WO2013139035A1

WO2013139035A1 - Methods and apparatus for doppler estimation enhancement in lte systems

Info

Publication number: WO2013139035A1
Application number: PCT/CN2012/072916
Authority: WO
Inventors: Neng Wang; Jilei Hou; Xiaohui Liu
Original assignee: Qualcomm Incorporated
Priority date: 2012-03-23
Filing date: 2012-03-23
Publication date: 2013-09-26

Abstract

A method, an apparatus, and a computer program product for wireless communication are provided. In a first configuration, the apparatus estimates a Doppler frequency based on a hypothesized SNR range determined based on an SNR, and estimates a channel based on the estimated Doppler frequency. In a second configuration, the apparatus estimates a Doppler frequency based on at least two CIRs determined based on at least two OFDM symbols containing CRS within a subframe, and estimates a channel based on the estimated Doppler frequency. In a third configuration, the apparatus estimates a Doppler frequency, compares the Doppler frequency based on a threshold, determines whether to re-estimate the Doppler frequency based on the comparison, and estimates a channel based on a final estimated Doppler frequency.

Description

METHODS AND APPARATUS FOR DOPPLER ESTIMATION

ENHANCEMENT IN LTE SYSTEMS

BACKGROUND

Field

[0001] The present disclosure relates generally to communication systems, and more particularly, to Doppler estimation enhancement in Long Term Evolution (LTE) systems.

Background

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

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

SUMMARY

In an aspect of the disclosure, a method, a computer program product, and an apparatus are provided. The apparatus estimates a Doppler frequency based on a hypothesized signal to noise ratio (SNR) range determined based on an SNR. In addition, the apparatus estimates a channel based on the estimated Doppler frequency.

In an aspect of the disclosure, a method, a computer program product, and an apparatus are provided. The apparatus estimates a Doppler frequency based on at least two channel impulse responses determined based on at least two orthogonal frequency division multiplexing (OFDM) symbols containing common reference signals (CRS) within a subframe. In addition, the apparatus estimates a channel based on the estimated Doppler frequency.

In an aspect of the disclosure, a method, a computer program product, and an apparatus are provided. The apparatus estimates a Doppler frequency, compares the Doppler frequency based on a threshold, determines whether to re-estimate the Doppler frequency based on the comparison, and estimates a channel based on a final estimated Doppler frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 is a diagram illustrating an example of a network architecture.

[0008] FIG. 2 is a diagram illustrating an example of an access network.

[0009] FIG. 3 is a diagram illustrating an example of a DL frame structure in LTE.

[0010] FIG. 4 is a diagram illustrating an example of an UL frame structure in LTE.

[0011] FIG. 5 is a diagram illustrating an example of a radio protocol architecture for the user and control planes.

[0012] FIG. 6 is a diagram illustrating an example of an evolved Node B and user equipment in an access network.

[0013] FIG. 7 is a block diagram illustrating an interaction between Doppler estimation and channel estimation. [0014] FIG. 8 is a diagram illustrating a Doppler frequency versus an optimal coefficient for an infinite impulse response (IIR) filter within CRS channel estimation for various SNRs.

[0015] FIG. 9 is a block diagram for illustrating exemplary methods of Doppler estimation enhancement.

[0016] FIG. 10 is a flow chart of a first method of wireless communication.

[0017] FIG. 11 is a flow chart of a first method of wireless communication.

[0018] FIG. 12 is a flow chart of a first method of wireless communication.

[0019] FIG. 13 is a flow chart of a first method of wireless communication.

[0020] FIG. 14 is a conceptual data flow diagram illustrating the data flow between different modules/means/components in an exemplary apparatus.

[0021] FIG. 15 is a diagram illustrating an example of a hardware implementation for an apparatus employing a processing system.

DETAILED DESCRIPTION

[0022] The detailed description set forth below in connection with the appended drawings is intended as a description of various configurations and is not intended to represent the only configurations in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. In some instances, well known structures and components are shown in block diagram form in order to avoid obscuring such concepts.

[0023] Several aspects of telecommunication systems will now be presented with reference to various apparatus and methods. These apparatus and methods will be described in the following detailed description and illustrated in the accompanying drawings by various blocks, modules, components, circuits, steps, processes, algorithms, etc. (collectively referred to as "elements"). These elements may be implemented using electronic hardware, computer software, or any combination thereof. Whether such elements are implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. [0024] By way of example, an element, or any portion of an element, or any combination of elements may be implemented with a "processing system" that includes one or more processors. Examples of processors include microprocessors, microcontrollers, digital signal processors (DSPs), field programmable gate arrays (FPGAs), programmable logic devices (PLDs), state machines, gated logic, discrete hardware circuits, and other suitable hardware configured to perform the various functionality described throughout this disclosure. One or more processors in the processing system may execute software. Software shall be construed broadly to mean instructions, instruction sets, code, code segments, program code, programs, subprograms, software modules, applications, software applications, software packages, routines, subroutines, objects, executables, threads of execution, procedures, functions, etc., whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise.

[0025] Accordingly, in one or more exemplary embodiments, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or encoded as one or more instructions or code on a computer-readable medium. Computer- readable media includes computer storage media. Storage media may be any available media that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media.

[0026] FIG. 1 is a diagram illustrating an LTE network architecture 100. The LTE network architecture 100 may be referred to as an Evolved Packet System (EPS) 100. The EPS 100 may include one or more user equipment (UE) 102, an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) 104, an Evolved Packet Core (EPC) 110, a Home Subscriber Server (HSS) 120, and an Operator's IP Services 122. The EPS can interconnect with other access networks, but for simplicity those entities/interfaces are not shown. As shown, the EPS provides packet-switched services, however, as those skilled in the art will readily appreciate, the various concepts presented throughout this disclosure may be extended to networks providing circuit-switched services.

[0027] The E-UTRAN includes the evolved Node B (eNB) 106 and other eNBs 108.

