The regulatory agencies in Europe and Japan require the implementation of avoidance techniques in some bands to reduce interference to licensed systems. Accordingly, ultra-wideband impulse radio (UWB-IR) based Wireless Personal Area Network (WPAN) standard IEEE 802.15.4a has suggested using linear combination of pulses to reduce interference to coexisting primary systems. In this paper, we consider the implementation of linear combination of pulses for a peaceful coexistence, and assess the UWB-IR system performance in the presence of an active narrowband system. For that, we study the possible transmitter and receiver structures that can be adapted for the physical layer of the IEEE 802.15.4a standard. The study shows that while the bit-error rate (BER) performances of coherent and noncoherent receiving structures may be slightly degraded with the use of linear combination of pulses when there is no active primary system, the performances can be significantly improved with appropriate filtering techniques at the receiver when the primary system is active.