Training-based channel estimation for multiple-antennas

Abstract

The burden of designing multiple training sequences for systems having multiple transmit antennas, is drastically reduced by employing a single sequence from which the necessary multiple sequences are developed. The single sequence is selected to create sequences that have an impulse-like autocorrelation function and zero cross correlations. A sequence of any desired length N t can be realized for an arbitrary number of channel taps, L. The created sequences can be restricted to a standard constellation (that is used in transmitting information symbols) so that a common constellation mapper is used for both the information signals and the training sequence. In some applications a a training sequence may be selected so that it is encoded with the same encoder that is used for encoding information symbols. Both block and trellis coding is possible in embodiments that employ this approach.

We claim: 
     
       1. A space-time diversity transmitter that includes (a) n transmitting antennas, where n is greater than one, (b) first encoder responsive to an applied information bit stream, said encoder developing n symbol streams, and (c) a constellation mapper responsive to said n symbol streams, for mapping symbols of each of said n symbol streams into a standard signal constellation to create a corresponding mapped stream, said constellation mapper applying each of the created n mapped streams to a different one of said n antennas in blocks of N t  mapped symbols that are synchronized in time to each other, the improvement comprising:
 a training generator for generating either n sequences of signals or a sequence of signals that contains n subsequences of signal; 
 a second encoder for creating n training symbol sequences from said signals created by said training generator, each containing N t  symbols, where N t  represents a training sequence length; and 
 said constellation mapper is configured to map said n training symbol sequences onto a standard constellation to develop n mapped training sequences, and to apply the n mapped training sequences to said n antennas at times when said constellation mapper stops applying said n mapped stream to said n antennas; 
 where said n training symbol sequences have an impulse-like autocorrelation function and zero cross correlation. 
 
     
     
       2. The transmitter of  claim 1  where said n training symbol sequences are selected to enable a receiver that receives said n training symbol sequences to determine characteristics of a medium through which said n antennas communicate to said receiver. 
     
     
       3. The transmitter of  claim 2  where said n training sequences have a common length N t  that is related to transmitted symbols memory, L−1, of said medium, N t  being at least equal to 2L−1. 
     
     
       4. The transmitter of  claim 1  where said constellation mapper employs a constellation taken from a set that includes binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), and 8-point phase shift keying (8-PSK). 
     
     
       5. The transmitter of  claim 1  where n=2, said generator creates a sequence s composed of sequence d 1  followed by sequence d 2 , and said second encoder creates a first training sequence that is −d 1  concatenated with {tilde over (d)} 2 *, and a second sequence that is d 2  concatenated with d 1 * where {tilde over (d)} 2 * corresponds to the sequence d 2  with its elements in reverse order and converted to their respective complex conjugates, and the sequence d 1 * corresponds to the sequence d 1  with its elements converted to their complex conjugate values. 
     
     
       6. The transmitter of  claim 5  where said sequence d 1  and said sequence d 2  contain N t /2 N t  symbols each, where N t  is related to number of channel parameters, L, that a receiver which seeks to receive signals from an antenna of said transmitter estimates. 
     
     
       7. The transmitter of  claim 1  where n=2, said generator creates a sequence d, and said second encoder creates a first training sequence that is −d concatenated with {tilde over (d)}*, and a second sequence that is d concatenated with d*, where the sequence d* corresponds to the sequence d with its elements converted to their complex conjugate values and the sequence {tilde over (d)}* corresponds to the sequence d with its elements in reverse order and converted to their respective complex conjugate values. 
     
     
       8. The transmitter of  claim 1  where n2 n=2, said generator creates a sequence d, and said second encoder creates a first training sequence that is −d concatenated with {tilde over (d)}*, and a second sequence that is {tilde over (d)} concatenated with d*, where the sequence d* corresponds to the sequence d with its elements converted to their complex conjugate values and the sequence {tilde over (d)}* corresponds to the sequence d with its elements in reverse order and converted to their respective complex conjugate values. 
     
     
       9. The transmitter of  claim 1  where n=2, said generator creates a sequence d, and said second encoder creates a first training sequence that is −d concatenated with d, and a second sequence that is {tilde over (d)}* concatenated with d*, where the sequence d* corresponds to the sequence d with its elements converted to their complex conjugate values and the sequence {tilde over (d)}* corresponds to the sequence d with its elements in reverse order and converted to their respective complex conjugate values. 
     
     
       10. The transmitter of  claim 1  where said first encoder and said second encoder embodied in a single module that creates either said n symbol streams or said n training symbol sequences. 
     
