Iterative OFDM ARQ Receivers with Imbedded Pilots

An optimized Automatic Repeat Request (ARQ) iterative receiver for Orthogonal Frequency Division Multiplexing (OFDM) transmission, suitable for application in rapidly varying multipath channels is presented. The receiver is an optimized Maximum Aposteriori (MAP) Turbo proces- sor that exploits information from all available transmis- sions. 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 Expectation Maximization (EM) algorithm and the avail- able received vectors, including all antennas and transmis- sions of the OFDM word. We show that this configuration achieves high gains compared to a standard ARQ system with packet combining and a standard soft decision decod- ing of a higher constraint length, = 7 convolutional code when binary modulation is employed on the link, while the gains diminish for non-binary modulations.