CHANNEL- AND QoS-AWARE SCHEDULER DESIGN FOR WiMAX NETWORKS

In this section some design guidelines are given for a “channel-aware” and “QoS-aware” scheduler to be implemented at the BS of a WiMAX PMP network for the delivery of DL traffic to a set of distributed SSs with active connections of different traffic nature. The scheduling algorithm, which is running at the WiMAX base station, needs to fulfill the following requirements:

§  Efficient link utilization: The scheduler shall take opportunistic decision and not assign a transmission opportunity to a flow with a currently low-quality link, because the transmission will be wasted.

§  Delay bound: The algorithm shall be able to provide delay bound guarantees for individual flows, to support delay-sensitive applications; besides, it shall prevent too late packet transmissions from wasting bandwidth.

§  Fairness: The algorithm shall redistribute available resources fairly among flows; thus providing short-term fairness to error-free flows and long-term fairness to error-prone flows.

§  Throughput: The algorithm shall provide guaranteed short-term throughput to error-free flows and guaranteed long-term throughput to all flows.

§  Implementation complexity: A low-complexity algorithm is necessary to take quick scheduling decisions.

§  Scalability: The algorithm shall operate efficiently when the number of flows sharing the channel increases.

To match all the above-mentioned needs, (1) a class-based wireless scheduling is recommended to fulfill the WiMAX service class differentiation needs, and also to simplify interworking with the Internet (supporting class-based differentiated services); (2) “per class” service differentiation must be achieved, and simple “per flow” mechanisms (e.g., lead/lag counters per each flow) must be provided to guarantee fair service to traffics within a class; (3) channel awareness must be exploited to make efficient use of wireless resources, and a compensation technique for missed transmission opportunities must be provided to guarantee proportional fairness in sharing the bandwidth; (4) useless compensation must be avoided through simple measures, e.g., periodic buffer cleaning of over-delayed packets could be used; and (5) simple expedients must be provided to avoid monopolization of the scheduler by a lagging flow after the recovery of its channel, and to guarantee graceful throughput degradation of leading flows; this could come at a very low cost by using combination of lead/lag counters and queue parameters.

In Figure 1, the reference channel- and QoS-aware scheduling architecture is illustrated. It is considered to be implemented at the MAC layer of a WiMAX BS, which manages local traffic queues for packet delivery over the DL channel. The illustrated framework must be able to provide per-class differentiated QoS and fair service to flows in the same class through the operation of the following QoS-support modules:

§  An error-free service scheduler that decides how to provide service to traffic flows based on an error-free channel assumption.

§  A lead/lag counter for each traffic flow that indicates whether the flow is leading, in sync with, or lagging its error-free service model and in which extent.

§  A compensation technique that is used to improve fairness among flows. A lagging flow is compensated at the expense of a leading flow when its link becomes error free again. The system maintains credit/debit counters for each lagging/leading flow.

§  Separate per-class packet queues used to support rtPS, nrtPS, and BE traffic flows.

§  A means for monitoring and predicting the channel state of each backlogged flow.

Figure 1: Channel-aware QoS scheduling architecture in the BS.

Channel Acquisition | WiMAX Operation

Channel acquisition is the process of finding and acquiring access to a communication channel. When WiMAX devices initialize (e.g. when they are turned on), they begin a channel scanning process. Channel scanning is the process of searching through multiple radio channels to find signals that indicate a channel is available on which to communicate. The WiMAX device will typically have a stored list of frequency channels for it to scan in order to reduce the amount of scanning time. These frequency channels may be preprogrammed by or for a WiMAX system operator so the WiMAX device will initially try to connect to a specific WiMAX system.

When the WiMAX device has found one or more WiMAX radio channels, the device will decode the channel and look for packets of data that have a frame control header that contains a downlink channel description (DCD) message and an uplink channel description (UCD) message. The DCD message contains parameters that are necessary or that will assist it to access the device in receiving information from the downlink channel on the communication system. The UCD message provides the device with the parameters that are necessary to access the communication system.