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We consider a single-antenna Gaussian multiple-access channel (MAC) with amultiple-antenna amplify-and-forward (AF) relay, where, contrary to manyprevious works, also the direct links between transmitters and receiver aretaken into account. For this channel, we investigate two transmit schemes:Sending and relaying all signals jointly or using a time-divisionmultiple-access (TDMA) structure, where only one transmitter uses the channelat a time. While the optimal relaying matrices and time slot durations arefound for the latter scheme, we provide upper and lower bounds on theachievable sum-rate for the former one. These bounds are evaluated by MonteCarlo simulations, where it turns out that they are very close to each other.Moreover, these bounds are compared to the sum-rates achieved by the TDMAscheme. For the asymptotic case of high available transmit power at the relay,an analytic expression is given, which allows to determine the superior scheme.

Achievable Sum-Rates in Gaussian Multiple-Access Channels with MIMO-AF-Relay and Direct Links