【作者】 乔永伟；

【导师】 吕铁军；

【作者基本信息】 北京邮电大学 ， 信号与信息处理， 2009， 博士

【摘要】 超宽带(UWB,Ultra-Wideband)脉冲无线电是一种可以和其它通信系统共享频谱资源的新型无载波通信技术。为了实现频谱共享,UWB系统通常采用极低功率谱密度的超短脉冲作为信息的载体;为了实现信息的有效解调,UWB系统通常采用多次重传的方式来提高接收信噪比。考虑到多址接入及FCC(Federal CommunicationsCommission,联邦通信委员会)频谱掩模的限制,UWB系统为每一用户分配一个唯一的伪随机序列,利用这些伪随机序列来调制用户的数据信息,实现多用户通信并平滑发射信号的频谱。UWB系统具有隐蔽性好、抗多径和窄带干扰能力强、传输速率高、系统容量大、功耗低、可以和其它通信系统共享频谱等一系列优点。然而利用UWB技术也面临着一些严峻的挑战,其中定时同步及信号解调是制约UWB技术发展的两项关键技术。定时同步是实现信号能量捕获的前提,可以说没有定时同步,信号解调就无从谈起。由于UWB系统中承载信息的是占空比极低的超短脉冲,且该系统常工作在稠密多径环境中,所以实现精确的定时同步是一项艰巨的任务。传统的RAKE接收技术需要估计信道信息且对定时误差异常敏感(纳秒量级的定时抖动就会引起接收机性能的急剧下降),在UWB通信系统中应用RAKE技术面临着复杂度高、抗定时抖动性差的尴尬局面。针对这些问题,本论文对UWB系统的同步及对应的解调算法进行了较深入的理论研究,提出了两种低复杂度的UWB同步算法和三种能有效对抗定时抖动的、不需要信道估计的UWB同步解调联合算法:1、基于DS(Direct Sequence,直接序列)码匹配滤波的无数据辅助UWB同步算法:将观测信号和其一个帧周期的延迟信号相乘,利用帧长度的积分清零器(I&D,Integrate-and-Dump)对相乘后的信号进行积分、采样。将离散采样值通过DS码匹配滤波器即可建立用于搜索同步参数的目标函数。DS码的伪随机性,使得目标函数类似冲激函数,且同步时刻对应目标函数最大值的出现时刻。由于UWB通信网络中各用户的DS码是相互独立的(具有近似正交性),多用户干扰(MUI,Multiple User Interference)通过DS码匹配滤波器后不会产生明显的峰值,所以本同步算法可以直接应用到多用户通信环境中。2、非搜索UWB同步算法:现有的UWB同步算法大都是先使用某种方法构建目标函数,然后搜索该目标函数的最大(小)值,用最大(小)值的下标刻画同步参数。由于目标函数值的获得通常需要大量繁琐的计算,所以避免搜索过程是降低UWB同步算法复杂度的有效途径。基于这一考虑,本文提出了一种基于巴克码的非搜索UWB同步算法。在算法实现中,首先在发送端发送周期延拓的4位巴克码序列,在接收端对观测信号进行延迟、相关、采样等操作得到一系列离散样本值,然后利用这些样本值和最小二乘准则计算出符号头部能量与符号尾部能量,最后利用这两个能量值得到同步参数的闭式解。3、SR-UWB系统的同步及解调算法:RAKE解调技术需要先通过信道估计建立解调模板,然后将解调模板和接收信号作相关得到判决变量,最后利用判决变量检测出发送信息。由于UWB信道通常是密集多径的,所以精确估计UWB信道具有极大的计算复杂度。另外,UWB信号的持续时间短、占空比低的特点使得RAKE接收机对定时误差异常敏感,纳秒量级的定时抖动就会引起接收机性能的急剧下降。为避开复杂的信道估计过程及增加UWB接收机的抗定时抖动性,本文提出了两种适用于SR-UWB(Single Reference UWB,单参考UWB)系统的、不需要信道估计的、能有效对抗定时抖动的同步解调联合算法。算法一先通过重叠相加、能量检测等方法建立目标函数,然后搜索目标函数的最大值,用最大值的下标表示同步参数,并顺带构造出解调模板,最后利用这一解调模板和同步参数估计值及TH(Time-Hopping,跳时)码的先验信息构造判决变量,将判变量通过符号检测器实现信息解调。算法二则更加充分地利用了SR-UWB系统独特的信号结构和TH码的先验信息。在实际实现中,首先利用I&D器件得到一系列离散样本值,通过对这些样本值的平方、平均操作建立目标函数,搜索目标函数的最大值位置即可获得同步参数,再利用同步参数选择I&D器件的输出样本值,将所选的样本值通过符号检测器即可实现信息解调。由于算法二不需要另外构造解调模板,且在同步阶段就利用了TH码的先验信息,所以算法二的性能要优于算法一。4、DTR-UWB系统的同步解调联合算法:为了在接收端避免信道估计过程,研究者们提出了TR-UWB(Transmitted Reference UWB,传输参考UWB)系统。该系统通过发送不承载信息的参考脉冲,利用参考脉冲的冲激响应作为信息脉冲的解调模板的方式,避免了信道估计过程。显然,参考脉冲的发送势必会降低通信速率和能量利用率。针对这一缺陷,人们又提出了DTR-UWB(Differential TransmittedReference UWB,差分传输参考UWB)系统。本文针对DTR-UWB系统设计了一种同步解调联合算法。通过对观测信号的加权平均及延迟相关操作得到一系列离散帧率(采样间隔为一个帧周期)的采样值,利用这些样本值可同时实现同步捕获和信息解调。加权平均操作提高了接收信噪比,从而提升了同步解调性能;帧率采样及采样值的重复利用有效地降低了系统复杂度。5、基于巴克码的同步解调联合算法:利用4位巴克码特有的自相关特性,本文提出了冗余模板(RDT)解调算法和非冗余模板(NRDT)同步解调联合算法。这两种算法都是直接从观测信号中提取解调模板,然后和接收信号作相关,利用符号检测器实现信息解调。虽然RDT算法不需要同步过程,具用极低的复杂度,但是RDT中存在不含有用信息的噪声段,该段的存在影响了接收机的性能。为了剔除冗余噪声段,本文又在RDT算法的基础上提出了NRDT算法。该算法首先利用能量检测的方法获得同步参数,然后利用同步参数剔除RDT中的噪声段。由于NRDT算法剔除了冗余段,所以和RDT方案相比,NRDT方案有更低的误码率。由于这两种方案的解调模板是从接收信号中提取的,在定时误差出现的情况下,仍然能捕获到大部分的信号能量,因而它们都有很强的抗定时抖动性。理论分析及计算机仿真都证明本文所提同步及解调算法比同类算法具有更优的性能。更多还原

【Abstract】 Ultra-wideband impulse radio (UWB-IR) is a promising technology for data communications in short-range indoor applications. It is characterized by the transmission of ultra-short pulses (in the sub-nanosecond scale) at low power spectral density. In order to keep the signal spectrum within the Federal Communications Commission (FCC) mask and obtain adequate signal energy for reliable detection, a single information symbol is represented by several pulses, each located in its own frame. To accommodate multiple-access and smooth transmit