터보등화기반 전송효율 향상을 위한 최적의 전송기법 연구

Abstract

The transmission of acoustic waves is limited because of various factors such as water temperature, salinity, depth in the underwater acoustic communication with a multi-path channel environment. Also, the performance is limited because the underwater acoustic communication uses low frequency band relative to wireless communication. In the underwater acoustic communication, the performance greatly depends on the characteristics of multi-path channel and the distortion of the received signal is degraded due to reverberation and multi-path. Such channel characteristics cause ISI(Inter-symbol interference). It is necessary to study on equalizer technique and channel coding to correct error by multi-path and ISI because the optimal communication method has to be designed based on characteristics of multi-path channel. There are various methods in order to countermeasure the multi-path channel. Among of them, one well proven method to counteract ISI is the decision feedback equalized (DFE), which has been used in many underwater communication links. The other way to cope with ISI, iterative equalizer is used which constitutes an outer loop in the receiver. An inner loop consists of iterative codes. Among the iterative coding techniques, turbo codes are optimal coding technique in aspect to performance and packet size. Turbo codes have error-flooring phenomenon, that is serious problem in underwater channel. In order to eliminate error-flooring phenomenon, turbo pi codes which add the shift register in the encoder. In this thesis iterative coding based equalization was proposed for single carrier underwater communication channels. As an outer code, DFE is used in this thesis. As an inner code, the turbo pi codes are used. The contents of this thesis are mainly deal with three parts. First, the performance of turbo equalization was analyzed. The performance of iterative turbo equalizer was studied by using the real data obtained in the underwater. It was confirmed that the performance is improved as increasing the number of iterations. Second, It was decided what kinds of packet structure are most optimal in underwater channel. Four kinds of packets are considered. As a result, consecutive packet structure are most optimal in aspect to performance and throughput. Third, in order to improve throughput, it is suggested which method is optimal among of high coding rates and high order modulation. Among high coding rates with QPSK and turbo coded 8PSK(Octal Phase Shift Keying), it was concluded high coding rates with QPSK(Quadrature Phase Shift Keying) is effective method in underwater communication. Finally, in aspect to performance and throughput, it was concluded that high coding rates with QPSK based on turbo equalization is most optimal transmission system in underwater communication.