Here is an overview of Cellular Communications I gave back in 2001. It is slightly out of date, but has good info.

1. Wireless Overview
Cellular
National Wireless Engineering Conference
October 31, 2000
John Skeffington, Sun
Microsystems
JFS May, 2001
JFS 10/11/00

2. Agenda
• Historical Perspective
• Building a Cellular System
• Current Landscape
• Future Landscape
JFS May, 2001
JFS 10/11/00

3. Historical Perspective (US)
• Evolved from Two-way Radio Systems
• Mobile Phone System (MTS)
– Operator Interaction Required
– 15 - 25 Simultaneous Users on System
• Improved Mobile Phone System (IMTS)
– Incorporated Direct Dial
– Still in Service until Early 1990’s
• Advanced Mobile Phone Service
(AMPS)
– 1st Commercial Phone Call Placed in 1983
JFS May, 2001
JFS 10/11/00

4. Definitions
• Wireless - Large coverage area, high mobility
(pedestrian and automotive) usage.
• Cellular - A communications system where a
small number of radio frequencies are carefully
managed and re-used in a geographic area.
– Full duplex, two frequencies used for each conversation
• PCS - Personal Communications System - A
spectrum of frequencies allocated by the FCC
for newer cellular type technologies.
JFS May, 2001
JFS 10/11/00

5. Agenda
• Historical Perspective
• Building a Cellular System
• Current Landscape
• Future Landscape
JFS May, 2001
JFS 10/11/00

6. Pre Cellular Service
Pre-Cellular Scenario - Metropolitan area using seven
channels (14 frequencies): seven simultaneous calls
JFS May, 2001
JFS 10/11/00

7. Basic Components
• Mobile Switch
• Central Switching Office Building
• Base Stations
• Leased locations for Base Stations
• Connections to Landline
• Antennas, Cable, Towers, etc.
• Mobile Phones
• Customers
JFS May, 2001
JFS 10/11/00

8. Basic Analog Cellular
System
Billing
Base System
Transceiver
System
Base
Transceiver Mobile PSTN
System
Switching
Center
Base
Transceiver
System
Subscriber
JFS May, 2001
JFS 10/11/00

9. Assumptions for “My Mobile”
• Mobile Switching Center in place
• Base Station (BTS) equipment cost $250,000
• Base Station Construction cost $200,000
• BTS rental $1000 per month
• One T1 line required per BTS, $1200 per month
• Subscriber usage is 50 minutes per month
• 5% of subscribers use system at any given time
• BTS upgrade is $100,000
• Very simplistic financial estimating
JFS May, 2001
JFS 10/11/00

10. My Mobile Service
Three Cell System
3 BTS $3600/mon 3 BTS $ 750,000
3 T1 $3000/mon BTS const $ 600,000
21 simultaneous calls
420 subscribers Total Capital $1,350,000
$0.31 per minute
JFS May, 2001
JFS 10/11/00

11. Handoffs
• Handoff: Major enabling technology that creates
mobile communications, transfer of call from one
cell to another
– First generation handoffs (AMPS)
– MSC initiated, mobile had “passive” role
– MSC would query neighboring cells to determine when to
handoff based on received power getting lower.
– “Intelligence” of handoffs contained to MSC
– Hard Handoff: “break before make” connection
– Second generation handoff
– Mobile initiated, mobile has active role
– Database assisted
– Soft Handoff: “make before break” connection
JFS May, 2001
JFS 10/11/00

12. Basic Hard Handoff
• Caller is active in cell
A
• Caller is driving on
“blue” road
• System detects lower
signal level in cell A B
as caller moves to
edge
• System checks other
cells to determine A
which has stronger
signal
• Cell B is determined C
to be stronger signal
• System directs mobile
to tune to Cell B
frequency JFS May, 2001
JFS 10/11/00

13. Basic Hard Handoff
• Mobile tunes to cell B
while still on call
• Call is now
established in cell B
• System detects lower
signal level in cell B B
as caller moves to
edge
• System checks other
cells to determine A
which has stronger
signal
• Cell C is determined C
to be stronger signal
• System directs mobile
to tune to Cell C
frequency JFS May, 2001
JFS 10/11/00

