基于小波包函数的正交MC-CDMA及其性能

本文将小波包变换理论应用于多载波码分多址(MC-CDMA)系统,提出一种基于小波包函数的正交调制方法.系统中,发端数据串并转换成若干子信道,每个子信道的数据进行直序扩频,不同子信道上的数据以chip调制不同的小波包函数作为基带传输波形.小波包函数的一系列优良性质,使系统能有效地抗多径干扰,抑制子信道串扰、多址干扰等.本文对系统在多径Rayleigh慢衰落信道中的性能,进行了理论分析和计算机仿真.结果表明,本系统比基于DFT的MC-CDMA有较大的性能改善.     

基于复小波包的MC-CDMA系统及上行链路性能分析

文章在分析多载波扩频码分多址的基础上，利用优化生成的复小波包及变换，研究了一种基于复小波包的多载波CDMA系统在多径瑞利衰落环境下的上行链路性能。仿真结果表明该系统要优于通常的MC-CDMA系统和基于实小波包变换的MC-CDMA系统，具有良好的抗衰落能力。并采用多天线分集接收技术进一步完善系统性能，从而增强了系统抗多径干扰和多址干扰的能力，且得到了更低的误码率。     

基于复小波包基函数调制的跳频多址通信系统

该文给出了一种生成复数小波包基函数的方法，提出了一种基于复小波包基函数调制的跳频多址通信系统的模型。该系统在多径衰落时变信道下采用判决反馈均衡器接收的性能与基于实小波包基函数调制的同类系统相比有明显改善，且通过采用空间分集接收，可显著提高系统的误码率性能。该系统易于支持多速率话音和数据业务，具有频谱利用率高，抗单音干扰能力较强和保密性高等优点。     

基于优化多带复小波和Turbo编码的多载波CDMA系统及性能

在分析多载波CDMA(MC-CDMA)技术原理的基础上,利用优化的多带复小波作为多载波调制以及turbo 编码作为信道编码,提出一种基于优化多带复小波和turbo编码的MC-CDMA系统,研究了其在瑞利衰落信道下的误比特率性能。该系统能克服通常MC-CDMA系统插入循环前缀(CP)所带来频谱效率的降低,并能充分利用turbo 码的良好抗信道衰落能力,进一步提高系统性能。理论分析和仿真结果表明该系统性能要好于通常MC-CDMA系统,具有与采用CP的MC-CDMA系统相比较的优势。同时turbo编码的应用显著增强了系统抗多径衰落和多址干扰的能力。     

小波包分复用系统子载波的构造及调制实现

小波包分复用(WPDM)系统中作为子载波的小波包基函数必须是连续解析的,但实际中很难得到,通常需要对其进行逼近.通过滤波器系数采用迭代卷积的方法来对小波包基函数进行离散逼近,构造出的子载波满足小波包基函数的正交性.同时用此方法实现了多载波调制,并将该系统与正交频分复用(OFDM)系统在多径时延信道下进行了比较.仿真结果表明,WPDM系统在抑制ISI和ICI方面以及对定时偏差的影响上更具优势.     

一种能有效抑制窄带干扰的小波包多载波扩频系统

本文提出一种能有效抑制窄带干扰的基于小波包的多载波扩频接收系统.该系统通过小波包分析来自适应调整优化参数并进行抗干扰处理,使系统性能在高功率窄带干扰下得到改善.处理过程可与传统的基于小波包的多载波扩频系统解调解扩方式很好的相匹配,系统复杂度很低.研究与仿真结果表明,这种方案简单实用,具有良好的性能.在相同的处理条件下,性能要大大优于基于离散傅立叶变换(DFT)的多载波扩频系统.     

基于小波包变换和判决反馈RBF网络的组合均衡器

本文提出了一种基于小波包变换和判决反馈RBF网络的组合非线性均衡器的结构和算法.首先将信号进行小波包分解,再将分解后的信号分量送入带有判决反馈结构的RBF神经网络进行均衡.一方面,小波包具有很强的去相关能力,可以提高均衡器的收敛速度;另一方面,RBF神经网络具有较强的非线性模式分类能力,可降低均衡器的均方误差.在仿真实验中,针对无线通信数字信号传输过程中由于多径效应和信道衰落而产生的码间干扰(ISI)问题,比较了最小均方(LMS)算法和组合均衡器算法的均衡效果,结果表明,组合均衡算法具有更快的收敛速度,更低的误码率.     

基于优化多带复小波的多载波CDMA及反向链路性能

在分析多载波码分多址的原理基础上,利用优化生成的多带复小波,提出了一种基于优化多带复小波的多载波CDMA系统反向链路模型,并采用基于多天线的空间分集合并(SDC)技术进一步完善所提系统.该系统可利用优化多带复小波的优良特性来避免通常多载波CDMA由于插入循环前缀所带来的频谱效率的下降,在此基础上我们研究了所提系统采用空间分集合并技术时在瑞利衰落信道下的反向链路性能;给出相应的误比特率分析.理论分析和仿真结果表明采用基于最大比合并的SDC技术可显著提高所提系统抗空间衰落和各种干扰的能力,而且基于SDC技术的所提系统要好于同样基于SDC技术的通常多载波CDMA系统和基于实小波包的多载波CDMA系统.     

