There are 3 types of beamforming in 5G NR:
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Firstly, Eigen based beamforming (EBB) - SRS Based
In this type is based on UL SRS to generate DL weight vectors that used in beam steering toward UE whenever UE moves in a cell. This solution is very suitable to TDD as take advantage of channel reciprocity. -
Secondly, Grid of beams (GOB) - SSB/PBCH Beams or beamformed CSI
In this type UE can have beams based on SSB/PBCH beams or using CSI consolidated within SSB.PBCH block to make beam refinement.- UE measure and select best SSB/PBCH block and reported SSBRI (SSS/PBCH block resource indicator)_Maximum 8 beams .
- UE measure and select best CSI Reference signal and reported best one using CRI (CSI RS Resources indicator). One SSB is covered by 4 csi-rs beams (called refined beams)._Maximum 8*4 = 32 Beam .
Applicable to both TDD & FDD.
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Third, Grid of beams (GOB) - non-beamformed CSI RS:
In this type, UE measure non-beamformed CSI RS and and UE determine PMI (Precoding Matrix Indicator) index that will create best beam toward UE. UE use PMI report to inform gNB selected Precoding Matrix.
Applicable to both TDD & FDD.
Noting that precoding weights for beamforming are long term weights that make PMI beams is less accuracy than SRS beams.
Other difference, usually SRS beams used in good UL SINR (close to site) once UE moves far away from site, it switches to PMI beams because in this situation SRS reciprocity principle will not accurate at cell edge.
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