Research On Key Issues Of Impulse Radio Ultra-wideband For Short Range Transmission

Impulse Radio Ultra-wideband (IR-UWB) is one of the promising technologies for future short-range highdata rate communications (e.g. for wireless personal area networks) and longer range low data communications(e.g. wireless sensor networks). Despite its various advantages and potentials (e.g. low-cost circuitry, unlicensedreuse of licensed spectrum, precision ranging capability etc.), IR-UWB also has its own challenges. The goal ofthis dissertation is to identify and address some of these challenges, and provide a framework for practicalIR-UWB system design.In this dissertation, various modulation options for IR-UWB systems are reviewed in terms of their bit errorrate (BER) performances, spectral characteristics, hardware complexities, and data rates. Existing IR-UWBsystems employ Pseudo random Noise (PN) time hopping for multiple access purpose, combined with pulseposition modulation (PPM) for encoding the digital information. Recently, it has been suggested to use aperiodic(chaotic) codes in order to enhance the spreading-spectrum characteristics of IR-UWB system by removing thespectral features of the transmitted signal, thus resulting in a low probability of intercept. In addition, the absenceof spectral lines may translate into a reduced interference towards other services such as GPS (GlobalPositioning System). Pseudo-chaotic time hopping (PCTH), a modulation scheme for IR-UWB system exploitsconcepts from symbolic dynamics to generate aperiodic spreading sequences that, in contrast to fixed (periodic)PN sequences, depending on the input data. The Pseudo-chaotic encoder (PCCE) operates on the input data in away that resembles as a convolution code（CC）. Its output is then used to generate the time hopping sequenceresulting in a random distribution of the inter-pulse intervals, and thus a noise-like spectrum. Significantspreading demands a large number of levels in the transmitter. This, in general, would require at the receiver aconvolutional decoder with a large number of states. PCTH signal can be decoded with a Viterbi detector ofreduced complexity with a limited number of states. Moreover, detectors of different complexity (andperformance) may coexist while decoding the same transmitted PCTH signal. This scalability property, which isnot presented in conventional convolutional coding, adds flexibility in terms of the receiver design. Thisdissertation firstly presents a novel kind of time hopping code designs introduced by the Pseudo ChaoticConvolutional (PCC) code assignment technique combined with various modulation options for IR-UWBsystems communication and applications (networking and positioning), also develops to a class of CC TH codedesigns by studying the optimal convolutional characteristics of the PCC TH code.The first concern of this dissertation introduces the spectrum characteristics by random spike fields ofIR-UWB signals describe as the result of various operations on the basic event stream or spike field, such asfiltering, jittering, delaying, thinning, clustering, sampling and modulating. More precisely, IR-UWB signals areobtained in a modular way by adding specific features to a basic model. This modular approach greatly simplifies the computations and allows treating highly complex model such as the ones occurring in IR-UWB ormultipath transmissions. A new modulation scheme PAM/PPM has been proposed for TH IR-UWB system. Itwas shown that a PAM/PPM TH-UWB system is an attractive alternative to PPM TH-UWB systems. In thisdissertation, it is shown that some spikes exist in power spectral density (PSD) of PPM TH-UWB transmittedsignal. Analytical and numerical simulation results show that spikes severely degrade PPM TH-UWB systemBER performance and must be properly removed. The proposed novel PAM/PPM scheme could remove thespikes from the PSD of PPM TH-UWB transmitted signal and yields better performance.High-capacity, high-data-rate, simple, power-efficient, low-cost, and small UWB receiver designs are achallenging task. Fully coherent receivers like optimal matched filtering, typically employed by Rake reception,perform well but at the expense of extremely high computational and hardware complexity. In general, acoherent receiver requires several parameters (side information) concerned with the received signal, radiochannel, and interference characteristics. Note that, in multipath channel, the number of multipath components isvery large (can be a few hundred). Therefore, estimating the delays and coefficients from the received multipathcomponents is an extremely challenging task. Therefore, receivers that relax these would also be preferable.Noncoherent (or lightly coherent) receiver designs in UWB relax the amount of information that needs to beestimated accurately for the detection of the transmitted bits. Some of the noncoherent receiver designs includingtransmitted reference (TR) based UWB, energy detector, and differential detector. Common to all theseapproaches is that the channel estimation and received pulse estimation are not necessary. In this dissertation,several receiver designs for IR-UWB signals will be studied. As mentioned above, the proposed transceiversrequirements and the related trade-offs