Cellular concepts and system design fundamentals

Cellular concepts and system design fundamentals
Reference: Wireless Communications: Principles and Practice: Theodore Rappaport

1. UNIT 2
CELLULAR CONCEPT
Dr Kamal Kr. Sharma
Professor, SEEE
“Provide additional radio capacity with
no additional Increase in Radio
Spectrum”

2. INTRODUCTION
•Earlymobileradiosystemwasto
achievealargecoverageareasby
usinghighpoweredtransmitterwith
anantennamountedonatalltower
•Inthiscaseitisimpossibletoreusethosesame
frequenciesthroughoutthesystem
•Sinceanyattemptstoachievefrequencyreuse
wouldresultininterference

3. Cont..
•Cellularconceptisasystemlevelideawhichcallsfor
replacingasingle,highpowertransmitterwithlowpower
smalltransmitterswitheachprovidingcoveragetoonlya
smallportionofservicearea
•Eachbasestationisallocatedaportionoftotalnoofchannels
availabletoentiresystem
•Nearbybasestationareassigneddifferentgroupsofchannels
sothatalltheavailablechannelsareassignedtoarelatively
smallno.ofneighboringbasestations
•NearbyBSareassigneddifferentgroupsofchannelsothat
interferencebt.BSisminimized

4. Cellular Concept

8. Deployment of Base stations

9. THE CELLULAR CONCEPT
Cluster of 7 cells
Cells
•seven groups of channel from A to G
•footprint of a cell -actual radio coverage
•omni-directional antenna v.s. directional antenna

10. possible radio coverage of the cell
idealized shape of the cell
cell
segmentation of the area into cells
CELLULAR NETWORK
–use of several carrier frequencies
–not the same frequency in adjoining cells
–cell sizes vary from some 100 m up to 35 km depending on user
density, geography, transceiver power etc.
–hexagonal shape of cells is idealized (cells overlap, shapes depend on
geography)
–if a mobile user changes cells
handover of the connection to the neighbor cell

11. FREQUENCY REUSE
•Eachcellularbasestationisallocatedagroupofradiochannelswithina
smallgeographicareacalledacell.
•Neighboringcellsareassigneddifferentchannelgroups.
•Bylimitingthecoverageareatowithintheboundaryofthecell,the
channelgroupsmaybereusedtocoverdifferentcells.
•Keepinterferencelevelswithintolerablelimits.
•Frequencyreuseorfrequencyplanning
“The design process of selecting and allocating channel groups for
all of the cellular base station within a system is
FREQUENCY REUSE/PLANNING ”
Hexagonal cell shape
is perfect over square or triangular cell shapes in cellular architecture
because it cover an entire area without overlapping i.e. they can cover the
entire geographical region without any gaps.

12. •Consider a cellular system which has a total of Sduplex channels.
•Each cell is allocated a group of kchannels, .
•The Schannels are divided among Ncells.
•The total number of available radio channels
•The Ncells which use the complete set of channels is calledcluster.
•The cluster can be repeated Mtimes within the system. The total
number of channels, C, is used as a measure of capacity
•The capacity is directly proportional to the number of replication M.
•The cluster size, N, is typically equal to 4, 7, or 12.
•Small Nis desirable to maximize capacity.
•The frequency reuse factor is given by Sk kNS MSMkNC  N/1

13. •Hexagonal geometry has
–exactly six equidistance neighbors
–the lines joining the centers of any cell and each of its neighbors are
separated by multiples of 60 degrees.
•Only certain cluster sizes and cell layout are possible.
•The number of cells per cluster, N, can only have values which satisfy
•Co-channel neighbors of a particular cell, ex, i=3and j=2.22
jijiN 
Why Hexagon

14. CLUSTER SIZES AND CELL LAYOUTA
B
C
A
C
A
C A
B
C
A F
E
G
D
E
F
D E
The factor N is called the cluster size and is givenN=i
2
+ij+j
2
Eg for i=1,j=1
Eg for i=2,j=1

15. CELL REUSE
EXAMPLE (N=19)
Method of locating co-channel cells in a cellular system. In this example, N= 19 (i.e., I= 3, j= 2). (Adapted
from [Oet83] © IEEE.)
To find the nearest co-channel
neighbor of a particular cell
1.Move ‘i’ cells along any
chain of hexagons
2.Then turn 60 degrees
counter-clockwise and
3.Move‘j’ cells.

