GNU Radio is a free and open source software development toolkit that provides signal processing blocks to implement software radios. It runs on Linux machines and works with varied SDR hardware platforms from the $20 RTL-SDR through the $1,500+ USRP products.
Many online tutorials we’ve read either take an overly simplistic approach, or assume that the readers have a knowledge of DSP concepts above that possessed by the SDR novice.
We found a series of four tutorials from California State University, Northridge that introduces GNU Radio with a concept basis ideally suited to beginners. Rather than assume that the reader knows the purpose of filters, the meaning of interpolation and decimation, and the effect of multiplying two signals, these tutorials explain concepts such as these through experiments using various GNU Radio companion signal blocks. You don’t even need SDR hardware, as these tutorials include downloadable signal files!
Strictly speaking, these tutorials cover the GNU Radio Companion which is a GUI IDE for developing GNU Radio applications and is the way most users approach GNU Radio, moving to the command line as their skill level increases.
You’ll also find other GNU Radio related docs at the CSUN site. Check it out.