CN101326783A - Average-tap energy based thresholding for channel estimation in multi antenna systems - Google Patents

Abstract

Techniques to facilitate estimating the frequency response of a wireless channel in an OFDM system are provided. The method and systems allow for combining signal information across multiple communication channels at one or more channel tap delays in order to determine appropriate taps for channel information.

Description

Average tap energy based thresholding for channel estimation in a multi-antenna system

CROSS-REFERENCE TO RELATED APPLICATIONS

The benefit of U.S. provisional application No.60/749,241 entitled "AVERAGE-TAP ENERGYBASED THEREFRESHOLDING FOR CHANNEL ESTIMATION IN MULTIANTENNA SYSTEMS", filed on 9/12/2005, the entire contents of both of which are hereby incorporated by reference.

Technical Field

The present disclosure relates generally to wireless communications, and more specifically to techniques for estimating a propagation channel in a wireless communication system.

Background

Currently, there are a large number of wireless communication systems that provide different types of communication services, such as voice, packet data, and so on. These systems may be multiple-access systems capable of supporting multi-user communications by sharing the available system resources. Examples of these multiple access systems include: code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, and Orthogonal Frequency Division Multiple Access (OFDMA) systems.

OFDM effectively partitions the overall system bandwidth into multiple (N) orthogonal subbands. These subbands are also referred to as tones (tones), bins (frequency bins), and subchannels. With OFDM, each subband is associated with a respective subcarrier on which data may be modulated. Thus, each sub-band may be viewed as an independent transmission channel that may be used to transmit data.

In a wireless communication system, an RF modulated signal from a transmitter may reach a receiver via multiple propagation paths. For an OFDM system, the N subbands may experience frequency selective fading due to fading and multipath effects.

It is often desirable to accurately estimate the wireless channel response between a transmitter and a receiver in order to efficiently decode the transmitted data on the available subbands. In an OFDM system, the propagation channel is estimated by transmitting several pilot tones in the frequency domain. The receiver extracts these pilot measurements in the frequency domain and performs an IFFT operation to obtain an estimate of the channel impulse response in the time domain. The length of the impulse response is typically limited to the length of the cyclic prefix of the OFDM symbol. Because these pilot measurements may be corrupted by noise at the receiver, there will be energy in all impulse response taps (taps). However, in the estimated impulse response, not all taps correspond to actual channel taps. Some taps have energy due to noise only. A method of reducing the effects of noise in an impulse response tap is: the taps most likely to be caused by noise are identified and zeroed out, thereby suppressing the noise contribution from these taps. However, if the signal-to-noise ratio (SNR) is low, the taps due to noise alone are likely to have more energy than the actual channel taps. In this case, taps due only to noise will be extracted instead of the actual channel taps, resulting in an inaccurate estimate of the channel impulse response.

Accordingly, there is a need in the art for techniques to more efficiently estimate channel response in a multi-channel OFDM system.

Disclosure of Invention

Techniques for estimating a frequency response of a wireless channel in a communication system (e.g., an OFDM system) having multiple subbands are provided herein. In an aspect, a receiver having multiple receive antennas may estimate multiple channels connected to all used receive antennas. In some cases, the multiple channels may be independent. In an aspect, the receiver calculates an average tap energy for a given possible tap position averaged over multiple antenna channels. Using this estimated tap energy distribution, the receiver is able to estimate the possible tap positions by selecting tap positions for which the corresponding average tap energy exceeds a predetermined energy level and reaches a predetermined number of multipath components.

In one embodiment, a method for estimating a frequency response of a wireless channel (e.g., in an OFDM system) is provided. According to the method, communication signals are received over a plurality of wireless communication channels. Signal energy of the communication signal at a plurality of channel taps is measured. From the measured energy at a plurality of channel taps, a signal energy value at a given tap position is determined, and a subset of the channel taps is selected based on the determination. In other aspects, a computer program product may contain instructions which, when used by a computer, are capable of performing the functions of the method.

Various aspects and embodiments of the disclosure are described in further detail below.

