无线信道中OFDM技术和QAM技术传输的性能差异

本文介绍了OFDM技术。OFDM是一种多载波调制技术,能够把有符号间干扰的信道转换成无符号间干扰的并行子信道,所以,OFDM技术能够有效的对抗信道的频率选择性衰落．它已经得到了广泛的应用。本文比较了语音文件在OFDM系统和QAM系统中的传输效果,得出在相同条件下OFDM调制要优于QAM调制的结论。     

OFDM系统中的ICI干扰消除技术

针对高速移动环境下多普勒频偏造成移动信道的快衰落和正交频分复用OFDM系统中子载波间干扰（ICI）问题,本文给出了OFDM系统中的多种ICI干扰消除技术。     

OFDM—CDMA检测技术性能分析

传统的CDMA技术在无线通信方面因其诸多优点而备受青睐,但在抗码间串扰和多址干扰等方面存在的问题使其总体性能受到限制。而随着正交频分复用（OFDM）技术的日趋成熟,其强大的抗多径性能正得到越来越多的关注。这里介绍了一种最新的无线通信技术：OFDM－CDMA,对其发射和接收系统模型进行了完整的描述并对接收系统中的检测技术性能进行了比较。分析表明OFDM－CDMA兼备了CDMA和OFDM技术的优点,在频谱利用率、抗码间干扰、抗多径和多址干扰等方面都优于传统的CDMA技术。     

OFDM无线通信中高峰均比及其抑制算法研究

凭借很强的抗符号间干扰(ISI)和抗衰落能力,采用正交频分信道的OFDM技术的无线通信越来越受到人们的重视。但是作为一种多载波调制方式,OFDM技术同样存在着高峰均比(PAPR)的问题,高的PAPR会导致信号畸变,造成子信道间的相互干扰,从而影响OFDM的性能。针对此,重点介绍了OFDM系统中的PAPR比问题的概念,并对降低PAPR的主要算法进行了论述。     

基于交叉复用技术的OFDM-WDM 光传输系统的性能分析

基于正交频分复用(OFDM)与波分复用(WDM)的基本原理,构建了一套基于OFDM技术的WDM传输系统的系统模型,并对其相关系统性能进行了理论分析。基于维纳相位噪声模型,文章对OFDM系统中相位噪声引起的公共相位误差和子载波间干扰进行了分析,在此基础上深入分析了基于交叉复用OFDM技术的OFDM子载波间干扰的消除方法,降低相位噪声对系统性能的影响。最后结合理论分析结果对系统模型的相关性能进行模拟计算,分析结果表明:采用交叉复用OFDM技术在降低相位噪声对OFDM系统性能的影响的同时,提高了系统的信号处理能力。     

基于SLM的OFDM系统PAPR减小算法研究

OFDM技术是第四代移动通信的核心技术。OFDM系统因其传输速率高、频带利用率高以及很强的抗因多径带来的符号间干扰(ISI)与子载波间干扰(ICI)的能力而得到了广泛的应用。OFDM系统急需解决的问题之一是高峰值平均功率比(PAPR)问题。本文分析了OFDM系统高PAPR的成因,分析了降低OFDM系统PAPR的选择性映射(SLM)算法,并用数据仿真证明了SLM算法有效性。     

OFDM/OQAM技术在时频弥散信道中的应用

正交频分复用/交错正交幅度调制(Orthogonal Frequency Division Multiplexing/Offset Quadrature Amplitude Modulation,OFDM/OQAM)与传统OFDM相比,OFDM/OQAM通过引入具有优良时频聚焦特性的脉冲成形滤波器,使其可同时具备抗符号间干扰和子载波间干扰的能力,且带外辐射较低,有很好的技术前景。介绍了OFDM/OQAM系统的基本原理及其局限性,并进一步分析了因其固有干扰对关键技术产生的影响,以及在时频弥散信道中的应用前景。     

OFDM系统降低峰均功率比的主要技术及其比较

正交频分复用（OFDM）技术在无线移动通信系统中具有抗多路径衰落和频带间干扰等优点。然而,其主要瓶颈在于OFDM信号的峰均功率比（PAPR）很大,因而容易导致OFDM信号的交调失真和系统性能的下降。阐述了降低OFDM系统峰均比的三类技术：预畸变技术、编码类技术、概率类技术,并对其性能做了比较。     

一种基于OFDM的多载波通信系统的设计与仿真

OFDM(正交频分复用)是一种无线环境下的高速传输技术,它对由于无线信道的多径传播引起的ISI(符号间干扰)有很强的抵抗能力,已经引起了人们的广泛关注.文中首先介绍了OFDM技术的应用、发展以及基本原理,然后在计算机上用MATLAB对其性能进行了仿真,仿真中将OFDM与传统的单载波调制方式--16-QAM(正交幅度调制)对抗多径干扰的性能进行了比较,结果表明OFDM技术更适合于在有多径干扰存在的无线信道中传输.     

一种适用于快衰落信道的ICI抑制方案

针对高速移动环境下多普勒频偏造成信道的快衰落和正交频分复用(OFDM)系统中子载波间干扰(ICI)的问题,提出了一种适合快衰落环境的OFDM系统子载波间干扰抑制算法。此算法用线性变化模型来近似一个OFDM符号周期内的信道冲激响应,并以此为基础采用迭代MMSE均衡方法抑制载波间干扰。分析和仿真结果表明,此方法能有效地保证载波间的正交性,从而改善了OFDM系统的误码率(BER)性能。     

Sidelobe suppression in OFDM-based spectrum sharing systems using adaptive symbol transition

In this letter, we introduce a new method for orthogonal frequency division multiplexing (OFDM) sidelobe suppression. An extension is added to OFDM symbols that is calculated using optimization methods to minimize adjacent channel interference (ACI) while keeping the extension power at an acceptable level. Using this technique, interference to adjacent signals is reduced significantly at the cost of a small decrease in the useful symbol energy. The proposed method can be used by cognitive radio (CR) systems to shape the spectrum of OFDM signals and to minimize interference to licensed users (LU), or to reduce the size of guard bands used in conventional OFDM systems.     

