How Wireless Works… Matthew C. Valenti Lane Department of CSEE.

How Wireless Works…

Matthew C. Valenti

Lane Department of CSEE

2/14/2007

And Why Sometimes it Doesn’t…

The Wireless Revolution

0.1

10

1000

1983

1986

1989

1992

1995

1998

2001

2004

100

Mil

lion

s of

Sub

scri

bers

1 Wireles

s subscr

ibers (c

ellular

and PCS)

Wireline access lines

1st generationAnalog (FM)

2nd

generation

3rd

generation

wireless growth in US alone Source: www.ctia.org

231.8 million Feb. 07

2007

2/14/2007

Wireless Is BIG Business

In 2006, Americans used 857 billion minutes of talktime. 1.6 million years total. Average of 10 minutes/day/customer. 42% of Americans use wireless as their primary phone.

U.S. Revenues and Industry Average bill of $49.30/month/customer. Over $135 billion/year industry. 2.5% of US workforce in wireless industry In the next 5 years, wireless will be a bigger US industry

than automotive or agriculture. Worldwide Usage

2.3 billion wireless subscribers worldwide 986 million handsets sold in 2006.

Worldwide Wireless

2/14/2007

Cellular: 824-894 MHz PCS: 1.85-2.0 GHz

2/14/2007

How Crowded is the Spectrum? Wireless Spectrum

231 million subscribers in US 220 MHz available for Wireless (110 each direction) So just give each subscriber about 1 Hz of dedicated

bandwidth (0.5 Hz each direction), right? Problem: cellular signal occupies:

200 kHz (GSM) 550 distinct channel pairs. Time division multiple access divides channels into 8

subchannels. So 4400 “conversations”

1.25 MHz (CDMA) 88 distinct channel pairs. Code division multiple access divides each channel into 64

subchannels So 5632 “conversations”

2/14/2007

Something’s Got to Give…

The Cellular Concept

Transmit power drops off with distance. When you are far-enough away you can re-use

the channel. Similar concept to frequency re-use for radio and

television stations.

Low power transmitter, Frequency is re-used

Ch #1 Ch #2 Ch #3 Ch #1

The Cellular Concept

Lower power transmitters provide coverage to a small portion of the service area. Frequency is reused

Set #1 Set #2 Set #3

Ch #1

Set #3

Set #4

Set #2

Set #1

Cell Patterns

Idealized Cells Idealized Coverage

Reality!

Footprint

The Cellular Concept

Break the metropolitan area into small areas

Each area is approximated with a hexagonal cell.

A base station is located at the center of each cell.

Each cell is assigned only a fraction of the total number of channels.

Cells that are sufficiently far apart can reuse the same frequency.

In the US, there are currently 200,000 base stations (cells).

Cluster #1Cluster #2

Cluster #3

AB

CD

E

FG

F

F

E

E

D

D

C

C

B

B

A

A

G

G

2/14/2007

Sectorized Antennas

Further interference reduction by using sectorized antennas.

Hand Off

Ch #1Ch #2

Mobile must be transferred between cells as it moves-Hard handoff-Soft handoff (CDMA)-Softer handoff (sectorized antennas)Possibility for a dropped call.

2/14/2007

The Challenges ofWireless

Fading Due to relative motion between TX and RX.

Multipath Due to signal reflections.

Diffraction Signal bending around objects (mountain, buildings)

Shadowing Obstructions that attenuate signal (foliage)

Interference Other signals Main limitation in built-up areas.

Noise Thermal excitement of electrons in receiver. Background noise in space.

2/14/2007

Facing the Challenges

Source Coding Companding: Reduces BW needed by voice.

Channel Coding Forward Error Correction Coding. By adding parity bits to transmitted data, errors can be

corrected. Spread Spectrum Communication Use of Multiple Antennas Advanced Receiver Processing

Equalizer: Undoes multipath This is the type of stuff that EE’s working in the

communications industry work on!

2/14/2007

To Learn More (A Lot More)…

Wireless Networking CPE 462 11:00 AM – 12:15 PM, Tues-Thur. Woerner EE 327 and STAT 215 prereq’s.

Wireless Communication Systems EE 562 2:00 - 3:15 PM, Tues-Thur. Valenti EE 461 and/or EE 513 prereq’s.