5G/NR - Cell Search  

 

 

 

Cell Search/SIB1 Detection

In any cellular communication system (2G, 3G, 4G, 5G), there are specific procedure called Cell Search. Even though the detailed mechanism varies a little bit depending on each specific Cellular Technology, the basic concept and purpose is same. They are same in 5G/NR as well.

 

38.300 - 5.2.5.3 defined 5G/NR as follows.

    Cell search is the procedure by which a UE acquires time and frequency synchronization with a cell and detects the Cell ID of that cell. NR cell search is based on the primary and secondary synchronization signals, and PBCH DMRS, located on the synchronization raster.

I think this is well phrased definition that shows both overal defintion that can be applied to every cellular technology and NR specific technology. As I highlighted with different color, the first sentence is the definition of celllar technology for every technology and the second sentence is the definition that are specific for NR.

 

Based on this definition, NR Cell Search procedure can be broken down to several steps as follows : (Even though it may seem simple process, this is very complicated process if you go in detail. First, try to understand overall procedure as described below and then follow through the linked pages for further details for each of the steps).

    i) UE tunes to a specific frequency (How UE select this specific freuqency to tune to ?)

    ii) UE tries to detect SSB and decode PSS, SSS. If UE fails at this step, it goes to step i) and passes at this step, go to next

    iii) Once UE successfully detect PSS/SSS, UE tries to decode PBCH.

    iv) Once UE successfully detect PBCH, it decode MIB

    v) Based on MIB.pdcch-ConfigSIB1, find the location of CORESET0 (CORESET for PDCCH/DCI for SIB1 transmission) and SearchSpace information. This CORESET0 finding is done by predefined parameters as described here.

    vi) Blind decode DCI 1_0 in the SearchSpace.

    vii) Based on the contents of the DCI 1_0. (NOTE : In order to interpret the DCI 1_0 for SI RNTI, UE need to have some additional table like TimeDomainResourceAllocation which is usually configured in RRC message (e.g, SIB1, RRCConfiguration etc). But this step is before decoding any RRC message. It implies that UE should know of TimeDomainResourceAllocation from predefined tables in 3GPP. Refer to [NOTE 1] in SI-RNTI DCI 1_0 for further details )

    viii) Detect and decode PDSCH carrying SIB1.

    ix) decode SIB1 and other SIBs (if SIB1 carries information on other SIBs).

    x) Once the SIB1 is decoded, UE can figure out various other fundamental information like channel bandwidth, center frequency of the channel etc as shown here.

 

NOTE : step ii) and iii) can be catagorized as a single step called SSB detection. This process is described in detail in my SSB/PBCH page.

Matlab Reference on Cell Search

Reference

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