Citation
Abstract
Convolutional codes chosen for greatest free distance or lowest error probability on a binary channel are not necessarily good codes for an M-ary channel. A rate k/v coder generates a 2°-ary output symbol for each k input bits. If a binary channel is used, the appropriate measure of distance between these symbols is Hamming distance (number of bit disagreements). But if either a 2°-ary orthogonal channel, or a 2°-ary simplex channel is used, the distance between any two different symbols is unity (number of symbol disagreements). Other distance measures are appropriate on other M-ary channels. Good rate 1/2 and 1/3 codes have been found by computer search for the orthogonal 4-ary and 8-ary channels. The result is a reduction of error probabilities by about a factor of two below previously tabulated codes. The computer technique used is described. At a fixed constraint length, further performance improvement results from increasing v, up to a limit at v = K (constraint length) on the orthogonal 2°-ary channel.
Details
- Volume
- 42-24
- Published
- December 15, 1974
- Pages
- 60–77
- File Size
- 1.2 MB