A Simplified Sphere Decoding-Based Detector for Generalized SCMA Codebooks

International audienceSparse code multiple access (SCMA) is one of the promising schemes to meet high connectivityand spectral efficiency in the future wireless networks. The iterative detectors, for example message passingalgorithm (MPA), can provide near optimal multiuser detection (MUD) performance but becomes infeasiblewhen the codebook size is large or the overloading factor is high. Recently, sphere decoding (SD) has beenconsidered in the MUD of SCMA by rewriting the generalized transmission into a linear system. In this work,we first review the state-of-the-art SD-based detectors for SCMA: sphere decoding for SCMA (SD-SCMA)and generalized SD-SCMA (GSD-SCMA). We not only explain the state-of-the-art in a comprehensiveway, but also exploit the sorted QR decomposition and Schnorr-Euchner enumeration to accelerate thetree search. Although GSD-SCMA overcomes the codebook constraint of SD-SCMA, its computationalcomplexity is extremely sensitive to the overloading factor. To satisfy the trade-off between complexity andMUD performance, we propose two pruning algorithms, PRUN1 and PRUN2, and introduce the simplifiedGSD-SCMA (SGSD-SCMA). In the paper, error probabilities of the proposed pruning algorithms arederived. Simulation results show that the proposed detector outperforms the iterative detectors and SD-basedstate-of-the-art when the overloading factor is moderate and the codebook size is large