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8/13/2019 Cdma Handoffs
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HandoffHandoffHandoffHandoffHandoff is the process by which a mobile station
maintains communication, when traveling from thecoverage area of one base station to that ofanother
Handoff keep the call established during thefollowing conditions:
Subscriber crosses the boundaries of a cell
Subscriber experiences noise or otherinterference above a specified threshold
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CDMA Idle Handoff(Cont.)CDMA Idle Handoff(Cont.)CDMA Idle Handoff(Cont.)CDMA Idle Handoff(Cont.)
PN 104
PN 108
A
Pagin
gCha
nnelMsg
s
Syn
cCha
nnelMs
g104 108
Ec/Io
>3dB
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CDMA Soft HandoffCDMA Soft HandoffCDMA Soft HandoffCDMA Soft Handoff
Soft Handoff:The mobile station starts communications with a targetbase station without interrupting communications with the currentserving base station
Make-before-break Directed by the mobile not the base station,Undetectable by user
Improves call quality
Can involve up to three cells simultaneously and use all signals
Mobile station combines the frames from each cell
Cell Site
B
Cell Site
A
Cell Site
A
Cell Site
B
CDMA
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CDMA Soft Handoff MechanicsCDMA Soft Handoff MechanicsCDMA Soft Handoff MechanicsCDMA Soft Handoff Mechanics
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 Each end of the link chooses what works best, on a frame-by-frame basis
Users are totally unaware of handoff
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CDMA Softer HandoffCDMA Softer HandoffCDMA Softer HandoffCDMA Softer Handoff Softer Handoff is between sectors of
the same cell,that means multiple
sectors of one BTS simultaneouslyserve a handset
Communications are maintained
across both sectors until the mobilestation transition has completed
MSC is aware but does not
participate
All activities are managed by the cell
site
Signals received at both sectors can
be combined for improved quality
alpha
beta
gamma
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CDMACDMACDMACDMA----totototo----CDMA Hard HandoffCDMA Hard HandoffCDMA Hard HandoffCDMA Hard Handoff
Between cells operating on different frequencies
Between cells that could be on the same frequency, but which
are subordinated to different MSC
A(1)
PSTN
MSC
BSC
B(2)
A
PSTN
MSC
BSC
B
MSC
BSC
T1 or E1 LinksTIA/EIA-41D
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Pilot SetsPilot SetsPilot SetsPilot Sets Pilot sets:the handset considers pilots in sets
Active: Pilots of sectors actually in use (max 6 pilots)
Candidate:Pilots not currently in the Active Set, but received by themobile with sufficient strength to indicate that the correspondingforward traffic Channels could be successfully demodulated (max 5pilots)
Neighbors: Pilots not currently on the Active or Candidate Sets, it toldto mobile by system,as nearby sectors to check(at least 20 pilots)
Remaining:All other possible pilots used by the current system on thecurrent CDMA frequency,integer multiplies of PILOT_INC ,excluding
the pilot in other sets All pilots in a set have the same frequency assignment
These sets can be updated during handoff by the base station
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Rules of Soft HandoffRules of Soft HandoffRules of Soft HandoffRules of Soft Handoff The MS assists the BS in the handoff process
by measuring and reporting the strengths ofreceived pilots using a Pilot Strength
Measurement Message
Handset sends PSMM to the system whenever: It notices a pilot in neighbor or remaining set
exceeds T_ADD
An active set pilot drops below T_DROP forT_TDROP time
A candidate pilot exceeds an active by
T_COMP
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Pilot Strength MeasurementPilot Strength MeasurementPilot Strength MeasurementPilot Strength MeasurementMessageMessageMessageMessage
The Pilot Strength Measurement Message is used by the mobilestation to direct the Base Station Controller(BSC) in the handoff
process.This message uses in the Reverse Traffic Channel
A PSMM is sent to the system under one of the following conditions: If MS finds a pilot in Neighbor or Remaining sets exceeds T_ADD
If an active set pilot drops below T_DROP after T_TDROP time
If a Candidate pilot exceeds an active pilot by T_COMP
A
B
C
Pilot Channel
Timing
TrafficChanne
ls
PSMM
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Extended Handoff Direction MessageExtended Handoff Direction MessageExtended Handoff Direction MessageExtended Handoff Direction Message
The Extended Handoff Direction Message(EHODM) is used by the
Base Station Controller(BSC) to tell the mobile station on which
base stations the BSC has allocated traffic channels for that user
An EHODM Message uses in the forward traffic channel
When the mobile station receives the EHODM message from
BSC,then it will updates its Active Set and sends a HCM to the BSC.
