Self-interference suppression in full-duplex MIMO relays

1. A method for self-interference suppression in a relay for wireless communication, wherein said relay is a part of a communication channel between a transmitter and a receiver in a wireless communication system, said relay being arranged for receiving a signal r(n) transmitted from said transmitter, and transmitting a self-interference suppressed transmit signal x r (n) to said receiver, said communication channel having a transmit channel, defined by a transmit channel matrix (H 1 ) between said transmitter and said relay, a receive channel, defined by a receive channel matrix (H 2 ) between said relay and said receiver, and a self-interference channel between an relay output and a relay input of said relay, the self-interference channel being defined by a self-interference channel matrix (H 0 ), the method comprising:

receiving at the relay input of said relay a receive signal r(n) transmitted from said transmitter over said transmit channel;

determining a receive suppression filter matrix W r , in said relay, based on said transmit channel matrix (H 1 ) and said self-interference channel matrix (H 0 );

determining a transmit suppression filter matrix W t , in said relay, based on said receive suppression filter matrix W r , said receive channel matrix (H 2 ), and said self-interference channel matrix (H 0 );

determining a self-interference suppressed transmit signal x r (n) by applying said receive suppression filter matrix W r , a relay amplification matrix G and said transmit suppression filter matrix W t , to said received signal r(n); and

transmitting said self-interference suppressed transmit signal x r (n) from said relay over said receive channel to said receiver.

2. The method according to claim 1 , wherein said receive signal r(n) is given by

r ( n )= H 1 x s ( n )+ H 0 x r ( n )+ n R ( n )

wherein x r (n) is the transmitted relay signal, and n R (n) is the noise contribution at the relay.

3. The method according to claim 1 , wherein said receive suppression filter matrix W r is determined by the optimal receive suppression matrix which is given by

W r,opt =U H ( H 0 H 0 H ) −1/2

wherein the columns of U are the corresponding generalized eigenvectors of the matrix (H 0 H 0 H ) −1/2 H 1 H 1 H (H 0 H 0 H ) −1/2 .

4. The method according to claim 1 , wherein said transmit suppression filter matrix W t is determined by the optimal transmit suppression matrix which is given by

W t,opt =( {tilde over (H)} 0 H {tilde over (H)} 0 ) −1/2 Ū

wherein {tilde over (H)} 0 =W r,opt H 0 and matrix Ū is obtained by applying a generalized eigenvalue decomposition of the matrix ({tilde over (H)} 0 H {tilde over (H)} 0 ) −1/2 H 2 H H 2 ({tilde over (H)} 0 H {tilde over (H)} 0 ) −1/2 , wherein the columns of matrix Ū are the corresponding eigenvectors.

5. The method according to claim 1 , wherein said determining of said self-interference suppressed transmit signal x r (n) is done according to x r =W t GW r r(n).

6. A relay adapted for self-interference suppression in a wireless communication system, wherein said relay is configured to be a part of a communication channel between a transmitter and a receiver in a wireless communication system, said communication channel having a transmit channel, defined by a transmit channel matrix (H 1 ), between said transmitter and said relay, a receive channel, defined by a receive channel matrix (H 2 ), between said relay and said receiver, and a self-interference channel between a relay output and a relay input of said relay, the self-interference channel being defined by a self-interference channel matrix (H 0 ), said relay comprising:

a receiver unit adapted to receive a receive signal r(n) transmitted from said transmitter over said transmit channel;

a first processing unit adapted to determine a receive suppression filter matrix W r , based on said transmit channel matrix (H 1 ) and said self-interference channel matrix (H 0 );

a second processing unit adapted to determine a transmit suppression filter matrix W t , based on said receive suppression filter matrix W r , said receive channel matrix (H 2 ), and said self-interference channel matrix (H 0 );

a third processing unit adapted to determine a relay amplification matrix G; and

a fourth processing unit adapted to produce a self-interference suppressed transmit signal x r (n) by applying said receive suppression filter matrix W r from said first processing unit, said transmit suppression filter matrix W t from said second processing unit, and said relay amplification matrix G from said third processing unit to said receive signal r(n), wherein said fourth processing unit is further adapted to supply said self-interference suppressed transmit signal x r (n) to a transmitter unit; wherein

said transmitter unit is adapted to receive said self-interference suppressed transmit signal x r (n) from said fourth processing unit, and adapted to transmit said self-interference suppressed transmit signal x r (n) to said receiver over said transmit channel.

7. The relay according to claim 6 , wherein said receive signal r(n) received by said receiver is given by

r ( n )= H 1 x s ( n )+ H 0 x r ( n )+ n R ( n )

wherein x r (n) is the transmitted relay signal, and n R (n) is the noise contribution at the relay.

8. The relay according to claim 6 , wherein said first processing unit is further adapted to determine said receive suppression filter matrix W r by determining the optimal receive suppression matrix according to

W r,opt =U H ( H 0 H 0 H ) −1/2

wherein the columns of U are the corresponding generalized eigenvectors of the matrix (H 0 H 0 H ) −1/2 H 1 H 1 H (H 0 H 0 H ) −1/2 .

9. The relay according to claim 6 , wherein said second processing unit is further adapted to determine said transmit suppression filter matrix W t by calculating the optimal transmit suppression matrix according to

W t,opt =( {tilde over (H)} 0 H {tilde over (H)} 0 ) −1/2 Ū

wherein {tilde over (H)} 0 =W r,opt H 0 and the matrix Ū is obtained by the generalized eigenvalue decomposition of the matrix ({tilde over (H)} 0 H {tilde over (H)} 0 ) −1/2 H 2 H H 2 ({tilde over (H)} 0 H {tilde over (H)} 0 ) −1/2 , and its columns are the corresponding eigenvectors.

10. The relay according to claim 6 , wherein said producing, in said fourth processing unit, of said self-interference suppressed transmit signal x r (n) is done according to x r =W t GW r r(n).