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 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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Early Mobile Systems

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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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Cellular Concept

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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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FDMA-Frequency Division Multiple Access

Time

Frequency

-Requires no synchronization or central timing, channels independent.

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TDMA-Time Division Multiple Access: Fixed Slots

Time

Frequency

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CDMA-Code Division Multiple Access: Frequency Hopping

Time

Frequency

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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

Forward Link

Reverse Link

Mobile UnitBase Station

Control or Setup Channels

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Cellular System

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Making calls from a cellphone1. Mobile sends call initiation request, its ID number to

base station on reverse control channel2. Base station receives and relays to the MSC3. MSC validates user, instructs base station to move

mobile to voice channels4. Base station pages mobile with instructions5. Mobile moves to voice channel6. MSC connects mobile to PSTN7. Voice transmission/reception between mobile and

base station begins

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Making calls to a cellphone1. MSC receives call from PSTN , sends page to base stations2. Base stations send page on forward control channel to

mobiles3. Mobile receives page and acknowledges on reverse control

channel; sends validation information4. MSC validates mobile, asks base station to put user on

voice channel pair5. Base station sends voice channel information on FCC,

mobile moves to voice channel6. 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

N=19

(i= 3, j=2)

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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 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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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