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Handoff ManagementHandoff ManagementProf. S.-I. Sou
EE, NCKU
Fall, 2008
2
Reference
[1] Wireless and Mobile Network Architectures,Y-Bing Lin and Imrich Chlamtac,Wiley Computer Publishing。� Chapters 3 and 4
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Outlines
• Introduction to handoff
• Handoff detection
• Channel assignment
• Link Transfer
• Radio link transfer� Hard Handoff � Soft Handoff
• Summary
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Introduction to HandoffIntroduction to Handoff
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Handoff
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Two Aspects of Mobility in a PCS Network
• Handoff � Link transfer, or Handover� A mobile user moves from one coverage area of
an old BS to the coverage area of a new BSduring the conversation.
� The radio link to the old BS is disconnected and a radio link to the new BS should be established to continue the conversation.
• Roaming� When a mobile user moves from one system to
another, the user location should tell the PCS system.
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BS Coverage Area
• BS coverage area:irregular.
• In the cell boundary:� Signal from a neighboring BS �� Signal from the serving BS �
• Otherwise: Forced termination
Overlap coverage should be considered in cell planning.
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Handoff Cost
• Handoffs are expensive.
• Specifically for the system with small cell sizes
• Small cell size for� To increase the capacity of the systems � To reduce power requirements of MSs.
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Issues for Handoff Management
• Handoff detection� Who and how
• Channel assignment
• Radio link transfer
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Handoff DetectionHandoff Detection
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Handoff Detection Concept
• Handoff detection:� Why is the handoff detected necessary?� Who initiates the handoff process?
• Handoff detection is based on link measurement.
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Link Measurement
• Signal measurements are used to determine the quality of a channel:� WEI (Word Error Indicator)� RSSI (Received Signal Strength Indication)� QI (Quality Indicator)
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Strategies for Handoff Detection
• Who makes a decision for handoff?
• Three handoff detection schemes:� Mobile-Controlled Handoff (MCHO)� Network-Controlled Handoff (NCHO)� Mobile-Assisted Handoff (MAHO)� Others
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• MCHO is used in DECT and PACS.
• Part I. The MS continuously monitors the signals of the surrounding BSs.
• Part II. The MS initiates the handoff process when some handoff criteria are met.
Mobile-Controlled Handoff (MCHO)
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Network-Controlled Handoff (NCHO)
• Used in CT-2+ and AMPS
• Part I. The surrounding BSs measure the signal from the MS.
• Part II. The network initiates the handoff process when some handoff criteria are met.
• MSC controls the handoff.
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Mobile-Assisted Handoff (MAHO)
• Used in GSM, IS-136 and IS-95
• Part I.. The network asks the MS to measure the signal from the surrounding BSs.
• Part II. The network makes the handoff decision based on the reports from the MS.
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Channel Assignment for Channel Assignment for
Handoff CallsHandoff Calls
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Channel Assignment
• Purpose:to achieve a high degree of spectrum utilization for a given grade of service� Ex:To reduce forced terminations
• Four channel assignment scheme:� Basic Scheme: Non-prioritized Scheme� Reserved Channel Scheme� Queuing Priority Scheme� Sub-rating Scheme
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Forced Terminations
• Blocked call:Initial access requests fail� For new call� No available channels on the visited BS
• Forced terminations:Handoff requests fail� For handoff call� No available channel on the selected BSs
• Which one is serious, new call blocking or force terminating?
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Some trade-offs
• Service quality
• Spectrum utilization
• Implementation complexity of the channel assignment algorithm
• Number of database lookups
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Flowchart for Non-prioritized Scheme
New or handoff call arrival
Channel available?
Channel assigned
yes
no Channel blocked
Ongoing call
Channel released
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Flowchart for Reserved Channel Scheme
New call arrival
Normal channel available?
Channel assigned
Handoff call arrival
Normal channel available?
