基于Matlab的OFDM系统设计及分析.doc

仁 人 教 育目录设计总说明IThe total designed to show thatV1绪论11.1课题背景11.2 无线通信11.2.1 无线通信概述11.2.2 无线信道特性21.3 OFDM系统介绍31.3.1 OFDM的概述31.3.2 OFDM的应用41.3.3 OFDM的关键技术51.3.4 OFDM系统的优点及缺点71.4 MATLAB特点与功能92 OFDM系统的基本原理112.1 OFDM技术原理112.2 基于IFFT/FFT 的OFDM 系统模型122.3 OFDM信号的频谱特性142.4 串并转换153 OFDM系统在MATLAB上的仿真分析163.1 0FDM 系统调制与解调解析163.2 加窗183.3 AWGNA信道下的仿真203.3.1加入高斯白噪声前后分析对比203.3.2 BER性能曲线214对系统误码率的改善分析234.1循环前缀234.2 OFDM系统的峰值平均功率比274.3信道估计284.3.1信道估计概述284.3.2基于导频的信道估计方法294.3.3信道的插值方法294.3.4仿真结果及分析305总结325.1完成的主要工作325.2不足与展望32参考文献34附录35致谢501基于Matlab的OFDM系统设计及分析设计总说明正交频分复用是一种历史悠久的多载波调制技术，最早在20世纪中期的时候，在通信领域，研究者们做了大量的卓有成效的研究工作，对多载波调制理论进行了研究，并论证了利用多载波调制技术能够对整体系统性能进行优化。当时许多专家学者对这频域非常密集的多载波调制的OFDM系统产生了巨大的关注。然而那个时候的数字信号处理技术还没有得到足够的成长，OFDM也因此没有得到普遍的关注和使用。在20世纪90年代，随着数字信号处理技术的日渐成熟，OFDM技术在告诉数据传输方面得到了人们的重视，已经普遍应用于数字音频广播（如DAB和DBA）、无线局域网、电话上的数字传输（VDSL和ADSL）和电力线通信（homeplugav和BPL）等领域。2004年，已经作为3G核心之一的WiMAX影响力的扩大强烈迫使传统电信领域的标准化组织3GPP启动了它的长期演进计划，以OFDM为基础的多址接入技术在这次以空中接口技术的较量中，通过下行OFDMA和上行SC-FDMA奠定巨大优势获得胜利，这也意味着未来的一段时间移动通讯系统中将出现OFDM一统天下的局面。 正交频分复用（OFDM）技术能够有效的避免有符号间干扰信道，因为其简易高效，现已成为现今无线高速通讯整体中不可或缺的主要技术之一。本文最开始介绍了无线通信的发展史以及到现在的移动通信系统中的OFDM采用多种新技术所以具备更高的带宽利用率和克服符号间干扰与突发噪声的能力，也因其子载波彼此之间存在正交性，能让其频谱互相叠加，对比一般的频分复用，OFDM能够尽可能地操控频谱资源去扩大系统容量，最主要的是它的灵活开放性能很好地符合多媒体通讯的特点，将包含数据、语音、影像等多种多样的多媒体业务高效完善地传达出去。并简略阐述了仿真必须用到的MATLAB的可靠的数值和符号运算功能，简易的符号语言，可视化的特点。文章对OFDM 系统进行了概述以及当今社会普遍应用的主要原因在于它能够将宽带有符号间干扰信道改造为一系列衰落信道，这也表明，带宽利用率是非常高的，和高速离散傅里叶变换（DFT）技术来实现调制和解调，且选用联合编码技术，使其能够有很强的抗衰减特点及可与多种接入方式结合使用的优势与需要补足的地方，如OFDM对系统定时和频率偏移敏感和存在较高的峰值平均功率比，同时由于无线信道的不稳定性，且发射端载波和本地振荡器的频率偏差，所以产生的频率存在一定偏差使OFDM整体出现问题。同时，就OFDM系统的基本原理进行了讨论，事实上，它是用N个子载波信道切割成相应的子信道，因为这些子信道调制，实现多个数据的频域并行传输，这点和OFDM的原理差不多，唯一的区别就是OFDM技术很好的抓住了其控制特点，使其整个体系中每个子信道的载波相互正交，频谱彼此叠加，所以频谱利用率有所提升并减少了每一个子载波之间的影响。得到了OFDM为了保证不同子信道上承载的数据在接收端可以正常分离，必须保证自信道之间的“正交性”，也就是说并行传输子信道并不是无限随意划分的。所谓的正交性在时域应体现为： 在频域体现为：OFDM所采用的信号设计形式是满足正交性要求的最佳信号设计，满足频域奈奎斯特采样定律的最密的采样，它的正交性是根据在某一个积分时间内所有子载波都是整周期而且不同载波之间周期个数不同来实现的。这样，就利用了子载波在时域的正交性可以完全屏蔽其他子载波的干扰，恢复了本子载波承载的数据信息。从OFDM信号在频域的图像可以看出，每个子载波的频谱实际上就是频域的抽样函数，在频域的最佳采样点无子载波间干扰。其实更本上OFDM调制和解调都是在数字基带上进行数字信号处理实现的，因此t =为起始时间可以得出OFDM的符号表达式：然后OFDM等效复基带信号通过逆傅里叶变换（IDFT）的定义，能够得到它的DFT转换：从而可以得出由DFT和IDFT来分别代替OFDM系统的调制和解调。其中通过上式中OFDM 符号看出其功率谱的带外衰减速率很慢，我们不妨对其每一个都进行加窗调制，符号周期边的幅值逐渐减小为零，信号的幅度也逐渐减小，即使对信号的误码率有一定的影响。通过以上OFDM 系统参数数学公式和加窗技术的原理及运用IDFT 和DFT 完成了基于IFFT/FFT 的OFDM 系统设计，即在接收端把时域信号通过快速傅里叶变换(FFT)转化为频域信号，由此我们在程序编码上配置调制解调相关参数和帧结构等相关参数可以得到OFDM在AWGN信道下的BER性能曲线仿真图，并粗略分析系统误码率与信噪比之间的关系，而其代码中也不会包含信道估计，循环前缀等模块。接下来通过上述的AWGN信道编写的程序代码的基础上进行添加修改下着重探讨了了理想同步条件下,保护时隙(CP)、循环前缀加入前后和各种信道内插方式在多径瑞利衰落信道下对OFDM系统整体性能的影响。其中如果一个符号的长度严格等于Ts，即N个采样点，那么经过符号间干扰信道之后连续的两个符号之间的样点之间必然发生重叠，而且符号之间的干扰还会影响每一个子载波间的正交性，带来严峻的后果。因此，系统中往往需要增添保护时隙在两个OFDM符号之间。 一种直接的思路是在两个符号间加上一段时间为0的区域，这虽然消除了ISI，但是这种方法实际上无法克服符号间干扰所带来的子载波干扰。即在FFT积分时间的子载波的延迟路径已经不在整个周期中，子载波之间的正交性受其严重的损伤。我们为了解决这一问题，在目前实用化的OFDM系统中都采用了添加循环前缀（CP）的方法。所谓的循环前缀其实是将一个符号的后面若干个(Ncp）样点按顺序移到这个符号前面，代替保护间隔，其作用是在FFT积分时间内，每一条延迟径上的各个子载波都仍然是整周期的，保证了子载波间的正交性。