Sunday, February 20, 2011

Base Station Initiated Handoff | WiMAX HANDOFF CONTROL

The serving BS may decide to no longer manage an MS and initiate handoff for it. This occurs generally when the serving BS can no longer provide the required QoS or when it detects that the MS is moving out of its coverage area. Although the causes of a BS-initiated handoff are similar to the causes of an MS-initiated handoff, it is useful to let the BS decide to centralize the handoff procedure. In fact, the MSs are generally tiny equipments with limited power and computing resources; therefore, it is important to implement the handoff process at the BS level. 

The serving BS continues broadcasting the MOB_NBR-ADV message for the served MSs, but it orders the MS that needs to perform handoff via a MOB_BSHO-REQ message to start scanning the neighboring BSs. The MOB_BSHO-REQ message transmitted on the basic connection defines a list of recommended target BSs along with service level predictions and channel details. Upon receiving that message, the BS starts the scanning procedure and sends back a MOB_BSHO-RSP message to the serving BS indicating a list of recommended BSs. 

The rest of the handoff process is similar to the MS-initiated handoff case. In fact, the MS waits for the list of the target BSs and then sends a HO-IND message to its serving BS. Upon receiving the fast ranging IE, the MS sends the RNG-REQ ranging request message to the target BS to register with it. The BS-initiated handoff process, described earlier, is depicted by the flow chart in Figure 1.

Figure 1: The BS initiated handoff at the MS level.

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