伪卫星远近效应分析与研究

伪卫星技术在改善和增强卫星导航系统定位性能等方面起到极其重要的作用。在应用中,常常会遇到时间同步、远近效应和几何精度衰减因子等问题。介绍了伪卫星应用中远近效应问题和常用的解决方案,重点分析了采用时分多址技术来进行伪卫星信号设计的问题。通过研究证明,采用时分多路复用更适合远近效应问题的解决。对伽利略伪卫星的脉冲调制进行了研究,得出了适合伽利略信号的脉冲调制占空比参数。     

矢量跟踪算法在抗远近效应中的应用

针对室内伪卫星独立定位系统中由于远近效应而导致跟踪环路失锁甚至定位失败的问题,提出了一种有效抑制远近效应的算法矢量跟踪环路（VTL）算法。通过分析 VTL 环路结构,建立了线性化的系统模型,并推导了详细的工作流程。VTL 以定位误差为状态量,以伪距、伪距率误差为观测量,结合更新后的定位结果,得到稳定的环路跟踪输入。仿真对比分析了标量跟踪环路（STL）和矢量跟踪环路对中断信号的持续跟踪性能。搭建了室内伪卫星独立定位系统,将采集到的中频信号在 VTL 软件接收机中进行定位解算,实验结果证明VTL 相对于STL 可以有效地抑制远近效应。     

跳时信号体制地基伪卫星远近效应分析

远近效应反映了传播距离变化下信号间干扰对信号捕获、跟踪和电文解析的影响,是影响地基伪卫星定位系统定位精度和有效范围的关键因素。针对地基伪卫星系统在高精度定位中的应用,分析了远近效应对系统性能的影响,提出基于跳时改善远近效应的方法,分析了TH/DS-CDMA伪卫星信号体制改善远近效应的作用原理和改善程度,并提出在帧信号体制下系统跳时相关参数的一般设计方法。针对基于北斗参数的伪卫星仿真了系统测距误差和可用范围的分布特性。仿真结果表明,跳时体制可大幅提升地基伪卫星信号的测距性能和有效覆盖范围,跳时对载波相位测量的影响和在星地联合运行下的参数设计有待进一步研究。     

卫星导航接收机的抗干扰技术分析

介绍了卫星信号可能的受干扰形式,并从卫星星上抗干扰技术和地面抗干扰技术2个方面介绍了抗干扰措施。提出了能够有效抑制干扰的抗干扰技术——频域滤波和空时滤波级联技术,通过频域的干扰抑制处理和空时滤波2个方面详细分析了该技术抑制干扰的特性。针对四元阵天线接收机方案进行了仿真,仿真结果证明了该技术在卫星接收机中应用能够有效的抑制干扰。     

GPS/BD伪卫星脉冲信号捕获特性分析

脉冲图案设计是当前解决伪卫星远近效应的有效方式。针对当前伪卫星信号捕获不易的问题,在GPS/BD双系统伪卫星平台的基础上,对伪卫星脉冲信号的捕获性能从脉冲信号伪卫星间和伪卫星对接收机的影响进行仿真分析。仿真结果表明,伪卫星信号占空比和是影响卫星信号接收的主要原因,伪卫星间的干扰主要与跟踪环路被激励和部分自相关偏差有关。     

一种基于伪卫星信号正交性的远近效应消除技术

该文针对导航领域伪卫星应用中的远近效应问题,提出了一种根据伪卫星信号正交性消除远近效应的新思想。在对伪卫星信号之间的正交性进行理论分析的基础上,提出了正交投影算法,该算法从接收信号中提取并减去在强信号分量上的投影,以减轻强信号对弱信号的捕获干扰,且具有无需改动接收机的硬件结构和估计信号分量幅度等优点。通过仿真与传统的滑动相关方法进行了捕获性能比较,结果表明,在捕获概率为90%的条件下,正交投影算法能将抗远近效应的能力提高约9 dB。     

RF干扰对卫星导航接收机性能影响分析及研究

对卫星接收机的干扰主要包括人为干扰和无意干扰,干扰的方式主要为压制式和欺骗式,一定的范围下,对卫星导航接收机的影响主要表现在对跟踪的状态的改变,即对跟踪的载噪比、伪距和载波相位的影响,从而使卫星导舷接收机的定位精度下降或失锁而达到干扰的目的。     

一种基于卡尔曼滤波处理的北斗卫星无源组合导航自适应野值剔除方法

在组合卫星导航定位中,由于导航接收机存在各种噪声和干扰,使得伪距出现较大的变动,直接导致定位结果出现野值。本文在卡尔曼滤波技术的基础上,提出了一种自适应野值剔除方法,对动态数据野值判断、识别与剔除,有效地抑制卫星导航中动态数据中的野值对滤波的影响。     

