What channel model to choose while simulating a system in passband with frequencies over GHz range?
However, when I add an AWGN channel, I get pretty low BER of 10^-5 in passband as compared to around 10^-4 BER in baseband under same AWGN channel of SNR 10 dB. Why is it happening?


You've got a bug in either passband channel or baseband channel model, or in how you transform your noise or your signal – end of story!


  I thought that there would be more attenuation in passband as attenuation increases with respect to frequency


If your passband model does that, then your baseband model has to do the same – they are mathematically equivalent.

If your passband model is frequency-selective, then your baseband model has to do the same.