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Radio Propagation Fundamentals
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LTE RPESSRadio Propagation Fundamentals
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Nokia Siemens Networks Academy
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Module Objectives
After completing this module, the participant should be able to:
Understand basic radio propagation mechanisms Understand fading phenomena Calculate free space loss
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Module Contents
Propagation mechanisms
Multipath And Fading
Propagation Slope And Different Environments
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Module Contents
Propagation mechanisms Basics: deciBel (dB) Radio channel Reflections Diffractions Scattering
Multipath And Fading
Propagation Slope And Different Environments
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deciBel (dB) Definition
Power
Voltages
dB PP
PlinP dB
=
=10 100
10log [ ].( )
dB EE
ElinE dB
=
=20 100
20log [ ].( )
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deciBel (dB) Conversion
Calculations in dB (deciBel) Logarithmic scale
always with respect to a reference dBW = dB above Watt dBm = dB above mWatt dBi = dB above isotropic dBd = dB above dipole dBV/m = dB above V/m
Rule-of-thumb: +3dB = factor 2 +7 dB = factor 5 +10 dB= factor 10 -3dB = factor 1/2 -7 dB = factor 1/5 -10 dB = factor 1/10
-30 dBm = 1 W-20 dBm = 10 W
-10 dBm = 100 W-7 dBm = 200 W-3 dBm = 500 W0 dBm = 1 mW+3 dBm = 2 mW+7 dBm = 5 mW
+10 dBm = 10 mW+13 dBm = 20 mW+20 dBm = 100mW
+30 dBm = 1 W+40 dBm = 10W
+50 dBm = 100W
LTE: UE: max. 23 dBm
eNB: typ. 43 / 46 dBm
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Radio Channel Main Characteristics
Linear In field strengthReciprocal UL & DL channel same (if in same frequency)Dispersive In time (echo, multipath propagation) In spectrum (wideband channel)
amplitude
delay time
direct path
echoes
Remember:Multipath Effects Normal / Extended CP
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Propagation Mechanisms (1/2)
Free-space propagation Signal strength decreases
exponentially with distance
ReflectionSpecular reflection
amplitude A a*A (a < 1)phase f - fpolarisation material dependant
phase shift
Diffuse reflectionamplitude A a *A (a < 1)phase f random
phasepolarisation random
specular reflection
diffuse reflection
D
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Propagation Mechanisms (2/2)
Absorption Heavy amplitude attenuation Material dependant phase shifts Depolarisation
Diffraction Wedge - model Knife edge Multiple knife edges
A A - 5..30 dB
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Scattering Macrocell
Scattering local to mobile Causes fading Small delay and angle spreads Doppler spread causes time varying
effectsScattering local to base station No additional Doppler spread Small delay spread Large angle spreadRemote scattering Independent path fading No additional Doppler spread Large delay spread Large angle spread
Scattering to mobile
Scattering to base station
Remote scattering
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Scattering Microcell
Many local scatterers: Large angle spreadLow delay spreadMedium or high Doppler spread
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Module Contents
Reflections, Diffractions And Scattering
Multipath and Fading Delay Time dispersion Angle Angular Spread Frequency Doppler Spread Fading Slow & Fast
Propagation Slope And Different Environments
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Multipath propagation
Radio signal propagates from A to B over multiple paths using different propagation mechanisms
Multipath Propagation Received signal is a sum of multipath signals
Different radio paths have different properties Distance Delay/Time Direction Angle Direction & Receiver/Transmitter Movement Frequency
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Delay Time dispersion
Multipath delays due to multipath propagation 1 s 300 m path difference
LTE CP to mitigate multipath effects CP (normal or extended) covers some 1.4 km or 20 km delay respectively Standardized delay profiles in 3GPP specs:
TU3 typical urban at 3 km/h (pedestrians) TU50 typical urban at 50 km/h (cars) HT100 hilly terrain (road vehicles, 100 km/h) RA250 rural area (highways, up to 250 km/h)
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t
P
4.3.2.
1.1.
