QPSK - BER graph

I have QPSK signal u(t) = (u + σu) * cos(ωt + φ+σφ)

I need to plot 3D graph of BER(σu,σφ) and here is the problem. I need to use surf or mesh function. I'm using meshgrid to create X and Y, but I don't know how the Z function should look like.

From what I know this graph should look similar to this (sorry for that ugly image, but right now I have access only to Paint.)