Abstract:
A method and apparatus for estimating separate channel frequency responses for two channels in an orthogonal frequency division multiplexing system with two transmitters is disclosed. The channel frequency responses are estimated using specifically selected training symbols that are broadcast from the two transmitters. The training symbols are specifically selected so as to improve the estimation of the channel frequency responses for each channel, while requiring the same amount of training symbols as in an estimation of the channel frequency response of a single channel.

Description:
FIELD OF THE INVENTION 
     The invention relates to channel estimation, and more particularly to a method and apparatus for estimating two propagation channels in an Orthogonal Frequency Division Multiplexing (OFDM) system with two transmitter antennas using specifically selected training information. 
     BACKGROUND OF THE INVENTION 
     The growing area of personal communications systems is providing individuals with personal terminals capable of supporting various services such as multimedia. These services require the use of increased bit rates due to the large amount of data required to be transferred. The use of increased bit rates generates problems in conventional single carrier systems due to inter-symbol interference (ISI) and deep frequency selective fading problems. 
     One solution to these problems utilizes orthogonal frequency division multiplexing (OFDM) within the radio mobile environment to minimize the above-mentioned problems. Within OFDM, a signal is transmitted on multi-orthogonal carriers having less bandwidth than the coherence bandwidth of the channel in order to combat frequency selective fading of the transmitted signal. The inter-symbol interference is mitigated by the use of guard intervals. OFDM systems are presently adopted in Europe for digital audio broadcasting and have been proposed for use in digital TV broadcasting systems. It is used also in asymmetric digital subscriber lines (ADSL) to transmit high rate data. OFDM has also been selected as the modulation method for wireless local area network (WLAN) standards in United States, Europe and Japan. 
     Transmitter diversity is an effective technique to mitigate multipath fading. One significant advantage of transmitter diversity is that the receiver needs only one antenna with Radio Frequency (RF) receiving chain. Since RF components are quite expensive the cost of the receiver can be reduced with transmitter diversity compared to a system using receiver diversity, that needs two or more antennas and corresponding receiving RF chains. Recently Space-Time Codes (STC) have been introduced as a method to achieve transmitter diversity system. Space-Time codes encode information over multiple antennas to achieve diversity advantage, however decoding of STC needs an estimate of the propagation path from each transmitter antenna to the receiver antenna. 
     Since radio channels often are subjected to multipath propagation, the receiver needs to comprise some sort of equalizer to eliminate this phenomenon. The equalizer requires an estimated frequency response of the transmission channel, i.e., a channel estimation. Existing channel estimation methods are based on adaptive signal processing wherein the channels are assumed to vary slowly. The estimated channel parameters at a particular time depend on the received data and channel parameters at a previous time. In the case of fast varying channels, such as in high data rate mobile systems, these methods must be modified to reduce the estimation time. 
     Single channel estimation is a well known problem and numerous methods exist to solve that problem. However, their extension to estimating multiple channels in an OFDM system has not been discussed. For example, Space-Time coded communication systems use multiple transmit antennas to achieve transmitter diversity gain, but require each propagation channel to be separately estimated. A trivial way to use existing single channel estimation algorithms is to separate transmission of the training information in time for each transmit antenna. Then, the existing algorithms can be used for each antenna as each antenna is transmitting training information. 
     A drawback of separating the training information in time is that it reduces the amount of information used to estimate each channel, provided that a fixed amount of training data is available. Time divisioning the training data between two antennas decreases the quality of the estimate of each channel. Another option is to double the amount of training data, which in turn increases the system overhead. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome the deficiencies described above by providing a method and apparatus for estimating separate channel frequency responses in a communication system with two transmitters. The channel frequency responses are estimated using specifically selected training symbols that are broadcast from the two transmitters. The invention has the advantage of retaining the same amount of training symbols as required in a single channel estimation case while improving the channel estimate for each channel. 