16. A
A
A
A
A
A
A
i
j i=1, j=2 , N=1+2+4=7
CLUSTER SIZES AND CELL LAYOUT

17. ADVANTAGES
•Solvestheproblemofspectralcongestionand
usercapacity.
•Offerveryhighcapacityinalimitedspectrum
withoutmajortechnologicalchanges.
•Reuseofradiochannelindifferentcells.
•Enableafixnumberofchannelstoservean
arbitrarilylargenumberofusersbyreusingthe
channelthroughoutthecoverageregion.

18. CAPACITY EXPANSION IN CELLULAR
SYSTEM
Techniques to provide more channels per coverage
area is by
•Cell splitting
•Cell sectoring
•Coverage zone approaches
•Microcell Zoaning

19. FrequencyBorrowing
•RFbandwidthisthemostimportantconstraint
inwirelesssystems.
•Sotoincreasethecapacity,frequencyof
RadioSignalsandwirelesssystemsshallbe
increased.
•Todothis,frequenciesaretakenfrom
adjacentcellsbycongestedcells.

20. Channel Assignment Strategies
•Frequency reuse scheme
–increases capacity
–minimize interference
•Channel assignment strategy
–fixed channel assignment
–dynamic channel assignment
•Fixed channel assignment
–each cell is allocated a predetermined set of voice channel
–any new call attempt can only be served by the unused channels
–the call will be blockedif all channels in that cell are occupied
•Dynamic channel assignment
–channels are not allocated to cells permanently.
–allocate channels based on request.
–reduce the likelihood of blocking, increase capacity.

21. •Cell splitting increases the capacity of cellular system
since it increases the number of times the channel are
reused
•Cell splitting -defining new cells which have smaller
radius than orginal cells by installing these smaller
cells called MICROCELLS between existing cells
•Capacity increases due to additional number of
channels per unit area
“Cellsplittingisprocessofsubdividingacongestedcell
intosmallercellseachwithitsownbasestation(with
correspondingreductioninantennaheightandtxpower)”
CELL SPLITTING

22. CELL SPLITTING
Split congested cell into smaller cells.
–Preserve frequency reuse plan.
–Reduce transmission power.
microcell
Reduce Rto R/2

23. CellSplitting
The unit area of RF coverage for cellular
network is called a cell.
In each cell, a base station transmits from a
fixed cell site location, which is often centrally
located in the cell.
In base stations where the usage of cellular
network is high, these cells are split into
smaller cells.

24. Delhi
ISBT
Gur/
g
Bypa
ss
S/Ex
NOI
DA
N/Ex

25. CellSplitting(con’t)
•Theradiofrequenciesarereassigned,and
transmissionpowerisreduced.
•Anewcellsitemustbeconstructedwhenacell
issplit
•Cellsplittingisoneoftheeasyandlesscostly
solutionwhenincreasingthecapacityofcellular
network.
•Splittingthecellsintosmalleronesalsoleadto
anewsolutioncalledcellsectoring.

26. CellSectoring
•Sectorizationconsistsofdividingan
omnidirectional(360degree)viewfromthecell
siteintonon-overlappingslicescalledsectors.
•Whencombined,sectorsprovidethesame
coveragebuttheyareconsideredtobe
separatecells.
•Alsoconsideredasoneofeasyand
inexpensivecapacityincreasingsolution.

27. Sectoringmethods

28. Sectoring
•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
•Larger frequency reuse factor, larger capacity

30. DAYANANDASAGAR
Repeater
•Extend coverage range
•Directional antenna or distributed
antenna systems

31. Microcells
•As the splitting of cell idea evolves, the
usage of smaller cells become efficient and it
leads the creation of microcells.
•The aim of creating microcells are increasing
the capacity of cellular network in areas
where population is high.

32. Microcells(con’t)
Typical comparison can be made like this;
Cells typically range in size from two to twenty
kilometers in diameter.
Microcells range from about a hundred meters to a
kilometer in diameter.

33. Micro Cell ZoneConcept
•Largecontrolbasestationisreplacedbyseveral
lowerpoweredtransmittersontheedgeofthe
cell.
•Themobileretainsthesamechannelandthe
basestationsimplyswitchesthechanneltoa
differentzonesiteandthemobilemovesfrom
zonetozone.
•Sinceagivenchannelisactiveonlyina
particularzoneinwhichmobileistraveling,base
stationradiationislocalizedandinterferenceis
reduced.

