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正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)是一种能够有效对抗电力线通信中的频率选择性衰落、多径和窄带干扰的技术,而OFDM技术对同步偏差十分敏感,微小的同步偏差也会影响OFDM系统的性能.针对电力线通信中OFDM系统的同步问题,给出一种基于恒包络零自相关(CAZAC)序列的时频同步的方法.MATLAB仿真表明,所提算法在减少运算量的基础上能很好地完成符号同步和频偏同步,明显改善电力线中OFDM的同步性能. 相似文献
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SER performance evaluation and optimization of OFDM system with residual frequency and timing offsets from imperfect synchronization 总被引:1,自引:0,他引:1
This paper deals with the effects of residual timing and frequency offsets on the symbol error rate (SER) performance of an orthogonal frequency division multiplexing (OFDM) system. The synchronization of an OFDM system generally consists of a coarse frequency and timing acquisition stage and a refine stage. Due to the presence of Gaussian noise, channel distortions and implementation losses of synchronization and equalization algorithms, residual frequency and timing offsets always exist for an OFDM receiver. The residual frequency and timing offsets are proven to be Gaussian distributed, with their corresponding variances determined. The reception process of an OFDM signal with frequency and timing offsets is analyzed. A closed-form analytical result on the SER of an OFDM system with residual synchronization errors is derived. Computer simulations and analyses show that the frequency and timing offsets affect the OFDM subcarriers differently. With this observation, a new technique is proposed to minimize the SER of the OFDM systems by adjusting the distribution of transmission power among the subcarriers. 相似文献
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同步对于OFDM(正交频分复用)系统非常重要。针对S&C算法和Park算法定时模糊的问题,提出一种改进的符号同步算法。该算法先通过接收信号与训练序列进行互相关确定OFDM符号的可能起始位置,然后再滤波进一步确定OFDM符号准确的起始位置。仿真结果表明:改进算法的定时度量在低信噪比和多径衰落信道环境下都具有脉冲型峰值,较传统方法有较小的均方误差。 相似文献
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We present a narrowband interference (NBI) canceller that suppresses spectral leakage in an orthogonal frequency-division multiplexing (OFDM)-based system caused by a narrowband (NB) signal. We assume that the spectrum of the NB signal is within the spectrum of the OFDM signal. This can be the case, e.g., on digital subscriber lines (DSL) and in new unlicensed frequency bands for radio transmission. The canceller makes linear minimum mean-square error estimates of the spectral leakage by measuring the NBI on a few modulated or unmodulated OFDM subcarriers. It uses a model of the NB signal's power spectral density as a priori information. Using a frequency invariant design, it is possible to cancel NBI from signals that are changing their frequency location with significantly reduced complexity overhead. The operational complexity of the canceller can be lowered by using the theory of optimal rank reduction and using the time-bandwidth product of the NB signal. Analytical performance evaluations, as well as Monte Carlo simulations, show that, without perfect a priori information, this canceller can suppress the spectral leakage from a strong NB signal (e.g., with equal power as the OFDM signal) to well below the background noise floor for typical applications where it causes negligible signal-to-noise ratio and symbol error rate degradation. 相似文献
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Timing recovery for OFDM transmission 总被引:29,自引:0,他引:29
Baoguo Yang Letaief K.B. Cheng R.S. Zhigang Cao 《Selected Areas in Communications, IEEE Journal on》2000,18(11):2278-2291
Orthogonal frequency division multiplexing (OFDM) is an effective modulation technique for high-rate and high-speed transmission over frequency selective fading channels. However, OFDM systems can be extremely sensitive and vulnerable to synchronization errors. In this paper, we present a scheme for performing timing recovery that includes symbol synchronization and sampling clock synchronization in OFDM systems. The scheme is based on pilot subcarriers. In the scheme, we use a path time delay estimation method to improve the accuracy of the correlation-based symbol synchronization methods, and use a delay-locked loop (DLL) to do the sampling clock synchronization. It is shown that by using this scheme, the mean square values of the symbol timing estimation error can be decreased by several orders of magnitude compared to the common correlation methods in both the AWGN and multipath fading channels. In addition, the scheme can track the symbol timing drift caused by the sampling clock frequency offsets 相似文献
