Impact of the Wireless Channel on the Performance of Ultrawideband Communication Systems

Ultrawideband (UWB) wireless systems employ signals with bandwidths in excess of 500 MHz or with relative bandwidth more than 20%. The radiated signals have low power spectral density. A decade ago, UWB wireless systems were deemed to be the technology that will deliver 'Gigabit-wireless' for short range communications. However, the performance of current systems is significantly below the initial expectations. This thesis explores the UWB wireless channel and shows how its properties limit the performance of current UWB systems. Furthermore, it is shown that if the knowledge of the channel is fully exploited a significant performance improvement of UWB systems can be achieved. The thesis begins with exploration of the channel properties. Unlike previous work, that has investigated either the 'classical narrowband' channel with bandwidth <100 MHz or the UWB channel with bandwidth >1 GHz, this work studies the transition between the narrowband channels with bandwidth of 1 MHz to the extremely wideband channels with bandwidths of up to 10 GHz. The thesis concludes that for signals with bandwidth <1 GHz UWB antennas and antenna arrays can be described by the classical means of gain and array factor, i.e. they treat such signals as 'narrowband'. In contrast, wireless propagation for signals with bandwidth >100 MHz has properties 'like UWB channels' with bandwidths in the GHz range. Additionally, the thesis suggests a correction to the IEEE802.15.4a model for channel impulse response because as will be shown in the thesis many multipaths in the model are manifestations of the antenna impulse response. Hence multiple multipaths in the IEEE802.15.4a model actually represent a single multipath component. This reduces the number of multipath components in the model by approximately factor of five. The understanding of the transition between narrowband and ultrawideband channel is used to improve the spectral efficiency of impulse radio systems which traditionally use signals with bandwidth >1 GHz. It is shown that the optimum signal bandwidth for impulse radio systems is in the range 150-450 MHz. Such systems balance the robustness against frequency selective fading with the reduction of duty cycle. Hence, the data-rate of impulse radio systems can be significantly improved. The frequency selective fading is shown to be the main limiting factor for the performance of the commercial UWB WiMedia systems with OFDM. It is shown that adaptive loading of OFDM subcarriers, which is compatible with the frequency selectivity of the channel, is more suitable for UWB OFDM systems than the use of strong Forward-Error-Correction measures. The introduction of the adaptive OFDM is not a significant change to the design of the scheme because the commercial WiMedia standard already foresees pilot OFDM symbols for channel estimation. The adaptive OFDM for UWB has already been considered by some authors. Unlike previous works, this thesis explores the performance of such a system in a large number of measured wireless channels. Finally, the thesis studies the MIMO techniques for UWB systems. Suitable schemes for fixed and adaptive OFDM are discussed. A realistic simulation using measured wireless channel shows that a 4×1 system with a low complexity beam-steering and adaptive OFDM can deliver a data-rate of 400 Mbps over a range of 9 m. This performance is for a system with bandwidth 528 MHz (like in the WiMedia standard). A further increase can be achieved with the increase of the system’s bandwidth.