cellular mobile communications-ii the cellular concept-system design fundamentals dr. nasir d. gohar...

Cellular Mobile Cellular Mobile Communications-IICommunications-IIThe Cellular Concept-System Design FundamentalsThe Cellular Concept-System Design Fundamentals

Dr. Nasir D. Dr. Nasir D. GoharGohar

Lecture Notes “Wireless Cellular” by Professor A. SmithLecture Notes “Cellular Systems-An Introduction” by Professor Reynold Cheung Other Internet Resources


IntroductionIntroduction Frequency ReuseFrequency Reuse Channel Assignment StrategiesChannel Assignment Strategies Handoff StrategiesHandoff Strategies Interference and System CapacityInterference and System Capacity Trunking Theory and Grade of Trunking Theory and Grade of ServiceService Improving Coverage and System Improving Coverage and System Capacity Capacity


Conventional Radio System & its LimitationsConventional Radio System & its Limitations Single Hi Power Transmitter and Large Antenna Single Hi Power Transmitter and Large Antenna

Towers/Masts Towers/Masts Large Coverage Area/Larger Size Radios with Large BatteriesLarge Coverage Area/Larger Size Radios with Large Batteries Limited No. of ChannelsLimited No. of Channels Poor Quality of Service Poor Quality of Service [Bell Mobile NY City, in 1970s 12 Ch/1K Sqr [Bell Mobile NY City, in 1970s 12 Ch/1K Sqr

Miles]Miles] Still in use for Some Public/Private Organizations [PMR Systems]Still in use for Some Public/Private Organizations [PMR Systems]


Solution-Frequency ReuseSolution-Frequency Reuse


The Cellular ConceptThe Cellular Concept

The Cellular Idea Developed by Bell Labs (1960s-1970s)

Divide the service Area into Several Smaller CellsPut at least as many Towers as the # of Cells and Reduce the

Transmitter PowerReuse the Allocated Frequency Spectrum (Channels) as many

Times as Possible Avoiding Interference

Gains but with PainsGreater System Capacity at the Cost of Large Infra-StructureOptimal Frequency Spectrum Utilization attained by making

System more ComplicatedUser Equipment Design made Smarter at the cost of Circuit

Complexity and Processing Power


The Cellular ConceptThe Cellular ConceptThe Cell ShapeThe Cell Shape All cells should have same All cells should have same shape and equal areashape and equal area Possible Choices: Possible Choices: Rectangle, Rectangle, Triangle, and HexagonTriangle, and Hexagon For a given value of S, A3 For a given value of S, A3 Provides the Max. CoverageProvides the Max. Coverage

Area with fewest number of Area with fewest number of CellsCells Actual Radio Coverage Area of Cell is amorphous (irregular shaped)- obtained by field measurements or by using prediction models through computer simulation This is known as footprint

R S L

C E L L

ACTUAL CELL


The Cellular ConceptThe Cellular ConceptFrequency Reuse Factor- Frequency Reuse Factor- N = no. of distinct channel groups = maximum cluster size

Typical Values of N are Typical Values of N are 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, ……


The Cellular ConceptThe Cellular ConceptFrequency Reuse/Planning - Frequency Reuse/Planning -

Selection and Allocation of Channel Selection and Allocation of Channel Groups (out of the Groups (out of the Allotted Spectrum Allotted Spectrum SS) for all the Cellular Base Stations ) for all the Cellular Base Stations within a Systemwithin a System

Each Cell is allocated a unique Each Cell is allocated a unique group of k Channelsgroup of k Channels If N is the Frequency Reuse If N is the Frequency Reuse Factor (Cluster Size), then Factor (Cluster Size), then S = kNS = kN If a cluster is replicated M times If a cluster is replicated M times in the system, then System in the system, then System Capacity as the measure of total Capacity as the measure of total number of duplex channels is number of duplex channels is given as given as C = MkN = MSC = MkN = MS


The Cellular ConceptThe Cellular Concept Placement of Base StationsPlacement of Base Stations

Center-Excited Cell:Center-Excited Cell: Base Station Base Station Transmitters/Receivers [Transceivers] Transmitters/Receivers [Transceivers] are placed at the location which is are placed at the location which is probably the most closest to the Center probably the most closest to the Center of the Cellof the Cell

Normally, Omni-Directional Antennas are used

Edge-Excited Cell:Edge-Excited Cell: Base Station Base Station Transceivers are placed three out of six Transceivers are placed three out of six vertices [Corners] of the hexagonal-vertices [Corners] of the hexagonal-shaped cellsshaped cells

Sectored Directional Antennas are used

NOTE: Most Systems Allow 1/4 of the Cell Radius as Proxy Margin for the Distance of the Actual Place from the Ideal Place [Center of the Cell

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Edge-Excited Cells


The Cellular ConceptThe Cellular Concept Locating Co-Channel CellLocating Co-Channel Cell

Observation:Observation: The Geometry of The Geometry of the Hexagons is such that the the Hexagons is such that the number of cells per cluster, N, can number of cells per cluster, N, can only have values such that only have values such that

N = i*i +i*j+j*jN = i*i +i*j+j*j,, i and j are i and j are non-negative integers.non-negative integers.

Method to Find the Nearest Method to Find the Nearest Co-Channel NeighborCo-Channel Neighbor

Move I Cells along any chain of Move I Cells along any chain of hexagon, then,hexagon, then, Turn 60 degree counter-Turn 60 degree counter-clockwise andclockwise and Move J Cells.Move J Cells.

