Efficient multiuser cooperation strategies using QAM space-time block codes

Practical space-time coding strategies are proposed for multiple users cooperating to communicate to a destination in a wireless channel. For a system with m single antenna users and a destination with N receive antennas, a two-phase cooperative scheme is proposed that achieves a diversity order of m - 1 + N for the error probability. This effectively proves the achievability of the diversity order implied by the outage probability for this model. This two-phase scheme employs a short full diversity space-time block code over QAM symbols. Even though the suboptimum decoder used by the destination assumes that the inter-user communication is successful, the corresponding analysis does account for the decoding errors in the inter-user communications phase. The key to the diversity order analysis is an appropriate modification of the rule for deciding whether or not a user acts as a relay for another user. The performance analysis technique presented here also extends to another practical cooperation strategy, proposed specifically for the case of m = 2 and N = 1, that is inspired from an optimal diversity-multiplexing tradeoff curve achieving protocol.