Timing Offset Estimation Research Articles

Low complexity timing synchronisation for high speed wireless local area networks

In this paper, we propose a low complexity coarse timing offset (CTO) and fine timing offset (FTO) estimation algorithm for the MIMO-OFDM-based 802.11n wireless local area network (WLAN) systems. For synchronisation purposes, the preamble of the high throughput mixed mode has a short training field (STF) and a long training field (LTF). Using the STF, an efficient coarse and fine timing estimation algorithm has been proposed for the mixed mode. Since the same STF structure is also used for the green field mode, the proposed technique is applicable for both modes. Effectively, a single coarse and fine timing estimation algorithm can be implemented for all the high throughput modes of 802.11n enabled systems. The performance of the proposed CTO estimator is compared with the performance of the conventional autocorrelation-based technique. Extensive simulation results show that the proposed CTO estimator performs better at lower signal-to-noise ratio (SNRs).

다중경로 페이딩 환경에서 OFDM 시스템을 위한 개선된 다중단계 타이밍 옵셋 추정기법

This paper proposes an improved multi-stage timing offset estimation scheme for orthogonal frequency division multiplexing (OFDM) systems in multipath fading channel environment. The conventional multi-stage timing offset estimation scheme is very sensitive to the random multipath components. By exploiting the sample standard deviation of the cross-correlation values, the proposed scheme achieves a robustness to the random multipath components. Simulation results demonstrate that the proposed scheme has a higher correct estimation probability and has a better mean square error (MSE) performance than the conventional scheme in multipath fading channels.

Data-aided synchronization algorithm dispensing with searching procedures for UWB communications

Rapid and accurate synchronization constitutes a major challenge in deploying Ultra-Wideband impulse radio (UWB-IR) for indoor wireless communications. This paper develops a data-aided (DA) synchronization algorithm dispensing with searching procedures for UWB-IR communications. By adopting a stream of alternating orthogonal pulses, frame-level synchronization can be achieved without channel information. Without utilizing any searching procedure, the timing offset estimator is obtained in a closed form. Because exhaustive search can be avoided, both computational complexity and synchronization time are reduced. Simulations and comparisons are provided to illustrate the advantages of our design over the alternatives.

Comments on "Timing Estimation and Resynchronization for Amplify-and-Forward Communication Systems

This comment first shows that the Cramer-Rao lower bound (CRLB) derivations in the above paper are not exact. In addition, contrary to the claims in the above paper, the assumptions of perfect timing offset estimation and matched-filtering at the relays affect the generality of the analytical results and cannot be justified.

ML Estimation of Timing and Frequency Offsets Using Distinctive Correlation Characteristics of OFDM Signals Over Dispersive Fading Channels

Orthogonal frequency-division multiplexing (OFDM) is a promising technology for communication systems. However, the synchronization of OFDM over dispersive fading channels remains an important and challenging issue. In this paper, a synchronization algorithm for determining the symbol timing offset and the carrier frequency offset (CFO) in OFDM systems, based on the maximum-likelihood (ML) criterion, is described. The new ML approach considers time-dispersive fading channels and employs distinctive correlation characteristics of the cyclic prefix at each sampling time. The proposed symbol timing estimation is found to be a 2-D function of the symbol timing offset and channel length. When compared with previous ML approaches, the proposed likelihood function is optimized at each sampling time without requiring additional pilot symbols. To practically realize the proposed method, a suboptimum approach to the ML estimation is adopted, and an approximate but closed-form solution is presented. Nonlinear operations of the approximate solution can be implemented using a conventional lookup table to reduce the computational complexity. The proposed CFO estimation is also found to depend on the channel length. Unlike conventional schemes, the proposed method fully utilizes the delay spread of dispersive fading channels (which usually reduces the accuracy of estimations). Furthermore, the Cramer-Rao lower bound (CRLB) on the CFO estimate is analyzed, and simulations confirm the advantages of the proposed estimator.