循环前缀的引入虽然对抗了符号间干扰，但是带来了传输效率的下降，因此在OFDM系统设计时循环前缀的长度需要适当的选择一个参数，其长度一定要大于最大多径时延时，才能担保子载波间正交性。在添加循环前缀后，很明显，整个系统都会产生功率和信息速率的损耗，但它能够克服符号间干扰和多径产生的符号间干扰，本文也会通过仿真的实际结果重点说明其两面性的长短。同时研究OFDM系统峰均功率比更高的原因，主要是该符号是由一个单一的调制的副载波信号。因此，它可能会导致一个非常高的信号峰值功率，具有很高的峰值均匀功率比。因此对比单载波系统，OFDM系统发射机的发送信号的瞬间时值极度不稳定。这势必要求系统内的如A/D、D/A转换器等一些器件具备非常大的线性动态范围；然而同时也会由于这些器件的非线性对动态范围较广的信号引起非线性失真，以及彼此之间的干扰信道引起的谐波,从而降低整个OFDM体系的性能。在接收端的同时，由于无线信道的波动范围很广和不可预测性，且随着多种原因的不稳定性，从而致使收到的信号幅值、相位和频率的失真，从而难以解析。这些问题对接收机的设计造成了很大的阻碍，因此在接收机中，信道估计器是一个不可或缺的部分。信道估计器的设计在OFDM系统中首要直接就面临着两个困难：一是导频信息的选择，二是要有较低的复杂度。在最新建立于正交频分复用的无线通信系统，因为其传输速率高，它必然运用相干检测技术来得到更高的性能，因此一般选取非盲估计收获很不错的估计结果，使其能够很好的追寻无线信道的改变，提升接收端信号的质量。此课题所研究的基于导频信道估计的信道估计方法也是其中一种很实用的方法。由于其分为常值内插、线性内插和DFT插值，其中常值内插是相当容易的方式，但是考虑到达成的困难度，信道估计准则选取LS估计准则，简单讨论LS算法下不同插值方式下对信道估计的仿真。在基于IFFT/FFT的系统的OFDM系统内加入保护间隔之前之后的模拟和循环前缀等模块后，在完全不一样的信道条件基础上，,对比保护时隙、循环前缀、采用LS估计方法对OFDM系统误码率影响,得到了自己此次设计中想要的结论。最后对整个毕业设计归纳总结，并提出本文存在的不足的地方和对OFDM技术将来工作的研究展望。关键词:正交频分复用;系统仿真;误码率影响MATLAB Simulation and Performance Analysis of OFDM SystemThe total designed to show that Orthogonal frequency division multiplexing is a multi-carrier modulation technology with a long history, as early as the middle of the 20th century, in the field of communication, the researchers have done a lot of fruitful work, studied the multicarrier modulation theory, and demonstrates the use of multicarrier modulation technique to optimize the overall performance of the system. At that time, many experts and scholars on the frequency domain of very intensive multicarrier modulation OFDM system to produce a great deal of attention. At that time, however the growth of digital signal processing technology has not been enough, OFDM and therefore didnt get the attention and widely used.In the 1990 s, with the maturing of digital signal processing technology, OFDM technology in tell data transmission has got the attention of the people, has been widely applied in the digital audio broadcasting (DAB) and DBA), wireless local area network (LAN), the digital transmission (VDSL and ADSL) on the phone and power line communication (homeplugav and BPL), etc.In 2004, has been one of the 3 g core WiMAX influence to expand a strong force of traditional telecom organization for standardization in the field of 3 GPP launched its long-term evolution plan, on the basis of OFDM multiple access technology in the air interface technology, through the downlink OFDMA uplink SC - FDMA and lay a huge advantage to win, it also means a period of time in the future mobile communication system will appear OFDM the situation of unify the whole country.Orthogonal frequency division multiplexing (OFDM) technology can effectively avoid the intersymbol interference channel, because of its simple and efficient, has now become the indispensable one of the main technology of high-speed wireless communication overall. This paper at first introduced the wireless communication in the history of development, as well as to the present