ice A subsequently refrains from transmitting to its targeted receiving device B, which is unnecessary because the trans- mission from A to B using highly directional beams does not interfere with the transmission from C to D. If the direction of the communication is known, directional carrier sensing may be useful; however, this would create a different problem as shown in Fig. 8.25. The effects of directionality on carrier sensing and transmission are studied in NR-U to ensure optimal system operation and performance [34]. 888 Chapter 8 ((9)) Figure 8.25 (A) Omnidirectional carrier sensing. (B) Two simultaneous directional links [34]. The NR supports CSI-RS with 1, 2, and 4 OFDM symbols and up to 32 antenna ports. It further supports periodic, aperiodic, and semi-persistent CSI-RS transmission patterns. The CSI-RS can be used for RRM measurement in the connected mode for mobility, the RSRP measurement for beam management, and CSI measurement for scheduling. The CSI-RS can be transmitted in conjunction with the SS/PBCH block and/or CORESET/RMSI using time or frequency multiplexing following an LBT process. If frequency-domain multiplexing is used, a CSI-RS resource is configured on the resources that are not occupied by SS/PBCH block and CORESET. Due to the uncertainty of channel availability for transmission of CSI-RS and the corresponding measurement report, the periodic CSI measurement and report is not feasible. Furthermore, the interference condition in the unlicensed spectrum may fluctuate severely and is unpredictable due to the larger delays between the measure- ment and the report as well as beamforming-based transmission. In LTE-based eLAA, block interlaced frequency division multiple access (B-IFDMA) was introduced in the uplink transmission to ensure compliance with OCB and maximum PSD requirements while maintaining a transmit power level that can support the desired cell COV- erage. In NR-U, given similar regulatory requirements, the B-IFDMA serves as the baseline design for the uplink transmission.