Voltage source in series with dependent curent source. How to solve for voltage source?
First of all your small-signal diagram is wrong. If this is a CC amp then collector should also be at AC ground. And this means that Cu capacitor is connected between the base and the ground. And the resistance seen by this capacitor is simply equal to: 
$$R_u=R_a||(r_{\pi}+(\beta+1)R_E)$$