第17章电子信息类专业英语(李白萍)

Unit Seventeen Others (Ⅱ)
Analogue Radio-relay Systems The first microwave radio-relay system was established in 1947 by Bell Telephone Laboratories (BTL) between New York and Boston (300km). It carried 480 channels operating in the 4 GHz band, using frequency modulation and heterodyne repeater scheme. The fact that most of the present microwave systems use the same modulation and repeater method is a clear indication of the excellent foresight of BTL.[3]
Unit Seventeen Others (Ⅱ)
Digital Radio-relay Systems The digital systems for which preliminary researches were conducted in the 1940s were unfortunately superseded by the rapidly developed frequency modulation (FM) systems, mainly because of the lack of high-speed pulse technique for multiplexing and the ineffectiveness in frequency spectrum utilization.[4] The study on microwave digital transmission systems was resumed in the early 1960s. According to various investigations on the microwave digital systems, their features can be summed up as follows:Unit Sຫໍສະໝຸດ venteen Others (Ⅱ)
Radio communication operating at medium or high frequencies was first applied to the fields, such as broadcasting, maritime mobile and intercontinental communication, where its superiority was evident. It was only after the advent of microwave communication technology that it became possible for radio technology to be applied in full scale in the field where cable transmission had been dominant. Thus, the microwave radio-relay system using analogue modulation grew into a very important transmission medium in the overall telecommunications networks in many countries. Recently, a rapid advance in semiconductor and pulse circuit technologies gave impetus to the research and development of digital radio-relay systems.[2] There is now a marked trend that the telecommunication network is moving gradually towards digital systems.
Unit Seventeen Others (Ⅱ) Passage A Microwave Radio System
Forty years have passed since the research and development of microwave technology were launched in the United States, the United Kingdom, Germany, France and other countries. At the beginning, research was mainly directed towards the development of radar technique. The results were first applied for military purposes. The development of the microwave radio-relay system was based on radar technique, and it now plays an important role as the transmission medium for multiplex telephone and television signals.[1]
Unit Seventeen Others (Ⅱ)
the realization of low-cost high-speed PCM terminal equipment contributes a great deal to the reduction of the total system cost; the almost complete pulse regenerative repeating can be achieved provided the carrier-to-noise power ratio is above the threshold level, and the accumulation of noise due to multiple repeating is negligible. Therefore high-quality long-haul transmission is attainable; the flexibility in transmitting various kinds of signals, such as voice, data, and video, is available without mutual interactions among different signals.
Unit Seventeen Others (Ⅱ)
The first full-scale commercial use of a digital radio-relay system with the above advantages commenced in 1969 by the introduction of a 2 GHz digital radio transmission system carrying 240 telephone channels, which was developed in Japan.[5] Since then, development has continued in various countries all over the world, especially in the United States, Canada, Italy, United Kindom, France and Japan. Frequency bands used for digital systems are mainly 2, 11 and 13 GHz at the present stage. The 2 GHz band is generally applied to small capacity systems about 10 Mbps, and the 11 or 13 GHz band to medium capacity systems at 50 or 100 Mbps.[6] These frequencies are generally applied to short-haul radio-relay systems. One example of a long haul-digital system is the 6 GHz band system of 45 Mbps which has been operational for some years in the United States. There is a marked tendency that the future telecommunications network in general will gradually evolve into the integrated digital network.
Unit Seventeen Others (Ⅱ)
On the other hand, in Europe, which consists of many small or medium countries, microwave systems in the network seems to be slightly less important. In many developing countries, microwave radio-relay systems play the key role in the network. It is quite logical because the microwave system has the following features: the initial investment required for the construction of transmission routes is low; the construction can be completed within a relatively short period; it is suitable for television transmission.