The general process of 5G NR cell search is:
1 Blind search for SSB
2 Determine the start position of the system frame and cell PCI according to the PSS and SSS in the SSB,
3 Determine the position of the DMRS, and then determine the position of the PBCH time-frequency resource,
4 After obtaining the PBCH data, perform frequency offset estimation and phase compensation, etc. , Channel estimation and equalization, decoding, etc., and finally parse out MIB data.
1. Overview of SSB
In NR, synchronization signals PSS, SSS, and PBCH (Synchronization Signal and PBCH block, referred to as SSB) together constitute SSB. Among them, PSS represents Primary Synchronization Signals (PSS), SSS represents Secondary Synchronization Signals (SSS), and PBCH (Physical Broadcast Channel) represents a physical broadcast channel. In LTE, PSS, SSS, and PBCH are also used for synchronization, but they are not called SSB. As shown in Figure 1 below, it is the fixed position distribution of PSS, SSS, and PBCH in the frame in the time-frequency domain in LTE.
Different from the design structure in which synchronization sequences and PBCH are sent separately in LTE, they are sent as a whole SSB on 4 symbols at the same time in NR. SSB occupies 4 OFDM symbols in the time domain and 240 subcarriers in the frequency domain (20 PRB), numbered from 0 to 239, as shown in Figure 2.
As shown in Figure 2, PSS is located in the middle 127 sub-carriers of symbol 0. SSS is located in the middle 127 subcarriers of symbol2; in order to protect PSS and SSS, the subcarriers at both ends of them are set to zero. PBCH is located at symbol1/3 and symbol2, where symbol1/3 occupies all subcarriers from 0 to 239, and symbol2 occupies all subcarriers except the subcarrier occupied by SSS and the subcarrier Set 0 of the protection SSS. DM-RS is located in the middle of PBCH, on symbol1/3, 60 on each symbol, separated by 4 subcarriers. This method of packet transmission enables the UE to start the process of channel estimation and analysis of the PBCH physical channel after detecting the synchronization sequence, which speeds up the UE cell access speed and reduces the network delay of the system.
to be continued. . . .