Radio Propagation Fundamentals Concept of Electromagnetic Wave Propagation Mechanisms Modes of Propagation Propagation Models Path Profiles Link Budget Fading Channels
Electromagnetic (EM) Waves EM Wave is a wave produced by the interaction of time varying electric and magnetic field Electromagnetic fields are typically generated by alternating current (AC) in electrical conductors The EM field composes of two fields (vectors) Electric Vector E Magnetic Vector H Electromagnetic waves can be Reflected and scattered Refracted Diffracted Absorbed
Electromagnetic Waves Properties E and H vectors are orthogonal In free space environment, the EM-wave propagates at the speed of light (c) The distance between the wave crests is called the wavelength (λ) The frequency (f) is the number of times the wave oscillates per second (Hz) The relation that combined the EM-wave frequency and wavelength with the speed of light is : λ0 = c / f
Propagation Mechanisms The propagation of radio wave is affected by Frequency Terrain Atmospheric Effects Multipath Effects
Frequency Effect Propagation of radio depends on frequency band At low frequencies (long wavelengths) propagating radio waves tend to follow the earth s surface At higher frequencies they tend to travel in straight lines At HF radio waves are reflected by the ionosphere At frequencies above 6 GHz radio wave is more affected by Gas absorption and precipitation At frequencies close to 10 GHz the effects of precipitation begins to dominate Gas absorption starts influencing at 22 GHz where the water vapour and at 62 GHz where the oxygen shows characteristic peak Nominal FrequencyClassification VLF- Very Low Frequency LF - Low Frequency MF - Medium Frequency HF - High Frequency VHF - Very High Frequency UHF - Ultrahigh Frequency SHF - Superhigh Frequency EHF - Extremely High Frequency Frequency 3-30 khz 30-300 khz 300-3,000 khz Wavelength 100,000-10,000 meters 10,000-1,000 meters 1,000-100 meters 3-30 MHz 100-10 meters 30-300 MHz 10-1 meters 300-3,000 MHz 100-10 centimeters 3-30 GHz 10-1 centimeters 30-300 GHz 10-1 millimeters
Terrain effect Reflection and scattering The radio wave propagating near the surface of earth is influenced by: oelectrical characteristics of earth otopography of terrain including man-made structures
Loss and refraction The gaseous constituents and temperature of the atmosphere influence radio waves by: o Absorbing it energy o Variations in refractive index which cause the radio wave reflect, refract and scatter Atmospheric effect
Rain Attenuation When radio waves interact with raindrops the Electromagnetic wave will scatter The attenuation depends on frequency band, especially for frequencies above 10 GHz The rain attenuation calculated by introducing reduction factor and then effective path length The rain attenuation depends on the rain rate, which obtained from long term measurement and vey short integration time The earth is divided into 16 different rain zones
Multipath effect Multipath effect occurs when many signals with different amplitude and /or phase reach the receiver Multipath effect is caused by reflection and refraction Multipath propagation cause fading
Modes of Propagation Ground wave - -long range (LF: 30-300 khz, MF: 300-3000kHz, etc) Sky wave HF bands 3 to 30 MHz Line of Sight (LOS)----higher frequencies
Propagation Models Free Space Path loss (FSL) 2-Ray Ground Reflection Model CCIR propagation Model
Free Space Loss (FSL) Free-space path loss (FSL): the loss in signal strength of an electromagnetic wave that would result from a line-of-sight path through free space, with no obstacles nearby to cause reflection or diffraction P t P r
Free Space Loss (FSL) Isotropic radiator of power P t power flow through surface at distance d = P t / 4π d 2 watts/m2 [power/ surface area of sphere] Power intercepted by antenna of effective area A, related to the gain by A=λ 2 / 4π Received power P r = A. P t / 4πd 2 Linear path loss: FSL= P t /P r = (4πd/λ) 2 For unity gain antennas and loss in db, using f = c/λ Pt FSL( db) 10log( P r ) 10log 4 d 2 2 10 ( ) 10log10( ) FSL( db) 32.44 20log 10 d( km) 20log 10 f ( MHz) Where: d is distance between the Transmitter and Receiver f is frequency of EM waves propagation 4 df c
2-Ray Ground Reflection Model L 40log d 20log 2 ray 10 10 h t h r
CCIR Model CCIR Model: An empirical formula for the combined effects of free-space path loss and terrain-induced path loss was published by the CCIR (Comité Consultatif International de Radio-Communication) L CCIR a( h a( h 2 ( db) 69.55 26.16log 2 ) ) (1.1log 10 10 f ( MHz) 0.7) h f ( MHz) 13.82log 2 (1.56log 10 10 h 1 f ( MHz) 0.8) (44.9 6.55log 10 h 1 )log 10 d( km) Where f: frequency, h 1 base station height h 2 mobile antenna height
Microwave Link Path Profile
Fresnel Zone Fresnel Zone is defined as the zone shaped as ellipsoid with its focal point at the antennas on both ends of the path If there is no obstacle within first Fresnel zone, the obstacle attenuation can be ignored and the path is cleared Equation of path of ellipsoid
Path Profile: Clearance of Path Design objective: Full clearance of direct lineof-sight and an ellipsoid zone surrounding the direct line-of-sight The ellipsoid zone is called the Fresnel Zone
Link Budget Calculation The performance of any communication link depends on the quality of the equipment being used. Link Budget is a way of quantifying the link performance The received power is determined by Transmit power, Transmitting antenna gain and Receiving antenna gain If that power, minus the free space loss of the link path, is greater than the minimum received signal level of the receiving radio, then a link is possible. The difference between the minimum received signal level and the actual received power is called the link margin. The link margin must be positive, and should be maximized (should be at least 10dB or more for reliable links).
dbm Link Budget Antenna1 Antenna 2 Path Loss Tx Radio Cable1 Cable2 Rx Radio P TX L 1 G 1 G 2 FSL L 2 P RX Rx Power Fade Margin (FM) Rx Threshold Distance R Thresh
Link Budget P RX P TX ( L L ) ( G G ) 1 2 1 2 FSL FM P R RX thresh Where P TX = Received Power (dbm) P RX = Transmitted power (dbm) L 1, L 2 = cable, feeder loss (db) G 1, G 2 = antenna gain (dbi) FSL=Free space loss (db)
Link budget on 1.4 GHz link Path length 12 km Frequency 1.4 GHz Feeder loss 4 db FS path loss 117 db Total loss 121 db Antenna gain 47.2 db Tx outpout +30 dbm Rx signal level -43.8dBm Rx threshold for BER of 10-6 -97dBm Fade margin 53.2 db