Carrier Frequency Offset Estimation Scheme Research Articles

Pilot-Assisted Carrier Frequency Offset Estimation for OFDM systems with Multiple Preambles Over Fast Fading Channels

This paper proposes a carrier frequency offset (CFO) estimation scheme for OFDM systems over fast fading channels. In the proposed scheme, exploiting the multiple preambles with the identical values, we estimate the CFO over fast fading channels. In particular, we improve the performance of the CFO estimator by applying the overlapped windows to preambles. Through simulations, we validate the proposed estimation scheme by showing the effectiveness of the proposed estimator.

Estimation of OFDM Carrier Frequency Offset Robust to Offset Variation

In this paper, we propose a novel carrier frequency offset (CFO) estimation scheme for orthogonal frequency division multiplexing (OFDM) systems. The proposed scheme has much wider estimation range compared with that of the conventional scheme, making it more robust to the fractional FO variation. Numerical results demonstrate that the proposed scheme has better estimation performance than the conventional scheme for wider fractional FO range.

Time and Frequency Synchronization for Multicarrier Transmission on Hexagonal Time-Frequency Lattice

In this paper, a novel synchronization acquisition approach, including preamble structure, time and frequency synchronization algorithms, is proposed for hexagonal multicarrier transmission (HMCT) system. Specifically, a novel signal acquisition scheme based on time-frequency subspace projection (TFSP) and its simplified form are proposed for reliable signal acquisition. Time and frequency synchronization is divided into three steps: 1) the coarse time offset (TO) and coarse fractional carrier frequency offset (CFO) estimation; 2) fine TO and integer CFO estimation; and 3) fine fractional CFO estimation. The closed-form Cramér-Rao lower bound of the proposed CFO estimation scheme is given. Theoretical analyses and simulation results show that the proposed preamble structure, time and frequency synchronization algorithms for HMCT system can obtain robust performance over doubly dispersive (DD) channel, and the simulation results show excellent agreement with theoretical analyses.

Blind Carrier Frequency Offset Estimation for OFDM Systems with Constant Modulus Constellations

This paper proposes a blind carrier frequency offset (CFO) estimation scheme for OFDM systems. In the proposed scheme, the covariance matrix of the received signal is obtained through the circular shifts of OFDM blocks in the time-domain. From the fact that the covariance matrix has a banded structure in the absence of CFO, we estimate CFO through minimizing the powers of the elements that are outside the band. We show that the proposed scheme is better than the conventional schemes through simulations.

비정규 충격성 잡음에서 OFDM 기반 인지 무선 시스템을 위한 주파수 옵셋 추청 기법

주파수 부족 문제를 해결하기 위하여 무선 통신 환경을 인지하여 활용할 수 있는 인지 무선(cognitive radio: CR) 시스템이 크게 주목받고 있다. 이러한 CR 시스템을 위한 변조 기법으로서 (orthogonal frequency division multiplexing: OFDM) 기술이 가장 유력하게 제안되고 있다. 하지만 CR 시스템이 사용되는 주파수 대역에는 인위적인 충격성 잡음, 간섭 등으로 인한 비정규 잡음이 존재하며, 이로 인해 정규 잡음을 기반으로 하는 OFDM 주파수 옵셋 추정 기법들은 심각한 성능 저하를 겪는다. 본 논문에서는 OFDM 기반 CR 시스템을 위한 최대 우도 주파수 옵셋 추정 기법을 제안하였으며, 또한 적은 개수의 시행 값을 사용함으로써 낮은 복잡도를 갖는 준최적 주파수 옵셋 추정 기법을 제안하였다. 모의실험을 통해 제안한 추정 기법들이 기존 추정 기법보다 비정규 충격성 잡음 환경에서 강인하고, 우수한 성능을 가짐을 확인하였다. Cognitive radio (CR) systems have received significant interest as a promising solution to the spectral shortage problem through efficient use of the frequency spectrum by opportunistically exploiting unlicensed frequency bands. Orthogonal frequency division multiplexing (OFDM) is widely regarded as a highly promising candidate for CR systems. However, the frequency bands used by CR systems are expected to suffer from non-Gaussian noise, which considerably degrades the performance of the conventional OFDM carrier frequency offset (CFO) estimation schemes. In this paper, robust CFO estimation schemes for OFDM-based CR systems in non-Gaussian channels are proposed. Simulation results demonstrate that the proposed estimators offer robustness and substantial performance improvement over the conventional estimator.

