The paper deals with a specific kind of BCI application implemented with the aim of recovering the reaching ability of mild impaired stroke survivors. The overall idea is to take advantage of the plasticity of the brain to make the subject artificially learn alternative neural paths to control the arm movement again, by-passing the injured area thanks to a BCI system with an EEG-related force provided as a real-time feedback during the training period. Preliminary results have shown improvements in the kinematics of the upper limb motion of a first patient that performed this experimental rehabilitative program. Then, this BCI application is expected to enter soon the daily clinical practise as a useful tool besides the standard rehabilitation therapy.

Motor rehabilitation after stroke injury is highly important since the number of people suffering this disease is constantly increasing. Brain-Computer Interfaces (BCIs) have been recently used in the recovery of motor functions: in particular, the closed loop involving sensorimotor brain rhythms, assist-ive-robot training and proprioceptive feedback in an operant learning fashion might be potentially one of the most effective ways to promote the neural plasticity of the ipsilesional brain hemisphere and to restore motor abilities. This study aimed at implementing such a scheme: one chronic stroke patient was recruited and underwent the experiment using both the damaged and the healthy arm, considered as control during the following analysis. Kinematic and neurophysiological outcomes confirmed the efficacy of this treatment and supported the hypothesis that a contingent force feedback can improve motor functions of the upper limb.

Cognitive radio (CR) networks can re-use the RF spectrum licensed to the primary user (PU) network by carefully controlling the interference to the PUs. However, due to lack of explicit support from the PU system, CR sensing algorithms often face difficulty in acquiring CR-to-PU channels accurately. Moreover, the sensing algorithms cannot detect silent PU receivers, which nevertheless have to be protected. In order to achieve aggressive spectrum re-use even in such challenging scenarios, a CR power control problem with probabilistic interference constraints is formulated. Both log-normal shadowing and small-scale fading uncertainties are taken into account through suitable approximations. In particular, a weighted sum-rate maximization problem is considered, whose Karush-Kuhn-Tucker points are obtained via sequential geometric programming. Numerical tests verify the performance of our novel approach.

Cognitive radio (CR) networks can re-use the RF spectrum licensed to a primary user (PU) network, provided that the interference inflicted to the PUs is carefully controlled. However, due to lack of explicit cooperation between CR and PU systems, it is often difficult for CRs to acquire CR-to-PU channels accurately. In fact, if the PU receivers are off, the sensing algorithms cannot obtain the channels for the PU receivers, although they have to be protected nevertheless. In order to achieve aggressive spectrum re-use even in such challenging scenarios, power control algorithms that take channel uncertainty into account are developed. Both log-normal shadowing and small-scale fading effects are considered through suitable approximations. Accounting for the latter, centralized network utility maximization (NUM) problems are formulated, and their Karush-Kuhn-Tucker points are obtained via sequential geometric programming. For the case where CR-to-CR channels are also uncertain, a novel outage probability-based NUM formulation is proposed, and its solution method developed in a unified fashion. Numerical tests verify the performance merits of the novel design.

The robustness of the linear minimum mean square error (LMMSE) channel estimator is studied with respect to the reliability of the estimated channel correlation matrix used for its implementation. The analysis is of interest in practical applications of multiple-input multiple-output (MIMO) systems, where a perfect estimate of the channel correlation matrix is not available. The channel estimation mean square error (MSE) is analytically analyzed assuming a general structure for the estimated channel correlation matrix used to implement the LMMSE channel estimator. The obtained results are successively detailed to the case of channel correlation matrices derived by sample correlation estimation methods. It is observed that the use of a coarse estimate of the channel correlation matrix can lead to a severe degradation on the LMMSE channel estimator performance, whereas the simpler least-square (LS) channel estimator may provide comparatively better results. Nevertheless, it is shown that a robust approach, although suboptimal, relies on implementing the LMMSE channel estimator by assuming transmissions over uncorrelated channels, since, with such an assumption, the resulting estimation MSE is certainly smaller than for the LS channel estimator.

In this paper, we investigate the effects of imperfect knowledge of the channel covariance matrix on the performance of a linear minimum mean-square-error (MMSE) estimator for multiple-input multiple-output (MIMO) channels. The estimation mean-square-error (MSE) is analytically analyzed by providing both a very tight lower bound and an upper bound. The proposed analysis is useful for the understanding of how estimation accuracy of the channel covariance matrix impacts on system performance, depending on the average signal-to-noise ratio (SNR) and specific propagation conditions. Conclusions are fully supported by numerical results.

This paper deals with the impact of channel estimation errors on the capacity of correlated fading MIMO channels. In literature the effect of channel uncertainty at the receiver is often investigated with the assumption that channel estimation errors are identically distributed and independent of both SNR and channel correlation values. In this paper we extend the analysis by considering correlation among the channel estimation errors as typically happen when the MIMO channel to be estimated is spatially correlated. We propose and study upper and lower bounds on the MIMO channel capacity giving emphasis to the impact of the resulting channel estimation error, through its covariance matrix. The role played by the effective SNR and spatial correlation is studied by detailing the results to MIMO systems employing linear MMSE channel estimators.

