发射分集OFDM系统中子载波频域均衡接收机设计

针对发射分集下的OFDM系统,提出了一种子载波频域均衡接收机设计方法。在保持子载波数恒定的条件下减少循环前缀的长度,并推导出最优分集合并系数。仿真结果表明,在发射分集条件下,本文提出的子载波频域均衡器有效地抑制了OFDM系统中由于循环前缀小于信道最大时延扩展所造成干扰和各个信道信号之间的互干扰,获得了较好的系统性能。     

An Efficient Cyclic Prefix Reconstruction Technique for MIMO Single-Carrier Frequency-Domain Equalization

In this letter, an efficient cyclic prefix reconstruction (CPR) technique with turbo equalization is developed for a multi-input multi-output (MIMO) single-carrier frequency-domain equalization (SC-FDE) system. The proposed method consists of pre-processing estimation (PPE) and residual inter-carrier interference suppression (RICIS). The PPE is employed to compute initial values of MIMO turbo equalization, and the RICIS is used to mitigate residual intercarrier interference (ICI) after each iteration of the CPR. By applying the proposed method to MIMO SC-FDE system with insufficient cyclic prefix (CP), we can significantly improve its error performance, obtaining both the benefits of multiplexing gain and spectral efficiency gain     

一种基于PN序列和循环前缀的OFDM同步算法

同步是OFDM（Orthogonal Frequency Division Multiplexing）系统的关键技术之一。使用重复的PN（pseudo-noise）序列和循环前缀作同步的新方法,与仅利用循环前缀的方法相比,有估计精度高和范围大的优点。在给出其推导过程的基础上给出了仿真结果,证明了分析的正确性。     

Two-stage interference immune blind equaliser which exploitscyclostationary statistics

Since cyclostationarity is inherent in many man-made signals, there is much to be gained in the signal processing and communication areas if this property is properly recognised and exploited. In particular, the authors propose a novel two-stage scheme which exploits second-order cyclostationary statistics for co-channel interference removal and blind equalisation. The first stage adopts the blind adaptive (BA) FRESH filter proposed by Zhang, to remove co-channel interference and channel noise. The second stage is based on a blind equalisation algorithm, proposed by Tong, which operates on the filtered cyclic signal to remove intersymbol interference. Simulation studies have demonstrated that this two-stage blind equalisation scheme has a fast rate of convergence and excellent performance in a high SNR environment     

OFDM系统中多普勒频偏估计技术仿真研究

研究了移动信道中的最大多普勒频移估计技术。分析了一种基于循环前缀的多普勒频移估计算法,该算法通过计算正交频分复用（OFDM）符号已有的循环前缀的相关函数来获得最大多普勒频移。采用广义平稳非相关散射下的多径时变瑞利信道模型,对各个算法进行了仿真试验,比较了各个算法的性能。仿真结果表明,利用多普勒分集技术,可以显著降低误码率,提高系统在移动环境下的性能。     

An Improved Frequency Synchronizer for Mobile OFDMSystems

1IntroductionOrthogonal Frequency Division Multiplexing(OFDM) hasgained strong position in the next generation widebandwireless systems for its advantages over frequency-selec-tive fading channels . The principal weakness of OFDMis its sensitivity to Carrier Frequency Offset ( CFO)caused by Doppler drifts and/or oscillator instabili-ties[1 ~3]. Many frequency synchronization algorithmsfor OFDM have been proposed, which can be dividedinto three main categories .First groupis based on s…     

基于正交多相序列的OFDM信道估计算法

提出了一种基于正交多相序列的OFDM信道估计算法。该算法采用相关性能良好的正交多相序列作为发送信号的循环前缀,在接收端,提取循环前缀,进行相关计算,最终得到系统频域响应的估计值。通过仿真,验证了在多径衰落信道中,提出的算法比频域梳状导频插入的时域滤波算法及采用小m序列作为循环前缀的信道估计算法具有更高的信道估计精度。     

循环前缀在OFDM系统中的应用

本文详细论述了OFDM系统中循环前缀在削弱符号间干扰（ISI）、载波间干扰（ICI）以及在实现时频同步中的应用。在OFDM符号间插入循环前缀，可以减小ISI和ICI，并为实时时频偏移估计提供足够信息，使得系统同步结构简单且工作高效。     

Performance of Frequency-Time MMSE Equalizer for MC-CDMA over Multipath Fading Channel

