FC15 Radio Propagation
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Transcript of FC15 Radio Propagation
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Fundamentals ofCommunications
EE3158
Professor Ian [email protected]
www.ctr.kcl.ac.uk/members
15: Radio Channels
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Lecture 15 2
Radio Channels
Frequency & Wavelength Classification & Use
Modes of Propagation
Propagation Mechanisms
Atmospheric Attenuation
Propagation Models
Fading ChannelsBateman p 94
Multipath NoiseBateman p 90
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Lecture 15 3
Frequency & Wavelength
Fundamental relationship
c f
where c = velocity of light 3x108 metres/sec
f = frequency (Hz)
= wavelength (m)
Frequency 1 MHz 3 MHz 30 MHz 100 MHz 300 MHz 1 GHz 3 GHz
Wavelength 300 m 30 m 10 m 3 m 1 m 30 cm 10 cm
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Lecture 15 4
Classification of Radio Use
3 30 kHz Very low frequency (VLF)
long range navigation, sonar
30 300 kHz
Low frequency (LF)
navigational aids, beacons, broadcast
300 3000 kHz
Medium frequency (MF)
maritime radio, commercial AM radio 3 30 MHz
High frequency (HF)
short wave radio for distance communications
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Lecture 15 5
Classification of Radio Use 2
30 MHz 300 MHz Very high frequency (VHF)
FM radio, emergency services, taxi, navigation
0.3 3 GHz
Ultra high frequency (UHF)
UHF television, mobile communications (900 MHz , 2 GHz)
3 30 GHz
Super high frequency (SHF)
satellite communications, radar systems, microwave links
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Lecture 15 6
Modes of Propagation
Ground wave medium wave broadcast
Sky wave HF bands 3 to 30 MHz
Line of Sight (LOS) higher frequencies
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Lecture 15 7
Propagation Mechanisms
At low frequencies (long wavelengths)propagating radio waves tend to follow theearths surface.
At higher frequencies they tend to travel instraight lines.
At HF (3 30 MHz) radio waves are reflected bythe ionosphere:
a series of layers of charged particles, ionised byradiation from the sun, at between 30 and 250 milesabout the earths surface and known as D, E and Flayers. Coalesce at night gives longer skip distances.
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Lecture 15 8
Radio Propagation 2
Above 300 MHz propagation is by line of sight. Higher still, above 3 GHz say, atmospheric gases
(mainly oxygen), water vapour and precipitation(rain!) absorb and scatter radio waves.
23 GHz water vapour resonance
62 GHz oxygen absorption
Care needed in design of microwave links and
ground to satellite links.
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Lecture 15 9
Atmospheric Attenuation
a) attenuation caused by atmospheric gases
note molecular resonance peaks
b) attenuation caused by rain can increase path loss by an order of magnitude ( 10 x)
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Lecture 15 10
Propagation Models
Free space path loss isotropic (equal in all directions) radiator of power Pt
power flow through surface at distance d
= Pt / 4pd2 watts/m2 [power/surface area of sphere]
power intercepted by antenna of effective area A,related to the gain by Gr = 4pA/
2
received power Pr= A.Pt Gt / 4pd2[Gt is transmit antenna gain]
whence Pr/ Pt = Gr Gt (/4pd)
2
for unity gain antennas and loss in dB, using f = c/
L = 32 + 20log fMHz + 20log dkm
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Lecture 15 11
Propagation Models2
Inverse square law received power decreases by 6dB each time we
double the distance. The transmission loss alsoincreases as the square of the frequency, double the
frequency increase the loss by 6dB. Real world effects, presence of
atmosphere
earth
trees
buildings
hills close to the transmission path.
