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1、译文： 正交频分复用技术简介 正交频分复用是一种多载波调制技术。其主要思想是：将信道分成若干正交子信道，将高速数据信号转换成并行的低速子数据流，调制到在每个子信道上进行传输。正交信号可以通过在接收端采用相关技术来分开，这样可以减少子信道之间的相互干扰。每个子信道上的信号带宽小于信道的相关带宽，因此每个子信道上的可以看成平坦性衰落，从而可以消除码间串扰。而且由于每个子信道的带宽仅仅是原信道带宽的一小部分，信道均衡变得相对容易。由于这种技术具有在杂波干扰下传送信号的能力，因此常常会被利用在容易受外界干扰或者抵抗外界干扰能力较差的传输介质中。目前正交频分复用技术已经被广泛应用于广播式的音频、视频领域
2、和民用通信系统，主要的应用包括：非对称的数字用户环路、欧洲电信标准协会的数字音频广播、数字视频广播、高清晰度电视、无线局域网等。正交频分复用并不是才发展起来的新技术，其应用已有40余年的历史，在上个世纪60年代就已经有人提出了使用平行数据传输和频分复用的概念。70年代，韦斯坦和艾伯特等人应用离散傅里叶变换和离散傅里叶逆变换的方法研制了一个完整的多载波传输系统，叫做正交频分复用系统。正交频分复用是一种特殊的多载波传输方案，它应用离散傅里叶变换和离散傅里叶逆变换的方法解决了产生多个互相正交的子载波以及从子载波中恢复原信号的问题。这就解决了多载波传输系统发送和传送的难题。应用快速傅里叶变换和快速傅里
3、叶逆变换更是使多载波传输系统的复杂度大大降低。从此正交频分复用技术开始走向实用。但是应用正交频分复用系统仍然需要大量繁杂的数字信号处理过程，而当时还缺乏数字处理功能强大的元器件，发射机和接收机振荡器的稳定性以及射频功率放大器的线性要求等因素也是正交频分复用技术实现的制约条件。因此正交频分复用技术迟迟没有得到迅速发展。 80年代，集成电路获得了突破性进展，大规模集成电路让快速傅里叶变换和快速傅里叶逆变换的实现不再是难以逾越的障碍，一些其它难以实现的困难也都得到了解决，自此正交频分复用走上了通信的舞台，逐步迈向高速数字移动通信的领域。进入90年代，由于技术的可实现性，正交频分复用的应用涉及到了利用
4、移动调频和单边带信道进行高速数据通信，陆地移动通信，高速数字用户环路，非对称数字用户环路，高清晰度数字电视和陆地移动广播等各种通信系统。1999年，国际电气与电子工程师协会通过了一个的无线局域网标准IEEE802.lla，其中正交频分复用调制技术被采用为物理层标准，使得传输速率可以达54Mbps。这样，可提供25Mbps的无线ATM接口和10Mbps的以太网无线帧结构接口，并支持语音、数据、图像业务。这样的速率完全能满足室内、室外的各种应用场合。欧洲电信组织的宽带射频接入网的局域网标准HiperiLAN2也把正交频分复用定为它的物理层标准调制技术。 正交频分复用有许多关键技术。（1）时域和频域
5、同步。正交频分复用系统对定时和频率偏移敏感，特别是实际应用中可能与频分多址、时分多址和码分多址等多址方式结合使用时，时域和频率同步显得尤为重要。与其它数字通信系统一样，同步分为捕获和跟踪两个阶段。在下行链路中，基站向各个移动终端广播式发同步信号，所以，下行链路同步相对简单，较易实现。在上行链路中，来自不同移动终端的信号必须同步到达基站，才能保证子载波间的正交性。基站根据各移动终端发来的子载波携带信息进行时域和频域同步信息的提取，再由基站发回移动终端，以便让移动终端进行同步。具体实现时，同步将分为时域同步和频域同步，也可以时频域同时进行同步。（2）信道估计。在正交频分复用系统中，信道估计器的设计
6、主要有两个问题：一是导频信息的选择。由于无线信道常常是衰落信道，需要不断对信道进行跟踪，因此导频信息也必须不断的传送。二是既有较低的复杂度又有良好的导频跟踪能力的信道估计器的设计。在实际设计中，导频信息选择和最佳估计器的设计通常又是相互关联的，因为估计器的性能与导频信息的传输方式有关。（3）信道编码和交织。为了提高数字通信系统性能，信道编码和交织是通常采用的方法。对于衰落信道中的随机错误，可以采用信道编码；对于衰落信道中的突发错误，可以采用交织。实际应用中，通常同时采用信道编码和交织，进一步改善整个系统的性能。在正交频分复用系统中，如果信道衰落不是太深，均衡是无法再利用信道的分集特性来改善系统
7、性能的，因为正交频分复用系统自身具有利用信道分集特性的能力，一般的信道特性信息已经被正交频分复用这种调制方式本身所利用了。但是正交频分复用系统的结构却为在子载波间进行编码提供了机会，形成编码正交频分复用。编码可以采用各种码，如分组码、卷积码等，卷积码的效果要比分组码好。（4）降低峰均功率比。由于正交频分复用信号时域上表现为N个正交子载波信号的叠加，当这N个信号恰好均以峰值占相加时，正交频分复用信号也将产生最大峰值，该峰值功率是平均功率的N倍。尽管峰值功率出现的概率较低，但为了不失真地传输这些高峰均功率比的信号，发送端对高功率放大器的线性度要求很高且发送效率极低，接收端对前端放大器以及模数转换器
8、的线性度要求也很高。因此，高的峰均功率比使得正交频分复用系统的性能大大下降甚至直接影响实际应用。为了解决这一问题，人们提出了基于信号畸变技术、信号扰码技术和基于信号空间扩展等降低正交频分复用系统峰均功率比的方法。 作为通信方面的应用，正交频分复用存在很多技术优点。（1）在窄带带宽下也能够发出大量的数据。正交频分复用技术能同时分开至少1000个数字信号，而且在干扰的信号周围可以安全运行，这种能力将直接威胁到目前已经开始流行的码分多址技术的进一步发展和壮大，正是由于具有了这种特殊的信号穿透能力使得正交频分复用技术深受欧洲通信营运商以及手机生产商的喜爱和欢迎。(2) 正交频分复用技术能够持续不断地监
9、控传输介质上通信特性的突然变化。由于通信路径传送数据的能力会随时间发生变化，所以正交频分复用能动态地与之相适应，并且接通和切断相应的载波以保证持续地进行成功的通信。(3) 正交频分复用可以自动地检测到传输介质下哪一个特定的载波存在高的信号衰减或干扰脉冲，然后采取合适的调制措施来使指定频率下的载波进行成功通信。(4) 正交频分复用技术特别适合使用在高层建筑物、居民密集和地理上突出的地方以及将信号散播的地区。高速的数据传播及数字语音广播都希望降低多径效应对信号的影响。(5) 正交频分复用技术的最大优点是对抗频率选择性衰落或窄带干扰。在单载波系统中，单个衰落或干扰能够导致整个通信链路失败，但是在多载