system for mobile communications OFDM USES the many kinds of new technology so with higher bandwidth utilization, and overcome the intersymbol interference and burst noise strength, also because of its carrier orthogonality between each other, each other can make its spectrum superposition, compared with the general frequency division multiplexing, OFDM can be as much as possible manipulation of the spectrum resources to expand the system capacity, the mainest is it flexible open performance conforms to the characteristics of multimedia communication, well will contain data, voice, image and so on a variety of multimedia business effective and perfect pass out. And briefly expounds the simulation must be used in the reliable numerical and symbolic operation function of MATLAB, a simple sign language, the characteristics of the visual.Articles on the OFDM system were summarized, and the main reason for general use in todays society is that it can be a broadband has transformed into a series of intersymbol interference channel fading channel, this also shows that the bandwidth utilization rate is very high, and the discrete Fourier transform (DFT) technology to realize the modulation and demodulation, and selects the joint coding technology, enable it to have a strong resistance attenuation characteristics and can be used with multiple access methods in combination with the advantages and the need to complement, such as OFDM is sensitive to timing and frequency offset and the existing high peak average power ratio, at the same time due to the instability of the wireless channel, the sender and the carrier and the local oscillator frequency deviation, so there is a certain deviation of frequency OFDM the whole problem. At the same time, it discusses the basic principle of OFDM system, in fact, it is the carrier channel cut into corresponding person with N sub-channels, because these sub channel modulation, implement multiple frequency domain parallel data transmission, which is similar to the principle of OFDM, the only difference is the OFDM technology good caught its control characteristic, make the whole system of orthogonal to each other, each channel in the carrier frequency spectrum superposition of each other, so to improve spectrum efficiency and to reduce the influence between each child carrier. In different sub-channels of OFDM in order to ensure the bearing data can be separated at the receiving end, must ensure the orthogonality between self-confidence, that is to say, parallel transmission is not infinite sub-channels randomly divided. The orthogonality of the so-called time domain should be embodied in:In the frequency domain embodied in:OFDM signal design form is adopted by the best signal that could satisfy the requirement of orthogonality design, satisfy the frequency domain Nyquist sampling law is the most dense sampling, its orthogonality is according to all the