5.3 Transport Channel

5.3.1  Types of Transport Channels

A transport channel is used at the physical layer to provide services for the upper layer. It defines the mode and features of data transmission on air interfaces.

It is divided into two types: Dedicated channel and common channel. The main difference between them is: Resources in the common channel are shared by all users or a group of users within a neighborhood, while resources in the dedicated channel are defined by the dedicated code on specific frequency, and they are used only for individual users.

5.3.2  Dedicated Transport Channel

Only one kind of dedicated transport channel exists, that is, Dedicated Channel (DCH). The Dedicated Channel (DCH) is an uplink or downlink channel. In the whole or part of the neighborhood, DCH uses beam-forming antenna to transmit.

5.3.3  Common Transport Channel

Totally, there are the following six transport channels: BCH, FACH, PCH, RACH, CPCH and DSCH.

l   BCH (Broadcast Channel): It is a downlink transport channel, which is used to broadcast the dedicated information in the system or in a cell. The BCH always transmits in the whole cell, and it has a separate transport format.

l   FACH (Forward Access Channel): it is a downlink transport channel. In the whole or part of the cell, FACH uses beam-forming antenna to transmit.

l   PCH (Paging Channel): It is a downlink transport channel, which always transmits in the whole cell. The transmission of the PCH associates with that of page indication generated by the physical layer so as to support the effective dormant mode program.

l   RACH (Random Access Channel): it is an uplink transport channel. It always receives information in the whole cell. It features collision and adventure, using open loop power control.

l   CPCH (Common Packet Channel): it is an uplink transport channel, and it associates with the dedicated channel of a downlink. This dedicated channel is used to provide CPCH power control of the uplink and CPCH control commands (such as emergency).It features initial collision and adventure, using open loop power control.

l   DSCH (Downlink Shared Channel): it is a downlink transport channel shared by some UEs, associating with one or more DCHs. In the whole or part of the cell, DSCH uses beam-forming antenna to transmit.

5.3.4  Indicator

The WCDMA protocol defines a series of indicators for the transport channel. In fact, an indicator is a fast lower-layer signaling entity. It does not occupy any entity block but is implemented directly in the physical layer by the physical channel.

There are the following related indicators: Acquisition Indication (AI), Access Preamble Indication (API), Channel Assignment Indication (CAI), Collision Detection Indication (CDI), Paging Indication (PI) and Status Indication (SI). Indicators can be either binary or ternary. Their mapping to the indicator channel is determined by the physical channel. The physical channel used to transmit indicators is called Indicator Channel (ICH).

5.3.5  Mapping from the Logical Channel to the Transport Channel

The transport channel serves the logical channel.