Issue link: https://resources.mouser.com/i/1315957
27 | • Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes of operation There are two mechanisms to enhance spatial multiplexing, which 5G NR employs to optimize its MIMO framework: • The first is to create a framework to support multiple transmission layers through multiple antenna ports for channel transmission. • The second is to measure the channel to determine its rank, and then precode information and feed it back to the transmitter to optimally precode the transmission layers across the available antenna ports. Downlink MIMO In downlink MIMO, for single-user MIMO (SU-MIMO), the framework supports eight layers per UE, with up to four layers per code word. For multiuser MIMO (MU-MIMO), up to 12 orthogonal demodulation reference signal (DM- RS) ports exist across all UEs. To determine the channel rank and MIMO precoding coefficients in the downlink, the network can use the Channel State Information (CSI) framework or the Sounding Reference Signal (SRS) framework. The CSI framework consists of two parts: CSI acquisition and CSI reporting. CSI acquisition relies on the CSI Reference Signal (CSI-RS) resource setting, which configures the CSI-RS resources. CSI reporting is done through the CSI report setting, which configures the resources on which the CSI reports. The network uses trigger states to configure a link between the CSI resource and CSI report settings. Alternatively, for TDD systems, the network can use the reciprocity of the downlink and uplink channels that transmit in the same frequency band and use the SRS framework. Using the SRS framework, the UE transmits SRSs on multiple antenna ports. The NR Node B (gNB) measures the channel quality and rank according to the uplink channel and uses this information to determine the channel rank and MIMO precoding coefficients in the downlink. Uplink MIMO The uplink MIMO can be either codebook-based or non- codebook-based. For the codebook-based uplink MIMO, the base station measures the uplink channel on the antenna ports of the SRS