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

In recent years, the constant-envelope orthogonal frequency-division multiplexing (CE-OFDM) has been considered as a candidate waveform in broadband satellite systems as it has a 0-dB peak-to-average power ratio (PAPR). However, the carrier frequency offset (CFO) estimation scheme for CE-OFDM broadband satellite systems directly affects system performance. In this paper, we analyze the network architecture and the propagation environment of CE-OFDM broadband satellite systems, and we propose a data-aided CFO estimation strategy based on the frequency domain pilot symbols. The Cramer–Rao bound (CRB) of our CFO estimator is given by mathematical analysis, and the effect of the number of pilot symbols on the estimation performance is analyzed. The pilot symbol-based CFO estimator is composed of a phase demodulator and a discrete Fourier transform (DFT) module, and it can obtain a large estimation range under a small pilot overhead. The simulation results show that the CE-OFDM broadband satellite systems can achieve a good bit error rate (BER) performance by using the proposed strategy to estimate and compensate the CFO.

Orthogonal frequency division multiplexing (OFDM) has been adopted in various wireless communication standards to exploit its high spectral efficiency and robustness against multipath fading channels. Along with the potential benefits of OFDM transmissions, a stringent requirement on orthogonality among subcarriers makes OFDM highly susceptible to synchronization errors, i.e., time and frequency offsets. Addressing the carrier frequency offset (CFO) estimation problem, the paper proposes a pilot-aided CFO estimation scheme for both coarse and fine carrier frequency synchronization in single-input single-output (SISO) OFDM receivers. By exploiting the maximum likelihood (ML) principle, the proposed ML-based CFO estimation algorithm functions properly over a wide range of CFO values and outperforms existing correlation-based schemes.

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.

A novel blind carrier frequency offset (CFO) estimation method for orthogonal frequency division multiplexing (OFDM) systems using constant modulus constellations is proposed in this study. CFO is estimated by minimizing a hybrid cost function which is formulated using properties of the covariance matrix and the magnitude of two nearby OFDM symbols. The performance of the proposed method is compared with that of the conventional estimation techniques under noisy time varying multipath channels. The performance comparison for different Doppler frequencies is also investigated. The superiority in performance of the proposed method over conventional methods is illustrated based on Monte Carlo simulations.

In this paper, we propose a pilot aided carrier frequency offset (CFO) estimation schemes for the Faster-than Nyquist (FTN) transmission system. The FTN transmission would increase the data rate by overlapping the pulse closer than the Nyquist rate. For this reason, there have been some efforts to apply FTN theory to second generation of digital video broadcasting satellite (DVB-S2) standard. However, the FTN produce inter symbol interference (ISI), which would deteriorate the receive performance even in an additive white Gaussian noise (AWGN) channel. In addition, CFO that exist due to transmitter and receiver oscillator mismatch is a major concern in communication system. In this paper, we present the maximum likelihood (ML) CFO estimator for the FTN transmission system. The mean square error (MSE) performance of the CFO estimator utilizing DVB-S2 pilot block sequences is also presented. Through the Monte Calro simulation, the BER performance degradation due to CFO error is investigated both uncoded and coded FTN system.

The paper proposes a novel estimation scheme of carrier frequency offset (CFO) based on Added Preamble Sequence Semi-Blind (APSSB) scheme. The proposed CFO estimator is based on APSSB OFDMA model in which Gold sequence is added to the output of IDFT modulator and exploited for CFO estimation at the receiver. CFO estimators based on APSSB scheme and preamble are compared by both SINR analysis and simulation. We propose that the APSSB scheme should be employed for fine frequency synchronization in OFDMA especially in delay- insensitive service. In this paper, we propose the APSSB OFDMA model in which APSSB sequence is added to the output of IFFT modulator. The APSSB sequence is exploited for CFO estimation by correlation detection with the local APSSB sequence at the receiver. Comparison of SINR is made between CFO estimation schemes based on APSSB and preamble. More accurate CFO estimation by APSSB scheme doesn't cause date rate decrease because data and APSSB sequence are transmitted simultaneously. Furthermore, CFO estimation of APSSB scheme is more accurate than preamble scheme when APSSB symbol number for CFO estimation is large enough to exceed the threshold. Furthermore, the estimation time of APSSB scheme is more flexible than preamble scheme. The APSSB scheme has the property of better accuracy as well as larger delay. We propose the APSSB scheme should be employed for fine frequency synchronization in delay- insensitive service.

