一种新的抗远近效应的盲DS-CDMA接收机

本文研究多径频率选择性衰落信道下直接序列扩频码分多址(DS-CDMA)信号的处理问题,提出了一种新型的盲接收机.该接收机不需预知多址干扰用户的扩频码,不需预知信道参数,只需已知期望用户的扩频码和粗略的定时,就可以完成用户的盲检测.同时,该接收机通过降维自适应去相关滤波和基于判决指导的自适应多径合并,来获得低成本和良好的抗衰落、抗远近效应性能.仿真结果验证了本文提出的接收机的优良性能.     

无信道估计和均衡器系数计算的直接盲多用户检测

在仅知道期望用户扩频码和传输信号属于有限字符集的条件下，文章把色散信道同步短码DS—CDMA系统盲多用户检测转化为带约束二次规划问题，并通过ε近似算法求解此问题来恢复期望用户信号。本方法既不依赖高阶统计量，也无需信道估计和均衡器系数计算，较好地避免了信道估计误差的影响，并具有适应时变信道的潜力。仿真表明本方法性能明显优于子空间方法和恒模算法。     

卫星无线电定位通信系统用户容量模型的研究

由地球同步卫星、地面中心站和用户机组成的集中式卫星定位系统中,用户到中心站的入站链路采用码分随机多址接入信道;出站链路采用扩频/TDM广播信道。本文建立了入站信道的用户容量模型,获得用户容量、系统参数、设备要求之间的关系;为增大出站广播信道的通信容量、采用同步码分多路复用技术,推导出计算数据误码率的公式。由计算机模拟,给出一系列曲线和图表以作为系统设计依据。     

基于负熵准则的FastICA盲多用户检测的研究

该文给出了一种基于负熵准则的FastICA盲多用户检测方法。修改了FastICA算法中的非2次函数,引入4次幂函数,把基于负熵的非高斯性测度转化为信号峰度的形式,这样降低了计算量。同时,算法充分考虑了各个用户信号的统计独立性,在下行链路干扰用户的扩频码未知情况下,把目标用户的扩频码作为训练序列,并用于初始化FastICA算法的分离向量,使用随机梯度法进行优化计算,能够获得优异的符号估计性能。对算法的计算复杂度的分析可以看出,计算量随着接收数据长度和用户数的增加而增加。通过与传统匹配滤波器,MMSE检测算法比较,表明在同步CDMA信道中,MAI较低时算法检测性能与MMSE检测器的性能接近,随着MAI增加,算法的性能明显优于MMSE算法。     

二维扩频系统中时频码片级差分检测的性能分析

时变频率选择性衰落信道中,针对二维扩频系统提出了一种时频码片级差分检测方法,并进行了误码率分析,同时进行了仿真验证.分析和仿真结果表明,时频码片级差分检测同时具有抵抗信道频率选择性和时间选择性衰落的能力,在选择性衰落信道中的性能优于平坦衰落信道中的性能.通过性能对比发现,时频码片级差分检测相对多载波扩频码片级差分检测能够更加有效地降低了载波间干扰对系统性能的影响.     

频域扩频码片级差分检测的误码率性能分析

该文提出了一种频域扩频系统中的差分检测方法:频域扩频码片级差分检测.通过利用判决变量的矩生成函数,并采用鞍点近似的分析方法,得到了频域扩频码片级差分检测的误码率性能的解析表达式,同时进行了仿真验证.分析和仿真结果表明,频域扩频码片级差分检测具有抵抗频率选择性衰落的能力,在信道的相干带宽不小于两个码片对应的频带宽度的前提下,随着信道多径时延扩展的增加,其误码率性能有进一步的提高.     

带有时偏的MC-CDMA上行链路的盲信道估计

为了能在带有时偏的MC-CDMA上行链路中估计信道，提出了一种新的信道估计方法。该方法能消除时偏对信道估计产生的影响，从而无需系统同步就能准确估计出信道。另外，由于扩频序列的信息对接收端是已知的，因此该信道估计方法可以利用扩频码的信息来完成盲估计。最后的仿真结果显示，该信道估计方法有良好的性能。     

基于FFT伪码捕获方法及性能

扩频伪码的捕获是扩频通信系统的核心关键技术。文章介绍了一种基于FFT得伪码捕获方法,并分析了该方法在高斯白噪声信道下PN码同步检测和虚警概率,从而进一步研究其解扩性能提供了理论依据。     

基于球形译码的联合多用户检测

针对采用码分多址接入（CDMA,Code Division Multiple Access）的多输入多输出系统（MIMO,Multiple Input Multiple Output）,建立多个用户同步上行接入基站的信道模型,收发端都采用2根天线。在基站端,由于扩频码非完全正交产生多个用户之间干扰和信道干扰影响信号检测的问题,分析并给出了多用户信号的最大似然函数,利用球形译码算法实现多用户的联合检测（Joint MUD,Joint Multiple User Detection）,并行处理信道干扰和多址干扰。仿真结果表明,提出的接收机有较好性能,同时降低了运算的复杂度。     

