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Classifications of Transmission Media Unguided Media General

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Classifications of Transmission Media Unguided Media General Classifications of Transmission Media Transmission Medium Transmission Media, Antennas and Physical path between transmitter and receiver Propagation Guided Media Waves are guided along a solid medium E.g., copper twisted pair, copper coaxial cable, optical fiber Chapter 5 Unguided Media Provides means of transmission but does not guide electromagnetic signals Usually referred to as wireless transmission E.g., atmosphere, outer space Unguided Media General Frequency Ranges Microwave frequency range Transmission and reception are achieved by 1 GHz to 40 GHz means of an antenna Directional beams possible Configurations for wireless transmission Suitable for point-to-point transmission Used for satellite communications Directional Radio frequency range Omnidirectional 30 MHz to 1 GHz Suitable for omnidirectional applications Infrared frequency range 11 14 Roughly, 3x10 to 2x10 Hz Useful in local point-to-point multipoint applications within confined areas Terrestrial Microwave Satellite Microwave Description of common microwave antenna Description of communication satellite Parabolic "dish", 3 m in diameter Microwave relay station Used to link two or more ground-based microwave Fixed rigidly and focuses a narrow beam transmitter/receivers Achieves line-of-sight transmission to receiving Receives transmissions on one frequency band (uplink), antenna amplifies or repeats the signal, and transmits it on Located at substantial heights above ground level another frequency (downlink) Applications Applications Long haul telecommunications service Television distribution Short point-to-point links between buildings Long-distance telephone transmission Private business networks 1 Broadcast Radio Introduction to Antenna Description of broadcast radio antennas An antenna is an electrical conductor or Omnidirectional system of conductors Antennas not required to be dish-shaped Transmission - radiates electromagnetic energy Antennas need not be rigidly mounted to a precise into space alignment Reception - collects electromagnetic energy Applications from space Broadcast radio In two-way communication, the same VHF and part of the UHF band; 30 MHZ to 1GHz antenna can be used for transmission and Covers FM radio and UHF and VHF television reception Radiation Patterns Types of Antennas Radiation pattern Isotropic antenna (idealized) Graphical representation of radiation properties of an Radiates power equally in all directions antenna Depicted as two-dimensional cross section Dipole antennas Beam width (or half-power beam width) Half-wave dipole antenna (or Hertz antenna) Measure of directivity of antenna Quarter-wave vertical antenna (or Marconi Reception pattern antenna) Receiving antenna’s equivalent to radiation pattern Parabolic Reflective Antenna Antenna Gain Antenna Gain Antenna gain Relationship between antenna gain and effective area Power output, in a particular direction, compared to that produced in any direction by a 4πA 4πf 2 A G = e = e perfect omnidirectional antenna (isotropic λ2 c2 antenna) G = antenna gain Effective area Ae = effective area f = carrier frequency Related to physical size and shape of antenna 8 c = speed of light (» 3 ´ 10 m/s) λ = carrier wavelength 2 Propagation Modes Ground Wave Propagation Ground-wave propagation Sky-wave propagation Line-of-sight propagation Ground Wave Propagation Sky Wave Propagation Follows contour of the earth Can Propagate considerable distances Frequencies up to 2 MHz Example AM radio Sky Wave Propagation Line-of-Sight 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 Voice of America 3 Line-of-Sight Propagation Line-of-Sight Equations Transmitting and receiving antennas must be Optical line of sight within line of sight d = 3.57 h Satellite communication – signal above 30 MHz not reflected by ionosphere Effective, or radio, line of sight Ground communication – antennas within effective line of site due to refraction d = 3.57 Κh Refraction – bending of microwaves by the atmosphere d = distance between antenna and horizon (km) h = antenna height (m) Velocity of electromagnetic wave is a function of the density of the medium K = adjustment factor to account for refraction, When wave changes medium, speed