What is CD-SSB in 5G NR?

To access an NR network, a UE needs to carry out initial access functionality which includes cell search and random access. To enable the UE to acquire DL time and frequency synchronization, an SS consisting of the primary SS (PSS) and the secondary SS (SSS) is periodically transmitted in the DL of each cell. After synchronization, the UE can decode the physical broadcast channel (PBCH) which carries the master information block (MIB) that the UE needs to decode in order to receive the remaining system information broadcast by the network. In NR, the PSS, SSS, and PBCH are jointly referred to as SS block (SSB) which occupies 20 resource blocks (RBs). After decoding the PBCH, the UE can move forward to decode the system information block type 1 (SIB1) which contains the system information that the UE needs to know before accessing the network. For example, SIB1 contains information about random access configuration that the UE needs in order to carry out random access procedure. Since the SSB has an associated SIB1 transmission, it is referred to as cell-defining SSB (CD-SSB).