Power control for fading multiple access channels with user cooperation

For a fading Gaussian multiple access channel with user cooperation, we obtain the optimal power allocation policies that maximize the rates achievable by block Markov superposition coding. The optimal policies result in a coding scheme that is simpler than the one for a general multiple access channel with generalized feedback. This simpler coding scheme also leads to the possibility of formulating an otherwise non-concave optimization problem as a concave one. Using the channel state information at the transmitters to adapt the powers, we demonstrate significant gains over the achievable rates for existing cooperative systems.