Cellular Telephones

1
Cellular Telephones

• How Do They Work and Are They Safe?
• Meagan Morrell
• May 3, 2000

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What Is a Cell Phone?

• A cell phone is nothing but a radio.
• They work like a two-way walkie-talkie that acts
  like a phone.
• Difference being, a walkie-talkie you can either
  talk or listen with a cell phone you can talk
  and listen at the same time.

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Analog vs. Digital Cell Phones

• The difference between analog and digital phones
  is how the audio signals are transmitted between
  the phone and base station.
• In either system, the audio at the microphone
  always starts out as a voltage level that varies
  continuously over time.
• High frequencies (high pitch) cause rapid changes
  and low frequencies cause slow changes.

4
Analog Systems

• In this type of system the audio is modulated
  directly onto a carrier. This acts like FM
  radio.
• Interference (RF noise or some other anomaly that
  affects the transmitted signal) gets translated
  directly into the recovered signal there is no
  check that the signal makes sense.

5
Digital Systems

• In this type of system the audio is converted to
  digitized samples at about 8000 samples per
  second.
• The digital samples are numbers that represent
  the time-varying voltage level at specific points
  in time. These samples (numbers) are transmitted
  in binary code.

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Digital Systems Continued

• At the other end, the samples are converted back
  to voltage levels and smoothed out so that you
  receive about the same audio signal that was
  sent. (Some loss does occur.)
• With this system the binary code that is sent can
  not be easily confused or distorted during
  transmission, plus extra data is typically
  included in the transmission to help detect and
  correct any errors. (A check does occur in
  digital systems.)

7
When a Cell Phone Places a Call

• After entering a number and pressing SND
• The phone checks all 21 control channels and
  determines which has the strongest signal.
• It chooses this signal to place the call.
• The phone now transmits a very short message
  (about ¼ second) that contains the MIN (Mobile
  Identification Number), its ESN (Electronic
  Serial Number), and the number being dialed.
• After the cellular service provider verifies
  validity, the base station sends a Channel
  Assignment to the phone (also a ¼ second burst).
• Conversation can now begin.

8
Cellular Radiotelephone Service

• A cellular system operates by dividing a large
  geographical service area into cells and
  assigning the same channels to multiple,
  nonadjacent cells.
• This allows channels to be reused, increasing
  spectrum efficiency (25 MHz for each cellular
  system).
• All cells are connected to a Mobile Telephone
  Switching Office (MTSO) by landline or microwave
  links.

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What does the MTSO do?

• The MTSO controls switching between Public
  Switched Telephone Network (PSTN) and the cell
  site for all calls, it also processes mobile unit
  status data.
• It also uses the control channel to tell the
  mobile/portable unit which voice channel has been
  assigned to the call.

10
Safety Issues

• It is still unknown to whether cellular
  telephones are the cause of any health issues.
• There has been an equal amount of data found that
  can prove and disprove that there is a
  correlation between health and the radiation form
  electromagnetic fields that comes from cellular
  use.
• A thought to leave you with about safety, a
  microwave and a cellular phone both work on GHz
  frequencies but a microwave cooks food.

11
Where Radiation Comes From

• This is a graphical representation of the
  radiation emitted by a cell phone. (From
  http//www.lessemf.com/cellphon.html)
• Notice the strong radiation coming from the body
  of the phone, especially the earpiece.

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How To Minimize Your Risk

• Shorter conversations.
• Speak as little as possible inside the car
  because it amplifies the radiation.
• Dont place mobile phones near babies, even on
  and not in use cell phones emit microwaves.
  Children tend to be more sensitive to things than
  adults and it goes for radiation as well.

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Minimizing Risks Continued

• Always pull out the antenna so it is further away
  from your head. (The microwaves of the phone
  heat up human cells.)
• You can also buy shields for your phone to reduce
  radiation directed at you. (Graphical
  representations follow.)

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Shielding Performance
Radiation Scan of a Normal Cell Phone Before
After Phone/Shield Added
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Conclusion

• What has been covered
• Basic cellular use and how cell phones work.
• Differences between Analog and Digital signals.
• Safety issues and are cell phones health
  conscious.
• Where radiation actually comes from and how to
  protect yourself and minimize your possible risks.