14. Basic Hard Handoff
• Mobile tunes to cell C
while still on call
• Call is now
established in cell C
• System detects lower
signal level in cell C B
as caller moves to
edge
• System checks other
cells to determine A
which has stronger
signal
• Cell A is determined C
to be stronger signal
• System directs mobile
to tune to Cell A
frequency JFS May, 2001
JFS 10/11/00

15. My Mobile Service
Eight Cell System
1 MSC
8 BTS $9600/mon 5 BTS $1,250,000
8 T1 $8000/mon BTS const $1,000,000
56 simultaneous calls Total Capital $2,250,000
1120 subscribers
$0.31 per minute
JFS May, 2001
JFS 10/11/00

16. Definitions
• Down Link, Forward Link - Radio connection from base
station to mobile
• Up Link, Reverse Link - Radio connection from mobile
to base station
• Channel: A physical (or logical) radio frequency(s)
– Traffic (voice) channel - A channel dedicated to voice usage
– Control (overhead) channel - A channel dedicated to control
functions
– Access Channel - Channel used for mobile to gain access to system (reverse
link)
– Paging Channel - Channel used for messaging from system to mobiles
(forward link)
JFS May, 2001
JFS 10/11/00

17. Forward / Reverse Links
Down Link
Down Link
Up Link Up Link
0.6 watt portable
3 watt portable
JFS May, 2001
JFS 10/11/00

18. Power Control
●System works best when all mobiles appear to be
same power at base station
●Mobility of mobiles requires power control to be
dynamic
●Base station tells mobiles whether to power up or
down
●Revisions made to original AMPS specs to allow
additional (lower) power levels
● CDMA power control happens every 20 msec
JFS May, 2001
JFS 10/11/00

19. Why is Power Control
Needed?
● Many Mobiles, many Distances
● Power Level Inversely Proportional to Square of Distance
● Mobile at One Mile = One Watt
● Same Mobile at Two Miles = 1/4 Watt
● Same Mobile at Three Miles = 1/9 Watt
4.5
4
3.5
3
2.5
power
2
1.5
1
JFS May, 2001
0.5
JFS 10/11/00
0

20. Where do I Place Sites?
●
Site Location Factors
●
Link Budget
●
Method of estimating power levels anywhere within coverage
●
Technical
●
Terrain and Clutter
●
Equally spaced “cell centers”
●
Business
●
Lease cost
● Acquisition and Construction cost
JFS May, 2001
JFS 10/11/00

21. System Design Tools
• Tools predict propagation
of radio signal based on
link budget
• Creates many different
output formats
Ideal Cell Locations
Predicted Signal Levels JFS May, 2001
JFS 10/11/00

22. System Manangement
JFS May, 2001
JFS 10/11/00

23. 28 Cell Cellular System
1 MSC
28 BTS $33600/mon
28 T1 $28000/mon 20 BTS $5,000,000
196 simultaneous calls BTS const $4,000,000
3920 subscribers Total Capital $9,000,000
$0.31 per minute
JFS May, 2001
JFS 10/11/00

24. Frequency Re-use
Metropolitan area using seven channels: 28 simultaneous
calls. Seven cell re-use pattern, same frequency never
closer than two cell radii.
5 2 7
7 3 1
2
1 4
7 3 6
6 4 5
1
6 4 5 2
7
5 2 7 3
7 3 1
Numbers represent frequency being used
Hexagons represent cell coverage areas
Shading represents frequency grouping JFS May, 2001
JFS 10/11/00

25. High Usage Cells
High Usage Cells (hot spot) develops based on statistics from system
1 MSC
28 BTS $33600/mon
28 T1 $28000/mon
196 simultaneous calls
3920 subscribers
$0.31 per minute
JFS May, 2001
JFS 10/11/00

26. Increasing Capacity
Omni-directional cell
●
Omni-directional cell transmit equal power in
all directions
●
Cells can be split into sectors using directional
antennas
●
Requires additional BTS equipment
●
Very similar to multiple cells in same location
Sectorized cell
●
Careful frequency planning required
●
Most common is three sector, but six sectors
have been implemented in many cities
●
Sectorizing and adding “voice channels” very
profitable to equipment provider
JFS May, 2001
JFS 10/11/00

27. Capacity Increased
1 MSC
28 BTS $33600/mon
34 T1 $34000/mon
238 simultaneous calls 3 BTS upgrades $300,000
4760 subscribers
$0.28 per minute
JFS May, 2001
JFS 10/11/00

