OFDM 现代经济中一项极具潜力的通信技术 

OFDM: 现代经济中一项极具潜力的通信技术

一直以来, 无线广播网络的发展都限制于实际的传输速率（少于11Mbps）。现在，由于正交频分复用 (OFDM)的出现，无线网络标准的像 IEEE 802.11a和 802.11 g将无线局域网的传输速率定义在50Mbps左右。

不仅仅是WLANs， 厂商更喜欢快捷简便，而且下一代无线网络采用 OFDM技术，将使OFDM技术在大城市网络（MAN）的使用与码分多址技术网络具有相同的覆盖率，或者超过CDMA。 的确, 正当以 CDMA 为基础的第三世代 (3 G)无线网络声称为终极无线技术网络服务进入美国市场时,Flarion 正在朝着OFDM第四的代无线技术发展。

并且,不仅是厂商，IEEE 802.16的一个委员会也在为OFDM技术在MANs网络建设制定一个标准。

OFDM 实际上在 1960 年代后期就已经出现了，但是在无线技术高速发展的阶段并没有采用OFDM技术。智能商务联盟的家庭网络技术开发的主管，Navin Sabharwal说, “基于OFDM技术的产品在数字电视与无线网络之间架设了一座桥梁，使数字电视具有可移动性”.

多媒体的应用，使大众对网络传输速度的要有了更高的要求，要求更快的传输速率，这成为OFDM技术发展的主要推动力。同时， MDR's Allen Nogee, Senior Analyst 等一些资深的分析家认为，“OFDM技术有很好的前景，尤其表现在廉价高效这方面！”

OFDM技术的发展并不是一帆风顺的。工业分析家认为，OFDM可能是非常完美的下一代通信技术，但是它确实还存在有不少的问题需要解决。举个例子来说明4G网络的发展，“虽然OFDM被证明是最好的通信技术，但是，现有的以CDMA技术为基础的公司是不愿意采用OFDM技术的，而且可能会抵制OFDM技术，政治因素影响着新技术的发展。”

甚至出现更难堪的事情-- 尽管 OFDM 技术开发组织的努力，在技术上，仍建立很多与OFDM技术不相溶的标准。 OFDM技术：

OFDM 技术在开发的40年里,仅仅用在极为不恰当的地方，如ADSL（非对称数字用户环线）宽带和数字音频、视频的传输，只是在近些年来，无线通信才采用的OFDM技术。

我们需要更深入的了解 OFDM设备是如何工作。

无线电收发设备，采用了支持OFDM技术的集成电路，通过扩频通信的手段，采用快速傅立叶变换（FFT）处理器，数据可以不断的在平行的信道上进行传输，很显然，OFDM技术的使用，使无线通信网络建设上升到一个更高的层次。

OFDM技术数据包是通过窄带在不同频率上进行传输的。“OFDM技术的主要思想就是在频域内将给定信道分成许多正交子信道，在每个子信道上使用一个子载波进行调制，并且各子载波并行传输。这样，尽管总的信道是非平坦的，具有频率选择性，但是每个子信道是相对平坦的，在每个子信道上进行的是窄带传输，信号带宽小于信道的相应带宽，因此就可以大大消除信号波形间的干扰。由于在OFDM系统中各个子信道的载波相互正交，于是它们的频谱是相互重叠的，这样不但减小了子载波间的相互干扰，同时又提高了频谱利用率。” 道格拉斯琼斯博士说道。

OFDM技术区别与以往的数据传输，主要在与数字信号的调制，琼斯继续说到，“采用快速的傅立叶变换（FFT）运算法则，可以更精确的对同时采用的不同的频率进行控制，使得数据能够在平行的信道上进行可靠的传输。”凭着在发射和接受设备上的FFT的使用，OFDM的系统。“非常有效地分隔可能会相互干扰的信号频率，与次通道没有产生冲突的最大带宽限制，因为在传输的时候信号频率之间是相互正交的。”

OFDM技术更好表现在于抗多径干扰能力，琼斯解释到，“接受到的信号并不只是单单我们想要接受的信号，而是接受到想要的信号在不同情况下延迟后的一个综合体，到底有多少个信号重叠，这就要决定于到底有多少反射体对信号进行反射，像筑物、飞机等等，对信号的反射，信号经过不同的反射物后，信号到达接受机的时间就回有不同的延时，就会接受到一个综合的信号。”一个简单的间隔设置，就能够避免对多径失真的修正，而不需要复杂的错误订正！

琼斯解释说，OFDM技术采用频率本身的特点解决的多径传输的错误。OFDM在平行的信道上进行传输信号，甚至减慢传输每比特数据传输的速率，来解决多径传输的问题。举个例子来说，比方说，我们要在一秒中传输1,000,000比特数据，我们可以每一微秒传输其中的一比特的信息，任何延时的信号延时超过1微秒就会与第二个信号叠加而产生干扰。如果采用在1000个不同频率的OFDM信道上传输这1000比特的信号，我们就能更慢的传输这些数据，经过1微秒多径延迟的信号只能重叠传输信号中的1/1000，因此几乎不会引起信号的干扰。 除了解决多径延迟引起的干扰，OFDM技术还必须解决其他的很多问题，其中最重要的是要解决频率的偏移以及相位的偏移。

这些基本问题，都是工程设计的问题---易受频率偏差的影响.由于子信道的频谱相互覆盖,这就对他们之间的正交性提出了严格的要求.由于无线信道的时变性,在传输的过程中出现无线信号的频谱偏移或发射机与接受机本地振荡器之间存在的频率偏差都会使OFDM系统子载波之间的正交性遭到破坏,产生子信道的信号相互干扰(ICI),这种对频率偏差的敏感是OFDM系统主要的缺点之一。 为了解决这个问题，IEEE802.11a规定了在每个数据包的开头含有52个子信道其中的4个导频载波，通过二进制相移键控调制产生一个“导频信号”。这个导频信号不断地跟踪接受机和发送机的参考相位。一旦有频率的相位偏移，压控振荡器就回自动纠正频率与初速电流状态相同来解决这个干涉问题。

OFDM: a great potential in the modern economy of communication technology All along, the radio network development are limited to the actual transfer rate (less than 11Mbps). Now, thanks to orthogonal frequency division multiplexing (OFDM) the emergence of wireless networking standards such as IEEE 802.11a and 802.11 g wireless LAN transmission rate at 50Mbps or so defined.

