On Clustering and Channel Disparity in Non-Orthogonal Multiple Access (NOMA)

TL;DR

This paper reviews clustering challenges in NOMA, clarifies misconceptions about user grouping based on channel disparity, and discusses open problems in resource allocation for improved system performance.

Contribution

It provides a critical review of NOMA clustering strategies, clarifies common misconceptions, and highlights open research problems in resource allocation.

Findings

Clustering users with low channel disparity is not necessarily detrimental.

Similar power allocation among users can be effective in NOMA.

Numerical examples clarify misconceptions about NOMA clustering.

Abstract

Non-orthogonal multiple access (NOMA) allows multiple users to share a time-frequency resource block by using different power levels. An important challenge associated with NOMA is the selection of users that share a resource block. This is referred to as clustering, which generally exploits the channel disparity (i.e. distinctness) among the users. We discuss clustering and the related resource allocation challenges (e.g. power allocation) associated with NOMA and highlight open problems that require further investigation. We review the related literature on exploiting channel disparity for clustering and resource allocation. There have been several misconceptions regarding NOMA clustering including: 1) clustering users with low channel disparity is detrimental, 2) similar power allocation is disastrous for NOMA. We clarify such misunderstandings with numerical examples.