Thesis
OFDM-based software modem for shallow water acoustic communications
Washington State University
Master of Science (MS), Washington State University
2012
Handle:
https://hdl.handle.net/2376/100732
Abstract
The use of acoustic signals at ultrasonic frequencies is becoming more prominent in underwater communications, an emerging field with applications that include remote monitoring of autonomous underwater vehicles and environmental sensor networks. In this thesis we describe a system to improve the speed and reliability of underwater communications by mitigating problems such as signal reflections off the surface or bottom, the decaying and smearing of signals over long distances, and Doppler shifts due to the relative motion of transmitters and receivers. To counteract these problems, this thesis considers a single-input, multiple-output (SIMO) orthogonal frequency-division multiplexing (OFDM) system that employs a vertical array of 8 hydrophones at the receiver. OFDM assigns each data symbol to a distinct frequency within a given length of time, simplifies the computational complexity at the receiver and greatly reduces interference between data symbols. Other techniques, such as a variety of signal shapes and block lengths, adaptive tracking of waveform sampling times, and soft-decision decoding, are explored to further improve reliability of transmitted bits. Our results show that, for acoustic communications through a lake of approximately iv 70-foot depth, we can signal error-free at an information rate of 2 kilobits per second (kbps) at an 800-meter range, transmit 1 kbps at a 2-km range with a BER of about 2 × 10−3 , and transmit 2 kbps at a 2-km range with a BER of about 3 × 10−2 . A variety of pulse shapes may be used to achieve low BER, and block lengths that maintain a block duration under 1 second are recommended to avoid exceeding the channel coherence time limit. However, our method for adaptive tracking of sampling times requires modification before it can be used to improve system performance in low-SNR situations.
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Details
- Title
- OFDM-based software modem for shallow water acoustic communications
- Creators
- Christopher Edward Alex
- Contributors
- Benjamin Belzer (Degree Supervisor)Thomas Fischer (Degree Supervisor)
- Awarding Institution
- Washington State University
- Academic Unit
- Electrical Engineering and Computer Science, School of
- Theses and Dissertations
- Master of Science (MS), Washington State University
- Publisher
- Washington State University; Pullman, Wash. :
- Identifiers
- 99900525098001842
- Language
- English
- Resource Type
- Thesis