Abstract:
A method for providing maximum likelihood detection with decision feedback interference cancellation is provided. The method includes estimating a current symbol with previous symbol interference (PSI) removed based on estimated previous symbols. A next symbol is estimated with PSI removed based on the estimated current symbol and/or the estimated previous symbols. The current symbol is re-estimated with PSI removed based on the estimated previous symbols and next symbol interference (NSI) removed based on the estimated next symbol. This method of providing maximum likelihood detection with decision feedback interference cancellation may be used in direct sequence spread spectrum systems with relatively short block spreading, such as IEEE802.11b Wireless LAN standard, or in any other suitable systems.

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention relates generally to communication systems and, more particularly, to a method and system for providing maximum likelihood detection with decision feedback interference cancellation. 
   BACKGROUND OF THE INVENTION 
   A symbol, in this document, means a continuous time waveform with a fixed duration, called “symbol duration,” or a discrete time waveform with a fixed number of samples, called chips at the Nyquist sampling rate. It is assumed that there exists a finite set of symbols and that there exists a one-to-one and onto mapping from the transmitted information bits to the symbol set. 
   A received signal through a multi-path channel may be resolved into a number of replicas of the transmitted signal with different delays. The power-delay profile of a multi-path channel shows the number of replicas, their relative delays according to the earliest one, and their average powers. 
   An equivalent channel impulse response for a multi-path channel may be calculated accurately at the sampling rate in the receiver. In that case, the time interval between two channel response taps becomes a multiple of the reciprocal of the sampling rate. The tap corresponding to the highest amplitude is called the cursor. The taps prior to the cursor are called the pre-cursors and the taps after the cursor are called the post-cursors. 
   In general, the power-delay profile is supposed to be an exponentially decaying curve. That is, the pre-cursors disappear on average. However, the pre-cursor part can exist with a low probability. A pessimistic example is that the channel impulse response may be symmetric around the cursor and have a length of two symbol durations. The energy of the s th  symbol may then be spread over the previous symbol and the next symbol. Hence, the observation interval in the detection of the s th  symbol becomes the three-symbol interval comprising s−1, s, and s+1 in order to use all received energy belonging to the symbol s. This also implies the energies of the neighbor symbols are spread over the observation interval of a received symbol; hence, inter-symbol interference occurs in this situation. Hereinafter, the interference caused by previous symbols is called Previous Symbol Interference (PSI), and the interference caused by next symbols is called Next Symbol Interference (NSI). 
   Another case is the minimum phase channel impulse response, that is, the channel impulse response without the pre-cursor part. The s th  symbol in this situation is spread over the interval of the S+1 th  symbol. Therefore, the optimal observation time becomes the two-symbol interval comprising s and s+1. 
   A detection method proposed in the literature is based on Maximum Likelihood (ML) (See J. G. Proakis, Digital Communications, 3rd ed., New York: McGraw-Hill, 1995). ML detection is reduced to the minimum distance problem if the additive noise in the received samples is independent zero-mean Gaussian, which is widely accepted. 
   For example, the modulator output may be a continuous stream of symbols x(s), which are selected from a finite symbol set. Linear channel experiences frequency selective distortion onto the transmitted signal; hence, the symbol energies are spread over time. One solution in the sense of ML uses a matched filter and a Viterbi algorithm with a search depth proportional to the expected maximum root mean square multi-path spread. The number of states in the Viterbi algorithm is equal to the number of elements in the symbol set. Thus, the Viterbi algorithm can become complex if the number of elements in the symbol set is high. 
