4. Handoff Management

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Handoff ManagementHandoff ManagementProf. S.-I. Sou

EE, NCKU

Fall, 2008

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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.

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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

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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.