In recent years, underwater acoustic communication have received much attention as their applications, have begun to shift from military toward commercial. The growing interest in underwater acoustic communications come as a response to the rapidly growing needs for wireless underwater communications, brought in part by the broadening of applications, such as military and commercial ones.
Commercial applications which have received much attention lately are pollution monitoring in environmental systems, remote control in off-shore oil industry, and collection of scientific data recorded at benthic station without the need for retrieving the instruments. Many developing applications include of commercial and military are now calling for real-time communication with submarines and autonomous underwater vehicles, not only in point -to-point links, but also in network configurations.
As the underwater acoustic communications channel is highly time-varying, it can cause undesirable distortions, such as multipath and Doppler effect.
Especially, underwater acoustic communication between fast- moving underwater vehicle is difficult due to the low speed of sound. The low speed of acoustic waves induces channel delay spreads of tens or hundreds of milliseconds, and is also the origin of extreme Doppler shifts compared to radio telecommunications.
Various data transmission strategies are being proposed by distinguished scientists to tackle these questions for optimal performance of underwater acoustic communications, including single carrier transmission with sparse-channel equalization, multi-input multi-output techniques and high efficiency error correction coding. Most of these approaches are exploited by conventional frequency-constant carrier signals for communications, showing that fighting severe inter symbol interference(ISI) in such intricate time variation and significant Doppler effect is an extremely hard problem.
Prior to 1990, underwater acoustic communication systems mainly relied on noncoherent modulation and energy detection, such as M-ary Frequency Shift Keying(MFSK). MFSK systems provide intrinsically robust to underwater acoustic channel distortions, and still are preferred methods for most commercially available acoustic modems.
Instead of frequency-constant carrier signals, chirp signal, a low Doppler sensitive signal, is commonly used in radar and sonar because of its good temporal resolution of autocorrelation function and large processing gain. Chirp signal have also been used in underwater acoustic communication. Chirp signals can be divided into two main types : Linear Frequency Modulation (LFM) and Non-Linear Frequency Modulation (Non-LFM). The instantaneous frequency of LFM changes linearly with the time. but the instantaneous frequency of Non-LFM changes non-linearly.
In this dissertation, underwater acoustic communication system using Non-LFM is proposed. The information is carried by the carrier amplitude, frequency, or phase in the most common underwater acoustic communications. However, the proposed method includes the information within frequency variation of carrier wave for a symbol. Especially, as carrier wave the Non-LFM signal, one of the chirp signal, is used and it is robust in the Doppler effect. The proposed method was analyzed and compared to CSK(Chirp Slope Keying) using LFM signal(LCSK) by simulation and lake trial. When the Doppler shift existed, the error probability of the proposed method is reduced by 5~12% than LCSK. The performances of the proposed method are evaluated in simulation and lake trial.