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CDMA 1x Handoff Algorithm

ISSUE 3.0

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Learning Guide

What is Handoff in mobile network?

What is the Search Window?

How to optimize handoff parameters?


Upon completion of this course, you will be able

to:

Know handoff principle and classification.

Know search window and key parameters.

Master handoff tuning in network.


Chapter 1 Basic ConceptChapter 1 Basic Concept

Chapter 2 Handoff in CDMAChapter 2 Handoff in CDMA

Chapter 3 Optimization CommandChapter 3 Optimization Command


What is handoff

BTS_ABTS_B


Why do we need handoff

To provide the continuous service for subscribers

In wireless communications, during HO process, there is a high probability of call drop, which directly effects the system performance （ 90% of radio call drop occurs during handoff). Proper handoff algorithm can reduce

system call drop and increase network capacity.

Notice


How many kinds of handoff in CDMA

Soft handoff

It is a process of establishing a link with a target sector before

breaking the link with the serving sector.

Softer handoff

Similar to the soft handoff, but the softer handoff is occurred

among multi-sectors in the same base station.

Hard handoff

Hard handoff occurs when the two sectors are not

synchronized or are not on the same frequency. Interruption in

voice or data communication occurs but this interruption does

not effect the user communication.






Soft Handoff

It is a process of establishing a link with a target sector before

breaking the link with the serving sector.

Soft handoff branches are selected by BSC.


Soft Handoff

CDMA soft handoff is driven by the handset

Handset continuously measures available pilots and report

to the system

System assigns sectors (up to 6 max.), informs handset

Handset assigns its fingers accordingly


Softer Handoff

Similar to the soft handoff, but the

softer handoff is occurred among

multi-sectors in the same base

station.

Softer handoff: Signal of different

branches are combined in BTS.


Questions

Which channel signal strength from BTS should be compared by

MS during HO process?

How does MS identify which signal is better than others?


Searcher Correlator

Pilot Sets

Pilots of sectors actually in use

Pilots mobile requested, but not yet being used by system

Pilots being sent to mobile by system, as nearby sectors to measure

Any pilots used by system but not already in the other sets

Active Set Active Set

Candidate Set

Candidate Set

Neighbor Set

Neighbor Set

Remaining Set

Remaining Set


Handoff Parameters

Handset sends PPilot SStrength MMeasurement MMessage to the

system whenever

It notices a pilot in neighbor or remaining set exceeds T_ADDT_ADD

An active set pilot drops below T_DROPT_DROP for T_TDROPT_TDROP time

A candidate pilot exceeds an active by T_COMPT_COMP


How to implement Soft Handoff

T_ADDT_ADD

T_DROPT_DROP

Pilot 1

Pilot Strength

Pilot 2

T_TDROPT_TDROP

NeighborSet

CandidateSet

ActiveSet

NeighborSet

TIME1 2 3 4 5 6

ActiveSet

T_COMPT_COMP


Hard Handoff

MS assistedHard handoff

Pseudo pilotHard handoff

Hand down

Hard handoffDirect

Hard handoff

Hard Handoff


MS Assisted Hard Handoff

CFSRQM: Candidate Frequency Search Request Message

CFSRSM: Candidate Frequency Search Response Message

CFSCNM: Candidate Frequency Search Order Message

CFSRPM: Candidate Frequency Search Report Message

CFSRPM(Report)

CFSCNM(Search Order)

F1 (serving)

F2 (candidate)

GHDM(HHO)

CFSRQM(Search Parameters)

CFSRSM(Acceptance)


Pseudo Pilot Hard Handoff

F0F0

F3F3

F2F2

F1F1

F0F0

F1F1

F2F2

F0F0

Sector 1Sector 1 Sector 2Sector 2 Sector 3Sector 3

Pilot BeaconF3

Pilot BeaconF3

Pilot BeaconF1

Pilot BeaconF1

Pilot BeaconF2

Pilot BeaconF2


F0F0

Hand Down Hard Handoff

F3F3

F2F2

F1F1

F0F0

F1F1

F2F2

F0F0

Sector 1Sector 1 Sector 2Sector 2 Sector 3Sector 3


Direct Hard Handoff

F2F2 F1F1






How to search pilot

The purpose of SRCH-WIN set by MS is to capture the multi-path

components, which arrived before or after the current signal. Based on

experience, the SRCH-WIN should be set wide enough so that it can

capture the signal, which has the maximum time delay due to multi-path.