EXAMPLE:Finding Co-Channel Neighbor, N= 7, I = 2, j=1

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The Cellular ConceptThe Cellular Concept - System Design Example-System Design Example-0101A A total of 33 MHz bandwidth is allocated to a particular FDD Cellular Phone System. If the Simplex total of 33 MHz bandwidth is allocated to a particular FDD Cellular Phone System. If the Simplex Voice/Control Channel bandwidth is 25 KHz, Find the total # of Channels available per Cell if the Voice/Control Channel bandwidth is 25 KHz, Find the total # of Channels available per Cell if the System uses (a) 4-Cell Frequency Reuse (b) 7-Cell Frequency-Reuse Plan. If 1 MHz out of the total System uses (a) 4-Cell Frequency Reuse (b) 7-Cell Frequency-Reuse Plan. If 1 MHz out of the total allocated bandwidth is used for Control Channels, determine an equitable distribution of the Control allocated bandwidth is used for Control Channels, determine an equitable distribution of the Control and Voice Channels in each Cell in case of each Frequency-Reuse Plan.and Voice Channels in each Cell in case of each Frequency-Reuse Plan.

SOLUTION:SOLUTION:Total allocated bandwidth = 33 MHz, Total allocated bandwidth = 33 MHz,

Duplex Channel bandwidth = 25x2=50 KHzDuplex Channel bandwidth = 25x2=50 KHz

Total # of Available(Voice/Control) Channels = 33,000/50 = 660 Channels.Total # of Available(Voice/Control) Channels = 33,000/50 = 660 Channels.

(a) N= 4, so total # of Channels/Cell = 660/4 = 165 Channels(a) N= 4, so total # of Channels/Cell = 660/4 = 165 Channels

(b) N=7, so total # of Channels/Cell = 660/7 = 95 Channels(b) N=7, so total # of Channels/Cell = 660/7 = 95 Channels

In Case of 1 MHz bandwidth allocated for Control Channels, total # of Control In Case of 1 MHz bandwidth allocated for Control Channels, total # of Control Channels = 1000/50=20 Channels per Systems. Channels = 1000/50=20 Channels per Systems.

Out of 660 Channels, 20 are used as Control and remaining 640 as Voice Channels.Out of 660 Channels, 20 are used as Control and remaining 640 as Voice Channels.

(a) N=4, Each Cell can have 20/4=5 Control Channels and 640/4=160 Voice (a) N=4, Each Cell can have 20/4=5 Control Channels and 640/4=160 Voice Channels. But, each Cell needs only one Control Channel, so, each cell will be Channels. But, each Cell needs only one Control Channel, so, each cell will be assigned one Control Channel and 160 Voice Channel.assigned one Control Channel and 160 Voice Channel.

(b) N = 7, Each Cell can have 20/7 = 3 Control Channels and 640/7=91 Voice (b) N = 7, Each Cell can have 20/7 = 3 Control Channels and 640/7=91 Voice Channels[Plus 3 Extra], but it needs only 1 Control Channel, so, we can assign 4 Channels[Plus 3 Extra], but it needs only 1 Control Channel, so, we can assign 4 Cells with 91 Voice Channels and one Control Channels, and 3 Cells with 92 Voice Cells with 91 Voice Channels and one Control Channels, and 3 Cells with 92 Voice Channels and one Control Channels.Channels and one Control Channels.



Effect of Cell Size – Trade offsEffect of Cell Size – Trade offsPluses of Smaller Cell Size:Pluses of Smaller Cell Size:

Higher M (More Replications of Cell Cluster) Higher System Capacity Channel reuse Higher System Capacity Lower power requirements for mobiles

Negatives of Smaller Cell Size:Negatives of Smaller Cell Size: Additional base stations requiredMore frequent handoffsExtra possibilities for interference



Effect of Cluster SizeEffect of Cluster Size Each Cluster have Unique Group of Channels

which are Repeated over Clusters Keeping Cell Size Same

Large N- weaker interference, but lower capacity Small N- higher capacity, more interference, need to maintain certain S/I level

Frequency Reuse Factor: 1/N Each Cell within a Cluster Assigned 1/N of the

total Available Channels


The Channel Assignment StrategiesThe Channel Assignment Strategies Objective:Objective: Maximize the System Capacity Maximize the System Capacity

while Minimizing the Interference [A while Minimizing the Interference [A Constrained Optimization Problem]Constrained Optimization Problem]

Classification:Classification: Fixed Channel AssignmentFixed Channel Assignment Dynamic Channel AssignmentDynamic Channel Assignment

Choice has Impact on System Choice has Impact on System

PerformancePerformance HandoffHandoff Call InitializationCall Initialization MSC Processing LoadMSC Processing Load

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The Channel Assignment StrategiesThe Channel Assignment Strategies Fixed Channel AssignmentFixed Channel Assignment

Each Cell is Assigned a Each Cell is Assigned a Predetermined Set of [say Predetermined Set of [say x=1/N] Voice Channelsx=1/N] Voice Channels

Any Request for a New Call Any Request for a New Call Initialization beyond x Initialization beyond x [ Assuming all x Channels of [ Assuming all x Channels of the Cell are in Use] will be the Cell are in Use] will be BlockedBlocked

Any Request for a Handoff Any Request for a Handoff [ Assuming all x Channels of [ Assuming all x Channels of this Candidate Cell are in this Candidate Cell are in Use] will not be treated. [MS Use] will not be treated. [MS may have to Wait, Call can may have to Wait, Call can Drop even]Drop even]

Several Solution to the Several Solution to the Problem:Problem:

