IEEE C802.16m-08/592r1

Project / IEEE 802.16 Broadband Wireless Access Working Group <
Title / Expedited Handover Cancel Procedure
Date Submitted / 2008-0710-0731
Source(s) / Yuji Kojima, Keiichi Nakatsugawa, Masato Okuda
Fujitsu / E-mail: {kojima.yuuji, nakatsugawa, okuda}@jp.fujitsu.com
Re: / TGm SDD: Other
IEEE 802.16m-08/02440,” Call for Comments and Contributions on Project 802.16m System Description Document (SDD)”.
Target topic: Mobility Management
Abstract / This contribution provides the method of expediting a handover cancel procedure.
Purpose / This contribution provides the method of expediting a handover cancel procedure.
Notice / This document does not represent the agreed views of the IEEE 802.16 Working Group or any of its subgroups. It represents only the views of the participants listed in the “Source(s)” field above. It is offered as a basis for discussion. It is not binding on the contributor(s), who reserve(s) the right to add, amend or withdraw material contained herein.
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Expedited Handover Cancel Procedure

Yuji Kojima, Keiichi Nakatsugawa, Masato Okuda

Fujitsu

1. Introduction

In IEEE 802.16m System Requirements Document [1], the reduction of the handover latency is required. Also, in System State Transition Diagramof Draft IEEE 802.16m SDD [2], Fast Network Re-entry procedure after handover is defined.

This contribution provides the method of expediting a handover cancel procedure.

2. Methods of expediting Handover Procedure

In general, the method of the reduction of the handover latency is categorized as the following.

Omit the messages exchanged during handover due to sharing the context related to the MS between the Serving BS and the Target BS.

Pre-allocate the dedicated resource for MS's network re-entry during the handover preparation phase before the handover action phase.

In the case of handover procedure from the Serving BS to the Target BS, regarding to the former item, HO (handover) process optimization (omitting SBC-REQ/RSP, PKM phase, REG-REQ/RSP, etc) is defined in IEEE Std. 802.16e-2005 [3]. In addition, regarding to the latter item, the allocation method of a non-contention-based ranging opportunity for the MS (i.e. Fast_Ranging_IE) is defined.

In the other hand, in the case of handover cancel procedure by which the MS attempting HO can return to the Serving BS, regarding to the former item, the HO cancel message (i.e. MOB_HO-IND message with the HO cancel option) is defined to allow the MS to resume normal operation communication without the other messages exchanged. However, regarding to the latter item, IEEE Std. 802.16e-2005 [3] doesn’t provide the corresponding method such as the above non-contention-based ranging opportunity. The MS is required to perform the bandwidth request ranging for sending the HO cancel message, as shown in Fig.1. This issue should've been considered. It should be fixed in the 802.16m amendment.

Fig.2 shows an example of the proposed HO cancel procedure, but doesn't restrict the other solution of the expedited HO cancel procedure.

The Serving BS retains the MS context and accepts HO cancel message until expiration of resource retain timer or until successful HO to the target BS. The resource retain timer starts upon receiving HO-IND message. If HO-IND message is omitted, the resource retain timer starts when the last message during HO preparation phase is transmitted, or when serving BS stops allocating resources to MS for transmission.

Fig.1. HO cancel procedure in the case of 802.16e / Fig.2. Example of the proposed HO cancel procedure

3. Evaluations

Fig.3 shows the typical timeline of the proposed HO cancel procedure compared with the conventional procedure in the case of 16m Frame Structure with two switching points in TDD duplex mode. In this case, the proposed HO cancel procedure can reduce HO interruption time for 10ms. Assuming that a contention occurs during ranging procedure in the case of conventional procedure, the reduced HO interruption time becomes longer. In SRD [1], the maximum HO interruption time is required to be 27.5ms (Intra-frequency), 40ms (Inter-frequency within a spectrum band) or 60ms (Inter-frequency between spectrum bands). The reduced HO interruption time (10ms) can be considered as a large value compared with the above requirements.

In the case where the MS moves along the cell boundary between the Serving BS and the Target BS, such as the Trajectory 2 defined in Section 8.1.1.1.2. of EMD [4], HO cancel often occurs. Therefore, it is important to reduce HO interruption time during HO cancel procedure in order to improve the overall user experience.

On the other hand, the ranging code may be a valuable resource although the total number of the separate ranging codes and ranging slots are not specified in the current SDD [2]. Inclusion of Station Identifier in a bandwidth request indicator is considered in the current SDD [2]. If CDMA code is used as non-contention based random access method to transmit the bandwidth request indicator including Station Identifier, the Serving BS is not required to allocate the additional dedicated ranging code for the MS. In this case, the Serving BS retains the already-allocated CDMA code as a component of the MS context until expiration of resource retain timer or until successful HO to the target BS. If the Serving BS receives the bandwidth request indicator including Station Identifier after the completion of HO preparation phase, the Serving BS can regards it as the bandwidth request for HO cancel message. Therefore, the Serving BS can allocate the uplink bandwidth for HO cancel message without the redundant message exchange such as step 5, 6 in Fig.1.

Anyway, the already-allocated bandwidth ranging code for the MS before HO preparation phase can be reutilized as the dedicated ranging code for HO cancel message in some cases.

Fig.3. Timeline of HO cancel procedure

34. Proposed Texts

[Change the text as follows in Section 10.3.2.2.4 of SDD [2].Insert the following text into a suitable place of Section representing Mobility Management]

------Start of the text proposal ------

IEEE 802.16m supports the handover cancel procedure by which the MS attempting HO can resume normal operation communication to the Serving BS without the exchange of any messages except the HO cancel message. This handover cancel procedure can include allocating the dedicated resource for the MS to send the HO cancel message.

10.3.2.2.4 HO cancellation

After HO is initiated, the MS may cancel HO at any phase during HO procedure by sending a HO cancellation message to the serving BS. After the HO cancellation message is processed, the MS and serving BS resume their normal operation. The dedicated ranging code assignment for transmitting HO cancellation message is FFS. A dedicated ranging code at the serving BS may be assigned to the MS during HO preparation to facilitate non-contention-based bandwidth request ranging. Otherwise, the already assigned dedicated ranging code for the bandwidth request indicator including the MS's Station Identifier before HO preparation may be utilized during HO cancellation procedure. If the dedicated ranging code is assigned to the MS by the serving BS, the MS shall transmit the dedicated ranging code to the serving BS for HO cancellation. Upon receiving the dedicated ranging code, the serving BS shall allocate uplink resources for HO cancellation message.

------End of the text proposal ------

References

[1] IEEE 802.16m System Requirements, IEEE 802.16m-07/002r46

[2] The Draft IEEE 802.16m System Description Document, IEEE 802.16m-08/003r35

[3] IEEE Std. 802.16e-2005, IEEE Standard for Local and metropolitan area networks, Part 16: AirInterface for Fixed and Mobile Broadband Wireless Access Systems, Amendment 2: Physical andMedium Access ControlLayers for Combined Fixed and Mobile Operation in Licensed Bands, andIEEE Std. 802.16-2004/Cor1-2005, Corrigendum 1, December 2005

[4] IEEE 802.16m Evaluation Methodology Document (EMD), IEEE 802.16m-08/004r3

[5] “Comments on 2nd Draft HO SDD Text”, IEEE C80216m-MAC-08/087