0 Ultra - Wideband ( UWB ) Communications Channel – Theory and Measurements

The most recent increase in demand within the wireless user community for short-range, very high rate data and video transmission devices has motivated the growth of a new generation of broadband wireless access communication systems, i.e. Ultra-Wideband (UWB) radio (1)-(4). UWB technology has been employed for several decades in military and commercial communications applications like high-speed mobile Local Area Networks (LAN), imaging and surveillance systems, ground penetration radars, automotive sensors, medical monitors and recently Wireless Personal Area Networks (WPAN) (5). Spread-spectrum communication systems using ultra-short impulses have seen a renewed interest because of its fine resolution in delay to the order of a tenth of nanosecond though at the cost of a ultra wide frequency band. Low transmission power and large bandwidth together render the power spectral density of the transmitted signal extremely low, which allows the frequency-underlay of a UWB system with other existing radio systems. Hence, the short range radio UWB will play a critical role in the local/home (pico-cell) level of the broadband networks due to its unprecedented, broad bandwidth. Indoor wireless systems operate in the areas where usually there is no Line-of-Sight (LOS) radio path between the terminals, the transmitter, and the receiver, and where due to obstructions (furniture, partitions, walls, etc.), multi-diffraction, multi-reflections, and multi-scattering effects occur. These lead to not only additional losses (with regarding those obtained in LOS), but also multipath fading of the signal strength observed at the receiver. Basically, one of the most important aspects of any radio channel-modeling activity is the investigation of the distribution functions of channel parameters. Typically, these distributions are obtained from measurements or simulations based on almost exact or simplified descriptions of the environment. However, such methods often only yield insights into the statistical behavior of the channel and are not able to give a physical explanation of observed channel characteristics. Due to the extremely broad bandwidth, the channel is highly dispersive, even for an individual path. Physics-based models (2) are usually required to understand the multipath pulses waveforms that are necessary for optimal reception. There exist very good fundamental investigations on the UWB propagation channel characterization and modeling in the literature (6)-(11). More particularly, the references (9) and (11) give an excellent overview of the UWB channels and the authors in (10) 2