Thomas D. Xenos

Improved Antenna Array Adaptive Beamforming with Low Side Lobe Level Using a Novel Adaptive Invasive Weed Optimization Method

An improved adaptive beamforming technique of antenna arrays is introduced. The technique is implemented by using a novel Invasive Weed Optimization (IWO) variant called Adaptive Dispersion Invasive Weed Optimization (ADIWO) where the seeds produced by a weed are dispersed in the search space with standard deviation specifled by the fltness value of the weed. The adaptive seed dispersion makes the ADIWO converge faster than the conventional IWO. This behavior is verifled by applying both the ADIWO and the conventional IWO on well-known test functions. The ADIWO method is utilized here as an adaptive beamformer that makes a uniform linear antenna array steer the main lobe towards the direction of arrival (DoA) of a desired signal, form nulls towards the respective DoA of several interference signals and achieve low side lobe level (SLL). The proposed ADIWO based beamformer is compared to a Particle Swarm Optimization (PSO) based beamformer and a well known beamforming method called Minimum Variance Distortionless Response (MVDR). Several cases have been studied with difierent number of interference signals and difierent power level of additive zero-mean Gaussian noise. The results show that the ADIWO provides su-cient steering ability regarding the main lobe and the nulls, works faster than the PSO and achieves better SLL than the PSO and MVDR.

Design of a Novel Antenna Array Beamformer Using Neural Networks Trained by Modified Adaptive Dispersion Invasive Weed Optimization Based Data

A new antenna array beamformer based on neural networks (NNs) is presented. The NN training is performed by using optimized data sets extracted by a novel invasive weed optimization (IWO) variant called modified adaptive dispersion IWO (MADIWO). The trained NN is utilized as an adaptive beamformer that makes a uniform linear antenna array steer the main lobe toward a desired signal, place respective nulls toward several interference signals, and suppress the side lobe level (SLL). Initially, the NN structure is selected by training several NNs of various structures using MADIWO-based data and by making a comparison among the NNs in terms of training performance. The selected NN structure is then used to construct an adaptive beamformer, which is compared to MADIWO-based and ADIWO-based beamformers, regarding the SLL and the ability to properly steer the main lobe and the nulls. The comparison is made, considering several sets of random cases with different numbers of interference signals and different power levels of additive zero-mean Gaussian noise. The comparative results exhibit the advantages of the proposed beamformer.

A comparative study of the biological effects of various mobile phone and wireless LAN antennas

This paper presents a comprehensive electromagnetic and thermal analysis of radiation and its impact on human beings, due to the use of various types of commonly used mobile phones and communication antennas. This is one of the first studies that deals with a wide-range comparative investigation of modern cell phones, unlike the majority of existing work, which do not extend beyond the obsolete generic phone case. The rather severe, although overlooked, case of wireless local area network antennas is also considered, due to their increasing use and the large times of exposure associated with them.

On the day-to-day variation of foF2 and M(3000)F2

Abstract Studying the relative deviations of the hourly daily values of the critical frequency foF2 from their corresponding monthly median we are lead to define as “disturbed” those days for which the absolute value of the 3-hour average of the relative deviation at least once exceeds 0.3. The relative deviations of foF2 and of the propagation factor M(3000)F2 are evaluated for quiet days in this sense. Furthermore some examples of disturbed ionosphere are examined and their dependence on season, solar activity and location is pointed out.

Synthesis of a Near-Optimal High-Gain Antenna Array With Main Lobe Tilting and Null Filling Using Taguchi Initialized Invasive Weed Optimization

A near-optimal base-station antenna array synthesis suitable for broadcasting applications is presented. The array is required to provide a high-gain radiation pattern with a main lobe slightly tilted from the horizontal plane and null filling inside an angular sector under the main lobe. To satisfy the above requirements, a novel invasive weed optimization (IWO) variant called Taguchi initialized IWO (TI-IWO) is proposed in this paper. In TI-IWO, the Taguchis optimization method is employed to initialize effectively the positions of the weeds used by the IWO method. In this way, the fitness function starts from lower values and, thus, the TI-IWO method finds better near-optimal solutions than the conventional IWO method. The proposed method has been applied to linear arrays. Due to its easy implementation in practice, a uniform-amplitude excitation distribution is considered to be applied on the array elements. Two cases of isotropic source arrays are studied under specific requirements for maximum possible gain, main lobe tilting, and null filling. Also, the TI-IWO method is applied to optimize realistic cases of collinear wire dipole arrays in front of a mast under the same requirements and an additional one concerning the impedance matching of all the dipoles.

