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Underwater acoustic communication is a technique of sending and receiving message below water.〔I. F. Akyildiz, D. Pompili, and T. Melodia, "Underwater Acoustic Sensor Networks: Research Challenges," Ad Hoc Networks (Elsevier), vol. 3, no. 3, pp. 257-279, March 2005.〕 There are several ways of employing such communication but the most common is using hydrophones. Under water communication is difficult due to factors like multi-path propagation, time variations of the channel, small available bandwidth and strong signal attenuation, especially over long ranges. In underwater communication there are low data rates compared to terrestrial communication, since underwater communication uses acoustic waves instead of electromagnetic waves. == Types of modulation used for Underwater Acoustic Communications == In general the modulation methods developed for radio communications can be adapted for underwater acoustic communications (UAC). However some of the modulation schemes are more suited to the unique underwater acoustic communication channel than others. Some of the modulation methods used for UAC are as follows: * Frequency Shift Keying (FSK) * Phase Shift Keying (PSK) * Frequency Hopped Spread Spectrum (FHSS) * Direct Sequence Spread Spectrum (DSSS) * Frequency and Pulse Position Modulation (FPPM and PPM) * Multiple Frequency Shift Keying (MFSK) The following is a high level discussion on the different types of modulation and their utility to UAC. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Underwater acoustic communication」の詳細全文を読む スポンサード リンク
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