"Most of the communication systems that are in use today are licensed systems. Due to the spectrum getting crowded with many licensed systems, there is a need to consider future communication systems different from the current licensed systems in use. Ultra-wideband (UWB) communications is one of them. In UWB communications, due to the very low power spectral density (PSD) of the transmitted signal, the UWB system can operate as an underlay system in the same frequency band. Despite the low PSD of such systems, the European and Japanese regulatory agencies have made the implementation of detect-and-avoid (DAA) techniques mandatory in some common bands so as to protect the license rights of the licensed users. Therefore, the challenge here is to implement the new UWB systems so that they can coexist with the licensed communication systems peacefully.The main objective of this research is to investigate the coexistence of UWB systems with licensed systems. The research has two main parts; the implementations of detection and avoidance techniques considering practical scenarios. In the implementation of detection, the realistic case of active multiple systems with possible dependencies will be considered. This approach is different from the general approach that assumes the independence of the systems. In the implementation of avoidance, the linear combination of pulses with limited pulses will be considered as suggested by the IEEE 802.15.4a UWB standard. This approach is different from the general approach that assumes having enough number of pulses (i.e., filter coefficients). The main expected result of this research is to make suggestions for the coexistence of primary and secondary systems as a result of the considering the realistic cases. The research results are expected to provide benefit especially to the international communications research and development community and the European regulatory agency."