Abstract:
Channel estimation for high mobility OFDM channels is achieved by identifying a set of channel path delays from an OFDM symbol stream including carrier data, inter-channel interference noise and channel noise; determining the average channel impulse response for the identified set of channel path delays in each symbol; storing the average channel impulse responses for the identified channel path delays; generating a path delay curvature for each channel path delay in each symbol based on stored average channel impulse responses for the identified channel path delays; estimating the carrier data in the symbols in the OFDM symbol stream in the presence of inter-channel interference noise and channel noise from the OFDM symbol stream and the average impulse responses for the identified channel path delays; reconstructing the inter-channel interference noise in response to the path delay curvature, the identified set of channel path delays and estimated carrier data; and subtracting the reconstructed inter-channel interference noise from the OFDM symbol stream to produce a symbol stream of carrier data and channel noise with suppressed inter-channel interference noise.

Description:
This application claims benefit of and priority to U.S. Provisional Application Ser. No. 60/852,607 filed Oct. 18, 2006 incorporated herein by this reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to an improved channel estimator system and method for high mobility OFDM channels. 
     BACKGROUND OF THE INVENTION 
     Binary phase shift keying (BPSK) is a conventional data modulation scheme that conveys data by changing, the phase of a reference carrier signal, for example, during each BPSK symbol period carrier data in the form of either a positive or negative sine wave is transmitted. A positive sine wave represents a data “1”, a negative sine wave a data “0”. When the symbol stream arrives at the receiver it is decoded by multiplying with a positive sine wave. The multiplying of it by another positive sine wave produces a average positive level; if the symbol period contains a negative sine wave the multiplexing by a positive sine wave produces an average negative level. Orthogonal Frequency Division Multiplexing (OFDM) employs the same idea but instead of one carrier wave per bit, the bit stream to be transmitted is split into several parallel low-rate bit streams, two, ten or any number; presently over 8 k (8192). Each low-rate bit stream is transmitted over one sub-channel by modulating a sub-carrier using a standard modulation scheme, for example BPSK. The sub-carrier frequencies are chosen so that the modulated data streams are orthogonal to each other. The demodulation at the receiver is done in the same way with the symbol period sine waves being multiplied selectively by a positive sine wave of each of the frequencies transmitted. By virtue of orthogonality it is possible to distinguish between the various carrier sine waves. OFDM is thus a much higher density data encoding technique. OFDM has shortcomings but works well especially where the transmitter and received are fixed or not moving fast with respect to each other and so the transmitter channel between them remains constant or fairly constant. That is, the amplitude and phase of the various sine waves transmitted over that channel within a symbol period do not vary significantly over the symbol period. However in high mobility situations where the channel does change over the time of a symbol period, e.g. video streaming to a receiver on a moving vehicle or train, different sine waves can experience different channel paths resulting in variations in their phase and/or amplitude. Such variations referred to as inter-carrier or inter-channel interference (ICI) noise interferes with the orthogonality of the sine waves and can cause errors in the data decoding causing “1”s to appear to be “0”s and “0”s to appear as “1”s. This ICI noise accompanies but is different then the conventional channel noise that accompanies the carrier data. 
     BRIEF SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide an improved estimator system and method for high mobility OFDM channels. 
     It is a further object of this invention to provide such an improved estimator system and method which makes efficient use of memory and power. 
     It is a further object of this invention to provide such an improved estimator system and method which is power adaptive to channel conditions. 
     The invention results from the realization that a channel estimation for high mobility OFDM channels can be achieved by identifying a set of channel path delays from an OFDM symbol stream including carrier data, inter-channel interference noise and channel noise; determining the average channel impulse response for the identified set of channel path delays in each symbol; storing the average channel impulse responses for the identified channel path delays; generating a path delay curvature for each channel path delay in each symbol from the stored average channel impulse responses for the identified channel path delays; estimating the carrier data in the symbols in the OFDM symbol stream in the presence of inter-channel interference noise from the OFDM symbol stream and said average impulse responses for the identified channel path delays; reconstructing the inter-channel interference noise in response to the identified set of channel path delays and estimated carrier data; and subtracting the reconstructed inter-channel interference noise from the OFDM symbol stream to produce a symbol stream of carrier data and channel noise with suppressed inter-channel interference noise. 
     The subject invention, however, in other embodiments, need not achieve all these objectives and the claims hereof should not be limited to structures or methods capable of achieving these objectives. 
