【作者】 张丙峰；

【导师】 高振明；

【作者基本信息】 山东大学 ， 通信与信息系统， 2008， 博士

【摘要】 为实现未来宽带无线信道下的高速率的可靠传输,多载波调制技术因其良好的抗多径能力和克服码间干扰(ISI)的能力而获得了深入研究和广泛应用。以正交频分复用(OFDM)为代表的多载波调制技术已经被多个标准采用,并作为第四代宽带移动通信(4G)系统的备选关键技术之一。然而,由于采用了子信道重叠的信道分割技术,OFDM在提高了频谱利用率的同时也存在一些内在缺点,如它的正交性在传输中易受破坏,引起了符号干扰和子信道间干扰,降低了系统性能,从而需要大量的开销(循环前缀(CP)和虚载波等)、较为严格的载波与定时同步等来保证系统性能。针对OFDM的这些问题,本文研究了一种新的多载波调制技术——滤波多音调制(FMT)技术及其在宽带无线通信中的应用。滤波多音调制是基于滤波器组(filter banks)的多载波技术。采用了子信道不重叠的信道分割技术,子信道具有很高的频谱约束性,对系统频率偏差不敏感,从而克服了OFDM易受频率偏差影响的弱点。但由于采用了不满足理想重构条件的有限冲激响应(FIR)滤波器作为子信道滤波器,FMT系统引入了码间干扰(ISI),因此系统接收端需要引入子信道均衡技术,以消除滤波器组的影响。本文系统介绍了FMT基本概念。由于FMT是基于滤波器组调制的技术,因此我们从多采样率数字信号处理和滤波器组技术的角度,对FMT的基本概念进行了描述,给出了严格采样FMT系统和非严格采样FMT系统的有效实现形式。搭建了完整的FMT仿真实验系统,通过该系统全面研究了影响FMT系统的一些重要参数和各种算法实现。在FMT系统中,影响系统性能的关键参数是原型滤波器的性能。本文对FMT系统滤波器组的设计进行了系统的研究。由于FMT系统性能与滤波器组的性能息息相关,因此滤波器组的设计成为FMT技术的关键之一。根据FMT的特点,提出FMT系统中滤波器组设计的几项性能指标,给出满足这些指标的滤波器设计方法和实例,同时创新性地提出了一种优化的FMT原型滤波器的设计方案。仿真试验表明,采用该准则设计出的原形滤波器与其他方法相比具有较高的可达比特率和较好的误比特率性能。从多采样率数字信号处理和滤波器组技术的角度给出了严格采样FMT系统和非严格采样FMT系统的有效实现形式。对FMT和OFDM两种多载波调制从不同的方面进行了比较。不采用均衡技术的非严格采样FMT系统的性能进行了仿真。FMT技术能否应用于无线信道环境下的首要问题之一,就是其同步系统的复杂度问题。因此,我们对FMT在同步系统进行了详细研究。分析了定时偏差、频率偏移和相位偏差对FMT系统的影响,对比OFDM系统的同步性能。对FMT系统的同步算法进行了仿真。考察了多径对FMT系统的影响。研究多载波传输系统常见定时算法和FMT传输系统的定时算法,对现有的定时算法进行改进,提出了新的FMT定时同步算法,并分析了其性能。仿真结果表明,全盲估计的相关算法不需附加数据,提高了系统的频谱效率。改进的H.Minn算法结构简单,额外开销小,仅需一个训练符号,而且抗多径能力强,定时估计精度较高。对于多载波传输系统的载波频率同步技术,设计了一种基于训练符号的FMT突发传输系统载波频率捕获方案,并分析了其性能。仿真结果表明,采用辅助数据的频率同步算法,改进了SCA算法,仅用一个训练符号完成频率同步。这就使得训练序列的冗余降低一半,而且在快衰落环境中更具有鲁棒性,尤其适用于类似WLAN的突发数据传输的应用环境。信道均衡的设计。因为FMT系统(尤其严格采样FMT系统)是采用频带严格受限的滤波器组实现的,由于实际应用中不满足完全重建条件,因此不可避免地引入了ISI。而有效克服ISI的方式是均衡技术。本文根据FMT的特点提出不同的均衡方案,并通过仿真实验比较它们的性能,提出不同环境,不同要求下的不同的均衡方案,同时研究了基于辅助数据均衡(DAE)技术,创新提出了结合因子图的DAE二次判决算法,将其应用于FMT中,取得了良好的均衡效果。针对FMT系统内的ISI问题,提出了高效的MMSE-DFE均衡方法,并研究了用预编码(THP)在FMT系统中的应用,并仿真了THP在FMT系统中的性能。将网格编码与预编码结合,考虑网格预编码带来的编码增益和成形增益,仿真得到了其在多径衰落信道中的性能。以PSK和QAM为调制方式,比较了FMT中MMSE-DFE、THP及网格预编码在AWGN和多径衰落信道环境中的性能,在保证不扩展带宽和传输速率不变的情况下,提高了FMT系统的性能。信道估计算法。宽带无线信道下的突发数据传输一般采用基于导频符号的信道估计方法。本文在研究了OFDM基于导频序列的信道估计的算法的基础上,结合FMT系统的特点,将基于导频符号的信道估计算法应用到FMT系统中。提出并验证了基于导频符号的时域LS信道估计算法、频域LS和LMMSE信道估计算法和时频二维信道估计算法在FMT信道估计中的性能,给出了两种不同的实现结构,并基于该结构将信道估计结果用于FMT子信道均衡器中,取得了较好的实验结果。研究表明FMT系统完全适合插入导频的信道估计方法。估计性能稳定,效果较好。论文的创新点包括:(1)本文提出FMT系统中滤波器组设计的几项性能指标,给出满足这些性能指标的滤波器设计方法和实例。提出了一种优化的FMT原型滤波器的设计方案。由于原有提出的一种FMT的优化设计方案,其方法仅仅适用于严格采样的FMT系统。本文给出的FMT原型滤波器优化设计的一般准则及优化算法,不仅适用于严格采样的FMT系统,而且适用于非严格采样的FMT系统。(2)论文完成了FMT系统相关的均衡器的设计,就FMT的特点提出不同的均衡方案,并将预编码和网格编码技术应用到FMT系统,保证不扩展带宽和传输速率不变的情况下,提高了FMT系统的性能。特别是将基于辅助数据的均衡技术引入FMT系统,创新性地提出了结合因子图的辅助数据的均衡算法,解决了其物理可实现问题。(3)研究了FMT系统的同步性能,并与OFDM系统进行比较。分析了定时偏差、频率偏移和相位偏差对FMT系统的影响,对比OFDM系统的同步性能。(4)通过研究OFDM系统的信道估计算法,给出了适合FMT系统的基于导频的信道估计方法。通过仿真实验得出结论,FMT系统非常适用基于导频的信道估计方法,估计性能稳定。应指出,将FMT技术应用到宽带无线通信系统中,仍许多问题有待于进一步研究,例如:1 FMT峰均比问题的研究。众所周知峰均比过大是制约OFDM在高速无线通信领域应用的主要问题之一。FMT技术是否存在峰均比问题呢?如果存在,该如何克服?直观来看,FMT由于采用的子信道数较少(32,64,128等),其峰均比问题应不如OFDM严重。如果能通过FMT系统自身参数的调整(如设计原型滤波器时将减蟹寰纫沧魑杓频牟问?加以克服,那么FMT的应用前景将更加光明。2原型滤波器的自适应设计问题。可否根据不同信道环境设计不同的原型滤波器呢?从非严格采样FMT系统的有效实现可以看出,其每个子信道的冲激响应对应不同的时间基准是变化的,能否根据这个特性设计出针对不同信道环境的最优原型滤波器呢?3 FMT作为频谱不重叠的多载波调制技术,非常有利于无线频谱资源的管理。采用FMT的多址接入方法及异步多用户通信相关技术需进一步研究。另外针对FMT抗载波频偏能力较强的特点,如何简化其同步系统等都应进一步的展开研究。更多还原