34. Micro CellZone
•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

35. Example
2.33 times
capacity gain

36. •Decrease the co-channel interference and keep the cell radius R
unchanged
–Replacing single omni-directional antenna by several directional
antennas
–Radiating within a specified sector

37. Microcell Zone Concept
•Antennas are placed at the outer edges of the cell
•Any channel may be assigned to any zone by the base station
•Mobile is served by the zone with the strongest signal.
•Handoff within a cell
–No channel re-
assignment
–Switch the channel to
a different zone site
•Reduce interference
–Low power
transmitters are
employed

38. Handoff Strategies

39. Mobile Handoff Strategies
•When a mobile moves into a different cell while a conversation is in
progress, the MSC automatically transfers the call to a new channel
belonging to the new base station.
•Handoff operation
–identifying a new base station
–re-allocating the voice and control channels with the new base station.
•Handoff Threshold
–Minimum usable signal for acceptable voice quality (-90dBm to -100dBm)
–Handoff margin cannot be too large or too
small.
–If is too large, unnecessary handoffs burden the MSC
–If is too small, there may be insufficient time to complete handoff
before a call is lost. usable minimum,, rhandoffr PP   

41. •Handoff must ensure that the drop in the measured signal
is not due to momentary fading and that the mobile is
actually moving away from the serving base station.
•Running average measurement of signal strength should be
optimized so that unnecessary handoffs are avoided.
–Depends on the speed at which the vehicle is moving.
–Steep short term average -> the hand off should be made quickly
–The speed can be estimated from the statistics of the received
short-term fading signal at the base station
•Dwell time: the time over which a call may be maintained
within a cell without handoff.
•Dwell time depends on
–propagation
–interference
–distance
–speed

42. •Handoff measurement
–In first generation analog cellular systems, signal strength
measurements are made by the base station and supervised by the
MSC.
–In second generation systems (TDMA), handoff decisions are
mobile assisted, called mobile assisted handoff (MAHO)
•Intersystem handoff: If a mobile moves from one cellular
system to a different cellular system controlled by a
different MSC.
•Handoff requests is much important than handling a new
call.

44. Practical Handoff Consideration
•Different type of users
–High speed users need frequent handoff during a call.
–Low speed users may never need a handoff during a call.
•Microcells to provide capacity, the MSC can become burdened if high
speed users are constantly being passed between very small cells.
•Minimize handoff intervention
–handle the simultaneous traffic of high speed and low speed users.
•Large and small cells can be located at a single location (umbrella cell)
–different antenna height
–different power level
•Cell dragging problem: pedestrian users provide a very strong signal to
the base station
–The user may travel deep within a neighboring cell

46. •Handoff for first generation analog cellular systems
–10 secs handoff time
– is in the order of 6 dB to 12 dB
•Handoff for second generation cellular systems, e.g., GSM
–1 to 2 seconds handoff time
–mobile assists handoff
–is in the order of 0 dB to 6 dB
–Handoff decisions based on signal strength, co-channel interference, and
adjacent channel interference.
•IS-95 CDMA spread spectrum cellular system
–Mobiles share the channel in every cell.
–No physical change of channel during handoff
–MSC decides the base station with the best receiving signal as the service
station
• 

47. Interference and System Capacity
•Sources of interference
–another mobile in the same cell
–a call in progress in the neighboring cell
–other base stations operating in the same frequency band
–noncellular system leaks energy into the cellular frequency band
•Two major cellular interference
–co-channel interference
–adjacent channel interference

48. R
D

49. Co-channel Interference and System Capacity
•Frequency reuse -there are several cells that use the same set of
frequencies
–co-channel cells
–co-channel interference
•To reduce co-channel interference, co-channel cell must be separated
by a minimum distance.
•When the size of the cell is approximately the same
–co-channel interference is independent of the transmitted power
–co-channel interference is a function of
•R: Radius of the cell
•D: distance to the center of the nearest co-channel cell
•Increasing the ratio Q=D/R, the interference is reduced.
•Qis called the co-channel reuse ratio

50. •For a hexagonal geometry
•A small value of Q provides large capacity
•A large value of Q improves the transmission quality -smaller level of
co-channel interference
•A tradeoff must be made between these two objectivesN
R
D
Q 3

51. •Let be the number of co-channel interfering cells. The signal-to-
interference ratio (SIR) for a mobile receiver can be expressed as
S: the desired signal power
: interference power caused by the ithinterfering co-channel cell base
station
•The average received power at a distance dfrom the transmitting
antenna is approximated by
or
nis the path loss exponent which ranges between 2 and 4. 0i 


0
1
i
i
i
I
S
I
S iI n
r
d
d
PP










0
0 








0
0 log10)dBm()dBm(
d
d
nPP
r
close-in reference pointTX
0d
0P:measued power

52. •When the transmission power of each base station is equal, SIR for a
mobile can be approximated as
•Consider only the first layer of interfering cells




