LTE CHANNEL STRUCTURE AND MAPPING

LTE CHANNEL STRUCTURE

(3GPP TS 36.211 and 36.212)
The physical layer provides transport channels to the L2. These transport channels differ in their characteristics how data is transmitted and are mapped to different logical channels provided by the MAC layer. Logical channels describe which type of data is conveyed.

LOGICAL CHANNELS

The logical channels can be divided into control channels and traffic channels. The control channels are used for transfer of control plane information and the traffic channels are used for the transfer of user plane information. The following logical channels are supported for LTE:

Control Channels

• Broadcast Control Channel (BCCH): A downlink channel for broadcasting system control information.
• Paging Control Channel (PCCH): A downlink channel that transfers paging information. This channel is used when the network does not know the location cell of the UE.
• Common Control Channel (CCCH): This channel is used by the UEs having no RRC connection with the network. CCCH would be used by the UEs when accessing a new cell or after cell reselection.
• Multicast Control Channel (MCCH): A point-to-multipoint downlink channel used for transmitting MBMS scheduling and control information from the network to the UE, for one or several MTCHs. After establishing an RRC connection this channel is only used by UEs that receive MBMS.
• Dedicated Control Channel (DCCH): A point-to-point bidirectional channel that transmits dedicated control information between a UE and the network. Used by UEs having an RRC connection

Traffic Channels

• Dedicated Traffic Channel (DTCH): A Dedicated Traffic Channel (DTCH) is a point-to-point channel, dedicated to one UE, for the transfer of user information. A DTCH can exist in both uplink and downlink.
• Multicast Traffic Channel (MTCH): A point-to-multipoint downlink channel for transmitting traffic data from the network to the UEs using MBMS.

TRANSPORT CHANNELS

An effort has been made to keep a low number of transport channels in order to avoid unnecessary switches between different channel types, which are found to be time consuming in UMTS. In fact there is currently only one transport channel in downlink and one in uplink carrying user data, i.e., channel switching is not
needed. For LTE, the following transport channels are provided by the physical layer:

Downlink:

• Broadcast Channel (BCH): A low fixed bit rate channel broadcast in the entire coverage area of the cell. Beam-forming is not applied.
• Downlink Shared Channel (DL-SCH): A channel with possibility to use HARQ and link adaptation by varying the modulation, coding and transmit power. The channel is possible to broadcast in the entire cell and beamforming may be applied. UE power saving (DRX) is supported to reduce the UE power consumption. MBMS transmission is also supported.
• Paging Channel (PCH): A channel that is broadcasted in the entire cell. DRX is supported to enable power saving.
• Multicast channel (MCH): A separate transport channel for multicast (MBMS). This channel is broadcast in the entire coverage area of the cell. Combining of MBMS transmissions from multiple cells (MBSFN) is supported.

Uplink:

• Uplink Shared channel (UL-SCH): A channel with possibility to use HARQ and link adaptation by varying the transmit power, modulation and coding. Beamforming may be applied.

• Random Access Channel (RACH): A channel used to obtain timing synchronization (asynchronous random access) and to transmit information needed to obtain scheduling grants (synchronous random access). The transmission is typically contention based. For UEs having an RRC connection there is some limited support for contention free access.

LTE CHANNEL STRUCTURE AND MAPPING

PHYSICAL CHANNELS

The physical layer offers services to the MAC layer in the form of transport channels. User data to be  transmitted is delivered to the physical layer from the MAC layer in the form of transport blocks. 

The MAC  layer at the transmitter side also provides the physical layer with control information necessary for  transmission and/or reception of the user data. The physical layer defines physical channels and physical signals. A physical channel corresponds to a set of physical resources used for transmission of data and/or control information from the MAC layer. A physical signal, which also corresponds to a set of physical

resources, is used to support physical-layer functionality but do not carry any information from the MAC layer.

Physical channels

•  Physical Downlink Shared Channel (PDSCH) – transmission of the DL-SCH transport channel

•  Physical Uplink Shared Channel (PUSCH) – transmission of the UL-SCH transport channel

•  Physical Control Format Indicator Channel (PCFICH) – indicates the PDCCH format in DL

•  Physical Downlink Control Channel (PDCCH) – DL L1/L2 control signaling

•  Physical Uplink Control Channel (PUCCH) – UL L1/L2 control signaling

•  Physical Hybrid ARQ Indicator Channel (PHICH) – DL HARQ info

•  Physical Broadcast Channel (PBCH) – DL transmission of the BCH transport channel.

•  Physical Multicast Channel (PMCH) – DL transmission of the MCH transport channel.

•  Physical Random Access Channel (PRACH) – UL transmission of the random access preamble as given by the RACH transport channel.

Physical signals

•  Reference Signals (RS) – support measurements and coherent demodulation in uplink and downlink.

•  Primary and Secondary Synchronization signals (P-SCH and S-SCH) – DL only and used in the cell search procedure.

•  Sounding Reference Signal (SRS) – supports UL scheduling measurements