lecture 2_cellular [compatibility mode]
TRANSCRIPT
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Lecture 2:
The Cellular Concept System DesignFundamentals
ng L Khoa
Class 2
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OutlineOutline
Cellular Concept
Handoff
Cochannel Interference
Trunking and Grade of Service
Call procedure
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Cellular ConceptCellular Concept
The limited capacity of the first mobile radio-telephone services was related
to the spectrum usednot much sharing and a lot of bandwidth dedicated to
a single call.
good coverage interference: impossible to reuse the same frequency
The cellular concept addressed many of the shortcomings of the first mobile
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Frequency reuse
Wasted spectrum allocated to a single user
In 1968, Bell Labs proposed the cellular telephony concept to the FCC
It was approved and then the work began! FCC allocated spectrum (took away TV UHF channels 70-83) in the 825-
845 MHz and 870-890 MHz bands
AT&T put up a developmental system in Chicago
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Cellular Network ArchitectureCellular Network Architecture
Mobile
Switchin
Public
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Center networkand Internet
Mobile
Switching
Center
Wired network
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Cellular ConceptCellular Concept
developed by Bell Labs 1960s-70s
areas divided into cells
a system approach, no major technological changes
a few hundred meters in some cities, 10s km at country side
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each gets portion of total number of channels
neighboring cells assigned different groups of channels,
interference minimized
hexagon geometry cell shape
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Frequency reuse conceptFrequency reuse concept
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Frequency ReuseFrequency Reuse
Adjacent cells assigned different frequencies to avoid
interference or crosstalk
Objective is to reuse frequency in nearby cells 10 to 50 frequencies assigned to each cell
transmission power controlled to limit power at that frequency
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the issue is to determine how many cells must intervene between
two cells using the same frequency
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Frequency ReuseFrequency Reuse
each cell allocated a group k channels
a cluster has N cells with unique and disjoint channel
groups, N typically 4, 7, 12
total number of duplex channels S = kN
Cluster repeated M times in a system
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C = MkN = MS
Smaller cells higher M higher C
+ Channel reuse higher capacity
+ Lower power requirements for mobiles Additional base stations required
More frequent handoffs
Greater chance of hot spots
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Effect of cluster size NEffect of cluster size N
channels unique in same cluster, repeated over clusters
keep cell size same
large N : weaker interference, but lower capacity small N: higher capacity, more interference need to maintain
certain S/I level
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requency reuse actor: N each cell within a cluster assigned 1/N of the total available
channels
In most of the current networks, frequency reuse factor is 1.
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Design of cluster sizeDesign of cluster size
In order to connect without gaps between adjacent cells (to
Tessellate)
N = i2
+ ij + j2
where i and j are non-negative integers Example i = 2, j = 1
N = 22 + 2(1) + 12 = 4 + 2 + 1 = 7
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Next page example move i cells along any chain or hexagon.
then turn 60 degrees counterclockwise and move j cells.
Example 3.1 in page 61
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ExampleExample
N=19
(i=3, j=2)
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Channel Assignment StrategiesChannel Assignment Strategies
Fixed Channel Assignments
Each cell is allocated a predetermined set of voice channels.
If all the channels in that cell are occupied, the call is blocked, andthe subscriber does not receive service.
Variation includes a borrowing strategy: a cell is allowed to
borrow channels from a neighboring cell if all its own channels are
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occupied. This is supervised by the Mobile Switch Center: Connects cells to
wide area network; Manages call setup; Handles mobility
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Channel Assignment StrategiesChannel Assignment Strategies
Dynamic Channel Assignments
Voice channels are not allocated to different cells permanently.
Each time a call request is made, the serving base station requests
a channel from the MSC.
The switch then allocates a channel to the requested call based on
a decision algorithm taking into account different factors:
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frequency re-use of candidate channel and cost factors. Dynamic channel assignment is more complex (real time), but
reduces likelihood of blocking
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Handover/handoffHandover/handoff
Reasons for handover
Moving out of range
Load balancing Cell, BSC (base station controller), MSC (mobile switching
center)
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Handover scenarios Intra-cell handover (e.g., change frequency due to
narrowband interference)
Inter-cell, intra-BSC handover (e.g., movement across cells) Inter-BSC, intra-MSC handover (e.g., movement across
BSC)
Inter MSC handover (e.g., movement across MSC)
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Four Types of HandoffFour Types of Handoff
MS MS MS MS
12 3 4
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MSC MSC
BSC BSCBSC
BTS BTS BTSBTS
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HandoffsHandoffs
important task in any cellular radio system
must be performed successfully, infrequently, and imperceptible
to users. identify a new base station
channel allocation in new base station
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high priority than initiation request (block new calls rather thandrop existing calls)
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HandoffHandoff
=handoff threshold -
Minimum acceptable
signal to maintain the call
too small:
Insufficient time
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before call is lost
More call losses
too large:
Too many handoffs
Burden for MSC
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Styles of HandoffStyles of Handoff
Network Controlled Handoff (NCHO)
in first generation cellular system, each base station constantly monitorssignal strength from mobiles in its cell
based on the measures, MSC decides if handoff necessary mobile plays passive role in process
burden on MSC
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present in second generation systems mobile measures received power from surrounding base stations and
report to serving base station
handoff initiated when power received from a neighboring cell exceeds
current value by a certain level or for a certain period of time faster since measurements made by mobiles, MSC dont need monitor
signal strength
Mobile Controlled Handoff
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Types of HandoffTypes of Handoff
Hard handoff -
(break before
make)
FDMA, TDMA
mobile has
radio link with
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only one BS atanytime
old BS
connection is
terminatedbefore new BS
connection is
made.
