Signal detection in wireless communication ensures signal quality by measuring a radio link’s characteristics. Devices like the user equipment (UE) analyze downlink channel quality and report it to the network. The network leverages this information to optimize parameters such as power levels and handover thresholds, improving subsequent transmissions. These measures enhance service reliability and user experience by dynamically adapting to real-time conditions.
Channel sounding is the process of measuring and evaluating specific signals to determine radio channel quality. For the downlink, this involves the Channel State Information Reference Signal (CSI-RS), and for the uplink, the Sounding Reference Signal (SRS). CSI-RS enables precise measurement of the channel’s state, while SRS facilitates uplink channel assessments. Details about the CSI structure in NR are outlined in 3GPP TS 38.214, section 5.2, which covers procedures for UE channel state information reporting.
In LTE Release 8, downlink channel information is obtained through Cell-Specific Reference Signal (CRS) measurements by UEs, which are continuously transmitted across the entire bandwidth in each 1 ms subframe. This ensures CRS availability for measurement after UE access.
LTE Release 10 introduced Channel State Information Reference Signal (CSI-RS) as a supplement. Unlike CRS, CSI-RS is not continuously transmitted, and UEs rely on network signaling for CSI-RS-related information. Without notification, UEs do not anticipate CSI-RS transmissions, making it more efficient for modern network demands.
The introduction of CSI-RS in LTE enhanced the spatial multiplexing capabilities, allowing downlink transmissions with more than four layers, which was not possible with CRS defined in LTE Release 8. CSI-RS is a more flexible and efficient channel sounding tool than CRS. Its functions were later extended to support advanced features like interference estimation and coordinated multi-point (CoMP) transmission, improving overall network performance and capacity in demanding scenarios.
In NR, the design minimizes always-on signals, unlike LTE’s CRS. The only always-on signal in NR is the SSB, transmitted on a limited bandwidth at longer intervals, making it unsuitable for downlink channel sounding. CSI-RS in NR replaces CRS for channel sounding and offers enhanced functionalities, such as supporting beam management and mobility as a supplement to SSB. This approach optimizes resource usage and flexibility in NR deployments.