Rab Nawaz Jadoon · disconnected, and a radio link in the new base station is required to continue the conversation. ... The signaling during a handoff causes a delay in the transfer

Department of Computer Science

DCS

COMSATS Institute of

Information Technology

Rab Nawaz JadoonAssistant ProfessorCOMSATS IIT, Abbottabad

Pakistan

Mobile Communication


Mobility

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Movement ORChanging position


Mobility in Cellular Networks▲ Mobility management is one of the major

functions of a Mobile network that allows mobile phones to work.

The aim of mobility management is to track where the subscribers are, allowing calls, SMS and other mobile phone services to be delivered to them.

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Aspects of Mobility

▲ Two aspects of mobility characterize a cellular telephony network

Handoff (Small Scale Mobility)

Roaming (Large Scale Mobility)

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Handoff (Small Scale Mobility)

▲ When a mobile user is in a conversation, a radio link connects the mobile phone to a base station.

If that user moves to another base station’s coverage area, the radio link to the old base station is disconnected, and a radio link in the new base station is required to continue the conversation.

This process is called automatic link transfer or handoff.

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Roaming (Large Scale Mobility)

▲ Roaming helps ensure that a traveling wireless device (typically a cell phone) is kept connected to a network without breaking the connection.

when travelling outside the geographical coverage area of the home network, by means of using a visited network.

For example; should you travel beyond your cell phone company's transmitter range, your cell phone would automatically hop onto another phone company's service, if available. Using another phone company's service can be done by using the

subscriber identity in the visited network.

Roaming is technically supported by mobility management, authentication, authorization and accounting billing procedures (known as AAA or 'triple A').

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Handoff▲ When a user moves from the coverage area of

one BS to the adjacent one, a handoff has to be executed to continue the call.

There are two main parts to the handoff procedure:

The first is to find an uplink-downlink channel pair from the new cell to carry on the call, and

The second is to drop the link from the first BS.

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Types of Handoffs

▲ Two Basic types,

Hard Handoff

Break-before-make

Soft Handoff

Make-before-break

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Issues Involved in Handoffs

▲ Optimal BS Selection

The BS nearest to an MT may not necessarily be the best in terms of signal strength.

Especially on the cell boundaries, it is very difficult to clearlydecide to which BS the MT must be assigned.

▲ Ping-pong effect:

If the handoff strategy had very strictly demarcated boundaries, there could be a series of handoffs between the two BSs whose cells touch each other.

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Cont…

▲ Data loss:

The interruption due to handoff may cause a loss in data.

While the delay between relinquishing the channel in the old cell, and resuming the call in the new cell, may be acceptable for a voice call, it may cause a loss of few bits of data.

▲ Detection of handoff requirement:

Mobile-Initiated Handoff

Handoffs may be mobile-initiated, in whichcase the MT monitors the signal strength received from the BS and requests a handoff when the signal strength drops below a threshold.

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Cont…

Network Initiated Handoff

In the case of network-initiated handoff, the BS forces a handoff if the signals from an MT weaken.

The BS inquires from all its neighboring BSs about the signal strength they receive from the particular MT, and deduces to which BS the call should be handed over.

The mobile-assisted scheme is a combination of the network and mobile-initiated schemes.

It considers the evaluation of signal strength from the mobile, but the final handoff decision is made by the BS.

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Handoff Quality▲ Some of the measures of handoff quality are as

follows:

Handoff Delay

The signaling during a handoff causes a delay in the transfer of an on-going call from the current cell to the new cell.

If the delay is too large, the signal may fall below the minimum carrier to interference ratio (C/I) required for continuation of the call, and the call may get dropped.

The handoff protocol should aim to minimize this delay.

Duration of Interruption

the channel pair from the current BS is canceled, and then the channel pair from the next BS is used to continue the call. This can cause an interruption in the call.

This interruption should be minimized.

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Cont…Handoff Success

The probability of a successful handoff, that is, continuation of the call when a mobile user crosses a cell boundary, is called the handoff success rate.

The handoff strategies should maximize the handoff success rate.

Probability of unnecessary Handoff

Unnecessary handoffs, as in the ping-pong effect, increase the signaling overhead on the network and lead to unwanted delays and interruptions in calls.

It should be minimized.

Subrating scheme

Subrating temporarily divides an occupied full-rate channel into two channels at half the original rate: one serves the existing call, and the other serves the handoff request.

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Improved Handoff Strategies

▲ Prioritization:

In order to reduce handoff failure, handoffs are given priority over new call requests.

A certain number of channels may be reserved in each cell explicitly for handoffs.

There is a trade-off here between probability of dropping a call due to handoff failure, and bandwidth utilization.

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Cont…

▲ Relative Signal Strength

There is a minimum time for which an MT must be in a cell before it can request a handoff, calledthe dwell time. The dwell timer keeps track of how long the MT has been in

the cell.

Dwell time has to be decided depending on the speed of the mobile users in the region.

A very large dwell time may not allow a handoff when it really becomes necessary for a rapidly moving mobile.

The concept of dwell time reduces the ping-pong effect as frequent handoffs will not occur on the border of two cells.

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Cont…

▲ Predictive handoff

The mobility patterns of users can be predicted by analysis of their regular movements, using the location tracking data.

This can help in theprediction of the requirements of channels and handoffs in different cells.

▲ Adaptive handoff

Users may have to be shifted across different layers, from micro- to macro-cellular or pico-cellular to micro-cellular, if their mobility differs during the call.

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Other high level types of Handoffs

▲ Inter Base Station Handoff

The base stations involved in the handoff might be connected to the same MSC

▲ Inter System Handoff

Two different MSCs (Inter system handoff or inter-MSC handoff.

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Inter-BS Handoff Procedure

• After the mobile phone detects the need for handoff, it temporarily suspends the conversation and initiates the handoff by signaling on an idle channel in the new base station.

• Then it resumes the conversation on the old base station.

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• On receipt of the signal, the switch transfers the encryption information to the selected idle channel of the new base station and sets up the new conversation path to the mobile phone through that Channel.

• The switch bridges the new path with the old path and informs the mobile phone to transfer from the old channel to the new channel.

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Cont…

• After the mobile phone has transferred to the new base station, it signals the network and resumes conversation by using the new channel.

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• On receipt of the handoff-completion signal, the network removes the bridge from the path and releases resources associated with the old channel.

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Inter System Handoff

▲ A communicating mobile user moves out ofthe base station served by MSC1 and enters the area covered by MSC2.

The handoff follows these steps:

Step 1

MSC1 requests MSC2 to perform handoff measurement. MSC2 then selects a candidate base station, BS2,for handoff.

That is, MSC2 finds a base station that covers the mobile phone and has a free radio channel to cover the call.

MSC2 returns the signal-quality parameter values and other information to MSC1.

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Cont…

Step 2

MSC1 checks if the mobile phone has made too many handoffs or if inter system trunks are not available. If so, MSC1 exits the procedure.

Otherwise, MSC1 asks MSC2 to set up a voice channel. Suppose that a voice channel is available in BS2.

MSC2 asks MSC1 to start the radio link transfer.

Step 3

MSC1 sends the mobile phone a handoff order.

The mobile phone tries to synchronize to BS2.

After the mobile phone connects to BS2, MSC2 informs MSC1 that the handoff is successful.

MSC1 then connects the call path (trunk) to MSC2 and completes the handoff.

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Roaming

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Department of Computer Science 23

Documents

Rab Nawaz Jadoon · disconnected, and a radio link in the new base station is required to continue the conversation. ... The signaling during a handoff causes a delay in the transfer