COMT 391

1

Cellular/PCS, Wireless Data Technical Issues

COMT 391 Wireless

COMT 391

2

Overview

• Cell Layout• Site Survey• Signaling• Mobile Operation

COMT 391

3

System Block DiagramCellular/PCS

Mobile BTS BSC MTSO PSTN

Data Bases

forward

reverse

BTS Base Tranceiver Station

BSC Base Station Controller

MTSOMobile Telephone Switching Office

Radio Link Bearer Trunk

Signaling Trunk

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4

Frequency Reuse Pattern

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5

Example: AMPS

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

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AMPS cont.

• 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

COMT 391

7

Antenna Placement - high

• Good Coverage• Co-Channel Interference Problems

COMT 391

8

Buildings as Boundaries

• Cell Limits Defined• Possible Adjacent Channel

Interference

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9

Interference

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

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10

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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11

Call Setup from Mobile

• Mobile receives AC numbers over PC

• Mobile finds the strongest AC• Mobile signals (dials) over the AC

to the Base Station and the MTSO• MTSO transmits the Bearer

Channel No.

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12

Mobile-Originated Call, AMPS

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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13

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 sends voice channel number

to mobile

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14

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.

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15

Call Handoff cont…

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

• Mobile tunes to the new channel.• Previous base station clears the

call path.

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16

Mobile Handset

• ESN - Electronic Serial Number• Programmable NAM (Number

Assignment Module)• Contains among other things

– System ID– First control channel to scan– System selection preferences– MIN (Mobile Identification Number)

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17

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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18

Hand-Off

MTSO

BSBS

MSMS

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19

Roaming

Mobile

MTSO

Mobile

Mobile

Mobile

Mobile

MTSO

Mobile

Mobile

Mobile

Mobile

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Roaming Details

MTSO

BS

MS

MTSO

HLR

HLR VLR

Request

Response

Billing Information

Service Profile

Current Location

Temporary Directory Number

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21

Intersystem Hand-Off

MTSO

BSBS

MSMS

MTSO

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22

The “Shoe-Lace” Effect

BS

BS

BS

BS

BS

BS

BS

BS

MS

MS

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23

Multi-System Tandem

MTSO

BSBS

MSMS

MTSOMTSOMTSO

MTSO

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24

The “Trombone” Effect

MTSO

BS

MS

MTSO

Fixed Station

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25

Security/Fraud Issues

• Security– Privacy of Conversations– Denial of Service

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

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26

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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27

Access MethodsFr

eq

uen

cy

TimeFr

eq

uen

cy

TimeFr

eq

uen

cy

Time

FDMA

TDMA

CDMA

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28

Standards Content

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

Structure• Signaling Structure

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29

Wireless LAN

• Short range• Single channel (“half-duplex”)• Packet-based

– Signaling packets– Bearer packets

• Authentication against back-end data base