Systematic Multi-Frequency Positioning Algorithms and Performance Analysis for GNSS Receivers

The known multi-frequency positioning algorithms neglected the correlations of the measurement errors and were lack of performance analysis. This paper introduces systematic dual-frequency and triple-frequency positioning algorithms, concerning the correlations of the multi-frequency signals, and defines multi-frequency dilution of precision parameters to compare the positioning accuracies. The variance of the multipath error and receiver measurement error is the key factor for multi- frequency positioning accuracy. Under normal conditions, dual-frequency accuracy is worse than single-frequency accuracy with the variance larger than 3.18m^2, and triple- frequency accuracy is worse than single-frequency accuracy with the variance larger than 4.56m^2. Multi-frequency GNSS receivers should be designed and used to reduce multipath effect as much as possible.     

A multipath mitigation discriminator for GPS receiver

A novel technique for mitigating the multipath-induced code delay estimation error in Global Positioning System (GPS) is proposed. In contrast to conventional methods that aim to eliminate multipath signals, the proposed method exploits them to enhance the direct signal without affecting the accuracy of GPS code delay estimates. To achieve this, coherent accumulation of the received GPS signals is first done by transforming the received data into frequency domain and the parameters of multipath signals are then estimated by sparse reconstruction algorithm. Subsequently, a modified local reference signal is employed in delay lock loop (DLL) of the GPS receiver, which mitigates the pseudo-range estimation error and increases the correlation value of direct GPS signal. Simulation results demonstrate the performance and robustness of the proposed method.     

Theory of False Lock in Costas Loops

This paper is concerned with new results on data related false lock of Costas loops caused by arm filter distortion of the data. The false lock phenomena is demonstrated to occur for random as well as periodic data and the relative levels of lock are predicted. It is shown that, for the case of random data with Manchester symbols, strong lock points exist when the incoming carrier frequency and the loop VCO frequency differ bynR_{s}/2 (n = 1, 2, 3, ...)Hz where Rsis the data symbol rate. In addition to the loop error signal the lock detection signal(I^{2}Q^{2})is also analyzed for false lock effects. The theory also applies to the case of NRZ data symbols with the main result being strong false lock points also occur at VCO frequency offsets ofnR_{s}/2 (n = l, 2, 3, ldot)but at different levels than the Manchester data. The results of these analyses lead to schemes capable of detecting false lock.     

A New Positioning Method for Location-Based Services in Wireless LANs

Positioning techniques are crucial to achieve location-based services. Location estimation systems using received signal strength in wireless LANs （Local area network） have attracted great attention for the advantages of low cost, ubiquitous coverage and ease of deployment. Existing methods neglect the difference between contributions of access points to location estimation results. Signals from far access points travel long distance and thus more uncertainty is added to the receive signal strength. This paper proposed a positioning method based on the Dempster-Shafer evidence theory. Different belief was assigned to signal strength from different access points according to its importance to location estimation. We carry out a series of experiments to evaluate the method and compare with other related work. Experimental results show that the approach gives a higher degree of positioning accuracy.     

State Estimation for Linear Constrained Systems with Unknown Input

This paper deals with the filtering problem for linear discrete constrained dynamic systems with unknown input. The constraint matrix and constraint vector in this system are allowed to vary in the value and in the dimension. The original full state is separated into two parts, and the estimate of the state is reduced to find the optimal estimate of a singular system. The estimable condition is researched and a recursive estimator for the original full state is presented. A rigorous mathematical induction is given to compare the performance of our approach to that of the existing method without constraint. A numerical example is presented to demonstrate the effectiveness of the new method.     

多径信道下OFDM系统小数倍频偏估计算法改进研究

针对多径信道环境下OFDM系统小数倍频偏估计精度不足的问题,提出一种基于循环前缀(CP)估计的改进算法.该算法重点在于准确搜索出受到多径干扰的数据范围,利用未受到多径干扰的CP数据进行ML频偏估计,并对估计值加以修正,使得估计结果更加精确.仿真结果表明,改进算法相对于原算法具有更好的估计性能.     