Borrowing StrategyBorrowing Strategy Reserve Some Channels for Reserve Some Channels for

HandoffHandoff

Dynamic Channel Dynamic Channel AssignmentAssignment No Permanent Assignment No Permanent Assignment

of Voice Channels to any of Voice Channels to any CellCell

Any Request for a New Call Any Request for a New Call Initialization / Handoff will Initialization / Handoff will be met by a Dynamic be met by a Dynamic Allocation of a Channel Allocation of a Channel from the Central Pool of from the Central Pool of Available Channels by MSCAvailable Channels by MSC

Channel Allocation is done Channel Allocation is done by using an Algorithm that by using an Algorithm that takes into Account;takes into Account;

Probability of Future Blocking Probability of Future Blocking (in the Cell)(in the Cell)

Frequency of Use of Candidate Frequency of Use of Candidate ChannelsChannels

Reuse Distance of the ChannelReuse Distance of the Channel


The Channel Assignment StrategiesThe Channel Assignment Strategies Advantages/Disadvantages of Dynamic Advantages/Disadvantages of Dynamic

Channel AllocationChannel AllocationReduction of Blocking ProbabilityReduction of Blocking ProbabilityReduction of Call Drop Probability During Hand OffReduction of Call Drop Probability During Hand OffImprovement of System Trunking Capacity [Traffic Improvement of System Trunking Capacity [Traffic

Intensity/Channel]- Intensity/Channel]- All Channels are Accessible by all All Channels are Accessible by all CellsCells

All that above-mentioned Benefits are Obtained at the All that above-mentioned Benefits are Obtained at the Cost ofCost of

Storage and Computational Load on MSCStorage and Computational Load on MSC MSC must Collect real-time Channel Occupancy DataMSC must Collect real-time Channel Occupancy Data Traffic Distribution InformationTraffic Distribution Information Radio Signal Strength Indications (RSSI) of all the Radio Signal Strength Indications (RSSI) of all the

ChannelsChannels


The Handoff StrategiesThe Handoff Strategies What is Handoff?What is Handoff?

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

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 High priority than initiation request( block new

calls rather than drop existing calls)


The Handoff StrategiesThe Handoff Strategies Improper Handoff SituationImproper Handoff Situation

Δ too small: Insufficient time to complete handoff before call is lost More call losses

Δ too large: too many handoffs burden for MSC


The Handoff StrategiesThe Handoff Strategies Proper Handoff SituationProper Handoff Situation


The Handoff StrategiesThe Handoff Strategies Handoff StylesHandoff Styles

Network Controlled Handoff (NCHO)

In first generation cellular system each base station constantly monitors signal strength from mobiles in its cell Based on the measures, MSC decides if handoff

necessary Mobile plays passive role in process Heavy Burden on MSC

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The Handoff StrategiesThe Handoff Strategies Handoff StylesHandoff Styles

Mobile Assisted Handoff (MAHO) 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 doesn’t need monitor signal strength


The Handoff StrategiesThe Handoff Strategies Handoff TypesHandoff Types

Hard Handoff - (break before make) FDMA, TDMA (1G and 2G Systems) Mobile has radio link with only one BS at anytime Old BS connection is terminated before new BS connection is made

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 B connection is broken Mobile unit remains in this state until one base

station clearly predominates


The Handoff StrategiesThe Handoff Strategies First Generation Cellular SystemsFirst Generation Cellular Systems

Locate Receiver at Base Station Plus MSC Decides Handoff Locate Receiver at Base Station Plus MSC Decides Handoff

Requests Requests

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The Handoff StrategiesThe Handoff Strategies

Second Generation Cellular Systems Second Generation Cellular Systems (TDMA)(TDMA)

Mobile Assisted Hand Off [MAHO]Mobile Assisted Hand Off [MAHO] Mobile Measures the Pr from all Surrounding Base Mobile Measures the Pr from all Surrounding Base

StationsStations Reports all these Measurement Result to the Serving Reports all these Measurement Result to the Serving

Base StationBase Station Hand Off Initiated when Prs < Prc [by Certain Level or Hand Off Initiated when Prs < Prc [by Certain Level or

for sometime]for sometime] Hand Off Times are Much Shorter [ 1-2 Sec]Hand Off Times are Much Shorter [ 1-2 Sec] = 0 to 6 dB= 0 to 6 dB


The Handoff StrategiesThe Handoff Strategies Second Generation Cellular Systems (CDMA)Second Generation Cellular Systems (CDMA)Unique Handoff Strategy [Soft Handoff ]

Radio Shares the Same Channel in all CellsNo Physical Change of Assigned Channel OccursJust Different Base Station Handles the ongoing CallMSC Continuously Monitors the Data about Received

Signal Level of the Mobile at Various Base Stations, and, Makes Decision which Version of the Mobile Signal is Better at any Time Point


The Handoff StrategiesThe Handoff Strategies Inter-System HandoffMobile is at the Border of the System [Home

Service Provider’s Service Area]MSC of the Serving Cell Talks to the MSC of the

Neighboring System or Vice VersaSeveral Issues are Resolved Before Handoff

Can Take Place Call Type Roaming is Allowed or Not? Compatibility Issues [Standards] User Authenticity and Call Charges Issues


Prioritizing HandoffsPrioritizing HandoffsGuard Channel Method

A Fraction of the Total Available Channels is Reserved for Handoffs

In case of Fixed Channel Assignment, it Affects System Capacity [C = M k N]Good in in Case of Dynamic Channel Assignment

