Mobile Concepts

8/3/2019 Mobile Concepts

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Dr. D. Pesch, Electronics Eng., CIT 2000

Mobile TelecommunicationMobile Telecommunication

ConceptsConcepts

Telecommunications

MSc in Software Development


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Key Characteristics of MobileKey Characteristics of Mobile

Communication SystemsCommunication Systems

Limited frequency spectrum

results in limited number of radio channel, which causes

limited teletraffic capacity

requires cellular architecture and frequency reuse

Mobility of terminals and subscribers

creates hostile radio propagation environment

requires handover of radio connections in cellular systems

complicates call setup and routing

requires enhanced security features to acertain identity of

terminal and subscriber


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Mobile Radio PropagationMobile Radio Propagation

Radio signals propagate according to three

mechanisms

reflection

diffraction

scattering

Result of the three mechanisms are three nearly

independent phenomena path loss

shadowing

multipath fading


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The Cellular Concept and Frequency ReuseThe Cellular Concept and Frequency Reuse

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Macrocell system using

a 7-cell reuse pattern

Generally, a tessellated

reuse pattern of size N

can be constructed if

N=i2+ij+j2

where i and j are non-

negative integers and

i j.

Possible cluster sizes are

N = 1, 3, 4, 5, 7, 9, 12, ...


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Cellular Network ArchitectureCellular Network Architecture

PSTN

ISDN

MS

BTS

VLR HLR

VLRMSC

GMSC

Cellular system

MSC


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Interference and NoiseInterference and Noise

Interference

Co-channel interference

Adjacent channel interference

Noise

Thermal noise

Man-made noise, e.g. ignition noise, etc.

Threshold effect - probability of outage

carrier-to-interference ratio less threshold, e.g. carrier-to-noise ratio less threshold

Two parameters affect interference

transmitter power and co-channel reuse factorD/R

{ }th


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Channel AllocationChannel Allocation

Channel allocation is performed to

allocate a radio channel to a new call request

allocate a radio channel to a handover request (inter- or intracell)

Classification of channel allocation techniques

Basic

Borrowing

Hybrid

Fixed

Flexible

Dynamic

Scheduled

Predictive

Centralised

Decentralised

Distributed


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Location Management and TrackingLocation Management and Tracking

Location management determines the location of a

mobile station for

call delivery

emergency call position locating

Two granularities of location

based on location area based on absolute position with a rms error of below 125m


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Spectral Efficiency and Grade of ServiceSpectral Efficiency and Grade of Service

Spectral efficiency Erlangs/km2/Hz

Gc offered traffic per channel (Erlangs/channel)

Nc number of radio channels per cell

Wsys total system bandwidth

A area per cell (km2)

Grade of Service (GoS) is usually defined in terms of new call blocking

and handover dropping probability

Pb new call blocking probability

Pd handover dropping probability (due to no free radio channels)

The best grade of service would be GoS = 0.

AW

GN

sys

CCS

=

db PPGoS += 10


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Radio PropagationRadio Propagation

Radio Signals propagate according to three

mechanisms

Reflection

Diffraction

Scattering


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Radio PropagationRadio Propagation

reflection

direct

path

transmitter

receiver

diffraction

scattering


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Mobile Radio PropagationMobile Radio Propagation

Three mechanisms characterise mobile radio

propagation

Path Loss

Shadowing

multipath fading


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Path LossPath Loss

Free Space Path Loss

PtGt effective isotropically radiated power (EIRP)

Calculation of the path loss in typical environments

can be based on

Empirical

Diffraction

or Ray Tracing models

2

crttr

d4GGPP

=


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Example Path Loss According to OkumuraExample Path Loss According to Okumura--

HataHata ModelModelPath loss in an urban environment of a small for medium size city such as Cork

fc = 900MHz

hb = 30m

hm = 1.5m

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Distance in km

Path

LossindB


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Example Path Loss According to COST231Example Path Loss According to COST231

WalfishWalfish--Ikegami Model in LOS DirectionIkegami Model in LOS Direction

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0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Pathlossin

dB

Distance in km

fc = 900MHz


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MultipathMultipath FadingFading

Multipath or fast fading

constructive and destructive addition of incident wave components

Doppler shift due to moving receiver

Causes short fades, which can be up to 30 - 40dB, in the

received signal envelope

Major cause for transmission errors in mobile radio systems

Three main types of multi-path fading (according to their

statistical distribution) Rayleigh fading

Ricean fading

Nakagami fading (only of statistical significance)


