OFDM正交频分复用.docx

1、OFDM正交频分复用OFDM正交频分复用原文载于维基百科，由zslcn周生烈编译摘注评，西邮学子整理用于通信与电子信息工程专业英语Unit3 Expanding reading B OFDMOrthogonal frequency-division multiplexing (OFDM), essentially identical to coded OFDM (COFDM) and discrete multi-tone modulation (DMT), is afrequency-division multiplexing(FDM) scheme used as a digital mu

2、lti-carriermodulationmethod. A large number of closely spacedorthogonalsub-carrier signalsare used to carrydata. The data is divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme (such asquadrature amplit

3、ude modulationorphase-shift keying) at a lowsymbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.OFDM本质上与编码OFDM（COFDM）和离散多音调制（DMT）是一样的。这是一种频分复用（FDM）方案,用来作为数字多载波调制方法。大量紧挨的正交子载波信号用来承载数据；数据分成若干个并行的数据流或信道，每个信道一个子载波；每个子载波使用通常的调制方案（如正交幅度

4、调制或相移键控）,以低的信符速率(symbol rate)对其进行调制；从而维持整个数据速率,以类似于相同带宽的常规单载波数据速率,进行传输。Orthogonal frequency-division multiplexing (OFDM) is a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme forwidebanddigital communication, whetherwirelessor overcopperwires,

5、 used in applications such as digital television and audio broadcasting,DSLbroadband internet access.正交频分复用（OFDM）是一种在多个载波频率上对数字数据编码的方法。OFDM已经发展成为宽带数字通信的一种流行方案；无论是无线还是在有线上，这种方案广泛应用于 如数字电视和音频广播、DSL宽带互联网接入.The primary advantage of OFDM over single-carrier schemes is its ability to cope with severechann

6、elconditions (for example,attenuationof high frequencies in a long copper wire, narrowbandinterferenceand frequency-selectivefadingdue tomultipath) without complex equalization filters. Channelequalizationis simplified because OFDM may be viewed as using many slowly modulatednarrowbandsignals rather

7、 than one rapidly modulatedwidebandsignal. The low symbol rate makes the use of aguard intervalbetween symbols affordable, making it possible to eliminateintersymbol interference(ISI) and utilize echoes and time-spreading (that shows up asghostingon analogue TV) to achieve adiversity gain, i.e. asig

8、nal-to-noise ratioimprovement. This mechanism also facilitates the design ofsingle frequency networks(SFNs), where several adjacent transmitters send the same signal simultaneously at the same frequency, as the signals from multiple distant transmitters may be combined constructively, rather than in

9、terfering as would typically occur in a traditional single-carrier system.OFDM比单载波方案优越,主要在于OFDM有能力应付恶劣的信道条件（例如，在长铜线中的高频衰减、窄带干扰、和由于多径引起的频率选择性衰落）,免去了对均衡滤波器过于复杂的需求。之所以能简化信道均衡,是因为OFDM可以看作为 使用许多被慢调制的窄带信号,而不是一个被快调制的宽带信号。低信符速率使有可能不会花费太大的代价,在信符间使用保护间隔,来避免信符间干扰（ISI）、消除回声和延时扩展（在模拟电视上显示为重影），以达到分-集增益，也就是改善信号信噪比

10、,而不会太多降低频谱效率。这种机制也有利于单一频率网络(SFN)的设计。SFN是一种这样的网络,其中几个相邻发信机,同时发送相同频率的同样信号,好像这些发自多个远隔的发信机的信号,在结构上组合在一起。而通常在一个传统的单载波系统中却是很易发生干扰的。In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers areorthogonalto each other, meaning thatcross-talkbetween the sub-channels is eliminated and inter-car

11、rier guard bands are not required. This greatly simplifies the design of both thetransmitterand thereceiver; unlike conventionalFDM, a separate filter for each sub-channel is not required.在OFDM中,子载波频率是这样选择的,即各子载波彼此频率正交,意味着子信道之间的串扰是可以被抵消的,而且也不需要载波间的保护频段。这就大大简化了发信机和收信机的设计；不再像常规的FDM那样，已没有必要为每一个子信道安排一个分

