I have a simulink model of zigbee over a Rayleigh fading channel.

The modulation in the transmitter part is O-QPSK modulated baseband.

Most of the examples didn't add the RF frequency after the baseband modulator and I can't understand why.

And what is the effect of cancelling this part in simulink?

However, when I try to simulate the RF band after baseband modulator using up converter, the output of it was real and the Rayleigh multipath fading block didn't accept it.

• You almost never want to simulate the actual RF signal, because the sampling rate is too high and it's completely unnecessary. Search for "complex envelope" on this site for more details. – MBaz Feb 9 '17 at 14:53

You're forgetting the most basic truth of baseband signal processing: Any causal, stable, linear channel $h$ can be just as well represented as baseband system as it can be represented as RF passband system. That is a direct result of both the Fourier transform and the Convolution being a linear operation.