The Study Of UWA Communication Technology Based-on Orthogonal Frequency Division Multiplexing

The limited channel bandwidth and ISI introduced by channel multipath propagation are the obstacles for underwater acoustic (UWA) communications. Orthogonal frequency division multiplexing (OFDM) is a bandwidth-efficient modulation scheme with robutness to frequency selective fading. To attain reliable data transmission over UWA channels, the key techniques of OFDM aimed for UWA communications are studied in this thesis. And the results of simulation study and lake trial proved its excellent performance. The main content of this thesis are as follows:1.An improved realization method of OFDM system applied for UWA communication was proposed. In this method, by inserting zeros before and after the data to be transmitted, the data can be directly modulated to the proper band after IFFT, and only real parts or imagine parts of the result are needed. By this method the circuits are simplified and the degradation caused by the offset between the oscillators of transmitter and receiver is avoided.2.Reducing the peak-to-average power ratio (PAR) is studied, and two new algorithms: improved Partial Transmit Sequence(PTS) algorithm and fraction-exponent companding transform algorithm are presented. The former is based on PTS algorithm and the randomicity of the distribution of the information. The latter deals with the signal by companding with fraction exponent. Both of them have good performance, at the same time the complexity is reduced.3.The degradation of OFDM system caused by frequency offset is studied and a method utilizing cyclic prefix (CP) is presented. By this method, the phase offset of the signal in a symbol duration is estimated from CP, then the received signal is phase-compensated. And the influence of frequency offset is reduced.4.The UWA Spread-Spectrum(SS) communication is also studied. Several SS systems based on OFDM was rendered. In the systems, the SS code was distributed to many orthogonal subcarriers for parallel transmission. Thesimulation results proved that the multicarriers SS systems formed better than traditional one on realization and flexibility.5.A UWA OFDM voice communication system based on special voice processor and DSP was designed. And the hardware is finished.6. The experiment of UWA communication based on OFDM was carried out in Songhua Lake in Jilin province. It showed that the communication method could perform very well and the influence of multipath was reduced effectively. In the experiments, the bit rate reached 8.3kbits/s within the distance of 7000m with the bit error rate between 10~-2 -10~-5.