     This invention features a channel estimator system for high mobility OFDM channels including a path delay estimator circuit, responsive to an OFDM symbol stream including carrier data, inter-channel interference noise and channel noise for identifying a set of channel path delays and an average channel estimator circuit, responsive to the OFDM symbol stream and the identified set of channel path delays, for determining the average channel impulse response for the identified set of channel path delays in each symbol. A storage circuit stores the average channel impulse responses for the identified channel path delays and a curve generator circuit, responsive to the stored average channel impulse responses, generates a path delay curvature for each channel path delay in each symbol. There is a carrier data estimator circuit, responsive to the OFDM symbol stream and the average impulse responses from the average channel estimator circuit, for estimating the carrier data in the symbols in the OFDM symbol stream in the presence of inter-channel interference and channel noise and a regenerator circuit, responsive to the curve generator, and path delay estimator circuit and carrier data estimation circuit, for reconstructing the inter-channel interference noise. A subtraction circuit subtracts the reconstructed inter-channel interference noise from the OFDM symbol stream resulting in a symbol stream of carrier data and channel noise with suppressed inter-channel interference noise. 
     In a preferred embodiment the path delay estimator circuit may identify a set of channel path delays which are above a predetermined energy threshold. The regenerator circuit may include a local OFDM symbol generator, responsive to the estimated carrier data from the carrier estimator circuit to generate, locally, OFDM symbol replicas. The regenerator circuit may include an ICI distortion generator for shifting an OFDM symbol replica by each associated channel path delay, multiplying it by the associated path delay curvature and summing the shifted, multiplied symbol replicas to produce local inter-channel interference noise. The curve generator circuit may include a selection circuit for selecting from the storage circuit the average channel gains of neighboring OFDM symbols. The curve generator circuit may include a rate determining circuit for determining the rate of change of the neighboring average channel gains. The curve generator circuit may include a model selection circuit for identifying a best fit average curve for the stored channel impulse responses. The carrier data estimator circuit may include an FFT circuit for performing FFT on a received OFDM symbol. The carrier data estimator circuit may include a vector generating circuit for creating a vector with zeros and inserting average path gains in associated delay locations. The carrier data estimator circuit may include a second FFT circuit for performing FFT on the vector. The carrier data estimator circuit may include an averaging circuit for calculating noise level. The carrier data estimator circuit may include an equalization circuit for calculating equalization coefficients in response to the second FFT circuit and the averaging circuit and applying them to the associated symbol. The carrier data estimator circuit may include a slicer circuit for matching the equalized symbols to a predefined grid of levels. The path delay estimator circuit may include an FFT circuit for performing FFT on an OFDM symbol. The path delay estimator circuit may include a normalizing circuit for extracting the channel frequency response for known carriers and inserting zeros for unknown carriers. The path delay estimator circuit may include an IFFT for performing IFFT on the channel frequency response. The path delay estimator circuit may include a noise estimator circuit for determining the channel noise level. The path delay estimator circuit may include a threshold setting circuit for setting a threshold for groups of channel path delays in accordance with their energy levels. The path delay estimator circuit may include a threshold circuit for selecting channel path delays meeting a predetermined threshold. The average channel estimator circuit may include an FFT circuit for performing an FFT on an OFDM symbol. The average channel estimator circuit may include a normalizing circuit for extracting the channel frequency response for known carriers. The average channel estimator circuit may include an estimator circuit for determining average path gains based on least squares and known noise. 
     This invention also features a channel estimation method for high mobility OFDM channels including identifying a set of channel path delays from an OFDM symbol stream including carrier data, inter-channel interference noise and channel noise; determining the average channel impulse response for the identified set of channel path delays in each symbol; storing the average channel impulse responses for the identified channel path delays; generating a path delay curvature for each channel path delay in each symbol based on stored average channel impulse responses for the identified channel path delays; estimating the carrier data in the symbols in the OFDM symbol stream in the presence of inter-channel interference noise and channel noise from the OFDM symbol steam and the average impulse responses for the identified channel path delays; reconstructing the inter-channel interference noise in response to the path delay curvature, the identified set of channel path delays and estimated carrier data; and subtracting the reconstructed inter-channel interference noise from the OFDM symbol stream to produce a symbol stream of carrier data and channel noise with suppressed inter-channel interference noise. 
