What is the secrecy rate?
Here is a pretty clear paper explaining secrecy rate and uses over fading channels.

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.105.2844&amp;rep=rep1&amp;type=pdf

See equation 3 for the formal definition. One way to think about it is like a tug-of-war between the main channel and wiretapping channel. If the main channel has a higher capacity then the secrecy rate is positive, otherwise it is negative.