subcarrier in a certain integral time is the whole cycle and cycle number is different between different carrier. So, it is using the orthogonality of the subcarrier in time domain can be completely shielded other sub-carrier interference, restored the book data information carrier. Can be seen from the OFDM signal in the frequency domain image, each subcarrier frequency spectrum is actually the sample function of frequency domain, in the frequency domain of the best sampling points without interference between subcarrier. More on the OFDM modulation and demodulation is carried out on the digital baseband digital signal processing, so t = as the starting time can be concluded that OFDM symbol expressions:Then OFDM complex equivalent baseband signal by inverse Fourier transform (IDFT) definition, to get its DFT transformation:Thus can draw by DFT and IDFT respectively instead of modulation and demodulation of OFDM system. Among them through the type on OFDM symbol to see its power spectrum out-of-band attenuation rate is slow, we might as well to each add window modulation, symbol of the periodic boundary amplitude decreases to zero, the signal amplitude decreases, even if the error rate of the signal has the certain influence. OFDM system parameters through the above mathematical formula and the principle of window technology and using IDFT based on IFFT/FFT and DFT of the OFDM system design, namely the time-domain signal at the receiving end through the fast Fourier transform (FFT) is converted into frequency domain signal, thus we configure modem related parameters on the program code and frame structure parameters can be related to OFDM BER performance under AWGN channel simulation curve graph, and a rough analysis of the relationship between the system bit error rate and signal to noise ratio, and the code will not be included in channel estimation, cyclic prefix and other modules.Then through the program code written in AWGN channel on the basis of adding modified item is emphatically discussed, under the condition of ideal synchronization protection time slot (CP), and a variety of before and after cyclic prefix to join channel interpolation method in multipath Rayleigh fading channel in OFDM system under the influence of the overall performance. One if the symbol is equal to the length of the strict Ts, namely N sampling points, then after the intersymbol interference channel between two consecutive symbol of sample must be overlap, and the interference among the symbols can also affect the orthogonality between each child carrier, serious consequences. Therefore, systems often need to add protection time slot between two OFDM symbol. A direct approach is combined with a period of time between two symbols of 0, it eliminates ISI, but this method is actually unable to overcome sub-carrier interference brought by the intersymbol interference. In FFT subcarrier delay path integral time isnt