10、波系统中，仅仅有很小一部分载波会受到干扰。对这些子信道还可以采用纠错码来进行纠错。(6) 可以有效地对抗信号波形间的干扰，适用于多径环境和衰落信道中的高速数据传输。当信道中因为多径传输而出现频率选择性衰落时，只有落在频带凹陷处的子载波以及其携带的信息受影响，其他的子载波未受损害，因此系统总的误码率性能要好得多。(7) 通过各个子载波的联合编码，具有很强的抗衰落能力。正交频分复用技术本身已经利用了信道的频率分集，如果衰落不是特别严重，就没有必要再加时域均衡器。通过将各个信道联合编码，则可以使系统性能得到提高。(8) 正交频分复用技术抗窄带干扰性很强，因为这些干扰仅仅影响到很小一部分的子信道。(9
11、) 信道利用率很高，这一点在频谱资源有限的无线环境中尤为重要；当子载波个数很大时，系统的频谱利用率趋于2Baud/Hz。 虽然正交频分复用有上述优点，但是同样其信号调制机制也使得其信号在传输过程中存在着一些劣势。(1)对相位噪声和载波频偏十分敏感。这是正交频分复用技术一个非常致命的缺点，整个正交频分复用系统对各个子载波之间的正交性要求格外严格，任何一点小的载波频偏都会破坏子载波之间的正交性，引起符号间干扰，同样，相位噪声也会导致码元星座点的旋转、扩散，从而形成信道间干扰。而单载波系统就没有这个问题，相位噪声和载波频偏仅仅是降低了接收到的信噪比，而不会引起互相之间的干扰。(2)峰均比过大。正交频
12、分复用信号由多个子载波信号组成，这些子载波信号由不同的调制符号独立调制。同传统的恒包络的调制方法相比，正交频分复用调制存在一个很高的峰值因子。因为其信号是很多个小信号的总和，这些小信号的相位是由要传输的数据序列决定的。对某些数据，这些小信号可能同相，而在幅度上叠加在一起从而产生很大的瞬时峰值幅度。而峰均比过大，将会增加模数转换器和数模转换器的复杂性，而且会降低射频功率放大器的效率。同时，在发射端，放大器的最大输出功率就限制了信号的峰值，这会在正交频分复用频段内和相邻频段之间产生干扰。(3)所需线性范围宽。由于正交频分复用系统峰值平均功率比大，对非线性放大更为敏感，故正交频分复用调制系统比单载波
13、系统对放大器的线性范围要求更高。原文：Orthogonal frequency division multiplexing technology introductionOFDM is a multi carrier modulation technique. The main idea is: divides the channel into several orthogonal sub channels, the high-speed data signal into a low flow sub data parallelism, modulation to transmit in ea
14、ch sub channel. Orthogonal signal can be separated by the use of related technologies at the receiving end, thus reducing the mutual interference between the channel. Relative bandwidth signal bandwidth of each sub channel is shorter than the channel, so each sub channel can be seen flat fading chan
15、nel, which can eliminate intersymbol interference. And because the bandwidth of each channel is only a small part of the channel bandwidth, channel equalization becomes relatively easy. Because of this technology has the ability to transmit signals in clutter, it is often used in the transmission me
16、dium is vulnerable to outside interference or resist interference ability is poorer in. At present, the orthogonal frequency division multiplexing technique has been widely applied to broadcast audio, video and civil communication system, including the main application: digital subscriber loop, non
17、symmetry of the European Telecommunications Standards Institute of digital audio broadcasting, digital video broadcasting, high-definition television, wireless local area network.The new technology of orthogonal frequency division multiplexing is not development, its application has a history of 40
18、years, 60 years in the last century, it has been put forward the concept of using parallel data transmission and frequency division multiplexing.In 70, Weinsein and Albert et al. Application of discrete Fu Liye transform and the discrete Fu Liye transform is developed for multi carrier transmission
19、system of a complete, called orthogonal frequency division multiplexing system. Orthogonal frequency division multiplexing is a special multi carrier transmission scheme, which using the discrete Fu Liye transform and discrete Fu Liye transform solution to restore the original signal to generate a p
