Abstract:
A method for providing wireless transmission diversity wherein an error correcting codeword is divided into first and second segments at first and second transmitting units. The first segments are transmitted from each of the first and second transmitting units and received at the first and second transmitting units, respectively. The received first segments are decoded and, responsive to the decoding, transmission of a second segment is made from each of the first and second transmitting units.

Description:
RELATED APPLICATION(S) 
     This Application claims priority from and incorporates herein by reference the entire disclosure of U.S. Provisional Application Serial No. 60/355,493 filed Feb. 7, 2002. 
    
    
     TECHNICAL FIELD 
     The present invention relates to the mitigation of multipath fading in wireless channels, and more particularly, to the use of transmission diversity for mitigating the effects of multipath fading in wireless channels. 
     BACKGROUND OF THE INVENTION 
     Multipath fading involves the dynamic reduction of the signal level of a radio communication signal at specific locations due to the combining of incoming signals that travel multiple, alternative paths. Multipath fading occurs because the path links between transmitters and receivers differ, and the incoming multipath signals cancel each other at the specific points where the signal levels are inverted. One technique for mitigating the effects of multipath fading in wireless channels is diversity. Diversity refers to a number of methods by which multiple copies of a signal that experience independent fading are provided to a receiver. 
     One form of diversity that has received considerable attention in recent years is transmit diversity. Transmit diversity uses multiple antennas to transmit copies of a signal to a receiver through several independent fading paths. The various known transmit diversity schemes involve different designs for the transmitted signals to enable the receiver to process the signals with a minimum of added complexity. Some systems introduce a new class of channel codes, known as space-time trellis codes, designed for multiple transmit antennas to provide both diversity and coding gain. The decoding complexity of these systems is comparable to that of existing trellis codes. 
     However, these and other types of transmit diversity methods are not applicable to an uplink of a cellular or other types of wireless systems because the size of mobile units typically precludes the use of multiple antennas. Previously proposed user cooperation methods also suffer from several shortcomings. First, they all involve some form of repetition which from a channel coding point of view may not be the best use of available bandwidth. Also, existing schemes either admit forwarding of erroneous estimates of a partner&#39;s symbols, or include propagation of a partner&#39;s noise. Error propagation diminishes the performance of transmissions, especially when the channel between partners is poor. Previous schemes also require that either the instantaneous bit error rate (BER) or signal-to-noise (SNR) of the channel between the partners be known at the base station for optimal maximum likelihood detection or decoding. In practice, it may be difficult to store sufficient information to reproduce the analog signal. Therefore, there has arisen a need for a cooperative transmission system and method that would be useful in cellular system environments and other similar types of environments. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the foregoing and other problems with a method for providing wireless transmission diversity wherein an error correcting codeword is divided into first and second segments at each of a first transmitting unit and a second transmitting unit. The first segments of the error correcting codewords are transmitted from each of the first and second transmitting units such that they are received at each of the first and second transmitting units. The received first segments are decoded at each of the first and second transmitting units, and responsive to this decoding, transmission of the second segment is made from each of the first and second transmitting units. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the method and apparatus of the present invention may be obtained by reference to the following Detailed Description when taken in conjunction with the accompanying Drawings wherein. 
