Modeling and characterization of urban radio channels for mobile communications

Results of this thesis contribute in modeling and characterization of radio channels for future mobile communications. The results are presented mainly in three parts: a) modeling of propagation mechanisms, b) methodology of developing a propagation model, c) characterization of urban radio channel. One of the main propagation physical phenomena that have an important role in diverting signals to non line of sight scenarios is the diffraction process. This thesis proposes diffraction coefficients that have better agreement with finite difference time domain solution and rigorous diffraction theory than the coefficient commonly used in propagation predictions for mobile communications. The importance of diffuse scattering has also been investigated and showed that this physical process may have a key role in urban propagation, with a particular impact on the delay spread and angular spread of the signal at the receiver. This thesis proposes wideband propagation models for main and perpendicular streets of urban street grids. The propagation models are ray-based and are given in explicit mathematical expressions. Each ray is characterized in terms of its amplitude, delay, and angle of arrival, angle of departure for vertical and horizontal polarizations. Each of these characteristics is given in a closed mathematical form. Having wideband propagation model in explicit expression makes its implementation easy and computation fast. Secondary source modeling approach for perpendicular streets has also been introduced in this thesis. The last part of the thesis deals with characterization of urban radio channels for extracting parameters that help in successful design of mobile communication systems. Knowledge of channel characteristics enables reaching optimum trade off between system performance and complexity. This thesis analyzes measurement results at 2 GHz to extract channel parameters in terms of Rake finger characteristics in order to get information that helps to optimize Rake receiver design for enhanced-IMT2000 systems. Finger life distance has also been investigated for both micro- and small cell scenarios. This part of the thesis also presents orthogonality factor of radio channel for W-CDMA downlink at different bandwidths. Characterization of dispersion metrics in delay and angular domains for microcellular channels is also presented at different base station antenna heights. A measure of (dis-) similarity between multipath components in terms of separation distance in delay and angular domains is introduced by the concept of distance function, which is a step toward in development of algorithm extraction and analysis multipath clustering. In summary, the significant contributions of the thesis are in three parts. 1) Development of new diffraction coefficients and corrections of limitations of existing one for accurate propagation predictions for mobile communications. 2) Development of wideband propagation models for urban street grid. The novelty of the model is the development in explicit mathematical expressions. The developed models can be used to study propagation problem in microcellular urban street grids. 3) Presenting channel parameters that will help in the design of future mobile communication systems (enhanced-IMT2000), like number of active fingers, finger life distance, and orthogonality factors for different bandwidths. In addition, a technique based on multipath separation distance is proposed as a step toward in development of algorithms for extraction and analysis of multipath clusters.reviewe

Computer simulations of the structure, stability and phase transitions of diatomic molecules physisorbed on ionic surfaces : the CO/MgO(001), N2/MgO(001) and N2/NaCl(001) systems

Metropolis Monte Carlo simulations, using semiempirical potentials, are performed to study the structures, stability, and phase transitions of layers of CO and N 2 molecules physisorbed on the MgO(100) surface as well as a monolayer of N 2 physisorbed on NaCl(001) surface. In agreement with experiments [35-37], our simulations show that the c (4 x 2) structure of CO on the MgO(001) surface is the most stable structure below 41 K. The unit cell contains three CO molecules; two bridging molecules tilted in opposite directions by the same polar angle of 31 ̕with respect to the surface normal and a third molecule perpendicular to the surface. At 41 K the c (4 x 2) phase undergoes a transition into a less dense disordered phase accompanied by the desorption of some molecules. The density of this disordered phase is the same as for the p (3 x 2) phase. A model to compare the stability of the c (4 x 2) and the p (3 x 2) phases is constructed and suggests that at sufficiently high pressures and temperatures the p (3 x 2) phase is more stable than the c (4 x 2) phase as found by Panella et al . [35]. We propose that a sequence of transitions to a set of (n x 2) structure with ever decreasing density is possible under suitable conditions of temperature and pressure. This sequence of transitions is an example of the devil's staircase phenomenon as has been suggested by LEED experiments. Nitrogen molecules adsorbed on MgO(001) were studied and were also found to form a sequence of structures. A ([Special characters omitted.] ) R 33.7 ̕structure is found to form the most stable phase at low temperatures (below 20 K). This structure undergoes a phase transition into a less dense phase at 20 K and in all likelihood evolves into the less dense ([Special characters omitted.] ) R 36.9 ̕structure. The ([Special characters omitted.] )R36.9 ̕structure is stable up to 25 K. These results are in agreement with HAS results. This system might also prove to be an example of the devil's staircase phenomena. A monolayer of nitrogen molecules adsorbed on an NaCl(001) surface are used to test the validity of the theory of critical phenomena and its applicability to molecular systems. The simulations predict that this system undergoes a continuous order-disorder transition near 25 K with a logarithmically divergent heat capacity. The values of the critical exponents for the order parameter and susceptibility are calculated and found to deviate significantly from the Ising values but still satisfy Rushbrooke's scaling law. This behaviour is typical of the universality class containing the XY model with cubic anisotropy where the critical exponents are functions of the anisotropy and are thus "nonuniversal

Terahertz Channel Characterization Inside the Human Skin for Nano-Scale Body-Centric Networks

This paper focuses on the development of a novel radio channel model inside the human skin at the terahertz range, which will enable the interaction among potential nano-machines operating in the inter cellular areas of the human skin. Thorough studies are performed on the attenuation of electromagnetic waves inside the human skin, while taking into account the frequency of operation, distance between the nano-machines and number of sweat ducts. A novel channel model is presented for communication of nano-machines inside the human skin and its validation is performed by varying the aforementioned parameters with a reasonable accuracy. The statistics of error prediction between simulated and modeled data are: mean (μ)= 0.6 dB and standard deviation (σ)= 0.4 dB, which indicates the high accuracy of the prediction model as compared with measurement data from simulation. In addition, the results of proposed channel model are compared with terhaertz time-domain spectroscopy based measurement of skin sample and the statistics of error prediction in this case are: μ = 2.10 dB and σ = 6.23 dB, which also validates the accuracy of proposed model. Results in this paper highlight the issues and related challenges while characterizing the communication in such a medium, thus paving the way towards novel research activities devoted to the design and the optimization of advanced applications in the healthcare domain

Ellipticity Statistics of Ultra Wideband MIMO Channels for Body Centric Wireless Communication

In this paper, ellipticity statistics of 2 × 2 ultra wideband multiple-input-multiple-output (MIMO) channel for body-centric wireless communication is evaluated by quantifying four different on body links namely; waist-back, waist-chest, waist-ankle and waist-wrist. Results show that at lower values of signal to noise (SNR), spatial multiplexing dependent capacity degrades as the eigen value dispersion decreases (i.e., lower ellipticity statistic), whereas it increases at higher values of SNR