changes rule of thumb K = 4/3 Wave bends at the boundary between mediums LOS Wireless Transmission Line-of-Sight Equations Impairments Maximum distance between two antennas Attenuation and attenuation distortion for LOS propagation: Free space loss: signal disperses with distance Noise 3.57( Κh1 + Κh2 ) Atmospheric absorption Multipath h1 = height of antenna one h2 = height of antenna two Refraction Thermal noise Acrobat Document Attenuation Free Space Loss Strength of signal falls off with distance over Free space loss, ideal isotropic antenna transmission medium P (4πd )2 ()4πfd 2 Attenuation factors for unguided media: t = 2 = 2 Received signal must have sufficient strength so that Pr λ c circuitry in the receiver can interpret the signal Pt = signal power at transmitting antenna Signal must maintain a level sufficiently higher than Pr = signal power at receiving antenna noise to be received without error λ = carrier wavelength d = propagation distance between antennas Attenuation is greater at higher frequencies, causing distortion c = speed of light (» 3 ´ 10 8 m/s) where d and λ are in the same units (e.g., meters) Amplifiers are introduced to amplify high frequences 4 Free Space Loss Free Space Loss Free space loss equation can be recast: Free space loss accounting for gain of other antennas 2 2 2 2 P ⎛ 4πd ⎞ L =10log t = 20log Pt (4π ) (d ) ()λd (cd ) dB ⎜ ⎟ = = = Pr ⎝ λ ⎠ 2 2 Pr GrGtλ Ar At f Ar At = −20log(λ)+ 20log(d )+ 21.98 dB Gt = gain of transmitting antenna G = gain of receiving antenna ⎛ 4πfd ⎞ r = 20log⎜ ⎟ = 20log()f + 20log ()d −147.56 dB At = effective area of transmitting antenna ⎝ c ⎠ Ar = effective area of receiving antenna Free Space Loss Categories of Noise Free space loss accounting for gain of other Thermal Noise antennas can be recast as Intermodulation noise Crosstalk LdB = 20log()λ + 20log ()d −10log (At Ar ) Impulse Noise = −20log()f + 20log ()d −10log (At Ar )+169.54dB Thermal Noise Thermal Noise Amount of thermal noise to be found in a Thermal noise due to agitation of electrons bandwidth of 1Hz in any device or Present in all electronic devices and transmission conductor is: media Uniformly distributed across the frequency N0 = kT ()W/Hz spectrum and hence is often referred to as white noise N0 = noise power density in watts per 1 Hz of bandwidth Cannot be eliminated -23 k = Boltzmann's constant = 1.3803 ´ 10 J/K Function of temperature T = temperature, in kelvins (absolute temperature) Particularly significant for satellite communication 5 Thermal Noise Noise Terminology Noise is assumed to be independent of frequency Intermodulation noise – occurs if signals with Thermal noise present in a bandwidth of B Hertz different frequencies share the same medium (in watts): Interference caused by a signal produced at a frequency that is the sum or difference of original frequencies N = kTB Due to the nonlinearity of the transmission sytem Crosstalk – unwanted coupling between signal or, in decibel-watts paths N =10log k +10 log T +10log B Impulse noise – irregular pulses or noise spikes = −228.6 dBW +10 log T +10log B Short duration and of relatively high amplitude Caused by external electromagnetic disturbances, or faults and flaws in the communications system A primary source of error for digital data transmission Other Impairments Multipath Propagation Atmospheric absorption – water vapor and oxygen contribute to attenuation Multipath – obstacles reflect signals so that multiple copies with varying delays are received Refraction – bending of radio waves as they propagate through the atmosphere The Effects of Multipath Multipath Propagation Propagation Reflection - occurs when signal encounters a Multiple copies of a signal may arrive at surface that is large relative to the wavelength of different phases the signal If phases add destructively, the signal level Diffraction - occurs at the edge of an impenetrable relative to noise declines, making detection body that is large compared to wavelength of radio more difficult wave Intersymbol interference (ISI) Scattering – occurs when incoming signal hits an One or more delayed copies of a pulse may object whose size in the order of the wavelength arrive at the same time as the primary pulse for of the signal or less a subsequent bit 6 Types of Fading Fast fading Slow fading Flat fading Selective fading Rayleigh fading Rician fading 7.
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