This post aims to simplify the fundamentals of Beam Management in 5G.
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Process 1: Beam Sweeping
SSB Broadcasting: gNB sends SSBs wide beams across the cell with unique IDs.
Initial Beam Selection: UE picks the best SSB based on RSRP in Idle Mode.
Contention-Free RACH: UE performs RACH on the chosen SSB, uniquely linked to an RO.
Beam Pair Linking (BPL): UE’s Tx beam is chosen based on the received Rx beam for MSG1. gNB monitors Rx beam, establishing Downlink and Uplink BPL.
Result: UE and gNB form a broader beam link for initial data transfer.
Note: Beam Sweeping is mainly used for Common channels. -
Process 2: Beam Refinement
Refining for Precision: The beam is refined from a broader SSB to a narrower CSI-RS for precise control, ensuring accuracy in data transfers.
Narrower Beam Generation: gNB creates narrower beams within the SSB, linking each to unique CSI-RS resources. Example: SSB beam A maps to CSI-RS 1 and CSI-RS 2.
Measurement and Switching: UE measures CSI-RS for each narrower beam, sending a report.
Result: The initial SSB beam switches to the most suitable CSI-RS, leading to improved SNR and overall throughput. -
Process 3: Beam Switching
RRC Reconfiguration: gNB informs UE of future Tx beam changes using assigned TCI ID and CSI-RS beams.
UE Configuration: gNB configures UE with TCI states linked to various beams (e.g., CSI-RS1 with TCI 1).
Measurement Reports: UE periodically sends CSI-RS reports to gNB, including beam-specific measurements.
Switching Beams: If conditions worsen for Beam 1, gNB switches from CSI-RS1 to CSI-RS2 based on received reports.
Result: UE switches to best serving beam based on CSI-RS measurements.
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