Towards the Optimal Amplify-and-Forward Cooperative Diversity Scheme

In a slow-fading channel, how to find a cooperative diversity scheme that achieves the transmit diversity bound is still an open problem. In fact, all previously proposed amplify-and-forward (AF) and decode-and-forward (DF) schemes do not improve with the number of relays in terms of the diversity-multiplexing tradeoff (DMT) for multiplexing gains r higher than 0.5. In this work, the class of slotted amplify-and-forward (SAF) schemes is studied. First, an upper bound on the DMT for any SAF scheme with an arbitrary number of relays N and number of slots M is established. Then, a sequential SAF scheme that can exploit the potential diversity gain in the high multiplexing gain regime is proposed. More precisely, in certain conditions, the sequential SAF scheme achieves the proposed DMT upper bound which tends to the transmit diversity bound when M goes to infinity. In particular, for the two-relay case, the three-slot sequential SAF scheme achieves the proposed upper bound and outperforms the two-relay nonorthorgonal amplify-and-forward (NAF) scheme of Azarian for multiplexing gains r les 2/3. Numerical results reveal a significant gain of our scheme over the previously proposed AF schemes, especially in high spectral efficiency and large network size regime.