Synchronization in impulse radio ultra-wideband communication systems

Ultra-wideband (UWB) communication has recently generated a significant amount of interest and is a primary physical layer candidate for wireless personal area network (WPAN) standards aimed at both high data rate and low data rate applications. Ultra-wideband spread spectrum systems employing impulse radio signaling are being considered for these applications. The UWB channel is a dense multipath channel with a large delay spread and highly resolvable multipath components, owing to the narrow pulses employed. In any communication system, the receiver requires knowledge of the timing information of the received signal to accomplish demodulation. The process of obtaining this timing information is known as synchronization. Spread spectrum systems typically achieve synchronization in two stages: acquisition and tracking. The acquisition stage attempts to achieve coarse synchronization to within a chip duration by evaluating the phases in the search space and the tracking stage employs a code tracking loop to maintain fine synchronization. Ultra-wideband systems employ long spreading sequences to eliminate spectral lines resulting from pulse repetition. Moreover, the short pulses employed result in a fine resolution of the search space. Due to these reasons, UWB acquisition systems face a large search space which results in a large amount of time for the system to successfully acquire the signal. This results in long preambles in a packet-based WPAN which adversely affects the network throughput. The dense resolvable multipath results in the existence of multiple phases which can be considered to be a good estimate of the true phase. In this dissertation, we define a hit set to be the set of phases which guarantee a nominal demodulation performance subsequent to acquisition. We focus on signal design and the development of efficient acquisition schemes which significantly reduce the mean detection time through search space reduction. One such design is the proposed hybrid direct sequence-time hopping (DS-TH) signaling format and a two-stage acquisition scheme which combats the large search space problem in impulse radio UWB systems. We study the acquisition problem in transmitted reference (TR) UWB systems with DS signaling and design a two-stage acquisition scheme which greatly reduces the mean detection time. We also consider the fine timing estimation problem in impulse radio systems in which the receiver does not have knowledge of the received pulse shapes or the channel. We derive the Cramer-Rao lower bound and maximum likelihood estimators for pilot-assisted and non-pilot-assisted cases, and compare their performance to sub-optimal methods with lower complexity.