Coding gain for FSK links

Anyone know an easy to understand paper or method for adding coding gain to a 915 MHz 2 level FSK link to improve range without increasing power. Data rate is very low, but battery life is important.

I have started to read some papers on coding gain (block,trellis, convolutional...my eyes are glazing over)--all far too complex for what I want. I want just a simple coding scheme to give me more range, maybe 6 to 10 dB more coding gain for a 512 bit or so message length.

Another way to improve S/N and link budget, without changing Pout and Coding Gain, is to increase the antenna gain and receiver sensitivity.
3dB’s on each side will give you a 6dB improvement.
This is the RF way, which sometimes is easier than Software way.