Drawings

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:

fig. 1 illustrates aspects of a multiple access wireless communication system in accordance with one embodiment;

fig. 2 illustrates aspects of a transmitter and receiver in a multiple access wireless communication system;

FIG. 3 shows an OFDM subband structure;

FIG. 4A shows the relationship between frequency response and impulse response;

FIG. 4B illustrates a Discrete Fourier Transform (DFT) matrix for all N subbands in an OFDM system; and

fig. 5 illustrates a process for estimating the frequency response of a wireless channel.

Fig. 6 shows a functional block diagram of an apparatus for estimating a radio channel frequency response in a communication system.

Fig. 7 illustrates a configuration of communication signals on a plurality of wireless communication channels.

Detailed Description

The channel estimation techniques described herein may be used for any communication system having multiple subbands. For clarity, these techniques are described with respect to an OFDM system, however, other multiple access schemes may use the same approach. Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be evident, however, that such embodiment(s) may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing one or more embodiments.

Referring to fig. 1, a multiple access wireless communication system is shown according to one embodiment. The multiple access wireless communication system 100 includes a plurality of cells, such as cells 102, 104, and 106. In the embodiment of fig. 1, each cell 102, 104, and 106 may include an access point 150 that includes multiple sectors. The multiple sectors are formed by multiple antenna groups, each responsible for communication with access terminals in a portion of a cell. In cell 102, antenna groups 112, 114, and 116 each correspond to a different sector. In cell 104, antenna groups 118, 120, and 122 each correspond to a different sector. In cell 106, antenna groups 124, 126, and 128 each correspond to a different sector.

Each cell includes a number of access terminals that may communicate with one or more sectors of each access point. For example, access terminals 130 and 132 are in communication base 142, access terminals 134 and 136 are in communication with access point 144, and access terminals 138 and 140 are in communication with access point 146.

As can be seen in fig. 1, each access terminal 130, 132, 134, 136, 138, and 140 is located in a different portion of its respective cell relative to every other access terminal in the same cell. Moreover, each access terminal can be a different distance from the corresponding antenna group with which it is communicating. Due to environmental and other conditions in the cell, the two factors provide a situation that results in different channel conditions between each access terminal and the corresponding antenna group with which it is communicating.

As used herein, an access point may be a fixed station used for communicating with the terminals and may also be referred to as, and include some or all the functionality of, a base station, a node B, or some other terminology. An access terminal may also be referred to as, and include some or all the functionality of, a User Equipment (UE), a wireless communication device, a terminal, a mobile station, or some other terminology.

Referring to fig. 2, one embodiment of a transmitter and a receiver in a multiple access wireless communication system is shown. At the transmitter system 210, traffic data for a number of data streams is provided from a data source 212 to Transmit (TX) data processor 214. In one embodiment, 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.

The coded data for each data stream may be multiplexed with pilot data using OFDM techniques. The pilot data is typically a known data type 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.

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). Then, TX MIMO processor 220 forwardsN_TA plurality of transmitters (TMTR)222a through 222t provide N_TA stream of modulation symbols.

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. Then, respectively from N_TN transmitted from transmitters 222a through 222t by antennas 224a through 224t_TA modulated signal.

At the receiver system 250, from N_REach antenna 252a through 252r receives the transmitted modulated signal and provides a received signal from each antenna 252 to a respective receiver (RCVR) 254. 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.

RX data processor 260 then proceeds from N_RA receiver 254 receiving N_RA stream of received symbols is processed based on a particular receiver processing technique to provide N_TA "detected" symbol stream. The processing by RX data processor 260 is described in further detail below. Each detected symbol stream includes a plurality of symbols that are estimates of the modulation symbols transmitted for the corresponding data stream. 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.

The channel response estimate generated by RX processor 260 may be used to perform space, space/time processing at the receiver, adjust power levels, change modulation rates or schemes, or other operations. RX processor 260 may further estimate the signal-to-noise-and-interference ratios (SNRs) of the detected symbol streams, and possibly other channel characteristics, and provides these quantities to a processor 270. RX data processor 260 or processor 270 may further derive an estimate of the "operating" SNR for the system. Processor 270 then provides estimated Channel State Information (CSI), which may comprise various types of information regarding the communication link and/or the received data stream. For example, the CSI may comprise only the operating SNR. The CSI is then processed by a TX data processor 214, modulated by a modulator 280, conditioned by transmitters 254a through 254r, and transmitted back to transmitter system 210, where TX data processor 214 also receives traffic data for a number of data streams from a data source 276.

