PN acquisition for DS/SS using a preloop parallel binary search phase estimator and a closed-loop selective search subsystem

An acquisition method of long pseudo-noise (PN) sequences in direct-sequence spread spectrum (DS/SS) systems is presented. The concurrent cross correlation of locally generated auxiliary signals with the incoming PN sequence produces binary search estimates leading to an initial approximation of the input signal's PN offset. The PN receiver then uses a closed-loop subsystem to determine the true offset of the incoming PN signal. New phase estimates are generated from successive corrections of likely errors in the binary search. The acquisition scheme's average acquisition time and its variance are analytically determined. Simulation results show that the proposed acquisition scheme is faster than the conventional serial receiver. A hybrid serial/parallel PN acquisition receiver is used for comparing the acquisition performance given similar hardware complexity. The hybrid receiver used the same number of parallel correlators as used in the proposed receiver's input phase estimator. Simulation results show that the proposed system provides an improved performance under normal operating conditions.     

Noncoherent hybrid parallel PN code acquisition for CDMA mobilecommunications

This paper proposes a noncoherent hybrid parallel pseudonoise (PN) code acquisition scheme for code-division multiple access (CDMA) mobile communication systems and analyzes the effect of multiple access interference (MAI) on the code acquisition performance for Rayleigh and Rician fading channels. The hybrid acquisition scheme combines parallel search with serial search to cover the whole uncertainty region of the input code phase. It has a much simpler acquisition hardware structure than the total parallel acquisition and can achieve the mean acquisition time slightly inferior to that of the total parallel acquisition in the case of severe MAI; on the other hand, it provides the flexibility in the tradeoff between the mean acquisition time and system complexity if no MAI is considered. The closed-form expressions of the detection and false-alarm probabilities and mean acquisition time are derived. Numerical analysis quantifies the severe performance degradation of code acquisition due to both MAI and channel fading, and demonstrates the dependence of the increase of mean acquisition time (due to MAI) on the number of users in the CDMA system, system design parameters, and channel fading statistics     

FFT辅助的部分相关码捕获技术的实现

FFT辅助的部分相关伪码快捕方法将沿码相位-多普勒频率的二维搜索简化为沿码相位的一维搜索,克服了多普勒频率的影响,大大缩短了PN码捕获时间,适用于中低轨卫星.文中介绍了该方法的基本原理、性能分析和实现方案.实验结果表明,在大多普勒频偏下FFT辅助的部分相关码捕获方法是一种性能优良的伪码捕获方法.     

基于分段相关－视频积累方法的多驻留伪码捕获系统优化设计

针对载波多普勒下的伪码捕获问题,分析了基于分段相关-视频积累方法的多驻留伪码捕获系统的最优性能。推导了正交双通道检测器的检测概率和虚警概率表达式,给出了多驻留系统的捕获时间均值表达式;通过采用遗传算法对系统进行优化设计,增大了系统的多普勒容限,降低了捕获时间;分析了驻留级数、多普勒和载噪比等对捕获系统性能的影响。研究成果可用于指导扩频系统接收机伪码捕获模块的设计。     

基于地空数据链路Rake接收技术研究

通常Rake合并与解调是根据导频信号来完成的,然而对于无导频的地空通信链路,如何实现Rake的分集接收,是一个值得深入研究的问题。本文提出了一种非相干解决方案,利用串-并混合搜索与串行搜索相结合的技术,优化了基于PN序列码捕获的接收机路径搜索算法;采用分区间加权方法,大大降低了实现接收机多径合并的复杂度,并提高了系统性能。仿真结果表明:它是克服信道衰落的一个重要手段,可以提高通信系统的整体性能,改善地空通信中"弱区"的影响。     

一种基于二分试探法的相干扩频快捕系统

利用辅助序列与接收PN码的线性相关性,在搜索方向判决准确的前提下,折半的查找范围对应折半的误差范围,达到应用二分法大步进缩小查找范围,从而有效的加快捕获速度,与常规步进滑动相关法相比,该捕获系统能够有效改善长PN码的捕获性能。     

组合导航接收机的码捕获技术

针对组合导航环境下的PN码捕获特点,采用分段并行捕获技术＋多次最大值判决策略,解决了高动态、大多普勒频移下PN码快速捕获的工程实现问题。从理论分析和仿真结果看是切实可行的。     

