Costas Tzaras

Multiple Time-Domain Diffraction for UWB Signals

The time-domain (TD) solution of the two-dimensional multiple-diffraction case is investigated. The proposed TD solution is based on the representation of the inverse Fourier transform of the corresponding frequency-domain (FD) solution in closed form, as it is given by the uniform theory of diffraction (UTD), and it incorporates the TD representation of the higher-order diffraction coefficients. An algorithm to predict the TD diffracted field after an arbitrary number of objects is also presented. In the proposed algorithm, different types of objects along the propagation path can be applied as well, such as absorbing knife-edges and metallic or nonperfectly conducting wedges. The comparison between the TD solution and the numerical inverse fast Fourier transform of the FD solution proves the validity of the proposed solution.

On the Capacity of Variable Density Cellular Systems under Multicell Decoding

The majority of multicell-decoding cellular models preserve a fundamental assumption which has initially appeared in Wyners model, namely the collocation of user terminals (UTs). Although this assumption produces more tractable mathematical models, it is unrealistic w.r.t. current practical cellular systems. In this paper, we alleviate this assumption by assuming uniformly distributed UTs. The model under investigation is the uplink channel of a planar cellular array in the presence of power-law path loss and flat fading. In this context, we employ a free probability approach to evaluate the effect of UT distribution on the optimal sum-rate capacity of a variable-density cellular system.

A Measurement-Based Model for Mobile-to-Mobile UMTS Links

In this paper we propose an empirical radio propagation model for the relaying system with low height terminals. The model differentiates line-of-sight (LOS) and non-line-of-sight (NLOS) propagation paths for increased accuracy by employing an easy-to-use expression. It takes into account the effect of transmitter height, receiver height, receiver location and environmental parameters and it is also complemented by shadow fading statistics, namely the distribution and autocorrelation function estimation. Finally, cell coverage for mobile-to-mobile systems is also discussed.

A Slope UTD Solution for a Cascade of Multishaped Canonical Objects

The case of multiple diffraction from a cascade of two dimensional multi-shaped canonical objects such as wedges, knife-edges and cylinders, is investigated. A heuristic slope uniform theory of diffraction (UTD) solution is presented that can give accurate and uniform field predictions at any area point and for any kind of environment. The solution is then compared against measurements taken in an anechoic chamber and a very good agreement is observed. Furthermore, an algorithm for implementing the proposed UTD solution to rural area field predictions is also presented, based on a novel algorithm for optimum fit of canonical shapes to the terrain irregularities. Comparisons between the model and measurements taken in real terrain profile show a very good agreement and uniform field predictions. The importance of the choice of the used canonical object to terrain modeling is also discussed since different diffraction patterns are observed for different combination of the canonical objects

Optimal information theoretic capacity of the planar cellular uplink channel

The majority of information-theoretic hyper-receiver cellular models preserve a fundamental assumption which has initially appeared in Wynerpsilas [1] model, namely the collocation of user terminals (UTs). Although this assumption produces more tractable mathematical models, it is unrealistic with respect to current practical cellular systems. In this paper, we alleviate this assumption by assuming uniformly distributed UTs. The model under investigation is a Gaussian cellular multiple access channel (GCMAC) over a planar cellular array in the presence of power-law path loss and flat fading. In this context, we evaluate the effect of UT distribution on the optimal sum-rate capacity by considering a variable-density cellular system. Furthermore, we compare the sum-rate capacity produced by the planar and the linear cellular array. Finally, the analytical results are interpreted in the context of a typical macrocellular scenario.

