DS-CDMA系统中PN码定时跟踪算法及其实现

定时跟踪是DS-CDMA系统中rake接收的关键技术,性能的好坏直接影响着扩频系统能否正常接收。本文首先对扩频系统中定时跟踪原理做出分析,然后重点介绍了PN码的“基于能量窗重心的定时跟踪”算法,并选择了这个算法来实现CDMA2000-1X行链路的PN码定时跟踪。最后,经过在典型城市环境下的现场实测,误帧率达到CDMA2000-1X的标准要求。     

CDMA系统的PN码技术

１CDMA系统中的PN码同步原理发射机和接收机采用高精确度和高稳定度的时钟频率源，以保证频率和相位的稳定性。但在实际应用中，存在许多事先无法估计的不确定因素，如收发时钟不稳定、发射时刻不确定、信道传输时延及干扰等，尤其在移动通信中，这些不确定因素都有随机性，不能预先补偿，只能通过同步系统消除。因此，在CDMA扩频通信中，同步系统必不可少。PN码序列同步是扩频系统特有的，也是扩频技术中的难点。CDMA系统要求接收机的本地伪随机码与接收的扩频编码相位完全同步，实现本地PN码与接收到的PN码在结构、频率和相位上完…     

基于虚拟多用户模型的长码直扩信号伪码估计

针对长码直接序列扩频(DSSS)信号的伪随机码(PN)盲估计问题,提出了一种新的周期长码直接序列扩频信号(PLC DSSS)模型.该模型等同于虚拟多用户短码扩频DS - CDMA系统,采用DS- CDMA的谱范数来估计同步偏移,该算法不需要更多假定限制,利用相关函数二阶矩、特征值分解和模糊酉矩阵方法就可以完成伪码序列的...     

基于混沌扩频CDMA的伽利略卫星定位系统编码技术

在伽利略系统中编码技术日益重要。文章在将混沌扩频技术应用到伽利略系统中后,对系统的信道特性进行了探讨,特别是利用混沌扩频的优秀随机性和正交性对系统进行了信道仿真。仿真结果表明信道衰减小于-40 dB,满足CDMA2000协议规范,从而简化了传统CDMA系统PN码产生的复杂性,统一了信号源和信道编码的混沌序列的使用。这种将混沌扩频CDMA技术代替传统PN序列的方法在伽利略系统中的应用具有实用性和可行性。     

扩频统一测控系统的PN码捕获

扩频体制的统一测控系统(TT&C)实现遥测、遥控、测距、跟踪、数字通信等多种功能统一以及多站测量和多目标测量的统一,其实质是利用PN码的相关性实现CDMA。多普勒效应和数据调制对PN码的同步捕获和跟踪的影响很大,此时PN码的捕获成为系统实现的关键。     

KM序列用作RAKE型CDMA系统扩频码的分析

本文提出将KM 序列用作RAKE型CDMA系统的扩频码.基于一个RAKE接收机模型,分析了CDMA系统性能并提出了适于作RAKE型CDMA系统扩频码的PN序列的特性.接着,分析了KM 序列作RAKE型CDMA系统扩频码的优劣.最后,运用计算机仿真,比较了m序列和KM 序列的互相关特性以及在相干接收机和RAKE接收机下,这两种序列和Gold序列对应的系统平均误码率,从而证实了KM 序列作为扩频码的可行性.     

一种零中频直接序列扩频数字接收技术

直接序列扩频技术在现代无线通信系统中有着广泛的应用,该信号的数字接收是实现对其侦察截获的关键技术之一。本文介绍了直接序列扩频技术原理及其在cdma2000移动通信系统中的应用,提出一种针对cdma2000直接序列扩频信号的零中频数字接收算法,并详细给出了码捕获及跟踪算法、载波同步算法的数学模型推导。实验结果显示,该算法成功地实现了码同步及载波同步,有较好的解调效果。     

