Downlink and uplink bandwidth allocations are completely different. The 802.16 standard has a MAC centralised architecture where the BS scheduler controls all the system parameters, including the radio interface. It is the role of this BS scheduler to determine the uplink and downlink accesses. The uplink and downlink subframe details were given in Chapter 9.
The downlink allocation of bandwidth is a process accomplished by the BS according to different parameters that are determinant in the bandwidth allocation. Taking into consideration the QoS class for the connection and the quantity of traffic required, the BS scheduler supervises the link and determines which SS will have downlink burst(s) and the appropriate burst profile. In this chapter, the uplink access mechanisms of WiMAX/802.16 are described. Chapter 11 describes scheduling and QoS.
In the uplink of each BS zone or, equivalently, WiMAX cell, the SSs must follow a transmission protocol that controls contention between them and enables the transmission services to be tailored to the delay and bandwidth requirements of each user application. This is accomplished while taking into account five classes of uplink service levels, corresponding to the five QoS classes that uplink transmissions may have.
Uplink access and bandwidth allocation are realised using one of the four following methods:
unsolicited bandwidth grants;
piggyback bandwidth request;
unicast polling, sometimes simply referred to as polling;
contention-based procedures, including broadcast or multicast polling, where contention based bandwidth request procedures have variants depending of the PHYsical Layer used:OFDM or OFDMA (see below).
The standard states that these mechanisms are defined to allow vendors to optimise system performance by using different combinations of these bandwidth allocation techniques while maintaining consistent interoperability. The standard proposes, as an example, the use of contention instead of individual polling for SSs that have been inactive for a long period of time. Next, the realisation of these methods is described, but first two possible differentiations of an uplink grant-request are introduced.