Case B
Understanding Case B
High-Throughput 30 kHz SCS Configuration
In 5G New Radio (NR), subcarrier spacing (SCS) is scalable. While 15 kHz is the baseline, 30 kHz SCS is widely deployed in mid-band spectrum, such as the 3.5 GHz C-band, because it halves the OFDM symbol duration, reducing transmission latency and doubling the subcarrier bandwidth to mitigate phase noise. Under the 30 kHz SCS framework in Frequency Range 1 (FR1), 3GPP defines two distinct SSB time-domain mapping patterns: Case B and Case C.
Case B is optimized to leave the beginning of slot boundaries clear of synchronization signals. Because 30 kHz SCS has a slot duration of 0.5 ms, a 5 ms half-frame contains 10 slots. Case B maps up to 8 candidate SSBs within this window. By grouping the SSBs differently than Case C, Case B alters the available symbol positions at the start of slots, which gives network operators alternative choices for scheduling downlink control information (PDCCH).
OFDM Symbol Mapping and Layout Rules
For Case B, the candidate SSB transmissions occupy starting symbol indices {4, 8, 16, 20} within a combined 28-symbol slot block (equivalent to two consecutive 30 kHz slots). The candidate SSBs occupy symbols 4-7, 8-11, 16-19, and 20-23. The maximum number of candidate SSBs ($L_{\text{max}}$) is 4 for carrier frequencies below 3 GHz, and 8 for frequencies between 3 GHz and 6 GHz. Because the first candidate SSB starts at symbol 4, symbols 0, 1, 2, and 3 are left entirely empty, allowing the network to transmit up to a 3-symbol wide PDCCH without overlapping the synchronization block.
Key Mathematical Relations
Technical Specifications Comparison
| Parameters | Case B | Case C | Design Trade-offs |
|---|---|---|---|
| Subcarrier Spacing (SCS) | 30 kHz | 30 kHz | Identical frequency spacing |
| Starting Symbols | {4, 8, 16, 20} + 28k | {2, 8} + 14k | Case B starts later in the slot |
| Empty Symbols at Slot Start | Symbols 0-3 | Symbols 0-1 | Case B allows larger PDCCH control region |
| Intra-Burst Gaps | None (consecutive blocks) | 2 symbols between SSBs | Case C provides gaps for TDD switching |
Frequently Asked Questions
What is the primary difference between Case B and Case C?
Both cases use 30 kHz subcarrier spacing, but they use different symbol starting positions. Case B starts candidate SSBs at symbol 4 (leaving symbols 0-3 empty for PDCCH control), whereas Case C starts at symbol 2 and features 2-symbol gaps between consecutive SSBs.
How many slots are in the 5 ms half-frame for Case B?
At 30 kHz SCS, the slot duration is 0.5 ms. Therefore, there are exactly 10 slots within a 5 ms half-frame, which can accommodate up to 8 candidate SSB locations.
What is the benefit of starting the SSB at symbol 4 in Case B?
Starting at symbol 4 ensures that symbols 0 to 3 are not occupied by the SSB. This allows the base station to transmit a full 3-symbol physical downlink control channel (PDCCH) to schedule user data without conflicting with initial search signals.