ORG010202 CDMA 1x Handoff Algorithm

ORG010202 CDMA 1x
Handoff Algorithm
ISSUE 1.2

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
Chapter 4 SummaryChapter 4 Summary

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 checks available pilots
 Handset tells system pilots it currently sees
 System assigns sectors (up to 6 max.), tells handset
 Handset assigns its fingers accordingly
 All messages sent by dim-and-burst, no muting!
 Each end of the link chooses what works best, on a frame-by-
frame basis!
 Users are totally unaware of handoff

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.

Softer Handoff
 Each BTS sector has unique PN offset & pilot
 Handset will ask for whatever pilots it wants
 If multiple sectors of one BTS simultaneously serve a handset,
this is called Softer Handoff
 Handset can’t tell the difference, but softer handoff occurs in
BTS in a single channel element
 Handset can even use combination soft-softer handoff on
multiple BTS & sectors

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 set
up & transmitting by system
Pilots told to mobile by system, as nearby
sectors to check
Any pilots used by system but not already in
the other sets
Active Set
Candidate
Set
Neighbor
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
Neighbor
Set
Candidate
Set
Active
Set
Neighbor
Set
TIME1 2 3 4 5 6
Active
Set
T_COMPT_COMP

Hard Handoff
MS assisted
Hard handoff
Pseudo pilot
Hard handoff
Hand down
Hard handoff
Direct
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 Beacon
F3
Pilot Beacon
F3
Pilot Beacon
F1
Pilot Beacon
F1
Pilot Beacon
F2
Pilot Beacon
F2

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).
PN4PN4
{
{
n chip offset with delayed signal necessaryn chip offset with delayed signal necessary
PN16PN16
{
n chip offset because of the search center of active setn chip offset because of the search center of active set
{
Expected 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 window
SRCH_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

 MOD HOMOD HO (Modify Handoff Parameters )
 This command is executed to modify handoff parameters.
 Index: CN -- Cell number, SCTID -- Sector ID, CRRID -- Carrier ID.

 MOD BSCHOMOD BSCHO (Modify Inter-BSC Handoff Parameters )
 This command is executed to modify handoff parameters at system level.

How To Optimize Hard Handoff
 In some cases, soft/softer handoff cannot be realized even the
frequencies of the pilots are the same. For example:
 Inter-BSC handoff without A3/A7 connection
 Handoff between different frame offsets on the same pilot
 In such cases, hard handoff is applied to ensure the uninterrupted
communication.

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.

Handoff Case
Ec/Io
All PN Offsets
0PN
0
-20
Rake FingersŒ

Reference PN
Active Pilot
168 500220
Mobile Rake RX
Srch PN??? W0
F1 PN168 W61
F2 PN168 W61
F3 PN168 W61


Handoff Case
 Just prior to this message, this particular
mobile already was in handoff with PN
168 and 220.
 This PSMM reports PN 500 has increased
above T_Add, and the mobile wants to
use it too.
PILOT STRENGTH MEASUREMENT MESSAGE(PSMM)
 The base station acknowledges receiving
the PSMM.
BASE STATION ACKNOWLEDGMENT

Handoff Case
 The base station sends a Handof Direction Message
authorizing the mobile to begin soft handoff with all
three requested PNs. The pre-existing link on PN 168
will continue to use Walsh code 61, the new link on
PN220 will use Walsh Code 20, and the new link on
PN500 will use Walsh code 50.
HANDOFF DIRECTION MESSAGE
MOBILE STATION ACKNOWLEDGMENT
 The mobile acknowledges it has received
the Handoff Direction Message.

Handoff Case
BASE STATION ACKNOWLEDGMENT
HANDOFF COMPLETION MESSAGE
 The mobile searcher quickly re-checks all
three PNs. It still hears their pilots!
 The mobile sends a Handoff Completion
Message, confirming it still wants to go
ahead with the handoff.
 The base station confirms it has received
the mobile’s Handoff Completion
message, and will continue with all of the
links active.

Handoff Case
MOBILE STATION ACKNOWLEDGMENT
NEIGHBOR LIST UPDATE MESSAGE
 In response to the mobile’s Handoff Completion Message, the
base station assembles a new composite neighbor list including
all the neighbors of each of the three active pilots.
 This is necessary since the mobile could be traveling toward any
one of these pilots and may need to request soft handoff with any
of them soon.
 The mobile confirms receiving the
Neighbor List Update Message. It is
already checking the neighbor list and will
do so continuously from now on.
 The handoff is fully established.

Handoff Case
Ec/Io
All PN Offsets
0 512PN
0
-20
T_ADD
Rake FingersŒ
Reference PN
Active Set
168 500220
Ž
T_DROP
Mobile Rake RX
Srch PN??? W0
F1 PN168 W61
F2 PN500 W50
F3 PN220 W20

Questions
 What is Handoff in mobile network?
 What is the Search Window?
 How to optimize handoff parameters?

ORG010202 CDMA 1x Handoff Algorithm

ORG010202 CDMA 1x Handoff Algorithm

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (20)

Similar to ORG010202 CDMA 1x Handoff Algorithm

Similar to ORG010202 CDMA 1x Handoff Algorithm (20)

More from Tempus Telcosys

More from Tempus Telcosys (15)

Recently uploaded

Recently uploaded (20)

ORG010202 CDMA 1x Handoff Algorithm