Where is SSB located in the time domain?

SSB Burst is contained inside a 5ms time frame and up to a max of 64 SSB beams are transmitted within the 5ms time period.  A radio frame is of 10ms and the SSB burst can happen in the first half of the second half of the radio frame. The goal of this article is to explain how UE finds the SSB location in the time domain relative to SFN and slot timing.

The intent of this article is the answer to find out which half radio frame is the SSB broadcasted?

To find the location of SSB you need two pieces of information

• Time Domain ( OFDM Symbol, Slot , Half-frame, SFN){ Yes all these}
• Frequency Domain ( Center Frequency of SSB i.e. Sub Carrier#0 of PRB#10)

In the case of Initial Scan or OOS mode when UE is finding for a Cell, a UE will be in work-horse mode scanning all the frequency bands and the GSCN Raster(Possible SSB location) in the frequency domain and in time domain it scans every GSCN Raster for 20ms until it detects an SSB Burst or an SSB.

The SSB burst is contained inside a 5ms time instance, 3GPP Spec defines that as a Half-frame, but does not specify whether it is in the first half of the radio frame or the second half of the radio frame.

3GPP specifications have defined the OFDMA start symbols of the SSB within the SSB Burst relative to SCS and Frequency range, but this location of OFDMA symbols is within the half-radio frame i.e. 5ms duration.

3GPP Specification 38.213 [Section 4.1] has defined the OFDMA start symbols for each of the frequency range and SCS. This is a separate topic in itself and will require more detailed information to cover.

Now the question is how will the UE knows whether the SSB burst is located in the first half radio frame or the second half of the radio frame. For a UE to decode SSB’s it needs to scan the particular GSCN raster for 20ms, by default if not specified UE assumes that the SSB periodicity is 20 ms and scans the GSCN raster for a particular band. UE scans each GSCN raster for 20ms if it’s not able to find any SSB’s it moves to the next GSCN raster until it finds an SSB burst or an SSB.

The UE when decodes the SS-PBCH block it gets the information of

• System Frame Number ( 0 -1023)
• Half-radio Frame ( first or second half-radio frame)
• SS-PBCH block index (0-63)

How UE is able to get the Half-frame index depends on Frequency range of the operating band [ 38.212 Section 7.1.2 Scrambling]

Below 3 GHz: UE can get this information from the PBCH DMRS scrambling sequence and also from the physical layer Payload of the PBCH

Between 2 GHz and 6GHz: physical layer Payload of the PBCH

Above 6 GHz: physical layer Payload of the PBCH

The Half-frame index that UE decodes from either PBCH payload or from PBCH DMRS sequence will indicate whether the SSB Burst is in the first half of the radio frame or the second half of the radio frame, This is important as UE will be able to find the radio frame boundary and will be time-synchronized at Slot and SFN level.

For a UE which is out of service and has no prior information of NR band information, it needs to perform a GSCN scan for Possible SSB locations. But what about in the case of RRC_CONNECTED mode, where a UE might be camped on NR Cell and it needs to measure neighbor cell which can be an Intra Frequency or Inter-Frequency cell with the same or different SSB location, In this case, Network can indicate two things to UE which helps UE in locating the SSB in time and frequency domain for a neighbor cell.

• The network can Configure Measurement Object with SMTC
  • SSB EARFCN will be implicitly indicated ( Frequency location )
  • SMTC will contain Periodicity and offset of SSB burst or SSB ( Time domain information )

• Measurement Gaps, allowing UE to tune away to measure Neighbor cells.
  • Intra Frequency Neighbor cell with SSB location in time and frequency domain
    • No measurement gaps needed.
  • Intra Frequency Neighbor cell with different SSB location in time and frequency domain
    • Measurement Gaps are required.
  • Inter-Frequency Neighbor cell same Frequency range
    • UE can be camped on FR1 Cell and neighbor cell is also FR1 cell in different band, UE will need Measurement gaps to measure SSB.
  • Inter-Frequency Neighbor cell different Frequency range
    • For example UE might be camped on FR1 cell, Neighbor cell can be FR2 cell, UE will not need Measurement gaps to measure the FR2 Cell SSB.

References

1. 3GPP TS 38.213
2. 3GPP TS 38.300
3. 3GPP TS 38.212