Hexacode

Abstract: An algorithm for maximum-likelihood soft-decision decoding of the binary (24,12,8) Golay code is presented. The algorithm involves projecting the codewords of the binary Golay code onto the codewords of the (6,3,4) code over GF(4)-the hexacode. The complexity of the proposed algorithm is at most 651 real operations. Along similar lines, the tetracode may be employed for decoding the ternary (12,6,6) Golay code with only 530 real operations. The proposed algorithm also implies a reduction in the number of computations required for decoding the Leech lattice. >

Abstract: Multilevel constructions of the binary Golay code and the Leech lattice are described. Both constructions are based upon the projection of the Golay code and the Leech lattice onto the (6,3,4) hexacode over GF(4). However, unlike the previously reported constructions, the new multilevel constructions make the three levels independent by way of using a different set of coset representatives for one of the quaternary coordinates. Based upon the multilevel structure of the Golay code and the Leech lattice, efficient bounded-distance decoding algorithms are devised. The bounded-distance decoder for the binary Golay code requires at most 431 operations. As compared to 651 operations for the best known maximum-likelihood decoder. Efficient bounded-distance decoding of the Leech lattice is achieved by means of partitioning it into four cosets of Q/sub 24/, beyond the conventional partition into two H/sub 24/ cosets. The complexity of the resulting decoder is only 953 real operations on the average and 1007 operations in the worst case, as compared to about 3600 operations for the best known in maximum-likelihood decoder. It is shown that the proposed algorithms decode correctly at least up to the guaranteed error-correction radius of the maximum-likelihood decoder. Thus, the loss in coding-gain is due primarily to an increase in the effective error-coefficient, which is calculated exactly for both algorithms. Furthermore, the performance of the Leech lattice decoder on the AWGN channel is evaluated experimentally by means of a comprehensive computer simulation. The results show a loss in coding-gain of less than 0.1 dB relative to the maximum-likelihood decoder for BER ranging from 10/sup -1/ to 10/sup -7/. >

Abstract: We introduce self-dual codes over the Kleinian four group K = Z_2 x Z_2 for a natural quadratic form on K^n and develop the theory. Topics studied are: weight enumerators, mass formulas, classification up to length 8, neighbourhood graphs, extremal codes, shadows, generalized t-designs, lexicographic codes, the Hexacode and its odd and shorter cousin, automorphism groups, marked codes. Kleinian codes form a new and natural fourth step in a series of analogies between binary codes, lattices and vertex operator algebras. This analogy will be emphasized and explained in detail.

Abstract: The authors present an algorithm for maximum-likelihood soft decision decoding of the binary (24, 12, 8) Golay code. The algorithm involves projecting the codewords of the binary Golay code onto the codewords of the (6, 3, 4) code over GF(4)-the hexacode. The complexity of the proposed algorithm is at most 651 real operations. Along similar lines the tetracode may be employed for decoding the ternary (12, 6, 6) Golay code with only 530 real operations. The proposed algorithm also implies a reduction in the number of computations required for decoding of the Leech lattice. >