CMPE 477 Wireless and Mobile Networks Lecture 3: Antennas and Propagation Antennas Propagation Modes Line of Sight Transmission Fading in the Mobile Environment
Introduction An antenna is an electrical conductor or system of conductors for radiating/collecting electromagnetic energy Transmission - radiates electromagnetic energy into medium Reception - collects electromagnetic energy from medium In two-way communication, the same antenna can be used for transmission and reception Radiation: An antenna radiates power in all directions, however does not perform well or the same in all directions.
Radiation Patterns Radiation pattern Graphical representation of radiation properties of an antenna The simplest pattern is produced by Isotropic Antenna Ideal antenna that radiates power the same in all directions. Depicted as two-dimensional cross section Reception pattern Receiving antenna s equivalent to radiation pattern
Antennas: simple dipoles Real antennas are not isotropic radiators but, e.g., dipoles with lengths /4 (Marconi) on car roofs or /2 as Hertzian dipole shape of antenna proportional to wavelength /4 /2 simple dipoles Example: Radiation pattern of a simple Hertzian dipole y y z x z x side view (xy-plane) side view (yz-plane) top view (xz-plane)
Antennas: directed and sectorized Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley) y x y z z x directed antenna side view (xy-plane) z side view (yz-plane) z top view (xz-plane) x x sectorized antenna top view, 3 sector top view, 6 sector parabolic antenna
Antenna Gain Antenna gain Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna) Effective area Related to physical size and shape of antenna Simply increasing the size of antenna does not guarantee an increase in effective area; however, other factors being equal, antennas with higher maximum effective area are generally larger.
Antenna Gain Relationship between antenna gain and effective area 2 4A e 4f Ae G 2 2 c G = antenna gain A e = effective area f = carrier frequency c = speed of light = carrier wavelength Type of Antenna Effective Area Power Gain Isotropic 2 /4ᴨ 1 Half-wave dipole Parabolic, Face Area A 1.5 2 /4ᴨ 1.5 0.56A 7A/ 2
Propagation Modes In wireless networks, the signal has no wire to determine the direction of propagation Three basic routes are followed by the wireless signals: Ground-wave propagation Sky-wave propagation Line-of-sight propagation
Ground Wave Propagation
Ground Wave Propagation Signals follow the contour of the earth and propagate long distances Found in signals up to 2MHz Why? Wavefront of the signal near the earth is due to the current produced by the electromagnetic wave on the earth s surface Diffraction Example: AM radio
Sky Wave Propagation
Sky Wave Propagation Signal reflected from ionized layer of atmosphere back down to earth Why? Caused by refraction: Mediums at different densities Signal can travel a number of hops, back and forth between ionosphere and earth s surface, travelling thousands of km. Examples Amateur radio CB (Citizens' Band) radio International Broadcasts, BBC Voice of America
Line-of-Sight Propagation
Line-of-Sight Propagation Transmitting and receiving antennas should be within line of sight Satellite communication signal above 30 MHz not reflected by ionosphere
LOS Wireless Transmission Impairments Impairments cause the received signal to be different than the transmitted signal or degrade the signal quality Result: Bit errors are introduced Impairments Attenuation and attenuation distortion Free space loss Noise Atmospheric absorption Multipath Refraction
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
Free Space Loss Even if there are no other source of attenuation or impairment, the signal attenuates with the distance Expressed in ratio For the ideal isotropic antenna P t P r 2 4d 4fd 2 P t = signal power at transmitting antenna P r = 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) c 2 2
Free Space Loss Free space loss equation can be recast in decibels: L db 10log Pt P r 4d 20log 20logd 21.98 db 20log 4fd 20log 20log c f 20logd 147.56 db
Free Space Loss Free space loss accounting for gain of other antennas P t P r 2 2 2 4 d d cd G G r t 2 A r A t f 2 A r 2 A t G t = gain of transmitting antenna G r = gain of receiving antenna A t = effective area of transmitting antenna A r = effective area of receiving antenna
Free Space Loss Free space loss accounting for gain of other antennas can be recast as L 20log 20log d 10log A A db t r f 20log d 10log A A t 169.54dB 20log r
Categories of Noise The received signal will consist of transmitted signal, modified by the various impairments and plus additional unwanted signals, referred as noise Thermal Noise Intermodulation noise Crosstalk Impulse Noise
Thermal Noise Thermal noise due to agitation of electrons Present in all electronic devices and transmission media Cannot be eliminated, puts an upper bound on system performance Uniformly distributed over the frequency spectrum Referred as white noise Function of temperature Particularly significant for satellite communication
Thermal Noise Amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is: N kt 0 W/Hz N 0 = noise power density in watts per 1 Hz of bandwidth k = Boltzmann's constant = 1.3803 10-23 J/K T = temperature, in kelvins (absolute temperature) Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz (in watts): N ktb
Noise Terminology Intermodulation noise occurs if signals with different frequencies share the same medium Interference caused by a signal produced at a frequency that is the sum or difference of original frequencies Crosstalk unwanted coupling between signal paths Impulse noise irregular pulses or noise spikes Short duration and of relatively high amplitude Caused by external electromagnetic disturbances, or faults and flaws in the communications system
Expression E b /N 0 Already discussed: SNR Related to SNR, quality of the digital communication performance Ratio of signal energy per bit to noise power density per Hertz E b N 0 S / R N S TR The bit error rate for digital data is a function of E b /N 0 Given a value for E b /N 0 to achieve a desired error rate, parameters of this formula can be selected As bit rate R increases, transmitted signal power must increase to maintain required E b /N 0 0 k
Other Impairments Atmospheric absorption water vapor and oxygen contribute to attenuation, peak attenuation around 22GHz. Multipath obstacles reflect signals so that multiple copies with varying delays are received Refraction bending of radio waves as they propagate through the atmosphere
Multipath Interference
Refraction
Multipath Propagation
Multipath Propagation Reflection - occurs when signal encounters a surface that is large relative to the wavelength of the signal Diffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave Scattering occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less
The Effects of Multipath Propagation Multiple copies of a signal may arrive at different phases If phases add destructively, the signal level relative to noise declines, making detection more difficult Intersymbol interference (ISI) One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit
Types of Fading fast fading rapid changes in strength over half wavelength distances eg. 900MHz wavelength is 0.33m see 20-30dB slow fading slower changes due to user passing different height buildings, gaps in buildings etc. over longer distances than fast fading flat fading affects all frequencies in same proportion selective fading different frequency components affected differently
Review Antenna Antenna gain Attenuation Crosstalk Diffraction Dipole Fading Fast Fading Flat fading Free Space Loss Ground Wave Propagation Isotropic Antenna Multipath Reflection Refraction Scattering Selective Fading Sky Wave Propagation Slow Fading Line of Sight Noise