Implementation of a spread-spectrum acoustic modem on an android mobile device

摘要

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 23.4bit/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 100 m 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 0dB for the rates 23.4 bit/s and 375.7bit/s respectively.