3 cellular networks & infarastructure

A Brief Overview
By
Engr. Abdul Razzaque Memon
Email: uldhdqpia@yahoo.com

Web: Http://www.uldhdqpia.webs.com

Frequency Carries/Channels
 The information from sender to receiver is carrier

over a well defined frequency band.
 This is called a channel

 Each channel has a fixed frequency bandwidth (in

KHz) and Capacity (bit-rate)
 Different frequency bands (channels) can be used
to transmit information in parallel and
independently.

by: Engr. Abdul Razzaque Memon

Example - Frequency Spectrum Allocation
in U.S. Cellular Radio Service
Reverse Channel
991 992

…

1023

1

2

Forward Channel
…

799

824-849 MHz

991 992

…

1023

1

2

…

799

869-894 MHz

Channel Number

Center Frequency (MHz)

Reverse Channel 1 <=N <= 799
991 <= N <= 1023

0.030N + 825.0
0.030(N-1023) + 825.0

Forward Channel 1 <=N <= 799
0.030N + 870.0
991 <= N <= 1023
0.030(N-1023) + 870.0
(Channels 800-990 are unused)
Channel bandwidth is 45 MHz

Frequency & Wavelength of Some
Technologies
 AMPS Phones:
 frequency ~= 800 Mhz
 wavelength ~= 37.5 cm
 GSM Phones:
 frequency ~= 900 Mhz
 wavelength ~= 33 cm
 PCS Phones
 frequency ~= 1800 Mhz (1.8 Ghz)
 Bluetooth:
 frequency ~= 2.4 Gz


Example
 Assume a spectrum of 90KHz is allocated over a base
frequency b for communication between stations A
and B
 Assume each channel occupies 30KHz.
 There are 3 channels
 Each channel is simplex (Transmission occurs in one
way)
 For full duplex communication:
Channel 1 (b - b+30)

 Use two different channels (front and reverse channels)
Station 
A
Channel 2 (b+30 - b+60)
Station B
Use time division in a channel
Channel 3 (b+60 - b+90)


Simplex Communication
 Normally, on a channel, a station can transmit

only in one way.

 This is called simplex transmision

 To enable two-way communication (called

full-duplex communication)

 We can use Frequency Division Multiplexing
 We can use Time Division Multiplexing


Duplex Communication - FDD
 FDD: Frequency Division Duplex

Mobile
Terminal
M

Forward Channel
Reverse Channel

Base Station
B

Forward Channel and Reverse Channel use different frequency
bands


Duplex Communication - TDD
 TDD: Time Division Duplex

Mobile
Terminal
M

M

B

M

B

M

B

Base Station
B

A singe frequency channel is used. The channel is divided into time
slots. Mobile station and base station transmits on the time slots
alternately.


What is Mobility
 Initially Internet and Telephone Networks is designed

assuming the user terminals are static

 No change of location during a call/connection
 A user terminals accesses the network always from a fixed
location

 Mobility and portability
 Portability means changing point of attachment to the

network offline
 Mobility means changing point of attachment to the
network online

Degrees of Mobility
 Walking Users
 Low speed
 Small roaming area
 Usually uses high-bandwith/low-latency access

 Vehicles





High speeds
Large roaming area
Usually uses low-bandwidth/high-latency access
Uses sophisticated terminal equipment (cell phones)

Very Basic Cellular/PCS
Architecture
Public Switched
Telephone Network

Mobility
Database
Base Station
Controller

Mobile
Switching
Center
(MSC)

Radio Network
Base Station
(BS)

Mobile Station

Mobile/Cellular System
Definitions:
 Mobile Station
 A station in the cellular radio service intended for use
while in motion at unspecified locations. They can be
either hand-held personal units (portables) or installed
on vehicles (mobiles)

 Base station
 A fixed station in a mobile radio system used for radio
communication with the mobile stations. Base
stations are located at the center or edge of a
coverage region. They consists of radio channels and
transmitter and receiver antennas mounted on top of
a tower.

Definitions:
 Mobile Switching Center
 Switching center which coordinates the routing of
calls in a large service area. In a cellular radio system,
the MSC connections the cellular base stations and
the mobiles to the PSTN (telephone network). It is
also called Mobile Telephone Switching Office
(MTSO)

 Subscriber
 A user who pays subscription charges for using a
mobile communication system

 Transceiver
 A device capable of simultaneously transmitting and
receiving radio signals

Definitions:
 Control Channel
 Radio channel used for transmission of call setup, call
request, call initiation and other beacon and control
purposes.

