RTL-SDR stick and:

> We present a novel design to implement a transmitter by utilizing GPIO pins on Raspberry pi board, which spares us from having additional RF hardware equipment

and then some magic happens apparently to implement "4G"? WTF is this? Academic spam?

It started with piFM [1] where the possibility was explored to modulate the internal PLL Raspberry Pi with FM and output it on pin GPIO4. This was then further refined and a PWM phase and amplitude modulation technique was applied which made it possible to generate SSB signals [2]. Based on this rpitx [3] was then created which made it possible to basically modulate any type of signal, enabling to use a raspberry pi for analog and digital broadcasts, and to replace key fobs for RF smart plugs and other wireless devices. So yes, it is possible..

[1] GPIO4 piFM transmitter, by Oliver Mattos and Oskar Weigl http://www.icrobotics.co.uk/wiki/index.php/Turning_the_Raspb...

[2] Direct SSB generation by frequency modulating a PLL, by Guido http://pe1nnz.nl.eu.org/2013/05/direct-ssb-generation-on-pll...

[3] Universal I/Q TX for rpi, Evariste Courjaud, https://github.com/F5OEO/rpitx