COMT 391

1

Cellular and PCS Technical Issues

COMT 391

COMT 391

2

Overview

• Cell Layout• Signaling• Mobile Handset Operation

COMT 391

3

System Block Diagram

Mobile BTS BSC MTSO PSTN

Data Bases

forward

reverse

BTS Base Tranceiver Station

BSC Base Station Controller

MTSOMobile Telephone Switching Office

Radio Link Voice Trunk

Data Trunk

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4

Frequency Reuse Pattern

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5

Standards

• AMPS - United States• TACS - UK• NTT - Japan• NMT - Scandanavia• MATS-E - France• C-450 - Germany• GSM - global

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North American AMPS system

• 50MHz spectrum• 30kHz per channel per direction• 832 total full-duplex channels• 4, 7, and 12 cell clusters used

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AMPS Specifics

• 832 channels (2 frequencies per channel)– Forward: 869-894 MHz– Reverse: 824-849MHz– Forward/Reverse Spacing: 45MHz– Adjacent Channel Spacing: 30kHz– 21 Control Channels per group

• Cell Sizes: 2-20km

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Antenna Placement - high

• Good Coverage• Co-Channel Interference Problems

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Buildings as Boundaries

• Cell Limits Defined• Possible Adjacent Channel

Interference

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Interference

• Co-Channel Interference• Adjacent Channel Interference• Outside Sources

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11

Signaling Channels

• Dedicated Control Channels (CC)– Mobile will scan for the strongest control

channel when first switched on– System Identification– Number of Paging and Access Channels

• Paging Channels (PC)– Used to Initiate Calls to a Mobile

• Access Channels (AC)– Used to Initiate Calls from a Mobile

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12

Call Setup from Mobile

• Mobile receives AC number over PC• User dialed digits are stored in the

Mobile• Mobile finds the strongest AC• Mobile sends the dialed digits over the

AC to the Base Station and the MTSO• MTSO transmits the Voice Channel No.

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Call Setup From Mobile cont…

• Mobile tunes to the selected voice channel

• Mobile loops the supervisory audio tone (shows voice channel active)

• MTSO sends digits to the PSTN• PSTN completes call

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14

Mobile-Originated Call

Mobile BTS BSC MTSO PSTN

RCC MIN, ESN, dialed no

validation

FCC Ch No, SAT code, Power

FVC SAT

RVC SAT

FVCRVC

Conversation

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Call Setup to Mobile

• PSTN delivers call to MTSO• MTSO transmits Mobile ID over all

Paging Channels• Mobile acknowledges on the strongest

Access Channel• MTSO selects the voice channel to use• MTSO starts supervisory tone and

sends voice channel number to mobile

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Setup to Mobile Cont.

• Mobile loops the supervisory tone• MTSO instructs mobile to begin

ringing• Mobile rings and sends 10kHz alert

tone to the MTSO• Alert tone stops when mobile picks

up, MTSO completes call connection

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17

Speech Channel Control Signals (AMPS)

• Signaling Tone (10kHz) from Mobile to MTSO– Alerting– Flash-hook, disconnect, etc.

• Supervisory Audio Tones (6kHz+/-30Hz) (there are three of these)– Base Station to Mobile, looped back

by the mobile

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18

Call Handoff

• Base Stations monitor the signal strength• If strength drops below hand-off level, all

base stations are asked to look for the mobile; strongest becomes new BS

• MTSO instructs the new base station to activate a voice channel to receive the handoff; BS starts transmitting the Supervisory Audio Tone.

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Call Handoff cont…

• Current base station at MTSO command sends signal to mobile giving the new channel assignment.

• Mobile send a signaling tone burst, and tunes to the new channel.

• Previous base station clears the call path.

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Mobile Handset

• ESN - Electronic Serial Number• Programmable NAM (Number

Assignment Module)• Contains among other things

– System ID– First control channel to scan– A/B system selection– MIN (Mobile Identification Number)

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21

Mobile Identification Number

• MIN1 - Mobile System Id (3 digits)– US: Area Code– International: Mobil Carrier

Identifcation

• MIN2 - Station Number– US: NXX and line number– International: internal mobile number

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22

Hand-Off

MTSO

BSBS

MSMS

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23

Roaming

Mobile

MTSO

Mobile

Mobile

Mobile

Mobile

MTSO

Mobile

Mobile

Mobile

Mobile

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Intersystem Hand-Off

MTSO

BSBS

MSMS

MTSO

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25

The “Shoe-Lace” Effect

BS

BS

BS

BS

BS

BS

BS

BS

MS

MS

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Multi-System Tandem

MTSO

BSBS

MSMS

MTSOMTSOMTSO

MTSO

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Automatic Roaming

MTSO

BS

MS

MTSO

HLR

HLR VLR

Request

Response

Billing Information

Service Profile

Current Location

Temporary Directory Number

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The “Trombone” Effect

MTSO

BS

MS

MTSO

Fixed Station

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Security/Fraud Issues

• Security– Privacy of Conversations– Denial of Service

• Fraud– Illegally obtained ESN/MINs– Roaming-related (“tumbling” ESNs)

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Classes of Standards

• Analog Cellular– FDMA - Frequency Division Multiple

Access

• Digital Cellular– FDMA– TDMA - Time Division Multiple Access– CDMA - Code Division Multiple Access

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Access MethodsFr

eq

uen

cy

TimeFr

eq

uen

cy

TimeFr

eq

uen

cy

Time

FDMA

TDMA

CDMA

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Standards Content

• Frequency Bands• Channel Assignments• Voice Encoding• Access Method and Frame

Structure• Signaling Structure