In this paper, a fine carrier frequency offset (CFO) estimator is proposed for ultra-wideband multi-band orthogonal frequency division multiplexing (UWB MB-OFDM) systems. The proposed scheme uses a frequency domain spreaded data symbol which is provided in the MB-OFDM system. To demonstrate the efficiency of the proposed CFO estimator, comparisons are made with other existing estimator in terms of mean square error (MSE) and bit error rate (BER). Indexing Terms : UWB, MB-OFDM, carrier frequency offset, frequency-domain spreading

In orthogonal frequency division multiplexing (OFDM) systems, most of the conventional sampling frequency offset (SFO) estimation methods work under the assumption of time-invariant or slow time-variant channels. In mobile environment, the time-variant channel significantly degrades the accuracy of SFO estimation. To solve the problem, we first analyze the properties of time-variant channels. If terminal moves within some tens of the wavelength of radio frequency (RF) signal, channel path delay almost remains unchanged. For most practical OFDM systems, our analysis indicates that channel path delay can be regarded as unchanged during the interval of some tens of OFDM symbols in time-variant channels. Based on the analysis, we propose a novel SFO estimation method for pilot-aided OFDM systems. Different from the conventional methods, the proposed method estimates SFO by detecting the variation of the symbol timing error caused by SFO. The detection is finished by implementing correlation between the channel impulse responses (CIRs) estimated by different OFDM symbols. Performance of the proposed method is simulated and compared with that of two conventional post-FFT methods. Numerical results show that, the proposed SFO estimation method performs better than the conventional methods not only in time-variant channels, but also at low SNRs and large residual carrier frequency offsets (CFOs).

This paper is to investigate the carrier frequency offset estimation for an orthogonal frequency division multiplexing (OFDM) communication system. Originating from the Fourier transform, OFDM uses the modulation of the amplitude and phase of subcarriers for communications. We propose a new approach that use the bit-error rate (BER) and the minimum square error (MSE) of the pilot as merits to achieve the carrier frequency offset (CFO) estimation of OFDM. The proposed method achieves lower root mean square error (RMSE) of CFO and BER on OFDM compared with conventional methods.

Multiple Input Multiple Output (MIMO) system with orthogonal frequency division multiplexing (OFDM) is a promising physical layer candidate for the current and future wireless standards. MIMO increases system throughput while OFDM maximizes spectrum efficiency. To reduce the impacts of frequency selective fading and increase the capacity, MIMO requires precise channel state information (CSI). The presence of carrier frequency offset (CFO) introduces intercarrier interference (ICI) among the subcarriers which degrades the system performance. MIMO channel estimation is done under the assumption of perfect frequency synchronisation between the transmitter and receiver, or zero CFO. However, such an assumption is incorrect for time-varying and frequency-selective wireless channels, and the presence of CFO has an impact on channel estimation accuracy. In the existing methods, CFO and channel estimations are carried out independently using separate training sequences which reduces the spectral efficiency. As a result, the pilot aided interpolation approach is proposed in this paper for the joint estimate and compensation of CFO and channel effects using the same set of pilot tones. Theresults show that the proposed method offers precise CFO and channel estimation which improves the bit error rate performance and carrier to interference power ratio (CIR) of MIMO-OFDM system.

