+

Cellular Networks

CPSC441, Winter 2010

+First Mobile Telephone System

One and only onehigh power base station with which allusers communicate.

Entire Coverage Area

NormalTelephone

System

Wired connection

+Problem with Original Design

Original mobile telephone system could only support a handful of users at a time…over an entire city!

With only one high power base station, users phones also needed to be able to transmit at high powers

Car phones were therefore much more feasible than handheld phones.

+The Core Idea: Cellular Concept

The core idea that led to today’s system was the cellular concept.

The cellular concept: multiple lower-power base stations that service mobile users within their coverage area and handoff users to neighboring base stations as users move. Together base stations tessellate the system coverage area.

+Main Principles

Small cells tessellate overall coverage area.

Users handoff as they move from one cell to another.

Frequency reuse.

+Tessellation

Three regular polygons that always tessellate: Equilateral triangle Square Regular Hexagon

TrianglesSquares

Hexagons

+Circular Coverage Areas

Antennas are omni-directional (usually)Circles don’t tesselate!Closest shape - hexagon Users located outside

some distance to thebase station receive weak signals.

Result: base station hascircular coveragearea.

Weak signal

Strong si

gnal

+Thus the Name Cellular

With hexagonal coverage area, a cellular network is drawn as:

The network resembles cells from a honeycomb thus the name cellular!

BaseStation

+Handoffs

Mobile users are “mobile” by definition!Continuous access to network required

Not a problem within a cellMoving between cells requires a handoff

mechanism!

+A Handoff

At some point, user’s signal is weak enough at B1

strong enough at B2

Messages between users and base stations required to coordinate handoff

B1B2

+Cellular Networks Components

+Cellular Networks Components

PSTN – Public Switched Telephone Network

MSC – Mobile Switching Centre Connects PSTN to BSS

BSS – Base Station System Interface between cellular network and mobile user

MS – Mobile Stations Cellphones, Carphones, etc.

+Frequency Reuse

Total spectrum allocated to the service provider is broken up into smaller bands

Adjacent cells assigned different frequencies to avoid interference or crosstalk

Spectrum is limited – need to reuse frequencyOptimal assignment is equivalent to graph

colouring problem – NP-Hard!

+Example of Frequency Reuse

Cells using the same frequencies

+Multiple Access Methods

Three widely-used policies:

Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA)

+FDMA

In FDMA, the band of frequency is broken up into smaller bands, i.e., subbands.

Each transmitter (user) transmits to the base station using radio waves in its own subband.

FrequencySubbands

Cell Phone User 1Cell Phone User 2::

Cell Phone User N

Time

+TDMA

In pure TDMA, base station does not split up its allotted frequency band into smaller frequency subbands.

Rather it communicates with the users one-at-a-time, i.e., “round robin” access.

…FrequencyBands

Time

Use

r 1

Use

r 2

Use

r 3

Use

r N

+Hybrid FDMA/TDMA

The TDMA used by real cellular systems (like AT&T’s) is actually a combination of FDMA/TDMA.

Base station breaks up its total frequency band into smaller subbands.

Base station also divides time into slots and frames.

Each user is now assigned a frequency and a time slot in the frame.

+Hybrid FDMA/TDMA (Cont’d)

Time

…

…

Use

r 1

Use

r 2

Use

r 10

Use

r 11

Use

r 12

Use

r 20

…

…

Use

r 31

Use

r 32

Use

r 40

Use

r 21

Use

r 22

Use

r 30

Assume a base station divides its frequency band into 4 subbands and time into 10 slots per frame.

…

…U

ser

1

Use

r 2

Use

r 10

Use

r 11

Use

r 12

Use

r 20

…

…

Use

r 31

Use

r 32

Use

r 40

Use

r 21

Use

r 22

Use

r 30

…

…

…

…

Frame

Frequency Subband 1

Frequency Subband 2

Frequency Subband 3

Frequency Subband 4

+CDMA

CDMA is a more complicated scheme.

Users communicate with BSS at the same time and using the same set of frequencies.

A desired user’s signal is deciphered using a unique code assigned to the user.

+References

LUCID Summer Workshop, July 27, 2004http://www.ece.lehigh.edu/~skishore/research/lucid/lucid_2.ppt

Song Zhang, CPSC601 Fall 2008, project presentation