A Novel Timing Synchronization Method for OFDM Systems

In this letter, a novel timing offset estimation method is presented for orthogonal frequency division multiplexing (OFDM) systems. The proposed method is developed on the basis of the cyclic structure of OFDM symbol and uses a new noise subspace based metric to estimate the timing offset. Simulation results show that the proposed method has a significantly higher probability of correct estimation of the timing offset than the other methods in multipath channels.

Some interesting results on compatible BER analysis issues related to multi-band timing and frequency synchronizers applicable for MB-OFDM based UWB communications

A closed form expression of bit-error-rate (BER) has immense importance for insightful understanding on the performance of a communication system. In this paper we provide the BER analysis of a convolution coded MB-OFDM Ultra-Wideband (UWB) system that captures the log-normal fading statistics of UWB channels and also considers the estimation error variances of timing and carrier frequency offset (CFO) estimation by our already published synchronizers ATS (Sen et al., 2008 [1,2]) and MBAFS (Sen et al., 2008 [3,4]) respectively. By invoking the moment generating function (MGF) for log-normal fading statistics and using the Gauss-Hermite quadrature integration the average BER for rate Rc coded QPSK modulation with ATS and MBAFS synchronizers and channel estimation by least square (LS) method is obtained. We compare the analytical result with simulation results in highest delay spread UWB channel model CM4 and validate the consistency of our theoretical analysis.

Blind Coarse Timing Offset Estimation for CP-OFDM and ZP-OFDM Transmission over Frequency Selective Channels

We present a blind coarse timing offset estimation technique for CP-OFDM and ZP-OFDM transmission over frequency selective channels. The technique exploits the cyclic prefix or zero-padding structure to estimate the starting position of the OFDM symbols without requiring any additional pilots. Simulation results are performed on various channel models with timing and frequency offsets. The presented technique is compared with the autocorrelation-based technique and various techniques in frequency selective channels. Our algorithm shows better performance results than those of the autocorrelation-based technique in NLOS channels, where the most predominant channel path is usually not the first arrival path.

Residual symbol timing offset estimation for DRM+ system with cyclic delay diversity

In this study, we propose an enhanced residual symbol timing offset(STO) estimation scheme in an orthogonal frequency division multiplexing (OFDM) based digital radio mondiale plus (DRM+) system with cyclic delay diversity (CDD). By proper selection of the amount of cyclic delay and pilot subset partitioning, a robust and low-cost residual STO estimator is derived. By computer simulation, the proposed estimator is shown to benefit from properly designed delay parameter and pilot subsets.

Time and frequency offset estimation for distributed multiple-input multiple-output orthogonal frequency division multiplexing systems

This study addresses the problem of time and frequency offset estimation in the case of the delays, and the frequency offsets of all the transmitter/receiver pairs are different for distributed multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. A new training structure is designed and an efficient time and frequency offset estimator is proposed. With the symmetric conjugate property of the preamble, the timing metric proposed has an impulse shape, making the time offset estimation accurate. Using the repetition property of the preamble, iterative frequency offset estimation algorithm proposed can acquire a high precision and a wide estimation range. To distinguish different transmit antennas, the preamble is weighted by pseudo-noise (PN) sequences with low cross-correlations. The analysis and the simulation results show that the proposed estimator offers an accurate estimation on the different time delays and frequency offsets caused by the distributed transmitters of the MIMO-OFDM system. This is a situation that the conventional estimators cannot handle as far.

Maximum Likelihood Timing and Carrier Frequency Offset Estimation for OFDM Systems With Periodic Preambles

Symbol timing offset (STO) and carrier frequency offset (CFO) estimation are two main synchronization operations in packet-based orthogonal frequency division multiplexing (OFDM) systems. To facilitate these operations, a periodic preamble is often placed at the beginning of a packet. CFO estimation has been extensively studied for the case of two-period preambles. In some applications, however, a preamble with more than two periods is available. A typical example is the IEEE802.11a/g wireless local area network system, which features a ten-period preamble. Recently, researchers have proposed a maximum likelihood (ML) CFO estimation method for such systems. This approach first estimates the received preamble using a least squares method and then maximizes the corresponding likelihood function. In addition to the standard calculations, this method requires an extra procedure to solve the roots of a polynomial function, which is disadvantageous for real-world implementations. In this paper, we propose a new ML method to solve the likelihood function directly and thereby perform CFO estimation. Our method can obtain a closed-form ML solution, without the need for the root-finding step. We further extend the proposed method to address the STO estimation problem as well as derive a lower bound on the estimation performance. Our simulations show that while the performance of the proposed method is either equal to or better than the existing method, the computational complexity is lower.

