Recent communications standards, such as vehicle-to-vehicle and fifth generation, include applications where the transmitted signal encounters rapid changes of propagation scenarios, resulting in wireless links characterized as non-stationary (NS) channels. Hence, channel models that correctly explain and represent the measured time-varying channel statistics, and their associated simulation methods for testing purposes, are all required. Although the body of works devoted to NS channel modeling is vast, due to the complexity and variety of this problem, the provided NS statistics are defined only within a limited observation time, and therefore, the generated channel realizations do not include the changes between scenarios. In light of this problem, this paper introduces a channel model that mimics the continuous change of the mobile propagation channel via a continual renewal of channel parameters, in which all stationary and NS channels are represented under a unified structure. Theoretical and simulation results provided in this paper confirm that the proposed model reproduces stationary models with high accuracy. In addition, NS channel realizations with predefined time-varying power spectral density and time-varying envelope distributions are also shown in this paper, providing a means for testing modern communications systems.

Current communication standards are designed to operate under either frequency or time selectivity. However, the rising demand of higher data rates combined with the mobility of the transceivers has led to the necessity of transmitting under doubly selective channels. In these channels, both single-carrier and multi-carrier approaches find high interference conditions, where the performance in terms of spectral efficiency and bit error rate is low. The state-of-the-art solutions involve high complexity and/or low compatibility making them infeasible. In this paper, a modified orthogonal frequency-division multiplexing (OFDM) communication system based on the virtual decomposition of the channel is proposed. The system is fully compatible with the standard OFDM transmitters and maintains several blocks of the standard OFDM receivers. This novel system substantially surpasses the performance of current approaches while requiring low computational complexity.

Preserving a reduced feedback load in Multiuser Diversity systems is crucial. By establishing either single or multiple thresholds only in the energy domain, the Selective Multiuser Diversity (SMUD) schemes achieve a reduced feedback load while maintaining a high capacity performance. This paper introduces an enhanced joint time-energy domain scheduling approach for SMUD systems that maximizes the total spectral efficiency and preserves a minimal feedback load with just a few thresholds in the joint domain. This joint domain capitalizes on the energy and the non-fade duration statistics in order to establish a feedback policy that is focused on those users who have good channel condition and preserve it for a predetermined amount of time, providing thus an automatically more rational and fair use of the resources without a sophisticated scheduling with large polling/decision times. The analysis of this enhanced approach for the general case of Nakagami fading channels is developed and expressions for the average system capacity and the average feedback load are presented. The tradeoff between the feedback load and the spectral efficiency gain is analyzed. Two main scenarios are studied; the first when there are two thresholds in the energy domain side and one in the time domain side; the second when there are two thresholds in both domains.

The minimum duration outage concept allows an improved analysis of the outage probability and it has been reported, up to now, only for Log-normal shadowing and Rayleigh fading using asymptotic results of the level crossing theory. Capitalizing on some approximate results for the Fade Duration Distribution, FDD, in this letter we show the analysis of the minimum duration outage for the general case of Nakagami fading. We also show how some elements of the physical layer of the system can be used to determine the minimum duration outage interval ¿min.

Based on the theory of optimum diversity combining, this paper presents original results for the error rate calculations for the Orthogonal Space-Time Block Codes (OSTBC) in generalized Gaussian MIMO channels. A simple bound for the symbol rate performance is derived, allowing to track contribution of channel parameters to the performance. Channel hardening is then investigated from the probability of error point of view. Results of the application of different signal constellation's choice are shown from the prospective of the trade- off between the error performance and spectral efficiency.

This paper presents some novel results for non-coherent decoding of the Differential Time-Space Modulated (DTSM) signal in MIMO channels. Noncoherent decoding algorithm is considered from prospective of autocorrelation processing structure. The analysis of its performance is carrier through development of an analytical expression for the symbol error probability. Characteristics of the diversity combining for MIMO channel, described by the four parametric model and "the hardening effect" are also presented.

This paper deals with the analysis of statistical properties of the capacity of spatially uncorrelated orthogonal space-time block coded (OSTBC) Nakagami-m multiple-input multiple-output (MIMO) channels. We have derived exact closed-form expressions for the probability density function (PDF), cumulative distribution function (CDF), level-crossing rate (LCR), and average duration of fades (ADF) of the channel capacity. We have also investigated the statistical properties of the approximated capacity of spatially correlated OSTBC Nakagami-m MIMO channels. The results are studied for different values of the fading parameter m, corresponding to different fading conditions. It is observed that an increase in the MIMO dimension1 or a decrease in the severity of fading increases the mean channel capacity. While, a significant decrease in the mean channel capacity is observed with an increase in the spatial correlation. The correctness of theoretical results is confirmed by simulations.

In their recent article Feng, Field and Haykin rediscovered an approach to the channel modelling based on stochastic differential equations (SDE). In the following we would like to bring readers attention to a body of literature which deals with similar subjects using SDE approach.

The second order statistics, such as the level crossing rate and the average fade duration of a generalized Gamma distribution are derived in this paper. Joint distribution of the process and its derivative and the marginal distribution of the derivative is developed. It is shown that apart from the Nakagami case the velocity of the process is dependent on the value of the process. The results are applied to derive some characteristics of error flow in channels with dual mobility, generalized Gamma fading and basic modulations.

