LTE Physical, Logical and Transport Channels

- overview, information, tutorial about the physical, logical, control and transport channels used within 3GPP, 3G LTE and the LTE channel mapping.

In order that data can be transported across the LTE radio interface, various "channels" are used. These are used to segregate the different types of data and allow them to be transported across the radio access network in an orderly fashion.

Effectively the different channels provide interfaces to the higher layers within the LTE protocol structure and enable an orderly and defined segregation of the data.

3G LTE channel types

There are three categories into which the various data channels may be grouped.

  • Physical channels:   These are transmission channels that carry user data and control messages.
  • Transport channels:   The physical layer transport channels offer information transfer to Medium Access Control (MAC) and higher layers.
  • Logical channels:   Provide services for the Medium Access Control (MAC) layer within the LTE protocol structure.

3G LTE physical channels

The LTE physical channels vary between the uplink and the downlink as each has different requirements and operates in a different manner.

  • Downlink:
    • Physical Broadcast Channel (PBCH):   This physical channel carries system information for UEs requiring to access the network. It only carries what is termed Master Information Block, MIB, messages. The modulation scheme is always QPSK and the information bits are coded and rate matched - the bits are then scrambled using a scrambling sequence specific to the cell to prevent confusion with data from other cells.

      The MIB message on the PBCH is mapped onto the central 72 subcarriers or six central resource blocks regardless of the overall system bandwidth. A PBCH message is repeated every 40 ms, i.e. one TTI of PBCH includes four radio frames.

      The PBCH transmissions has 14 information bits, 10 spare bits, and 16 CRC bits.
    • Physical Control Format Indicator Channel (PCFICH) :   As the name implies the PCFICH informs the UE about the format of the signal being received. It indicates the number of OFDM symbols used for the PDCCHs, whether 1, 2, or 3. The information within the PCFICH is essential because the UE does not have prior information about the size of the control region.

      A PCFICH is transmitted on the first symbol of every sub-frame and carries a Control Format Indicator, CFI, field. The CFI contains a 32 bit code word that represents 1, 2, or 3. CFI 4 is reserved for possible future use.

      The PCFICH uses 32,2 block coding which results in a 1/16 coding rate, and it always uses QPSK modulation to ensure robust reception.
    • Physical Downlink Control Channel (PDCCH) :   The main purpose of this physical channel is to carry mainly scheduling information of different types:

      • Downlink resource scheduling
      • Uplink power control instructions
      • Uplink resource grant
      • Indication for paging or system information
      The PDCCH contains a message known as the Downlink Control Information, DCI which carries the control information for a particular UE or group of UEs. The DCI format has several different types which are defined with different sizes. The different format types include: Type 0, 1, 1A, 1B, 1C, 1D, 2, 2A, 2B, 2C, 3, 3A, and 4.
    • Physical Hybrid ARQ Indicator Channel (PHICH) :   As the name implies, this channel is used to report the Hybrid ARQ status. It carries the HARQ ACK/NACK signal indicating whether a transport block has been correctly received. The HARQ indicator is 1 bit long - "0" indicates ACK, and "1" indicates NACK.

      The PHICH is transmitted within the control region of the subframe and is typically only transmitted within the first symbol. If the radio link is poor, then the PHICH is extended to a number symbols for robustness.
  • Uplink:
    • Physical Uplink Control Channel (PUCCH) :   The Physical Uplink Control Channel, PUCCH provides the various control signalling requirements. There are a number of different PUCCH formats defined to enable the channel to carry the required information in the most efficient format for the particular scenario encountered. It includes the ability to carry SRs, Scheduling Requests.

      The basic formats are summarised below:

      PUCCH Format Uplink Control Information Modulation Scheme Bits per Sub-frame Notes
      Format 1 SR
      Format 1a 1 bit HARQ ACK/NACK with or without SR
      Format 1b 2 bit HARQ ACK/NACK with or without SR
      Format 2 CQI/PMI or RI
      Format 2a CQI/PMI or RI and 1 bit HARQ ACK/NACK
      QPSK + BPSK
      Format 2b CQI/PMI or RI and 2 bit HARQ ACK/NACK
      QPSK + BPSK
      Format 3
      Provides support for carrier aggregation.
    • Physical Uplink Shared Channel (PUSCH) :   This physical channel found on the LTE uplink is the Uplink counterpart of PDSCH
    • Physical Random Access Channel (PRACH) :   This uplink physical channel is used for random access functions. This is the only non-synchronised transmission that the UE can make within LTE. The downlink and uplink propagation delays are unknown when PRACH is used and therefore it cannot be synchronised.

      The PRACH instance is made up from two sequences: a cyclic prefix and a guard period. The preamble sequence may be repeated to enable the eNodeB to decode the preamble when link conditions are poor.

LTE transport channels

The LTE transport channels vary between the uplink and the downlink as each has different requirements and operates in a different manner. Physical layer transport channels offer information transfer to medium access control (MAC) and higher layers.

  • Downlink:
    • Broadcast Channel (BCH) :   The LTE transport channel maps to Broadcast Control Channel (BCCH)
    • Downlink Shared Channel (DL-SCH) :   This transport channel is the main channel for downlink data transfer. It is used by many logical channels.
    • Paging Channel (PCH) :   To convey the PCCH
    • Multicast Channel (MCH) :   This transport channel is used to transmit MCCH information to set up multicast transmissions.

  • Uplink:
    • Uplink Shared Channel (UL-SCH) :   This transport channel is the main channel for uplink data transfer. It is used by many logical channels.
    • Random Access Channel (RACH) :   This is used for random access requirements.

LTE logical channels

The logical channels cover the data carried over the radio interface. The Service Access Point, SAP between MAC sublayer and the RLC sublayer provides the logical channel.

  • Control channels: these LTE control channels carry the control plane information:
    • Broadcast Control Channel (BCCH) :   This control channel provides system information to all mobile terminals connected to the eNodeB.
    • Paging Control Channel (PCCH) :   This control channel is used for paging information when searching a unit on a network.
    • Common Control Channel (CCCH) :   This channel is used for random access information, e.g. for actions including setting up a connection.
    • Multicast Control Channel (MCCH) :   This control channel is used for Information needed for multicast reception.
    • Dedicated Control Channel (DCCH) :   This control channel is used for carrying user-specific control information, e.g. for controlling actions including power control, handover, etc..

  • Traffic channels:These LTE traffic channels carry the user-plane data:
    • Dedicated Traffic Channel (DTCH) :   This traffic channel is used for the transmission of user data.
    • Multicast Traffic Channel (MTCH) :   This channel is used for the transmission of multicast data.

It will be seen that many of the LTE channels bear similarities to those sued in previous generations of mobile telecommunications.

By Ian Poole

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