         FIG. 1  illustrates a first example of an existing transmit diversity system; 
         FIG. 2  illustrates a second example of an existing transmit diversity system; 
         FIG. 3  illustrates a wireless cellular network for implementing the system and method of the present invention; 
         FIG. 4  illustrates the manner in which source data from a mobile station is segmented for coding and transmission according to the present invention; 
         FIG. 5  illustrates the cooperative transmission scheme according to the present invention; 
         FIG. 6  is a flow diagram illustrating the method of operation of the system illustrated in  FIG. 5 ; 
         FIG. 7  illustrates four cooperative cases for second frame transmission based on first frame decoding results; 
         FIG. 8  illustrates simulation results for an exemplary system; 
         FIG. 9  illustrates simulation results for an exemplary system; and 
         FIG. 10  illustrates simulation results for an exemplary system. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, and more particularly to  FIGS. 1 and 2 , signal diversity is achieved by a scheduling scheme that enables two users to send their information using both of their antennas.  FIG. 1  illustrates one existing method in which each user  5  cooperatively transmits one bit over two successive symbol periods  10  to a base station  15 . The users  5  transmit their own bits in the first symbol period  10   a . Each user  5  receives and detects the bit transmitted by the other user  5 , and transmits the resulting estimate of the received bit during the second symbol period  10   b.    
       FIG. 2 , illustrates another system utilizing a similar scheme for ad-hoc wireless networks as that illustrated in  FIG. 1  except that each transmission period  10  corresponds to a coded block of symbols rather than a single symbol period. Each user  5  either amplifies and forwards a partner users  5  received signal, or decodes the symbols and retransmits using a same code. The choice of cooperative strategy depends on the quality of the channel between the partnering users. Both of these schemes improves system performance in terms of capacity and outage probability despite a noisy channel between the users. 
     From a channel coding perspective, the cooperative schemes described in  FIGS. 1 and 2  may be viewed as a form of repetition coding. Given that repetition codes are poor codes, a scheme that incorporates cooperation within the framework of existing channel codes would be more desirable. Referring now to  FIG. 3 , there is illustrated a wireless cellular network  30  consisting of a number of mobile stations  35  (users) having communication channels  40  with a base station  45 . The cellular network  30  employs a multiple access protocols that allow the base station  45  (and other mobile stations  35  in the cooperative case) to separately detect each mobile station. However, the present invention does not depend upon the specifics of any particular protocol. Thus, any system such as CDMA, TDMA, FDMA, or any other multiple access protocol may be utilized. 
     Referring now also to  FIG. 4 , each mobile station  35  segments source data to be transmitted by the user into blocks  50  of K bits and includes a number of additional bits  60  for a CRC code. The blocks are encoded at  55  for block transmission from the mobile station  35  using existing channel codes. Various channel coding methods may be used within the coded cooperation framework. For example, the overall code may be a block or convolutional code, or a combination of both. Alternatively, other types of error correcting codes may be used. The mobile stations  35  use a BPSK modulation scheme with all users having the same transmit power. The channels  40  between mobile stations  35  and between each mobile station and the base station  45  are mutually independent and subject to flat, slow Rayleigh fading. The fading coefficients are assumed to remain constant over each input block. Furthermore, the receivers at the mobile stations  35  and base station  45  track the fading coefficients and employ coherent detection so that only the magnitudes of the fading coefficients need be considered. 
     The baseband-equivalent discrete-time signal transmitted by a particular user i as:
 