基于导频信号和最大似然估计算法的离散小波包调制的研究

目前,紧张的无线频谱资源已经使频谱利用率成为无线通信系统面临的重要问题。多载波调制技术能有效解决上述问题并以其高效的频谱利用率和良好的抗多径衰落性能成为4G的核心传输技术之一。本文提出一种基于导频信号和最大似然估计算法的离散小波包多载波调制系统。通过对导频信号的设计,获得信道状态信息,利用最大似然估计算出最优信道状态信息,最后利用基于迫零算法消除由多径衰落引起的码间干扰。通过理论分析和仿真验证,基于导频信号和最大似然估计的算法的离散小波包调制是值得考虑的多载波调制技术。     

基于恒模算法的DS-CDMA系统下行信号接收机

本文研究多径频率选择性衰落信道下,DS-CDMA系统下行信号检测技术。提出了一种新的基于恒模算法的检测方法。将均衡器的输出投影到一个未使用的扩频序列上,以结果为零作为约束条件,本文分别给出了批处理和自适应的线性约束恒模算法。本文方法克服了常规恒模算法无法保证收敛到期望用户的缺点,并且对周期和非周期扩频系统都有效。仿真结果验证了本文方法具有良好的检测性能,可以在克服信道衰落的同时抑制多址干扰。     

Grouped signed power-of-two algorithms for low-complexity adaptive equalization

With increasing demand for higher data rate, modern communication systems have grown more complex. Equalization has become more and more important as it is effective in mitigating the multipath fading often occurred in high-data-rate communication systems. However, the implementation complexity of adaptive equalizers is usually too high for mobile communication applications. In this paper, a novel adaptive equalization algorithm and its low-complexity architecture are proposed. This algorithm employs a new grouped signed power-of-two (GSPT) number representation. The GSPT algorithm and several enhanced versions are simulated as adaptive equalizers in a phase-shift keying communication receiver for several practical channels and the GSPT-based equalizers perform as well as the least mean square (LMS) equalizer. Moreover, for comparison, two GSPT-based equalizers and two other equalizers are implemented in field-programmable gate arrays. The GSPT-based equalizers require only about 25%-30% of the hardware resources needed in the LMS equalizer. Also the GSPT-based equalizers are more than twice as fast as the LMS equalizer.     

Efficient bad equalizer in two-ray multipath fading channels

An efficient bidirectional arbitrated decision feedback (BAD) equalizer is presented in single-carrier block transmission system in the Two-Ray multipath fading channels, where the output from the bidirectional equalizers are combined together directly using maximal ratio combining (MRC) rule to improve the signal-to-noise ratio (SNR) before demodulation. The computational complexity of the BAD equalizer presented is linear with the channel length, which is the same as conventional decision feedback equalizer (DFE) and is significantly lower than that of conventional BAD equalizer as well as the maximum likelihood (ML) algorithm. While the performance of the new scheme depends on the specific channel characteristics, it is shown by simulation results that the performance of the new BAD can surpass the one of DFE dramatically in the minimum or non-minimum phase Two-Ray multipath fading channels.     

Time-Domain Block and Per-Tone Equalization for MIMO–OFDM in Shallow Underwater Acoustic Communication

Shallow underwater acoustic (UWA) channel exhibits rapid temporal variations, extensive multipath spreads, and severe frequency-dependent attenuations. So, high data rate communication with high spectral efficiency in this challenging medium requires efficient system design. Multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO–OFDM) is a promising solution for reliable transmission over highly dispersive channels. In this paper, we study the equalization of shallow UWA channels when a MIMO–OFDM transmission scheme is used. We address simultaneously the long multipath spread and rapid temporal variations of the channel. These features lead to interblock interference (IBI) along with intercarrier interference (ICI), thereby degrading the system performance. We describe the underwater channel using a general basis expansion model (BEM), and propose time-domain block equalization techniques to jointly eliminate the IBI and ICI. The block equalizers are derived based on minimum mean-square error and zero-forcing criteria. We also develop a novel approach to design two time-domain per-tone equalizers, which minimize bit error rate or mean-square error in each subcarrier. We simulate a typical shallow UWA channel to demonstrate the desirable performance of the proposed equalization techniques in Rayleigh and Rician fading channels.     

