Performance evaluation of modified OFDM for underwater communications

Conventional orthogonal frequency division multiplexing (OFDM) can reduce errors in underwater channels but frequency selective fading can still degrade the data on carriers near deep fade regions. Recently spread-OFDM (SOFDM) which combines the properties of spread spectrum with multicarrier communication has become popular. In this paper spreading by Walsh-Hadamard (WH) codes, discrete Fourier transform (DFT), discrete cosine transform (DCT) and carrier interferometry (CI) codes have been compared for underwater acoustic communication using OFDM. The spreading technique not only helps overcome frequency-selective and multipath fading but also provides reduced peak to average power ratio (PAPR). The performance of this modified OFDM scheme is evaluated in terms of bit error rate (BER) and PAPR. Simulation results show that with CI-SOFDM it is possible to achieve a BER of 1e-4 at 15dB signal to noise ratio. It is also shown that CI-SOFDM offers best PAPR performance. To further improve the BER performance a two level orthogonal spreading by WH followed by CI-SOFDM is also evaluated for underwater communication. The proposed scheme exploits the spreading gain diversity of WH code and phase characteristic property of CI signal to achieve a further gain of 1dB at a BER of 1e-4.