摘要
同步技术是通信系统的关键技术之一,对于脉冲无线电超宽带系统则更是如此。其一、宽带导致时间搜索域上的高分辨率,因而为捕获过程带来更大的搜索区域;另外极低占空比的窄脉冲可能引起多径,在超宽带接收机端可能会有多于一个的多径满足同步要求引起误判。其二、由于收发端存在不可避免的时钟频偏,将为接收系统引入显著的误码率。其三、现存的网络时钟同步算法不能满足分布式超宽带网络的同步精度和网络开销等要求。针对密集多径条件下的超宽带同步困难的问题,研究了超宽带同步技术,包括物理层捕获、频偏估计、网络层时钟同步和超宽带平台的同步算法验证等四个方面。
建立了脉冲无线电超宽带系统的数学模型,研究了脉冲无线电超宽带系统的接收模式。从理论上对常用的三种接收模式基于发送相关、基于能量检测和基于相关检测进行了深入的分析和比较。详细分析了衡量同步捕获性能的两个重要参数:平均捕获时间和虚警检测率,仿真对比了在IEEE提供的四种超宽带信道模型下不同接收模式的平均捕获时间和虚警检测率等同步性能指标。
提出了一种采用变步长搜索策略的快速捕获算法。传统的采用非连续跳跃式搜索的快速捕获算法主要是针对于捕获时间的改进,其跳跃式搜索引起较大的失捕获概率。针对这个问题,提出了一种改进失捕获概率和平均捕获时间的快速捕获算法,变步长算法(Variable Step Algorithm,VSA)。从理论上推导了平均捕获时间和失捕获概率的公式,为研究变步长捕获算法的同步性能提供了理论依据。仿真比较了该算法与经典快速捕获算法的同步性能,仿真表明,变步长算法具有跳跃式搜索的特点,因此能够将搜索区域大幅度的减小,有助于减小平均捕获时间。并且该方案能够改善跳跃式搜索算法的失捕获情况。
其次,提出了一种采用分数域辅助的频偏估计方法。鉴于频偏对超宽带系统的重要影响,提出了采用切普信号作为特殊的帧同步脉冲来估计系统频偏。通过对切普信号频偏特性的理论分析,证明频偏不会影响切普信号在分数域上收敛的特性,切普信号是非常适合作为频偏系统的帧同步脉冲。并且提出了一种全新的采用分数域辅助的频偏估计系统。该系统的核心是分数域辅助的频偏估计算法,算法包括时域和分数域两部分,通过在超宽带信号上叠加切普信号并且通过选择适当的分数域达到估计系统频偏的目的。在此系统的基础上,进一步提出了利用切普信号进行超宽带脉冲的捕获预测方案。对提出算法的频偏估计性能进行了仿真分析,其估计偏差接近最大似然的克拉美劳界。并且仿真比较了正常捕获和分数域捕获预测的同步性能,证明后者的捕获时间可以得到改善。
然后,提出了一种适合于超宽带分布式网络的时钟同步算法。通过分析超宽带分布式网络的物理层特点,得出现有的网络时钟同步算法无法满足同步精度、网络开销等要求的结论,提出了双线估计算法(Dual Line Estimation Algorithm,DLEA),该算法能够提供时钟频偏的估计,能够考虑多次时延测量的结果,并且是基于成对模式。双线估计法包括数据搜集和频偏、时偏估计两部分,最后根据频偏和时偏的估计结果调整本地时钟。仿真证明,双线估计法的时钟同步精度明显优于传统的时钟同步算法,并具有降低网络开销的优势。
最后,在基于FPGA的超宽带硬件平台上验证第3章提出的变步长捕获算法。首先介绍了超宽带硬件平台的设计原理,其主要的设计包括发送机、接收端和同步模块,着重介绍了各部分中FPGA的设计及仿真。然后验证了第三章提出的变步长算法的同步实现方案,同步的设计包括捕获和验证两部分。经过硬件的调试,采用变步长同步方案的超宽带平台误码率可以达到10-6数量级,具有较好的同步接收性能。
The utilization of smart synchronization technology is a very sensitive and important issue in Impulse Radio Ultra-Wideband (IR-UWB) systems for many reasons. Firstly, small resolution search is caused by broadband, resulting in the inclusion of larger search area during acquisition process. Also, the use of extremely narrow pulse with low duty cycle have inherent multi-path effect, i.e. more than one multi-path will satisfy the synchronization threshold, yielding misjudgment. Secondly, frequency offset between receiver and transmitter is omnipresent, unavoidable, and introduces significant BER in IR-UWB system. Thirdly, the existing network clock synchronization algorithms can not meet the requirements of synchronization accuracy and overload from the distributed network of IR-UWB. In order to solve synchronization problems in IR-UWB system under intensive multi-path conditions, synchronization in IR-UWB system is studied in depth, including acquisition at the physical layer, frequency offset problem, time synchronization at the network layer and hardware realization of IR-UWB platform.
