Link Adaptation (LA) in 5G (NR) Networks (Continued)

In 5G (NR) wireless networks, mobile terminal equipment (UEs) can employ two types of link adaptation: inner-loop link adaptation and outer-loop link adaptation. Their characteristics are as follows:

• ILLA – Inner-loop link adaptation;
• OLLA – Outer-loop link adaptation.

I. ILLA (Inner-loop Link Adaptive) performs fast and direct adjustments based on the Channel Quality Indicator (CQI) reported by each UE. The UE measures downlink quality (e.g., using CSI-RS). It reports the CQI to the gNB, which maps the CQI (via a static lookup table) to the MCS index for the next transmission. This mapping reflects the link condition estimate for that time slot/TTI. ILLA applies a three-step process as follows:

• The UE measures the CSI-RS and reports CQI=11.
• The gNB maps CQI=11 to MCS=20.
• The MCS is used to calculate the transport block for the next time slot.

ILLA's advantage lies in its ability to adapt very quickly to channel changes; however, it has limitations in terms of false detections, CQI errors, and noise. Specifically, the BLER target value may shift if the channel is not ideal or the feedback is imperfect.

II. OLLA (Outer Loop Link Adaptive) uses a feedback mechanism to fine-tune the MCS target value to compensate for the actual link performance observed through HARQ ACK/NACK responses. For each transmission, the gNB receives either an ACK (success) or NACK (failure); where:

• If the BLER is higher than the set target value (e.g., 10%), OLLA adjusts downwards by a correction offset (Δoffset), i.e., reducing the aggressiveness of the MCS.
• If the BLER is lower than the target value, the offset is adjusted upwards, i.e., increasing the aggressiveness of the MCS.

The offset is added to the SINR→CQI mapping in ILLA, thus ensuring that the BLER eventually converges to the target value—even if the input signal is not ideal.

OLLA's advantage lies in its ability to maintain a robust and stable BLER and adapt to slowly changing system errors in the SINR/CQI report. Due to its slower response speed, the optimal setting of the step size (i.e., Δup and Δdown) requires a trade-off between stability and response speed. In the OLLA mechanism, feedback is used to fine-tune the MCS target to compensate for the actual link performance observed through HARQ ACK/NACK responses.

III. Comparison of 4G and 5G Link Adaptation The table below compares 4G and 5G link adaptation.

In 5G (NR) networks, Link Adaptive (LA) plays a crucial role in ensuring high-performance and reliable connectivity. Unlike the slower, fixed-table approach of 4G (LTE), 5G systems employ smarter and faster technologies, including AI/ML and real-time feedback. This enables the network to adapt to changing environments in real time and utilize radio resources more efficiently.