KM序列用作RAKE型CDMA系统扩频码的分析

本文提出将KM M序列用作RAKE型CDMA系统的扩频码 .基于一个RAKE接收机模型 ,分析了CDMA系统性能并提出了适于作RAKE型CDMA系统扩频码的PN序列的特性 .接着 ,分析了KM M序列作RAKE型CDMA系统扩频码的优劣 .最后 ,运用计算机仿真 ,比较了m序列和KM M序列的互相关特性以及在相干接收机和RAKE接收机下 ,这两种序列和Gold序列对应的系统平均误码率 ,从而证实了KM M序列作为扩频码的可行性     

基于快速OPAST算法的直扩信号PN码序列估计

在直接序列扩频(DSSS)通信对抗系统中,伪码(PN)序列估计是一个重要的研究课题.针对在某些情况下权值向量不收敛的问题,提出了一种基于快速正交投影逼近子空间跟踪(OPAST)算法和滑动窗技术的直扩信号PN码序列估计算法,对非同步接收DSSS信号按照宽窗口分段,利用快速OPAST算法提取主特征向量,滑动窗技术实现码同步.该算法迭代权值向量具有正交性以及良好的收敛性,同时解决了常见相位模糊问题.算法具有较低复杂度,数据存储量少,易于硬件实现与实时处理.仿真结果表明,在-10 dB的较低信噪比背景环境中,该快速OPAST算法可以正确有效地估计出PN码序列.     

相位搜索法扩频码捕获的仿真实现

扩频码同步是扩频通信系统中特别关键的技术,相位搜索法捕获是扩频码捕获中最基本的方法,硬件实现最简单,在短PN码捕获时有很大的优势.文中在MATLAB平台研究该捕获方法的实现,给出了整个同步过程的仿真模型,各部分的详细参数设置和关键部分的仿真结果.选择长度为31的PN码进行仿真,用5级移位寄存器产生本地PN码序列,不断调整其相位,直到本地PN码与接收的PN码序列初步对齐.仿真结果表明,该模型能够实现31位PN码的正确捕获.捕获时间与本地PN码和接收PN码之间的相位差有关.     

OCDD/CDMA: a new DS/CDMA with orthonormal code diversity detection

The paper presents a new spread spectrum communication system called orthonormal code diversity detection (OCDD)/CDMA system based on the novel concept of orthonormal-basis diversity which is a generalization of the existing spread spectrum diversity concepts such as path diversity and frequency diversity. The OCDD/CDMA system is similar to the conventional DS/CDMA system in the transmitter structure, but is different in the receiver structure as it employs the extended orthonormal basis-function set which is the union of the Walsh basis-functions multiplied by the PN sequences and, optionally, their delayed replicas. The received signal is matched to the extended basis functions, and the matched signal components are combined together after individual channel compensation. The proposed OCDD/CDMA system exhibits the bit error performance which is much improved over the conventional DS/CDMA system using maximal ratio combing. In addition, it is robust to the chip timing error, which becomes more crucial in the future DS/CDMA systems having a higher data rate and smaller chip interval. From the simulation results, we confirm that the OCDD/CDMA system is a unique spread spectrum communication technique that can effectively increase the diversity utilization in the slowly fading channel, overcoming the inherent problems in the DS/CDMA and OFDM/CDMA systems     

Pseudonoise code tracking loop for a CDMA system with imperfect power control

In this paper, performance of a pseudonoise (PN) code tracking loop is analysed and simulated for a direct‐sequence/code‐division‐multiple‐access (DS/CDMA) system with imperfect power control in a multipath fading channel. A non‐coherent first‐order delay‐locked loop (DLL) is considered as a PN code tracking loop. Power control error is modelled as a log‐normally distributed random variable. The multipath fading channel is modelled as a two‐ray Rayleigh fading model that is typically applied to land mobile communication environments. The tracking performance of DLL is evaluated in terms of tracking jitter and mean‐time‐to‐lose‐lock (MTLL). From the simulation results, it is shown that for smaller discriminator offset, tracking jitter performance is improved while MTLL performance is degraded. It is shown that large power control error and heavy shadowing substantially degrade the PN tracking performance. Therefore, an accurate power control scheme is very essential to compensate for fading and shadowing for a DS/CDMA system. The extension of analysis to higher‐order loop models is very straightforward. The analysis in this paper can be applied to design the PN code tracking loop for a DS/CDMA system. Copyright © 2001 John Wiley & Sons, Ltd.     

