【5G RRC】Cell Search and System Acquisition process

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I work for an internationally renowned terminal manufacturer and am responsible for the research and development of modem chips.
In the early days of 5G, he was responsible for the development of the terminal data service layer and the core network. Currently, he is leading the research on technical standards for 6G computing power networks.


The content of the blog mainly revolves around:
       5G/6G protocol explanation
       computing power network explanation (cloud computing, edge computing, end computing)
       advanced C language explanation
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Cell Search and System Acquisition Processes

       Cell search is a process initiated by the UE to obtain time and frequency synchronization with the cell and to decode the Cell ID of the cell. In 5G NR, the concept of cell search is similar to that in LTE, which is to use the primary synchronization signal (PSS, Primary Sync Signal) and secondary synchronization signal (SSS, Secondary Sync Signal) to decode into a physical cell ID (PCI, Physical Cell ID) .

UE can access 5G-NR cells in two ways:

• Non-independent mode (NSA, Non Stand Alone) deployment: EUTRA - NR dual connection [EN-DC];
• Independent (SA, Stand Alone) deployment: NR Only Cell;

       In EN-DC, the network provides cell search information, such as frequency (NR-ARFCN), Cell ID, RACH parameters, etc., through the RRC reconfiguration message of the LTE eNB. The terminal can try to access the NR cell through RACH.

Under SA, the NR cell search process steps are as follows:

• The UE tunes to a specific frequency;
• The UE tries to detect PSS and SSS signals to obtain frequency and time synchronization;
• Once the UE successfully detects PSS, SSS, it will get information about synchronization and Physical Cell ID (PCI), now UE is ready to decode PBCH;
• After successfully decoding PBCH, UE tries to decode PDCCH and PDSCH to obtain RMSI and OSI;

       The UE scans this frequency band (this frequency band is defined by 3GPP 38.104 section 5.4.3.3) on the synchronization raster to tune to a specific frequency. The synchronization raster indicates the frequency position of the synchronization block, and when there is no explicit synchronization block position signaling, the UE can use the frequency position of the synchronization block for system acquisition. The subcarrier spacing of the sync raster and sync block depends on each frequency band.

       In the next step, the UE decodes the primary and secondary synchronization signals to obtain the physical cell ID (PCI) and PBCH DMRS located on the synchronization raster. PSS, SSS and NR-PBCH are transmitted in one synchronization signal block (SS Block) . SSB consists of 4 symbols, one of which is PSS, one is SSS, and the other two are PBCH . Time synchronization and frequency synchronization at symbol-level and slot level can be realized through PSS and SSS .

       NR supports up to 1008 PCIs , twice that of LTE. There are a total of 336 unique cell groups, and each group contains 3 unique cells. The corresponding cell ID calculation formula is as follows:

       $N_{ID}^{cell}=3N{\_}_{ID}^{(1)}+N_{ID}^{(2)}$

in:

• $N_{ID}^{(1)}$: Secondary synchronization signal (SSS), whose value range is {0,1,…,335};
• $N_{ID}^{\left(2\right)}$: Primary synchronization signal (PSS), its value range is {0,1,2};

       SSB is transmitted in batches by forming an SS Burst (each beam has an SSB). SS Burst is used during beam scanning, and the direction of beam transmission is changed after each SSB transmission. UE uses beam scanning (Beam Sweeping) mechanism to measure and identify an optimal beam.

       The collection of SS Burst is called SS Burst Set. Both SS Burst and SS Burst Set can contain one or more elements, and the maximum number of SSBs in SS Burst depends on the frequency , it can be 4 (below 3 GHz), 8 (3 ~ 6 GHz) or 64 (6 ~ 52.6 GHz). The period of SSB is configured by the network, and the default transmission period is used for initial cell selection . The period of SS Burst Set is 20 ms by default in all frequency ranges (that is, 2 NR system frames). This interval is 4 times that of LTE (5 ms in LTE), and the purpose is to reduce the transmission overhead of "always-on". The frame and slot timing is defined by the identifier of the SSB and acquired by the UE.

       The gNB defines multiple SSB candidate positions in one radio frame, and the number of beams radiated in a certain direction corresponding to each position. Each SSB can be SSB indexidentified by a unique number called SSB, and which SSB is detected depends on where the UE is located. The UE measures the signal strength of the demodulation reference signal (PBCH DMRS) for each SSB (period of one SSB set) that it detects within a certain period. The UE can identify the SSB index with the strongest signal strength from the measurement results. The SSB with the strongest signal strength is the best beam for the UE .

The figure above shows an example of two UEs, UE #1 finds that SB #1 is the best beam, and UE #2 finds that SB #4 is the best beam:

• UE selects the best beam and decodes the content information of PBCH MIB, such as SFN, SSB index, raster offset, default DL numerology, RMSI configuration, DM-RS position and cell barring information, etc.;
• If the cell is not blocked, the UE tries to decode the subsequent PDCCH to obtain the frequency point position of the PDSCH resource, in preparation for receiving SBI1 information;
• The PDCCH configuration of RMSI is provided by PBCH. The control resource set (CORESET, COntrol REsource SET) configuration of RMSI is associated with the SSB in the SSB Burst Set. The 1-bit information field is used in the PBCH to indicate the SCS of RMSI and OSI, and other information used for the initial random access procedure;
• The UE reads the remaining minimum system information from SIB# 1, and decodes the PLMN ID, cell selection parameters, and RACH parameters;
• If the PLMN ID of the network matches the list of available PLMN IDs provided by the terminal, perform the cell selection process; otherwise, the UE will obtain another 5G cell and start the above process again;
• Another parameter that the UE tries to verify is that qRxminif the signal detected by the UE meets the qRxmin level, the cell selection process is successful, and the UE can try to initiate the RACH process to complete the uplink synchronization;
• If the cell selection fails, the UE will try to acquire another cell and restart the above procedure.

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