Achievable Rates of Massive MIMO NOMA Downlink with Limited RF Chains

Abstract

The impact of low-dimensional digital precoding on the achievable sum rate of a training-based massive multiple-input multiple-output (MIMO) non-orthogonal multiple access (NOMA) system with a limited number of radio-frequency (RF) chains is investigated. A low-dimensional digital precoder is cascaded with a high-dimensional analog precoder and thus reducing the number of RF chains required at the NOMA-enabled massive MIMO base-station. Uplink channel cascaded with the analog precoder is estimated at the BS via pilots sent by the clustered users. Each cluster is allocated with an orthogonal pilot sequence, and it is shared among users within a cluster. The achievable downlink sum rate is derived by capturing the effects of practical impediment, including channel estimation errors, intra-cluster pilot contamination, imperfect successive interference cancellation, and statistical/partial channel knowledge at the users for signal decoding. Thereby, the sum rate degradation caused by these transmission impairments and the impact of reduced number of RF chains at the BS are quantified. Moreover, the achievable sum rate and the number of users that can be served simultaneously in the same time-frequency resource block by massive MIMO NOMA are compared with those of massive MIMO orthogonal multiple access (OMA). Our results are used to draw system-design insights on fundamental trade-offs between the number of simultaneously served users, achievable sum rates and computational complexity. We conclude that massive MIMO NOMA is practically-viable for supporting massive access with low rates requirements, whereas massive MIMO OMA is more desirable when the high rate requirement is more prevalence than the demand for massive access.