Sparse or Dense: A Comparative Study of Code-Domain NOMA Systems

This paper is focused on code-domain non-orthogonal multiple access (CD-NOMA), which is an emerging paradigm to support massive connectivity for future machine-type wireless networks. We take a comparative approach to study two types of overloaded CD-NOMA, i.e., sparse code multiple access (SCMA) an...

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Bibliographic Details
Published in:IEEE transactions on wireless communications Vol. 20; no. 8; pp. 4768 - 4780
Main Authors: Liu, Zilong, Yang, Lie-Liang
Format: Journal Article
Language:English
Published: New York IEEE 01.08.2021
The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Subjects:
Channels
Comparative studies
Decoding
dense code multiple access (DCMA)
Design criteria
Domains
Error analysis
Fading
low-density parity check (LDPC) code
machine-type communications (MTC)
massive connectivity
message passing algorithm
Multiaccess communication
Multiuser detection
Nodes
NOMA
Non-orthogonal multiple access (NOMA)
Nonorthogonal multiple access
Receivers
sparse code multiple access (SCMA)
Sparsity
sphere decoding
Subcarriers
Uplink
Wireless networks
ISSN:1536-1276, 1558-2248
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Summary:This paper is focused on code-domain non-orthogonal multiple access (CD-NOMA), which is an emerging paradigm to support massive connectivity for future machine-type wireless networks. We take a comparative approach to study two types of overloaded CD-NOMA, i.e., sparse code multiple access (SCMA) and dense code multiple access (DCMA), which are distinctive from each other in terms of their codebooks having sparsity or not. By analysing their individual diversity orders (DO) in Rayleigh fading channels, it is found that DCMA can be designed with the aid of generalized sphere decoder (i.e., a nonlinear multiuser detector) to enjoy full DO which is equal to the maximum number of resource nodes in the system. This is in contrast to SCMA whose error rate suffers from limited DO equal to the codebook sparsity (i.e., the effective number of resource nodes occupied by each user). We conduct theoretical analysis for the codebook design criteria and propose to use generalized sphere decoder for DCMA detection. We numerically evaluate two types of multiple access schemes under "<inline-formula> <tex-math notation="LaTeX">4\times 6 </tex-math></inline-formula>" (i.e., six users communicate over four subcarriers) and "<inline-formula> <tex-math notation="LaTeX">5\times 10 </tex-math></inline-formula>" NOMA settings and reveal that DCMA gives rise to significantly improved error rate performance in Rayleigh fading channels, whilst having decoding complexity comparable to that of SCMA.
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ISSN:1536-1276
1558-2248
DOI:10.1109/TWC.2021.3062235