In his latest video out on YouTube, Matt from the Tech Minds channel gives us an overview of GNU Radio, and shows a few examples of how it can be used to receive, transmit and decode digital data.
GNU Radio is a popular open source DSP framework for software defined radios. With it you can graphically implement any sort of digital signal processing chain that you like, which can be used for decoding/encoding and demodulating/modulating signals.
GNU Radio can be extremely complex and powerful, but in the video Matt shows some simple starter example flowgraphs like an LSB demodulator, and a simple wav file source transmitter for the HackRF.
How To Make Your Own SDR Software With GNU Radio Companion