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