Wireless Radio Propagation& Antennas Fundamentals Dr. R. K. Rao
Propagation Modes • Ground Wave Ground wave propagation more or less follows the contour of the earth • Sky Wave Signal from an earth based antenna is reflected from the ionized layer of the upper atmosphere back down to earth • Line of Sight wave Communication is by line of sight
Wireless Propagation • Wireless propagation is the total of everything that happens to a wireless signal as the signal travels from Point A to Point B. • The study of how EM waves travel and interact with matter can become extremely complex. • There are several important simplifications which can be made. • In a vacuum, 2.4 GHz microwaves travel at the speed of light. • Once started, these microwaves will continue in the direction they were emitted forever, unless they interact with some form of matter. • In the atmosphere, the microwaves are traveling in air, not in a vacuum. • This does not significantly change their speed. • Similar to light, when RF travels through transparent matter, some of the waves are altered. • 2.4 & 5 GHz microwaves also change, as they travel through matter. • Amount of alteration depends heavily on the frequency of the waves and the matter.
Wireless Propagation Mental picture • Wave is not a spot or a line, but a moving wave. • Like dropping a rock into a pond. • Wireless waves spread out from the antenna. • Wireless waves pass through air, space, people, objects,…
Transmission Impairments • Attenuation • Free Space Loss • Noise • Atmospheric Absorption • Multi-path • Reflection • Refraction
Attenuation Same wavelength (frequency), less amplitude. • Attenuation is the loss in amplitude that occurs whenever a signal travels through wire, free space, or an obstruction. • At times, after colliding with an object the signal strength remaining is too small to make a reliable wireless link.
Attenuation and Obstructions • Shorter the wavelength (higher frequency) of the wireless signal, the more the signal it is attenuated. • Longer the wavelength (lower frequency) of the wireless signal, the less the signal is attenuated. Same wavelength (frequency), less amplitude.
Attenuation and Obstructions • The wavelength for the AM (810 kHz) channel is 1,214 feet • The larger the wavelength of the signal relative to the size of the obstruction, the less the signal is attenuated. • The shorter the wavelength of the signal relative to the size of the obstruction, the more the signal is attenuated.
Free-Space Waves • Free-space wave is a signal that propagates from Point A to Point B without encountering or coming near an obstruction. • The only amplitude reduction is due to “free space loss” . • This is the ideal wireless scenario.
Noise • The received signal will consist of the transmitted signal, modified by various distortions imposed by the transmission system, plus additional unwanted signal inserted between transmission and reception • Thermal Noise; Crosstalk; Impulse noise • Measure is Signal-to-Noise Ratio, Eb/No
Reflected Waves • When a wireless signal encounters an obstruction, normally two things happen: • Attenuation – The shorter the wavelength of the signal relative to the size of the obstruction, the more the signal is attenuated. • Reflection – The shorter the wavelength of the signal relative to the size of the obstruction, the more likely it is that some of the signal will be reflected off the obstruction.
Microwave Reflections • Microwave signals: • Frequencies between 1 GHz – 30 GHz (this can vary among experts). • Wavelength between 12 inches down to less than 1 inch. • Microwave signals reflect off objects that are larger than their wavelength, such as buildings, cars, flat stretches of ground, and bodes of water. • Each time the signal is reflected, the amplitude is reduced.
Reflection • Reflection is the light bouncing back in the general direction from which it came. • Consider a smooth metallic surface as an interface. • As waves hit this surface, much of their energy will be bounced or reflected. • Think of common experiences, such as looking at a mirror or watching sunlight reflect off a metallic surface or water. • When waves travel from one medium to another, a certain percentage of the light is reflected. • This is called a Fresnel reflection (Fresnel coming later).
Reflection • Radio waves can bounce off of different layers of the atmosphere. • The reflecting properties of the area where the WLAN is to be installed are extremely important and can determine whether a WLAN works or fails. • Furthermore, the connectors at both ends of the transmission line going to the antenna should be properly designed and installed, so that no reflection of radio waves takes place.
Microwave Reflections Multipath Reflection • Advantage: Can use reflection to go around obstruction. • Disadvantage: Multipath reflection – occurs when reflections cause more than one copy of the same transmission to arrive at the receiver at slightly different times.
