Apparatus and method for reduced complexity maximum likelihood MIMO detection

1. A method of estimating a plurality of transmit symbols s i , i=1 to n, in a transmit symbol vector s of a multiple-input multiple-output (MIMO) communication system, where n is a size of the transmit symbol vector s, the method comprising:

calculating a plurality of permuted MIMO channel matrices H i , i=1 to n, wherein a rightmost column of each permuted MIMO channel matrix H i is an i th column of an estimated MIMO channel matrix H;

determining a QR decomposition of each permuted MIMO channel matrix H i in the plurality of permuted MIMO channel matrices H i , i=1 to n, to generate a plurality of unitary matrices Q i , i=1 to n, and a plurality of upper triangular matrices R i , i=1 to n;

determining a plurality of modified receive symbol vectors y i , i=1 to n, each modified receive symbol vector y i calculated using a receive symbol vector z and a conjugate transpose of the unitary matrix Q i ;

estimating each transmit symbol s i , i=1 to n, of the transmit symbol vector s using the modified receive symbol vector y i and the upper triangular matrix R i ; and

using a receiver, determining a soft decision reliability metric for a bit of the transmit symbol s i of the transmit symbol vector s using a plurality of symbol values as candidates for the estimated transmit symbol s i , each symbol value σ j taken from a subset of a signal constellation,

wherein determining the soft decision reliability metric for a bit of the estimated transmit symbol s i further comprises:

calculating a set of error distances, one error distance for each different symbol value σ j in the subset of the signal constellation, by estimating the transmit symbol s i as the symbol value σ j ;

calculating an error distance of the estimated transmit symbol vector s containing the estimated transmit symbol s i set to the symbol value σ j using the modified receive symbol vector y i and the upper triangular matrix R i ;

determining a minimum error distance for a bit value of “one” for the bit of the estimated transmit symbol s i and a minimum error distance for a bit value of “zero” for the bit of the estimated transmit symbol s i from the set of calculated error distances; and

determining the soft decision reliability metric based on a difference between the minimum error distance with the bit value of “one” and the minimum error distance with the bit value of “zero”.

2. The method of claim 1 wherein calculating the error distance of the estimated transmit symbol vector s containing the estimated transmit symbol s i set to the symbol value σ j comprises:

estimating each transmit symbol in the estimated transmit symbol vector s iteratively using the symbol value σ j and a set of previously estimated transmit symbols in the estimated transmit symbol vector s;

calculating a set of partial error distances, each partial error distance calculated using an element of the modified receive symbol vector y i , a corresponding row of the upper triangular matrix R i , the symbol value σ j and a subset of the estimated transmit symbols in the estimated transmit symbol vector s; and

summing the set of partial error distances to generate the error distance of the estimated transmit symbol vector s.

3. The method of claim 2 wherein the signal constellation is a square QAM constellation of size M=2 2q , where q is a number of bits per symbol in each dimension of the signal constellation, and the subset of the signal constellation is a square QAM constellation of size (2 q−1 +1) 2 ≦K≦M.

4. The method of claim 3 wherein the subset contains a symbol in the signal constellation closest to a linear equalization estimate for the transmit symbol s i calculated using an element of the modified receive symbol vector y i and an element from the upper triangular matrix R i .

5. The method of claim 2 wherein (n−1) leftmost columns of each permuted MIMO channel matrix H i are ordered from left to right in increasing column rank.

6. The method of claim 1 wherein the plurality of transmit symbols s i , i=1 to n, in a transmit symbol vector s are estimated in parallel.

7. The method of claim 1 wherein a plurality of soft decision metrics, one for each estimated transmit symbol s i , i=1 to n, in a transmit symbol vector s are estimated in parallel.

8. An apparatus for estimating a plurality of transmit symbols s i , i=1 to n, in a transmit symbol vector s of a multiple-input multiple-output (MIMO) communication system, where n is a size of the transmit symbol vector s, the apparatus comprising:

a permutation processor to calculate a plurality of permuted MIMO channel matrices H i , i=1 to n, wherein a rightmost column of the permuted MIMO channel matrix H i is an i th column of an estimated MIMO channel matrix H;

a decomposition processor to determine a QR decomposition of each permuted channel matrix H i in the plurality of permuted MIMO channel matrices H i , i=1 to n, to generate a plurality of unitary matrices Q i , i=1 to n, and a plurality of upper triangular matrices R i , i=1 to n;

a calculator to determine a plurality of modified receive symbol vectors y i , i=1 to n, each modified receive symbol vector y i calculated using a receive symbol vector z and a conjugate transpose of the unitary matrix Q i ;

an estimator to estimate each transmit symbol s i , i=1 to n, of the transmit symbol vector s using the modified receive symbol vector y i and the upper triangular matrix R i ;

a soft decision reliability metric calculator to determine a soft decision reliability metric for a bit of the transmit symbol s i of the transmit symbol vector s using a plurality of symbol values as candidates for the estimated transmit symbol s i , each symbol value σ j taken from a subset of a signal constellation;

an error distance calculator to calculate a set of error distances, one error distance for each different symbol value σ j in the subset of the signal constellation, wherein the error distance calculator estimates the transmit symbol s i as the symbol value σ j and calculates an error distance of the estimated transmit symbol vector s containing the estimated transmit symbol s i set to the symbol value σ j using the modified receive symbol vector y i and the upper triangular matrix R i ;

a minimum error distance calculator to determine a minimum error distance for a bit value of “one” for the bit of the estimated transmit symbol s i and a minimum error distance for a bit value of “zero” for the bit of the estimated transmit symbol s i from the set of calculated error distances; and

a difference calculator to calculate a difference between the minimum error distance with the bit value of “one” and the minimum error distance with the bit value of “zero”.

9. The apparatus of claim 8 wherein the estimator further comprises:

an iterative estimator to estimate each transmit symbol in the estimated transmit symbol vector s iteratively using the symbol value σ j and a set of previously estimated transmit symbols in the estimated transmit symbol vector s;

a partial error distance calculator to calculate a set of partial error distances, each partial error distance calculated using an element of the modified receive symbol vector y i , a corresponding row of the upper triangular matrix R i , the symbol value σ j and a subset of the estimated transmit symbols in the estimated transmit symbol vector s; and

a summing block to sum the set of partial error distances to generate the error distance of the estimated transmit symbol vector s.

10. The apparatus of claim 9 wherein the signal constellation is a square QAM constellation of size M=2 2q , where q is the number of bits per symbol in each dimension of the signal constellation, and the subset of the signal constellation is a square QAM constellation of size (2 q−1 +1) 2 ≦K≦M.

11. The apparatus of claim 10 wherein the subset contains a symbol in the signal constellation closest to a linear equalization estimate for the transmit symbol s i calculated using an element of the modified receive symbol vector y i and an element from the upper triangular matrix R i .

12. The apparatus of claim 8 wherein the permutation processor orders (n−1) leftmost columns of each permuted MIMO channel matrix H i from left to right in increasing column rank.

13. The apparatus of claim 8 wherein the estimator estimates the plurality of transmit symbols s i , i=1 to n, in the transmit symbol vector s in parallel.

14. The method of claim 8 wherein the soft decision metric calculator calculates each soft decision reliability metric calculator calculates each soft decision reliability metric, one for each estimated transmit symbol s i , i=1 to n, in the transmit symbol vector s in parallel.