2.=>
f1
f1
f1
f1
BTS
1st floor
2nd floor
3rd floor
4th floor
Delay Spread
Multipath propagation
Channel impulse response
Delayed components in DAS
(Distributed antenna systems)
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Delay Spread
Typical values
Environment Delay Spread (s)
Macrocellular, urban 0.5-3
Macrocellular, suburban
0.5
Macrocellular, rural 0.1-0.2
Macrocellular, HT 3-10
Microcellular < 0.1
Indoor 0.01...0.1
HT: hilly terrain
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Angle Angular Spread
Angular spread arises due to multipath, both from local scatterers near the mobile and near the base station and remote scatterers
Angular spread is a function of base station location, distance and environment
Angular Spread has an effect mainly on the performance of diversity reception and adaptive antennas
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Macrocellular Environment= Macrocell Coverage Area
Microcellular Environment= Microcell Coverage Area
Microcell Antenna
Macrocell Antenna
Angular Spread
5 - 10 degrees in macrocellular environment>> 10 degrees in microcellular environment< 360 degrees in indoor environment
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Frequency Doppler Spread
With a moving transmitter or receiver, the frequency observed by the receiver will change (Doppler effect) Rise if the distance on the radio path is decreasing Fall if the distance in the radio path is increasingThe difference between the highest and the lowest frequency shift is called Doppler spread
fcvvfd ==
v: Speed of receiver (m/s)c: Speed of light (3*108 m/s)f: Frequency (Hz)
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Fading
Fading describes the variation of the total pathloss ( signal level) when receiver/transmitter moves in the cell coverage area
Fading is commonly categorised to two categories based on the phenomena causing it Slow fading: Caused by shadowing because of obstacles Fast fading: Caused by multipath propagation
Time-selective fading: Short delay + DopplerFrequency-selective fading: Long delaySpace-selective fading: Large angle
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time
power
2 sec 4 sec 6 sec
+20 dB
mean value
- 20 dB
lognormal fading
Rayleighfading
Fading Slow & Fast
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Slow Fading Gaussian Distribution
Measurement campaigns have shown that slow fading follows Gaussian distribution Received signal strength in dB scale (e.g. dBm, dBW)Gaussian distribution is described by mean value m, standard deviation 68% of values are within m 95% of values are within m 2Gaussian distribution used in planning margin calculations
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Slow Fading Gaussian Distribution
d
Normal / Gaussian Distribution
Standard Deviation, = 7 dB
0.00000
0.01000
0.02000
0.03000
0.04000
0.05000
0.06000
0.07000
-25 -20 -15 -10 -5 0 5 10 15 20 25
Normal / Gaussian Distribution
22
1
+
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Fast Fading
Different signal paths interfere and affect the received signal Rice Fading the dominant (usually LOS) path exist
Rayleigh Fading no dominant path exist
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Fast Fading Rayleigh Distribution
It can be theoretically shown that fast fading follows Rayleigh Distribution when there is no single dominant multipath component Applicable to fast fading in obstructed paths Valid for signal level in linear scale (e.g. mW, W)
+10
0
-10
-20
-300 1 2 3 4 5 m
level (dB)
920 MHzv = 20 km/h
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Fast Fading Rician Distribution
Fast fading follows Rician distribution when there is a dominant multipath component, for example line-of-sight component combined with in-direct components Sliding transition between Gaussian and Rayleigh Rice-factor K = r/A: direct / indirect signal energy
K = 0 RayleighK >>1 Gaussian
K = 0(Rayleigh)
K = 1K = 5
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Module Contents
Reflections, Diffractions And Scattering
Multipath And Fading
Propagation Slope And Different Environments Free Space Loss Received power with antenna gain Propagation slope
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Free Space Loss
Free space loss proportional to 1/d2
Simplified case: isotropic antenna Which part of total radiated power is found within surface A? Power density S = P/A = P / 4 d2
Received power within surface A : P = P/A * A Received power reduces with square of distance
dSurface A = 4 * d2
assume surfaceA= 1m2
2d4d
A = 4*AA = 16*A
A
d
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Received power with antenna gain
Power density at the receiving end
Effective receiver antenna area
Received power
Reff GA 4
2
=
ss GdPS 24
=
PP
G Gd
r
ss r=
4
2
PsAsGs
PrArGr
d
SAP effr =
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Propagation slope
The received power equation can be formulated as:
where
C is a constant is the slope factor
Free space = 2 Practical propagation = 2.5 ... 5
2
4
=
C
= dCGGPP rssr