Queuing Handoff Request Method Any Handoff Request, if can not be tackled

Immediately, it will be Placed in a Queue [ for sometime before the signal levels goes below the minimum acceptable and it has to be dropped]

Does not Guarantee 100% Success for all Handoff Requests


Practical Handoffs ProblemsPractical Handoffs ProblemsSpeed Difference Between Various MobilesSpeed Difference Between Various Mobiles

High Speed Vehicles vs PedestriansHigh Speed Vehicles vs Pedestrians Umbrella Cell Approach to Tackle High Speed and Low Speed Umbrella Cell Approach to Tackle High Speed and Low Speed

Traffic SimultaneouslyTraffic Simultaneously

Cell DraggingCell Dragging Pedestrian Users Providing a very Strong Signal due to Pedestrian Users Providing a very Strong Signal due to

Direct LOSDirect LOS

Large Umbrella Cellfor Hi peed Traffic

Small MicroCells for Low Speed Traffic


Cell Site ConfigurationCell Site Configuration

T 1 / E 1

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R x - 1

R x - n

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C o m b i n e rSplitter

Splitter

DuplexerD i v e r s i t y A n t e n n a

M a i n A n t e n n a

C h - 1

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Cell Site ConfigurationCell Site Configuration Mux/DeMux Equipment:Mux/DeMux Equipment: Demultiplexes a Base-band Signal [T1/E1] from Demultiplexes a Base-band Signal [T1/E1] from

MSC into 24/32 Signals and vice-versaMSC into 24/32 Signals and vice-versa

Transceivers:Transceivers: AMPS allows typically 48 Radio Channels per Cell / A Digital AMPS allows typically 48 Radio Channels per Cell / A Digital Cellular Radio System, Using TDMA, can Serve 48x3 Subscribers with same Cellular Radio System, Using TDMA, can Serve 48x3 Subscribers with same number of Channelsnumber of Channels

Splitter:Splitter: Used on Receive Path, it Amplifies the incoming Signal using LNA, Used on Receive Path, it Amplifies the incoming Signal using LNA, Filters and Splits the Base-band Signal into Corresponding ChannelsFilters and Splits the Base-band Signal into Corresponding Channels

Combiner:Combiner: Used on Transmit Path, after the Signals have been Modulated, Used on Transmit Path, after the Signals have been Modulated, upconverted, and Amplified by Transmitters of the Corresponding Channels, upconverted, and Amplified by Transmitters of the Corresponding Channels, these are Combined to form a Single Stream of High-Power Radio Signalsthese are Combined to form a Single Stream of High-Power Radio Signals

DuplexerDuplexer:: It filters the Radio Signal and keeps Rx/Tx Signals Confined to their It filters the Radio Signal and keeps Rx/Tx Signals Confined to their Respective Paths.Respective Paths.

Spatial Diversity:Spatial Diversity: Main and Diversity Antennas are used to Provide Spatial Main and Diversity Antennas are used to Provide Spatial Diversity for the Better CommunicationDiversity for the Better Communication


Cellular AntennasCellular AntennasAntenna:Antenna: Signal Processing Device that Transmits/Receives EM Signal Processing Device that Transmits/Receives EM

Signals SimultaneouslySignals Simultaneously

Antenna CategoriesAntenna Categories Passive Antenna:Passive Antenna: RP Controlled by Type and Construction of RP Controlled by Type and Construction of

the Device, Using Mechanical Means we can Guide the Signalthe Device, Using Mechanical Means we can Guide the Signal Active Antenna:Active Antenna: RP Controlled by Type and Construction as RP Controlled by Type and Construction as

well as DSP Technique of the Devicewell as DSP Technique of the Device

General Classification on RPGeneral Classification on RP Omni Directional Antenna:Omni Directional Antenna: Equal Radiation in all DirectionsEqual Radiation in all Directions

Directional Antenna:Directional Antenna: More Radiation in a Certain DirectionMore Radiation in a Certain Direction


Essential Antenna ParametersEssential Antenna Parameters Antenna Directivity & GainAntenna Directivity & Gain

Energy Concentration in Energy Concentration in One direction wrt other One direction wrt other directionsdirections

Gain = PGain = P11/P/P22

Antenna Beam WidthAntenna Beam Width Beam Width = Beam Width = 2 2

Antenna Front-to-Back Antenna Front-to-Back RatioRatio F2B ratio = 10 Log PF2B ratio = 10 Log Pmm / P / Pbb

Frequency ResponseFrequency Response Antenna BW = fH - fL where Antenna BW = fH - fL where

FH and fL are upper and FH and fL are upper and Lower 3 dB FrequenciesLower 3 dB Frequencies

Bore Sight

Isotropic

GAIN

Type BW

M. Antenna 70 MHzD. Antenna 25 MHzMob. Ant. 70 MHz

Frequency Response824-894 MHz824-849 MHz824-894 MHz

0 dB

- 3 dB

GAIN

FREQUENCYfL fo fH

Typical Antenna Parameters


Interference and System CapacityInterference and System CapacityWhat is Interference:What is Interference: Unwanted Signal Which Affects the Unwanted Signal Which Affects the

Speech Quality and System CapacitySpeech Quality and System Capacity

Sources of Interference: Several Including another Mobile Several Including another Mobile in the Same Cell, a Call in Progress in the Neighboring Cell, in the Same Cell, a Call in Progress in the Neighboring Cell, Other Base Stations Operating in Vicinity Using the Same Other Base Stations Operating in Vicinity Using the Same Frequency Band, or Some non-Cellular Device/System Leaking Frequency Band, or Some non-Cellular Device/System Leaking Energy in the Cellular Frequency Band. Two Major Ones are;Energy in the Cellular Frequency Band. Two Major Ones are; Co-Channel InterferenceCo-Channel Interference Adjacent-Channel InterferenceAdjacent-Channel Interference

