Impact of Channel Aging and Pilot Contamination on Decoding and Power Control Strategies for Uplink Massive MIMO-NOMA Systems

We quantify the impact of channel outdatedness and pilot contamination on the achievable spectral efficiency (SE) of uplink massive multiple input multiple output non-orthogonal multiple access (mMIMO-NOMA) system. Specifically, we compute novel closed-form expressions for achievable SE (i) for both ZF and MR decoders based on outdated channel estimates obtained using pilot sharing-based estimation schemes, namely, Scheme-I and Scheme-S, and (ii) for ZF decoder based on predicted channel obtained using Wiener linear predictor. The analysis accounts for time-variations in channel due to Doppler shift, correlation due to pilot sharing and imperfect successive interference cancellation. We formulate and solve optimization problems for max-min and proportional fairness power control using convex programming. We also study equal and inversion power control strategies. Intriguingly, among equal, inversion and max-min power control under channel aging, the ZF decoder based on Scheme-S (i) gives the highest per-user SE with max-min power control for smaller values of normalized Doppler shift (
f_DT_S
) and with inversion power control for larger values of
f_DT_S
, and (ii) gives the highest minimum user SE with max-min power control. Proportional fairness provides higher per-user SE than max-min power control while maintaining fairness to a significant extent.