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A software-defined radio (SDR) system is a radio communication system
which can tune to any frequency band and
receive any modulation across a
large frequency spectrum by means of programmable hardware which is
controlled by software.
An SDR performs significant amounts of signal processing in
a general purpose computer, or a
reconfigurable piece of digital electronics. The
goal of this design is to produce a radio that can
receive and transmit a new form of radio protocol only by running
new software. The scheme of any digital receiver consists of an
antenna circuit, followed by low-noise amplifiers and one or more intermediate
frequency conversion phases (IF-phases). Nowadays, the predominant trend is to
convert the information signal into a numerical form and to elaborate samples by
means of numerical techniques in order to execute all the main phases of a
digital receiver. This tendency brings to a technology named Software
Radio.
The hardware of a software-defined radio typically
consists of a super-heterodyne RF front end which
converts RF signals from (and
to) analog IF signals, and analog to digital
converter and digital to analog
converters which are used to convert a digitized IF signal from and
to analog form, respectively.
Software radios may be particularly utilized for the
military and cell phone services,
both of which must serve a wide variety of changing radio protocols in real
time.
Software-defined radio can currently be used to
implement simple radio modem technologies. In the long run, software-defined
radio is expected by its proponents to become the dominant technology in radio
communications. It enables the enabler of the cognitive
radio.
Many Software Radio (SR) designs have been proposed.
The main differences are in architecture and in the boundary point where
transition from analog to digital Signal Processing (SP) occurs. From a SP point
of view, it is best to push the digital processing as close to the antenna as
possible. With device densities and clock speeds increasing constantly, the
development of an operating system environment suitable for programming a host
of protocols and operating over a wider and wider RF bandwidth can be expected.
A likely scenario is that a reconfigurable hardware will become a common part of
the ¡°microprocessor¡±. This transition will allow morphing of the computational
environment based on the application for speed optimization.
The performance of the free
oscillator (piloting the ADC) is expected to be a crucial point for the
correctness of the SDR operations. Theoretical comparison between the
performance of SDR with quartz and rubidium clocks in a carrier recovery
system show that
optimum performance
will be achieved by using rubidium oscillators were employed both in
transmission and reception, as they present values of instability of the order
of 1E-13.
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Software Defined Radio Technology 
