Computation of soft bits for a turbo decoder in a communication receiver

1. A method on a communication receiver having a soft bit generator and a turbo decoder, the method for computing a threshold Sth i used in demodulating a quadrature amplitude modulated (QAM) signal received by the communication receiver to generate a plurality of soft bits per received symbol for input to the turbo decoder, the method including the steps of:

the soft bit generator:

computing a mean amplitude A of the received symbols;

multiplying the mean amplitude A by a constant C i for a square QAM constellation with 4 m points, such that

Sth i =A×C i

where m is a positive integer and I is a positive integer from 1 to (√{square root over (4 m-1 )})−1;

computing one or more of the soft bits from the threshold Sth i ; and

outputting the computed soft bits to the turbo decoder.

2. A method according to claim 1 , wherein the mean amplitude A is computed from a block of K received symbols, where K is a positive integer.

3. A method according to either one of claims 1 or 2 , wherein the value of K is inversely proportional to the speed of change in channel conditions.

4. A method according to either one of claims 1 or 2 , wherein the constant C i is computed according to

C i =2× I×Δ

where Δ is a normalising parameter for a square QAM constellation with 4 m points.

5. A method according to claim 4 , wherein the QAM signal is a 16QAM signal and the constant

C

i

⁢

⁢

equals

⁢

⁢

2

10

.

6. A method according to claim 4 , wherein the QAM signal is a 16QAM signal and the constant Ci equals 0.5.

7. A method according to claim 1 , wherein the mean amplitude A of the received symbols is computed according to

A =max( AI,AQ )+0.5min( AI,AQ )

where AI and AQ are respectively the averages of orthogonal I and Q components of each received symbol.

8. A method according to either one of claims 1 or 2 , wherein the mean amplitude A of the received symbols is computed according to

A=AI+AQ

where AI and AQ are respectively the averages of orthogonal I and Q components of each received symbol.

9. A method according to claim 1 , wherein log 2 4 m soft bits are computed from the threshold Sth i .

10. A device within a communication receiver for computing a threshold Sth i used in demodulating a quadrature amplitude modulated (QAM) signal received by the communication receiver to generate a plurality of soft bits per received symbol for input to a turbo decoder, the device including:

means for computing a mean amplitude A of the received symbols and multiplying the mean amplitude A by of the received symbols and multiplying the mean amplitude A by a constant C i for a square QAM constellation with 4 m points, such that

Sth i =A×C i

where m is a positive integer and i is a positive integer from 1 to (√{square root over (4 m-1 )})−1;

means for computing one or more of the soft bits from the threshold Sth i ; and

means for outputting the computed soft bits to the turbo decoder.

11. A device within a communication receiver for generating soft bits per received symbol for input to a turbo decoder used in demodulating a quadrature amplitude modulated (QAM) signal received by the communication receiver, the device including:

means for computing a mean amplitude A of the received symbols and multiplying the mean amplitude A by a constant C i for a square QAM constellation with 4 m points, such that

Sth i =A×C i

where m is a positive integer and i is a positive integer from 1 to (√{square root over (4 m-1 )})−1; and

means for computing one or more of the soft bits from the threshold Sth i ; and

means for outputting the computed soft bits to the turbo decoder.