Here is a YouTube video about Software Radio Basics in a Mathematical Sense, and is part of a growing collection of videos I have found to be useful. The reason is taking a look at Complex Signals, Digital Downconverters (DDCs), Receiver Systems & Decimation and Digital Upconverters (DUCs) Transmitter Systems & Interpolation and understanding science related to software defined radio.
This tutorial, "Software Radio Basics in a Mathematical Sense" is free education you can watch, which will help you deepen your knowledge and be better prepared for licence exams, schoolwork, or operating radio equipment.