Case A
Understanding Case A
5G NR SSB Time-Domain Mapping
In 5G New Radio (NR), initial cell search, synchronization, and beam sweeping rely on the periodic transmission of Synchronization Signal and Physical Broadcast Channel (SS/PBCH) blocks, commonly referred to as SSB. To accommodate diverse frequency bands, duplex modes, and propagation conditions, 3GPP defines multiple time-domain mapping configurations known as Cases A through E. Each case specifies a particular subcarrier spacing (SCS) and OFDM symbol indices within a slot where SSBs can be transmitted.
Case A is tailored for the baseline 15 kHz subcarrier spacing, which matches the legacy LTE subcarrier grid. It operates exclusively in Frequency Range 1 (FR1) bands. The SSB transmissions occur within a 5 ms half-frame, which serves as the transmission window for the SSB burst. Because 15 kHz SCS has a slot duration of 1 ms (containing 14 OFDM symbols), a 5 ms half-frame contains 5 slots. Case A defines the exact symbols within these slots that are reserved for synchronization.
Symbol Mappings and Timing Rules
For Case A, the first symbol index of each candidate SSB in a slot is defined by the set of indices {2, 8}. Since each SSB spans 4 consecutive OFDM symbols, the candidate SSBs occupy symbols 2, 3, 4, 5 and symbols 8, 9, 10, 11 of the configured slots. The maximum number of candidate SSBs ($L_{\text{max}}$) within the 5 ms window is determined by the carrier frequency: for frequencies below 3 GHz, $L_{\text{max}}$ is 4, requiring candidate slots 0 and 1; for frequencies between 3 GHz and 6 GHz, $L_{\text{max}}$ is 8, requiring candidate slots 0, 1, 2, and 3. Symbols 0, 1, 6, 7, 12, and 13 in these slots remain free, providing headroom for downlink control channels (PDCCH) or gap times.
Key Mathematical Relations
Technical Specifications Comparison
| Configuration Case | Subcarrier Spacing (SCS) | Maximum SSBs (L_max) | OFDM Symbol Index (n) | Frequency Range Limit |
|---|---|---|---|---|
| Case A | 15 kHz | 4 or 8 | {2, 8} + 14k | FR1 (up to 6 GHz) |
| Case B | 30 kHz | 4 or 8 | {4, 8, 16, 20} + 28k | FR1 (up to 6 GHz) |
| Case C | 30 kHz | 4 or 8 | {2, 8} + 14k | FR1 (up to 6 GHz) |
| Case D | 120 kHz | 64 | {4, 8, 16, 20} + 28k | FR2 mmWave (up to 240 GHz) |
Frequently Asked Questions
What subcarrier spacing does Case A use?
Case A uses a subcarrier spacing (SCS) of 15 kHz. This matches the legacy LTE subcarrier grid, making it highly compatible with existing low-frequency FDD and TDD bands.
How many candidate SSBs can be transmitted in Case A?
The maximum number of candidate SSBs (L_max) is frequency-dependent. For carrier frequencies below 3 GHz, L_max is 4. For carrier frequencies between 3 GHz and 6 GHz, L_max is 8.
Which OFDM symbols are used by Case A in a slot?
Within a 14-symbol slot, Case A maps candidate SSBs to starting symbols 2 and 8. Because each SSB spans 4 symbols, they occupy symbols 2-5 and symbols 8-11.