Cellullar Concept & Design Fundamentals Waheed
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Transcript of Cellullar Concept & Design Fundamentals Waheed
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CELLULAR CONCEPTS&
SYSTEM DESIGN FUNDAMENTALS
Dr. Muhammad WaheedPhD, RRM in 4G & Beyond Networks
Certified Lte [email protected]
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OUTLINE
Introduction
Frequency Reuse/Planning
Channel Assignment StrategiesHandoff Strategies
Interference & System Capacity
Trunking & GOS Improving Coverage & Capacity in Cellular
Systems
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INTRODUCTION
Early Mobile Radio System Design objective
Large coverage
Limitations No Frequency reuse
Interference limitations
Challenges Increased service demands
non-availability of spectrum allocations in proportion todemands
Requirement Radio system restructuring
Cellular Systems, to achieve: high capacity
With limited spectrum
While having large coverage
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FREQUENCY REUSE/PLANNING
Cell Small geographic area
Base station (BS) in each cell is allocated group of radiochannels
Adjacent Cells BSdifferent group of channels. Cellular Base Stations
Antennae designed for limiting coverage within cell.
Avoiding interference to cells using same group ofchannels
Frequency reuse/planning In cellular system design
Channel grouping
Group allocation to cellular BSs
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FREQUENCY REUSE/PLANNING
Cluster Group of cells
Frequency reuse factor If cluster size =N
Total number of duplex channels =S
If each cell has uniform same group of channels= k
Freq, reuse factor =1/N
Total no. of available radio channels =S=kN
System Capacity If no. of clusters using same S radio channels=M
Capacity = C=MkN=MS
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FREQUENCY REUSE/PLANNING
Hexagon geometry To connect cells without gaps b/w adjacent cells
Geometry of hexagon
No. of cells per cluster, Ncan have value satisfying:
Nearest co-channel cell:
1. Move icells along any chain2. Turn 60 degree counter-clockwise and move j cells
If i =2, j = 2, ..gives N=12
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FREQUENCY REUSE/PLANNING
Example:
FDD System
Total Bandwidth 10MHz Paired(10+10)
Channel bandwidth= 100k
Total available chls =10,000K/100k=100
1. No. of Chls per cell, for N = 7 = 100/7=14
2. No. of Chls per cell, for N = 12 = 100/12=8
3. No. of Chls per cell, for N = 19 = 100/19=5
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CHANNEL ASSIGNMENT STRATEGIES
Classification Fixed
Dynamic
Fixed Channel Assignment Predetermined set of channels for voice/data
Service subject to unused channel availability Call blocking
Channel borrowing (MSC supervise)
Dynamic Channel Assignment Chls not allocated permanently
Call requestBS request MSC for chl allocation MSC allocates chls .avoiding blocking.interference
MSC collects Radio Signal Strength Indicator (RSSI)
Increase computational load of system Increased chl utilization
Decreased blocking probability
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HANDOFF STRATEGIES
HandoffActive state
When mobile moves into a different cell whileconversation/data session is in progress
MSC automatically transfers the call/session to new
channel belonging to new BS.Voice/Data , Control chls are allocated are that of new BS
Idle State.When mobile moves into a different cell while its in idle
state.
Handoff strategies. Priority
Prioritize Handoffs over new initiations requests
Allocate un-allocated chls in a cell site for H/O
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HANDOFF STRATEGIES .. ..
Initiation thresholds Need optimum signal levels at which H/O can be initiated
Minimum usable signal level at MS/BS
For BS is normally between -90dBm and -100dBm
And slightly lower on BS side*
Initiation thresholds
System/Operator Policies Different systems, operators have different policies
For managing Handoffs
Treating HO as new calls
HO failure equals blocking probability
* Network EIRP threshold is lower & antenna gain is higher than MS
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HANDOFF STRATEGIES ..
Prioritizing Handoffs Guard Channel concept
Fraction of total available chls in cell are reserved for handoffs
Static , reduces cell traffic/throughput.
Dynamic , efficient spectrum utilization
Queuing Handoff requests To decrease probability of forced termination of call
Due to lack of chls available for HO
Time between the.. time the received signal level dropshandoff threshold &
Time the call is terminated due to insufficient signal level
Doesnt guarantee zero probability of forced termination Since large delays cause signal drop below the level required
to maintain communication
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HANDOFF STRATEGIES ..
Practical Consideration Mobility/Speed User travelling Cars/high speed
Pedestrian/Slow speed.
Dynamic , efficient spectrum utilization
Cell Size Micro .capacity Macro .Coverage
UmbrellaWide coverageHighways
LoS/Low penetration losses Cell dragging
HO doesnt occur, user move into other cells Traffic management issues
Different technologies Hard handoff
Different Chls after HOChanellized wireless systemsGSM
Soft Handoff Different BS after HO.Spread spectrum systems.CDMA
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INTERFERENCE & SYSTEM CAPACITY
Interference Major factor limiting.cellular system performance
Sources Other BS/Transceivers, MS, Calls, Sessions in the same cell, energy in
the band
Types
Co-channel
Adjacent channel
Competitors transmitters are often significant source of out of
band interference
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INTERFERENCE & SYSTEM CAPACITY ..
Co-Channel Interference & System Capacity Co-channel Cells
Frequency re-use systems
Cells using same set of frequencies/chls
Co-channel cells
Co-Channel Interference
Interference between signals form Co-channel cells
Thermal noise can be overcome improving SINR Co-channel interference cannot be combated by increasing carrier power of
transmitter
Increasing carrier tx powerincreases interference in neighboring co-channel cells
To reduce co-channel interference..physical separation/sufficient isolation
Co-channel reuse ratiorelated to cluster size:
Where Dis distance b/w centers of co-channel cells of radius R
Small Q large capacitycluster size Nis small
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INTERFERENCE & SYSTEM CAPACITY ..
Channel Planning for wireless Systems 1G/2G/GSM
Frequency planning is difficult
Sectoring is used to improve SINR
Leads to smaller cell size
Coverage region and interference levels are well defined
CDMA
Cluster size N =1
Has dynamic , time varying coverage region
Cell Breathing:
Coverage region varies depending upon instantaneous no. of
users
Difficult decisions
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INTERFERENCE & SYSTEM CAPACITY ..
Adjacent Channel Interference & System Capacity Interference form signals which are adjacent in
frequency to desired signal.
Result from improper receive filters which allow
nearby frequencies to leak into pass band.
Serious if adjacent chl user is transmitting very
closenear-fareffect.
Adjacent Chl interference can be minimized
Through careful filtering &
Channel assignments
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INTERFERENCE & SYSTEM CAPACITY ..
Power Control For reducing interference
BS controls MS transmit power
To ensue MS transmits at minimum power level required
for quality common reverse link
Improve battery life of MS
Reduces reverse channel S/I in the system
CDMA
Power is important to allow every user in every cell to
share same radio channel
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TRUNKING AND GRADE OF SERVICE (GOS)
Trunking allows a large number of users to sharerelatively small no. of channels
Pool of chls.each user is assigned chl on per call
basis.
Chl returned to pool upon call termination
Grade of Service (GoS)
GoS is measure of ability of a user to access a
trunked system during busiest hour
GoS is a benchmark used to define the desired
performance of a particular trunked system
by specify a desired likelihood of a user obtaining chl
access out of available in trunked system.
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THANKS