SCHEDULING SETUP IN WiMAX

IEEE 802.16e has been developed to serve mobile SSs through a centralized BS by employing point-to-multi point (PMP) as well as through the optional mesh mode architecture of a wireless network topology. In the former operating mode, the downlink from a BS to an SS operates on the basis of PMP. However, in the mesh mode, there are no separate DL and UL subframes and there can be SSs that are not directly connected to the BS but only through intermediary SSs, which is in contrast to the PMP mode. Hence, a larger number of SS can be supported in the mesh mode than in the PMP mode or equivalently, mesh mode can offer the least number of BSs for economy. Furthermore, in the mesh mode of WiMAX the SSs can consume less power, thereby efficiently using battery life, as it is not mandatory to be always connected to the BS. The intermediate SSs will greatly reduce the power consumption of far off users.

Three types of scheduling are supported in the mesh mode of WiMAX; they are coordinated distributed, uncoordinated distributed, and centralized scheduling. In coordinated scheduling, all nodes coordinate in their two-hop neighborhood and broadcast their schedules (available resources, requests and grants) to all of their neighbors; whereas, in uncoordinated scheduling there are direct uncoordinated requests and grants between two nodes. The main difference between the two types of distributed scheduling methods is in the use of the control subframe: transmitting collision-free scheduling messages in the coordinated type and with possible collision in the uncoordinated type. In the centralized method, the resources are distributed centrally and this is similar to the PMP case.

The performance of coordinated distributed scheduling has been investigated. It has been reported that this mechanism has a scalability problem that leads to poor performance in dense networks and aggravates QoS provisioning. To overcome these problems, the XmtHoldoffTime has been made adaptive at every node, which has been shown to improve contention, and thus enhance the throughput in dense meshes. A combined distributed and centralized scheduling scheme has been proposed for mesh networks in WiMAX; wherein, through simulation studies, it has been shown that the minislot^[*] utilization can be significantly improved with the proposed scheme.

For synchronization of distributed and centralized control mesh networks, the WiMAX standard provides network configuration (MSH-NCFG) and network entry (MSH-NENT) packets as a basic level of communication between various nodes. The scheduling of transmission for the next MSH-NCFG is done by a mesh-election procedure. It is carried out among all eligible competing and local nodes. The NetEntry scheduling protocol provides slots for transmission of MSH-NENT packets by new nodes that are not yet fully functional members of the mesh .

In contrast to mesh mode, a detailed QoS architecture has been defined for the PMP mode. Scheduling services refer to data-handling mechanisms supported by the MAC scheduler for data transport on each connection. A single scheduling service will be associated with each data connection. Each of the data services will be characterized by a set of parameters that will quantify the QoS aspects of its behavior. These QoS parameters are managed by dynamic service addition (DSA), dynamic service change (DSC), and dynamic service deletion (DSD) message dialogues, where each of these signaling schemes can be initiated by either a BS or an SS.

It can be seen that scheduling mechanisms for a PMP mode are also applicable to a mesh mode; however, since all transmission between two nodes is managed by a link, PMP scheduling is not directly applicable to the mesh mode. By default, at the time of connection establishment, each mesh SS is assigned a unique node identifier; a Service Adaptive QoS has been proposed for mesh mode, which assigns five node IDs to each SS instead of a single ID. These five virtual nodes correspond to five traffic classes, and each node requests bandwidth individually and the mesh mode BS handles these requests on the basis of their scheduling services. Hence, mesh mode WiMAX can be treated by scheduling the services of the PMP mode. Therefore, subsequently in this chapter, we shall only consider scheduling in the PMP mode for WiMAX networks.