CDMA Reference Blockset

IS-95A Reverse Traffic Channel Detection Demo

The IS-95A Reverse Traffic Channel Detection demo simulates modulation and spreading of the data symbols at the mobile station transmitter, as well as despreading and noncoherent demodulation at the base station receiver. At the transmitter, the symbols are modulated by a Walsh modulator and spread, and a randomized gating is applied to the transmit bursts. At the receiver, the noncoherent rake demodulator recovers the data. The bit error rate for the data is displayed in the simulation.

You can open the model by following the instructions in Opening a Demo, or by typing is95revchdetection2 at the MATLAB prompt.

Library Blocks in the Demo

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

• IS-95A Rev Ch Detector
• IS-95A Rev Ch Walsh Modulation and Spreading, which is inside the Spreading and Modulation subsystem
• IS-95A Short Code Generator, which is inside the Data Source subsystem

How the Demo Works

Among the components used at the transmitter, the Data Source subsystem provides a source for random data bits, the data rate for the channel, and the short pseudonoise (PN) code used for the in-phase and quadrature spreading of the signal. In particular, the IS-95A Short Code Generator library block generates the short PN code.

The Spreading and Modulation subsystem contains several blocks that are responsible for the Walsh modulation, and the spreading with the long and short PN codes. The IS-95A Rev Ch Walsh Modulation and Spreading library block contains the IS-95A Rev Ch Burst Randomizer library block, which processes the long code and generates a gating signal based on the long code and the data rate of the input frame. The IS-95A Rev Ch Walsh Modulation and Spreading library block also groups the input data in 6-bit groups, maps each group of 6 bits to a 64-symbol Walsh code, upsamples these 64 symbols by a factor of 4 to bring the result to chip rate, and then spreads the upsampled symbols with the gated long code. This gating ensures that the transmission is only performed for a fraction of the frame duration (half the time for half rate, and so on). Finally, other portions of the Spreading and Modulation subsystem spreads the data in quadrature by the PN code.

The Transmit Filter block generates the I and Q waveforms. The Q waveform is delayed by a 1/2-chip duration relative to the I waveform.

The Rayleigh Multipath and AWGN Channel subsystem simulates the propagation through multiple paths of a Rayleigh fading channel. Complex white Gaussian noise is added to the channel output; this noise represents the interference generated by other base stations that are using the same frequency band.

In the receiver section, the incoming signal is first filtered by the Receive Filter block, which implements a filter matched to the transmit filter. The filters in this demo are designed to maximize the signal power within the desired frequency band.

Then the filtered signal is sent to the IS-95A Rev Ch Detector library block, which contains the reverse channel rake receiver. The rake receiver consists of three rake fingers that are set to different delays to handle up to three multipaths. Each active rake finger performs the despreading of the input data with the short PN sequence, followed by despreading with the long code. This is followed by the correlation with the entire set of 64 Walsh codes. The energies in the I and Q components are added, and the results from the fingers are added together. This is processed by the Walsh demodulator, which generates decisions in groups of 6 bits, the size used for modulation. The Walsh demodulator outputs both soft decisions and bipolar-valued hard decisions. both are gated by the data burst randomizer signal.

The Error Rate Calculation block compares the transmitted bits and the received decisions, and produces the raw (that is, without channel coding) bit error rate. This rate is displayed in the model window during the simulation.

Visible Results of the Demo

The demo shows the raw bit error rate (BER) for the reverse channel. The BER depends on the channel conditions and the number of rake receiver fingers active. As the signal-to-noise ratio or the channel conditions are changed, the effect of these on the raw BER can be seen by running the model for these different conditions.

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 2 seconds. Because the frame duration in IS-95A is 0.02 second, this model processes 100 frames. This gives a sufficient time for the BER measurements to converge at reasonable SNR settings. To run the simulation for a longer or shorter time, 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 data rate, double-click on the Data Source icon. Then double-click on the Mobile Station Transmitter Data Rate icon and change the Data Rate parameter. Choices are Full, Half, Quarter, and One-Eighth rates.

Long Code Mask.   To change the long code mask, double-click on the Spreading and Modulation icon. Then double-click on the IS-95A Rev Ch Walsh Modulation and Spreading icon, and enter a new value for the Long code mask parameter. This parameter can be any nonnegative integer less than 2⁴²-1. The block uses the binary representation of this number to generate the code. The value for the Long code mask parameter is also required in the IS-95A Rev Ch Detector library block. Experiment with using the same or different values in the two blocks. This illustrates why the receiver does not demodulate the signal not intended for it, although they are transmitted in the same frequency band. For more information, see the IS-95A specification.

Doppler Frequency in Channel.   To change the Doppler frequency, double-click on the Rayleigh Multipath and AWGN Channel icon. Then double-click on the Multipath Rayleigh Fading icon, and change the Doppler frequency parameter. Reasonable Doppler frequencies representative of the cellular mobile environment are in the 0 to 200 Hz range.

Signal-to-Noise Ratio.   Change the Es/No parameter in the AWGN Channel block in the channel subsystem. The detection performance improves with an increase in the signal-to-noise ratio.

Random Seed.   To run the simulation with different seeds, change the Initial seed parameters in one or more of these:

• Random Data Frame Generator
• Multipath Rayleigh Fading
• AWGN Channel

The first item is inside the Data Source subsystem and the last two blocks are inside the Rayleigh Multipath and AWGN Channel subsystem.

Channel Paths and Rake Fingers.   Changes in the number or delay of fading channel paths and rake fingers must correspond to each other. To change the number or delay of fading channel paths, double-click on the Rayleigh Multipath and AWGN Channel icon. Then double-click on the Multipath Rayleigh Fading icon and change the Delay vector parameter. The vector length of this parameter is the number of paths and the vector elements measure the delay of each path in seconds.

To change the number or delay of rake fingers, double-click on the Initial Phases and Finger Enables icon and change the Constant value parameter. The three consecutive pairs of elements of this length-six vector indicate the delay and status of the three rake fingers. The delay is measured in samples, not in seconds. A status of zero disables the finger and a status of one enables the finger. By default, this demo enables two rake fingers, with delays of 96 and 116 samples, respectively. These delays incorporate the filter delays of 96 samples, as well as the two default channel path delays of 0 and 2x10^-6 seconds. (To convert a delay from seconds to samples, multiply by the chip rate of 1.2288 Mcps and then by the oversampling rate of 8.)

Further Explorations

The demo serves as the starting point for building simulation models with different characteristics and conditions. 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:

• Introducing components from the reverse channel codec
• Replacing the transmit and receive filters by systems developed using the DSP Blockset with different oversampling rates, coefficients, or precision -for instance, to evaluate the effects of fixed-point implementation
• Introducing an analog-to-digital converter to observe the performance degradation at the receiver

Note    Some changes involve consistency considerations. For instance, a change in the oversampling rate requires corresponding changes in the channel, rake receiver, and the filters.

IS-95A Forward Traffic Channel Detection Demo

IS-95A Forward Traffic Channel Codec Demo