利用双正交小波变换优化G3-PLC系统性能

为了提高G3-PLC(G3-Power Line Communication)在复杂信道环境下通信的可靠性,提出了基于双正交小波变换的正交频分复用（Discrete Wavelet Transform-Orthogonal Frequency Division Multiplexing,DWT-OFDM）。通过双正交小波变换减小复杂信道环境下G3-PLC通信系统中OFDM载波间干扰,并通过合理减少循环前缀,优化传输效率,降低峰均比。在DWT-OFDM的G3-PLC系统中,通过不同小波基性能分析与对比,利用bior6.8双正交小波基进行系统性能优化。将DWT-OFDM的G3-PLC系统在实际电力线噪声库环境下进行实验分析,仿真结果表明,在平均误码率为10-3量级时,性能有5 dB左右提升,通信效率由40%提高至54%;结合限幅法,峰均比由11 dB降低至8 dB,通信性能极大优于基于FFT-OFDM的G3-PLC系统。     

变换域通信系统基函数生成方法改进

变换域通信系统通过信道估计在变换域设计信号波形避开干扰,它没有采用载波调制,而是用一个类似噪声的基函数进行信息调制.通过引入干扰系数丰富了基函数的频谱结构,提高了基函数的自正交性,使基函数更适合正交调制,增强了变换域通信系统的抗干扰能力.特别是在干扰信号较强的情况下,能够保证系统的信息传输畅通.通过仿真比较改进前后基函数的自相关性,验证了前面的理论.     

基于LWT的可见光OFDM-IM系统研究

针对索引调制正交频分复用(OFDM-IM)技术峰均比过高,对相位噪声和载波频偏敏感,系统复杂度高等问题,提出基于小波提升变换(LWT)的可见光OFDM-IM系统。首先选择正交小波基作为子载波,通过对信号的分裂、预测和更新,将信号分为高频信号和低频信号,再结合可见光信道构成LWT-OFDM-IM系统,最后通过理论分析和蒙特卡罗方法对系统的可靠性、峰均比特性和最佳小波分解层数进行了仿真验证,结果表明：当子载波N=256,子块中子载波数量L=4,激活子载波数量k=2,系统误码率为10^-4时,LWT-OFDM-IM比FFT-OFDM-IM改善约8 dB,相较于DWT-OFDM-IM提高约4 dB。当系统互补累计分布函数(CCDF)为10^-1数量级时,LWT-OFDM-IM的峰均比较FFT-OFDM-IM改善约2.3 dB。随着小波层数的递增,系统的误码性能越好,当小波层数为3层时,大约在10 dB附近,误码率可到10^-5量级。     

对多载波调制信道不等带宽分配的研究

本文把小波变换应用于多载波调制系统，提出了对信道进行不等带宽分配，克服多个窄带干扰影响的思想，有效地提高了信道的利用率和系统的稳定性，这种技术应用于ＨＦＣ网的反向信道中，可以有效提高系统的抗噪声性能。     

DSSS中的窄带干扰抑制技术

直接序列扩频(DSSS)通信具有抗干扰能力强的特点。采用窄带干扰抑制技术,可以有效提高DSSS系统的性能和顽存力。介绍了DSSS通信中的基于时域和变换域的干扰抑制技术。     

FSO-WOFDM系统的实验研究

在无线光正交频分复用（FSO-OFDM）系统中,因需要加入循环前缀而消耗近20%的带宽,降低了数据传输效率。为了进一步减少带宽损耗,改善系统性能,采用小波包变换代替FSO-OFDM中的傅里叶变换,设计了一种基于小波变换的无线光正交频分复用（FSO-WOFDM）系统。该系统无需加入循环前缀,利用小波基函数的正交性来抑制子载波间的干扰。根据FSO-WOFDM系统原理,研究了信道估计方法、湍流强度、小波基对FSO-WOFDM系统误码率的影响,对比分析了FSO-WOFDM与FSO-OFDM的系统性能,并通过实验验证了FSO-WOFDM系统的可行性及其优良的抗混合噪声干扰能力。     

变换域通信系统QAM调制及抗干扰性能仿真

针对变换域通信系统现有调制方式频谱效率较低的情况,提出了在变换域通信系统中实现QAM调制(TD-QAM)的方法。基于MATLAB,验证了TD-QAM的正确性,仿真了TD-QAM在部分频带干扰下的性能。结果表明,所提出的TD-QAM差错概率与常规数字QAM调制一致,同时具备较强的抗部分频带干扰的能力。     

TDCS大规模分区多址通信方法

针对传统的变换域通信系统多址干扰严重,抑制用户数和误码率性能的问题,提出了一种分区方法降低多址干扰,适用于大规模变换域多址通信.提出多用户的区域划分方法,将用户分为区内用户和干扰较小的区外用户,并推导出误码率计算公式;提出分区向量计算方法,引入CAN算法,提高分区向量的自相关性;从频谱效率和高斯信道下的误码率等角度,分...     

