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    Avoiding Static for Spread Spectrum

    Fresnel Zone Clearance

    A discussion on the propagation characteristics of a microwave beam always brings up the subject of Fresnel zones and Fresnel zone clearances. Fresnel zones can be viewed as a series of concentric ellipsoids surrounding the entire microwave path. 

    If you sliced through a path at any point, the cross-section would be a series of concentric circles. 

    From a propagation point of view, the first Fresnel zone is defined as the "surface" containing every point for which the sum of the distances from that point to the two ends of the path is exactly one half of a wavelength longer than the direct end-to-end path (see Figure 1). 

    Figure 1

    The First Fresnel Zone

     

    The "nth" Fresnel zone is defined in the same manner, except that the difference is in n half-wavelengths. 

    Fresnel zone clearance is important both from reflection and obstruction standpoints. From a practical standpoint, a 60% clearance of the first Fresnel zone (0.6F1) radius is a requirement for all microwave paths. As you will note, the radius of the first Fresnel zone will be the greatest at the mid-path point. For example, the first Fresnel zone radius at the mid-point of a 10-mile path is approximately 73.6 feet. If you were to draw a line from the center of one antenna to the center of the other, following the 60% clearance requirement (0.6F1), mid-path clearance for this path needs to be just over 44 feet from the antenna centerline. 

    You should always assume that a microwave signal will be blocked by trees and other forms of foliage. Many microwave links have been installed during the winter months, with microwave paths literally running through the barren branches of deciduous trees. Come spring, with new growth, these paths are often rendered useless

    Through the Glass

    Numerous point-to-point microwave links have at least one antenna inside of a building, transmitting and receiving through a glass window. Some of these windows are even impregnated with wire mesh, but are relatively transparent to selected microwave frequencies.
    The transparency of glass to microwave energy can vary dramatically, and is not something that can be readily determined by visible light.
    The actual transmission capability of glass is dependent upon a variety of factors,to include metal content and coating, as well as the specific frequency of the microwave signal.
    Glass which seemingly has little or no effect on visible light may severely attenuate microwave signals.
    Conversely, glass which dramatically attenuates visible light may have little or no effect on microwave energy.
    The only way to determine whether a window is suitable for microwave transmission is to have the glass manufacturer provide appropriate information on its transmission attributes, or to have the glass tested. Never assume that a window is transparent to microwave energy without having evidence of its microwave transmission characteristics.

    Inference Issues

    Spread spectrum microwave radio systems are among the most interference tolerant communication networks in use today. Spread spectrum signals are very difficult to detect and, by their nature, are highly resistant to jamming and interference.
    But as more and more signals are spread, the "noise level" in the band increases accordingly. Once the noise floor reaches a certain level, effective communication in the band is effectively negated.
    In the U.S., the 2.4GHz band is license free, making it very difficult to know whether or not another spread spectrum radio is operating in a manner which could possibly interfere with one's own link. While these links are usually able to spread narrow band interference, other spread spectrum signals in the 2.4 GHz band could possibly interfere if they are of the proper frequency and amplitude.
    It is extremely difficult to predict the effect of an interfering signal unless specific information is known about the interferer. In general, other spread spectrum signals in the 2.4 GHz band tend to rise the band's noise floor.
    For this reason, even when working with paths which are very short and not subject to any sort of fading condition, a fade margin of 15 dB or greater should always be maintained for the path.

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    Steve Miller is the execuitve vice president of Wireless, Inc. 

    COPYRIGHT NOTICE: 

    "Reproduced with permission of the author and Wireless, Inc." 

    "Reproduced with permission from Telecom Asia, November 97 Pages 53/54/56. Copyright by Advanstar Communications Inc. Advanstar Communications Inc. retains all rights to this article. This article may only be viewed. User may not activley save any text or graphics/photos to local hard drives or print pages to paper or duplicate this article in whole or in in part, in any medium. Advanstar Communications Inc. home page is located at http://www.advanstar.com" 

    Reprinted from TELECOM ASIA, November 1997 AN ADVANSTAR ~ PUBLICATION Printed in U.S.A 

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