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Radio wave Propagation
Chapter 5Chapter 5
Chapter OutlinesChapter Outlines
• The Friis Transmission Equation• Propagation Path Loss
• Propagation• Ionosphere propagation• Troposphere Propagation• Urban Propagation
• Propagation phenomena• Fading and Attenuation• Antenna Noise Temperature
• Link Budget Analysis
IntroductionIntroduction
Wireless communications involves the transfer of information between two points without direct connection sound, infrared, optical or RF energy.
Most modern wireless systems rely on RF or microwave signals, usually in the UHF to millimeter wave freq range.
But why high freq? spectrum crowding, need for higher data rates majority of today’s wireless systems operate at freq ranging from 800MHz to few GHz. E.g. broadcast radio and TV, cellular telephones, DBS TV service, WLAN, GPS and RFID.
Introduction (Cont’d..)Introduction (Cont’d..)
Characterizing the wireless systems:
Point to point radio systems single transmitter with single receiver use high gain antennas in fixed positions to max received power and minimize interference with other radios (nearby frequencies).
Point to multipoint systems connect a central station to a large number of possible receivers commercial AM and FM radio and broadcast tv Uses an antenna with broad beam to reach many listeners and viewers.
Introduction (Cont’d..)Introduction (Cont’d..)
Multipoint to multipoint systems simultaneous communication between individual users (maybe not in fixed location) generally not connect two users directly, but rely on a grid of base stations to provide desired interconnections between users. E.g. cellular telephone systems and WLAN.
Can also be characterize in terms of directionality of communication:
Simplex system communication occurs in one direction, from tx to rx. E.g. broadcast TV, radio and paging systems.
Introduction (Cont’d..)Introduction (Cont’d..)
Half Duplex system communication in two directions, but not simultaneously. E.g. early mobile radios (walkie-talkie) ..which rely on push to talk function with different intervals of transmitting and receiving.
Full Duplex systems simultaneous two-way transmission and reception. E.g. cellular telephone and point to point radio systems require ‘duplexing’ techniques : 1. using separate freq bands for transmit and receive, 2. users to transmit and receive in certain predefined time intervals.
5.1 The Friis Transmission Equation5.1 The Friis Transmission Equation
The Friis transmission equation describes how well the energy is exchanged between transmitter and receiver. Consider a pair of horn antennas with the same polarization and aligned each other.
The radiated power density from Horn 1 at the location of Horn 2 is :
1
1max21
4,, D
R
PRP
rad
The power received by Horn 2 is product of this power
density and capture area A2, written as :
2
2max21
4,, 1
12 R
ADPARPP radrec
The Friis Transmission Equation (Cont’d..)The Friis Transmission Equation (Cont’d..)
The power received at Horn 1 resulting from power emitted by Horn 2 :
2
1max
42
21 R
ADPP radrec
The Friis Transmission Equation (Cont’d..)The Friis Transmission Equation (Cont’d..)
The reciprocity property – the transmission pattern is the same as receive pattern, and the ratio of received power to radiated power will be the same, regardless which pair is transmitting or receiving.
2
1
1
2
rad
rec
rad
rec
P
P
P
P
Therefore, or1max2max 21ADAD
2
max
1
max 21
A
D
A
D
Since the directivity and area are independent of each other, the ratio must be equal to constant :-
2max 4
A
D
The Friis Transmission Equation (Cont’d..)The Friis Transmission Equation (Cont’d..)
Generally,
We find,
,,4 2 rtrad
rec ADR
PP
r – receivert – transmitter
The ratio is also valid even the antennas are not in line :
2
4
,
,
eA
D
The Friis Transmission Equation (Cont’d..)The Friis Transmission Equation (Cont’d..)
Replace the effective area with receiving area to get :
2
4,,
RDD
P
Prt
rad
rec
Finally consider,
To get:
rrrtttrecroutintrad DeGDeGPePPeP ,,,
2
4,,
RGG
P
Prt
in
out
2
4
RGGPP rttr
Simpler Form
The Friis Transmission Equation (Cont’d..)The Friis Transmission Equation (Cont’d..)
