Frequency domain blind channel estimation without phase ambiguity for QAM-OFDM systems

It is well known that, in the case of frequency domain blind channel estimation, the channel gains are correctly estimated but the channel phases are not necessarily estimated correctly: the channel estimates are often accompanied with phase ambiguity because the phase range for numerical calculation is restricted to (-π, π]. In this paper, a new frequency-domain blind channel estimation without phase ambiguity is proposed for QAM-OFDM systems of which transmitting and receiving stations are in LoS (Line of Sight) under the conditions: i) the absolute value of phase rotation is less than π/2 at all subcarriers, and ii) it is less than π/4 for at least one subcarrier among them. From several simulation results for 50 sub-carriers with 16 QAM using 2048 OFDM blocks, it is confirmed that, when the SNR (Signal-to-Noise Ratio) in baseband region is over 20[dB], the QAM-OFDM channel is estimated at high precision without phase ambiguity. The restored symbols resulting from the channel estimates give the bit-error-rate of about 10-3 or less at SNR≥20[dB] for the case that the ratio of the root-mean-square delay spread to the OFDM symbol length is 1/16.