Page 19 - Optical Communications Essentials

Basic Concepts of Communication Systems

Basic Concepts of Communication Systems 9

Intuitively, one can see that if the digitization samples are taken frequently enough
relative to the rate at which the signal varies, then to a good approximation the sig-
nal can be recovered from the samples by drawing a straight line between the sam-
ple points. The resemblance of the reproduced signal to the original signal depends
on the fineness of the quantizing process and on the effect of noise and distortion
added into the transmission system. According to the Nyquist theorem, if the sam-
pling rate is at least 2 times the highest frequency, then the receiving device can
faithfully reconstruct the analog signal. Thus, if a signal is limited to a bandwidth of
B Hz, then the signal can be reproduced without distortion if it is sampled at a rate
of 2B times per second. These data samples are represented by a binary code. As
noted in Fig. 1.6, eight quantized levels having upper bounds V 1 , V 2 , ..., V can be
3
described by 3 binary digits (2 8). More digits can be used to give finer sampling
n
levels. That is, if n binary digits represent each sample, then one can have 2 quan-
tization levels.

1.5. Electromagnetic Spectrum
To understand the distinction between electrical and optical communication
systems and what the advantages are of lightwave technology, let us examine
the spectrum of electromagnetic (EM) radiation shown in Fig. 1.7.

1.5.1. Telecommunication spectral band
All telecommunication systems use some form of electromagnetic energy to trans-
mit signals from one device to another. Electromagnetic energy is a combination
of electrical and magnetic fields and includes power, radio waves, microwaves,
infrared light, visible light, ultraviolet light, x rays, and gamma rays. Each of
these makes up a portion (or band) of the electromagnetic spectrum. The fun-
damental nature of all radiation within this spectrum is the same in that it can
be viewed as electromagnetic waves that travel at the speed of light, which is

Figure 1.7. The spectrum of electromagnetic radiation.

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