Analysis of Fine Frequency Synchronization for OFDM in Time-Varying Channels

This paper evaluates the performance of a pilot-assisted fine carrier frequency offset (CFO) estimation scheme for orthogonal frequency division multiplexing (OFDM) in time-varying channels. An analytical closed-form expression of the mean square error (MSE), of the post-FFT based CFO synchronization scheme, is presented in terms of time-variant fading channels. To verify our analysis in this paper, simulations are carried out within the framework of mobile WiMAX systems.

CFO Estimation and Compensation in SC-IFDMA Systems

Single carrier interleaved frequency division multiple access (SC-IFDMA) has been recently receiving much attention for uplink multiuser access in the next generation mobile systems because of its lower peak-to-average transmit power ratio (PAPR). In this paper, we investigate the effect of carrier frequency offset (CFO) on SC-IFDMA and propose a new low-complexity time domain linear CFO compensation (TD-LCC) scheme. The TD-LCC scheme can be combined with successive interference cancellation (SIC) to further improve the system performance. The combined method will be referred to as TD-CC-SIC. We shall study the use of user equipment (UE) ordering algorithms in our TD-CC-SIC scheme and propose both optimal and suboptimal ordering algorithms in the MMSE sense. We also analyze both the output SINR and the BER performance of the proposed TD-LCC and TD-CC-SIC schemes. Simulation results along with theoretical SINR and BER results will show that the proposed TD-LCC and TD-CC-SIC schemes greatly reduce the CFO effect on SC-IFDMA. We also propose a new blind CFO estimation scheme for SC-IFDMA systems when the numbers of subcarrier sets allocated to different UEs are not the same due to their traffic requirements. Compared to the conventional blind CFO estimation schemes, it is shown that by using a virtual UE concept, the proposed scheme does not have the CFO ambiguity problem, and in some cases can improve the throughput efficiency since it does not need to increase the length of cyclic prefix (CP).

Robust carrier frequency offset estimation for OFDM systems in doppler shift channels

In this paper, a robust carrier frequency offset (CFO) estimation scheme is proposed for OFDM systems in Doppler shift channels. We first present a training symbol (TS) pattern that has the same value so as to provide a wide CFO estimation range and to ensure the accuracy of the CFO estimation. Using the TS, we then present maximum-Likelihood (ML) and averaging least square (LS) algorithms for performing acquisition and tracking steps of the CFO estimation. The performance of the proposed technique is evaluated with respect to a mean-square-error (MSE) and a bit-error-rate (BER) in a typical urban (TU) channel.

New joint frame synchronisation and carrier frequency offset estimation method for OFDM systems

AbstractWe propose a new joint frame synchronisation and carrier frequency offset estimation scheme for burst transmission mode OFDM systems. This scheme uses a central‐symmetric and comb‐like (CSCL) training sequence, which eases the power detection at the receiver without increasing the total training sequence power. Fine frame synchronisation as well as carrier frequency offset acquisition with a maximum acquisition range of $\pm {{N} \over {4\times {\rm SF}}}$ times the sub‐carrier spacing can also be performed based on the proposed CSCL training sequence, where N is the discrete Fourier transform (DFT) length and SF is an integer‐valued spreading factor used to generate CSCL. The post‐acquisition residual carrier frequency offset can be further estimated and corrected via a fine adjustment algorithm. In order to reduce performance loss due to the high peak‐to‐average power ratio (PAPR) of the CSCL training sequence, a time‐domain constant‐envelope (CE) training sequence is also proposed. The superior estimation accuracy of the proposed algorithm over that of the Moose algorithm and the SS (Shi and Serpedin) algorithm is proved by computer simulation. Copyright © 2008 John Wiley & Sons, Ltd.

