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ORG000004 CDMA1X Handoff Algorithm
Issue 3.0
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Internal Use
Introduction
In CDMA system Handoff has close
relationship with network quality
(conversation quality, call-drop
performance, etc.).
It also effects the system coverage,
capacity and interference.
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Internal Use
Chapter 1 Basic Concept of CDMA Handoff
Chapter 2 Handoff in CDMA
Chapter 3 Here is Handoff
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Internal Use
What is handoff?
When an MS moves from one sector/cell to the service area of
other sector/cell, it disconnects itself from the old sector/cell andmakes connection with the new sector/cell. The sector/cell from
which it disconnects is called “source” and the sector/cell with
which it is being connected is called “target”. In CDMA system,
there are three kinds of handoffs, these are:
1: Hard handoff
2: Soft handoff
3: Softer handoff
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Internal Use
Why need handoff?
To ensure continuous conversation.
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.
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Internal Use
Handoff types in CDMA
Hard handoff Break-make
Soft handoff
Make-break
Softer handoff
Make-break
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Soft handoff
Soft handoff branches are selected by BSC.
Better frame selected
in CFMR (BSC)
Abis Link
BTS BBTS A
Pilot A Pilot B
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Softer handoff
Softer handoff: Signal of different branches are combined in BTS.
CFMR in BSC
BTS
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Handoff in CDMA
In CDMA network, a user need to switch within
Same frequency
Different frequency
Soft handoff is possible only within the same
frequency i.e. intra-frequency handoff
Hard handoff is required for inter-frequency handoffs Softer handoff is possible only within the same
frequency and BTS
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Handoff in CDMA
Not all the intra-frequency handoffs are soft handoff For example, if the frequencies of the target pilot and the
current pilot are the same, but:
Belonging to the different BSCs
Soft handoff path unavailable between two BSCs Frame offset unavailable for destination pilot
Different SID/NID
then hard handoff is required.
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The pilot set, carries the active pilots. An active pilot is a pilotwhose paging or traffic channels are actually being monitoredor used.
The pilot set, not in the active set but
potential to be demodulated
The pilot set, not included in the
active set or the candidate set but
being possible to be added in the
candidate set
Other pilot sets
Active Set
Candidate
Set
Neighbor
Set
Remaining
Set
What are the pilot sets?
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Internal Use
Handoff parameters
T_ADD:
Threshold for new pilot to be added in
active or candidate set.
Range: 0~63 (unit: -0.5db)
Recommended value: 28 (-14dB)
T_DROP:
Threshold for serving pilot to be removed
from active or candidate set
Range: 0~63 (unit: -0.5dB),
Recommended value :32 (-16dB)
Active Set
CandidateSet
Neighbor
Set
>T_ADD<T_DROP
>T_ADD <T_DROP
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Internal Use
T_TDROP:
It is a timer, which triggers when the Ec/Io drops below
T_DROP. If this timer expires and Ec/Io is still below T_DROP,
the pilot is moved from Active set to the Candidate set or
Neighbor set. In other words, it is an Active or Candidate Set
drop timer.
Range: 2~5
Recommended value: 3
Handoff parameters
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Internal Use
T_COMP:
Active versus candidate pilot strength comparison threshold
Range: 0~15 (unit: 0.5dB)
Recommended value: 5 (2.5dB)
Note: We need to choose appropriate value of T_COMP because low value
causes much signaling and high value will increase the pilot interference.
Handoff parameters
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Internal Use
Pilot P1
Pilot P2
Pilot P0
t0
T_COMP×0.5dB
t1 t2
T_ADD
Pilot strength
P0-Strengh of Pilot P0 in Candidate Set.
P1,P2-Stength of Pilot P1,P2 in Active Set.
t0-Pilot strength Measurement Message Sent, P0>T_ADD
t1-Pilot strength Measurement Message Sent, P0>P1+T_COMP*0.5dB
t2 -Pilot strength Measurement Message Sent, P0>P2+T_COMP*0.5dB
Handoff parameters
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Internal Use
Handoff Completion Message
Extended Handoff Direction Message
Mobile Station Acknowledgment Order
Base Station Acknowledgment Order
Pilot Strength Measurement Message
Base Station Acknowledgment Order
Selector arranges channel elements/Walsh codes in requested sector
Handset verifies which assigned PNs it can now hear.
Neighbor List Update Message
Mobile Station Acknowledgment Order
12
3
4
Ec/Io better than T_ADD then sends a PSMM to BS
Once the HO is completed, the BS sends new neighbor list message.
Handoff flows
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Internal Use
Time
Ec /Io
Sector
A Sector
B
T_TDROP
T_ADD
T_DROP
Soft Handoff Region
T_ADD & T_DROP is the standards used to add or drop a pilot.
T_TDROP is a timer.
1 2 3 4 5
Soft handoff procedure
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Internal Use
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 maximumtime delay due to multi-path.
How to find pilot?
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Internal Use
Types of search window
Currently an MS uses three kinds of search windows, these are:
SRCH-WIN-A, used for searching the pilots in the active
set and the candidate set
SRCH-WIN-N , used for searching the pilots in the
neighbor set
SRCH-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.
