Hybrid Iterative Detection and Decoding of Near-Instantaneously Adaptive Turbo-Coded Sparse Code Multiple Access

Reduced-complexity hybrid detection and decoding (HDD) schemes are conceived for turbo-coded sparse code multiple access (SCMA) by analysing its convergence behavior using extrinsic information transfer (EXIT) charts, which outperforms the state of the art joint detector and decoder (JDD). As a benefit of its carefully controlled complexity, the resultant system is eminently suitable for ultra-reliable low-latency communication (URLLC) in multiuser scenarios, since it requires a low number of turbo iterations between the detector and the decoder. In particular, a pair of HDD schemes are proposed. HDD-I and HDD-II achieve a complexity reduction of up to 25% and 36%, respectively, at a similar bit error rate (BER) performance as that of JDD. Additionally, we propose an adaptive turbo-coded SCMA system for mitigating the influence of multipath propagation so that the system's bits per symbol (BPS) throughput may be improved under favorable channel conditions by using the most appropriate near-instantaneous user load, modulation order and coding rate. Our adaptive system design principle can also be readily used for other channel coding schemes.