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Introduction to WiMAX Handover

1

January 21, 2011 by Isybel Harto

Handover Scenarios

Strictly speaking, the BS defined in IEEE 802.16e is a transceiving entity at media access control layer (MAC) or physical layer (PHY). In a specific cellular network structure, a BS entity (base transceiver station, BTS) actually includes multiple MAC and PHY transceiving entities. This is represented in the implementation. For clear description, a BS refers to the minimum unit for handover, a sector, unless otherwise stated in this document.

Figure 1-1 shows the handover scenarios:


When an MS moves, the BSs around it also change. The handover scenarios described herein are distinguished by the location relationship between the target BS and the serving BS in the handover indication message of an MS during the handover action. One site generally manages 1 to 6 BSs. An ASN-GW has a point-to-point physical connection with a BTS. Therefore, handovers can be categorized as intra-GW handover and inter-GW handover. As shown below, when an MS moves, it gradually triggers handover scenarios 1, 2, 3, and 4. Scenarios 1 and 2 are intra-GW handover (intra-GW HO). Scenarios 3 and 4 are inter-GW handover (inter-GW HO).

The WASN9770 V200R001 only considers intra-GW handover (scenarios 1 and 2). The subsequent release will support inter-GW handover.

Intra-GW Handover

Intra-GW handover refers to the handover between BSs managed by a same ASN-GW. According to the location of the local BTS of the handover related BSs, Intra-GW can be categorized in two scenarios:

2)      Handover scenario 1 is defined as the handover between different BSs of a same BTS, which mean R1 air interface handover occurs.

This handover occurs between different BSs of a same BTS. When the system determines that an MS should hand over to another BS than the current BS, the MS needs to reestablish the R1 interface with the target BS. This process is not accompanied by the handover or migration of other interfaces (for example, the R6/R4/R3 interface). Once the handover succeeds, the R1 link between the MS and the original serving BS is deleted.

Figure 1-2 Handover scenario 1

2)      Handover scenario 2 is defined as the handover between BSs of different BTSs of a same GW, that is, R1 air interface handover and R6 interface handover occur.

This handover occurs between the BSs of different BTSs in a same ASN-GW. The R6 channel between the serving BS and the ASN-GW is different from that between the target BS and the ASN-GW. When the MS moves to the target BS, it triggers the reestablishment of the R1 air interface link with the target BS. Meantime, the R6 path between the target BS and the anchor GW is established.

When the handover succeeds, the R1 link between the MS and the original serving BS is deleted. Meantime, the R6 path between the original serving BS and the anchor GW is also deleted.

This handover process is not accompanied by the handover or migration of other interfaces (for example, R4/R3 interface).

Figure 1-3 Handover scenario 2

R1 air interface occurs with R6 interface handover.

As shown in Figure 1-3, before the handover, the network side data is carried by the R6 path between the serving BS and the ASN-GW. After the handover, the MS moves to the target BS and the network side data is carried by the R6 path newly established between the target BS and the ASN-GW. Meantime, the original R6 channel between the original serving BS and the ASN-GW is deleted.

Inter-GW Handover

Inter-GW handover refers to the handover between BSs of different ASN-GWs. According to network handover paths, Inter-GW handover can be categorized as follows:

2)      Handover scenario 3 is defined as the handover between BSs of different ASN-GWs. R1 air interface handover and R6 interface handover occur. Meantime, the R4 interface changes.

This handover occurs between BSs managed by adjacent ASN-GWs. When an MS moves to a target BS, if the target BS and the serving BS are managed by respective ASN-GWs, the MS reestablishes a new R1 air interface link with the target BS and the target BS establishes a new R6 data path with the target GW. To ensure that no data is lost and data path change is the smallest when the MS moves, an R4 data path need first be established between the target GW and the Server GW. When the MS is handed over to the target BS, the network side data delivered from the R3 interface is first cached on the serving GW and then delivered to the target GW through the R4 path. The target GW then delivers the network side data to the target BS. The serving GW also serves as the anchor GW to provide data forwarding.

This handover process is not accompanied by R3 interface migration.

Figure 1-4 Handover scenario 3-1

Air interface handover occurs, accompanied by R6 handover and the establishment of an R4 path.

As shown in Figure 1-4, before and after the handover, the handover of air interface link occurs between the MS and the BS, accompanied by the R6 path handover between the BS and the ASN-GW. Before the handover, network side data is carried by the R6 path between the serving BS and the serving GW. After the handover, when the MS moves to the target BS, the network side data is carried by the R6 path newly established between the target BS and the target GW. Meantime, an R4 data path is established between the target GW and the serving GW. The serving GW/anchor GW forwards data to the R4 interface.

Figure 1-5 Handover scenario 3-2

When handover scenario 3-1 is complete, the original serving GW/anchor GW serves as an anchor GW only and the target GW serves as the serving GW. The MS continues moving to the BS managed by another ASN-GW. Assume that there is no R3 interface relocation. Then, the serving GW and the anchor GW are not on the same NE entity. As shown in Figure 1-5, before and after the handover, the air interface link between the MS and the BS changes. Meantime, R6 path handover occurs between the BS and the serving GW. An R4 data path is established between the target GW and the anchor GW. The anchor GW forwards data to the R4 interface. After the handover, the R4 data path between the anchor GW and the serving GW is deleted.

1)      Handover scenario 4 is defined as the handover between BSs of different GWs. R1 air interface handover and R6 interface handover both occur. Meantime, R3 interface relocation occurs.

This handover occurs between the BSs managed by adjacent GWs. If the target BS and the serving BS are managed by respective GWs when an MS moves to a target BS, the MS reestablishes a new R1 air interface link with the target BS and a new R6 data path is established between the target BS and the target GW. The serving GW determines that the R3 interface should relocate to the target GW.

This handover process affects the mobile IP (MIP) function to a large extent. Due to R3 interface relocation, the FA function on the ASN-GW needs to relocate to the target GW and the tunnel between the HA and the foreign agent (FA) is relocated. The MIP tunnel is reestablished between the HA and the target GW FA. Meantime, the change of Care of Address (CoA) triggers a new MIP registration.

Figure 1-5 Handover scenario 3-3

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One thought on “Introduction to WiMAX Handover

  1. eeeboy says:

    thanks for this wonderful document !

    I am currently working in the core network team of an WiMAX operator, we are using Huawei equipments for both core (WASN9770 V3R3) and Access Networks.

    In my network context, I have one query to you- In case of Intra GW hand over, will client IP (last mile IP) be changed ? I need this answer for both of the scenario, i.e. when handover occurs between from one BS to another BS on the same BTS (only R1 data is deleted) and when handover occurs between one BTS to another BTS (R1 and R6, both data deletes).

    Thanks in advance. 🙂

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