ling and procedures specified in the IEEE 802.16-2009 standard. An MS performs handover from a legacy BS to a new BS by using either zone-switching or direct handover process. The zone-switching handover is applicable to new base stations supporting coexisting legacy and new system. The direct handover is applicable to new base stations which only support new mobile stations. A new BS may also decide to keep a new MS in the legacy zone when coexisting with legacy systems. As shown in Figure 6-18, the zone-switching handover procedure begins with a mobile station's decision to handover from the serving legacy BS to the LZone of a target new BS. The handover decision, initiation, and cancellation follow the same procedures described earlier. The zone-switching is initiated either by the MS or the new BS, and the final decision is made by the new BS. In the case of XiCipher-based Message Authentication Code tuple is used to protect control messages in IEEE 802.16-2009 standard. For example, if a BS intends to transmit a control signal with a CMAC tuple to an MS, the CMAC must comprise an MS authentication code for the MS to authenticate whether the control message is authentic. Serving Target New BS Serving Legacy BS Legacy BS Target New BS LZone MZone LZone MZone MOB_MSHO-REQ MOB_MSHO-REQ MOB_BSHO-RSP MOB_BSHO-RSP MOB HO-IND MOB HO-IND (Target BS-ID) (Target BS-ID) RNG-REQ RNG-REQ RNG-RSP RNG-RSP RNG-RSP (including Zone-Switching Parameter) Data Path Established Synchronization with MZone RNG-RSP (including Zone-Switching Parameter) Ranging Purpose Indication = Zone Switch Synchronization with MZone I RNG-RSP AAI_RNG-REQ Ranging Purpose Indication = Zone Switch Data Path Established AÄI INRG-RSP Data Path Established The Target BS Instructs the MS to Switch Zone during Network Re-entry The Target BS Instructs the MS to Switch Zone after Network Re-entry FIGURE 6-18 The handover procedure from legacy to new base stations [2] CHAPTER 6 The IEEE 802. 16m MAC CPS (Part I) an MS-initiated zone-switching process, the MS 