Dear community friends, today I want to share with you is cell search and downlink synchronization.
LTE search process
In order to access the cell, the device needs to locate the cell and synchronize it.
Figure 1 LTE search process
What is cell search?
Principles of cell search
1. Cell search is a process in which the UE implements downlink time-frequency synchronization with the E-UTRAN and obtains the serving cell ID.
2. The cell search consists of two steps:
Step 1: The UE demodulates the primary synchronization signal to implement symbol synchronization and obtains the cell group ID.
Step 2: The UE demodulates the synchronization signal to implement frame synchronization, and obtains the CP length and the cell group ID.
Cell ID
According to the LTE protocol, a cell ID at the physical layer is divided into two parts: cell group ID and intra-group ID. According to the latest protocol, there are 168 cell groups at the physical layer, and each cell group consists of three IDs. Therefore, there are 504 independent cell IDs (168 x 3).
Initializing cell search
After the UE is powered on, it starts initial cell search and network search. Generally, when the UE is powered on for the first time, the UE does not know the bandwidth and frequency of the network.
The UE repeats the basic cell search process and attempts to demodulate synchronization signals from frequencies on the entire spectrum. This process is time-consuming but generally does not have a strict time requirement. The subsequent initialization time of the UE may be shortened by using some methods. For example, the UE stores the information about the available networks and preferentially searches for these networks after being powered on.
When the UE searches for an available network and implements time-frequency synchronization with the, the UE obtains the serving cell ID. That is, after the cell search is complete, the UE demodulates the downlink broadcast channel PBCH and obtains system information such as the system bandwidth and number of transmit antennas.
After the preceding process is complete, the UE demodulates the downlink control channel PDCCH and obtains the paging cycle allocated by the network to the UE. Then, the eNodeB wakes up from the IDLE state in a fixed paging period to demodulate the PDCCH and monitor the paging. If there is paging for the UE, the eNodeB demodulates the specified downlink shared channel PDSCH resources and receives the paging.
Figure 2 Cell Search and Downlink Synchronization
Downlink synchronization
Downlink synchronization is the first step that a UE needs to complete after entering a cell. Only after the downlink synchronization is complete, the UE can start to receive other channels (for example, broadcast channels) and perform other activities.
Downlink synchronization consists of three steps:
Step 1: The UE correlates the received signals with three known primary synchronization sequences, finds the maximum correlation peak value, and obtains the primary synchronization sequence and the primary synchronization channel position of the cell, thereby achieving OFDM symbol synchronization. The PSC transmits data every 5 ms. Therefore, the UE cannot determine the beginning of the entire frame. In addition, the primary synchronization sequence of the cell is part of the cell ID.
Step 2: The UE correlates 270 known secondary synchronization sequences with the received signal at a specific location to find the secondary synchronization sequence of the cell. The SSC transmits every 5 ms, but the two SSCs in a frame transmit different sequences. The UE obtains frame synchronization based on this feature. The secondary synchronization sequence is also part of the cell ID.
Step 3: The downlink synchronization is complete, and the UE has obtained the cell ID.
Figure 3 Cell Search and Downlink Synchronization
Figure 4 Cell Search and Downlink Synchronization
Figure 5 Cell Search and Downlink Synchronization
NOTE：
1. Cell search is a process in which a UE synchronizes time and frequency with a cell, obtains a physical cell ID, and obtains cell signal quality and other cell information based on the physical cell ID. During cell selection or reselection, the UE searches for cells on all frequencies.
2. In an LTE system, a synchronization channel is a signal dedicated for cell search and is classified into a primary synchronization channel and a secondary synchronization channel. The UE performs cell search on the synchronization channel as follows:
The UE detects the primary synchronization channel and obtains 5 ms clock synchronization. The physical cell ID is mapped to the primary synchronization signal. Therefore, the UE obtains the physical cell ID through the primary synchronization channel.
The UE detects the secondary synchronization signal and completes frame synchronization, that is, completes cell time synchronization. The cell ID group and the secondary synchronization signal have a one-to-one mapping relationship. Therefore, the secondary synchronization UE determines the number of the cell ID group to which the physical cell ID belongs.
That's all I've shared about cell search and downlink synchronization, and I'll share random access in my next post.
You are welcome to read and comment.
