Link Engineering and RF Path Planning
Overview

The installation of wireless networks requires the same basic planning as for any wired network. The main difference is that due to the nature of the wireless signal, some additional planning is required. This planning includes site selection and RF path analysis. There might also be the need to investigate local zoning laws, as well as government regulations, when erecting towers. The planning of a wireless link involves collecting information by doing a physical site survey and making decisions. These link-engineering tasks are outlined in Figure .

When designing a building-to-building connection, do not forget about the Fresnel zone. Figure lists some important details about line of sight and the Fresnel zone. The Fresnel zone is an elliptical area immediately surrounding the visual path, as illustrated in Figure . It varies, depending on the length of the signal path and the frequency of the signal. The Fresnel zone can be calculated, and it must be taken into account when designing a wireless link. Figure shows some ways to improve the Fresnel effect.

The following tools can be helpful in making an accurate alignment:

  • Balloon – The tether should be marked at three meter (ten feet) intervals, so a height can be established. This value will help determine the overall height of the tower or mast needed, as pictured in Figure .
  • Binoculars or a telescope – These are needed for the more distant links. Remember that the balloon must be visible from the remote site.
  • GPS – For very distant radio links, this tool allows the installer to aim the antennas in the correct direction.
  • Strobe light – This can be used instead of the balloon. Use this at night to determine where to align the antenna and at what height.

A main consideration in a building-to-building design is the Fresnel zone, which we can think of as line of sight. For WLANs, line of sight is more than a direct line between the two antennas. Line of sight is more of an ellipse that is clear of obstacles, and it should account for the future growth of trees.

Because of the Fresnel zone ellipse, the antennas must be mounted high enough to provide for clearance at the midpoint of the Fresnel zone.