 Forward Channel
 Radio channel used for transmission of information
from the base station to the mobile

 Reverse Channel
 Radio channel used for transmission of information
from mobile to base station


Definitions:
 Simplex Systems
 Communication systems which provide only one-way
communication

 Half Duplex Systems
 Communication Systems which allow two-way
communication by using the same radio channel for
both transmission and reception. At any given time,
the user can either transmit or receive information.

 Full Duplex Systems
 Communication systems which allow simultaneous
two-way communication. Transmission and reception
is typically on two different channels (FDD).

Definitions:
 Handoff
 The process of transferring a mobile station from one
channel or base station to an other.

 Roamer
 A mobile station which operates in a service area
(market) other than that from which service has been
subscribed.

 Page
 A brief message which is broadcast over the entire
service area, usually in simulcast fashion by many base
stations at the same time.

Mobile Communications Network


CELLUAR
TELEPHONY

WHY NEED CELLULAR SYSTEM
FOR MOBILES
Capacity Limitation
used a single high power radio
transmitter to cover a large area.
few channels for many people
1976 Bell Mobile Phone service in
New York had 12 channels, serving
543 customer, waiting list of 3,700
and market of 10 million!!


Cellular system / Network
 Apart from the capacity limitation of these
early systems, the other characteristic was
that the carrier frequency was only re-used
many tens or hundreds of kms away, so
that no ‘co–channel’ interference would
arise.
[co–channel = same frequency)


CELLUAR
TELEPHONY

Cellular systems
Cellular systems are based on the

concept of dividing the geographic
service area into a number of cells
and placing a low power transmitter in
each of these, usually at the
geographic centre.


Cellular systems – cont. . .
 The transmit frequencies are re-used across
these cells and the system becomes
interference rather than noise limited.
 Needs a method for handling the call as the user
crosses the cell boundary i.e. call Handover


Mobile Cellular Telephone System


Cellular Telephony
 Characterized by
 High mobility provision
 Wide-range
 Two-way tetherless voice communication
 Handoff and roaming support
 Integrated with sophisticated public switched

telephone network (PSTN)
 High transmit power requires at the handsets
(~2W)

3 cellular networks & infarastructure

CELLUAR
TELEPHONY

What is a Cell?
 Cellular Telephony meets demand of

multiple users in a metropolitan
area (called the market) by breaking
the whole market into several
smaller regions known as CELLS.
 Each cell has its own transmission
tower and set of assignable
communication channels. A Cell is an
area covered by a BASE STATION.


CELLUAR
TELEPHONY

Figure showing mobile
telephone tower


CELLUAR
TELEPHONY

Question?

 why hexagon is used

as a cell? Why not a
triangle? a square?
a circle?


CELLUAR
TELEPHONY

Answer – Yes Hexagonal
 Because to avoid gaps

between cells.
 If a customer is near the
boundary of a cell, it can’t be
undertaken by the cell next to
it,because of the gap between
two cells, therefore
communication breakdown
occurs.
 And also to remain equal
distance from a base station to
a mobile user.


FREQUENCY REUSE
 The same frequency is used by many cells (separated by a

distance).

 Spectral efficiency (or capacity) is greatly increased.

See next slide . . .


Example of 2G Digital Cellular
The Global System for Mobile communication . . . .

Network Architecture


Mobile Station (MS)
 The Cellular (GSM) committee has introduced an important

powerful innovation by using a Smart Card in conjunction with a
mobile telephone. Thus GSM subscribers are provided with a
Subscriber Identity Module card (SIM-Card) with its unique
identification at the very beginning of the service.
 "Confidential information -- may not be copied or disclosed
without permission".
 The Mobile Station (MS) includes radio equipment and the man
machine interface (MMI) that a subscriber needs in order to
access the services provided by the Cellular (GSM) network.


Mobile Station (MS)


Mobile Station (MS)
 The mobile station includes provisions for data

communication as well as voice.

 Each mobile station has an International Mobile

Equipment Identity (IMEI) that is permanently stored in
the mobile unit. Upon request, the MS sends this number
over the signaling channel to the network.
 The IMEI is used to identify mobile units that are
reported stolen or operating incorrectly.


Um

Radio-Interface


3 cellular networks & infarastructure

3 cellular networks & infarastructure

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