On the performance of a maximum likelihood method with delayed correlation for the coarse carrier frequency offset estimation of OFDM signals with multiple preamble symbols

Estimation of carrier frequency offset (CFO) is an important issue in the design of a wireless receiver that employs orthogonal frequency division multiplexing (OFDM) techniques. In this paper, using the ten short training symbols specified in the signal format of the IEEE 802.11a WLAN, we investigate the performance of a coarse CFO estimation scheme for OFDM signals with multiple preamble symbols. This scheme, which we call DC-ML, employs the maximum likelihood (ML) method with delayed correlation (DC). For AWGN channels under moderate signal to noise ratio (SNR) conditions, we develop an analysis to evaluate the variance of estimation error (VEER). The analysis is corroborated in light of simulations, and compared with the formulated Cramer-Rao lower bound (CRLB). VEER of the DC-ML in a multipath environment is studied via simulations. Numerical results show that a certain parameter combination can result in minimum VEER. Simulation results justify that the probability of estimation error (PEER) approximates Gaussian distribution in both AWGN and multipath scenarios. We also present a Two-Branch DC-ML (TBDC-ML) scheme, which comprises two correlation branches of DC-ML, and an associative ambiguity resolution algorithm. Numerical examples reveal that TBDC-ML outperforms DC-ML in both VEER and PEER. Assuming that the estimation error resulting from the two branches is jointly Gaussian, we derive the joint probability density function (pdf) and validate it via simulations.

CFO estimation schemes for differential OFDM systems

This paper proposes two blind carrier frequency offset (CFO) estimation schemes for differentially modulated orthogonal frequency division multiplexing (OFDM) systems. The proposed schemes estimate the fractional part of the CFO with only two consecutive OFDM blocks, and they exploit two implicit properties associated with differentially modulated OFDM (DOFDM) systems, i.e., the channel keeps constant over two consecutive OFDM blocks, and the DOFDM systems employ an M-ary phase-shift keying constellation. One of the schemes is based on the finite alphabet (FA) constraint and the other one is based on the constant modulus (CM) constraint. They provide a trade-off between the performance and computational complexity. The constrained Cramer-Rao lower bound is also derived. Several numerical examples are presented to validate the efficacy of the proposed schemes.

OFDM Carrier Synchronization Based on Time-Domain Channel Estimates

Carrier frequency synchronization is critical to the quality of signal reception in OFDM systems. This paper presents an approximate maximum-likelihood (ML) carrier frequency offset (CFO) estimation scheme based on time-domain channel estimates which retain the CFO information in the form of phase rotation. The proposed ML CFO estimate is investigated under static as well as time-varying fading channels. Statistical properties of the estimator are examined and Cramer-Rao lower bound (CRLB) is derived. Theoretical analysis and numerical simulations show that the proposed CFO estimator renders excellent performance with lower computational complexity. The proposed CFO estimate also has an advantage of allowing for more flexible pilot patterns.

A blind carrier frequency offset estimation scheme for OFDM systems with constant modulus signaling

This paper presents a new blind carrier frequency offset (CFO) estimation scheme for orthogonal frequency division multiplexing (OFDM) systems with constant modulus (CM) signaling. Both single-input single-output (SISO) systems and multiple-input multiple-output (MIMO) systems with orthogonal space-time block coding are considered. The proposed scheme is based on the reasonable assumption that the channel frequency response changes slowly in the frequency domain, which implies that the channel frequency response on two consecutive sub- carriers is approximately the same. Based on this assumption, cost functions are derived in closed-form, which minimize the difference between the signal power of two neighboring subcarriers. The identifiability of the proposed scheme is mathematically proved, which implies that minimizing the derived cost function gives an approximate estimate of the CFO. We demonstrate that the proposed scheme provides an excellent trade-off between complexity and performance as compared to prominent existing estimation schemes.

AN ELABORATE FREQUENCY OFFSET ESTIMATION FOR OFDM SYSTEMS

In this paper, a carrier frequency offset estimation scheme in orthogonal frequency division multiplexing is proposed. We focus on increasing the correlation of each PN sequence code repeated in one preamble. We aim at getting an efficiency like using many preambles. The proposed method can improve the performance of the system by estimating fine frequency offset. Also, the proposed method enhances reliability by maximizing the number of correlations compared with the established method in time domain.