28. System Monitoring
28 base stations in service 24 x 7
●
System reports statistics every 10 minutes
●
●
40 items each base station
●
6,720 per hour, 161,280 per day
●
Each T1 status reported
●
168 per hour, 4,032 per day
Lots of data to look at !
●
JFS May, 2001
JFS 10/11/00

29. Additional Hot Spot
1 MSC
28 BTS $33600/mon
34 T1 $34000/mon
238 simultaneous calls
4760 subscribers
$0.28 per minute
JFS May, 2001
JFS 10/11/00

30. More Capacity Needed
1 MSC
28 BTS $33600/mon
40 T1 $40000/mon
280 simultaneous calls 3 BTS upgrades $300,000
5600 subscribers
$0.26 per minute
JFS May, 2001
JFS 10/11/00

31. Additional Capacity
Added
1 MSC
28 BTS $33,600/mon
49 T1 $49,000/mon
343 simultaneous calls
6860 subscribers 3 BTS upgrades $300,000
$0.24 per minute
JFS May, 2001
JFS 10/11/00

32. Entire System Sectorized
1 MSC
28 BTS $33,600/mon
93T1 $93,000/mon
651 simultaneous calls
13020 subscribers 22 BTS upgrades $2,200,000
$0.19 per minute
JFS May, 2001
JFS 10/11/00

33. Analog Cellular System
Voice Mail
System
Billing
System
Base Connection to
Transceiver other networks
System
Base
Transceiver Mobile PSTN
System
Switching
Center
Customer Care
System
Base
Transceiv Subscriber
er
System
Provisioning
System
JFS May, 2001
JFS 10/11/00

34. Network Sizing
Voice Mail
System
Billing
• Public Switched System
Telephone Base
Transceiver
Connection to
other networks
Network is at System
Center of Mobile
Network
Mobile
• Interconnect Base Switching
Transceiver PSTN
Between devices
System
Center
needs to be sized
properly
Customer Care
System
Base
Transceiv Subscriber
er
System
Provisioning
System
JFS May, 2001
JFS 10/11/00

35. Network Sizing
Most networks sized for 2% blocking
●
Unit of measure for telco sizing
●
● Erlang
● Equivalent to one phone call for 60 minutes
● Used to size a path through the network
● One Erlang:
– 1 - 1 hour call
– 2 - 30 minute calls
– 4 - 15 minute calls
– 60 - 1 minute calls
JFS May, 2001
JFS 10/11/00

36. First Generation Cellular
Advanced Mobile Phone System (AMPS)
●
● American Standard, 824 MHz – 894 MHz
● Initially 666, Later Expanded to 832 Channels
● Split Between Two Carriers
● A Carrier (non- wireline carrier)
● B Carrier (wireline provider)
● 42 Control Channels (21 for each carrier)
● First Commercial Cellular Phone Call Made in 1983
● Designed as "Higher Power" (3 watt) Car Based System
● Proprietary System for Infrastructure Vendors
JFS May, 2001
JFS 10/11/00

37. Other First Generation
Nine Incompatible Standards used in Europe
●
●
Total Access Communication System (TACS) – UK
●
Nordic Mobile Telephone (NMT)
Japanese Total Area Cellular System JTACS
●
Proprietary System for Infrastructure Vendors
●
JFS May, 2001
JFS 10/11/00

38. Capacity Increases in 1 G
Networks
Increase Spectrum
●
Cell Splitting (requires
●
careful frequency
planning)
●
Three Sector
●
Six Sector
●
Add Additional Cells
Move to Digital Standard
●
●
Significant Improvement with similar Infrastructure
Investment
JFS May, 2001
JFS 10/11/00

39. Generic Digital System
Voice Mail
System
IWF
Base Base
Transceiver Transceiver
System System
Internet
Base Mobile
Station Connection to
Controller Switching other networks
Center
PSTN
Subscriber
Base Provisioning Customer Care
Transceiver
System System System
JFS May, 2001
JFS 10/11/00

40. Second Generation Systems
GSM – Global System for Mobile
●
● Channelized, 6 Time Slots / Channel
● Time Division Multiple Access (TDMA) Based
● Most Widely Deployed, Most Predicted Growth
● Common Digital Standard Across Europe, Africa, Middle East, US
● SIM cards widely used
● Defined Interfaces Between Infrastructure Elements
● Multiple Vendors Used in Same Network
● First Commercial Service in 1991
● Frequency Planning Crucial
● Major Vendors: Ericcson, Nokia, Nortel, Motorola, Lucent
JFS May, 2001
JFS 10/11/00

