Receiver Enhancement for Differentially Encoded Underwater Acoustic OFDM Incorporating Amplitude and Phase Mismatches

Differential phase shift keying encoded underwater acoustic orthogonal frequency division multiplexing (OFDM) systems have been extensively investigated in recent years due to their robust performance and efficient use of bandwidth with no overhead on pilot subcarriers for channel estimation. Conventional differential detection schemes usually assume that the channel frequency responses on consecutive OFDM subcarriers are identical, which would not be true for practical underwater acoustic channels. To address this issue, this article presents an advanced symbol demodulator consisting of two key modules. First, the receiver estimates the amplitude and phase mismatches on neighboring subcarriers, which utilizes the finite-alphabet property of the signal constellation and hence does not incur any pilot overhead. Second, with the estimated amplitude and phase mismatches, the receiver computes the bit likelihood ratio based on the Generalized Likelihood Ratio Test (GLRT) Principle. The proposed detection schemes are further extended to multisymbol differential detection involving measurements on more than two neighboring subcarriers for differential decoding. Simulation and sea trial data decoding results demonstrate considerable performance improvement of the proposed schemes over conventional detection schemes.