Friday, February 25, 2011

Network-Initiated Handoff | WiMAX HANDOFF CONTROL

The network can initiate handover depending on its current status. Such a decision can be made after the evaluation of the payload of different BSs or the data throughput at the reference points. In profile A, ASN GWs may initiate handover of MSs under their control. The network-initiated handoff procedure depicted by Figure 1 begins by a prehandover operation, during which the serving ASN GW collects status information from the BSs and the MSs to decide whether a network-initiated handover is required. If it is the case, the ASN GW sends a HO_Directive message to the serving BS to order it to handoff some MSs to other BSs while providing it with a list of recommended BSs and starting a timer. 

The ASN GW may also specify how many payload should be migrated to other BSs to achieve load balancing as it may indicate the list of the recommended MSs that need to be handed over. The serving BS should respond by a HO_Directive_Rsp message to make the ASN GW stop the timer. The serving BS selects some candidate MSs based on the information maintained by it and the list given by the HO_Directive message and then it may order some candidates to achieve scanning to get their neighbors’ information. The serving BS will then select some suitable MSs for handover and send separately a HO_Req message relative to each MS to the serving ASN-GW. The following procedure is the same as the process of MS-initiated handoff described earlier. When the process of handover preparation is finalized by the network, the serving BS will send a MOB-BSHO_REQ message to each MS to order it to hand over to the target BS.

Figure 1: The preparation phase of a network-initiated handoff.

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