脉冲超宽带短距传输关键问题研究
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摘要
脉冲超宽带无线电（IR-UWB）是未来无线个域网络（WPAN）短距离高速率通信和无线传感器网络（WSN）长距离低速率通信的重要技术解决方案。尽管具有低成本电路、许可执照频谱的复用、精确定位能力等潜力，IR-UWB技术还存在诸多的研究挑战。论文主要目的是找出并解决目前存在的挑战问题，为实际IR-UWB系统提供框架设计。
本论文着重给出了目前不同调制选取的IR-UWB系统的比特误码率性能（BER）、功率谱特征、系统复杂度和传输数据率等研究结果。现存IR-UWB系统主要采用伪随机噪声（PN）跳时（TH）码结合脉冲位置调制（PPM）方式的多址接入方式。近来提出一种采用非周期性混沌编码的方法可以移除发射信号的离散功率谱值，能够提高IR-UWB系统的扩频频谱特征而与其他设备如全球定位系统（GPS）间具有很低的干扰性，即伪混沌跳时（PCTH）调制方案，借助符号动力学概念产生输入数据依赖性的非周期扩频序列。由混沌序列马尔科夫链特征可知，经伪混沌卷积编码器（PCCE）运算编码后的输入数据实质上具有卷积码（CC）表现。编码器输出后产生的跳时序列使得脉冲间隔呈现随机分布特征，因此具有类噪声的功率谱特征。明显的扩频特征需要发射机大量级数处理，而接收机的卷积编码器也需要处理大量状态值。而PCTH信号仅需复杂度较低的维特比（Viterbi）解码器处理少量状态值，而CC编码并不具有这种可扩展性，为接收机提供一种灵活性设计。而PCTH调制方案扩展版的多址接入（MA）方式是为每用户提供唯一签名的脉冲链而替代单用户系统的单脉冲形式。本文着重讨论这类新颖具有伪混沌卷积驱动（PCC）类跳时（TH）码结合不同调制方式的IR-UWB系统通信、组网和定位应用，同时依据PCC TH码的最优卷积特征给出类似的CC TH码设计方法。
首先，本文给出IR-UWB信号频谱特征的数学随机尖峰原理，数学分析表明尖峰域的主要成因（例如，滤波、抖动、时延、细化、簇、采样和调制等）可描述为一系列模块化的数学运算。确切地说，IR-UWB信号功率谱可由基本模型添加特定特征而进行模块化计算。这种模块化方法可简化IR-UWB系统复杂模型或多径模型的功率谱计算。接下来，给出不同调制方式功率谱计算的基本模型，经过研究分析为TH-UWB系统提出一种新颖的脉冲幅度调制（PAM）和脉冲位置调制（PPM）的联合调制方式（PAM/PPM）。本文结果表明PAM/PPM TH-UWB系统可作为替代PPM TH-UWB系统的合适选择。从功率谱角度来说，PPM TH-UWB发射信号功率谱（PSD）具有一些尖峰值，已有分析结果表明这些尖峰严重影响系统BER性能，需要合理处理，而本文提出的PAM/PPM方式可以移除这些尖峰值并且能够提高系统整体性能。
具有高容量、高数据率、简单、有效功率控制、低成本和小巧等特点的IR-UWB接收机设计也是一项高挑战性的任务。全相关接收机，例如采用典型的Rake接收技术最优匹配滤波器的性能最优，而缺点是具有极高的复杂度和硬件实现难度。通常，相关接收机需要已知接收信号、信道和干扰因素的参数信息。值得注意的是，由于多径信道的多径分量巨大，而从接收多径分量中估计信道时延和相关系数是一项巨大耗时的工作。因此，接收机最好能够忽略这些计算。近期对于这种不需要上述参数信息的非相关接收机设计成为主要研究热点。非相关接收机设计主要包括三类：发射参考（TR）UWB接收机、能量检测器（ED）和差分检测器。这些方法的共同点在于不需要信道估计和接收脉冲估计。本论文着重研究以下几种IR-UWB系统接收机设计和性能分析。给出前述提出的发射机性能、容量、计算复杂性等实际设计需求权衡，进一步讨论上述系统接收机设计的信道因素、自干扰或互干扰、调制方式等。具体包括：第一，给出不同调制方式和PCC跳时信道编码结合系统的Rake接收机的最优匹配滤波实现以及三种抽头选取下Rake接收机的性能分析。第二，考虑非相关接收的TR-UWB接收机，本文研究了TR-UWB与时间反转（TR）方法相结合的新颖定位系统，命名为(TR)2-UWB定位系统，通过研究结果表明，接收端的时空能量聚焦特性带来接收机性能的提升。第三，参考简单低成本的非相关接收机形式即ED接收机，主要利用接收信号的接收能量作为比特判决的设计方法，本文首次提出在能量检测器前加入基于高阶统计量（HOS）的盲检测机制来提高直接混沌通信（DCC）UWB系统性能的新颖方法。