0
1
i
i
n
i
n
D
R
I
S 
00
3)/(
i
N
i
RD
I
S
n
n

•Example: AMPS requires that SIR be
greater than 18dB
–Nshould be at least 6.49 for n=4.
–Minimum cluster size is 76
0
i

53. •For hexagonal geometry with 7-cell cluster, with the mobile unit being
at the cell boundary, the signal-to-interference ratio for the worst case
can be approximated as 44444
4
)()2/()2/()(2




DRDRDRDRD
R
I
S

54. Adjacent Channel Interference
Next to anotherchannel
•Results from signals that are adjacent in the frequency to
the desired signal.
•Results from imperfect receiver filters that allow nearby
frequencies to leak in
•Why the prices of handset go down
–because the hardware put in there is cheaper and the filters that we put in there also do not have
two stringent requirements. That is, the sharp cut off do not exist.desired signal
receiving filter
response
desired signal
interference
interference
signal on adjacent channel
signal on adjacent channel
FILTER

55. Theproblemcanbesevereiftheinterfererisveryclosetothe
subscriber’sreceiver.
SoifmyfriendandIaregoinginthesamecarandbywhatever
coincidencewebothareassignedadjacentchannels,thenwewill
havecrosstalkoriftheinterferenceisinachannelwhichisused
forcontrol,thenoneofthecallsmightgetdroppedorsomeother
problems

56. Near Far Effect
Anothereffectofadjacentchannelinterferenceiscalledthe
nearfareffect.
whenaninterfererclosetothebasestationradiatesinthe
adjacentchannel,whilethesubscriberisactuallyfaraway
fromthebasestation.
Soifmyinterferinghandsetisclosetothebasestation,where
asIam,asasubscriberfarawayfromthebasestation,my
signalwillgetalotofinterferenceatthebasestation.

57. Fairly
weak

58. Adjacentchannelinterferencecanbeminimizedthrough
•carefulfiltering–Expensivefiltersatbasestations
•channelassignment.
•Keepthefrequencyseparationbetweeneachchannelinagivencellas
largeaspossible.
–if a subscriber is at a distance d 1 and the interferer is at a distance d
2,then the interference value will be determined by d 1 and d 2 and the
signal to interference ratio prior to filtering is given by
S/I = (d1/d2)
-n

59. Theothermethodtoreducetheadjacentchannelinterferenceisby
SmartFrequencySeparation.
Thatis,youhaveafrequencybandwhichhassub-bandstobeallocatedtodifferent
userscalledchannels

60. Power Control for Reducing Interference
•Ensure each mobile transmits the smallest power necessary to maintain
a good quality link on the reverse channel
–long battery life
–increase SIR
–solve the near-far problem

61. •Set-upTime:Thetimerequiredtoallocateatrunkedradiochannel
toarequestinguser.
•BlockedCall:Callwhichcannotbecompletedattimeofrequest,
duetocongestion.Alsoreferredtoasalostcall.
•HoldingTime:Averagedurationofatypicalcall.DenotedbyH(in
seconds).
•TrafficIntensity:Measureofchanneltimeutilization,whichisthe
averagechanneloccupancymeasuredinErlangs.Thisisa
dimensionlessquantityandmaybeusedtomeasurethetime
utilizationofsingleormultiplechannels.DenotedbyA.
Definitions of Common Terms Used In Trunking
Theory

62. GradeofService(GOS):Ameasureofcongestionwhichis
specifiedasthe
•Probabilityofacallbeingblocked(forErlangB)
•Probabilityofacallbeingdelayedbeyondacertainamountof
time(forErlangC).
RequestRate:Theaveragenumberofcallrequestsperunittime.
DenotedbyAseconds.
Load:Trafficintensityacrosstheentiretrunkedradiosystem,
measuredinErlangs.
Definitions….

63. Trunking and Grade of Service
•Erlangs: One Erlangs represents the amount of traffic density carried
by a channel that is completely occupied.
–Ex: A radio channel that is occupied for 30 minutes during an hour carries
0.5 Erlangs of traffic.
•Grade of Service (GOS): The likelihood that a call is blocked.
•Each user generates a traffic intensity of Erlangs given by
H: average duration of a call.
: average number of call requests per unit time
•For a system containing Uusers and an unspecified number of
channels, the total offered traffic intensity A, is given by
•For Cchannel trunking system, the traffic intensity, is given as HA
u  uUAA cA CUAA
uc / uA