20、lurality of mutually orthogonal subcarriers and sub carrier in question. This would solve the problem of multi carrier transmission system transmission and transfer. Application of the fast Fu Liye transform and fast Fu Liye transform is the complexity of multi carrier transmission system is greatly
21、 reduced. From the orthogonal frequency division multiplexing technology to practical. But the application of orthogonal frequency division multiplexing system still needs a lot of digital signal processing procedure is complicated, and there was a lack of digital processing powerful components, res
22、tricting factors and orthogonal frequency division multiplexing technology of the transmitter and the receiver oscillator stability and linear power amplifier requirements. Therefore, the technology of orthogonal frequency division multiplexing has not obtained the rapid development.In 80, the integ
23、rated circuit to be a breakthrough, realize large-scale integrated circuit to make the fast Fu Liye transform and fast Fu Liye transform is not insurmountable obstacles, some other difficulties of implementation have been solved, since orthogonal frequency division multiplexing on the communication
24、stage, gradually moving towards high speed digital mobile communication field.Enter 90 age, the realization of the technology, application of orthogonal frequency division multiplexing involves the use of mobile FM and single sideband channel high speed data communication, mobile communication, high
25、 speed digital subscriber loop, asymmetric digital subscriber loop, high-definition digital TV and land mobile radio communication system. In 1999, the International Association of electrical and electronic engineers through the IEEE802.lla wireless LAN standard one, the orthogonal frequency divisio
26、n multiplexing modulation technique is adopted as the standard of physical layer, the transmission rate can be up to 54Mbps. In this way, Ethernet wireless frame structure of wireless ATM interface and 10Mbps interface provides 25Mbps, and support for voice, data, video service. This rate can satisf
27、y the various applications of indoor, outdoor. LAN standard HiperiLAN2 broadband radio access network European Telecommunications Organization of the orthogonal frequency division multiplexing for physical layer modulation technology it.Orthogonal frequency division multiplexing many key technology.
28、(1) the time domain and frequency domain synchronization. Orthogonal frequency division multiplexing system is sensitive to the timing and frequency offset, in particular may be used in actual application combined with FDMA, TDMA and CDMA, FDMA, time and frequency synchronization is very important.