A Major Bottle-Neck in System Capacity:A Major Bottle-Neck in System Capacity: A Trade-off A Trade-off has to be made between System Capacity and Speech Qualityhas to be made between System Capacity and Speech Quality


Co-Channel Interference and Co-Channel Interference and System CapacitySystem Capacity

Co-Channel Interference: The Signals of the Co-Channel Cells, the Cells Using

the Same Set of Frequencies in the given Coverage Area, Interfere with Each Other

Co-Channel Reuse Factor If the Base Station Power is the Same,and Cell Radius is also Almost then Same, then Q, the Co-Channel Reuse Factor is defined as

Q = D / R = √3 N Signal to Interference Ratio

S/I = (D/R)n = (√3 N )

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Co-Channel Interference and System CapacityCo-Channel Interference and System Capacity Co-Channel Interference:

Co-Channel Reuse FactorQ = D / R = √3 N Consider two adjacent co-channel cells,

centered at C1 = (u1, v1) and C2 = (u2, v2)

C1 = (u1 Cos 30o, v1+u1Sin 30

o)

C2 = (u2 Cos 30o, v2+u2Sin 30

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Distance between them is D

In terms of X-axis and V-axis Coordinate systems, it is

D = [(u2-u1)2+(v2-v1)

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In case of (u1, v1) moved to the origin, we have D = [(u2)2+(v2)2+(v2)(u2)]1/2

In case of normalization, D = (i2+j2+ij)1/2

Distance between any two adjacent Cells = 2 R Cos 30o

= √3 R

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Co-Channel Interference and System CapacityCo-Channel Interference and System Capacity Co-Channel Interference:

Co-Channel Reuse Factor

The Co-Channel Interference is a function of Q, Co-Channel Reuse Factor?

Under normalized condition,

D = (i2+j2+ij)1/2

But, the actual distance is D = √3 R (i2+j2+ij)

1/2

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Ratio of large hexagon area to a single cell area

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D/R = √ 3N


Co-Channel Interference and System CapacityCo-Channel Interference and System Capacity Q = D / R = √3 N and

S/I = (D/R)n = (√3 N )

n

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N D/R = 3 N S/I Ch. Cap per Cell

3 3 -11 1384 3.46 -13 1047 4.58 -18 599 5.2 -20 4612 6 -23 34


Co-Channel Interference and System CapacityCo-Channel Interference and System Capacity Relationship between Co-Channel Reuse Ratio Q and SIR

SIR = (Q)n/Number of Interfering Cells in the First Tier, n

is the Path Loss Exponent (2-5 depending on the type of area)

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Adjacent Cell Interference and Adjacent Cell Interference and System CapacitySystem Capacity

Adjacent-Channel Interference [ACI]:Adjacent-Channel Interference [ACI]: An Interference Arising from Energy Spill-Over between Two Adjacent Channels

ACI results from the Imperfect behavior of the Rx Filters Allowing nearby Frequencies to Leak into the Pass-band

ACI can be minimized through careful filter design and Channel Assignment [ by keeping the Inter-Channel Frequency Difference as large as Possible]

Some Channel Assignment Schemes keeps this Difference in a cell by at least N Channel Bandwidths, where N is the Cluster Size

A1,8,15,22

F6,13,20,2

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Trunking and Grade of ServiceTrunking and Grade of Service Trunking Theory

Developed by a Danish Mathematician, A.K. Earling in the Late 19th Century

It helps in Establishing a Trunked System which can Provide Communication Services to a Large Group of Users with Limited Number of Available Circuits / Channels in the System [based on a certain GOS]

A Sharing Concept: Large Number of Users Share a Small Number of Channels in a Cell/System

Based on a Statistical Behavior of Users All PSTN/Cellular Radio Systems Exploit Trunking to Cover a

Large User Community with their Limited Number of Circuits / Frequency Spectrum


Trunking and Grade of ServiceTrunking and Grade of Service Trunking Terminology

Call Set up Time: Time Required to Allocate a Trunked Radio Channel to a Requesting User

Blocked/Lost Call: A Call that can not be Completed at the Time of the Request, due to Congestion

Holding Time: Average Duration of a typical Call, denoted by H

Request Rate: Average Number of Call Requests/Unit Time [denoted by ]

Traffic Intensity : Average Channel Occupancy, measured in Earlings

A Channel that Remains Occupied Cent/Cent [All the Time] is said to Carry a Traffic of One Earling. For Example, if a Channel remained busy for 1 hr/hr or 1 min/min , its Traffic Intensity is 1 Earling. And, a Channel that remained under use for 30 mins in an hr, its Traffic Intensity is 0.5 Earling [Denoted by A]


Trunking and Grade of ServiceTrunking and Grade of Service Trunking Terminology – Cont’dTrunking Terminology – Cont’d

Load: The System-wide Traffic Intensity The System-wide Traffic Intensity

Grade of Service (GOS): A Measure of Congestion, in terms A Measure of Congestion, in terms of Probability of Call being Blocking or Call being Delayed [ 0 < of Probability of Call being Blocking or Call being Delayed [ 0 < GOS < 1]GOS < 1]

Traffic Intensity Offered by a User: Au = H (in Earlings)

Total System Traffic / Load = A = Au, U, where U is the total number of System Users