The eNB 106 provides user and control planes protocol terminations toward the UE 102. The eNB 106 may be connected to the other eNBs 108 via an X2 interface (e.g., backhaul). The eNB 106 may also be referred to as a base station, a base transceiver station, a radio base station, a radio transceiver, a transceiver function, a basic service set (BSS), an extended service set (ESS), or some other suitable terminology. The eNB 106 provides an access point to the EPC 110 for a UE 102. Examples of UEs 102 include a cellular phone, a smart phone, a session initiation protocol (SIP) phone, a laptop, a personal digital assistant (PDA), a satellite radio, a global positioning system, a multimedia device, a video device, a digital audio player (e.g., MP3 player), a camera, a game console, or any other similar functioning device. The UE 102 may also be referred to by those skilled in the art as a mobile station, a subscriber station, a mobile unit, a subscriber unit, a wireless unit, a remote unit, a mobile device, a wireless device, a wireless communications device, a remote device, a mobile subscriber station, an access terminal, a mobile terminal, a wireless terminal, a remote terminal, a handset, a user agent, a mobile client, a client, or some other suitable terminology.

[0028] The eNB 106 is connected by an SI interface to the EPC 110. The EPC 110 includes a Mobility Management Entity (MME) 112, other MMEs 114, a Serving Gateway 116, and a Packet Data Network (PDN) Gateway 118. The MME 112 is the control node that processes the signaling between the UE 102 and the EPC 110. Generally, the MME 112 provides bearer and connection management. All user IP packets are transferred through the Serving Gateway 116, which itself is connected to the PDN Gateway 118. The PDN Gateway 118 provides UE IP address allocation as well as other functions. The PDN Gateway 118 is connected to the Operator's IP Services 122. The Operator's IP Services 122 may include the Internet, the Intranet, an IP Multimedia Subsystem (IMS), and a PS Streaming Service (PSS).

[0029] FIG. 2 is a diagram illustrating an example of an access network 200 in an

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

The modulation and multiple access scheme employed by the access network 200 may vary depending on the particular telecommunications standard being deployed. In LTE applications, OFDM is used on the DL and SC-FDMA is used on the UL to support both frequency division duplexing (FDD) and time division duplexing (TDD). As those skilled in the art will readily appreciate from the detailed description to follow, the various concepts presented herein are well suited for LTE applications. However, these concepts may be readily extended to other telecommunication standards employing other modulation and multiple access techniques. By way of example, these concepts may be extended to Evolution-Data Optimized (EV-DO) or Ultra Mobile Broadband (UMB). EV-DO and UMB are air interface standards promulgated by the 3rd Generation Partnership Project 2 (3GPP2) as part of the CDMA2000 family of standards and employs CDMA to provide broadband Internet access to mobile stations. These concepts may also be extended to Universal Terrestrial Radio Access (UTRA) employing Wideband-CDMA (W-CDMA) and other variants of CDMA, such as TD-SCDMA; Global System for Mobile Communications (GSM) employing TDMA; and Evolved UTRA (E-UTRA), IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, and Flash-OFDM employing OFDMA. UTRA, E- UTRA, UMTS, LTE and GSM are described in documents from the 3 GPP organization. CDMA2000 and UMB are described in documents from the 3GPP2 organization. The actual wireless communication standard and the multiple access technology employed will depend on the specific application and the overall design constraints imposed on the system. The eNBs 204 may have multiple antennas supporting MIMO technology. The use of MIMO technology enables the eNBs 204 to exploit the spatial domain to support spatial multiplexing, beamforming, and transmit diversity. Spatial multiplexing may be used to transmit different streams of data simultaneously on the same frequency. The data steams may be transmitted to a single UE 206 to increase the data rate or to multiple UEs 206 to increase the overall system capacity. This is achieved by spatially precoding each data stream (i.e., applying a scaling of an amplitude and a phase) and then transmitting each spatially precoded stream through multiple transmit antennas on the DL. The spatially precoded data streams arrive at the UE(s) 206 with different spatial signatures, which enables each of the UE(s) 206 to recover the one or more data streams destined for that UE 206. On the UL, each UE 206 transmits a spatially precoded data stream, which enables the eNB 204 to identify the source of each spatially precoded data stream.

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

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

FIG. 3 is a diagram 300 illustrating an example of a DL frame structure in LTE. A frame (10 ms) may be divided into 10 equally sized sub-frames. Each sub-frame may include two consecutive time slots. A resource grid may be used to represent two time slots, each time slot including a resource block. The resource grid is divided into multiple resource elements. In LTE, a resource block contains 12 consecutive subcarriers in the frequency domain and, for a normal cyclic prefix in each OFDM symbol, 7 consecutive OFDM symbols in the time domain, or 84 resource elements. For an extended cyclic prefix, a resource block contains 6 consecutive OFDM symbols in the time domain and has 72 resource elements. Some of the resource elements, as indicated as R 302, 304, include DL reference signals (DL-RS). The DL-RS include Cell-specific RS (CRS) (also sometimes called common RS) 302 and UE-specific RS (UE-RS) 304. UE-RS 304 are transmitted only on the resource blocks upon which the corresponding physical DL shared channel (PDSCH) is mapped. The number of bits carried by each resource element depends on the modulation scheme. Thus, the more resource blocks that a UE receives and the higher the modulation scheme, the higher the data rate for the UE.

[0035] FIG. 4 is a diagram 400 illustrating an example of an UL frame structure in

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

[0036] A UE may be assigned resource blocks 410a, 410b in the control section to transmit control information to an eNB. The UE may also be assigned resource blocks 420a, 420b in the data section to transmit data to the eNB. The UE may transmit control information in a physical UL control channel (PUCCH) on the assigned resource blocks in the control section. The UE may transmit only data or both data and control information in a physical UL shared channel (PUSCH) on the assigned resource blocks in the data section. A UL transmission may span both slots of a subframe and may hop across frequency.

