Sparse channel estimation for orthogonal frequency division multiplexed signals

A computationally efficient channel estimation technique for use within an orthogonal frequency division multiplexing (OFDM) communication system determines coefficients of a channel transfer function by calculating the dot products of a pilot vector and a plurality of interpolation vectors. One dot product is preferably calculated for each subcarrier of interest within the system. The pilot vector is extracted from an OFDM symbol received from a communication channel. In a preferred approach, a number of interpolation vectors are precalculated and stored within a communication device for subsequent use during channel estimation and equalization operations. The technique is highly flexible and can be implemented using, for example, a variable user block size or a variable pilot vector size.

FIELD OF THE INVENTION

The invention relates generally to communication systems and, more particularly, to techniques and structures for performing channel estimation in such systems.

BACKGROUND OF THE INVENTION

After a communication signal has traveled through a communication channel, equalization is often performed on the received signal to remove channel effects from the signal. One of the channel effects that often needs to be removed is intersymbol interference (ISI). In a wireless communication system, ISI is typically present in the form of multipath interference. That is, a transmit signal travels through the wireless channel via multiple different paths that each have a different channel delay. For example, one signal component may travel in a direct path from the transmitter to the receiver while one or more other signal components are reflected from objects in the surrounding environment toward the receiver. As can be appreciated, the signal component that travels directly to the receiver will typically be the first to arrive at the receiver and have the largest amplitude. The reflected components will typically arrive at the receiver sometime later and have smaller amplitudes. Although smaller in amplitude, the reflected signals can interfere with the direct signal making it more difficult to accurately detect the data therein. Equalization is thus used in the receiver to reduce or eliminate the negative channel effects from the received signal to improve the likelihood of accurate detection.

In most equalization techniques, an estimate of the present channel response is first determined. The channel estimate is then used to process the received signal to remove the negative channel effects. The channel estimation process is often a computationally complex and time consuming process. That is, performance of such processes will often consume a large percentage of system resources and may introduce undesirable delays in the receiver processing. As can be appreciated, it is generally desirable to reduce computational complexity and processing delays within a communication system. This is especially true within handheld and portable communication units that have limited processing capabilities and a limited supply of power (e.g., batteries). Therefore, there is a need for channel estimation techniques and structures that are computationally efficient while still providing accurate estimates.

DETAILED DESCRIPTION

The present invention relates to computationally efficient techniques and structures for providing channel estimation within a communication system implementing orthogonal frequency division multiplexing (OFDM). The techniques and structures are most useful when only a subset of the subcarriers within each OFDM symbol are of interest. In a system using subcarrier division multiplexing, for example, where subsets of the data subcarriers are assigned to users on a dynamic basis, a communication device associated with a particular user will only be interested in the subcarriers assigned to that user. In one approach, interpolation vectors are first obtained for each of the subcarriers of interest. A dot product is then calculated between each of the interpolation vectors and a pilot vector extracted from a received OFDM symbol. Each dot product results in an equalization coefficient for a corresponding subcarrier of interest. The equalization coefficients are then used to modify the subcarriers of interest within the received OFDM symbol to reduce or remove undesirable channel effects (e.g., frequency selective fading) from the symbol. In one approach, only a subset of the pilot symbols within each OFDM symbol are used to form the pilot vector. The inventive principles are particularly well suited for software implementation (e.g., soft-PHY architectures), although hardware and hybrid software/hardware realizations can also be provided.

