  The 5th generation mobile communication systems aim to support massive access
for future wireless applications. Unfortunately, wireless resource scarcity in
random access (RA) is a fundamental bottleneck for enabling massive access. To
address this problem, we propose a virtual carrier sensing (VCS) based RA
scheme in massive MIMO systems. The essence of the proposed scheme lies in
exploiting wireless spatial resources of uplink channels occupied by assigned
user equipments (UEs) to increase channel resources for RA. With the proposed
scheme, RA UEs are able to exploit the spatial resources that are approximately
orthogonal to those of assigned UEs, thus sharing the uplink channel resource
with assigned UEs without causing significant interference to them.
Specifically, to ensure RA UEs avoid serious interference with assigned UEs,
base station (BS) sends tailored virtual carriers to RA UEs on behalf of
assigned UEs. RA UEs then conduct VCS to determine whether or not the uplink
channel resource is available for RA. Closed-form approximations for
probability of channel availability and uplink achievable rate with the
proposed scheme are derived. Theoretical analysis and simulation results show
that the proposed scheme is able to significantly increase channel resources of
RA for massive access.