[0037] A set of resource blocks may be used to perform initial system access and achieve UL synchronization in a physical random access channel (PRACH) 430. The PRACH 430 carries a random sequence and cannot carry any UL data/signaling. Each random access preamble occupies a bandwidth corresponding to six consecutive resource blocks. The starting frequency is specified by the network. That is, the transmission of the random access preamble is restricted to certain time and frequency resources. There is no frequency hopping for the PRACH. The PRACH attempt is carried in a single subframe (1 ms) or in a sequence of few contiguous subframes and a UE can make only a single PRACH attempt per frame (10 ms).

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

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

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

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

[0042] FIG. 6 is a block diagram of an eNB 610 in communication with a UE 650 in an access network. In the DL, upper layer packets from the core network are provided to a controller/processor 675. The controller/processor 675 implements the functionality of the L2 layer. In the DL, the controller/processor 675 provides header compression, ciphering, packet segmentation and reordering, multiplexing between logical and transport channels, and radio resource allocations to the UE 650 based on various priority metrics. The controller/processor 675 is also responsible for HARQ operations, retransmission of lost packets, and signaling to the UE 650.

[0043] The transmit (TX) processor 616 implements various signal processing functions for the LI layer (i.e., physical layer). The signal processing functions includes coding and interleaving to facilitate forward error correction (FEC) at the UE 650 and mapping to signal constellations based on various modulation schemes (e.g., binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), M-phase-shift keying (M-PSK), M-quadrature amplitude modulation (M- QAM)). The coded and modulated symbols are then split into parallel streams. Each stream is then mapped to an OFDM subcarrier, multiplexed with a reference signal (e.g., pilot) in the time and/or frequency domain, and then combined together using an Inverse Fast Fourier Transform (IFFT) to produce a physical channel carrying a time domain OFDM symbol stream. The OFDM stream is spatially precoded to produce multiple spatial streams. Channel estimates from a channel estimator 674 may be used to determine the coding and modulation scheme, as well as for spatial processing. The channel estimate may be derived from a reference signal and/or channel condition feedback transmitted by the UE 650. Each spatial stream is then provided to a different antenna 620 via a separate transmitter 618TX. Each transmitter 618TX modulates an RF carrier with a respective spatial stream for transmission.

[0044] At the UE 650, each receiver 654RX receives a signal through its respective antenna 652. Each receiver 654RX recovers information modulated onto an RF carrier and provides the information to the receive (RX) processor 656. The RX processor 656 implements various signal processing functions of the LI layer. The RX processor 656 performs spatial processing on the information to recover any spatial streams destined for the UE 650. If multiple spatial streams are destined for the UE 650, they may be combined by the RX processor 656 into a single OFDM symbol stream. The RX processor 656 then converts the OFDM symbol stream from the time-domain to the frequency domain using a Fast Fourier Transform (FFT). The frequency domain signal comprises a separate OFDM symbol stream for each subcarrier of the OFDM signal. The symbols on each subcarrier, and the reference signal, is recovered and demodulated by determining the most likely signal constellation points transmitted by the eNB 610. These soft decisions may be based on channel estimates computed by the channel estimator 658. The soft decisions are then decoded and deinterleaved to recover the data and control signals that were originally transmitted by the eNB 610 on the physical channel. The data and control signals are then provided to the controller/processor 659.

[0045] The controller/processor 659 implements the L2 layer. The controller/processor can be associated with a memory 660 that stores program codes and data. The memory 660 may be referred to as a computer-readable medium. In the UL, the controller/processor 659 provides demultiplexing between transport and logical channels, packet reassembly, deciphering, header decompression, control signal processing to recover upper layer packets from the core network. The upper layer packets are then provided to a data sink 662, which represents all the protocol layers above the L2 layer. Various control signals may also be provided to the data sink 662 for L3 processing. The controller/processor 659 is also responsible for error detection using an acknowledgement (ACK) and/or negative acknowledgement (NACK) protocol to support HARQ operations.

[0046] In the UL, a data source 667 is used to provide upper layer packets to the controller/processor 659. The data source 667 represents all protocol layers above the L2 layer. Similar to the functionality described in connection with the DL transmission by the eNB 610, the controller/processor 659 implements the L2 layer for the user plane and the control plane by providing header compression, ciphering, packet segmentation and reordering, and multiplexing between logical and transport channels based on radio resource allocations by the eNB 610. The controller/processor 659 is also responsible for HARQ operations, retransmission of lost packets, and signaling to the eNB 610. [0047] Channel estimates derived by a channel estimator 658 from a reference signal or feedback transmitted by the eNB 610 may be used by the TX processor 668 to select the appropriate coding and modulation schemes, and to facilitate spatial processing. The spatial streams generated by the TX processor 668 are provided to different antenna 652 via separate transmitters 654TX. Each transmitter 654TX modulates an RF carrier with a respective spatial stream for transmission.

[0048] The UL transmission is processed at the eNB 610 in a manner similar to that described in connection with the receiver function at the UE 650. Each receiver 618RX receives a signal through its respective antenna 620. Each receiver 618RX recovers information modulated onto an RF carrier and provides the information to a RX processor 670. The RX processor 670 may implement the LI layer.

[0049] The controller/processor 675 implements the L2 layer. The controller/processor 675 can be associated with a memory 676 that stores program codes and data. The memory 676 may be referred to as a computer-readable medium. In the UL, the control/processor 675 provides demultiplexing between transport and logical channels, packet reassembly, deciphering, header decompression, control signal processing to recover upper layer packets from the UE 650. Upper layer packets from the controller/processor 675 may be provided to the core network. The controller/processor 675 is also responsible for error detection using an ACK and/or NACK protocol to support HARQ operations.

[0050] FIG. 7 is a block diagram 700 illustrating an interaction between Doppler estimation and channel estimation. Doppler estimation may be employed in conjunction with channel estimation. As shown in FIG. 7, the CRS channel estimation module 702 adjusts channel estimation parameters, such as a temporal filter coefficient in an IIR filter, based on a Doppler frequency fo provided by the Doppler estimation module 706. In addition, the UE-RS channel estimation module 704 adjusts estimator parameters based on the Doppler frequency fo provided by the Doppler estimation module 706. The CRS estimation module 702 provides a channel impulse response (CIR) to the Doppler estimation module 706, which uses the CIR to determine the Doppler frequency f_D.