12、开的滤波器。OFDM requires very accurate frequency synchronization between the receiver and the transmitter; with frequency deviation the sub-carriers will no longer be orthogonal, causing inter-carrier interference (ICI) (i.e., cross-talk between the sub-carriers). Frequency offsets are typically caused b

13、y mismatched transmitter and receiver oscillators, or byDoppler shiftdue to movement. While Doppler shift alone may be compensated for by the receiver, the situation is worsened when combined withmultipath, as reflections will appear at various frequency offsets, which is much harder to correct. Thi

14、s effect typically worsens as speed increases, and is an important factor limiting the use of OFDM in high-speed vehicles. Several techniques for ICI suppression are suggested, but they may increase the receiver complexity.OFDM需要在收信机和发信机之间有非常准确的的频率同步；子载波的频率偏差将导致不再正交,造成载波间干扰（ICI）（即子载波之间的串扰）。频率偏移通常是由于

15、发信机和收信机的振荡器不匹配所引起的，或者,由于移动而引起多普勒频移。虽然单独的多普勒频移可以在收信机中得到补偿，但当同时有多径结合时,情况就会恶化；因为各种频率偏移都会出现反射，这就更难校正了。这种效应通常随速度增加而恶化,成为限制OFDM在高速行驶的车辆中使用的一个重要因素。对于ICI的抑制有若干种技术建议，但他们可能会增加收信机的复杂性。orthogonality allows for efficient modulator and demodulator implementation using the FFT algorithm on the receiver side, and i

16、nverse FFT on the sender side. Although the principles and some of the benefits have been known since the 1960s, OFDM is popular for wideband communications today by way of low-costdigital signal processingcomponents that can efficiently calculate the FFT.正交允许在接收端采用FFT算法,并在发送端使用反向FFT,以高效地执行解调器和调制器功能

17、。虽然其原理及其得益早在20世纪60年代以来 就已众所周知，然而,直到今天才普及OFDM宽带通信,是由于采用了低成本的数字信号处理组件；使用它,才可以有效地计算FFT。One key principle of OFDM is that since low symbol rate modulation schemes (i.e., where the symbols are relatively long compared to thechanneltime characteristics) suffer less fromintersymbol interferencecaused bymul

18、tipath propagation, it is advantageous to transmit a number of low-rate streams in parallel instead of a single high-rate stream. Since the duration of each symbol is long, it is feasible to insert aguard intervalbetween the OFDM symbols, thus eliminating the intersymbol interference.OFDM的关键原理之一,是因为

19、低信符速率的调制方案（即信符时间间隔 与信道时间特性相比,相对较长),从而由于多径传播引起的信符间干扰少了，因此，用多个并行发送的低速率流,来取代一个单一的高速率流,是有利的。由于每个信符的持续时间很长,在OFDM信符之间插入保护间隔也就可行,从而消除了信符间干扰。The guard interval also eliminates the need for apulse-shaping filter, and it reduces the sensitivity to time synchronization problems.保护间隔也消除了脉冲整形滤波器的需要，并减少了时间同步问题的敏感

20、性。A simple example:If one sends a million symbols per second using conventional single-carrier modulation over a wireless channel, then the duration of each symbol would be one microsecond or less. This imposes severe constraints on synchronization and necessitates the removal of multipath interfere

21、nce. If the same million symbols per second are spread among one thousand sub-channels, the duration of each symbol can be longer by a factor of a thousand (i.e., one millisecond) for orthogonality with approximately the same bandwidth. Assume that a guard interval of 1/8 of the symbol length is ins

22、erted between each symbol. Intersymbol interference can be avoided if the multipath time-spreading (the time between the reception of the first and the last echo) is shorter than the guard interval (i.e., 125 microseconds). This corresponds to a maximum difference of 37.5 kilometers between the leng

