problems about transfer function of TX and RX

Hi,

I currently designed a transmitter and a receiver. The transmitter includes a multiplexer such as 4 to 1, and a PLL supplying the clock. The receiver includes a CDR and a 1 to 4 DeMultiplexer.
Does anyone know how to write the transfer function of such TX and RX?
And can I simulate the impulse response of TX and RX in Cadence without knowing the transfer functions?

Thanks a lot!

YLuo