Coherent detection of turbo coded OFDM signals transmitted through frequency selective Rayleigh fading channels

This work addresses the problem of coherently detecting turbo coded orthogonal frequency division multiplexed (OFDM) signals, transmitted through frequency selective Rayleigh fading channels. A single transmit and receive antenna is assumed. The channel output is distorted by a carrier frequency and phase offset, besides additive white Gaussian noise (AWGN). A new frame structure for OFDM, consisting of a known preamble, cyclic prefix and data is proposed. A filter that is matched to the preamble is used for start-of-frame (SoF) detection. A two-step ML detector for frequency-offset estimation is proposed, which has a much lower complexity compared to the single step ML detector. Turbo decoding and data interleaving is used to significantly enhance the bit-error-rate (BER) performance of the coherent receiver. Simulation results show that the BER performance of the practical coherent receiver is close to the ideal coherent receiver, for data length equal to the preamble length, and attains a BER of about 4× 10^-5 at an SNR of just 8 dB. It is also shown that the probability of erasure is less than 10^-6 for a preamble length of 512 QPSK symbols. The proposed algorithms are well suited for implementation on a DSP-platform.