   An ML detection method under multi-path (frequency selective fading) is the rake receiver if the symbol set satisfies the following orthogonality properties: (i) two different symbols in the set are orthogonal to any delayed version of themselves; (ii) a symbol in the set is orthogonal to any non-zero delayed version of itself; (iii) the symbols in the symbol set have identical energy; and (iv) the symbol duration is long relative to multi-path spread. If these properties fail in some degree, then the rake receiver cannot completely cancel inter-symbol interference (ISI) and the correlator bank does not provide equivalent metrics for Maximum Likelihood detection due to inter-chip interference (ICI). Thus, the rake receiver cannot provide path diversity in an ideal manner. The imperfection of the orthogonality properties (i) and (ii) is explained as a self-noise (See J. G. Proakis, Digital Communications, 3rd ed., New York: McGraw-Hill, 1995). If all four of the orthogonality properties are satisfied, then the rake receiver can resolve the multi-path and provide the path diversity. However, if N is small, then the imperfection in the orthogonality properties is unavoidable. 
   U.S. Pat. No. 6,233,273 claims an improved rake receiver structure with an Embedded Decision feedback Equalizer (DFE) for direct sequence spread spectrum (DSSS) transmissions. A DFE is employed to remove inter-symbol interference with or without feed-forward taps located between the channel-matched filter and the correlator bank. The channel-matched filter and the feed-forward taps can be convolved to be a single filter. The term “canceling inter-chip interference” is used in the patent along with “canceling inter-symbol interference.” However, the cancellation of inter-chip interference conflicts with the path diversity achieved by the rake receiver for DSSS transmissions. Moreover, although the feedback taps can be designed to cancel inter-symbol interference only, this is not completely valid for the feed-forward taps. The filtering through feed-forward tries to cancel inter-chip interference, which again conflicts with the path diversity that is a goal of the design of rake receiver. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, a method and system for providing maximum likelihood detection with decision feedback interference cancellation are provided that substantially eliminate or reduce disadvantages and problems associated with conventional systems and methods. 
   According to one embodiment of the present invention, a method for providing maximum likelihood detection with decision feedback interference cancellation is provided. The method includes estimating a current symbol with previous symbol interference (PSI) removed based on estimated previous symbols. The next symbol is estimated with PSI removed based on the estimated current symbol and/or the estimated previous symbols. The current symbol is estimated with PSI removed based on the estimated previous symbols and next symbol interference (NSI) removed based on the estimated next symbol. 
   According to another embodiment of the present invention, a method for providing maximum likelihood detection with decision feedback interference cancellation is provided. The method includes receiving a filter output for a current symbol. The filter output for the current symbol is delayed for a symbol duration to generate a delayed filter output. PSI is removed from the delayed filter output to generate a delayed filter output with PSI removed. The current symbol is estimated based on the delayed filter output with PSI removed. A filter output for a next symbol is received. PSI is removed from the filter output for the next symbol to generate a filter output with PSI removed. The next symbol is estimated based on the filter output with PSI removed. NSI is removed from the delayed filter output with PSI removed to generate a delayed filter output with PSI and NSI removed. 
   According to yet another embodiment of the present invention, a system for providing maximum likelihood detection with decision feedback interference cancellation is provided that includes an estimated channel-matched filter, a delay block, first and second PSI removers, first, second and third symbol estimators, and an NSI remover. 
   The delay block is operable to receive a filter output for a current symbol and to delay the filter output for the current symbol for a symbol duration to generate a delayed filter output. The estimated channel-matched filter is coupled to the delay block. The estimated channel-matched filter is operable to receive an input signal, to generate the filter output based on the input signal, and to provide the filter output to the delay block. 
   The first PSI remover is coupled to the delay block. The first PSI remover is operable to receive the delayed filter output and to remove PSI from the delayed filter output to generate a delayed filter output with PSI removed. The first symbol estimator is coupled to the first PSI remover. The first symbol estimator is operable to receive the delayed filter output with PSI removed and to estimate the current symbol based on the delayed filter output with PSI removed. 
   The second PSI remover is coupled to the first symbol estimator. The second PSI remover is operable to receive a filter output for a next symbol, to receive the estimation of the current symbol from the first symbol estimator, and to remove PSI from the filter output for the next symbol to generate a filter output with PSI removed. The second symbol estimator is coupled to the second PSI remover. The second symbol estimator is operable to receive the filter output with PSI removed and to estimate the next symbol based on the filter output with PSI removed. 
