qrcoder c# Optical Wireless Mesh Networks in Objective-C

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Optical Wireless Mesh Networks
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Lens Detector (PN diode, APD) Receive aperture Receive Optics and Aperture
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Field of view FSO system
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FSO system The angle defines the FoV of the receiver Receive Field of View FoV of the FSO receiver
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Figure 93 Receive optics and receive field of view
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four times the amount of light A larger aperture also has the benefit of mitigating the effect of atmospheric scintillation due to averaging over a greater area of the receiver However, as in the case of transmitter optics, the downside of using a large aperture is the size, weight, and cost of the system Unlike transmitter optics where multiple transmit beams can be used to transmit more power, using multiple receive optics to increase the amount of received signal collected is not always efficient because of the challenges in combining the signals received from multiple receivers This is unlike RF receivers, where multiple antennas are used at a great advantage to system performance Such advantages in RF systems are derived mostly in cases of non-line-of-sight and point-to-multipoint communications systems; both of these scenarios do not apply to optical wireless systems as discussed in this chapter, however Receive Field of View (FoV) Field of view (FoV) is the region within which the receiver can see It is the counterpart of beam divergence and is defined by the angle of the cone (measured in degrees) within which the transmitter has to be located in order for the receiver to receive the signal As shown in Figure 93, the receiver can see the transmitter within its FoV, identified by the circle, and thus can receive the signal from the transmitter within the FoV The downside of having a larger FoV may not be apparent until the impact of background light is considered By virtue of being able to see everything within its FoV,
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a receiver collects all the light it sees within its FoV For the most part, the light consists of the signal transmitted by the FSO system at the other end of the link However, the collected light also consists of all the background light that exists within the FoV The background light thus collected acts as noise that, when sufficient, can degrade the performance of the optical wireless link Therefore, a system with a larger FoV collects more background noise than a system with a smaller FoV, though it may collect the same amount of signal, thus reducing the overall signal-to-noise ratio The amount of background noise can also be reduced significantly by optically filtering the received signal Narrowband optical filters are routinely used in optical wireless products to knockout unwanted background light from the receiver However, the ratio of background light received by receivers with different FoVs remains the same For example, regardless of the amount of filtering used, an FSO system collects four times as much background light as a similar system with half the FoV Additionally, doing optical filtering poses its own limitations For example, using too narrow a filter, which is often costly, may also knock off signals from wider spectrum sources such as LED Finally, making FoV smaller poses the same challenges as reducing beam divergence It requires precision components, precision manufacturing, and complex alignment As discussed in the preceding sections, it is often desirable to use FSO systems with narrow divergence and FoV However, even a small scale mispointing of such a narrow beam can easily disrupt the FSO link established by the beam There are several reasons for such involuntary mispointing FSO equipment is generally installed in open environments such as buildings and on poles that are likely to exhibit small movements For example, buildings are subject to daily sway due to thermal expansion and contractions and poles exhibit oscillations under heavy winds In other cases, FSO systems often get installed too close to sources of vibration such as large air conditioners causing the FSO systems to resonate along with the vibrating equipment All of these involuntary movements can cause beam mispointing There are two common ways to compensate for mispointing due to involuntary movement for FSO links: (1) passively by means of a relatively large beam divergence and FoV and (2) actively by means of tracking Larger beam divergence and FoV are not highly desirable, as discussed in the preceding sections On the other hand, the complexity of tracking required to compensate for all types of mispointing may make it impractical for certain applications The right solution is often a combination of both methods Movements that produce large magnitude mispointing, such as building expansion, happen at much slower speeds, in the order of several minutes to a few hours Compensation for such a large mispointing solely by passive means would require a relatively large divergence and FoV However, such slow movements are suited to being corrected by means of active tracking using much simpler mechanisms than would be required to compensate for fast movements On the other hand, movements that are fast (in the order of milliseconds such as the ones produced by vibrations) cause much smaller magnitude mispointings Compensation for such fast and small mispointings solely by means of active tracking may not be commercially viable for certain applications However, they can be compensated for much more reliably by passive means
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