We present here the results of a teaching project funded by Universitat Autònoma de Barcelona (UAB) and developed during the last academic year. The application context is a 2nd year subject of the Bachelor Degree in Telecommunication Systems Engineering and the Bachelor Degree in Electronic Engineering for Telecommunication, named Fundamentals of Communications and with a student load of 10,5ECTS. The subject is challenging for the students due to three main factors: i) the amount of concepts involved (10,5 ECTS);
ii) time-frequency (Fourier transform) analysis of the communication systems and
iii) mathematical reasoning.

The main hypothesis in this work is: since concepts involved in the analysis of communication systems are abstract, experimenting with Software Defined Radios (SDRs) will improve the learning curve.

SDRs are small USB devices with an antenna that can acquire the signals present in the air, that is, they can be adjusted to demodulate the signals at a given carrier frequency and provide the corresponding base-band digital samples. These samples can afterwards be processed in a standard Personal Computer (PC). Whereas simple SDRs can be used only as receivers, more sophisticated SDRs can do both tasks, i.e. they can also transmit. The pedagogical side is that available open-source software tools, like GNURadio used in this work, allow to easily program the receiver/transmitter (by block-based coding) and visualize the involved signals at each point in the diagram, both in time and frequency domain. Seven different schemes have been considered and provided to the students:
i) FM radio receiver;
ii) analog band-pass system;
iii) visualization of analytical and base-band equivalent signals;
iv) analog pass-band system;
v) AM receiver;
vi) digital band-pass system (2 versions) and
vii) digital pass-band systems.

With the first scheme, students check the SDR system and realize that it works by listening local FM stations while they look at the spectrum to identify the different senders and frequencies. The second scheme lets the students experiment (both visually and listening to a transmission in loop mode) concepts such as attenuation, distortion or Signal to Noise Ratio (SNR). The third and fourth scheme work the essential topics in pass-band transmissions, that is modulation/demodulation and the signals involved. With the fifth scheme, the students take the FM receiver and modify it to listen to an AM transmission. Finally, with the last two schemes the students can look at the signals involved in digital transmissions and experiment with practical measurements such as eye diagrams or received signal constellations. The main goal of the sessions was to motivate the students. As a representative example, a challenge was posed in the AM session. Using their knowledge of the AM spectrum shape (from previous lectures), the students were asked to find the hidden signal among FM signals and listen to it.

Our initiative was evaluated with a survey, where 127 out of 225 students (56%) participated. We asked to rate between 1 and 5 the following questions:
Q1) SDR sessions helped in understanding the subject;
Q2) I would like to have more SDR sessions;
Q3) SDR sessions were useful to see the applicability of the subject.

Results obtained are as follows: a mean of 3,65 (1,12 sd) for Q1, 3,75 (1,17 sd) for Q2 and 3,99 (1,12 sd). In summary, the students were satisfied with the initiative and agreed that it reinforced the lectures.

Keywords: SDR, communications, experimental learning.