41. Second Generation Systems
Time Division Multiple Access (TDMA)
●
● Channelized, 3 Time Slots/Channel
● Primarily Deployed in US
● AT&T Wireless (Dual Band, Dual Mode Phones, IS-154)
● SBC (Cellular One in Chicago, IS-54)
● Some Markets Will be Transitioned to TDMA from CDMA due to mergers
● Frequency Planning Crucial
● First Commercial Service in 1990
● Typically One Infrastructure Vendor per Market
● Major Vendors: Ericsson, Lucent
JFS May, 2001
JFS 10/11/00

42. Second Generation Systems
Code Division Multiple Access (CDMA)
●
●
Spread Spectrum Coded Transmission, Non Channelized
●
One Frequency for Entire System
●
Primarily Used in US, Canada, Australia, Korea and parts of
China
●
SprintPCS, Verizon, PrimeCo
●
Typically One Infrastructure Vendor per Market (not per
Carrier)
●
Complex Power Control
●
First Commercial Service in Late 1995
●
Major Vendors: Nortel, Lucent, Motorola, Ericsson, Nokia
JFS May, 2001
JFS 10/11/00

43. Mobile Call Origination Flow
Mobile Registration Mobile
Communication Communication
s System
Device Control and System Parameters
Access Request
Phone Call
Access Confirmation
Dialed Digits
Traffic Channel Information
Call Setup Information
Call Setup Information
Call Termination
Release Resources
JFS May, 2001
JFS 10/11/00

44. Mobile Call Origination
Mobile to Land Call
Flow Example
Base
Transceiver
System
Base
Transceiver Mobile PSTN
System
Switching
Center
Base
Transceiver
System
Subscriber
Mobile User
Initiates Call
JFS May, 2001
JFS 10/11/00

45. Mobile Call Origination
Mobile to Land Call
Flow Example
Base
Transceiver
System
Base
Transceiver Mobile PSTN
System
Switching
Center
Base
Transceiver
System
Subscriber
JFS May, 2001
JFS 10/11/00

46. Mobile Call Origination
Mobile to Land Call
Flow Example
Base
Transceiver
System
Base
Transceiver Mobile PSTN
System
Switching
Center
Validate
Base
Transceiver Subscriber
System
Subscriber
JFS May, 2001
JFS 10/11/00

47. Mobile Call Origination
Mobile to Land Call
Flow Example
Base
Transceiver
System
Valid Subscriber
Base
Transceiver Mobile PSTN
System
Switching
Center
Base
Transceiver
System
Subscriber
JFS May, 2001
JFS 10/11/00

48. Mobile Call Origination
Mobile to Land Call
Flow Example
Base
Transceiver
System
Base
Transceiver Mobile PSTN
System
Switching
Center
Base
Transceiver
System
Subscriber
Phone Rings
JFS May, 2001
JFS 10/11/00

49. Mobile Call Origination
Mobile to Land Call
Flow Example
Base
Transceiver
System
Base
Transceiver Mobile PSTN
System
Switching
Center
Base
Transceiver
System
Subscriber Phone Answered
JFS May, 2001
JFS 10/11/00

50. Mobile Call Origination
Mobile to Land Call
Flow Example
Base
Transceiver
System
Base
Transceiver Mobile PSTN
System
Switching
Center
Base
Transceiver
System
Subscriber
Call Established
JFS May, 2001
JFS 10/11/00

51. Mobile Call Termination Flow
Mobile Registration Mobile
Communication Communication
s Control and System Parameters System
Device
Page for Mobile
Phone Call
Page Response
Call Setup Information
Call Setup Information
Call Termination
Release Resources
JFS May, 2001
JFS 10/11/00

52. Mobile Call Termination
Land to Mobile Call
Flow Example
Base
Transceiver
System
Base
Transceiver Mobile PSTN
System
Switching
Center
Base
Transceiver
System
Subscriber
Land Line User
Initiates Call
JFS May, 2001
JFS 10/11/00

