System design for ultra wideband wireless communications

Abstract

Ultra wideband (UWB) technology has become a promising candidate for wireless personal area networks (WPAN) due to its favorable properties such as fine time resolution and low energy density, etc. However, to implement UWB system, there exist a number of challenges in both physical (PHY) and multiple access (MAC) layers. In this thesis, we investigate the system design for UWB communications and ranging systems, focusing on channel modeling, receiver and multiple access design. In the first contribution, parsimonious correlated non-stationary models for real baseband UWB data are proposed. The novel models treat the UWB data as a random process, consisting of three components to represent the non-stationarity, large scale fluctuation and local variation, respectively. Their capability in data regeneration and receiver design is illustrated by applying to real UWB data measured by TimeDomain and Intel Corporation. This channel modeling provides the basis for the following work of receiver design. In channel modeling, we show that it is reasonable to treat UWB signals as correlated random process. Based on this, in the second contribution, coherent eigen-based receiver is derived for UWB communications system. The template utilized by eigen-receiver is the eigen-function with the largest eigenvalue for the covariance function of received UWB signals, that is the dimension on which the random process projects most of its energy. In this sense, the proposed eigen-receiver is the best coherent correlation receiver with deterministic template because it can capture most energy from the received signal on average. Our third contribution is the non-coherent dimension-diversity receiver. A deterministic template can only capture partial energy from the random UWB signal, since a random signal is of multi-dimensions. We construct a receiver with multiple orthogonal branches to fully capture the signal energy. The local reference signals are derived from the orthogonal expansion of UWB signals. The random projections of a UWB signal onto different orthogonal basis functions are non-coherently combined, leading to the concept of dimension diversity. Besides performance advantage verified by numerical results based on real UWB data, non-coherent nature of the dimension-diversity receiver decreases its requirement on channel estimation and system synchronization. The first three parts of the thesis focus on the physical layer, while the fourth contribution of this thesis is the multiple access design for UWB impulse radio system. Time hopping (TH) pulse position modulation (PPM) is widely believed promising and even necessary for UWB impulse radio. Unfortunately, user energy of each frame in TH-PPM is concentrated on one TH chip, making user collision fatal for the system. In order to more evenly spread user energy, we propose the novel code division multiple access (CDMA) PPM system. The better performance of CDMA-PPM under multi-user circumstances comes from the fact that its large processing gain obtained by spreading user energy evenly can compensate user collision. The proposed system is superior to the conventional one in terms of error performance, as evidenced by theoretical analysis and computer simulations. The last contribution of this thesis is for UWB ranging systems. Apply the idea of the dimension diversity to UWB ranging system; we derive the multi-dimensional detector for UWB ranging application. The advantage of the proposed detector over correlation detector comes from its ability to capture multi-dimensional information from the distorted signal. Advantage of multi-dimensional detectors is illustrated by both theoretical analysis and simulation results through applying to real UWB data.