2
4
RGGPP rttr
The R could be known as distance, d
2
10)()()(
4log
d
GGPP dBrdBtdBtdBr
In decibels,
)log(20)(10log2045.32 Mhzkm fdFSPL
Free space path loss,
Effective isotropic radiated power, EIRP
The Friis Transmission Equation (Cont’d..)The Friis Transmission Equation (Cont’d..)
This result is known as Friis transmission equation, which addresses on how much power is received by an antenna.
Practically, it can be interpreted as the max possible received power, whereby with lot of factors to reduce the received power in actual radio system:
• impedance mismatch at either antenna
• polarization mismatch between the antennas
• propagation effects leads to attenuation or depolarization
• multipath effects partial cancellation of the received field.
The Friis Transmission Equation (Cont’d..)The Friis Transmission Equation (Cont’d..)
The received power decreases as 1/R2 as the separation between transmitter and receiver increases.
It seems large for large distance, but it is much better than the exponential decrease in power due to losses in a wired communication link (coax lines, waveguides, even fiber optic lines) the attenuation power on Tline varies as e-2αz , with α is attenuation constant of the line at large distance, the exp function decreases faster than an algebraic dependence like 1/R2 .
For long distance communication, radio links perform better than wired links.
15
Example 1Example 1
Consider a pair of half wavelength dipole antennas,
separated by 1 km and aligned for maximum power
transfer as shown. The transmission antenna is
driven with 1 kW of power at 1 GHz. Assuming
antennas are 100% efficient, determine the receiving
antenna’s output power.
Solution to Example 1Solution to Example 1
For 100% efficiency and antennas optimally aligned,
2
maxmax 4
RDD
P
Prt
in
out
For the λ/2 dipole antennas we have Dmaxt = Dmaxr = 1.64
and at 1 GHz, λ = 0.3m,
9
2
32 105.1
1014
3.064.1
in
out
P
P
Solution to Example 1 (cont’d..)Solution to Example 1 (cont’d..)
In terms of decibels,
dBdBP
P
in
out 88105.1log10 9
So finally,
WkWPout 5.11105.1 9
2
4,,
RGG
P
Prt
t
r
The Friis Transmission Equation (Cont’d..)The Friis Transmission Equation (Cont’d..)
The Friss transmission equation can also be known as (in terms of receive and transmit) :
Whereby, the product of PtGt can be interpreted
equivalently as the power radiated by an isotropic
antenna with input power PtGt, or effective
isotropic radiated power (EIRP):
wattttGPEIRP
For a given frequency, range and receiver antenna gain, the received power is proportional to EIRP of transmitter, and can only be increased by increasing the EIRP increase transmit power, or transmit antenna gain or both.
The Friis Transmission Equation (Cont’d..)The Friis Transmission Equation (Cont’d..)
In any RF or microwave system, impedance mismatch will reduce the power delivered from a source to a load, where the Friss formula can be multiplied by the impedance mismatch factor,
2211 rtimp
Max transmission between two antennas requires both antenna be polarized in the same direction. E.g. if a transmit antenna is vertically polarized, max power will be delivered to a vertically polarized receive antenna, while zero power would be delivered to a horizontally polarized received antenna.
The Friis Transmission Equation (Cont’d..)The Friis Transmission Equation (Cont’d..)
The polarization mismatch effects is measured by multiplying the Friss formula by the polarization loss factor,
2ˆˆ ripol eee
5.2 Introduction to Propagation5.2 Introduction to Propagation
For frequencies below, about 100 MHz, other propagation effects can be important:
• ground surface waves
• atmosphere ducting
• ionosphere reflection
Generally, propagation effects have the effect of reducing the received signal power, thus limit the usable range or maximum data rate of a wireless system.
