Patent Publication Number: US-7587191-B2

Title: High-quality detection based on sequential interference cancellation techniques

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to multi-user detection for communication systems. More particularly, the invention relates to a successive interference cancellation multi-user detector for wireless communication systems. 
   BACKGROUND OF THE INVENTION 
   In a multiple access wireless communications network, a plurality of users/transmitters communicate simultaneously with a single receiver in a given wireless spectrum. Given the shared nature of the wireless spectrum, however, signals transmitted by different users may collide at the receiver causing a loss of transmitted information and of network resources. This problem is known as multiple access interference (MAI) and is a main factor that limits the capacity and performance of a multiple access network. 
   In some cases, to remedy the MAI problem, the available spectrum in a network is multiplexed in time and/or frequency among available users/transmitters in the network. Each user may then receive a dedicated access time and/or frequency multiplexed channel to communicate with the receiver. Signals transmitted by different users in the network are then said to be orthogonal (in time and/or frequency), and, as a result, cause no interference to each other. 
   While significantly reducing potential MAI in a network, time and/or frequency multiplexing, on the other hand, often result in a far from optimal usage of the capacity of a network. For example, in a time division multiple access (TDMA) system, unless each user always has information ready to transmit during its allocated time slot, time opportunities to communicate with the receiver will be wasted resulting in a degraded throughput performance of the system. 
   In recent years, spread-spectrum-based code division multiple access (CDMA) has taken a greater role in multiple access networks. By using a unique spreading code for each user, CDMA eliminates the need for orthogonality in time and/or frequency among signals in a network. Typically, the unique spreading codes ensure low signal cross-correlation over a wideband, and allow for CDMA signals to be successfully decoded at a receiver in the presence of permissible interference. 
   Nonetheless, a low signal cross-correlation, as required by CDMA, is very challenging to maintain in a wireless spectrum due to unpredictable channel conditions. In fact, the random time offsets between signals that occur in a wireless spectrum make it difficult to ensure that CDMA signals are completely orthogonal. This may result in reduction of network capacity and throughput. Furthermore, since CDMA signals are typically spread over a very wide bandwidth, possibly the entire network&#39;s spectrum, transmit power level considerations impose further limitations on the capacity of CDMA systems. 
   In the last few years, however, multi-user detection (MUD) has received considerable attention as an area of research holding the key to improving the capacity and to alleviating some technical requirements of CDMA systems. Many algorithms for performing multi-user detection have been put forth. These range from the high-complexity optimum detectors to many forms of sub-optimum lower complexity detectors. 
   While a good number of the current solutions have shown to be too complex for actual implementation, a common limitation of these solutions is that they rely on spread-spectrum modulation to keep the cross-correlation between signals low. Consequently, this limits the applicability of these techniques to spread spectrum systems such as CDMA. 
   What is needed therefore is a multi-user detection technique having low implementation complexity and that supports detection for highly correlated signals. In other words, a multi-user detector is needed for both spread spectrum and non-spread spectrum systems that enables multiple access communication in a code, time, or frequency-multiplexed system. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention relates to a system and method for successive interference cancellation in a multi-user wireless communication system. 
   In one aspect of the invention, a method of decoding signals from a plurality of superimposed signals is provided. In an embodiment, the plurality of superimposed signals are ranked in order of received signal strength. The strongest signal is then decoded and a signal estimate thereof generated and subtracted from the plurality of superimposed signals. The method repeats iteratively, decoding and canceling the strongest signal, until all signals are decoded. 
   In another aspect of the invention, a system for decoding signals from a plurality of superimposed signals is provided. In an embodiment, the system is a wireless multi-user receiver. 
   Further embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
     The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. 
       FIG. 1  is a block diagram that illustrates an example of a typical multiple access communication model. 
       FIG. 2  is a block diagram that illustrates a multi-user receiver chain according to an embodiment of the present invention. 
       FIG. 3  is a block diagram that illustrates a multi-user detector according to an embodiment of the present invention. 
       FIG. 4  is a block diagram that illustrates an iterative interference cancellation decoder according to an embodiment of the present invention. 
       FIG. 5  is a block diagram that illustrates a first stage of the iterative cancellation decoder of  FIG. 4 . 
       FIG. 6  is a flowchart that illustrates an iterative interference cancellation technique according to the present invention. 
   

   The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Typical Multiple Access Communication Model 
     FIG. 1  is a block diagram that illustrates an example of a typical multiple access communication model  100 . In the example of  FIG. 1 , K users/transmitters are simultaneously accessing the same communication channel in order to communicate with a single receiver  120 . The corresponding bit transmitted in bit period i, for a user k, is represented by b k (i). In an embodiment, b k (i) belongs to the set {+1, −1}. 
