Link adaptation has different applications in the downlink and in the uplink. The principle is always the same: choosing the most suitable Modulation and Coding Scheme (MCS) in order to have the highest possible data rate while fulfilling the CINR (quality) requirements.
The downlink burst profile is determined by the BS according to the quality of the signal that is received by each SS. To reduce the volume of uplink traffic, the SS monitors the CINR and compares the average value against the allowed range of operation. This region is bounded by threshold levels indicated in the DCD message for each defined burst profile. If the received CINR goes outside the allowed operating region (see Chapter 9), the SS requests a change to a new burst profile using one of two methods: the RNG-REQ message or the DBPC-REQ message. The SS applies an algorithm to determine its optimal burst profile in accordance with the threshold parameters established in the DCD message. This algorithm is not specified in the standard and can be proposed by the vendor or the operator.
The messages exchanged between the SS and the BS for a burst profile change are not exactly the same whether an SS is moving to a more or less robust burst profile. Figure 11.5 shows the case where an SS is moving to a more robust type. Figure 11.6 shows a transition to a less robust burst profile.
In the uplink, the burst profile is also (as for the downlink) decided by the BS. The RNG-RSP Message is used for that purpose as described in Section 11.1.3
IEEE 802.16-2004, IEEE Standard for Local and Metropolitan Area Networks, Air Interface for Fixed Broadband Wireless Access Systems, October 2004.