一种快速的PN码同步捕获方法

针对直扩系统中长PN码同步捕获速度慢的问题,利用推导的串接短PN码与长PN码具有相似相关性的结论,对直扩系统中PN码同步捕获的并行匹配滤波捕获方法进行改进,得到了简单可行的串并结合捕获方法。然后,结合一个周期内PN码相关值,采用迭代的方式,选取简单、实用的自适应门限,实现了突发通信中PN码的快速同步捕获。采用该自适应门限的捕获方法无需估计噪声,计算量小。仿真实验表明所提方法捕获概率高,抗噪性能强。     

基于伪码互相关特性的GPS信号捕获算法

高动态GPS接收机研究的核心是信号的捕获与跟踪算法,在接收机硬件实现时,系统性能与资源消耗直接受捕获算法的影响。结合时域滑动相关法和基于FFT的频域捕获算法各自的优点,提出一种基于伪码互相关特性的快速捕获算法,在不影响系统性能的前提下,既加快了捕获速度,又降低了资源消耗,实现了捕获速度和资源消耗二者的平衡。     

数字相关器在MSK扩频通信中的应用

讨论并分析了采用数字相关器的MSK扩频系统发射和接收基本原理。在PN码元的捕获系统中使用了一种改进的附加数字相关器。分析及实验结果表明,对于MSK扩频系统,低信噪比情况下能够正确解码,处理信号的形式灵活,免除了—般扩频系统中最难解决的PN码同步,并为位同步的提取带来了极大的方便;与数字相关器相比,能够简化捕获运算,且有效改善干扰。     

Simplified matched filter receiver designs for spread spectrumcommunications applications

The design of receiver architectures for use in direct sequence spread spectrum systems is reviewed. As it is difficult to construct the long matched filters for these systems, the authors propose a programmable receiver architecture which achieves the required high processing gain while, at the same time, simplifying the filter hardware requirements. Results are presented for a prototype receiver, based on digital signal processing (DSP) components, which can be reprogrammed for serial, serial-parallel or fully parallel operation while offering minimal degradation from theoretical performance     

大频偏下直接序列扩频系统的码捕获技术

关于大频偏下直接序列扩频系统的码捕获,一般的处理方法是在直扩信号中引入导频信号,接收端通过对导频信号的处理进行频偏估计,并消除频偏对PN码相关值的影响(典型的有“全球星”卫星移动通信系统)。然而,依据现代军事通信低截获、低检测性的要求,这种插入导频法已不再适用于军事通信。为解决此问题,文章介绍了直接序列扩频系统的一种新型码捕获技术即利用FFT消除频偏对PN码相关值影响的串并相关捕获,并进一步深入、具体的分析了这种捕获系统的原理,研究了此种捕获算法在加性高斯白噪声信道条件下的性能。     

并行数字相关器的FPGA实现

扩频码的相关解扩是扩频通信接收机的关键技术之一,主要介绍了数字相关器在全球定位系统（GPS）信号捕获中的应用,并进行了FPGA实现。在设计中,采用了16路并行相关运算的方式加快相关解扩运算速度。在GPS信号时域捕获的理论推导基础上,给出了数字下变频电路结构,分析了载波NCO的频率精度,详细讨论了扩频码相关解扩单元阵列的计算方法和电路结构与参数。最后,通过ModelSim算法仿真和Xilinx Virtex-5 LX220FPGA测试,取得了较好的捕获效果。     

扩频系统中自适应窄带干扰抑制滤波器对PN码捕获性能的影响

本文通过详细的理论推导,分析了扩频系统中自适应窄带干扰抑制滤波器对PN码捕获性能的影响;并根据滤波器理论,给出了自适应窄带干扰抑制滤波器的幅频响应、群延迟响应和相应的仿真结果.理论分析与仿真结果表明,自适应干扰抑制滤波器在抑制窄带干扰的同时,也抑制了部分扩频信号,从而形成较大的相关旁瓣,对PN码捕获性能产生严重的影响.     

A new approach to rapid PN code acquisition using iterative message passing techniques

Iterative message passing algorithms on graphs, which are generalized from the well-known turbo decoding algorithm, have been studied intensively in recent years because they can provide near-optimal performance and significant complexity reduction. In this paper, we demonstrate that this technique can be applied to pseudorandom code acquisition problems as well. To do this, we represent good pseudonoise (PN) patterns using sparse graphical models, then apply the standard iterative message passing algorithms over these graphs to approximate maximum-likelihood synchronization. Simulation results show that the proposed algorithm achieves better performance than both serial and hybrid search strategies in that it works at low signal-to-noise ratios and is much faster. Compared with full parallel search, this approach typically provides significant complexity reduction.     

