Where is SSB Located in Time Domain?

The SSB Burst is contained inside a 5ms time frame, and up to a maximum of 64 SSB beams are transmitted within this 5ms time period. Since a basic radio frame spans 10ms, the SSB burst can occur in either the first half or the second half of the radio frame.

The goal of this article is to explain how the User Equipment (UE) finds the exact SSB location in the time domain relative to the System Frame Number (SFN) and slot timing. Determining whether the SSB is broadcasted in the first half or the second half of the radio frame is crucial for time synchronization.

Essential Location Information

To find the correct location of an SSB, a UE needs two key pieces of information:

1. Time Domain: OFDM Symbol, Slot, Half-frame, and System Frame Number (SFN).
2. Frequency Domain: Center Frequency of the SSB (e.g., Sub-Carrier #0 of PRB #10).

In the case of an Initial Scan or Out-Of-Service (OOS) mode when the UE is searching for a cell, it acts in an exhaustive search mode. It scans all available frequency bands and the corresponding GSCN Raster (possible SSB locations) in the frequency domain. In the time domain, it scans each GSCN Raster for 20ms until it successfully detects an SSB Burst or a single SSB.

The SSB Burst Position

The SSB burst is contained inside a 5ms time instance, which the 3GPP Specification defines as a Half-frame. However, this alone does not specify whether it resides in the first half or the second half of the radio frame.

3GPP specifications define the OFDMA start symbols for the SSB within the burst relative to the Sub-Carrier Spacing (SCS) and Frequency Range. But again, these symbols exist relative to the 5ms boundary.

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

So, how does the UE know if the SSB burst is in the first or second half of the radio frame?

For a UE to decode SSBs, it must scan a particular GSCN raster for at least 20ms. By default, if not explicitly specified, the UE assumes the SSB periodicity is 20ms and scans the GSCN raster accordingly. If no SSBs are found after 20ms, it simply moves to the next raster in its search list.

PBCH Payload and Scrambling

When the UE successfully decodes the SS-PBCH block, it extracts the following vital information:

• System Frame Number (0 - 1023)
• Half-radio Frame (First or Second Half-Radio Frame)
• SS-PBCH block index (0 - 63)

How the UE obtains the Half-frame index depends on the Frequency Range of the operating band (38.212 Section 7.1.2 Scrambling):

• Below 3 GHz: The UE can extract this information from the PBCH DMRS scrambling sequence as well as the physical layer payload of the PBCH.
• Between 3 GHz and 6 GHz: It is obtained strictly from the physical layer payload of the PBCH.
• Above 6 GHz (FR2): It is obtained strictly from the physical layer payload of the PBCH.

The decoded Half-frame index indicates whether the SSB Burst sits in the first or second half of the radio frame. This is critically important because it allows the UE to pinpoint the radio frame boundary and achieve full time-synchronization at the Slot and SFN levels.

SSB Measurements in Connected Mode

For a UE that is out of service with no prior information about NR bands, a blind GSCN scan is necessary. But what happens in RRC_CONNECTED mode? Here, a UE might already be camped on an NR Cell but needs to measure neighboring cells (either intra-frequency or inter-frequency), which might have completely different SSB locations.

In this scenario, the network can indicate two things to help the UE locate the SSB in both time and frequency domains for neighboring cells:

1. Configure Measurement Object with SMTC
   • The SSB EARFCN will be implicitly indicated (Frequency location).
   • The SMTC will contain the Periodicity and temporal offset of the SSB burst or SSB (Time domain information).
2. Configure Measurement Gaps
   • Allows the UE to tune away its receiver to properly measure neighboring cells.

Neighbor Cell Scenarios

• Intra-Frequency Neighbor (Same SSB location in time and frequency domain)
  • No measurement gaps needed.
• Intra-Frequency Neighbor (Different SSB location in time and frequency domain)
  • Measurement gaps are required.
• Inter-Frequency Neighbor (Same Frequency Range)
  • E.g., The UE is camped on an FR1 Cell, and the neighbor is an FR1 cell in a different band.
  • Measurement gaps are required to measure the SSB.
• Inter-Frequency Neighbor (Different Frequency Range)
  • E.g., The UE is camped on an FR1 Cell, and the neighbor is an FR2 cell.
  • Measurement gaps are typically NOT required if the UE has independent RF chains to measure the FR2 Cell SSB.

References

• 3GPP TS 38.213
• 3GPP TS 38.300
• 3GPP TS 38.212