     According to one embodiment of the present invention, a method and apparatus for estimating separate channel frequency responses for channels in an orthogonal frequency division multiplexing system with two transmitters is disclosed. First and second training symbols (A 1 , A 2 ) and data from a first transmitter are transmitted to a receiver using a first channel. Third and fourth training symbols (B 1 , B 2 ) and data from a second transmitter are transmitted to the receiver using a second channel. First and second received symbols are received at the receiver. The first and second received symbols are then combined. A first channel estimate and a second channel estimate are then derived from the combined received symbols, wherein the first and second received symbols comprise the training symbols, wherein the first and third training symbols form a first symbol pair and the second and fourth training symbols form a second symbol pair. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     For a better understanding of these and other objects of the present invention, reference is made to the detailed description of the invention, by way of example, which is to be read in conjunction with the following drawings, wherein: 
     FIG. 1 is a block diagram of a typical OFDM transmitter according to the prior art; 
     FIG. 2 is an illustration of a typical OFDM signal within an OFDM channel bandwidth showing the frequency domain positioning of OFDM sub-carriers and their modulated spectra, according to the prior art; 
     FIG. 3 is a block diagram of a typical OFDM receiver according to the prior art; 
     FIG. 4 is a block diagram of an OFDM communication system with two transmit antennas and one receive antenna according to one embodiment of the invention; 
     FIG. 5 illustrates the transmission of training information according to one embodiment of the invention; and 
     FIG. 6 is a flow chart illustrating a channel estimation process according to one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Orthogonal frequency division multiplexing is a robust technique for efficiently transmitting data over a channel. The technique uses a plurality of sub-carrier frequencies (sub-carriers) within a channel bandwidth to transmit the data. These sub-carriers are arranged for optimal bandwidth efficiency compared to more conventional transmission approaches, such as frequency division multiplexing (FDM), which waste large portions of the channel bandwidth in order to separate and isolate the sub-carrier frequency spectra and thereby avoid intercarrier interference (ICI). By contrast, although the frequency spectra of OFDM sub-carriers overlap significantly within the OFDM channel bandwidth, OFDM nonetheless allows resolution and recovery of the information that has been modulated onto each sub-carrier. Additionally, OFDM is much less susceptible to data loss due to multipath fading than other conventional approaches for data transmission because intersymbol interference is prevented through the use of OFDM symbols that are long in comparison to the length of the channel impulse response. Also, the coding of data onto the OFDM sub-carriers can take advantage of frequency diversity to mitigate loss due to frequency-selective fading. 
     The general principles of OFDM signal transmission can be described with reference to FIG. 1 which is a block diagram of a typical OFDM transmitter according to the prior art. An OFDM transmitter  10  receives a stream of baseband data bits  12  as its input. These input data bits  12  are immediately fed into an encoder  14 , which takes these data bits  12  in segments of B bits every T g +T s  seconds, where T s  is an OFDM symbol interval and T g  is a cyclic prefix or guard interval. The encoder  14  typically uses a block and/or convolutional coding scheme to introduce error-correcting and/or error-detecting redundancy into the segment of B bits and then sub-divides the coded bits into 2N sub-segments of m bits. The integer m typically ranges from 2 to 6. 
     In a typical OFDM system, there are 2N+1 OFDM sub-carriers, including the zero frequency DC sub-carrier which is not generally used to transmit data since it has no frequency and therefore no phase. Accordingly, the encoder  14  then typically performs 2 m -ary quadrature amplitude modulation (QAM) encoding of the 2N sub-segments of m bits in order to map the sub-segments of m bits to predetermined corresponding complex-valued points in a 2 m -ary constellation. Each complex-valued point in the constellation represents discrete values of phase and amplitude. In this way, the encoder  14  assigns to each of the 2N sub-segments of m bits a corresponding complex-valued 2 m -ary QAM sub-symbol c k =a k +jb k , where −N 1 ≦k≦N 1 , in order to create a sequence of frequency-domain sub-symbols that encodes the B data bits. Also, the zero-frequency sub-carrier is typically assigned c 0 =0. The encoder  14  then passes the sequence of subsymbols, along with any additional zeros that may be required for interpolation to simplify filtering, on to an inverse discrete Fourier transformer (IDFT) or, preferably, an inverse fast Fourier transformer (IFFT)  16 . 
