Downlink Channels — PDSCH, PDCCH, PBCH
PDSCH (Physical Downlink Shared Channel) carries user data — uses LDPC, MCS Table 1/2/3. PDCCH (Physical Downlink Control Channel) carries DCI — uses Polar coding. PBCH (Physical Broadcast Channel) carries MIB — in SSB.
PDSCH Deep Dive
Resource mapping (Type A vs B), DMRS Type 1/2 + Type single-symbol/double-symbol, MCS tables (Table 1 64-QAM, Table 2 256-QAM, Table 3 URLLC), TBS calculation, LDPC base graphs (BG1/BG2), PRB bundling, layer mapping, precoding.
PDCCH & DCI
CORESET configuration (1-3 OFDM symbols + freq region). Search spaces (Type 0/0A/1/2/3/UE-specific). DCI formats (0_x for UL, 1_x for DL, 2_x group-common). Polar coding with CRC-aided successive-cancellation list decoder.
PBCH & SSB
Synchronization Signal Block: PSS + SSS + PBCH on 240 SCs × 4 sym. L = 4/8/64 SSB burst within 5 ms half-frame. UE uses for cell-search, time-frequency sync, PCI detection, MIB acquisition.
Reference Signals
DMRS (UE-specific data demod). CSI-RS (channel sounding for MIMO feedback). PT-RS (phase noise tracking). TRS (tracking RS). PRS (positioning). All numerology-aligned with the BWP.
MCS & Modulation
QPSK (Qm=2) cell-edge. 16-QAM (Qm=4) mid-cell. 64-QAM (Qm=6) good signal. 256-QAM (Qm=8) high SNR. 1024-QAM (Qm=10) Rel-17 FR2-2 only. CQI-driven adaptive selection.
HARQ & Retransmission
Up to 16 parallel HARQ processes per UE. ACK/NACK on PUCCH. Soft combining (Chase or IR) on retransmissions. RV cycling 0→2→3→1 for optimal IR.