IR-UWB系统的多址信道接入主要依据PN码码分多址（CDMA）方式来减小多址干扰（MAI）。因此，合理的TH码设计不仅能够抑制多径干扰（MPI）和MAI干扰，还能够带来良好的功率谱形状。早期的直接扩频（DS）和跳频（FH）多址接入序列设计可修正并延伸到TH-IR组网应用，例如，线性同余码（LCCs）,双曲同余码（HCCs），置换序列等。基于PCC码设计原理，本文着重研究可适用于IR-UWB系统的卷积类CC码设计方法，研究结果表明这种具有最大自由距离设计的卷积类（CC）TH码可减小由MPI和MAI引起的脉冲碰撞。本文研究了一种利用PCC跳时信道编码和联合调制方式结合的改进多址接入方案受加性高斯白噪声（AWGN）、多径干扰（MPI）、窄带干扰（NBI）和多址干扰（MAI）影响的性能分析，进一步给出传统PPM TH-UWB系统和改进多址系统的闭式表达以及性能对比结果。
由于IR-UWB技术的低功耗和低数据率（LDR）特性适用于短距传输组网应用，如IEEE802.15.4a任务小组标准。根据802.15.4a标准的需求，媒体接入控制（MAC）层设计需要考虑这种具有新特点的物理层（PHY）设计。根据特定情况的LDR UWB组网需求，现有基于ALOHA协议的(UWB)2MAC协议能够保证抗MAI鲁棒性的理想多址接入方式。(UWB)2MAC协议主要依赖TH码作为公用信道和数据信道码分配方式，而系统的抗MAI鲁棒性取决于TH码的互相关性，采用合理的码设计将减小脉冲间碰撞，因此，TH码码分方案将影响到相同区域的分布式组网性能。因此，本文接着给出一种基于分组碰撞多址模型的TH-UWB系统物理层设计方法，提出利用PCC TH码和联合调制结合的改进系统来减小脉冲碰撞以及提高组网性能的新方法。
IR-UWB系统定位应用需要高精度检测接收信号的路径边缘，而目前的基于到达角度（DOA）的空间谱估计定位方法并不是最优的。最近，将物理声学的时间反转技术应用于无线通信领域定位应用已经成为热点问题，由于时间反转技术具有时间和空间聚焦特性，因此将时间反转方法与IR-UWB系统相结合不仅能够提高系统的性能还可以在空间聚焦定位成像，并且时间反转技术与DOA空间谱估计相结合能够在恶劣环境下提高对主动或者被动（电磁）EM目标的定位精度，最后，本文给出这种新颖的(TR)2-UWB系统的定位成像性能分析，仿真结果表明利用IR-UWB宽带信号源与TR技术相结合进一步提高了TR算法的定位成像性能。
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.
本论文着重给出了目前不同调制选取的IR-UWB系统的比特误码率性能（BER）、功率谱特征、系统复杂度和传输数据率等研究结果。现存IR-UWB系统主要采用伪随机噪声（PN）跳时（TH）码结合脉冲位置调制（PPM）方式的多址接入方式。近来提出一种采用非周期性混沌编码的方法可以移除发射信号的离散功率谱值，能够提高IR-UWB系统的扩频频谱特征而与其他设备如全球定位系统（GPS）间具有很低的干扰性，即伪混沌跳时（PCTH）调制方案，借助符号动力学概念产生输入数据依赖性的非周期扩频序列。由混沌序列马尔科夫链特征可知，经伪混沌卷积编码器（PCCE）运算编码后的输入数据实质上具有卷积码（CC）表现。编码器输出后产生的跳时序列使得脉冲间隔呈现随机分布特征，因此具有类噪声的功率谱特征。明显的扩频特征需要发射机大量级数处理，而接收机的卷积编码器也需要处理大量状态值。而PCTH信号仅需复杂度较低的维特比（Viterbi）解码器处理少量状态值，而CC编码并不具有这种可扩展性，为接收机提供一种灵活性设计。而PCTH调制方案扩展版的多址接入（MA）方式是为每用户提供唯一签名的脉冲链而替代单用户系统的单脉冲形式。本文着重讨论这类新颖具有伪混沌卷积驱动（PCC）类跳时（TH）码结合不同调制方式的IR-UWB系统通信、组网和定位应用，同时依据PCC TH码的最优卷积特征给出类似的CC TH码设计方法。
首先，本文给出IR-UWB信号频谱特征的数学随机尖峰原理，数学分析表明尖峰域的主要成因（例如，滤波、抖动、时延、细化、簇、采样和调制等）可描述为一系列模块化的数学运算。确切地说，IR-UWB信号功率谱可由基本模型添加特定特征而进行模块化计算。这种模块化方法可简化IR-UWB系统复杂模型或多径模型的功率谱计算。接下来，给出不同调制方式功率谱计算的基本模型，经过研究分析为TH-UWB系统提出一种新颖的脉冲幅度调制（PAM）和脉冲位置调制（PPM）的联合调制方式（PAM/PPM）。本文结果表明PAM/PPM TH-UWB系统可作为替代PPM TH-UWB系统的合适选择。从功率谱角度来说，PPM TH-UWB发射信号功率谱（PSD）具有一些尖峰值，已有分析结果表明这些尖峰严重影响系统BER性能，需要合理处理，而本文提出的PAM/PPM方式可以移除这些尖峰值并且能够提高系统整体性能。
