WCDMA Physical Layer Design (FDD)

This tutorial covers the FDD mode WCDMA physical layer design as per the UTRA specification documents 3GPP TS 25.212. It explains both the uplink and downlink transmission modules, and also provides a link for comparison with the TDD physical layer version.

The physical layer provides different transport channels to carry the MAC layer’s logical channels. Similarly, the MAC layer provides different logical channels to the RLC sublayer of Layer 2 of the WCDMA protocol stack.

The logical channel is identified by the type of information it carries. The transport channel is identified by how the information is transferred over the air interface. Physical channels are defined within the Physical layer itself, which is controlled via the RRC layer in WCDMA.

There are two duplexing modes: TDD and FDD. Based on the duplex method employed, there are two distinct physical layers. In TDD mode, a physical channel is characterized by a timeslot. However, in FDD mode, a physical channel in the downlink is characterized by code and frequency. In the FDD uplink, a channel is characterized by the relative phase of I/Q components.

The basic unit exchanged between Layer 1 (L1) and the MAC layer is referred to as a Transport Block. The following are block diagrams illustrating the modules used in physical layer uplink and downlink transmissions in FDD mode.

WCDMA Physical Layer performs the following functions:

• Macro-diversity distribution and combining, including the execution of soft handover operations.
• Error detection of transport channels using CRC (Cyclic Redundancy Check) and reporting to the upper layer using indication messages.
• Forward Error Correction encoding and decoding of the transport channels.
• Multiplexing operation on transport channels and demultiplexing operation of CCTrCHs (coded composite transport channels). CCTrCH is essentially a combination of all the transport channels to be multiplexed onto the physical channel.
• Rate matching of coded transport channels to physical channels.
• Mapping of CCTrCHs onto physical channels.
• Power weighting and combining of physical channels.
• Modulation/spreading and demodulation/despreading of the physical channels.
• Frequency and time synchronization (i.e., chip/bit/slot/frame synchronization) of the received frame from the NodeB.
• Radio-related measurements, which include FER (Frame Error Rate), Signal-to-Interference Ratio, Interference Power, and indication of the same to upper layers.
• Inner loop power control.
• RF processing to make the baseband I/Q data suitable for transmission over the air, interfacing with NodeB RF carriers.

WCDMA Physical Layer Uplink Transmission

The following modules are involved in the uplink of the WCDMA physical layer transmitter:

• CRC Attachment
• Transport Block concatenation/Code Block Segmentation
• Channel Coding
• Radio Frame Equalization
• First Interleaving
• Radio Frame Segmentation
• Rate Matching
• TrCH Multiplexing
• Physical Channel Segmentation
• Second Interleaving
• Physical Channel Mapping

WCDMA Physical Layer Downlink Transmission

The following modules are involved in the downlink of the WCDMA physical layer transmitter:

• CRC Attachment
• Transport Block concatenation / code block segmentation
• Channel coding
• Rate matching
• First insertion of DTX indication
• First interleaving
• Radio frame segmentation
• Transport Channel multiplexing
• Second insertion of DTX indication
• Physical channel segmentation
• Second interleaving
• Physical channel mapping