Time-domain equalization for underwater acoustic OFDM systems with insufficient Cyclic Prefix

Single Input Single Output (SISO) Orthogonal Frequency Division-Multiplexing (OFDM) over short to medium range shallow water channels suffers from low bandwidth efficiency. This happens due to large Cyclic Prefix (CP) and relative small number of sub-carriers. To increase the bandwidth efficiency, a time domain channel shortening equalizer (CSE) can be inserted before the Fast Fourier Transform (FFT). The CSE shortens the channel impulse response (CIR) so that a smaller CP is needed. This paper analyses the performance of four time domain CSEs: 1) Minimum Mean Square Error (MMSE) Unit Tap Constraint (UTC) 2) MMSE Unit Energy Constraint (UEC) 3) Maximum Shortening Signal to Noise Ratio (MSSNR) 4) Minimum Inter Symbol Interference (Min ISI) and Frequency Domain Decision Feedback Equalizer (FD-DFE). Analysing simulated and real data, the MMSE UTC equalizer shows the best performance in terms of Bit Error Rate (BER). When bit-loading is applied, the BER of Min ISI approach has comparable performance to UTC.