Recently, the necessity of underwater acoustic communication increase, and demands for transmitting and receiving various data such as voice or high resolution data are increasing as well. However, the performance of underwater acoustic communication is influenced on underwater channel characteristic. Many obstacles exist in underwater acoustic communication. The performance of coherent acoustic communication systems is highly dependent upon channel characteristics including multipath, spatial and temporal coherence, Doppler effects and ambient noise. An important feature of the process of the sound field formation in oceanic waveguide is the interference caused by the surface and bottom reflections and by the layered structure of the water bulk. This means multipath effect. The interference effects lead to the ISI (Inter-Symbol-Interference). The performance of underwater acoustic communication is sensitive to the ISI due to multipath signal propagation.
A popular technique to maintain coherent demodulation of digital modulation schemes is known as pilot-symbol-assisted channel estimation. The main idea of pilot-symbol-assisted channel estimation is to add known pilot symbols (or training symbols) into a data stream. In this paper, I verify the performance of acoustic communication based on pilot-symbol-assisted channel estimation technique through the sea trial.
However, receiver for channel estimation must satisfy the condition that pilot symbol length is longer than the delay spread. If the delay spread is longer than pilot symbol length, data demodulation is impossible because equalizer has not optimized tap coefficient and in the opposition case. And in the opposition case, it obtains a good performance but generates an overhead and decrease data rate.
This thesis proposed underwater acoustic communication using channel estimation based on superimposed pilot for decrease an overhead. Channel estimation technique based on superimposed pilot transmit the data symbol and pilot symbol in same time and same frequency. In the superimposed pilot channel estimation, the receiver correlates the received signal with the pilot symbol, and obtains the channel estimate. The system with the superimposed channel estimation has a high spectral efficiency because of the simultaneous transmission of the pilot symbol and the data symbol. Especially, superimposed pilot technique is possible to estimate the time-variant channel without additional overhead and increase the data rate.
In this thesis, the performance of the conventional pilot-symbol-assisted channel estimation and the superimposed pilot channel estimation was compared in underwater acoustic communication system. Furthermore, the thesis compares the performance according to the superimposed pilot length and power.
As a result, minimize the overhead of underwater communication system and increase the data rate about 10 % ~ 30%.