CDMA Reference Blockset

IS-95A Reverse Traffic Channel Transmitter Demo

The IS-95A Reverse Traffic Channel Transmitter demo simulates the mobile station transmitter, which is the transmitting section of the IS-95A reverse link. These are the main components of the demo:

• CRC generator
• Convolutional encoder
• Symbol repeater
• Interleaver
• Modulator and spreader
• Transmit filter

The mobile transmitter model is important when designing a receiver, because the ability to simulate the transmitted waveform and its characteristics, such as the peak-to-mean ratio, affects the power control components of the design.

You can open the model by following the instructions in Opening a Demo, or by typing is95revchtransmitter2 at the MATLAB prompt.

Library Blocks in the Demo

The IS-95A Reverse Traffic Channel Transmitter demo uses these library blocks from the CDMA Reference Blockset:

• IS-95A CRC Generator
• IS-95A Rev Ch Convolutional Encoder
• IS-95A Rev Ch Interleaver/Deinterleaver
• IS-95A Rev Ch Repeater/Derepeater
• IS-95A Rev Ch Walsh Modulation and Spreading (inside Spreading and Modulation subsystem)
• IS-95A Short Code Generator

How the Demo Works

The Mobile Station Transmitter Data Rate masked subsystem provides the data rate, while the Random Binary Frame Generator masked subsystem generates random data bits for the system to transmit. The raw data corresponds to the speech frame that is generated at the vocoder. The IS-95A CRC Generator library block generates the CRC (cyclic redundancy check) bits and appends them to the frame data depending on the data rate. The result of the CRC addition is then convolutionally encoded for error protection by the IS-95A Rev Ch Convolutional Encoder library block. Because the modulation symbol rate is constant regardless of the data rate, the IS-95A Rev Ch Repeater/Derepeater library block (denoted IS-95A Rev Ch Repeater in this demo) repeats the convolutionally encoded data by a factor depending on the data rate to bring the output symbols to a constant symbol rate. This data is then interleaved by the IS-95A Rev Ch Interleaver/Deinterleaver (denoted IS-95A Rev Ch Interleaver in this demo) for protection against error bursts. Meanwhile, the IS-95A Short Code Generator generates the short PN code, which is used for quadrature spreading.

The IS-95A Rev Ch Walsh Modulation and Spreading library block, which is inside the Spreading and Modulation subsystem, performs the Walsh modulation and spreading with the long code. The Spreading and Modulation subsystem also performs the short PN spreading. The Filtering and Unbuffering subsystem performs the pulse shaping to generate the I and Q waveforms that are displayed in the Scope I and Q window.

Visible Results of the Demo

The demo has a built-in Scope block (denoted Scope I and Q in this demo) to display the in-phase and quadrature components of the modulated waveform. The waveform quality may be examined using the display. Also when the rate is less than full rate, the displays show that the transmit waveform is gated off for part of the duration. Under the default parameter settings, the nature of the transmitter I and Q waveforms is shown below.

Changing Demo Parameters

This section suggests some ways that you can alter the parameters in the demo after you understand the model.

Simulation Duration.   The simulation duration in this demo is set to 0.1 second. Because the frame duration in IS-95A is 0.02 seconds, this model processes five frames. To obtain long term statistical measurements of waveform properties, you need to increase the duration of the simulation. To do this, select Parameters from the Simulation menu, and change the Stop time to 0.02 times the number of frames you want to simulate.

Data Rate.   To change the source data rate, double-click on the Mobile Station Transmitter Data Rate icon, and choose either Full, Half, Quarter, or One-Eighth rates from the menu.

Rate Set.   Change the Rate set parameter in the IS-95A CRC Generator library block and the IS-95A Rev Ch Convolutional Encoder library block. To do this, double-click on these blocks' icons and choose either Rate Set I or Rate Set II from the Rate set menu.

Random Seed.   The simulation uses a random seed to generate random data at the source. To run the simulation with a different random data sequence, double-click on the Random Binary Frame Generator icon and change the Initial seed parameter.

Further Explorations

The demo serves as the starting point for building simulation models with different objectives relevant to a specific application. Parameters in the demo's components may be changed for different configurations. For instance, you can alter the channel type in all components (on the main level or within subsystems) that include a Channel type parameter, to obtain a simulation for a different configuration.

Other possible changes that you can make include:

• Adding components such as a specific digital-to-analog converter or power amplifiers, to evaluate their performance
• Evaluating waveform quality, for example, by measuring error vectors
• Replacing the transmit filters with blocks from the DSP Blockset or adding extra filters to model specific analog filter components
• Introducing performance measurement blocks to obtain statistics for the waveform properties

In addition, this transmitter may be used for design and evaluation of base-station receiver algorithms for objectives such as acquisition.

IS-95A Reverse Traffic Channel Codec Demo

Selected Bibliography