This is the first part for 5G Throughput Optimization Basics that explains the basics of 5G Data Scheduling and Link Adaptation.
It starts from the interaction of CQI and MCS as both of them are required for data scheduling. It further explains how CQI is used to estimate MCS and throughput and what are the factors affecting each of them. In the end, it touches the BLER control algorithm with details about BLER Target and its implications on the scheduling algorithms.
5G Throughput Optimization Basics #1 – Data Scheduling & Link Adaptation
The eNB/gNB needs to know about the UE’s channel conditions to perform scheduling
This is done using the CQI (Channel Quality Indicator) feedback that the UE calculates based on its downlink SINR
Once the eNB/gNB get the CQI, they convert it to a MCS (Modulation and Coding Scheme) and schedule the user on basis of this MCS
For the Data Scheduling, there are two algorithms that work together
The first one is based on CSI reports where the UE receives CSI-RS/CSI-IM and estimate CSI feedback (CQI,RI,PMI) and then sends this CSI feedback to the gNB. The gNB uses this CSI feedback to assign MCS and MIMO layers to the UE.
The second part of the algorithm serves as an outer loop where the gNB keeps checking the BLER statistics for each UE and then modifies the MCS based on BLER statistics. This ensures that a target service is achieved by the UE especially since the CSI feedback might differ between different UEs
Link Adaptation has two parts
The first part estimates MCS based on received CQI
The second part adjusts the MCS based on IBLER (outer loop)
This adjustment can be tuned by changing the IBLER target
A higher IBLER target will result in higher MCS and a lower IBLER target will result in lower MCS
However, if the IBLER target is too high then the throughput will degrade due to retransmission overhead.
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