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Non-line-of-sight (NLOS) or near-line-of-sight is radio transmission across a path that is partially obstructed, usually by a physical object in the innermost Fresnel zone.212443 Many types of radio transmissions depend, to varying degrees, on line of sight (LOS) between the transmitter and receiver. Obstacles that commonly cause NLOS conditions include buildings, trees, hills, mountains, and, in some cases, high voltage electric power lines. Some of these obstructions reflect certain radio frequencies, while some simply absorb or garble the signals; but, in either case, they limit the use of many types of radio transmissions, especially when low on power budget. Lower power levels at receiver give less space for correctly picking the transmission. Low levels can be caused by at least three basic reasons: low transmit level, for example Wi-Fi power levels; far-away transmitter, such as 3G more than away or TV more than away; and obstruction between the transmitter and the receiver, leaving no good path. NLOS lowers the effective received power. Near Line Of Sight can usually be dealt with better antennas, but Non Line Of Sight is usually dealt with alternative paths or multipath propagation methods. How to achieve effective NLOS networking has become one of the major questions of modern computer networking. Currently, the most common method for dealing with NLOS conditions on wireless computer networks is simply to circumvent the NLOS condition and place relays at additional locations, sending the content of the radio transmission around the obstructions. Some more advanced NLOS transmission schemes now use multipath signal propagation, bouncing the radio signal off other nearby objects to get to the receiver. Non-Line-of-Sight (NLOS) is a term often used in radio communications to describe a radio channel or link where there is no ''visual'' line of sight (LOS) between the transmitting antenna and the receiving antenna. In this context LOS is taken * Either as a straight line free of any form of visual obstruction, even if it is actually too distant to see with the unaided human eye * As a virtual LOS i.e., as a straight line through visually obstructing material, thus leaving sufficient transmission for radio waves to be detected There are many electrical characteristics of the transmission media that affect the radio wave propagation and therefore the quality of operation of a radio channel, if it is possible at all, over an NLOS path. The acronym NLOS has become more popular in the context of wireless local area networks (WLANs) and wireless metropolitan area networks such as WiMAX because the capability of such links to provide a reasonable level of NLOS coverage greatly improves their marketability and versatility in the typical urban environments where they are most frequently used. However NLOS contains many other subsets of radio communications. The influence of a visual obstruction on a NLOS link may be anything from negligible to complete suppression. An example might apply to a LOS path between a television broadcast antenna and a roof mounted receiving antenna. If a cloud passed between the antennas the link could actually become NLOS but the quality of the radio channel could be virtually unaffected. If, instead, a large building was constructed in the path making it NLOS, the channel may be impossible to receive. Beyond Line-Of-Sight (BLOS) is a related term often used in the military to describe radio communications capabilities that link personnel or systems too distant or too fully obscured by terrain for LOS communications. These radios utilize active repeaters, groundwave propagation, tropospheric scatter links, and ionospheric propagation to extend communication ranges from a few miles to a few thousand miles. ==Radio waves as ''plane electromagnetic waves''== From Maxwell's equations〔Pozar, David M. (2005); ''Microwave Engineering, Third Edition'' (Intl. Ed.); John Wiley & Sons, Inc.; pp 5-9. ISBN 0-471-44878-8.〕 we find that radio waves, as they exist in free space in the far field or ''Fraunhofer'' region behave as ''plane waves''.〔Ramo, Whinnery and Van Duzer;"Fields and Waves in Communication Electronics"; John Wiley & Sons, Inc; pp 322-324. ISBN 0-471-58551-3〕〔Morton, A H; "Advanced Electrical Engineering"; Pitman Publishing Ltd.; pp 387-389. ISBN 0-273-40172-6.〕 In plane waves the electric field, magnetic field and direction of propagation are mutually perpendicular.〔A. J. Baden Fuller;"Microwaves, Second Edition"; Pergammon Press; p 47. ISBN 0-08-024228-6.〕 To understand the various mechanisms that allow successful radio communications over NLOS paths we must consider how such plane waves are affected by the object or objects that visually obstruct the otherwise LOS path between the antennas. It is understood that the terms radio far field waves and radio plane waves are interchangeable. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Non-line-of-sight propagation」の詳細全文を読む スポンサード リンク
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