Traffic Intensity per Channel: If C is the Total number of Channels in a System, and given Load A, then the Traffic Intensity / Channel, Ac = A/C

Maximum System Traffic Capacity: Equal to the number of Available Channels [in Earlings]

POISSON DISTRIBUTION:A Statistical Process that Applies to a Sequence of Events which take place at regular intervals of time or throughout a continuous interval of time. It has got many Applications such as number of customers arriving at a Gasoline Station, Number of Air Planes arriving at an Airport, or Number of Phone Calls arriving at a Switch [MTX]. Let C be total # of Trunks [Channels], and A be the Offered

Traffic in Earlings, then the Probability of all the C Channels are busy or in other words Probability of Blocking is defined by the following Poisson Distribution;

P(C;A) = P(Blocking) =AC e

-A Where e

-A = 1/C

kA

k = GOS

C! k!

POISSON DISTRIBUTION:A Statistical Process that Applies to a Sequence of Events which take place at regular intervals of time or throughout a continuous interval of time. It has got many Applications such as number of customers arriving at a Gasoline Station, Number of Air Planes arriving at an Airport, or Number of Phone Calls arriving at a Switch [MTX]. Let C be total # of Trunks [Channels], and A be the Offered

Traffic in Earlings, then the Probability of all the C Channels are busy or in other words Probability of Blocking is defined by the following Poisson Distribution;

P(C;A) = P(Blocking) =AC e

-A Where e

-A = 1/C

kA

k = GOS

C! k!


Trunking and Grade of ServiceTrunking and Grade of ServiceTrunking SystemsTrunking Systems

Blocked Calls Cleared (BCC) Trunking Systems: Blocked Calls Cleared (BCC) Trunking Systems: Any Call Request at any time will be Served Immediately if

Some Free Channel is Available in the PoolA Call will be Blocked [and Lost] if no Free Channel is

Available at the Time of RequestThe Blocked User is Free to Try Again Later at any Time.

Assumptions:Assumptions: Calls Arrive According to Poisson Distribution


Trunking and Grade of ServiceTrunking and Grade of ServiceTrunking Systems-Cont’d

Blocked Calls Cleared (BCC) Trunking Systems Assumptions[Continued]:

There is Infinite Number of Users*Memoryless Arrivals of Requests -> Any User, including the blocked

one, can make a request at any timeProbability of a User Occupying a Channel is Exponentially

Distributed -> Longer Calls are Less likely to OccurFinite Number of Available Channels in the Pool

Such a System is called an Earling B System which is governed by Earling B Formula [EBF], as given in the Equation 3.16 in the book.

* Practical Trunking Radio Systems always have finite number of users, however, it is typical that the number of Users in a system always outnumber the available channels by orders of magnitudes. So, EBF, gives us a modest measure of GOS as the actual systems, where users are finite, will face less chances of a call blockage


Trunking and Grade of ServiceTrunking and Grade of ServiceTrunking Systems

Blocked Calls Cleared (BCC) Trunking Systems-Cont’d

Table-1: Capacity of an Earling B System

No. of Channels 0.02 0.01 0.005 0.002 0.001

2 0.223 0.153 0.105 0.065 0.0464 1.09 0.869 0.701 0.535 0.4395 1.66 1.36 1.13 0.9 0.762

10 5.08 4.46 3.96 3.43 3.0920 13.2 12 11.1 10.1 9.4124 16.6 15.3 14.2 13 12.240 31 29 27.3 25.7 24.570 59.1 56.1 53.7 51 49.2

100 88 84.1 80.9 77.4 75.2

System Capacity [Earlings] for GOS



Blocked Calls Cleared (BCC) Trunking Systems-Cont’d

EXAMPLE-01: Consider a small Cellular radio System of 4 Channels. There are all together 20 Subscribers and each Subscriber is expected to generate a traffic of 0.1 Earling. Determine the Probability of Blockage [That at any Time all the 4 channels get Busy.

SOLUTION: C = 4, U = 20, and Au = 0.1. Now, A = 0.1 * 20 = 2 Earlings

P(4, 2) = 24 * e-2 = 0.094!

EXAMPLE-02: GOS required is 0.02, and it is expected that 1000 calls are generated per hr with avg. call duration of 2 min., calculate the total number of channels required by the system per cell.

SOLUTION: GOS = 0.02, and Total Traffic per cell = 1000 x 2/60 = 33 Earling

Looking at the Table, with this GOS, we see we require 42 Channels per Cell.



Blocked Calls Delayed (BCD) Trunking SystemsBlocked Calls are Provided with a Queue to hold the Call Request

unless a Channel becomes Available.GOS of BCD Trunking Radio System is defined as a Probability that a

Call is blocked and delayed for a time longer than t seconds.

P(C;A, delay > t sec) = P(C;A, delay > 0)*P(C;A, delay >t sec | delay >0)

= P(C;A, delay > 0)*exp(-(C-A)t /H)

Average Delay D for all the Calls in the System is given by

D = P(C;A, delay >0) * H/(C-A)Earling C Chart is used to study the Relationship between various

parameters such as Traffic Intensity A, C, and P(C;A,delay>0)

Trunking Efficiency [TE]Trunking Efficiency (%) = [Traffic Intensity (A) / C] * 100

EXAMPLE-03: C = 48, GOS = 0.02, we Calculate A = 38.4 Earlings[Using Earling B Chart], now TE = [38.4/48]*100 = 80 %


Trunking and Grade of ServiceTrunking and Grade of Service Earling B Chart - Earling B Chart - Gives Relationships among GOS, C, and A for BCC Trunking Radio Systems


Trunking and Grade of ServiceTrunking and Grade of Service Earling C Chart - Earling C Chart - Gives Relationships among GOS, C, and A for BCD Trunking Radio Systems


Trunking and Grade of ServiceTrunking and Grade of Service


Improving Coverage and System Improving Coverage and System CapacityCapacity Review: What is System Capacity?Review: What is System Capacity?