自干扰幅度及相位误差对全双工系统的影响

在加性白高斯噪声无线传播信道和自干扰信道中,分析了自干扰射频抵消幅度及载波相位估计误差对二进制相位调制同时同频全双工系统误码率的影响.分析结果表明:相同信干比条件下,误码率随幅度估计相对误差、载波相位估计数值误差的绝对值减小而减小;当载波频率为1.9 GHz、信干比为-120 dB、误码率为10-2,自干扰载波相位估计数值误差为10-5度时,同时同频全双工系统损失发射功率0.2 dB.     

基于变换域通信的OFDM系统窄带干扰抑制方法

为解决宽带OFDM系统易受传统窄带信号干扰的问题,采用基于变换域通信(TDC)的OFDM系统,通过在变换域中的电磁环境采样结果,对OFDM信号进行设计,避免使用受干扰频谱传送信息。该方法在强窄带干扰得到抑制的同时,信噪比也不容易受到变换域滤波的影响。在加性高斯白噪声信道中的仿真结果表明,基于TDC的干扰抑制方法能够有效地抑制宽带OFDM系统的窄带干扰。     

基于双门限的变换域通信系统基函数设计

针对单门限变换域通信系统往往无法兼顾抗干扰和隐蔽通信的问题,提出了一种基于双门限变换域通信系统基函数幅度谱的设计方法。该方法首先在干扰存在情况下进行单门限系统最优误码率门限与最差误码率门限的判断,然后通过参数灵活调整两个门限的值,将大于高门限和低于低门限的值分别设为0和1,位于双门限之间的频谱进行倒置,得到的基函数通过对数据进行调制实现通信。仿真结果表明:相比于单门限变换域通信系统,通过参数调节后的双门限变换域通信系统可以同时具有良好的抗干扰及隐蔽通信性能。     

基于Simulink的直扩系统频域窄带干扰抑制研究

直接序列扩频系统（Direct Sequence Spread Spectrum,DSSS）自身具有抗窄带干扰的能力,但当存在强窄带干扰时,需要有效的窄带干扰抑制技术来提高系统性能。研究了基于FFT的频域窄带干扰抑制算法,并对其中的加窗、重叠复用及门限生成等关键技术进行了分析。利用Matlab的可视化工具Simulink建立了仿真模型,并在给定的仿真条件下进行了仿真,验证了基于FFT重叠变换的频域陷波技术抑制干扰的有效性,为直接序列扩频系统在干扰环境下的应用提供了依据。     

一种多输入多输出变换域通信系统的设计及其抗干扰性能研究

传统多输入多输出技术可以提高信道容量,但不能解决信号抗干扰问题;而变换域通信系统已经被证明是一种良好的抗干扰通信系统。通过引入变换域处理技术,提出了一种多输入多输出变换域通信系统(MIMO-TDCS),旨在突破传统MIMO系统的应用瓶颈。对其收发信机模型和抗干扰机制作了简要介绍,并通过仿真窄带干扰下的误码率性能,表明MIMO-TDCS是一种可靠的抗干扰通信手段。     

一种新的TDCS基函数随机相位产生方法及性能分析

针对变换域通信系统对随机相位随机性与相关性的要求,提出了一种采用混沌映射m序列控制法的基函数相位映射方法,相比于传统一维m序列产生基函数,该方法所产生基函数的随机性提高了N倍(N为m序列的长度);同时由于采用混沌映射,产生了大量互相关系数很小的基函数,具有更加良好的相关性。仿真结果表明,采用混沌映射m序列控制法的基函数相位映射方法,提高了基函数的相关性,有效地降低了系统的误码率。     

Interference mitigating based on FRactional Fourier Transform in Transform Domain Communication System

The method of FRactional Fourier Transform （FRFT） is introduced to Transform Domain Communication System （TDCS） for signal transforming in the paper after theoretical analysis. The method yields optimal Basis Function （BF） by FRFT with optimal transform angle. The TDCS using the proposed method has wider usable spectrum, stronger robustness and better ability of anti non-stationary jamming than using usual methods, such as Fourier Transform （FT）, Auto Regressive （AR）, Wavelet Transform （WT）, etc. The main simulation results are as follows. First, the Bit Error Rate （BER） Pb is close to theoretical bound of no jamming no matter in single tone or in linear chirp interference. Second, the interference-to-signal ratio J/E is at least 12dB more than that of Direct Spread Spectrum System （DSSS） under the same BER if the spectrum hopping-to-signal ratio is 1：20 in chirp plus hopping interfering. Third, the Eb/N0 （when estimation difference is 90% between transmitter and receiver） is about 3.5dB or about 0.5dB （when estimation difference is 10% between transmitter and receiver） more than that of theoretical result when no estimation difference under Pb = 10^-2.     

