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Outdoor Installations
Line of Sight and Fresnel Zone
Essential condition for correctly operating outdoor wireless link is direct visibility between transmitter and receiver points, called Line of Sight (LoS), or Readio Frequency Line of Sight (RF LoS). In case LoS is available, we can achieve long range wireless links, crossing distances in multiple km.
RF LoS means, that there is no obstacle in LoS and in the Fresnel Zone. Fresnel Zone, named for physicist Augustin-Jean Fresnel, is one of a number of concentric ellipsoids which define volumes in the radiation pattern of a circular cross section. The highest radius of the Fresnel zone is in the middle of the link. Fresnel provided a means to calculate where the zones are, where obstacles will cause most reflections between the transmitter and the receiver.
In its simplified translation, once there is obstacle in Fresnel zone, the wireless link will behave like there is no direct visibility between Transmitter and Receiver points. There is a simple application available in section "Downloads", which may help you to calculate the Fresnel Zone for your wireless link. The way how to secure the RF LoS and secure free Fresnel Zone is the right placement of antennas on both sides of the link. Please refer to following examples how Fresnel Zone can be disrupted:
Indoor Installations
The transmission of high frequency RF signal is influenced by the environment. 2,4 GHz or 5 GHz waves ar rather to be reflected by the walls then go through them. Signal-to-noise ratio is essential to get the best performance of wireless link. It is about the difference between signal level and noise level on specific frequency. "Noise" contains an interferrence with another transmission on the same frequency /e.g. another wireless device in the area/, but also contains our own transmitted signal reflected by the walls, furniture, windows etc and than received again by the receiver of the same (our) device. As the device expects answer for transmitted signal, but not the reflection of transmitted signal itself, it is considered as noise.
Once we use unappropriate high gain antenna indoor, we actually can possibly decrease performance of the wireless transmission as result of stronger signal interferred by more /even stronger/ reflections considered as noise. Bottom line, signal-to-noise ratio could be even worse as it was before antenna gain increase. Less is more sometimes works as well. The ways how improve the indoor link performance include correct placement of transmitting point, increasing the number of transmitters and/or using of directional sector antennas rather than omniderectional. 