Orthogonal frequency division multiplexing (OFDM) is a multi-carrier transmission technique that uses a plurality of orthogonal subcarriers to transmit information within an available spectrum. Because the subcarriers are orthogonal to one another, they can be spaced much more closely together within the available spectrum than, for example, the individual channels in a conventional frequency division multiplexing (FDM) system. That is, the orthogonality of the subcarriers prevents inter-subcarrier interference within the system. In a typical OFDM system, orthogonality is achieved by using subcarriers that each have a spectrum with a null at the center frequency of each of the other subcarriers. Before transmission, each of the subcarriers is modulated with a low rate data stream. Thus, the transmitted symbol rate of the OFDM system is low and the transmitted OFDM signal is highly tolerant to multipath delay spread within the channel. For this reason, many modern digital communication systems are turning to OFDM as a modulation scheme for signals that need to survive in environments having multipath reflections and/or strong interference. Many wireless communication standards have already adopted OFDM including, for example, IEEE Standard 802.11a, the digital video broadcasting T standard (DVB-T), and the high performance radio local area network standard (HiperLAN). In addition, several industry consortia, including the Broadband Wireless Internet Forum and the OFDM Forum, are proposing OFDM for fixed wireless access systems.

Before a description of the inventive principles is undertaken, a discussion of the basic operating characteristics of a conventional OFDM communication system is presented. It should be appreciated, however, that the inventive principles can be implemented in any communication system utilizing OFDM techniques and are not limited to use within systems or devices having the specific architectures described below.

FIG. 1is a block diagram illustrating a conventional OFDM transmitter10. In a typical scenario, the transmitter10will be part of a transceiver unit that is capable of supporting duplex communication within a wireless communication system. As illustrated, the transmitter10includes: a modulator12, an inverse fast Fourier transform (IFFT) unit14, a parallel to serial converter16, a cyclic extension unit18, a radio frequency (RF) transmit unit20, and an antenna22. The modulator12receives a plurality of symbols (S0, S1, S2, . . . , SN−1) that need to be transmitted by the transmitter10. The modulator12uses each of the input symbols to modulate a corresponding subcarrier of the OFDM system to generate a symbol modulated subcarrier (e.g., S0SC, S1SC, S2SC, . . . , S(N−1)SC) at an output thereof. As described above, each of the subcarriers of the OFDM system is orthogonal to each of the other subcarriers to keep inter-subcarrier interference to a minimum. The modulator12can use any of a variety of modulation types to modulate the subcarriers (e.g., binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), quadrature amplitude modulation (QAM), differentially coded star QAM (DSQAM), and others). In addition, the modulator12can use a different modulation type for each individual symbol or for different groups of symbols if desired.

The input symbols (S0, S1, S2, . . . , SN−1) are used to generate a single OFDM symbol to be transmitted by the transmitter10. The symbol modulated subcarriers (S0SC, S1SC, S2SCSC, . . . , S(N−1)SC) form a frequency domain representation of the OFDM symbol. The symbol modulated subcarriers are applied to the inputs of the IFFT14to generate a time domain representation of the OFDM symbol. As shown, the time domain representation of the OFDM symbol consists of a plurality of time domain samples (S0, S1, S2, . . . , SN−1). Any form of inverse discrete Fourier transform (IDFT) can be used to perform the inverse transform operation. The IFFT is preferred, however, because it is the most computationally efficient method available. As is well known, the number of time domain samples generated by the IFFT14is equal to the number of frequency components input thereto (i.e., N).

The samples output by the IFFT14are applied to the parallel to serial converter16which generates a sample stream representing the OFDM symbol. This serial OFDM symbol is transferred to the cyclic extension unit18which adds a cyclic extension (or guard interval) to the OFDM symbol. The cyclic extension is added to the OFDM symbol to prevent the occurrence of inter-symbol interference in the channel that can be caused by the channel's memory (i.e., multipath reflections). The cyclic extension usually consists of a plurality of samples (e.g., NGsamples) that are copied from the end of the serial OFDM symbol and placed at the beginning of the symbol. The number of samples will typically depend upon the memory of the channel. It is typically desirable to use a cyclic extension having a length that is no more than 10% of the length of the OFDM symbol to maintain efficient (e.g., low overhead) operation.

The cyclic extension unit18outputs each OFDM symbol and its corresponding cyclic extension in a continuous stream to the RF transmit unit20.FIG. 2is a diagram illustrating the stream output by the cyclic extension unit18in a typical application. The RF transmit unit20is operative for converting the OFDM symbol stream into a radio frequency signal for transmission into the wireless channel. To perform this function, the RF transmit unit20may include, for example, a digital to analog converter, a frequency conversion unit (e.g., an up converter), a power amplifier, and/or any other equipment required to generate an RF transmit signal. The output of the RF transmit unit20is delivered to the antenna22which transmits a radio frequency communication signal24into the channel. It should be appreciated that other processing functionality, such as error coding circuitry, may also be included within the OFDM transmitter10.

