1. Introduction to the Advanced
Mobile Phone System (AMPS)
EE421
Fall 2001
Dr. Julie Dickerson

2. 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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3. History
• 1946 First mobile phones deployed, FM
120KHz, half-duplex
• 1950 FM 60KHz, better RF filters available
• Mid 1960’s: 30KHz, full-duplex, IMTS
(Improved Mobile Telephone Service),
limited capacity: 12 channels, 543 paying
customers in New York City (1976)
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4. Early Mobile Systems
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5. Cellular Communications
• 1968 – Cell concept proposed by AT&T
• 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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6. Cellular Concept
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7. Digital Communications
• Early 1990’s digital systems begin with the
goal of increasing the number of users, D-
AMPS
• TDMA
• FDMA
• CDMA
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8. FDMA-Frequency Division
F Multiple Access
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Time
-Requires no synchronization or central timing,
channels independent.
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9. TDMA-Time Division Multiple
Access: Fixed Slots
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Time
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10. CDMA-Code Division Multiple
Access: Frequency Hopping
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Time
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12. 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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13. • 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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14. Wireless System Basics
Reverse Link
Forward Link
Control or
Setup Channels
Mobile Unit
Base Station
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15. Cellular System
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16. Making calls from a cellphone
1. Mobile sends call initiation request, its ID
number to base station on reverse control
channel
2. Base station receives and relays to the MSC
3. MSC validates user, instructs base station to
move mobile to voice channels
4. Base station pages mobile with instructions
5. Mobile moves to voice channel
6. MSC connects mobile to PSTN
7. Voice transmission/reception between mobile
and base station begins
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17. Making calls to a cellphone
1. MSC receives call from PSTN , sends page to base
stations
2. Base stations send page on forward control channel to
mobiles
3. Mobile receives page and acknowledges on reverse
control channel; sends validation information
4. MSC validates mobile, asks base station to put user on
voice channel pair
5. Base station sends voice channel information on FCC,
mobile moves to voice channel
6. Voice transmission/reception initiated; MSC connects
with PSTN.
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18. Forward/Reverse Channels
849-851 Air Mobile, air cellular
851-869 Private land mobile, public safety frequencies, trunk lines
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19. Cell Frequency Reuse
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20. Locating Cells
N=19
(i= 3, j=2)
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21. 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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22. Handoff
Scenarios
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23. Umbrella Cells
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24. 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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25. Co-Channel Interference
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26. 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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27. 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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28. 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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29. Cell Sectoring
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30. Cell Splitting
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31. AMPS Specs
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32. AMPS Voice Processing
Compander – compresses signal in amplitude, roughly 2:1
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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33. 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
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