C = M C = M kk N, N, Where Where NN is the Cluster Size, is the Cluster Size, kk is the number of is the number of Channels used per Cell, and Channels used per Cell, and MM is the Replications of the the is the Replications of the the Cluster in the given System Coverage Area.Cluster in the given System Coverage Area.In terms of Traffic Intensity, we know that total Traffic handled In terms of Traffic Intensity, we know that total Traffic handled by a System is by a System is A = U AuA = U Au, , UU is the number of users in the is the number of users in the System, and System, and AuAu is the traffic generated by a typical user. Max. is the traffic generated by a typical user. Max. Value that Value that AA can assume is can assume is CC [Upper Limit of the System]. [Upper Limit of the System].From From A = U AuA = U Au, we can easily Conclude that , we can easily Conclude that as as U increases, U increases, A will increaseA will increase, , so the Traffic Offered to the System will Increaseso the Traffic Offered to the System will Increase Leading to CongestionLeading to Congestion [Blockage of the Calls].[Blockage of the Calls].Given an Allocated Spectrum [Given an Allocated Spectrum [S = S = kk N N ] ] which is Fixed, We which is Fixed, We have to use some Cellular Design Techniques to Improve System have to use some Cellular Design Techniques to Improve System Capacity [ C or A as A is a Function of C].Capacity [ C or A as A is a Function of C].


Improving Coverage and System Improving Coverage and System CapacityCapacityCellular Design Techniques to Improve System Cellular Design Techniques to Improve System CapacityCapacity

Cell Splitting Technique:Cell Splitting Technique: This Technique Improves the System Capacity by Reducing This Technique Improves the System Capacity by Reducing the Cluster Coverage Area [ in other words Cell Area] to the Cluster Coverage Area [ in other words Cell Area] to Increase M, keeping Cluster Size N and Co-Channel Reuse Increase M, keeping Cluster Size N and Co-Channel Reuse Ratio, Q = D/R = SQRT(3N) Constant. It Maintains S/I by Ratio, Q = D/R = SQRT(3N) Constant. It Maintains S/I by Reducing the Base Station Tx Power, Antenna Height, and Reducing the Base Station Tx Power, Antenna Height, and Antenna Down-Tilting Mechanism.Antenna Down-Tilting Mechanism.

Sectoring Technique:Sectoring Technique:This Technique Improves the system Capacity by Reducing This Technique Improves the system Capacity by Reducing the Cluster Size, N, to get High value of M. Since it plays with the Cluster Size, N, to get High value of M. Since it plays with N, thus, Changing Q and S/I. In order to use same Cell Size N, thus, Changing Q and S/I. In order to use same Cell Size and Tx Power, it has to use Cell Sectoring to Avoid Co-and Tx Power, it has to use Cell Sectoring to Avoid Co-Channel Interference by using Directional Antennas. It Channel Interference by using Directional Antennas. It Improves System Capacity and S/I but at the Cost of Improves System Capacity and S/I but at the Cost of Decreased System Trunking Efficiency.Decreased System Trunking Efficiency.

Microcell Zone Technique:Microcell Zone Technique: This is the Latest Technique which Improve the System This is the Latest Technique which Improve the System Capacity and S/I without Compromising at System Trunking Capacity and S/I without Compromising at System Trunking Efficiency.Efficiency.


Improving Coverage and System Improving Coverage and System CapacityCapacity Cell Splitting TechniqueCell Splitting Technique

Any Congested Cell is Splitted Any Congested Cell is Splitted

into several Smaller Cells called into several Smaller Cells called

uCells.uCells. Cell [uCell] Coverage Area is Cell [uCell] Coverage Area is

Reduced -> Cluster Area is Reduced -> Cluster Area is

Reduced that in turn Increase M, Reduced that in turn Increase M,

so Improvement in System so Improvement in System

Capacity is Achieved.Capacity is Achieved. Placement of the uCells is Made Placement of the uCells is Made

such that it Maintains the System such that it Maintains the System

Frequency Reuse Structure/Plan. Frequency Reuse Structure/Plan.

Q and S/I is Maintained byQ and S/I is Maintained by Reduction in Tx Power, Antenna Reduction in Tx Power, Antenna Height, and Using Antenna Down-Height, and Using Antenna Down-Tilting Technique.Tilting Technique.

A

B

C

D

E

F

G

C

E

D

F

E

GF

B

G

C

D

FE

GB

C

D


Improving Coverage and System Improving Coverage and System CapacityCapacity Cell Splitting TechniqueCell Splitting TechniqueHow much Reduction of Tx Power?How much Reduction of Tx Power?

PPr r (old Cell Boundary) (old Cell Boundary) P Pt1 t1 RR-n-n

PPr r (new Cell Boundary) (new Cell Boundary) P Pt2 t2 (R/2)(R/2)-n-n

Equating them, we get PEquating them, we get Pt2 t2 = P = Pt1 t1 / 2/ 2nn

For Urban Environment, n = 4,For Urban Environment, n = 4,

so. Pso. Pt2 t2 = P = Pt1 t1 / 16 or 12 dB down./ 16 or 12 dB down.

