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GSM MOBILITY MANAGEMENT

ECE 2526 – MOBILE COMMUNICATION

Monday, January 28, 2019

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WHY MOBILITY MANAGEMENT IS

IMPORTANT?

1. The performance of a Mobile Communication Network is significantly affected by the way the network manages the mobility of users.

2. Two Important aspects of mobility management are:

(a) Handoff

(b) Roaming

3. Call drop-offs result from poor mobility management.

4. Regulators and Users use forced call drop-off as a significant indicator of a poor performance

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ILLUSTRATION OF MOBILITY

Subsriber from Cell A enters Cell B with the following consequences

1. Free Channels are available and handoff takes place

2. No free channel and subrating is used to create capacity

3. No free-channel is available and the call is put into a handoff queue

4. No free channel is available and the call is allowed to continue until the link fails.

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HANDOFF PRINCIPLE

1. Mobile Station (MS) is engaged in conversation

2. It crosses from one cell to the other

3. Radio link is established to the new BTS and the radio link to the old

BTS is disconnected

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INTER-BS HAND-OFF (1)

1.The MS initiates the hand-off

procedure by signaling the MSC

on the New BTS

2.Upon receipt of the transfer

request, the MSC sets up a new

conversation path to the MS

through the new BTS

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INTER-BS HAND-OFF (2)

3. The MSC transfers the

conversation of the MS

to the new channel.

4. The BCS removes the

resources associated

with old channel

HANDOFF SIGNAL STRENGTH

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As the MS moves from BS1 to BS2, the signal strength

from BS1 decreases and that of BS2 increases. At TL, the

system starts to check for candidate handoff bases

stations. At TH it handoff.

GSM UPLINK CRITERIA

GSM specifications have six criteria for

handoff in order of priority as follows:

1. Uplink quality

2. Downlink quality

3. Uplink level

4. Down link level

5. MS to BS distance

6. Power Budget

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GSM HANDOFF PROCESS

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HANDOFF TYPES

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NON-PRIORITIZED HAND-OFF

1. Handoff is initiated in the same manner

as a new call attempt.

2. If no channel is available, the handoff is

blocked and the call is held on the

current channel in the old cell until the

call is completed or when the failing link

is no longer available.

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FLOWCHART FOR NON-PRIORITIZED SCHEME

Is Channel

Available?

Channel is

AssignedHandoff

Arrival

yes

Handoff call

Blocked

No

Hand-off to

new BTS

Channel is

released

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RESERVED CHANNEL SCHEME

1. Handoff is initiated in the same manner as a new call attempt.

2. However, If no channel is available, the handoff is blocked and the call is held on the current channel in the old cell until the call is completed or when the failing link is no longer available.

3. However, some channels in each BTS are reserved for hand-off calls.

SAME AS NON-PRIORITIZED SAVE FOR THE FACT THAT EACH

BTS RESERVES SOME CHANNELS FOR HANDOFF

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QUEUING PRIORITY SCHEME

1. Handoff is initiated in the same manner as a new call attempt.

2. If no channel is available in the new BS during handoff, The new BS buffers the handoff request in a waiting queue.

3. The MS continues to use the channel with the old BS until either a channel in the new BS becomes available (and the handoff call is connected) or the MS moves out of the handoff area and the call drops.

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QUEUING PRIORITY HANDOFF SCHEMEFLOWCHART FOR QUEUING SCHEME

Channel is Idle

Is Channel

Available?

Is Channel

Available before

call expires?

Channel is

AssignedHandoff

Arrival

yes

yes

Handoff call

Blocked

No

No

Handoff to

New BTS

Channel is

released

Is the Waiting Queue

Empty?

yes

Insert Call in

Waiting Queue

No

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SUBRATING SCHEME

1. Handoff is initiated in the same manner as a new call attempt.

2. If no channel is available in the new BS, an occupied full-rate channel is temporarily divided into two channels at half the original rate:

3. When occupied channels are released, the subrated channels are immediately switched back to full-rate channels.

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Subrated pair have rate

upgraded

SUBRATING HAND-OFF

Is Channel

Available?

Is Channel

Available for

Subrating?

Channel is

AssignedHandoff

Arrival

yes

yesHandoff call

Blocked

No

No

Handoff to

New BTS

Channel is

released

Was released Channel

Subrated?

yes

Channel is Idle

No

Channel is

subrated?

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IMPLEMENTATION: RESERVED CHANNEL SCHEME

(RCS)

1. RCS is easy to implement

2. It reduces forced termination probability

more effectively than NPS

3. However, the new call blocking probability

is higher than NPS

4. Therefore RCS is desirable when reducing

forced call termination is more important

than reducing new call blocking

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1. Takes advantage of handoff area to buffer calls

2. They effectively reduce forced terminations at the

expense of increased new call blocking

3. The Scheme adds hardware and software complexity

to both BS and MS to manage waiting queues.

ADVANTAGES OF GUEUING PRIORITY SCHEME

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• Has the smallest forced call termination

probability compared to other schemes

• The advantage is gained at the expense of

increased hardware/software complexity.

• The Increased cost can be shared by

using channel subrating for emergency

services e.g. 411

IMPLEMENTATION: SUBRATING SCHEME

SCHEME

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IMPLEMENTING A SUBRATING SCHEME

• Simplest: Use adaptive Differential Pulse Code

Modulation (ADPCM) which operates at say 13Kbps and

5.6Kbps.

• Advantage: No impact on channel delay, current drain

or cost

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1. More Complex: 13 Kbps Codec could

be implemented along with the 5.6 kbps

Codec and the conversation switched

between the two.

2. Disadvantage: High Processing delay,

higher cost of the MS, increased power

drainage during subtraction use.

IMPLEMENTING A SUBRATING SCHEME

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REVIEW QUESTIONS

• Describe for different channel assignment

schemes.

• Under what circumstances is the reserved

channel scheme more desirable than the others.

• What do you understand by the term ‘Call

Dropping’?

• Describe three measurements that may be

performed to decide on handoff.

• Describe three handoff detection strategies.

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CDMA SOFT HANDOFF

1. CDMA BSs transmit pilot signals that assist

MSs to track/synchronize the down-link signals.

2. The MS measures the strength of the pilot

signals of BSs in the neighbourhood.

3. If the strength of a new BS exceeds a

threshold, then the procedure of adding a new

link to the MS is initiated.

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CDMA-SOFT HANDOFF (ADDING A BS)

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DROPPING A BS

• If a signal strength between a BS and MS falls below a predetermined threshold, the MS:

1. Sends a pilot strength signal to the Old BS and requests for removal

2. Old BS sends relevant call record information to the MS by exchanging interface primary transfer pair.

3. The new BS and MSC exchange the hand-offinformation

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DROPPING OLD BASE STATION (BS)

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REVIEW QUESTIONS

• Handoff can be performed in order to achieve load balancing. Discuss the advantages of doing this.

• How can emergency calls be introduced into the Mobile Controlled Handoff Subtracting Link Transfer?

• Why are pilot strength measurements important in soft handoff systems?