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1
GSM MOBILITY MANAGEMENT
ECE 2526 – MOBILE COMMUNICATION
Monday, January 28, 2019
2
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
3
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.
4
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
7
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
8
GSM HANDOFF PROCESS
9
HANDOFF TYPES
10
11
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
13
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
14
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.
15
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
16
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.
17
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?
18
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
20
• 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
21
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
22
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
23
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.
24
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.
25
CDMA-SOFT HANDOFF (ADDING A BS)
26
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
27
DROPPING OLD BASE STATION (BS)
28
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?