     
       11. The transmitter of  claim 1  where said second encoder is a trellis encoder. 
     
     
       12. The transmitter of  claim 11  where
 (a) said n training sequences are selected to enable a receiver that receives said n training sequences to determine characteristics of a transmission medium between said transmitter and said receiver, and 
 (b) said generator develops said symbols sequence to comprise N t +(n−2)L+m symbols long, where L is the number of channel parameters that describe a channel within said transmission medium from one of said n transmitter antennas to a receiving antenna of said receiver, m is number of symbols stored in a memory of said trellis encoder, and N t  is not less than 2L−1. 
 
     
     
       13. The transmitter of  claim 11  where said training sequence mapper employs symbols from a set composed of elements e i2πp     k     / 8, p k =0,1,2, . . . , 7. 
     
     
       14. The transmitter of  claim 13  where said generator develops a symbols sequence s(k), and said second encoder develops a first sequence s 1  (k)=s(k), and a second sequence s 2 (k)=(−1) p     k−1   s(k−1). 
     
     
       15. The transmitter of  claim 14  where said sequence s(k) is composed of a first segment, s even , that comprises symbols from a set composed of e i2πp     k     /8 , p k =0, 2, 4, 6, and a second segment, s odd , where s odd =αs even , and α=e iπk/4  for any k=1, 3, 5, 7. 
     
     
       16. The transmitter of  claim 15  where said s even , and a corresponding α are any one of the following: 
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                   S even   
                   α 
                 
                     
                     
                 
                     
                   −1 1 1 1 1 −1 −I −1 1 1 −1 1 1 
                   exp(i5π/4) 
                 
                     
                   1 1 −1 1 I I 1 −I I −1 −1 −1 1 
                   exp(i3π/4) 
                 
                     
                   1 −1 −1 −I I −I 1 1 1 −I −1 1 1 
                   exp(iπ/4) 
                 
                     
                   1 −1 −1 −I 1 −1 1 −I −I − I −1 1 1 
                   exp(iπ/4) 
                 
                     
                   1 I 1 1 I −1 −1 I 1 −1 1 I 1 
                   exp(iπ/4) 
                 
                     
                   1 I 1 I −1 −1 1 −1 −1 I 1 I 1 
                   exp(i3π/4) 
                 
                     
                   1 −I 1 1 −I −1 −1 −I 1 −1 1 −I 1 
                   exp(i7π/4) 
                 
                     
                   1 −I 1 −1 I 1 −1 −I 1 1 1 −I 1 
                   exp(i5π/4) 
                 
                     
                   −1 1 1 1 −1 −1 −I −1 −1 1 −1 1 1 
                   exp(i3π/4) 
                 
                     
                   −1 I −1 −I 1 −I I I 1 I 1 −I 1 
                   exp(i7π/4) 
                 
                     
                   −1 −I −1 I 1 I −I −I 1 −I 1 I 1 
                   exp(iπ/4) 
                 
                     
                   −1 −I −1 I −1 I −I −I −1 −I 1 I 1 
                   exp(i3π/4). 
                 
                     
                     
                 
             
                
                
                
               
               
                
                
                
                
                
                
                
                
                
                
                
                
                
               
            
           
         
       
     
     
       17. A space-time diversity transmitter that includes (a) n transmitting antennas, where n is greater than one, (b) first encoder responsive to an applied information bit stream, said encoder developing n symbol streams, and (c) a constellation mapper responsive to said n symbol streams, for mapping symbols of each of said n symbol streams into a standard signal constellation to create a corresponding mapped stream, said constellation mapper applying each of the created n mapped streams to a different one of said n antennas in blocks of N t  mapped symbols that are synchronized in time to each other, the improvement comprising:
 a training generator for generating either n sequences of signals or a sequence of signals that contains n subsequences of signal; 
 a second encoder for creating n training symbol sequences from said signals created by said training generator, each containing N t  symbols, where N t  represents a training sequence length; and 
 said constellation mapper is configured to map said n training sequences onto a standard constellation to develop n mapped training sequences, and to apply the n mapped training sequences to said n antennas at times when said constellation mapper stops applying said n mapped stream to said n antennas; 
 where said n training sequences have an impulse-like autocorrelation function and zero cross correlation, and where n2 n=2, said generator creates a sequence s, and said second encoder creates a first training sequence that is equal to −s concatenated with s, and a second training sequence that is equal to s concatenated with s. 
 
     
     
       18. The transmitter of  claim 17  where said sequence s contains N t /2 symbols, where N t  is related to number of channel parameters, L, that a receiver which seeks to receive signals from an antenna of said transmitter estimates.