Page 57 - DCAP311_DCAP607_WIRELESS_NETWORKS

Unit 3: Radio Frequency and Light Signal Fundamentals

used. PSK modulators are often designed using the QAM principle, but are not considered as Notes
QAM since the amplitude of the modulated carrier signal is constant. QAM is used extensively as
a modulation scheme for digital telecommunication systems. Arbitrarily high spectral efficiencies
can be achieved with QAM by setting a suitable constellation size, limited only by the noise level
and linearity of the communications channel.
QAM modulation is being used in optical fiber systems as bit rates increase; QAM16 and QAM64
can be optically emulated with a 3-path interferometer.

3.4.4 Spread Spectrum

Spread-spectrum communications technology was first described on paper by an actress and
a musician! In 1941 Hollywood actress Hedy Lamarr and pianist George Antheil described a
secure radio link to control torpedos. They received U.S. Patent #2.292.387. The technology was
not taken seriously at that time by the U.S. Army and was forgotten until the 1980s, when it
became active. Since then the technology has become increasingly popular for applications that
involve radio links in hostile environments.
Typical applications for the resulting short-range data transceivers include satellite-positioning
systems (GPS), 3G mobile telecommunications, W-LAN (IEEE® 802.11a, IEEE 802.11b, IEEE
802.11g), and Bluetooth. Spread-spectrum techniques also aid in the endless race between
communication needs and radio-frequency availability—situations where the radio spectrum is
limited and is, therefore, an expensive resource.
Different spread-spectrum techniques are available, but all have one idea in common: the
key (also called the code or sequence) attached to the communication channel. The manner of
inserting this code defines precisely the spread-spectrum technique. The term “spread spectrum”
refers to the expansion of signal bandwidth, by several orders of magnitude in some cases, which
occurs when a key is attached to the communication channel.
The formal definition of spread spectrum is more precise: an RF communications system in which
the baseband signal bandwidth is intentionally spread over a larger bandwidth by injecting a
higher frequency signal (Figure 1). As a direct consequence, energy used in transmitting the
signal is spread over a wider bandwidth, and appears as noise. The ratio (in dB) between the
spread baseband and the original signal is called processing gain. Typical spread-spectrum
processing gains run from 10dB to 60dB.
To apply a spread-spectrum technique, simply inject the corresponding spread-spectrum code
somewhere in the transmitting chain before the antenna (receiver). (That injection is called the
spreading operation.) The effect is to diffuse the information in a larger bandwidth. Conversely,
you can remove the spread-spectrum code (called a despreading operation) at a point in the receive
chain before data retrieval. A despreading operation reconstitutes the information into its original
bandwidth. Obviously, the same code must be known in advance at both ends of the transmission
channel. (In some circumstances, the code should be known only by those two parties.)
Figure 3.2. Spread-spectrum Communication System

Source: http://www.maximintegrated.com/app-notes/index.mvp/id/1890

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