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WCDMA End-to-End Physical Layer

This model shows part of the frequency division duplex (FDD) downlink physical layer of the third generation wireless communication system known as wideband code division multiple access (WCDMA).

WCDMA is one of five air interfaces for the third generation of wireless communications developed within the framework of the International Mobile Telecommunications (IMT)-2000, as defined by the International Telecommunication Union (ITU). The WCDMA technology is officially known as IMT-2000 Direct Spread.

The specifications of the WCDMA system are developed by the Third Generation Partnership Project (3GPP), Release 1999, which is a joint effort among standards bodies from Europe, Japan, Korea, USA, and China.

The WCDMA air interface is a direct spread technology. This means that it spreads encoded user data at a relatively low rate over a much wider bandwidth (5 MHz), using a sequence of pseudorandom units called chips at a much higher rate (3.84 Mcps). By assigning a unique code to each user, the receiver, which has knowledge of the code of the intended user, can successfully separate the desired signal from the received waveform.

Structure of the Example

The physical layer is in charge of providing transport support to the data generated at higher layers. This data is exchanged between the higher layers and the physical layer in the form of transport channels. There can be up to eight transport channels processed simultaneously. Each transport channel is associated with a different transport format that contains information on how the data needs to be processed by the physical layer. The physical layer processes this data before sending it to the channel.

The model has seven main subsystems, whose functions are summarized in the following table.

WCDMA DL Tx Channel Coding Scheme. The WCDMA DL Tx Channel Coding Scheme subsystem processes each transport channel independently according to the transport format parameters associated with it. This subsystem implements the following functions:

  • Cyclic redundancy code (CRC) attachment

  • Transport block concatenation and segmentation

  • Channel encoding

  • Rate matching

  • First interleaving

  • Radio frame segmentation

The different transport channels are then combined to generate a coded combined transport channel (CCTrCH). The CCTrCH is then sent to the WCDMA Tx Physical Mapping subsystem.

WCDMA Tx Physical Mapping. This subsystem implements the following functions:

  • Physical channel segmentation

  • Second interleaver

  • Slot builder

The output of this subsystem constitutes a dedicated physical channel (DPCH), which is passed to the WCDMA BS Tx Antenna Spreading and Modulation subsystem.

WCDMA BS Tx Antenna. The WCDMA BS Tx Antenna subsystem performs the following functions:

  • Modulation

  • Spreading by a real-valued orthogonal variable spreading factor (OVSF) code

  • Scrambling by a complex-valued Gold code sequence

  • Power weighting

  • Pulse shaping

WCDMA Channel Model. The WCDMA Channel Model subsystem simulates a wireless link channel containing additive white Gaussian noise (AWGN) and, if selected, a set of multipath propagation conditions. You can modify the multipath profile with the Propagation conditions environment parameter, as described under Exploring the Example.

WCDMA UE Rx Antenna. The received signal at the WCDMA UE Rx Antenna subsystem is the sum of attenuated and delayed versions of the transmitted signals due to the so-called multipath propagation introduced by the channel. At the receiver side, a Rake receiver is implemented to resolve and compensate for such effect. A Rake receiver consists of several rake fingers, each associated with a different received component. Each rake finger is made of chip correlators to perform the despreading, channel estimation to gauge the channel, and a derotator that, using the knowledge provided by the channel estimator, corrects the phase of the data symbol. The subsystem coherently combines the output of the different rake fingers to recover the energy across the different delays.

WCDMA Rx Physical Channel Demapping and Channel Decoding Scheme. The WCDMA Rx Physical Channel Demapping and the WCDMA DL Rx Channel Decoding Scheme subsystem decode the signal by performing the inverse of the functions of the WCDMA DL Tx Channel Coding Scheme subsystem, described above.

Exploring the Example

You can view or change parameters in the model by double-clicking the block labeled Model Parameters. This displays the Block Parameters dialog.

The Power for [DPCH, P-CPICH, PICH, PCCPCH, SCH] in dB parameter consists of a row vector containing the powers in decibels corresponding to the different physical channels.

The Show Transport Channel Settings check box enables you to specify the parameters corresponding to the WCDMA Tx Channel Coding Scheme subsystem, the WCDMA Tx PhCh Mapping subsystem, and its corresponding subsystems at the receiver side. When the box is selected, the dialog displays the following parameters:

The Show Antenna Settings check box enables you to specify the parameters corresponding to the WCDMA BS Tx Antenna and WCDMA UE Rx Antenna subsystems. When the box is selected, the dialog displays the following parameters:

The Show Channel Model Settings check box enables you to specify the parameters corresponding to the WCDMA Channel Model subsystem:

Results and Displays

The following blocks calculate various error rates in the example:

  • BLER (Block Error Rate) Calculation shows the block error rate of the combined transport channels.

  • BER (Bit Error Rate) Calculation shows the results of the BER computation block associated with each transport channel separately.

The following scopes display the signal in various ways. To view the scopes, double-click the icons when the simulation is running.

  • Time scopes show the bit stream before spreading, after spreading, and after combining the different weighted physical channels. They show both the real and the imaginary part separately. They also display both the real and the imaginary part of the output of the channel estimator for the first rake finger.

  • Power spectrum plots show the power spectrum of the signal before spreading, after spreading, after pulse shaping, and at the input of the receiver antenna.

  • Scatter plots show the signal constellation at the output of the data correlator, after phase derotation, and after amplitude correction.

Accompanying Models

The following two models offer standalone implementation of some of the subsystems included in this example model:

commwcdmamuxandcoding.mdlcommwcdmamuxandcoding.mdl: shows the WCDMA DL Tx Channel Coding Scheme with Physical Channel Mapping and WCDMA Physical Channel Demapping with the Rx Channel Decoding Scheme.

commwcdmaspreadandmod.mdlcommwcdmaspreadandmod.mdl : shows WCDMA BS Tx Antenna and WCDMA UE Rx Antenna.

Selected Bibliography

http://www.3gpp.org

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