FIG. 3is a block diagram illustrating a conventional OFDM receiver28. Like the transmitter10ofFIG. 1, the receiver28will typically be part of a transceiver unit that is capable of supporting duplex communications within a wireless communication system. As illustrated, the receiver28includes: an antenna30, an RF receive unit32, a synchronization unit34, a serial to parallel converter36, an FFT unit38, and a demodulation unit40. The antenna30receives an RF communication signal24from the channel. The RF receive unit32converts the received RF signal to a format required for subsequent processing. The RF receive unit32may include, for example, a low noise amplifier, one or more frequency conversion units (e.g., a down converter), an analog to digital converter, and/or any other functionality required to achieve the desired signal format. The RF receive unit32transfers the received signal to the synchronization unit34which synchronizes the signal in a manner that allows the individual OFDM symbols within the signal to be recognized and the cyclic extensions to be discarded. The OFDM symbols are delivered one after the other to the serial to parallel converter36which converts each symbol into a parallel group of time domain samples (r0, r1, r2, . . . , rN−1). The samples are input into the FFT unit38which generates a plurality of frequency domain symbol modulated subcarriers (R0SC, R1SC, R2SC, . . . , R(N−1)SC). The symbol modulated subcarriers are then demodulated by the demodulator40to produce a plurality of symbols (R0, R1, R2, . . . , RN−1).

The impulse response of the wireless channel (i.e., the channel impulse response or CIR) is typically assumed to be time-invariant for the duration of one OFDM symbol. Thus, the time domain convolution of the transmitted time domain signal with the CIR is equivalent to the multiplication of the spectrum of the transmitted signal with the frequency domain transfer function H(f) of the channel (which is simply the Fourier transform of the CIR). Each of the frequency domain symbol modulated subcarriers (R0SC, R1SC, R2SC, . . . , R(N−1)SC) that are received by the receiver28, therefore, is the product of a corresponding symbol modulated subcarrier (S0SC, S1SC, S2SC, . . . , S(N−1)SC) of the transmitter10and an associated coefficient of the frequency domain transfer function H(f) of the channel, plus some additive channel noise (e.g., additive white Gaussian noise). This can be expressed in equation form as follows:
RnSC=SnSC×Hn+nn
where RnSCis the nth symbol-modulated subcarrier received in the receiver, SnSCis the nth symbol-modulated subcarrier transmitted by the transmitter, Hnis the frequency domain channel transfer function coefficient corresponding to the nth subcarrier, and nnis a white Gaussian noise sample corresponding to the nth subcarrier. As coherent detection is assumed for the system, the received data symbols RnSCneed to be de-faded in the frequency domain. Thus, an estimate of the frequency domain transfer function H(f) of the channel needs to be made.

In a typical approach, the channel transfer function is estimated using pilot symbols that are included within the OFDM symbols transmitted by the transmitter10. The pilot symbols are usually located at fixed frequency intervals within the OFDM symbols.FIG. 4is a diagram illustrating the frequency spectrum of an OFDM symbol42having a plurality of subcarriers. As shown, every fourth subcarrier within the OFDM symbol42includes a pilot symbol44that can be used for channel estimation. The other subcarriers within the OFDM symbol42are used to carry user data symbols through the channel. The spacing of the pilot symbols44within a particular OFDM symbol will typically depend upon the specific system being implemented. Similarly, the overall number of subcarriers within each OFDM symbol will also be system specific. The location and content of the pilot symbols within the transmitted OFDM symbol will typically be known within the receiver.

