A NOVEL INTEGER FREQUENCY OFFSET ESTIMATOR FOR OFDM

One of the principal disadvantages of Orthogonal Frequency Division Multiplexing （OFDM） is very sensitive to carrier frequency offset. The integer frequency offset has no effect on the orthogonality among the subcarriers, but it causes a circular shift and phase rotation of the received data symbols sequence, resulting in a Bit Error Rate（BER） of 0.5. In this paper, a novel integer frequency offset estimator for OFDM is derived based on maximum likelihood estimation technique and exploration of the differential relation between two consecutive OFDM data symbol sequences in frequency domain. Its performance is compared with the conventional method by computer simulations for the additive white Gaussian noise channel and a multipath fading channel. Simulation results show that the performance of the proposed estimator is better than the conventional estimator.     

Lossless Digital Watermarking Scheme for Image Maps

A lossless digital watermarking scheme,which was zero- perturbation on the content and graphics for the digital image maps,is proposed.During the simulation,the scheme has been utilized to model the copyright protection program as a commerce application.Compared to the traditional digital watermarking schemes,our scheme operates in redundancy areas of maps and is scalable to topology changes.Experimental results show that,with respect to the geometric attacks and image transformed,the performance of our scheme is better than the classical algorithms based space or frequency domain with much lower complexity..     

Interleaving scheme for multicarrier CDMA system

In MultiCarrier Code-Division Multiple-Access (MC-CDMA) system,the received signals scattered in the frequency domain are combined to get frequency diversity gain.However,the frequency diversity gain is limited because of correlation between subcarriers.A novel interleaving scheme for MC-CDMA system is proposed in this paper.A circular shifting register is introduced into each subcarrier branch to decrease the correlation between subcarriers.By using interleaving,frequency diversity gain of system is increased.System structure and model with interleaver are discussed.In the case of multiple users,Parallel Interference Cancellation (PIC) technique is also introduced.Computer simulations demonstrate the performance of proposed scheme,and the performance comparison of MC-CDMA with interleaver and conventional MC-CDMA system is shown as well.     

Timing Advanced Estimation Algorithm of Low Complexity Based on DFT Spectrum Analysis for Satellite System

In satellite mobile communication system, relative movement of the satellite and the terminal will cause a large Doppler offset. Timing advanced estimation with Zadoff-Chu sequence is sensitive to the frequency offset. When the frequency offset is larger than one times subcarrier spacing, the value of peak cannot be detected at the receiving end. To suppress the larger Doppler frequency shift, this paper proposes a novel timing advanced estimation scheme(TAE-MCD) for satellite communication system. In this algorithm, t r a n s m i t t e d s i g n a l i s d i v i d e d i n t o Z C sequence and its conjugate sequence. Using multiplication and DFT operation to find the estimated peak at the receiving end, and make subtraction with the obtained sequences at last. The scheme can not only inhibit the adverse effects of large Doppler frequency shift in timing estimation effectively, but also reduce the computational complexity at the receiving end and improve the work efficiency of the hardware. Simulations results show that TAEMCD outperform the existing timing advanced estimation methods, on the condition of no additional time and frequency resource are needed.     

Single tone jamming elimination scheme of multicarrier spread spectrum system

The proposed scheme is based on Discrete Fourier Transform (DFT) domain processing. The key technology of this scheme is jamming parameters‘ accurate estimation and jamming reconstruction. Compared with the “threshold exciser”scheme.the proposed scheme can eliminate more jamming energy on the whole frequency band with the minimum loss of useful signal energy. As shown in the research and simulation, the proposed scheme is much better than the “threshold exciser” scheme, especially in the case of high power jamming whereas the “threshold exciser” scheme might be invalid.     

A weighted general discrete fourier transform for the frequency-domain Blind Source Separation of Convolutive Mixtures

This letter deals with the frequency domain Blind Source Separation of Convolutive Mixtures （CMBSS）. From the frequency representation of the ＂overlap and save＂, a Weighted General Discrete Fourier Transform （WGDFT） is derived to replace the traditional Discrete Fourier Transform （DFT）. The mixing matrix on each frequency bin could be estimated more precisely from WGDFT coefficients than from DFT coefficients, which improves separation performance. Simulation results verify the validity of WGDFT for frequency domain blind source separation of convolutive mixtures.     