Data-aided symbol timing and CFO synchronization for filter bank multicarrier systems

In this paper we consider the problem of data-aided joint symbol timing and carrier-frequency offset (CFO) estimation for filter bank-based multicarrier (FBMC) systems. As all multicarrier systems, FBMC systems are very sensitive to synchronization errors, since CFO and symbol timing errors cause interference between successive symbols and adjacent subcarriers which can lead to a severe performance degradation. Therefore, reliable and accurate synchronization algorithms must be designed for these systems. The approach herein presented is based on the deployment of appropriate training sequences. In particular, we propose a new joint symbol timing and CFO synchronization algorithm based on the least squares approach and exploiting the transmission of a training sequence made up of identical parts. The performance of the derived estimators, assessed by computer simulations, is compared with that of two data-aided synchronization algorithms previously proposed in the literature.

Time and Frequency Synchronisation in 4G OFDM Systems

This paper presents a complete synchronisation scheme of a baseband OFDM receiver for the currently designed 4G mobile communication system. Since the OFDM transmission is vulnerable to time and frequency offsets, accurate estimation of these parameters is one of the most important tasks of the OFDM receiver. In this paper, the design of a single OFDM synchronisation pilot symbol is introduced. The pilot is used for coarse timing offset and fractional frequency offset estimation. However, it can be applied for fine timing synchronisation and integer frequency offset estimation algorithms as well. A new timing metric that improves the performance of the coarse timing synchronisation is presented. Time domain synchronisation is completed after receiving this single OFDM pilot symbol. During the tracking phase, carrier frequency and sampling frequency offsets are tracked and corrected by means of the nondata-aided algorithm developed by the author. The proposed concept was tested by means of computer simulations, where the OFDM signal was transmitted over a multipath Rayleigh fading channel characterised by the WINNER channel models with Doppler shift and additive white Gaussian noise.

Timing synchronization using cross ambiguity function for MIMO OFDM systems with distributed antennas

In this letter, we propose a novel timing offset estimation method for multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) systems with distributed antennas using cross ambiguity function (CAF). The proposed algorithm has an impulse-shaped timing metric which allowed it to reduce the uncertainty due to the timing metric plateau. Simulation results show that the mean square error (MSE) performance of the proposed method is significantly improved.

An efficient initial ranging algorithm for WiMAX (802.16e) OFDMA

Ranging is one of the most important processes in the mobile WiMAX standard. Power adjustment, timing offset estimation, and synchronization between a base station (BS) and all users within a cell are done through the ranging process referred to as initial ranging. In this paper, we discuss the details of initial ranging together with some of the proposed algorithms, as well as a novel algorithm to carry out a successful ranging process. Performance curves and computational complexity comparisons are presented. The system performance is evaluated for both additive white Gaussian noise (AWGN) channels and practical multipath fading channels with multiuser interference. It is shown that the proposed algorithm offers a reduced complexity ranging method that can be employed in practical WiMAX-based BSs.

