Rate allocation for 2-user MAC with MMSE turbo equalization

We consider the problem of rate allocation in frequency-selective 2-user Gaussian multiple access fading channels employing turbo equalization. The turbo equalization framework used in this paper contains a soft cancellation frequency domain minimum mean squared error equalizer and two a posteriori probability channel decoders. Using the relationship between the rate of any code and the area under its corresponding decoder extrinsic information transfer (EXIT) function, we derive an upper bound on the rate region of the 2-user turbo system, given the EXIT characteristic of the equalizer for a particular channel realization. With the rate region upper bound, we then study the problem of maximizing the sum rate of both users, and provide an approximate solution to this optimization problem. Based on the obtained result, a practical code selection algorithm for rate allocation at both transmitters is proposed. In addition, we discuss the extension of the proposed algorithm to an outage-based rate allocation approach. Numerical results of capacity calculations and throughput simulations are presented to demonstrate the performance enhancement achieved by the proposed rate allocation technique over automatic repeat request with fixed coding rate.