     Upon receiving the sequence of OFDM frequency-domain sub-symbols from the encoder  14 , the IFFT  16  performs an inverse Fourier transform on the sequence of sub-symbols. In other words, it uses each of the complex-valued sub-symbols, c k , to modulate the phase and amplitude of a corresponding one of 2N+1 sub-carrier frequencies over a symbol interval T s . The sub-carriers are given by e −2πjf   k   t , and therefore, have baseband frequencies of f k =k/T s , where k is the frequency number and is an integer in the range −N≦k≦N. The IFFT  16  thereby produces a digital time-domain OFDM symbol of duration T s  given by          U        (   t   )       =       ∑     k   =     -   N       N            c   k            exp        (       -   2        π                   f   k        t     )       .                                
     As a result of this discrete-valued modulation of the OFDM sub-carriers by frequency-domain sub-symbol intervals of T s  seconds, the OFDM sub-carriers each display a sinc x=(sin x)/x spectrum in the frequency domain. By spacing each of the 2N+1 sub-carriers 1/T s  apart in the frequency domain, the primary peak of each sub-carriers sinc x spectrum coincides with a null of the spectrum of every other sub-carrier. In this way, although the spectra of the sub-carriers overlap, they remain orthogonal to one another. FIG. 2 illustrates the arrangement of the OFDM sub-carriers as well as the envelope of their modulated spectra within an OFDM channel bandwidth, BW, centered around a carrier frequency, f ct . Note that the modulated sub-carriers fill the channel bandwidth very efficiently. 
     Returning to FIG. 1, the digital time-domain OFDM symbols produced by the IFFT  16  are then passed to a digital signal processor (DSP)  18 . The DSP  18  performs additional spectral shaping on the digital time-domain OFDM symbols and also adds a cyclic prefix or guard interval of length T g  to each symbol. The cyclic prefix is generally just a repetition of part of the symbol. This cyclic prefix is typically longer than the OFDM channel impulse response and, therefore, acts to prevent inter-symbol interference (ISI) between consecutive symbols. 
     The real and imaginary-valued digital components that make up the cyclically extended, spectrally-shaped digital time-domain OFDM symbols are then passed to digital-to-analog converters (DACs)  20  and  22 , respectively. The DACs  20  and  22  convert the real and imaginary-valued digital components of the time-domain OFDM symbols into in-phase and quadrature OFDM analog signals, respectively, at a conversion or sampling rate of f ck     —     r  as determined by a clock circuit  24 . The in-phase and quadrature OFDM signals are then passed to mixers  26  and  28 , respectively. 
     In the mixers  26  and  28 , the in-phase and quadrature OFDM signals from the DACs  20  and  22  are used to modulate an in-phase intermediate frequency signal (IF) and a 90° phase-shifted (quadrature) IF signal, respectively, in order to produce an in-phase IF OFDM signal and a quadrature IF OFDM signal, respectively. The in-phase IF signal that is fed to the mixer  26  is produced directly by a local oscillator  30 , while the 90° phase-shifted IF signal that is fed to the mixer  28  is produced by passing the in-phase IF signal produced by the local oscillator  30  through a 90° phase-shifter  32  before feeding it to the mixer  28 . These two in-phase and quadrature IF OFDM signals are then combined in a combiner  34  to form a composite IF OFDM signal. In some prior art transmitters, the IF mixing is performed in the digital domain using a digital synthesizer and digital mixers before the digital-to-analog conversion is performed. 
     This composite IF OFDM signal is then passed into radio frequency transmitter  40 . Many variations of RF transmitter  40  exist and are well known in the art, but typically, the RF transmitter  40  includes an IF bandpass filter  42 , an RF mixer  44 , an RF carrier frequency local oscillator  46 , an RF baseband filter  48 , an RF power amplifier  50 , and an antenna  52 . The RF transmitter  40  takes the IF OFDM signal from the combiner  34  and uses it to modulate a transmit carrier of frequency f ct , generated by the RF local oscillator  46 , in order to produce an RF OFDM-modulated carrier that occupies a channel bandwidth, BM. Because the entire OFDM signal must fit within this channel bandwidth, the channel bandwidth must be at least (1/T s )·(2N+1) Hz wide to accommodate all the modulated OFDM sub-carriers. The frequency-domain characteristics of this RF OFDM-modulated carrier are illustrated in FIG.  2 . This RF OFDM-modulated carrier is then transmitted from antenna  52  through a channel, to an OFDM receiver in a remote location. In alternative embodiments of RF transmitters, the OFDM signal is used to modulate the transmit carrier using frequency modulation, single-sided modulation, or other modulation techniques. Therefore, the resulting RF OFDM-modulated carrier may not necessarily have the exact shape of the RF OFDM-modulated carrier illustrated in FIG. 2, i.e., the RF OFDM-modulated carrier might not be centered around the transmit carrier, but instead may lie to either side of it. 