     In a preferred embodiment the identified set of channel path delays may be above a predetermined level of energy. Reconstructing the inter-channel interference noise may include generating, locally, OFDM symbol replicas from the estimated carrier data. Reconstructing the inter-channel interference noise may further include shifting an OFDM symbol replica by each associated channel path delay, multiplying it by the associated path delay curvature and summing the shifted, multiplied symbol replicas to produce local inter-channel interface noise. Generating a path delay curvature may include averaging the channel gains of neighboring OFDM symbols. Generating a path delay curvature may further include determining the rate of change of the neighboring average channel gains. Generating a path delay curvature may further include identifying a best fit average free curve for the stored channel impulse responses. Estimating the carrier data may include performing FFT on a received OFDM symbol. Estimating the carrier data may further include creating a vector with zeros and inserting average path gains in associated delay locations. Estimating the carrier data may further include performing FFT on the vector. Estimating the carrier data may further include calculating noise level. Estimating the carrier data may further include calculating equalization coefficients in response to the second FFT and applying them to the associated symbol. Estimating the carrier data may further include matching the equalized symbols to a predefined grid of levels. Identifying a set of path delays may include performing FFT on an OFDM symbol. Identifying a set of path delays may further include extracting the channel frequency response for known carriers and inserting zeros for unknown carriers. Identifying a set of path delays may further include performing IFFT on the channel frequency response. Identifying a set of path delays may further include determining the channel noise level. Identifying a set of path delays may further include setting a threshold for groups of channel path delays in accordance with their energy levels. Identifying a set of path delays may further include selecting channel path delays meeting a predetermined threshold. Determining the impulse response may include performing an FFT on an OFDM symbol. Determining the impulse response may further include a extracting the channel frequency response for known carriers. Determining the impulse response may further include determining average path gains based on least squares and known noise. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which: 
         FIG. 1  is a schematic, time domain, representation of two OFDM symbols; 
         FIG. 2  is a schematic, frequency domain, representation of the OFDM symbols of  FIG. 1 ; 
         FIG. 3  is a schematic diagram showing an example of multiple paths occurring in a channel between a transmitter and receiver; 
         FIG. 3A  is a graphical illustration of the gain and delay associated with each path in  FIG. 3 ; 
         FIG. 4  is a schematic block diagram of one embodiment of a channel estimator system according to this invention; 
         FIG. 5  is a diagram of a flow chart of the path delay estimator circuit of  FIG. 4 ; 
         FIG. 5A  is a graphical illustration of the forcing of zeros in the unknown data carriers, referred to in  FIG. 5 ; 
         FIG. 5B  is a graphical illustration of the windowing and thresholding of the channel impulse responses, referred to in  FIG. 5 ; 
         FIG. 6  is a diagram of a flow chart of the average channel estimator circuit of FIG.  4 ; 
         FIG. 7  is a diagram of a flow chart of the carrier data estimator circuit of  FIG. 4 ; 
         FIG. 7A  is a graphical illustration of the insertion of average path gains and zeros for unknown carriers in an N size vector, referred to in  FIG. 5 ; 
         FIG. 7B  is a graphical illustration of the slicing of equalized data to set thresholds, referred to in  FIG. 7 ; 
         FIG. 8  is a diagram of a flow chart of the curve generator estimator circuit of  FIG. 4 ; 
         FIG. 8A  is a graphical illustration of curve modeling and filtering operation, referred to in  FIG. 8 ; 
         FIG. 8B  is a graphical illustration of a high-order curve modeling and filtering operation, referred to in  FIG. 8 ; 
         FIG. 9  is a diagram of a flow chart of the regenerator ICI circuit of  FIG. 4 ; 
         FIG. 9A  a graphical illustration of the building of an N size vector and insertion of carrier data estimation, pilots and zeros, referred to in  FIG. 9 ; and 
         FIG. 9B  is a graphical illustration of the distortion or adjusting of an OFDM symbol according to the associated delay and gain referred to in  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer. 
     There is shown in  FIG. 1  an OFDM symbol stream  10  including two symbols  12  and  14  each of which includes a cyclical prefix section  16  and carrier data section  18 . Each carrier data section  18 ,  FIG. 2 , includes a plurality of carrier data a 0 , a 1 , a 2 , a 3- -a n-1 , a n  where the filled circles represent pilot carrier data whose amplitude and phase are known and the empty circles represent unknown carrier data. The OFDM symbol stream is typically propagated along a channel from a transmitter  20 ,  FIG. 3 , to a receiver  22 . Because of reflection from objects  24  in the area the channel may have multiple paths, the most direct path  28  with a phase of m 0  and additional paths  30 ,  32 , and  34  having phases of m 1 , m 2 , m 3 , respectively. Each path has its own gain or attenuation as shown in  FIG. 3A , where each path has associated it with it a gain or amplitude h 0 , h 1 , h 2 , h 3 , and an associated phase shift m 0 , m 1 , m 2 , m 3 . If the transmitter  20  and  22  move relatively fast with respect to one another, inter-channel interference (ICI) noise develops due to the loss of orthogonality because the carrier data sine wave arrives at the receiver  22  along four paths with different phases and different amplitudes. This can result in inaccuracies in determining the nature of the data, possibly reading ones as zeros and zeros as ones. 