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Orthogonal frequency-division multiplexing (OFDM) systems are highly sensitive to synchronization errors. We introduce an algorithm for the blind estimation of symbol timing and carrier frequency offset in wireless OFDM systems. The proposed estimator is an extension of the Gini-Giannakis (see IEEE Trans. Commun., vol.46, p.400-411, 1998) estimator for single-carrier systems. It exploits the cyclostationarity of OFDM signals and relies on second-order statistics only. Our method can be applied to pulse shaping OFDM systems with arbitrary time-frequency guard regions, OFDM based on offset quadrature amplitude modulation, and biorthogonal frequency-division multiplexing systems. We furthermore propose the use of different subcarrier transmit powers (subcarrier weighting) and periodic transmitter precoding to achieve a carrier frequency acquisition range of the entire bandwidth of the OFDM signal, and a symbol timing acquisition range of arbitrary length. Finally, we provide simulation results demonstrating the performance of the new estimator 相似文献
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OFDM系统的实现还存在着一些方面问题,如:同步问题尤其是频偏,峰值功率与平均功率比值过高,器件的非线性化等。OFDM系统对同步偏差敏感是本文讨论的重点,尤其是频偏。因此怎样获得准确的符号定时和载波频偏估计对OFDM系统性能至关重要。本文围绕OFDM同步过程中的符号定时和载波频率偏差问题进行描述,在分别介绍同步过程中捕获和跟踪两个阶段各自一些典型算法基础上,对一种基于保护间隔/循环前缀(PI/CP)的联合实现符号定时和载波同步最大似然估计(MLE)算法进行叙述并仿真。 相似文献
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Sanguinetti L. Morelli M. Imbarlina G. 《Wireless Communications, IEEE Transactions on》2009,8(9):4470-4475
We consider an orthogonal frequency-division multiplexing (OFDM) system and address the problem of carrier frequency estimation in the presence of narrowband interference (NBI) with unknown power. This scenario is encountered in emerging spectrum sharing systems, where coexistence of different wireless services over the same frequency band may result into a remarkable co-channel interference, and also in digital subscriber line transmissions as a consequence of the cross-talk phenomenon. A possible solution for frequency recovery in OFDM systems plagued by NBI has recently been derived using the maximum-likelihood criterion. Such scheme exhibits good accuracy, but involves a computationally demanding grid-search over the uncertainty frequency range. In the present work, we derive an alternative method that provides frequency estimates in closed-form by resorting to the expectation-maximization algorithm. This makes it possible to achieve some computational saving while maintaining a remarkable robustness against NBI. 相似文献
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基于最大似然算法的OFDM同步技术研究 总被引:1,自引:0,他引:1
同步问题是卫星通信中的一个关键问题,直接关系到通信系统的性能。文中简要介绍了OFDM的同步技术,主要讨论了符号定时同步和载波同步。最大似然估计算法在OFDM符号定时同步和载波频率同步中得到了广泛的应用,通过最大似然估计方法,避免了基于导频符号的同步估计带来的频率和功率资源的浪费,通过仿真可以看到,能够很好地实现OFDM系统的符号同步和载波同步。 相似文献
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On the synchronization techniques for wireless OFDM systems 总被引:3,自引:0,他引:3
Bo Ai Zhi-xing Yang Chang-yong Pan Jian-hua Ge Yong Wang Zhen Lu 《Broadcasting, IEEE Transactions on》2006,52(2):236-244
The latest research works on the synchronization scheme for either continuous transmission mode or burst packet transmission mode for the wireless OFDM communications are overviewed in this paper. The typical algorithms dealing with the symbol timing synchronization, the carrier frequency synchronization as well as the sampling clock synchronization are briefly introduced and analyzed. Three improved methods for the fine symbol timing synchronization in frequency domain are also proposed, with several key issues on the synchronization for the OFDM systems discussed. 相似文献
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本文提出了一种在OFDM系统中基于导频符号的自适应符号同步新算法,它利用FFT的特性,采用频域同步方法,对符号定时进行估计。该算法计算量小,插入的导频数少,并且同步捕获速度、精度高低,可以灵活调节。仿真结果表明,该算法能简单而有效地获取符号同步,自适应性强。 相似文献
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Outage Analysis of OFDM‐Based Cognitive AF Relay Network in the Presence of Narrowband Interference 下载免费PDF全文
Orthogonal frequency‐division multiplexing (OFDM) is one of the most widely used technologies in current wireless communication systems and standards. Cognitive radio (CR) provides a robust solution to the problem of spectrum congestion as it offers opportunistic usage of frequency bands that are not occupied by primary users. Due to the underlying sensing, spectrum shaping, scaling, and interoperable capabilities of OFDM, it has been adapted as a best transmission technology for CR wireless systems. However, the performance of an OFDM‐based CR wireless system is affected by the existence of narrowband interference (NBI) from other users. Further, due to carrier frequency offset in NBI sources, NBI energy may spread over all subcarriers of an OFDM signal. In this paper, a fixed Amplify‐and‐Forward (AF) relay that operates at a frequency band that is different from that of direct mode is introduced to suppress the effect of NBI. Analytical expressions are derived for outage probability in direct, AF‐relay, and incremental relaying modes. The outage performance of the proposed AF relay–based CR network is proven to be better than that of direct mode. 相似文献