At transmitter system 210, the modulated signals from receiver system 220 are received by antennas 224, conditioned by receivers 222, demodulated by a demodulator 240, and processed by a RX data processor 242 to recover the CSI reported by the receiver system. The reported CSI is then provided to processor 230 and used to (1) determine the data rates and coding and modulation schemes to be used for the data streams and (2) generate various controls for TX data processor 214 and TX MIMO processor 220.

At the receiver, various processing techniques may be used to process N_RReceive signals to detect N_TA stream of transmit symbols. Exemplary techniques may include: (i) spatial and space-time receiver processing techniques (also known as equalization techniques); and (ii) "successive nulling/equalization and interference cancellation" receiver processing techniques (also referred to as "successive interference cancellation" or "successive cancellation" receiver processing techniques).

From N_TA transmitting antenna and N_RThe MIMO channel formed by the receiving antennas can be decomposed into N_SA separate channel of which N_S≤min{N_T，N_R}. Said N is_SEach of the individual channels may also be referred to as a spatial subchannel (or a transmission channel) of the MIMO channel and corresponds to a dimension.

For full rank MIMO channels, where N_S＝N_T≤N_RFrom N_TEach of the transmit antennas transmits a separate data stream. The transmitted data streams may experience different channel conditions (e.g.,different fading and multipath effects) and may achieve different signal-to-noise-and-interference ratios (SNRs) for a given amount of transmit power.

Referring to fig. 3, a subband structure 300 that may be used for an OFDM system is shown. The OFDM system has a full system bandwidth of W MHz, wherein the system bandwidth is divided into N orthogonal subbands using OFDM. Each sub-band has a bandwidth of W/N MHz. In a typical OFDM system, only M of the total N subbands are used for data transmission, where M < N. The M used subbands are also referred to as data subbands. The remaining N-M subbands are not used for data transmission, but rather are guard subbands to allow the OFDM system to meet spectral mask requirements. The M used subbands include subbands F through F + M-1.

For OFDM, data to be transmitted on each subband is first modulated (i.e., symbol mapped) using a particular modulation scheme selected for that subband. The signal value is set to zero for each of the N-M unused subbands. For each symbol period, N symbols (i.e., M modulation symbols and N-M zeros) are transformed into the time domain using an Inverse Fast Fourier Transform (IFFT) to obtain a "transformed" symbol that includes N time-domain samples. The duration of each transform symbol is inversely proportional to the bandwidth of each subband. For example, if the system bandwidth is W-20 MHz and N-256, the bandwidth of each sub-band is 78.125KHz (or W/N MHz) and the duration of each transform symbol is 12.8 μ sec (or N/W μ sec).

OFDM can provide certain advantages, such as being able to resist frequency selective fading, which is characterized by different channel gains at different frequencies across the system bandwidth. As is well known, frequency selective fading is accompanied by intra-symbol interference (ISI), a phenomenon whereby each symbol in a received signal constitutes distortion to subsequent symbols in the received signal. The ISI distortion degrades performance by affecting the ability to correctly detect the received symbol. For OFDM, frequency selective fading can be resisted by repeating a portion of each transformed symbol (or appending a cyclic prefix to each transformed symbol) to form a corresponding OFDM symbol and then transmitting the OFDM symbol over a wireless channel.

The length (i.e., the amount of repetition) of the cyclic prefix for each OFDM symbol depends on the delay spread of the system. The delay spread of a given transmitter is the difference between the earliest and latest arriving signal instances at the receiver of the signal transmitted by the transmitter. The delay spread of the system is the expected worst case delay spread for all terminals in the system. To effectively combat ISI, the cyclic prefix is typically longer than the delay spread of the system.

Each transform symbol has a duration of N sample periods, where each sample period has a duration of (1/W) μ sec. A cyclic prefix may be defined to include Cp samples, where Cp is a suitable integer selected based on the delay spread of the system. Specifically, Cp is selected to be greater than or equal to the number of wireless channel impulse response taps (L) (i.e., Cp ≧ L). In this case, each OFDM symbol will include N + Cp samples, and each symbol period will span N + Cp sample periods.