   Waveforms  102 , represented by s 1 (t), . . . , s K (t), are used to modulate corresponding bits b 1 (i), . . . , b K (i). As can be understood by a person skilled in the relevant art, various types of modulating waveforms can be used at the transmitters to modulate bits b 1 (i), . . . , b K (i). For example, in a Frequency Division Multiple Access (FDMA) scheme, the modulating waveforms are the carrier frequencies with appropriate pulse shaping. In a Code Division Multiple Access (CDMA) scheme, however, the modulating waveforms correspond to the spreading codes assigned to each user. 
   Due to the lossy nature of the wireless channel, transmitted signals suffer attenuation as they propagate to the receiver. This is illustrated by the channel loss factors  104 , represented as w 1 , . . . , w K , in  FIG. 1 . Typically, channel loss factors w 1 , . . . , w K  represent the relative received amplitude levels of the k users. Since users are typically at different physical distances from receiver  120  and may also employ different transmit power levels and experience different channel conditions, channel loss factors differ from one user to another. 
   In addition to attenuation, transmitted signals experience different delays before reaching the receiver. Delays  106 , represented as τ 1 , . . . , τ K , in  FIG. 1  represent the relative transmission delays for the K users in communication model  100 . Typically, transmission delays comprise one or more delays including, for example, a propagation delay. As a result, the transmitted signals are typically not received synchronously at the receiver. 
   Given the communication model  100  described above, the input at receiver  120 , due to K users each transmitting a block of N bits, can then be written as: 
   
     
       
         
           
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   where T is the symbol period of the modulating waveforms s 1 (t), . . . , s K (t). 
   s(t) represents the noiseless input to receiver  120 . Typically, however, one or more noise elements are added during reception, and, assuming an additive noise channel, the corresponding received signal  110  can be described as: r(t)=s(t)+n(t), where n(t) represents additive noise. It must be noted here that received signal r(t), as described above, represents a baseband received signal. Therefore, as can be understood by a person skilled in the relevant art, one or more steps of baseband down-conversion may have been omitted for the ease of illustration in  FIG. 1 . 
   A Multi-User Receiver 
     FIG. 2  is a block diagram that illustrates a multi-user receiver path  200  according to an embodiment of the present invention. Receiver path  200  includes a baseband converter  210 , a sampler  220 , an energy detector  230 , a correlator  240 , a memory buffer  250 , and an IIC detector  260 . Operation of the elements of receiver path  200  in the reception of a signal of interest will now be described with reference to  FIG. 2 . 
   Baseband converter  210  receives a bandpass signal  205  from a wireless antenna (not shown in  FIG. 2 ). Typically, before reaching baseband converter  210 , signal  205  undergoes a number of steps including bandpass filtering and amplification. Bandpass filtering allows the receiver to only pass content of the intercepted signal located within a desired range of frequencies. Amplifying steps are typically also needed as the bandpass filtered signal is often too weak for frequency down-conversion. 
   Baseband converter  210  converts bandpass signal  205  to a baseband signal  215 . As can be understood by a person skilled in the relevant art, various down-conversion techniques can be used for baseband converter  210 . In one embodiment, baseband converter  210  is a heterodyne baseband converter. In another embodiment, baseband converter  210  employs a direct-conversion scheme. 
   At the output of baseband converter  210 , signal  215  is a baseband signal having frequency content located around the frequency of zero Hertz. Sampler  220  samples signal  215  according to a sampling rate to generate a sampled signal  225 . Typically, the sampling rate is selected to be high enough for a distortion-less reconstruction of signal  215 . In an embodiment, sampler  220  over-samples signal  215  at 6 times the information bit rate. As can be understood by a person skilled in the relevant art, sampler  220  may employ any of a number of well known sampling techniques. For example, sampler  220  uses a switched sampling or a zero-order-hold sampling technique. 
   Following sampling, a sampled signal  225  is fed into energy detector  230 . Energy detector  230  typically includes a number of circuits designed to sense energy increases or decreases in received signal  225 . In an embodiment, energy detector  230  continuously senses for any energy increase or decrease in received signal  225  according to the information bit rate of received signal  225 . Furthermore, energy detector  230  triggers correlator  240  upon detecting an energy increase in signal  225  that may indicate the beginning of data reception. At the output of energy detector  230 , signal  235  is unchanged relative to signal  225 . 
