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Cellular Telephones
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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The Cells
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images/cells.gif
The Cells (cont.)
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  • Properties:
    • Appropriate size for low power transmissions (26 km2 = 10 mi2)
    • Non-adjacent cells can use same frequencies (i.e., each cell can use 1/7 of the available frequencies)
  • Handoff:
    • The tower you are communicating with changes as you move from cell to cell
    • Handoff occurs based on signal strength
Frequency Allocation and Channels
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  • Frequency Allocation:
    • A carrier typically has 832 frequencies
    • 2 are used per call (full duplex)
    • 42 are used for control
  • Channels:
    • 832 - 42 = 790 frequencies are available for voice channels
    • 790 / 7 = 112 (approximately) frequencies are available per cell
    • 112 / 2 = 56 voice channels per carrier
Advanced Mobile Phone System (AMPS)
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  • Created in 1983
  • Frequencies between 824 MHz and 894 MHz
  • Each channel is 30Khz wide
Frequency Division Multiple Access (FDMA)
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Each call is on its own frequencies
Time Division Multiple Access (TDMA)
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  • Assigns each call a certain portion of time on a designated frequency
  • Uses analog-to-digital conversion and compression
  • A channel (30 kHz wide and 6.7 milliseconds long) is split time-wise into three time slots
  • Operate in either the 800-MHz (IS-54) or 1900-MHz (IS- 136) frequency bands
Global System for Mobile Communications (GSM)
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  • Established in Europe in the mid-1980s
  • Similar to TDMA
  • Uses encryption
  • 900-MHz and 1800-MHz bands in Europe and Asia, 1900- MHz band in the U.S.
Code Division Multiple Access (CDMA)
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  • Uses analog-to-digtal conversion
  • Gives a unique code to each call and spreads it over the available frequencies (spread spectrum)
  • Each phone has its own spreading code
  • Multiple calls (8-10) are overlaid on a channel
  • Operates in both the 800-MHz and 1900-MHz frequency bands
Personal Communications Services (PCS)
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  • Smaller cells
  • Operates in the 1850-MHz to 1990-MHz bands
  • Similar to TDMA but with 200-kHz channel spacing and eight time slots
There's Always More to Learn
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