When an OFDM symbol is received by the receiver28, the pilot symbols are extracted from the received signal. The extracted pilot symbols include information about the frequency domain transfer function of the channel (e.g., coefficients) at the frequencies of the pilot carrying subcarriers. To obtain information about the frequency domain transfer function of the channel at the frequencies of the data carrying subcarriers, interpolation techniques are often used. In the simplest approach, linear interpolation is performed from pilot to pilot. This method provides reasonable performance as long as the inverse of the realized delay spread does not approach the pilot spacing. Often, however, the linear approach falls short of the performance levels required in modem communication systems.

In an “optimal” interpolation method, zero-padded FFTs (or DFTs) are used to fill in the missing transfer function coefficients. This method will typically result in the most accurate channel estimate available, but it is very computationally complex. For example, in a typical procedure, the pilots from a received OFDM symbol are input to an FFT to generate an array of values at an output of the FFT. The number of values within the array output by the FFT is equal to the number of pilots input to the FFT (i.e., M). A plurality of zeros is then added to the array of values to increase the total number of values to the number of subcarriers within the OFDM symbol (i.e., from M to N). This process is known as zero-padding. An inverse N-point FFT is then performed on the zero-padded array to achieve interpolated channel transfer function coefficients for all of the subcarriers. These coefficients can then be used to perform channel equalization for the data carrying subcarriers within the received OFDM symbol on a subcarrier by subcarrier basis (e.g., by simple division).

In conceiving the present invention, it was appreciated that many situations exist where only a subset of the subcarriers within a particular OFDM symbol are of interest. For example, in an OFDM system where orthogonal frequency division multiple access (OFDMA) is being utilized, individual users are each assigned subsets of the subcarriers on a dynamic basis. Thus, the communication equipment associated with a particular user (e.g., a handheld communicator) will only be concerned with the corresponding subcarriers assigned to that user and not the entire subcarrier array. In a similar example, a basestation in an OFDM system implementing OFDMA will typically receive OFDM signals from multiple user terminals concurrently. The base station must then estimate the channel associated with each user terminal separately. Each channel estimation, therefore, is only concerned with the subcarriers of interest for the corresponding user terminal. Other situations also exist where only a subset of the subcarrier array is of interest. In these situations, it will often be inefficient to perform the computationally complex optimal interpolation technique described above to determine equalization coefficients for the subcarriers of interest. Therefore, in accordance with the present invention, techniques are presented that are capable of providing a significant reduction in the computational complexity associated with channel estimation (i.e., with respect to the optimal interpolation approach) when only a subset of subcarriers within an OFDM symbol are of interest. The techniques of the present invention will typically provide a performance level between that of the linear interpolation method and the optimal interpolation method (often closer to the optimal method).

The reduced complexity channel estimation techniques of the present invention are derived from the optimal interpolation approach described above. In the optimal approach, a pilot vector having a length M (extracted from a received OFDM symbol) is transformed into a vector of length N using zero padded FFTs (or DFTs). An M-point FFT is first performed on the pilot vector xnas follows:

Xf=∑n=0M⁢⁢1⁢⁢xn·ⅇ-j2π⁢⁢fn/M
where Xfare the frequency domain coefficients output by the FFT, M is the number of pilots in the pilot vector, n is the time index, and f is the frequency index. The resulting group of coefficients Xfis then zero-padded to length N (i.e., total number of subcarriers within the OFDM symbol) with the zeros inserted in the high frequency (center) terms. An N-point inverse FFT (or DFT) is then performed on the zero-padded array to generate a plurality of interpolated equalization coefficients xn′ that can be described as follows:

x^n=∑f=0(M/2)-1⁢⁢Xf·ⅇj2π⁢⁢fn/N+∑f=N-(M/2)N-1⁢⁢Xf-N+M·ⅇj2π⁢⁢fn/N
The subscript of X has been modified in the second summation of this equation to conform to the original, non-zero padded indicies described above. Substituting the second equation into the first and expanding the result yields:

In accordance with one aspect of the present invention, a plurality of interpolation vectors are predetermined (using the above equation or some variant thereof) and stored within a communication device. When an OFDM symbol is received by the communication device, a pilot vector is extracted from the OFDM symbol. Interpolation vectors are then retrieved for each subcarrier of interest within the communication device and a dot product is calculated between the pilot vector and each of the retrieved interpolation vectors. Each dot product results in an equalization coefficient for a corresponding subcarrier of interest. This basic approach will be referred to herein as the vector interpolation method (VIM). The equalization coefficients resulting from the dot products are then used to equalize the associated subcarrier signals within the OFDM symbol (e.g., by division). As long as the subcarriers of interest remain unchanged, the same interpolation vectors can be used to process each OFDM symbol received from the channel.

