matlab_tutorial_three - ECE 561 Digital Communications...

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ECE 561 – Digital Communications Systems MATLAB Tutorial #3 In this tutorial we will simulate a detector for a binary symbol set. We will start with a signal set for which a “1” is represented by a positive one volt pulse and a “0” is represented by a negative one volt pulse, or bipolar pulses. We can generate a random sequence of pulses using the Simulink model shown below. The Random Integer Generator is set for M = 2 (binary) and a sample time of 1E-3. This will provide 1mS pulses and a 1Kbps data rate. The Unipolar to Bipolar Converter found in the Utility Blocks Library of the Communications Blockset. This is an alternative method for generating bipolar pulses than the one used in a previous MATLAB assignment. M needs to be set to 2 for this block also. The sampling rate generated by the generator is also 1E-3. In order to observe an accurate spectrum we need to sample more frequently. If we want to observe frequency components up to 10KHz, we need to sample at 20KHz, or at a sampling interval of 5E-5. The rate transition block is set to that sampling frequency. To demonstrate that this model operates correctly a simulation is run for 20E-3 seconds (discrete, fixed step) and the waveform shown below results. Note that the rate transition block causes a delay in the output. Thus, the waveform does not begin until after 1mS.
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The model shown below is a correlation receiver that is used to detect the bipolar pulses generated by this pulse modulator. In a correlation receiver, the input to the receiver is multiplied by s 1 – s 2 . In this case, s 2 = -s 1 = -1, and so the difference is 2, a constant. This is generated with the constant block. The output is then fed into the Integrate and Dump block from the Comm Filters Library in the Communications Blockset. This block integrates over a specified period of samples and then outputs the result. The integration begins over after the period is complete. Set the integration period in this block to (1E-3)/(5E-5) or 20. This is the number of samples in a single pulse. Leave the Output Intermediate Values box unchecked so that the output will only change at the
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matlab_tutorial_three - ECE 561 Digital Communications...

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