     In order to receive the OFDM signal and to recover at a remote location the baseband data bits that have been encoded into the OFDM sub-carriers, an OFDM receiver must perform essentially the inverse of all of the operations performed by the OFDM transmitter described above. These operations can be described with reference to FIG. 3 which is a block diagram of a typical OFDM receiver according to the prior art. 
     The received signal is first filtered in a receiver filter  302  so as to limit the bandwidth of the received signal. The band limited received signal is then sent to a channel estimator  304 , wherein the channel estimator comprises a processor. The channel estimator processes the band limited received signal to produce an estimate of the channel frequency response (Ĥ ; k k) Of the transmit channel. In this example, the channel estimator also performs frame synchronization in a known manner and produces an estimate of the frame timing ({circumflex over (T)} F ). 
     The estimate of the frame timing ({circumflex over (T)} F ) is sent to S/P processor  306  which converts the serial data input stream from the receive filter  302  and frame timing from the channel estimator into a parallel stream by framing N symbols. The S/P  306  outputs a received cyclically extended OFDM frame. The cyclic prefix attached to the OFDM data frame is then removed in processor  308 . With proper synchronization, the inter-frame interference is removed. The received OFDM data frame is then sent to the Discrete Fourier Transformer DFT  310 . The DFT  310  implements the OFDM demodulator with N sub-carriers using the discrete Fourier transform, wherein the input corresponds to the time domain and the output to the frequency domain. The DFT  310  outputs the transmitted modulated symbols affected by the channel frequency response to a channel equalizer  312 . 
     The channel equalizer  312  receives the estimated channel frequency response and the transmitted modulated signals. The channel equalizer  312  performs frequency domain zero-forcing equalization of the OFDM sub-carriers. Only sub-carriers with magnitudes above a certain predetermined threshold value are equalized, since magnitudes below the predetermined threshold value are considered unreliable. The channel equalizer  312  outputs recovered modulated signals. The recovered modulated signals are converted from N-symbol parallel data streams (frames) into a serial stream in a P/S processor  314 . The serial stream is then inputted into a base band demodulator  316 . The base band demodulator  316  demodulates the recovered modulated signals and maps one input symbol into k binary symbols according to the base band signaling scheme. The base band demodulator outputs received binary data to a data sink  318  which applies application specific processing to the received data. 
     FIG. 4 shows a model of an OFDM communication system  400  with two transmit antennas and one receive antenna. This system has two separate propagation channels H 1  and H 2 . The goal of this embodiment of the present invention is to estimate the channel frequency response of both of these channels using the structure of the training information. A first transmitter  402  prepares information to be transmitted, for example in the manner set forth above with respect to FIG. 1, and the information is sent to a transmit filter  404  and then transmitted to the receiver  414  through the physical channel H 1  ( 406 ). During transmission, noise is unavoidably added to the transmitted signal. A second transmitter  408  prepares information to be transmitted and sends the information to a transmit filter  410 . The information is then transmitted to the receiver  414  through the physical channel H 2  ( 412 ). During transmission, noise is unavoidably added to the transmitted signal. When the signals are received at the receiver  414 , the signals are filtered in a receive filter  416  and are then processed in a processor  418 . One of the operations of the processor  418  is to estimate a channel frequency response of channels H 1  and H 2 . 
     FIG. 5 shows how the training information is transmitted by the two transmitters  402  and  408 . The first transmitter  402  transmits OFDM training symbols A 1  and A 2 , and the second transmitter  408  transmits OFDM training symbols B 1  and B 2 . The goal of the receiver is to separate the OFDM symbols so that all the information in A 1  and A 2  can be used to estimate the channel frequency response of channel H 1  and all of the information in B 1  and B 2  can be used to estimate the channel frequency response of channel H 2 . 
     The operation of one embodiment of the present invention will now be described with reference to FIG.  6 . As will be explained below in more detail, the transmitters  402  and  408  select the appropriate training symbols in step  602  and transmit the training symbols and data over physical channels H 1  and H 2 , respectively, in step  604 . The transmitted training symbols and data are then received at the receiver  414  in step  606 . The first received symbol R 1  at the receiver in frequency-domain during the transmission of the training symbols A 1  and B 1 , with additive noise N 1  is 
     