Overall Uncertainty Estimation in Multiple Narrow-Band In Situ Electromagnetic Field Measurements

The possible adverse effects on human health of exposure to nonionizing electromagnetic fields have led international organizations and several countries to adopt exposure limits regarding public or occupational safety. To ensure compliance with these limits, specifically designed measurements with suitable instruments have to be performed. Since uncertainty is always present even when expensive instrumentation is employed, the final results from computations of the measured data deviate from the respective true values. The divergence depends on the instrumentation employed, the used methodology, or even the random effects, e.g., human factors or environmental conditions. In this paper, a method for evaluating total uncertainty when multiple narrow-band electromagnetic field measurements are performed is proposed. This is very common when exposure in residential areas is assessed.

PARABOLIC EQUATION SOLUTION OF TROPOSPHERIC WAVE PROPAGATION USING FEM

In this work,the parabolic equation applied on radi owave and microwave tropospheric propagation,properly manipulated,and resulting in a one-dimensional form,is solved using the Finite Element Method (FEM). The necessary vertical tropospheric profile characteristics are assigned to each mesh element,while the solution advances in small and constant range segments,each excited by the solution of the previous step. This is leading to a marching algorithm,similar to the widely used Split Step formulation. The surface boundary conditions including the wave polarization and surface conductivity properties are directly applied to the FEM system of equations. Since the FEM system returns the total solution,a technique for the separation of the transmitted and reflected waves is also presented. This method is based on the application of the Discrete Fourier Transform (DFT) in the space domain,which allows for the separation of the existing wave components. Finally,abnormal tropospheric condition propagation is being employed to assess the method,while the results are compared to those obtained using the Advance Refractive Prediction System (AREPS v.3.03) software package.

On the Spatial Averaging of Multiple Narrowband Electromagnetic Field Measurements: Methods and Uncertainty Estimation

Methods of computational prediction can be used or specific measurements have to be performed for the estimation of human exposure to nonionizing electromagnetic fields at a certain place. According to the exposure standards, the predicted or the measured values have to be spatially averaged in an area representing the dimensions of the human body and compared with the adopted maximum permissible levels of exposure. In this paper, methods for spatial averaging nonionizing electromagnetic fields are discussed. Moreover, methods for estimating spatial average uncertainty of the total field strength values and of the summation of exposure quotients when narrowband measurements are performed in a multiple-frequency environment are proposed. Furthermore, since maximum field values have to be averaged according to the most widely accepted exposure standards, the traffic-dependent power level variation issue is also addressed. In particular, for GSM systems, an extrapolation method to the maximum possible field levels using data from real-time measurements when the frequency allocation per operator base station and the spatial distribution of the exposure sources are unknown is also proposed. Finally, an uncertainty estimation procedure associated to the extrapolated levels is presented.

On the uncertainty estimation of electromagnetic field measurements using field sensors: a general approach

One of the most common and popular practices on measuring the non-ionising electric and/or magnetic field strength employs field meters and the appropriate electric and/or magnetic field strength sensors. These measurements have to meet several requirements proposed by specific guidelines or standards. On the other hand, performing non-ionising exposure assessment using real measurement data can be a very difficult task due to instrumentation limits and uncertainties. In addition, each measuring technique, practice and recommendation has its own drawbacks. In this paper, a methodology for estimating the overall uncertainty for such measurements, including uncertainty estimation of spatial average values of electric or magnetic field strengths, is proposed. Estimating and reporting measurement uncertainty are of great importance, especially when the measured values are very close to the established limits of human exposure to non-ionising electromagnetic fields.

Optimal Wideband LPDA Design for Efficient Multimedia Content Delivery Over Emerging Mobile Computing Systems

An optimal synthesis of a wideband log-periodic dipole array (LPDA) is introduced in this paper. The LPDA optimization is performed under several requirements concerning the standing wave ratio, the forward gain, the gain flatness, the front-to-back ratio, and the sidelobe level, over a wide frequency range. The LPDA geometry that complies with the aforementioned requirements is suitable for efficient multimedia content delivery. The optimization process is accomplished by applying a recently introduced method called invasive weed optimization (IWO). The method has already been compared to other evolutionary methods and has shown superiority in solving complex nonlinear problems in telecommunications and electromagnetics. In this paper, the IWO method has been chosen to optimize an LPDA for operation in the frequency range of 800-3300 MHz. Due to its excellent performance, the LPDA can effectively be used for multimedia content reception over future mobile computing systems.