WCDMA Radio Interface

The radio interface is comprised of three protocol layers:

• Physical Layer (Layer 1)
• Data Link Layer (Layer 2), which is divided into the following sub-layer:
• Medium Access Control (MAC)
• Radio Link Control (RLC)
• Package Data Convergence Protocol (PDCP)
• Broadcast/Multicast Control (BMC)
• Network Layer (Layer 3), which is divided into three sub-layer
• Radio Recourse Control (RRC)
• Mobility Management (MM)
• Call Control (CC)

In addition, layer 3 is divided into Control (C) Plane and Use (U) Plane. The PDCP and BMC exist in the User Plane only.

Physical Layer offers data transport service to MAC and higher Layer. The Physical Layer transport service describe how and with what characteristic data is transfer over the radio interface.
FDD Mode of operation Physical Channel defined by:

• Carrier Frequency
• Spreading Code
• Carrier Phase

TDD Mode of operation Physical Channel defined by:

• Carrier Frequency
• Spreading Code
• Time Slot

Transport Channels, are always uni-directional, and are either common (i.e share among other users) or dedicated (i.e allocated a specific user).

The common transported channels are:

• Broad cast channel (BCH)
• Forward Access Channel (FACH)
• Paging Channel (PCH)
• Random Access Channel (RACH)
• Common Packet Channel (CPCH)
• Downlink Share Channel (DSCH)
• Uplink Share Channel (USCH)

The dedicated transport channel is the Dedicated Channel (DCH) .

1. BCH is a downlink common transport channel, and it uses to broadcast system and cell specific control information. BCH transmit over the entire coverage area of a cell. BCH is used by mobile station for system synchronization.
2. FACH is a downlink common transport channel, it is used to carry control information to mobile station when the system know in which cell the mobile station is located. FACH may also carry short user  data packet in order to avoid the signaling overhead (that is, additional unnecessary signaling demand) associated with allocating dedicated traffic channel. FACH may transmit over entire cell or a part of a cell by using adaptive antenna array.
3. PCH is a downlink common transport channel, uses to carry control information to mobile station when the system have not determined in which cell the mobile station is located. PCH transmits over the entire cell, so all mobile station located within the cell maybe alerted to incoming calls.
4. RACH is an uplink common control channel, is used to carry control information from the mobile station. The RACH is always received in entire cell. So that the base station can response to service request from all mobiles station in its cell.
5. CPCH is a contention-base uplink common transport channel, is used for transmission of bursty data traffic. This channel only exist in FDD mode, is shared by all mobiles station in a cell, and is fast power  control.
6. DSCH is a downlink common transport channel shared by several mobile station in a cell. DSCH is associated with one or several downlink DCHs. It is transmitted over all or a part of the cell, using bean forming adaptive antenna.
7. Dedicated Channel:

• The DCH is a downlink or uplink dedicated transported channel, is use to carry user data or control information between the network the mobile station. The DCH may transmit over all or a part of the cell by mean of adaptive antenna arrays.

Indicators are fast low signaling entities that are transmitted without sending information block over transport channel. The indicators defined in the current version of the specification are:

• Acquisition Indicator (AI)
• Access Preamble Indicator (API)
• Channel Assignment Indicator (CAI)
• Collision Detection Indicator (CDI)
•  Page Indicator (PI)
• Status Indicator (SI)

Indicators maybe either two value (Boolean) or three value. Their mapping to indicator channel  is channel specific.

A Key Feature of WCDMA Air Interface is its ability of transport multiple parallel services with different quality requirement on one connection. This is an important feature of 3G system and is not found in 2G system.
Several DCHs can be coded and multiplexed together to produce coded composite transport channel (CCTrCH) is fed to a data multiplexing/splitting unit, where it is divided into the physical channel data streams.

Each transport channel has an associated transport format, which indicate its coding  and multiplexing configuration.
The transport format information is transmitted in transport format combination indicator (TFCI) which is time multiplex onto physical channel. This allow the transport of multiple service on different quality requirement on a single connection.