In 5G New Radio (NR), SSB stands for Synchronization Signal Block. The SSB is a fundamental building block for initial access procedures in 5G networks. It enables User Equipments (UEs) to detect the presence of a 5G network, achieve time and frequency synchronization, and obtain essential information required to connect to the network.

The SSB is periodically broadcast by the gNB (5G base station) over the air interface. This periodic transmission allows UEs to perform cell search and selection procedures as they enter the network coverage area or when they need to reselect a cell. The design of SSB periodicity and transmission ensures that UEs can quickly and efficiently find the necessary signals for synchronization and initial access, even in challenging radio conditions or when moving at high speeds.

In summary, the SSB in 5G NR is a set of signals that facilitates initial UE synchronization with the network, providing the necessary information for UEs to proceed with further system access procedures. Without the SSB, UEs would not be able to detect, synchronize, or access the 5G network.

Components of SSB

The SSB consists of the following elements, each serving a specific purpose in the synchronization and initial access process:

• Primary Synchronization Signal (PSS): Helps UEs achieve time synchronization with the cell. The PSS aids in identifying the cell's symbol timing and frame boundary. It also helps the UE determine the physical layer identity (N_ID²) of the cell.
• Secondary Synchronization Signal (SSS): Provides additional synchronization information, including the cell ID within a cell group (N_ID¹), enabling the UE to identify and distinguish between different cells. It also assists in completing the cell search process initiated by the PSS and helps determine the frame timing and cyclic prefix length.
• Physical Broadcast Channel (PBCH): Carries the Master Information Block (MIB), which includes essential system information that the UE needs for initial system access, such as the System Frame Number (SFN), subcarrier spacing for the downlink, and information about the SSB's periodicity and location.
• Demodulation Reference Signal (DMRS) for PBCH: Associated with the PBCH, this reference signal is used for channel estimation to accurately demodulate the PBCH. It helps the UE compensate for channel impairments and ensures reliable decoding of the MIB.

SSB Transmission and Beamforming

In 5G NR, SSBs are often transmitted using beamforming techniques. Multiple SSBs can be transmitted in different spatial directions (beams) to cover the entire cell area. Each SSB is mapped to a specific beam, and the UE detects the SSB with the strongest signal to determine the best beam for initial access. This approach improves coverage and signal quality, especially in high-frequency bands (such as mmWave) where signal attenuation is significant.

SSB Index and Frequency Location

Each SSB is assigned an index, and its frequency and time location within a radio frame are determined by network configuration. The number of SSBs, their periodicity, and their mapping to beams depend on the deployment scenario and frequency band. For example, in FR1 (sub-6 GHz), up to 4 SSBs or 8 SSBs may be transmitted, while in FR2 (mmWave), up to 64 SSBs can be used.

Summary Table

Component	Purpose
Primary Synchronization Signal (PSS)	Time synchronization, cell identity (NID2)
Secondary Synchronization Signal (SSS)	Frame timing, cell group identity (NID1), cyclic prefix
Physical Broadcast Channel (PBCH)	Broadcasts MIB (system information)
DMRS for PBCH	Channel estimation for PBCH demodulation

Key Points

• SSB is essential for UE to detect and synchronize with a 5G cell.
• SSB contains PSS, SSS, PBCH, and DMRS for PBCH.
• SSBs are transmitted periodically and may use beamforming for coverage.
• The SSB provides the minimum information required for a UE to access the 5G network.

Further Readings

• SS Block