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）
超寬帶無線技術(1)
說到無線通信和聯網技術時，肯定不乏選擇。目前已有多種無線上網形式，如手機、3G、Wi-Fi、WiMax、藍牙、電力線和802.11a、 b、 g和n，你可能會認為其他技術沒有了發展余地。但技術仍在大踏步前進，在今后幾年，我們將看到一種新的、不同的無線技術。
這個無線電領域中的新東西就是超寬帶，也稱UWB或數字脈沖無線。它有助于在無線技術裝備起來的家庭和辦公室中傳送電視節目、電影、游戲和百萬兆字節的數據文件。UWB比目前的無線局域網技術速度更快，提供了一種無干擾的短距離、高帶寬管道。
UWB的起源
Gerald F. Ross在上世紀六十年末和七十年代初首次演示了用于雷達和通信應用的UWB波形的可行性。此項技術最初由（美國）國防高級研究計劃局開發，當時叫基帶、無載波、脈沖通信或者時域信號傳輸，直至1989年美國國防部將它命名為超寬帶。
在有些方面，UWB技術回到了無線電初現曙光和馬可尼的早期火花隙發射的時代。UWB也是擴頻無線電（也稱跳頻）技術的繼承者，這是一項二次世界大戰的技術，該技術將廣播分散到不同的無線電頻率上，每次只用一個頻率，以避免干擾。而UWB在同一時間使用所有可用的頻率。
UWB不是直接替代其他形式的無線通信，但它做了一些其他技術無法與之匹敵的事情。UWB發射機在一個很寬的無線電頻率上發送幾十億個短脈沖。這些射頻脈沖非�？臁谎永m萬億分之一秒到幾納秒，每個脈沖實際上只用了射頻載波的幾個周期。
這樣短的持續時間給予UWB波形一些特別的屬性。相對而言，它們不受多路徑消除效應的影響，例如當返回的強反射波與直接路徑信號不同相位時，降低了接收機中的信號強度。UWB脈沖如此短，以至于反射路徑信號到達之前直接路徑信號已經收到和送出。由于UWB脈沖非常短，所以它們可以使用極寬的頻譜，這就允許信號使用極低的功率，就將無線電頻率之間的干擾降至最低，降低了對人的健康危害，而且常常低于常態的噪聲水平，因此使之很難被探測到。
從技術上講，UWB可以定義成任何一種占據頻譜超過中心頻率20%或者帶寬至少為500MHz的無線技術。為了占據非常寬的帶寬，現代的UWB技術使用不同的調制技術，其中包括正交頻分多路復用。
2002年，美國聯邦通信委員會批準UWB在3.1 GHz至10.6 GHz的頻率范圍內、以受限的發射功率進行商業應用。原則上，UWB可以設計成使用幾乎任何一部分的射頻頻譜。（未完待續）
文章來源于領測軟件測試網 http://www.kjueaiud.com/
