In the reverse link of OFDMA, different users (subcarriers) may have different frequency offsets, which causes severe performance degradation. It is also difficult to estimate/compensate these frequency offsets in the reverse link. Even if the frequency offsets are correctly estimated, the compensation of one subcarrier (or user) may worsen the inter-carrier interference from the frequency offsets of other subcarriers (users). One of the key ideas in this letter is to merge frequency offset compensation into the FFT matrix. The new inverse transform matrix is used in lieu of regular FFT. A single MMSE-based transform is used to perform both inverse Fourier transform and frequency offset compensation. Compared to the conventional methods such as circular convolution, this approach has lower computational complexity with comparable performance.

A scheme that jointly estimates carrier frequency offset and channel is proposed for the orthogonal frequency division multiplexing (OFDM) system. In the proposed scheme, the carrier frequency offset (CFO) and the channel state information (CSI) are first estimated by an minimum mean square error (MMSE) estimator and an maximum likelihood (ML) estimator, respectively. By exchanging the estimation information between these two estimators, the final estimation of CFO and CSI is then obtained by an iterative method. In the iterative process, the effect of imperfect CSI is considered. It can improve the estimation precision for a shorter preamble and accelerate the iterative convergence rate. To reduce the complexity of the proposed scheme, a procedure is adopted to eliminate the inverse operation of covariance matrix that is recalculated at each iteration. In addition, a sufficient condition for the convergence of the proposed method is deduced. The numerical simulation results show that the BER performance of our scheme is better than that of joint MLE for a shorter preamble and is comparable to that of joint MLE for a longer preamble. Furthermore, the average iterative time of our method is reduced by half as compared to the MLE methods without considering the effect of imperfect CSI.

In this letter, we propose an integer carrier frequency offset (IFO) estimator in the presence of symbol timing error for an ultra-wideband multi-band orthogonal frequency division multiplexing (UWB MB-OFDM) system. The proposed IFO estimator uses frequency-domain spreaded data symbol which is provided in the MB-OFDM system. To demonstrate the accuracy of the proposed IFO estimator, comparisons are made with conventional estimators via computer simulation.

A method that jointly estimates the carrier frequency offset (CFO), channel and symbol timing for orthogonal frequency division multiplexing (OFDM) is proposed in this letter. Based on the characteristic of cyclic training symbols in the frequency domain, the joint estimation is divided into three separate estimations. The CFO and equivalent channel impulse response (CIR) are first estimated by an iterative joint maximum likelihood estimation (JMLE), then the symbol timing offset (STO) is obtained by the assistance of equivalent CIR, finally the CIR is calculated based on the equivalent CIR after known STO and CFO. In our proposed method, the effect of imperfect CIR is considered in the CFO estimator. Moveover, a procedure, which eliminates the inverse operation of a covariance matrix at each iterative process, was adopted to reduce the complexity of our proposed method. Simulations show that the proposed method is capable of retaining the same bit error rate as joint CFO and CIR maximum likelihood estimation without symbol timing error.

This letter proposes a computationally efficient way of jointly estimating the residual frequency offset (RFO) and sampling frequency offset (SFO) by using a continual pilot (CP) defined in OFDM-based DVB-T system. In order to devise an unbiased joint frequency estimator in the current DVB-T system, a CP subset is selected to offset the effects of RFO and SFO simultaneously.

Orthogonal frequency division multiplexing (OFDM) is an attractive technique to accomplish wired or wireless broadband communications. Since it has been adopted as the terrestrial digital-video-broadcasting standard in Europe, it has also subsequently been embedded into many broadband communication standards. Many techniques for frame timing and frequency synchronization of OFDM systems have been studied as a result of its increasing importance. We propose a new technique of simultaneously synchronizing frame timing and frequency utilizing matched filters. First, a new short preamble consisting of short sequences multiplied by a DBPSK coded sequence is proposed. Second, we show that the new short preamble results in a new structure for matched filters consisting of a first matched filter, a DBPSK decoder, and a second matched filter. We can avoid the adverse effects of carrier frequency offset (CFO) when frame timing is synchronized because a DBPSK decoder has been deployed between the first and second matched filters. In addition, we show that the CFO can be directly estimated from the peak value of matched filter output. Finally, our simulation results demonstrate that the proposed scheme outperforms the conventional schemes.

In this paper, we present a new all-digital carrier recovery loop for high-order quadrature amplitude modulation (QAM) signal constellations. The proposed approach is a blind phase-frequency detector structure that consists of a phase detector, a phase offset estimator, a frequency offset estimator, and a digital control oscillator. Compared to previous related approaches, the proposed algorithm provides a wider acquisition range and a more accurate estimation of frequency and phase offsets. These features are demonstrated by simulation results of the DOCSIS (Data-Over-Cable Service Interface Specifications) cable modem system.

