Toward 5G Wireless Interface Technology: Enabling Nonorthogonal Multiple Access in the Sparse Code Domain

Designers of fifth-generation (5G) wireless networks are focusing on enabling highly reliable low-latency communications that support a high data rate and allow massive connectivity. Nonorthogonal multiple access (NOMA), an essential enabling technology tailored to accommodate a wide range of communication requirements, shows potential as a tool for helping 5G networks to fulfill these promised capabilities. By coordinating connections for massive numbers of devices within the same resource block on power domain, frequency domain, or code domain, NOMA is superior to conventional orthogonal multiple access in terms of network connectivity, system throughputs, and other characteristics. Sparse code multiple access (SCMA) is a kind of multicarrier code-domain NOMA and has been studied extensively. The challenge in designing a high-quality SCMA system is crafting feasible encoding and decoding schemes to meet the desired requirements. In this article, we describe recent progress in designing multidimensional codebooks, a practical low-complexity decoder, and grant-free multiple access for SCMA systems. Our particular focus is on showing how the designs of the multidimensional constellation and factor graphs (FGs) have formed the foundation of SCMA codebooks. In addition, we review various lowcomplexity SCMA decoders with a special emphasis on sphere decoding. Also, we introduce SCMA grant-free transmission based on the framework of belief propagation (BP) and discuss the problem of collision resolution.