e (UE transmits SRS, gNB computes precoding weights) Typically intended for arrays having 32 TXRUs or Supports arrays with arbitrary number of TXRUs less with no beam selection (no CRI) Maximum of 32 ports per CSI-RS CSI-RS (8 ports) CSI-RS (32 Ports) CSI-RS (8 ports) CSI-RS (8 ports) CSI-RS (8 ports) RI/PMI (32)/CQI RICQI CRI/PMI/CQI Downlink MIMO operation in above 6 GHz Single-panel array Multipanel array Combination of analog beamforming and digital precoding at baseband Combination of analog beamforming and digital precoding at baseband Analog beamforming is typically one RF beamforming weight-vector per Analog beamforming is typically one RF beamforming weight-vector per polarization per panel polarization (a single cross-polarized beam) One cross-polarized beam per subpanel Supports two TXRUs and single-user MIMO only Number of TXRUs = 2 X number of panels Digital precoding options: None (rank-2 all the time); CSI-RS based (RI/PMI/ Digital precoding options: CSI-RS based (RI/PMI/CQI); SRS-based (RI/CQI); CQI); and SRS-based (RI/CQI) and SU-MIMO and MU-MIMO (typically one UE per cross-polarized beam) SU-MIMO SU-MIMO MU-MIMO TXRUs at the gNB Up to 4 UEs at a time 2 ports/UE 1< rank <2 2 TXRUs at the gNB One UE at time 1< rank< 2 8 TXRUs at the gNB One UE at time 8 ports/UE 1< rank< 8 Figure 4.71 Summary of NR downlink multi-antenna operation [26]. inter-stream interference would be inevitable, and it would be the dominant factor to degrade the demodulation performance. In this case, a nonlinear receiver may be needed. When the received signal power difference among the streams is larger than 3 dB, the codeword-level successive interference cancellation or SIC receiver can satisfactorily perform. However, when the received signal power is almost the same for different data streams, the performance of SIC will deteriorate. In that case, a parallel interference can- cellation receiver would be a more suitable choice. In NR, the improved DM-RSs and tracking reference signals can be used to estimate the timing of