TIME-FREQUENCY 2-D LMS BASED LONG-RANGE CHANNEL PREDICTION FOR WIRELESS OFDM SYSTEMS

Adaptive modulation can optimize the spectrum efficiency and system performance with the channel state information achieved by the long-range channel prediction.To avoid re-estimating channel correlation function as the channel statioharity varies and to track the channel adaptively, LMS (Least-Mean-Square) based long-range channel prediction is discussed in the existing literature, but it needs long observation interval to reach the convergence.Given that all OFDM (Orthogonal Frequency Division Multiplexing) subcarriers have the identical time-domain correlation and sta- tionarity during the same time interval,this paper proposed a 2-D LMS based predictor which updates the filter weights in both time and frequency domain.The proposed scheme can effectively decrease the observation intervals and significantly speed up the convergence than the conventional LMS and Parallel LMS (PLMS).Complexity analysis and simulation results prove that the proposed scheme can improve the BER (Bit Error Rate) performance and spectrum efficiency with negligible complexity increase.     

STATISTICAL APPROXIMATION BASED FINE FREQUENCY SYNCHRONIZATION FOR OFDM SYSTEMS

The paper proposes a novel approach for fine frequency synchronization of OFDM syn- chronization systems in multi-path channels. Maximum Likelihood (ML) function of frequency offsets including integral and decimal parts in frequency domain is developed according to the law of great number to eliminate the noise impact of the signal. When the timing delay close to the actual time, the proposed function produces a deep valley indicating frequency offset when large Valley-Square- Error (VSE) appears. Coarse timing offset can also be detected when function’s Valley-Square-Error (VSE) is maximized. Simulation results shows that the proposed algorithm gives very robust estimation of frequency offset, and a coarse timing offset estimation.     

Cell search algorithms for the 3G long-term evolution

This article presents downlink initial synchronization and cell identification algorithms for long term evolution (LTE) of third-generation (3G) mobile communication systems, which are based on synchronization channel (SCH) and cell specific pilot symbols, respectively. The key features of the proposed scheme are: it can improve performance of the frequency synchronization through oversampling of the SCH, it can support a large number of target cells by modulating a cell-specific pilot sequence over two symbols within a subframe, and it can guarantee cell identification performance by maximally ratio combining the frequency domain differential cross-correlation. Simulations show that the proposed scheme has a potential use in 3G LTE.     

A frequency offset estimation algorithm for OFDM systems

A new fine carrier frequency offset estimation algorithm in Orthogonal Frequency Division Multiplexing （OFDM） system is proposed. The correlation item is the training sequence instead of the received signal in the new algorithm. Simulation results show that the performance of the new algorithm is 4dB-9dB better than that of Schmidl＇s algorithm. Coarse frequency offset estimation is also discussed in this paper, which is the improvement of Zhang＇s method. The estimation range using the improvement method is twice as that using the Zhang＇s method. Based on the hardware of the receiver and the improved algorithm, a method using Fast Fourier Transform （FFT） is proposed to implement the coarse frequency estimation. The chip area of OFDM system can be reduced by using the proposed method.     

Block-wise frequency offset estimation scheme in MIMO-OFDM systems

This paper proposes a robust residual carrier frequency offset (CFO) tracking scheme with an extended estimation range, as well as an improved estimation performance in a multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) system. An analytical expression for the mean square error (MSE) of the frequency synchronization scheme is reported, and simulation results are used to verify the theoretical analysis developed in this paper.     

Parallel interference cancellation for carrier frequency offset in OFDM assisted with recursive least squares algorithm

Parallel interference cancellation (PIC) assisted with recursive least squares (RLS) algorithm is proposed to cancel the interference due to the carrier frequency offset (CFO) in orthogonal frequency division multiplexing (OFDM) system. The proposed algorithm is composed of two stages, which are RLS scheme and PIC scheme. RLS scheme is selected to compensate the frequency offset in the time domain in the first stage, and the interference induced by residual frequency offset is canceled by the PIC scheme in the frequency domain in the second stage. The result of bit error rate (BER) shows that its performance is robust for cancellation as comparison criteria, even though the frequency offset is 0.45. The 16QAM constellation is also simulated to observe the improvements from the proposed suppression schemes.     