     In accordance with this invention the inter-channel interference (ICI) noise is suppressed by generating a replica ICI noise function and subtracting it from the signal in channel noise: thus where the incoming signal is represented by S+f(S)+n where S is the OFDM carrier data, f(S) is the ICI noise and N is the general channel noise this invention contemplates the generation of a replica ICI noise f′(S) and subtracting it from the incoming signal S+f(S)+n resulting in an output of simply S+n. 
     In one embodiment,  FIG. 4 , channel estimator  36  according to the invention includes a path delay estimator circuit  40  which responds to OFDM symbol stream  38  and estimates the path delays m 0 -m n ; the certain identified ones of the estimated path delays are delivered both to ICI regenerator circuit  42  and average channel estimator circuit  44 . Average channel estimator circuit  44  responds to the identified set of channel path delays from path delay estimator circuit  40  and the OFDM symbol stream on line  38  and determines the average channel impulse response  h   0 ,  h   1 , . . . ,  h   n  for the identified set of channel path delays in each symbol. Those average channel impulse responses for the identified channel path delays are stored in storage circuit  46  and then used by curve generator circuit  48  to generate a path delay curvature for each channel path delay in each symbol. Carrier data estimator circuit  50  also responds to the average impulse responses from the average channel estimator circuit and the OFDM symbol stream on input line  38  to locally estimate the carrier data (a 0 , a 1 , . . . an) in the OFDM symbol stream in the presence of inter-channel interference and channel noise. Regenerator ICI circuit  42  responds to the locally produced estimated carrier data from carrier data estimator circuit  50  and the path delay curvature for each channel path delay for curve generator circuit  48  and adjusts their phase in accordance with the path delay estimator circuit output  40  to reconstruct a replica ICI noise. This replica ICI noise on line  52  is then subtracted from the incoming OFDM symbol stream on line  38  in subtraction circuit  54  resulting in a symbol stream of carrier data and channel noise with suppressed inter-channel interference noise. 
     Channel estimator system  36  in one embodiment may be constructed using a programmable device such as a Digital Signal Processor (DSP) programmed to operate as indicated in  FIGS. 5-9 . 
     Path delay estimator circuit  40 .  FIG. 5 , first extracts the next OFDM symbol  60  and a Fast Fourier Transform (FFT)  62  is performed. The results are then normalized in a normalizing circuit using the known carriers. Thus, where, for example, a known carrier data a 0  is known and its frequency response H 0  can be determined, the carrier can be normalized by dividing a 0 H 0  by the known a 0  to obtain the channel frequency response H 0  alone  64 . Zero&#39;s are now forced in positions of all the unknown carriers  66  as shown graphically in  FIG. 5A ; the known or pilot carriers are shown as filled circles  70 ; the empty circles  72  represent the unknown carriers in which the zeros are forced, and the inverse fast Fourier transform (IFFT)  68  is performed. This is done for a number of iterations, K, over a number of symbols to obtain an average H 0  and successively an average H 1 , H 2 , H 3 . The noise level is then estimated in a noise estimator circuit  78  to determine the channel noise level. After the Kth iteration,  76 , the noise level  78  is estimated and then a window including a group of channel impulse responses are monitored to determine their energy level and accordingly a threshold is set for the particular group  80 . Then those channel impulse responses above the threshold level are identified and become the identified set of channel path delays  82 . This is shown more graphically in  FIG. 5B  where, for example, channel impulse responses  90 ,  92 ,  94  and  96  are viewed in window  98  to determine the energy level of that group of impulse responses  90 - 96 . Based on that energy level a first threshold level  100  is set. The noise level is shown at  102 . Anything above threshold  100  is then selected as the identified channel path delays and the delays m o , m 1 , m 2 , m 3  can be determined. In a second group  104 ,  106 ,  108 ,  110 , viewed through a second window  112 , a lower energy is detected resulting in a second lower threshold  114  being set. 
