NR - CSI Beam Management

This slide (Animation) shows very high level picture of how beam management works based on UE report. Since this is very high level view, it would miss out a lot of details. This is just for bulding up intuition, not for the detailed process. For the details of the process and configuration, refer to this note. I would suggest you to refer back and forth to the note and this animation until you get the understanding both in terms of the details and intuition.

In theory, this kind of beam management is expected to happen in all deployment. But as far as I heard from field, this kind of detailed beam management would not be applied in low frequency (i.e, FR1) and this is likely to happen in FR2 in most case.

It is assumed that an appropriate wide beam and narrow beam for the UE during the initial call setup. SSB/PDCCH are usually transmitted with widebeam. Usually this kind of widebeam is cell specific, not for specific UE. The direction of widebeam is determined by gNB's internal schedule. User Traffic (PDSCH) is usually transmitted with Narrowbeam dedicated for each specific UE. Usually (but not mandatorilly) gNB determins this direction from measurement report from UE.

At the slot with CSI RS , CSI beam transmitted periodically (or semi or aperiodically) by gNB. UE knows the periodicity and exact position based on RRC message. Usually the direction of the beam for each UE would vary. At this slot, UE performs measurement for the CSI beam configured for it. Ideally gNB would transmit the beam with different directions across multiple csi period.

Usually some time after the measurement(a few slots later), UE send measurement report to gNB. gNB would transmit the beams with slightly different directions across several consecutive CSI RS period and UE perform the measurement report accordingly.

Let's suppose UE changed a position as shown below.

At the slot with CSI RS , CSI beam transmitted periodically (or semi or aperiodically) by gNB. UE knows the periodicity and exact position based on RRC message. Usually the direction of the beam for each UE would vary. At this slot, UE performs measurement for the CSI beam configured for it. Ideally gNB would transmit the beam with different directions across multiple csi period.

Then the best beam reported would change accordingly and gNB would notice this beam change based on CSI report from the UE.

Once gNB notice the best beam change for the UE based on the UE report, it would (not mandatory) change the narrow beam for the UE.