the reference-signal sequence of the cell (given by the physical layer cell identity) ._ 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. In aspects, different and/or additional reference signals may be employed. The eNB may also transmit a Physical Broadcast Channel (PBCH) in symbol periods 0 to 3 in slot 1 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.

[0061] Figure 4 shows two exemplary subframe formats 410 and 420 for the downlink with the normal cyclic prefix. The available time frequency resources for the downlink may be partitioned into resource blocks. Each resource block may cover 12 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.

[0062] Subframe format 410 may be used for an eNB equipped with two antennas. A CRS may be transmitted from antennas 0 and 1 in symbol periods 0, 4, 7 and 11. A reference signal is a signal that is known a priori by a transmitter and a receiver and may also be referred to as a pilot. A CRS is a reference signal that is specific for a cell, e.g., generated based on a cell identity (ID). In Figure 4, for a given resource element with label R _a , 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 420 may be used for an eNB equipped with four antennas. A CRS may be transmitted from antennas 0 and 1 in symbol periods 0, 4, 7 and 11 and from antennas 2 and 3 in symbol periods 1 and 8. For both subframe formats 410 and 420, 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 410 and 420, resource elements not used for the CRS may be used to transmit data (e.g., traffic data, control data, and/or other data).

[0063] The PSS, SSS, CRS and PBCH in LTE are described in 3 GPP TS 36.211, entitled "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation," which is publicly available.

[0064] An interlace structure may be used for each of the downlink and uplink for FDD in LTE. For example, Q interlaces with indices of 0 through Q - 1 may be defined, where Q may be equal to 4, 6, 8, 10, or some other value. Each interlace may include subframes that are spaced apart by Q frames. In particular, interlace q may include subframes q, q + Q , q + 2Q , etc., where q e { 0, Q - 1 } . [0065] The wireless network may support hybrid automatic retransmission (HARQ) for data transmission on the downlink and uplink. For HARQ, a transmitter (e.g., an eNB) may send one or more transmissions of a packet until the packet is decoded correctly by a receiver (e.g., a UE) 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.

[0066] A UE may be located within the coverage area 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, pathloss, etc. Received signal quality may be quantified by a signal-to-noise-and-interference 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.

LOW COMPLEXITY CHANNEL ESTIMATION BASED ON A PARAMETRIC MODEL

[0067] Minimum mean square error (MMSE) can be used for channel estimation. In performing channel estimation, an MMSE function may use the cross-correlation between the channel response of data tones and pilot tones, as well as the autocorrelation of channel responses of pilot tones. Robust MMSE may also employ assumptions on the correlation matrices, including assuming a rectangular spectrum for frequency correlations and a 0 ^th order Bessel function of the first kind for temporal correlations. MMSE may be complex to implement. Aspects of the present disclosure may provide for low complexity channel estimation.

[0068] A parametric model with an orthogonal basis may provide for a low complexity method of performing channel estimation. A parametric model may use, for example, a lookup table to generate the profile of a channel. A lookup table may be used to replace, for example, the complicated calculations used in MMSE. In an aspect, channel estimation using a parametric model may entail an analysis step and a synthesis step. The analysis step may generate a parameters vector Θ and may be expressed according to the equation: θ = Δ ^-1 · A _P · X [0069] The synthesis step may generate a channel estimate H and may be expressed according to the equation:

H = B _D · β · Θ where vector β is an MMSE cleaning factor and Θ is the output from the analysis step. In an aspect, vector β may be expressed according to the equation:

[0070] Signal subspace may be dimensioned dynamically using a priori information such as delay spread, Doppler spread, and signal to noise ratio (SN ). In an aspect, multiple A _P s may be designed offline, and a specific A _P may be selected according to the parameter selection results in each channel estimation. Each of the designed A _P s may be orthogonal. In an aspect, one or more A _P s may be designed offline, and a specific Ap may be selected according to the parameter selection results in each channel estimation. The selected A _P may be orthogonal. By employing an A _P which is orthogonal, a performance gain may be realized by eliminating the diag(A _P A ^T p) term in the MMSE cleaning factor β. Additionally, memory usage and/or power consumption may be decreased by using a single matrix for channel estimation and eliminating computations by using constants instead of dynamic loading and/or computing.

