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