Generalized multiuser QAM based cooperation strategy and an optimal two-user scheme

A two user cooperation strategy based on QAM information symbols, known as the self-information canceling linear (SCL) scheme, was recently proposed by the authors. The SCL scheme achieves the full diversity order of 2 and the full multiplexing gain of 1 inherent in the system. However, the diversity versus multiplexing gain tradeoff (DMT) of the SCL scheme was not shown to be optimal. In the first part of this work, we propose a new two user cooperation strategy that achieves the optimal DMT curve for the two user cooperative diversity system. This scheme is obtained by a suitable modification of the original SCL scheme. The modified SCL scheme is also a practical cooperation strategy based on QAM information symbols and the present analysis accounts for the inter-user communications errors. The modified SCL scheme is, thus, established as a specific coding scheme to achieve the information theoretic limits within the DMT framework. In the second part of this work, we generalize the original SCL scheme to the case of m users for any m > 2. The generalization leads to a practical multiuser cooperative scheme based on QAM symbols that provides the full diversity order of m, fair rate allocation among the users and potential for achieving the full multiplexing gain.