Uplink capacity of a variable density cellular system with multicell processing

In this work we investigate the information theoretic capacity of the uplink of a cellular system. Assuming centralised processing for all base stations, we consider a power-law path loss model along with variable cell size (variable density of Base Stations) and we formulate an average path-loss approximation. Considering a realistic Rician flat fading environment, the analytical result for the per-cell capacity is derived for a large number of users distributed over each cell. We extend this general approach to model the uplink of sectorized cellular system. To this end, we assume that the user terminals are served by perfectly directional receiver antennas, dividing the cell coverage area into perfectly non-interfering sectors. We show how the capacity is increased (due to degrees of freedom gain) in comparison to the single receiving antenna system and we investigate the asymptotic behaviour when the number of sectors grows large. We further extend the analysis to find the capacity when the multiple antennas used for each Base Station are omnidirectional and uncorrelated (power gain on top of degrees of freedom gain). We validate the numerical solutions with Monte Carlo simulations for random fading realizations and we interpret the results for the real-world systems.

Transmission and Reflection Coefficients in Time-Domain for a Dielectric Slab for UWB Signals

The time-domain (TD) solutions for the reflection and transmission through a dielectric slab case are presented. In contrast to other existing solutions, the proposed ones can incorporate correctly the multiple internal reflections inside the slab in simple and easy-to-use closed forms. The comparison of the proposed solutions against the numerical inverse Fourier transform of the received waveforms as these were calculated in the frequency domain reveals an extremely good agreement and validates the correctness of our TD forms.

Information theoretic capacity of cellular multiple access channel with shadow fading

In this paper, we extend the well-known model for the Gaussian Cellular Multiple Access Channel originally presented by Wyner. The first extension to the model incorporates the distance-dependent path loss (maintaining a close relevance to path loss values in real world cellular systems) experienced by the users distributed in a planar cellular array. The density of base stations and hence the cell sizes are variable. In the context of a Hyper-receiver joint decoder, an expression for the information theoretic capacity is obtained assuming a large number of users in each cell. The model is further extended to incorporate the log-normal shadow fading variations, ensuring that the shadowing models are fairly comparable to the free space model. Using these fair models the effect of the shadow fading standard deviation on the information theoretic capacity of the cellular system is quantified. It is observed that a higher standard deviation results in lower capacity if the mean path loss is appropriately adjusted in order to model the mean loss due to the physical obstacles causing the shadow fading. The results validate that larger cell sizes and a higher standard deviation of shadowing (with appropriately adjusted mean path loss) results in lower spectral efficiency.

Uplink capacity of MIMO cellular systems with multicell processing

Multiple antennas are known to increase the link throughput by providing a multiplexing gain which scales with the number of antennas. Especially in cellular systems, multiple antennas can be exploited to achieve higher rates without the need for additional base station (BS) sites. In this direction, this paper investigates the multi-antenna capacity scaling in a cellular system which employs multicell processing (hyper-receiver). The model under investigation is a MIMO Gaussian cellular multiple-access channel (GCMAC) over a planar cellular array in the presence of power-law path loss and flat fading. Furthermore, the considered cellular model overcomes the assumption of user collocation utilized by previous models by incorporating uniformly distributed user terminals (UTs). The asymptotic eigenvalue distribution (a.e.d.) of the covariance channel matrix is calculated based on free-probabilistic arguments. In this context, we evaluate the effect of multiple BS/UT antennas on the optimal sum-rate capacity by considering a variable-density cellular system. Finally, the analytical results are interpreted in the context of a typical real-world macrocellular scenario.

The effect of user distribution on a linear Cellular Multiple-Access Channel

The Gaussian cellular multiple-access channel (GCMAC) has been the starting point for studying the Shannon-theoretic limits of cellular systems. In 1994, a simple infinite GCMAC was initially introduced by Wyner and was subsequently extended by researchers to incorporate flat fading environments and power-law path loss models. However, Wyner-like models, preserve a fundamental assumption, namely the symmetry of User Terminals (UTs). In this paper, we investigate the effect of this assumption on the sum-rate capacity limits, by examining the case of distributed and thus asymmetric UTs. The model under investigation is a GCMAC over a linear cellular array in the presence of power-law path loss and flat fading. In this context, we study the effect of UT distribution for cell-centre and cell-edge UTs and we show that its effect is considerable only in the case of low cell density.