A New OFDM Synchronization Symbol for Carrier Frequency Offset Estimation

This letter proposes a new data-aided carrier frequency offset (CFO) estimation scheme for orthogonal frequency division multiplexing (OFDM) communications suitable for frequency-selective channels. The proposed method is based on the transmission of a specially designed synchronization symbol that generates a particular signal structure between the received observation samples at the receiver. This structure is exploited to derive a closed-form expression of the CFO. The proposed solution offers a wide acquisition range with reduced computational load. Simulations over frequency-selective channels confirm the superiority of the proposed method compared to recent data-aided synchronization algorithms

Joint Frame Synchronization and Frequency Offset Estimationin OFDM Systems

A new joint frame synchronization and carrier frequency offset estimation scheme in orthogonal frequency division multiplexing (OFDM) systems is proposed in this paper, where both frame synchronization and carrier frequency offset estimation can be performed by using only ONE training symbol. Frame synchronization and carrier frequency offset acquisition are performed simultaneously in the proposed scheme. Reliable frame synchronization is obtained in the proposed scheme even in low SNR. The maximum carrier frequency offset acquisition range of the proposed scheme can be up to one half of the total signal bandwidth. The same training symbol can also be utilized to carrier frequency offset Fine Adjustment, which estimates the remaining carrier frequency offset after acquisition with higher accuracy. The performance comparison of the proposed Fine Adjustment algorithm and Schmidl's algorithm by using computer simulation illustrates and verify the superior performance of the proposed algorithm with regard to estimation accuracy.

EM-Based Iterative Receiver Design With Carrier-Frequency Offset Estimation for MIMO OFDM Systems

In this letter, we study the design of expectation-maximization (EM)-based iterative receivers for multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing systems with the presence of carrier-frequency offset (CFO). Motivated by the spirit of maximum-likelihood estimation in the EM algorithm, we first present a pilot-aided CFO estimation scheme that allows fast Fourier transform-based fast implementation. Then this CFO estimation is incorporated into the initialization step of the iterative receiver. Experimental results show the effectiveness of our receiver design in combating CFO.

High accuracy frequency offset correction with adjustable acquisition range in OFDM systems

A new carrier frequency offset estimation scheme in orthogonal frequency-division multiplexing (OFDM) systems is proposed in this paper. Both the carrier frequency offset acquisition and tracking are based on a fixed-length training-symbol-block, which consists of multiple small identical training symbols. When each training symbol is shortened, the number of training symbols in the training-symbol-block should be increased accordingly to keep the total training-symbol-block length fixed. The proposed scheme extends Moose's estimator, where the estimation error is only dependent on total training symbol energy and cannot be reduced any more, once the total training symbol energy is determined. The proposed scheme can shorten each training symbol in a training-symbol block and select an appropriate estimator simultaneously, which can lead to further reduction of estimation error and increase of acquisition range, even with the total training-symbol-block energy being fixed. Performance analyzes for the proposed scheme in both the additive white Gaussian noise channel (AWGN) and the multipath channel are also presented in this paper. All estimators in the proposed scheme are conditionally unbiased, and simulation results demonstrate that they can work well both in the multipath channel and in the AWGN channel.

Complex Efficient Carrier Frequency Offset Estimation Algorithm in OFDM Systems

A new carrier frequency offset estimation scheme in the orthogonal frequency division multiplexing (OFDM) system is proposed. The proposed algorithm is an extension of the Michele Morelli (M&M) algorithm. By dividing one training symbol into L>1 identical small blocks, the carrier frequency offset estimation range up to /spl plusmn/L/2 times subcarrier spacing can be obtained. The proposed algorithm can utilize the correlativity among all small blocks of a training symbol more sufficiently than the M&M algorithm, and thus it is more accurate and robust. Without increasing the estimation errors, the computational complexity of the proposed algorithm can be further reduced by increasing parameters H/sub l/ and H/sub h/ with a positive value simultaneously.

Frequency Offset Estimation With Fast Acquisition in OFDM System

A new carrier frequency offset estimation scheme in orthogonal frequency-division multiplexing (OFDM) system is proposed. The carrier frequency offset estimation includes acquisition and tracking, and the acquisition range is as large as one half of overall signal bandwidth. The proposed tracking estimator is a maximum-likelihood estimator, and in AWGN channel, the Cramer-Rao lower bound is met at high signal-to-noise ratio (SNR); in multipath channel, the tracking algorithm works well at moderate SNR. Timing synchronization can be also performed during the course of acquisition.