多码环架构对扩频捕获性能的优化

为了提高扩频系统的捕获性能,可通过在扩频接收机中采用多码环架构或提高PN码捕获电路精度这两种方式达到目的.提出了在扩频接收机中采用多码环架构的设计方法,采用这种方式可以在降低码捕获电路精度要求、简化码捕获电路设计并减少近50%的系统资源消耗的基础上成倍地提高捕获性能.     

Serial search code acquisition using smart antennas with single correlator or matched filter

This paper investigates three code acquisition methods for direct sequence spread spectrum systems (DS/SS) utilizing smart antennas. The methods are suitable especially for receivers that consist of a smart antenna followed by a single correlator or a matched filter (IMF). The first method is the known fixed beam strategy where the whole angular uncertainty region is divided into small cells using conventional beamforming techniques. Therein, the receiver searches through all angular and delay cells via a serial search procedure. In the second method, the fixed division is made using advanced beamforming techniques, which provide improved tolerance against interference. In the third strategy, the direction-of-arrival (DOA) of incoming signals are estimated and utilized in the acquisition process. An advantage of this DOA estimation-based strategy, when compared to fixed beamforming methods, is a decrease in the region of uncertainty. Disadvantages are increased computational complexity and signal-to-noise ratio (SNR) limitations. The acquisition strategies are compared when the serial search acquisition of the code phase is made using either the correlator or the matched filter. The results indicate that a single antenna receiver gives the best acquisition performance when SNR is high. However, single antenna methods are sensitive to interference. On the other hand, DOA estimation-based methods offer shorter mean acquisition times than fixed beam methods, especially when the number of arriving signals is small.     

Code acquisition in coherent optical pulse CDMA systems utilizingcoherent correlation demodulation

A code acquisition technique based on active optical correlation detection is introduced for coherent optical pulse code-division multiple-access (CDMA) systems implemented through coherent correlation demodulation. Passive matched filter acquisition, commonplace in most radio CDMA systems, does not translate directly into the optical domain since synchronization between the incoming optical signal and a local optical code of the intended user at the receiver has to take place in several picosecond duration. Parallel and serial acquisition schemes under the same search algorithm are described and the effect of optical device noise, partial autocorrelation, and multiuser interference on acquisition performance is theoretically analyzed. Numerical results are given for Gold codes. In general, the degree of parallelism inherent in code acquisition techniques for radio systems allows a tradeoff between acquisition speed and hardware cost; however, in optical acquisition schemes, increasing the degree of parallelism with the concomitant hardware cost penalty does not necessarily reduce the time to acquisition. This is due to optical power budget limitations brought about by the need to split the optical signal, although optical amplifier technology can improve the performance of these parallel search schemes. The expressions for the partial correlation and multiuser interference derived in the paper can also be utilized for the analysis of other radio and optical acquisition schemes     

Multiple-shift code acquisition of optical orthogonal codes in optical CDMA systems

In this paper, we introduce a new and advanced algorithm, namely, multiple-shift algorithm for code acquisition in optical code-division multiple access (CDMA) systems using unipolar optical orthogonal codes (OOCs) as signature sequences. We analyze the performance of the newly proposed algorithm and obtain a bound on its performance and show its advantage in reducing the mean time of synchronization when compared with other synchronization methods. The algorithm can be used with many different receiver structures, like active or passive correlator with or without hardlimiter(s). However, in this paper, we only consider the simple active correlator structure for further discussions and analysis.     

非频率选择性衰落信道下捕获性能的分析

对基于数字匹配滤波器(DMF)捕获系统在非频率选择性衰落信道下的捕获性能进行了深入讨论,利用状态转移图推导出单次驻留判决方式时平均捕获时间的表达式。对平坦慢衰落信道和快衰落信道下的平均捕获时间进行了数字分析,得出的主要结论是：无论信道是否存在衰落,基于DMF的捕获系统都具有很快的捕获速度;基于DMF的捕获系统在衰落信道下的捕获性能明显要比非衰落信道差;衰落速度越快,平均捕获时间就越长,系统的捕获性能就越差。因此当信道存在非频率选择性衰落时,可以在接收端采用分集接收(比如空间分集)来改善系统的捕获性能。