Reduced complexity soft demapper for multi antenna communication system

1. An apparatus comprising:

a demapper configured to receive a plurality of bit streams received by a plurality of radio-frequency (RF) antennas, the demapper being configured to compute a first reliability metric associated with a first stream of the plurality of bit streams and to compute a second reliability metric associated with a second stream of the plurality of bit streams; and

a channel decoder configured to process the first and the second reliability metrics to recover decoded data,

wherein the demapper is a reduced complexity demapper and is configured to use a symbol subset of the first stream of the plurality of bit streams to compute, by employing look-up tables, the second reliability metric for the second stream of the plurality of bit streams.

2. The apparatus of claim 1 , wherein the first stream comprises a strong stream and the second stream comprises a weak stream, and wherein the demapper is configured to generate a plurality of candidate pair of points from the strong stream and the weak stream.

3. The apparatus of claim 2 , wherein a candidate point from the strong stream is taken from a region of constellation points, wherein at least one point from the weak stream matches to the candidate point from the region of constellation points.

4. The apparatus of claim 1 , wherein the demapper is configured to support quadrature amplitude modulation (QAM) constellations up to a constellation size M of 4096, and wherein the symbol subset comprises a subset of a full QAM constellation grid where a solution is most likely, wherein the subset comprises a spherical or a rectangular space, and wherein a subset dimension is less than a number of streams.

5. The apparatus of claim 3 , wherein the first stream comprises a strong stream and the second stream comprises a weak stream, wherein the demapper is configured to use one or more pre-computed look-up tables including bits of real and imaginary portions of symbols of each of the weak and strong streams, and wherein the demapper is configured to setup the one or more pre-computed look-up tables for bits of symbols of the weak stream simultaneously.

6. The apparatus of claim 5 , wherein the first reliability metric comprises a first log likelihood ratio (LLR) for a bit of the first stream, and the second reliability metric comprises a second LLR for a bit of the second stream.

7. The apparatus of claim 6 , wherein the demapper is configured to reduce an order of computation complexity of determining LLR to a level of 2*N*log (N), wherein N is a QAM constellation size.

8. The apparatus of claim 7 , wherein the demapper is configured to determine the LLR based on register values, wherein the register values include minimum bit log likelihood values.

9. The apparatus of claim 8 , wherein the register values are updated based on corresponding values from the one or more pre-computed look-up tables, a predetermined bias value and a Gmin value, wherein the Gmin value is a measure of a distance covering a geometry of the subset.

10. The apparatus of claim 9 , wherein the demapper is configured to scale a triangular decomposition matrix in a way that one of a main diagonal term is a real constant and another main diagonal term is a real parameter.

11. A method of demapping with reduced computational complexity, the method comprising:

receiving, by a plurality of radio-frequency (RF) antennas, a plurality of bit streams;

computing, by a demapper, a first reliability metric associated with a first stream of the plurality of bit streams;

computing, by the demapper, a second reliability metric associated with a second stream of the plurality of bit streams by using a symbol subset of the first stream of the plurality of bit streams, wherein the first and the second reliability metrics are computed with a reduced complexity by employing look-up tables; and

processing, by a decoder, the first and the second reliability metric to recover decoded data.

12. The method of claim 11 , wherein the symbol subset comprises a subset of a full quadrature amplitude modulation (QAM) constellation grid where a solution is most likely, wherein the subset comprises a spherical or a rectangular space, and wherein a subset dimension is less than a number of streams, wherein the full QAM constellation grid includes up to 4096 grid points.

13. The method of claim 11 , wherein the first stream comprises a strong stream representing an interfering bit stream and the second stream comprises a weak stream representing a desired stream, wherein demapping further comprises storing bits of real and imaginary portions of symbols of each of the strong stream and the weak stream in separate independent look-up tables.

14. The method of claim 11 , wherein computing a reliability metric comprises computing a log likelihood ratio (LLR) for a bit of a stream, and wherein the LLR is a log of a ratio of a first probability of the bit being a 0 (P0) to a second probability of the bit being a 1 (P1), and wherein demapping reduces an order of computation complexity of determining the LLR to a level of 2*N*log (N), wherein N is a QAM constellation size.

15. The method of claim 14 , wherein demapping comprises determining the LLR based on register values, wherein the register values include minimum bit log likelihood values.

16. The method of claim 15 , further comprising updating the register values based on corresponding values from the look-up tables, a predetermined bias value and a Gmin value, wherein the Gmin value is a measure of a distance covering a geometry of the subset.

17. The method of claim 16 , wherein demapping comprises scaling a triangular decomposition matrix in a way that one of a main diagonal term is a real constant and another main diagonal term is a real parameter.

18. A wireless communication device comprising:

multiple antennas configured to receive a plurality of bit streams; and

a baseband processor comprising a demapper and a channel decoder,

wherein:

the demapper is configured to compute a first reliability metric associated with a first stream of the plurality of bit streams, and to compute a second reliability metric associated with a second stream of the plurality of bit streams by using a symbol subset of at least a first stream of the plurality of bit streams,

the first reliability metric and the second first reliability metric are computed with a reduced complexity using look-up tables, and

the channel decoder is configured to process the first and the second reliability metrics to determine decoded data.

19. The wireless communication device of claim 18 , wherein the first stream comprises a strong stream representing an interfering bit stream and the second stream comprises a weak stream representing a desired stream, wherein the demapper is configured to store bits of real and imaginary portions of symbols of each of weak and strong streams in separate independent look-up tables.

20. The wireless communication device of claim 19 , wherein the demapper is configured to reduce an order of computation complexity of determining an LLR to a level of 2*N*log (N), wherein N is a quadrature amplitude modulation (QAM) constellation size, and wherein the demapper is configured to determine the LLR based on register values including minimum bit log likelihood values, wherein the register values are updated based on corresponding values from the look-up tables, a predetermined bias value and a Gmin value, and wherein the Gmin value is a measure of a distance covering a geometry of a subset of the QAM constellation.