[0051] The Doppler estimation module 706 may determine a likely SNR and Doppler frequency through the likelihood function with hypothesis ^{~ ii}'^S, where I is the theoretical noisy spectrum at the frequency^ under the hypothesis p, S-.<i-'¾3 is a periodogram based on a linear combination of multiple CIRs, and N is the number of samples. The number of samples N=T/N_S, where T is a subframe window length in milliseconds and N_s is a number of OFDM symbols sampled per subframe for determining one or more CIRs.

[0052] A hypothesized SNR with a wide range, such as [0, 30 dB], may provide a globally optimum solution. However, such a wide range may result in an erroneous Doppler frequency estimation. Further, in the current Doppler estimation, the CIR is sampled once in each subframe (N_s=l), and therefore the periodogram S i¾ i_s based on only one CIR per subframe.

[0053] FIG. 8 is a diagram 800 illustrating a Doppler frequency versus an optimal coefficient a for an IIR filter within CRS channel estimation for various SNRs. For low Doppler frequency (e.g., less than 150 Hz), a small change in the Doppler frequency fo results is a large change in an IIR filter coefficient a. The Doppler frequency fo samples are uniform (i.e., uniformly spaced in Hz), and therefore for a low Doppler frequency fo, the setting for the IIR filter coefficient a may be suboptimum. The suboptimum IIR filter coefficient setting negatively affects CRS channel estimation and the CIRs fed to the Doppler estimation.

[0054] FIG. 9 is a block diagram 900 for illustrating exemplary methods of Doppler estimation enhancement. In a first configuration, the Doppler frequency estimation module 906 may receive a narrower SNR range to which to apply the likelihood function ⁱ _-( ¾ . In such a configuration, the SNR estimation module 902 estimates an SNR. The SNR estimation module 902 may estimate the SNR in a frequency tracking loop (FTL) module or in another module. The SNR estimation module 902 provides the estimated SNR to an SNR range hypothesis determination module 904. The SNR range hypothesis determination module 904 sets an SNR range to [SNR-A, SNR+B], where the SNR is the estimated SNR, and A and B are greater than zero. For example, if A and B are both 5 dB, the SNR range hypothesis determination module 904 may return a hypothesized SNR range of [SNR - 5 dB, SNR + 5 dB]. The narrower range may provide better binning of likely Doppler frequencies when the likelihood

is applied for the hypothesis p having discrete SNR values within the hypothesized SNR range. [0055] In a second configuration, the Doppler frequency estimation module 906 may receive one or more additional CIRs by which to determine the periodogram

^ ¾) j_{n suc}h _a configuration, in addition to obtaining a CIR based on the 1^st OFDM symbol of a sub frame, the CRS channel estimation module 912 may obtain a CIR for each of one or more of the other reference signal OFDM symbols of a subframe. The reference signal OFDM symbols of a subframe 910 may include the 1^st, 2^nd, 5^th, 8^th, 9^th, and/or 12^th OFDM symbols of the subframe 910 for normal cyclic prefix (CP) length. In other systems, the reference signal OFDM symbols may include other OFDM symbols. For one and two antenna ports with normal CP length, the reference signal OFDM symbols of a subframe 910 may include the 1^st, 5^th, 8^th, and 12^th OFDM symbols of the subframe 910. For four antenna ports with normal CP length, the reference signal OFDM symbols of a subframe 910 may include the 1^st, 2^nd, 5^th, 8^th, 9^th, and 12^th OFDM symbols of the subframe 910. The oversampled CIRs improve the periodogram ½_" £- and the performance of the Doppler frequency estimation. Based on the improved Doppler frequency estimate, the Doppler frequency estimation module 906 may adjust the temporal filter coefficient a in an IIR filter within the CRS channel estimation module 912 or provide information to the CRS channel estimation module 912 for adjusting the temporal filter coefficient a.

[0056] In a third configuration, the Doppler frequency estimation module 906 may be coupled with a Doppler multistage controller module 908 for performing multistage Doppler estimation. In such a configuration, the Doppler multistage controller module 908 provides a subframe window length to the Doppler frequency estimation module 906. The Doppler frequency estimation module 906 estimates the Doppler frequency fo based on the provided window length and returns the estimated Doppler frequency fo to the Doppler multistage controller module 908. The Doppler multistage controller module 908 compares the Doppler frequency /o to a threshold and based on the comparison determines whether to re-estimate the Doppler frequency fo. If the Doppler multistage controller module 908 determines to re-estimate the Doppler frequency fo, the Doppler multistage controller module 908 provides a larger window length to the Doppler frequency estimation module 906. The larger window length T increases a Doppler resolution, as the spacing between hypothesized discrete Doppler frequency f_D values is equal to 1000/Γ. The greater Doppler frequency resolution allows the Doppler frequency estimation module 906 to better estimate the Doppler frequency fo. The Doppler frequency estimation module 906 re-estimates the Doppler frequency fo, and provides the re-estimated Doppler frequency fo to the Doppler multistage controller module 908. If the Doppler multistage controller module 908 implements only a two stage process, then based on the improved Doppler frequency estimate, the Doppler frequency estimation module 906 may adjust the temporal filter coefficient a in an IIR filter within the CRS channel estimation module 912 or provide information to the CRS channel estimation module 912 for adjusting the temporal filter coefficient a. If the Doppler multistage controller module 908 adjusts or provides information for adjusting the temporal filter coefficient a within the IIR filter in the CRS channel estimation module 912, then the Doppler frequency estimation module 906 may not provide such functionality to the CRS channel estimation module 912.

