Download - 3 cellular networks & infarastructure

A Brief Overview By

Engr. Abdul Razzaque Memon

Email: [email protected]

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


991 992 … 1023 1 2 … 799 991 992 … 1023 1 2 … 799

824-849 MHz 869-894 MHz

Reverse Channel Forward Channel

Channel Number Center Frequency (MHz)

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

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

0.030N + 825.00.030(N-1023) + 825.0

0.030N + 870.00.030(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) wavelength ~= 16.6 cm

Bluetooth: frequency ~= 2.4 Gz wavelength ~= 12.5 cm


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:

Use two different channels (front and reverse channels)

Use time division in a channel


Channel 1 (b - b+30)

Channel 2 (b+30 - b+60)

Channel 3 (b+60 - b+90)

Station A Station B

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


Base StationB

Mobile Terminal

M

Forward ChannelReverse Channel

Forward Channel and Reverse Channel use different frequency bands

Duplex Communication - TDD TDD: Time Division Duplex


Base StationB

Mobile Terminal

M

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

M B M B M B

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


Base Station(BS) Mobile Station

Base Station Controller

Mobility Database

Mobile Switching

Center(MSC)

Radio Network

Public SwitchedTelephone Network

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.



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



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



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).



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

BS MSC PSTN

PBX

Residence

Mobile Station(Phone)

LandLine

LandLine

Cell Site

Base stationsand other cell

sites

Two- to Three-Mile Radius

BS = Base StationMSC = Mobile Switching Center

WHY NEED CELLULAR SYSTEM FOR MOBILESCapacity Limitationused 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!!


CELLUAR TELEPHONY


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)

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.


CELLUAR TELEPHONY


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

FrequencyA

FrequencyB

FrequencyC

FrequencyC

FrequencyF

FrequencyE

FrequencyA

FrequencyD

FrequencyA

FrequencyB

BaseStation

Tolloffice

Tolltrunk

Localloop

Landline

CentralOffice

Landline topublic

switchedtelephonenetwork

FrequencyG

FrequencyA

FrequencyF

FrequencyD

FrequencyE

FrequencyG

FrequencyB

FrequencyC

Masterswitching

center(MTSO)

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)


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.


CELLUAR TELEPHONY

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


Download - 3 cellular networks & infarastructure

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