Nonlinear equalization of multipath fading channels withnoncoherent demodulation

A nonlinear decision-based adaptive equalizer compatible with differentially coherent phase shift keying (PSK) is proposed for frequency-selective fading channels. This equalization scheme is appropriate whenever conventional equalizers are not capable of tracking phase variations in selective fading channels. The received signal is first converted to a baseband signal and then sent through a differential detector. A nonlinear processor before the equalizer generates the needed nonlinear terms that are weighted and summed in the equalizer. Nonlinear intersymbol interference at the output of the differential detector is dealt with by minimizing an error signal between the output of the equalizer and the detected data. The adaptation algorithm can be any algorithm currently used for conventional equalizers. Our simulation results confirm that for channels with spectral nulls, equalization is achieved successfully with the proposed scheme, whereas, linear equalizers, either with coherent or noncoherent detection, fail     

Low-complexity equalization for π/4 DQPSK signals based on themethod of projection onto convex sets

An equalizer adaptation technique for compensation of degradations caused by multipath Rayleigh fading channels to π/4 differential quadrature phase shift keying (DQPSK)-modulated signals is presented. The technique is applied to linear and nonlinear transversal-filter-type equalizers. It is based on the method of projection onto convex sets (POCS), realized in a particular form of the iterative least mean squares (LMS) procedure. The convergence speed of the proposed equalizer coefficient adaptation technique and its computational complexity depend on the newly introduced look-back parameter. Both can be tailored to the characteristics of the channel. For achieving convergence speeds comparable to speeds of recursive least squares (RLS) techniques, the computational load of the presented equalization is of the order of the load required of RLS techniques. However, its algorithmic implementation is notably simpler and its code and storage size requirements are smaller. The technique is numerically stable, and it is suitable for low-power implementations in digital signal processors or custom very large-scale integration (VLSI) circuits. Performed simulations verify good performance of the technique in various channel conditions for 900-MHz multipath fading radio channels     

Prediction of State Transitions in Rayleigh Fading Channels

This paper presents a channel-sampling scheme that allows robust estimation of channel transitions into and out of the fade state. The sampling scheme is based upon a second-order autoregressive (AR-2) model for the Rayleigh fading channel and a corresponding state-space representation in terms of fade and nonfade states. The threshold parameter that segments the fading signal into states is derived as a function of signal-to-noise ratio and a bit-error-rate performance metric. The sampling rates correspond to the values at which the mutual information of the process occupying a particular state, conditioned on two past observations of the channel, is maximized. The performance of the proposed sampling scheme in a model-based prediction algorithm is presented. The state of the received envelope is predicted with 98% accuracy. When applied to the estimation of channel gain, the sampling scheme in conjunction with a Kalman-filter-based AR-2 predictor yields one-step forecasts that accurately track the fading signal. Equalization of multipath channels using the estimated channel impulse response shows improved error performance over traditional recursive least squares equalizers     

Instantaneous Mixture Channel Selection for Blind Equalization Using Cumulant Features in MIMO Systems

Cognitive radio combined with multiple-input multiple-output (MIMO) communications provides high data rate, efficiency and high reliability. One of the most important challenges in MIMO communication is combating MIMO multipath channel. MIMO blind equalizers and channel estimators combat MIMO multipath channels without the use of training or pilot sequences. First, the multipath channel is converted into instantaneous mixture channel (IMC), using second-order statistics of the data. Then using higher-order statistics, these mixtures are separated. However, proper selection of IMC is a major challenge. In this paper, a novel blind algorithm for choosing the best IMC is proposed. The proposed algorithm is based on the cumulant value of the received signal.     

一种新的适用于多径信道的基于LMK准则的盲自适应多用户检测及其收敛性分析

基于最小平均峰度(Least MeanKurtosis,LMK)准则],该文提出了一种适用于同步直扩码分多址(DS/CDMA)系统多径慢衰落信道中的线性盲自适应多用户检测算法。这种算法基于高阶统计量(High Order Statistics,HOS)特性,算法复杂度相对较低。文中分析了算法在多径信道中无噪声情况下的收敛特性,指出在这种情况下,算法具有全局最小点,此时算法满足解相关条件。仿真结果表明,文中给出的检测器具有较强的多址干扰抑制性能。     

基于DFT滤波器组的多载波调制系统设计

采用DFT滤波器组来代替OFDM系统中的IDFT/DFT模块,形成基于滤波器组的收发器。设计适于多经衰落信道的DFT滤波器组收发器。其均衡与OFDM系统一样,是在接收端采用单抽头的均衡器。实验采用随机多径信道,仿真结果表明所提出的滤波器组收发器可以获得较好的频谱特性,以及较满意的SIR（信号与干扰比值）。     

Artificial Neural Network Channel Estimation Based on Levenberg-Marquardt for OFDM Systems

The many advantages responsible for the widespread application of orthogonal frequency division multiplexing (OFDM) systems are limited by the multipath fading. In OFDM systems, channel estimation is carried out by transmitting pilot symbols generally. In this paper, we propose an artificial neural network (ANN) channel estimation technique based on levenberg-marquardt training algorithm as an alternative to pilot based channel estimation technique for OFDM systems over Rayleigh fading channels. In proposed technique, there are no pilot symbols which added to OFDM. Therefore, this technique is more bandwidth efficient compared to pilot-based channel estimation techniques. Also, this technique is making full use of the learning property of neural network. By using this feature, there is no need of any matrix computation and the proposed technique is less complex than the pilot based techniques. Simulation results show that ANN based channel estimator gives better results compared to the pilot based channel estimator for OFDM systems over Rayleigh fading channel.