User Manual

Section 2 Microwave Path Engineering Basics TRACER 6000 Series Integrated System Manual
16 Copyright © 2005 ADTRAN, Inc. 612806420L1-1E
1. LINE-OF-SIGHT
The TRACER 6410 and TRACER 6420 systems are designed for operation in the license-free 2.400 to
2.483 GHz and 5.725 GHz to 5.850 GHz industrial, scientific, and medical (ISM) bands, respectively.
Radio wave propagation in these bands exhibits microwave characteristics which are ideally suited for
point-to-point, line-of-sight communications. Line-of-sight requires that the transmitting antenna and
receiving antenna are able to “see” each other, and that the straight-line path between the two antennas is
free of obstructions such as buildings, trees, mountains, and in longer paths, even the curvature of the
earth. For maximum signal strength, the area around the visual line-of-sight where microwave signals
reflect (Fresnel zone) must also be free of obstructions. Fresnel zones are discussed in more detail on
page 25.
Terminology
2. DECIBELS
Understanding the decibel (dB) format is key when discussing microwave path engineering because the
received signal power is often expressed in decibel format. In general, any quantity can be expressed in
decibels. If the quantity x is a power level (in watts), the decibel equivalent is defined as
If the quantity x, expressed in milliwatts (mW), is referenced to a mW, then the decibel-milliwatt (dBm) is
used instead of a generic decibel.
Using the decibel format simplifies power calculations by reducing multiplication and division operations
into addition and subtraction operations.
3. CALCULATING THE FADE MARGIN
The fade margin (F ) is a value in decibels (dB) that represents the amount of signal reduction that can be
tolerated before the link exceeds the specified bit error rate (BER). Fade margin is simply the difference
between the available signal power at the receiver (P
R
) and the receiver sensitivity (P
sens
).
Point-to-Point Wireless communication from a single site to another individual
site. Contrast with point-to-multipoint.
Line-of-Sight An unobstructed, direct path exists between the transmitting and
the receiving antennas.
It is imperative to determine whether the proposed microwave path is suitable (at a
minimum) for ideal, nondistorted signals before attempting installation.
x
dB
10 log
10
x() ⋅ =
(dB)
x
dBm
10 log
10
x
1mW
-------------
⎝⎠
⎛⎞
⋅ =
(dBm)
FP
R
P
sens
–=
(dB)
= P
T
+ G
T
+ G
R
- L - L
P
- P
sens