The N subbands of an OFDM system may experience frequency selective fading due to different channel conditions (i.e., different effects caused by fading and multipath effects) and may be associated with different complex channel gains. In general, an accurate estimate of the channel response is needed in order to properly process (e.g., decode and demodulate) the data at the receiver.

May be based on time domain channel impulse responsehOr corresponding frequency domain channel frequency responseHTo characterize a wireless channel in an OFDM system. Channel frequency responseHIs the channel impulse responsehDFT transform of (1). This relationship can be expressed in a matrix form as follows:

H＝Whequation (1)

Wherein,his an (N x 1) vector corresponding to the wireless channel impulse response between the transmitter and receiver in an OFDM system;

His an (N × 1) vector corresponding to the radio channel frequency response; and

Wis an (N × N) matrix, which is used for the vectorhPerforming a DFT transform to obtain a vectorH。

Matrix arrayWIs defined such that the (n, m) th item w_n，mComprises the following steps:

<math> <mrow> <msub> <mi>w</mi> <mrow> <mi>n</mi> <mo>,</mo> <mi>m</mi> </mrow> </msub> <mo>=</mo> <mfrac> <mn>1</mn> <msqrt> <mi>N</mi> </msqrt> </mfrac> <msup> <mi>e</mi> <mrow> <mo>-</mo> <mi>j</mi> <mn>2</mn> <mi>&pi;</mi> <mfrac> <mrow> <mrow> <mo>(</mo> <mi>n</mi> <mo>-</mo> <mn>1</mn> <mo>)</mo> </mrow> <mrow> <mo>(</mo> <mi>m</mi> <mo>-</mo> <mn>1</mn> <mo>)</mo> </mrow> </mrow> <mi>N</mi> </mfrac> </mrow> </msup> <mo>,</mo> </mrow> </math> wherein N is an element of {1.. N }, m is an element of {1.. N } equation (2)

(Vector)hIncluding one non-zero term for each tap of the channel impulse response. Thus, if the channel impulse response includes L taps, where L < N, the vectorhThe first L terms of (a) will be L non-zero values, while the last (N-L) terms will be zero. However, even if the L non-zero values are in the vectorhAny selection within N items may be equally applicable to the techniques described herein.

Referring to FIG. 4A, a diagram illustrates a channel frequency responseHAnd channel impulse responsehThe relationship between them. Vector quantityhIncluding N time-domain values of the wireless channel impulse response from the transmitter to the receiver. By applying the vectorhLeft-hand multiplication DFT matrixWCan convert the vectorhTransformation into the frequency domain. Vector quantityHN frequency-domain values of complex channel gain comprising N subbands.

Referring to FIG. 4B, a matrix is illustratedWWhich is an (N × N) matrix composed of the elements defined in equation (2).

It is believed that the impulse response of the wireless channel can be characterized by L taps, where L is typically much smaller than the number of total subbands in the system (i.e., L < N). That is, if the transmitter applies a pulse to the wireless channel, L time-domain samples (at a sampling rate of W) will be sufficient to characterize the wireless channel response based on the pulse excitation. The number of taps L of the channel impulse response depends on the delay spread of the system, with longer delay spreads corresponding to larger values of L, i.e., each tap can be considered a delay and incremented for each of a plurality of taps.

Channel frequency response because only L taps are needed for the channel impulse responseHIn a subspace of dimension L (instead of N). More specifically, the frequency response of the wireless channel may be fully characterized based on the channel gains of as few as L of the appropriately selected subbands, rather than the channel gains of all N subbands.

Fig. 5 is a flow diagram illustrating a process 500 for estimating a frequency response of a wireless communication channel and selecting a subset of communication channels from a plurality or fixed range of signal paths extending in time. At 510, signals received over a plurality of wireless communication channels are input, e.g., for further processing at a receiver. It should be noted that the receiver can be combined with almost any type of device, such as a cellular phone, a personal computer, a handheld computer or other node in the transmission process, such as a base station. At 520, the energy of the communication signal at the plurality of channel taps is measured. In one embodiment, the communication signal may be a pilot signal. At 530, a signal energy value at a given tap position is determined from the measured energy at a plurality of channel taps, and at 540, a determination is made as to whether the received signal path is above (or below) a predetermined threshold. If the signal path is below the threshold, the tap associated with that signal path element is cleared, discarded, etc. At 540, if the signal path is above the threshold, processing proceeds to 550, and the communication taps associated with the signal path are selected as a subset of the communication taps and applied to the reconstruction of the communication channel.