[0071] In an aspect, parameter comparison to a threshold value may be utilized to select one or more parameters for channel estimation. Performing such comparison to short-term parameters to adjust the number of subcarriers (Np) for channel interpolation can allow for a capturing of short-term channel characteristics. In an aspect, parameter comparison to a threshold value can be used to determine whether a parameter is treated as a parameter in noise subspace or a parameter in signal subspace. For example, a parameter could be processed as if in noise subspace if the value is below a threshold value and processed as if in signal subspace if the value is above a threshold value. Parameters in noise subspace can be used for _m computation, while parameters in signal subspace can be used for channel reconstruction. Comparisons to a threshold value may be performed before or after parameter whitening.

[0072] Figure 5 illustrates example operations for performing channel estimation based on a parametric model with one or more orthogonal matrices in accordance with an aspect. A method 500 may begin at step 502, where a UE can receive a reference signal from an eNodeB. At step 504, the UE can select one or more parameters for channel estimation from the reference signal, wherein selecting one or more parameters comprises applying an orthogonal analysis matrix to the reference. At step 506, the UE can take one or more actions to perform channel estimation based on the selected parameters.

[0073] In an aspect, the reference signal received from an eNodeB and used to select one or more parameters for channel estimation can be a UE-specific reference signal (UERS).

[0074] In an aspect, selecting one or more parameters may be based on at least one of a signal-to-noise ratio (SN ), delay spread, or Doppler spread.

[0075] In an aspect, a UE in MIMO mode can calculate SNR and interference-to- noise ratio (INR) separately.

[0076] In an aspect, selecting one or more parameters may utilize a lookup table.

[0077] Figure 6 illustrates a comparison of throughput performance for transmissions using a fixed orthogonal Ap, a pseudo inversion using a fixed approximately orthogonal Ap selected by the maximal size of signal subspace, and a dynamically generated Ap according to parameter selection.

[0078] Figure 7A illustrates an example of throughput gains that may be realized by comparing parameters to a threshold to determine which parameters to use for channel estimation. Figure 7B illustrates an example of comparisons to a threshold that may also provide for robust performance against model mismatches (e.g., overestimation of a delay spread).

PARAMETER SELECTION IN UERS-BASED CHANNEL ESTIMATION

[0079] Signal subspace dimension can be determined by delay spread, Doppler spread, and operating signal to noise ratio (SNR). Processing UERS and CRS can share the estimation of delay spread and Doppler spread. Processing the operating SNR may differ between UERS and CRS, as receipt of a UERS may result in a higher received SNR from beamforming gains.

[0080] Noise ratios can be used to determine parameters for UERS channel estimation. Received SNR from a UERS can be used to determine parameters for UERS channel estimation in both single user multiple input, multiple output (SU- MIMO) and multiple user multiple input, multiple output (MU-MIMO) modes. In SU- MIMO mode, a wireless device may perform parameter selection based solely on the SN from a received UERS. In MU-MIMO mode, parameters may be selected based on both the SNR for the signal layer and the INR (interference to noise ratio) for the interference layer.

[0081] Figure 8 illustrates example operations for performing parameter selection in UERS based channel estimation in accordance with an aspect. As shown, operations 800 may begin at step 802, where a UE receives at least one UERS from an eNodeB. At step 804, the UE can select one or more parameters for channel estimation based on an SNR estimation of the at least one UERS. At step 806, the UE can take one or more actions to perform channel estimation based on the selected one or more parameters.

[0082] Figure 9 illustrates a comparison of throughput performance using a UERS- based approach and a CRS-based approach.

PARAMETER SELECTION WITH SWITCHING BETWEEN SU-MIMO AND

MU-MIMO MODES

[0083] For a user, whether transmissions are being performed on a single-user (SU) or multi-user (MU) basis is transparent. A receiver assuming a default of either SU or MU mode may result in false alarms or misdetection and corresponding performance degradation.

[0084] Figure 10 illustrates example operations for performing parameter selection with switching between SU-MIMO and MU-MIMO modes in accordance with an aspect. Operations 1000 may begin at step 1002, where a receiver selects at least one pilot subcarrier N _p assuming that transmissions are occurring in MU-MIMO mode. At step 1004, the receiver may generate an analysis matrix A _p , also assuming MU-MIMO transmission mode. At step 1006, parameters may be selected for a signal layer and interferer layer based on the selected at least one pilot subcarrier N _p . At step 1008, the receiver may detect whether operations are being performed in single user, multiple-in, multiple-out (SU-MIMO) or multiple user, multiple-in, multiple-out (MU-MIMO) mode based on the selected parameters. In an aspect, MU-MIMO detection may be performed after whitening based on at least one of a likelihood ratio (e.g., of operation in MU- MIMO mode) and an interference-to-noise ratio (INR). MU-MIMO detection may be performed based solely on a likelihood ratio (e.g., of operation in MU-MIMO mode). [0085] Likelihood ratio-based MU-MIMO detection may be performed by calculating L _M U-MIMO and UMU-MIMO according to the following equations:

