Transport Channel Types of LTE
Transport channels are used as the basic transmission service offered by layer 1 to MAC. MAC will use transport channels to multiplex and demultiplex logical channels onto and from them. In contrast to logical channel types, referring to the type of information transmitted, transport channel types are used to indicate the transport characteristics. This means that certain transport channel type is associated with certain bit rates (transport block sizes, number of blocks), a transmission time interval (the time it takes to send one transport block set), delay, support for HARQ, support for beam-forming, support for DRX/DTX, and so on.
Transport channels are always unidirectional, in the downlink List as the following:
BCH (Broadcast Channel): The BCH is a transport channel with fixed transport format. It is used to transmit the BCCH in the entire cell. It will only support QPSK modulation and no HARQ or beam-forming is allowed.
PCH (Paging Channel): The PCH is used to carry the PCCH. In contrast to BCH there might be beam-forming applied to PCH, but still no HARQ is available. Also the channel supports at least QPSK and 16QAM as modulation scheme and DRX.
DL-SCH (DL Shared Channel): This is the major transport channel in the
downlink direction. It is used to carry mainly DCCH and DTCH. But also BCCH,
MCCH and if required MTCH can be sent on it. The channel supports HARQ,
beam-forming and all modulation schemes QPSK, 16QAM and 64QAM. The DLSCH will support DRX and DTX on UE side to reduce power consumption of end terminals.
MCH (Multicast Channel): The MCH is used for broadcast and multicast MBMS services. It thus carries MTCH and MCCH. It will allow at least QPSK and 16QAM as modulation scheme, 64QAM is under investigation. Obviously HARQ cannot be supported as MTCH/MCCH are point-to-multipoint channels. This channel has a unique special property, as UEs are able to combine MCH signals from different cells using the same frequency (MBSFN= Multicast Broadcast Single Frequency Networks). In this case all MBSFN cells must use the same MCH configuration and must be synchronized with each other.
In the uplink there are only two transport channel defined:
RACH (Random Access Channel): The RACH is used as initial access request by the UE to the network. Currently it does not contain logical information, rather the RACH is formed by a special layer 1 preamble that acts as a channel request message. It is under investigation whether the RACH should be able to carry logical channel information.
UL-SCH (UL Shared Channel): The UL-SCH is the only uplink transport channel able to carry logical channel data. Thus CCCH, DTCH and DCCH run over this channel. It supports HARQ and at least QPKS and 16QAM.