OFDM水声通信系统相关问题研究

发布时间：2018-04-18 11:00

  本文选题：水声通信 + 正交频分复用　； 参考：《哈尔滨工程大学》2014年博士论文

【摘要】：水声信道的多径时延会引起严重的码间干扰;水声信道的窄带宽、快速时变性及水下通信设备和水介质的相对运动等因素会产生明显的多普勒效应;水声信道的介质构成、水体运动形式及水下环境的复杂性等因素会造成大的水下噪声。水声信道的上述特点最终导致了水声信号的严重失真和快速衰减。因此如何解决水声通信系统中与多径现象、多普勒效应及水下噪声相关的问题,已成为水声通信领域所面临的重要课题。正交频分复用(OFDM)技术是一种多载波并行传输技术,由于其不但具有良好的抗多径性能和高的频谱利用率,而且可以根据信道条件对子载波进行灵活调制及功率分配,因而已被广泛应用于陆上无线电磁通信和水声通信系统中。与陆上无线电磁通信相比较,将OFDM技术应用于水声通信系统中会出现更为严重的问题。首先,OFDM信号容易出现高的峰均比(PAPR),这更加不利于水下无线低功率通信设备的正常使用;第二,OFDM系统对于载波频率偏移非常敏感,而水声信道中显著的多普勒效应更容易导致这种频偏;另外,由于水声信道的时变特性和高噪声,要求OFDM系统在进行相干解调时,信道估计器不但要有更好的信道跟踪能力以满足系统实时性的要求,而且还要有更强的抗噪声能力。因此,解决高峰均比、多普勒频移及信道估计等相关问题是OFDM水声通信系统所面临的重要任务。本文结合水声信道和水声通信设备的特点,针对OFDM水声通信系统中出现的高峰均比、多普勒频移及信道估计等相关问题作了进一步的研究,以提高水声通信系统的高速实时处理性能为目标,提出了创新算法,并通过理论分析和计算机仿真验证分析了这些算法的性能。论文的主要研究工作包括以下四部分:首先,分别从带宽、噪声、多径及多普勒效应等方面分析介绍了水声信道的特性;再针对上述特性对水声信道进行了建模和仿真;最后概括了 OFDM水声通信系统的优缺点,并提出了信号的峰均比、多普勒频移及信道估计等三个与OFDM水声通信系统密切相关的问题。其次,考虑到水下无线通信设备的低功率特点,在传统限幅算法的基础上,针对限幅噪声问题对OFDM水声通信系统的峰均比进行了研究,提出了一种计算复杂度较低的降低限幅噪声的新算法。该算法对超过限幅门限的数据采用分段式的不同的限幅幅度,再用限幅记录保存这一处理结果,然后接收端可以利用这一记录恢复相应数据的原始幅度。从而大幅降低限幅噪声对于系统性能的影响。理论分析和仿真结果均表明,这种限幅算法在较低误码率的前提下能够有效提升水声发射机功率放大器的工作效率。第三,针对OFDM水声通信系统对于多普勒频移敏感的问题,从多普勒因子(DSF)估计、采样率转换及载波频移(CFO)估计三方面提出了一种多普勒频移估计和补偿的新算法。首先在接收端对接收数据进行过采样,然后利用线性调频信号估计出多普勒因子的初值;再利用上述过采样数据结合多普勒因子的估计初值,通过线性插值的方法进行采样率转换;最后根据系统精度要求,以上述多普勒因子估计初值作为中心数据,列出若干多普勒因子的候选值,结合空载波能量代价函数对多普勒因子进行快速搜索实现二次估计,以完成由多普勒效应引起的载波频移(CFO)的估计。通过理论及计算机仿真对新算法的性能进行了分析,结果表明该算法和传统的多普勒频移估计及补偿算法相比具有较低的运算复杂度,能够有效改善OFDM水声通信系统的多普勒频移问题。最后,结合水声信道的快速时变性和高噪声特性,针对频域最小二乘(LS)信道估计算法(性能受噪声影响大)和离散傅立叶变换(DFT)算法(性能受空载波影响大)各自的误差问题,提出了一种LS-DFT联合信道估计新算法。首先在频域对导频子载波进行LS信道估计,再通过DFT对上述LS导频信道估值作降噪处理,得到另一组频域导频信道估值,然后将两组导频信道估值作误差对比和数据替换,取误差较小者组成一组新的导频信道估值,最后在频域作线性插值运算,以获得数据子载波处的信道估值。无论是理论分析还是计算机仿真结果,均表明这种联合信道估计算法能有效克服LS算法和DFT算法各自的缺点,且该算法能以小的计算复杂度从整体上有效提升OFDM水声通信系统信道估计的实时性能和抗噪声性能。
[Abstract]:Multipath underwater acoustic channel will cause serious intersymbol interference; narrow bandwidth of underwater acoustic channel, fast time-varying and underwater communication equipment and water medium relative motion and other factors will have Doppler effect; composition of underwater acoustic channel medium, water sports and water environmental factors such as the complexity of the cause the underwater noise. The characteristics of underwater acoustic channel eventually led to a serious and rapid attenuation of underwater acoustic signal distortion. So how to solve the underwater acoustic communication system with multipath effect, Doppler effect and underwater noise related issues, has become an important task in the field of underwater acoustic communication. Orthogonal frequency division multiplexing (OFDM) technology is a parallel multi carrier transmission technology, because it not only has good anti multipath performance and high spectrum utilization, but also flexible modulation and power allocation according to the channel condition of sub carrier Therefore, has been widely used in terrestrial wireless electromagnetic communication and underwater acoustic communication system. And the wireless electromagnetic communication compared to the application of OFDM technology in underwater acoustic communication systems, there will be more serious problems. First, the OFDM signal is easy to appear the high peak to average ratio (PAPR), which is not conducive to the normal the use of low power wireless communication equipment under water; second, OFDM system is very sensitive to carrier frequency offset, and the Doppler effect in the underwater acoustic channel is more likely to result in the frequency offset; in addition, due to the time-varying characteristics of underwater acoustic channel and high noise, the requirement of OFDM system in coherent demodulation, channel estimator not only have better tracking capability to meet the real-time requirement of the system, but also has stronger anti noise ability. Therefore, to solve the problems related to the high peak to average ratio, Doppler frequency shift and channel estimation in OFDM underwater acoustic communication system An important task facing. Combining with the characteristics of underwater acoustic channel and underwater acoustic communication equipment, for OFDM in underwater acoustic communication system PAPR, made further research on issues related to the Doppler shift and the channel estimation, in