Have a look at mscohere, and its help and documentation. It should allow you to calculate the coherence. My version resides in the signal processing toolbox. If you don't have that toolbox, I think you'd have to write your own versions of
spectrogram, and related functions, that will be a bit of a bother but shouldn't be toooooo hard if your application allows you to be a bit less general than Mathworks has to be.