simplifying expressions – FullSimplify not working

 [Phi]=[Pi]/8
 G=Cos[[Phi]]*M+ I*Sin[[Phi]]*N

In the answer to the above code I am getting an answer without any reduction as follows

s

I have tried FullSimplify, PowerExpand etc. Nothing works. Is there a way that I get the answer in fractional form after finding the value of Cos[[Pi]/8]