 S   i ( n )=√{square root over ( E   b )} ·b   j ( n )  (1)
 
where E b  is the transmitted energy per unit and b i (n)ε{−1, +1} is the information-bearing component of the signal. The corresponding signal received by user j (where j=0 denotes the base station) is:
 
 r   ij ( n )=α ij   s   i ( n )+ z   j ( n )=α ij √{square root over ( E   b )}· b   i ( n )+ z   j ( n )  (2)
 
where α ij  is the fading coefficient magnitude between users i and j and z j (n) is the AWGN receiver noise sample. The noise samples have zero mean and variance N j /2, and are mutually independent. The quality of each channel is quantified by the average SNR with respect to the fading distribution:
 
     
       
         
           
             
               
                 
                   
                     
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     Referring now to  FIGS. 5 and 6 , there is more fully illustrated the system of the present invention and a flow chart describing the operation of the system. Two mobile stations  70 , including a transmitter and receiver, cooperatively transmit their codewords with the same transmit power and information rate as a comparable noncooperative system. The mobile stations  70  share their antennas such that a portion of each mobile station&#39;s codebits are received at the base station  75  through a different, independent fading channel from the others. In this way, diversity is achieved in a manner similar to channel coding with time interleaving, but without associated delay in the case of a slowly fading channel. Two noteworthy features of the proposed system are that the operation and performance are independent of the multiple access protocol. Thus, the scheme works equally well with CDMA, TDMA, FDMA or any other multiple access system. Also, previously reported cooperation diversity schemes work well when the interuser channel is as good or better than the channels to the base station. However, significant gains are possible with the present system even when an interuser channel signal to noise ratio (SNR) 20 dB below the channels to the base station is present. 
     According to the system and method of the present invention, the mobile stations  70  segment their source data into blocks at step  76  and augment the blocks with a cyclic redundancy check (CRC) code at step  78  such that there are a total of K bits per source block (including the CRC bits). The mobile stations  70  next encode at step  80  the source blocks to be transmitted to the base station  75  and the other mobile station  70  using an error correcting code, such that for an overall rate R code, there are N=K/R total code bits per codeword. As described previously, various error correcting codes may be used including block or convolutional codes, a combination thereof or any other known error correcting code. The N code bits of the codeword are divided at step  90  into two successive time segments or frames. The division or partitioning of the codewords for the two frames may be achieved through puncturing, product codes, or other forms of concatenation. The first segment of N 1 =K/R 1  bits forms the corresponding codeword for the rate R 1  code, and the second segment are the additional N 2 =N−N 1  bits for the rate R codeword. In the first frame  100   a , the mobile stations  70  transmit at step  105  their own first set of N 1  bits. They also receive and decode at step  110  the partner mobile station&#39;s transmission. If mobile station  70   a  successfully decodes mobile station&#39;s  70   b  data, as determined by using the CRC code at inquiry step  115 , mobile station  70   a  computes and transmits mobile station&#39;s  70   b  second set of N 2  bits in the second frame  100   b  at step  120 . Otherwise, mobile station  70   a  transmits its own second set of bits at step  125 . Mobile station  70   b  acts similarly, and each mobile station always transmits a total of N bits per source block  85 . 
     The level of cooperation is defined as N 2 /N, the percentage of the partner&#39;s bits transmitted relative to the total number of bits. A smaller percentage implies a more powerful code for the first frame  100   a  and increased probability that a user successfully decodes the bits of their partner. However, this also means a smaller N 2 , thus reducing the degree of diversity. The effects of varying the level of cooperation will be more fully discussed in a moment. 
     The mobile stations  70  act independently in the second frame  100   b , with no knowledge of whether their first frame  100   a  was correctly decoded by their partner. As a result, there are four possible cooperative cases for the transmission of the second frame  100   b . These cases are illustrated in  FIG. 7 . In Case  1 , both mobile stations successfully decode their partners, so that they each send their partner&#39;s second set of code bits in the second frame  100   b , resulting in the fully cooperative scenario depicted in  FIG. 5 . In Case  2 , neither mobile station successfully decodes their partner&#39;s first frame  100   a , and the system reverts to the non-cooperative case for that pair of source blocks. In Case  3 , mobile station  70   b  successfully decodes mobile station  70   a , but mobile station  70   a  does not successfully decode mobile station  70   b . Consequently, neither mobile station transmits the second set of code bits for mobile station  70   b  in the second frame  100   b , but instead both transmit the second set of data for mobile station  70   a . These two independent copies of mobile station  701  bits are combined via maximal ratio combining at the base station  75  prior to decoding. Case  4  is identical to Case  3  with the roles of mobile station  70   a  and mobile station  70   b  reversed. Clearly the base station  75  must know which of these four cases has occurred in order to correctly decode the received bits. 