and

N _{p l} +N _p2

^MU-MIMO

N V _{pp ll} + + 11 Fp

[0086] Interference-to-noise ratio based MU-MIMO detection may be performed by calculating L _M U-MIMO and UMU-MIMO according to the following equations:

and f^MU—MIMO COTLStCLTLt

[0087] Regardless of the method of performing MU-MIMO detection, a receiver may determine that transmissions are occurring in MU-MIMO mode if L _MU _ _MIM0 ≥ ^MU-MIMO and that transmissions are occurring in SU-MIMO mode otherwise. Both LR and INR-based MU-MIMO detection may be performed before or after parameter whitening. If MIMO mode detection is performed before parameter whitening:

[0088] If MIMO mode detection is performed after parameter whitening:

[0089] Figures 1 1A-1 1B illustrate examples of throughput performance when correctly and incorrectly detecting the MIMO mode being used for transmissions in a wireless network. As shown in Figure 1 1 A, MU-MIMO mode detection using a fixed threshold value can achieve performance close to a rank-2 receiver for multi-user transmissions. As shown in Figure 1 1B, SU-MIMO mode detection using a fixed threshold can achieve performance close to an SU rank-1 receiver for SU rank-1 transmissions. In both SU-MIMO and MU-MIMO mode detection, the performance penalty of a false alarm is smaller than the performance penalty of mode misdetection, particularly at medium to high geometries.

NOISE ESTIMATION IN UERS CHANNEL ESTIMATION

[0090] As illustrated in Figure 12, Interference structures can be different between CRS and UERS in some multi-cell cases. For example, when UERS is impacted by narrow band interference, the impact on the CRS is limited because the CRS is transmitted on a wider band. In another example, a UERS may not experience interference, but a CRS might be impacted by high interference due to collisions. Regardless, a UERS with the same interference structure as the corresponding data could be used to perform noise estimation.

[0091] Figure 13 illustrates example operations for performing UERS-based noise estimation in accordance with an aspect. Operations 1300 may begin at step 1302, where a receiver can receive at least one UERS. At step 1304, noise estimation may be performed based on one or more of the at least one UERS.

[0092] Figures 14A-B show the position of UE reference signals within an OFDM symbol. In Figure 14A, the use of two UE-specific reference signals can be used to perform noise estimation for SU rank-2 and MU-MIMO transmissions. As shown in Figure 14B, one UERS may be used for noise estimation for SU rank-1 transmissions. Noise estimation may be based on at least one of the UERS and may further be based on the observed signal generally.

[0093] Figure 15 illustrates an example reference design 1500 of a UERS receiver. Parameter selection block 1502 may be configured to select an analysis matrix A _p , a synthesis matrix B, weighting for signal subspace β, and weighting for noise subspace a. A lookup table may receive inputs corresponding to Doppler spread, delay spread, and signal-and-interference-to-noise ratio (SINR). In an aspect, SINR may be generated from the equation: where SE _UERS represents the signal energy of the UE reference signal and NE _CRS represents the noise energy of the cell-specific reference signal.

[0094] Channel parameter estimation block 1504 may generate a set of one or more parameters for channel estimation. Certain parameters may be treated as signal parameters, and certain parameters may be treated as noise parameters. The determination of whether parameters are treated as signal or noise parameters may be based, for example, on a comparison to a threshold value.

[0095] Based on the parameters generated at block 1504, a UERS receiver can detect whether a transmission is an SU or MU transmission in block 1506 and treat parameters as if in noise subspace as appropriate.

