Frequency selective propagation effects on spread-spectrum receiver tracking

This paper presents results of investigations of frequency selective scintillation effects, due to transionosphetic propagation, on the performance of spread-spectrum receivers. Emphasis is placed on pseudonoise (PN) direct sequence systems. Pertinent features of the disturbed ionospheric propagation channel are described together with the characteristics of resulting satellite signal scintillations. Effects of frequency selective scintillations on PN spread-spectrum code correlation and tracking are illustrated in some detail. The design of the receiver PN code tracking loop is an important factor in determining the significance of these effects. Of the designs that have been investigated, a dedicated (non-time-shared) noncoherent code tracking loop appears to be least susceptible to losing code lock in the presence of frequency selective scintillation. Possible design modifications to further enhance the robustness of PN code tracking are discussed.     

Tail Extrapolation in MLSE Receivers Using Nonparametric Channel Model Estimation

This paper presents a method for determining the probability of rare events, in particular for probability density function (pdf) and bit error rate (BER) estimation. The derivation of the method is based on the presumption that the pdf is a member of a family of distributions very often named as the generalized exponential (GE) class of distributions. Based on high reliability estimations obtained in short simulation/measurement times, the low probably events are estimated accurately by extrapolation. The suggested method can be applied to some distributions that are different from GE distributions, such as noncentral chi-square distributions, to extrapolate to low probability events, with some extrapolation error. It can also be applied to BER estimation. The method is in particular helpful for estimating channels suffering from both severe signal distortion causing undesired intersymbol interference (ISI) of several symbols, and from severe noise. Such conditions prevail, for example, in metro and long haul high-speed optical fiber communication systems. So the method may be implemented in particular in maximum-likelihood sequence estimation (MLSE) optical receivers using nonparametric channel model estimation. A special use of the extrapolation method is explained for practical systems using trellis branch metrics derived from the estimated pdf to decode the transmitted sequence of symbols.     

K-means clustering-based data detection and symbol-timing recovery for burst-mode optical receiver

Burst-mode receivers are key components of optical transmission systems, including passive optical networks, and have received much attention in recent years. We present new, efficient methods for burst optical signal detection in burst-mode data transmission using a modified K-means clustering technique. We also develop a data-aided feedforward symbol-timing recovery method based on a polynomial interpolation and maximum-likelihood estimation theory. A performance criterion considering the error caused by the interpolation approximation is derived for this method. The proposed detection and timing recovery approaches can be implemented effectively and rapidly; therefore, they are very suitable for burst-mode receivers. We also provide some numerical examples to demonstrate the performance of the proposed methods.     

A Noncoherent Delay-Lock PN Tracking Loop: Impact of RFI and Nonideal Arm-Filter Characteristics

A noncoherent delay-lock PN tracking loop is investigated. The approach used is one in which simulation and analysis are combined to generate tracking performance. Performance data are presented for PN phase jitter standard deviation and compared to theoretical results. The impact of radio frequency interference (RFI) and gain/phase imbalances on PN tracking performance is determined and performance data are given. It is suggested that self-noise terms (introduced via the correlation operation between the incoming signal and local PN code) and nonideal arm-filter characteristics (resulting in gain/phase imbalances) may be significant factors in the investigation of PN tracking performance for those cases in which PN code timing synchronization is a major concern.     

Receiver Performance for Heterodyne Optical Communication

A model for the heterodyne optical channel that is consistent with experimental observations is introduced, and a general maximum-likelihood sequence estimation receiver that accounts for laser phase instabilities is proposed using the generalized log-likelihood functional. The performance of various suboptimal "estimator-correlator" receivers, obtained from optimal, nonimplementable receiver equations, is studied by simulation and compared to that of an incoherent receiver for practical parameter values. It is observed that, for hard decisions, the proposed receiver performs significantly better than an incoherent receiver for bit intervals larger than about half the coherence time of the laser oscillators, with larger gains to be expected when soft decisions on long sequences are made. A way of reducing the complexity of the soft-decisions receiver for long sequences is discussed.     

大频偏条件下PN码捕获改进算法

针对大频偏条件下PN码捕获的问题,提出了一种改进的PN码捕获算法,即基于谱线插值的联合码捕获算法.首先对现有的码捕获技术进行了分析,然后在部分相关值作FFT的码捕获算法的基础上,提出了基于谱线插值的联合码捕获算法.算法的核心思想是对部分相关值作FFT处理,并利用插值公式计算出载波频偏的精确估计值,使得后续的载波跟踪环更有利于载波跟踪.介绍了新的码捕获系统的工作原理,推导了载波频偏估计的插值公式,并将非相干累加技术应用于码捕获系统,从而提高了低信噪比条件下载波频偏估计的精度.最后,通过仿真证明了此算法对现有GPS接收机性能有5dB的改善.     

