Time Domain Configuration for SS/PBCH Block

Last updated on Mar 16, 2023

The time domain configuration for the synchronization signal/physical broadcast channel (SS/PBCH) block in 5G NR is an important aspect of the physical layer (PHY) design. The time domain configuration determines the timing and frequency structure of the SS/PBCH block and is critical for enabling efficient and reliable communication between the base station and user equipment (UE). In this article, we will discuss the technical details of the time domain configuration for the SS/PBCH block in 5G NR.

Overview of Time Domain Configuration

The time domain configuration for the SS/PBCH block in 5G NR is specified by several parameters, including the subcarrier spacing (SCS), the slot duration, and the SS/PBCH block periodicity. These parameters are defined in the 5G NR standard and are used by the base station to configure the timing and frequency structure of the SS/PBCH block.

Subcarrier Spacing

The subcarrier spacing (SCS) is a key parameter in the time domain configuration for the SS/PBCH block. The SCS determines the spacing between the subcarriers in the frequency domain and has a direct impact on the bandwidth and spectral efficiency of the system. The SCS can be configured to be 15 kHz, 30 kHz, 60 kHz, 120 kHz, or 240 kHz.

The choice of SCS depends on several factors, including the bandwidth available for the system and the desired trade-off between spectral efficiency and receiver complexity. For example, a larger SCS results in a wider bandwidth and higher spectral efficiency, but also increases receiver complexity due to the need for more precise synchronization and channel estimation.

Slot Duration

The slot duration is another important parameter in the time domain configuration for the SS/PBCH block. The slot duration determines the time interval for which data is transmitted and received on the downlink and uplink channels. In 5G NR, the slot duration is fixed at 0.5 ms, which is shorter than the slot duration in previous generations of cellular networks.

The shorter slot duration in 5G NR enables higher data rates and lower latency by reducing the round-trip time for transmitting and receiving data. However, it also places greater demands on the synchronization and channel estimation capabilities of the system.

SS/PBCH Block Periodicity

The SS/PBCH block periodicity is a critical parameter in the time domain configuration for the SS/PBCH block. The SS/PBCH block is transmitted periodically on the downlink channel, and the periodicity determines the interval at which the SS/PBCH block is transmitted. In 5G NR, the SS/PBCH block is transmitted every 20 ms, which corresponds to the length of one radio frame.

The SS/PBCH block periodicity is important for enabling UE to synchronize with the network and decode the system information carried by the PBCH. By transmitting the SS/PBCH block periodically, the base station enables UE to search for and detect the SS/PBCH block at regular intervals, which helps to ensure that the UE maintains accurate time and frequency synchronization with the network.

Time Division Duplexing

In addition to the parameters discussed above, the time domain configuration for the SS/PBCH block must also take into account the duplexing scheme used in the system. In 5G NR, time division duplexing (TDD) and frequency division duplexing (FDD) are both supported, and the timing and frequency structure of the SS/PBCH block must be configured accordingly.

In TDD mode, the downlink and uplink transmissions share the same frequency band and are separated in time. The base station and UE take turns transmitting and receiving data on the same frequency band, with the time intervals for downlink and uplink transmissions determined by a predefined pattern. The time domain configuration for the SS/PBCH block in TDD mode must take into account the downlink/uplink pattern and ensure that the SS/PBCH block is transmitted at the appropriate times.

In FDD mode, the downlink and uplink transmissions are separated in frequency, with the base station and UE transmitting and receiving on different frequency bands. The time domain configuration for the SS/PBCH block in FDD mode must ensure that the SS/PBCH block is transmitted on the downlink frequency band and received by the UE on the uplink frequency band.

Time Domain Structure of SS/PBCH Block

The time domain structure of the SS/PBCH block is defined by the subcarrier spacing, slot duration, and SS/PBCH block periodicity. The SS/PBCH block consists of two parts: the synchronization signal (SS) and the physical broadcast channel (PBCH). The SS carries information about the cell identity and timing synchronization, while the PBCH carries system information, such as the system bandwidth and modulation scheme.

The SS/PBCH block is transmitted on the downlink channel, with the SS and PBCH transmitted in separate time-frequency resources. The SS is transmitted in the first slot of every radio frame, while the PBCH is transmitted in the last four slots of every radio frame.

The SS is transmitted in two parts: the primary synchronization signal (PSS) and the secondary synchronization signal (SSS). The PSS is transmitted in the first symbol of the first slot of the radio frame and carries information about the physical cell identity group (PCIG) to which the cell belongs. The PCIG is a group of cells that share the same physical layer characteristics, such as the same subcarrier spacing and slot duration.

The SSS is transmitted in the last symbol of the first slot of the radio frame and carries information about the physical cell identity (PCI) of the cell. The PCI is a unique identifier for the cell and is used by the UE to identify and synchronize with the cell.

After the SS is transmitted, the PBCH is transmitted in the last four slots of the radio frame. The PBCH carries system information, such as the system bandwidth and modulation scheme, and is transmitted in a predefined pattern that depends on the system configuration. The PBCH is modulated using quadrature phase shift keying (QPSK) or 16 quadrature amplitude modulation (QAM), depending on the system bandwidth.

Conclusion

In conclusion, the time domain configuration for the SS/PBCH block in 5G NR is a critical aspect of the PHY design. The time domain configuration determines the timing and frequency structure of the SS/PBCH block and is important for enabling efficient and reliable communication between the base station and UE. The subcarrier spacing, slot duration, and SS/PBCH block periodicity are key parameters in the time domain configuration, and must be carefully selected to ensure optimal system performance. By transmitting the SS/PBCH block periodically, the base station enables UE to synchronize with the network and decode the system information carried by the PBCH.