LTE defines two types of channels, logical channels and Physical channels. Logical channels are defined by the type of information that they carry and that are mapped to transport channels and from there to physical channels. Physical channels are defined by their physical properties, i.e., time, subcarrier etc.

1) Logical Channels:

The logical channels are similar to those in WCDMA, they are of two types, Traffic channels, Control Channels. Their functions are described as follows:

• Traffic channels

  a) Dedicated Traffic CHannel (DTCH): It is a dedicated traffic channel to one user that carries the user data for all ULs, as well as for those downlink data that are not multicast/broadcast.

  b) Multicast Traffic CHannel (MTCH): It is a point to multipoint traffic channel that carries the user data for multicast/broadcast downlink transmission.

• Control channels

  a) Broadcast Control CHannel (BCCH): It is a downlink channel that carries system information data that are broadcasted to the MSs in a cell. Note the difference from the MTCH, which also broadcasts to MSs, but carries user data.

  b) Paging Control CHannel (PCCH): It is a downlink channel that transfers paging information and system information change notifications to MSs in multiple cells. Specifically it is used when it is not known exactly in which cell the MS currently is located.

  c) Common Control CHannel (CCCH): It transmits control data for the Random Access (RA), i.e., when a connection is started.

  d) Dedicated Control CHannel (DCCH): It is used for the transmission of control information that relates to a specific MS (as opposed to the system information relevant for all MSs, which is broadcast in the BCCH).

  e) Multicast Control CHannel (MCCH): It carries the control information related to multicast/broadcast services.

2) Transport Channels:

The Logical channels are mapped onto the transport channels as illustrated in Figure 8: They are as follows:

a)Broadcast CHannel (BCH): It transmits a part of the BCCH (the remainder is on the DL- SCH described below)in the entire coverage area of the cell. It has a fixed predefined transport format, so that any MS can listen to it easily.

b) Paging CHannel (PCH): It transmits the PCCH in the entire coverage area of the cell, It supports discontinuous reception for power saving of MS/UE. It is mapped to physical resources which can also be used for traffic /other control channels.

c) Multicast Channel (MCH): It is used to support broadcast/multicast transmission in the entire coverage area of the cell. It has a semi static scheduling and transport format.

• d) Down Link Shared Channel (DL- SCH) and Up Link Shared Channel (UL- SCH): They carry the user data, as well as most of the control information (except the one already mentioned above).Theses channels also support Hybrid ARQ, dynamic link adaptation by varying the modulation, coding and transmit power. They also support dynamic and semi-static resource allocation and discontinuous reception to enable power saving of UE/MS. Optionally they can be used for beam forming and broadcasting in the entire coverage area. The data on transport channels are organized into transport blocks; in each transmission time interval (usually a sub frame = 2 time slots), one transport block is transmitted. A transport format is associated with each transport block. Finally, these transport channels are mapped onto physical channels; there are also physical channels that do not carry any transport channel, but are purely used for PHY functionality.

3) Physical Channels:

Transport channels are mapped into physical channels to carry user and control data. In this section the function of both downlink as well as uplink physical channels are presented.

• Downlink Physical Channels

  a) Physical Broadcast CHannel (PBCH ): It carries the BCH. The coded BCH transport blocks are mapped to four sub frames within a 40 ms interval. 40 ms timing is blindly detected i.e. there is no explicit signaling for indicating 40 ms timing. Each subframe is assumed to be self decodable i.e. BCH can be decoded in a single reception assuming good channel conditions. It supports QPSK modulation.

  b) Physical Downlink Shared CHannel (PDSCH ): It carries the DL-SCH, i.e., user data, some control data for the downlink, as well as the PCH. It supports QPSK,16 QAM and 64 QAM modulation formats.

  c) Physical Multicast CHannel (PMCH): it carries the MCH, which contains the multicast payload, as well as some of the control information for multicast. It supports QPSK, 16 QAM and 64 QAM modulation formats.

  d) Physical Downlink Control CHannel (PDCCH): It carries control information, such as scheduling that is required for reception of the PDSCH. This channel does not carry any transport channel.

  e) Physical Control Format Indicator CHannel (PCFICH ): It carries control information about the PDCCH. This channel does not carry any transport channel.

  f) Physical HARQ Indicator CHannel (PHICH ): It carries the feedback bits indicating whether a retransmission of transport blocks is necessary. This channel does not carry any transport channel.

  g) Synchronization Signal (SS ): The SS carries information about the timing of the cell, as well as the cell ID. LTE actually provides two SSs, the Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS). In contrast to other systems, these signals are not called “channels,” but perform similar functions as, e.g., the synchronization channels in WCDMA. To understand the functionality of the SS, keep in mind that there are 504 cell IDs defined for LTE, which are divided into 168 ID groups. The PSS is transmitted in the last symbol of the first slot of subframes 0 and 5 of every frame, extending over 72 subcarriers. The SSS signal is transmitted in the symbol directly before every PSS signal. The SSS carries information about the cell ID group: the signal also extends over 72 subcarriers.

• Uplink Physical Channels

  a) Physical Uplink Shared Channel (PUSCH): It is the uplink counterpart to the PDSCH.

  b) Physical Uplink Control Channel (PUCCH): It carries mainly three types of information: (i) channel state feedback; (ii) resource requests (remember that the BS performs the scheduling, i.e., assigns all the resources also for the up link; thus the MS must request resources when it has data to transmit), and (iii) HARQ feedback bits.

  c) Physical Random Access CHannel (PRACH): it is used for the random access, i.e., MS communicating to the BS before a connection with scheduling has been established.

Complete mapping of Logical to Transport to Physical channels for both downlink and uplink is illustrated in Figure 9.