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Lecture 15 12
Direct and Reflected Waves
two antennas height h1 and h2 separated by d, where
d>>h1/h2. path difference = 2 h1h2/d (use Pythagorus & binomial expansion)
phase difference = 2 h1h2/d .2p/ 4p h1h2/ d
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Lecture 15 13
Direct and Reflected Waves 2
As we change the antenna height or the distance (a movingmobile) we will get constructive and destructive interferencebetween our direct and reflected wave causing fading, thedepth of which will depend on the magnitude of thereflection coefficient, r, for the reflected wave. [r=1 below]
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Lecture 15 14
Real Channel
In a practical mobile radio cell the received signal is the sum of manyreflected or multipathcomponents. If each of these is independentthen the statistics of their sum is described by a Rayleigh distribution.
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Lecture 15 15
Rayleigh Distribution
mean nearly equal to standard deviation (s)
larger tail than normal distribution
deep fade p > 3s
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Lecture 15 16
Propagation Models3
CCIR developed a propagation model for broadcastradio and television:
Ldb = 40log(d) - 20 log(h1h2)
an inverse 4th power law [all distances in metres]
modified to include the effects of: surface roughness
line of sight obstacles
buildings and trees
Ldb = 40log(d) - 20 log(h1h2) + b where b represent these additional losses and usually
established empirically.
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Lecture 15 17
Practical Results
Mobile channels are not optimised for line of sightreception, the path loss is continually changing.
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Lecture 15 18
Practical Results 2
within the city (clutter) environment we see not onlyfast (Rayleigh) fading but a second distancedependent fade with Gaussian like characteristics.
Slow or log-normal fading
L = (10 x n)log(d) + a(d)
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Lecture 15 19
Delay Spreads
Discussion so far relates to the transmission of anunmodulated carrier. For a digital mobile system weare concerned with the delay spread of our channelresulting from the multipath reflected signals. A
single transmitted pulse will be spread in time whenit reaches the receiver and if this spread iscomparable with the symbol length we will get InterSymbol Interference ISI.
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Lecture 15 20
Mobile Channel Summary
inverse 4th power law for path loss is a goodapproximation
many empirical models proposed and used
e.g. Okumura and Hata
continually varying loss - fast fading / Rayleighstatistics
shadow fading with distance (slow fading) caused bybuildings and other obstacles
significant delay spread caused by multipathreception requiring channel equalisers
A pretty hostile environment!
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Lecture 15 21
Noise Limited Channels
Thermal noise occurs in both media andcommunications equipment.
It arises from random electron motion and ischaracterised by a uniform distribution of energy over
the frequency spectrum with a Gaussian distributionof levels.
Amount of thermal noise in 1 Hz of bandwidth:
Pn = kT (W/Hz)
where k is Boltzmanns constant 1.3803x 10-
23
J/K T is absolute temperature
In a specified bandwidth B
Pn = kTB (W)
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Lecture 15 22
Noise Limited Channels 2
Thermal noise sets the lower limit of sensitivity of areceiving system.
example, at 17oC or 290 oK in a bandwidth of 1 MHz
Pn = kTB
= 1.3803x10-23 x 290 x 1.0x106 = 4.0x10-15 Watts
Often expressed with respect to 1 milliwatt (10-3 W)
i.e. 10 log (4.0x10-15/10-3) or -114 dBm dBm power reference 1 milliwatt
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Lecture 15 23
Power and Loss
Example: a transmitter of 2 W output on 30 m masttransmits to a mobile receiver height 2 m over adistance of 15 km. If the bandwidth is 200 kHz,temperature 17oC, what is the receiver signal tonoise ratio?
Tx power is 2 W = 10 log (2/10-3) = 33 dBm
Path loss Ldb = 40log(d) - 20 log(h1h2) - slide 16
= 40 log (15,000) -20 log (30 x 2) = 132 dB
Rx power = 33-132 =-99 dBm kTB = 1.3803x10-23 x 290 x 2.0x105 = 8.0x10-16 W
= -121 dBm. Thus S/N is -99 - (-121) = 22 dB
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Lecture 15 24
Radio Channel Topics
frequency / wavelength relation modes of propagation
simple one path reflection model
that mobile channels:
loss can be approximated by inverse 4th power low
fade
have delay spread
thermal noise limitation - kTB
simple loss power / calculations