How Wireless Works

1
How Wireless Works

• Matthew C. Valenti
• Lane Department of CSEE

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And Why Sometimes it Doesnt
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The Wireless Revolution
1000
100
Millions of Subscribers
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1
0.1
1983
1986
1989
1992
1995
1998
2001
2004
2007
wireless growth in US alone Source www.ctia.org
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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.

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Worldwide Wireless
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PCS 1.85-2.0 GHz
Cellular 824-894 MHz
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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

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Somethings Got to Give
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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.

Ch 1
Ch 2
Ch 3
Ch 1
Low power transmitter, Frequency is re-used
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The Cellular Concept
Set 1
Set 2
Set 3
Set 2
Set 3
Ch 1
Set 4
Set 1
Lower power transmitters provide coverage to a
small portion of the service area. Frequency is
reused
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Cell Patterns
Idealized Cells
Idealized Coverage
Footprint
Reality!
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The Cellular Concept
Cluster 1
Cluster 2

• 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).

A
F
B
A
B
F
G
E
G
C
E
C
D
A
D
F
B
G
E
C
D
Cluster 3
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Sectorized Antennas

• Further interference reduction by using
  sectorized antennas.

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Hand Off
Ch 2
Ch 1

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

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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.

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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 EEs working in
  the communications industry work on!

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To Learn More (A Lot More)

• Wireless Networking
• CPE 462
• 1100 AM 1215 PM, Tues-Thur.
• Woerner
• EE 327 and STAT 215 prereqs.
• Wireless Communication Systems
• EE 562
• 200 - 315 PM, Tues-Thur.
• Valenti
• EE 461 and/or EE 513 prereqs.