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Thread: SSB Frequency-Domain Position

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    Post SSB Frequency-Domain Position

    In an NR system, a synchronization signal and PBCH block (SSB) carries the primary synchronization signal (PSS), secondary synchronization signal (SSS), and physical broadcast channel (PBCH). Each SSB consists of 240 contiguous subcarriers. A UE searches for cells based on received PSS and SSS signals and obtains necessary information from the received broadcast system information. The UE then selects the most appropriate cell to camp on using the received information.

    In SA or hybrid SA and NSA networking, the SSB Description method parameter must be set to SSB_DESC_TYPE_GSCN.
    In NSA networking, the SSB Description method parameter must be set to SSB_DESC_TYPE_GSCN or SSB_DESC_TYPE_NARFCN.

    Synchronization raster and numbering
    The synchronization raster indicates the frequency positions of the synchronization block that can be used by the UE for system acquisition when explicit signaling of the synchronization block position is not present.

    When NR is activated in NSA, UE does not need to do blind search for SSB since the frequency, subcarrier spacing etc are all configured by LTE RRC Connection Reconfiguration message. But in SA, UE need to blindly detect SSB since UE need to detect this before it gets any RRC (MIB, SIB). If UE needs to search SSB based on ARFCN raster, it will take too long time since ARFCN raster is very narrow. So, it would be good to define a SSB searching frequency in wider steps. This is the usage/purpose of GSCN.

    ARFCN step is 5Khz or 15kHz or 60Khz depending on frequency range. If UE has to scan the spectrum in this granularity for cell search, it will take very long time to find a cell. In case of GSCN, as you see in the following table, the granularity is 50 or 150 or 250 Khz under 3Ghz and it has the granularity of 1.44 Mhz in above 3 Ghz frequency range and below 24.25Ghz, and the granularity jumps to 17.28 MHz if the frequency goes over 24.25 Ghz


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