A cyclic prefix of sufficient length is inserted (longerthan the maximum delay spread of the channel) in Multi-Carrier CDMA (MC-CDMA) transmission over dispersive channels such that thechannel, as seen by subcarrier, is a simple multiplicativedistortion in the frequency domain, that can be compensated for by aone-tap Frequency Equalizer (FE). If, in addition to the spectralinefficiency caused by insertion of the cyclic prefix, the delayspread should exceed the designed prefix length, the FE will attempt tocompensate for the mismatched signal model and the system performancewill degrade significantly. In this paper, we derive a multi-input,multi-output (MIMO) model for received signal in synchronousMC-CDMA system over a multipath channel. Based on this model, aFrequency-Time MMSE Equalizer (FTE) is proposed. The performanceof the FTE is studied as a function of key system parameters, suchas prefix length and number of subcarriers, as well assensitivity to channel estimation errors. The numerical result showsthat system performance degradation due to insufficient guardtime can be recovered with the FTE; i.e., full spectralefficiency can be achieved at the price of additional receivercomplexity.     

Turbo均衡在Non-CP SC-FDE系统中的应用

针对无循环前缀的单载波频域均衡(Non-CP SC-FDE)系统中的符号间干扰(ISI),提出了一种Turbo均衡与循环重构联合迭代算法。通过循环重构技术消除ISI,并采用频域均衡频域判决反馈(FDE-FDDF)方式,将Turbo均衡算法与循环重构算法结合在一起进行迭代。仿真结果表明,Non-CP SC-FDE系统使用该算法能实现与传统有CP系统近似的误码性能,在大幅提高传输效率的同时能够有效消除多径干扰。     

加窗的PAM-DMT室内白光通信系统研究

将脉冲幅度调制/离散多音频调制(PAM-DMT)应用于白光LED通信系统中,提出了将时域加窗技术与PAM-DMT相结合的思想,并且加入了循环前缀和后缀,做了系统仿真.结果表明,此系统能够有效抑制多径效应对系统性能的影响,提高系统的频谱利用率,降低系统的误比特率.     

A Carrier Synchronization Algorithm for IEEE 802.11a System

1　Introduction　OFDMisaneffectivetransmissionschemetocombatmultipathfading[1～2 ] .Ithasbeenappliedinmanycommunicationfields,suchasDigitalAudioBroadcast (DAB ) [4] ,AsymmetricalDigitalSub scriberLoop (ADSL) [5] andIEEE 80 2 1 1 .a[6] .OneoftheprincipaldisadvantagesofOFDMisthesensi tivitytofrequencyoffset.TheCFOdestroystheor thogonalityofthesubcarriers,inducesInter CarrierInterference (ICI)anddegradesthesystem perfor mancesignificantly[7～8] .　Manymethodshavebeen proposedtoestim…     

Training sequence versus cyclic prefix-a new look on single carriercommunication

Orthogonal frequency-division multiplexing (OFDM), with the help of a cyclic prefix, enables low complexity frequency domain equalization, but suffers from a high crest factor. Single carrier with cyclic prefix (SC-CP) has the same advantage with similar performance, but with a lower crest factor and enhanced robustness to phase noise. The cyclic prefix is overhead, so we put more information in it by implementing this cyclic prefix as a training sequence (TS). This new training aided frequency domain equalized single carrier (TASC) scheme offers us additional known symbols and enables better synchronization and (potentially) channel estimation, with the same performance as SC-CP     

Modulated Coded OFDM Systems with Special Precoder

Orthogonal frequency-division multiplexing (OFDM) has been considered as a strong candidate for next-generation wireless communication systems to achieve high rate data transmission in a mobile environment. However, the performance of OFDM systems may be degraded when inter-symbol interference (ISI) channels have spectral nulls, and the data rate overhead due to the insertion of cyclic prefix is high when ISI channels have many taps. Recently, the precoded OFDM systems and vector OFDM (VOFDM) systems were proposed to combat these two problems, respectively. We propose a novel modulated coded OFDM system with special precoder that is robust to spectral nulls and with reduced cyclic prefix length. The precoder can be easily formulated by channel coding and digital modulation. This precoding scheme uses the redundancy information introduced by channel coding (such as zeros inserted in precoded OFDM system). It is able to remove the spectral nulls of an ISI channel without knowing the ISI. Simulation results show that our proposed OFDM system performance is better than precoded OFDM and VOFDM system.     