高动态地基伪卫星接收机载波跟踪技术

针对地基伪卫星系统接收信号接收载噪比和频率激励特点,分析了三阶锁相环的载波跟踪能力,在高动态条件和接收机接近离去伪卫星等过程中,有限的载噪比和接收信号的高阶频率激励将导致三阶锁相环失锁。为提高伪卫星系统接收机在高动态条件下的载波跟踪能力,提出一种惯导/伪卫星接收机深耦合载波跟踪方法,基于惯导输出的高频定位、速度信息和伪卫星位置信息估计接收信号的频率偏移并进行补偿。仿真实验表明,接收机在不高于10 g加速度直线运动下接收邻近伪卫星信号,深耦合算法具有相对更为稳定和精确的载波跟踪能力。     

基于蜂窝构型的空基伪卫星定位网络

伪卫星是基于非星基平台提供附加导航信号的设备,可与现有导航卫星共同工作或独立工作,能有效地改善导航性能。当伪卫星独立工作时,其空间分布对导航定位性能和应用有很大影响。针对基于中高空飞行器平台伪卫星网络的定位特点,提出一种高/低空混合蜂窝构型,并对可见性、远近效应和精度因子(DOP)分布进行了仿真。仿真结果表明,蜂窝构型具有覆盖范围大、拓展方便和DOP好的优点,能够提高覆盖地域的定位性能。     

导航系统抗远近效应接收机的研究

研究了导航系统抗远近效应接收机,首先给出该接收机结构,然后给出判断远近效应的方法,最后研究了消除远近效应的方法.仿真结果表明,该接收机具有良好的抗远近效应性能,适合采用直扩技术的陆基导航系统应用.     

混合干扰抵消技术在导航接收机中的应用

在扩频体制的无线电导航系统中,接收机同时会收到多个导航台发射的地波和天波信号,这样接收点同时存在天波干扰和远近效应问题。提出基于天地波识别的远近效应判定算法和混合干扰抵消方法,在捕获结果都为地波的前提下,判断是否存在远近效应,利用串行干扰抵消跟踪获得较准确的初始数据估计,在此基础上用并行干扰抵消使信息数据更加精确。分析了干扰的因素,建立了实现原理框图,从而得出系统性能。仿真结果表明,利用改进的干扰抵消技术不仅可以提高系统的识别能力,还可以提高对于导航电文的接收性能。     

Training analysis of adaptive interference suppression fordirect-sequence code-division multiple-access systems

This paper studies the transient behavior of an adaptive near-far resistant receiver for direct-sequence (DS) code-division multiple-access (CDMA) known as the minimum mean-squared error (MMSE) receiver. This receiver structure is known to be near-far resistant and yet does not require the large amounts of side information that are typically required for other near-far resistant receivers. In fact, this receiver only requires code timing on the one desired signal. The MMSE receiver uses an adaptive filter which is operated in a manner similar to adaptive equalizers. Initially there is a training period where the filter locks onto the signal that is sending a known training sequence. After training, the system can then switch to a decision-directed mode and send actual data. This work examines the length of the training period needed as a function of the number of interfering users and the severity of the near-far problem. A standard least mean-square (LMS) algorithm is used to adapt the filter and so the trade-off between convergence and excess mean-squared error is studied. It is found that in almost all cases a step size near 1.0/(total input power) gives the best speed of convergence with a reasonable excess mean-squared error. Also, it is shown that the MMSE receiver can tolerate a 30-40 dB near-far problem without excessively long convergence time     

Performance analysis of CDMA with imperfect power control

The standard correlation receiver for code-division multiple access (CDMA) systems is susceptible to the near-far problem. Power control techniques attempt to overcome near-far effects by varying transmitted power levels to ensure that all signals are received with equal power levels. Since these algorithms cannot perfectly compensate for power fluctuations in a mobile communications channel, the capacity of the system is reduced for a given bit-error rate (BER). This paper examines the performance of a CDMA system using imperfect power control by extending analytical techniques that account for multiple access interference. Single cell capacity is compared with systems employing perfect power control     

Blind adaptive interference suppression for the near-far resistantacquisition and demodulation of direct-sequence CDMA signals

Two key operations required of a receiver in a direct-sequence (DS) code division multiple access (CDMA) system are the timing acquisition of transmissions that are starting up or have lost synchronization, and the demodulation of transmissions that have been acquired. The reliability of both these operations is limited by multiple-access interference, especially for conventional matched filter-based methods, whose performance displays an interference floor and is vulnerable to the near-far problem. Recent work has shown that, provided timing information is available for a given transmission, it can be demodulated reliably using blind or training-sequence-based adaptive interference suppression techniques. These techniques are near-far resistant, unlike the matched filter demodulator, and do not require explicit knowledge of the interference parameters, unlike nonadaptive multiuser detectors. In this paper, we present a blind adaptive interference suppression technique for joint acquisition and demodulation, which has the unique feature that the output of the acquisition process is not simply the timing of the desired transmission, but a near-far resistant demodulator that implicitly accounts for knowledge of the timing and amplitudes of all transmissions to suppress the multiple-access interference. The only knowledge required by the scheme is that of the desired transmission's signature sequence, so that it is amenable to a decentralized implementation. On the other hand, it can be efficiently implemented as a centralized scheme in which the bulk of the computations for the adaptation are common to all transmissions that need to be acquired or demodulated     

