Cellular and PCS Technical Issues

1
Cellular and PCS Technical Issues

• COMT 391

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Overview

• Cell Layout
• Signaling
• Mobile Handset Operation

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System Block Diagram
forward
Mobile
BTS
BSC
MTSO
PSTN
reverse
Data Bases
BTS Base Tranceiver Station BSC Base Station
Controller MTSO Mobile Telephone Switching Office
Radio Link
Voice Trunk
Data Trunk
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Frequency Reuse Pattern
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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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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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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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Mobile-Originated Call
Mobile
BTS
BSC
MTSO
PSTN
RCC
MIN, ESN, dialed no
validation
FCC
Ch No, SAT code, Power
SAT
FVC
SAT
RVC
FVC
Conversation
RVC
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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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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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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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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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Hand-Off
MS
MS
BS
BS
MTSO
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Roaming
MTSO
Mobile
MTSO
Mobile
Mobile
Mobile
Mobile
Mobile
Mobile
Mobile
Mobile
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Intersystem Hand-Off
MS
MS
BS
BS
MTSO
MTSO
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The Shoe-Lace Effect
MS
BS
BS
BS
BS
BS
BS
BS
BS
MS
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Multi-System Tandem
MS
MS
BS
BS
MTSO
MTSO
MTSO
MTSO
MTSO
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Automatic Roaming
MS
BS
Request
MTSO
MTSO
Response
HLR
HLR
VLR
Current Location Temporary Directory Number
Billing Information Service Profile
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The Trombone Effect
MS
BS
MTSO
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 Methods
FDMA
TDMA
CDMA
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Standards Content

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