The cross-layer design is a promising strategy to maximize the spectral efficiency in communication systems. In this paper we focus on packet-oriented systems and we propose to combine adaptive modulation and coding (AMC) at physical layer and hybrid automatic repeat request (ARQ) at the data link layer. In the proposed multilevel hybrid ARQ (M-HARQ) strategy, the transmitted symbols are decomposed into a set of parallel sub-channels with an independent ARQ protocol over each sub-stream. It follows a layered structure where each sub-channel can be decoded only once the lower sub-channels have been resolved, thus subchannels are sequentially detected. The number of parallel streams is adaptively arranged according to the channel state information (CSI). Considerable throughput gain is achieved using the same encoder/decoder structure for all the sub-channels. Moreover, M-HARQ permits an adaptive and flexible implementation even when CSI is not available. The drawback is an increase in the decoding delay, which might be tolerable in some applications
['Roberto Bosisio', 'Umberto Spagnolini', 'Yeheskel Bar-Ness']
Multilevel type-II HARQ with adaptive modulation control