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The LTE Channel Architecture defines E-RAB channels, Radio Bearer (RB) channels, Signaling Radio Bearer (SRB) channels, Logical Channels, Transport Channels, and Physical Channels. In general, each category behaves as a service access point between adjacent protocol layers.

E-RAB - An E-RAB channel carries one or more service data flows between a UE and the EPC.

Radio Bearer - A Radio Bearer channel transports the data packets of an E-RAB from the eNodeB toward the UE. Each E-RAB has a one-to-one mapping with a radio bearer.

Signaling Radio Bearer - A Signaling Radio Bearer (SRB) channel transports signaling packets between the RRC Sublayer and the PDCP Sublayer.

Logical Channel - A Logical Channel transports control or data traffic between the RLC Sublayer and the MAC Sublayer. Logical control channels are mapped to signaling radio bearer channels, while logical traffic channels are mapped to radio bearer channels. Logical Channels describe transmission reliability (RLC Acknowledged Mode, etc.).

Transport Channel - A Transport Channel forwards control or data traffic between the MAC Sublayer and the Physical Layer. Each Logical Channel is mapped to a transport channel. Transport Channels describe how the information will be formatted before being transmitted (coding, transport block size, etc.).

Physical Channel - A Physical Channel provides the transmission media (resource elements) through which the information is actually transmitted. Each Transport Channel is mapped to a physical channel.

3GPP TS 36.211 Physical Channel and Modulation

3GPP TS 36.321 Medium Access Control (MAC) Protocol Specification

3GPP TS 36.322 Radio Link Control (RLC) Protocol Specification

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Logical Channels provide control and data transport between the RLC and MAC Sublayers. Signaling traffic is carried by control channels (xCCH), and data traffic is carried by traffic channels (xTCH). Control channels are mapped to SRB channels, and traffic channels are mapped to user plane radio bearer channels.

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Logical channels are in a one-to-one fashion associated with radio bearers. Logical channel types are used to distinguish the type of information transmitted within the attached radio bearer. The two major groups of logical channel types are therefore control channels for signaling and traffic channels for IP user data.

Currently the following logical channel types are defined for EUTRAN signaling:

BCCH (Broadcast Control Channel): The BCCH is used to transmit system information regarding access and non-access stratum. It allows the UE to retrieve cell and network configuration parameters (e.g. PLMN code, cell identity, cell re-selection parameters, etc.) required for normal operation within EUTRAN.

PCCH (Paging Control Channel): The PCCH is used to transmit the paging messages from RRC. Hence it is a downlink point-to-multipoint channel a UE is using when it is in LTE_IDLE mode.

CCCH (Common Control Channel): The CCCH is an uplink and downlink channel. RRC signaling channel used by UEs to do the initial access signaling when it is in RRC_IDLE state and wants to enter RRC_CONNECTED state. The UE will send only one message (RRC CONNECTION REQUEST) and the rest of the communication takes place on DCCH.

DCCH (Dedicated Control Channel): The DCCH is a bidirectional RRC signaling channel used for point-to-point (dedicated) RRC and NAS signaling procedures. It is the main signaling channel to be used by RRC_CONNECTED UEs.

MCCH (Multicast Control Channel): The MCCH is associated with MBMS. It allows the eNB to inform UEs that want to listen to broadcast or multicast service traffic about availability of such services and about the associated MBMS radio bearer (point-to-multipoint) radio bearers.

On the traffic channel side we have currently only two types defined:

DTCH (Dedicated Traffic Channel): The DTCH is used for user radio bearers carrying IP traffic. The eNB connects DTCHs with their associated S1-U tunnel to the SAE GW. DTCH can be bidirectional, uplink only or downlink only. DTCH are of course point-to-point.

MTCH (Multicast Traffic Channel): The MTCH is a point-to-multipoint traffic channel for MBMS. It carries IP traffic for broadcast or multicast services driven by the MBMS feature.

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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 we have 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 DL-SCH 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.

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The graphic shows the mapping between the LTE Logical Channels and Transport Channels.

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The physical layer uses resource blocks (e.g. 12 subcarriers with 6 or 7 OFDM symbols) 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.

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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).

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The graphic shows the mapping between LTE Transport Channels and Physical Channels. The PDCCH, PCFICH, PHICH, and PUCCH Physical Channels are not mapped to Transport Channels.

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