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Wireless Networking Handbook
(Publisher: Macmillan Computer Publishing)
Author(s): Jim Geier
ISBN: 156205631x
Publication Date: 09/01/96

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  The beam should not be directed near or through electrical power lines or tree branches. Consider tree growth, wind load on trees, and power lines when installing the system. Power lines also sag during warm weather and tighten up during cold weather. Make provisions to discourage nest building in the optical path by birds and insects.
  Make sure the transmission path is at least ten feet above pedestrian or vehicular traffic—preventing accidental viewing of the laser beam and keeping the signal from being interrupted. Make allowances for any unusual effects that traffic may cause, such as dust clouds.
  Make sure the transmission path is not shooting through or near exhaust vents, which can cause steam to be blown into the path. This has the same effect as fog on the laser beam.
  LACE units are fully weatherproofed and are intended to be mounted outdoors. They may be mounted inside buildings, however, and the signal passed through glass windows. When light particles hit a glass surface, some of the light is reflected. With a clear glass window, approximately four percent of the light is reflected per glass surface. If the glass is tinted, the amount of light reflected and absorbed increases. In the case of reflective coatings, the laser light will reflect off of the coating and the light will never be detected at the receiver. Another problem when shooting through glass occurs when it rains. Water droplets on the glass in front of the transmit lens act as additional lenses and can cause the beam to diffuse. Mounting the laser near the top of the window will reduce this problem somewhat, especially if the window has an awning. When passing signals through glass, it is advisable to keep the beam as close to perpendicular to the glass as possible to minimize reflection losses which can reduce signal strength (see fig. 10.3). As the angle of the beam to the glass increases, more and more light is reflected until the critical angle is reached (approximately 42 degrees). Above the critical angle, all the light is absorbed into the glass and no transmission occurs.
  Avoid East-West orientations. Although LCI uses optical filters in the receiver and has a small angle of acceptance, direct sunlight can overload the units for several minutes a day for a few days per year.


Figure 10.3  The critical angle of a laser beam penetrating glass.

  Heat from roof tops, air duct vents, air conditioners, glass-faced buildings, and so on can cause a condition known as shimmer. Shimmer will cause the light beam to bend and appear to dance around the receiver. If sufficient heat is present, the beam will deflect enough to miss the receiver altogether, usually for a few milliseconds at a time, and burst error will occur. When mounting on roof-top locations, the preferred location is at the leading edge of the roof with the front of the laser at least six inches over the edge. Placing the laser at this location minimizes the effect of roof heating, heat rising up the side of the building, and snow accumulation in front of the unit. The location also provides access to the rear of the unit for easier set-up and alignment.
  The movement of LACE units caused by a strong mechanical vibration could cause the system to intermittently go in and out of alignment. Avoid mounting LACE near vibrating machinery such as air conditioning units, compressors, and motors.
  The LACE units are designed to operate within the temperature range of -10 degrees Fahrenheit to +120 degrees Fahrenheit. Although the units may operate at further extremes, do not do so over an extended period of time. If sustained periods of extraordinarily severe temperatures are normally experienced at the installation, the LACE units may be mounted in environmental housings with additional heating or fan cooling as necessary.
  The laser beam produced by LACE is not subject to the interference produced by EMI sources. If LACE units are placed within proximity of such sources, however, the LACE electronics may “pick-up” this interference which would then be impressed on the signals to and from the LACE equipment. LACE should be mounted away from large microwave dishes, antennas, radio stations, or any unusual electronic equipment that may be radiating electromagnetic signals.
  The laser transmission system is employed in point-to-point, line-of-sight applications. To help alleviate the problems with beam wander due to shimmer, the laser beam is purposely diverged by the transmit lens to give a two meter footprint at the receiver. Diverging the laser also helps with maintaining alignment. The received light is focused by a collecting lens onto the photodetector. This lens system has a field of view of only five milliradians, providing some selectivity by cutting down on background light sources. As shown in figure 10.4, the receiver acceptance angle is greater than the transmitter divergence angle, making alignment of the transmitter more critical than alignment of the receiver.


Figure 10.4  Laser system transmitter divergence and receiver acceptance angles.

  LACE units are designed to project a two meter diameter beam at the receiver. This provides some latitude for beam movement. Unit movement, however, should be kept to an absolute minimum to ensure peak performance. A movement of only 1 mm at the transmitter can divert the beam off of the receiver if the units are installed one kilometer apart.
  Ideally, LACE units should be mounted on the corner of the building to which it will be attached and preferably to masonry construction. This will provide the most stable arrangement. When transmitting signals over 300 meters, it is not advisable to mount LACE units anywhere except at the corner of the structure. On buildings where a thin metal skin is covering the building, the base for the mounts must be made to the supporting structure or to the metal substructure.
  Do not mount LACE units on structures which can sway, such as trees, fences, towers, poles, or buildings exceeding 40 stories in height. Always avoid moveable camera mounts.
  Do not mount LACE units to wooden structures. The expansion/contraction properties of these materials through precipitation and temperature make them good sources for movement and should be avoided. For example, high humidity will cause the units to go out of alignment due to the wood expanding.
  Make sure that when LACE is mounted there are no ledges in front of the laser that might be used by roosting birds. Ledges can also cause a problem in rain or snow. Water bouncing up from the ledge onto the optics or snow buildup in front of the optics will diminish performance.
  Once a stable mounting position has been chosen to mount the laser, the actual design of the mount needs to be considered. The mount should always provide a flat surface with contact on the entire bottom surface of the LACE equipment. LACE units should be mounted to supports with a 6"×6"×3/8" metal plate welded to brackets which attach to the supporting structure. These mounts must be substantial enough to resist any movement. Typical light-duty CCTV camera mounts are not acceptable. To test the mount and support structure for a LACE system, push on the mount and the LACE unit with about five to ten pounds of force. This should not disrupt the alignment. If, for example, a person on a ladder leaning against a wall supporting LACE is enough to disrupt alignment, the structure is not strong enough to provide a good, long-term mount.


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