Pilot-Based Unsourced Random Access with a Massive MIMO Receiver:\n Interference Cancellation and Power Control

Abstract

In this work we treat the unsourced random access problem on a Rayleigh\nblock-fading AWGN channel with multiple receive antennas. Specifically, we\nconsider the slowly fading scenario where the coherence block-length is large\ncompared to the number of active users and the message can be transmitted in\none coherence block. Unsourced random access refers to a form of grant-free\nrandom access where users are considered to be a-priori indistinguishable and\nthe receiver recovers a list of transmitted messages up to permutation. In this\nwork we show that, when the coherence block length is large enough, a\nconventional approach based on the transmission of non-orthogonal pilot\nsequences with subsequent channel estimation and Maximum-Ratio-Combining (MRC)\nprovides a simple energy-efficient solution whose performance can be well\napproximated in closed form. Furthermore, we analyse the MRC step when\nsuccessive interference cancellation (SIC) is done in groups, which allows to\nstrike a balance between receiver complexity and reduced transmit powers.\nFinally, we investigate the impact of power control policies taking into\naccount the unique nature of massive random access, including short message\nlengths, uncoordinated transmission, a very large amount of concurrent\ntransmitters with unknown identities, channel estimation errors and decoding\nerrors. As a byproduct we also present an extension of the MMV-AMP algorithm\nwhich allows to treat pathloss coefficients as deterministic unknowns by\nperforming maximum likelihood estimation in each step of the MMV-AMP algorithm.\n