计算机专业时文选读--超宽带无线技术（一）
添加时间: 2007-4-13 3:49:12 作者: 计算机等级考试认证参考 阅读次数:47 来源: http://www.d9soft.com
Ultrawideband(1)
There’s certainly no lack of choices when it comes to wireless communications and networking technologies. With all the currently available forms of wireless access ——cell phones, 3G, Wi-Fi, WiMax, Bluetooth, power lines, and 802.11a, b, g and n——you wouldn’t think there’s room for anything more. But technology marches forward, and in the next couple of years, we’re going to be seeing a new and different wireless technology.
The new kid on the radio block is ultrawideband, also known as UWB or digital pulse wireless. It will help deliver television programs, movies, games and multimegabyte data files throughout our wireless homes and offices. UWB is faster than current wireless LAN technologies and provides a short-range, high-bandwidth pipe that eliminates interference.
Origins of UWB
Gerald F. Ross first demonstrated the feasibility of UWB waveforms for radar and communications applications in the late 1960s and early 1970s. Originally developed by the Defense Advanced Research Projects Agency, the technology was called baseband, carrier-free, impulse communications or time-domain signaling, until the U.S. Department of Defense named it ultrawideband in 1989.
In some respects, UWB technology goes back to the dawn of radio and Guglielmo Marconi’s early spark-gap transmissions. UWB is also a successor to spread-spectrum radio (also called frequency-hopping), a World War II technology that splits a broadcast across many different radio frequencies, using one at a time to avoid jamming. In contrast, UWB uses every frequency available to it, all at the same time.
UWB isn’t a direct substitute for any other form of wireless communications, but it does some things that no other technology can match. A UWB transmitter sends billions of short-duration pulses across a wide spectrum of radio frequencies. These RF bursts come so fast——lasting only from a few trillionths of a second to a few nanoseconds——that each actually uses only a few cycles of an RF carrier wave.
This short duration gives UWB waveforms some unique properties. They are relatively immune to multipath cancellation effects, such as when a strong reflected wave arrives out of phase with the direct path signal, reducing the signal strength in the receiver. UWB pulses are so short that the direct signal has come and gone before the reflected path arrives, so no cancellation takes place. Because UWB pulses are so short, they can use very wide frequency spectra; this allows signals to use very low power, which minimizes interference with and from other radio frequencies, reduces health hazards and often falls below the normal noise floor, thus making it harder to detect.
Technically, UWB is defined as any radio technology whose spectrum occupies more than 20% of the center frequency, or a bandwidth of at least 500 MHz. Modern UWB systems use various modulation techniques, including Orthogonal Frequency Division Multiplexing, to occupy these extremely wide bandwidths.
In 2002, the Federal Communications Commission approved the commercial use of UWB transmissions in the range from 3.1 GHz to 10.6 GHz, at a limited transmission power. UWB systems can, in principle, be designed to use nearly any part of the RF spectrum.
This will allow the streaming of high-definition video between media servers and high-definition monitors, as well as the extremely fast transfer of files between servers and portable devices.（To Be Continued）
超宽带无线技术(1)
说到无线通信和联网技术时，肯定不乏选择。目前已有多种无线上网形式，如手机、3G、Wi-Fi、WiMax、蓝牙、电力线和802.11a、 b、 g和n，你可能会认为其他技术没有了发展余地。但技术仍在大踏步前进，在今后几年，我们将看到一种新的、不同的无线技术。
这个无线电领域中的新东西就是超宽带，也称UWB或数字脉冲无线。它有助于在无线技术装备起来的家庭和办公室中传送电视节目、电影、游戏和百万兆字节的数据文件。UWB比目前的无线局域网技术速度更快，提供了一种无干扰的短距离、高带宽管道。
UWB的起源
Gerald F. Ross在上世纪六十年末和七十年代初首次演示了用于雷达和通信应用的UWB波形的可行性。此项技术最初由（美国）国防高级研究计划局开发，当时叫基带、无载波、脉冲通信或者时域信号传输，直至1989年美国国防部将它命名为超宽带。
在有些方面，UWB技术回到了无线电初现曙光和马可尼的早期火花隙发射的时代。UWB也是扩频无线电（也称跳频）技术的继承者，这是一项二次世界大战的技术，该技术将广播分散到不同的无线电频率上，每次只用一个频率，以避免干扰。而UWB在同一时间使用所有可用的频率。
UWB不是直接替代其他形式的无线通信，但它做了一些其他技术无法与之匹敌的事情。UWB发射机在一个很宽的无线电频率上发送几十亿个短脉冲。这些射频脉冲非常快——只延续万亿分之一秒到几纳秒，每个脉冲实际上只用了射频载波的几个周期。
这样短的持续时间给予UWB波形一些特别的属性。相对而言，它们不受多路径消除效应的影响，例如当返回的强反射波与直接路径信号不同相位时，降低了接收机中的信号强度。UWB脉冲如此短，以至于反射路径信号到达之前直接路径信号已经收到和送出。由于UWB脉冲非常短，所以它们可以使用极宽的频谱，这就允许信号使用极低的功率，就将无线电频率之间的干扰降至最低，降低了对人的健康危害，而且常常低于常态的噪声水平，因此使之很难被探测到。
从技术上讲，UWB可以定义成任何一种占据频谱超过中心频率20%或者带宽至少为500MHz的无线技术。为了占据非常宽的带宽，现代的UWB技术使用不同的调制技术，其中包括正交频分多路复用。
2002年，美国联邦通信委员会批准UWB在3.1 GHz至10.6 GHz的频率范围内、以受限的发射功率进行商业应用。原则上，UWB可以设计成使用几乎任何一部分的射频频谱。
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