Introduction to the Advanced Mobile Phone System AMPS

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Introduction to the Advanced Mobile Phone System
(AMPS)

• EE421
• Fall 2001
• Dr. Julie Dickerson

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Information Sources

• Theodore Rappaport, Wireless Communications
  Principles Practice, Prentice-Hall, 1996
• Gallagher Snyder, Mobile Telecommunications
  Networking with IS-41, McGraw-Hill
• Tutorial on Mobile Communications from the
  International Engineering Consortium - on course
  web site

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History

• 1946 First mobile phones deployed, FM 120KHz,
  half-duplex
• 1950 FM 60KHz, better RF filters available
• Mid 1960s 30KHz, full-duplex, IMTS (Improved
  Mobile Telephone Service), limited capacity 12
  channels, 543 paying customers in New York City
  (1976)

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Early Mobile Systems
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Cellular Communications

• 1968 Cell concept proposed by ATT
• 1983 FCC allocates 40 MHz of spectrum in 800
  MHz band, 30 kHz for each one-way channel (for
  full-duplex) analog
• Channels split between two carriers
• More frequencies allocated, 1.8-1.9 GHz, etc.
  over time
• Bandwidth limited

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Cellular Concept
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Digital Communications

• Early 1990s digital systems begin with the goal
  of increasing the number of users, D-AMPS
• TDMA
• FDMA
• CDMA

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FDMA-Frequency Division Multiple Access
-Requires no synchronization or central timing,
channels independent.
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TDMA-Time Division Multiple Access Fixed Slots
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CDMA-Code Division Multiple Access Frequency
Hopping
Frequency
Time
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(No Transcript)
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Terminology

• Base Station
• Fixed station used for radio communication with
  mobiles. Located at the center or edge of
  coverage region. Consists of radio channels,
  transmit/receive antennas.
• Control Channels
• Radio channels used for transmission of call
  setup, request, initiation and other control
  purposes
• Full Duplex
• Communication system that allows simultaneous
  two-way communication, transmission reception
  usually on two different frequencies (FDD)

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• Forward Channel
• Radio channel for transmission from base station
  to mobile
• Reverse channel
• Radio channel for transmission from mobile to
  base station
• Handoff
• Process of transferring a mobile from one channel
  or base station to another
• Mobile Switching Center
• Switching center that coordinates call routing in
  a large service area. MSC connects cellular base
  stations and the mobiles to the PSTN (also called
  Mobile Telephone switching office (MTSO)

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Wireless System Basics
Reverse Link
Forward Link
Control or Setup Channels
Mobile Unit
Base Station
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Cellular System
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Making calls from a cellphone

• Mobile sends call initiation request, its ID
  number to base station on reverse control channel
• Base station receives and relays to the MSC
• MSC validates user, instructs base station to
  move mobile to voice channels
• Base station pages mobile with instructions
• Mobile moves to voice channel
• MSC connects mobile to PSTN
• Voice transmission/reception between mobile and
  base station begins

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Making calls to a cellphone

• MSC receives call from PSTN , sends page to base
  stations
• Base stations send page on forward control
  channel to mobiles
• Mobile receives page and acknowledges on reverse
  control channel sends validation information
• MSC validates mobile, asks base station to put
  user on voice channel pair
• Base station sends voice channel information on
  FCC, mobile moves to voice channel
• Voice transmission/reception initiated MSC
  connects with PSTN.

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Forward/Reverse Channels
849-851 Air Mobile, air cellular 851-869 Private
land mobile, public safety frequencies, trunk
lines
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Cell Frequency Reuse
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Locating Cells
N19 (i 3, j2)
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Channel Assignment

• Need to assign frequencies to users/cell
• Fixed
• Each cell has predetermined number of channels
• If all channels used, cell is blocked, no
  service
• Cells can borrow channels from neighboring cells
• Dynamic
• No permanent allocation
• Frequency band requested from MSC, assigned using
  reuse distance, blocking probabilities, use of
  channel, signal strength

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Handoff Scenarios
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Umbrella Cells
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Interference and Capacity

• Interference causes
• cross-talk, poor quality,(voice channel)
• blocking and missed calls (control channel)
• Co-channel interference
• Frequency reuse in nearby cells
• Adjacent channel interference
• Signal in adjacent frequency band
• Signals from other cell companies

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Co-Channel Interference
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Adjacent Channel Interference

• Comes from imperfect filters that allow frequency
  leakage into the band
• Serious problem if interferer is nearby, near-far
  effect
• Nearby mobile transmits on a frequency near to
  that of a weak mobile
• Base station receivers need high-Q filters to
  reject adjacent channel interference.

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Power Control

• Each mobile should use the minimum amount of
  power to have good quality.
• Base station controls power
• CDMA power toggles up and down continuously, like
  delta modulation
• TDMA, AMPS adjusted by base station

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Increasing Capacity

• Cell Sectoring
• Divide cell up into angular sections, typically 3
  or 6
• Increases reuse
• Cell Splitting
• Subdivide cell into smaller cells, increases the
  number of channels, keep same structure
• Needs power control

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Cell Sectoring
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Cell Splitting
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AMPS Specs
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AMPS Voice Processing
Compander compresses signal in amplitude,
roughly 21 ratio Deviation Limiter assures that
the max. deviation is the /- 12
kHz Postdeviation limiter filter LPF,
attenuated signal to keep in band and avoid
interfering with SAT tones
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Other Cell Specs

• IS-54 Digital cellular in US, DQPSK
• IS136 PCS
• IS 95 A CDMA spread spectrum
• Global system for mobiles (GSM)
• TDMA with channelization