cellullar concept & design fundamentals waheed

5/22/2018 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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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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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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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




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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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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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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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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

cellullar concept & design fundamentals waheed

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