Center for Pervasive Communications and Computing

In this technical report, we provide diversity analysis for two transmission schemes in (J;Ja;Ra;N) multi-access relay networks (MARNs), where J users, each equipped with Ja antennas, communicate to one N-antenna receiver through Ra single-antenna relays. Both transmission schemes allow all users' symbols to be transmitted concurrently through the source-relay and relay-receiver links. Therefore, both schemes have the potential of high transmission rate in multi-user relay networks. In the first scheme, called DSTC joint-user ML decoding, the relays perform distributed space-time coding (DSTC) to improve the reliability of the system, and the receiver jointly decodes all users' symbols using the ML decoding. Through rigorous analysis, this scheme achieves a symbol rate of 1/2 symbols/user/channel use, in conjunction to a diversity gain of Ra*min(Ja;N), which is the maximum diversity achievable in this network. But the decoding complexity of this scheme is exponential in the number of users. To reduce the decoding complexity, we consider a second scheme, DSTC-ICRec, in which the relay operations are the same, but the receiver first conducts interference cancellation (IC) to decouple multi-user signals, then decodes each user's symbols independently. We show analytically that in (2; 1; 2; N) and (2; 2;2;N) MARNs, DSTC-ICRec achieves a diversity of 1 and min(2;N-1), respectively, at a symbol rate of 1/2 symbols/user/channel use. Since the maximum achievable diversity gains in these two networks are 2 and 2*min(2;N), respectively, DSTC-ICRec has a lower diversity gain, compared to DSTC joint-user ML decoding, but its decoding complexity is much lower due to the IC.