Wimax standards and Security The Wimax

Centralized Scheduling

The schedule using centralized scheduling is determined in a centralized manner than in the distributed scheduling mode. The network connections and topology are the same as in the distributed scheduling mode. However, the scheduled transmissions for the SSs shall be defined by the BS. The BS determines the flow assignments from the resource requests from the SSs. Sub- sequently, the SSs determine the actual schedule from these flow assignments by using a common algorithm that divides the frame proportionally to the assignments. Thus, the BS acts just like the BS in a PMP network except that not all of the SSs have to be directly connected to the BS, and the assignments determined by the BS extends to those SSs not directly connected to the BS. The SS resource requests and the BS assignments are both transmitted during the control portion of the frame. Centralized scheduling ensures that trans- missions are coordinated to ensure collision-free scheduling over the links in the routing tree to and from the BS, typically in a more optimal manner than the distributed scheduling method for traffic streams (or collections of traffic streams that share links), which persist over a duration that is greater than the cycle time to relay the new resource requests and distribute the updated schedule.

Only TDD is supported in mesh mode. Contrary to the basic PMP mode, there are no clearly separate downlink and uplink subframes in the mesh mode. Stations shall transmit to each other either in scheduled channels or in random access channels as in PMP mode. All the basic functions like scheduling and network synchronization are based on the neighbor infor- mation that all the nodes in the mesh network shall maintain. Each node (BS or SS) maintains a physical neighborhood list.

When using coordinated distributed scheduling, all the stations in a net- work shall use the same channel to transmit schedule information in a format of specific resource requests and grants in MSH-DSCH messages. A station shall indicate its own schedule by transmitting a MSH-DSCH regularly. The MSH-DSCH messages shall be transmitted during the control portion of the frame. An SS that has a direct link to the BS shall synchronize to the BS while an SS that is at least two hops from the BS shall synchronize to its neighbor SSs that are closer to the BS.
When using centralized scheduling, the BS shall act as a centralized sched- uler for the SSs. Using centralized scheduling, the BS shall provide schedule configuration (MSH-CSCF) and assignments (MSH-CSCH) to all SSs. The BS determines the assignments from the resource requests received from the SSs. Intermediate SSs are responsible for forwarding these requests for SSs (listed in the current routing tree as specified by the last MSH-CSCF modified by the last MSH-CSCH update) that are further from the BS (i.e., more hops from the BS) as needed. All the SSs shall listen and compute the schedule. Further, they shall forward the MSH-CSCH message to their neighbors that are further away from the BS.

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