Physical Channel Types of LTE
The physical layer uses resource blocks to transmit binary coded information in QPKS, 16QAM or 64QAM or OOK modulation form. Physical channels determine how data is processed and then mapped via dynamical scheduling onto resource blocks. Thus physical channels also in EUTRAN represent the available physical resources. Like transport channels, also physical channels are unidirectional. There is usually a fixed linkage between transport channel types and physical channel type used to transmit the transport blocks. Next to the physical channels that are used to carry transport channels, there are also physical channels that carry physical layer control data and physical signals
that are mainly used for synchronization and measurement purposes.
On the downlink side we have the following:
PBCH (Physical Broadcast Channel): The PBCH is the physical channel used to carry BCH (BCCH), in other words on this channel the system information can be found. It will use a fixed mapping onto resource blocks. There will be one PBCH per cell.
PDSCH (Physical Downlink Shared Channel): The PDSCH can carry DL-SCH or PCH. Thus this channel type will allocate most of the capacity in a cell. It is still under investigation how many PDSCHs a cell can have and how many PCH and DL-SCH a single PDSCH can carry.
PMCH (Physical Multicast Channel): This channel type is used to carry MCH. It implements the option of MBSFN where a UE receives the PMCH from several cells operating in the same area on the same frequency band. All the PMCH will be jointly decoded by the UE. This would allow a UE to do cell re-selection without interrupting MBMS services.
PDCCH (Physical Downlink Control Channel): This is a pure physical layer control channel. It contains the scheduler signaling to inform the UEs about the coming downlink (and maybe also uplink) resource block assignments to PDSCH. The PDCCH will be sent in each subframe shortly before PDSCH starts.
PCFICH (Physical Control Format Indicator Channel): The PCFICH is like the PDCCH a pure physical layer control channel. It indicates how many OFDM symbols are used to encode the PDCCH. So the order of decoding for a UE is to read first the PCFICH to get the PDCCH format. Then the PDCCH is decoded to find out which resource blocks to the PDSCH of interest for the UE.
DL Synchronization Signal: There are two DL synchronization signals – a
primary and a secondary one. Both consist of predefined code sequence to be
used for cell detection and initial time and frequency synchronization. Both
synchronization sequences are transmitted within slot 0 and slot 10 of each radio frame (frame type 1). The synchronization signals always use 2 subcarrier centered around the DC subcarrier. For the primary synchronization signal there will be three distinct code sequences defined, which one is applicable for a cell is determined by the cell identity. This can be used as some form of cell color code to distinguish direct neighbors.
DL Reference Signal: Reference signals allow coherent decoding and permanently repeated channel estimation. Therefore in each slot (and each
resource block) some OFDM symbols are reserved for reference signals and
cannot be used for data transmission. The reference signals itself are created from an orthogonal and a pseudo-noise sequence. This allows efficient channel and phase detection at the receiver side.
In the uplink there some less physical channels defined:
PUSCH (Physical Uplink Shared Channel): The PUSCH is the major uplink channel, because on it we will find the transport blocks of UL-SCH on which all radio bearers uplink occur. Like PDSCH also the PUSCH is dynamically assigned to uplink resource blocks. This happens via so called UL Assignments which are not completely specified yet. The PUSCH supports DTX, HARQ and at least QPSK and 16QAM. 64QAM is under investigation for this channel.
PUCCH (Physical Uplink Control Channel): This is a pure physical layer control channel. One idea is to use this channel for UL capacity requests and HARQ ACK/NACK indications by the UE as well as CQI (Carrier Quality Indication) feedback information to optimize CDS and MIMO.
PRACH (Physical Random Access Channel): The PRACH carries the RACH. Currently this means, that the PRACH simply transmits preamble sequences that act as initial access request for UL and DL resources when the UE is RRC_IDLE.
Demodulation Reference Signal: Some OFDM symbols of the resource block assigned to PUCCH/PUSCH are reserved for the demodulation reference signal. Like in downlink also here the reference signal is a predefined code sequence that allows the eNB coherent decoding of the transmitted SC-FDMA signal. The eNB derives from it phase and frequency correction, channel estimation and time synchronization.
Sounding Reference Signal: The sounding reference signal is a very long
reference sequence that is transmitted by the UE only on explicit request by the network. It is used for a very detailed channel estimation especially when MIMO is in use. This information is required for efficient decoding, but also for efficient channel dependent scheduling (CDS).