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.

Scheduling Services

Overview

Scheduling services are the medium access control functions (data flow control) that define how and when devices will receive and transmit on a communication system. The types of services that WiMAX can provide range from guaranteed bandwidth with low delay unsolicited grant service (UGS) to random access best effort (BE) service. WiMAX systems use a grant management system to coordinate the request for new services and changes to existing services (such as requesting more bandwidth). The WiMAX system uses a combination of time division multiple access, polling and contention based flow control to provide specific types of services to users.

Time division multiple access (TDMA) is a process of sharing a single radio channel by dividing the channel into time slots that are shared between simultaneous users of the radio channel. When a subscriber communicates on a WiMAX system using TDMA, he/she is assigned a specific time position on the radio channel. By allowing several users to use different time positions (time slots) on a single radio channel, TDMA systems can guarantee a constant data rate with a minimal amount of flow control overhead.

Polling is the process of sending a request message (usually periodically) for the purpose of collecting events or information from a network device. The receipt of a polling message by a device starts an information transfer operation for a specific time period. Polling may be performed with specific units (unicast), to groups of units (multicast) or to all units (broadcast).

Unicast polls are requests for data transmission or responses to commands that are only sent between a sender (polling device) and receiver (polled device). When a subscriber station is responding to a unicast polling message, no other devices are allowed to transmit.

Multicast polls are requests for data transmission or responses to commands that are sent from a polling device to several receiving devices which are part of a multicast group. When a device receives a multicast polling message for its group, it will respond if it has data to send. When a subscriber station is responding to a multicast polling message, others may also have information to transmit. For multicast poll messages, subscriber stations must use contention based access (on the contention slot) to send their data.

Broadcast polls are requests for data transmission or responses to commands that are sent from a polling device to all devices that are able to receive its broadcasted polling message. When a device receives a broadcast polling message, it will respond if it has data to send. For broadcast poll messages, subscriber stations must use contention based access (on the contention slot) to send their data.

The amount of time between polling messages is called the polling cycle. The time between polling cycles is a balance between delay (more polling messages is less delay) and overhead (more polling messages increases the percentage of data that is used for control messages).

Figure 1 illustrates the different types of polling that are used in the WiMAX system. A device that is part of a multicast group, has received a multicast polling message, must compete for access to send its data. Finally, for a broadcast polling message, any device that has data will compete for access to send its data.

Figure 1: WiMax Polling Types

Contention based access control is the independent operation (distributed access control) of communication devices (stations). In a contention-based system, communication devices randomly request service from channels within a communication system. Because communication requests occur randomly, two or more communication devices may request service simultaneously. The access control portion of a contention based session usually involves requiring the communication device to sense for activity before transmitting and listening for message collisions after sending its service request. If the requesting device does not hear a response to its request, it will wait a random amount of time before repeating the access attempt. The amount of time waited between retransmission requests increases each time a collision occurs.

The WiMAX system defines time periods that subscriber stations can use for contention based access. When subscriber units desire to initiate requests to the system that are not scheduled from a polling message, they must access the process during the contention time slots period. The contention time slot period is periodically broadcast on the downlink channel along with other channel access control information.

Figure 2 shows how contention based access control can be performed on a WiMAX system. Channel descriptors are periodically broadcasted on the downlink radio channel that provides the time intervals for the contention slots. Subscriber devices that use contention based access must compete during these time periods. The WiMAX subscriber station will initially attempt to access the system at a relatively low power level. If the subscriber station does not hear a response to its request, it will wait a random amount of time, increase its transmitted power level and attempt access again. The subscriber station will continue to wait increasing amounts of time each time and increases its transmitted power level each time an access attempt fails until it receives a response from the system.

Figure 2: WiMax Contention Based Access Control

The WiMAX system uses a grant management process for the requesting and allocation (granting) of resources (such as transmission time or bandwidth). Subscriber stations can request changes to the type of services they require (e.g. increases or decreases in bandwidth) by transmitting a bandwidth request header and the system can decide to grant, adjust or not authorize the grant request.

The WiMAX system can grant resources based on a connection or based on a specific subscriber station. A grant per subscriber station is the allocation of transmission bandwidth that affects the transmission for all the connections associated with a subscriber station. A grant per connection is the assignment of bandwidth which only affects the transmission for a specific connection on a subscriber device.

Bandwidth requests can be in aggregate or incremental form. An aggregate request is a message that defines the amount of a resource (such as transmission bandwidth) that is requested to provide for a combined group of applications or services. An incremental request is a message that defines the additional amount of a resource (such as transmission bandwidth) that is requested to provide for an application or service. Bandwidth request messages may be sent as stand alone messages or they may be piggybacked in the payload of another packet of data.