       
         R 1 =H 1 ·A 1 +H 2 ·B 1 +N 1   
       
     
     and the second received symbol R 2  during transmission of the training symbols A 2  and B 2 , with additive noise N 2  is 
     
       
         R 2 =H 1 ·A 2 +H 2 ·B 2 +N 2   
       
     
     To achieve noise reduction, the signals R 1  and R 2  are added together in step  608   
     
       
           R =R 1 +R 2 =H 1 ·A 1 +H 2 ·B 1 +H 1 ·A 2 +H 2 ·B 2 +N 1 +N 2   
       
     
     After reordering the terms 
     
       
           R =H 1 ·(A 1 +A 2 )+H 2 ·(B 1 +B 2 )+N 1 +N 2 . 
       
     
     To estimate H 1  it is necessary to remove the effects of H 2  on the received signal R and vice versa. As a result, B 1 +B 2  should be equal to zero while preserving A 1 +A 2 , and vice versa. One solution according to one embodiment of the present invention is to select A 1 , A 2 , B 1  and B 2  as follows: 
     A 1  is a set of complex numbers, one number for each subcarrier 
     
       
         A 2 =A 1   
       
     
     
       
         B 1 =A 1   
       
     
     
       
         B 2 =A 1   
       
     
     and 
     
       
         |A 1 | 2 =1 
       
     
     In this case, the sum of R 1  and R 2  is 
     
       
           R =R 1 +R 2 =H 1 ·A 1 +H 2 ·A 1 +H 1 ·A 1 −H 2 ·A 1 +N 1 +N 2 =2·H 1 ·A 1 +H 2 ·(A 1 −A 1 )+N 1 +N 2 =2·H 1 ·A 1 +N 1 +N 2   
       