基于矩阵填充和三阶相关的长短码DS-CDMA信号多伪码盲估计

由于长短码直扩码分多址(LSC-DS-CDMA)信号包含了多个用户的长码和短码,已有的直扩码分多址信号的盲伪码估计方法不再适用。为此该文提出一种基于矩阵填充和三阶相关的伪码估计方法。首先从理论上将结构复杂的LSC-DS-CDMA信号构建为多用户短码扩频的缺失矩阵模型,将复合码矩阵估计建模为盲源信号分离问题;然后将矩阵填充理论应用于复合码矩阵估计,提出基于奇异值阈值算法和快速独立成分分析算法的各用户复合码序列估计方法;最后利用m序列的移位相加性特性,提出延迟三阶相关算法,从各用户复合码序列中估计其包含的长短伪码序列。仿真表明,当信噪比高于-2 dB时,该文算法的长短伪码估计平均误码率低于0.1%。     

伪码测距中码跟踪环的FPGA实现

码跟踪环的设计是伪码/载波相位联合测距系统的一个关键环节.分析了伪码跟踪环路的结构,利用查表法、二阶环路分别对NCO和环路滤波器等关键部分进行了设计,并给出了相应的电路结构.系统仿真与FPAG片上测试结果表明,实现了伪码准确、实时跟踪,跟踪到的伪码相位误差小于载波相位的周期长,设计的跟踪环具有一定工程应用价值.     

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.     

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。     

Noncoherent MLSE detection of M-DPSK for DS-CDMA wireless systems

This paper focuses on maximum-likelihood sequence estimation of noncoherent M-ary differential phase-shift keying (M-DPSK) receivers for code division multiple-access (CDMA) systems, which make use of direct-sequence spread-spectrum modulations. A typical frequency-selective Rayleigh environment with multipath diversity at the receiver is considered. In this scenario, the optimum noncoherent decision metric, which requires an estimation of the channel tap weights envelope, is derived. Then, in order not to increase the receiver implementation complexity, a joint channel and data estimation strategy is proposed, which does not require the transmission of a known training sequence (blind estimation). In this case, the decision metric becomes a simple equal gain combining of multiple-symbol square-law detection decision metrics. For this suboptimum noncoherent detector, useful bounds on the bit error probability are provided through a theoretical analysis. Nonconstant and constant multipath intensity profiles are both considered for this purpose. Simulations are also carried out in order to verify the accuracy of the theoretical bounds     

Optical CDMA via temporal codes

The authors provide an analysis of the performance of optical orthogonal codes in an optical code division multiple access (CDMA) network by considering the probability distribution of the interference patterns. It is shown that the actual performance is close to a previous estimate. A less structured temporal code in which the code words are allowed to overlap at two pulse positions is also considered. The bit error probability for this class of codes is obtained for two cases: with and without optical hard-limiting at the receivers. It is shown that this code may increase the number of users in the network considerably without a significant loss in the performance     

Comparative study of space-diversity techniques for MLSE receiversin mobile radio

Several space-diversity techniques combined with maximum-likelihood sequence estimation (MLSE) are considered for time-division multiple-access (TDMA) digital mobile radio. Under the assumption that diversity paths fade independently, the Nth order diversity maximum-likelihood (ML) receiver is analyzed. Two categories of diversity receivers with MLSE are taken into account: the receivers performing diversity on the signal samples and those applying diversity inherently in the sequential algorithm. The simulation study was performed for standard global system for mobile communication (GSM) channel models. The results confirm the fact that even second-order diversity is a very powerful means improving the TDMA system performance     

基于数字匹配滤波器与多径合并的CDMA码跟踪环

PN码跟踪环是DS-CDMA蜂窝移动通信系统的关键技术。本文针对第三代(3G)移动通信系统，着眼于对下行公共导频信道的充分利用，提出了一种基于数字匹配滤波器(DMF)和多径误差合并的全数字式码跟踪环。导频信道的DMF输出，一方面可用于信道估计与多径搜索，另一方面用于提取码定时误差，二者结合可以进行码定时误差的多径RAKE合并。理论推导与仿真结果表明，该跟踪环性能良好，具有较高的实用价值。     