卷积与RS级联编码直扩系统性能仿真

为了研究编码增益与扩频增益的有效结合,设计了一种卷积码与RS码级联编码直扩系统,并搭建了该级联编码直扩系统的仿真模型,仿真分析了级联编码模块对于所设计的直扩系统性能的影响.仿真结果表明,与未采用任何编码的直扩系统相比较,所设计的级联编码的直扩系统性能在信噪比达到-1.4 dB后表现更优.而在RS码与卷积码之间加入交织器后,选取合适的交织器参数可以使直扩系统的性能得到进一步的提升.     

Multidimensional parallel combinatory transform domain communication system

Transform domain communication system is an adaptive anti‐interference system that can dynamically generate interference‐orthogonal waveforms. As a spread spectrum system, its low spectrum efficiency limits potential practical applications. In TDCS, cyclic code shift keying is an orthogonal modulation, whose symbols can be regarded as scalars in different dimensions. However, the constellation is restricted by signal time‐bandwidth product. In this study, extending constellation is proven to be a valid approach to improve spectrum efficiency. The combinatory of multidimensional vectors is designed mathematically to increase the variety of the scalars with the minimum orthogonality damage. On the basis of the design, an efficient modulation scheme, namely, multidimensional parallel combinatory transform domain communication system, is proposed, whose fundamental modulation symbols can be represented by the combinatory of a series of cyclic code shift keying symbols with phase shift keying modulation. Compared with the existing methods, multidimensional parallel combinatory transform domain communication system can achieve a considerably higher spectrum efficiency. The bit error rate performance is also improved to various extents. Theoretical spectrum efficiency and bit error rate of the proposed method are derived for additive white Gaussian noise and typical multipath fading channels. System with different spectral conditions and channel estimation methods is also simulated and analyzed to confirm the practical usability.     

变换域通信系统：原理、技术与发展趋势

变换域通信系统（TDCS）是指能适应快速变化的无线通信环境,主动避开干扰,具有低截获（LPI）传输信号波形的一种智能无线电技术,它以其良好的自适应性和兼容性受到越来越广泛的关注,有望成为实现认知无线电（CR）的一项潜在的候选技术。TDCS的提出为解决不断增长的无线通信应用需求与日益紧张的无线频谱资源之间的矛盾提供了一种有效的途径。简述了TDCS的基本原理,对TDCS涉及的变换域技术、频谱估计和数据调制等关键技术进行了总结分析,并结合当前的发展状况对该技术未来的发展趋势进行了预测。     

A Novel Multi-carrier Order Division Multi-access Communication System Based on TDCS with Fractional Fourier Transform Scheme

In this paper, we propose a novel fractional Fourier transform (FrFT) based multi-carrier order division multi-access communication system, in which each user is uniquely identified by an FrFT order. Transform domain communication system (TDCS) with FrFT scheme is also proposed to synthesize the wide-band baseband waveforms in all FrFT domains with different users’ FrFT orders, which enhances the interference avoidance capability of this system under most of interference. Therefore, multiple independent data streams can be transmitted by using FrFT–OFDM in the same time and different FrFT domains. However, chirp bases, as the new kind of carriers with different modulated rates, are merely mutually approximately orthogonal. There is a problem of energy leakage between multiple chirp carriers, which possibly causes the multiple chirp carriers inter-shielding to influence the FrFT–OFDM demodulation performance. An efficient allocation algorithm of multiple chirp carriers by presetting the carrier parameters is proposed to solve this problem. Based on MC-CDMA, a variable bit rate system structure is proposed for TDCS with FrFT scheme under different channel environments. In order to simplify the process of modulation and demodulation of TDCS with FrFT scheme, a whole new cyclic shift key modulation mode in FrFT domain is also proposed. Both theories and simulations confirm strictly the validity of the proposed system.     

Spread spectrum techniques for indoor wireless IR communications

Multipath dispersion and fluorescent light interference are two major problems in indoor wireless infrared communications systems. Multipath dispersion introduces intersymbol interference at data rates above 10 Mb/s, while fluorescent light induces severe narrowband interference to baseband modulation schemes commonly used such as OOK and PPM. This article reviews the research into the application of direct sequence spread spectrum techniques to ameliorate these key channel impairments without having to resort to complex signal Processing techniques. The inherent properties of a spreading sequence are exploited in order to combat the ISI and narrowband interference. In addition, to reducing the impact of these impairments, the DSSS modulation schemes have strived to be bandwidth-efficient and simple to implement. Three main DSSS waveform techniques have been developed and investigated. These are sequence inverse keying, complementary sequence inverse keying, and M-ary biorthogonal keying (MBOK). The operations of the three systems are explained; their performances were evaluated through simulations and experiments for a number of system parameters, including spreading sequence type and length. By comparison with OOK, our results show that SIK, CSIK, and MBOK are effective against multipath dispersion and fluorescent light interference because the penalties incurred on the DSSS schemes are between 0-7 dB, while the penalty on OOK in the same environment is more than 17 dB. The DSSS solution for IR wireless transmission demonstrates that a transmission waveform can be designed to remove the key channel impairments in a wireless IR system.