Everything the air interface actually carries rides on a handful of named physical channels. Learn what each one does, which direction it runs, and where it sits on the time–frequency grid, and a 5G log stops being an alphabet soup — PDSCH and PUSCH move the data, a ring of control channels tells them how.
Above the physical layer, 5G thinks in transport channels — logical pipes like ‘shared data’ or ‘broadcast’. The physical layer’s job is to carry those over the air, and the named physical channels are how: each one is a defined use of the resource grid, with its own modulation, coding and position in time and frequency. Get the channel map straight and the rest of the PHY falls into place.
PDSCH (Physical Downlink Shared Channel) is the workhorse — it carries user data and paging, LDPC coded, scheduled slot by slot, and protected by HARQ. PDCCH (Physical Downlink Control Channel) carries the DCI: the scheduling decisions and grants that tell a UE where its PDSCH is and how to decode it, and where to put its uplink. It sits in a control region (CORESET) at the front of the slot and uses Polar coding. PBCH (Physical Broadcast Channel) carries the minimum system information a UE needs to camp, and travels inside the SSB (synchronisation signal block) alongside the sync signals.
PUSCH (Physical Uplink Shared Channel) is the uplink counterpart of PDSCH — the channel that carries the user’s data up to the network, LDPC coded and under HARQ, and it can piggyback uplink control (UCI) when the UE is already transmitting data. PUCCH (Physical Uplink Control Channel) carries that control on its own when there is no PUSCH: HARQ ACK/NACK, channel-state feedback (CSI) and scheduling requests (SR). It lives at the band edges to leave the middle clear for data. PRACH (Physical Random Access Channel) carries the preamble a UE sends to first get the network’s attention and align its timing — the doorbell before any of the rest can happen.
None of the data channels can be demodulated without pilots. DM-RS (demodulation reference signal) is embedded in PDSCH and PUSCH so the receiver can estimate the channel and undo it. CSI-RS lets the UE measure the downlink channel and report what modulation and how many layers it can take. SRS (sounding reference signal) does the same job in reverse, letting the gNB measure the uplink. PT-RS tracks phase noise, which matters most at mmWave. The reference signals are not glamorous, but without them the channels they share the grid with are undecodable.
The transport-to-physical mapping is worth holding in your head: on the downlink, DL-SCH maps to PDSCH, BCH to PBCH, and the scheduler’s DCI goes on PDCCH. On the uplink, UL-SCH maps to PUSCH, the UE’s control (UCI) goes on PUCCH or rides PUSCH, and the random-access preamble goes on PRACH. Data channels (PDSCH, PUSCH) are LDPC coded and run HARQ; control channels (PDCCH, PUCCH) use Polar and small block codes, because control is short and must be decoded fast and reliably.
Each channel is a different processing chain with a different deadline. PDCCH has to be blind-decoded early in the slot before anything else can proceed; PDSCH and PUSCH drive the heavy LDPC and HARQ paths; PUCCH formats are small but latency-critical; PRACH detection is a correlation problem running continuously. A receiver that is well-architected for one can still miss its budget on another. Laying the channels out against the slot clock — what has to be decoded when, and with which engine — is a large part of designing the baseband.
PDSCH is the downlink shared channel (data from the network to the UE); PUSCH is the uplink shared channel (data from the UE to the network). Both carry LDPC-coded user data under HARQ; PUSCH can also carry uplink control information when the UE is already transmitting.
User data on the uplink, LDPC coded and protected by HARQ, and optionally uplink control information (UCI) such as HARQ feedback or channel-state reports piggybacked onto the same transmission when the UE has a grant.
PUSCH carries uplink data (and can carry control alongside it). PUCCH carries uplink control on its own — HARQ ACK/NACK, channel-state information and scheduling requests — when the UE has no data grant. PUCCH sits at the band edges to keep the centre free for data.
It carries Downlink Control Information (DCI): the scheduling grants that tell a UE where its downlink data is, how to decode it, and where and how to transmit on the uplink. It sits in a control region at the start of the slot and uses Polar coding.
Reference signals (pilots). DM-RS is embedded in data channels so the receiver can estimate and undo the channel. CSI-RS lets the UE measure the downlink and report channel quality. SRS lets the gNB measure the uplink. Without them the data channels cannot be demodulated.
The Physical Random Access Channel. A UE sends a preamble on it to initiate access and align its uplink timing before it has any dedicated resources — the first step in attaching to a cell.
The grid these channels sit on.
KNOWLEDGEHow PDSCH and PUSCH recover from errors.
KNOWLEDGELDPC for data, Polar for control.
KNOWLEDGEPBCH and the synchronisation block.
TECHNOLOGYWhere the channels sit in the PHY.
KNOWLEDGEWho schedules the channels.
Blind-decoding PDCCH early, driving PDSCH/PUSCH through LDPC and HARQ, and catching PRACH — all against the slot clock — is the real architecture problem. Tell us the numerology and the throughput target.