Optimized Iterative OFDM ARQ Receivers with Imbedded Pilots

In this paper, we propose an optimized ARQ iterative receiver for OFDM transmission, suitable for application in rapidly varying multipath channels. The receiver is an optimized MAP Turbo processor that exploits information from all available transmissions in an optimized way, using only the minimum required number of pilots for initial channel estimation. Our model does not account for CFO or transmitter non-linearities. The system uses an imbedded pilot structure in the frequency domain to initially estimate the channel. Further improvement of these estimates takes place based on the EM algorithm and the available received vectors, including all antennas and transmissions of the OFDM word. We show that this configuration achieves high gains compared to a standard ARQ system with packet combining and soft decision decoding of a higher constraint length, nu = 7 convolutional code. Finally, the potential of the approach in more realistic models (e.g. with CFO or nonlinear transmission) is established due to the minimum number of pilots employed herein.