53. Mobile Call Termination
Land to Mobile Call
Flow Example
Base
Transceiver
System
Base
Transceiver Mobile PSTN
System
Switching
Center
Validate
Base Subscriber
Transceiver
System
Subscriber
JFS May, 2001
JFS 10/11/00

54. Mobile Call Termination
Land to Mobile Call
Flow Example
Base
Transceiver
System
Valid Subscriber
Base
Transceiver Mobile PSTN
System
Switching
Center
Base
Transceiver
System
Subscriber
JFS May, 2001
JFS 10/11/00

55. Mobile Call Termination
Land to Mobile Call
Flow Example
Base
Transceiver
System
Base
Transceiver Mobile PSTN
System
Switching
Center
Base
Transceiver
System
Subscriber
Page Mobile
JFS May, 2001
JFS 10/11/00

56. Mobile Call Termination
Land to Mobile Call
Flow Example
Base
Transceiver
System
Base
Transceiver Mobile PSTN
System
Switching
Center
Base
Transceiver
System
Subscriber
Page Response
JFS May, 2001
JFS 10/11/00

57. Mobile Call Termination
Land to Mobile Call
Flow Example
Base
Transceiver
System
Base
Transceiver Mobile PSTN
System
Switching
Center
Update
Subscriber
Database
Base
Transceiver
System
Subscriber
Page Response
JFS May, 2001
JFS 10/11/00

58. Mobile Call Termination
Land to Mobile Call
Flow Example
Base
Transceiver
System
Base
Transceiver Mobile PSTN
System
Switching
Center
Base
Transceiver
System
Subscriber
Page Response
Call Established
JFS May, 2001
JFS 10/11/00

59. Spectrum Comparison
30 kHz AMPS
no time slots
180 kHz
30 kHz TDMA
3 time slots
180 kHz
GSM
8 time slots (6 voice, 2 control)
200
kHz
CDMA
1.23 MHz
JFS May, 2001
JFS 10/11/00

60. Comparison of Current
Systems
AMPS GSM TDMA CDMA
Physical Channels
per Carrier 1 8 3 Variable
Channel Spacing
(kHz) 30 200 30 1,230
Cellular Efficiency
(conversations 2.3 5.0 – 6.6 7.0 12.1 – 45.1
/cell/MHz)
Capacity
Compared to - 2.1 – 2.8 3 5.2 – 19.6
AMPS
Capacity
Compared to GSM - - 1 - 1.4 1.8 – 6.8
Capacity
Compared to - - - 1.7 – 6.4
TDMA
Reference David J. Goodman, Wireless Personal Communications Systems, 1997 Addison Wesley Longman, Inc.
JFS May, 2001
JFS 10/11/00

61. Agenda
• Historical Perspective
• Cellular Overview
• Current Landscape
• Future Landscape
JFS May, 2001
JFS 10/11/00

62. Chicago Wireless Market
• Seven Carriers now in service
– Cingular - TDMA (AMPS & IS-54)
– Verizon - CDMA(AMPS, IS-95A/B, Jstd-8)
– Nextel - TDMA based
– PrimeCo - CDMA(Jstd-8)
– Sprint PCS - CDMA (Jstd-8)
– AT&T Wireless - TDMA(IS-136)
– VoiceStream -GSM
JFS May, 2001
JFS 10/11/00

63. Issues Facing Today’s
Carriers
• Network Sizing
– Coverage vs. Capacity
– Quality of Service
• Hot Spots
– Concentrated areas of usage
– Special Events, Sporting Events, Trade Shows
– Emergency Requirements
• E911
JFS May, 2001
JFS 10/11/00

64. Network Sizing (14.4 kbps
Terminals)
14.4 kbps
Base
Transceiver Base
System Transceiver IWF
System
Internet
Base Mobile
Station
Controller Switching
Center
2-3 Voice
Channels One T1 per 64 PSTN
per DS0 Channel Base
(64/3=21DS0) Station
Base
Subscriber
Transceiver
System
64 Voice Channels JFS May, 2001
JFS 10/11/00

65. Current Situation
• Short Message Service (SMS)
– Currently Deployed Across All Standards
– 180 - 240 Characters per Message
• Wireless Internet Access
– Wireless Application Protocol (WAP)
– In Service now
– Designed for Low Bandwidth
– Compatibility Problems
JFS May, 2001
JFS 10/11/00

66. Agenda
• Historical Perspective
• Cellular Overview
• Current Landscape
• Future Landscape
JFS May, 2001
JFS 10/11/00

