Cdma Handoffs

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

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Cdma Handoffs