This paper presents a novel modeling technique of Ultra-Wideband Impulse Radio for the simulation of wireless sensor networks, to evaluate this technology from a systems point of view that includes the effect of communication protocols. The adopted approach, named Maximum Pulse Amplitude Estimation, considers the characteristics of the propagation channel, the transmitter and receiver architectures, the modulation type, the robustness to multiple access interference and the impact of error correction coding. It differs from the state of the art by working at the symbol level instead of developing an analytical model. While this technique increases the processing time, it offers more flexibility in terms of modulations, channel models and receiver architecture. Our approach enabled us to implement the first network simulation model of the UWB PHY specified in the IEEE 802.15.4A standard. It is also the first UWB-IR network simulation model to consider an energy detection receiver. Several channel models have been implemented, offering trade-offs between simulation speed and accuracy. The performance of this energy detection transceiver is compared with MATLAB models. The packet error rate is evaluated as a function of distance with several channel models, and in the presence of interferers. It is shown that our approach leads to results in line with MATLAB models, that the choice of channel model greatly impacts the simulation speed and that the IEEE 802.15.4A UWB PHY allows some limited degree of protection against multiple access interference even when using a low performance energy detection receiver architecture.