In the third configuration, the Doppler multistage controller module 908 may implement more than a two-stage process. The Doppler multistage controller module 908 may compare the re-estimated Doppler frequency /_fl to a threshold, and based on the comparison, determine whether to re-estimate the Doppler frequency fo again based on an even larger window length. The thresholds may vary in each stage of the process. Further, the provided window length may be a function of the received Doppler frequency fo. An example best demonstrates the process. Assume the Doppler multistage controller module 908 provides a window length T equal to 8 ms to the Doppler frequency estimation module 906. The Doppler frequency estimation module 906 determines the Doppler frequency f_D to be 125 Hz. The Doppler multistage controller module 908 compares the Doppler frequency /_Oto a threshold equal to 150 Hz. Because 125 Hz is less than 150 Hz, the Doppler multistage controller module 908 determines to re-estimate the Doppler frequency fo. The Doppler multistage controller module 908 provides a window length T equal to 16 ms to the Doppler frequency estimation module 906. The Doppler frequency estimation module 906 determines the Doppler frequency fo to be 62.5 Hz. The Doppler multistage controller module 908 compares the Doppler frequency /o to a threshold equal to 90 Hz. Because 62.5 Hz is less than 90 Hz, the Doppler multistage controller module 908 determines again to re-estimate the Doppler frequency fo. The Doppler multistage controller module 908 provides a window length T equal to 32 ms to the Doppler frequency estimation module 906. The Doppler frequency estimation module 906 determines the Doppler frequency fo to be 93.75 Hz. The Doppler multistage controller module 908 compares the Doppler frequency u to a threshold equal to 32 Hz. Because 93.75 Hz is greater than 32 Hz, the Doppler multistage controller module 908 determines not to re-estimate the Doppler frequency f_D. Accordingly, based on a Doppler frequency _ _D=93.75 HZ, the Doppler multistage controller module 908 adjusts or provides information for adjusting the temporal filter coefficient a within the IIR filter in the CRS channel estimation module 912.

[0058] FIG. 10 is a flow chart 1000 of a first method of wireless communication. The method may be performed by a UE. As shown in FIG. 10, in step 1002, a UE may estimate an SNR. In step 1004, the UE may determine a hypothesized SNR range based on the estimated SNR. In step 1006, the UE estimates a Doppler frequency based on the hypothesized SNR range determined based on the estimated SNR. In step 1008, the UE estimates a channel based on the estimated Doppler frequency. In step 1010, the UE communicates with an eNB based on the channel estimation. The determined hypothesized SNR range may be [SNR-A, SNR+B], where the SNR is the estimated SNR, and A and B are greater than zero. The Doppler frequency may be estimated further based on at least two CIRs determined based on at least two OFDM symbols containing CRS within a subframe.

[0059] For example, in step 1002, a UE may estimate the SNR to be 5 dB. In step

1004, the UE may determine a hypothesized SNR range to be [0, 10 dB], having applied A=5 dB and B=5 dB. To estimate a Doppler frequency fo, the Doppler frequency estimation module 906 determines the likelihood function for

where the SNR value within p is one of a set of discrete SNR values within the hypothesized SNR range of [0, 10 dB].

[0060] FIG. 11 is a flow chart 1100 of a second method of wireless communication.

The method may be performed by a UE. As shown in FIG. 11, in step 1102, a UE may determine at least two CIRs based on at least two OFDM symbols containing CRS within a subframe. In step 1104, the UE estimates a Doppler frequency based on the at least two CIRs determined based on the at least two OFDM symbols containing CRS within the subframe. In step 1106, the UE may estimate a channel based on the estimated Doppler frequency. In step 1108, the UE may communicate with an eNB based on the channel estimation. The at least two OFDM symbols may include at least one of a 1^st, 2^nd, 5^th, 8^th, 9^th, or 12^th OFDM of the subframe. The Doppler frequency may be estimated further based on a hypothesized SNR range determined based on an estimated SNR.

[0061] FIG. 12 is a flow chart 1200 of a third method of wireless communication.

The method may be performed by a UE. As shown in FIG. 12, in step 1202, a UE estimates a Doppler frequency. In step 1204, the UE compares the Doppler frequency based on a threshold. For example, the UE may compare the Doppler frequency to a threshold. The threshold may vary in each stage of the process. In step 1206, the UE determines whether to re-estimate the Doppler frequency based on the comparison. If the UE determines not to re-estimate the Doppler frequency, in step 1210, the UE estimates a channel based on the estimated Doppler frequency. If the UE determines to re-estimate the Doppler frequency, the UE re- estimates the Doppler frequency in step 1208. After re-estimating the Doppler frequency in step 1208, in a two-stage Doppler estimation process, the UE may go to step 1210 and estimate a channel based on a final estimated Doppler frequency. Alternatively, if the UE implements more than a two-stage Doppler estimation process, the UE may return to step 1204 in order to compare the re-estimated Doppler frequency based on a threshold.

[0062] The Doppler frequency may be initially estimated based on a first window length and re-estimated based on a second window length greater than the first window length. The Doppler frequency may be re-estimated when the Doppler frequency is less than the threshold. Re-estimating the Doppler frequency when the Doppler frequency is less than a threshold allows for greater Doppler resolution for lower Doppler frequencies. A greater Doppler resolution provides improved Doppler frequency estimation, and therefore improved CRS channel estimation through the use of a more optimum temporal filter coefficient a in an IIR filter within the CRS channel estimation module 912.

[0063] The Doppler frequency may be re-estimated when the Doppler frequency is greater than the threshold. Such a configuration may apply to other modules or other systems in which greater Doppler frequency resolution is needed for higher Doppler frequencies. As discussed supra, the UE may re-estimate the Doppler frequency upon determining to re-estimate the Doppler frequency, and repeat the steps of the comparing in step 1204, the determining in step 1206, and the re- estimating in step 1208 based on a comparison of the re-estimated Doppler frequency based on the threshold in step 1204. The Doppler frequency may be estimated in step 1202 or re-estimated in step 1208 based on a hypothesized SNR range determined based on an estimated SNR. The Doppler frequency may be estimated in step 1202 or re-estimated in step 1208 based on at least two CIRs determined based on at least two OFDM symbols containing CRS within a subframe.

[0064] Referring again to step 1204, the UE compares the Doppler frequency based on a threshold. The UE may compare the Doppler frequency to the threshold. In another configuration, the UE compares a difference between a previously estimated Doppler frequency and a currently estimated Doppler frequency to a threshold. Other comparisons are possible. The threshold itself may vary in each stage of the process.

