System modeling of the mutual coupling of multiple UWB antennas

Cognitive radio has been attracting significant interest and has the potential of shaping the future of wireless communication systems. Since the concept of cognitive radio is still at the stage of being developed, there is no consensus on what kind of wireless technologies to employ for realizing it. There are a number of requirements a wireless system has to satisfy in order to be considered as a suitable candidate for cognitive radio. These requirements include no spurious interference to licensed systems, adjustable pulse shape, bandwidth, and transmitted power, supporting various throughputs, providing adaptive multiple access, and ensuring the security of information. Ultra Wideband (UWB) seems to be one of the potential candidates because it has an inherent potential to fulfill some of the key cognitive radio requirements. However, it is not claimed that a cognitive wireless system with only the UWB capability can handle all the requirements of the ideal cognitive radio. Advances in reconfigurability RF front-ends, particularly reconfigurable multiple antennas afford a new “hardware” dimension for optimizing the performance of wireless communication systems. In particular, the association of Multiple Input Multiple Output (MIMO) systems, which exploit rich scattering environments by the use of multiple antennas, with UWB technology is more and more studied and seems to be a very promising approach for enhancing capacity, increasing range, raising link reliability and improving interference cancellation [1]. In the promising theoretical studies, the signals transmitted by multiple antenna elements are generally supposed to be independent or uncorrelated. But in reality, the current induced on one antenna produces a voltage at the terminals of nearby elements, termed as mutual coupling [2]. A strong mutual coupling degrades the antenna system efficiency where diversity performance is required.