Introduction to Propagation (Cont’d)Introduction to Propagation (Cont’d)
Propagation path lossPropagation path loss
• Propagation path loss due to movement of the mobile away from RBS (Radio Base Station)
• Propagation over land or sea follows law (propagation power law where has a theoretical value 4 (for flat earth))
d/1
Plane Earth
Free Space
4
1
dPr
2
1
dPr
PowerReceived
Distance
Types of Propagation
• Ionosphere Propagation– Using ionospheric propagation, signals
leave the transmitting antenna and travel away from the antenna
– Signal reflected– Affected by day and night – Also known as sky wave propagation
Types of Propagation
• Tropospheric propagation– Radio waves can propagate over the horizon when the lower atmosphere of the earth
bends, scatters, and/or reflects the electromagnetic fields.– In the lower part of the earth's atmosphere
electromagnetic energy may be reflected/refracted within and between air masses of different density.
–
– Such propagation is usually of short duration and not suitable for reliable signal transmission.
Urban propagation
• Urban environment– Propagation of electromagnetic waves in
urban areas in the frequency range above 300 MHz is influenced mainly by the buildings.
– Phenomena like multi-path propagation, reflection, diffraction and shadowing have a significant influence on the received power.
– Need to consider other path loss model apart from FSPL such as,
• Hata-Okumura Model
• COST 231 Walfisch Ikegami Model
Urban Propagation ScenarioUrban Propagation Scenario
A point to point radio link with a single line of sight propagation
path
A cellular telephone channel having multiple propagation
paths.
5.3 Propagation Phenomena5.3 Propagation Phenomena
Attenuation decrease in signal power due to losses in the propagation path.
Material Frequency Loss, dB
Concrete Block Wall
1300 MHz 13
Sheetrock 2 x 3/8” 9.6 GHz 2
Plywood 2 x 3/4” 9.6 GHz 4
Concrete Wall 1300 MHz 8-15
Chain Link Fence 1300 MHz 5-12
Loss Between Floors
1300 MHz 20-30
Corner in Corridor 1300 MHz 10-15
Rain Attenuation
Propagation PhenomenaPropagation Phenomena
0 20 40 60 80 100 120 140 1600
0.5
1
1.5
2
2.5
Rain rate (mm/h)
Att
enua
tion
0.01
% p
er k
m (
dB/k
m)
5.8 GHz
Antenna Noise TemperatureAntenna Noise Temperature
NOISE
Noise is any unwanted received signal independent of the transmitted signal and man action
EXTERNAL NOISE
Cosmic noise, Atmospheric noise
INTERNAL NOISE
Cable noise (waveguide or copper), receiver noise (thermal noise)
Antenna Noise Temperature (Cont’d..)Antenna Noise Temperature (Cont’d..)
Natural and manmade sources of background noise.
Antenna noise temperature is the sum of all noise source at the antenna Noise power given by:
kTBN 0
Where k = Boltzman’s constant = J/K = 228.6
dBW/k/HzT = physical temperature of source in kelvin degreeB = noise bandwidth in which the noise power is
measured in Hz
231039.1
Antenna Noise Temperature (Cont’d..)Antenna Noise Temperature (Cont’d..)
Antenna Noise Temperature (Cont’d..)Antenna Noise Temperature (Cont’d..)Normally, we have the simple case to measure an
available output noise power N0, given by:
kTBN 0
Illustrating the concept of background temperature. (a) A resistor at temperature T. (b) An antenna in an anechoic chamber at temperature T. (c) An antenna viewing a uniform sky background at temperature T.
Chamber room…Chamber room…
Antenna Noise Temperature (Cont’d..)Antenna Noise Temperature (Cont’d..)
But when the antenna beamwidth is broad enough that different parts of the antenna pattern see different background temperatures, the temperature now is called as effective noise
temperature, Te seen by the antenna. This
antenna brightness temperature takes into account the distribution of background temperature, directivity and the power pattern function of the antenna
System noise temperature,
Ts = Tin + Te
where Ts = system temperature
Tin = noise temperature of antenna and cable
Te = Thermal noise (at Rx)
Antenna Noise Temperature (Cont’d..)Antenna Noise Temperature (Cont’d..)
Loss of waveguide between antenna and receiver
Antenna Noise Temperature (Cont’d..)Antenna Noise Temperature (Cont’d..)
Gskya TTT
GWGW
aIN L
TL
TT
/0
/
11
eINs TTT
So, the system noise temperature
BkTN s0
Noise power given by:
If given noise in term of noise figure, to find noise temperature
Antenna Noise Temperature (Cont’d..)Antenna Noise Temperature (Cont’d..)