The problem related to the fade and non-fade duration distributions (FDD and NFDD) of randomly distributed signals has been studied for more than forty years. However, for non-gaussian processes, closed form general solutions for these distributions are still unknown. In this paper we use an orthogonal series expansion to analyze the FDD and NFDD (also known as downcrossing and upcrossing duration distributions respectively) for the rather general case of Nakagami distributed random processes. Our results show that for asymptotic and non-asymptotic threshold levels the crossing duration distributions can be approximated, in a simple closed form way, using the gamma distribution. We back up this approximation with some goodness of fit metrics and show how the parameters of such distribution can be computed a priori analytically.

This paper studies the statistical properties of the channel capacity of spatially correlated Nakagami-m multiple- input multiple-output (MIMO) channels. We have derived closed- form expressions for the probability density function (PDF), the cumulative distribution function (CDF), the level-crossing rate (LCR), and the average duration of fades (ADF) of the lower bound on the channel capacity. In order to study the impact of the spatial correlation on the channel capacity, the analysis of the statistical properties of the channel capacity is carried out for different receiver antenna spacings. It is observed that the antenna spacing has a significant influence on the spread and maximum value of the PDF and LCR. The proposed method can be employed to study the statistical properties of the capacity of MIMO channels in different fading environments. The correctness of the analytical expressions is confirmed by simulations.

During almost fifty years the fade duration distribution (FDD) of random signals has been an open research topic. However, for Nakagami fading channels, and in general for the great majority of non-Gaussian processes, closed form general solutions are still unknown. In this paper we apply an orthogonal series expansion that allows to characterize the FDD. Our results show that the FDD in Nakagami wireless channels can be approximated, for both asymptotic and non-asymptotic threshold levels (C), by a simple closed form function: the Gamma distribution. This approximation is validated in different ways. We show how it is possible to compute (a priori) the parameters of the approximate Gamma distribution through the analytical fade statistics. We also examine the influence of different autocorrelation functions on the FDD.

The fade duration distribution (FDD) of randomly varying wireless channels has been a research topic for more than forty years. However, for non-Gaussian channels the FDD is up to now an unsolved problem and closed form general solutions are still unknown. In this paper we use an orthogonal series expansion to analyze the FDD for the rather general case of Nakagami fading channels. Our results show that the fade duration distribution, for both asymptotic and no-asymptotic threshold levels, can be approximated by a simple closed form function: the Gamma distribution. This approximation is validated by means of several statistical goodness of fit tests. We show that the parameters of the approximated Gamma distribution can be computed a priori through the fade duration statistics, namely the average and variance. We compare the analytical fade statistics with simulation results.

This paper introduces a novel noncoherent linear channel equalization structure for time dispersive channels subjected to non negligible carrier frequency offsets for double differential phase shift keying (DDPSK) modulation. An optimization criterion for the adjustment of the coefficients of this equalization structure, termed cross-differential minimum mean square error (CDMMSE), is also proposed. In contrast to a previous work in the literature where the equalizer coefficients needed to be iteratively calculated without assurance of global convergence, the proposed CDMMSE criterion accepts a closed form solution. It will be shown that this solution coincides with that of the optimum coherent linear minimum mean square error (MMSE) equalizer when the transmitted symbols possess constant modulus. Simulation results are presented that corroborate the predicted performance of the proposed approach

The second order statistics, such as the level crossing rate and the average fade duration of a generalized Gamma distribution are derived in this paper. Joint distribution of the process and its derivative and the marginal distribution of the derivative is developed. It is shown that apart from the Nakagami case the velocity of the process is dependent on the value of the process. The results are applied to derive some characteristics of error flow in channels with dual mobility, generalized Gamma fading and basic modulations

In this paper we are presenting the generalized multiple-input multiple-output (MIMO) Kronecker wideband channel simulator by considering the prolate spheroidal wave functions (PSWF) set as a universal basis both for the spatial and temporal correlation properties of the channel.

This paper introduces a new formulation for the blind channel estimation problem. In particular, it is found that any possible realization of the discrete time channel impulse response can be expressed as a linear combination of a set of predetermined eigenvectors. These are obtained from a certain “universal” autocorrelation matrix that depends only on the pulse shaping filter and the maximum delay spread of the multi-path channel, and not on the specific characteristics of the power delay profile. This new formulation shows that an interrelation exists between the different sub-channel responses of the single-input multiple-output equivalent model of the over-sampled baseband signal. Thus, the number of free parameters equals the number of signal space eigenvectors and not the number of coefficients in the multi-channel model. Therefore the optimization associated with the CR algorithm [1] can be reformulated as a reduced dimensional minimization problem with a notoriously enhanced estimation performance.

In this paper a new and constructive method useful to simulate Wide Band Channels with non-separable scattering function via the filtering approach is provided. This method is based on the approximation of the Channel by the eigenfunctions taken from the Karhunen-Loève expansion of the Channel's. ensemble autocorrelation function at Δt=0. Channel realizations are performed via the filtering of an independent gaussian random process vector by some filter matrix, which is obtained from some manipulations of the correlation statistics of eigenfunctions. An example to clarify this proposal is also presented.

The correlated random process is represented as a solution of a stochastic differential first-order equation (SDE). The case of processes with a probability density function defined on a semi-infinite or finite range is considered. The limitation of the range of the simulated process requires modification of the specific structure of the SDE. The approach presented provides excellent results in modeling significant non-Gaussian processes with approximately an exponential correlation function. It is validated by direct numerical simulation of a uniformly distributed correlated process.