23、ths of the paths.一个简单的例子：如果某站在无线信道上,使用传统的单载波调制,发送每秒一兆个信符，则每个信符的持续时间是1微秒或更少。这严重制约了同步，同时还必须消除多径干扰。如果将该每秒一兆个信符扩展到一千个正交子信道中,每个信符的时间间隔就延长1000倍（即1毫秒），而总带宽近似相同。假定,在每个信符之间插入1/8信符长度的保护间隔。如果多径时间展延（接收第一个信符和最后一个回声信符之间的时间）小于保护间隔（即125微秒,这相当于路径长度最大差值为37.5公里),信符间干扰就可以避免。Thecyclic prefix, which is transmitted during

24、 the guard interval, consists of the end of the OFDM symbol copied into the guard interval, and the guard interval is transmitted followed by the OFDM symbol. The reason that the guard interval consists of a copy of the end of the OFDM symbol is so that the receiver will integrate over an integer nu

25、mber of sinusoid cycles for each of the multipaths when it performs OFDM demodulation with the FFT.在保护间隔期间发送循环前缀。它是由表示OFDM信符结束的符号拷贝组成，并跟在OFDM信符之后被发送。之所以要这样做,是因为由OFDM信符结束部分的副本组成的保护间隔,能在收信机使用FFT执行OFDM解调时,对于每个多径,将会在整数个正弦周期上进行整合Thecyclic prefix, which is transmitted during the guard interval, consists o

26、f the end of the OFDM symbol copied into the guard interval, and the guard interval is transmitted followed by the OFDM symbol. The reason that the guard interval consists of a copy of the end of the OFDM symbol is so that the receiver will integrate over an integer number of sinusoid cycles for eac

27、h of the multipaths when it performs OFDM demodulation with the FFT.在保护间隔期间发送循环前缀。它是由表示OFDM信符结束的符号拷贝组成，并跟在OFDM信符之后被发送。之所以要这样做,是因为由OFDM信符结束部分的副本组成的保护间隔,能在收信机使用FFT执行OFDM解调时,对于每个多径,将会在整数个正弦周期上进行整合(保证对所需信号的完整提取)。The effects of frequency-selective channel conditions, for example fading caused by multipat

28、h propagation, can be considered as constant (flat) over an OFDM sub-channel if the sub-channel is sufficiently narrow-banded (i.e., if the number of sub-channels is sufficiently large). This makes frequency domainequalizationpossible at thereceiver, which is far simpler than the time-domain equaliz

29、ation used in conventional single-carrier modulation. In OFDM, the equalizer only has to multiply each detected sub-carrier (each Fourier coefficient) in each OFDM symbol by a constant complex number, or a rarely changed value.关于频率选择性对信道环境的影响,例如由多径传播引起的衰落,如果子信道的带宽足够窄(也就是子信道的数目足够大），可以认为,在OFDM子信道上是不变的

30、，于是就有可能在接收端实现频域均衡，这比用于传统单一载波调制的时域均衡要简单得多。在OFDM中,均衡器只要对 检测到的 每个OFDM信符的子载波（每个傅里叶系数）乘以一个复常数,或一个更改很少的值 就可以了。Our example:The OFDM equalization in the above numerical example would require one complex valued multiplication per subcarrier and symbol (i.e.,complex multiplications per OFDM symbol; i.e., one

31、million multiplications per second, at the receiver). The FFT algorithm requiresthis is imprecise: over half of these complex multiplications are trivial, i.e. = to 1 and are not implemented in software or HW. complex-valued multiplications per OFDM symbol (i.e., 10 million multiplications per secon

32、d), at both the receiver and transmitter side. This should be compared with the corresponding one million symbols/second single-carrier modulation case mentioned in the example, where the equalization of 125 microseconds time-spreading using aFIR filterwould require, in a naive implementation, 125 multiplications per symbol (i.e., 125 million multiplications per second). FFT techniques can be used to reduce the number of multiplications for anFIR filterbased time-domain equalizer to a number comparable with OFDM, at the cost of delay between reception