the whole cycle, the subcarrier orthogonality between its serious damage. We have to solve this problem, in the current practical OFDM systems are adopted the approach of adding cyclic prefix (CP). So-called cyclic prefix is actually a symbol behind a number of sample point (Ncp) moved to the front of the symbol according to the order instead of protection interval, and its role is in the FFT integral time, each delay diameter on each subcarrier is still a whole cycle, ensure the orthogonality between the subcarrier. The introduction of the cyclic prefix although against the intersymbol interference, but lead to a drop in efficiency, so when the length of the cyclic prefix in OFDM system design need to select a proper parameter, when most of its length must be greater than the diameter delay, to guarantee the subcarrier orthogonality between. After adding cyclic prefix, obviously, the whole system will produce loss of power and information rate but it can overcome the intersymbol interference and multipath intersymbol interference, this paper will through the simulation of the actual results based on the length of the two sides. Research OFDM system peak power is higher than at the same time, is the symbol is dominated by a single modulated subcarrier signal. Therefore, it may lead to a very high signal peak power, has the very high peak power ratio. So comparing single carrier system, OFDM system transmitter signal instantaneous value extremely unstable. This will inevitably request in the system, such as A/D, D/A converter and some other devices with very large linear dynamic range; But also because of these devices with wider range of nonlinear dynamic nonlinear distortion caused by the signal, and the interference between each channel caused by harmonics, thus reducing the whole performance of the OFDM system. On the receiving end at the same time, because of the wireless channel fluctuation range is very wide and unpredictability, and as the instability of a variety of reasons, thus received signal amplitude, phase and frequency distortion, which is difficult to parse. These problems to the design of receiver caused great obstacles, so in the receiver, channel estimation is an indispensable part. The design of the channel estimator in OFDM systems directly to face two difficulties: first one is the choice of pilot information, 2 it is to have a lower complexity. In the newly established in orthogonal frequency division multiplexing wireless communication system, because of its high transmission rate, it must use coherent detection technology to get a higher performance, therefore generally choose the blind estimation harvest good estimation results, enable it to very good after the change of the wireless channel, improve the quality of the receiver signal. This topic research of channel estimation method based on pilot channel estimation is one of the very practical method. Because