Page 55 - DCAP311_DCAP607_WIRELESS_NETWORKS

Unit 3: Radio Frequency and Light Signal Fundamentals

Interference from other sources of light, however, can still be a problem for systems that use light Notes
signals. For example, the installation of a point-to-point infrared transmission system aimed in
an easterly or westerly direction can receive substantial interference from infrared light found
within sunlight because the sun is low to the horizon. This interference can be high enough in
some cases to completely disrupt transmission of data on the infrared link. When installing these
types of systems, be certain to follow the manufacturer’s recommendations when orienting the
antennae.
Obstructions such as buildings, mountains, and trees offer substantial amounts of attenuation to
light signals as they propagate through the air. Most of these objects are composed of materials
that readily absorb and scatter the light. As a result, be sure that the path between the end points
of a light-based communications system are completely clear of obstacles.

Even if the communications path is open, weather can still impress large amounts of attenuation
to light signals. The problem with weather is that it varies. For example, heavy fog might be
present, and then the skies might be completely clear the following hour. This makes planning
link budgets for light-based systems, especially those operating near the range limits, extremely
difficult. Planners must be certain that the attenuation imposed by weather will not disrupt
communications.

3.4 Modulation

In electronics and telecommunications, modulation is the process of varying one or more
properties of a periodic waveform, called the carrier signal, with a modulating signal which
typically contains information to be transmitted. This is done in a similar fashion to a musician
modulating a tone (a periodic waveform) from a musical instrument by varying its volume,
timing and pitch. The three key parameters of a periodic waveform are its amplitude (“volume”),
its phase (“timing”) and its frequency (“pitch”). Any of these properties can be modified
in accordance with a low frequency signal to obtain the modulated signal. Typically a high-
frequency sinusoid waveform is used as carrier signal, but a square wave pulse train may also
be used.
In telecommunications, modulation is the process of conveying a message signal, for example
a digital bit stream or an analog audio signal, inside another signal that can be physically
transmitted. Modulation of a sine waveform is used to transform a baseband message signal
into a passband signal, for example low-frequency audio signal into a radio-frequency signal (RF
signal). In radio communications, cable TV systems or the public switched telephone network for
instance, electrical signals can only be transferred over a limited passband frequency spectrum,
with specific (non-zero) lower and upper cutoff frequencies. Modulating a sine-wave carrier
makes it possible to keep the frequency content of the transferred signal as close as possible to
the centre frequency (typically the carrier frequency) of the passband.
A device that performs modulation is known as a modulator and a device that performs the
inverse operation of modulation is known as a demodulator (sometimes detector or demod). A
device that can do both operations is a modem (from “modulator–demodulator”).
The aim of digital modulation is to transfer a digital bit stream over an analog bandpass channel,
for example over the public switched telephone network (where a bandpass filter limits the
frequency range to between 300 and 3400 Hz), or over a limited radio frequency band.
The aim of analog modulation is to transfer an analog baseband (or lowpass) signal, for example
an audio signal or TV signal, over an analog bandpass channel at a different frequency, for
example over a limited radio frequency band or a cable TV network channel.

Analog and digital modulation facilitate frequency division multiplexing (FDM), where several
low pass information signals are transferred simultaneously over the same shared physical
medium, using separate passband channels (several different carrier frequencies).

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