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Cognitive radio technology is the key technology
that enables an xG network to use spectrum in a
dynamic manner. The term, cognitive radio, can
formally be defined as follows:
a‘‘Cognitive Radio’’ is a radio that can change its
transmitter parameters based on interaction with
the environment in which it operates.
From this definition, two main characteristics of
the cognitive radio can be defined :
• Cognitive capability: Cognitive capability refers
to the ability of the radio technology to capture
or sense the information from its radio environment.
This capability cannot simply be realized
by monitoring the power in some frequency band
of interest but more sophisticated techniques are
required in order to capture the temporal and
spatial variations in the radio environment and
avoid interference to other users. Through this
capability, the portions of the spectrum that
are unused at a specific time or location can be
identified. Consequently, the best spectrum
and appropriate operating parameters can be
selected.
• Reconfigurability: The cognitive capability provides
spectrum awareness whereas reconfigurability
enables the radio to be dynamically
programmed according to the radio environment.
More specifically, the cognitive radio can
be programmed to transmit and receive on a
variety of frequencies and to use different transmission
access technologies supported by its
hardware design
.
The ultimate objective of the cognitive radio is to
obtain the best available spectrum through cognitive
capability and reconfigurability as described
before. Since most of the spectrum is already
assigned, the most important challenge is to share
the licensed spectrum without interfering with the
transmission of other licensed users as illustrated
in Fig. 3. The cognitive radio enables the usage of
temporally unused spectrum, which is referred to
as spectrum hole or white space . If this band is
further used by a licensed user, the cognitive radio
moves to another spectrum hole or stays in the same
band, altering its transmission power level or modulation
scheme to avoid interference as shown in
Fig. 3.
In the following subsections, we describe the
physical architecture, cognitive functions and reconfigurability capabilities of the cognitive radio
technology
