ADTRAN 12804108L1A, Системное руководство

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TRACER 4108/4208 System Manual Section 2 Microwave Path Engineering Basics
612804208L1-1B © 2003 ADTRAN, Inc. 15
1. LINE-OF-SITE
The TRACER 4108 and 4208 systems are designed for operation in the license-free 2.400 to 2.4835 GHz
and 5.725 GHz to 5.850 GHz industrial, scientific and medical (ISM) bands, respectively. Radio wave
propagation in these bands exhibit 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. In addition, 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 22.
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 is referenced to a milliwatt (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 indicating the amount of extra signal power available to the receiver to
operate at a maximum 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 if the proposed microwave path is suitable (at a minimum) for
ideal, nondistorted signals before attempting installation.
xdB 10 log10 x
( )
⋅
=
(dB)
xdBm 10 log10
x
1mW
-------------
 
 
⋅
=
(dBm)
F PR Psens – = (dB)