  We consider a Gaussian multiple-access channel (MAC) with an
amplify-and-forward (AF) relay, where all nodes except the receiver have
multiple antennas and the direct links between transmitters and receivers are
neglected. Thus, spatial processing can be applied both at the transmitters and
at the relay, which is subject to optimization for increasing the data rates.
In general, this optimization problem is non-convex and hard to solve. While in
prior work on this problem, it is assumed that all transmitters access the
channel jointly, we propose a solution where each transmitter accesses the
channel exclusively, using a time-division multiple-access (TDMA) scheme. It is
shown that this scheme provides higher achievable sum rates, which raises the
question of the need for TDMA to achieve the general capacity region of MACs
with AF relay.
