.5 resource elements per PRB per antenna port. Unlike LTE-A, NR offers high degree of flexibility when configuring CSI-RS. Depending on the number of antenna ports, CSI-RS may be constructed by aggregation of one or mul- tiple basic units transmitted within a slot (see Figure 3.5.12). Multiple antenna ports within each basic unit of the CSI-RS can be supported by using different orthogonal cover codes. In the time domain, CSI-RS may start at any OFDM symbol of a slot as long as there is no collision with other reference signals. Depending on the number of configured antenna ports, CSI-RS spans 1, 2, or 4 OFDM symbols. CSI-RS can be periodic, semi-persistent, or aperiodic. Periodic and semi-persistent CSI-RS can be transmitted with periodicity of {4, 5, 8, 10, 16, 20, 32, 40, 64, 80, 160, 320, 640} slots depending on the configuration. Semi-persistent CSI-RS is transmitted based on the activation/de-activation command from the MAC control element, while aperiodic CSI-RS transmission is based on the DCI and is limited to a single occasion unlike periodic and semi-persistent CSI-RS, which are typically transmitted over multiple instances. Unlike LTE, which only supports wideband CSI-RS, the CSI-RS bandwidth can be con- figurable and indicated to the UE in contiguous units of four PRBs. CSI-RS transmission can be UE-specific or cell-specific and supported by the unified UE-specific configuration procedure of CSI-RS. CSI-RS is modulated by QPSK using a pseudo-random sequence that depends on the configuration parameter and the OFDM symbol index. For cell-specific CSI-RS transmission that enables CSI-RS sharing among UEs possibly supporting different bandwidth, the reference for the CSI-RS sequence mapping is point A (cf. Figure 3.5.11). Unlike CSI-RS for larger number of antennas, CSI-RS with one antenna port has a uni- form pattern and has a density of three resource elements per PRB. The increased density of CSI-RS is required to support Layer 1 reference signal received power (L1-RSRP) measure- ments used for the ac