Maximum diversity order in cooperative MAC for general modulation schemes

We consider two sources that cooperatively send their messages to a destination over Rayleigh-fading channels. Assuming a decode-and-forward scheme, we obtain conditions to achieve the maximum diversity order for a general class of modulation schemes. This class includes the BPSK modulation where the mutual information is bounded, and Gaussian signaling where the mutual information is unbounded. With mutual information that is bounded, clearly the maximum achievable rate is limited; interestingly, our results quantify the exact conditions for the diversity order to be fundamentally limited.