Wimax standards and Security The Wimax

2.10.1 MAC Enhancement for Coexistence
802.16h is in the process of providing MAC enhancements to support commu- nication using license-exempt and uncoordinated bands. We list below some of the enhancements included in (IEEE LE-TG, 2006). A complete description of these enhancements was not ready until the time of writing this document.

1. 
   Capability negotiation: is a mechanism provided at the MAC layer for the BS to learn about its associated SS capabilities and functionalities for supporting coexistence licensed-exempt band.
   
2. 
   Extended channel numbering structure: is used to define the channel bandwidth for better interference management. This pro- cedure provides enhancement to channelization and definition of channel numbers. It defines three channelization schemes— extended channel number, which specifies channel number refer- ence; base channel reference, which defines the frequency range; and channel spacing, which defines channel spacing value in 10 kHz increments.
   
3. 
   Measurement and reporting: a process for defining mechanisms and messages at the MAC layer to measure and report interference level and bandwidth band usage.
   

2.11 IEEE 802.16j
IEEE 802.16’s Relay Task Group is in charge of developing amendments to extend the IEEE 802.16e-2005 to support multihop relay operation. IEEE 802.16’s Mobile Multihop Relay Study Group was in charge of IEEE 802.16j project since July 2005. The study group was disbanded in March 2006 and the project was assigned to the Relay Task Group, which continues to work on the project that is still in the predraft phase.
802.16j (IEEE Relay-TG, 2006) is intended to improve legacy 802.16 network’s coverage, throughput, and system capacity. 802.16j extends the network infrastructure of legacy 802.16 to include three relay types: fixed relays, nomadic relays, and mobile relays. 802.16j is required to enable the operation of the relay nodes over the licensed band. The OFDMA PHY air interface is the PHY layer specification chosen by the group for 802.16j oper- ation. 802.16j is supposed to define the necessary MAC layer enhancements while at the same time it does not change the SS specifications. However, exis- tence of mobile relay types requires that the relaying process should be carried out by the MS as well. To provide an efficient relaying process, MS should be chosen efficiently and should have some knowledge of the network status, mobility characteristics of other MSs, and the traffic. Thus, conventional MS may not serve as a mobile multihop relay (MMR), since relay stations (RS) are required to pretend to be a BS for MS and to be an MS for BS. Hence, 802.16j defined the three RS types capable of supporting PMP links, MMR links, and aggregation of traffic from multiple RSs. To facilitate RSs communication with BS, this requires changes to BS to support MMR links and aggregation of traf- fic from multiple RSs. To achieve MMR requirements, 802.16j enhances the normal frame structure at the PHY layer and adds new messages for relay at the MAC layer (Marks, 2006).
We remark that the optional 802.16-2004 mesh mode is different from
802.16j. Actually, 802.16j is initiated to overcome mesh mode limitations because mesh mode replaces the PMP frame structure by point-to-point struc- ture. Consequently, conventional 802.16-2204 PMP devices are not able to communicate with mesh devices. Thus, one of the main objectives of 802.16j is to design MMR without modifications to SSs. Hence, to retain the PMP backward compatible frame structure, 802.16j unlike mesh mode defines the network architecture to be tree based with BS as the root.