On scalar ambiguity in blind channel estimation for OFDM systems
A new two-dimensional blind channel estimation scheme for coherent detection of OFDM signals in a mobile environment is presented. The channel estimation is based on the A Posteriori Probability (APP) calculation algorithm. The time-variant channel transfer function is completely recovered without phase ambiguity with no need for any pilot or reference symbols. The two-dimensional channel estimation is performed by applying a concatenation of two one-dimensional APP estimators for frequency and time direction in combination with an iterative estimation and decoding loop. The phase ambiguity problem is solved by using higher order modulation schemes with asymmetrical arrangement. The proposed approach maximizes the spectral efficiency by avoiding any reference or pilot symbols and minimizes the BER by using coherent demodulation. We investigate the performance of our algorithm with respect to the BER and study the convergence of the iterative estimation and decoding loop using Extrinsic Information Transfer (EXIT) charts.