order to improve the speed of real-time underwater communication system can reason as the goal, put forward the innovative algorithm and analysis of the performance of these algorithms through theoretical analysis and computer simulation. The main research work of the thesis includes the following four parts: first, from the bandwidth, noise, multipath and Doppler effect analysis and introduces the characteristics of underwater acoustic channel; and then according to the characteristics of underwater acoustic channel modeling and simulation; finally summarized the advantages and disadvantages OFDM underwater communication system, and puts forward the PAPR and is closely related to the three and OFDM underwater acoustic communication system and channel estimation of Doppler frequency shift. Then Considering the characteristics of low power, wireless communication equipment under water, based on the traditional algorithm of amplitude limit, aiming at limiting the noise problem of OFDM underwater acoustic communication system PAPR is studied, a new algorithm is proposed for low complexity to reduce the clipping noise. The algorithm adopts segmented different limiting amplitude of the data exceeds the limit threshold, and limiting record the results of this treatment, then the receiver can recover the corresponding data of the original amplitude using this record. To reduce the effect of clipping noise on system performance. Theoretical analysis and simulation results show that this algorithm can limit to effectively enhance the acoustic transmitter power amplifier at low bit error rate under the premise of efficiency. Third, for OFDM underwater acoustic communication system sensitive issue for the Doppler frequency shift, from the Doppler factor (DSF) estimation, sampling rate Conversion and carrier frequency offset (CFO) estimation and propose a new algorithm of Doppler frequency shift estimation and compensation in three aspects. First, in the receiver to receive data over sampling, and then use the estimated initial Doppler factor of linear FM signal; and then use the over sampling data combined with initial estimates of the Doppler factor, sampling the rate of conversion by linear interpolation method; finally, according to the accuracy requirement of the system, the data center to initial estimates as Doppler factor, Doppler factor lists a number of candidate values, combined with the empty carrier energy cost function for quick search to achieve two times to estimate the Doppler factor, carrier frequency to complete the shift caused by the Doppler effect (CFO) estimation through the performance of the new algorithm theory and computer simulation are analyzed. The results show that the algorithm and the Doppler frequency shift estimation and compensation algorithm It has low computational complexity and can effectively improve the Doppler frequency shift of the OFDM underwater acoustic communication system. Finally, combined with the acoustic channel fast time-varying and high noise characteristics based on frequency domain least squares (LS) channel estimation algorithm (performance affected by noise) and the discrete Fu Liye transform (DFT) algorithm (performance by the air carrier effect of their error), proposed a new LS-DFT joint channel estimation algorithm. Firstly, the pilot subcarriers in the frequency domain for LS channel estimation, and then through the DFT on the LS pilot channel estimation of noise reduction processing, get another set of frequency domain pilot channel estimation, and then two groups of pilot channel estimation error comparison and data substitution error is smaller consisting of a new set of pilot channel estimation, the linear interpolation in the frequency domain, to obtain data subcarrier channel estimation. Both theory analysis and calculation The simulation results show that the joint channel estimation algorithm can effectively overcome the shortcomings of LS algorithm and DFT algorithm, and the algorithm can effectively enhance the real-time performance and noise immunity of OFDM underwater acoustic communication system in a small computational complexity.

【学位授予单位】：哈尔滨工程大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TN929.3
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本文编号：1768083