spectrum, pseudo random timing-hoping (TH) spread codes or/and direct-sequence (DS) spread codes are often employed. Interest in UWB is motivated by several features including: ample multipath diversity, low-complexity baseband transceivers, a potentially large user capacity, and potential to overlay existing narrowband systems. To harness these benefits, however, UWB-IR faces several challenges, among which synchronization and demodulation are particularly critical at the receiver end.Synchronization is a performance-critical factor in most communication systems, especially in UWB transmission. It stems from the fact that the transmitted pulses are very narrow and have low power. The dense multipath channel, through which the pulses bearing information propagate, is unknown at the receiver during the synchronization phase. Moreover, the bit error rate (BER) of UWB is very sensitivity to mistiming, the timing errors as small as fractions of a nanosecond can seriously degrade the system performance. To cope with these challenges, a number of synchronization and demodulation algorithms have been proposed in this dissertation. Summarized as follows:Relying on the prior knowledge of the DS codes, a non-data aided synchronization algorithm is proposed for DS-UWB systems. It remains operational in practical UWB settings with unknown multipath propagation. With the aid of the integrate-and-dump (I&D) over a frame duration and peak-picking from the output of the DS codes matching filter, the proposed approach can achieve frame-level timing acquisition within one symbol duration. The maximum significantly differs from other values due to the pseudo random property of the DS codes. Therefore, the proposed scheme has high immunity to noise effects.A data-aided synchronization scheme dispensing with searching procedure is proposed in this dissertation. With the help of judicious training symbols and symbol-level samples, the energy values of the truncated tail and head of the received symbol waveform can be obtained. With the two energy values in hand, frame-level timing acquisition can be acquired in closed form. It is worth noting that with the perfect knowledge of the multi-path channel, the proposed scheme can achieve timing synchronization at any desirable resolution. The proposed algorithm only exploits symbol-level samples and without searching process, thus it turns out to not only speed up synchronization, but also enjoys low-complexity.Regarding demodulation, RAKE is the first receiver considered for UWB systems. Unfortunately, this classical approach will cause high re- ceiver complexity since a large number of fingers are required to estimate. Furthermor, RAKE is very sensitivity to mistiming, the timing errors as small as fractions of a nanosecond can significantly degrade the system performance. To bypass channel estimation and enhance the robustness to mistiming, the single reference UWB (SR-UWB) system is proposed. Relying on the unique signal structure of SR-UWB, two joint synchronization and demodulation algorithms are proposed without employing any training sequences (non-data aided). With the aid of overlap-add method and energy detection technique, the first approach can acquired timing