一种复合伪随机码跟踪环路设计及误差分析

该文针对当存在较大的载波相位跟踪误差时可能导致传统的复合码相干型跟踪环路失效的问题提出一种复合码非相干型跟踪环路。通过给出基于二进制偏移载波(BOC)调制的复合码的构造和相关特性分析,设计了具有非相干型鉴别器的跟踪环路,并分析了环路的跟踪误差。分析和仿真结果表明该环路解决了在存在较大载波相位误差时跟踪环路失效的问题,且具有良好的跟踪性能。     

PN code synchronization effects on narrow-band interferencerejection in a direct-sequence spread-spectrum receiver

The impact of imperfect synchronization on the performance of prediction-error interference rejection filters in noncoherent direct-sequence (DS) spread-spectrum communications is considered. Bit-error-rate analysis (BER) analysis (BER) of binary DPSK (differential phase-shift keying) data modulation used in conjunction with direct-sequence spread-spectrum is used as a performance measure. A first-order noncoherent delay-lock loop is used for the pseudonoise (PN) code tracking. Conditional BER results for the DS/DPSK systems for fixed values of the code tracking error are obtained. The average BER of the system is then evaluated by averaging the conditional BER expressions over the probability density function of the code tracking error. Results include the effects of both a single fading tone and a narrowband Gaussian interferer on the overall system performance. Interferer offset frequency is considered in determining the code tracking loop noise as well as the receiver BER     

带残余载波的周期长码直扩信号PN码盲估计

针对下变频后含有残余载波的周期长码直扩信号PN(Pseudo-Noise)码盲估计难题,在已知码片速率和PN码周期前提下,该文提出一种结合矩阵特征值分解和数字锁相环(Digital Phase Locked Loop, DPLL)的PN码盲估计方法。该方法首先将带有残余载波的周期长码直扩信号等效建模为虚拟多用户短码直扩信号模型,利用矩阵特征值分解和模糊酉矩阵的方法估计出含有残余载波的PN码,然后利用DPLL对残余载波的频率和相位进行跟踪和估计并最终消除残余载波,最后根据特定约束条件（如m序列、Gold序列）去除分段相位模糊,最终估计出PN码序列。理论分析和仿真结果表明,提出的方法能够有效地工作在较低信噪比下,且表现出良好的性能。      

Transmission and Reception Concepts for WLAN IEEE 802.11b

State-of-the-art wireless local area network (WLAN) IEEE 802.11b terminals employ complementary code keying (CCK) as modulation format. In this paper, receiver concepts tailored for CCK transmission over frequency-selective fading channels are presented in a unified and systematic framework. First, optimum maximum-likelihood (ML) detection for CCK signaling is considered. Second, for complexity reduction, minimum mean-squared error block decision-feedback equalization (MMSE-DFE) is investigated and reduced-state sequence estimation (RSSE) is considered on the basis of an Ungerbock-like set partitioning of the multidimensional CCK code wordset. In order to improve the reliability of CCK transmission over fading channels, time-reversal space-time block codes (TR-STBCs) combined with receive diversity are applied. Simulation results of the considered suboptimum receivers are compared with a performance approximation for optimum detection. Our results demonstrate the excellent performance of the advocated equalization schemes and the significant gains that can be achieved with TR-STBCs and receive diversity in typical WLAN environments     

非周期长码直扩信号同步及伪码序列盲估计

非合作直接序列扩频系统中伪随机码的估计与同步是正确获取信息的关键。现有的研究成果多集中在短码或周期长码直扩信号的解扩,该文针对无伪码先验知识条件下NPLC-DSSS信号的失步时间估计问题,提出一种基于相关矩阵元素分布建模的方法,该方法以信息码宽分段的信号构建自相关矩阵,并以该矩阵的Frobenius 范数与失步时间之间的对应关系,实现NPLC-DSSS信号失步时间的精确估计。在此基础上,通过引入判决辅助思想构建了循环迭代结构实现了NPLC-DSSS信号伪码序列的盲估计。最后,推导了该伪码序列盲估计问题的Cramer-Rao理论界。数值分析结果表明,文中所述算法在相同的信噪比和数据量条件下具有更好的估计精度,且对伪码的估计性能接近理论界。     

A note on some efficient estimates of the noise variance for first-order Reed-Muller codes (Corresp.)

The maximum-likelihood estimate based on order statistics for both a single-sample and anM-sample estimate are derived for the first-order Reed-Muller code sent over a zero-mean white Gaussian noise channel. The estimation method is subject to an equipment-complexity constraint. The constraint imposed requires that the estimates be obtained from successive comparisons of the correlations in a serial system, without the use of arithmetic operations. The maximum-likelihood estimates obtained here are compared to that of Posner [1], Posner et aL [2], and also the unrestricted maximum-likelihood estimate.