A Comparative Study of Spread OFDM with Transmit Diversity for Underwater Acoustic Communications

This paper compares the effectiveness of different spread OFDM techniques with transmit diversity used for underwater acoustic communications. Spreading by Walsh-Hadamard codes, discrete Fourier transform, discrete cosine transform and carrier interferometry (CI) codes have been combined with the application of space-time and space-frequency transmit diversity in OFDM for underwater acoustic communication. The spreading technique helps overcome frequency-selective and multipath fading and also provides reduced peak-to-average power ratio (PAPR). The application of space time and space frequency coding techniques improves the data rate and reliability of the spread OFDM scheme in terms of bit error rate (BER). Simulation results show that all the spread OFDM schemes show better BER performance in underwater environment when transmit diversity schemes are used. CI spread OFDM with space-frequency block coding shows a BER of 1×10-6\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$1\times 10^{-6}$$\end{document} at a signal-to-noise ratio (SNR) of 13.6\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$13.6$$\end{document} dB and is proposed as one of the best scheme. This gain in SNR may be used to further increase the data transmission rate.