FBMC-PAM is a recently introduced filter bank multicarrier technique which exploits pulse amplitude modulation and operates at the same symbol rate as OFDM, unlike FBMC-OQAM which operates at twice that symbol rate. The scheme is efficiently implemented in the frequency domain and, in the system, the receiver is equipped with a frequency domain equalizer. In the present paper, uplink access is considered, focusing on two issues, single carrier modulation and preamble-based channel estimation. It is shown that the scheme can handle asynchronous users, thanks to the spectral separation brought by the filter bank and flexible frequency domain equalizers, and the performance is demonstrated by simulations. An FBMC-PAM-based physical layer can be envisaged for the uplink of future wireless systems.

In spite of widely recognized benefits, the acceptance of filter bank multicarrier (FBMC) as an alternative to cyclic prefix orthogonal frequency division multiplexing (CP-OFDM) for the physical layer of future mobile and wireless communication systems is restrained by the introduction of offset-quadrature amplitude modulation (OQAM). The proposed approach, based on the so-called lapped transform, combines a simple yet efficient perfect reconstruction real filter bank with QAM modulation. Real processing of the real and imaginary components of the QAM data samples introduces a symmetry of the used carriers and the spectrum of the multicarrier signal can be either continuous or fragmented. Implementing the receiver with an extended FFT allows for frequency domain channel equalization and the performance achieved is sufficient to avoid the need for a cyclic prefix. The scheme is appropriate for asynchronous access and the filter provides a high level of spectral protection, as required by users in cognitive radio scenarios.

The paper is devoted to the performance analysis of a new receiver structure recently introduced for OFDM/OQAM systems. Such a structure is based on the frequency-despreading at the receiver and it is designed to minimize the processing delay of the receiver structure. Its performance analysis and the comparison with the performance of the standard receiver structure show the capability of the new structure to operate with a small number of subcarriers.

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.

In this paper we deal with the problem of data- aided symbol timing estimation for orthogonal frequency division multiplexing systems based on offset-QAM modulation (OFDM/OQAM). Specifically, a data-aided joint phase offset and symbol timing estimator, based on the least squares (LS) approach, exploiting the conjugate-symmetry property of a properly designed training symbol, is derived. Since the phase estimate is in closed form, the symbol timing estimate requires only an one-dimensional maximization procedure. The derived LS estimator does not require the knowledge of the prototype filter and, moreover, its performance, is independent of the carrier frequency offset.

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.

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.

We consider the problem of semiblind carrier frequency offset (CFO) estimation for OFDM systems. Specifically, we consider a synchronization scheme based on the insertion of pilot tones in the OFDM block. In this case, if the number of subcarriers is sufficiently large, the received signal vector can be modeled as a complex Gaussian random vector with a nonzero mean. Then, by exploiting the joint probability density function for complex Gaussian random vectors, semi- blind maximum likelihood CFO estimators are derived. The performance of the proposed semiblind algorithms is assessed via computer simulations and compared with that of some blind CFO estimators previously proposed in the literature.

In this letter, the problem of blind symbol-timing estimation with M-ary phase-shift keying (M-PSK) signals is considered. An estimator exploiting the constant modulus (CM) property of M-PSK constellations and the structure of the received signal when the convolution of the transmitter's signaling pulse and the receiver filter satisfies the Nyquist criterion is proposed. Since the derived estimator, termed the CM estimator, requires a maximization with respect to a continuous variable, a closed-form approximate CM (ACM) algorithm, suitable for digital implementation, is proposed. Computer simulation results show that with small excess bandwidth factors, the derived ACM algorithm outperforms previously proposed algorithms at moderate and high signal-to-noise ratios

In this paper we investigate the problem of blind carrier frequency offset (CFO) estimation in orthogonal frequency division multiplexing systems based on offset QAM. Specifically, by modeling the received signal as a complex Gaussian random vector and under the hypothesis of a non dispersive channel, we derive the unconditional maximum likelihood CFO estimator. Moreover, due to its computational complexity we consider, successively, the more feasible best linear unbiased estimator. The performance of the proposed algorithms is assessed via computer simulation and compared with that of recently proposed CFO estimators exploiting second order cyclostationarity

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

The problem of blindly estimating the parameters of a Doppler channel for noncircular transmissions in multiple-access communication systems is addressed. A nondata-aided algorithm based on the cyclostationarity features of the received signal is proposed to estimate amplitude, phase, time delay, and frequency shift of each user. Under mild assumptions on the disturbance and user signals, the proposed method provides estimates of the unknown parameters that are mean-square consistent. Moreover, the proposed algorithm is asymptotically near-far resistant, and is not based on the usual assumption of white and/or Gaussian noise.

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.

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.

This letter deals with the array detection of a random signal subject to a disturbance modeled as an additive mixture of Gaussian and spherically invariant noise. A detector based on a large sample size approximation of the locally optimum decision statistic is proposed. The implementation of the detection structure requires the knowledge of the correlation matrix of the disturbance at each array element, but is otherwise independent of the noise distribution of the spherically invariant noise component.

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.

The letter deals with noncoherent diversity detection of bandpass signals subject to slow and nonselective fading and embedded in correlated non-Gaussian noise modeled as a spherically invariant random process. At first, an asymptotic sufficient statistic for arbitrary fading laws on the diversity branches is derived; then, the asymptotically optimum detector for Rayleigh-distributed amplitude fluctuations is synthesized and its performance is assessed.

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.>