Before revision:  We further establish the equivalence between sorting SNRs and sorting channel gains in the jointly optimal solution , which allows simple implementation of optimal subcarrier pairing at the relays. Simulation results are presented to demonstrate the performance gain of the jointly optimal solution over some suboptimal alternatives  .
After revision:  We further establish the equivalence between sorting SNRs and sorting channel gains in the jointly optimal solution . It significantly reduces the computational complexity in finding the jointly optimal solution. The solution for optimizing power allocation is also provided. Numerical results are provided to demonstrate substantial performance gain of the jointly optimal solution over some suboptimal alternatives .