ADTRAN 6000 Series system manual Line-Of-Sight, Terminology, Decibels, Calculating the Fade Margin

Section 2 Microwave Path Engineering Basics TRACER 6000 Series Integrated System Manual

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

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.

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

xdB = 10 ⋅ log10(x) (dB)

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.

x		= 10 ⋅ log		x	(dBm)
	dBm		10	⎛ ------------⎞
				⎝ 1mW⎠

Using the decibel format simplifies power calculations by reducing multiplication and division operations into addition and subtraction operations.

3.CALCULATING THE FADE MARGIN

It is imperative to determine whether the proposed microwave path is suitable (at a minimum) for ideal, nondistorted signals before attempting installation.

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 (PR ) and the receiver sensitivity (Psens ).

F = PR – Psens = PT + GT + GR - L - LP - Psens (dB)

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