Wireless Technology

1
Wireless Technology
2
Radio Transmission

• Oscillating electron generates electromagnetic
  waves with the frequency of the oscillation
• Many electrons must be excited in an antenna for
  a strong signal

3
Cellular Telephones

• Original Mobile Telephones
• One transmitter/receiver
• Limited number of channels
• For good service can support about 20 subscribers
  per channel

Transmitter/ Receiver
Mobile Phone
4
Cellular Telephones

• Divide Region into Cells
• One cellsite per cell
• Channels can be reused in non-adjacent cells

Yes
No
Can Reuse Ch. 232?
Uses Channel 232
No
No
Yes
Yes
No
No
Channel 232 Used in 4 cells
No
5
Cellular Telephones

• Reuse
• Without reuse, only 20 users per channel for good
  service
• If reused 3 times, 60 subscribers per channel
• Reuse Rule (Rough)
• Reuse factor Number of cells / 7
• If 20 cells, reuse factor is about 3

6
Handoffs

• When you move to another cell within the same
  system
• You are transferred automatically to that cells
  cellsite

7
Roaming

• Take your cellphone to another city
• Use it there to send and receive
• Not always possible technically
• May be limited procedurally because of high rates
  of cellular fraud in some areas
• Dont confuse this with handoff, which takes
  place within a cellular system between cells

8
Control

• Mobile Telephone Switching Office
• Controls cellsites, handoffs, etc.
• Calls go to/from MTSO
• Connects to POP at LEC to link to traditional
  telephone (wireline) carriers

POP at LEC
MTSO
9
Placing a Call

• Enter number, hit send
• Cellphone broadcasts request
• Several cellsites receive, send to MTSO
• MTSO assigns cellphone to cellsite with loudest
  signal
• MTSO sends message to cellphone, telling it what
  incoming, outgoing channels to use

10
Receiving a Call

• MTSO has each cellsite broadcast cellphones ID
  number
• Cellphone transmits a response
• Responses from cellsites go to MTSO
• MTSO selects loudest cellsite
• MTSO sends message to cellphone, giving channels
  and telling the cellphone to ring

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First Generation Cellular

• Analog or Digital Operation
• Initially analog initially analog in U.S. using
  the AMPS standard
• Europe and the rest of the world started with a
  large number of incompatible analog systems but
  settled on the digital GSM standard

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First-Generation Cellular

• Large Cells
• Usually only 20-40 per city
• Limits channel reuse
• Limited Number of Channels
• In U.S., 832 two-way channels
• No Compression
• Each voice signal required a full two-way channel

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First-Generation Cellular

• How Many Subscribers Can You Support?
• 20 cells
• Channel reuse is about 3 (20/7)
• 832 channels
• With channel reuse, 2,496 effective channels
• 20 users per available channel
• So only about 50,000 subscribers per city
• Engineering tricks can extend, but only somewhat

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Second-Generation Cellular

• Personal Communication Service (PCS)
• Or Personal Communication Network (PCN)
• More channels
• About 2,500
• Smaller cells permit more channel reuse
• Dont just say smaller cells be explicit about
  channel reuse

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Potential System Capacity (Roughly)

• Category 1st Gen 2nd Gen
• Cells/City 30 100
• Channel reuse (cells/7) 4 14
• Channels 800 2,500
• Effective channels 3,200 35,000
• With compression 3,200 105,000
• Subscribers (x20/channel) 64,000 2,000,000
• No compression in 1st generation

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Second-Generation Cellular

• Smaller cells
• PCS Cellphones
• Do not have to transmit as far because cells are
  smaller
• Cellphones can be less expensive because use less
  power

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Second-Generation Cellular

• Most of World
• Standardizing on DCS Technology
• Based on GSM and usually called GSM
• U.S.
• FCC did not specify a standard
• Different carriers use different technologies
• Some have standardized on GSM
• Your cellphone may not work with another carrier
• Limits roaming

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Generations Recap
1st
2nd
Analog/Digital
Both AD
Digital
Cells
Large
Small
Channels (Approx.)
800
2500
Compression
No
Yes
U.S. Standardization
AMPS
Poor
International Standards
GSM
DCS
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TDMA

• A technology for delivering digital wireless
  service using time-division multiplexing (TDM).
• TDMA works by dividing a radio frequency into
  time slots and then allocating slots to multiple
  calls.
• In this way, a single frequency can support
  multiple, simultaneous data channels.
• TDMA is used by the GSM digital cellular system.
  

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CDMA

• Every channel uses the full available spectrum.
• Individual conversations are encoded with a
  pseudo-random digital sequence. The mobile phone
  is then instructed to decipher only a particular
  code to pluck the right conversation off the air.
  
• Increased privacy is inherent in CDMA technology.
  CDMA phone calls will be secure from the casual
  eavesdropper.
• Excellent voice quality and longer battery life.

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Third-Generation (3G)

• Smarter Devices
• Devices will have the power of a small PC
• Greater Number of Uses
• Data, including internet access
• Graphics and even video