Ales Prokes

Atmospheric effects on availability of free space optics systems

The availability of free space optics (FSO) systems in dependence on weather conditions and on FSO link parameters, such as transmitted optical power, beam divergence, receiver sensitivity or link path distance, is discussed. A number of phenomena in the atmosphere, such as absorption, scattering, and turbulence, can affect beam attenuation, but in the case of wavelengths typical of FSO systems operation, only scattering and turbulence are appropriate to be taken into consideration. We model the power loss caused by turbulence by using the Rytov scintillation theory. Attenuation due to scattering, which can be expressed as a function of the link distance, wavelength, and meteorological visibility, is calculated from visibility data collected at several airports in Europe. Statistical evaluation of the attenuation caused by scattering and the power link margin calculated from FSO link parameters are used for calculating the link availability.

In-Vehicle mm-Wave Channel Model and Measurement

This contribution documents and discusses recent wideband radio channel measurements carried out in the intra-vehicle environment. Channels in the millimeter-wave (MMW) frequency band have been measured in 55-65 GHz using open-ended rectangular waveguides. We present a channel modeling approach based on a decomposition of spatially specific Channel Impulse Responses (CIRs) into the large and small scale fading. The decomposition is done by a Hodrick-Prescott filter. We parametrize the small scale fading utilizing Maximum-likelihood estimates for the parameters of a generalized extreme value (GEV) distribution. The large scale fading is described by a two dimensional polynomial curve. We also compare simulated results with our measurement exploiting two-sample Kolmogorov-Smirnov test.

Detection performance of cooperative spectrum sensing with hard decision fusion in fading channels

In this paper, we investigate the detection performance of cooperative spectrum sensing (CSS) using energy detector in several fading scenarios. The fading environments comprise relatively less-studied Hoyt and Weibull channels in addition to the conventional Rayleigh, Rician, Nakagami-m and log-normal shadowing channels. We have presented an analytical framework for evaluating different probabilities related to spectrum sensing, i.e. missed detection, false alarm and total error due to both of them, for all the fading/shadowing models mentioned. The major theoretical contribution is, however, the derivation of closed-form expressions for probability of detection. Based on our developed framework, we present performance results of CSS under various hard decision fusion strategies such as OR rule, AND rule and Majority rule. Effects of sensing channel signal-to-noise ratio, detection threshold, fusion rules, number of cooperating cognitive radios (CRs) and fading/shadowing parameters on the sensing performance have been illustrated. The performance improvement achieved with CSS over a single CR-based sensing is depicted in terms of total error probability. Further, an optimal threshold that minimises total error probability has been indicated for all the fading/shadowing channels.

Estimation of free space optics systems availability based on meteorological visibility

A number of phenomena in the atmosphere such as scattering, absorption and turbulence can cause large variation in laser beam attenuation. The influence of absorption and turbulence can in some cases be significantly reduced by an appropriate design of free space optics (FSO) systems. Thus, in a typical continental area, where rain, snow or fog occur, scattering is the most important phenomenon causing power losses. Attenuation due to scattering can be expressed as a function of the link distance, wavelength and meteorological visibility. The paper deals with the estimation of FSO availability based on statistical evaluation of visibility data collected at several airports in Europe. The availability is evaluated in dependence on link distance, wavelength and power link margin.

Frequency-Domain In-Vehicle UWB Channel Modeling

This paper aims to present a simple but robust model characterizing the frequency-dependent transfer function of an in-vehicle ultrawideband (UWB) channel. A large number of transfer functions spanning the UWB (3–11 GHz) are recorded inside the passenger compartment of a four-seated sedan. It is found that the complex transfer function can be decomposed into two terms, the first term being a real-valued long-term trend that characterizes frequency dependence with a power law and the second term forming a complex correlative discrete series that may be represented via an autoregressive (AR) model. An exhaustive simulation framework is laid out based on empirical equations characterizing trend parameters and AR process coefficients. The simulation of the transfer function is straightforward as it involves only a handful of variables; however, it is in good agreement with the actual measured data. The proposed model is further validated by comparing different channel parameters, such as coherence bandwidth, power delay profile, and root-mean-square delay spread, obtained from raw and synthetic data sets. It is also shown how the model can be compared with existing time-domain Saleh–Valenzuela-influenced models and related IEEE standards.

Simulation of random effects in transmission line models via stochastic differential equations

The paper deals with a method for simulation of transmission line (TL) models with randomly varied parameters, based on the theory of stochastic differential equations (SDE). The random changes of both excitation sources and TL model parameters can be considered. Voltage and/or current responses are represented in the form of the sample means and proper confidence intervals to provide reliable estimates. The TL models are based on a cascade connection of RLGC networks enabling to model nonuniform TLs in general. To develop model equations a state-variable method is used, and afterwards a corresponding vector SDE is formulated. A stochastic implicit Euler numerical scheme is used while using the MATLAB® language environment for all the computations. To verify the results the deterministic responses are also computed by the help of a numerical inversion of Laplace transforms procedure.

In-vehicle UWB channel measurement, model and spatial stationarity

This contribution documents an ultra-wide band (UWB) channel measurement performed in an in-vehicle environment for the frequency range 3-11 GHz. An emphasis is placed into an evaluation of a spatial consistency of measured channel characteristics in terms of Pearson correlation between measured channel impulse responses (CIRs). Moreover, the measured CIRs are reproducible via a two-part exponentially decaying envelope-delay profile (EDP). The small scale variation of received signal is parametrized utilizing a random process obeying the generalized extreme value (GEV) distribution. Validation of the channel model is demonstrated utilizing a two sample Kolmogorov-Smirnov (K-S) test.

Generalized sampling theorem for bandpass signals

The reconstruction of an unknown continuously defined function from the samples of the responses of linear time-invariant (LTI) systems sampled by theth Nyquist rate is the aim of the generalized sampling. Papoulis (1977) provided an elegant solution for the case where is a band-limited function with finite energy and the sampling rate is equal to times cutoff frequency. In this paper, the scope of the Papoulis theory is extended to the case of bandpass signals. In the first part, a generalized sampling theorem (GST) for bandpass signals is presented. The second part deals with utilizing this theorem for signal recovery from nonuniform samples, and an efficient way of computing images of reconstructing functions for signal recovery is discussed.

UWB measurements for spatial variability and ranging: Parked car in underground garage

The randomness in ultrawideband channel for small positional variations around a car, parked in an underground garage, is reported. Keeping the receiver fixed at the conventional rooftop antenna location, the transmitter location is changed to study the variations against distance, angle, and height. The path loss is found to be monotonically increasing with distance, but varies randomly with angle and height and thereby renders signal-strength-based ranging inaccurate for such scenarios. On the other hand, the first peak can reliably provide distance estimate irrespective of transmitter angle or height. The number of clusters in the delay profile reduces with distance, but the shape of the profile remains fairly consistent across angle. The Saleh–Valenzuela parameter values also vary with distance and height, but their average values are close to the IEEE 802.15.3 standard.

CLEAN algorithms for intra-vehicular time-domain UWB channel sounding

A comparison of two variants of CLEAN, a time-domain serial subtractive deconvolution algorithm, is presented. Appropriate statistical metrics for assessing the relative merit of the deconvolution technique are identified in the context of intra vehicle ultra wide band transmission, and the better variant was selected based on its performance over a standard IEEE channel simulation testbed. The chosen method is then applied to extract important channel characteristics for a real-world channel sounding experiment performed inside a passenger car.