谱相关理论用于直接序列扩谱信号的检测与估计

本文论述了谱相关理论在直接序列扩谱信号的检测与估计中的应用，提出了利用谱相关函数法与集平均算法相结合的方法来实现对低信噪比直接序列扩谱信号的检测及其载频与伪码速率的估计，并利用相关处理方法来估计的伪码周期，信息码序列及伪码序列，通过计算机模拟证实了该方法的有效性。     

基于谱相关的直扩信号载频及伪码速率估计技术

将谱相关理论应用到直接序列扩频信号的关键参数估计中，针对BPSK调制的直扩信号，分析了其谱相关密度函数的特征，提出了基于集平均的谱相关估计算法。该算法在非零循环频率上搜索特征谱线，从而在较低信噪比下完成对直扩信号载频和PN码速率的估计。最后通过仿真验证了该算法的性能。     

直扩信号检测的相关积累技术

分析直接序列扩谱信号与噪声在统计特征上的差别,在此基础上,提出利 用相关积累的思路来检测直扩信号载频和码速的方法。这种相关积累技术充分 利用了伪噪声和噪声之间的统计差别,以较长的积累时间,来获得低信噪比下检 测扩谱信号的能力。在仿真验证中,对于 ＧＰＳ的 Ｐ码信号,在信噪比为－２６ ｄＢ(带内信噪比为－１２ ｄＢ）时,相关积累 ０．５～１ｓ钟后,扩谱信号的载频和码速被准 确估计出来,其误差在１１ ｋＨｚ以内。     

一类新型二元序列族的相关函数研究

扩频序列设计是直接序列扩频通信系统的关键课题之一。本文在一类具有二值自相关函数的二相序列基础上，构造了一类新的二相序列族，计算了新序列族的相关函数，结论表明：该二相序列族在Welch界意义下是最佳的，可适用于直接序列CDMA通信系统中。     

新型离散Walsh函数构造及其发生器

介绍一种二维离散Walsh函数的新型构造方法和相关特性,并给出采用这种方法设计的发生器的电路结构。在码分多址扩频通信（CDMA扩谱通信）中,将离散Walsh码序列作为扩谱码可提高系统性能,有效地克服多路瑞利衰减,使32个、64个或更多的用户共用一个频率进行无线通信。     

直扩信号检测的相关积累技术

分析直接序列扩谱信号与噪声在统计特征上的差别，在此基础上，提出了利用相关积累的思路来检测直扩信号载频和码速的方法，这种相关积累充分利用了伪噪声和噪声之间的统计差别，以较长的积累时间，来获得低信噪比下检测扩谱信号的能力，在仿真验证中，对于GPS的P码信号，在信噪比为－26dB（带内信噪比为－12dB)时，相关积累0．5－1s钟后，扩谱信号的载频和码速被准确估计出来，其误差在100kHz以内。     

混合色噪声下：可消除谱估计伪峰的多正弦信号率估计互谱奇异值分解方法

本文依据对正弦谐波信号的扩阶互相关函数矩阵的特征分析,提出消除扩阶方程产生的谱估计伪峰的理论及方法,仿真结果表明本文所提出的互谱异值－整体最不二乘法（SVD－LS）可完全消除谱估计伪峰,从而提高了谱分辨率及噪声抑制能力。     

扩频码速率异步时直扩系统码分多址能力

直接序列扩频码分多址系统是目前应用最广泛的扩频系统,影响系统容量的主要因素是系统内多址信号的干扰。分析了采用Gold码的码速率异步直扩系统多址干扰,对扩展积分周期和单一积分周期均进行了数值仿真。仿真结果表明,码速率异步的Gold序列互相关函数不再具有三值特性;随着积分长度的扩展,互相关函数的统计值逐渐递减,即多址干扰减小;多址干扰与码偏移量大小有关,在某些特定码偏移量,积分长度为最小公倍数周期时,互相关函数接近为单值,而这些码偏移量正负对称出现;积分长度为单一周期时,多址干扰随码偏移量的变化趋势与扩频码长度无关;在相同码偏移量的情况下,随着扩频码的长度增加而减小。     

