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.
Abstract: Purpose/Objective: This work investigates techniques to mitigate the impact of acoustic communication signals on marine life, by minimising source level (SL) and designing waveforms with characteristics proven to reduce animal discomfort in bioacoustics studies. Methods: High-ratio spread spectrum transmission is employed with bandwidth-time products exceeding 1000. Signalling is based on families of near orthogonal pseudo-noise waveforms, generated by bandpass filtering of binary M-sequences. This enables reception of data, at very low SNR, over a radius many times greater than the radius of discomfort experienced by marine mammals. Computationally efficient receivers with novel synchronisation structures needed to be developed to operate at very low SNR and with severe Doppler effects. Results: Simulations show the proposed scheme is able to achieve 45 bit/s at –18dB SNR and 140 bit/s at –12dB SNR. Experimental system performance was assessed during realistic experiments in the North Sea, verifying performance over ranges up to 10km with transmitted SL of <170dB re 1 μPa @ 1m and with Doppler effects induced by relative motion exceeding 2m/s. Conclusion: The system developed compares favourably, in terms of SNR performance and channel utilisation, with previously reported work aimed at covert communication but offers reduced transmitter/receiver complexity and discomfort to animals. Significance: The work offers a way forward to more bio-friendly acoustic modem devices for operation in regions with sensitive fauna and/or increasingly strict environmental controls.
Abstract: Anthropogenic underwater noise has been shown to have a negative impact on marine life. Acoustic data transmissions have also been shown to cause behavioural responses in marine mammals. A promising approach to address these issues is through reducing the power of acoustic data transmissions. Firstly, limiting the maximum acoustic transmit power to a safe limit that causes no injury, and secondly, reducing the radius of the discomfort zone whilst maximising the receivable range. The discomfort zone is dependent on the signal design as well as the signal power. To achieve these aims requires a signal and receiver design capable of synchronisation and data reception at low-received-SNR, down to around −15 dB, with Doppler effects. These requirements lead to very high-ratio spread-spectrum signaling with efficient modulation to maximise data rate, which necessitates effective Doppler correction in the receiver structure. This thesis examines the state-of-the-art in this area and investigates the design, development and implementation of a suitable signal and receiver structure, with experimental validation in a variety of real-world channels. Data signals are designed around m-ary orthogonal signaling based on bandlimited carrierless PN sequences to create an M-ary Orthogonal Code Keying (M-OCK) modulation scheme. Synchronisation signal structures combining the energy of multiple unique PN symbols are shown to outperform single PN sequences of the same bandwidth and duration in channels with low SNR and significant Doppler effects. Signals and receiver structures are shown to be capable of reliable communications with band of 8 kHz to 16 kHz and transmit power limited to less than 170.8 dB re 1 μPa @ 1m, or 1W of acoustic power, over ranges of 10 km in sea trials, with low-received-SNR below −10 dB, at data rates of up to 140.69 bit/s. Channel recordings with AWGN demonstrated limits of signal and receiver performance of BER 10−3 at −14 dB for 35.63 bit/s, and −8.5 dB for 106.92 bit/s. Piloted study of multipath exploitation showed this performance could be improved to −10.5 dB for 106.92 bit/s by combining the energy of two arrival paths. Doppler compensation techniques are explored with experimental validation showing synchronisation and data demodulation at velocities over ranges of ±2.7m/s. Non-binary low density parity check (LDPC) error correction coding with M-OCK signals is investigated showing improved performance over Reed-Solomon (RS) coding of equivalent code rate in simulations and experiments in real underwater channels. The receiver structures are implemented on an Android mobile device with experiments showing live real-time synchronisation and data demodulation of signals transmitted through an underwater channel.
Abstract: Underwater acoustic modems can be expensive and inflexible. Software-defined underwater modems provide flexibility to modify the protocols and modulation schemes. The motivation of this research is towards producing minimal cost, low-power systems. The focus in this paper being the concept of a surface receiver consisting of a hydrophone plugged into a mobile device such as a smart phone or tablet. Applications could be where data or diagnostics are required from a distributed network of underwater sensors in the field that incorporate an integrated acoustic modem. Spread-spectrum signals with large bandwidth-time products are considered including binary orthogonal keying (BOK) using linear frequency modulated (LFM) chirps (Chirp-BOK) and pseudorandom noise m-ary orthogonal code keying (PN M-OCK). Bandwidth of 8 kHz to 16 kHz is utilised for simulated performance of the modulation schemes. The modulation schemes target low-power, low-received-SNR applications with rates between 23.4 bit/s and 375.7 bit/s, targeting a BER of 10-4 at received-SNR of -14 dB to -6 dB respectively. A receiver structure design is implemented on an Android mobile device with experimental validation carried out in a marina over a 100m range. The receiver application was able to successfully demodulate, error-free, all packets received in real-time with received SNR of 34 dB. The receiver modulation scheme was user-selectable at run-time. Recordings from the marina trials were combined with AWGN for varying SNR. These were played into the mobile device for real-time demodulation and showed the mobile device and receiver application produced the target BER of 10-4 for SNRs of -12 dB to 0 dB for the rates 23.4 bit/s and 375.7 bit/s respectively.
Abstract: Low-power, low received signal-to-noise-ratio (SNR) signals have potential for reducing the impact on marine life from acoustic communications. Here we explore the use of bandlimited pseudo-noise m-ary orthogonal code keying (M-OCK) scheme using m-sequences. Analysis and simulation of receiver structure for synchronisation and data demodulation performance is carried out. Performance of M-OCK is compared with m-ary quadrature amplitude modulation with direct-sequence spread-spectrum (M-QAM DSSS). Real-world channel experiments are carried out with transmission power for the M-OCK sequences limited to less than 1 W acoustic power (170.8 dB re 1 μPa at 1 m) and transmission range varied from 100 m to 10 km in the North Sea. Synchronisation at 10 km is achieved with effective received signal-to-noise-ratio of less than -9.96 dB, and data demodulation of 140.7 bit/s raw throughput with pre-coding bit-error-rate (BER) 0.5 × 10-1 (symbol-error-rate (SER) 0.1) and 46.9 bit/s raw throughput with pre-coding BER 0.9 × 10-3 (SER 1.95 × 10-3). Error-free synchronisation and data demodulation is achieved at ranges up to 2 km, demonstrating data rates in excess of 140 bit/s.