Method and system for a turbo trellis coded modulation scheme for communication systems
View Patent ↗Method and System for evaluating a turbo-coding scheme suitable for VDSL2 systems. The scheme is a Turbo-Trellis Coded Modulation TTCM that makes use of a Parallel Concatenated Convolutional Code (PCCC) including an inner Latin square/rectangular matrix of Random sequence pattern Interleaver (LRI) of approximately 2044 bits. In the presence of the outer Reed Solomon Code but without an outer interleaver (fast mode), the method and system provides 7.1 dB of average net coding gain, i.e., 2.6 dB of coding gain improvement relative to the standard 16 states 4-dimensional (4-D) trellis code used in current ADSL2 systems. The substantial coding gain achieved through the method and system, leads to an increase in the VDSL2 rate by 38% at 6 kft against local exchange carriers, such as ILEC (SBC) conditions with approximately 1.5 ms of latency. By encoding only two bits per tone (i.e., 1 bit per dimension), the turbo code implementation complexity is kept reasonable. Finally, the coded modulation scheme supports very small constellations, allowing transmitting spectral efficiency as low as 0.5 bit/sec/Hz over Binary Phase Shift Keying (BPSK) constellations at a signal-to-noise-ration (SNR) very close to the Shannon bound.
1. A method of performing turbo trellis coded modulation (TTCM) on an n bit information word in a discrete multitone (DMT) system comprising:
passing at least one least significant bit of the n bit information word into a redundant bit turbo-engine to generate an m bit code word, wherein passing the at least one information bits into the redundant bit turbo-engine comprises splitting the at least one least significant bit into duplicate first and second serial data streams, passing the first serial data stream through a first systematic recursive encoder and passing the second serial data stream through a second systematic recursive encoder preceded by an interleaver;
alternately selecting bits output from the first and second systematic recursive encoders to form an m-bit serial data stream;
converting the m-bit serial data stream into the m-bit code word through serial to parallel conversion;
appending the m-bit code word to the n bit information word at the least significant bit position to create an (n+m) bit codeword v; and
preparing the codeword v for transmission over a communications channel, wherein the (n+m) bit code is mapped onto two identical 2 dimensional 8-QAM constellation mappings.
2. The method according to claim 1 , wherein n is also the spectral efficiency of the turbo trellis coded modulation.
3. The method according to claim 2 , wherein n.gtoreq.2.
4. The method according to claim 3 , wherein 2 least significant bits are passed to the redundant bit turbo-engine to generate a 2 bit code word and the 2 bit codeword is appended to the least significant bit position of the n bit information word to create a 2+n bit codeword v.
5. The method according to claim 4 , wherein preparing the codeword v for transmission over a communications channel comprises mapping the codeword v onto a 2 dimensional 2 n+2 points QAM constellation.
6. The method according to claim 1 , wherein the interleaver is a Latin square/rectangular matrix of random sequence pattern interleaver (LRI) with a size=2044 bits.
7. The method according to claim 5 , wherein the constellation mapping comprises mapping to at least a 4 bits QAM constellation.
8. The method according to claim 5 , wherein mapping the codeword v onto a 2 dimensional 2 n+2 points QAM constellation comprises mapping the codeword v to a 2 dimensional signal set partitioned into 16 cosets labeled by the four least significant bits of the codeword v.
9. The method according to claim 7 , wherein a minimum squared Euclidian distance (MSED) between points in the same coset is 16 d.sub.0.sup.2, where 16d 0 2 is the MSED between point in the unpartitioned constellation.
10. The method according to claim 8 , wherein the constellation is a b=4 bits QAM (16 QAM) constellation and each coset is formed by a single constellation point.
11. The method according to claim 8 , wherein the constellation is a b>4 bits QAM constellation, each coset is formed by 2 b−4 points and is determined by the turbo coded bits only, and the coset labels are obtained from a square lattice with MSED d 0 2 between points that is constructed by duplication a 4.times.4 pattern obtained when the constellation is a 4 bits QAM such that the MSED between points with the same label remains equal to 16 d 0 2 .