OFDM系统中基于SNR估计的改进DFT信道估计算法

为提高目前无线通信系统中正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)技术下的信道估计性能,分析了基于离散傅里叶变换(Discrete Fourier Transform,DFT)的信道估计算法存在的缺陷.针对传统DFT信道估计算法没有考虑循环前缀内噪声的...     

时域均衡在OFDM水声通信系统中的应用

为了对抗水声信道时延扩展大于循环前缀长度时引起的码间串扰问题,在基本的OFDM水声通信系统接收端进行DFT变换前加入相对较短的时域均衡器来限制信道冲激响应的长度。分析了基于MMSE准则的时域均衡算法的实现过程以及算法的复杂程度,对该算法在水声环境下的性能进行了仿真分析,并比较了影响系统性能的因素。结果表明通过加入时域均衡器能够有效对抗信道严重的时间离散性,改善OFDM水声通信系统在循环前缀不足时的系统性能。     

Subspace-based blind channel estimation for OFDM by exploiting virtual carriers

Reliable channel estimation is indispensable for orthogonal frequency-division multiplexing (OFDM) systems employing coherent detection and adaptive loading in order to achieve high data rate communications. Several options exist in practical OFDM systems-including training symbols, cyclic prefix, virtual carriers, pilot tones, and receiver diversity-to facilitate channel estimation. In this paper, a subspace blind channel estimation method based on exploiting the presence of virtual carriers is proposed for OFDM systems over a time-dispersive channel. The method can be applied to conventional OFDM systems with cyclic prefix as well as OFDM systems with no cyclic prefix. The reduction/elimination of cyclic prefix thereby provides the OFDM systems the potential to achieve higher channel utilization than most previously reported cyclic prefix based estimators. Sufficient channel identifiability condition is developed as well. Comparison with two other recently reported subspace methods is presented via computer simulations to support the effectiveness of the proposed method.     

一种基于循环前缀的OFDM时频同步新算法

OFDM符号是由多个子载波信号叠加而成,为确保各子载波之间的正交性,就对载波间的同步有很严格的要求,否则将造成信道间干扰和符号间干扰,严重影响OFDM系统的性能。提出一种新的基于循环前缀的OFDM时频同步算法,通过仿真,将新算法和传统的ML算法及集相关算法相比较。仿真结果表明,新算法定时捕获更快,且能更准确地确定无IBI干扰的循环前缀采样部分,具有更好的频偏和定时估计性能。     

循环前缀块传输系统中的一种MIMO盲信道估计算法

信道估计一直是无线通信领域的研究热点之一,信道参数估计的好坏对系统的整体性能有着至关重要的影响。针对采用循环前缀的多输入多输出(MIMO-CP)系统,本文提出了一种基于子空间的盲信道估计方法,该算法利用了循环前缀所引起的冗余信息。基于子空间的盲信道估计算法都是通过对接收块的自相关矩阵进行奇异值分解(SVD)来实现信道估计的,因此需要利用尽可能多的接收块来得到准确的自相关矩阵的估计值,这就意味着会产生过久的判决延迟以及不能准确对快变信道进行跟踪。利用MIMO-CP系统中系统矩阵特有的块循环特性,对于连续的两个接收数据块以及对应的循环前缀部分组成的向量,可以重新构造一组新的向量而不改变系统的信道矩阵,因此可以通过较少的接收块来得到准确的自相关矩阵的估计值,该方法十分适用于对快变信道的盲估计。文章通过仿真分析了在不同的重复系数以及不同的接收块下该算法的性能并且比较了该算法与现有的“预编码”、“虚拟子载波”等盲信道估计算法的性能。仿真结果表明,提出的算法利用较少的数据块个数就得到了一个可靠的信道估计值和很好的误码率性能。      

Exploiting input cyclostationarity for blind channel identificationin OFDM systems

Transmitter-induced cyclostationarity has been explored previously as an alternative to fractional sampling and antenna array methods for blind identification of FIR communication channels. An interesting application of these ideas is in OFDM systems, which induce cyclostationarity due to the cyclic prefix. We develop a novel subspace approach for blind channel identification using cyclic correlations at the OFDM receiver. Even channels with equispaced unit circle zeros are identifiable in the presence of any nonzero length cyclic prefix with adequate block length. Simulations of the proposed channel estimator along with its performance in OFDM systems combined with impulse response shortening and Reed-Solomon coding are presented