Normalizing the level of an incoming signal is a critical step in many wireless communications systems and is necessary before further processing can happen in the receiver. This is particularly necessary in digital modulation schemes which encode information in the signal amplitude (e.g. see LIQUID_MODEM_QAM16 in section-modem-digital ).

http://liquidsdr.org/doc/agc/

This tutorial explains how to write and simulate a phase-locked loop in the C programming language.

http://liquidsdr.org/blog/pll-howto/