Coded modulation for partially coherent systems

• US 20050074068A1
• Filed: 10/02/2003
• Published: 04/07/2005
• Est. Priority Date: 10/02/2003
• Status: Active Grant

• Current Assignee: Nokia Corporation
• Original Assignee: Nokia Corporation
• Inventors: Borran, Mohammad Jaber, Aazhang, Behnaam
• Granted Patent: US 7,088,784 B2
• Time in Patent Office: Days
• US Class Current: 375/264
• CPC Class Codes: H04L 1/0054 Maximum-likelihood or sequential decoding, e.g. Viterbi, Fano, ZJ algorithms
• CPC Class Codes: H04L 1/006 Trellis-coded modulation
• CPC Class Codes: H04L 27/3433 using an underlying square constellation

Abstract

A signal constellation is optimized for trellis coded modulation in fast fading channels, where the receiver does not have perfect knowledge of the channel parameters. Specifically, the signal constellation is partitioned into 2n mutually exclusive subsets, each preferably defining two points. Points within each subset are separated from one another by a distance between conditional distributions, preferably a Kullback-Leibler (KL) distance. For a block m=k₁+k₂ of information bits input into a trellis coder 30, the k, bits are trellis encoded into n bits (n>k₁) and used to select a subset of the constellation. The k₂ bit(s) is/are used to select a particular point within the subset. Because the inter-subset distance between points is a KL distance that is effectively greater than a Euclidean distance, error at the receiver is substantially reduced, especially at higher SNR. Using a KL distance ensures statistics of channel fading are inherent within the signal constellation.

Claims

1. A method for encoding a plurality of bits, comprising:

based on a plurality of bits, selecting one of at least two mutually exclusive subsets of a signal constellation and a point within said selected subset; and

modulating the selected point using a carrier waveform,

wherein the selected subset includes at least two constellation points that are separated from one another by a distance based on a conditional distribution.

2. The method of claim 1 wherein the distance based on a conditional distribution is one of a Kullback-Leibler distance and an expected Kullback-Leibler distance.

3. The method of claim 1, wherein selecting a subset of a signal constellation and a point within said selected subset comprises, based on a plurality m=k₁+k₂ of bits, using k₁ of the bits to select said subset and k₂ of the bits to select the point within said subset, wherein m, k₁ and k₂ are non-zero integers.

4. The method of claim 3 wherein using k₁ of the bits to select said subset comprises encoding the k₁ bits into n encoded bits, and selecting one of 2ⁿ mutually exclusive subsets with the n encoded bits, wherein n is greater than k₁

5. The method of claim 4 wherein k₂=1 and n=k₁+1.

6. The method of claim 5 wherein n is selected from the set consisting of three, four and five, wherein the k₁ bits are encoded using a 2/3 convolutional code when n=3, the k, bits are encoded using a 3/4 convolutional code when n=4, and the k₁ bits are encoded using a 4/5 convolutional code when n=5.

7. The method of claim 1 wherein the constellation points define concentric circles, and every point lying within a circle is from a different subset from every other point lying on that circle.

8. The method of claim 7 wherein every point on a circle is from a different subset from every other point lying on that circle and from every other point lying on an adjacent circle.

9. The method of claim 8 wherein n=3 and each subset defines two points.

10. The method of claim 1 further comprising transmitting the carrier, receiving the carrier over a fading channel, and decoding the symbol using a Viterbi algorithm.

11. A transmitter for transmitting a series of input bits comprising:

an encoder having an input for receiving a plurality of input bits;

a mapper having an input coupled to an output of the encoder; and

a computer-readable storage medium coupled to the mapper for storing at least one signal constellation,

wherein the mapper selects a subset of said signal constellation and a point within the selected subset based on the plurality of input bits, said selected subset including at least two constellation points that are separated from one another by a distance based on a conditional distribution.

12. The transmitter of claim 11 wherein the distance based on a conditional distribution is one of a Kullback-Leibler distance and an expected Kullback-Leibler distance.

13. The transmitter of claim 11, wherein the plurality of input bits comprises m=k₁+k₂ of bits, of which k₁ of the bits are used to select said subset and k₂ of the bits are used to select the point within said subset, wherein m, k₁ and k₂ are non-zero integers.

14. The transmitter of claim 13 wherein the encoder encodes k, of the bits into n encoded bits, and the mapper selects one of 2ⁿ mutually exclusive subsets using the n encoded bits, wherein n is greater than k₁

15. The transmitter of claim 14 wherein k₂=1 and n=k₁+1.

16. The transmitter of claim 15 wherein n is selected from the set consisting of three, four and five, wherein the k, bits are encoded using a 2/3 convolutional code when n=3, the k₁ bits are encoded using a 3/4 convolutional code when n=4, and the k, bits are encoded using a 4/5 convolutional code when n=5.

17. The transmitter of claim 11 wherein the constellation points define concentric circles, and every point lying within a circle is from a different subset from every other point lying on that circle.

18. The transmitter of claim 17 wherein every point on a circle is from a different subset from every other point lying on that circle and from every other point lying on an adjacent circle.

19. The transmitter of claim 18 wherein n=3 and each subset defines two points.

20. The transmitter of claim 12 further comprising a receiver, said receiver said receiver using a Viterbi algorithm to decode a received symbol into a subset and a point within the subset according to the constellation.

21. A method for encoding a plurality of m=k₁+k₂ input bits comprising:

selecting a subset of a signal constellation based on the k₁ input bits;

selecting a point within the selected subset based on the k₂ input bits, wherein at least two points within the selected subset are spaced from one another by a distance based on a conditional distribution of at least one of said at least two points; and

modulating the selected point using a carrier waveform,

wherein m, k₁ and k₂ are non-zero integers, and at least one of k₁ and k₂ are greater than one.

22. The method of claim 21, wherein selecting a subset of a signal constellation based on the k₁ input bits comprises encoding the k₁ input bits into n encoded bits and selecting one of 2ⁿ subsets using the n encoded bits, wherein n is an integer greater than k₁ that is derived from the k₁ bits and a previously input plurality of bits.

23. The method of claim 22, wherein each subset consists of two points and the signal constellation consists of 2^m+1 points.