【Abstract】 Multi-carrier modulation technique has recently evoked considerable interests and widespread applications for its attractive qualities in resisting multi-path effects and mitigating ISI. Taken as an example, the OFDM technique has been adopted for some formed standards and even regarded as a candidate for the coming 4G-wideband mobile communication systems. However, OFDM has some intrinsic shortcomings: its orthogonality is susceptible to be destroyed in practical transmission, thus considerable overheads (CP and virtual carriers), strict synchronization and etc. are needed. In order to avoid these disadvantages of OFDM, a new multi-carrier modulation technique, named Filtered Multi-tone Modulation (FMT) is studied for its applications in wireless communication in this thesis, with some related aspects being investigated intensively.FMT is also a multi-carrier modulation technique based on filter banks technology. The main character of the technique is that its high-spectral constraint of each sub-carrier results in its non-sensitivity to frequency offset where OFDM enduring. However, FMT has inter-symbol interferences (ISI) which are caused by the non-ideal sub-channel filters. ISI has to be eliminated by per-sub-channel equalizers.We first introduced the basic concept of FMT system in this dissertation. FMT belongs to filter banks technology. Thus we give description of FMT system based on multi-rate signal processing and filter banks. Efficient realizations of both critically sampling system and non-critically sampling system are deduced. We also built up a FMT simulation system by Matlab. Through it, we studied how the relevant coefficients and algorithms influence the system’s performance.For FMT system, one of the most key parameters is the prototype filter. Thus, we studied the design technology of the filter banks. Based on the characteristic of the FMT system, we first gave the design target of the prototype filter. Then, we gave some examples which could satisfy these requirements. Finally, we delivered a new scheme on optimizing the prototype filter. Simulation results showed that the optimum prototype filter could achieve better error probability performance and achievable bit rates simultaneously.Whether FMT system could be used in the wireless communication environment is depend on the complexity of its synchronization scheme. Therefore we studied this problem deeply. We analyzed how the timing offset, the carrier frequency offset and the carrier phase affect the system performance and compared them with the OFDM system. We simulated those algorithm efficiencies and studied the influence of the multi-path channel on the FMT system.We treated some familiar timing synchronization schemes of multi-carrier systems and schemes of FMT system. We gave a new timing synchronization scheme of FMT system in wireless communication and simulated the performance of some of the schemes. The simulation results showed that blind timing estimation algorithm could achieve better spectrum efficiency with the tradeoff of complexity. The improved H.Minn algorithm could achieve precision estimation with features of simple, little extra cost, one training symbol and strong anti-multi-path ability. We also investigated frequency synchronization schemes of multi-carrier systems. We modified SCA to accomplish frequency synchronization with only one training symbol, instead of two, without performance degradation. The redundancy of the training sequence was reduced to half. This reduction is critical especially in burst data transmission, for example, in WLAN.We also considered how to design the per-sub-carrier equalizers. FMT system should add per-sub-carrier equalizers at the output of the receiver to compensate ISI