Post on 04-Apr-2018
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FADINGS
signal level (dB)
log (distance)
global mean value
log normal fadinglocal mean valueslow fadingshadowinglong-term fading
rayleigh fadingfast fadingshort-term fading
so many names
to make lifeworst!
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FAST FADING
signal level (dB)
log (distance)
present due to the fact that the mobile antenna is lowerthan the surrounding structures such as trees and buildings.
peak-to-peak distance is ~ lambda/2 (in GSM ~ 17 cm) affects the signal quality and can lead to signal level belowthe receiver sensitivity.
SOLUTIONS: use more power at the transmitter (providing a fading
margin). use space diversity.
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SLOW FADING
signal level (dB)
log (distance)
if we smooth out the fast fading, the signal variationreceived is called the local mean or the slow fading. caused by obstructions near the mobile such asbuildings, bridges and trees and this may cause a rapid
change of the local mean (in the range of 5 to 50 meters). because slow fading reduces the average strengthreceived, the total coverage from the transmitter is reduced.
SOLUTION: fading margin must be used.
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INTERFERENCE
co-channel interference
adjacent channel interferenceCo-channel interference is the term used for the interference in a cellby carriers with the same frequency present in other cells.
Adjacent carrier frequencies, i.e. frequencies shifted +/- 200 kHz with
respect to the carrier, can not be allowed to have too strong signalstrengths either. Even though they are at different frequencies, partof the signal can interfere with the wanted carriers signal and causequality problems.
f1f1
co-channel
wanted carrier
adjacent
carrier
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CO-CHANNEL INTERFERENCE
CI
C
I
C/I > 0 dB
dB Carrier, f1 Interferer, f1
distanceGSM Specification: C/I => 9 dBEricsson Planning Criterion:C/I => 12 dB (without frequency hopping)C/I => 9 dB (with frequency hopping)
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ADJACENT CHANNEL INTERFERENCE
CA
A
C
C/A< 0 dB
dB Carrier, f1 Adjacent, f2f2=f1 +/- 200 kHz
distanceGSM Specification: C/A > -9 dBsince we could not cell plan a negative valueEricsson Planning Criterion:C/A > 3 dB
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ISI
InterSymbol Interference caused by excessive time dispersion. it may be present in all cell re-use pattern.
it can be thought of as a co-channel interference. in this case, the interferer is a time delayed reflectionof the wanted carrier. GSM specification: C/R > 9 dB however, if the time delay is smaller than 15microseconds, i.e. 4 bits or approximately 4.4 km, the
equalizer can solve the problem.
D0
D1
D2
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TRAFFIC THEORY Traffic and Channel Dimensioning
Traffic theory attempts to obtain useful estimates of the numberof channels needed in a cell...
= HOW MANY CUSTOMERS?= HOW LONG WILL THEY TALK?= WHEN?
FACTORS AFFECTING THE CELLULAR SYSTEM CAPACITY: The number of channels available for voice and/or data.
The amount of traffic the subscribers are generating. The grade of service the subscribers are encountering in the system
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The number of channels available for voice and/or data.
Assume 1 cell has 2 carriers:2 x 8 - 2 = 14
14 traffic channels2 physical channels are
needed for signalling
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The amount of traffic the subscribers are generating.
What is traffic? the usage of channels
holding time per time unit
the number of call hours per hour
measured in the unit Erlang (E)
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The amount of traffic the subscribers are generating.
1 Erlang = 1 call that last 1 hour!
...studies show that the averagetraffic per subscriber during thebusy hour is typically 15 - 20 mE...
Typical actual values:
Philippines (Smart E-TACS)7 mE/subscriber - Metro Manila8 mE/subscriber - Provincial
Malaysia and Sweden25 mE/subscriber
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The amount of traffic the subscribers are generating.
visualize 15 mE !1 E = 1 hour of use15 mE = ? hour of use
15 mE / 1 E = 0.015~ 1.5% of 60 min = 0.9 min x 60 sec15 mE = 54 seconds of use
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The grade of service the subscribers are encountering in the system.
How much trafficcan one cell carry?
That depends on the number of
traffic channels and the acceptableprobability that the system is
congested, the so called
Grade of Service (GoS)...
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The grade of service the subscribers are encountering in the system.
Grade of Serviceis the
Grade of NO SERVICE !
- unsuccessful call set-up- GoS = 2% means
98% can make a call2% blocking probability
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Alien !!!
The grade of service the subscribers are encountering in the system.
Erlangs B-Model - loss system
no queues number of subscribers much higher than number of traffic channels no dedicated (reserved) traffic channels Poisson distributed (random) traffic blocked calls abandon the callattempt immediately
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The grade of service the subscribers are encountering in the system.
Erlangs B-Model relates: number of traffic channels, n the GoS
the traffic offered, A
Example:2 carriers
14 TCHGoS = 2%from the Erlang Table...Traffic Offered, A = 8.2003 Erlangs
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In GSM, a call goes through two different devices.
