【作者】 夏梦璐；

【导师】 徐文；

【作者基本信息】 浙江大学 ， 通信与信息系统， 2012， 博士

【摘要】 水声通信是海洋中通过声信号实现的无线通信。由于水中声传播速度慢、可利用带宽窄等特点,水声通信面临众多不同于陆地无线电通信的技术难点。特别是浅海环境,由于其边界及介质起伏效应,造成多径时延扩展、多普勒频移扩展严重,且表现出快速时变特性,使传统的高速通信信号处理技术性能不佳,甚至无法工作。论文针对浅海起伏环境下具有延时、多普勒扩展及时变特点的信道,以声传播环境-声通信信道-通信信号处理为主轴,采用基于水声传播互易性这一物理机制发展的时间反转处理方法,结合单载波、多载波相干水声通信信号体制,发展模型与数据相结合的信道估计和均衡技术,旨在深入研究浅水水声通信信号传播物理模型的同时,探讨现实起伏环境中实现可靠高速数据传输的水声通信信号处理方法及系统设计。因地理位置、温度、盐度等的不同,声速在不同浅海环境中呈现不同的梯度模式,水声通信的信道冲击响应也千差万别。目前并没有一个通用的浅海水声通信信道统计模型可用于描述所有浅海环境,这就要求我们在研究水声信道时将模型和数据相结合,利用模型的一般性和数据的特殊性设计稳定可靠的水声通信系统。时间反转技术是基于声在时不变环境中传播的互易性提出的一种信号处理方法,即若环境时不变,将接收到的信号时间反转发射,信号将在最初的发射位置实现空间聚焦和时间压缩。论文的主要思想就是基于声信号时反的特性开展理论和实验研究,并应用于相干水声通信中。然而浅海起伏环境中,时变是造成通信接收机性能下降的主要因素。这就引出论文的一个重要研究方向：起伏环境对水声通信信道的影响,以及如何在起伏环境中跟踪信道变化。论文首先建立了基于散射信道物理模型的状态空间时变信道模型,并采用卡尔曼滤波和扩展卡尔曼滤波实现信道的估计和跟踪。由于信道时延扩展严重,随时间变化明显,造成估计算法运算量巨大。论文提出了估计并跟踪时反信道——经过被动时反处理后的相关信道函数。根据理论分析和实验数据观察,时反信道时延扩展明显小于信道冲击响应,且结构相对稳定,对时反信道的跟踪明显可降低待估计参数的更新频率和算法计算量。对时反信道特性的研究为设计被动时反均衡方法提供了基础。由于信道的时变,被动时反后的时反信道并不是理想的单位冲击函数,仍然存在着时间扩展及通信信号间的码间干扰。因此在被动时反处理后可级联一个简单的时域判决反馈均衡器,用于消除残余的码间干扰。当环境时变严重时,近似单位冲击函数的时反信道会随时间发生扩展,这种情况就需要更新用于时反操作的参考信道响应。在设计接收系统时,论文提出通过检测估计时反信道的卡尔曼滤波的误差变化,判断是否需要更新参考信道响应。实验数据的处理和分析验证了方案的可行性。论文同时研究了单载波频域均衡系统在水声通信中的应用,在揭示其数学表达式与被动时反结合时域均衡非常相似的基础上,论文将两者从理论和实验数据两个方面进行了比较。两者在时不变环境中,是等价的。而在时变环境中,单载波频域均衡需要更频繁地更新信道冲击响应函数。频域均衡的另一个缺点是由于基于快速傅立叶变换实现,其每一块用于快速傅立叶变换的数据前需要加入循环前缀,以保证序列可周期扩展,且循环前缀的长度必须大于信道响应扩展的最大长度。这种信号形式产生很多信号冗余,降低了平均通信速率。利用时反信道的特点,论文进一步尝试了将被动时反技术与多载波通信体制相结合,设计了被动时反结合正交频分复用(Orthogonal Frequency Division Multiplexing, OFDM)通信系统。基于被动时反处理后的时反信道时延扩展小的特点,设计符号时间宽度短、子载波较宽的OFDM信号,使得OFDM对多普勒效应和随机多普勒的宽容性增强。仿真和实验数据分析展示了方案的有效性。更多还原