A

B

C

D

E

F

G

C

E

D

F

E

GF

B

G

C

D

FE

GB

C

D


Improving Coverage and System Improving Coverage and System CapacityCapacitySystem Growth in Cell SplittingSystem Growth in Cell SplittingIn the beginning, only the Congested Cells are Splitted.In the beginning, only the Congested Cells are Splitted.Different Cell Sizes in Use Demands for Different Different Cell Sizes in Use Demands for Different

Frequency/Channel Groups and Tx Power/ Antenna Height.Frequency/Channel Groups and Tx Power/ Antenna Height.Size of Channel Groups Depends on the Splitting Stage.Size of Channel Groups Depends on the Splitting Stage.Bigger Cells are used for High Speed TrafficBigger Cells are used for High Speed TrafficAs the Demand Increases in Micro-cells and With More Cells As the Demand Increases in Micro-cells and With More Cells

get Splitted, , uCell Channel Group is made Relatively get Splitted, , uCell Channel Group is made Relatively

Bigger.Bigger.As Splitting gets Completed, all Cells are of the same Size As Splitting gets Completed, all Cells are of the same Size

and Use the Entire Frequency Spectrum according to the and Use the Entire Frequency Spectrum according to the

Frequency Reuse Plan. Frequency Reuse Plan.


Improving Coverage and System Improving Coverage and System CapacityCapacityCell Splitting Technique:Cell Splitting Technique:


Improving Coverage and System Improving Coverage and System CapacityCapacitySectoring Technique:Sectoring Technique:

Improves the System Capacity by Improves the System Capacity by

Reducing the Cluster Size, Reducing the Cluster Size,

N(keeping the Cell Size same), to N(keeping the Cell Size same), to

get more Cell Cluster Replications.get more Cell Cluster Replications.As N is decreased, Q = D/R = As N is decreased, Q = D/R =

SQRT(3N), is also decreased. SQRT(3N), is also decreased. As the Cell Size is kept the same, the Tx As the Cell Size is kept the same, the Tx

Power of the BS is to be kept the same. Power of the BS is to be kept the same.

That will Increase the Chances of Co- That will Increase the Chances of Co-

Channel Interference. Channel Interference. To Improve S/I (by reducing I), we use To Improve S/I (by reducing I), we use

Cell Sectoring and Use Directional Cell Sectoring and Use Directional

Antennas.Antennas.120 Degree Sectoring [3 Freq. Groups]120 Degree Sectoring [3 Freq. Groups]60 Degree Sectoring [6 Freq. Groups]60 Degree Sectoring [6 Freq. Groups]

3-1

3-2

3-3

1-1

1-21-3

4-1

4-2

4-3

2-1

2-2

2-3

3-1

3-2

3-3

1-1

1-21-3

4-1

4-2

4-3

2-1

2-2

2-3

12

345

6


Improving Coverage and System Improving Coverage and System CapacityCapacitySectoring Technique:Sectoring Technique:

Each Cell in the Cluster is Divided into Each Cell in the Cluster is Divided into

3/6 Sectors3/6 SectorsEach Sector uses a Smaller Group of Each Sector uses a Smaller Group of

ChannelsChannelsReduced Trunking EfficiencyReduced Trunking EfficiencyWith Sectoring and Use of Directional With Sectoring and Use of Directional

Antennas, Interference is Reduced.Antennas, Interference is Reduced.S/I is Improved.S/I is Improved.Number of Handoff is IncreasedNumber of Handoff is IncreasedMany Modern Systems, Within a Cell, Many Modern Systems, Within a Cell,

Inter-Sector Handoff are dealt by the Inter-Sector Handoff are dealt by the

Mobile sets without Involving MSC.Mobile sets without Involving MSC.In Short, Increase in System Capacity In Short, Increase in System Capacity

and S/I is Achieved at the Cost of and S/I is Achieved at the Cost of

Reduction of Trunking Efficiency.Reduction of Trunking Efficiency.


Improving Coverage and System Improving Coverage and System CapacityCapacitySectoring Technique:


Improving Coverage and System Improving Coverage and System CapacityCapacityMicrocell Zone Technique:Microcell Zone Technique:

Improves the System Capacity as Improves the System Capacity as

well as S/I ratio without Sacrificing well as S/I ratio without Sacrificing

the Trunking Efficiency of the the Trunking Efficiency of the

System.System.

All Microcell Zones [3 or more in a All Microcell Zones [3 or more in a

Cell] use a Single Base Station but Cell] use a Single Base Station but

Different Tx/Rx Equipment with Different Tx/Rx Equipment with

Reduced Tx Power.Reduced Tx Power.All the Channels are Placed in a Pool All the Channels are Placed in a Pool

at the BS and Equally Shared by all at the BS and Equally Shared by all

uCell Zones [ No subdivision of uCell Zones [ No subdivision of

Channels as against in Sectoring]Channels as against in Sectoring]The Antennas of Each Zone Tx/Rx The Antennas of Each Zone Tx/Rx

are Directed inwards and Placed at are Directed inwards and Placed at

the Outer Edge of the Cell.the Outer Edge of the Cell.Cell Maintains its Coverage Area, Co-Cell Maintains its Coverage Area, Co-

Channel Interference is Reduced.Channel Interference is Reduced.

cellular mobile communications-ii the cellular concept-system design fundamentals dr. nasir d. gohar...

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cellular base stations

painsgreater system

service area

allotted spectrum s

given value of s

large batterieslimited

distinct channel groups

allocation of channel