Bandwidth-efficient, low-latency adaptive coded modulation schemes for time-varying channels

In wireless systems supporting slowly moving users, adaptive trellis-coded modulation (TCM) schemes have demonstrated large bandwidth efficiency gains over their nonadaptive counterparts. In systems with highly mobile users, the adaptive bit-interleaved coded modulation (BICM) achieves a moderate bandwidth efficiency gain over previously proposed adaptive schemes and nonadaptive schemes with similar complexity. However, adaptive BICM requires a bit interleaver, which results in long latency. In this paper, adaptive coded modulation (ACM) schemes which do not employ interleaving and do not use uncoded bits are considered for time-varying channels. Two such ACM schemes are proposed. One of the ACM schemes uses a forward trellis search algorithm (FTS) to adapt to the current channel fading. Numerical results demonstrate that the proposed FTS-ACM scheme achieves a comparable bandwidth efficiency gain to adaptive BICM. FTS-ACM is particularly attractive for low latency transmission applications.