   The NSI remover is coupled to the first PSI remover and to the second symbol estimator. The NSI remover is operable to receive the delayed filter output with PSI removed, to receive the estimation of the next symbol from the second symbol estimator, and to remove NSI from the delayed filter output with PSI removed to generate a delayed filter output with PSI and NSI removed. The third symbol estimator is coupled to the NSI remover. The third symbol estimator is operable to receive the delayed filter output with PSI and NSI removed and to estimate the current symbol based on the delayed filter output with PSI and NSI removed. 
   Technical advantages of one or more embodiments of the present invention include providing an improved method for providing maximum likelihood detection involving knowing the previous and next symbols. In a particular embodiment, because of symbol-by-symbol detection, the estimates of the previous symbols are known already at the detection of the current symbol. The estimation of the next symbol may be achieved in three steps with the help of a one-symbol delay: (i) pre-estimation of the current symbol without next symbol interference (NSI) cancellation; (ii) pre-estimation of the next symbol without NSI cancellation; and (iii) estimation of the current symbol with estimated previous symbol interference (PSI) and NSI cancellation. In this way, the technique may be implemented in a multi-path condition in which the most of the energy of a received symbol is spread over the previous neighbor symbol and the next neighbor symbol. 
   The resulting performance improvement depends on the distance properties of the symbols used in the transmission. If the average distance is not large, then the performance may degrade due to decision feedback interference cancellation mechanisms. However, this problem may be solved by designing the system according to the distance properties of the symbols used in the transmission and signal quality, or link quality, using these to determine whether or not to disable either NSI cancellation or both NSI and PSI cancellation. Hence, on average, the capacity of the transmission system can be increased. 
   Other technical advantages will be readily apparent to one skilled in the art from the following figures, description, and claims. 
   Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, wherein like reference numerals represent like parts, in which: 
       FIG. 1  is a block diagram illustrating a receiver operable to provide maximum likelihood detection in accordance with one embodiment of the present invention; 
       FIG. 2  is a block diagram illustrating the first previous symbol interference remover of  FIG. 1  in accordance with one embodiment of the present invention; 
       FIG. 3  is a block diagram illustrating the second previous symbol interference remover of  FIG. 1  in accordance with one embodiment of the present invention; 
       FIG. 4  is a block diagram illustrating the next symbol interference remover of  FIG. 1  in accordance with one embodiment of the present invention; 
       FIG. 5  is a block diagram illustrating the symbol estimator of  FIG. 1  in accordance with one embodiment of the present invention; and 
       FIG. 6  is a flow diagram illustrating a method for providing maximum likelihood detection in the receiver of  FIG. 1  in accordance with one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 through 6 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged communication system. 
     FIG. 1  is a block diagram illustrating a receiver  10  operable to provide maximum likelihood detection in accordance with one embodiment of the present invention. The receiver  10  comprises an estimated channel-matched filter  12 , a delay block  14 , previous symbol interference (PSI) removers  16  and  18 , a next symbol interference (NSI) remover  20 , and symbol estimators  22 ,  24  and  26 . 
   The estimated channel-matched filter  12  is operable to receive an input signal  30  that comprises a plurality of symbols transmitted over a wireless interface by a transmitter. Each symbol transmitted may be received at different times due to multi-path distortion, resulting in potential interference from other symbols transmitted both before and after any one particular symbol. For example, the transmitted symbols may be reflected off walls, ceilings, and the like while being transmitted within an indoor WLAN system, resulting in the symbols being received as the input signal  30  at different times corresponding to the different distances for the various paths between the transmitter and the receiver  10 . 
   The estimated channel-matched filter  12  is operable to combine the delayed versions of the input signal  30  which are multiplied by the corresponding taps of the estimated channel-matched filter and to generate a filter output  32  that substantially comprises each transmitted symbol spread symmetrically so that PSI and NSI after matched filtering are equivalent in length and amount. For purposes of discussion, each filter output  32  may be said to comprise the “current symbol” with potential interference from a “previous symbol” or symbols and/or from a “next symbol” or symbols. In addition, as used herein, “each” means every one of at least a subset of the identified items. 
