What is PSS and SSS in LTE?
In this article, we are going to explore two important reference signals in LTE: PSS (Primary Synchronization Signal) and SSS (Secondary Synchronization Signal). These signals are crucial for the initial synchronization process when a user equipment (UE) connects to the LTE network.
As we have discussed in previous articles, LTE (Long-Term Evolution) uses a complex system of signals to ensure efficient communication between the User Equipment (UE) and the eNodeB (evolved NodeB). Among these signals, the PSS and SSS play a key role in the network synchronization procedure, which is necessary for the UE to establish a connection with the network and start communicating.
Primary Synchronization Signal (PSS)
The PSS is one of the key synchronization signals transmitted by the eNodeB. It is used by the UE for initial synchronization with the network. The PSS helps the UE to identify the cell and obtain the frequency synchronization. It is transmitted periodically and is typically found in the first symbol of a subframe.
The primary function of the PSS is to allow the UE to detect the cell’s timing and synchronization. When the UE is powered on, it scans the radio environment for available LTE cells. It first searches for the PSS, which enables it to roughly estimate the frequency and timing of the eNodeB transmission. The PSS is uniquely identified by three distinct sequences, each corresponding to one of the three different LTE frequency bands.
In simpler terms, think of the PSS as a “locator” signal, allowing your device to understand the time and frequency characteristics of the cell it is trying to connect to. Without the PSS, your device would not be able to know where or when the cell is transmitting, making it impossible to establish a connection.
Secondary Synchronization Signal (SSS)
Once the UE has detected the PSS, the next step is to find the SSS. The SSS works alongside the PSS and is used by the UE to further refine the cell’s identification process. While the PSS provides the timing and frequency information, the SSS helps the UE to determine the physical layer cell identity, which is essential for distinguishing different cells in the same frequency band.
The SSS is also transmitted periodically and follows the PSS in the transmission frame structure. While the PSS helps with basic synchronization, the SSS provides additional information about the cell’s identity, enabling the UE to accurately identify the specific cell it is communicating with. Together, the PSS and SSS enable the UE to synchronize with the eNodeB and properly establish a communication link.
How PSS and SSS Work Together
Now that we know the individual functions of the PSS and SSS, it’s important to understand how they work together during the synchronization process. The PSS and SSS are transmitted in two separate phases of the cell search procedure:
- In the first phase, the UE detects the PSS, which gives it the timing and frequency synchronization it needs.
- In the second phase, the UE detects the SSS, which provides the cell identity and allows the UE to distinguish between different cells operating in the same frequency band.
After the UE has detected both the PSS and SSS, it is fully synchronized with the eNodeB. This synchronization enables the UE to proceed with further network procedures such as the Random Access Procedure (RAP), which is required to establish a connection to the LTE network.
To summarize, the PSS and SSS are essential components of the LTE synchronization process. The PSS provides the timing and frequency synchronization, while the SSS helps with cell identity detection. These signals are transmitted by the eNodeB to ensure that the UE can connect to the network seamlessly and efficiently.
In earlier discussions, we’ve looked at how LTE optimizes connectivity, and the PSS and SSS are integral to this by enabling devices to quickly and accurately sync with the network. These synchronization signals are the starting point for all LTE communication, ensuring that the connection is stable and reliable from the moment the UE begins its search for the network.