混沌扩谱CDMA的系统模型及其性能分析

混沌序列在具有很好随机性的同时,一些序列还具有KroneckerDelta自相关和近似正交的互相关特点,因此被认为是实现直序扩谱CDMA系统较理想的扩谱序列之一,成为近年来研究的热点。文中从异步DS-CDMA系统的模型入手,分析影响CDMA系统性能的关键问题,说明混沌扩谱CDMA系统的性能优点。为了实现混沌扩谱CDMA系统的数值仿真与性能验证,以切比雪夫映射产生的混沌序列为例进行混沌扩谱CDMA系统的性能分析,并与基于Gold和Kasami两种序列的传统DS-CDMA系统进行比较。     

利用三阶相关特征信息的周期长码扩频信号伪码盲估计

针对非合作通信下长码直扩码分多址信号的伪随机序列估计问题,本文首先利用同阶数、不同本原多项式产生的m序列的三阶相关函数峰不同的特点,构造三阶相关函数的特征信息矩阵。然后推导得到长码直扩码分多址信号三阶相关向量与特征信息矩阵的匹配理论值。最后估计接收信号延迟三阶相关函数,得到其三阶相关向量,与特征信息矩阵匹配,估计得到各扩频长码。理论分析和仿真结果表明该方法能够有效估计单用户和多用户的长码直接序列扩频信号伪随机序列。      

GMW序列和WG序列及WG序列间的互相关特性

对于奇数n,研究了Gordon-Mills-Welch（GMW）序列与Welch-Gong（WG）序列（及其某一采样序列）间的互相关函数以及WG序列间的互相关函数。研究表明：GMW序列与WG序列（及其某一采样序列）间的互相关函数以及WG序列间的互相关函数最终均与m-序列与其采样序列间的互相关函数相关;并且在一定条件下,WG序列间的互相关函数可为3-值或5-值的。另外,给出了GMW序列与WG序列间互相关函数的最大峰值。     

Blind joint information and spreading sequence estimation for short-code DS-SS signal in asynchronous and synchronous systems

The problem of blind despreading of short-code direct sequence spread-spectrum signal in asynchronous or synchronous system is considered. A novel blind estimation algorithm for time offset, spreading and information sequences is presented in this paper. Firstly, the received signal is divided into two-information-period-length temporal vectors overlapped by one-information-period and accumulates these vectors one by one to form the accumulated matrix. The algorithm exploits an unbiased estimation of user’s time offset to realize synchronization blindly, and an operation of singular value decomposition is applied to the intercepted matrix, which is intercepted from the accumulated matrix according to user’s time offset. Lastly, the spreading sequence is recovered from a right singular vector blindly; therefore, the shortcomings of traditional algorithms, such as phase ambiguity when make use of two vectors to form the spreading sequence, are avoided. And the information sequence is then recovered blindly from the left singular vector. Simulation results show that the new method is capable of retrieving time offset, the spreading and information sequences with higher accuracy than previous methods.     

采用广义正交序列的多载波DS-CDMA系统在瑞利衰落信道中的性能分析

在CDMA系统中，采用正交序列可以有效地减小多址干扰的影响。然后，当由于多径衰落的影响使码片间的相对移位增加时，系统中用户扩频序列间的相互正交性减弱，导致性能大大恶化。本文基于新近提出的广义正交序列，分析了采用该序列的多载波直接序列码多分址系统在瑞利衰落信道中的误比特性能。数值结果表明，当扩频序列间相互移位在零相关区之内时，扩频序列仍可保持正交，因而大大提高了系统误比特性能。     

Performance evaluation of DS-CDMA systems on multipath propagationchannels with multilevel spreading sequences and RAKE receivers

Performance of small-cell wireless direct sequence code-division multiple access (DS-CDMA) systems with multilevel spreading sequences is investigated. An analytical methodology to evaluate both outage probability and mean bit error probability is presented; and its results are compared to those obtained with a semianalytical Monte Carlo based approach. Multipath propagation, the exact correlation properties of spreading codes, and both the simple correlator and RAKE receivers are taken into account in the analysis, which is oriented to asynchronous and synchronous environments. The spreading sequences investigated include multilevel sequences with complex isomorphic mapping, as well as some well-known two-level sequences. The numerical results show the impact of multilevel sequences on system performance, the improvements found in the absence of multiple access interference by using them rather than binary sequences, and the agreement between analytical and semianalytical evaluation     

