There was a 5G mobile and it was turned on. The first procedure to happen is cell search and cell selection when the phone is turned on. This allows the phone to find a cell. Cell search is the procedure by which the UE requires the time and frequency synchronization with the cell and detects the cell ID of that cell. The cell in 5G is defined by the physical cell ID. It was the cell search for the physical cell ID. The UE has to synchronize with the cell before it can do any wireless communication. Each cell transmits a synchronization signal consisting of two parts on the downlink periodically to help the UE to find any cell while entering the 5G system.
They are of different types
i) Primary Synchronization Signal (PSS)
ii) Secondary Synchronization Signal (SSS)
iii) Physical Broadcast Channel (PBCH)
The above two can be used to deduce the physical cell ID. We need some major information instead of having information related to synchronization and the cell ID, in able to communicate with the cell. That information is transmitted over PBCH (physical broadcast channel). If we combine the physical synchronization signal, secondary synchronization signal and physical broadcast channel, they will make or refer to as SSB (synchronization signal block). The cell in 5G can be defined as an area over which a certain SSB is available. This SSB are transmitted over the cell or they can be in the beam form. There is one SSB defined per beam if beamforming is used full stops when there are lots of SSB for the beam is used then it is together called as SSB burst set.
1. When the 5G phone is powered on and enters into the coverage. Then the first step will be the primary synchronization signals or PSS, for finding the cell and corresponding physical cell ID. The device will suppose to scan known frequency bands. The 5G support more bands than 4G can support. In 5G, synchronization signals can be transmitted every 20 milliseconds. This means the device has to wait for 20 milliseconds while they check for another possible location. We can see, that it can increase the time spent scanning each possible frequency band. So to overcome this, a different procedure is used in 5G. In 5G, the carrier raster and sync raster are disconnected which means they are decoupled. In 5G, the carrier raster is 100 KHz. Now the carrier frequency can be specified with the resolution same as the carrier raster i.e. 100 kHz. Here the synchronization signals do not place in all possible locations with the same precision.
Synchronization signals are seen in some designated spot and that spot will have 1.2 or 1.4 megahertz spacing. Due to this, the frequency scanning process will not have to look into all possible locations. Although they are transmitted every 20 milliseconds while in 4G it was 5 milliseconds but the number of locations they have to look for synchronization signal becomes very less. Now, this becomes the efficient way for UE to scan and find out the PSS. A 1.2 megahertz sync raster is used for the frequency bands below the 3 GHz and the 1.4 megahertz carrier sync raster is used for the frequency band above the 3 GHz. Now the PSS can take any value out of the three possible values (0-2).
2. When the UE scans the PSS it will now know the timing of transmission of SSS (secondary synchronization signal). It can take almost 336 possible values. When the UE detects the SSS, that means it will get the physical cell ID by using a formula. Now we have 3 possible values of PSS and 336 possible values of SSS this will all make around 1008 physical channel cell IDs. The PSS are having only three possible values but it has to go and scan the different bands and different raster locations to get the PSS. In the case of SSS, it has a high possible value but it already knows the PSS, so they need only one possible location to find the SSS. This means the number of values is high but the number of locations to find is only one. So it is not complex as finding the PSS.
3. Now the scanning or detection of PSS and SSS is done successfully. After that, the decoding of the physical broadcast channel is to be done for getting the minimum system information. When the device gets a physical channel cell ID. The next step is to get the minimum system information. It is holding the basic information required for initial access and or getting other system information. The minimum system information is divided into 2 parts
i) MIB (master information block)
ii) SIB1 (system information block 1)
The PBCH carries this MIB, which has a limited number of essential and most frequently transmitted parameters that are necessary to get the other information of the cell. It consists of information like system bandwidth, system frame number, cell barred, system carriers spacing for key messages, etc. The MIB provides the parameter to the device which are required to get the SIB1. This consist of information required for monitoring the control channels to understand when and where we can find SIB1 in the data channel. By getting the information from MIV, the UE can decode the SIB1. This SIB1 can also be known as RMSI (remaining minimum system information). It carries the essential information required by the UE to access the cell. This information can consist of self-selection information, cell access information, and information about the other SIBs.
4. Other SIBs (SIB2, SIB3….): This holds the information which is not necessarily needed to know why the UE while accessing the 5G system. This includes information like reselection parameter, earthquake warning parameter etc. These SIBs can also be periodically broadcast as SIB1 or they can be transmitted only when there was a demand. Once the device has done the SIB and MIB decoding. Then they decide whether to camp in the cell or not. If everything is successful, then the device or you decide to camp on the cell.