Davide Mattera

Widely linear decision-feedback equalizer for time-dispersive linear MIMO channels

The paper deals with the transmission over multiple-input/multiple-output channels exhibiting time-dispersion. A minimum mean-square error equalizer based on widely linear processing combined with the decision-feedback (DF) strategy is implemented via finite-impulse-response filters. The proposed equalizer provides considerable performance gain at the expense of a limited increase in computational complexity. The performance analysis has been carried out accounting for mismatch conditions always present in practice. The results confirm the stronger sensitivity of the DF-based equalizers with respect to the feedforward-based ones when system parameters are not accurately known.

Efficient sparse FIR filter design

We consider the problem of designing a sparse FIR filter and show that it can be cast into a problem of determining a sparse solution of a linear system of equations. Previously proposed design algorithms for FIR filter utilize an intelligent search over all possible structures for sparse filter. We propose a new filter design method based on a simpler algorithm for finding a sparse solution of the linear system. Simulation experiments show significant improvements over classical nonsparse methods.

An explicit algorithm for training support vector machines

The support vector machine (SVM) constitutes one of the most powerful methods for constructing a mathematical model on the basis of a given number of training examples. SVM training requires that we solve a quadratic optimization problem; this step is usually performed by means of existing software packages. Such a black-box approach may be undesirable. In this paper we introduce a simple iterative algorithm for SVM training which compares well with some typical software packages, can be simply implemented, and has minimal memory requirements. It addresses the problem of regression estimation and utilizes ideas similar to those proposed by J. Platt (1998) for training binary SVM.

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.

MMSE WL Equalizer in Presence of Receiver IQ Imbalance

In this correspondence, with reference to the transmission over a linear time-dispersive channel, we address the problem of the in-phase and quadrature-phase (IQ) imbalance compensation when a single-carrier modulation scheme is used. Low-cost fabrication technologies and high data-rate transmissions render the conventional receivers very sensitive to the imperfections of their analog stage and, hence, proper countermeasures need to be adopted. Since the IQ imbalance renders the received signal rotationally variant, we propose to resort to the widely linear (WL) filtering in the synthesis of the receiver. More specifically, the synthesis of the WL receiver is performed by assuming perfect knowledge of the IQ imbalance parameters, but a new blind algorithm for IQ imbalance compensation, which outperforms the existing blind technique, is also proposed.

Simple and robust methods for support vector expansions

Most support vector (SV) methods proposed in the recent literature can be viewed in a unified framework with great flexibility in terms of the choice of the kernel functions and their constraints. We show that all these problems can be solved within a unique approach if we are equipped with a robust method for finding a sparse solution of a linear system. Moreover, for such a purpose, we propose an iterative algorithm that can be simply implemented. Finally, we compare the classical SV approach with other, recently proposed, cross-correlation based, alternative methods. The simplicity of their implementation and the possibility of exactly calculating their computational complexity constitute important advantages in a real-time signal processing scenario.

Lapped-OFDM as an Alternative to CP-OFDM For 5G Asynchronous Access and Cognitive Radio

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.

A filter bank multicarrier scheme running at symbol rate for future wireless systems

The filter bank multicarrier techniques considered for efficient transmission in communication systems generally rely on the offset-QAM modulation, which implies doubling the processing speed in transceivers. In contrast, the scheme presented here, based on the complex lapped transform, is running at the same symbol rate as OFDM. It does not require a cyclic prefix and, in the multi-user context, it provides a high level of spectral separation between users. In addition, a high performance per user carrier frequency offset compensation can be implemented in the receiver in a straightforward manner. Theoretical derivations are completed by simulations for channel equalization and carrier frequency offset compensation. With its reduced computation speed and its level of performance, the proposed scheme should strengthen the case of FBMC as an alternative to OFDM for asynchronous access in future wireless networks and cognitive radio.

Performance analysis of some timing offset equalizers for FBMC/OQAM systems

The requirements of future wireless networks lead to reconsidering the physical layer and, particularly the respective merits of the major multicarrier transmission techniques, OFDM and FBMC. In comparison, equalization has been a sticking issue for filter bank multicarrier systems, because of performance and implementation aspects. In contrast, equalization is straightforward to implement in OFDM systems and it is easy to figure out the performance achieved, thanks to the presence of the cyclic prefix. Here, OFDM and FBMC/OQAM systems are compared for timing offset equalization, which is representative of global channel equalization and appears as such in some use cases, e.g. asynchronous access. The single tap equalizer and the multitap time-domain equalizer are analyzed for a specific, yet representative, FBMC scheme. It is shown that multitap time-domain timing-offset sub-channel equalization brings improvements over single-tap equalization but its discrete time nature limits its performance. There is no such limitation in the frequency domain and a recently proposed implementation of the FBMC concept, named frequency spreading, is analyzed. Equipped with this receiver, an FBMC system can withstand any timing offset and, then, unlike OFDM, it is able, in a multicarrier context, to support asynchronous users while keeping spectral efficiency. HighlightsThe performance of single-tap and multi-tap time domain timing offset sub-channel equalizers for filter bank based multicarrier systems using Offset QAM is assessed.The performance of frequency-domain timing-offset equalization achieved by a recently proposed structure named frequency-spreading FBMC is presented and analyzed.Multi-tap time-domain sub-channel equalization significantly improves the performance of the single-tap equalizer but also the delay.Frequency-domain timing-offset equalization is equivalent to the best considered multitap equalizer but without additional delay.Frequency-spreading FBMC can withstand any timing offset and, then, can easily accommodate asynchronous users.

Frequency domain CFO compensation for FBMC systems

The frequency-domain compensation of the deleterious effects introduced by the residual carrier frequency offset (CFO) still present on the received signal after rough time-domain compensation in a filter bank multicarrier (FBMC) receiver is considered. The compensation method depends not only on the particular FBMC modulation technique but also on the chosen receiver structure. The paper is dedicated to the case where the recently proposed frequency-despreading receiver, one of the most promising solutions, is used in the OFDM/OQAM transceiver. Specifically, this receiver exploits some samples of an extended FFT and, as shown in the paper, it can nearly perfectly compensate the CFO effects on the received signal by adapting the weights of the linear combination of the considered samples. A detailed analysis of the frequency-despreading structure and of the proposed CFO-compensation technique is presented by using as performance measure the signal-to-interference ratio and the bit-error-rate. HighlightsA simple procedure for nearly perfectly compensating the deleterious effects of the carrier-frequency offset on the filterbank multicarrier signal at the input of the frequency-despreading receiver is proposed.A complete analysis of the considered receiver, equipped with the proposed procedure, is carried out both with reference to uplink/downlink and to flat/dispersive channels.The analysis shows that the proposed approach achieves a significant performance advantage in comparison with the uncompensated structure.Unlike other strategies already proposed in the literature for the same frequency-spreading receiver and for the standard receiver, the complexity increase is always minimal.The proposed approach provides great flexibility since each user can achieve the most useful trade-off between complexity increase and performance.