TCH & SDCCH
SDCCH procedures: location updating periodic registration IMSI attach IMSI detach call setup SMS (Short Message Services) facsimile other supplementary services
SDCCH TCH
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Ericsson Rule on GoS Dimensioning:
GoSSDCCH
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CHANNEL UTILIZATION (EFFICIENCY)
VS
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CHANNEL UTILIZATION (EFFICIENCY)
VS
To calculate the channel utilization, thetraffic offered is reduced by the GoS (yieldingthe traffic served), and dividing that valueby the number of channels.
Traffic Offered - GoS (Traffic Offered)Channel Utilization =
Number of Channels
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SITE EQUIPMENT
BSC
BTS
Base Station System (BSS)
The BSS consists of a Base Station Controller (BSC) with a numberof base stations connected to it. The BSS is mainly responsible forall radio related functions in the system. In the GSM specifications, thedetonation BTS (Base Transceiver Station) is used for the base station.
site equipment
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SITE REQUIREMENTS
Permits
Access Roads
Material Transport
and Storage Space Requirements
Antenna SupportStructures
AC Mains Supply
Transmission Access
Antenna Feeder Routes
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RBS EQUIPMENT
EricssonNokiaNortelLucent
MotorolaAlcatel
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COMBINERS
transmitter 2
transmitter 3
transmitter n
transmitter 1
combiner
Combiners are needed to enable more thanone transmitter to be connected to one
common transmitting antenna.
In GSM, two different TX combiners can beused -
FILTER COMBINER HYBRID COMBINER
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SENSITIVITY
BTScabinet Feeder & Jumpers ALNA
Without ALNA
RX ref point 2
With ALNA
RX ref point 1
System Cell Planning Worst CaseSensitivity SensitivityGSM900 - 107 dBm - 105 dBmGSM1800 with ALNA - 109 dBm - 107 dBmGSM1800 w/o ALNA - 106 dBm - 104 dBm
Base station receiver sensitivityValid for both Ericssons RBS2000 and RBS200/205
Micro Base Station: Cell Planning Power = 1.6W, 32 dBmWorst Case Sensitivity = - 104 dBm
Base Station
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SENSITIVITY
System MS Power Cell Planning Worst CaseClass Power PowerGSM900 2 39 dBm (8W) 37 dBmGSM900 3 37 dBm (5W) 35 dBmGSM900 4 (handheld) 33 dBm (2W) 31 dBmGSM900 5 (handheld) 29 dBm (0.8W) 27 dBm
GSM1800 1 30 dBm (1W) 28 dBmGSM1800 2 24 dBm (0.25W) 22 dBm
Mobile Station Power Classes
Mobile Station Reference Sensitivity
System MS Type Cell Planning Worst CaseSensitivity Sensitivity
GSM900 Handheld - 104 dBm - 102 dBmGSM900 All other types - 106 dBm - 104 dBmGSM1800 Handheld - 102 dBm - 100 dBm
No loss or antenna gain should be used for the MSs.
MS antenna gain: 0 dBi
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ANTENNAS
Basic Antenna Types
Omnidirectional Antennas Unidirectional Antennas
Special Antennas
Multi Antenna Systems
Diversity
Space Diversity
Polarization Diversity
Antenna Tilt
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DIVERSITY
There is a need for receiver diversity in cellular systemsto improve the uplink.
Space Diversity
TX1/RXA TX1/RXA
BTS Equipment
CommonTX/RXAntenna
dd
Horizontal Separation, dd
for diversity = 12-18 (wavelength)
for isolation = 30 dB = 2 (wavelength)[antennas with 65 degrees beamwidth,
all gain values]
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DIVERSITY
There is a need for receiver diversity in cellular systemsto improve the uplink.
Polarization Diversity using dual-polarized antennas
vertical + horizontal polarization +/- 45 degrees polarization
vertical
array
horizontalarray
antennahousing
connectors
feeders
+45degrees
- 45degrees
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DIVERSITY
There is a need for receiver diversity in cellular systemsto improve the uplink.
Polarization Diversity using dual-polarized antennas
1.5 dB downlink loss
TX1/RXA TX1/RXA
Required isolation >30 dB between
the two antenna parts...
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DIVERSITY
There is a need for receiver diversity in cellular systemsto improve the uplink.
SPACE DIVERSITYVSPOLARIZATION DIVERSITY
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ANTENNA TILT
When the antenna is mounted vertically,the main lobe of the antenna radiationpattern will follow a horizontal linestarting at the centerpoint of the antenna.
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ANTENNA TILT
For reasons, such as co-channel interferenceand time dispersion problems, it can be interestingto tilt the antenna, and let the main lobe point
a few degrees downward.
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NOMINAL CELL PLAN
Traffic and Coverage Analysis
Nominal Cell Plan
Surveys
System Design
Implementation
System Tuning
TRAFFIC DATA
Based on ERICSSON
Initial PlanningSystem Growth
cell database stationlocations
channelallocations
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