n is confined to a set of PRBs where the correspond- ing physical channel is transmitted. SRS: signals to assist reciprocity-based precoding in the downlink as well as to acquire CSI for the uplink. NR also supports new types of reference signals not supported by LTE (3GPP TS 38.211): CSI-RS for tracking: tracking reference signals for time and frequency offset tracking and estimation of the channel delay spread and Doppler spread. PT-RS: phase tracking reference signals for fine time domain granularity phase estima- tion that may occur, e.g. due to phase noise impairments in mmWave bands. To facilitate orthogonal multiplexing and sharing of reference signals among UEs with different bandwidth capabilities, the modulation sequence for reference signals in NR is typically performed starting from common point A (see Figure 3.5.10). The actual sequence for the UE is determined based on the offset parameter k signaled to the UE as part of the reference signal configuration. For forward compatibility, the range of the offset parameter k is also selected to be a relatively large value to accommodate the introduction of UEs with larger bandwidth capabilities. In the following text, each NR reference signal is discussed in more detail. 3.5.7.1 CSI-RS Similar to LTE-A, CSI-RS in NR can be used for downlink CSI acquisition (3GPP TS 38.211). Based on channel measurements from CSI-RS, UE reports to gNB the preferred parameters for downlink transmission, for example, number of MIMO layers, precoding, and modu- lation and coding scheme. When CSI-RS is used for CSI feedback purposes, the time and 3.5 NR Physical Layer Common sequence from Sequence for RS modulation reference Point A Carrier bandwidth Figure 3.5.11 Modulation sequence for reference signal. frequency density of CSI-RS is low incurring small overheads. More specifically, CSI-RS in NR has a density of one resource element per PRB per antenna port. Additional over- head reduction for CSI-RS is achieved by transmitting CSI-RS every other PRB with total overheads of 0