To make the channel-aware scheduler effectively operate, information about the quality of the wireless links toward each SS must be gathered by the BS. To this aim, the wireless channel state is continuously monitored by each SS, which gets CINR (carrier to interference and noise ratio) measurements, updates estimates of the CINR’s mean and standard deviation values, and reports them back to the BS through specific signalling messages provided by the IEEE 802.16 standard, e.g., through BR messages (normally used by SSs for requesting bandwidth), which can also include the CINR report.
The compensation block is in charge of collecting received information on the channel qualities and making the BS aware of the status of each link toward the SSs. Two assumptions hold in our reference scenario:§ Wireless channel quality of each connection changes on a per-frame basis, but it remains constant for a frame duration; i.e, the BS assumes that the CINR report received by the SS at the frame beginning is valid for the entire subsequent frame duration. This is equivalent to modelling a block-fading channel, which is a feasible assumption for the slowly varying radio links of the fixed WiMAX PMP scenario.
§ Perfect channel state information is available at the BS. This means that the CINR reports must be correctly received by the BS over an error-free feedback channel.
The monitored channel quality at the SS is compared against the allowed values of signal-to-noise ratio and receiver sensitivity for each transport mode (burst profile) at a given bit error rate (BER). As soon as the received power level becomes lower than the receiver sensitivity threshold for a given DL transport mode and target BER, then a more robust profile must be chosen. This change in the transport mode can proceed until the most robust modulation/coding scheme (BPSK modulation with 1/2 coding rate) is chosen. A channel is considered “good” as long as a more robust transport mode can be selected, which is capable to cope with the channel adverse conditions. Otherwise, the channel is finally considered as “bad.”
