HOW IS LINK ADAPTATION USED IN LTE DOWNLINK ?

For unicast traffic, link adaptation is used and is controlled by the eNodeB. A common modulation and channel code is used in the frequency domain per UE and codeword. The modulation and coding scheme (MCS) is selected based on Channel Quality Indicator (CQI) feedback and buffer content.

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. Few operating points which are used are given as as:

• Delay Requirements
• Throughput Requirements
• Block Error Rate (BLER)

Low Delay Requirements

To reach low delay (few retransmissions), a margin to the interference variations can be included. However, this leads to limited throughput, as often an unnecessary robust MCS is used.

High Throughput

To reach a high throughput, a low margin (even negative margin) is used. This instead leads to a larger number of retransmissions, and hence a larger delay. The risk of throughput loss or large delays in case of negative margins is reduced by the use of incremental redundancy for retransmissions.

BLER

BLER can be used as an operating point by the eNodeB to identify the link quality and perform adaptation. If BLER values go beyond a certain threshold, eNodeB can apply certain MCS schemes for link adaptation.