ARQ with Subcarrier Assignment for OFDM Systems

We consider an automatic-repeat-request (ARQ) scheme in OFDM systems. When a transmission fails, the transmitter repeats the data symbols in the retransmission, but on different subcarriers. The receiver then performs maximal- ratio combining (MRC) on the data symbols in the original and ARQ transmissions, before symbol detection is carried out. Our goal is to determine subcarrier assignment of the data symbols in the retransmission, such that utility functions which determine system-level performances are maximized. We show that when the utility function is Schur concave, the optimum solution is to assign the ARQ subcarrier with the largest SNR to the original subcarrier with the smallest SNR. Further, we propose heuristic solutions when some imposed constraints are relaxed.