Research Of Space-Time Coding Technology For Underwater Acoustic Communication Systems

Space-time coding (STC) is a space diversity technique used in the transmitters which can provide diversity gain and coding gain without losing band-width via combining coding and modulation together. It can overcome the problem of fading and interference in the communication systems, and give benefit in term of date rate and reliability. As one of STC, space-time block code (STBC) has drawn more attention because of its low complexity and performance in flat-fading channels.Characterized as a time- varying and space-varying channel, underwater acoustic (UWA) channel is a typical frequency-selective fading channel. Usually, frequency diversity and space diversity techniques are used to mitigate the fading in acoustic communication systems. This thesis tries to use STC in UWA communication systems to mitigate the fading. But many difficulties will be met when STC techniques are used in UWA channel because of its hostile ocean transmission conditions, especially of multipath propagation. The performance of STBC in UWA channels was studied in this thesis.The thesis introduces the development of underwater acoustic communications, the development of space-time code and characteristics of underwater acoustic channel at first, then discusses the coding ideas, the encoding and decoding criterion, the classification and the performance characteristics of STC. The thesis investigates the encoding and decoding criterion of STBC, and proposes a STBC-FSK scheme which use frequency shift keying (FSK) to modulate STBC signals. Transmitting-delay and receiving-delay schemes are proposed when STBC-FSK scheme is used in UWA channel. The performance of STBC-FSK schemes with two transmitting transducers, one and two receiving transducers is evaluated by MIMO UWA channel models. Numeral simulations show feasibility and performance of STBC-FSK in UWA communication channel with short distance and no obvious multipath interferences. At last, several STC schemes are discussed which are able to mitigation multipath interferences.