   The delay block  14  is operable to receive the filter output  32  and to delay the filter output  32  for a symbol duration to the first PSI remover  16  as a delayed filter output  34 . 
   The first PSI remover  16  is operable to receive the delayed filter output  34  and to remove interference due to a previous symbol, or PSI, from the delayed filter output  34 . According to one embodiment, the first PSI remover  16  is operable to remove PSI from the delayed filter output  34  by using the previously estimated symbols at the output  48  of the receiver  10 . The first PSI remover  16  is also operable to generate a first PSI output signal  36  that comprises the delayed filter output  34  with PSI removed and to provide the first PSI output signal  36  to the first symbol estimator  22  and to the NSI remover  20 . 
   The first symbol estimator  22  is operable to receive the first PSI output signal  36  and to estimate the current symbol. However, the accuracy of the estimate of the current symbol by the first symbol estimator  22  may be limited because the first PSI output signal  36  may comprise interference due to a next symbol, or NSI, and/or additional interference due to the possibility of an incorrect estimation of the previous symbols. The first symbol estimator  22  is also operable to generate a first symbol estimator signal  38  that comprises the current symbol estimation for the second PSI remover  18 . 
   The second PSI remover  18  is operable to receive the filter output  32  and the first symbol estimator signal  38 . Because the filter output  32  is not delayed with respect to the second PSI remover  18 , the second PSI remover  18  is operable to receive the filter output  32  corresponding to the next symbol while also receiving the first symbol estimator signal  38  corresponding to the current symbol. Thus, the second PSI remover  18  is operable to remove PSI from the filter output  32  comprising the next symbol by using the estimated symbols provided by the first symbol estimator  22  through the first symbol estimator signal  38 . The second PSI remover  18  is also operable to generate a second PSI output signal  40  that comprises the filter output signal  32  comprising the next symbol with PSI removed and to provide the second PSI output signal  40  to the second symbol estimator  24 . 
   According to one embodiment, the second PSI remover  18  is also operable to receive an additional signal  42  from the first PSI remover  16 . For this embodiment, the second PSI remover  18  is operable to remove PSI from the filter output  32  comprising the next symbol by using an estimate of the previous symbol obtained through the receiver output  48 , as well as the estimate of the current symbol obtained through the first symbol estimator signal  38 . 
   The second symbol estimator  24  is operable to receive the second PSI output signal  40  and to estimate the next symbol. The second symbol estimator  24  is also operable to generate a second symbol estimator signal  44  that comprises the next symbol estimation for the NSI remover  20 . The estimation of the next symbol may be limited by remaining interference due to subsequent symbols and/or the possibility of an incorrect estimation of the current symbol and previous symbols, which can result in additional interference. 
   The NSI remover  20  is operable to receive the first PSI output signal  36  and the second symbol estimator signal  44 . The NSI remover  20  is operable to remove NSI from the first PSI output signal  36  by using the estimate of the next symbol provided by the second symbol estimator  24  through the second symbol estimator signal  44 . 
   The NSI remover  20  is also operable to generate an NSI output signal  46  that comprises the first PSI output signal  36  comprising the current symbol with both PSI and NSI removed and to provide the NSI output signal  46  to the third symbol estimator  26 . It will be understood that the NSI output signal  46  may comprise interference due to symbols prior to the previous symbols and subsequent to the next symbol; however, this interference may be ignored due to the decaying characteristics of the channel impulse response. Moreover, because of the nature of the decision feedback interference cancellation structure, additional interference can exist due to incorrect estimations of the previous symbol and/or the next symbol. In addition, the accuracy of the channel estimation in the receiver  10  may also affect the performance. 