The pairwise error probability (PEP) for multiple- input multiple-output (MIMO) radio interfaces is investigated by means of a novel formulation based on compound matrices. The proposed approach is suitable for any MIMO system having average upper-bounded PEP written as$[\det ( {\mmb I}+\gamma \, {\mmb A})]^{-\zeta }$, where$ {\mmb A}$is a Hermitian matrix,$\zeta $an integer number, and$\gamma $the signal-to-noise ratio (SNR); that bound frequently results in MIMO single- and multicarrier transmissions. It is shown that the minimization of the bounded PEP should consider the whole set of nonzero compound matrices of$ {\mmb A}$. In particular, the SNR of interest marks the compound matrix that mainly drives the system performance. Both diversity advantage and coding gain are given as continuous functions of the variable$\gamma $, hence, their asymptotic behaviors are taken as important case of studies. The interaction effects between channel code and propagation environment are also discussed. It is shown how the eigenvectors and eigenvalues of the autocorrelation channel matrix may be considered for code design. It is also proved the maximization of the code rank is not always a necessary requirement for performance improvement being its optimal value fixed by the channel structure and SNR of interest. Finally, the analysis is applied to space–time trellis-coded transmissions over spatially correlated slow Rayleigh-fading channels.

Time division duplex (TDD) systems based on orthogonal frequency division multiplexing (OFDM) have recently been proposed as potential candidates for next generation (4G) cellular networks. In this paper, as a multiple access scheme we propose an adaptive frequency division multiple access (AFDMA), where resources are assigned to users exploiting channel knowledge at the transmitter. In order to reduce the system overhead, the algorithm can also work on group of sub-carriers and OFDM symbols. The proposed solution is shown, through numerical results, to be superior to conventional allocation strategies, namely time division multiple access (TDMA) and FDMA. Furthermore, we also investigate a new power control algorithm based on a single-user discrete bit loading technique. Numerical results are reported for both downlink and uplink transmissions in a multi-cell environment supporting real-time traffic

In this paper, we study the effects of phase noise in a multicarrier system. We show that the phase noise introduces interference components, and we evaluate their second order statistics. The analysis is then applied to two particular multicarrier architectures: discrete multitone (DMT) modulation, and filtered multitone (FMT) modulation. For the DMT case, our results are in agreement with other related work that considers DMT only. For FMT modulation, we demonstrate that the phase noise power spectral density as well as the frequency response of the prototype filters play a key role on the resulting interference power value. Finally, we show that the analytical performance results are close to the simulation results.

Phase noise can be a major source of performance degradation in low-cost, low-bit-rate digital systems operating in the millimeter wavelength band. The use of code-division multiple access (CDMA) for several new applications is envisaged, and the study of the effects of phase noise shows that even CDMA systems exhibit a performance degradation comparable to that of standard modulation formats. The use of CDMA with a differential modulation at the chip level improves the system performance with a simpler demodulator at the expense of the enhancement of multiuser interference. In this paper we propose a new approach to CDMA modulation, and a possible solution is suggested to obtain a tradeoff between the performance degradation due to the phase noise and to the multiuser interference.

We consider a multitone multiple access wireless system where users are multiplexed by the assignment of sub-sets of the available tones. The information signals of distinct users are multicarrier modulated and propagate through asynchronous frequency selective fading channels, i.e., uplink. In this scenario intersymbol, intercarrier, and multiple access interference arise. We consider simple single user detection and study its performance in terms of outage probability. In particular we investigate the effect on outage probability of the choice of the sub-channel transmit filters and the tone allocation strategy.

The orthogonal frequency-division multiplexing (OFDM) modulation format has been proposed in Europe as the standard for broadcasting both audio and television digital signals and for wide-band wireless communication systems (e.g., HIPERLAN II). The performance of the OFDM scheme is severely affected by the nonlinearity of the high-power amplifier at the transmitter end and by the phase noise of the oscillators. In this paper, we investigate the joint effects induced on the OFDM signal by the amplifier nonlinearity and by the phase noise. An accurate statistical description of each contribution to the signal distortion is provided. Theoretical results show perfect agreement with those obtained by simulation and they can be used to derive the OFDM system performance, without the need to run extensive simulations.

The effects of oscillator phase noise on the performance of digital transmission systems employing QAM are investigated. The model used for the phase noise is a Wiener-Levy process which allows a moment characterization of the filtered phase noise. The penalty introduced by phase noise on the error probability is evaluated and compared with the approximation of a constant phase error.

