Another dimension where CSI variations are used is the ACM. However, instead of granting or depriving access to the resources based on the CSI, ACM fine-tunes the transmission parameters based on the CSI. Particularly, the coding rate and the modulation order. For instance, if the channel conditions are good, the modulation order is increased while the coding rate is decreased. Similarly, if the channel is bad, a lower modulation order is used along with a higher coding rate. These adaptations give the user the ability to continuously exploit the channel to its best. Hence, achieving a throughput as close to the Shannon capacity as possible.
We should remark here that the benefits of ACM, in addition to those of multiuser diversity in general, do not require perfect (full) or even instantaneous CSI in order to attain the full potentials of the channel. In fact, it has been reported in the literature that using delayed or even incomplete CSI has demonstrated reasonable reliability in both, OFDMA and SC-FDMA systems .