LTE Communication Channels
The information flows between the different protocols are known as channels and signals. LTE uses several different types of logical, transport and physical channel, which are distinguished by the kind of information they carry and by the way in which the information is processed.
Logical Channels : Define whattype of information is transmitted over the air, e.g. traffic channels, control channels, system broadcast, etc. Data and signalling messages are carried on logical channels between the RLC and MAC protocols.
Transport Channels : Define howis something transmitted over the air, e.g. what are encoding, interleaving options used to transmit data. Data and signalling messages are carried on transport channels between the MAC and the physical layer.
Physical Channels : Define whereis something transmitted over the air, e.g. first N symbols in the DL frame. Data and signalling messages are carried on physical channels between the different levels of the physical layer.
Logical channels define what type of data is transferred. These channels define the data-transfer services offered by the MAC layer. Data and signalling messages are carried on logical channels between the RLC and MAC protocols.
Logical channels can be divided into control channels and traffic channels. Control Channel can be either common channel or dedicated channel. A common channel means common to all users in a cell (Point to multipoint) while dedicated channels means channels can be used only by one user (Point to Point).
Logical channels are distinguished by the information they carry and can be classified in two ways. Firstly, logical traffic channels carry data in the user plane, while logical control channels carry signalling messages in the control plane. Following table lists the logical channels that are used by LTE:
|Channel Name||Acronym||Control channel||Traffic channel|
|Broadcast Control Channel||BCCH||X|
|Paging Control Channel||PCCH||X|
|Common Control Channel||CCCH||X|
|Dedicated Control Channel||DCCH||X|
|Multicast Control Channel||MCCH||X|
|Dedicated Traffic Channel||DTCH||X|
|Multicast Traffic Channel||MTCH||X|
Transport channels define how and with what type of characteristics the data is transferred by the physical layer. Data and signalling messages are carried on transport channels between the MAC and the physical layer.
Transport Channels are distinguished by the ways in which the transport channel processor manipulates them. Following table lists the transport channels that are used by LTE:
|Downlink Shared Channel||DL-SCH||X|
|Uplink Shared Channel||UL-SCH||X|
|Random Access Channel||RACH||X|
Data and signalling messages are carried on physical channels between the different levels of the physical layer and accordingly they are divided into two parts:
Physical Data Channels
Physical Control Channels
Physical data channels
Physical data channels are distinguished by the ways in which the physical channel processor manipulates them, and by the ways in which they are mapped onto the symbols and sub-carriers used by Orthogonal frequency-division multiplexing (OFDMA). Following table lists the physical data channels that are used by LTE:
|Physical downlink shared channel||PDSCH||X|
|Physical broadcast channel||PBCH||X|
|Physical multicast channel||PMCH||X|
|Physical uplink shared channel||PUSCH||X|
|Physical random access channel||PRACH||X|
The transport channel processor composes several types of control information, to support the low-level operation of the physical layer. These are listed in the below table:
|Downlink control information||DCI||X|
|Control format indicator||CFI||X|
|Hybrid ARQ indicator||HI||X|
|Uplink control information||UCI||X|
Physical Control Channels
The transport channel processor also creates control information that supports the low-level operation of the physical layer and sends this information to the physical channel processor in the form of physical control channels.
The information travels as far as the transport channel processor in the receiver, but is completely invisible to higher layers. Similarly, the physical channel processor creates physical signals, which support the lowest-level aspects of the system.
Physical Control Channels are listed in the below table:
|Physical control format indicator channel||PCFICH||X|
|Physical hybrid ARQ indicator channel||PHICH||X|
|Physical downlink control channel||PDCCH||X|
|Relay physical downlink control channel||R-PDCCH||X|
|Physical uplink control channel||PUCCH||X|
The base station also transmits two other physical signals, which help the mobile acquire the base station after it first switches on. These are known as the primary synchronization signal (PSS) and the secondary synchronization signal (SSS).