Suppose that one were to use code by that what I mean is that you add certain amount of redundancy, if you add redundancy and get N code bits then the code rate is given to you by "k" by "n". If you do that, and you again were to follow and find out what the B are and what a signal to noise-ration is, you will draw a curve, and the difference of this particular curve from that of PSK is called coding gain, hence coding gain is related to the code rate, the amount of redundancy added, and naturally you have a code therefore you must worry about what kind of complexity you are having to get this particular code implemented.