具有高容量、高数据率、简单、有效功率控制、低成本和小巧等特点的IR-UWB接收机设计也是一项高挑战性的任务。全相关接收机，例如采用典型的Rake接收技术最优匹配滤波器的性能最优，而缺点是具有极高的复杂度和硬件实现难度。通常，相关接收机需要已知接收信号、信道和干扰因素的参数信息。值得注意的是，由于多径信道的多径分量巨大，而从接收多径分量中估计信道时延和相关系数是一项巨大耗时的工作。因此，接收机最好能够忽略这些计算。近期对于这种不需要上述参数信息的非相关接收机设计成为主要研究热点。非相关接收机设计主要包括三类：发射参考（TR）UWB接收机、能量检测器（ED）和差分检测器。这些方法的共同点在于不需要信道估计和接收脉冲估计。本论文着重研究以下几种IR-UWB系统接收机设计和性能分析。给出前述提出的发射机性能、容量、计算复杂性等实际设计需求权衡，进一步讨论上述系统接收机设计的信道因素、自干扰或互干扰、调制方式等。具体包括：第一，给出不同调制方式和PCC跳时信道编码结合系统的Rake接收机的最优匹配滤波实现以及三种抽头选取下Rake接收机的性能分析。第二，考虑非相关接收的TR-UWB接收机，本文研究了TR-UWB与时间反转（TR）方法相结合的新颖定位系统，命名为(TR)2-UWB定位系统，通过研究结果表明，接收端的时空能量聚焦特性带来接收机性能的提升。第三，参考简单低成本的非相关接收机形式即ED接收机，主要利用接收信号的接收能量作为比特判决的设计方法，本文首次提出在能量检测器前加入基于高阶统计量（HOS）的盲检测机制来提高直接混沌通信（DCC）UWB系统性能的新颖方法。
IR-UWB系统的多址信道接入主要依据PN码码分多址（CDMA）方式来减小多址干扰（MAI）。因此，合理的TH码设计不仅能够抑制多径干扰（MPI）和MAI干扰，还能够带来良好的功率谱形状。早期的直接扩频（DS）和跳频（FH）多址接入序列设计可修正并延伸到TH-IR组网应用，例如，线性同余码（LCCs）,双曲同余码（HCCs），置换序列等。基于PCC码设计原理，本文着重研究可适用于IR-UWB系统的卷积类CC码设计方法，研究结果表明这种具有最大自由距离设计的卷积类（CC）TH码可减小由MPI和MAI引起的脉冲碰撞。本文研究了一种利用PCC跳时信道编码和联合调制方式结合的改进多址接入方案受加性高斯白噪声（AWGN）、多径干扰（MPI）、窄带干扰（NBI）和多址干扰（MAI）影响的性能分析，进一步给出传统PPM TH-UWB系统和改进多址系统的闭式表达以及性能对比结果。
由于IR-UWB技术的低功耗和低数据率（LDR）特性适用于短距传输组网应用，如IEEE802.15.4a任务小组标准。根据802.15.4a标准的需求，媒体接入控制（MAC）层设计需要考虑这种具有新特点的物理层（PHY）设计。根据特定情况的LDR UWB组网需求，现有基于ALOHA协议的(UWB)2MAC协议能够保证抗MAI鲁棒性的理想多址接入方式。(UWB)2MAC协议主要依赖TH码作为公用信道和数据信道码分配方式，而系统的抗MAI鲁棒性取决于TH码的互相关性，采用合理的码设计将减小脉冲间碰撞，因此，TH码码分方案将影响到相同区域的分布式组网性能。因此，本文接着给出一种基于分组碰撞多址模型的TH-UWB系统物理层设计方法，提出利用PCC TH码和联合调制结合的改进系统来减小脉冲碰撞以及提高组网性能的新方法。
IR-UWB系统定位应用需要高精度检测接收信号的路径边缘，而目前的基于到达角度（DOA）的空间谱估计定位方法并不是最优的。最近，将物理声学的时间反转技术应用于无线通信领域定位应用已经成为热点问题，由于时间反转技术具有时间和空间聚焦特性，因此将时间反转方法与IR-UWB系统相结合不仅能够提高系统的性能还可以在空间聚焦定位成像，并且时间反转技术与DOA空间谱估计相结合能够在恶劣环境下提高对主动或者被动（电磁）EM目标的定位精度，最后，本文给出这种新颖的(TR)2-UWB系统的定位成像性能分析，仿真结果表明利用IR-UWB宽带信号源与TR技术相结合进一步提高了TR算法的定位成像性能。
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.
引文
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