Multipath Reflection • Reflected signals 1 and 2 take slightly longer paths than direct signal, arriving slightly later. • These reflected signals sometimes cause problems at the receiver by partially canceling the direct signal, effectively reducing the amplitude. • The link throughput slows down because the receiver needs more time to either separate the real signal from the reflected echoes or to wait for missed frames to be retransmitted. • Solution discussed later.
Modeling Multi-path environment • Channel is often modeled as: Rayleigh Fading Channel: When there are multiple indirect paths between transmitter and receiver and no distinct dominant path such as LOS- applicable to outdoor environment Rician Fading Channel: When there is a direct LOS path in addition to number of indirect multi-path signals –applicable to indoor environment
Diffraction • Diffraction of a wireless signal occurs when the signal is partially blocked or obstructed by a large object in the signal’s path. • A diffracted signal is usually attenuated so much it is too weak to provide a reliable microwave connection. • Do not plan to use a diffracted signal, and always try to obtain an unobstructed path between microwave antennas. Diffracted Signal
Weather - Precipitation Precipitation: Rain, snow, hail, fog, and sleet. • Rain, Snow and Hail • Wavelength of 2.4 GHz 802.11b/g signal is 4.8 inches • Wavelength of 5.7 GHz 802.11a signal is 2 inches • Much larger than rain drops and snow, thus do not significantly attenuate these signals. • At frequencies 10 GHz and above, partially melted snow and hail do start to cause significant attenuation.
Weather - Precipitation • Rain can have other effects: • Get inside tiny holes in antenna systems, degrading the performance. • Cause surfaces (roads, buildings, leaves) to become more reflective, increasing multi-path fading. • Tip: Use unobstructed paths between antennas, and do not try to blast through trees, or will have problems.
Collapsed tower Weather - Ice • Ice buildup on antenna systems can: • Reduce system performance • Physically damage the antenna system
Weather - Wind • The effect of wind: • Antenna on the the mast or tower can turn, decreasing the aim of the antenna. • The mast or tower can sway or twist, changing the aim. • The antenna, mast or tower could fall potentially injuring someone or something.
Refraction Sub-Refraction Refraction (straight line) • Refraction (or bending) of signals is due to temperature, pressure, and water vapor content in the atmosphere. • Amount of refractivity depends on the height above ground. • Refractivity is usually largest at low elevations. • The refractivity gradient (k-factor) usually causes microwave signals to curve slightly downward toward the earth, making the radio horizon father away than the visual horizon. • This can increase the microwave path by about 15%, Normal Refraction Earth
Refraction • Radio waves also bend when entering different materials. • This can be very important when analyzing propagation in the atmosphere. • It is not very significant in WLANs, but it is included here, as part of a general background for the behavior of electromagnetic waves.
Antenna Directivity • Antennas radiate wireless power • Accept wireless signal energy from the transmission line connected to a transmitter • Launch that wireless energy into free-space
Antenna Directivity • Antennas focus wireless energy like a flashlight reflector (focusing element) focuses light from a flashlight bulb. • Without the focusing element, the bulb radiates light energy in all direction. • No direction receives more light than any other direction.
Antenna Directivity Theoretical Isotropic Antenna • Light energy from an unfocused flashlight bulb is similar to the wireless energy radiated from a theoretical isotropic antenna. • Like a light bulb, an isotropic antenna radiates wireless energy equally in all directions and does not focus the energy in any single direction.
Antenna Directivity • A flashlight focuses the light into a beam that comes out the front of the flashlight. • The flashlight (reflector) does not amplify the power or total amount of light from the bulb. • The flashlight simply focuses the light so all of it travels in the same direction.
Antenna Directivity • By focusing the light, the flashlight provides more directivity (beam focusing power). • An antenna provides directivity for the wireless energy that it focuses. • Depending upon the design of the antenna, antennas focus and radiate their energy more strongly in on favored direction. • When receiving, antennas focus and gather energy from their favored direction and ignore most of the energy arriving from all other directions.