The VIM offers a computational flexibility that was not previously available using the optimal interpolation approach. For example, the VIM allows flexibility in block size (i.e., the number of subcarriers computed) and pilot vector size that was not previously available. Using the VIM, an OFDM communication system can be implemented that allows the number of subcarriers assigned to each user to be dynamically varied during system operation. Similarly, an OFDM system can be provided that allows the total number of subcarriers and/or pilot symbols within each OFDM symbol to be dynamically varied. Theoretically, the optimal interpolation method can be performed with only a subset of the pilot symbols within an OFDM symbol to reduce computational complexity. However, to dynamically vary the size of the pilot vector used to perform optimal interpolation during system operation, block processing elements (e.g., FFTs) would have to be available for each expected vector size. As dedicated hardware is typically used to perform the FFTs using the optimal approach, this could easily become cost prohibitive. For this reason, systems implementing the optimal approach typically employ the entire array of pilots to perform the interpolation. For a given number of pilots used, the optimal method and the VIM will provide identical performance. However, when the number of subcarriers to be interpolated is a subset of those traversed by the pilot tones used in the channel estimation, a significant computation benefit is achieved by using the VIM. Thus, the VIM allows channel estimation to be performed only in the region of the subcarriers of interest in a relatively simple and dynamic fashion, usually with a reduced computational requirement. In addition, the VIM is particularly well suited for implementation within a soft-PHY architecture.

FIG. 5is a block diagram illustrating an OFDM equalization subsystem50in accordance with one embodiment of the present invention. The subsystem50will typically be implemented as part of a wireless receiver (e.g., receiver28ofFIG. 3) in an OFDM communication system. In a multi-user basestation scenario, a separate equalization subsystem50can be provided for each user currently communicating with the basestation. It should be appreciated that the individual blocks within the block diagram do not necessarily correspond to discrete hardware structures. For example, one or more of the blocks (or all of the blocks) may be implemented in software within a digital processing device.

As illustrated, the equalization subsystem50includes: a subcarrier tracking unit52, a pilot vector unit54, an interpolation vector retrieval unit56, a computation unit58, an equalizer60, and a memory62. The subcarrier tracking unit52tracks the current subcarriers of interest for the subsystem50. The pilot vector unit54extracts a number of pilot symbols from a recently received OFDM symbol64to form a pilot vector. The interpolation vector retrieval unit56retrieves a plurality of interpolation vectors corresponding to the subcarriers of interest identified by the subcarrier tracking unit52from the memory62. As will be described in greater detail, the interpolation vector retrieval unit56may use information about the pilot symbols within the pilot vector to determine which interpolation vectors to retrieve. The computation unit58uses the pilot vector from the pilot vector unit54and the interpolation vectors from the interpolation vector retrieval unit56to generate a plurality of equalization coefficients for delivery to the equalizer60. The equalizer60then uses the equalization coefficients generated by the computation unit58to equalize each of the subcarriers of interest within the corresponding OFDM symbol64. This procedure is repeated for each OFDM symbol received.

As described above, the subcarrier tracking unit52is operative for tracking the subcarriers of interest within the equalization subsystem50. To do this, the subcarrier tracking unit52will typically need to determine which subcarriers are presently assigned to a user associated with the subsystem50. In a basestation that is servicing multiple users within the OFDM system, a separate equalization subsystem50may be provided for each currently connected user. The subcarrier tracking unit52within each subsystem50would thus track the subcarriers assigned to the corresponding user. The subcarrier tracking unit52will output an indication of the present subcarriers of interest to the interpolation vector retrieval unit56and possibly the pilot vector unit54. In at least one approach, the number and location of the subcarriers of interest associated with a particular user can change with time. The subcarriers of interest associated with a particular user are not necessarily adjacent to one another within the OFDM symbol.

