On the Sum-Rate of Contention Resolution in Massive MIMO With NOMA

In this paper, we investigate the random access (RA) problem in a massive MIMO scenario. To deal with the continuous increase in the number of users, and the difficulty associated with sharing pilots among such users in order to organize channel access, a strongest-user collision resolution (SUCRe) protocol is modified to include a non-orthogonal multiple access (NOMA) approach. The proposed NOMA-RA protocol allows to resolve collisions between users who try to access the medium using the same pilot signal. Numerical results show that the proposed NOMA-RA strategy achieves a better performance in terms of sum-rate with lower average delay in comparison with the traditional SUCRe scheme, even considering imperfect channel estimation and successive interference cancellation. For instance, in a scenario when 10 pilot sequences are available and 20 devices compete to access the channel, comparing with the SUCRe protocol, the average delay is reduced by 30% while the sum-rate is improved by 33% and 22% considering, respectively, perfect and imperfect SIC decoders. In addition, the percentage of users that fail to access the BS in this situation drops from 66.5% with SUCRe to 38.5% with the proposed NOMA-RA scheme.