Ultra-Low-Cost and Ultra-Low-Power, Miniature Acoustic Modems Using Multipath Tolerant Spread-Spectrum Techniques

Abstract

To enable long-term, large-scale, dense underwater sensor networks or Internet of Underwater Things (IoUT) this research investigates new novel waveforms and experimental prototypes for robust communications on ultra-low-cost and ultra-low-power, miniature acoustic modems. Spread-spectrum M-ary orthogonal signalling (MOS) is used with symbols constructed from subsequences of long pseudorandom codes. This decorrelates multipath signals, even when the time-spread spans many symbols, so they present as random noise. A highly cost-engineered and miniaturised prototype acoustic modem implementation was created, for the 24 kHz–32 kHz band, with low receive power consumption (12.5 mW) and transmit power of <1 W. Simulations show that the modulation scheme achieves 640 bit/s at −4.5 dB with AWGN or the equivalent level of multipath energy. Experimental validation of the hardware shows successful point-to-point communication at ranges of >3 km in lakes and >2 km in the sea including severe multipath. In lake testing of a 7-node, multi-hop, sensor network with TDA-MAC protocol, packet delivery was near 100% for all nodes. Trials of acoustic sensor nodes in the North Sea achieved 99.5% data delivery over a 3-month period and a wide range of sea conditions. Modulation and hardware have proven reliable in a variety of underwater environments. Competitive range and throughput with low cost and power are attractive for large-scale and long-term battery-operated networks. This research has delivered a viable and affordable communication technology for future IoUT applications.