How Your Cellphone Works

1
How Your Cellphone Works

• Matthew C. Valenti
• Lane Department of CSEE

2
And Why Sometimes it Doesnt
3
How many cellular subscribers are there
worldwide? In the US?
4
The Wireless Revolution
1000
100
Millions of Subscribers
10
1
0.1
1983
1986
1989
1992
1995
1998
2001
2004
wireless growth in US alone Source www.ctia.org
5
Largest Carriers in US

• Cingular, 49.1 million (GSM)
• Verizon, 43.8 million (CDMA)
• Sprint, 24.8 million (CDMA)
• T-mobile, 17.3 million (GSM)
• Nextel, 16.2 million (iDen)
• In the US, the wireless industry brings in
  100B/yr.

6
Wireless Worldwide

• Over 1 billion subscibers.
• Penetration Rates for Select Countries
• Taiwan 101
• U.K 100
• Italy 96
• Netherlands 94
• Germany 84
• France 76
• Japan 65
• United States 59
• Russia 50
• India 9
• China 38

Growing Fast
7
PCS 1.85-2.0 GHz
Cellular 824-894 MHz
8
How Crowded is the spectrum

• 176.4 million subscribers
• 220 MHz available for Wireless
• So just give each subscriber 0.8 Hz of dedicated
  bandwidth (0.4 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

9
Somethings Got to Give
10
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
11
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
12
175,368 cell towers in the US
13
Cell Patterns
Idealized Cells
Idealized Coverage
Footprint
Reality!
14
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.

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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Frequency Reuse
2
1
2
5
1
5
4
4
7
3
7
3
6
6
2
5
1
4
7
3
N 7
6
16
Cell Pattern N 4
2
3
1
2
2
4
1
3
3
1
4
2
4
3
1
4
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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)

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Wireless Propagation

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

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But all is not lost

• 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.
• Advanced Receiver Processing
• Equalizer Undoes multipath
• This is the type of stuff that EEs working in
  the communications industry work on!

21
To Learn More (A Lot More)

• Wireless Networking
• CPE 493g
• 9-10 AM, M-W-F
• ESB 801
• Valenti
• EE 327 and STAT 215 prereqs.
• Wireless Communication Systems
• EE 562
• Reynolds
• 12 PM 1 PM M-W-F
• ESB 201
• Reynolds
• EE 461 and/or EE 513 prereqs.