   The third symbol estimator  26  is operable to receive the NSI output signal  46  and to estimate the current symbol. The third symbol estimator  26  is also operable to generate a third symbol estimator signal, or receiver output,  48  that comprises the current symbol estimation for the receiver  10 . In addition, the third symbol estimator signal  48  may be used as a previous symbol estimation by the first PSI remover  16  when removing PSI from a subsequent symbol. It will be understood that any two or all three of the symbol estimators  22 ,  24  and  26  may be implemented with a single symbol estimator that may be used in a time-sharing manner to perform the functions of the corresponding symbol estimators  22 ,  24  and/or  26 . 
     FIG. 2  is a block diagram illustrating the first previous symbol interference (PSI) remover  16  in accordance with one embodiment of the present invention. The first PSI remover  16  comprises a symbol regenerator  100 , a PSI-I filter  102 , and a differential combiner  104 . As described above in connection with  FIG. 1 , the first PSI remover  16  is operable to receive the delayed filter output  34 , in addition to the third symbol estimator signal  48 , and to generate a first PSI output signal  36  based on the delayed filter output  34  and the third symbol estimator signal  48 . 
   The symbol regenerator  100  is operable to re-generate the previous symbols based on the third symbol estimator signal  48  with substantially no distortion or additive noise and to provide the previous symbols to the second PSI remover  18  and to the PSI-I filter  102  as the additional signal  42 . The PSI-I filter  102  comprises a finite impulse response filter. The PSI-I filter  102  is operable to generate a filtered signal  108  for the differential combiner  104 . The differential combiner  104  is operable to subtract the filtered signal  108 , which is the estimated PSI for the current symbol, from the delayed filter output  34  in order to generate the first PSI output signal  36 . 
     FIG. 3  is a block diagram illustrating the second previous symbol interference (PSI) remover  18  in accordance with one embodiment of the present invention. The second PSI remover  18  comprises a symbol regenerator  110 , a PSI-II filter  112 , and a differential combiner  114 . As described above in connection with  FIG. 1 , the second PSI remover  18  is operable to receive the filter output  32  and the first symbol estimator signal  38  and to generate a second PSI output signal  40  based on the filter output  32  and the first symbol estimator signal  38 . 
   The symbol regenerator  110  is operable to re-generate the current symbol based on the first symbol estimator signal  38  with substantially no distortion or additive noise and to provide that current symbol to the PSI-II filter  112  as a symbol regenerator signal  116 . The PSI-II filter  112  comprises a finite impulse response filter. The PSI-II filter  112  is operable to receive the symbol regenerator signal  116  from the symbol regenerator  110  and to generate a filtered signal  118  for the differential combiner  114 . The differential combiner  114  is operable to subtract the filtered signal  118 , which is the estimated PSI for the next symbol, from the filter output  32  in order to generate the second PSI output signal  40 . In order to improve accuracy, the PSI-II filter taps corresponding to the previous symbols can be updated according to the additional signal  42  from the first PSI remover  16 . 
     FIG. 4  is a block diagram illustrating the next symbol interference (NSI) remover  20  in accordance with one embodiment of the present invention. The NSI remover  20  comprises a symbol regenerator  120 , an NSI filter  122 , and a differential combiner  124 . As described above in connection with  FIG. 1 , the NSI remover  20  is operable to receive the first PSI output signal  36  and the second symbol estimator signal  44  and to generate the NSI output signal  46  based on the first PSI output signal  36  and the second symbol estimator signal  44 . 
   The symbol regenerator  120  is operable to generate the next symbol based on the second symbol estimator signal  44  with substantially no distortion or additive noise and to provide that next symbol to the NSI filter  122  as a symbol regenerator signal  126 . The NSI filter  122  comprises a finite impulse response filter. The NSI filter  122  is operable to receive the symbol regenerator signal  126  from the symbol regenerator  120  and to generate a filtered signal  128  for the differential combiner  124 . The differential combiner  124  is operable to subtract the filtered signal  128  from the first PSI output signal  36 , which is the estimated PSI for the next symbol, in order to generate the NSI output signal  46 . 
