GNURADIO Demonstration

 

Download and Installation

  • Downloading and intalling gnuradio is straightforward. In ubuntu, you can get a near latest version by typing in command prompt
    $sudo apt install gnuradio
  • if you need help after repeated attempts, we can help you install it (contact by email).


Experimental Setup

  • After installing, type gnuradio-companion to open gnuradio GUI. Feel free to play around with the different devices. Many examples are available in the public domain and even youtube videos are available.
  • In this experiment, you will generate a communication signal of bandwidth 80KHz. Open the .grc file within the gui and click on the play/execute button.


Experiments


Exp Nr. Details
1 ISI GRC Flowgraph If you are not able to get the grc file, the flowgraph is depicted here (Flowgraph). We will first generate a QPSK waveform, using a sinc pulse-shaping filter. The entire flowgraph uses an oversampling factor of 4, i.e. four samples per symbol.

The Interpolating FIR filter essentially acts as a channel filter, which we have learnt in the class to have a tapped delay line form. The taps are separated by symbol duration. However, we have oversampled by a factor of four. To maintain the filter at symbol level, we upsample the filter coefficients by padding 3 zeros between adjacent taps of the delay line filter.

The receiver of this communication diagram is essentially the polyphase clock synchronization block. This block takes in four samples per symbol, and outputs an optimal sample according to the timing synchronization acquired. Notice that the input samples per symbol is set as 4, whereas the output samples per symbol(SPS) is set as 1, essentially decimating the communication signal to the correct sample per symbol. The taps are set by the command, firdes.root_raised_cosine(32,32,0.25,0.35,32*11*4), which essentially is a multirate implementation with 32 parallel filters. The third parameter is 1/SPS, fourth being the excess bandwidth, and the last one is the total number of taps in the pulse-shaper (the pulse-shaper is required at the receiver)

2 TO DO Notice that the QPSK constellation contains only four points, whereas the shown output has a collection of samples near each QPSK constellation point. This anomaly occurs primarily due to a mismatch of transmit and receive filters. You are required to improve the quality of the receiver by specifying the correct filter to the polyphase block employed.

We have termed that the bandwidth is 80 KHz. Observe the output spectrum and comment on the actual bandwidth encountered in the scope.

Send suggestions and/or comments to: bsraj att ee . Better to use the Right to Information (RTI) act in the class than later.