[0065] FIG. 13 is a flow chart 1300 of a fourth method of wireless communication.

The method may be performed by a UE. As shown in FIG. 13, a UE may estimate a first Doppler frequency based on a first window length. In step 1306, the UE may compare the first Doppler frequency to a threshold. If in step 1306 the Doppler frequency is greater than the threshold, in step 1308, the UE may estimate a channel based on the first Doppler frequency. Subsequently, in step 1314, the UE may communicate with an eNB based on the channel estimation. If in step 1306 the Doppler frequency is less than the threshold, in step 1310, the UE may re-estimate a second Doppler frequency based on a second window length greater than the first window length. In step 1312, the UE may estimate a channel based on the second Doppler frequency. Subsequently, in step 1314, the UE may communicate with an eNB based on the channel estimation.

[0066] For example, assume the UE re-estimates the Doppler frequency if the

Doppler frequency estimate is less than 150 Hz (i.e., threshold=150 Hz). If in step 1302, the UE estimates a Doppler frequency of 125 Hz with a window length Γ=8 ms, in step 1306 the UE will determine to re-estimate the Doppler frequency because the Doppler frequency f_D=125 Hz is less than 150 Hz. Assume in step 1310 that the UE estimates a Doppler frequency of 187.5 Hz with a window length T=16 ms. In step 1312, the UE may estimate a channel based on the Doppler

HZ, and in step 1314, the UE may communicate with an eNB based on the channel estimation. [0067] FIG. 14 is a conceptual data flow diagram 1400 illustrating the data flow between different modules/means/components in an exemplary apparatus 1402. The apparatus may be a UE. The apparatus may include a receiving module 1410 that receives a signal from an eNB 1450. The apparatus may further include an SNR estimation module 1404 and/or a CRS channel estimation module 1414. The receiving module 1410 may provide the signal to the SNR estimation module 1404 and/or the CRS channel estimation module 1414. The SNR estimation module 1404 may estimate an SNR and provide the estimated SNR to a SNR range hypothesis determination module 1406, which provides a hypothesized SNR range to the Doppler frequency estimation module 1412. The CRS channel estimation module 1414 may provide one or more CIRs to the Doppler frequency estimation module 1412. The Doppler frequency estimation module 1412 estimates a Doppler frequency f_D. Based on the estimated Doppler frequency, the Doppler frequency estimation module 1412 may provide parameter adjustments to the CRS channel estimation module 1414. The apparatus may further include a Doppler multistage controller module 1408, which provides a subframe window length to the Doppler frequency estimation module 1412. The Doppler frequency estimation module 1412 estimates a Doppler frequency f_D based on the received window length and provides the estimated Doppler frequency fo to the Doppler multistage controller module 1408. The Doppler multistage controller module 1408 may have the Doppler frequency estimation module 1412 re-estimate the Doppler frequency f_D based on a larger window length. Based on the estimated Doppler frequency fo, the Doppler multistage controller module 1408 may provide parameter adjustments to the CRS channel estimation module 1414. The CRS channel estimate module 1414 may provide channel estimation information to the receiving module 1410 and/or the transmission module 1416. The receiving module 1410 and/or the transmission module 1416 communicates with the eNB 1450 based on the channel estimation.

[0068] The apparatus may include additional modules that perform each of the steps of the algorithm in the aforementioned flow charts of FIGs. 10-13. As such, each step in the aforementioned flow charts of FIGs. 10-13 may be performed by a module and the apparatus may include one or more of those modules. The modules may be one or more hardware components specifically configured to carry out the stated processes/algorithm, implemented by a processor configured to perform the stated processes/algorithm, stored within a computer-readable medium for implementation by a processor, or some combination thereof.

[0069] FIG. 15 is a diagram illustrating an example of a hardware implementation for an apparatus 1402' employing a processing system 1514. The processing system 1514 may be implemented with a bus architecture, represented generally by the bus 1524. The bus 1524 may include any number of interconnecting buses and bridges depending on the specific application of the processing system 1514 and the overall design constraints. The bus 1524 links together various circuits including one or more processors and/or hardware modules, represented by the processor 1504, the modules 1404, 1406, 1408, 1410, 1412, 1414, 1416 and the computer-readable medium 1506. The bus 1524 may also link various other circuits such as timing sources, peripherals, voltage regulators, and power management circuits, which are well known in the art, and therefore, will not be described any further.

[0070] The processing system 1514 may be coupled to a transceiver 1510. The transceiver 1510 is coupled to one or more antennas 1520. The transceiver 1510 provides a means for communicating with various other apparatus over a transmission medium. The processing system 1514 includes a processor 1504 coupled to a computer-readable medium 1506. The processor 1504 is responsible for general processing, including the execution of software stored on the computer-readable medium 1506. The software, when executed by the processor 1504, causes the processing system 1514 to perform the various functions described supra for any particular apparatus. The computer-readable medium 1506 may also be used for storing data that is manipulated by the processor 1504 when executing software. The processing system further includes at least one of the modules 1404, 1406, 1408, 1410, 1412, 1414, and 1416. The modules may be software modules running in the processor 1504, resident/stored in the computer readable medium 1506, one or more hardware modules coupled to the processor 1504, or some combination thereof. The processing system 1514 may be a component of the UE 650 and may include the memory 660 and/or at least one of the TX processor 668, the RX processor 656, and the controller/processor 659.