     FIG. 5  is a block diagram illustrating the symbol estimator  22 ,  24  and/or  26  in accordance with one embodiment of the present invention. The symbol estimator  22 ,  24 ,  26  is operable to receive an input signal, such as the first PSI output signal  36 , the second PSI output signal  40 , or the NSI output signal  46 , and to generate an output signal, such as the first symbol estimator signal  38 , the second symbol estimator signal  44 , or the third symbol estimator signal  48 , based on the input signal. 
   The symbol estimator  22 ,  24 ,  26  comprises a plurality of correlators  150 , a plurality of differential combiners  152 , and a peak detector  154 . Each correlator  150  is operable to receive the input signal, which comprises an incoming symbol, and to correlate the incoming symbol with a different symbol in the symbol set, such as x 1 , x 2 , . . . , x M , in order to generate a correlator output  156 . According to one embodiment, the correlators  150  may be implemented using a Fast Walsh Transform, as in the IEEE802.11b WLAN standard. However, it will be understood that the correlators  150  may be implemented in any suitable manner without departing from the scope of the present invention. 
   Each differential combiner  152  is operable to receive one of the correlator outputs  156  and to receive an offset correction  158  and to subtract the offset correction  158  from the correlator output  156  in order to generate a peak detector input  160 . It will be understood that, if the symbol set is ideally orthogonal, the symbol estimator  22 ,  24 ,  26  may be implemented without the differential combiners  152  and offset corrections  158 . In this embodiment, the correlator outputs  156  may be directly applied to the peak detector  154 . Indeed, these offset corrections  158  come from minimum distance detection while no inter-symbol interference but frequency selective fading exists (as in the case where the symbols are transmitted through a multipath channel with sufficiently spaced intervals to prevent inter-symbol interference). 
   The peak detector  154  is operable to receive the peak detector inputs  160  and to detect the largest input from the set of peak detector inputs  160  in order to generate the output signal for the symbol estimator  22 ,  24 ,  26 . The output signal comprises the incoming symbol with the maximum likelihood of transmission, assuming that the additive noise is zero-mean and Gaussian. 
   The performance of this invention depends on the signal-to-noise ratio, the distance properties of the symbol set, and the multipath spread. According to an appropriate link quality measure, the NSI remover  20 , or both the PSI and NSI removers  16 ,  18  and  20  can be disabled or enabled adaptively. Error propagation can be expected. However, if the distance properties of the symbols are sufficiently large, then the error propagation can be ignored. 
     FIG. 6  is a flow diagram illustrating a method for providing maximum likelihood detection in the receiver  10  in accordance with one embodiment of the present invention. The method begins at step  200  where the estimated channel-matched filter  12  receives the input signal  30 . At step  202 , the estimated channel-matched filter  12  generates the filter output  32  based on the input signal  30 . 
   At step  204 , the delay block  14  delays the filter output  32  for a symbol duration before providing the delayed filter output  34  to the first PSI remover  16 . At step  206 , the first PSI remover  16  removes the PSI from the delayed filter output  34 . At step  208 , the first symbol estimator  22  estimates the current symbol based on the delayed filter output  34  with the PSI removed by the first PSI remover  16 . 
   At step  210 , the second PSI remover  18  receives the filter output  32  for the next symbol from the estimated channel-matched filter  12 , in addition to the current symbol estimation from the first symbol estimator  22 . At step  212 , the second PSI remover  18  removes the PSI from the filter output  32  corresponding to the next symbol. At step  214 , the second symbol estimator  24  estimates the next symbol based on the filter output  32  corresponding to the next symbol with the PSI removed by the second PSI remover  18 . 
   At step  216 , the NSI remover  20  removes the NSI from the delayed filter output  34  with the PSI removed. At step  218 , the third symbol estimator  26  estimates the current symbol based on the delayed filter output  34  with the PSI and the NSI removed, at which point the method comes to an end. 
   Although the present invention has been described with several embodiments, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.