A multiple trellis-coded hybrid-ARQ scheme for land mobile communication channels

A multiple trellis-coded scheme that adapts itself to varying channel conditions is proposed and simulated over a land mobile radio channel. The scheme can be classified as a type-II hybrid ARQ protocol that makes use of incremental redundancy. A multiple trellis code is one in which each trellis branch is associated with k⩾2 transmitted baud. Simon and Divsalar (1988) have described a method for constructing multiple trellis codes suited for fading environments. Their design technique is used to create rate-compatible punctured trellis codes-analogous to the rate-compatible punctured convolutional codes of Hagenauer (1988)-that vary their redundancy in response to feedback from the demodulator to the transmitter. Specifically, we use a multiple trellis code of multiplicity k=3, designed for fading channels, as our “mother” code. Depending on the channel conditions, this code is punctured to get another code of multiplicity k=2 with a higher bandwidth efficiency. Both the mother and the punctured codes are shown to satisfy the design procedure of Simon et al. The proposed protocol does not require the transmitter to anticipate the channel quality before transmission. Moreover, the protocol requires only a low-rate feedback channel, with significant delay acceptable. The performance of the adaptive code is compared to a “static” code of the same bandwidth efficiency and complexity designed by Schlegel and Costello (1989). Simulations show that the adaptive code gives a coding gain of about 3 dB over the Schlegel-Costello code at a bit error probability of 10^-5