基于MATLAB的OFDM系统仿真分析

正交频分复用（OFDM）是一种多载波宽带数字调制技术，它具有频带利用率高和抗多径干扰能力强等优点，因而适合于高速率的无线通信系统。分析了OFDM技术的实现原理，用MATLAB软件对OFDM的传输性能进行了仿真模拟并对结论进行了分析。     

OFDM系统仿真与性能分析

正交频分复用（OFDM）是一种特殊的多载波方案,它既是一种调制技术,也是一种复用技术。由于其具有良好的抗频率选择性衰落或窄带干扰、频谱利用率高和成本低的优点,应用越来越广泛。采用Simulink搭建的OFDM仿真系统,既能有效测试OFDM的性能指标,也能研究和理解OFDM特点和具体应用。     

一种OFDM系统强窄带干扰消除方法

OFDM系统由于具有子载波的正交性从而可以充分利用频谱资源,鉴于当OFDM受到窄带干扰而影响较多的子载波时,会出现系统误码率性能严重恶化的现状。文中提出了一种OFDM系统窄带干扰消除技术,该技术在接收端检测出受干扰子载波,利用反馈信道告知发送端,并在发送端将受干扰的子载波不加调制的信息发送到信道中,而在信道中该部分子载波只受干扰信号的影响,接收端将干扰信息重构并储存,从而完成干扰消除确保正常通信。仿真和分析结果表明,该方法在不浪费频谱资源的情况下,能有效抑制OFDM系统的窄带干扰。     

Semi-Blind Time-Domain Equalization for MIMO-OFDM Systems

In this paper, a semi-blind time-domain equalization technique is proposed for general multiple-input-multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems. The received OFDM symbols are shifted by more than or equal to the cyclic prefix (CP) length, and a blind equalizer is designed to completely suppress both intercarrier interference (ICI) and intersymbol interference (ISI) using second-order statistics of the shifted received OFDM symbols. Only a one-tap equalizer is needed to detect the time-domain signals from the blind equalizer output, and one pilot OFDM symbol is utilized to estimate the required channel state information for the design of the one-tap equalizer. The technique is applicable irrespective of whether the CP length is longer than, equal to, or shorter than the channel length. Computer simulations show that the proposed technique outperforms the existing techniques, and it is robust against the number of shifts in excess of the CP length.     

OFDM调制新方案及其脉冲成形滤波器的设计

本文提出一种新的正交频分复用(OFDM)调制方案,结合了两种OFDM系统的特点,既利用了脉冲成形技术的优势,又能采用保护间隔.分析了频率偏移存在时脉冲成形滤波器与载波间干扰的关系,并给出一个设计实例的仿真.数值分析和仿真结果表明,该设计能显著抑制载波间干扰.     

基于OFDM的POF通信PAPR抑制算法仿真与分析

OFDM技术能有效解决塑料光纤（POF）信道中频率选择性衰落及码间干扰等问题。这里在POF通信中引入OFDM技术,建立OOFDM通信模型,分析了高峰均比（PAPR）对POF系统的影响。提出采用脉冲整形技术改变原始数据相位和幅度的分布来抑制PAPR,通过仿真分析了不同基带调制方式对PAPR抑制性能影响,并与几种常见PAPR抑制方法进行比较,最后对模型的整体性能进行仿真。仿真结果表明该技术计算复杂度较小,效率比其他方法高很多,结合QPSK信号调制,降低峰均比效果明显,能有效降低BER值,提高信号质量。     

Robust channel estimation and ISI cancellation for OFDM systems with suppressed features

A feature-suppressed orthogonal frequency-division multiplexing (OFDM) system and the corresponding channel estimation and intersymbol interference (ISI) mitigation techniques are investigated in this paper. Cyclic prefix (CP) and pilot tones, which are commonly used in civilian OFDM systems for ISI mitigation and channel estimation, create distinctive waveform features that can be easily used for synchronization and channel estimation purposes by intercepting receivers. As a result, CP and pilot tones are eliminated in the proposed feature suppressed OFDM system to reduce the interception probability. Instead, a set of specially designed OFDM symbols, driven by different pseudorandom sequences, are employed as preambles to avoid unique spectral signature. These preambles are inserted into the OFDM data symbol stream periodically and in a round-robin manner. In addition, a random frequency offset is introduced to each preamble to further mask the multicarrier signature. New challenges arising from these feature suppression efforts are studied, including robust channel estimation and demodulation techniques in the presence of frequency offset and severe interference. Based on our interference analysis, an iterative ISI and intercarrier interference (ICI) estimation-cancellation-based technique is proposed for both channel estimation and OFDM data demodulation. Our channel estimator performs joint frequency offset and channel impulse response estimation based on the maximum-likelihood (ML) principle. To reduce its complexity, we employ a number of techniques, which include approximation of the ML metrics, as well as fast Fourier transform pruning. The performances and feasibility of the proposed feature suppressed OFDM system and the channel estimator are analyzed and verified through numerical simulations.