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Title: Active HO optimization from eHRPD to LTE

Abstract: This contribution supersedes “C22-20110523-014r1 ZTE C.S0087-eAT-based-redirection” which provides one approach for active HO optimization from eHRPD to LTE.

Recommendation: Review and Adopt for C.S0087-A v2.0

Source: Yuanfang Yu, Ting Lu, Yonggang Fang, Xiaowu Zhao Yu.yuanfang, lu.ting@zte.com.cn

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eHRPD to LTE active Handoff• Current C.S0087 only defines idle handoff from eHRPD to LTE

• However, redirecting eATs during active mode might be helpful for the following scenarios:– Load balancing– Handoff from eHRPD to LTE when LTE is available.

• Proposals– Option 1: Refer to QC/ZTE/CT joint contribution– Option 2: Redirection controlled by AT with AN assist in this

contribution

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UE PGWEUTRAHSGWeAN/ePCF

UE active on eHRPD

OtherRATNeighborList (redirect indicator and traffic

channel)UE measures LTE signal which satisfy the redirect

conditions

UE performs Handoff attach on ETURAN

eHRPD binding revoke

HSGW maintains partial context

Proposal – Redirection controlled by AT with AN assist

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Redirection controlled by AT with AN assist

• AN sends redirect indicator through OtherRATNeighborList message while AN wants UEs to handoff from eHRPD to LTE– AN provide the reasonable redirect threshold to UE, once the

strength of LTE channel meets the condition, UE will redirect to LTE

– Key points• AN decides the timing of setting redirect indicator and the

threshold for redirect in OtherRATNeighborList message– AN decides its redirect indicator based on its requirement, e.g. high traffic

load, special application, energy saving operation and so on, but final redirect decision is made by UE based on its conditions

• AN unicast OtherRATNeighborList message through Traffic channel

• UE performs redirect when conditions meet, otherwise UE still in eHRPD

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Enhanced OtherRATNeighborList message

Field Length (bits)MessageID 8OtherRATSignature 6NumOtherRAT 4NumOtherRAT occurrences of the following field {RATType 4NeighborRATRecordLength 10NeighborRATRecord 8*NeighborRATRecord

Length}RedirectIndicatorIncluded 1NumOtherRAT occurrences of { RedirectToThisRAT 0 or 1 ReturnIfFail 0 or 1}Reserved 0 – 7 (as needed)

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Analysis for comments• Comment 1: benefits of unicast OtherRATNeighborList

message– AN could inform its redirection requirement to eAT.

– The unicast OtherRATNeighborList message could transfer a set of handoff threshold parameters which may be differ from that including in broadcast OtherRATNeighborList message to an active eAT to assist it doing active handoff.

– The modification to eHRPD network is minimize.

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Analysis for comments

• Comment 2: Ping pong issue– Since initial LTE deployment only

covers hot spots area, it can thought eHRPD has bigger coverage and better signal than LTE, therefore no matter LTE->eHRPD or eHRPD->LTE mainly depends on LTE signal strength, as the figures show, it is reasonable that the threshold1 for triggering LTE->eHRPD handoff is smaller than threshold2 for eHRPD->LTE handoff. There is a gap between these two thresholds, in this proposal, when unicast OtherRATNeighborList, if the threshold for eHRPD->LTE handoff might be at the range between threshold1 and threshold2, the pingpong can be avoided.

eHRPD

LTE

The border of handoff from LTE to Ehrpd, Assuming LTE->eHRPD handoff happens whi l e LTE signal strength < 5

eHRPD

LTE

The border of handoff from Ehrpd to LTE, Assuming eHRPD->LTE handoff happens whi le LTE signal strength>10

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Analysis for comments

• Comment 3: conflict between unicast and broadcast otherRATNeighborList message– Unicast message always has higher priority than broadcast message, so

when UE in connected state receives unicast OtherRATNeighborList, it should ignore the broadcast OtherRATNeighborList;

– When UE performs LTE measurement in this contribution, it still maintains in eHRPD connected state, the above criteria always works;

– Once LTE signal strength meets the redirection condition (refer to comment 2), UE will redirect to LTE, it does not need to enter eHPRD idle to receive HRPD overhead messages;

– If LTE signal strength does not meet the redirection condition, UE should maintain eHRPD connection. If UE return the eHRPD idle state after ending of service, it can receive broadcast OtherRATNeighborList and overrides all parameters before.

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Pros and Cons of Proposal

• Pros– Minor modifications for AT and AN to support redirection function

– Sending TCH OtherRATNeighborList message with redirect indication allows UE to get LTE paras and perform LTE measurement more quickly than broadcast mechanism

– not handling LTE mesurement results can simplify AN’s procedure and not reporting LTE measurement result to AN can simplify AT’s procedure

• Cons– Since final redirection decision depends on UE’s measurement results, it is not

mandatory redirection