【作者】 徐海源；

【导师】 张锡祥； 周一宇；

【作者基本信息】 国防科学技术大学 ， 信息与通信工程， 2007， 博士

【摘要】 在现代雷达和通信系统中,相位编码是广泛应用的一种调制方式。针对不同的应用需求,相位编码信号所采用的具体编码形式又各不相同,这就要求侦察接收机必须根据侦察对象的特点采取相应的处理方法,以达到最佳的截获性能。本文对雷达和数字通信中常用的几类相位编码信号的侦察处理问题进行了研究和探索,主要工作如下:1.针对低截获概率（LPI）雷达中常用的基于线性调频（LFM）导出的一类多相编码信号的检测和参数估计问题,利用这类信号在模糊平面和时频平面内的线性特征,提出了基于Radon-Ambiguity变换（RAT）和改进的Radon-Wigner变换（RWT）的联合检测与参数估计方法。以P4码信号为例推导了参数估计的克拉美.罗下限（CRLB）,并通过仿真表明,基于RAT和RWT的参数估计性能相近,在低信噪比下均比较接近CRLB。2.针对卫星通信中常用的非平衡QPSK（UQPSK）信号的参数估计问题,推导了UQPSK信号的谱相关函数,提出了基于谱相关的载频、码速率、初相以及I、Q通道功率比的估计方法。对功率比的估计性能进行了分析,并通过仿真验证了性能分析的正确性。针对采用长、短周期伪码（PN码）扩频的UQPSK直扩信号的扩频周期估计问题,采用相关函数二阶矩方法进行了分析。推导了UQPSK直扩信号的相关函数二阶矩,详细分析了长、短PN码信号的扩频周期检测性能与各种参数的关系,并进行了仿真验证。3.针对数字通信中相位编码等线性调制信号采用的一类平方根升余弦成形滤波器的参数估计问题,提出了基于IFFT（逆快速傅立叶变换）和最小二乘的估计方法,并对滚降系数的估计性能进行了理论分析。仿真表明所提出的滚降系数和码速率估计方法具有较高的估计精度,并且具有低信噪比适应能力。4.针对卫星测控/数传链路中常用的BPSK、QPSK、OQPSK和UQPSK信号的调制分类问题,本文根据这四种信号的复码元序列特性提出了基于高阶矩的特征C₁和C₂,实现了层次分类,所提出的分类特征对载频偏移和相位偏移具有不变性。对分类算法的性能进行了详细的理论分析,并通过仿真验证了算法的有效性。5.针对长码直扩信号的参数估计问题,本文对扩频码长P与扩频增益L之比K是否为整数的情况分别进行了研究。对于K为非整数的长码直扩信号,提出了基于对相关函数二阶矩进行后处理的符号速率估计方法。结合理论推导和仿真验证表明,K为非整数的长码直扩信号的相关函数二阶矩不仅在PN码周期整数倍处出现峰值,而且还隐含了符号周期的信息。通过一系列处理后,能够在较低信噪比条件下得到符号速率的有效估计。对于K为整数的长码直扩信号,推导了其相关函数二阶矩,得出了与短码直扩信号和K为非整数的长码直扩信号不同的结论;提出了基于最大范数的符号周期和同步偏移估计方法,并进行了改进以提高低信噪比适应能力;最后,提出了对协方差矩阵进行分块特征值分解的PN码序列估计方法。更多还原

【Abstract】 Phase-coded signals are widely used in modern radar and communication systems. The phase characteristics are usually different in various phase-coded systems, so intercept receivers must resort to sophisticated signal processing techniques to achieve best interception performance according to the characteristics of the objects. Analysis and processing of several phase-coded signals widely used in radar and digital communications is intensively studied in this dissertation. The main research works are listed as follows.1. To investigate the joint detection and parameter estimation of polyphase codes derived from linear frequency modulation （LFM） used in low probability of intercept （LPI） radars, the linear characteristics in ambiguity function and time-frequency distributions are utilized and algorithms based on Radon-Ambiguity transform （RAT） and modified Radon-Wigner transform （RWT） are proposed. The Cramer-Rao Lower Bound （CRLB） of parameter estimation for P4 code is deduced as an example. Simulation results show that the two methods have similar parameter estimation performance which is close to CRLB at low SNRs.2. Parameter estimation methods of unbalanced quaternary phase shift keyed （UQPSK） signal widely used in satellite communications are investigated. The spectral correlation function of a UQPSK signal is derived. Signal parameters such as carrier frequency, chip rate, carrier phase and channel power ratio are estimated by means of the special cyclic spectrum amplitudes and phases. The estimation performance of channel power ratio is analyzed and verified by computer simulations. To explore the pseudo-noise （PN） code period estimation of UQPSK direct sequence spread spectrum （DSSS） signal with long and short period spreading codes, the method based on the second order moment of a correlation estimator is introduced. The relationship between periodic detection performance and signal parameters is derived, and simulation results show the validity of theoretical analysis.3. For estimating the parameters of the root raised cosine filter in any linear digital modulation scheme in digital communications, algorithms based on the inverse fast Fourier transform （IFFT） and least squares estimation are proposed. The estimation performance of roll-off factor is analyzed in detail. Simulations show that the proposed roll-off factor and chip rate estimators have good performance at low SNRs.4. To investigate the automatic modulation classification of Phase Shift Keying （PSK） signals widely used in TTC （Tracking, Telemetry and Control） and data links of satellite, such as BPSK, QPSK, OQPSK （Offset QPSK）, and UQPSK, two classification features C₁ and C₂ are proposed based on the characteristics of the complex code sequnces. The proposed features are invariant with carrier frequency offset and carrier phase offset. A hierarchical scheme is used to classify the four signals based on C₁ and C₂. The classification performance is analyzed in detail and is verified by simulations.5. Parameter estimation of a long-code DSSS signal is studied. Denote the ratio of spreading code length P to processing gain L as K, we find that the long-code DSSS signals with K integral or not have different characteristics. An algorithm based on further processing of the autocorrelation estimator is proposed to estimate the symbol rate of a long-code DSSS signal with K not integral. Theoretical analysis and simulations show that PN code period and symbol period are hidden in the curves of the second order moment of the autocorrelation estimator. Simulation results show that the proposed algorithm is effective on symbol rate estimation at low SNRs. When K is an integer, the second order moment of the autocorrelation estimator of the long-code DSSS signal is derived, and which is shown to be different to that of a short-code DSSS signal and a long-code DSSS signal with K not integral. The maximum norm approach is introduced to estimate PN code period and timing offset. Finally, the covariance matrix is divided into several low rank matrices and eigenvalue decomposition approach is introduced to each matrix to estimate the PN sequence.更多还原