Mobile-Initiated Handoff | WiMAX HANDOFF CONTROL

An MS may decide to change its serving BS after losing signal quality or after detecting that a higher QoS can be disserved by another BS. In such situations, the MS will initiate the handoff. Nevertheless, IEEE 802.16e specifications did not specify the methodology of deciding whether or not to perform the handoff; they only focused on the mechanisms that should be implemented to collect information about the neighboring BSs for taking the handoff decision. In fact, each BS should transmit on the broadcast connection a mobile neighbor advertisement message MOB_NBR-ADV informing the listening MSs of the characteristics of any neighboring BS. Such a message includes the identifier of each neighboring BS, its frequency, the supported services, and its available radio resources such as its available channels. Upon getting such information, each MS should be able to take the handoff decision in the light of scanning of possible target BSs. The scanning procedure begins when the MS sends its serving BS a MOB_SCN-REQ message to inform the serving BS that it wishes to scan the neighboring BSs. The message indicates a length of time in frames for this interval and the type of association that will be used for scanning. The aim of association is to enable the MS acquiring and recording ranging parameters and service availability information to select the proper BS target. 

The specifications define three levels of possible associations, which are the association without coordination, the association with coordination, and the network-assisted association reporting. With the association without coordination, the target BS has no knowledge of the MS. Association with coordination means that the serving BS will coordinate association with the requested target BS and then respond to the requesting MS. With the network-assisted association reporting, the serving BS coordinates association with the requested target BS, a RNG-RSP message is sent back over the backbone to the serving BS. The latter collects all received RNG-RSP messages from all scanned BSs and then sends them to the MS in the form of a MOB_ASC_REPORT message. 

Next, the serving BS responds with a MOB_SCN-RSP message specifying the length of the approved scan and the association type that will be used. Upon receiving that message, the MS may scan its neighbors by synchronizing with a given BSs DL transmissions and estimating the quality of the physical channel. After performing scanning, the MS sends a MOB_MSHO-REQ message to its serving BS on the basic connection. That message includes a list of BSs recommended by the MS as targets. Upon receiving such message, the serving BS sends HO-prenotification messages to all BSs specified in the MOB_MSHO-REQ message and waits for the corresponding HO-prenotification-response messages to analyze them. Next, the serving BS generates the MOB_BSHO-RSP message indicating a list of target stations and sends it back to the MS on the basic connection. The MS may now perform or cancel the handoff; it informs its serving BS via a MOB_HO-IND message sent on the basic connection. If it performs handoff, the MS will inform its serving BS that it is leaving it while providing the parameters of the target BS and then it registers with the target BS. The target BS may, upon receiving the HO-prenotification message, include a fast ranging information element (IE) that provides the MS with a noncontention-based initial ranging opportunity to minimize the handoff process latency.