A New Blind Joint Timing and Frequency Offset Estimator for OFDM Systems Over Multipath Fading Channels

A blind joint timing and frequency synchronization algorithm is derived for orthogonal frequency-division multiplexing (OFDM) signals transmitting over multipath fading channels. The proposed estimator requires neither the knowledge of the signal-to-noise ratio (SNR) nor the power delay profile of the fading channels, except that the maximum delay spread <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> should be known and should be shorter than the guard interval. In the proposed estimator, the transmitted time-domain OFDM samples are modeled as an independent Gaussian random sequence, and the received time-domain OFDM samples are partitioned into a few subsets in such a way that the neighboring entries in each subset are uncorrelated. This is achieved by picking out OFDM samples that are spaced <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> samples apart into each subset. By modeling the channel gains as unknown deterministic variables, a joint symbol timing and frequency offset (FO) estimation algorithm is derived based on these subsets. Simulation results show that, in terms of lock-in probability, the proposed blind ST estimator achieves better performance than the estimators studied by Van De Beek and Speth , as well as the approximate maximum-likelihood estimator described by Lee and Cheun when the SNR is sufficiently high. For FO estimation, in terms of mean square error (MSE), the proposed estimator outperforms the estimators studied by Van De Beek and Speth at medium and high SNR. In terms of bit error rate (BER), the proposed estimator is superior to the estimators studied by Van De Beek and Speth Furthermore, the simulation results show that the performance of the proposed estimator is tolerant to the variation in the number of delay paths. This means that the proposed estimator will still adequately perform, even when the number of delay paths is time varying and is not estimated at the receiver. Finally, simulation results show that the use of virtual subcarriers has negligible performance loss in the proposed estimator.

Data-aided efficient synchronization for UWB signals based on minimum average error probability

One of the biggest challenges in ultra-wideband (UWB) radio is the accurate timing acquisition for the receiver. In this article, we develop a novel data-aided synchronization algorithm for pulses amplitude modulation (PAM) UWB systems. Pilot and information symbols are transmitted simultaneously by an orthogonal code division multiplexing (OCDM) scheme. In the receiver, an algorithm based on the minimum average error probability (MAEP) of coherent detector is applied to estimate the timing offset. The multipath interference (MI) problem for timing offset estimation is considered. The mean-square-error (MSE) and the bit-error-rate(BER) performances of our proposed scheme are simulated. The results show that our algorithm outperforms the algorithm based on the maximum correlator output (MCO) in multipath channels.

Statistical approximation based fine frequency synchronization for OFDM systems

The paper proposes a novel approach for fine frequency synchronization of OFDM synchronization 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.

A new ranging method for OFDMA systems

We present a new initial ranging method for OFDMA systems such as IEEE 802.16-2004. First, a new orthogonal ranging signal design is proposed by which an efficient, low-complexity multi-user ranging signal detection is developed. Based on the ranging signal detector results, power estimation for the detected ranging subscriber stations is performed. Then, by utilizing the proposed orthogonal ranging signal design together with the redundancy introduced by cyclic prefixes, a new iterative multi-user timing offset estimator is presented. Compared with the existing methods utilizing the CDMA-type ranging codes (in frequency-domain) defined in IEEE 802.16-2004, the proposed method achieves a better performance and greater robustness against multi-user interference and multi-path fading channels

ML-based symbol timing and frequency offset estimation for OFDM systems with noncircular transmissions

This paper deals with the problem of joint symbol timing and carrier-frequency offset (CFO) estimation in orthogonal frequency-division multiplexing (OFDM) systems with noncircular (NC) transmissions. Maximum-likelihood (ML) estimators of symbol timing and CFO have been derived under the assumption of nondispersive channel and by modeling the OFDM signal vector as a circular complex Gaussian random vector (C-CGRV). The Gaussian assumption is reasonable when the number of subcarriers is sufficiently large. However, if the data symbols belong to an NC constellation, the received signal vector becomes an NC-CGRV, i.e., a CGRV whose relation matrix (defined as the statistical expectation of the product between the vector and its transpose) is not identically zero. Hence, in this case, previously mentioned estimators, termed MLC estimators, are not ML estimators. In this paper, by exploiting the joint probability density function for NC-CGRVs, ML estimators are derived. Moreover, since their implementation complexity is high, feasible computational algorithms are considered. Finally, refined symbol timing estimators, apt to counteract the degrading effects of intersymbol interference (ISI) in dispersive channels, are suggested. The performance of the derived estimators is assessed via computer simulation and compared with that of MLC estimators and that of modified MLC (MMLC) estimators exploiting only ISI-free samples of the cyclic prefix