[0071] In a first configuration, the apparatus 1402/1402' for wireless communication includes means for estimating a Doppler frequency based on a hypothesized SNR range determined based on an SNR, and means for estimating a channel based on the estimated Doppler frequency. The apparatus may further include means for estimating the SNR, and means for determining the hypothesized SNR range based on the estimated SNR. The apparatus may further include means for communicating with an eNB based on the channel estimation. In a second configuration, the apparatus includes means for estimating a Doppler frequency based on at least two CIRs determined based on at least two OFDM symbols containing CRS within a subframe, and means for estimating a channel based on the estimated Doppler frequency. The apparatus may further include means for determining the at least two CIRs based on the at least two OFDM symbols containing CRS within the subframe. The apparatus may further include means for communicating with an eNB based on the channel estimation. In a third configuration, the apparatus includes means for estimating a Doppler frequency, means for comparing the Doppler frequency based on a threshold, means for determining whether to re-estimate the Doppler frequency based on the comparison, and means for estimating a channel based on a final estimated Doppler frequency. The apparatus may further include means for re-estimating the Doppler frequency upon determining to re-estimate the Doppler frequency, and means for repeating the steps of the comparing, the determining, and the re- estimating based on a comparison of the re-estimated Doppler frequency based on the threshold. The apparatus may further include means for communicating with an eNB based on the channel estimation. The aforementioned means may be one or more of the aforementioned modules of the apparatus 1402 and/or the processing system 1514 of the apparatus 1402' configured to perform the functions recited by the aforementioned means. As described supra, the processing system 1514 may include the TX Processor 668, the RX Processor 656, and the controller/processor 659. As such, in one configuration, the aforementioned means may be the TX Processor 668, the RX Processor 656, and the controller/processor 659 configured to perform the functions recited by the aforementioned means.

It is understood that the specific order or hierarchy of steps in the processes disclosed is an illustration of exemplary approaches. Based upon design preferences, it is understood that the specific order or hierarchy of steps in the processes may be rearranged. Further, some steps may be combined or omitted. 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.

The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects. Thus, the claims are not intended to be limited to the aspects shown herein, but is to be accorded the full scope consistent with the language claims, wherein reference to an element in the singular is not intended to mean "one and only one" unless specifically so stated, but rather "one or more." Unless specifically stated otherwise, the term "some" refers to one or more. All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed as a means plus function unless the element is expressly recited using the phrase "means for."

WHAT IS CLAIMED IS:

Claims

CLAIMS What is claimed is:

1. A method of wireless communication, comprising:

estimating a Doppler frequency based on a hypothesized signal to noise ratio (SNR) range determined based on an SNR; and

estimating a channel based on the estimated Doppler frequency.

2. The method of claim 1, further comprising:

estimating the SNR; and

determining the hypothesized SNR range based on the estimated SNR.

3. The method of claim 2, wherein the determined hypothesized SNR range is [SNR- A, SNR+B], where SNR is the estimated SNR, and A and B are greater than zero.

4. The method of claim 1, wherein the Doppler frequency is estimated further based on at least two channel impulse responses (CIRs) determined based on at least two orthogonal frequency division multiplexing (OFDM) symbols containing common reference signals (CRS) within a subframe.

5. The method of claim 1, further comprising communicating with an evolved Node B (eNB) based on the channel estimation.

6. A method of wireless communication, comprising:

estimating a Doppler frequency based on at least two channel impulse responses (CIRs) determined based on at least two orthogonal frequency division multiplexing (OFDM) symbols containing common reference signals (CRS) within a subframe; and

estimating a channel based on the estimated Doppler frequency.

7. The method of claim 6, further comprising determining the at least two CIRs based on the at least two OFDM symbols containing CRS within the subframe.

8. The method of claim 6, wherein the at least two OFDM symbols comprise at least one of a 1^st, 2^nd, 5^th, 8^th, 9^th, or 12^th OFDM of the subframe.

9. The method of claim 6, wherein the Doppler frequency is estimated further based on a hypothesized signal to noise ratio (SNR) range determined based on an estimated SNR.

10. The method of claim 6, further comprising communicating with an evolved Node B (eNB) based on the channel estimation.

11. A method of wireless communication, comprising:

estimating a Doppler frequency;

comparing the Doppler frequency based on a threshold;

determining whether to re-estimate the Doppler frequency based on the comparison; and

estimating a channel based on a final estimated Doppler frequency.

12. The method of claim 11, wherein the Doppler frequency is initially estimated based on a first window length and is re-estimated based on a second window length greater than the first window length.

13. The method of claim 11, wherein the Doppler frequency is re-estimated when the Doppler frequency is less than the threshold.

14. The method of claim 11, wherein the Doppler frequency is re-estimated when the Doppler frequency is greater than the threshold.

15. The method of claim 11, further comprising:

re-estimating the Doppler frequency upon determining to re-estimate the Doppler frequency; and

repeating the steps of the comparing, the determining, and the re-estimating based on a comparison of the re-estimated Doppler frequency based on the threshold.

16. The method of claim 11, wherein the Doppler frequency is at least one of estimated or re-estimated based on a hypothesized signal to noise ratio (SNR) range determined based on an estimated SNR.

17. The method of claim 11, wherein the Doppler frequency is at least one of estimated or re-estimated based on at least two channel impulse responses (CIRs) determined based on at least two orthogonal frequency division multiplexing (OFDM) symbols containing common reference signals (CRS) within a subframe.

18. The method of claim 11, further comprising communicating with an evolved Node B (eNB) based on the channel estimation.

19. An apparatus for wireless communication, comprising:

means for estimating a Doppler frequency based on a hypothesized signal to noise ratio (SNR) range determined based on an SNR; and

means for estimating a channel based on the estimated Doppler frequency.

20. The apparatus of claim 19, further comprising:

means for estimating the SNR; and

means for determining the hypothesized SNR range based on the estimated

SNR.

21. The apparatus of claim 20, wherein the determined hypothesized SNR range is [SNR-A, SNR+B], where SNR is the estimated SNR, and A and B are greater than zero.

22. The apparatus of claim 19, wherein the Doppler frequency is estimated further based on at least two channel impulse responses (CIRs) determined based on at least two orthogonal frequency division multiplexing (OFDM) symbols containing common reference signals (CRS) within a subframe.

23. The apparatus of claim 19, further comprising means for communicating with an evolved Node B (eNB) based on the channel estimation.

24. An apparatus for wireless communication, comprising:

means for estimating a Doppler frequency based on at least two channel impulse responses (CIRs) determined based on at least two orthogonal frequency division multiplexing (OFDM) symbols containing common reference signals (CRS) within a subframe; and

means for estimating a channel based on the estimated Doppler frequency.

