This paper evaluates the bit error rate performance of Impulse Radio-Ultra-Wideband (IR-UWB) communication system under the impact of both multiple access and narrowband interferences operating. The Narrowband Interference (NBI) signal is modelled as the IEEE802.11a orthogonal frequency division multiplexing-based wireless local area network signal, which can be approximated by the sum of independent asynchronous tone interferers with arbitrary frequencies. Multiple Access Interference (MAI) is assumed a zero mean Gaussian process, where it has been investigated in both perfect and imperfect power control scenarios. The bit error rate performance is evaluated in three different channel models: ideal channel model (additive white Gaussian noise channel), Nakagami-m multipath fading channel model and the IEEE802.15.3a UWB channel model. It will be shown that the performance of a UWB system is severely degraded due to the presence of both types of interference, yet the NBI has more impact on the performance of UWB communication systems compared to the multiple access one.