Wimax standards and Security The Wimax

Basic Handover Operation

HO is essential for supporting MS mobility in mobile cellular environments, and it enables an MS to change its air interface from one BS to another. Figure 6.8 illustrates a basic HO procedure in the IEEE 802.16e system. An HO observes the following procedures: (1) cell reselection, (2) HO decision and initiation, (3) HO cancellation, (4) synchronization to target BS down- link, (5) use of scanning and association results, (6) ranging, (7) termination with the serving BS, (8) drops during HO, and (9) network entry/reentry. The messages related with the HO process are as follows:

• 
  MOB_MSHO-REQ is issued by an MS to initiate an HO. It contains the information about the recommended neighboring BSs.
  
• 
  MOB_BSHO-RSP is sent by a BS in response to reception of the MOB_MSHO-REQ message. It delivers the information about the recommended neighboring BSs for HO.
  
• 
  MOB_BSHO-REQ is issued by a BS that wants to initiate an HO. The MS receiving this message scans the recommended neighboring BSs specified in this message.
  

Cell reselection


Handover decision and initiation


Termination with serving BS


Synchronization to target BS downlink
Ranging


Network entry/reentry


FIGURE 6.8
Handover operation.

• 
  MOB_HO-IND is transmitted from the MS to its serving BS to inform of the final HO indication, which may result in serving BS release, HO cancellation, or HO rejection.
  

1. Cell Reselection

Cell reselection refers to the process of an MS scanning or association with one or more BSs to determine their availability and suitability as an HO target. To perform cell reselection, the MS uses the information acquired from an

MOB_NBR-ADV message and the serving BS’s scheduled scanning intervals. Therefore, cell reselection process does not involve terminating an existing connection with a serving BS.

2.