25. The apparatus of claim 24, further comprising means for determining the at least two CIRs based on the at least two OFDM symbols containing CRS within the subframe.

26. The apparatus of claim 24, wherein the at least two OFDM symbols comprise at least one of a 1^st, 2^nd, 5^th, 8^th, 9^th, or 12^th OFDM of the subframe.

27. The apparatus of claim 24, wherein the Doppler frequency is estimated further based on a hypothesized signal to noise ratio (SNR) range determined based on an estimated SNR.

28. The apparatus of claim 24, further comprising means for communicating with an evolved Node B (eNB) based on the channel estimation.

29. An apparatus for wireless communication, comprising:

means for estimating a Doppler frequency;

means for comparing the Doppler frequency based on a threshold;

means for determining whether to re-estimate the Doppler frequency based on the comparison; and

means for estimating a channel based on a final estimated Doppler frequency.

30. The apparatus of claim 29, wherein the Doppler frequency is initially estimated based on a first window length and is re-estimated based on a second window length greater than the first window length.

31. The apparatus of claim 29, wherein the Doppler frequency is re-estimated when the Doppler frequency is less than the threshold.

32. The apparatus of claim 29, wherein the Doppler frequency is re-estimated when the Doppler frequency is greater than the threshold.

33. The apparatus of claim 29, further comprising:

means for re-estimating the Doppler frequency upon determining to re- estimate the Doppler frequency; and

means for repeating the steps of the comparing, the determining, and the re- estimating based on a comparison of the re-estimated Doppler frequency based on the threshold.

34. The apparatus of claim 29, wherein the Doppler frequency is at least one of estimated or re-estimated based on a hypothesized signal to noise ratio (SNR) range determined based on an estimated SNR.

35. The apparatus of claim 29, wherein the Doppler frequency is at least one of estimated or re-estimated based on at least two channel impulse responses (CIRs) determined based on at least two orthogonal frequency division multiplexing (OFDM) symbols containing common reference signals (CRS) within a subframe.

36. The apparatus of claim 29, further comprising means for communicating with an evolved Node B (eNB) based on the channel estimation.

37. An apparatus for wireless communication, comprising:

a processing system configured to:

estimate a Doppler frequency based on a hypothesized signal to noise ratio (SNR) range determined based on an SNR; and

estimate a channel based on the estimated Doppler frequency.

38. The apparatus of claim 37, wherein the processing system is further configured to: estimate the SNR; and

determine the hypothesized SNR range based on the estimated SNR.

39. The apparatus of claim 38, wherein the determined hypothesized SNR range is [SNR-A, SNR+B], where SNR is the estimated SNR, and A and B are greater than zero.

40. The apparatus of claim 37, wherein the Doppler frequency is estimated further based on at least two channel impulse responses (CIRs) determined based on at least two orthogonal frequency division multiplexing (OFDM) symbols containing common reference signals (CRS) within a subframe.

41. The apparatus of claim 37, wherein the processing system is further configured to communicate with an evolved Node B (eNB) based on the channel estimation.

42. An apparatus for wireless communication, comprising:

a processing system configured to:

estimate a Doppler frequency based on at least two channel impulse responses (CIRs) determined based on at least two orthogonal frequency division multiplexing (OFDM) symbols containing common reference signals (CRS) within a subframe; and estimate a channel based on the estimated Doppler frequency.

43. The apparatus of claim 42, wherein the processing system is further configured to determine the at least two CIRs based on the at least two OFDM symbols containing CRS within the subframe.

44. The apparatus of claim 42, wherein the at least two OFDM symbols comprise at least one of a 1^st, 2^nd, 5^th, 8^th, 9^th, or 12^th OFDM of the subframe.

45. The apparatus of claim 42, wherein the Doppler frequency is estimated further based on a hypothesized signal to noise ratio (SNR) range determined based on an estimated SNR.

46. The apparatus of claim 42, wherein the processing system is further configured to communicate with an evolved Node B (eNB) based on the channel estimation.

47. An apparatus for wireless communication, comprising: a processing system configured to:

estimate a Doppler frequency;

compare the Doppler frequency based on a threshold;

determine whether to re-estimate the Doppler frequency based on the comparison; and

estimate a channel based on a final estimated Doppler frequency.

48. The apparatus of claim 47, wherein the Doppler frequency is initially estimated based on a first window length and is re-estimated based on a second window length greater than the first window length.

49. The apparatus of claim 47, wherein the Doppler frequency is re-estimated when the Doppler frequency is less than the threshold.

50. The apparatus of claim 47, wherein the Doppler frequency is re-estimated when the Doppler frequency is greater than the threshold.

51. The apparatus of claim 47, wherein the processing system is further configured to: re-estimate the Doppler frequency upon determining to re-estimate the Doppler frequency; and

repeat the steps of the comparing, the determining, and the re-estimating based on a comparison of the re-estimated Doppler frequency based on the threshold.

52. The apparatus of claim 47, wherein the Doppler frequency is at least one of estimated or re-estimated based on a hypothesized signal to noise ratio (SNR) range determined based on an estimated SNR.

53. The apparatus of claim 47, wherein the Doppler frequency is at least one of estimated or re-estimated based on at least two channel impulse responses (CIRs) determined based on at least two orthogonal frequency division multiplexing (OFDM) symbols containing common reference signals (CRS) within a subframe.

54. The apparatus of claim 47, wherein the processing system is further configured to communicate with an evolved Node B (eNB) based on the channel estimation.

55. A computer program product, comprising:

a computer-readable medium comprising code for:

estimating a channel based on the estimated Doppler frequency.

56. A computer program product, comprising:

a computer-readable medium comprising code for:

estimating a channel based on the estimated Doppler frequency.

57. A computer program product, comprising:

a computer-readable medium comprising code for:

estimating a Doppler frequency;

comparing the Doppler frequency based on a threshold;

estimating a channel based on a final estimated Doppler frequency.