Reserved channel available?
yes yes
yes
no
no no
Channel blocked
Ongoing call
Channel released
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Flowchart for Queuing Priority Scheme (for Handoff Calls)
Handoff call arrival
Channel available?
Channel assigned
New call channel released
Is a channel available before the call expires?
Is the waiting queue empty?
yes
yes
yes
no
no
no
Call blocked
Ongoing call
Channel releasedInsert the call into the waiting queue
The channel is idle
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Flowchart for Subrating Scheme (for Handoff Calls)
Handoff call arrival
Channel available?
Channel assigned
New call channel released
Is there a subrated
channel pair?
Is the release channel
subrated?
yes
yes
yes
no
no
no
Call blocked
Ongoing call
Channel released
Each channel of the subrated pair is upgraded to full-rate channels
The other channel of the subratedpair is upgraded
Channel available for subrating?
yes
Channel is idle
no
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Radio Link TransferRadio Link Transfer
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Radio Link Transfer
• The radio link to the old BS is disconnected.
• A radio link to the new BS should be established.
• Link transfer procedure � Hard handoff� Soft handoff
MSC
Old BS New BS
1. disconnected
2. established
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Five Distinct Link Transfer Cases (1/3)
1. Intra-BTS handoff or intra-cell handoff
2. Inter-BTS handoff or inter-cell handoff
3. Inter-BSC handoff
4. Inter-MSC handoff or intersystem handoff
5. Intersystem handoff between two PCS networks
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Inter-BSC Handoff
MSC 1
Old BS
New BS
BSC 2BSC 1
(a) Before handoff (b) After handoff
MSC 1
Old BS
New BS
BSC 2BSC 1
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Inter-MSC Link Transfer
MSC B
BS 1
MSC A
BS 2
PSTNPSTN
(a) Before handoff
trunk MSC B
BS 1
MSC A
BS 2
PSTNPSTN
(b) After handoff
trunk
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Hard HandoffHard Handoff
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Hard Handoff
• MS connects with only one BS at a time.
• Interruption in the conversation occurs
• Used in TDMA and FDMA systems
• We will study the signaling of handoff:� MCHO Link Transfer � MAHO/NCHO Link Transfer
MSC
Old BS
New BS
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MCHO Inter-BS Handoff Message Flow
1. link suspend message
2. handoff request message
3. handoff acknowledgement message
4. link resume message
5. Voice is resumed on old link
6. New BS acqufrescypher key
7. Network bridgesthe new and old BSs
8. Handoff execution message
9. access release message
The MS initializes the synchronization with the new BS.The MS returns to the old BS and then comes back to the
new BS. Cypher starts at this point.
10. handoff execution message
11. handoff complete message
12. Voice is resumed on new link
MS Old BS New BS
Voice isinterrupted
Voice isinterrupted
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MAHO Inter-BS handoff message flow
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Comparisons between MCHO and MAHO/NCHO
• In MAHO/NCHO� The handoff command to the MS is sent over the
failing link. � If MS does not receive this message, the handoff
procedure fails.
• In MCHO� The handoff request message is sent by the MS
to the new BS on a new & more reliable link.� No measurement report should be sent via air
interface.
35
Soft HandoffSoft Handoff
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Soft Handoff
• MS connects to multiple BSssimultaneously.
• BSs use the same frequency.
• BSs must be synchronized.
• The network must combine the signals form the multiple BSs simultaneously.
• Soft handoff is more complicated than hard handoff.
MSC
BS 1 BS 2
37
DSSS/CDMA
• Direct Sequence Spread Spectrum (DSSS)
• Every signal is processed with PN sequence (pseudo-noise sequence).� Each PN sequence is a unique orthogonal code.� User signals are distinguished by different PN
sequences.� An MS communicates with several BSs using the
same PN sequences.� The signaling are typically combined (or bridged)
at the MSC (MS).
• In soft handoff, the link transfer procedure is no time-critical.