FC15 Radio Propagation

8/2/2019 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.


20/24

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

FC15 Radio Propagation

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