Multi-user Multi-carrier Underwater Acoustic Communications

We address the design of a multi-user system where each user employs multi-carrier modulation in the form of orthogonal frequency division multiplexing (OFDM). We present a transmission and detection method that combines OFDM with direct-sequence spread spectrum. Coding is employed across carriers in a manner that provides multi-user interference suppression along with subsequent channel estimation and data detection. We consider both coherent and differentially coherent data detection, each integrated with a custom-designed technique for Doppler frequency shift compensation. Using the recordings from the Mobile Acoustic Communication Experiment (MACE'10), where signals were transmitted in the 10.5 kHz - 15.5 kHz acoustic band over a shallow-water channel, we demonstrate the performance of the system in terms of the mean squared error for a varying number of users. We conclude that the proposed system is able to operate in the multi-user interference scenario while maintaining low computational complexity.