PRACTICAL HANDOFF CONSIDERATION

Practical handoff considerations

AJAL.A.J Assistant Professor –Dept of ECE,

(FISAT) TM

MAIL: [email protected]

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A Taxonomy By Forms Of Energy

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Taxonomy is the practice and science of classification.

Why Mobile Communication ?

People now a days are demanding instant Communication, at the same time they do not want to stick to their Desk or Office.

What they want is, Communication at any place & at any time, even while they are on move.

DemonstrationDemonstration

Cellular Wireless NetworkCellular Wireless Network

Multi-Agency Seamless Mobility

TrunkingTrunking

Trunking: the channel is allocated on demand and recycle after usage

Tradeoff between the number of channels and blocking probability

Grade of service– Likelihood of a call is blocked or the delay greater

than a threshold during the busiest time.

Trunking theory– Erlang, a Danish Mathematician studied how a large

population could be accommodated by a limited number of servers.

– Erlang capacity: the percentage of line/channel occupied over time


Practical handoff

considerations / Problem

in µcell system

►Umbrella cell

►Cell dragging

Umbrella cell per spot beam

architectureto achieve a Single Frequency

Network outdoor & indoor

Terminal’s rake receiver combines the Satellite & terrestrial repeaters signalsas echos of the same signal

Coveragehole

Coveragehole

Terrestrial repeaters: gap filler coveragesame carrier, same codes

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Practical handoffs consideration

Several problems arise to design a system for wide range of mobile velocities► High speed vehicles pass

through a cell in a matter of seconds• With micro cells addition, the MSC

can quickly become burdened

► Pedestrian users may never need a handoff during a call

► Issues• Schemes to handle high speed and

low speed users simultaneously• Ability to obtain new cell sites

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Additional capacity is provided through addition of new cell sites,

Difficult to obtain new cell sites Install additional channels and BS

at same location of an existing cell By using different antenna heights

and power levels, possible to provide large and small cells, which are co-located at single location called umbrella cell

► Provide large coverage area to high speed users minimizing number of handoffs

► Small coverage to slow speed users ► Speed can be estimated by BS or MSC

by RSSI

CUST ME MER

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@ holistic approach

This is Mobile Broadband

Everywhere coverageThis is Mobile Broadband

This is Mobile Broadband

Full mobility

High data speedsThis is Mobile Broadband

Confidential Inform

ation

WiMAX – Anytime, AnywhereWiMAX – Anytime, Anywhere

EnterpriEnterprisese

SOHOSOHO ResidentResidentialial

MobileMobile

InternInternet et

AcceAccessss

VoicVoicee

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Handoff :• Cellular system tracks mobile stations in order to maintain their communication links.

• When mobile station goes to neighbor cell, communication link switches from current cell to the neighbor cell.

Hard Handoff :• In FDMA or TDMA cellular system, new communication establishes after breaking current communication at the moment doing handoff. Communication between MS and BS breaks at the moment switching frequency or time slot.

Hard handoff : connect (new cell B) after break (old cell A)

switching

Cell B Cell A

Handoff (1/2)

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Σ

Cell B Cell

A

Soft handoff : break (old cell A) after connect (new cell B)

transmitting same signal from both BS A and BS B

simultaneously to the MS

Soft Handoff :• In CDMA cellular system, communication does not break even at the moment doing handoff, because switching frequency or time slot is not required.

Soft Handoff (2/2)

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Mobility/Handoff in Umbrella Cells

Avoids multiple handoffs.

Types of cells

Depending on density of population, various types of cells are used.

Macrocells Microcells Selective cells Umbrella cells

Selective cells

It is not always useful to define a cell with a full coverage of 360 degrees.

In some cases, cells with a particular shape and coverage are needed. These cells are called selective cells.

A typical example of selective cells are the cells that may be located at the entrances of tunnels where a coverage of 360 degrees is not needed. In this case, a selective cell with a coverage of 120 degrees is used.

NEED OF THE HOUR

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umbrella cell

Umbrella cells

An umbrella cell covers several microcells.

The power level inside an umbrella cell is increased comparing to the power levels used in the microcells that form the umbrella cell.