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Internal Use
Pilot searching
SRCH-WIN-AThe MS can utilize this 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 first,
based on the multi-path signal of the pilot reaches earliest.
SRCH-WIN-A
The earliest arrived pilot
PILOT PHASE
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Internal Use
Pilot searching
SRCH-WIN-N
Used to monitor neighbor set , the size of this window is larger than SRCH-WIN-A. The distance is measured in term of chips (1
chip approximately covers 244m).
PN4 { {
n chip offset with delayed signal necessaryPN16
{
n chip offset because of the search center of active set
{
Expected window
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Internal Use
Pilot searching
SRCH-WIN-R
Used to track the remaining pilots. Typically it should be equal or
larger than SRCH-WIN-N at the start of the project. After
optimization, this window is set to zero.
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Internal Use
PILOT SEARCHER VIEWED IN SEQUENCE: Typical Elapsed Time = 4 seconds
A A A C N
R
A A A C A A A C A A A C A A A C A A A C A A A CN N N N N N
A A A C N A A A C A A A C A A A C A A A C A A A C A A A CN N N N N
A A A CN A A A C A A A C A A A C A A A C A A A C A A A CN N N N N N
N A A A C A A A C A A A CN N N R A A A C N A A A C A A A C A A AN N
C A A A C A A A CN N N
R
A A A C N A A A C A A A C A A AN N C A A AN
C A A A CN N Only 3 of 112 remaining set pi lots h ave been c hecked thu s far!
CURRENT PILOT SET CONTENTS A A A
CN N N N N N N N N N N N
R R R R R R R R R R R R
R R R R R R R R R R R R
R R R R R R R R R R R R
R R R R R R R R R R R R
R R R R R R R R R R R R
R R R R R R R R R R R RR R R R R R R R R R R R
R R R R R R R R R R R R
R R R R R R R R R R R R
R R R R
3
112
112
A
N
R
R
R
R
R
R
R
NN
N
N
N
NN N
A
A
The Pilot Searcher’s Measurement Process
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Internal Use
Delay budget and size of search window
SRCH_WIN_A Width of
SRCH_WIN_A
(chips)
Delay budget
(ms)
Availab
-le 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 5126 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
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Internal Use
Question
Whether data service supports soft handoff?
Consideration the fact, high power may affect the
system, whether soft handoff can be performed
after reducing the rate?
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Internal Use
Chapter 1 Basic Concept of CDMA Handoff
Chapter 2 Handoff in CDMA
Chapter 3 Here is Handoff
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Internal Use
CDMA handoff analogy
The objectives of handoff are:
Reduce call-drop rate to guarantee QoS Reduce interference to increase network capacity Balance the distribution of traffic load of the network and
reduce the congestion of the hot spots
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Internal Use
Types of handoffs
In a CDMA network following handoff can occur: Intra-frequency handoff
Soft handoff/softer handoff
Hard handoff
Inter-frequency handoff
MS assisted hard handoff
Pseudo pilot hard handoff
Handdown hard handoff
Direct hard handoff
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Internal Use
Optimize the soft handoff by
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
Useful command:
MOD HO
MOD BSCHO
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Internal Use
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Internal Use
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Internal Use
Intra-frequency 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.
I f H d H d ff b
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Internal Use
Intra-frequency Hard Handoff by
Reasonable planning of handoff region Reasonable setting of same-frequency HHO service carrier
threshold, same-frequency HHO target carrier threshold, and same-
frequency HHO relative threshold
Useful command:
MOD HHOSF
MS i t d H d H d ff
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Internal Use
MS assisted Hard Handoff
MS-assisted hard handoff is occurred when:
MS measures the current pilot strength is below than the
threshold value. And the target pilot cannot allocate the same
frame offset. Then MS sends PSMM to BS, to trigger the hard
handoff. BS receives abundant FER over forward or reverse link. It
means the quality of link is poor. If there is neighbor inter-
frequency pilot reported by the MS, which meets the setting
conditions of the system, then the system will trigger hard
handoff.
MS i t d H d H d ff
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Internal Use
MS assisted Hard Handoff
CFSRPM(Report)
CFSCNM
(Search Order)
F1 (serving)
F2 (candidate)
GHDM
(HHO)
CFSRQM
(Search Parameters)
CFSRSM(Acceptance)
CFSRQM: Candidate Frequency Search Request
CFSRSM: Candidate Frequency Search Response
CFSCNM: Candidate Frequency Search Order
CFSRPM: Candidate Frequency Search Report
O ti i MS A i t t H d H d ff b
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Internal Use
Optimize MS Assistant Hard Handoff by
The following parameters would be optimized: MS assisted HHO service carrier threshold
MS assisted HHO target carrier threshold
MS assisted HHO relative threshold
MS assisted HHO start search threshold
MS assisted HHO stop search thresholdUseful command:
MOD HHOMA
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Internal Use
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P d Pil t H d H d ff
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Internal Use
Pseudo Pilot Hard Handoff
O ti i th P d Pil t H d H d ff b
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Internal Use
Optimize the Pseudo Pilot Hard Handoff by
The following parameters would be optimized:Pseudo pilot hard HO T_ADD
Pseudo pilot hard HO absolute threshold
Pseudo pilot hard HO relative threshold
Useful command:
MOD HHOBPLT
ADD BCDMACHTRG
RMV BCDMACHTRG
LST BCDMACHTRG
H d D H d H d ff
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Internal Use
Hand Down Hard Handoff
Once serving BTS is offering frequency (F1) different to the target BTS (F0),
and serving BTS also can handle calls on F1. Then MS will observe Hand
down handoff.