Just WLANs, vendors prefer quick and easy, and next-generation wireless network using OFDM technology, OFDM technology in the big cities will network (MAN) network using CDMA technology and has the same coverage, or more than CDMA. Indeed, when the CDMA-based third generation (3 G) wireless network technology are claimed to be the ultimate wireless network services into the U.S. market, Flarion is moving in the fourth-generation OFDM wireless technology development.

And not just firms, IEEE 802.16 is also a committee for the OFDM technology in the construction of MANs network to develop a standard.

OFDM is actually in the late 1960s had already seen, but in the stage of rapid development of wireless technology does not use OFDM technology. Smart Business Alliance home networking technology development director, Navin Sabharwal said, \bridge, make digital TV with mobility.\

Multimedia applications, so that the public network transmission speeds have a higher demand, requiring a faster transfer rate, which became the main driving force for the development of OFDM technology. Meanwhile, MDR's Allen Nogee, Senior Analyst and some senior analysts, \and efficient in this respect!\

OFDM technology is not easy. Industry analysts believe, OFDM can be perfect the next generation of communications technology, but it does still exist many problems to solve. To give an example to illustrate the development of 4G networks, \been proven to be the best communication technology, however, to the existing CDMA technology-based companies are reluctant to use OFDM technology, and may resist the OFDM technology, political factors affecting the development of new technologies. \

Even more embarrassing things - even though the efforts of OFDM technology development organization, technically, OFDM technology is still not set up a lot of compatibility with the standard. OFDM technology:

OFDM technology in the development of 40 years, used only in very inappropriate places, such as ADSL (Asymmetric Digital Subscriber Line) broadband and digital audio and video transmission, but in recent years, wireless communications using OFDM technology only.

We need a better understanding of how work OFDM devices.

Radio equipment, using OFDM technology to support the integrated circuit, by means of spread spectrum communication, using fast Fourier transform (FFT) processors, data can be continuously transmitted in the parallel channel, obviously, OFDM technology use, construction of the wireless communication network to a higher level.

OFDM technology in data packets through a narrow-band transmission on different frequencies. \

into many orthogonal subchannels, each subchannel using a sub-carrier modulation, and each sub-carrier transmission in parallel, so that, despite the overall channel non-flat, with frequency selective, each subchannel is relatively flat in each subchannel is narrow-band transmission on the signal bandwidth is less than the corresponding bandwidth of the channel, and therefore can greatly eliminate the interference between the signal waveform due to the OFDM system each sub-channel carrier orthogonal, so their spectrum is overlapping, it will not only reduce the interference between sub-carriers, while improving the spectral efficiency. \

OFDM technology differences between the previous data transmission, mainly in the digital signal modulation, Jones continued, %use of fast Fourier transform (FFT) algorithms can be more accurate for the same time using different frequencies to control, making the data In parallel to the channel for reliable transmission. \and receiving equipment on the use of the FFT, OFDM systems. \effective in separating the signal may interfere with each frequency sub-channels do not conflict with the maximum bandwidth limit, because the transmission time between the signal frequency is orthogonal to each other.\

Better performance of the OFDM technology is anti-multipath interference, Jones explains, \signal in different situations of a complex delay , in the end the number of signal overlap, which would be decided in the end how many reflections of the signal reflector, such as buildings, airplanes, etc., the signal reflection, signal through different reflector, the signal reaches the receiver time went back to a different delay, will receive an integrated signal. \simple interval, we can avoid multipath distortion correction, without the need for complex error revised!

Jones explained that, OFDM technology uses the frequency characteristics of their own to solve the multi-path transmission error. OFDM transmission in the parallel channel signal, or even slow down the transmission rate of data transmission per bit, to solve the multipath problem. For example, say we have to be transmitted in a second 1,000,000 bits of data, we can transfer each of microseconds which one bit of information, any delay of the signal delay and the second will be more than 1 microsecond signals superimposed interference. If you use different frequencies in the 1000 the 1000-bit transmission channel OFDM signal, we can transfer data more slowly, after a 1 microsecond delay multipath signals can transmit signals in overlapping 1 / 1000, so almost does not cause signal interference.

In addition to addressing the delay caused by multipath interference, OFDM technology must also address many other issues, the most important thing is to solve the frequency offset and phase offset.

These basic problems are susceptible to engineering design problems --- the impact of the frequency deviation due to each sub-channel spectrum coverage, which between them raised a strict orthogonality requirements because of time-varying wireless channel , occurred during the transmission of wireless signals in the spectrum or the transmitter and receiver offset local oscillator frequency offset between the sub-carrier OFDM system will cause the orthogonality between the destruction, resulting in each sub-channel signal interference (ICI), the frequency deviation of the sensitivity of OFDM system is one of the

major drawbacks.

To solve this problem, IEEE802.11a provides at the beginning of each packet contains 52 sub-channel one of the four pilot carrier by binary phase shift keying modulation to generate a \the reference phase. Once the frequency of the phase shift, went back to auto-correct the VCO frequency with the same muzzle velocity of the current state intervention to solve this problem.