The concatenation of coset (trellis) codes with multichannel modulation is studied. It is found that at a conceptual level, this concatenation is straightforward. Multichannel modulation partitions the digital communication channel into a set of memoryless subchannels, each of which can be separately encoded and decoded. However, the approach is also found to be unacceptable in practice because of latency and complexity. Encoding/decoding methodologies that can be used to reduce latency significantly and that also use the same encoder on all subchannels are studied. It is shown that using multidimensional coset codes (four or eight dimensions) to achieve a granularity in bit distribution that will closely approximately the continuous distribution that is optimum is best. A loading algorithm is introduced for assigning bits and energies to the subchannels. It easily and quickly produces an overall modulated signal that is near optimum. The technique is illustrated using example channels based on a high-bit-rate digital subscriber line at 1.6 Mb/s.
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