     For the base station  75  to correctly interpret the received bits, each user must indicate whether the partner&#39;s data was decoded successfully from the first frame  100   a . One approach is to have each user send one additional bit in the second frame to indicate whether the partner was successfully decoded. This bit would have to be strongly protected via repetition coding, which introduces a tradeoff between the rate loss incurred and the impact on performance of imperfect knowledge at the base station. 
     An alternative approach, in which the base station simply decodes according to each of the four cooperative cases in succession, according to their relative probabilities of occurrence, until the CRC code indicates correct decoding. This strategy maintains the overall system performance and rate at the cost of some added complexity at the base station. Under most conditions this added complexity is 10% or less. 
     The performance of this cooperative scheme was evaluated via simulations in which the bit error rate was considered for each user at the base station  75 . To produce these results we implemented the user cooperation scheme using rate-compatible punctured convolutional (RCPC) codes. In this implementation, the overall rate R code is selected from a given RCPC code family (for example, the mother code). The code word for the first segment is then obtained by applying the puncturing matrix corresponding to rate R 1 , and the additional code bits for the second segment are those bits that are punctured in the first frame. For the simulations, the family of RCPC codes with memory M=4 and puncturing period P=8 was used and rate R=¼ (the mother code for this family) was selected. The source block size was K=128 bits. All receivers are assumed to maintain perfect channel state information, and the fading coefficients were varied independently for each data block in a Monte Carlo fashion to obtain average BER&#39;s over the channel fading distributions. The simulation for results for these various case are illustrated in  FIGS. 8 ,  9  and  10 . 
     When two mobile stations  70  have statistically similar channels to the base station  75 , i.e., their average received SNR&#39;s are equal, there is a marked improvement for both users over the noncooperative system.  FIG. 8  illustrates that the improvement decreases as the interuser channel quality worsens, since cooperation occurs for a smaller percentage of the blocks. However, the cooperation provides a significant gain even when the interuser channel quality is well below that of the user channels to the base station  75 . For example, when the interuser channel has an average SNR of 0 dB, cooperation provides a diversity gain of almost 3 dB when the user channels to the base station  75  have an average SNR of 15 dB. 
       FIG. 9  illustrates that the gain from cooperation is relatively insensitive to the level of cooperation until it falls below 25%. Since the rate ¼ code has four code bits per branch, the RCPC code puncturing pattern for 25% cooperation corresponds to exactly 1 bit for each branch being transmitted by the partner. Thus, below 25% cooperation there will be some branches that have no diversity at all. This is likely the cause of the increase in BER from 25% to 12.5% cooperation shown in  FIG. 9 . 
     When the two users have statistically dissimilar channels to the base station  75 ,  FIG. 10  shows that the BER for the users with the worst channel improves significantly with cooperation, as does the combined BER for both users. More interestingly, the user with the better channel also improves, a result that is not necessarily intuitive. In the simulation results shown in  FIG. 10 , User  1 , with an uplink SNR of 20 dB, improves with cooperation diversity even while the partner&#39;s channel is as much as 20 dB worse. Thus, even the user with a better channel has motivation to cooperate. This is an effect that has not been observed in existing cooperation schemes. 
     Thus, the above system enables two users to share their antennas to achieve transmission diversity in the uplink of a cellular network. Cooperation is incorporated with channel coding so that, for a slow fading environment, each user&#39;s codeword is divided into two parts which are transmitted to the base station  75  over independent fading channels. Each user employs the CRC code to determine when he has correctly decoded the partner&#39;s data and can thus successfully cooperate. The proposed cooperative scheme does not require additional transmit bandwidth, and decoding complexity is the same as that for a noncooperative system. Additionally, the scheme does not depend on any particular multiple access protocol. Simulation results indicate that this scheme provides significant improvement in BER for both users, even when the interuser channel is much worse that either user channel to the base station, and when one user has a significantly better channel than the other to the base station. 
     The previous description is of a preferred embodiment for implementing the invention, and the scope of the invention should not necessarily be limited by this description. The scope of the present invention is instead defined by the following claims.