   The function of correlator  240  is to detect the beginning of information data frames in a received signal. In an embodiment, a synchronization/training bit sequence, also known at the receiver, is attached at the beginning of every transmitted message. Correlator  240  is then designed such that its correlation output is at a maximum when the transmitted training sequence is received and is correctly aligned with a known synchronization/training bit sequence at the receiver. Accordingly, correlator  240 , after locking into the training sequence, determines the beginning of information data frames in a received signal. In a multiple access model, correlator  240  detects the beginning of information data frames contained in a superposition of received signals. 
   At the output of correlator  240 , a signal  245  is a superposition of information data frames. Referring to  FIG. 2 , signal  245  is buffered in its entirety into memory buffer  250 . Memory buffer  250  then outputs periodically according to the information bit rate the content  255  of its buffer to the IIC detector  260 . In an embodiment, the content of memory buffer  250  is entirely outputted to IIC detector  260  before any new content is buffered in. 
   IIC detector  260  receives a superposition of information data frames from memory buffer  250 , and, for each information data frame contained in the superposition, detects a corresponding data bit stream. A description of the operation of IIC detector  260  follows. 
   An Iterative Interference Cancellation Detector 
     FIG. 3  is a high-level block diagram of the multi-user IIC detector  260  of  FIG. 2  according to an embodiment of the present invention. As shown in  FIG. 3 , IIC detector  260  includes a sorter circuit  310  and an IIC decoder  320 . The operation of IIC detector  260  will now be described with reference to  FIG. 3 . 
   At the input of IIC detector  260 , signal  255  is a superposition of information data frames. The beginning and the signal level of each of the information data frames contained in the superposition of information data frames is known at IIC detector  260 . 
   As a preliminary step to iterative interference cancellation in IIC decoder  320 , sorter circuit  310  ranks based on the received signal strength the information data frames contained in signal  255 . Accordingly, the strongest signal is ranked first and the weakest signal is ranked last. The output  330  of sorter circuit  310  is then provided to IIC decoder  320 . 
   In an embodiment of the present invention, IIC decoder  320  works by successively decoding the strongest signal in a superposition of signals, regenerating the signal, and subtracting it off from the superposition of signals, then repeating this technique for the next strongest signal until every signal is successfully decoded. 
   The signals are decoded in decreasing order of signal strength because: 1) it is easiest to achieve acquisition and demodulation on the strongest signal: the strongest signal has the best chance for correct decoding, and, as a result, the best chance for the most reliable cancellation; and (2) the cancellation of the strongest signal results in the maximum interference reduction for the remaining signals. 
     FIG. 4  is a block diagram that illustrates the iterative interference cancellation IIC decoder  320  of  FIG. 3  according to an embodiment of the present invention. The operation of IIC decoder  320  will now be described with reference to  FIG. 4 . 
   As mentioned above, IIC decoder  320  takes a serial approach to canceling interference. As a result, each stage i of IIC decoder  320  is concerned with the decoding and cancellation of the i-th strongest signal in the superposition of signals  400   a  of  FIG. 4 . 
   In an embodiment of the present invention, stage  1  of IIC decoder  320  includes a demodulator  410   a , a modulator  412   a , an estimation block  414   a , a time delay device  416   a , and a summer circuit  418   a . Subsequent stages  2 , . . . , K of IIC decoder  320  include the same elements as stage  1 . The operation of a stage of IIC decoder  320  will now be described with respect to the elements of stage  1 . 
   In the exemplary embodiment of  FIG. 4 , a superposition of K signals  400   a  is received at the input of stage  1  of IIC decoder  320 . Demodulator  410   a  is configured to demodulate the strongest signal s 1  contained in the superposition of signals  400   a . In an embodiment, demodulator  410   a  includes a correlation-type demodulator and a bit decoder. In the example of  FIG. 4 , demodulator  410   a  decodes signal s 1  to generate a first bit stream d 1 (t). Bit stream d 1 (t) matches exactly the bit stream encoded in transmitted signal s 1  if no decoding errors are made. Typically, decoding errors can be attributed to high MAI levels. 
   After decoding s 1 , it remains to subtract it off from the superposition of signals  400   a . Using the first bit stream d 1 (t), an estimate of signal s 1  is reconstructed. In a first step, the first bit stream d 1 (t) is re-modulated using modulator  412   a . In an embodiment, modulator  412   a  applies the exact same bit modulation and encoding techniques as used by the transmitter of signal s 1 . 
   At the output of modulator  412   a , the re-modulated version of signal s 1  corresponds to a pre-transmission version of signal s 1 . In other words, the re-modulated version at the output of modulator  412   a  does not take into account the channel effects on transmitted signal s 1 . To account for channel effects, the re-modulated signal is acted upon by the estimation block  414   a . Estimation block  414   a  estimates channel conditions and applies appropriate corrections to the amplitude, phase, and frequency of the re-modulated signal to generate the first signal estimate s′ 1 . In other words, estimation block  414   a  attempts to minimize the difference between the first signal estimate s′ 1  and the actual received signal s 1 . 