While, for purposes of simplicity of explanation, the methodologies are shown and described herein with reference to operation numbers or designations, it is to be understood and appreciated that the processes described herein are not limited by the order of the operations, as some operations may occur in different orders and/or concurrently with other operations from that shown and described herein. For example, those skilled in the art will understand and appreciate that a methodology could alternatively be represented as a series of interrelated states or events, such as in a state diagram. Moreover, not all illustrated acts may be required to implement a methodology in accordance with the subject methods disclosed herein.

Referring to fig. 6, a functional block diagram for estimating a radio channel frequency response in a communication system 600 is shown. Communication signals received over a plurality of wireless communication channels are provided by the receiving device 602. The energy of the communication signal is measured at a plurality of channel taps by a measuring means 604. The determining means 606 determines the signal value at a given channel tap position from the output of the measuring means 604. The selection means 608 selects a subset of the channel taps based on the output of the determination means 606.

Referring to fig. 7, a diagram 700 of communication signals over multiple wireless communication channels is shown. Transmit antennas 702a-702d transmit communication signals. In a wireless communication system, RF modulated signals from transmitter 224 may reach receiver 252 through multiple propagation paths. For an OFDM system, the N subbands may experience frequency selective fading due to fading and multipath effects. In an OFDM system, the propagation channel is estimated by transmitting several pilot tones in the frequency domain. Receiver 252 extracts these pilot measurements in the frequency domain and performs an IFFT operation to obtain an estimate of the channel impulse response in the time domain. Since these pilot measurements can be corrupted by noise at the receiver, there will be energy in all of the multiple impulse response taps 704-708. However, not all taps in the estimated impulse response correspond to actual channel taps. Some taps have energy due to noise only. The different impulse response taps reflect multiple time delays in the signal arriving at the receiver 252. As discussed, these delays are typically due to different signal propagation paths caused by fading, multipath, etc. effects. As used herein, a given channel tap position is used to describe the energy received on a given channel at a given point in time (or window). For example, channel taps 704a-704d represent channel taps at relatively similar locations. That is, the channel taps 704a-704d are relatively close in time to each other. In the embodiment shown in 700, communication signals transmitted via antennas 702a-702d may arrive at receiver 254 with different delays. Although the signals transmitted by antennas 702a-704d have separate physical channels, they have similar power delay profiles.

The virtual channel is a linear sum of the same physical channels obtained using orthogonal transformation. Thus, the virtual channels will have similar channel tap positions. Thus, in one embodiment, channel taps 704a-704d, 706a-706d, and 708a-708d each have similar channel tap positions. By averaging the energies of 704a-704d, the resulting value can be used to determine whether the tap is primarily noise energy, or whether the energy is from a desired signal. Typically, most of the channel taps contain a portion of the noise energy. However, by averaging the energy of most or all antennas at similar channel tap positions, the generated values can be used to determine channel taps with signal energy without being affected by noise components at one tap position. Channel tap positions 706a-706d, 708a-708d, etc. are processed as described for 704a-704 d. An energy value is generated based on the determined number of channel tap positions. In one embodiment, the value is obtained by averaging the same channel tap positions for multiple channels. For example, the channels established by antennas 702a-702d may be compared to a threshold. Channel tap positions having values above the threshold may be used to establish a subset of channel taps, while channel tap positions having values below the threshold will not be used to establish a subset of channel taps. Resulting in a more accurate estimate of the wireless channel response between the transmitter 224 and the receiver 252.

The channel estimation techniques described herein may be implemented in different ways. For example, these techniques may be implemented in hardware, software, or a combination thereof. For a hardware implementation, the elements used to implement any one or combination of the techniques may be implemented within one or more Application Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof.

For a firmware and/or software implementation, the techniques may be implemented with instructions (e.g., procedures, code, functions, and so on) that perform the functions described herein and constitute a computer program product. The instructions or software code may be stored in a computer readable medium of a computer program product. The memory may be implemented within the processor or external to the processor.