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Types of HandoffTypes of Handoff
Soft handoff (make before break)
CDMA systems
mobile has simultaneous radio link with more than one BS at any
time new BS connection is made before old BS connection is broken
mobile unit remains in this state until one base station clearlyredominates
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http://www.youtube.com/watch?v=mYdk89hMyss
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Types of HandoffTypes of Handoff
Vertical handoff
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Prioritizing handoffPrioritizing handoff
Dropping a call is more annoying than line busy
Guard channel concept
Reserve some channels for handoffs Waste of bandwidth
But can be dynamically predicted
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Queuing of handoff requests There is a gap between time for handoff and time to drop.
Better tradeoff between dropping call probability and network
traffic.
Reduce the burden for handoff
Cell dragging
Umbrella cell
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Umbrella CellUmbrella Cell
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Interference and System CapacityInterference and System Capacity
major limiting factor in performance of cellular radio systems
sources of interference:
other mobiles in same cell
a call in progress in a neighboring cell
other base stations operating in the same frequency band
Non-cellular s stem leakin ener into the cellular fre uenc
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band effect of interference:
voice channel: cross talk
control channel: missed or blocked calls
two main types:
co-channel interference
adjacent channel interference
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CoCo--Channel InterferenceChannel Interference
cells that use the same set of frequencies are called co-channel
cells.
Interference between the cells is called co-channel interference.
Co-channel reuse ratio: Q = D/R=sqrt(3N)
R: radius of cell
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D: distance between nearest co-channel cells
Small Q small cluster size N large capacity
large Q good transmission quality
tradeoff must be made in actual cellular design
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CoCo--channel Reuse Ratiochannel Reuse Ratio
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SINR Power
propagation
factor 2-6=
+
=0
1
2i
i
iI
S
I
S
=
=
0
0
log10)()(d
ddBmPdBmP
d
dPP
tr
tr
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Cochannel Interference
SINR
Power: propagation factor 2-6
Sun, nuclear bomb
Approximation
=
+
=0
1
2i
i
iI
S
I
S
=
=
0
0
log10)()(d
ddBmPdBmP
d
dPP
tr
tr
)3()/(0
NRDRSi
===
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AMPS example
=4, S/I=18dB, N needs to be larger than 6.49.
Reuse factor 1/N small
Relations: cochannel interference, link quality, reuse factor
Example 3.2
00
1 )(Di i
=
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Worst Case Interference
S/I ~ R-4/[2(D-R)-4 + 2(D+R)-4 + 2D-4]
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Adjacent Channel InterferenceAdjacent Channel Interference
Interference resulting from signals where are adjacent in
frequency to the desired signal.
Due to imperfect receiver filters that allow
nearby frequencies to leak into pass band.
Can be minimized by careful filtering and assignments, and by
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keeping frequency separation between channel in a given cell as
large as possible, the adjacent channel interference may be
reduced considerably.
Example 3.3
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Channel Planning and Power ControlChannel Planning and Power Control
Cell planning
Control channel 5%
Voice/data channel
f1/f2 cell planning
Breathing cell
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Power Control
Open loop
Close loop
800Hz in CDMA2000, 1500Hz in WCDMA
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TrunkingTrunking
Trunking: the channel is allocated on demand and recycle after usage
Tradeoff between the number of channels and blocking probability
Grade of service Likelihood of a call is blocked or the delay greater than a thresholdduring the busiest time.
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Erlang, a Danish Mathematician studied how a large population could beaccommodated by a limited number of servers.
Erlang capacity: the percentage of line/channel occupied over time
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Terms of Trunking TheoryTerms of Trunking Theory
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Trunking TheoryTrunking Theory
Each user generates a traffic intensity of Au Erlangs.
Au=H, where H is the average duration of a call and is the average
number of call requests per unit time for each user
A=UAu, where U no. of users and A is the total offered traffic intensity Ac=UAu/C, where C is the number of channels
Offered traffic >= the traffic carried by the trunked system
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First type of trunked system
Blocked calls cleared: no queuing for call requests, no setup time
M/M/m/m: memory-less, Poisson arrival, exponential service, finite
channels.