Third-order delay-locked loop: mean time to lose lock and optimalparameters

A systematic approximation for the mean time to lose lock (MTLL) of a coherent third-order PN-code tracking loop has been derived. Such loops are essential in various spread spectrum systems (Global Positioning System, for example). The computation of the MTLL is based on the singular perturbation method. The application of this method to the coherent delay-locked loop (DLL) yields an approximate expression for the MTLL. Therefore, with the proposed loop model the authors are able to analyze this third-order system at a level that gives a well understanding of the nonlinear loop behavior and the exit phenomenon. The influence of a loop offset due to an acceleration rate (jerk) between transmitter and receiver on the optimal filter parameters is described by comparing MTLL and tracking error performance. As intuitively might be expected it turns out that acceleration rate and code rate are exchangeable in the sense that a lower code rate allows a higher acceleration rate (and vice versa) for the same signal-to-noise ratio in order to maintain the same performance. In a case study, GPS code tracking for objects with high jerk is briefly discussed     

基于FIR模型的多径抑制方法研究

多径效应是卫星导航系统的主要误差源之一。在现有研究中,基于最大似然估计准则的方法被认为是最佳的多径抑制技术,但是当存在多路信号且路径数量未知时,其计算尤为复杂。本文提出了一种基于FIR模型的多径抑制方法,利用基准相关函数和最小均方误差准则对接收信号进行参数估计和多径信号对消。该方法具有实现简单,易于实时信号处理,计算量与多径路数无关等优点。仿真结果表明,其性能略优于基于最大似然估计准则的MEDLL（Multipath Estimating Delay Lock Loop）技术。      

用于CDMA中的智能天线技术研究

讨论了应用于CDMA中的智能天线技术,并且利用高阶统计量和累积分布函数的概念,分析了在多径干扰条件下智能天线的性能.     

Multipath error mitigation based on wavelet transform in L1 GPS receivers for kinematic applications

GPS positioning issue has been experienced a surge of interest for multipath mitigation in the past few years. Multipath disturbance is a challenging error in high precision GPS positioning particularly in kinematic mode where the moving receiver undergoes different multipath circumstances. Diverse hardware and software approaches have been implemented to reduce this error in both static and kinematic modes. We have proposed an algorithm to alleviate C/A code multipath in kinematic mode. Our proposed scheme falls into software group where wavelet transform (WT) has been used as the main basis. Of course, we have used stationary wavelet transform (SWT) as the key idea of our proposed method that makes it possible to investigate all frequency sub-bands. Consequently, it would be possible to extract multipath disturbance since it is considered to be a low frequency term. After applying SWT to double difference (DD) residuals, the multipath error is extracted and used to correct DD observations. In this paper, we have designed three experiments to study our proposed method efficiency under different conditions in comparison with existing algorithms.     

Phase Lock for Correlation Detection of a Periodic Pulse

The detection of the presence of a periodic pulse with uncertain pulse rate and low duty cycle is a common biotelemetry problem. Modern phase-lock and correlation techniques have provided more efficient means of making the presence decision than that provided by energy detection methods. A local signal is cross correlated with the input in a phase-locked loop (PLL) to provide phase reference. Upon lock, a local duplicate signal is generated and correlated with the input. A decision threshold operates on the correlator output. The PLL with filter F(s) = (s+a)/s and three appropriate nonlinearities are analyzed for acquisition time and maximum frequency error that will lock.     

Some Simulation Results for the Time to Indicate Phase Lock

A digital simulation of a second-order phase-lock loop (PLL) acquiring lock in the presence of noise is reported. Since the system simulated includes lock-indicating circuitry, a distinction is made between the time to phase lock and the time to indicate lock. The parameter of interest is the time to indicate lock. The major results of the simulation are estimates of 1) the mean time to indicate lock, and 2) the probability distribution function for the mean time to indicate lock. Results are presented for a wide range of signal-to-noise ratios and for input frequency offsets ranging from one-half to three times the phase-lock loop noise bandwidth. In each case 100 independent trials of the simulation were conducted to obtain the estimate. It is found that the mean time to indicate lock is relatively independent of signal-to-noise ratio (S/N) down to aboutS/N = 10dB. For greater values ofS/Nthe mean time to indicate lock may be closely approximated by the time to indicate lock in the absence of noise. The results are compared with Viterbi's time to frequency lock and found in good agreement for the highS/Ncases. Most of the difference is attributed to the time constant of the lock indicating circuitry. The system simulated is considered representative of many practical phase-lock receiving systems where there is a requirement for indicating acquisition of lock. Hence, the data presented in this paper can be used to good advantage for estimating the probability of acquiring and indicating lock in a specified time.     

The Optimization of Combined Beamforming with Space-Time Block Coding

The combination of beamforming and Alamouti space-time block coding may not be optimal in rich scattering since it only uses two equal energy loading beams. A new scheme combining beamforming with spacetime block coding is put forward in this paper. The new scheme requires only the knowledge of channel correlation at the transmitter, the optimization design turns out to be an eigen-beamformer with multiple beams pointing to orthogonal directions along the eigenvectors of the channel correlation matrix and combine with appropriate spacetime block coding. In order to minimize a tight upper bound on the symbol error rate the optimal energy loading algorithm is proposed in this paper. We assume that the channel receive vectors observed on different receive antennas are mutually uncorrelated, but have the same correlation matrix, thus the new scheme can be extended to multiple receivers. The combination of beamforming and Alamouti space-time block coding is a special case of the new scheme. Based on Laplace transform, this paper provides an algorithm to calculate the probability density function of the equivalent signal to noise ratio, thus the closedorm expressions for the symbol error rate of the new scheme are derived. Simulation results demonstrate the superiority of the new scheme.     

