  We study multi-user transmission and detection schemes for a multi-access
relay network (MARN) with linear constraints at all nodes. In a $(J, J_a, R_a,
M)$ MARN, $J$ sources, each equipped with $J_a$ antennas, communicate to one
$M$-antenna destination through one $R_a$-antenna relay. A new protocol called
IC-Relay-TDMA is proposed which takes two phases. During the first phase,
symbols of different sources are transmitted concurrently to the relay. At the
relay, interference cancellation (IC) techniques, previously proposed for
systems with direct transmission, are applied to decouple the information of
different sources without decoding. During the second phase, symbols of
different sources are forwarded to the destination in a time division
multi-access (TDMA) fashion. At the destination, the maximum-likelihood (ML)
decoding is performed source-by-source. The protocol of IC-Relay-TDMA requires
the number of relay antennas no less than the number of sources, i.e., $R_a\ge
J$. Through outage analysis, the achievable diversity gain of the proposed
scheme is shown to be $\min\{J_a(R_a-J+1),R_aM\}$. When {\small$M\le
J_a\left(1-\frac{J-1}{R_a}\right)$}, the proposed scheme achieves the maximum
interference-free (int-free) diversity gain $R_aM$. Since concurrent
transmission is allowed during the first phase, compared to full TDMA
transmission, the proposed scheme achieves the same diversity, but with a
higher symbol rate.
