What channel model to choose while simulating a system in passband with frequencies over GHz range?

I was wondering as to what kind of channel model do we use to test for BER vs SNR in a passband RF system. The system that I designed is basically a wideband frequency hopped OFDM system where I use Digital up-converter (DUC) to up convert the signal to a frequency in GHz range and use the Digital down converter (DDC) to down convert the signal back to baseband in the receiver side. Now my system works fine. 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? I thought that there would be more attenuation in passband as attenuation increases with respect to frequency. Or is it that my way of modelling the channel isn't good. Btw, I have tried adding Rayleigh SISO fading channel along with AWGN but in that case I achieve around 0.4 BER which is quite high. I just don't know anything about BER vs SNR plots in passband. All the plots that I found are mostly in baseband.

So my question is

1. How do I model the channel in passband?

2. How does a typical BER vs SNR look like in passband?

Thank you!