Dinkov Georgi Wireless Networks Prof. Bala
Basic Wave Propagation Overview The problem of wave propagation is central to mobile communications. It forms the lowest (physical) layer of the mobile communication framework. Proper understanding of the nature of wave propagation is important in order to grasp the limitations of the mobile concept. Transmission/Reception Information is exchanged between the transmitter and receiver in the following way. The transmitter launches radio signal into the propagation medium. The wave travels through the medium as ground, troposphere or ionosphere wave and exhibits phenomena like reflection, refraction and scattering. If the energy of wave (depending of the power of transmission) is high enough, it reaches a detection point (the receiver) where it might be necessary to remove noise added during the journey before the receiver gets the original radio wave that was actually sent by the transmitter. The wireless environment has a significant level of randomness built into it. The path of the wave while traveling through the medium can be a simple line-of-sight or it can be a complicated reflection path. Relative motion of the objects in the medium through which the wave travels affects the fading of the wave and thus the predictability and performance of transmission. Models Modeling the behavior of electromagnetic waves is the major challenge in mobile communications. The most commonly used Models are: Statistical Modeling Because of the inherent randomness of radio wave behavior, statistical theory and probability distributions are used to model wave propagation. Empirical Modeling With this model, attempts are actually made to find direct equations that describe the behavior of radio waves in a typical wireless environment. In general, the equations concern the topology of environment.
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Physical Phenomena In general, there are three physical phenomena that can significantly affect the performance of wireless communications. Rayleigh (Fast) Fading Delay Spread / Time Dispersion Long Term Fading
Propagation Paths Free space propagation Free space propagation occurs only when the waves are not influenced by the earth or its atmosphere. That kind of propagation is rare and generally occurs only in outer space (i.e. communication between two spaceships, although the ships themselves influence the wave somewhat). Ground Waves Ground wave travels in contact with earth's surface by scattering off objects on its way. Troposphere Waves These are waves that are kept close to the surface of the earth by refraction in the lower atmosphere Ionosphere/Sky Waves These waves arise as a result of waves that leave the antenna at angles >0 and then get reflected by the ionosphere and again the surface. earth's This successive reflection allows communications over very long distances, especially during sunspot activity, or during specific hours of day (usually late morning and early evening). Propagation mechanisms Reflection phenomenon that occurs when a wave collides with an object whose size is very large in comparison to the wavelength (buildings, earth surface). Diffraction phenomenon that occurs when the wave bends around object, which have very sharp irregularities. Scattering phenomenon that occurs when the wave encounters object whose dimensions are much smaller than the wavelength (signs, leaves). Noise sources Three major sources of noise exist and these are Galactic Noise, Atmospheric Noise, Man-made noise. Galactic noise comes from the suns and stars (15-100 MHz). Atmospheric noise is generated by the ionized layers of atmosphere and is major problem for MF and HF communication. Man-made noise is a result of power lines, industrial activity and fluorescent equipment. Antennas Antennas are a very important part of mobile communications because the efficient transmission of the signal is the main goal of the mobile system. There are several kinds of antennas but the most widely used ones are the Pencil Beam (microwave radio relay or satellite
communications) and Omnidirectional / Toroidal (wireless cellular communications). Most mobile communications require small antennas, which are derivatives of the dipole antenna. The Frequency Spectrum Low Frequency (10-300KHz) Medium Frequency (300-3000KHz) High Frequency (3-30MHz) Very High Frequency (30-300MHz) Ultra High Frequency (300-3000MHz) Super High Freque...