3G communication, such as Wideband Code Division Multiple Access/ Universal Mobile Telecommunications System (WCDMA/UMTS), is becoming more and more popular all over the world. Most of the existing cellular devices provide built-in Wireless Personal Area Network (WPAN) capability. Due to its high bandwidth, UWB is considered as the predominant WPAN standards in the near future. Therefore, it is natural to consider the co-existence issue between the UWB technology and WCDMA/UMTS technology. Although much of the previous literature has addressed this issue, the research on UWB-3G coexistence is inconclusive, and even contradictory. While some studies show that UWB can be highly detrimental to 3G systems , , others have concluded that both systems can gracefully coexist ,. Furthermore, most of the analyses in the existing literature consider the distance between the victim receiver and the UWB interference is larger than one wavelength without recognizing that propagation characteristic is very different for collocated devices due to the near-field affect. This research considers the theoretical and practical aspects of the coexistence between UWB and UMTS/WCDMA system, with particular emphasis on UMTS and MB-OFDM systems. A series of simulation results are provided to quantify the effect of UWB interference on the performance of a UMTS/WCDMA system.