UMTS/WCDMA Logical, Transport and Physical Channels

WCDMA is the main air interface standard for the 3G UMTS mobile network. The mobile station and base station communicate by means of several physical channels that are transmitted on a given frequency assignment. The "Downlink" refers to a radio link for the transmission of signals from the base station to a UE (mobile station) while the "Uplink" refers to a radio link for the transmission of signals from a UE (mobile station) to the base station.

There are three types of channels in the WCDMA technologies: Physical Channel, Transport Channel and Logical channel. The Transport Channels are interface between MAC and Layer 1, while Logical Channels are interface between MAC and RLC. The logical and transport channels define WHAT data are transported, while the physical channels define HOW and with what physical characteristic the data are transport.

Transport channels can be further subdivided into Common Transport Channels; and dedicated transport channels. Common transport channel types are:

• Random Access Channel (RACH): A contention based uplink channel used for transmission of relatively small amounts of data, e.g. for initial access or non-real-time dedicated control or traffic data.
• Common Packet Channel (CPCH): A contention based channel used for transmission of bursty data traffic. This channel only exists in FDD mode and only in the uplink direction. The common packet channel is shared by the UEs in a cell and therefore, it is a common resource. The CPCH is fast power controlled.
• Forward Access Channel (FACH): Common downlink channel without closed-loop power control used for transmission of relatively small amount of data.
• Downlink Shared Channel (DSCH): A downlink channel shared by several UEs carrying dedicated control or traffic data.
• Uplink Shared Channel (USCH): An uplink channel shared by several UEs carrying dedicated control or traffic data, used in TDD mode only.
• Broadcast Channel (BCH): A downlink channel used for broadcast of system information into an entire cell.
• Paging Channel (PCH): A downlink channel used for broadcast of control information into an entire cell allowing efficient UE sleep mode procedures. Currently identified information types are paging and notification. Another use could be UTRAN notification of change of BCCH information.
• High Speed Downlink Shared Channel (HS-DSCH): A downlink channel shared between UEs by allocation of individual codes, from a common pool of codes assigned for the channel.

Dedicated transport channel types are:

• Dedicated Channel (DCH): A channel dedicated to one UE used in uplink or downlink.
• A general classification of logical channels is into two groups; Control Channels (for the transfer of control plane information) and Traffic Channels (for the transfer of user plane information).

Control Channels:

• Broadcast Control Channel (BCCH): A downlink channel for broadcasting system control information.
• Paging Control Channel (PCCH): A downlink channel that transfers paging information. This channel is used when the network does not know the location cell of the UE, or, the UE is in the cell connected state (utilising UE sleep mode procedures).
• Common Control Channel (CCCH): Bi-directional channel for transmitting control information between network and UEs. This channel is commonly used by the UEs having no RRC connection with the network and by the UEs using common transport channels when accessing a new cell after cell reselection.
• Dedicated Control Channel (DCCH): A point-to-point bi-directional channel that transmits dedicated control information between a UE and the network. This channel is established through RRC connection setup procedure.
• Shared Channel Control Channel (SHCCH): Bi-directional channel that transmits control information for uplink and downlink shared channels between network and UEs. This channel is for TDD only.

Traffic Channels:

• Dedicated Traffic Channel (DTCH): A Dedicated Traffic Channel (DTCH) is a point-to-point channel, dedicated to one UE, for the transfer of user information. A DTCH can exist in both uplink and downlink.
• Common Traffic Channel (CTCH): A point-to-multipoint unidirectional channel for transfer of dedicated user information for all or a group of specified UEs.