Interference in DS-CDMA systems with exponentially vanishingautocorrelations: chaos-based spreading is optimal

A novel estimation of the minimum achievable interference in direct sequence-code division multiple access (DS-CDMA) systems is introduced, which holds when spreading sequences with exponentially vanishing autocorrelation are employed. This can be applied to many of the recently proposed improvements to classical maximum-length or Gold sequences, such as chaos-based spreading. Asymptotic, infinite-bandwidth results are also provided, clarifying the maximum attainable gain. Empirical evidence shows that this theoretical maximum is achieved by some chaos-based sequences which are therefore optimal     

Spreading codes for direct sequence CDMA and wideband CDMA cellularnetworks

This article presents an overview of the spreading techniques for use in direct sequence CDMA cellular networks. We review the theoretical background for sequences used in CDMA and wideband CDMA, and discuss the main characteristics of the maximal length, Gold (1967, 1968), and Kasami sequences, as well as variable- and fixed-length orthogonal codes. We also describe different methods of multiple spreading for channelization and scrambling in CDMA and W-CDMA realizations     

Optimal spreading sequence design based on PR-QMF theory

Based on filter bank theory, a new scheme for spreading sequence generation is proposed for direct sequence code division multiple access (DS-CDMA) systems. It is shown that optimal spreading sequences produce much less multiple access interference (MAI) than Gold codes with comparable lengths     

Performance of SCHT Sequences in Asynchronous CDMA System

In this letter, the performance of a new set of spreading sequences for a direct sequence (DS) code division multiple access (CDMA) system is examined. These new sequences are derived from sequency-ordered complex Hadamard transform (SCHT). Various correlation properties are presented and the performance of the new sequence is analyzed by comparing it with the other well-known existing spreading sequences, such as UCHT, WHT, Gold, small set of Kasami and m-sequences. According to the simulation results, it is shown that the SCHT sequences provide smaller mean square cross correlation values and offer better average bit error rate for an asynchronous CDMA system over the multipath fading channel.     

Performance enhancement of DS/CDMA system using chaotic complex spreading sequence

In this paper, a novel direct sequence/spread spectrum (DS/SS) communication system is proposed. This scheme exploits the two-dimensional complex valued chaotic Ikeda map as the spreading sequences. With this double spreading DS/SS system, the effect of multiple access interference can be mitigated by choosing the spreading sequences with appropriate cross-correlation properties. The performance of the system is assessed and demonstrated in a multiuser environment by means of computer simulation with additive white Gaussian noise, Rayleigh fading, and selective fading channel conditions. These studies reveal that the proposed system significantly outperforms the Gold code DS/SS-BPSK system in synchronous channel conditions. In asynchronous case, the improvement is substantial for low signal-to-noise ratios.     

Adaptive Interference Suppression for the Downlink of a Direct Sequence CDMA System with Long Spreading Sequences

A simple approach for adaptive interference suppression for the downlink (base-to-mobile link) of a direct sequence (DS) based cellular communication system is presented. The base station transmits the sum of the signals destined for the different mobiles, typically attempting to avoid intra-cell interference by employing orthogonal spreading sequences for different mobiles. However, the signal reaching any given mobile passes through a dispersive channel, thus destroying the orthogonality. In this paper, we propose an adaptive linear equalizer at the mobile that reduces interference by approximately restoring orthogonality. The adaptive equalizer uses the pilot's spreading sequence (which observes the same channel as the spreading sequence for the desired mobile) as training. Simulation results for the linear Minimum Mean Squared Error (MMSE) equalizer are presented, demonstrating substantial performance gains over the RAKE receiver. Long spreading sequences (which vary from symbol to symbol) are employed, so that the equalizer adapts not to the time-varying spreading sequences, but to the slowly varying downlink channel. Since the inter-cell interference from any other base station also has the structure of many superposed signals passing through a single channel, the adaptive equalizer can also suppress inter-cell interference, with the tradeoff between suppression of intra- and inter-cell interference and noise enhancement depending on their impact on the Mean Squared Error (MSE).