introduced by the non-ideal prototype filter. We gave some different equalization schemes, such as decision feedback equalizer and pre-equalizer, etc. We introduced data-aided equalizer to FMT system and presented a two-decision algorithm based on factor diagram. We also studied TCM scheme, which combined the trellis code with pre-equalizer, in FMT system and compared its performance with other equalization schemes under AWGN channel and multi-path fading channel. All the equalization scheme could achieve better performance and in the meantime guarantee without expanding band-width and data-rate varying.Channel estimation based on training sequence is considered as a performing scheme for burst data transmission on wireless communication system. Base on OFDM channel estimation algorithm, we proposed a set of channel estimation schemes based on pilot symbols. We gave the realization models of LS estimation algorithm in the time domain, the LS and LMMSE estimation algorithm in the frequency domain and two dimensions algorithm in time-frequency domain. The simulation results showed that these algorithms could achieve better performance in FMT system.The innovation research of this dissertation include in:1. We proposed the design targets of the prototype filter and gave an optimum design method of the prototype filter. Some papers present an optimizing scheme of the prototype filter which could only be used to design prototype filter for critically sampling FMT system. In this dissertation, we proposed a general optimizing scheme which could be adopt to design prototype filter for both critically sampling and non-critically sampling FMT system.2. We accomplished the relevant equalizer design of FMT system. We design different equalizers for different environments and introduced pre-equalizer and trellis coding technique in FMT system. We also introduced the DAE equalizer to FMT system and presented a two-decision algorithm based on factor diagram to overcome the non-causally of traditional DAE equalizer.3. We studied the FMT synchronization problem and compare its performance with OFDM. We also analyzed timing offset, frequency offset and phase offset on FMT system.4. Based on the channel estimation algorithm of OFDM, we presented channel estimation algorithms suitable for FMT system. These algorithms are based on training sequences and could achieve robust performance.However, there are still some problems should be solved when FMT is used in wireless environment, such as:1. How to solve high peak-average power ratio (PAR). It is well known that high PAR is one of the most difficult questions which should be overcomed first in OFDM system. Whether FMT has high PAR has no result at present. If FMT has PAR problem, what can we do?Intuitively, FMT has few sub-carriers (16,32,64 etc.). Thus PAR of FMT would be low than OFDM. If we could find a scheme which could guarantee lower PAR for FMT, the prospective of FMT will be bright.2. How to design the prototype filter self-adaptively? If we could adjust the coefficients of the prototype filter based on the different channel, we could get the optimal system. 3. As a non-overlapping multi-carrier modulation technique, FMT is suitable for wireless resource management. Therefore, it is valuable to research multi-access and asynchronous multi-user communication for FMT system.Moreover, it is deserved to further simplify synchronization algorithm.更多还原