Calibration Failure in TDD Massive MIMO

Article items will include:

What is beam-forming operation steps?

What is calibration failure problem ?

What is recommended solution ?

Show Benchmark Result

let us discuss each item in details,

What is beam-forming operation steps?

In TDD system 4G/5G , ducting interference is severely have bad impact on main KPIs and UE perception. As usually all KPIs including user throughput is badly impacted.

One of main important operation or function in TDD massive MIMO to get gain for beamforming is called “channel calibration” that shown in above figure (calibration step).

Above flow shows all operations needed from site side that needed to get beamforming (beams shapes) toward UE.

1.???? Channel calibration: that implement by sending test signal in specific position and time in TDD subframe so can calibrate Tx/RX paths for HW.

2.???? SRS or PMI weight calculation: Site receiving test signal from UE side called SRS (sounding reference signal) or PMI (precoding matrix indicator) that used to induct weights used to make beam steering toward UE.

3.???? Beamforming implementation: Now site have traffic narrower beams that can direct to serving UEs.

4.???? Finally, demodulation/decoding from UE side.

More details for HW calibration process?

Calibration process consist of Tx-signal and Rx-signal that will send in specific location and time within UL TDD subframe so site can detect/measure how is correlation between transmitted Tx-signal and received Rx-signal. Often these test signals sending on GAP interval of special subframe (DwPTS+GAP+UpPTS). Supposed SINR measured for Tx-signal & Rx-signal should have good positive_sign SINR so Calibration can success .

What is calibration failure problem ?

UL interference cause calibration process to fail during interference window so if calibration fail, beamforming will not do and UEs did not benefit from beamforming gain.

Below is an example for calibration success versus Ul interference for sample site:

Yellow color : Show Calibration SR start degradation with increasing UL interference.

Green color: after shifting Calibration from GAP (high interfered) to UL Subframe (less interfered), Calibration start to recover.

What is recommended solution ?

often UL interference pattern will starting high level for early UL symbols then became less once you move away to far symbols, i.e GAP have higher interference than UlPTS and UlPTS have higher interference than UL subframe and so on. So idea here to overcome interference effect on calibration that was dine be default in GAP, is to move that calibration test from GAP to be in UL subframe that should be have lower interference as mention before.

Benchmark Result

After shifting calibration position from GP (high interfered) to UL subframe (low interfered).