Cambium Networks PMP 450 AP, Планирование и руководство по установке

Страница: 163 / 287

Chapter 3: System planning Link planning
Page
3-33
Link planning
This section describes factors to be considered when planning links, such as range, obstacles path loss
and throughput. LINKPlanner is recommended.
Range and obstacles
Calculate the range of the link and identify any obstacles that may affect radio performance.
Perform a survey to identify all the obstructions (such as trees or buildings) in the path and to assess
the risk of interference. This information is necessary in order to achieve an accurate link feasibility
assessment.
The 450 Platform Family is designed to operate in Non-Line-of-Sight (NLoS) and Line-of-Sight (LoS)
environments. An NLOS environment is one in which there is no optical line-of-sight, that is, there are
obstructions between the antennas.
OFDM technology can often use multi-pathing to an advantage to overcome nLOS, especially in cases
where the Fresnel zone is only partially blocked by buildings, “urban canyons”, or foliage. OFDM tends
to help especially when obstacles are near the middle of the link, and less so when the obstacles are
very near the ODU.
However, attenuation through walls and trees is substantial for any use of the 5.4 GHz and 5.8 GHz
frequency bands. The lower frequency radio waves of 900 MHz radios provide greater penetration
through walls, trees and other obstacles, making it optimal for most non-line-of-sight applications. Even
with OFDM, these products are not expected to penetrate walls or extensive trees and foliage.
Path loss
Path loss is the amount of attenuation the radio signal undergoes between the two ends of the link. The
path loss is the sum of the attenuation of the path if there were no obstacles in the way (Free Space
Path Loss), the attenuation caused by obstacles (Excess Path Loss) and a margin to allow for possible
fading of the radio signal (Fade Margin). The following calculation needs to be performed to judge
whether a link can be installed:
capability seasonal fade excess space free
L L L L L
< + + +
_
Where: Is:
space free
L
_
Free Space Path Loss (dB)
excess
L
Excess Path Loss (dB)
fade
L
Fade Margin Required (dB)
seasonal
L
Seasonal Fading (dB)
capability
L
Equipment Capability (dB)