A mathematical model of IR-UWB system is established, and different receiving modes for IR-UWB system are studied. Receiving modes based on transmitted reference, energy detection and correlation detection are theoretically analyzed and compared. Two important synchronization parameters, i.e., Mean Acquisition Time (MAT) and False Alarm Rate (FAR) are analyzed. In latter simulations, MAT and FAR for different receiving modes are compared in four IEEE _UWB channels.
Firstly, a rapid acquisition algorithm adopting variable step searching strategy is proposed. Classic rapid acquisition algorithms are targeted at improving MAT, without acquisition accuracy considerations. To address this issue, a novel rapid acquisition algorithm, called Variable Step Algorithm (VSA), aimed at improving acquisition accuracy and MAT at the same time, is proposed. Theoretical deduction of MAT and FAR formulae for VSA is performed which provide analytical grounds for VSA synchronization performance comparisons. Empirical algorithms and VSA are compared with respect to synchronization performance. Simulations reveal that VSA outperforms classic algorithms with a shorter acquisition time and more accurate precision.
Secondly, an anti-jitter timing method incorporating Fractional Fourier domain is proposed. Given the important role of frequency offset in IR-UWB system and its effect on frequency offset estimation in time domain, a new frequency offset estimation system adopting Fractional Fourier domain is proposed. The system kernel lies in frequency offset estimation algorithm, which includes both time domain and Fractional domain. The estimation algorithm adds chirp signals to UWB pulses and chooses the appropriate Fractional domain in order to achieve frequency offset estimation and acquisition forecast. A scheme for UWB acquisition forecasting in this system is also proposed and tested. In thorough simulative analysis, forecasted acquisition adopting Fractional Fourier domain is compared with regular acquisition in time domain, which confirmed that MAT and FAR can be improved by using proposed scheme.
Then, a novel clock synchronization algorithm suitable for IR-UWB distributed network is proposed. Through the analysis of ultra-wideband network physical layer characteristics, the existing network clock synchronization algorithm might not meet network overhead and accuracy requirements. Thus, a novel time synchronization algorithm providing clock frequency offset, considering several delay measurement results, and based on pair-wised model is proposed, referred to as Dual Line Estimation Algorithm (DLEA). The new algorithm is implemented in two steps, i.e., data collection and frequency offset estimation. Then, the local clock is to be adjusted according to the estimations. Simulations reveal that DLEA synchronization accuracy is superior to the traditional clock synchronization algorithm, and has the advantage of lower network cost.
Finally, we have verified the synchronization algorithm on the FPGA-based UWB hardware platform. Core design principles for UWB hardware platform are introduced; including the design of its main transmitter, receiver and synchronization module with due focus on FPGA design and simulation setup. Proposed VSA is also verified in FPGA, and the design includes acquisition and verification modules. After implementing the hardware debugger, error frame rate for UWB platform reaches the order of magnitude 10-6, with a good synchronization performance.
引文
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