【Abstract】 Underwater acoustic communication achieves wireless data transfer in the ocean via propagation of sound. It faces numerous technical challenges different from radio communications in air, due to special channel features such as limited bandwidth and relatively slow propagation speed. Particularly in shallow water environments, fluctuations along the boundaries and in the media cause severe multipath time-delay spread and Doppler frequency-shift spread, both of which can be rapidly time-varying, resulting in poor performance or even failure of conventional high-rate communication techniques.Facing the above-mentioned doubly-spread and rapidly time-varying channels, this thesis adopts an approach that takes into account environmental variations, sound propagation channel, and communication signal processing all together. Especially, time-reversal processing exploiting sound propagation physics is incorporated into coherent single-carrier and multi-carrier underwater acoustic communications, from which some model-date fused channel estimation and equalization techniques are developed. As such, we aim to not only obtaining deep understanding of the physical model of the shallow water communication signal propagation, but also improving signal processing methods and system design for reliable high-rate acoustic data transfer in realistic fluctuating environments.The channel impulse response (CIR) of shallow water acoustic communication differs from place to place partially because sound speed profiles follow different patterns due to the temperature and salinity variations. Currently there is no universal CIR model that works in all the shallow water conditions. That requires a combination of model and data, which takes advantage of both the models’ generality and data’s specialty. Time reversal is a signal processing method developed based on the reciprocity of sound propagation in a time-invariant environment. Specifically in the context of array receiving, when the environment is stationary, if the received signals at the array from initial transmission are time reversed and re-transmitted, one can obtain the original signal at the original transmitting position via spatial focusing and time compression. This thesis studies relevant aspects of the time-reversal processing both theoretically and experimentally, and apply it to coherent underwater acoustic communications. In a fluctuating shallow water environment, however, time-varying CIR is the main factor causing communication performance degradation. This motivates the thesis research to study the influence of the environmental fluctuation on communication channel and then track the CIR variations. We first develop a state space time-varying model for a channel with moving scattering clusters:we then apply the standard Kalman filter and extended Kalman filter to estimate and track the channel CIR. Since the CIR has a large time delay spread and changes with time, the required computational effort is significant. Thus we propose to track the so-called time-reversed channel (TRC), which is the channel correlation function after applying time-reversal processing. Theoretical analysis and data observation show that TRC has a smaller time-delay spread and more stable structure compared to CIR. Hence tracking the time-reversed channel requires less computation as well as lower update frequency.Understanding the characteristics of the time-reversed channel provides a basis for equalizer design in passive time reversal communications. Due to time-varying, time-reversed channel does not show as an ideal impulse response; instead the response still has some spread, i.e., there still exists some residual inter-symbol interferences (ISI) in communications. Hence passive time reversal processing can be followed by a simple decision feedback equalizer to remove the residual ISI. As the channel varies with time more rapidly, the time-reversed channel will spread even more; it is thus necessary to update the CIR used to implement the passive time reversal algorithm. In this thesis, errors in Kalman filtering are used to track the variation of the time-reversed channel, i.e., if the predicted error goes beyond a predetermined threshold, the CIR used for passive time reversal is then updated. Experimental data processing and analysis verify that the proposed approach is feasible.This thesis also investigates the use of single-carrier frequency domain equalizer (SC-FDE) in underwater acoustic communications. Having disclosed that the mathematical expression of the SC-FDE is quite similar to that of passive time reversal combined with temporal decision feedback equalizer, we compare those two methods from both theoretical and experimental data perspectives. It is shown that when the environment is stationary, they perform equally well. In a dynamic environment. SC-FDE needs to update the CIR more frequently. Besides. implementation of the SC-FDE is based on fast Fourier transform which requires a cyclic extension for each transform data block to warrant periodic extension of data sequences; further length of the cyclic prefix has to be longer than the CIR spread. This signal sequence formation reduces the average communication data rate.Given the favorable features of the time-reversed channel, we further exploit processing the passive time reversal incorporated into the Orthogonal Frequency Division Multiplexing (OFDM) system. Considering the smaller delay spread of the time-reversed channel, we can design shorter OFDM symbols so that the sub-carrier can have a larger bandwidth; the combined OFDM system can thus better handle inter-carrier interferences. Simulations and experimental data analysis demonstrate the effectiveness of the scheme.更多还原