Study And Design Of OFDM Underwater Acoustic Communication System Based On FPGA

With the development of underwater communication technology, underwater communication develops towards low power, high speed and high error performance. Due to the complexity of underwater channel, there are many challenges such as large power consumption, high complexity, susceptible to the underwater channel environment, expensive and ect. in underwater communication systemIn this thesis, an underwater acoustic modem system was studies and designed based on Orthogonal Frequency Division Multiplexing technology(OFDM),which comprises of an adaptive variable threshold frame synchronization module, scrambler(descrambler) module, convolutional code(viterbi decode), interleaver(deinterleaver) module, mapping(inverse mapping)module, FFT/IFFT(Fast Fourier Transformation/ Inverse Fast Fourier Transform) module, zero padding(delete zero) module and add(delete) cyclic prefix module. The system was implemented based on FPGA(Field Programmable Gate Array), which demonstrated a lower power consumption comparing with traditional DSP solution. A simplified implementation method of OFDM was used, which only need to pad zero before the OFDM modulation and send the real component of the data after the OFDM modulation.An adaptive variable threshold symbol synchronization method was proposed, in order to reduce the error bit rate(BER) further. Zadoff_Chu complex sequence was used as preamble, due to its excellent characteristics of auto-correlation and cross-correlation. Cross-correlation of the received sequences and local Zadoff_Chu sequences was used as the sentence of symbol detection. At the same time, the cross-correlation of the received sequences and the orthogonal sequences was set as the adaptive dynamic variable threshold.Only when the sentence of symbol detection is greater than the variable threshold and the sentence of symbol detection reach the maximum value, then is symbol synchronization arrived. The Matlab simulation results show that the symbol synchronization module can still achieve good synchronization when the SNR for the receiver reaches-10 d B. Use segmented related principle can greatly reduce the symbol synchronization realize resource overhead, time-division multiplexing and assembly line method were used, which improves the system throughput, at the same time multiplier resources were reduced by 75%.In this thesis, the design of schematics as well as PCB layout were accomplished. An high speed communication software was developed for convenient test. System was measured in the basin. The result showed that the system could communicate at the rate of 2.91 kbps, BER of 35.93 10-′ at the distance of 4.5 meters, which met the requirements of communication systems. Compared with Park synchronization algorithm, the BER in the proposed system was decreased by 5.01%...