Near-far resistance of multiuser detectors in asynchronous channels

Consideration is given to an asynchronous code-division multiple-access environment in which receiver has knowledge of the signature waveforms of all the users. Under the assumption of white Gaussian background noise, the authors compare detectors by their worst case bit error rate in a near-far environment with low background noise, where the received energies of the users are unknown to the receiver and are not necessarily similar. Conventional single-user detection in a multiuser channel is not near-far resistant, and the substantially higher performance of the optimum multiuser detector requires exponential complexity in the number of users. The authors explore suboptimal demodulation schemes which exhibit a low order of complexity while not exhibiting the impairment of the conventional single-user detector. It is shown that there exists a linear detector whose bit-error-rate is independent of the energy of the interfering users. It is also shown that the near-far resistance of optimum multiuser detection can be achieved by a linear detector. The optimum linear detector for worst-case energies is found, along with existence conditions, which are always satisfied in the models of practical interest     

MMSE interference suppression for direct-sequence spread-spectrumCDMA

We consider interference suppression for direct-sequence spread-spectrum code-division multiple-access (CDMA) systems using the minimum mean squared error (MMSE) performance criterion. The conventional matched filter receiver suffers from the near-far problem, and requires strict power control (typically involving feedback from receiver to transmitter) for acceptable performance. Multiuser detection schemes previously proposed mitigate the near-far problem, but are complex and require explicit knowledge or estimates of the interference parameters. In this paper, we present and analyze several new MMSE interference suppression schemes, which have the advantage of being near-far resistant (to varying degrees, depending on their complexity), and can be implemented adaptively when interference parameters are unknown and/or time-varying, Numerical results are provided that show that these schemes offer significant performance gains relative to the matched filter receiver. We conclude that MMSE detectors can alleviate the need for stringent power control. In CDMA systems, and may be a practical alternative to the matched filter receiver     

一种空时盲自适应RAKE接收机

多址干扰是直扩码分多址系统中的一个主要问题.多用户检测技术和阵列天线已成为解决这一问题的两种主要方法,将这两种方法结合起来的空时处理能大大提高系统的容量和性能.本文提出了一种空时盲自适应RAKE接收机.研究结果表明,该接收机具有较强的抑制多址干扰和克服"远-近"效应的能力,且能快速收敛.     

An Adaptive MMOE-PIC Detector for Asynchronous DS-CDMA Communications

We propose a modified linear parallel interference cancelation (PIC) structure using the adaptive minimum mean output-energy (MMOE) algorithm for direct-sequence code-division multiple-access (DS-CDMA) systems. The complexity of the proposed receiver structure is shown to be linear in the number of users and hence, lower complexity than the centralized minimum mean-squared error (MMSE) multiuser detector. It is demonstrated that the proposed receiver structure can significantly reduce the long training period required by the standard adaptive MMOE receiver in near-far environments. Both numerical and theoretical results show that the proposed receiver performs close to the optimum MMSE receiver whereas the conventional adaptive MMOE detector suffers from high BER’s due to the imperfect filter coefficients. Also our results show a three fold increase in the number of users when the MMOE-PIC is used relative to the conventional MMOE receiver. Furthermore, the transient behavior of the proposed MMOE-PIC receiver due to abrupt changes in the interference level is examined. It is shown that the proposed adaptive receiver offers much faster self recovery, with less signal-to-interference ratio (SIR) degradation, than the standard MMOE in sever near-far scenarios.     

Adaptive Multiuser Detection Based on RBF Networks in Impulsive Noise CDMA Channels

With increasing need for capacity in mobilecellular communications, code division multiple accesshas been viewed as the technique to meet this goal.However, the conventional receiver suffers frommultiuser interference and the near-far effect. Toalleviate this problem, various multiuser receivers havebeen proposed. They have been shown to enhance theperformance of the conventional receiver with respect to capacity and interference rejection. Butthese receivers, intended for Gaussian noise channels,can fail in other environments such as impulsive noiseconditions. A possible solution is to exploit the advantages of neural networks. A neuralnetwork's ability to reject non-Gaussian noise can beutilized to implement multiuser receivers that rejectboth impulsive noise and multiuser interference.Following this logic, this paper considers nonadaptivemultiuser receiver and adaptive multiuser receiver basedon radial basis function neural networks. Simulations ofthese receivers show results that are superior to previously studied multiuser receivers, withrespect to practicality and performance.