Optimize Hand Down Hard Handoff by
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Internal Use
Optimize Hand Down Hard Handoff by
The following parameters would be optimized:Handdown hard HO Ec/Io intensity absolute threshold
Handdown hard HO max. loop delay threshold
Useful command:
MOD HNDDWN
ADD HNDDWNTRG
RMV HNDDWNTRG
LST HNDDWNTRG
MOD HNDDWNTRG
Direct Hard Handoff
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Internal Use
Direct Hard Handoff
It is applicable to the sectors with simple network structure, but different frequencies;
Optimize Direct Hard Handoff
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Internal Use
Optimize Direct Hard Handoff
The following parameters would be optimized:Direct hard HO Ec/Io strength absolute threshold
Direct hard HO maximum loop delay threshold
Useful command:
MOD DRCT
ADD DRCTTRG
RMV DRCTTRG
LST DRCTTRG
MOD DRCTTRG
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Internal Use
Chapter 1 Basic Concept of CDMA Handoff
Chapter 2 Handoff in CDMA
Chapter 3 Here is Handoff
The Call is Already Established
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Internal Use
The Call is Already Established
E c / I o
All PN Offsets
0PN
0
-20
Rake Fingers
3
Reference PN
Active Pilot
168 500220
Mobile Rake RX
Srch PN??? W0
F1 PN168 W61
F2 PN168 W61
F3 PN168 W61F3 PN220 W20
Mobile Requests the Handoff!
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Internal Use
Mobile Requests the Handoff!
PILOT STRENGTH MEASUREMENT MESSAGE(PSMM)
BASE STATION ACKNOWLEDGMENT
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.
The base station acknowledgesreceiving the PSMM.
Ok! Handoff is Accepted
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Internal Use
Ok! Handoff is Accepted
HANDOFF DIRECTION MESSAGE
The base station sends a Handoff
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, PN220 will continue to use Walsh
Code 20, and the new link on PN500 willuse Walsh code 50.
The mobile acknowledges it has received
the Handoff Direction Message.
MOBILE STATION ACKNOWLEDGMENT
Handoff is Done!
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Internal Use
Handoff is Done!
The mobile searcher quickly re-checksall three PNs. It still hears their pilots!
The mobile sends a Handoff Completion
Message, confirming it still wants to go
ahead with the handoff.
BASE STATION ACKNOWLEDGMENT
HANDOFF COMPLETION MESSAGE
The base station confirms it has
received the mobile’s Handoff
Completion message, and will
continue with all of the links
active.
Time to Update Neighbor List
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Internal Use
Time to Update Neighbor List
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 isalready checking the neighbor list and
will do so continuously from now on.
The handoff is fully established.
Oops!!!
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Internal Use
Oops!!!
E c / I o
All PN Offsets
0 512PN
0
-20
T_ADD
Rake Fingers
Reference PN
Active Set
168 500220
3T_DROP
Mobile Rake RX
Srch PN??? W0
F1 PN168 W61
F2 PN500 W50
F3 PN220 W20
BSC soft handoff flow
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Internal Use
BSC soft handoff flow
MS BTS_D
Pilot StrengthMeas. Msg Abis-IS-2000FCHRvs(Pilot Strength)
Abis-BTSSetup
Abis-Connect
Abis-Connect Ack
Abis-BTSSetupAck Abis-IS-2000FCHFwd(ForwardFra)
ForwardTrafficFrame Abis-IS-2000FCHRvs(Idle)
Abis-IS-2000FCHFwd(EHDM)
MSAckOrder Abis-IS-2000FCHRvs(MSAck)
Handoff CompletionMsg Abis-IS-2000FCHRvs(HCM)
Abis-IS-2000FCHFwd(BaseAck)BaseAckOrder
BaseAckOrder
BTS_S
Abis-IS-2000FCHFwd(BaseAck)
Handoff DirectionMessage
Hard handoff flow
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Hard handoff flow
MS Source BS MSC Time
a
b
c
d
e
f
g
Handoff Required
Handoff Request
Handoff Command
Handoff Direction Message
MS Ack Order
Target BS
Null forward traffic channel frames
Handoff Request Ack
Handoff Commenced h
j
i
k
l
m
n
Reverse Traffic Channel Frames or Traffic Channel Preamble
Handoff Completion Message
Clear Command
Clear Complete
BS Ack Order
Handoff Complete
x