29、As with other digital communication systems, synchronization is divided into two stages to capture and track. In the downlink, base station to each mobile terminal to broadcast type synchronous signal, therefore, the downlink synchronization is relatively simple, is easy to realize. In the uplink, s
30、ignals from different mobile terminal must be synchronized to the base station, in order to ensure the orthogonality between subcarriers. The base station according to the extraction of each mobile terminal to the sub carrier to carry the information of the time domain and frequency domain synchroni
31、zation information, then the base station transmitted back to the mobile terminal, so that the mobile terminal synchronization. Concrete implementation, synchronization will be divided into synchronous time synchronization and frequency domain, time domain and frequency domain at the same time synch
32、ronization can also be.(2) Channel estimation. In orthogonal frequency division multiplexing system, the design of channel estimator has two main problems: one is the selection of pilot information. Since the wireless channel is often a fading channel, need to continue to track the channel, the pilo
33、t information must also be continuous conveyor. The two is to design both the channel estimator with low complexity and good pilot tracking capability. In practical design, the design of pilot information selection and optimal estimator is usually related to each other, because the transmission perf
34、ormance and pilot information estimator of.(3) Channel coding and interleaving. In order to improve the performance of digital communication system, channel coding and interleaving is a commonly used method. For fading random errors in the channel, the channel coding for fading; burst error channel,
35、 can the interleaving. In practical application, usually at the same time, channel coding and interleaving, to further improve the performance of the whole system. In orthogonal frequency division multiplexing system, if the channel fading is not too deep, the equilibrium is no longer use diversity
36、channel to improve the performance of the system, because the orthogonal frequency division multiplexing system itself has the ability to utilize the channel diversity characteristics, information the general channel characteristics have been orthogonal frequency division multiplexing this modulatio
37、n is used. But the structure of orthogonal frequency division multiplexing system is encoded in inter carrier provides opportunities, the formation of coded orthogonal frequency division multiplexing. Code can be used in a variety of code, such as code, convolutional code, convolutional codes to sco
38、re good results.(4) Reducing the peak to average power ratio. The orthogonal frequency division multiplexing signal in time domain on the performance of the superposition of N orthogonal sub carrier signal, when the N signal is in the peak of overtime, orthogonal frequency division multiplexing sign
39、al will produce the maximum peak, the peak power is N times the average power. Although the probability of peak power of the lower, but in order not to transmit the signal distortion of the peak to average power ratio, the sending end on linear high power amplifier high degree requirements and the t
40、ransmission efficiency is very low, the receiving end of the front-end amplifier and ADC linearity requirements are very high. Therefore, the performance of high peak to average power ratio of the orthogonal frequency division multiplexing system is greatly decreased or even directly affect the actu
41、al application. In order to solve this problem, people put forward the signal distortion techniques, signal scrambling techniques and signal space expansion based on reduction of orthogonal frequency division multiplexing system peak average power ratio based method. As an application of communicati
42、on, OFDM has many technical advantages. (1) In the narrow bandwidth can be a lot of data sent. Orthogonal frequency division multiplexing technology can also separated at least 1000 digital signal, and can safely run around in interference signal, this ability will be a direct threat to the further
43、development of the current has begun to CDMA technology popular and expands, it is because of the special signal penetration capability of the orthogonal frequency division multiplexing technology by European telecommunications operators and mobile phone manufacturers welcomed and accepted.(2) A sud
44、den change in orthogonal frequency division multiplexing technology can constantly monitor the communication characteristics of the transmission medium. Because of the ability to transmit data communication path will change over time, so the orthogonal frequency division multiplexing can dynamically
45、 adapt, and connect and disconnect the corresponding carrier to ensure ongoing successful communication.(3) Orthogonal frequency division multiplexing can automatically detect the transmission medium under which a specific carrier has high signal attenuation and interference pulse, and then take the
46、 appropriate measures to make the specified carrier frequency modulation of the successful communication.(4) Orthogonal frequency division multiplexing technique is especially suitable for use in high-rise buildings, highlighting the densely populated and geographical place and signals of spreading
47、area. Data communication and digital audio broadcasting high hope to reduce multipath effects on signal.(5) The biggest advantages of orthogonal frequency division multiplexing technique is to combat frequency selective fading or narrowband interference. In single carrier systems, a single fading or
48、 interference can cause the entire communication link failure, but in a multi carrier system, only a very small part of carrier interference. The sub channel can also use error correcting codes for error correction.(6) Can effectively resist the interference between the signal waveform, high speed d
49、ata transmission in multipath environment and fading channels. When the channel because of multipath transmission frequency selective fading, only fell on the band depression sub carrier and carry information affected, subcarrier other unimpaired, therefore better BER performance than the general.(7) Through the joint coding of each sub carrier, has a strong ability of anti fading. Orthogonal frequency division multiplexing technology itself ha
