| China is in a sharp rise in the ocean development,as an important component of ocean development underwater acoustic communication has caused widely concern.With limited available bandwidth,severe multipath fading underwater acoustic channel restrict the development of underwater acoustic communication.Multiple antennas can be transmited by Multiple input multiple output(MIMO)technology at the same time under the condition of the same bandwidth.Channel data stream through various antennas is independent,so it can increase the spectral efficiency at the same bandwidth.Meanwhile MIMO technology can also improve the robustness of the communication system by diversity.The fixed frequency bandwidth is divided into a plurality of segments by Orthogonal Frequency Division Multiplexing(OFDM).The frequency selective fading channel is converted into a flat channel in the frequency domain.So the influence caused by multipath fading channels to the communication system will effectively be suppressed.As one of the core technology of MIMO systems,space-Time Coding(STC)technology well link encoding,modulation and diversity systems to establish spatial correlation and overcome spatial interference.Firstly,the characteristics of underwater acoustic channel are analyzed by Lotus Lake experimental,and the basic principles of MIMO communications technology,MIMO space-time system and OFDM technology is explained.The principles of Alamouti space-time block coding and differential Alamouti coding based on two transmit antennas is decribed.Error performance of Alamouti coding and differential Alamouti coding is given by simulation results in MIMO-OFDM underwater acoustic communication system.In this paper orthogonal space-time block coding is designed for real signals and complex signals.The performance of omplex signals orthogonal space-time block coding system in MIMO-OFDM underwater acoustic communication can be analyzed by Lotus Lake measurement channel simulation under different antennas conditions.The system achieving higher diversity order with increasing of the transmit antennas is verified.Robust high-speed underwater acoustic communication system is the focus of today’s underwater acoustic communication studies,how to improve the efficiency of communication systems is a prerequisite for the development of high-speed underwater acoustic communication.When transmitting antenna is greater than two complex signal coding design of Orthogonal space-time block coding will not be able to achieve full transmission rate.Therefore,this paper analyzes coding scheme QOSTBC proposed byTirkkonen and Jafarkhani.Because the program is to exchange for full rate by the expense of part of diversity gain,Givens matrix rotation theory is applied to improve Jafarkhani coding scheme to further improve the performance of space-time block coding in this paper,so full rate diversity is achieved.In this paper,three QOSTBC coding schemes is simulated by computer simulation experiment、tank experiment and the Songhua River experiments.The results showed that:efficient and stable communication can be achieved by MIMO-OFDM underwater acoustic communication system based on QOSTBC encoding scheme,providing a feasible solution for high-speed underwater acoustic communication. |
