Iterative blind data detection in constant modulus OFDM systems

In this paper, we consider blind data detection for OFDM transmission over block fading channels. Specifically, we show how constant modulus data of an OFDM symbol can be blindly detected using output symbol and associated cyclic prefix. Our approach relies on decomposing the OFDM channel into two subchannels (cyclic and linear) that share the same input and are characterized by the same channel parameters. This fact enables us to estimate the channel parameters from one subchannel and substitute the estimate into the other, thus obtaining a nonlinear relationship involving the input and output data only that can be searched for the maximum likelihood estimate of the input. This shows that OFDM systems are completely identifiable using output data only, irrespective of the channel zeros, as long as the channel delay spread is less than the length of the cyclic prefix. We also propose iterative methods to reduce the computational complexity involved in the maximum likelihood search of input.