like performance, capacity, and computational complexity regardingpractical designs will be discussed. The impact of IR-UWB radio channel, self-and other user-interference andmodulation options on the receiver design will be explained. First, an optimal matched filtering and itsimplementation using Rake receivers will be provided to the various modulation options for PCTH UWBsystems. Then, various simpler and less coherent versions of the Rake reception that trade off performance forcomplexity will be overviewed. Another popular receiver that employs reference pulses associated with thetransmitted data pulses, referred to as TR-based UWB, will be examined for the Time Reversal (TR) positioningIR-UWB system, named as (TR)2-UWB system. It will be seen that the TR scheme has some similarities withthe Rake reception, and these common points will be exploited to unify these approaches. Techniques to enhancethe performance of the TR scheme and TR positioning implementations will be discussed. Finally, receivers thatonly receive the energy of the received signal over transmission intervals and subsequently make decisions basedon the received energy (energy detector) will be studied. The energy detector (ED) will be combined with DirectChaotic Communication (DCC) UWB system and possible improvements of the detection performance byintroducing a blind detection mechanism based on High Order Statistics (HOS) will be explained.Multiple accessing for IR-UWB makes it possible to accommodate multiple users on the same UWBchannel. In order to reduce multiple access interference (MAI) pseudo random codes are often used as in code division multiple access (CDMA) systems. Careful design of the TH codes minimizes the effects of multipathinterference (MPI) and MAI, controls the power spectrum for good coexistence with other technologies. Earlierresearch on DS and FH multiple access sequence design can be applied to TH-IR networks, with somemodifications. The methods of implementation of FH sequences (such as linear congruence codes (LCCs),hyperbolic congruence codes (HCCs), permutation sequences) to TH-UWB were already presented. A kind ofCC TH code design already presented for IR-UWB systems is presented, which yields bounded pulse collisionsand can be successfully applied to IR-UWB. One-coincidence CC codes construction that maximized freedistance between adjacent symbols has been proposed and a similar approach can be considered for TH-UWB tominimize the inter-pulse interference from the user’s own pulses due to multipath. The performance of theproposed multi-access scheme is examined with numerical simulations in the presence of the proposedmulti-access scheme is examined with numerical simulations in the presence of AWGN, multipath interference(MPI), narrow-band interference (NBI), and multi-access interference (MAI) and furthermore closed formrepresentations for the probability of error of both proposed and conventional multi-access PPM TH-UWBschemes are given.Recently, there has been a growing interest in the application of the IR-UWB technology to low-power,low-data-rate (LDR) networks, like in sensor networks, as witnessed by the creation of the IEEE802.15.4a TaskGroup. As a consequence, the MAC will need to be redesigned in order to satisfy the new requirement byefficiently using the features of the alternative PHY. The results of the above analysis formed the basis for thedefinition of a MAC protocol suitable for IR-UWB systems, which is specifically designed for the special caseof LDR UWB networks, thanks to the MUI robustness guaranteed by impulse radio, the (UWB)2MAC protocolbased on ALOHA protocol is a suitable solution for LDR UWB networks.(UWB)2takes advantage of datatransmissions of the multiple access capabilities warranted by the TH codes, and relies for access to the commonchannel on the high MAI robustness provided by the processing gain of UWB. Since the robustness of thesystem to MAI is determined by the cross correlation properties of the codes. The effect of code collisions can bemitigated by adopting appropriate code selection protocols. The task of assignment codes to differenttransmitters in the same coverage area is a challenging issue in the design of distributed networks.Direction of arrival (DOA) estimation and ranging using IR-UWB systems require accurate detection of theleading edge path of a received signal, which may not be the strongest. Time Reversal (TR) has beensuccessfully used for many years, mostly in acoustics. The idea of applying TR in wireless communications hasrecently gained much attention because of its properties related to temporal and spatial focusing. Combining TRand IR-UWB has proved to improve performance thanks to increased capacity of energy collection at thereceiver. Moving beyond acoustics, TR has also been recently proposed as an enhancement for DOA estimationalgorithms in solving the problem of locating active vs. passive EM targets in potentially harsh propagationenvironments. Base on the evident that TR improves positioning accuracy in presence of nonhomogeneouspropagation media, this dissertation investigated the possibility of introducing TR in the design of an IR-UWB DOA positioning system. The resulting positioning accuracy is analyzed for frequency selective propagationmedia.