Mario Tanda

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

Multiuser detection in flat fading non-Gaussian channels

This paper deals with the problem of multiuser detection in direct-sequence code-division multiple-access (CDMA) fading channels with impulsive noise. This issue arises in practical situations because in many realistic wireless channels, the ambient channel noise is impulsive, resulting from various natural and man-made impulsive sources. An M-estimator-based structure for noncoherent demodulation of differentially phase-shift keyed (DPSK) signals transmitted simultaneously via a CDMA flat-fading channel and embedded in impulsive noise, is proposed and analyzed. Analytical and numerical results show that, in highly impulsive noise, the performance gain afforded by the proposed multiuser detector can be substantial when compared to the linear decorrelating detector for DPSK, with little attendant increase in algorithmic complexity.

Blind symbol-timing and frequency-offset estimation in OFDM systems with real data symbols

In this letter, the problem of blind symbol-timing and frequency-offset estimation in orthogonal frequency-division multiplexing (OFDM) systems with real data symbols is considered. Maximum-likelihood (ML) estimators for the parameters of interest have been derived by modeling the OFDM signal as a proper complex Gaussian random process. However, when data symbols belong to a real constellation, the received signal becomes an improper complex random process, and hence, previously mentioned estimators, termed cyclic prefix (CP) estimators, are not ML estimators. In this letter, minimum mean-squared error estimators exploiting the conjugate-symmetry property exhibited by the OFDM signal with real data symbols, termed MSR estimators, that greatly outperform CP estimators in additive noise channels, are derived. Moreover, a modified MSR symbol-timing estimator that can assure satisfactory performance in multipath fading channels is proposed.

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.

Signal interception in non-Gaussian noise

The interception of weak signals in nonGaussian noise is discussed. The spectral correlation property exhibited by all cyclostationary signals is exploited to synthesize multi-cycle and single-cycle detectors which assure a superior tolerance (as compared to radiometric techniques) to one of the most challenging problems in interception, namely accommodating unknown and changing noise level and interference activity. The proposed detectors perform a maximum likelihood estimate of the noise level and use the estimate to form the detection statistic. To obtain some analytical information about the performance of the multi-cycle and single-cycle detectors, the deflection, which is a useful measure of the output (signal-to-noise ratio), especially appropriate for weak-signal SNR interception, has been evaluated. The receiver operating characteristics, carried out via computer simulation, confirm the superiority of the proposed cycle detector with respect to the traditional radiometer. >

Blind parameter estimation in multiple-access systems

A blind algorithm for the estimation of amplitude, phase, and relative time delay of each user in multiple-access communication systems is proposed. This algorithm is based on the cyclostationarity features of the received signals and provides estimates of the unknown parameters that are intrinsically immune to the effects of noise and interference, provided that a cycle frequency of the user signals exists which is not shared with the disturbance terms. The proposed estimators are, under mild assumptions, asymptotically unbiased and consistent, Moreover, the proposed algorithm is asymptotically near-far resistant and is not based on the usual assumption of white and/or Gaussian noise.

Blind Symbol Timing and CFO Estimation for OFDM/OQAM Systems

The paper deals with the problem of blind synchronization for OFDM/OQAM systems. Specifically, by exploiting the approximate conjugate-symmetry property of the beginning of a burst of OFDM/OQAM symbols, due to the presence of the time offset, a new procedure for blind symbol timing and CFO estimation is proposed. The performance of the derived blind estimators is analyzed by computer simulations; the results show that the proposed methods may provide acceptable performance for reasonable values of the signal-to-noise ratio.

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.