In this letter, the problem of attaining frequency synchronization for high rate WPANs is considered. An estimation of the frequency offset is essential step at the receiver because the frequency offset means that the phase of transmitted signal is corrupted. The proposed algorithm utilizes the difference in phase between received preamble and estimated preamble. It turns out that the proposed algorithm has low complexity and good performance. To enhance the capability, we use peak phase detector. The performance of the proposed algorithm is assessed by computer simulation, and the frequency offset variance is compared to Cramer Rao Bound.

In this paper, we propose a statistical method of time synchronization for computer clocks that have precisely frequency-synchronized oscillators. This method not only improves the accuracy of time synchronization but also prevents degradation in the frequency stability of the precise oscillators when the errors in the measured time offsets between computer clocks caused by network traffic possess a Gaussian distribution. Improved accuracy of time synchronization is achieved by estimating the confidence interval of the measured time offsets between the clocks. Degradation in frequency stability is prevented by eliminating unnecessary time correction for the computer clock, because time correction generally causes changes in the frequency accuracy and stability of the precise oscillators. To eliminate unnecessary time correction, our method uses an extended hypothesis test of the difference between the current mean and the mean at the last time adjustment to determine whether time correction is needed. Evaluation by simulating changes in the time offset of the existing ISDN clock synchronization system showed that this method achieves accurate time and stable frequency synchronization.

Recently, there has been increasing interest in providing high quality and efficient broadband services over wireless and mobile links. Space-time code is designed to exploit multiple-input multiple-output antenna systems and by doing so an enormous increase in the capacity of wireless systems can be achieved. In this letter, a synchronization technique is proposed to improve the performance of multiple-input multiple-output system. The interpolation method is employed to estimate the coarse and fine frequency offset at the same time without additional complexity.

In this paper, we propose a modified CP-AFC (Cross-Product Automatic Frequency Control) algorithm to enhance coherent signal detection for WCDMA reverse link receiver. We introduce a moving average filter at the FDD input to decrease the noise effect by increasing the number of cross-products, since pilot symbol in WCDMA is not transmitted continuously. We also add normalization algorithm to overcome the conventional CP-FDD's sensitivity to the variance of input signal amplitude and to increase the linear range of S-curve. For rapid frequency acquisition and tracking, we adopt a multi-stage tracking mode. We applied the proposed algorithm in the implementation of WCDMA base station modem successfully.

This paper presents a cell search scheme embedded with carrier frequency synchronization for inter-cell asynchronous orthogonal frequency-division multiplexing code-division multiplexing (OFDM-CDM) systems. Several subcarriers are dedicated to a differentially encoded synchronization channel (SCH). In the other subcarriers, data symbols and pilot symbols are two-dimensionally spread in the time-frequency domain. The cell search scheme consists of a three-stage cell search and a two-stage carrier-frequency synchronization, that is, coarse carrier-frequency acquisition, fast Fourier transform window-timing detection, SCH frame-timing detection, fine carrier-frequency synchronization, and cell-specific scrambling code (CSSC) identification. Simulation demonstrated that this scheme can identify the CSSC with high detection probability while precisely synchronizing the carrier frequency in severe frequency-selective fading channels.

This paper proposes a novel space diversity reception scheme suitable for packet-based orthogonal frequency division multiplexing (OFDM) wireless access systems that achieves large diversity gain by improving the accuracy of both carrier frequency synchronization and phase tracking. Phase tracking compensates the phase rotation caused by residual carrier frequency error and phase noise and is necessary for high data rate OFDM systems that use coherent detection. In the proposed scheme, the accuracy of carrier frequency synchronization is improved by combining the information of the carrier frequency offset detected on all diversity branches; the accuracy of phase tracking is improved by using pilot signals whose signal to noise ratio (SNR) is raised by maximal ratio combining of the pilot signals extracted from all branches. Computer simulation results show that the proposed scheme effectively improves the diversity gain even in severe environments such as those with low carrier to noise ratios (CNR) and large delay spreads.

The frequency-offset estimation of a wireless asynchronous transfer mode (ATM) modem based on the orthogonal frequency division multiplexing (OFDM) is discussed. For burst synchronization operating under the time-division multiple access (TDMA) medium access control (MAC), an OFDM preamble using a multistage frequency-offset estimation technique is proposed and shown to have a good accuracy with a large estimation range. And also an averaged decision-directed channel estimation (ADDCE) technique suitable for burst data is proposed.