基于贝叶斯模型的长码DS-CDMA信号扩频码与信息序列联合估计

 针对长码直接序列扩频码分多址(DS-CDMA)信号的盲解扩,在信号模型分析的基础上,提出了一种基于可逆跳跃的马尔可夫链蒙特卡罗(RJ-MCMC)扩频码和信息序列联合估计算法。该算法分别对建立的联合后验分布模型进行迭代抽样,并有效地在不同维数的子空间中跳转,从而构造一条马尔可夫链,使其平稳分布为待估参数的后验分布,最终拼接得到每个用户的完整扩频序列和信息序列估计。仿真结果表明:该方法在迭代二十几步时达到收敛;并且在功率相同和不同条件下,当信噪比(SNR)大于-9 dB时,估计序列与真实序列的相似度均超过0.95,信息序列的误码率低于0.01;同时算法对不同用户个数和不同调制样式具有较强的适应性,与Fast-ICA算法相比,估计性能平均提高了约3 dB。     

基于改进补零/差分的北斗弱信号捕获算法

北斗信号二次编码调制了速率为1kbps的NH码,频繁的比特跳变导致传统捕获算法的效果受到抑制,并且限制了积分时间的增长。为了实现北斗弱信号的捕获,提出了一种适合北斗弱信号的捕获算法,将补零算法和差分相干积分算法相结合并进行了优化设计。对补零算法和差分相干积分算法进行了改进,将差分延迟时间调整为20ms,有效消除了NH码的影响,并增加了积分时间,提高了信噪比。采用Monte Carlo仿真测试了在载噪比为20dBHz~38dBHz的条件下,此算法与传统算法的检测性能。结果表明,相比传统算法,此算法的捕获灵敏度提高了约10dB。此算法能够有效消除北斗NH码相位变化对信号捕获的影响,并提高信噪比,适用于弱信号条件下的北斗信号捕获。     

On the Acquisition Time for an Apollo Ranging Code

An Apollo ranging system is considered whose phase reference is obtained by a phase-locked loop for bit synchronization. The bit phase reference is noisy, and the error probability for the ranging code is shown to depend on the input signal energy per bit to noise density ratio. The procedure of computing the acquisition time for the ranging code is then presented and the acquisition time for a lunar ranging code is plotted versus the input signal-to-noise density ratio.     

A Method for Setting a Test Signal Level Relative to Noise

A method is described for adjusting the leval of an RF test signal generator relative to the noise level at the receiver output. The method compares a detected output to a threshold and counts the number of times noise and signal plus noise cross the threshold in a given number of tries. By setting the threshold at a given false alarm probability for noise alone and then adding the test signal and adjusting its level to give a specified detection probability, the signal-to-noise ratio can be calibrated to an accuracy that depends on the number of samples used to measure the probabilities. The false alarm and detection probabilities are given for best accuracy as well as the rms error in signal-to-noise ratio as a function of the number of samples used.     

一种改进的直方图概率多假设多目标跟踪方法

直方图概率多假设跟踪（H-PMHT）方法及其变形泊松分布直方图概率多假设跟踪（P-HPMHT）方法的一个主要缺点是其量测模型仅考虑了背景杂波而没有考虑传感器噪声,从而导致在低信噪比条件下检测概率较低。针对这一问题,提出了一种带传感器噪声模型的H-PMHT方法,通过将传感器噪声引入量测模型,从而明显提高了对低信噪比目标的跟踪检测能力。该方法的计算量与目标数保持线性关系,仍然适用于目标数目较多的情况。仿真实验表明：该方法在误跟踪比率为1‰,信噪比为6 dB时,检测比率可提升近20%,信噪比为3 dB时,可提升近10%。     

Poly-phase codes and optimal filters for multiple user ranging

A technique is introduced to select poly-phase codes and optimal filters of a pulse compression system that have specific temporal and frequency characteristics. In the particular problem under study, multiple vehicles are assigned unique codes and receiver filters that have nearly orthogonal signatures. Narrowband users, that act as interference, are also present within the system. A code selection algorithm is used to select codes which have low autocorrelation sidelobes and low cross correlation peaks. Optimal mismatched filters are designed for these codes which minimize the peak values in the autocorrelation and the cross correlation functions. An adjustment to the filter design technique produces filters with nulls in their frequency response, in addition to having low correlation peaks. The method produces good codes and filters for a four-user system with length 34 four-phase codes. There is considerable improvement in cross and autocorrelation sidelobe levels over the matched filter case with only a slight decrease in the signal-to-noise ratio (SNR) of the system. The mismatched filter design also allows the design of frequency nulls at any frequency with arbitrary null attenuation, null width, and sidelobe level, at the cost of a slight decrease in processing gain     

Closed-Form Expressions for Noncoherent Radar Integration Gain and Collapsing Loss

Closed-form formulas allow rapid determination of noncoherent integration gain and integration loss when the single-sample IF signal-to-noise ratio (SNR) is known. In addition, if the required SNR is known for any number of integrated pulses, the required SNR for any other number is easily determined. A closed-form expression is given for radar collapsing loss, expressed in terms of the equivalent integrated signal-to-noise ratio required to produce a given combination of false-alarm and detection probabilities. Alternatively, the single-sample signal-to-noise ratio of a set of samples may be used together with the closed-form expression for integration gain to get the equivalent integrated signal-to-noise ratio.     