   Still referring to  FIG. 4 , after the first signal estimate s′ 1  is generated, it is subtracted from the superposition of signals  400   a  using summer circuit  418   a . Since typically a time delay is incurred to decode the first signal s 1  and reconstruct its estimate, a time delay device  416   a  is used to delay signal  400   a , thereby guaranteeing that the inputs to summer circuit  418   a  are synchronized in time. The output of summer circuit  418   a  comprises a superposition of the remaining signals after the subtraction of the strongest signal, and constitutes the input to the second stage of the IIC decoder  320 . The process described above repeats in the multi-stage structure of IIC decoder  320  until all K signals are decoded. 
     FIG. 5  is a block diagram that illustrates another embodiment of the first stage of the IIC decoder  320  of  FIG. 4 . In the embodiment of  FIG. 5 , the superposition of signals  400   a  is first fed into a matched filter  510  to demodulate the strongest signal s 1 . In an embodiment, matched filter  510  has an impulse response matched to the modulating waveform of signal s 1 , thereby maximizing the signal-to-interference ratio of signal s 1  at the output. 
   Decision block  520  makes a hard decision on s 1  using the output of matched filter  510 . In an embodiment, decision block  520  decodes signal s 1  to generate a bit stream d 1 . Bit stream d 1  corresponds to the information content encoded in signal s 1 . 
   Similar to the embodiment of  FIG. 4 , signal s 1  is then reconstructed using bit stream d 1  before being subtracted from a time delayed version of signal  400   a . In the embodiment of  FIG. 5 , mixer circuit  530  re-modulates bit stream d 1  by multiplying it with a modulating waveform  550 . In an embodiment, modulating waveform  550  is a replica of the modulating waveform used by the transmitter of signal s 1 . 
   In another embodiment, phase offset compensation is also incorporated into the re-modulating step. In the embodiment of  FIG. 5 , this is reflected by the relative transmission delay term τ 1  in the modulating waveform expression. 
   Following re-modulation of bit stream d 1 , amplitude compensation is applied to the re-modulated signal  535  based on estimation of channel conditions. In the embodiment of  FIG. 5 , mixer  540  multiplies re-modulated signal  535  with an appropriate amplitude modulating waveform  560 . As can be understood by a person skilled in the relevant art, amplitude modulating waveform  560  applies appropriate amplification/attenuation factors to the various frequency components of re-modulated signal  535 . 
   The output of mixer circuit  540 , signal  545 , represents an estimate of signal s 1 . Stage  1  of the IIC decoder is completed by subtracting off signal  545  from a time delayed version of signal  400   a  as discussed above using time delay device  416   a  and summer circuit  418   a . The output  410   b  of stage  1  represents the input to stage  2  of the decoder. 
   A Multi-User Detection Method 
     FIG. 6  is a flowchart that illustrates an iterative interference cancellation technique for decoding a plurality of superimposed signals according to the present invention. The process of  FIG. 6  includes steps  610 ,  620 ,  630 ,  640 ,  650 , and  660 . 
   In step  610 , the plurality of superimposed signals are sorted in order of signal strength. In an embodiment of the present invention, an energy detector circuit is used to detect the signal levels of the plurality of superimposed signals. A sorter circuit then ranks the plurality of superimposed signals in decreasing order of signal strength. 
   In step  620 , the plurality of superimposed signals are demodulated with respect to a first signal having the highest signal strength among the plurality of superimposed signals. As a result of step  620 , the first signal is decoded and a corresponding first bit stream is generated. 
   In step  630 , using the first bit stream generated in step  620 , an estimate of the first signal is regenerated. In an embodiment, the first bit stream is re-modulated using a replica of the modulating waveform used by the transmitter of the first signal. Further, amplitude, frequency, and phase corrections are applied to the re-modulated signal in order to closely estimate the first signal. 
   In step  640 , the first signal estimate is subtracted from the plurality of superimposed signals. At the end of step  640 , a superposition of one or more remaining signals is generated. 
   At step  650 , the number of remaining signals in the superposition is evaluated. If the number of remaining signals is larger than 1, the process returns to step  610 , and steps  610 - 640  are repeated with respect to the strongest signal in the superposition of remaining signals. Otherwise, only the weakest signal in the plurality of superimposed signals remains, and the process moves to step  660  where the last remaining signal is decoded. In step  660 , the last remaining signal is decoded with no MAI due to other signals. 
   CONCLUSION 
   While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.