可变速率CDMA系统的话务模型

本文研究可变速率码分多址(CDMA)系统中的话务模型.由于可变速率CDMA系统中采用的正交扩频码是正交可变扩频因子(OVSF)码,信道的传输速率和可用信道的数目随着选择的码字不同而不同.针对用户的服务时间是时变的任意分布,我们运用M/G/K队列模型,用近似法获得用户平均延迟时间等话务模型的重要参数.文中给出了采用可变速率正交扩频码的CDMA系统的话务模型,以及相应的话务流量,平均延迟时间,用户平均停留时间等系统参数的计算式;得出采用可变速率的数据传输后,系统的平均延迟时间将比固定速率传输时增加了一倍的结论.     

Filterbank-based blind code synchronization for DS-CDMA systems in multipath fading channels

We present a filterbank approach to blind code synchronization for asynchronous direct-sequence (DS) code-division multiple-access (CDMA) systems. The key idea of the proposed scheme is to first pass the received signal through a bank of filters, which are designed to enhance signals of interest and suppress interference/noise, and then to derive the code timing from the filtered data. The only required knowledge by the proposed filterbank scheme is the spreading code of the desired user. It can be used in various environments, including frequency-nonselective and frequency-selective, time-invariant, and time-varying fading channels. It can deal with colored channel noise and unmodeled interference, such as inter-cell interference (ICI) and narrowband interference. It has relatively low complexity and can be readily implemented using standard adaptive algorithms. We show that under mild conditions, the proposed scheme yields statistically consistent [in signal-to-noise ratio (SNR)] code timing estimates, irrespective of the strength of the interference and with only a finite number of data samples. We also derive an unconditional Cramer-Rao bound (UCRB), which serves as a lower bound for all unbiased blind code synchronization schemes. Numerical results indicate that the proposed scheme compares favorably with a popular subspace-based method in terms of user capacity, near-far resistance, and robustness to time-varying fading and unmodeled interference.     

Blind space-time multiuser channel estimation in time-varying DS-CDMA systems

A multistep linear prediction (MSLP) approach is presented for blind channel estimation for short-code direct sequence code division multiple access signals in time-varying multipath channels using a receiver antenna array. The time-varying channel is assumed to be described by a complex exponential basis expansion model. First, a recently proposed MSLP approach to blind channel estimation for time-varying single-input multiple-output (SIMO) systems is extended to time-varying multiple-input multiple-output (MIMO) systems to define a "signal" subspace. Second, the knowledge of the spreading code of a desired user is exploited in conjunction with the signal subspace to estimate the time-varying channel of the desired user up to an unknown time-invariant scale factor. Equalization/detection for the desired user can be then carried out if the information sequence is differentially encoded/decoded. Sufficient conditions for channel identifiability are investigated. Three illustrative simulation examples are provided.     

Network Synchronization of an Orthogonal CDMA Satellite Communication System

This paper provides the network synchronization of an orthogonal CDMA geostationary satellite system for fixed service communications. It includes the synchronization procedures, the system architecture and the performance evaluation. The main objective is to provide network wide synchronization of all uplink orthogonal CDMA transmissions. This is achieved in steps; first by providing coarse synchronization using the uplink random access channel and then fine sync using innovative tracking control mechanisms. The uplink access channel receiver utilize a parallel/serial search method for rapid code acquisition, while the code tracking of the uplink orthogonal CDMA traffic channel is based on a delay feedback early-late gate in which the sych control resides in the receiver. The proposed system is designed to minimize the onboard complexity and satisfy the performance requirements. As shown in the performance section, the requirement that all uplink transmissions are synchronized to a reference time within 10% of the chip length can be achieved. In addition, the system analysis determines the design parameters values which optimize performance.     

A multistep linear prediction approach to blind asynchronous CDMAchannel estimation and equalization

A multistep linear prediction approach is presented for blind channel estimation, multiuser interference (MUI) suppression, and detection of asynchronous short-code direct sequence code division multiple access signals in multipath channels. Only the spreading code of the desired user is assumed to be known; its transmission delay may be unknown. We exploit the previously proposed multistep linear prediction approach for blind multiple-input multiple-output channel estimation in conjunction with the structure imposed by the desired user's spreading code sequence. With the knowledge of the desired user's code sequence, only the second-order statistics of the data are needed under certain sufficient conditions on the underlying multiuser MIMO transfer function. Based on the desired user's channel estimate, a linear minimum mean square error filter is designed for simultaneous equalization and MUI suppression. Three illustrative simulation examples are presented     