     Average channel estimator  44 ,  FIG. 6 , begins by extracting the OFDM symbol  120  and then performing FFT on it,  122 . The results are normalized by known carriers, step  124 , in the same way as previously, where the known carrier, a 0 , accompanied by the frequency response, H 0 , is normalized by being divided by a 0  to obtain the frequency response H 0 . The average path gains such as  90 - 96  shown in  FIG. 5B  are then estimated  126  using the Least Squares (LS) model and the known noise. Carrier data estimator circuit  50 ,  FIG. 7 , may be implemented by performing an FFT  130  on a received OFDM signal, then building a vector size N with zeros  132  and average path gains  134  inserted in the proper delay locations. This is shown in greater detail in  FIG. 7A  where the average path gains are shown at  138  and the unknown carriers which receive the zero insertions are shown at  140 . Following the insertion of the average path gains FFT is performed  136  to obtain the channel frequency response H 0 , H 1  . . . . The noise level is again calculated  138  using an averaging circuit based on H 0 , H 1 , H 2  . . . and the pilot carriers. After this the equalization coefficients 
               1     H   0       ,     1     H   1       ,     …   ⁢           ⁢     1     H   n               
are calculated using an equalization circuit and equalization is performed  140 . This can be done using the minimum mean square error (MMSE) method which is well known in the art. After this, slicing is performed  142  to match the equalized values to a predefined grid of level. For example, as shown in  FIG. 7B , there are a grid of levels +1, +2, +3, −1, −2, −3, and the equalized data  144  are assigned to thresholds consistent with their levels: equalized data  144   a  is assigned level three, while equalized data  144   b  is assigned level 1, equalized data  144   c  is assigned level −2.
 
     Curve generator circuit  48  may be implemented as shown in  FIG. 8 . Initially the average channel gains of the selected symbol P and neighboring symbols P+1, P+2. P−1, P−2 . . . are retrieved, selected using a selection or addressing circuit  170  from storage  46 . The curvature model is then determined using an FFT operation  172  and an estimation model is built  174  to estimate the tap function parameters. For example, if the best estimate is a line the model would be ax+b, if it were a parabola it would be ax 2 +bx+c, a third order curve it would be ax 3 +bx 2 +cx+d. After the estimation the system returns to inquire whether the last path delay in the set has been processed  176 . If it has the routine is finished. If not it returns to retrieve average channel gain symbols  170  from storage  46 . A selection circuit performs the retrieving of the average channel gains in  170  and the FFT operation  172  functions as a rate determining circuit for determining the rate of change of the neighboring average channel gains. Model selection is accomplished by building the estimation model  174 . The operation is shown graphically in  FIG. 8A  where the instant symbol P has average channel response ĥ 0  along with the neighboring symbols P+1, P+2, P+3 . . . P−1, P−2 . . . in order to obtain an indication of the best fit average free curve  180 . In this case a first order or straight line best fit is indicated. In  FIG. 8B , however, the curve  180   b  changes at a much higher rate and so it requires a higher order best fit average free curve, for example, a parabolic shape  182  whose average should be equal to the average channel response of the symbol P. The order of the best fit curve thus depends upon the rate of change of the average channel gain as determined by the FFT operation  172 . 
     Regenerator ICI circuit  42  may be implemented,  FIG. 9 , by building a vector size N with zeros  190  and then inserting carrier data a 0  estimation  192  and inserting the pilot data  194 . This is shown graphically in  FIG. 9A  where the inserted carrier data estimation and pilots are shown at  198  along with carrier data labeled a 0 -a n-3  and null carriers  200  indicated by zeros. After this  FIG. 9 , FFT is performed  202  and then ICI distortion is accomplished  204  and the results are summed  206 . The ICI distortion is accomplished by a local OFDM symbol replica generator  209  as shown in  FIG. 9B . OFDM symbol  210  represented as OFDM symbol sine wave  212  is multiplied by the ICI average free gain curve  214  associated path delay curvature. Each of the phases m 0  through m 3  is shifted. The shifted forms of OFDM symbol are multiplied  212  by each of the ICI average free gains h 0 , h 1 , h 2 , h 3 , represented as one curve at  214 . The multiplication occurs in multiplier  212  and each of the waves, phase shifted by their phase m 0 -m 3  is presented at  210   a ,  210   b ,  210   c ,  210   d , respectively. These are then summed  216  to generate the ICI replica  218 . 
     Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments. 
     In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant can not be expected to describe certain insubstantial substitutes for any claim element amended. 
     Other embodiments will occur to those skilled in the art and are within the following claims.