HOW IS LINK ADAPTATION USED IN LTE UPLINK?

Uplink link adaptation, i.e., selection of modulation and channel coding, is controlled by the network. The eNodeB measures the uplink channel quality and orders the UE to use a specific modulation and coding scheme (MCS) based on this. Other parameters may also be taken into account, such as UE power headroom, scheduled bandwidth, buffer content and acceptable delay.

Rapid interference variations make it difficult to predict the link quality accurately, and select MCS based on such knowledge. Instead, preliminary MCS selection is based on averaged link quality. Different operating points can then be used. To reach low delay (few retransmissions), a margin to the interference variations can be included.

In addition to the usage during initial access, the RA is also used when the UE has lost the uplink synchronization, is in an idle or a low-power mode, or handover for link adaptation.