Search Window


SRCH-WIN Classification

Currently MS uses three kinds of search windows, these are:

SRCH-WIN-ASRCH-WIN-A ， used for searching the pilots in the active set and

the candidate set

SRCH-WIN-NSRCH-WIN-N ， used for searching the pilots in the neighbor set

SRCH-WIN-RSRCH-WIN-R ， used for searching the pilots in the remain set

The MS fixes the width of the search window according to the system

message. An MS can search the multi-path components of the available

pilot signals within the setting.


The Complete Picture of Handoff & Pilot Sets


Pilot Searching

SRCH-WIN-ASRCH-WIN-A

The MS can utilize the search window to search the pilots in

the active set and the candidate set based on the PN code.

For pilots in the active set and candidate set, the MS fixes

the center of the search window at the moment, when the

multi-path signal of the pilot reaches earliest.

SRCH-WIN-ASRCH-WIN-A

The earliest arrived pilot

PILOT PHASE


Pilot Searching

SRCH-WIN-NSRCH-WIN-N

Uses to monitor neighbor set , the size of this window is

larger than SRCH-WIN-A. The distance is measured in term

of chips (1 chips approximately covers 244m).

The distance is measured in term of chips (1 chips approximately covers

244m).

PN4PN4PN4PN4{{

{{

n chip offset with delayed signal necessary n chip offset with delayed signal necessary n chip offset with delayed signal necessary n chip offset with delayed signal necessary PN16PN16PN16PN16

{{n chip offset because of the search center of active set n chip offset because of the search center of active set n chip offset because of the search center of active set n chip offset because of the search center of active set

{{ Expected windowExpected windowExpected windowExpected window


Pilot Searching

SRCH-WIN-RSRCH-WIN-R

Uses to track the remaining pilots. A typical requirement for

the size of windows is, it should be equal or larger than

SRCH-WIN-N.


The Pilot Searcher’s Measurement Process


Delay budget and size of search windowSRCH_WIN_A

Width of SRCH_WIN_A

(Chips)

Delay Budget

(ms)Available PN

0 4 T < 1.64 512

1 6 1.64 < T < 2.45 512

2 8 2.45 < T < 3.27 512

3 10 3.27 < T < 4.09 512

4 14 4.09 < T < 5.72 512

5 20 5.72 < T < 8.17 512

6 28 8.17 < T < 11.44 512

7 40 11.44 < T < 16.34 512

8 60 16.34 < T < 24.51 512

9 80 24.51 < T < 32.68 512

10 100 32.68 < T < 40.85 512

11 130 40.85 < T < 53.11 512

12 160 53.11 < T < 65.36 256

13 226 65.36 < T < 92.32 256

14 320 92.32 < T < 130.72 170

15 452 130.72 < T < 184.42 128


Questions

Whether data service supports soft handoff?

Considering the fact, high power may affect the system, whether

soft handoff can be performed after reducing the rate?


How To Optimize Soft Handoff

Reasonable coverage planning and BTS layout

Reasonable setting of T_Add, T_Drop, T_TDROP

Optimize the maximum branches in active handoff

Reasonable planning of handoff region

Reasonable setting of neighbor and neighbor priority


How To Optimize Soft Handoff MOD HOMOD HO (Modify Handoff Parameters )

This command is executed to modify handoff parameters.

Index: CN -- Cell number, SCTID -- Sector ID, CRRID -- Carrier ID.


How To Optimize Soft Handoff

MOD BSCHOMOD BSCHO (Modify Inter-BSC Handoff Parameters )

This command is executed to modify handoff parameters at system level.


Optimize Intra-frequency Hard Handoff

MOD HHOSFMOD HHOSF (Modify Same-Frequency HHO Parameters )

This command is executed to modify same-frequency hard

handoff parameters.

Index: CN -- Cell number, SCTID -- Sector ID, CRRID--

Carrier ID.


Optimize MS Assistant Hard Handoff

MOD HHOMAMOD HHOMA (Modify Mobile Assisted HHO Parameters )

This command is executed to modify the mobile assisted hard

handoff parameters.



Optimize Pseudo Pilot Hard Handoff

MOD HHOBPLTMOD HHOBPLT (Modify Pilot Beacon HHO Parameters )

This command is executed to modify pilot beacon hard handoff

parameters.

Index: CN -- Cell number, SCTID -- Sector ID, CRRID -- Carrier

ID.


Optimize Hand Down Hard Handoff

MOD HNDDWNMOD HNDDWN (Modify Hand down HHO Parameters )

This command is executed to modify handdown hard handoff

parameters.



Optimize Direct Hard Handoff

MOD DRCTMOD DRCT (Modify Direct HHO Parameters )

This command is executed to modify direct hard handoff parameters.



Questions

What is Handoff in mobile network?

What is the Search Window?

How to optimize handoff parameters?

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