New Centroid Algorithm Based upon Amplitude-Angle Signature

A method of estimating the centroid location of a target utilizing radar scan return amplitude versus angle information is presented. The method is compared with three thresholding estimators and a first moment estimator in a computer-simulated automatic landing system. This new method is the most robust and accurate during periods of low signal-to-noise ratio. In periods of high signal-to-noise ratio the method has less error than the thresholding methods and is similar in accuracy to the first moment estimator. Furthermore, the number of pulse transmissions required to obtain a desired level of performance in noise is much less than that needed for the thresholding methods and the first moment estimator employed in this simulation.     

GPS多径信号的自适应滤波估计方法

 研究全球定位系统(GPS)多径信号估计的问题。通过分析自适应滤波器的原理,建立了数字中频信号处理的数学模型,提出一种用自适应滤波实现GPS多径幅度、码相位和载波相位估计的方法。该方法采用不同延迟的伪随机序列对信号进行解扩、解调和累加,得到了作为期望信号的系列自适应滤波相关值。对该方法与其他3种方法进行了理论上的分析比较,得出本方法具有信噪比高、自适应滤波性能好、带有码相位信息和不存在载波模糊度问题等优点。根据各种滤波器算法的特点和本应用的需求,给出了选用递归最小二乘算法实现的方法。通过计算机仿真,验证了提出的方法能够在14 dB的信噪比下,以1个采样间隔的时间延迟分辨率和0.005周的载波相位估计精度估计出GPS L1的多径信号。     

白天大气层内星敏感器观星能力分析

利用星敏感器进行白天大气层内观星,首先要解决的问题是强天空背景的干扰。通过对白天天空背景和不同光谱恒星的特点进行分析,提出了光谱滤波和偏振成像加形态学滤波和多帧图像累加的技术路线,综合利用光学方法和图像处理方法提高观测对比度和信噪比。给出了白天观星的信噪比模型,并在不同天空背景偏振度条件下,得出了G、K、M光谱恒星的0至5等星的单帧图像信噪比和对应的使CCD(Charge Coupled Device,电荷耦合器件)接近饱和的极限曝光时间。结果表明:星等值越低,天空背景偏振度越大,则信噪比越高;同等条件下,信噪比由小到大依次为G、K、M光谱恒星;而对于低信噪比的图像,通过多帧累加后可满足观测的要求。     

大气湍流对APD直接检测接收机抗噪声性能影响

针对以往分析湍流信道对系统抗噪声性能影响的不足,给出了一种基于APD直接检测接收机的湍流噪声的处理方法.在APD直接检测接收机模型的基础上,推导了其信噪比和曼彻斯特编码系统的差错概率,并详细分析了湍流对接收机的抗噪声性能影响.结果表明,增大接收天线孔径可以减弱湍流的影响,但在天线孔径较小时,增加发射功率并不能有效地提高...     

Probability of Detecting a Fluctuating Target Immersed in Both Noise and Clutter

The probability of detecting either a Swerling 1 or Swerling 2 target immersed in both Rayleigh-distributed noise and log-normally distributed clutter is calculated. Results are presented which demonstrate the effect of noise-to-clutter ratio, signal-to-noise ratio, and number of pulses integrated on the detection statistics.     

A Postdetection Method of Measuring Predetection RF Signal-to-Noise Ratio

A method of determining the predetection signal-to-noise power ratio in a radio receiving system by measurement of average postdetection signal-plus-noise and noise-only voltages (dc output of the detector) is described. The principle has actually been known for many years, but does not seem to be well known or widely used, possibly because of some associated computational difficulties. Some digital computer tabulated results are presented which remove these difficulties, and measurement techniques are discussed. The calculation of expected signal-to-noise ratio for radio and radar systems is briefly reviewed.     

