Explain LTE downlink (DL) channels.
Downlink logical channels:
PCCH, BCCH, CCCH, DCCH and MCCH is called Control channels – These LTE control channels is used to carry the different control plane information.
DTCH and MTCH is called Traffic Channels – These traffic channels is used to carry the user plane data.
- Paging Control Channel (PCCH): PCCH transfers paging information and system information change notification. This control channel is used for paging information when location cell of UE is not known to the network.
- Broadcast Control Channel (BCCH): This control channel is used to provides system information to all mobile terminals connected to the eNodeB.
- Common Control Channel (CCCH): This control channel is used for random access information, e.g. for actions including setting up a connection.
- 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.
- Multicast Control Channel (MCCH): This control channel is used multicast data Information.
- 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.
Downlink transport channels:
- 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.
Downlink physical channels:
- 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): This channels is used to 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 sub frame 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.