ECE 5221 Personal Communication Systems
Prepared by:
Dr. Ivica Kostanic
Lecture 2: Basics of cellular system architecture
(Chapter 2.1 and 2.2)
Spring 2011
Florida Institute of technologies
Outline
Outline of cellular system architecture
Elements of the cellular system architecture
Support for mobility – Handoff
Automatic roaming
Important note: Slides present summary of the results. Detailed
derivations are given in notes.
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Page 2
Layout of the Cellular Comm. System
Other MSCs
PSTN
BTS
MSC
BSC
ISDN
EXTERNAL
NETWORKS
MS
HLR
AC
VLR
EIR
RADIO SYSTEMS
DATA-BASE
SUBSYSTEM
OA&M
SWITCHING
SYSTEMS
Circuit switched part of cellular system
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MSC - Mobile Switching Center
BSC - Base Station Controler
BTS
- Base Transceiver System
HLR
- Home Location Register
VLR
- Visitor Location Register
AC
- Authentication Center
EIR
- Equipment Identity Register
PSTN - Public Switched Telephone Network
ISDN - Integrated Service Digital Network
OA&M - Operation, Administration &
Maintenance System
Mobile Subscriber Unit
 Usually the only part of the system
that users are aware of
 Traditionally used for voice
CONTROL
SECTION
Receive Audio
Signal
Processing
 Becoming more data centric
 Performs any functions
ANTENNA
ASSEMBLY
TRANSCEIVER UNIT
Demodulation
RF
Amplifier
Filter
Antenna
RECEIVER
Display
Duplexer
Logic Unit
Control
o Voice processing
o CS/PS Call management
o Packet data transfer
Keyboard
Transmit
Audio Signal
Processing
Modulation
RF
Amplifier
Filter
TRANSMITTER
 May be multi-band
 May be multi-technology
Block diagram of voice
processing in mobile unit
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Base Station Controller
Two implementations
 Multiple BSCs
 BSC within MSC
BSC
PSTN
MSC
BSC
PSTN
MSC
BSC
Responsibility of BSC
 Radio resource management
 Handoff between cells
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Base Transceiver Station
Tx/Rx_A
 Base station
Duplexer
Filter
...
 May be configured as omnidirectional or sectored
...
Voice Links
Rx_A
Tx
Rx_B
Rx_A
Rx_B
Rx_A
...
to B
receivers
Control Scanning
Channel Receiver
Tx
Voice
Channels
...
 BTS connects to the core
network (BSC) through T1/E1
lines
Receiver
Multicoupler_B
to A
receivers
Rx_B
 The number of users that can
be served = number of BTS
resources
...
Tx
 Base station provides
communication resources over
given coverage area
Receiver
Multicoupler_A
from
transmitters
Rx_A
o Responsible for radio link
between system and the
mobile
Transmitter
Combiner
Rx_B
o Bank of radios
Signaling Links
...
to BSC
...
from BSC
...
unchannelized T1
Block diagram of a base
station
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Rx_B
Base Station Antenna Configuration
 Omnidirectional (Omni) Antennas
 Coverage pattern is a circle
 Usually located in lowtraffic
(rural) areas
 Directional Antennas
 Coverage directed to
a specific area
 Increase system capacity
 60º, 90º and 120º are
the most common
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Base Station Coverage Classification
Macrocells
Coverage depends on;
 transmit power
Microcells
 antenna gains
 antenna height
Picocells
Cell Type
Cell Radius
Operating
Environment
Installation
Mode
Subscriber’s
Speed
Mega Cell
100 – 500 km
Global
Macro Cell
<= 35 km
Suburban
Micro Cell
<= 1 km
Urban
Pico Cell
<= 50 m
In-building
Satellites
Top of the
building or tower
Lamp-post or
side of the
building
<= 100 km/h
Inside the
building
<= 500 km/h
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< = 10 km/h
Towers
Monopole
- 80’ - 150’
- small amount of
land required
- limited loading
Guyed Tower
- up to 1500’
- heavy loading
can be a problem
Self Supporting
- 100’ - 300’
- heavy loading
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Circuit vs. packet switching
 Legacy cellular technologies (1G and 2G)
 are circuit switched
 Dial-up type connections
 A single user occupies a channel for the
entire transmission
 Requires time-oriented billing
 Modern cellular networks (3G and 4G)
is packet switched technology
 GSM transmissions are bursty
 More appropriate for data services
 Bursty nature favors data services
 Continuous flow is not required
 Access is based on demand only
 Several users can be multiplexed
 Billing based on negotiated QoS and
usage
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Page 10
Cellular architecture CS/PS (2G->3G)
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Page 11
Handoff
 Fundamental requirement for mobility support
 Two types: hard handoff and soft handoff
 Hard Handoff … ‘Break before make!’
 Analog Handoff - obsolete
 MAHO (Mobile Assisted Handoff)
 Soft Handoff … ‘Make before break!’
 currently just in CDMA systems
 Soft handoff is MAHO
 Type of implemented handoff depends on air interface
technology
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MAHO
PSTN
Neighboring Cell
Switch
(2)
(1a)
RSSI report
Decides about
Handoff based
on reported
RSSIs
Sends
Channel
Assignment
Command to
new BS
Sends Retune
Command to
MS through
Serving Cell
Serving Cell
to Swich
RSSI report + voice
All modern
technologies use
MAHO
Scanns Spectrum
for users in need of
Reports RSSI Handoff
(1b)
Neighboring Cell
Measures
RSSI from the
Neighboring
BSs
Reports RSSI to
Swich through
Serving BS
RSSI report
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Hard HO and Soft HO
A1
A3
B1
A2
B3
Drop A2
B2
Add B3
Drop B3
Add C2
C1
C3
C2
Hard handoff
Type of handoff – function of air interface
Soft handoff more reliable
Soft handoff – resource intense
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Soft handoff
Automatic Roaming
MSC - Mobile Switching Center
HLR - Home Location Register
VLR - Visitor Location Register
MS - Mobile Station
BS - Base Station
SS7 - Signaling Network
Current location of a mobile – kept in a VLR
When mobile turns on – it registers
As a part of registration – HLR is updated with
the current mobile’s VLR
When the mobile is called, based on the HLR
entry, the call is located to the MSC of the mobile’s
current VLR
The mobile is paged within the area or sub-area
of current VLR
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