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Pilot Search WindowsPilot Search WindowsPilot Search WindowsPilot Search Windows
A search window is a range of PN offsets (measured in chips) wherethe mobile station searches for usable multipath components of thepilots in a set
Usable means that multipath components can be used fordemodulation of an associated forward traffic channel by MS
Search Window for pilots in the Active and CandidateSet:
Search Window for pilots in the NeighborSet:
Search Window for pilots in the Remaining Set:
Earliest arriving usable multipath component of the pilot
pilot PN offset
pilot PN offset
SRCH_WIN_N
SRCH_WIN_A
SRCH_WIN_R
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HAND OFFS IN CDMA
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Handoff Process in CDMA
What is hand off?
In cellular telecommunications, the termhandoff refers to the process of transferring anongoing call or data session from one channelconnected to the core network to another.
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Base Station
Base Station
Control
Equipment
Network InterfacePSTN
Forw
ard
Rev
erse
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Handoff
Base Station
Base Station
Base Station
Cell
Boundary
Control
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Types of Hand off
Soft Hand offSoft Soft Hand off
Softer Hand off
Hard Hand off
All types of soft hand offs are called as makebefore break connections, and these are done onlybetween cells having the same CDMA channel(frequency) assignments.
Hard hand off is called as break before makeconnection and these are done between cells
having different CDMA channel frequencies.
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Soft Hand off :
This is the condition when two cells in amobile network are in simultaneouscommunication with the mobile receiver.
This situation will continue till one of thepilot signals from a particular base station falls
below a chosen threshold level.
From this, the mobile will be transferred to
the remaining base station.
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Soft Soft Hand off :
The situation here is the same as thatdescribed above. However, the number of cellsinvolved in this case are 3. This number is
dictated by the provision of rake receiver in themobile. This has 3 fingers for the separatereception and processing of three stations at agiven time.
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Softer Hand off :
This hand off procedure is similar to thesoft hand off. But, comes into picture
when the mobile transition is from onesector to another in the same cell area.The mobile is served by two sectors
simultaneously till one drops off.
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Hard Hand off :
Needing frequency transition
This is similar to GSM. This occurs onlywhen the mobile transit from one area to
another where the allocated / used channelfrequency happens to be different.
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Pilot sets in Hand off Procedure
Hand offs are done by measuring pilotsignal strengths from neighboring cells.
Measurement message sent to basestation when a pilot of adequate strength,not belonging to the mobiles currentassignment of traffic channels is detected.
Hand off process is dependent on pilot
searches.
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Pilots can be grouped into 4 sets.1. ACTIVE SET
2. CANDIDATE SET
3. NEIGHBOURING SET
4. REMAINING SET
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1. Active Set :
The pilots associated with the forward
traffic channels assigned to the mobilestation.
2. Candidate Set :
These pilots which are NOT in thecurrent Active Set but have sufficientsignal strength to qualify for hand-off.
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3. Neighbour Set :
This contains a list of neighbouring pilotswhose signal strengths are strong enough to
make them candidate pilots. This list isentered through the data base. Even if a pilotfrom an adjoining cell is very strong, the
mobile will not look at if it is NOT mentioned inthe neighbor list.
4. Remaining Set :
The set of all pilots other than the threetypes mentioned above.
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Remaining SetNeighbour Set
Candidate Set
ACTIVE SET
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Parameters used in Hand-OffsThe following Parameters are very
important in Hand-off situations:1. T_ADDs :
Suggested value : -13 dB(Range : -31.5 to 0dB).
T_ADD is the Pilot Detection Threshold.
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2. T_COMPs :
Suggested Value : 2.5 dB
( Range : 0 to 7.5 dB )
This is the ACTIVE SET Vs CANDIDATESET COMPARISION THRESHOLD.
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3. T_DROPs :Suggested Value : -15 dB
( Range : -31.5 to 0 dB)
This is the PILOT DROP THRESHOLD.
4. T_TDROPs :
Suggested Value : 2 seconds
( Range : 0 to 15 seconds )
This is the PILOT DROP TIMER VALUE.
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Neighbour Set Remaining Set
P exceeds T_ADD
1. Move the pilot to candidate set
2. Send Measurement report to base station
P exceeds an active pilotby T_Comp X0.5
No YES
Keep pilot in candidate set
Send measurement report
Receive H/O Direction message
Move Pilot to Active set
Send Hand off com letion Messa e
yes
No
Search for Pilot