Wimax standards and Security The Wimax

RNG-REQ
RNG-RSP

RNG-REQ
RNG-RSP

RNG-REQ
RNG-RSP
BS

MS
Start frame

FIGURE 6.4
Operation of PSC III.

the MS to enter sleep mode again for the duration of the time indicated by the next periodic ranging TLV encoding in the RNG-RSP message. If downlink traffic addressed to the MS arrives during PSC III operation, the BS informs the MS of this fact by sending an RNG-RSP message that includes the next periodic ranging TLV with a value set to zero. If an MS receives the RNG-RSP message with this indication, it immediately deactivates PSC III and resumes normal operation with the BS to receive pending data.

3. Handover

The IEEE 802.16e system provides an HO function to support the mobility of MS. When the signal quality of the current BS worsens due to fading or interference due to mobility, the MS hands over to another BS that provides better signal quality and quality of service (QoS). The HO procedure in the IEEE 802.16e is mainly divided into two processes: network topology acqui- sition and HO operation. In addition, macro diversity HO (MDHO) and fast BS switching (FBSS) techniques are proposed as optional modes to support more seamless and faster HO.

1. Network Topology Acquisition

The object of the network topology acquisition is to collect information about a channel’s description and its physical quality from an MS’s neighboring BSs before an actual handover occurs. Information about the network topology is acquired by performing a network topology advertisement process and a scanning process. In addition, an MS can execute an association process during the scanning process, which is an optional initial ranging procedure formed between the MS and a target BS to which the MS wants to connect.

1.