Communications Blockset

WCDMA End-to-End Physical Layer Demo

The WCDMA End-to-End Physical Layer Demo, wcdma_phlayer, models 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 next generation of wireless communications being 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 being developed by the Third Generation Partnership Project (3GPP), Release 1999, which is a joint effort between 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 pseudo-random units called chips at 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.

This document highlights the following aspects of the demo:

• Overall Structure of the Physical Layer
• Parameters in the Demo
• Visible Results of the Demo
• References

Overall Structure of the Physical Layer

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 has associated a different transport format that contains information of how the data needs to be processed by the physical layer. The physical layer processes this data before sending it to channel.

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

Subsystem	Function
WCDMA DL Tx Channel Coding Scheme	Transport channel encoding and multiplexing
WCDMA Tx Physical Channel Mapping	Physical channel mapping
WCDMA BS Tx Antenna	Modulation and spreading
WCDMA Channel Model	Channel model
WCDMA UE Rx Antenna	Despreading and demodulation
WCDMA Rx Physical Channel Demapping	Physical channel demapping
WCDMA DL Rx Channel Decoding Scheme	Transport channel demultiplexing and decoding

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 to 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 together 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 Spreading and Modulation

The WCDMA BS Tx Antenna Spreading and Modulation 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 in Propagation conditions environment

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 of them associated to 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, as described above.

Parameters in the Demo

You can view or change parameters in the model by double-clicking the block labeled "WCDMA Demo: Initial Settings." 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 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:

Parameter	Description
DL Measurement channels	The down link (DL) measurement channels. There are four channels whose settings are specified by the standard: 12.2 Kbps 64 Kbps 144 Kbps 384 Kbps If you select one of these channels, the parameters listed below are greyed out. To change these parameter settings, select User Defined.
Transport block set size	Integer row vector representing the transport block set size as defined by the standard associated to each transport channel.
Transport block size	Integer row vector representing the transport bock size as defined by the standard associated to each transport channel.
TTI in ms	Integer row vector representing the transmission time interval (TTI) in ms as defined by the standard associated to each transport channel.
CRC Size	Integer row vector representing the CRC size in number of bits associated to each transport channel.
Type of error Protection	Integer row vector representing the coding scheme associated to each transport channel. The different options are: 1 for no coding 2 for convolutional encoding 3 for turbo coding.
Rate matching attribute	Integer row vector representing the rate matching attribute as defined by the standard associated to each transport channel
Position of TrCH in radio frame	Sets the position of the transport channels in the radio frame to be Fixed or Flexible as defined by the standard
Number of PhCH	Integer from 1 to 3 corresponding to the number of physical channels used.
Slot format (0..16)	Sets the corresponding slot format parameter as defined by the standard.

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 checked, the dialog displays the following parameters:

Parameter	Description
DPCH Code number	Integer number from 0 to the value of the spreading factor minus 1 corresponding to the index of the orthogonal code assigned to the DPCH channel.
Scrambling code	Vector of two elements corresponding to the index of the Scrambling Code assigned to the Base Station.
Number of filter taps for RRC	Number of filter coefficients for the root-raised cosine filter.
Number of coefficients for channel estimation filters	Number of filter coefficients for low pass filter implemented in channel estimation.
Oversampling factor	Integer value corresponding to the number of samples per symbol

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

Parameter	Description
Propagation conditions environment	Selects between the different pre-built propagation conditions environments.
Number of enable fingers	Integer from 1 to 4 that sets the number of enable fingers
SNR (in dB)	Value of the signal to noise ratio in decibels.
Relative delay of Rx signals (in s)	Vector corresponding to the delay (in s) of the different paths.
Average Power of Rx signals (in dB)	Vector corresponding to the Power (in dB) of the different path.
Speed of Terminal (in Km/h)	Value of the speed of the UE (User Equipment) in Km/h

Visible Results of the Demo

The following blocks calculate various error rates in the demo:

• 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 to each transport channel separately.

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

• Time Scopes shows the bit stream before spreading, after spreading and after combining the different weighted physical channels. It shows both the real and the imaginary part separately. It also displays both the real and the imaginary part of the output of the channel estimator for the first rake finger.
• Power spectrum shows 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 constellation at signal at the output of the data correlator, after phase derotation and after amplitude correction.

References

[1]  http://www.3gpp.org

WCDMA Coding and Multiplexing Demo

WCDMA Spreading and Modulation Demo