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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EE421 Fall 2001

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, GHz, etc. over time
Bandwidth limited
Needed method for better spectrum use, break coverage area into small cells
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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 Multiple Access
Time
Frequency
-Requires no synchronization or central timing, channels independent.
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9. TDMA-Time Division Multiple Access: Fixed Slots
Frequency
Requires timing synchronization
More advanced versions use channelization where time/frequency domains are split into smaller channels and time slots, then allocated to users, packetized
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EE421 Fall 2001

10. CDMA-Code Division Multiple Access: Frequency Hopping
At each time slot the signal is assigned a frequency band, signal moves around band, immune from narrowband noise interference. Users are defined pseudorandom codes. Time synchronization among users not required.
Direct Sequence Spread Spectrum – spreads the signal over a wide bandwidth to lower power.
Time
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11. 9/22/2018
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12. Terminology Base Station Control Channels Full Duplex
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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EE421 Fall 2001

13. Mobile Switching Center
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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EE421 Fall 2001

14. Wireless System Basics
Reverse Link
Forward Link
Control or Setup Channels
Mobile Unit
Base Station
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15. Cellular System 9/22/2018 EE421 Fall 2001
Base stations communicate with the Mobile Switching Center which controls subscriber verification, what tower subscriber is near, handoffs, links to Public Switched Telephone Network
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16. 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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17. 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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18. Forward/Reverse Channels
Air Mobile, air cellular
Private land mobile, public safety frequencies, trunk lines
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19. Cell Frequency Reuse 9/22/2018 EE421 Fall 2001
Cells with the same letter reuse the same frequencies, cell clusters are outlined in bold. The typical cluster size is N=7, with a frequency reuse factor of 1/7.
Many low power cells, increases the number of possible users by reusing frequencies, for example: we go from 12 conversations with one transmitter to 1200 conversations with 100 smaller transmitters. Note that user power control is critical here. – to prevent interference with adjoining cells.
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20. Locating Cells N=19 (i= 3, j=2) 9/22/2018 EE421 Fall 2001
Frequency reuse:
S channels =k N; k – group of k channels divided among N cells in a cluster (typically 4,7,12
C= total number of duplex channels = M S = M k N; M is the number of clusters in a system
Small cluster size means that co-channel cells are closer, but higher capacity in system
Large cluster size, less interference
N=I^2 + I*J + j^2 for no gaps in coverage
Example of frequency reuse calculation see notes
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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
Dynamic is more complex, needs channel occupancy, traffic distribution, signal strength indicators
Depends on MSC capabilities, coordination between MSCs
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EE421 Fall 2001

22. Handoff Scenarios 9/22/2018 EE421 Fall 2001
Passing call between cells, when, where, what channel.
When a user passes between cells, needs voice and control signals to be reallocated.
Based on power margins = P_handoff –P_minimum =delta, if delta too large, system is handing off frequently, if too low, calls can drop below minimum and be lost due to insufficient time to handoff.
Need to prioritize handoffs –don’t want to drop calls, better not to let a call be placed.
Newer systems (second generation): Mobile assisted handoffs- mobiles measure signal strength from nearby base stations and reports results to the active base stations, when power from adjacent exceeds that of active, handoff initiated. Good for microcells.
Can get complicated for microcells and high-speed users, when users roam into another company’s network
Need to make decision at right time, don’t want to switch due to momentary fading
Depends on power level, loading in adjacent cells, channel availability
Try to predict user dwell time in cells – works for highways which have known speed ranges and length; not good for microcells, complex environments.
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23. Umbrella Cells 9/22/2018 EE421 Fall 2001
High speed users use larger cells, too many handoffs for microcells and slow users- urban environments
MSC tries to estimate user speed
Use same tower for different power transmitters
Cell Dragging – a slow moving user can have a line-of sight signal be quite strong and signal strength does not decay quickly, this can “stretch” the cell
Newer systems-spread spectrum users share channels in different cells, received by multiple base stations,
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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
-Can’t overcome by increasing power, just causes more interference, need to separate (propogation isolation), Q=D/R, R- radius of cell, D istance between centers of co-channel cells
Q – co-channel reuse ratio (N=3,Q=3;N=7,Q=4.6;N=12, Q=6)
SIR – signal to interference ratio = S/I=signal power/(sum of all interferers)
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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 Cell Splitting
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 9/22/2018 EE421 Fall 2001
Can reduce co-channel interference due to directionality
Improves SIR
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30. Cell Splitting 9/22/2018 EE421 Fall 2001
More precise power control needed, can add small cells in dense areas, leave others be.
Requires new basestations,more handoffs
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31. AMPS Specs
BCH=block coding method for error detection and correction, non-binary, part of Reed-Solomon Codes
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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
SAT –supervisory tones during voice transmission to confirm that the base station/mobile are properly connected, handshake repeated at least every 250 ms. Mobile decodes and transmits back to base.
Wideband blank-and-burst encoding- wideband 10 kbps data streams, uses NRZ, Manchester (bi-phase coding), concentrated at 10kHz, sends a wide array of commands to users, uses BCH block codes less than 100 ms in duration
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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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EE421 Fall 2001