In this session, “Throughput Troubleshooting – Drive Test Analysis” we will discuss about practical throughput troubleshooting using Drive Test logs to root-cause various issues including Coverage or Interference problems, congestion or capacity bottlenecks and transport or backhaul anomalies.
Throughput Troubleshooting: Drive Test Analysis
- Coverage or Radio Issues
The first issue is simple. If coverage (RSRP) is bad, the throughput will be bad. If you look at the graph on the right, you can see that as the RSRP reduces, the throughput also reduces.
So, with Drive Test data, if there is low throughput and the RSRP is also low then that would typically indicate a bad coverage scenario.
Some of the steps to improve this can be by up-tilting or increasing the power of the base station or by adding more sectors/sites.
- Interference or SINR Issues
Typically, lower SINR also causes lower throughput as lower SINR leads to lower CQI which in turn leads to lower MCS and Modulation.
Now SINR can be low because of poor RSRP i.e. low coverage can result in low SINR or SINR can also be lower in case of higher interference.
If we look at two different clusters and we want to analyse from DT logs, which has higher interference, we can plot their SINRs against RSRP.
Here it can be seen that Cluster2 has lower SINR & higher interference when compared to Cluster1.
- Congestion or Capacity Issues
From Drive Test logs, it can be difficult to find congestion or capacity issues. However, one way to estimate the congestion can be RB utilization.
If you are using full buffer Drive Test (like FTP or UDP), you can plot RB utilization against TTIs (Slots or Subframes) to see how much of the resources were allocated per TTI.
If there is no congestion then usually for full buffer Drive Test, you should see 80% or higher RB Utilization but if you see sporadic and fluctuating RB Utilization then that can indicate a congestion on the site. This means that there can be many other users on the site which need resources so the DT UE could not get all the resources that it requires.
- Transport or Backhaul Issue
If there is an issue with the transport network or the backhaul of the site or if there is a core issue that is not related to RAN, then it can be estimated by Drive Test data based on Frame Usage or PDCCH Allocations or Scheduling Data.
The theory behind this concept is that if there is enough data in the Base Station’s buffer then it will always try to schedule that data over the air and this will result in nearly 100% frame usage or in other words all slots will carry some data.
However, if there is no data in the buffer then the Base Station will not schedule any data and the UE will not get any PDCCH allocation which means that it will see low Frame Usage.
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