Some Key Techniques Researches On The UWB-OFDM Systems

The ever-growing demand for higher quality media and faster content delivery where we work, live, and play drives the quest for higher data rates in wireless communication networks. UWB technology is uniquely qualified to address that requirement and was designed specifically to support high speed, short range wireless communications. When compared to other existing and nascent technologies capable of providing wireless connectivity, the performance benefits of UWB are compelling, UWB is recognized as the most potential physical layer technology with very high bit-rate, low-cost, low-power. Under the auspices of the MultiBand OFDM Alliance (MBOA), nearly all prominent providers of silicon and end products for consumer electronics, personal computers and mobile devices have endorsed an evolutionary approach called MultiBand Orthogonal Frequency Division Multiplexing (MB-OFDM, also UWB-OFDM) as the optimal UWB solution. UWB-OFDM has demonstrated several technical advantages, the system combines OFDM modulation technique with a multibanding approach, which divides the spectrum into several sub-bands, whose bandwidth is approximately 500 MHz. At present, the application of UWB-OFDM in short distance high speed wireless connections is still a new subject, a lot of theoretical problems and key techniques about UWB-OFDM should be studied in detail.This dissertation provides an overview of UWB-OFDM technology as the de facto standard of IEEE 802.15.3a High Rate WPAN, as well as some relative comparisons with DS-UWB technology. Some key techniques about UWB-OFDM are discussed in this dissertation including the UWB-OFDM channel model, blind symbol-timing synchronization, and multi-rate UWB-OFDM systems design. There are two general ways to use the bandwidth available for UWB. Impulse Radio was the original approach to UWB. It involves the use of very short-duration baseband pulses that use a bandwidth of several Gigahertz. Data could be modulated using either pulse amplitude modulation (PAM) or pulse-position modulation (PPM). A more recent approach to UWB is a multibanded system where the UWB frequency band from 3.1 - 10.6 GHz is divided into several sub-bands. Each of these sub-bands has a bandwidth greater than 500MHz with an OFDM modulation method. In chapter 2, the basic principle and architecture of UWB-OFDM systems are introduced, some relative comparisons with DS-UWB technology are provided, including frequency band allocation, range of system, receiver complexity, multi-path energy capture, flexibility in coexisting with other wireless systems, support of industries, and so on. UWB-OFDM technology is more fit for high rate WPAN, and DS-UWB technology is more fit for low rate WPAN.The goal of UWB channel modeling is to understand the characteristics of the propagation channel where UWB devices are operating in order to help implement an efficient high data rate communications system. UWB channel model must balance the trade-off between the need for a simple model and the accuracy of the model. In chapter 3, the propagation characteristics of UWB channels are summarized and typical channel models are introduced based on the work of pioneer researchers. In order to get actual channel models matched with specific system bandwidth where UWB devices are operating, the discretization issue of IEEE802.15.3a multipath channel models is investigated, and the misuse of this channel model is analysed, then two more reasonable design methods for the BP UWB channel are put forward based on the sampling rate conversion theory, which keep the intrinsic essence of the UWB channel in different frequency bands. The proposed methods are used in the design on the UWB-OFDM channel, the statistical characteristics of the UWB-OFDM channel are given at last. Final computer results demonstrate that the new methods preserve the essence of the original UWB channel in corresponding band at most. The proposed BP UWB channel design methods, together with baseband UWB channel design method provided by Intel, form whole UWB channel design methods.In chapter 4, a robust blind symbol-timing synchronization scheme applicable to UWB-OFDM systems is presented. The proposed method is based on ZP(Zero-padded Prefix) power collection in the UWB-OFDM symbols whose ZP average power distribute regularly in the slide windows. The synchronization schemes in Gaussian channel, Rayleigh channel and UWB channel are investigated. Average power distribution of the received signal in Rayleigh channel and UWB channel is analysed detailedly in order that symbol synchronizes accurately. A multi-slide-windows method is designed in order to improve the performance of synchronization. Finally computer results are also demonstrate the superior performance of the new method in both Gaussian channel, Rayleigh channel, and UWB channel.The emission power spectrum density of UWB is strictly limited by America Federal Communication Committee (≤-41.3dBm/MHz). It is a big challenge for improving the effective emission power under the limitation of FCC. In chapter 5, a kind of improvement project for UWB-OFDM systems is put forward, which supports multirate services without changing the existing IEEE802.15.3a standard and system complexity . Accordingly, two multirate schemes are designed, and corresponding multirate detection methods are given, which rely on the fact that the data source is statistically independent and the repeated data blocks have excellent relativity. Data multirate design improves the ability of UWB-OFDM system to support various rate services. For UWB-OFDM systems, it is comprehensive that sacrificing sufficient band to exchange the benefits of lowering BER, or lowering power, or enlarging range of systems.