67. JFS May, 2001
JFS 10/11/00

68. 3G Challenges
• Network Capacity
– Current Networks Sized to Provide 9.6 - 14.4 kbps
– Future Networks to Provide 1+ Mbps
– Capacity Planning (Where will it be needed?)
– Ubiquitous Coverage (in-building, in-residence)
– Cell Site Development Costs (alternatives to traditional cells?)
• Is 3G Viable - Auction Costs
• What is 3G?
– Current Standards Define Technical Aspects
– Is There a “Killer App”?
– Video (one and two way)?
– Instant Messaging?
JFS May, 2001
JFS 10/11/00

69. Evolving Wireless Standards
No Clear Winner - Yet!
2G 2G+ 3G
(up to 14.4 kbps) (up to 384 kbps) (up to 2 mbps)
GSM GPRS EDGE W-CDMA
TDMA IS-136+ ?
(64 kbps) IS-136HS
CDMA IS-95B 1X-EV
CDMA-2000
(64 kbps)
JFS May, 2001
JFS 10/11/00

70. Basic Soft/Softer Handoff
• Caller is driving on “blue” road
• Caller is active in cell A sector 1
and cell B sector 3
• Phone is constantly checking for 3 1
neighbor cells 2
• Phone communicates with network B
and adds sectors as they exceed B
power threshold 3 1
• Phone detects lower signal level in 2
cell A S1 as caller moves to edge A
• Cell A S1 is dropped from call
• Cell B S2 is added to call 3 1
C 2
C
JFS May, 2001
JFS 10/11/00

71. Basic Soft/Softer Handoff
• Caller is active in cell B sector 3
and cell B sector 2
• Phone is constantly checking for
neighbor cells 3 1
• Phone detects cell C S1 as 2
exceeding threshold, adds to call B
• Phone detects lower signal level in
cell B S3 as caller moves to edge 3 1
• Cell B S3 is dropped from call 2
A
3 1
C 2
C
JFS May, 2001
JFS 10/11/00

72. Basic Soft/Softer Handoff
• Caller is active in cell B sector 2
and cell C sector 1
• Phone is constantly checking for
neighbor cells 3 1
• Phone detects cell C sector 2 as 2
exceeding threshold, adds to call B
• Phone detects lower signal level in
cell B sector 2 as caller moves to 3 1
edge, cell B sector 3 is dropped 2
from call A
• Phone detects cell C sector 3 as
exceeding threshold, adds to call 1
3
C 2
C
JFS May, 2001
JFS 10/11/00

73. Basic Soft/Softer Handoff
• Caller is active in cell C sectors 1,
2 and 3 - softer handoff
• Phone is constantly checking for
neighbor cells 3 1
• Phone detects lower signal level in 2
cell C sector 1 as caller moves to B
edge, cell C sector 1 is dropped B
from call 1
3
• Phone detects cell A sector 2 as 2
exceeding threshold, adds to call A
• Phone detects lower signal level in
cell C sector 2,cell C sector 2 is 1
3
dropped from call
C 2
C
JFS May, 2001
JFS 10/11/00

74. Basic Soft/Softer Handoff
• Caller is active in cell C sector 3
and cell A sector 2
• Phone is constantly checking for
neighbor cells 3 1
• Phone detects lower signal level in 2
cell C sector 3 as caller moves to B
edge, cell C sector 3 is dropped B
from call 1
3
• Phone detects cell A sector 3 as 2
exceeding threshold, adds to call A
• Phone detects cell A sector 1 as
exceeding threshold, adds to call 1
3
C 2
C
JFS May, 2001
JFS 10/11/00

75. Basic Soft/Softer Handoff
• Caller is active in cell A sectors 1,
2 and 3 - softer handoff
• Phone is constantly checking for
neighbor cells 3 1
• Phone detects lower signal level in 2
cell A sector 2 as caller moves to B
edge, cell A sector 2 is dropped B
from call 1
3
2
A
3 1
C 2
C
JFS May, 2001
JFS 10/11/00

76. Basic Soft/Softer Handoff
• Caller is active in cell A sectors 1
and 3 - softer handoff
• Phone is constantly checking for
neighbor cell 3 1
2
B
B
3 1
2
A
3 1
C 2
C
JFS May, 2001
JFS 10/11/00