Antenna Radiated Patterns • Antennas exhibit directivity by radiating most of their power in one direction. • Major or Main Lobe – Main direction of the power from the antenna • Minor or Side Lobes – Small amount of power in other directions • Nulls – Where no power is radiated Top View Main Lobe Front Null Back Side Lobes
Antenna Radiated Patterns • Antennas provide the same directivity for transmitting and receiving. • Antennas radiate transmitter power in the favored direction(s) when transmitting. • Antennas gather signals coming in from the favored directions(s) when receiving. Top View Main Lobe Front Null Back Side Lobes
Antenna Radiated Patterns • When selecting antennas, remember: • When receiving, antenna directivity not only gathers incoming signals from the favored direction, but also reduces noise, interference, and unwanted signals coming in from other directions. Patch Antenna (Directional Antenna)
Antenna Radiated Patterns • An omnidirectional antenna radiates equally well in all horizontal directions around the main lobe, surrounding the antenna like a donut. Top View (H) Side View (V) Dipole Antenna (Omnidirectional Antenna)
Antenna Gain Like a flashlight, there is always a tradeoff between gain, which is comparable to brightness in a particular direction, and beamwidth, which is comparable to the narrowness of the beam. (coming) • Antenna gain – Measurement of the power in the main lobe of an antenna and comparing that power to the power in the main lobe of a reference antenna. • Gain - This refers to the amount of increase in energy that an antenna appears to add to an RF signal. • Measure in dBi or dBd • dBd – “d” is the gain measured relative to the gain of a dipole reference antenna. • dBi – “i” is the gain measure relative to the gain of a theoretical isotropic antenna.
Antenna Gain • The dBi is a unit measuring how much better the antenna is compared to an isotropic radiator. • An isotropic radiator is an antenna which sends signals equally in all directions (including up and down). • An antenna which does this has an 0dBigain. • The higher the decibel figure the higher the gain. • For instance, a 6dBi gain antenna will receive a signal better than a 3dBi antenna. +21 dBi or about 100 times the signal strength when comparing it to an isotropic antenna Top View
Antenna Gain Dipole antenna • A dBd unit is a measurement of how much better an antenna performs against a dipole antenna. • As a result a dipole antenna has a 0dBd gain. • Note: Wireless power never stops exactly on a sharp line like the lobe drawings show, but tapers off. • More later…
Antenna Beamwidth • Beamwidth – The width of the main beam (main lobe) of an antenna. • Measures the directivity of an antenna • The smaller the beamwidth in degrees, the more the antenna focuses power into its main lobe. • The more power of the main lobe, the further the antenna can communicate.
Antenna Beamwidth 15 dBi • Beamwidth is a measurement used to describe directional antennas. • Beamwidth is sometimes calledhalf-power beamwidth. • Half-power beamwidth is the total width in degrees of the main radiation lobe, at the angle where the radiated power has fallen below that on the centerline of the lobe, by -3 dB (half-power). -3 dBi 12 dBi 15 dBi
Remember, wireless power does not stop and start exactly along a straight line, but declines gradually with distance. • The smooth outlines of the main lobes show the approximate intensity of the wireless power at various distances away from the antenna. • The dotted lines pass through the half-power points – the points on each side of the center of the main lobe where the wireless power is one-half as strong as it is at the center of the lobe.
Line of Sight Attenuated Signal Diffracted Signal • When a wireless signal encounters an obstruction, the signal is always attenuated and often reflected or diffracted. • It is important to try and obtain a wireless line-of-sight whenever possible, especially in a wireless WAN environment (outdoor connections between building or different parts of a campus). • A wireless LOS typically requires visual LOS plus additional path clearance to account for the spreading of the wireless signal (Fresnel Zone – coming).
Visual LOS “I see you!” “And, I see you!” • There is a difference between visual LOS and wireless LOS. • This is because of the difference in wavelengths. • The wavelength of visual light is very small. • For example, the wavelength of a green light is only about 1/50,000th of an inch • Remember, the wavelength of a 2.4 GHz WLAN signal is about 4.8 inches. 1 Mile 1 Mile
LOS • A lightwave and a wireless wave are similar. • Both are forms of electromagnetic radiation. • Both must obey the same laws of physics as they propagate. • Wireless signals are like lightwaves that you cannot see. 1 Mile 1 Mile