This study deals with the joint channel and carrier frequency offset (CFO) estimation in a Multiple Input Single Output (MISO) communications system. This problem arises in OFDM (Orthogonal Frequency Division Multiplexing) based multi-relay transmission protocols such that the geo-routing one proposed by A. Bader et al in 2012. Indeed, the outstanding performance of this multi-hop relaying scheme relies heavily on the channel and CFO estimation quality at the PHY layer. In this work, two approaches are considered: The first is based on estimating the overall channel (including the CFO) as a time-varying one using an adaptive scheme under the assumption of small or moderate CFOs while the second one performs separately, the channel and CFO parameters estimation based on the considered data model. The two solutions are analyzed and compared in terms of performance, cost and convergence rate.

ABSTRACTWhile inter-symbol interference is not of concern, inter-carrier interference that is caused by carrier frequency offset (CFO) is more concerned in orthogonal frequency division multiplexing (OFDM) systems. The estimation of channel impulse response (CIR) is another important parameter of interest in the system. Recently, a number of approaches for the joint estimation of CFO and CIR in OFDM systems have been proposed in the literature, and most of them are traditional maximum-likelihood-based approaches. In this paper, we propose a number of mixture cost-reference particle filtering (CRPF) approaches for joint estimation of CIR and CFO, and a couple of them can be applied without the knowledge of both state and measurement noise statistics. Conventional least mean squares algorithm or recursive least squares algorithm is jointly employed with recently developed CRPF for nonlinear and multiple parameters estimation in OFDM systems. The computer simulation results show that mixture CRPFs are more robust against particle impoverishment (PI) problem than standard mixture particle filtering (PF) and mixture regularized PF (RPF) approaches although RPF was particularly developed to tackle the PI problem. In this paper, we investigate a number of mixture PF approaches including ones that do not need the information of noise statistics for the joint estimation of CFO and channel in OFDM systems.

In this paper, we develop a new subspace based multiuser carrier frequency offset (CFO) estimation scheme for multi-input multi- output (MIMO) orthogonal frequency division multiple access (OFDMA) uplink transmission. We exploit the rank reduction approach by equipping multiple antennas at the receiver, in which the CFO of each user is derived blindly using one dimension (1-D) search individually. The proposed scheme supports the generalized subcarrier assignment scheme and full loaded transmission with all subcarriers being allocated to users. Numerical results are provided to corroborate the proposed studies.

The rapid growth and development of wireless data transmission brings new trends, challenges and constraints in the telecommunications field. Novel multicarrier techniques are proposed to meet these requirements. The orthogonal frequency division multiplexing (OFDM) is an attractive technique that achieves high data rate and high spectral efficiency and, at the same time, suppresses multipath fading in wireless communications. However, the OFDM is very sensitive to mobility of the transmitter, receiver or surrounding objects. The mobility produces interchannel interference which significantly deteriorates transmission performance. The interchannel interference suppression represent research challenge. Carrier frequency offset (CFO) estimation and its compensation represent a way to improve OFDM system's performance. In this paper, we consider CFO estimation in the frequency domain by using well known iterative algorithms for precise estimation of spectral peak's central peak frequency. Simulations show that the proposed method outperforms standard CFO estimation methods in terms of accuracy. In simulations, both AWGN channel and multipath channel are considered.

OFDM (orthogonal frequency division multiplexing) is a promising technology for future broadband wireless communications due to its capability in combating multipath fading. Carrier frequency offset (CFO), which can induce the loss of orthogonality among OFDM sub-carriers and significant performance degradation, needs to be estimated and compensated for. In this paper, we propose a general CFO estimator based on the maximum likelihood (ML) estimation criterion. It will be shown that with the same estimator architecture, CFO can be obtained using either training OFDM symbols, pilot tones, null sub-carriers, or a combination of them. Furthermore, by taking advantage of the channel side information, the performance of CFO estimation can be significantly improved. To demonstrate the capability of the proposed CFO estimator, we will consider an OFDM system using the signal structure of the IEEE WLAN standard 802.11a. Compared with previous work using null sub-carriers alone, it will be shown that by taking advantage of the pilot tones, null sub-carriers, and the channel statistics, the performance of CFO estimation can be improved by about 2 dB