Although UWB is referred to the PHY underlying layer of the IEEE 802.15.3 (HR-WPAN), but i think that you mean IEEE 802.15.4 (LR-WPAN) no?

If it's LR-WPAN, there's a model implemented in NS-2 (No consistent model for NS-3) ... In the other hand if you have OPNET, you can benefit from a complete performing model of IEEE 802.11.4

Thanks Imad, Appreciate your reply. I am working on WSN-Wireless Sensor Networks and trying to use UWB (instead of conventional WiFi Standard.).

If you have adequate mathematical formulation any tool like MATLAB can solve your purpose