Tilde Fusco

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.

Blind Frequency-Offset Estimation for OFDM/OQAM Systems

This paper deals with the problem of blind carrier-frequency offset (CFO) estimation in orthogonal frequency-division multiplexing (OFDM) systems based on offset quadrature amplitude modulation (OFDM/OQAM). Specifically, by assuming that the number of subcarriers is sufficiently large, the received signal is modeled as a complex Gaussian random vector (CGRV). Since the OFDM/OQAM signal results to be a noncircular (NC) process (i.e., its relation function is different from zero), by exploiting the generalized probability density function for NC-CGRVs, the unconditional maximum-likelihood algorithm for CFO estimation in nondispersive channel is derived. Moreover, due to its computational complexity, a more feasible estimator is considered. The performance of the proposed estimators, assessed via computer simulation, is compared with that of recently proposed CFO estimators exploiting second-order cyclostationarity and with the Gaussian Crameacuter-Rao bound

Sensitivity of multi-user filter-bank multicarrier systems to synchronization errors

In this paper we investigate the sensitivity of multiuser filter-bank multicarrier (FBMC) systems to synchronization errors in both the uplink and the downlink scenarios. In particular, the performance degradation caused by synchronization errors is evaluated analytically and with numerical simulations and compared with that of orthogonal frequency division multiplexing (OFDM) systems. It is shown that, as all multicarrier systems, FBMC systems are very sensitive to synchronization errors, since timing errors and carrier frequency offsets produce intercarrier interference and interference between successive symbols. However, in an asynchronous multi-user scenario FBMC systems are more robust than OFDM systems to time and frequency misalignments among the users.

Blind synchronization for OFDM systems in multipath channels

In this paper we consider the problem of blind feedforward carrier-frequency offset (CFO) and symbol timing estimation for orthogonal frequency-division multiplexing systems in multipath channels. At first, a closed-form blind CFO estimator based on the least-squares (LS) approach is derived under the assumption that the start position of the FFT window is within the lock-in interval. Then, this assumption is removed and, a joint CFO and symbol timing LS estimator is obtained. Specifically, the proposed joint LS estimator does not require the knowledge of the channel impulse response and/or the insertion of virtual subcarriers. Computer simulation results show that the proposed estimators can outperform several blind CFO and symbol timing estimators previously proposed in the literature.

Joint symbol timing and CFO estimation for OFDM/OQAM systems in multipath channels

The problem of data-aided synchronization for orthogonal frequency division multiplexing (OFDM) systems based on offset quadrature amplitude modulation (OQAM) in multipath channels is considered. In particular, the joint maximum-likelihood (ML) estimator for carrier-frequency offset (CFO), amplitudes, phases, and delays, exploiting a short known preamble, is derived. The ML estimators for phases and amplitudes are in closed form. Moreover, under the assumption that the CFO is sufficiently small, a closed form approximate ML (AML) CFO estimator is obtained. By exploiting the obtained closed form solutions a cost function whose peaks provide an estimate of the delays is derived. In particular, the symbol timing (i.e., the delay of the first multipath component) is obtained by considering the smallest estimated delay. The performance of the proposed joint AML estimator is assessed via computer simulations and compared with that achieved by the joint AML estimator designed for AWGN channel and that achieved by a previously derived joint estimator for OFDM systems.

Blind CFO Estimation for Noncritically Sampled FMT Systems

In this correspondence, we consider the problem of blind carrier-frequency offset (CFO) estimation for noncritically sampled filtered multitone (FMT) systems. Specifically, a closed-form CFO estimator, based on the best linear unbiased (BLU) estimation principle, is derived. Although the proposed BLU estimator is derived under the hypothesis of additive white Gaussian noise channel, it demonstrates a remarkable robustness against multipath fading. Moreover, it does not require the knowledge of the symbol timing. The performance of the derived BLU estimator is compared with that of four recently proposed CFO estimators for FMT systems.

Joint symbol timing and CFO estimation in multiuser OFDM/OQAM systems

In this paper we consider the problem of data-aided synchronization in the up-link of a multiple access (MA) orthogonal frequency division multiplexing (OFDM) system based on offset quadrature amplitude modulation (OQAM). In particular, the joint maximum-likelihood (ML) phase offset, carrier-frequency offset (CFO) and symbol timing (ST) estimator exploiting a short known preamble embedded in the burst received from each of U users is considered. Under the assumption that the CFO of each user is sufficiently small the considered approach leads to U different approximate ML (AML) joint phase offset, CFO and ST estimators. Specifically, the phase and CFO estimators are in closed form while the AML ST estimator requires a one-dimensional maximization procedure. The performance of the proposed joint AML estimator is assessed via computer simulations both in AWGN and multipath channel, and for three different allocation schemes.

On the second-order cyclostationarity properties of long-code DS-SS signals

In this letter, the cyclostationarity properties of a general class of long-code direct-sequence spread-spectrum (DS-SS) signals are analyzed, and it is shown that these properties can be exploited in long-code DS code-division multiple-access systems. Specifically, it is shown that a continuous-time DS-SS signal exhibits cyclostationarity at cycle frequencies related to both the symbol rate and the chip rate. Moreover, it is pointed out that the discrete-time signal obtained by uniformly sampling a continuous-time long-code DS-SS signal exhibits cyclostationarity, provided that at least two samples per chip are taken. Finally, it is shown how such a cyclostationarity can be suitably exploited for blind signal-parameter estimation

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

Blind Symbol Timing Estimation for OFDM/OQAM Systems

In this paper we consider the problem of blind symbol timing estimation for pulse shaping orthogonal frequency-division multiplexing (OFDM) systems based on offset quadrature amplitude modulation (OQAM). In particular, under the assumption of low signal-to-noise ratio, the joint ML phase offset and symbol timing estimator for AWGN channels is derived. Since the phase estimate is in closed form, the symbol timing estimate requires only a one-dimensional maximization procedure with respect to a continuous parameter. Specifically, the cost function to be maximized to obtain the desired symbol timing estimate depends on both the unconjugate and the conjugate correlation functions of the transmitted OFDM/OQAM signal. The performance of the proposed blind estimator is assessed via computer simulations in AWGN and multipath channel.