大频差长地址码扩频系统快速捕获方案的设计与实现

该文基于离散时间信号处理分析了多普勒频移对捕获性能影响的表达式,给出部分匹配滤波器与FFT结合方案的原理。针对大频差长地址码扩频系统快速捕获问题,提出一种采用部分匹配滤波器与FFT结合算法的实现方案,在考虑捕获性能与实现复杂性间的折衷上,具有很好的灵活性。该方案应用FPGA与DSP芯片配合实现,应用结果表明该方案在移动卫星通信和直扩抗干扰通信中具有很好的应用前景。     

Modified tracking loop for DS spread spectrum systems

A nonconventional pseudo-noise (PN) code tracking loop for direct sequence (DS) spread spectrum systems is presented. It solves problems of component imbalance while maintaining hardware simplicity.<>     

PN acquisition and tracking performance in DS/CDMA systems withsymbol-length spreading sequences

Direct sequence spread spectrum code-division multiple-access (DS/CDMA) is receiving increasing attention for cellular communications systems. When the users are synchronized and special symbol-length sequences, such as Gold codes, are used, the mutual interference can be substantially reduced relative to a system with very long or purely random spreading sequences. It is shown that this approach degrades the performance of the code phase acquisition and tracking, however. This effect prevents the system from acquiring and tracking long before the data detection is affected by the multiuser interference     

A Generalized Analysis for a Dual Threshold Sequential Detection PN Acquisition Receiver

Spread spectrum techniques have become very useful in modern communications. A common spread spectrum technique is to directly modulate a pseudorandom noise (PN) code onto the carrier. Systems employing this technique are called direct sequence (DS) systems. A primary concern with direct sequence systems involves PN code synchronization (acquisition) of the incoming and local signals to within at least one PN symbol followed by a fine alignment (tracking) which must be maintained throughout transmission. This paper is concerned with the PN acquisition aspect of code synchronization. Specifically, a dual threshold sequential detection receiver is analyzed and a general expression is derived which addresses the probability that the sequential detection procedure ceases after an arbitrary number of samples. This expression is needed in order to support a characteristic function approach in deriving acquisition Probability as a function of time which will be the topic of a subsequent paper.     

扩频通信系统的软件无线电实现结构及性能分析

本文主要论述了直接序列(DS)扩频通信系统中的软件无线电实现结构.首先从软件无线电的基本概念出发,论述了直接扩频通信系统中采用软件无线电结构的可能性和必要性;其次详细论述了直接扩频通信系统中软件无线电结构的实现形式,包括发射方、接收方的实现结构,扩频编码的同步(包括扩频编码相位的捕获与跟踪)以及多址通信的实现等.对文章中提出的方法进行了计算机模拟试验,结果是令人满意的.     

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.     

直序扩频系统中扩频码同步技术的研究

针对扩频通信中伪随机序列的快速捕获问题,文中提出了扩频信号快速捕获方法,来解决低信噪比条件下伪码序列的捕获问题.首先对直接扩频通信系统的基础理论做了探讨,而后重点研究了两种基带扩频信号同步捕获方案--序列相位搜索捕获法和不连续相位滑动相关法,序列相位搜索捕获法主要适用于m序列码周期较短的情况,而不连续滑动相关法则克服了...     

DS-CDMA移动通信系统的捕获技术

PN (PseudoNoise)码捕获技术是DS CDMA (DirectSequence CodeDivisionMultipleAccess)蜂窝移动通信系统的关键技术。文中提出了一种基于能量窗的捕获技术 ,有效地利用了CDMA系统固有的多径分集能力 ,捕获对象是多径信号 ,研究结果表明 :对有效多径成分进行捕获有比单径捕获和跟踪有更好的性能。