Further, time-frequency bins are exemplary resources that may be allocated for signaling and data. In addition to time-frequency bins, the time-frequency bins may also include frequency subcarriers, transmission symbols, or other resources.

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. 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 invention. Thus, the present invention 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.

Claims (28)

1. A method of performing channel estimation in a wireless communication system, comprising:

receiving communication signals on a plurality of wireless communication channels;

measuring energy of the communication signal at a plurality of channel taps;

determining a signal energy value at a given channel tap position from the measured energy at the plurality of channel taps; and

based on the signal energy values, a subset of channel taps is selected.

2. The method of claim 1, wherein determining the signal energy value at a given channel tap position comprises: combining the signal energy measured at a plurality of channel taps, wherein the plurality of channel taps have the same position as the given channel tap position.

3. The method of claim 2, wherein the combining step comprises averaging.

4. The method of claim 1, wherein the determining step comprises averaging the measured energy at the plurality of channel taps.

5. The method of claim 1, wherein the selecting step comprises selecting when the signal energy value is above a threshold.

6. The method of claim 5, wherein thresholding is configurable.

7. The method of claim 1, further comprising:

assigning a value above the weight or a value below the weight to the signal energy value prior to the determining step.

8. The method of claim 1, wherein the measuring step comprises estimating a signal-to-noise ratio (SNR).

9. The method of claim 1, further comprising:

the unselected channel taps are discarded.

10. The method of claim 1, further comprising:

feedback is provided on the forward link or reverse link to improve the channel estimate.

11. A wireless communication device, comprising:

a processor configured to:

receiving communication signals on a plurality of wireless communication channels;

measuring energy of the communication signal at a plurality of channel taps;

determining a signal energy value at a given channel tap position from the measured energy at the plurality of channel taps; and

based on the signal energy values, a subset of channel taps is selected.

12. The device of claim 11, wherein the processor is configured to: determining the signal energy value at the given channel tap position by combining signal energies at a plurality of channel taps, wherein the plurality of channel taps have the same position as the given channel tap position.

13. The device of claim 12, wherein the processor is configured to: combining the signal energy by averaging signals at the plurality of channel taps, wherein the plurality of channel taps have the same position as the given channel tap position.

14. The device of claim 11, wherein the processor is configured to: averaging the energy of the communication signal measured at the plurality of channel taps.

15. The device of claim 11, wherein the processor is configured to: selecting when the signal energy value is above a threshold.

16. The device of claim 11, wherein the processor is configured to: so that the thresholding is configurable.

17. The device of claim 11, wherein the processor is configured to: the signal energy is given a value above the weight or a value below the weight.

18. The device of claim 11, wherein the processor is configured to: the measurement is made by estimating the signal-to-noise ratio (SNR).

19. The device of claim 11, wherein the processor is configured to: feedback is provided on the forward link or reverse link to improve the channel estimate.

20. A wireless communication device, comprising:

means for receiving communication signals on a plurality of wireless communication channels;

means for measuring energy of the communication signal at a plurality of channel taps;

means for determining a signal energy value at a given channel tap position from the measured energy at the plurality of channel taps; and

means for selecting a subset of channel taps based on the signal energy values.

21. The apparatus of claim 20, wherein the means for determining a signal energy value at a given channel tap position comprises: measuring a plurality of channel taps, wherein the plurality of channel taps have the same position as the given channel tap position.

22. The apparatus of claim 21, wherein the means for determining comprises: averaging the signal energy measured at the plurality of tap positions.

23. The apparatus of claim 20, wherein the means for determining comprises: averaging the energy of the communication signal measured at the plurality of channel taps.

24. The apparatus of claim 20, wherein the means for selecting comprises: selecting when the signal energy value is above a threshold.

25. The apparatus of claim 20, wherein the thresholding is configurable.

26. The apparatus of claim 20, wherein the means for selecting is based on giving the signal energy value a value above weight or a value below weight.

27. The method of claim 20, wherein the means for measuring comprises: a signal-to-noise ratio (SNR) is estimated.

28. A computer program product, comprising:

a computer-readable medium comprising:

instructions for measuring energy of the communication signal at a plurality of channel taps;

instructions for determining a signal energy value at a given channel tap position from the measured energy at the plurality of channel taps; and

instructions for selecting a subset of channel taps based on the signal energy values.

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