only when one of the phase zones relative to that line-of-sight is blocked by an obstacle that the phase difference between wavelets impacts a receiver. Remember that itʼs the first zone that creates the most significant impact on the received signal and degradation due to obstruction in the first zone is exacerbated by the fact that the relative effect of the second zoneʼs opposing influence is magnified when the first zone is blocked.
Fresnel Zones
The zones that have been described are referred to as “Fresnel Zones” and the first Fresnel zone (the inner-most zone) is commonly discussed in 802.11 literature. Unfortunately, there are very few reasonable, non-mathematical descriptions of exactly what a Fresnel zone is, and this leads to confusion. A Fresnel Zone may be thought of as a 3-dimensional oval volume defined along the axes
formed by the straight line between a transmitting antenna and a receiving antenna. The figure below shows a single Fresnel Zone volume.
Figure 6.8 The Oval Volume of a Fresnel Zone
Multiple Fresnel Zones are built up around the central line-of-sight axis as shown in the figure below.
Figure 6.9 Multiple Fresnel Zones Built Up Around the Central Axis
As mentioned, itʼs only the first Fresnel Zone (the innermost zone) that is of particular significance with regard to signal obstruction in an 802.11 wireless network.
Fresnel Zones are not Related to Antenna Gain or Directivity
The radius of the Fresnel Zone volume at any point along its axis is determined solely by the wavelength of the signal and is completely unrelated to how the energy is dispersed in the field. Therefore, the radius of a Fresnel Zone is unrelated to antenna beamwidth, directivity, or gain.
A directional antenna offers some particular total field energy in some particular direction. That field energy, whatever its value, is either unopposed or opposed by Fresnel Zone interaction when an obstacle gets too close to the straight line-of-sight between the antenna and the receiver. Consider
Math and Physics for the 802.11 Wireless LAN Engineer |
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Copyright 2003 - Joseph Bardwell