cellular basic
TRANSCRIPT
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Lecture 9: Cellular NetworkLecture 9: Cellular Network
BasicsBasics
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Cellular BasicsCellular Basics
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Terminologies: BS & MSTerminologies: BS & MS
Base station (BS) Access point (AP)
Mobile station (MS) SS (Subscriber station)
MT (mobile terminal) MN (mobile node)
Downlink Forward link
BSMS
Uplink Reverse link
MSBS
downlink
uplink
BS MS
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1G and 2G cellular systems1G and 2G cellular systems
1
st generation AMPS Analog Analog FM modulation FDMA
2nd generation
DAMPS(IS-54) U.S. Digital PSK modulation FDM/TDMA
GSM Europe, Asia
Digital PSK modulation FDM/TDMA
IS-95 CDMA U.S. Digital PSK modulation FDM/CDMA
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Abstract model for a cellular systemAbstract model for a cellular system
Components Gateway to the wireline system (backhaul)
Base station (BS)
Database Security
Air interface
Cellular phone
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(1) Gateway to the wireline system(1) Gateway to the wireline system
Backhaul connection MSC (Mobile Switching Center)
Switch that connects mobile telephone network
to wireline telephone network (PST
N) IWF(Interworking Function) Protocol translation
E.g. circuit-switch to packet-switch
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(2) Base station(2) Base station
BS includes 2 components Transmitters/receivers
BTS (base transceiver system)
Controllers for transceivers BSC (base station controller)
Typically, 1 BSC controls several BTS
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(3) Database(3) Database
Track user location and billing information HLR (home location register)
Track current user locations
VLR (visitor location register) Track roaming users
HLR/VLR
Two-tier location database for user locationmanagement
Concept of home
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(4) Security mechanism(4) Security mechanism
Identify a phone IMEI (International Mobile Equipment Identity)
Like your ROC ID
Equipment Identity Register (EIR)
A database One of the MSCs is used as EIR
Check the identity of cell phones Stolen phones, malicious phones
Not conform to the network Conform to the network
Authentication Center (AC or AuC) Actual encryption and verification
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(5) Air interface(5) Air interface
PHY and MAC layers of wirelesscommunications standards
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(6) Cellular phone(6) Cellular phone
MS (Mobile Station)
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Basic Cellular ConceptBasic Cellular Concept
Cell Typically, cells are hexagonal
In practice, it depends on available cell sitesand radio propagation conditions
Spectrum reuse Reuse the same EM spectrum in other
geographical location
Frequency reuse factor
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Frequency ReuseFrequency Reuse
Cluster A group of cells
Frequency reuse factor
(Total # of channels in a cluster) / (Total # ofchannels in a cell)
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TDMA/FDMA Spatial Reuse
TDMA/FDMA Spatial Reuse
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A frequency reuse exampleA frequency reuse example
Example Frequency reuse factor = 7
Cluster size =7
Question What are other possible
frequency reuse patterns?
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ClusterCluster
The hexagon is an ideal choice formacrocellular coverage areas, because itclosely approximates a circle and offers awide range of tessellating reuse clustersizes.
A cluster of size N can be constructed if, N = i2 + ij + j2.
i,j are positive integer
Allowable cluster sizes are N = 1,3,4,7,9,12,
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Determine frequency reuse patternDetermine frequency reuse pattern
Co-channel interference [CCI] one of the major factors that limits cellular
system capacity
CCI arises when the same carrier frequency isused in different cells.
Determine frequency reuse factor Propagation model
Sensitivity to CCI
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Reuse distanceReuse distance
Notations D :Reuse distance
Distance to cell using the same frequency
r : Cell radius
N : Frequency reuse factor
Relationship between D and r D/r=(3N)^0.5
N = i2 + ij + j2
Proof?
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D
L * iL * j
r
In this case: j=2, i=1
Compute D based on
law of cosineNijjirD
ijjiLD
jiLjLiLD
jLiLjLiLD
3)(3/
)(
)5.0(2
)3/2cos())((2)()(
22
2222
222222
222
!!
!
!
! T
rL 3!
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Cell splittingCell splitting
Smaller cells have greater system capacity Better spatial reuse
As traffic load grows, larger cells could
split into smaller cells
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SectorsSectors
Use directional antenna reduces CCI Why? Think about it! 1 base station could apply several
directional antennas to form several
sectors 3-sector cell
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Forward link and reverse linkForward link and reverse link
Forward link Also called downlink BSMS
Reverse link
Also called uplink MSBS
How forward link and reverse link are separated? FDD (more often)
Frequency Division Duplex
TDD Time Division Duplex
Why is it more difficult to engineer a TDD system?
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More about cellularMore about cellular
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Cell size & FRFCell size & FRF
Cell size should be proportional to1/(subscriber density)
Co-channel interference is proportional to
1/D r
1/N^0.5
Path-loss model
Total system capacity is proportional to 1/N
N : Frequency reuse factor
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Example: N=7Example: N=7
Frequency reuse factor N=7 N = i2 + ij + j2
(i,j)=(1,2) or (2,1)
Other commonly used patterns N=3 (1,1)
N=4 (2,0); (0,2)
N=1 is possible CDMA
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Compute total system capacityCompute total system capacity
Example 11-1 Total coverage area = 100 mile2 = 262.4 km2
Total 1000 duplex channels
Cell radius =1
km N=4 or N=7
Whats the total system capacity for N=4and N=7?
r
22 6.22
33rrA !!
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Evolving deploymentEvolving deployment
Early stage Intermediate stage Late stage
Multiple stages of deployment
Deployment evolves with subscriber growth
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Practical deployment issuesPractical deployment issues
Location to setup antenna Antenna towers are expensive
Local people do not like BSs Antenna/BS does not look like antenna/BS
Antenna Omni-directional
Directional antenna
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Wireless QoSWireless QoS Quality of Service (QoS)
Achieving satisfactory wireless QoS is an important designobjective
Quality measures Channel availability (wireless network is available when users need
it) Blocking probability Dropping probability
Coverage: probability of receiving adequate signal level atdifferent locations
Transmission quality: fidelity/quality of received signals BER FER
Application-dependent Voice Data Multimedia
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Wireless QoSWireless QoS
Admission control Blocking
Poor reception quality
Co-channels Frequency reuse factor Cell planning
Frequency planning
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WorstWorst--Case CCI on the Forward ChannelCase CCI on the Forward Channel
Co channel interference [CCI] is one of the primelimitations on system capacity. We use the propagationmodel to calculate CCI.
There are six first-tier, co-channel BSs, two each at(approximate) distances of D-R, D, and R+D and the
worst case (average) Carrier-to-(Co-Channel)Interference [CCI] is
R= cell radius
)()(2
1FFF
F
!0
RDDRD
R
Worst case CCI
on the forward channel
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OverlayOverlay
Dual-mode or dual-frequency phones Overlay different wireless accesstechnologies Different technologies
Same technology operating in different bands Increase system capacity Reduce blocking
Example: GSM 900/1800 TDMA+CDMA
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Overlaid cellsOverlaid cells
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HandoffHandoff
Handoff threshold: typically, -90~-100 dBm(1~10uW)
Need to prevent from ping-pong effect
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RoamingRoaming
MS moves to a new cell The new cell is served by a new MSC
i.e. handoff that results in MSC change
Different types of handoff Same cell (different sectors) Same BSC
Same MSC
Different operators