Tuesday, July 27, 2010

Use of Existing Message to Request and ACK Handoff

As mentioned earlier, the HO-REQ and HO-ACK messages have not been defined in the 802.16-2004 standard and defining the new messages in the standard is not our goal either. To avoid a change to the standard, we observe that it is possible to reuse an existing message, namely the Deregistration Command (DREG-CMD) with action code of 03, to serve the place of the HO-REQ and HO-ACK messages. That is, when the SS initiates the handoff, it sends a DREG-CMD (code = 03) message to its BS. If the BS agrees to the handoff, it returns another DREG-CMD (code = 03) to the SS. When the latter is received by the SS, it signifies that the handoff process is started.

We now explain why the DREG-CMD (code = 03) message can be applied as such. The standard specifies [5] that "the DREG-CMD message shall be transmitted by the BS on an SS's basic CID to force the SS to change its access state. Upon receiving a DREG-CMD, the SS shall take the action indicated by the action code." If the action code is 03, the "SS shall return to normal operation and may transmit on any of its active connections." First of all, BS does not expect to receive the DREG-CMD (code = 03) message from its SSs. If it is indeed received, how the BS would interpret the message has not been specified in the standard. Thus, it is acceptable if the BS chooses to interpret the message as a request for handoff (HOREQ). After the SS sends the first DREG-CMD (code = 03) message, the SS intends to begin a handoff, thus has a context to interpret the returned DREG-CMD from the BS as an ACK (HO-ACK).

The choice of the DREG-CMD (code = 03) message has an additional advantage. Namely, the message simply asks the SS to resume normal operations, thus it does not cause any adverse effects to the SS if it does not interpret the message in such a special way for supporting handoff. Furthermore, the message also enables correct operations for mixed SSs and BSs with or without the new handoff capability. For example, suppose that the SS has the handoff capability, but its BS does not. In this case, after receiving the first DREG-CMD (code = 03) from the SS, the BS will not send the second DREG-CMD (code = 03) to acknowledge (or approve) the handoff. Without the returned DREG-CMD, the SS simply continues its operations as defined in the original standard. In short, by initiating the handoff process from the SS, and by reusing the DREG-CMD (code = 03) message, we ensure that there are no problems arising from the misinterpretation of a message that arrives at an unexpected time due to a failure of synchronization, for example.

To prevent "ping-ponging" of an SS between an old and new BS, we propose the usual solution of a hysteresis threshold, such that a handoff will only be requested by the SS if the received signal strength from the new BS exceeds that from the old BS by at least this threshold. However, as the handoff scheme uses a "short" version of the initialization process, and in particular omits the authentication and key exchanges and request/grant of connection IDs (which are retained by the old BS and transmitted over the backhaul to the new BS), it is possible for the SS to abort the handoff at any stage before the MAC and PHY are reset at the old BS with BN-MSG4 in, simply by sending another DREG-CMD (code = 03) to the old BS.

With the protocol designed for supporting terminal mobility in the 802.16-2004 networks, mobile terminals now can no longer enjoy the "unlimited" supply of power as in the fixed wireless networks. In the following, we propose and study a mechanism to conserve battery energy for terminals in the 802.16-2004 networks.
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