What is signal hardening? What is error correction, diversity, & convolutional techniques in signal hardening?

Answer 1

The concept of "Signal Hardening" as a way to increase the efficiency of the available communications space.

Signal hardening such as channel coding, exploiting space- time and frequency diversity through signal expansion and finally explore some receiver adaptive techniques to maximize spectrum efficiency and to deal with multipath phenomena.
http://www.fiec.espol.edu.ec/investigacion/telecom/proposed-w.html

Answer 2

You are asking for a few college courses in a few lines. Basically they are all things used to make sure that the information transmitted is received with as much correct information as possible.