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This article discusses the basics of signal propagation in mobile communications and explores different propagation modes such as ground wave, sky wave, and line-of-sight. It covers topics such as attenuation, free space loss, multipath propagation, and atmospheric absorption.
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CSE 4215/5431:Mobile Communications Winter 2011 Suprakash Datta datta@cs.yorku.ca Office: CSEB 3043 Phone: 416-736-2100 ext 77875 Course page: http://www.cs.yorku.ca/course/4215 Some slides are adapted from the book website CSE 4215, Winter 2011
Signal propagation basics Many different effects have to be considered CSE 4215, Winter 2011
Signal propagation ranges Transmission range communication possible low error rate Detection range detection of the signal possible no communication possible Interference range signal may not be detected signal adds to the background noise sender transmission distance detection interference CSE 4215, Winter 2011
Signal propagation Propagation in free space always like light (straight line) Receiving power proportional to 1/d² in vacuum – much more in real environments(d = distance between sender and receiver) Receiving power additionally influenced by fading (frequency dependent) shadowing reflection at large obstacles refraction depending on the density of a medium scattering at small obstacles diffraction at edges refraction shadowing reflection scattering diffraction CSE 4215, Winter 2011
Real world example CSE 4215, Winter 2011
Propagation Modes • Ground-wave propagation • Sky-wave propagation • Line-of-sight propagation CSE 4215, Winter 2011
Ground Wave Propagation CSE 4215, Winter 2011
Ground Wave Propagation • Follows contour of the earth • Can Propagate considerable distances • Frequencies up to 2 MHz • Example • AM radio CSE 4215, Winter 2011
Sky Wave Propagation CSE 4215, Winter 2011
Sky Wave Propagation • Signal reflected from ionized layer of atmosphere back down to earth • Signal can travel a number of hops, back and forth between ionosphere and earth’s surface • Reflection effect caused by refraction • Examples • Amateur radio • CB radio CSE 4215, Winter 2011
Line-of-Sight Propagation CSE 4215, Winter 2011
Line-of-Sight Propagation • Transmitting and receiving antennas must be within line of sight • Satellite communication – signal above 30 MHz not reflected by ionosphere • Ground communication – antennas within effective line of site due to refraction • Refraction – bending of microwaves by the atmosphere • Velocity of electromagnetic wave is a function of the density of the medium • When wave changes medium, speed changes • Wave bends at the boundary between mediums CSE 4215, Winter 2011
Line-of-Sight Equations • Optical line of sight • Effective, or radio, line of sight • d = distance between antenna and horizon (km) • h = antenna height (m) • K = adjustment factor to account for refraction, rule of thumb K = 4/3 CSE 4215, Winter 2011
Line-of-Sight Equations • Maximum distance between two antennas for LOS propagation: • h1 = height of antenna one • h2 = height of antenna two CSE 4215, Winter 2011
LOS Wireless Transmission Impairments • Attenuation and attenuation distortion • Free space loss • Atmospheric absorption • Multipath (diffraction, reflection, refraction…) • Noise • Thermal noise CSE 4215, Winter 2011
Attenuation • Strength of signal falls off with distance over transmission medium • Attenuation factors for unguided media: • Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal • Signal must maintain a level sufficiently higher than noise to be received without error • Attenuation is greater at higher frequencies, causing distortion CSE 4215, Winter 2011
Free Space Loss • Free space loss, ideal isotropic antenna • Pt = signal power at transmitting antenna • Pr = signal power at receiving antenna • = carrier wavelength • d = propagation distance between antennas • c = speed of light (» 3 ´ 10 8 m/s) where d and are in the same units (e.g., meters) CSE 4215, Winter 2011
Free Space Loss • Free space loss equation can be recast: CSE 4215, Winter 2011
Free Space Loss • Free space loss accounting for gain of other antennas • Gt = gain of transmitting antenna • Gr = gain of receiving antenna • At = effective area of transmitting antenna • Ar = effective area of receiving antenna CSE 4215, Winter 2011
Free Space Loss • Free space loss accounting for gain of other antennas can be recast as CSE 4215, Winter 2011
Multipath propagation Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction Time dispersion: signal is dispersed over time interference with “neighbor” symbols, Inter Symbol Interference (ISI) The signal reaches a receiver directly and phase shifted distorted signal depending on the phases of the different parts multipath pulses LOS pulses signal at sender signal at receiver CSE 4215, Winter 2011
Atmospheric absorption • Water vapor and oxygen contribute most • Water vapor: peak attenuation near 22GHz, low below 15Ghz • Oxygen: absorption peak near 60GHz, lower below 30 GHz. • Rain and fog may scatter (thus attenuate) radio waves. • Low frequency band usage helps… CSE 4215, Winter 2011
Effects of mobility Channel characteristics change over time and location signal paths change different delay variations of different signal parts different phases of signal parts quick changes in the power received (short term fading) Additional changes in distance to sender obstacles further away slow changes in the averagepower received (long term fading) long term fading power t short term fading CSE 4215, Winter 2011
Fading channels • Fading: Time variation of received signal power • Mobility makes the problem of modeling fading difficult • Multipath propagation is a key reason • Most challenging technical problem for Mobile Communications CSE 4215, Winter 2011
Types of Fading • Short term (fast) fading • Long term (slow) fading • Flat fading – across all frequencies • Selective fading – only in some frequencies • Rayleigh fading – no LOS path, many other paths • Rician fading – LOS path plus many other paths CSE 4215, Winter 2011
Fading models CSE 4215, Winter 2011
Dealing with fading channels • Error correction • Adaptive equalization • attempts to increase signal power as needed • can be done with analog circuits or DSP CSE 4215, Winter 2011
