User's Manual Part 1

3: Detailed Description
APCD-LM043-8.0 (DRAFT C) 39
• More signal recovery from diffraction
• More signal recovery from reflections
Radio line of sight exists when there is a clear optical path between the CCU and EUM
antennas, as well as adequate clearance of the path over terrain, foliage, and buildings. This
clearance requirement is called the Fresnel clearance. The required clearance varies along
the path and reaches a maximum at the path midpoint. If you have a path with Fresnel
clearance, the loss between the antennas is generally equivalent to free-space loss and can
be readily calculated.
NLOS exists when the path between the CCU and EUM is obstructed, or partially obstructed,
by terrain, buildings, or foliage. NLOS is illustrated in Figure 24. Since radio waves reflect,
refract, and diffract, a non line of sight path does not necessarily mean the EUM-CCU radio
link does not have enough signal margin. It simply means that the path loss is greater than the
LOS path loss. Within the engineered NLOS coverage range of the CCU, the NLOS path,
using an indoor antenna, is acceptable for most EUM installations.
Figure 24 Typical NLOS Path
It is difficult to accurately predict NLOS path loss; however, a lot of field data has been
collected and factored into commercially available path-prediction software.
LMS4000 900 MHz radio coverage prediction depends on the following:
• CCU radio output power, transmission-line losses, and antenna height and gain
• Length of the path between the CCU and EUM
• Height of terrain, foliage, and buildings along the path between the CCU and EUM,
which determines the percentage of the path that is obstructed.
• EUM antenna height and gain, transmission-line losses, and receiver sensitivity
• If the EUM antenna is installed indoors, location of the EUM antenna within the end-
user premises, and the premises building type and wall construction
The EUM has been designed to work with the WaveRider indoor switched-beam diversity
antenna. Where greater range is required, outdoor EUM antennas are also available.
Generally, the higher the CCU antenna, the better the range, especially for LOS performance.
Ideally, the CCU antenna should be installed well above the average height of trees in the
vicinity of the CCU.