03) radio signal propagation
TRANSCRIPT
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Radio Signal Propagation
ZTE University
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Content
1. Radio wave propagation
2. Propagation models
3. Antenna systems
4. Diversity technique
5. Interference and interference reduction
6. Link budget
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Radio Link Propagation
Multi-path propagation
Radio path is a complicated propagation medium
Limited transmitting energy
The service range is determined by thetransmission power of mobiles
Battery life-time
Limited spectrum
Set upper limitation for data rate (Shannon´s
theorem)
Additional effort needed for channel coding
Frequency reused result in self- interference
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Reflections
Strong echoes can cause excessive transmission delay
No impact If the delay falls in the equalizer window
Cause self-interference if the delay falls out of the
equalizer windowdirect signal
strong reflected signal
equalizer window 16 s
amplitude
delay time
long echoes, out of equalizer window:
self-interference
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Fading(1)
Slow fading (Lognormal
Fading)
Shadowing due to large
obstacles on propagationdirection
Fast fading (Rayleigh
fading)
Serious interference from
multi-path signals
+10
0
-10
-20
-300 1 2 3 4 5 m
Level (dB)
920 MHz
v = 20 km/h
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Fading(2)
time
power
2 sec 4 sec 6 sec
+20 dB
mean
value
- 20 dB
lognormal
fading
Rayleigh
fading
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Signal Variations
Rayleighfading Lognormalfading Large scalevariation
Cause Superposition ofmultiplepropagationpaths withdifferent phase
Shadowing orreflection bycars, trees,buildings
Prop. path profile, terrain& clutter structure, Earthcurvature
Correlation< 10 ... 100m > 100m
Prediction unpredictable mostlypredictable(buildings!!)
predictable (maps, terraindatabase)
Planningmethod
apply statisticalthresholds forRayleigh fading
signals
considerlognormaldistribution
around localmean (use =
3 ... 10dB)
use maps or digitalterrain & clutterdatabases to predict
(50 ..200m pixelresolution)
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Propagation
Free- space propagation
Signal strength decreases with distance increases
Reflection
Specula R.
• Amplitude : A --> α*A (α< 1)
• Phase : --> -Ф
• Polarization : material determining phase shift
Diffuse R.
• Amplitude : A --> α*A (α<< 1)
• Phase : random•
specula reflection
diffuse reflection
D
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Propagation
Absorption
Heavy amplitude attenuation
Material determining phase shift
Diffraction
Wedge-model
Knife edge
Multiple knife edges
A A - 5..30 dB
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Mobile Radio Link
1. Radio wave propagation
2. Propagation models
3. Antenna systems
4. Diversity technique
5. Interference and interference reduction
6. Link budget
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Propagation Model
Historical CCIR- Model for Radio station
Not very accurate nor serious
Okumura- Hata
Empirical model
Measure and estimate additional attenuations
Applied for larger distance estimation (range: 5 ..
20km) Not suitable for small distance ( < 1km)
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Hata Model
Model used for 900 MHz
L A B f h a h
h d L
b m
b morpho
log . log ( )
( . . log ) log
1382
44 9 655with
f frequency in MHz
h BS antenna height [m]
a(h) function of MS antenna height
d distance between BS and MS [km] and
A= 69.55, B = 26.16 (for 150 .. 1000 MHz)
A= 46.3 , B = 33.9 (for 1000 ..2000MHz)
additional attenuation due
to land usage classes
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Land Usage Types
Urban small cells, 40..50 dB/Dec attenuation
Forest heavy absorption; 30..40 dB/Dec;
differs with season (foliage loss)
Open, farmland easy, smooth propagation conditions
Water propagates very easily ==> dangerous !
Mountain surface strong reflection, long echoes
Glaciers very strong reflection; extreme delay ,strong interferences over long distance
Hilltops can be used as barriers between cells, do
not use as antenna or site location
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Walfish- Ikegami Model
Model used for urban micro-cell propagation. Assume
regular city layout (“Manhattan grid”). Total path loss
consists of three parts:
Line-of-sight loss LLOS
Roof-to-street loss LRTS
Mobile environment loss LMS
h
w
b
d
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Mobil Radio Link
1. Radio wave propagation
2. Propagation model
3. Antenna system
4. Diversity technique
5. Interference and interference reduction
6. Link budget
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Coupling Between Antennas
main lobe
5 .. 10
Horizontal separation
Sufficient decoupling distance: 5-10λ
Antenna patterns superimposed if
distance too close
Vertical separation
Decoupling distance:1λ can provide good RX /TX
decoupling Minimum coupling loss
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Recommended decoupling
TX - TX: ~20dB
TX - RX: ~40dB
Horizontal decoupling distance depends on
Antenna gain
Horizontal rad. pattern
Omni-directional antenna
Use vertical separation for RX and TX
Use vertical separation (“fork”) for RX and diversity
RXVertical decoupling is much more effective
0,2m
Omni-directional.: 5 .. 20m
directional : 1 ... 3m
Installation Examples
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Installation Examples
Directional antenna
Antenna downtilt
Improve hotspot coverage
Reduce interference
5..8 deg
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Feeder
Feeder parameterType Diameter 1800MHz 900MHz
(mm) dB/100m
dB/100m
3/8” 10 14 10
5/8” 17 9 6
7/8” 25 6 4
1 5/8” 47 3 2
Use the short feeder whenever possible
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Distributed Antennas
Leaking feeder
Cables with very high loss per length unit
“distributed antenna” often used for tunnel coverage.
