【作者】 张洁；

【导师】 许茹；

【作者基本信息】 厦门大学 ， 通信与信息系统， 2007， 硕士

【摘要】 水声信道是一个极其复杂的时间-频率-空间变化,强多径干扰,带宽有限且噪声干扰严重的信道,在这样的信道环境下实现通信系统的高速、可靠传输是极具挑战性的。正交频分复用(OFDM)技术是一种多载波数字调制技术,它是一种实现高速无线数据通信的良好方法,但将OFDM技术应用于水声通信中,在国内的研究尚处于起步阶段。OFDM系统可以避免ISI并减小ICI,然而信号经过水声信道的传输后,各子载波的幅度仍然会产生服从瑞利分布的衰落,有些子载波由于深度衰落可能完全被噪声淹没。因此,为了保证系统的可靠传输,必须引入信道纠错编码,许多纠错编码技术被应用于OFDM系统中,如卷积码、R-S码、Turbo码等等。近年来,低密度奇偶校验码(LDPC)已经在纠错码领域引起广泛的注意,LDPC码是Gallager在1962年提出的,之后在Turbo码的巨大成功的带动下,Mackay等人重新研究了它,并发现LDPC码具有逼近香农限的特性。现在,LDPC码已经成为纠错领域的研究热点,将LDPC编码应用于OFDM系统在B3G移动通信中的应用已经有相关的研究和探讨,本文尝试探讨结合LDPC编码的OFDM系统在水声通信系统中的应用。本文的主要工作为:结合水声信道的特点,在MATLAB中建立一个LDPC-COFDM系统,然后在建立的信道模型下,进行以下几方面的仿真:①对结合不同构造方式和映射方式下的LDPC-COFDM系统性能进行分析;②LDPC码应用于OFDM系统与其它编码应用于OFDM系统的性能比较;③采用交织与否对LDPC-COFDM系统性能的影响;④采用实验室自行研制的OFDM水声通信系统DSP硬件平台,在实验室水池中实现LDPC-COFDM系统的水声传输,对实验结果进行分析。本研究的仿真及实际测试结果说明:选择性能优越的LDPC码作为OFDM水声传输通信系统的信道编码方案是可行的,经过进一步的深入研究获得良好的系统性能也是可期待的。更多还原

【Abstract】 It is well known that underwater acoustic (UWA) channel is an extremely complex channel with the characteristics of time-frequency-space variant, strong multipath effect, narrow available bandwidth and high background noise. It is the most difficult channel for wireless communications. Therefore, it is very challenging to accomplish high-speed and reliable communications in such channel.Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier digital modulation technique, which proved to be a good method for high-speed wireless data communications. Applying OFDM to underwater acoustic communications is just at the beginning in our country. OFDM system can eliminate ISI and reduce ICI. However, when the transmitted signals arrive at the receiver through the UWA channel, the signal envelope of each sub carrier may be very different. Some sub carrier may even be severely distorted by deep fading. So it requires the use of channel coding to protect the transmitted data and achieve reliable communications. Many error correction code technologies are applied into the OFDM system, convolution code, R-S code and Turbo code etc.Recently, low-density parity-check (LDPC) codes have attracted much attention particularly in the field of coding theory. LDPC code had discovered by Gallager in 1962. After the great success of Turbo codes, MacKay et al. rediscovered LDPC code and proved its performance is very close to Shannon Limit. Now, LDPC code has become a hot spot of research in the area of error correcting codes and will be applied in many areas. Applying the LDPC code into the OFDM system in the B3G mobile communications also has been studied. This paper tries to study about the performances of LDPC-COFDM used in underwater acoustic communication.In this paper, combined with the characteristic of UWA channel, a COFDM system is firstly set up with MATLAB. And then, the performances as followed have been simulated. The performance contrast of different construction LDPC used in COFDM. Analysis performance of different mapping of MPSK applied into LDPC-COFDM. The performance contrast of the LDPC code applied into OFDM system and other codes applied into OFDM system. And then contrast the influence applied interleaver or not in the LDPC-COFDM system. Finally, LDPC_COFDM systems is set up based on DSP hardware platform and implement underwater acoustic communication. From both simulation results and real test data, COFDM with LDPC codes as its channel coding scheme turns out to be a good choice in underwater acoustic communication.更多还原