Antennas and Propagation Antennas Propagation Modes Line of Sight - PowerPoint PPT Presentation

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 simple  /4  /2 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 y z directed x z x antenna side view (xy-plane) side view (yz-plane) top view (xz-plane) z z sectorized x antenna x 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 4 A 4 f A   e e G  2 2 c Type of Effective Power Gain G = antenna gain Antenna Area  2 /4 ᴨ A e = effective area Isotropic 1 1.5  2 /4 ᴨ Half-wave 1.5 f = carrier frequency dipole c = speed of light 7A/  2 Parabolic, 0.56A  = carrier wavelength Face Area A

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       2 2 P 4 d 4 fd   t  2 2 P c r 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)

Free Space Loss Free space loss equation can be recast in decibels:  d   P 4     t L 10 log 20 log  dB   P r          20 log 20 log d 21 . 98 dB    4 fd           20 log 20 log f 20 log d 147 . 56 dB   c

Free Space Loss Free space loss accounting for gain of other antennas           2 2 2 2 P 4 d d cd    t  2 2 P G G A A f A A r r t r t r 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 20 log 20 log d 10 log A A dB t r            20 log f 20 log d 10 log A t A 169 . 54 dB 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  k T W/Hz 0  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 k TB

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 S R S   k N N TR 0 0  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

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