Cyclic code shift keying: a low probability of intercept communication technique

A low probability of intercept (LPI), or low probability of detection (LPD) communication technique known as cyclic code shift keying (CCSK) is described. We discuss the basic concepts of CCSK and describe a system based on the use of random or pseudorandom codes for biphase modulation. We use simulation to show that the bit error rate (BER) for CCSK can be closely estimated by using existing equations that apply to M-ary orthogonal signaling (MOS). Also, we show that significantly fewer computations are required for CCSK than for MOS when the number of bits per symbol is the same. We show that using biphase modulation results in waveforms that have a large time-bandwidth product and very low input signal-to-noise ratio (SNR) and thus inherently have an LPI by a radiometer. We evaluate detection by a radiometer and show that LPI can be achieved by using codes of lengths greater than about 2/sup 12/ (i.e., by transmitting more than about 12 bits per symbol). Results illustrate the effect that the CCSK symbol length and error probability, and the radiometer integration time and probability of false alarm (PFA), have on detection by a radiometer. We describe a variation of CCSK called truncated CCSK (TCCSK). In this system, the code of length 2/sup k/ is cyclically shifted, then truncated and transmitted. Although shortened, the truncated code still represents k bits of information, thus leading to an increased data rate. We evaluate radiometer detection of TCCSK and it is shown that the probability of detection is increased compared with the detection of CCSK.     

差分GPS的原理和实现方法

GPS卫星无线电导航系统预计在1995年投入正式运行。GPS信号中的P码仅供美军和特别用户使用,C/A码则向全世界民用开放。C/A码受选择可用性(SA)影响,导航静度下降到一个适当水平。为提高GPS定位精度,各国都争相研究差分GPS(DGPS)。本文从GPS误差入手,主要就DGPS原理和实现方法进行讨论。     

A Practical PRSD System for Multiple Target Detection

A block diagram of a practical system using the sequential detection technique for radar detection of multiple targets is given. The system is designed to obtain better detection probability with a lower value of signal-to-noise ratio for a given false-alarm probability and average transmitted power. The total pulse duration is divided into a number of range elements which are tested for the presence or absence of a target before switching the antenna beam to the next position.     

Hansen's Method Applied to the Inversion of the Incomplete Gamma Function, with Applications

Hansen's method for obtaining the threshold for a speciried false alarm probability following noncoherent integration after square law detection is applied to finding the inverse of the incomplete gamma function. The method is algebraic and direct, circumventing the necessity for repeated evaluation of an integral or of a finite series. The method is applied to finding any specified percentile of the chi-square distribution and to finding the required signal-to-noise ratio for a specified detection probability of a Swerling II target.     

GPS code and carrier multipath mitigation using a multiantennasystem

Multipath is a major source of error in high precision Global Positioning System (GPS) static and kinematic differential positioning. Multipath accounts for most of the total error budget in carrier phase measurements in a spacecraft attitude determination system. It is a major concern in reference stations, such as in Local Area Augmentation Systems (LAAS), whereby corrections generated by a reference station, which are based on multipath corrupted measurements, can significantly influence the position accuracy of differential users. Code range, carrier phase, and signal-to-noise (SNR) measurements are all affected by multipath, and the effect is spatially correlated within a small area. In order to estimate and remove code and carrier phase multipath, a system comprising a cluster of five GPS receivers and antennas is used at a reference station location. The spatial correlation of the receiver data, and the known geometry among the antennas, are exploited to estimate multipath for each satellite in each antenna in the system. Generic receiver code and carrier tracking loop discriminator functions are analyzed, and relationships between receiver data, such as code range, carrier phase, and SNR measurements, are formulated and related to various multipath parameters. A Kalman filter is described which uses a combination of the available information from the antennas (receivers) in the multiantenna cluster to estimate various multipath parameters. From the multipath parameters, the code range and carrier phase multipath is estimated and compensated. The technique is first tested on simulated data in a controlled multipath environment. Results are then presented using field data and show a significant reduction in multipath error     

The 22 GHz resource sharing experiment of the DAVID mission

The resource sharing experiment (RSE) of the DAVID (DAta and Video Interactive Distribution) multiexperiment mission of the Italian Space Agency (ASI) is described. The experiment envisages adaptively varying the robustness of signals down-transmitted, to a set (16) of Earth terminals by acting on their coding and spreading. During the DAVID satellite passes, each terminal determines autonomously its signal-to-noise ratio (SNR) and transmits it to a central station which, by using this information, works out the parameters for the global system optimization and indicates, in real time, to the terminals which code and despreading factor they must utilize to receive the part of the signal addressed to them.     

Cross-Correlation Interference Effects in Multiaccess Optical Communications

The effects of the cross correlation between user codes in an opticalcode-division multiple-access communication system are investigated. The system model is a multiaccess satellite repeater in which the uplink and downlink channels are direct-detection, optical-polarization modulation links. The error probability is derived in terms of the cross correlation between the intended and interfering user codes. It is shown that the system error rate can be minimized by using code sequences in which the normalized second moment of the cross correlation between codes is small. The signalto- noise ratio (SNR) on the uplink is shown to be proportional to 1/K while the SNR on the downlink is proportional to 1/K1/2, where K is the number of users which are simultaneously accessing the system.