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CDMA Handoffs
CBB_T031_E2
ZTE University
CDMA-BSS Team
Objectives
In this section, the student will learn:What is Handoff
Types of handoffsIdle HandoffSoft HandoffSofter HandoffInter-system Soft HandoffHard Handoff
What is Handoff?
Handoff is the process by which a mobile station maintains communications with the Switching Center when traveling from the coverage area of one cell/sector to another.
Handoffs keep the call established during the following conditions:
Subscriber crosses the boundaries of a cell
Subscriber experiences noise or other interference above a specified threshold
A base station component experiences an out-of-service condition during a call
Handoff between BTS
Handoff between sectors of the same BTS
CDMA Handoff Types
Duringa Call
Idle Handoff
Soft Handoff
Softer Handoff
CDMA-to-CDMA Handoff
Inter-System Soft Handoff
CDMA-to-Analog Handoff
While in the Idle State
Idle Handoff
PN 104
PN 108
A
Paging C
hannel
Msgs
Sync C
hannel
Msg
104 108
Ec/Io
PN 104
PN 108
A Paging C
hannel
Msgs
Sync C
hannel
Msg
104 108
Ec/Io>3dB
Soft Handoff
Cell SiteB
Cell SiteA
PSTN
MSC
BSC
In Soft Handoff the MS starts communications with a target BS without interrupting communications with the current BS.
Mobile station can be in soft handoff with up to 3 cells, it canreceive signals combine the frames from each cell
Softer Handoff
Softer Handoff occurs between cell/sectors of the same BTS
Signals received at both sectors can be combined for improved quality
This handoff may happen frequently
MSC is aware but does not participate; all activities are managed by the cell site
alpha
beta
gamma
Inter-system Soft Handoff
Mobile Station starts communications with a new cell under a different BSC while still communicating with the cell controlled by the source BSC
Soft Handoffs over Hard Handoffs
MSC
BSCE1 Links
MSC
BSC
Af1Af1
Df1Df1
Bf1Bf1 E
f1Ef1
Cf1Cf1 F
f1Ff1
Uf1Uf1
Xf1Xf1
Vf1Vf1 Y
f1Yf1
Wf1Wf1 Z
f1Zf1
CDMA Soft Handoff CDMA Hard Handoff
E1 LinksTIA/EIA-41D
Hard Handoff
A(ƒ1)
PSTN
MSC
BSC
B(ƒ2)
A
PSTN
MSC
BSC
B
MSC
BSC
T1 or E1 LinksTIA/EIA-41D
Between cells operating on different frequenciesBetween cells that could be on the same frequency,
but which are connected to different MSC
Pilot SetsPilot Sets1. Active2. Candidate3. Neighbor4. Remaining
• There are 4 memorizer(pilot set) in handset, to save the related PN offset index.After power off, all memorizer will be cleared automatically.
–Active Set: Pilots associated with the forward traffic channels assigned to the mobile station (max 6 pilots)
–Candidate Set: Pilots not currently in the Active Set, but received by the mobile with sufficient strength to indicate that the corresponding Traffic Channels can be successfully demodulated (max 5 pilots)
–Neighbor Set: Pilots not currently on the Active or Candidate Sets, that are likely handoff candidates (at most 20 pilots)
–Remaining Set: All other possible pilots in the current system on the current CDMA frequency assignment
•These sets can be updated during handoff by the base station
• There are 4 memorizer(pilot set) in handset, to save the related PN offset index.After power off, all memorizer will be cleared automatically.
–Active Set: Pilots associated with the forward traffic channels assigned to the mobile station (max 6 pilots)
–Candidate Set: Pilots not currently in the Active Set, but received by the mobile with sufficient strength to indicate that the corresponding Traffic Channels can be successfully demodulated (max 5 pilots)
–Neighbor Set: Pilots not currently on the Active or Candidate Sets, that are likely handoff candidates (at most 20 pilots)
–Remaining Set: All other possible pilots in the current system on the current CDMA frequency assignment
•These sets can be updated during handoff by the base station
Candidate
Remaining
Active
Neighbor
Pilot whose Paging Channel is being monitored or
associated with the assigned Forward Traffic Channel
Pilots specified in the most recently received Extended
Neighbor List Message(AGE set at NGHBR_MAX_AGE)
Empty
Any other possible pilotin the system
(While in the Idle or in an Active Call)
Pilot Set Initialization
Pilot Set Maintenance
(While in an Active Call)
Candidate
Remaining
Active
Neighbor
Pilots listed the most recently received Extended
Handoff Direction Msg
Pilots specified in the most recently received Neighbor
List Update Message(or whose AGE is still less than NGHBR_MAX_AGE)
Pilots whose strength exceeds T_ADD
Any other possible pilotin the system
Pilot Strength MeasurementMessage (PSMM)
AB
C
Pilot Channel
Timing
Traffic Channels
PSMM
The Pilot Strength Measurement Message is used by the mobile station to direct the base station in the handoff process.
Mobile station reports the strength of the pilots associated with forward traffic channels currently being demodulated (and whether it would like to continue to receive traffic from them), as well as pilots from the neighbor and remaining list which are being received with sufficient strength so that traffic could be demodulated from them successfully.
Soft Handoff Signaling
PilotStrength
Pilot Strengthexceeds T_ADD
EHOD message received,Pilot Dropped into Neighbor Set,
EHOC message sent,NLU message received
Time
Pilot strength drops below T_DROP,Drop Timer started
T_TDROP
T_ADD
T_DROP
EHOD message received, Pilot Added to Active Set, EHOC message sent, NLU message received
N C NA(1)(2) (3) (4) (5)(6) (7)
Drop Timer expires
PSMM sent (request to add) , Pilot added to Candidate Set
PSMM Sent (request to remove)
T_ADD: pilot detection threshold (Typically 26 = -13dB)T_DROP: pilot drop threshold (Typically 30 = -15 dB)T_TDROP: drop timer value (Typically 3 = 4 sec)
Prevents unnecessary transmissions of PSMM when a mobile stationexperiences a fade