Towards Robust High Speed Underwater Acoustic Communications Using Chirp Multiplexing

Wide band chirp spread spectrum (CSS) has been studied in underwater acoustic (UWA) communications to guarantee low bit error rates with low transmission rates. On the other hand, high speed transmission schemes for UWA, especially Orthogonal-Frequency-Division-Multiplexing (OFDM) technology, can reach Mbit/s data rates but at the expense of reliability. In this paper, we propose a new transmission scheme based on multiplexing information bits to (quasi-)orthogonal chirp signals, called Quasi-Orthogonal Chirp Multiplexing (QOCM). It can be easily implemented on reprogrammable hardware as an extension to OFDM architectures. We evaluated the performance of the proposed scheme with simulation and tank experiments. Results show that QOCM scheme is able to achieve one order of magnitude better performance than an OFDM scheme under severe Doppler effect posed by simulation in long distance water transmission. Moreover, we observe in water tank experiment that the QOCM scheme is resilient against to the Doppler effects induced by relative speeds up to 5 knots, which corresponds to a Doppler shift of 214.16 Hz at a center frequency of 125 kHz.