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

• US 20070121763A1
• Filed: 09/22/2004
• Published: 05/31/2007
• Est. Priority Date: 09/26/2003
• Status: Active Grant

• Current Assignee: Lenovo Innovations Ltd Hong Kong (Lenovo Group Ltd.)
• Original Assignee: NEC Corporation
• Inventors: Bui, Thanh
• Granted Patent: US 7,680,214 B2
• Time in Patent Office: Days
• US Class Current: 375/340
• CPC Class Codes: H04L 25/03171 Arrangements involving maximum a posteriori probability [MAP] detection
• CPC Class Codes: H04L 25/03318 Provision of soft decisions
• CPC Class Codes: H04L 25/067 providing soft decisions, i.e. decisions together with an estimate of reliability H04L25/068 and H04L25/069 take precedence; sequence estimation techniques H04L25/03178
• CPC Class Codes: H04L 27/3809 Amplitude regulation arrangements

Abstract

A device (20) for computing a threshold Sthi used in demodulating a quadrature amplitude modulated (QAM) signal to generate a plurality of soft bits per received symbol for input to a turbo decoder, the device including: means (30) for computing the 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 (√4^m−1)−1.

Claims

1. A method for computing a threshold Sth_i used in demodulating a quadrature amplitude modulated (QAM) signal to generate a plurality of soft bits per received symbol for input to a turbo decoder, the method including the steps of:

computing the 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<sup>m−

1</sup>)})−

1.

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 any one of the preceding claims, 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/√

{square root over (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 any one of the preceding claims, wherein the mean amplitude A of the received symbols is computed according to

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

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

8. A method according to any one of claims 1 to 6, 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 for generating soft bits per received symbol for input to a turbo decoder used in demodulating a quadrature amplitude modulated (QAM) signal, the method including the steps of.

computing the threshold Sth_i according to any one of the preceding claims;

and

computing one or more of the soft bits from the threshold Sth_i.

10. A method according to claim 9, wherein log2 4 m soft bits are computed from the threshold Sth_i.

11. A device for computing a threshold Sth_i used in demodulating a quadrature amplitude modulated (QAM) signal to generate a plurality of soft bits per received symbol for input to a turbo decoder, the device including:

means for computing the 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<sup>m−

1</sup>)})−

1.

13. A communication receiver including a device according to either one of claims 11 or 12.

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

means for computing the 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<sup>m−

1</sup>)})−

1; and

means for computing one or more of the soft bits from the threshold Sth_i.