This is convex for N = 1, and can be entered as log2(a) - log(b+x)/log(2)
But it is neither convex nor concave for N > 1. For instance, let N = 2 and all the parameters and variables have the value 1. Then per Maple, the eigenvalues of the Hessian are -0.40856482 and 0.31839638
You should carefully re-read Why isn't CVX accepting my model? READ THIS FIRST! .
Edit: There was a typo in my Maple entry. So I withdraw the above assessment.