     
     In step  610 , the channel frequency response A 1  can now be estimated by multiplying R by            H   ^        1     =             A1   *     ·   R     +   N1   +   N2     2     =         H1   ·          A1        2       +       N1   +   N2     2       =     H1   +       N1   +   N2     2                                  
     A 1  conjugate and dividing by 2. Since noise is independent, its power is reduced by 2. 
     Similarly, the channel frequency response Ĥ ; 2  of channel Ĥ ; 2  can be estimated by subtracting R 1  and R 2 . 
       R =R 1 −R 2 +N 1 +N 2 =H 1 ·A 1 +H 2 ·A 1 −H 1 ·A 1 +H 2 ·A 1 +N 1 −N 2 =H 1 ·(A 1 −A 2 )+2H 2 ·A 1 +N 1 −N 2 +2·H 2 ·A 1 +N 1 −N 2   
     Now, the channel frequency response Ĥ ; 2  can be estimated by multiplying R by A 1  conjugate and            H   ^        2     =           A1   *     ·   R     2     =         H2   ·          A1        2       +       N1   -   N2     2       =     H2   +       N1   -   N2     2                       H   ^        2     =           A1   *     ·   R     2     =         H2   ·          A1        2       +       N1   -   N2     2       =     H2   +       N1   -   N2     2                                  
     dividing by 2. 
     One drawback with this solution is, if H 1 =H 2 , as in the additive White Gaussian Noise (AWGN) channel, the received signal during A 2  and B 2  is equal to A 1 −A 1 =0, so nothing is received. To remove this effect, the symbol pairs (A 1 ,B 1 ) and (A 2 ,B 2 ) should be orthogonal. In this case, they will not cancel each other, if the channels H 1  and H 2  happen to be equal. 
     According to another embodiment of the present invention, the following selection of the symbols A 1 , A 2 , B 1 , B 2  has all of the required properties to avoid the problems in the additive White Gaussian Noise channel. Symbol pairs (A 1 , B 1 ) and (A 2 , B 2 ) have a 90° phase-shift, so they are orthogonal and will not cancel each other out in an Additive Gaussian Noise channel. Also, the channel estimation can be performed using both A 1  and A 2  for H 1 , and B 1  and B 2  for H 2 . 
     A 1  is a set of complex numbers, one number for each subcarrier 
     
       
         A 2 =A 1   
       
     
     
       
         B 1 =e jπ/2 A 1   
       
     
     
       
         B 2 =e −jπ/2 A 1   
       
     
     and 
     
       
         |A 1 | 2 =1 
       
     
     With these training symbols, the channel estimation can be performed in the following manner for H 1 : 
     
       
           R=R 1 +R 2 +N 1 +N 2 =H 1 ·A 1 +H 2 ·e   jπ/2 ·A 1 +H 1 ·A 1 +H 2 ·e −jπ/2 ·A 1 +N 1 +N=2·H 1 ·A 1 +H 2 ·A 1 (e jπ/2 +e −jπ/2 )+N 1 +N 2 =2·H 1 ·A 1 +N 1 +N 2   
       
     
     Now, the channel frequency response Ĥ ; 1  can be estimated by multiplying R by A 1  conjugate and            H   ^        1     =           A1   *     ·   R     2     =         H1   ·          A1        2       +       N1   +   N2     2       =     H1   +       N1   +   N2     2                                  
     dividing by two: 
     The estimation of the channel frequency response of channel Ĥ ; 2  can then be performed as follows: 
     
       
           R=e   −jπ/2 ·R 1 +e jπ/2 ·R 2 =H 1 ·e −jπ/2 ·A 1 +H 2 ·A 1 +H 1 ·e jπ/2 ·A 1 +H 2 ·A 1 +N 1  +N 2 =H 1 ·A 1 ·(e −jπ/2 )+2·H 2 ·A 1 +N 1 +N 2 =2·H 2 ·A 1 +N 1 +N 2   
       
     
     Now, the channel frequency response of Ĥ ; 2  can be estimated with the following equation:            H   ^        2     =             A1   *     ·   R     +   N1   +   N2     2     =         H2   ·          A1        2       +       N1   +   N2     2       =     H2   +       N1   +   N2     2                                  
     Although preferred embodiments of the method and apparatus of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it is understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications, and substitutions without departing from the spirit or scope of the invention as set forth and defined by the following claims.