acquisition even if in the presence of inter-frame interference (IFI) and inter-symbol interference (ISI). With the synchronization parameter being available, the demodulation template can be easily constructed. Taking advantage of the synchronization parameter and the demodulation template, the decision statistic, which is then passed through a sign detector to result in the symbol estimates, can be attained by using of the correlation operation. In the second algorithm, in terms of the unique signal structure and the prior knowledge of the TH codes, the objective function can be established over a short observation interval. Peak-picking the objective function, the synchronization parameter can be acquired. After timing acquisition, the samples used to achieve synchronization can be re-utilized for the demodulation process by a zero-crossing comparator. As a result, the implementation complexity is markedly reduced in comparison with the first approach.Based on the prior knowledge of the DS codes, a joint blind synchronization and demodulation scheme is developed for differential transmitted reference UWB (DTR-UWB) systems. The proposed approach can achieve frame-level synchronization with the help of frame-rate samples. Taking advantage of the periodicity of the DS spread codes, the frame-level synchronization can be carried out even in one symbol interval. On the other hand, after timing acquisition, these frame-rate samples can be re-utilized also for the demodulation by a zero-crossing comparator. Thus the acquisition time and the implementation complexity are reduced considerably. Meanwhile, the noise cross noise effect is alleviated because a wide-sense average operation has been executed in synchronization phase, which is responsible for the enhancement of the detection performance. It is also worth noting that the proposed approach can be employed in multi-user scenarios without any modification.By the special auto-correlation pattern of the proposed training sequence (4 bits Baker codes), a redundance-included demodulation template (RDT) is constructed from the received signal. When the RDT is available, two approaches can be selected to detect transmitted information symbols. One is to demodulate transmitted symbols directly by the RDT, which does not require timing acquisition, and thereby has considerably lower complexity. The alternative is to first carry out timing acquisition by invoking simple energy detection, which is employed to shear the RDT to obtain a non-redundance-included demodulation template (NRDT), and then demodulate transmitted symbols by the NRDT. Although the second approach has higher complexity (depending on the level of timing error desired), it can obtain better BER performance compared with the first one owing to canceling the redundant noise. Furthermore, the demodulation template is derived from the received signal, so both approaches can avoid the effect of pulse distortions and cope with the timing error.Simulations and theoretical analyses validate that the proposed approaches which have been stated above enjoy better performances in comparison with the similar ones in terms of acquisition probability, normalized mean square error (NMSE) and BER.更多还原