The evaluation of optimum transmission ranges in a packet radio network in a fading and shadowing environment is considered. It is shown that the optimal probability of transmission of each user is independent of the system model and is p/sub o//spl sime/0.271. The optimum range should be chosen so that on the average there are /spl chi/(G/b)/sup 2//spl eta// terminals closer to the transmitter than the receiver, where G is the spread spectrum processing gain, b is the outage signal-to-noise ratio threshold, /spl eta/ is the power loss factor and /spl chi/ depends on the system parameters and the propagation model. The performance index is given in terms of the optimal normalized expected progress per slot, given by /spl thetav/(G/b)/sup 1//spl eta// where /spl thetav/ is proportional to the square root of /spl chi/. A comparison with the results obtained by using deterministic propagation models shows, for typical values of fading and shadowing parameters, a reduction up to 40% of the performance index.>

Digital radio links with bandlimited pulses exhibit a severe performance degradation when the transmitter high power amplifier operates near saturation. To cope with the increase of nonlinear intersymbol interference due to the amplifier nonlinearities, a discrete-time Volterra system can be used to process the transmitted data. We present an efficient technique for implementing adaptive data predistorters with memory based on a discrete-time Volterra system composed of digital linear filters and memoryless nonlinear devices working at the symbol rate. Third- and fifth-order structures are proposed and a system performance evaluation is presented for several realistic situations.>

This paper evaluates the effects of transmit amplifier nonlinearities on digital radio link performance. Special emphasis is given to links carrying 256-level modulations, for which the impact of even mild nonlinearities can be severe. Performance is measured in terms of the flat fade margin (F) normalized by its theoretical maximum (Fmax). Curves ofF / F_{max}versus amplifier input drive level are used to evaluate and compare numerous design strategies. A major contribution of this work is the development of a fast, accurate method of computer analysis, in contrast to the more cumbersome Monte Carlo simulations generally used for these assessments. Moreover, the new method is used to study a variety of techniques devised to combat the nonlinearity, particularly signal predistortion. It is also used to investigate various approaches to pulse shaping and filtering, to quantify the degradation in performance with increasing numbers of modulation levels, and to examine the possible benefits of linear equalization.

We analyze a digital radio link in which rectangular RF data pulses drive the high-power amplifier (HPA) and spectral shaping of the transmission is done after the HPA. The purpose of the analysis is to quantify detection performance for a variety of design possibilities. These include two approaches to predistortion in the transmitter and two ways of setting decision boundaries in the receiver. Also considered are different signal constellations and numbers of levels(M). We show that performance differences among practical system designs increase significantly withM. For example, the spread of minimum distance values among the various designs studied here grows from 4.4 dB forM = 16to 14.4 dB forM = 256.

The performance analysis of a fully digital acquisition circuit for direct sequence spread-spectrum systems has been made. The disturbance is considered Gaussian but with a colored spectral density. The effects of the disturbance correlation show remarkable effects on the detection probability and on the mean acquisition time at low signal-to-noise ratios. An example of an application is reported.

A closed form expression for the spectral density of the time jitter produced in a line regenerator in the presence of a general polynomial nonlinear circuit, band-limited baseband pulses, and an arbitrary tuned filter is developed. The method applies to a PAM digital signal with mutually independent symbols but with an arbitrary number of levels and symbol probabilities. As an example of the method, the spectrum of the time jitter produced in the presence of a fourth power law nonlinear circuit is reported.

The paper deals with the spectral analysis of both phase and frequency modulated digital signals with correlated messages, where correlation is obtained by encoding. Also, a possible general filtering of the baseband digital signal is taken into account. The method of computation is fast, since the computational complexity increases linearly with the baseband pulse duration. The main result is concerned with frequency modulation. It is shown that for some well-known baseband encoding schemes, such as bipolar, pair selected ternary, high density bipolar, and bipolar withnzeros substitution, the spectrum exhibits lines no matter what the value of the modulation index.

Spectral analysis is performed for a digital message produced by a variable-length encoder driven by a stationary memoryless source. In particular, closed form expressions are derived for both the continuous and the discrete part of the spectral density. The continuous part turns out to be a rational function ofz=\exp (j2\pi fT),whereTis the symbol period, whereas the discrete part exhibits in general spectral lines at multiple integers of(\lambda_{0}T)^{-1}, where\lambda_{0}is the greatest common divisor of the codeword lengths. As an application of the theory explicit formulas are derived for the spectra of BnZS and HDBncodes.

An approach to evaluate the error probability in conventional PAM digital data transmission systems with correlated symbols in the presence of intersymbol interference and additive noise is formulated in general and it is applied to the Gram-Charlier series expansion method. It is shown that the technique of conditioning few symbols before and/or after the symbol to be detected increases substantially the range of signal-to-noise ratios with an acceptable increase of numerical work. This technique also improves existing bounds on error probability, as, e.g, Glave's bound.

In digital communication systems, the error probability in the presence of intersymbol interference (II) and additive noise may be calculated to any desired degree of accuracy by well-known approximation methods which avoid the exponential computation growth (with the number of interferers) inherent in an exhaustive method, on the condition that a fast technique for computing II moments is available. Such a technique is indeed available at present, but it is strongly limited by the assumption that the data symbols are mutually independent. In this paper, this limitation is removed, and a fast procedure is given for computing H moments of correlated digital signals. The computations grow linearly with the number of interferers. The assumption made is that correlated symbols are produced by a general finite-state sequential machine. As illustrative examples, the fast procedure is applied to bipolar and Franaszek MS-43 codes.