12. The method according to claim 2 , wherein n=1.5.
13. The method according to claim 12 , wherein 3 least significant bits are passed to the redundant bit turbo-engine to generate a 3 bit code word and the 3 bit codeword is appended to the least significant bit position of the 3 bit information word to create a 6 bit codeword.
14. The method according to claim 13 , wherein preparing the (n+m) bit code word for transmission over a communications channel comprises mapping the (n+m) bit codeword onto two identical 2 dimensional 8-QAM constellations and loading the two 2 dimensional 8-QAM constellations onto two distinct tones such that the 8 constellation map points 000, 100, 110, 010, 011, 111, 101 and 001 are at x-y locations {2,1}, {1,2}, {−1,2}, {−2,1}, {−2,−1}, {−1,−2}, {1,−2} and {2,−1} respectively.
15. The method according to claim 2 , wherein n equals either 1 or ½.
16. The method according to claim 15 , wherein the one least significant bit is passed to the redundant bit turbo-engine to generate a 1 bit code word and the 1 bit codeword is appended to the least significant bit position of the 1 bit information word to create a 2 bit codeword.
17. The method according to claim 16 , wherein preparing the (n+m) bit code word for transmission over a communications channel comprises, if n=½, performing a 4 dimensional (two 2-D) mapping of the (n+m) bit codeword onto two separate BPSK constellations on two distinct tones, otherwise, if n=1, performing a 2 dimensional mapping of the (n+m) bit codeword onto a 2 dimensional QPSK constellation on a single tone.
18. A redundant bit turbo engine apparatus for performing turbo trellis coded modulation (TTCM) on an n bit information word in a DMT system comprising:
first and second serial data paths that each receive a duplicate serial data stream comprised of at least one least significant bit of then bit information word;
a serial to parallel converter for converting the m-bit serial data stream into an m-bit code word;
circuitry adapted to append the m-bit code word to the n bit information word at the least significant bit position to create an (n+m) bit codeword; and
a constellation mapper for mapping the (n+m) bit codeword on to at least one tone to prepare the (n+m) bit code word for transmission over a communications channel
wherein the first serial data path comprises a first systematic recursive encoder, and the second serial data path comprises a second systematic recursive encoder preceded by an interleaver, and a switch adapted to alternately select bits output from the first and second serial data paths to form an m-bit serial data stream, wherein the constellation mapper is adapted to map the (n+m) bit code onto two identical 2 dimensional 8-QAM constellation mappings.
19. The apparatus according to claim 18 , wherein n is also the spectral efficiency of the turbo trellis coded modulation.
20. The apparatus according to claim 19 , wherein n≧2.
21. The apparatus according to claim 20 , wherein the first and second serial data paths are adapted to receive a duplicate serial data stream comprised of 2 least significant bits of the n bit information word.
22. The apparatus according to claim 21 , wherein the constellation mapper is adapted to map the (n+m) bit code word onto a 2 dimensional 2 n+2 points QAM constellation on a single tone.
23. The apparatus according to claim 18 , wherein the interleaver is a Latin square/rectangular matrix of random sequence pattern interleaver (LRI) with a size=2044 bits.
24. The apparatus according to claim 19 , wherein n=1.5.
25. The apparatus according to claim 24 , wherein the first and second serial data paths are adapted to receive a duplicate serial data stream comprised of 3 least significant bits of the n bit information word and to each generate a 3 bit serialized code word.
26. The apparatus according to claim 24 , wherein the constellation mapper is adapted to load the two pairs of 2 dimensional 8-QAM constellations onto two distinct tones.
27. The apparatus according to claim 19 , wherein n equals either 1 or ½.
28. The apparatus according to claim 27 , wherein the first and second serial data paths are adapted to receive a duplicate serial data stream comprised of one least significant bit and to each generate a 1 bit code word.
29. The apparatus according to claim 19 , wherein the constellation mapper is adapted to perform a 4 dimensional (two 2-D) mapping of the (n+m) bit codeword onto two separate BPSK constellations on two distinct tones if n=½, and to perform a 2 dimensional mapping of the (n+m) bit codeword onto a 2 dimensional QPSK constellation on a single tone if n=1.