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MultipathMultipath FadingFading


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Cellular Concept and Frequency ReuseCellular Concept and Frequency Reuse


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Cellular GeometryCellular Geometry


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3-Cell 4-Cell 7-Cell

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9-Cell

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12-Cell

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KK--cell reuse clusterscell reuse clusters


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Cluster Size K = 3 Reuse PatternCluster Size K = 3 Reuse Pattern


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0

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4 10 20 50 100 200 500 1000

Channels per Cell

TeletrafficCapacity[Erlang]


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Cell SectoringCell Sectoring -- 120120oo SectorsSectors


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Cell SectoringCell Sectoring -- 6060oo SectorsSectors

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Channel Assignment SchemesChannel Assignment Schemes

Permanent FCA

FCA with fractional loading

FCA with reuse partitioning

FCA with channel borrowing

Centralised DCA

Decentralised DCA

Distributed DCA

Hybrid Channel Allocation


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CDF of CIR for permanent FCACDF of CIR for permanent FCA


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CDF of CIR with fractional loading andCDF of CIR with fractional loading and

several loading factorsseveral loading factors


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FCA and Channel BorrowingFCA and Channel Borrowing

Radio channels are assigned in a permanent fashion to

cells

If a cell experiences congestion it tries to borrow a

channel from a neighbour cell temporarily

Borrowed channels are held until the state of

congestion ceases

Borrowed channels may be barred from use in co-channel cells of the cell they were borrowed from

Channel borrowing requires communication between

cell sites


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Centralised DCACentralised DCA

Channel assignments to cells made upon request by a

central controller in the cellular network

Each cell requires communication link to central

controller

If channel request cannot be served central controller

may reshuffle assignment of channels to cells in order

to make channel available Channel assignments can be made based on co-

channel reuse distance approach or actual co-channel

interference situation


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Decentralised DCADecentralised DCA

Problem with central control is high signalling load

between cell sites and central controller

Decentralised control moves assignment control to a

group of cells or a cluster and thus reduces load in the

network

Requires less signalling load as information is shared

among group of cells Does not achieve optimum capacity in non-uniform

cases but provides feasible implementation


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Distributed DCADistributed DCA

Each cell (BS) decides which channel to allocate basedon the current co-channel interference on each channel

Requires continuous monitoring of co-channelinterference on each channel

Achieves excellent performance as it does not requireany coordination among cell sites and is based oncurrent co-channel interference situation rather than

average co-channel interference Implemeted in cordless telecommunication systems

such as DECT and PHS and also in wireless LANsystems


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Concept of Distributed DCAConcept of Distributed DCA


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Other Channel Assignment ConceptsOther Channel Assignment Concepts

Directed Retry - upon blocking call is directed to try

again at neighbour BS

Queuing based Channel Assignment - Call attempt is

queued until channel becomes available or timer

expires

Channel allocation with handover priority


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Aspects of Handover DesignAspects of Handover Design

Statistical evaluation of the received signal strength

(RSS) in order to determine right moment for

handover

Selection of target cell that will provide best new link

quality

Avoidance of the ping-pong effect

Radio resource reservation in target cell Hard and soft handover


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Handover Performance MetricsHandover Performance Metrics

Call blocking probability

Handover blocking probability

Handover probability

Call dropping probability (handover failure)

Probability of unnessecary handover

Rate of handover

Duration of interruption (hard handover)

Handover decision delay


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Handover AlgorithmsHandover Algorithms

Network Controlled Handover

Mobile Assisted Handover

Mobile Controlled Handover


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Handover InitiationHandover Initiation

Relative signal strength

Relative signal strength with threshold

Relative signal strength with hysteresis

Relative signal strength with hysteresis and threshold

Prediction techniques


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T1

T2

T3

Signal strength

from BS 1

Signal strength

from BS 2

BS 1 BS 2

HandoverHandover


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Handover ExecutionHandover Execution

Backward handover

Forward handover

Hard handover

Soft handover


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Power ControlPower Control

Constant received power control

Quality based power control


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LA 1

LA 2

LA 3

VLRMSC

The Concept of a Location AreaThe Concept of a Location Area


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Time Slot

Frequency Carrier

Frequency HoppingFrequency Hopping


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Site A

Site B

Hill

Hill

Motorway

Service area

of site B

Service area

of site A

Shadow section

Macro DiversityMacro Diversity

Mobile Concepts

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