【作者】 沈彩耀；

【导师】 于宏毅；

【作者基本信息】 解放军信息工程大学 ， 通信与信息系统， 2011， 博士

【摘要】 随着无线通信的使用范围越来越广泛,所面临的电磁环境变得更加复杂,使得某些应用环境中接收信号变得越来越微弱。例如深空通信和军事通信中,即使采用最大口径天线和最低噪声温度接收机也可能难以实现信号的有效接收。因此,如何保障低信噪比信号的接收是目前通信技术的一个重要研究内容。一种有效解决方法是通过多个天线对同一信号进行接收,利用信号的相干性和噪声的不相关性,将信号进行加权合成,从而提高接收信号质量。该方法可以提高天线的等效口径,接收性能能够超过现有的最大口径天线,给一些极低信噪比信号的接收提供了途径。同时由于多天线信号合成具有成本低、组阵灵活以及稳健性强等优点,因此在众多领域得到应用。本文主要围绕无需符号同步的随机布局多天线信号波形合成的关键技术进行研究,针对其中多天线信号相位差估计、时延对准技术、合成权值估计以及频率选择性慢衰落信道下的合成问题进行深入探讨和分析。由于采用多个天线接收同一目标信号,因此若不考虑信号间的参数差以及信道的影响,各接收信号具有相同的信号分量,这就为信号间的参数差估计性能提升带来可能,从而也提高的合成性能。鉴于以上的事实,本文在信号波形层面上就如何联合利用多天线信号进行参数估计,降低合成增益损失进行深入研究,主要工作和创新点如下:1、从参数估计角度对SUMPLE算法的合成权值相位特性进行理论分析,并针对不等信噪比该算法合成权值的优化问题,提出了一种合成权值幅度修正方法,有效地提高了该算法对低信噪比信号的抑制能力,可用于故障天线的检测。在深入研究SUMPLE算法的基础上,对该算法合成权值的相位补偿性能进行理论分析,从参数估计角度揭示了该算法具有较好合成性能的内在机理。由于各路信号经过补偿后具有相同的相位中心,因此将该算法用于多天线信号相位差估计中,并分析了相位差估计的理论性能。同时,为了降低信噪比较低信号的影响,提出了一种基于SUMPLE算法的权值幅度修正方法,并分析了修正因子对降低弱信号影响的有效性。理论分析与仿真结果表明,权值幅度修正算法能明显降低信噪比较低信号的权值幅度,有利于抑制质量较差信号的影响,减少合成增益损失,可用于天线组阵中故障天线的检测。2、针对低信噪比下不同时延多天线信号的参考选择问题,提出了一种基于准合成输出信号作为参考的时延对准算法,分析了该算法的理论收敛特性并进行仿真验证,有效降低了合成增益损失。从匹配滤波器界角度理论上分析了时延估计误差对信号合成的影响,并利用最大似然接收机进行仿真验证。旨在寻求有效的多天线信号联合时延估计算法,区别于传统的仅利用固定参考信号实现信号间的时延估计方法,提出了基于准合成输出信号作为参考时延对准算法,并分析了该算法的理论收敛特性以及实现的相关问题。结果表明,该算法以其它所有路的合成信号作为参考,提高参考信号的信噪比,时延估计性能在低信噪比条件下其估计性能改善更为明显,且优于冗余融合算法。3、推导了多天线联合接收BPSK/QPSK信号合成权值估计的CRLB,并利用合成权值具有相对性特点,提出了一种基于噪声自相关特性的合成权值盲估计算法,解决了信号合成中权值估计的通用性问题。根据各天线接收信号具有相同信号分量的特点,因此首先推导了多天线联合接收的BPSK/QPSK信号合成权值估计的CRLB,从参数估计性能下界的角度理论上证明了多天线联合的合成权值具有更好估计性能。同时,针对信号合成中权值估计的通用性问题,鉴于合成性能跟信号间合成权值的比值有关而与权值大小无关的特性,提出了一种基于噪声自相关特性的合成权值盲估计算法。该算法充分考虑信号间的关系,跳过估计单路信号的信噪比和功率,利用信号的二阶统计特性和噪声的自相关特性,完成合成权值的估计。该算法能适用于在未知信号信噪比、调制方式、信号带宽以及中心频率偏差下的信号合成,而且计算量简单,易于实现。4、针对频率选择性慢衰落信道下的低信噪比多天线信号合成问题,从频率分量最大信噪比的角度分析了最佳合成权值,提出了基于频率分量合并的多天线信号波形合成算法,显著改善了信号的解调性能。在对匹配滤波器研究的基础上,推导了频率选择性慢衰落信道下的多天线信号的最佳合成权值,并从频率分量信号的信噪比最大角度解释的合成权值的物理意义。针对频率选择性慢衰落信道下的低信噪比信号合成问题,提出了一种在未知信道下无需进行符号同步的多天线信号波形合成算法。在频域,该算法按照每频点信号最大信噪比准则对信号进行加权合成,提高了合成输出信号的每个频点的信号信噪比,改善了合成信号的衰落特性。该算法对信号调制方式透明且无需符号定时同步,能够有效地提高信号的接收质量,为频率选择性信道下的低信噪比信号合成提供了新的解决思路。5、针对固定参考信号的信道相位非线性失真影响信号解调性能的问题,提出一种基于平均相位参考的相位补偿算法,解决了频率选择性慢衰落信道下的参考相位信号选择问题。对于符号定时同步后跟随符号间隔均衡器的常用解调方式,首先从符号同步后的信号信噪比以及均衡器性能分析了信道相位的非线性失真对解调的影响。为了降低相位非线性失真对解调性能的影响,区别于固定信号为参考的方法,提出了一种基于所有路信号平均相位参考的相位补偿思路,即合成后信道相位为所有路信道相位的平均值。由于平均可以消除随机性,因此将有效地改善合成信号的相频特性,避免了固定参考信号相位非线性带来的可能严重影响解调性能的问题。同时针对相位差估计的模糊性问题,给出了相对平均相位的补偿算法,最后通过仿真验证了算法的有效性。更多还原