Tardos Fingerprinting is Better Than We Thought

Tardos has proposed a randomized fingerprinting code that is provably secure against collusion attacks. We revisit his scheme and show that it has significantly better performance than suggested in the original paper. First, we introduce variables in place of Tardos' hard-coded constants and we allow for an independent choice of the desired false positive (FP) and false negative (FN) error rates. Following through Tardos' proofs with these modifications, we show that the code length can be reduced by more than a factor of two in typical content distribution applications where high FN rates can be tolerated. Second, we study the statistical properties of the code. Under some reasonable assumptions, the accusation sums can be regarded as Gaussian-distributed stochastic variables. In this approximation, the desired error rates are achieved by a code length twice shorter than in the first approach. Overall, typical FP and FN error rates may be achieved with a code length approximately five times shorter than in the original construction.     

CMA-based code acquisition scheme for DS-CDMA systems

In direct-sequence code-division multiple access, a code synchronization must take place before the multiuser detector. As the initial synchronization stage, a code acquisition scheme is used to estimate the relative timing phase for the desired transmission within one chip interval. In this paper, a blind code acquisition scheme using adaptive linear filtering based on a linearly constrained constant modulus algorithm (CMA) is proposed. The uncertainty of a desired user's delay is initially discretized and translated into a number of hypotheses. The lock convergence property of CMA is exploited, where the filter at the steady state can lock onto the desired user while nulling all other interfering users (i.e., a decorrelator). For each delay hypothesis, the filter is initialized as the corresponding shifted spreading sequence of the desired user. It is shown that lock convergence always occurs for the correct hypothesis, while all incorrect hypotheses will be hovered around some saddle regions, given sufficiently small step sizes. Then, the correct hypothesis is the one which has the converged filter to yield the maximum lock onto the desired user, or a maximum output energy     

基于DMF的软判决信道同步电路设计

论文针对Turbo译码器设计时软判决信道帧同步问题，采用数字匹配滤波器(DMF)实现软判决信道的帧同步捕获。通过对DMF的几种实现结构的分析，采用倒置型FIR结构，按照CCSDS标准，设计了用于Turbo码编译码系统同步捕获电路。试验表明，设计电路满足Turbo编译码系统帧同步需要，并可在其它软判决信道帧同步设计中得到应用。     

Integral-observer-based chaos synchronization

In this paper, a new scheme based on integral observer approach is designed for a class of chaotic systems to achieve synchronization. Unlike the proportional observer approach, the proposed scheme is demonstrated to be effective under a noisy environment in the transmission channel. Based on the Lyapunov stability theory, a sufficient condition for synchronization is derived in the form of a Lyapunov inequality. This Lyapunov inequality is further transformed into a linear matrix inequality (LMI) form by using the Schur theorem and some matrix operation techniques, which can be easily solved by the LMI toolboxes for the design of suitable control gains. It is demonstrated with the Murali-Lakshmanan-Chua system that a better noise suppression and a faster convergence speed can be achieved for chaos synchronization by using this integral observer scheme, as compared with the traditional proportional observer approach.     

Low-complexity space-time processor for DS-CDMA communications

A novel wideband beamforming technique for cellular CDMA systems is presented in this paper. The proposed algorithm asymptotically provides the maximum SINR estimate of the signal with the desired code (SDC) by optimally combining desired signals from different paths and canceling strong multiuser access interference (MUAI). A two-dimensional (2-D) matched filter structure is used where not only different temporal samples of the matched filter output are processed but where those from matched filters connected to different antennas are processed as well. In contrast to previously proposed techniques, an exact code synchronization for the SDC is not required. The algorithm presented herein asymptotically provides the arrival time of the multipaths within a bit period and the optimum beamformers for extracting each of them. Space-time filters for combining the fingers across both space and time while canceling the MUAIs are constructed correspondingly. The instrumental property exploited by this technique is the fact that although the respective spectra of the SDC and MUAI components at the matched filter output are statistically identical, the respective spectra of their squared values differ. A simplified RAKE structure-based receiver is also proposed. The 2-D RAKE receiver considerably decreases the computations but requires a coarse SDC code synchronization. A technique to achieve coarse SDC code synchronization is also proposed     

A frame synchronization method for adaptive array antennas in digital mobile communications

This paper proposes a frame synchronization method for an adaptive array antenna (AAA) used in digital mobile communications. The proposed scheme, which is based on the maximum likelihood estimation (MLE), calculates error signals by subtracting a training sequence convolved with an estimated channel impulse response from the AAA outputs and searches for the timing that minimizes the mean squared errors. Because the proposed scheme can effectively exploit delayed paths of the desired signal, it can improve the synchronization performance on frequency-selective fading channels. Computer simulations show that the proposed scheme can operate properly even in interference-rich fading environments and that it can significantly improve the synchronization performance.