A freeway crossing very small cells produces an important number of handovers among the different small neighboring cells. In order to solve this problem, the concept of umbrella cells is introduced. When the speed of the mobile is too high, the mobile is handed off to the umbrella cell. The mobile will then stay longer in the umbrella cell. This will reduce the number of handovers and the work of the network.

Umbrella cell - Defn

By using

1.Different Antenna Heights (often on same building /

tower) &2. Different power levels

It is possible to provide large and small cells which are co-located at a single location.

This technique is called umbrella cell approach

Umbrella cells - advantages

used to provide :1. large area coverage to

high speed users

2.small area coverage to

low speed users

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Smaller Cells: Macro-cell vs Micro-cell vs Pico-cell vs Femto-cell/Wifi offload

BS

2-3 km

BS

500m

BS

200m

BS

50m

Pico-cell: enterprise-operated: indoor

Macro-cell

Micro-cell

Femto-cell: home/SOHO operated: indoor

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What does this approach ensures !

Umbrella cell approach ensures that

1. The No: of handoffs is minimized for

High speed users

&2.Provides additional microcell channels

for

pedestrian users

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Break Time For queries –

10 minutes

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•Using different antenna heights and different power levels it is possible to provide large and small cells which are co-located at a single location. This technique is called umbrella cell approach and is used to provide large area coverage to high speed users while providing small area coverage to users traveling at low speeds.

• The umbrella cell approach ensures that the number of handoffs in minimized for high speed users and provides additional microcell channels for pedestrian users.

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•Another practical handoff problem in microcell systems is known as

cell dragging. Cell

dragging results from pedestrian users that provide a very strong signal to the base station. The signal strength does not decay rapidly as the user travels away from the base station at a very low speed.

•The IS-95 code division multiple access (CMDA) spread spectrumCellular system provides a unique handoff capability that cannot be provided with other wireless systems.unlike channelized wireless systems that assign different radio channels during a handoff, spread spectrum mobiles share the same channel in every cell.

Leveraging the Ubiquity of Wireless

Cell dragging

Cell dragging

BS

LOS

scenario

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Cell dragging

Cell dragging► Problem in micro-cell due to high

signal strength of pedestrian users.► Occurs in urban areas when there

is a LOS path► Average signal strength does

not decay rapidly even if a user travels well beyond the designed range of cell

► The RSSI may be above the handoff threshold and thus handoff is not made

► This creates potential interference since a user has traveled deep within a neighboring cell and creates Traffic management problem

How to solve Cell Dragging Problem ?

► Handoff parameters, threshold must be adjusted carefully

ie :

1. Handoff Thresholds2. Radio Coverage Parameters

Must be adjusted carefully

Cell dragging

If there is line of sight path between MS and BS, even when the user has travelled well beyond the designed range of the cell, the received signal at the base station may be above the handoff threshold, thus a handoff may not be made. This creates a potential interference and traffic management problem, since the user has meanwhile travelled deep within a neighboring cell.

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@ IS-95 (CDMA) system

In IS-95 (CDMA) system► Provides unique handoff capability

that can not be provided in with other wireless systems

► Unlike channelized (hard handoff), SS mobiles share the same channel in every cell.

► Thus handoff does not assign channel but a different BS handles a communication task

► By simultaneously evaluating RSSI from single user, MSC decides which version of the signal is best

► This ability selects between instantaneous received signals from a variety of BS is called

soft handoff

CONCLUSIONPractical handoff considerations

There are several problems due to a wide range of mobile velocities.

The umbrella cell umbrella cell approach:► Different antenna heights and different

power levels.► “large” and “small” cells► Used to provide large are coverage to

high speed users while providing small are coverage to users traveling at low speed.

Cell draggingCell dragging problem in microcell systems.

► Results from pedestrians that provide a very strong signal in uplink (e.g., in urban environment when there is a LOS between the subscriber and the BS).

► Necessary handoff may not be made.► For good solution, handoff thresholds

must be adjusted carefully.

Prioritizing handoff – next session

Dropping a call is more annoying than line busy

Guard channel concept► Reserve some channels for handoffs► Waste of bandwidth► But can be dynamically predicted

Queuing of handoff requests► There is a gap between time for

handoff and time to drop.► Better tradeoff between dropping call

probability and network traffic.

Reduce the burden for handoff► Cell dragging ► Umbrella cell

Thanks for your attention

Education

PRACTICAL HANDOFF CONSIDERATION