一种新的OFDM系统载波频偏估计算法

在正交频分复用(OFDM)系统中,频率偏移会破坏子载波间的正交性,降低整个系统的性能。在研究传统频偏估计算法的基础上,利用简单的实数训练序列,进行OFDM系统的载波频偏估计,只需要一个训练序列,就可以有效地在时域内同时对小数倍和整数倍的频率偏移进行估计。仿真结果表明,基于实数训练序列的载波频偏估计算法不仅方法简单,易于实现,而且可提高频偏估计精度。     

一种新的OFDM系统载波频率偏移估计和跟踪方案

针对载波偏移使OFDM系统出现严重的子载波干扰和性能的下降,该文提出一种OFDM频偏估计和跟踪方案,载频捕获阶段频偏估计采用时域内插入等值导频的方法,载频跟踪阶段不需要导频,而是利用循环前缀来估计频率偏移并进行校正。仿真结果证实,该方案中导频辅助的频率偏移估计和载频跟踪不仅估计范围大,而且估计值准确,频偏校正后系统的性能能达到信道理想估计条件下的误比特率。     

Replica Correlation‐Based Synchronization with Low Complexity and Frequency Offset Immunity

This paper investigates the multifarious nature of the long‐term evolution (LTE) scheme and that of the modified LTE scheme for symbol timing synchronization (STS). This investigation allows us to propose a new replica correlation‐based STS scheme to overcome the inherent weaknesses of the other two schemes. The proposed STS signal combines a gold sequence and a half sine wave in the time domain, whereas conventional STS signals specify either binary sequences or complex sequences in the time domain or in the frequency domain. In the proposed scheme, a sufficient correlation property is realized by the gold sequence, and robustness against the frequency offset (FO) is achieved through the sine wave. Compared to the existing LTE‐related schemes, the proposed scheme can better achieve immunity to FO and reduction in detector complexity, as well as a low peak‐to‐average power ratio and a low detection error rate. Performance evaluations through analysis and simulation are provided in the paper to demonstrate these attributes.     

基于FPGA的OFDM系统同步算法的研究

符号、载波偏移严重影响着正交频分复用(OFDM)接收系统的性能.本文在分析和研究同步估计算法的基础上,设计了一种OFDM同步系统实现方案,并对符号偏移,载波频偏和采样钟偏移的估计及补偿算法和具体电路实现进行了相应优化.并采用VirtexII系列xc2v1000 FPGA进行实现验证,结果表明该同步系统不仅能有效地实现OFDM系统符号和载波同步,而且可降低硬件资源的消耗.     

Low-complexity blind carrier frequency recovery for OFDM signals over frequency-selective radio channels

This paper introduces a blind (i.e., data independent) algorithm for carrier frequency offset recovery in an orthogonal frequency-division multiplexing (OFDM) receiver operating over frequency-selective fading channels. The main idea behind this algorithm is to exploit the time-frequency-domain exchange inherent to the modulation scheme. Due to this feature, a carrier frequency offset has a similar impact on OFDM as a clock timing offset has in a quadrature amplitude modulation (QAM) system. The scheme we propose is a variant of Oerder-Meyr's (1988) feedforward clock recovery. Its performance is assessed by simulation, and the results are compared to those obtained from Van de Beek-Sandell-Borjesson's (1997) frequency synchronizer, which bears comparable complexity. The new scheme is shown to outperform the latter over frequency-selective fading channels, notably at medium to high signal-to-noise ratios. We also evaluated the efficiency of two different (time domain and frequency domain) offset correction strategies embedded in a particular OFDM receiver.     

IEEE802.16e系统上行链路初始测距算法研究

该文提出了一种适用于IEEE802.16e的OFDMA系统上行链路的初始测距频域改进算法。频域算法利用测距信号具有连续相位的特点,在频域将接收信号与备选码字共轭相关,按照接入用户信号的强弱依次检测出不同用户发送的码字和定时偏差,从而完成初始测距过程。改进频域算法在频域检测方法的基础上结合了串行干扰抵消的思想,提高了多用户同时接入系统时的正确检测概率,降低了虚警概率。通过仿真比较了改进算法和时域、频域算法的性能。结果表明,所提出的算法在计算复杂度和性能上兼有一定的优势。     

OFDM系统中基于载波干扰比最大的预编码方案

OFDM系统对频率的偏移非常敏感。一种简单的1/2码率的预编码方案可以有效地抑制载波间干扰(Inter-Carrier Interference,ICI),提高载波干扰比(Carrier-to-Interference Ratio,CIR)。该文提出了一种基于最大化载波干扰比原理的含参数的预编码方案。在估计系统频率偏移的情况下,这种方案可以通过简单的计算得到使载波干扰比最大的编码参数。实验证明,这种含有参数的预编码方案,可以有效地抑制载波间干扰,提高系统性能。