[0096] The UERS receiver may then perform channel parameter whitening at block 1508. Whitening may be performed to, for example, decorrelate signals from multiple receive antennas. For example, a whitening factor may be defined by the equation:

The whitening factor may be used to generate a whitened parameter matrix. For example, a whitened parameter matrix may be generated according to the equation: θ .Ζ . = W01:2,fc = Rnn ^/2 ^l:2,fe

[0097] After parameter whitening, the receiver may interpolate the UERS and generate a channel estimate at block 1510. The channel estimate may be defined by the equation: h = B - diag( ) · B ^WHT

[0098] In an aspect, noise estimation may be based at least on an observed signal. A differential can exist between channels in the time domain, the SINR calculated in the mode detection and SINR calculation block may be provided as input to the parameter selection block. In aspects, NE = E ¾i | ² J , where h = h + w , w~N(0, σ ² ). Parameters may be selected using the SINR _UERS for the current transmission time interval (TTI).

[0099] Figure 16 illustrates an example block diagram 1600 of a system for performing noise estimation based on one or more elements in a parameter matrix in accordance with an aspect.

[0100] In parameter selection block 1602, analysis matrix A _rs may be defined as a fixed 12x12 matrix. Weighting for noise subspace a _12xl may be defined as LUT(f _d , T, SINR™EK _S ) . Synthesis matrix B and weighting for signal subspace β may be defined according to the following equation:

(Sl68XWp(7/8 /?W _p (7/8)Xl) = LUT (f _d , T , S I N R _UERS )

SINRUERS may be derived from an IIR filter on SINRUERS-

[0101] MU mode detection block 1606 may be modified from the reference design to perform both MU mode detection and SINR calculation. SINR may be calculated according to the equation:

SINR = SE _UERS /NE _UERS where

]_ ^— i Nrx sr— i MaxNp _^

^{SEuERS =} N^∑ _i=1 ∑ _k=1 ^ ^{fc |}

and

[0102] In an aspect, noise estimation may be based at least on an observed signal. A differential can exist between channels in the time domain, the SINR calculated in the mode detection and SINR calculation block may be provided as input to the parameter selection block. Parameters may be selected using the SINRUERS for the current transmission time interval (TTI). In aspects, parameters may be selected using the SINRUERS for the previous transmission time interval (TTI).

[0103] UERS channel parameter whitening block 1608 may be modified to generate a whitening factor according to the equation:

[0104] Figure 17 illustrates an example block diagram 1700 of noise estimation based on a post-whitening SINR in accordance with an aspect. Post-whitening parameters may be compared to a threshold to capture short-term characteristics of a channel and select the subcarrier N _P based on the post-whitening SINR considering potential interference that could nullify gains. [0105] In aspects, unlike MU mode detection and SINR calculation block 1606, the MU mode detection and SINR calculation block 1706 may not provide the UERS SINR to the parameter selection block. Block 1706 may calculate an interference-to-noise ratio (INR) as a MU detection metric. INR may be described according to the equation:

INR = IE/NE where

]_ r-iIrX r-i ttXlVp ₂

IE =

NrsNrx —ti ₌₁ _i fc _=Wrs \e _{i k} \ and

2

[0106] At UERS channel parameter whitening block 1708, a post-whitening UERS SINR may be calculated and provided to parameter selection block 1702. After generating a whitened set of parameters, a thresholding function may be performed to generate a subset of parameters according to the following equation:

(§' ₇ , §' ₇ , §' _Nt ) = thresholding^ WH where

TH = 1

t ^r (Rnn)

[0107] N _p s may be defined as: or MU

[0108] A post- whitening SINR may be calculated according to the equation:

1 i Wrx ^— , Wp7

SINR = - > > \ §' _k \

6Nrx Z-, _i=1 Z-, _k=1 ^{l fc |}

[0109] After calculating a post- whitening SINR, UERS parameter whitening may be performed according to the reference design, and a channel estimate may be calculated according to the reference design. [0110] The various operations of methods described above may be performed by any suitable combination of hardware and/or software component(s) and/or module(s).

[0111] It is understood that the specific order or hierarchy of steps in the processes disclosed is an example of exemplary approaches. Based upon design preferences, it is understood that the specific order or hierarchy of steps in the processes may be rearranged while remaining within the scope of the present disclosure. 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.

[0112] Those of skill in the art would understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.

[0113] Those of skill would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present disclosure.

[0114] The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.

[0115] The steps of a method or algorithm described in connection with the embodiments disclosed herein 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 RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal. A phrase referring to "at least one of a list of items refers to any combination of those items, including single members. As an example, "at least one of: a, b, or c" is intended to cover: a; b; c; a and b; a and c; b and c; and a, b and c.

[0116] The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present disclosure. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the disclosure. Thus, the present disclosure is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.