Exploring quaternary linear frequency modulation (LFM) for improving long range underwater acoustic communication

For 80 km underwater acoustic (UWA) communication, Refs. 4 and 5 used time delay shift coding and Gold sequences respectively to deal with severe multipath interference and fading in shallow ocean channel. Thinking over their research results again and again, we came to feel that the performances of their methods against multipath interference and fading should be further improved if possible. So we used the well-known quaternary LFM in a new way to explore the possibility of better performance against multipath interference and fading. In the full paper, we explain in much detail our new way of using quaternary LFM; here we give only a briefing. In our new way, quaternary LFM signals, which occupy two different frequency bands, are used to represent messages in transmitter. The receiver estimates the messages by correlating the received signal with each LFM signal and selecting the largest value. Our new way of using quaternary LFM has three obvious good reasons for applying it to long range UWA communication: its good performance against multipath interference and fading, its relatively high data rate and its processing gain. Sea test results show that, over a distance of 30 km in a bandwidth of 200 Hz, its corresponding data rate reaches 11.8 bit/s without error. Computer simulation results using channel model based on sea test data show that its data rate can reach 20 bit/s in a bandwidth of 200 Hz over a distance of 80 km with a bit error rate of below 10^-4.