For TDD massive MIMO channel recpirocity is crucial.
Due to this channel calibration of TX and RX is done periodically for this.
Do you know how often is done chanel calibration is done in 5G TDD?
I could not find this in documentation.
Every 10 minutes? every hour? every 24 hours?
For example in TDD Huawei uses SRS to calculate PMI weights.
This is much more precise than using CSI-RS which allow only a specific codebook for PMI (not all values possible).
But for SRS PMI to work channel calibration is essential.