Poster Abstract: OFDM in Underwater Channels

Orthogonal Frequency Division Multiplexing (OFDM) is a wideband modulation scheme that has recently gained attention for underwater acoustic communications. The benefits of OFDM include its ability to overcome long channel delay spreads through the use of a guard interval, its ability to transform a frequency selective channel into multiple frequency non-selective channels, and its relatively easy implementation through the use of the fast fourier transform and its inverse. However, OFDM has several distinct challenges in the underwater channel environment. The underwater channel is known to be highly frequency selective with large delay and Doppler spreading as well as fast time variance. The frequency selectivity requires accurate estimation of the channel transfer function in order to recover the transmitted symbols. Large delay spreading requires a longer guard interval, decreasing the data rate. Finally, Doppler spreading and fast time variations will introduce added noise at the demodulator due to intercarrier interference (ICI) and inaccurate channel estimation. In this work in progress poster, we will show results of an experiment using zero padded (ZP) OFDM where good performance was achieved at ranges of 250m to 4km. The results will show that, counter-intuitively, the worst performance was achieved at 250m. The results highlight the sensitivity of the OFDM parameters to the specific conditions of the underwater channel.