In one embodiment of the present invention, the pilot vector generated by the pilot vector unit54includes all of the pilot symbols from the present OFDM symbol64. In a preferred approach, however, the pilot vector unit54includes selection functionality for dynamically selecting a subset of pilot symbols within the OFDM symbol64to be used to form the pilot vector (i.e., to perform the interpolation). By using a subset of pilot symbols, computational complexity can be reduced considerably. However, as discussed previously, a reduction in equalization performance will usually be experienced. In this regard, a tradeoff can be made between performance and computational efficiency. The subset of pilot symbols that is used within the vector interpolation calculations should envelope all of the subcarriers of interest being processed. For example, with reference toFIG. 6, if subcarriers66,68,70,72, and74are presently of interest, then at least pilot symbols A, B, C, and D should be used in the interpolation. The pilot vector unit54can determine which of the pilot symbols to use for a particular set of subcarriers of interest based on the interpolation vectors that are known to be available within the memory62.

The memory62will include a plurality of interpolation vectors for use during the channel estimation process. The interpolation vectors can all have the same length or a plurality of different length interpolation vectors can be provided. The interpolation vectors will typically be calculated a priori using the interpolation vector equation described above or a similar equation. In a system where the number and arrangement of the subcarriers of interest can vary, the length of the corresponding pilot vectors may also vary. In this scenario, interpolation vectors can be stored in the memory62for each subcarrier within each possible pilot vector arrangement. The interpolation vector retrieval unit56retrieves the appropriate interpolation vectors from the memory62based on the subcarriers identified by the subcarrier tracking unit52. If the pilot vector unit54dynamically selects pilot symbols for the pilot vector, the interpolation vector retrieval unit56will also use information about the present pilot vector to retrieve the appropriate interpolation vectors. As described previously, the retrieved interpolation vectors will each have the same length as the pilot vector assembled by the pilot vector unit54. In a preferred approach, the computation unit58calculates a separate dot product between the pilot vector and each of the retrieved interpolation vectors. The result of each dot product is the equalization coefficient for the corresponding subcarrier of interest. The equalizer60equalizes the subcarriers of interest within the present OFDM symbol64by dividing each subcarrier by the corresponding equalization coefficient. The equalized data symbols are then output for further processing.

FIG. 7is a flowchart illustrating a method for performing channel estimation and equalization in an OFDM communication system in accordance with one embodiment of the present invention. A plurality of subcarriers of interest are first identified (block70). A pilot vector is then extracted from a received OFDM symbol (block72). The pilot vector can include all of the pilot symbols from the OFDM symbol or a subset thereof. In at least one embodiment, pilot symbols are chosen for the pilot vector based on the quantity and location of the subcarriers of interest. An interpolation vector is obtained for each of the identified subcarriers of interest (block74). In a preferred approach, the interpolation vectors are retrieved from a memory within the corresponding communication unit. However, other methods for obtaining the interpolation vectors can also be used. A dot product is next calculated between the pilot vector and each of the interpolation vectors (block76). Each dot product results in an equalization coefficient for a corresponding subcarrier of interest. Each equalization coefficient is then applied to a corresponding subcarrier within the received OFDM symbol (block78). This process is repeated for each OFDM symbol received.

It should be appreciated that the principles of the present invention can be beneficially implemented in any receiver unit used within an OFDM-based communication system. The receiver unit can be located, for example, within a multi-user basestation, a single user handheld communicator, a satellite uplink, downlink, or crosslink transceiver, a transceiver supporting a terrestrial wireless link, mobile transceivers within ad hoc networks, and in a wide variety of other communication device types. The inventive principles have application in both wireless and wired systems, although wireless systems will typically derive the greatest benefit.