A hybrid-ARQ system using rate-compatible trellis codes designed for Rayleigh fading

This paper presents classes of rate-compatible trellis codes designed for c hannels with flat, slow Rayleigh fading. The codes are "multiple" TCM (MTCM ) codes as proposed by Divsalar and Simon - i.e., codes in which multiple s ymbols are associated with each transition through the trellis; by applying appropriate puncturing tables to low-rate MTCM codes,we obtain families of MTCM codes, all of which can be decoded with (essentially) the same decode r. By means of computer search, several such families are obtained so that each family member is at least as good as any comparable code in the litera ture. ("Good" here is defined in terms of minimum time diversity and minimu m squared product distance, the most important parameters for performance o ver Rayleigh fading channels). A protocol to implement these rate-compatibl e trellis codes in a type-II hybrid ARQ format is described. The proposed p rotocol selectively punctures symbols from the low-rate code based on the o bserved fading values observed on each branch. Upper bounds on the resultin g bit error rate are developed and the results are used to select the best adaptive code from several possibilities. Simulation results comparing the proposed scheme with fixed-rate codes of the same throughput show substanti al coding gain. Finally,a protocol modification limiting the variability of the code rate over a frame is described; this modification eliminates the need for excessive buffering, with a very small effect on performance.