Erland B formula GOS
K
Ablocking C
k
kC
AC
==
=0
!
!
)Pr(
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Erlang B ExampleErlang B Example
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Erlang BErlang B
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Blocked Calls DelayedBlocked Calls Delayed
Blocking calls are delayed until channels are available, queuing
Erlang C
=
+
=>
1
0 !)1(!
)0Pr(C
k
k
CAC
C
kACA
Adelay
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Exponential service distributions
Pr[delay>t]=Pr[delay>0]Pr[delay>t|delay>0]
=Pr[delay>0]exp(-(C-A)t/H)
The average delay D
D=Pr[delay>0]H/(C-A)
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Erlang CErlang C
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ExamplesExamples
Example 3.4
Example 3.5
Example 3.6 Example 3.7
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Approaches to Increasing Capacity
Frequency borrowing
frequencies are taken from adjacent cells by congested cells
Cell splitting cells in areas of high usage can be split into smaller cells
Cell sectoring
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cells are divided into a number of wedge-shaped sectors, each with
their own set of channels
Microcells
antennas move to buildings, hills, and lamp posts
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Cell Splitting
subdivide a congested cell into smaller cells
each with its own base station,
reduction in antenna and
transmitter power
more cells more clusters
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achieves capacity
improvement by essentially
rescaling the system.
Cell Splitting fromradius R to R/2
Table 3.4 example
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Cell Splitting ExampleCell Splitting Example Power reduction is 16 times and 12 dB for half cell radius for
propagation factor is 4.
antenna downtilting
Umbrella cell
Example 3.8
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SectoringSectoring
In basic form, antennas are omnidirectional
Replacing a single omni-directional antenna at base station with
several directional antennas, each radiating within a specified sector.
achieves capacity improvement by essentially rescaling the system.
less co-channel interference, number of cells in a cluster can be
reduced
EE 542/452 Spring 2008
Larger frequency reuse factor, larger capacity
S t i E lS t i E l
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Sectoring ExamplesSectoring Examples
Only two cochannel cell
S/I improvement 7.2dB
Capacity 12/7 First type handoff
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Urban area not good
Example 3.9
R tR t
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RepeaterRepeater Extend coverage range
Directional antenna or distributed antenna systems
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Micro Cell Zone
Superior to sectoring, any base station channel may be assigned
to any zone by the base station
Same channel
No handoff
Only the active zone
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Micro Cell Zone Concept
Large control base station is replaced by several lower powered
transmitters on the edge of the cell.
The mobile retains the same channel and the base station simply
switches the channel to a different zone site and the mobilemoves from zone to zone.
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which mobile is traveling, base station radiation is localized andinterference is reduced.
ExampleExample
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ExampleExample
Dz/Rz=7, but D/R is 3.
2.33 times
capacity gain
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B i f O tli f C ll l P
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Brief Outline of Cellular Process
Telephone call to mobile user
Step 1 The incoming telephone call to Mobile X is received at the MSC.
Step 2 The MSC dispatches the request to all base stations in the cellularsystem.
Step 3 The base stations broadcast the Mobile Identification Number(MIN), telephone number of Mobile X, as a paging message over the FCCthroughout the cellular system.
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monitors and responds by identifying itself over the reverse control channel.
Step 5 The base station relays the acknowledgement sent by the mobileand informs the MSC of the handshake.
Step 6 The MSC instructs the base station to move the call to an issuedvoice channel within in the cell.
Step 7 The base station signals the mobile to change frequencies to anunused forward and reverse voice channel pair.
At the same time, another data message (alert) is transmitted over the forwardvoice channel to instruct the mobile to ring.
T l h C ll Pl d b M bil
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Telephone Call Placed by Mobile
Step 1 When a mobile originates a call, it sends the base
station its telephone number (MIN), electronic serial number
(ESN), and telephone number of called party. It also transmits a
station class mark (SCM) which indicates what the maximumpower level is for the particular user.
Step 2 The cell base station receives the data and sends it to
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the MSC.
Step 3 The MSC validates the request, makes connection to
the called party through the PSTN and validates the base station
and mobile user to move to an unused forward and reverse
channel pair to allow the conversation to begin.
R i
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Roaming
All cellular systems provide a service called roaming. This
allows subscribers to operate in service areas other than the one
from which service is subscribed.
When a mobile enters a city or geographic area that is differentfrom its home service area, it is registered as a roamer in the
new service area.
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Registration MSC polls for unregistered mobiles
Mobiles respond with MINs
MSC queries mobiles home for billing info Calls
MSC controls call and bills mobiles home
HomeworkHomework
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HomeworkHomework
3.1
3.5
3.7
3.9
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.
3.15
3.27
3.28
3.29