Saturday, July 24, 2010

Handoff Protocol and Message Exchanges

Figure 1 shows the sequence of message exchanges for connection handoff. We note that as a SS can stay silent (with no transmission) at times, its BS may not recognize the need of handoff when the SS moves away for the BS. So SS initiated handoff is more appropriate than that initiated by BS. When a SS realizes a need for handoff (e.g., by checking error rate for the MAPs periodically broadcast from BS on the DL or by measuring the received signal strength), it sends a handoff request (HO-REQ) to its current BS (denoted as the old BS). In turn, the BS returns with a handoff acknowledgment (HO-ACK) message to signify that the SS can start the handoff process. It is important to note that both HO-REQ and HO-ACK messages are not defined in the 802.16-2004 standard. We include them here mainly to illustrate the handoff protocol and discuss later how one can replace these messages by an existing one defined in the standard.

Figure 1: Handoff protocol. 

Soon after the old BS responds to the SS's request for handoff, the old BS sends the Backhaul Network message 1 (BN-MSG1) to inform the PFA, which is the "anchor" point for the SS, of the MAC address, CIDs, encryption keys, and other service parameters associated with the SS. Upon receiving the MSG1, the PFA forwards BN-MSG2 messages, which contain information about the SS's MAC address, connections, and operational parameters, via the backhaul network to alert all BS's surrounding the old BS, to look out for the possible handoff of the SS. This list of neighboring BSs, which are the likely candidates for handoff, is maintained at the PFA, and is analogous to the neighbor list in code division multiple access (CDMA) systems.

Following the reception of the HO-ACK message, the SS proceeds to execute the functionalities in Figure 1. That is, it scans and synchronizes with a new channel of a neighboring BS (denoted as the new BS in the diagram). Then, it obtains the UL transmission parameters, completes the ranging and adjustment procedure, registers and sets up provisional connections with the new BS. Once the "short initialization process" is completed, the new BS sends the BN-MSG3 to inform the PFA of the completion of the handoff. In turn, the PFA sends the BN-MSG4 to reset PHY and MAC associated with the SS on the old BS. As the new connections are established between the SS and the new BS, the PFA starts to tunnel data to the new BS for forwarding to the SS.

Before continuing, we note that there is a key delay requirement for the handoff protocol to work properly. That is, the BN-MSG2 sent from the PFA must be received and processed by all BSs surrounding the old BS before the first ranging (RNG-REQ) message from the SS arrives. This is so because without receiving the BN-MSG2 message, the neighboring BSs will not be aware of the handoff, and thus follow the rest of the steps for the normal initialization process, instead of those of the "short" process for handoff. (On the other hand, the SS knows that it has to follow the short process because it has been told to do so by receiving the HO-ACK message from the old BS.) As the scanning and synchronization with a new channel may take at least tens of milliseconds to complete, the delay requirement does not appear to be a stringent one. Rather, with a typical high-speed IP backhaul network, it is expected that the BN-MSG2 message can reach and be processed by the neighboring BSs within a couple of tens of milliseconds, which should be short in comparison with the delay incurred in channel scanning and synchronization.
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