 carries no payload and can be sent standalone or concatenated with other MAC PDUs in the downlink or uplink. If the MS uses an anonymously assigned uplink resource to send the signaling header, the MS is required to include the STID in the content field of the signaling header. The reserved value of Flow ID used for the MAC signaling header is 0b0001. The structure of a MAC signaling header is shown in Figure 6-10. The Flow ID in the figure denotes the signaling header and the Type field identifies the type of signaling header. There different types of signaling headers specified by IEEE 02.16m standard, as shown in Table 6-5. The use case, as well as important parameters of each signaling header type, are described in Table 6-5. CHAPTER 6 The IEEE 802.16m MAC CPS (Part I) Feedback Type MSB of Feedback Content (4 Bits) (8 Bits) MSB Feedback Content MSB of CID (8 bits) (8 Bits) LSB of CID Header Checksum Sequence (8 Bits) (8 Bits) Feedback Header with CID Field Feedback Type MSB of Feedback Content (4 Bits) (8 Bits) Feedback Content (16 bits) LSB of Feedback Content Header Checksum Sequence (8 Bits) (8 Bits) Feedback Header without CID Field FIGURE 6-9 Legacy Type II feedback MAC signaling headers [1] 6.4 MOBILITY MANAGEMENT AND HANDOVER 6.4.1 Handover Mechanisms The IEEE 802.16-2009 standard defines three basic mechanisms for handover [6,7,9,10]: 1. Hard Handover (HHO): a process that is based on Received Signal Strength Indicator (RSSI) measurements conducted on the preamble. The MS continuously measures the RSSI of the serving BS and reports the values periodically to the serving BS. When the MS notices that the RSSI is below a certain threshold, it starts scanning procedure by sending a MOB_SCN-REQ message to the serving BS. The serving BS replies by sending a MOB_SCN-RSP message that contains, among other parameters, the scanning time interval. During the scanning period, the MS measures the RSSI for all the neighbor base stations. The neighbor base stations are 6.4 Mobility management and handover Flow ID S