0
1T
TF e 0)1( TFTe and
Where F = noise figure (nf)
T0 = ambient temperature
Antenna Noise Temperature (Cont’d..)Antenna Noise Temperature (Cont’d..)
The G/T ratio is another important parameter where the signal to noise ratio (SNR) at the input of a
receiver is proportional to G/Ts.
KdBT
GdBTG
s
log10/
The SNR at the input to the receiver can be calculated as:
s
rtt
s
ttr
i
i
T
G
RkB
PG
RBkT
PGG
N
S22
44
Antenna Noise Temperature (Cont’d..)Antenna Noise Temperature (Cont’d..)
Where SNR is proportional to G/T of the receive antenna. Only Gr/Ts is controllable at the receiver, and others are fixed by the transmitter design and location.
G/T can be maximized by increase the gain of antenna usually minimize reception of noise from hot sources at low elevation angles but higher gain requires larger and more expensive antenna, and high gain may not be desirable for application of omnidirectional coverage!!
• Signal to Noise Ratio
)()( nfkTBGRLGTPTdBSNR
Antenna Noise Temperature (Cont’d..)Antenna Noise Temperature (Cont’d..)
0)( NPRdBSNR or
Example 2Example 2
Suppose we have satellite system operates at 12.5GHz, with transmit carrier power 120W, transmit antenna gain 34dB, IF Bandwidth 20 MHz. The receiving dish have gain of 33.5dB, with receiver noise figure 1.1dB, locates 39000km from the satellite. The temperature noise between Tx-Rx are, Tsky = 50K and TG = 50K and Lw/g = 1dB. Find:
(a)EIRP of the transmitter
(b)G/T for the receive antenna
(c) Received carrier power at receiver terminal
(d)Signal to Noise Ratio (SNR)
Solution to Example 2Solution to Example 2
Convert the quantities in dB to numerical values:
34 dB = 2512, 1.1 dB = 1.29, 33.5 dB = 2239The operating frequency 12.5 GHz, so wavelength 0.024m.
In dB, convert Pt in dB, PT = 50.8dBm
dBmWGPEIRP tt 8.841001.32512120 5
dBmdBdBmGPEIRP TT 8.84348.50
So,
Solution to Example 2 (Cont’d..)Solution to Example 2 (Cont’d..)
To find G/T, first find noise temperature of the antenna
Gskya TTT KKKTa 1005050
KKKT
FTTTTT
KL
TL
TT
s
aeINs
GWGW
aIN
1.184)129.1(290100
1
1001
1
0
/0
/
So then G/T for the antenna is:
KdBdBTG 85.101.184
2239log10/
For Lw/g =1.29;
Solution to Example 2 (Cont’d..)Solution to Example 2 (Cont’d..)
The received carrier power is from Friis formula:
dBmW
R
GGPP rttr
9.871062.1
109.34
024.022391001.3
412
272
25
2
2
Or in dB
dBm
dBdBdBm
RGEIRPP RR
9.87
)109.3(4
024.0log105.338.84
4log10
2
7
2
dB
MHzK
W
N
P
KBT
G
REIRPSNR
o
r
A
r
1565.31
201.1841039.1
1062.1
1
4
23
12
2
The SNR at the receiver is:
Solution to Example 2 (Cont’d..)Solution to Example 2 (Cont’d..)
Or in dB
dB
mW
BKTdBm
NPRdBSNR
s
15
1log109.87
)( 0
5.4 Link Budget5.4 Link Budget
Link BudgetLink Budget
If the estimated received power is sufficiently large (typically relative to the receiver sensitivity), the link budget is said to be sufficient for sending data under perfect conditions.
Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) − Losses (dB)
The amount by which the received power exceeds receiver sensitivity iscalled the link margin.
Link margin = Received Power – Receiver sensitivity
SNR = Received power – channel noise
Maximum channel noise = Received power – minimum SNR
If received power is -30dBm and the system required to achieve a minimum data rate of 54 Mbps.Determine the maximum channel ratio.
Example 3Example 3
PropagationPropagation
End