There are 3 types of channels:
- Logical channels. Logical channels transfer data between RLC and MAC layer. This data is called RLC PDU. CCCH(Common Control channel), DCCH (Dedicated control channel) and DTCH(Dedicated traffic channel) are uplink logical channels mapped to ULSCH transport channel. CCCH and DCCH are used to transfer RRC signaling messages. DTCH is used to transfer application data.
- Transport channels transfer data between MAC layer and Physical layer. This data is called MAC PDU or transport blocks. ULSCH (Uplink shared channel) and RACH (Random access channel) transport channels are mapped to PRACH (Physical Random access channel) and PUSCH physical channels. RACH is used to transfer the index of the preamble signal. MAC layer completes the preamble selection procedure and passes it to physical layer which is sent on PRACH.
- Physical channels transfer data across air interface. PRACH (Physical Random access channel), PUSCH and PUCCH are uplink related physical channels. PUCCH is not associated with any transport channels, it is used to transfer uplink control information (UCI), which includes Scheduling requests (SR), HARQ acknowledgements and channel state information (CSI). This CSI can include: Rank Indicator(RI), layer Indicator(LI), Precoding matrix indicator (PMI), channel quality indicator (CQI), CSI reference signal indicator (CRI), SS/PBCH block resource indicator (SSBRI) and layer 1 RSRP.
Physical signals are used for physical layer measurements. These measurements include:
- Uplink channel estimation
- Downlink channel estimation
- Uplink beam management
- Selection of uplink precoding weights
- Phase error estimation
- Channel aware scheduling
- Link adaptation
There are 3 types of physical signals:
- DMRS: Demodulated reference signals. These reference signals are sequences which are known to base station, base station compares the with the known reference to estimate the impact of propagation channel. UE can then apply an inverse of the propagation channel during demodulation. These signals are used for PUCCH and PUSCH.
- SRS: Sounding reference signals used by base station for link adaptation and channel aware packet scheduling. When there is channel reciprocity, SRS is also used for downlink channel estimation, else SRS is used for Uplink beam management. SRS is also used to support selection of uplink precoding weights for both codebook based and non-codebook based transmissions.
- PTRS: Phase tracking reference signals used to compensate for phase noise generated by local oscillators at both transmitters and receivers. Phase noise is more significant for higher operating bands. PTRS is also used for channel estimation. PTRS uses less resource elements , so less accurate compared to DMRS, but creates a smaller overhead towards PUSCH.
