  In this paper, we consider the problem of multi-relay selection for
multi-stream cooperative MIMO systems with $M$ relay nodes. Traditionally,
relay selection approaches are primarily focused on selecting one relay node to
improve the transmission reliability given a single-antenna destination node.
As such, in the cooperative phase whereby both the source and the selected
relay nodes transmit to the destination node, it is only feasible to exploit
cooperative spatial diversity (for example by means of distributed space time
coding). For wireless systems with a multi-antenna destination node, in the
cooperative phase it is possible to opportunistically transmit multiple data
streams to the destination node by utilizing multiple relay nodes. Therefore,
we propose a low overhead multi-relay selection protocol to support
multi-stream cooperative communications. In addition, we derive the asymptotic
performance results at high SNR for the proposed scheme and discuss the
diversity-multiplexing tradeoff as well as the throughput-reliability tradeoff.
From these results, we show that the proposed multi-stream cooperative
communication scheme achieves lower outage probability compared to existing
baseline schemes.