【Abstract】 Due to the rapid extension of the wireless communication and the troublesome environment for electromagnetic propagation, the received signals become weaker in some applications. It is possible to be in despair even if using maximum antenna apertures and lower receiver noise temperatures such as in Deep Space Network and military communication. Therefore, it is important to pay attention to improve the quality of the received signal at low Signal-to-Noise Ratio level. An effective method for improving the SNR is to combine the signals from several antennas, and it is based on the characteristics about the correlation of the signal component and the non-correlation for noise component. This technique can offer better performance than a single maximum antenna. Forthermore, it holds many possibilities: increased operational robustness, implementation cost saving and more stable and flexible.For the purpose of the general applicability for the received signals and the adoptive equipments, this thesis mainly deals with signal waveform combining techniques on a randomly distributed multi-antenna array, especially about the phase difference estimate, time delay alignments, the combining weight estimate and the combining algorithm in the frequency selective channels among the signals. If taking no account of the different parameters and the channels, the received signals have the same signal component. As a result, it is likely to improve the performance about the parameters’estimation and reduce the combining loss consequently. Based on the fact that the same signal component exists in the received signals, the paper mainly analyzes and makes a deep research on the key techniques of the sample combining, which utilizes multi-antenna signals jointly to realize the more accurate parameter estimates. The main research results include:1、Analyzing phase compensating performance and phase difference characteristics of the combining weight from SUMPLE algorithm in view of parameter estimation, and a modified weight amplitude method based on SUMPLE algorithm is proposed. On the basis of the deep research on the SUMPLE algorithm, the principle that the algorithm can provide lower combining loss for weak signals is explained on account of the compensating phase performance. At the same time, the paper analyses the phase difference characteristics from the combining weight, and the corresponding theoretical performance and the required least antenna number are also derived. Moreover, in order to reduce the adverse effect from the weaker signal for non-uniform signal combining, a modified weight amplitude method is presented. The validity of modified factor is also analyzed. Theoretical analysis and simulation results show that the modified algorithm can obviously reduce the weight amplitude of the weaker signals, improve the arraying combining performance, and favor for confirming the failed antenna.2、A simple algorithm that aligns signals of different arrival times on a randomly distributed antenna array is presented, and the theoretical convergence characteristic is also analyzed. Analyzing the combining performance affected by the time delay estimation error using the matched filter bound in case of only two signals and it is confirmed by simulation relusts using an optimum maximum-likelihood receiver. To align the signals of different arrival times on a randomly distributed antenna array, a new algorithm for time delay alignment based on quasi-combined output as reference is proposed, which is different from most algorithms based on a single fixed reference. The paper also analyzes the convergence characteristic and the application issue in practice. Finally, results show that the algorithm achieves a substantial performance improvement by making use of the array combining output except self-signal as reference to improve the SNR of reference signal and the performance of time delay estimator, especially at low SNR level. Furthermore, the estimatiom performance of the algorithm is better than redundancy fusion algorithm.3、Cramer-Rao lower bounds for the estimation of combining weight of BPSK and QPSK modulated signals are derived, and a blind combining weight estimation algorithm based on the noise autocorrelation is also present. Theoretical results about estimation lower bound show that it is assured to achieve a substantial performance improvement due to the optimal usage of the mutual information between the signals, and reduce the loss of array combining output gain especially at low SNR level. At the same time, aiming at computing the optimum combining weight generally and practically, a blind combining weight estimation algorithm using signals’second order moment with the characteristic of the noise power spectral autocorrelation jointly is proposed, which discards the solely signal’s power and SNR. Theoretical analysis and simulations show that the algorithm can be used in many cases, such as unknown signal’s SNR, different modulation and signal bandwidth along with frequency offset, and have achieved preferable performance and low computational complexity which are proper for hardware implementation.4、An effective blind combining algorithm is presented to combine the signals at low SNR from multiple antennas without symbol timing synchronization in the frequency selective channels. The combining weights are studied based on matched filter in the frequency selective channels, and the optimum receiver structure is also presented. The combining algorithm can be interpreted as each frequency component for all the received signals varying co-phasing and maximal ratio combining. The combined signal distortion will be reduced besides improving the ratio of signal to noise for each frequency component. It is shown that the algorithm can achieve a substantial performance improvement, and doesn’t require the symbol timing synchronization and the signal modulation mode. It could be considered as a useful combining technique, especially when symbol timing synchronization is difficult to realize at low SNR in the frequency selective channels.5、To meliorate adverse effect on the demodulation performance caused by the single channel’s nonlinear phase characteristic, an average phase method from all of the branches is proposed. Because the demodulation scheme with Baud Spaced Equalizer after symbol timing synchronization is often used on the assumption of the unknown channel, the reduction about SNR after symbol timing synchronization and equalizer’s output performance is analyzed, which is brought by the channel’s nonlinear phase characteristic. Different from the fixed single signal, the new phase compensating algorithm utilizes all of the signals from the antenna array, and the resultant phase is converged to the average phase of the signals. As a result, it could improve the characteristic of the phase frequency and overcome the shortcoming from the fixed single method. Moreover, a relative average phase algorithm is also presented to avoid the multivalued phenomenon about the phase difference. Finally the validity of the phase compensating algorithm is affirmed by the simultations.更多还原