This kind of feeder is expensive
Optic fiber distribution system
Distribute RF signal radiate from discrete antenna
points at remote locations via (very thin) optic fiber.
50 Ohm
Propagation loss: 4 ... 40 dB/100m
coupling loss: ~ 60 dB (at 1m dist.)
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Repeaters
Repeater type
Narrow-band Repeater
Wide-band Repeater
The Repeater is used to relay signal into shadowed
area
Behind hill
Into valley
Into building
Note: The Repeater needs a host cell
decoupling ~40 dB needed
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Mobile Radio Link
1. Radio wave propagation
2. Propagation models
3. Antenna systems
4. Diversity technique
5. Interference and interference reduction
6. Link budget
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Diversity
Time diversity
Coding, interleaving
Frequency diversity
Frequency hopping
Space diversity
Multiple antennas
Polarization diversity
Dual-polarized
antennas
Multi-path diversity
Equalizer
t
f
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Mobile Radio Link
1. Radio wave propagation
2. Propagation models
3. Antenna systems
4. Diversity technique
5. Interference and interference reduction
6. Link budget
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Interference
Signal quality =
sum of all expected signals carrier (C )
sum of all unexpected signal interference (I)=
Notes: GSM specification : C / I >= 9 dB (Co-Channel)
expected signal
atmospheric
noise
other signals
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Effects of Interference
Affect signal quality
Cause bit error
Repairable errors : channel coding, error
correction
Irreducible errors : phase distortions
Interference situation is
Non- reciprocal : uplink <> downlink
Unsymmetrical : different situation at MS and
BTS
C/I
Co-Channel C/I : 9dB
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Signal Quality in GSM
RX QualityRXQUAL class : 0 ... 7
RXQUAL Mean BER BER range
class (%) from... to0 0.14 < 0.2%
1 0.28 0.2 ... 0.4 %
2 0.57 0.4 ... 0.8 %
3 1.13 0.8 ... 1.6 %
4 2.26 1.6 ... 3.2 %
5 4.53 3.2 ... 6.4 %6 9.05 6.4 ... 12.8 %
7 18.1 > 12.8 %
usable signal
unusable
signal
good
acceptable
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Interference sources
Multi-path (long echoes)
Frequency reuse
External interferenceNote : Interference has the same effect as poor coverage.
Reduce the interference
as possible.
M th d f d i
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Methods for reducing
Interference
Frequency planning
Suitable site location
Antenna azimuth, downtilt and height
good location
bad location
M th d f d i
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Methods for reducing
Interference
Frequency hopping
A diversity technique, frequency diversity include:
Less fading loss
De-coding gain
Interference averaging
Power control based on quality Evaluate signal level and quality
DTX
Silent transmission in speech pauses
M th d f d i
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Methods for reducing
Interference
Adaptive antenna
According to subscriber distribution, concentrate
signal energy to certain direction.
Adaptive channel allocation
Always assign the best available channel during call
setup.
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Frequency Hopping
Diversity technique
Frequency diversity can reduce fast fading effects
Useful for static or slow-moving mobiles
Cyclic base-band hopping
TRX hops cyclic between its allocated frequencies
RF hopping
Either cyclic or random hopping
Needs wideband combiner
Can use any frequency included in the MA
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Power Control
Save battery life-time
Minimize interferenceGSM : 15 steps and 2 dB for each
Use power control in both uplink and downlink
triggered by level or quality
time
signal
level target level
e.g. -85 dm
Power control isn’t allowed
on BCCH
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DTX
DTX (Discontinuous transmission)
Switch transmitter off in speech pauses and silence
periods, both sides transmit only silence
updates (SID frames) comfort noise generated bytranscoder.
VAD: voice activity detection
Transcoder is informed the use of DTX/ VAD
Battery saving and
interference reducing
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Mobile Radio Link
1. Radio wave propagation
2. Propagation models
3. Antenna systems
4. Diversity technique
5. Interference and interference reduction
6. Link budget
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Link Budget Calculation
Why we need a link budget?
Which will decide the coverage range?
The coverage range is limited by the weaker one.
Two-way communication needed
link usually limited by mobile transmitting power
Desired result: downlink = uplink
Link budget shouldbe balanced
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