Ray Tracing In 4G Wireless Communications


Ray Tracing In 4G Wireless Communications 

4G Communications 



In a typical urban or indoor environment, a radio signal transmitted from 
a fixed source will encounter multiple objects in the environment that produce reflected, diffracted, or scattered copies of the transmitted signal, as shown in the next figure These additional copies of the transmitted signal, called multipath signal components, can be attenuated in power, delayed in time, and shifted in phase and/or frequency from the LOS signal path at the receiver. The multipath and transmitted signal are summed together at the receiver, which often produces distortion in the received signal relative to the transmitted signal.

 
In ray tracing we assume a finite number of reflectors with known location
and dielectric properties. The details of the multipath propagation can then be solved using Maxwell’s equations with appropriate boundary conditions. However, the computational complexity of this solution makes it impractical as a general modeling tool. Ray tracing techniques approximate the propagation of electromagnetic waves by representing the wave fronts as simple particles. Thus, the reflection, diffraction, and scattering effects on the wavefront are approximated using simple geometric equations instead of Maxwell’s more complex wave equations. The error of the ray tracing approximation is smallest when the receiver is many wavelengths from the nearest scatterer, and all the scatterers are large relative to a wavelength and fairly smooth. 
Comparison of the ray tracing method with empirical data shows it to accurately model received signal power in rural areas, along city streets where both the transmitter and receiver are close to the ground, or in indoor environments with appropriately adjusted diffraction coefficients. Propagation effects besides received power variations, such as the delay spread of the multipath, are not always well-captured with ray tracing techniques. If the transmitter, receiver, and reflectors are all immobile then the impact of the multiple received signal paths, and their delays relative to the LOS path, are fixed.
However, if the source or receiver are moving, then the characteristics of
the multiple paths vary with time. These time variations are deterministic when the number, location, and characteristics of the reflectors are known over time. Otherwise, statistical models must be used. Similarly, if the number of reflectors is very large or the reflector surfaces are not smooth then we must use statistical approximations to characterize the received signal.
Hybrid models, which combine ray tracing and statistical fading, can also 
be found in the literature, however we will not describe them here. The most general ray tracing model includes all attenuated, diffracted, and scattered multipath components. This model uses all of the geometrical and dielectric properties of the objects surrounding the transmitter and
receiver. 
Computer programs based on ray tracing such as Lucent’s Wireless
Systems Engineering software (WISE), Wireless Valley’s Site Planner_R, and Marconi’s Planet_R EV are widely used for system planning in both indoor and outdoor environments. In these programs computer graphics are combined with aerial photographs (outdoor channels) or architectural drawings (indoor channels) to obtain a 3D geometric picture of the environment

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