tailieunhanh - Điện thoại di động mạng lưới Radio P14

Mobile Satellite Communication∗ In principle communications satellites provide the same connectivity as terrestrial (wireless and wireline) networks. The advantages of satellites, such as fast wide-area coverage, flexible transmission parameters and cost independence due to distance, are compared with the disadvantages, such as restricted channel capacity because of the frequencies available, orbital positions, need for line-of-sight connectivity and high initial investment besides relatively long signal propagation times. As a result, only certain application areas have been developed for satellites in the past. . | Mobile Radio Networks Networking and Protocols. Bernhard H. Walke Copyright 1999 John Wiley Sons Ltd ISBNs 0-471-97595-8 Hardback 0-470-84193-1 Electronic 14 Mobile Satellite Communication Fundamentals In principle communications satellites provide the same connectivity as terrestrial wireless and wireline networks. The advantages of satellites such as fast wide-area coverage flexible transmission parameters and cost independence due to distance are compared with the disadvantages such as restricted channel capacity because of the frequencies available orbital positions need for line-of-sight connectivity and high initial investment besides relatively long signal propagation times. As a result only certain application areas have been developed for satellites in the past. Application Areas Satellites are being widely used for distribution functions . for transmitting television and radio programmes as well as for data. Existing communications networks can be totally bridged through the use of satellite systems. Satellite communication which until recently was almost only exclusively used for navigation and aviation as well as in land vehicles is a branch that opens up a totally new world of applications. Satellite paging along with GPS Global Positioning System and GLONAS have recently been introduced for civil use. The interest in global personal communications is leading to big efforts in the development of new satellite systems that operate at the low orbital heights. Currently mobile satellite systems are being used mainly in areas where no other terrestrial communications systems are available on the open seas in the desert in rural regions etc. . These systems are also attractive to users who operate internationally and otherwise use different kinds of terrestrial mobile radio systems requiring them to carry terminals with different standards. A differentiation is made between worldwide regional and national systems depending on the coverage .