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Timing advance (TA) estimation at the base station (BS) and reliable decodingof random access response (RAR) at the users are the most important steps inthe initial random access (RA) procedure. However, due to the limitedavailability of physical resources dedicated for RA, successful completion ofRA requests would become increasingly difficult in high user density scenarios,due to contention among users requesting RA. In this paper, we propose to usethe large antenna array at the massive MIMO BS to jointly group RA requestsfrom different users using the same RA preamble. We then beamform the commonRAR of each detected user group onto the same frequency resource, in such a waythat most users in the group can reliably decode the RAR. The proposed RARbeamforming therefore automatically resolves the problem of collision betweenmultiple RA requests on the same RA preamble, which reduces the RA latencysignificantly as compared to LTE. Analysis and simulations also reveal that fora fixed desired SINR of the received RAR, both the required per-user RApreamble transmission power and the total RAR beamforming power can bedecreased roughly by 1.5 dB with every doubling in the number of BS antennas.

Timing Advance Estimation and Beamforming of Random Access Response in Crowded TDD Massive MIMO Systems