Case C
Understanding Case C
Optimized TDD Guard-Interval SSB Mapping
In mid-band 5G deployments operating in Frequency Range 1 below 6 GHz, Time Division Duplexing (TDD) is the dominant duplex mode. TDD requires strict synchronization and fast switching between downlink and uplink transmission directions. To facilitate this switching and protect slot boundaries, 3GPP introduced the Case C SSB transmission pattern. Like Case B, Case C operates with a 30 kHz subcarrier spacing. However, its symbol-level mapping is structured differently to accommodate the fast-switching requirements of mid-band TDD networks.
Case C defines a 14-symbol slot spacing mapping. In contrast to Case B, which groups four candidate SSBs into a single 28-symbol block with no gaps, Case C schedules two candidate SSBs per slot, separated by a 2-symbol guard interval. This structural gap aligns with the guard periods required for downlink-to-uplink transitions, preventing interference and reducing scheduling conflicts in active cells.
Symbol Layout and Guard Intervals
Under the Case C mapping, candidate SSBs are scheduled at starting symbol indices {2, 8} in each configured slot, repeating every 14 symbols (the length of one 30 kHz slot). The candidate SSBs occupy symbols 2-5 and symbols 8-11. This creates a 2-symbol gap (symbols 6 and 7) between the first and second SSB in a slot, and a 4-symbol gap (symbols 12, 13, 0, and 1 of the next slot) between successive slots. The maximum number of candidate SSBs ($L_{\text{max}}$) is 4 (for carrier frequencies below 3 GHz) or 8 (for frequencies between 3 GHz and 6 GHz), utilizing up to 4 consecutive slots.
Key Mathematical Relations
Technical Specifications Comparison
| Parameters | Case C | Case B | System Advantage |
|---|---|---|---|
| Slot Repeat Period | 14 symbols (1 slot) | 28 symbols (2 slots) | Case C fits standard slot periodicity |
| Candidate SSBs per Slot | 2 (symbols 2-5, 8-11) | Up to 4 (symbols 4-23) | Case C spreads SSBs evenly |
| Intra-Slot Empty Symbols | Symbols 6, 7, 12, 13 | None inside the block | Provides TDD switching and guard times |
| TDD Duplex Matching | High compatibility | Moderate compatibility | Case C aligns with UL/DL switching points |
Frequently Asked Questions
Why is Case C widely used in 5G TDD networks?
Case C is widely used because the 2-symbol gap between consecutive SSBs (symbols 6 and 7) matches the natural guard times required for downlink-to-uplink switching in TDD frame structures, minimizing slot structure fragmentation.
What are the candidate starting symbols for Case C?
The starting symbols for Case C candidate SSBs are symbols 2 and 8 in each slot. Each block occupies 4 symbols, meaning they sit on symbols 2-5 and 8-11.
How many slots does Case C occupy when L_max is 8?
When L_max is 8, the system requires 8 candidate positions. Since Case C maps exactly 2 candidate SSBs per slot, it occupies 4 consecutive 30 kHz slots (a total duration of 2.0 ms).