Design and analysis of a fast frequency-hopped DBPSK communicationsystem. I. System description and hop timing tracking loop analysis

Design and performance analyses of a fast frequency-hopped (FFH) spread spectrum communication system employing differential binary phase shift keying (DBPSK) modulation and differentially coherent demodulation are presented. The receiver utilizes a hop timing tracking loop to lock the hop clock. Through this tracking loop, the differentially coherent demodulation of an FFH signal is made possible. The authors present the system model of an FFH/DBPSK system. A digital, full-time early-late gate timing error estimator is investigated. The tracking loop is shown to be a Markov chain, and various performance characteristics of the tracking loop are studied     

一种新的GPS多径抑制方法

随着GPS(全球定位系统)技术的广泛应用,人们对GPS的定位精度提出了越来越高的要求,有的已经要求达到cm甚至mm级。差分GPS系统虽然可以消除大部分定位误差,提高定位精度,但是还是无法消除多径信号引起的定位误差。介绍了GPS中多径信号的影响,分析了TK算法和多径监视(MPI)算法抑制多径的优缺点。通过将这2种算法级联,提出了一种新的混合方法(TK-MPI算法),并且取得了更好的定位精度,特别是在低信噪比的情况下具有很好的效果。     

GPS系统中的多径效应分析

本文研究了ＧＰＳ系统中多径效应的伪唬测量误差，先分析镜面多径信号的时间延迟和由此引起的频带扩展，然后研究在用相干和非相干延时锁定环跟踪ＰＮ码信号时多径信号对Ｓ曲线的影响和由此带来的伪距测量误差（多径误差）以及减小多径误差的方法。     

An Efficient Approach to Odometric Non-systematic Error Modeling for Mobile Robots

Odometric non-systematic error modeling for mobile robot is the basis of localization. Most of the approaches to odometric non-systematic error modeling are designed for some special driving-type robots nowadays. And the long-term odometric errors without bound, which degrade the localization precision after long-distance movement, are not often capable of being compensated in real-time. Therefore, a general approach to odometric nonsystematic error modeling for mobile robot is proposed in regard to both synchronous drive roller robots and diiTerential drive roller robots. The approach assumes that the robot path is approximated to circular arcs. The function relationships, between the odometric process input and non-systematic errors, are derived on the basis of the odometric error transformation rules, further the accumulative errors of odometry in the localization process are compensated in real-time. The experiments show that the compensation of non-systematic error can reduce the odo- metric long-term errors efficiently, and improve the localization precision remarkably.     

GPS接收机载波跟踪环设计与分析

针对GPS接收机载波跟踪环环宽与跟踪的动态性能问题,在分析影响GPS信号动态性能的主要参数热噪声、晶振Allan相位噪声、晶振振动相位噪声和动态应力的基础上,通过对不同阶数的锁相环、锁频环跟踪门限分析与仿真,主要解决了如何设计GPS接收机的载波跟踪环路的带宽,并使系统性能达到最佳的问题,即使用环宽为18 Hz的二阶锁相环辅助环宽为10 Hz的三阶锁频环可以跟踪动态范围小于10 g、100 g/s的高动态信号。     

一种DSSS系统中窄带与多径干扰抑制级联算法

根据线性预测具有良好的抗窄带干扰性能但无法抑制多径干扰的特性,提出了一种窄带干扰和多径干扰抑制的级联算法即CNMS算法,理论分析表明本算法可以获得比线性预测算法更高的性能增益。仿真结果说明,CNMS算法在优于线性预测算法的同时还优于线性均衡算法。     

An Adaptive Designing Method of Broadcast Almanac Parameters

A model of the signal acquisition time influenced by the almanac was introduced to evaluate the almanac model. It shows that the six parameters almanac model outperforms the three parameters almanac model. The almanac truncation error was introduced to describe the error between the original data and the interface specification formatted terms. The parameter sen- sitivity and the truncation error were analyzed to validate the necessity to choose the proper definition of the almanac parameters, including the effective range and the scale factor. It demonstrates the parameter definition is the key designing element while the almanac representation is determined. Based on the research above, An Adaptive almanac designing method （AADM） of broadcast almanac parameters was presented. Simulation results reveal that the AADM can generate an optimum set of almanac parameters with the shortest signal acquisition time to a limited bit allocation.