its can be divided into constant interpolation and linear interpolation and DFT interpolation, the constant interpolation is fairly easy way, but given the difficulty, channel estimation criteria to select the LS estimate criterion, under the simple discussion of the LS algorithm for channel estimation under different interpolation methods in the simulation. In the OFDM system based on IFFT/FFT system incorporating protection interval before and after simulation and cyclic prefix module, on the basis of completely different channel conditions, and compare the protection time slot, cyclic prefix, the LS estimation methods for OFDM system ber, got myself want to in the design of the conclusion.Finally, the graduation design, the conclusion and put forward in this paper, the deficiencies of place and the research of OFDM technique in the future job prospects.Key words:Orthogonal frequency division multiplexing; System simulation; Ber influence VII1绪论1.1课题背景 纵观移动通信的发展史，第一代模拟系统仅提供语音服务，不能传输数据。第二代数字移动通信系统的数据传输速率也只有9.6bit/s，最高可达32kbit/s；第三代移动通信系统数据传输速率可达到2Mbit/s；而我们目前所致力研究的第四代移动通信系统可以达10Mbit/s至20Mbit/s。虽然第三代移动通信可以比现有传输速率快上千倍，但是仍无法满足未来多媒体通信的要求，第四代移动通信系统的提出便是希望能满足提供更大的频宽需求。第四代移动通信系统计划以OFDM(正交频分复用)为核心技术提供增值服务，它在宽带领域的应用具有很大的潜力。较之第三代移动通信系统，采用多种新技术的OFDM具有更高的频谱利用率和良好的抗多径干扰能力，它不仅仅可以增加系统容量，更重要的是它能更好地满足多媒体通信要求，将包括语音、数据、影像等大量信息的多媒体业务通过宽频信道高品质地传送出去。OFDM（Orthogonal Frequency Division Multiplexing）是目前已知的频谱利用率最高的一种通信系统，它将数字调制、数字信号处理、多载波传输等技术有机结合在一起，使得它在系统的频谱利用率、功率利用率、系统复杂性方面综合起来有很强的竞争力，是支持未来移动通信特别是移动多媒体通信的主要技术之一1。1.2 无线通信1.2.1 无线通信概述 到目前为止，无线通信大约经历了四个阶段。 (1)1G阶段 主要采用的是模拟技术和频分多址(FDMA)技术。由于受到传输带宽的限制，不能进行移动通信的长途温游，只能是一种区域性的移动通信系统2。第一代移动通信有很多不足之处，比如容量有限、制式太多、互不兼容、保密性差、通话质量不高、不能提供数据业务、不能提供自动漫游等。(2)2G阶段 主要采用的是数字的时分多址(TDMA)技术和码分多址(CDMA)技术。全球主要有GSM和CDMA两种体制。GSM技术标准是欧洲提出的，目前全球绝大多数国家使用这一标准。我国移动通信也主要是GSM体制。目前使用GSM的用户占国内市场的97。CDMA是美国高通公司提出的标准。由于第二代采用不同的制式，移动通信标准不统一，用户只能在同一制式覆盖的范围内进行漫游，因而无法进行全球漫游，由于第二代数字移动通信系统带宽有限，限制了数据业务的应用，也无法实现高速率的业务如移动的多媒体业务3。(3)3G阶段与从前以模拟技术为代表的第一代和目前正在使用的第二代移动通信技术相比，3G将有更宽的带宽，其传输速度最低为384K，最高为2M，带宽可达5MHz以上。目前全球有三大标准，分别是欧洲提出的WCDMA、美国提出的CDMA2000和我国提出的TDSCDMA。第三代移动通信网络能将高速移动接入和基于互联网协议的服务结合起来，提高无线频率利用效率6。提供包括卫星在内的全球覆盖并实现有线和无线以及不同无线网络之间业务的无缝连接。 (4)4G阶段由于第三代移动通信系统仍是基于地面标准不一的区域性通信系统，尽管其传输速率可高达2 Mb/s，但仍无法满足多媒体通信的要求，因此，第四代移动通信系统(4G)的研究随之应运而生。第四代移动通信技术的概念可称为广带(Broadband)接入和分布网络，具有非对称超过2 Mb/s的数据传输能力，对全速移动用户能提供150 Mb/s的高质量影像服务，将首次实现三维图像的高质量传输。他包括广带无线固定接入、广带无线局域网、移动广带系统和互操作的广播网络(基于地面和卫星系统)。其广带无线局域网(WLAN)能与B-ISDN和ATM兼容，实现广带多媒体通信，形成综合广带通信网(IBCN)，他还能提供信息之外的定位定时、数据采集、远程控制等综合功能7。1.2.2 无线信道特性 无线信道包括了电波的多径衰落，时延扩展，以及多普勒效应，在移动通信中，必须要充分考虑这些特性，并提出相关的解决方案。(1)时延扩展无线信道中电波的传播不是单一路径，而是许多路径来的众多反射波的合成。由于电波通过各个路径的距离不同，因而各个路径来的反射波到达时间不同，也就是各信号的时延不同。当发送端发送一个极窄的脉冲信号时，移动台接收的信号由许多不同时延的脉冲组成，这被称之为时延扩展。(2)多径衰落由于各个路径来的反射波到达时间不同，相位也就不同。不同相位的多个信号在接收端迭加，有时迭加而加强（方向相同），有时迭加而减弱（方向相反）。这样，接收信号的幅度将急剧变化，即产生了快衰落。此外，接收信号除瞬时值出现快衰落之外，场强中值（平均值）也会出现缓慢变化。主要是由地区位置的改变以及气象条件变化造成的，以致电波的折射传播随时间变化而变化，多径传播到达固定接收点的信号的时延随之变化。这种由阴影效应和气象原因引起的信号变化，称为慢衰落。这种衰落是由多种路径引起的，所