Software-defined radio (SDR) is a radio communication system where
components that have been typically implemented in hardware
(e.g. mixers ,filters , amplifiers , modulators /demodulators
, detectors , etc.) are instead implemented by means of software on
a personal computer or embedded system.
HISTORY OF SDR
• The term "Software Defined Radio" was coined in 1991
by Joseph Mitola, who published the first paper on the
topic in 1992
• One of the first public software radio initiatives was a U.S.
military project named SpeakEasy.
• The primary goal of the SpeakEasy project was to use
programmable processing to emulate more than 10
existing military radios, operating in frequency bands
between 2 and 2000 MHz.
Complete Base band processing digital – Reconfigurable
Software upgrading of commercial radios – Future proof
Generic hardware can be used for a variety of applications –
Software prototyping faster and cheaper than hardware
prototyping – Time to market
Libraries of software radio components are easily created and
shared – Reuse
Digital processing of signals is ideal, unencumbered by the non-
linearities that plague analog hardware-Reliability
MOTIVATION TOWARDS SDR
• Commercial wireless communication industry is currently
facing problems due to constant evolution of link-layer
protocol standards (2G, 3G, and 4G)
• existence of incompatible wireless network technologies in
different countries inhibiting deployment of global roaming
• problems in rolling-out new services/features due to wide-
spread presence of legacy subscriber handsets.
• The ideal SDR will cover all frequencies from 9kHz to 300GHz.
• •It will receive/transmit and modulate/demodulate all modulation
modes and bandwidths
• •It will configure itself automatically.
IDEAL TRANSMITTER AND
• The ideal receiver scheme would be to attach an analog-to-
digital converter to an antenna to directly convert RF to digital.
• A digital signal processor would read the converter, and then its
software would transform the stream of data from the converter
to any other form the application requires.
• An ideal transmitter would be similar.
• A digital signal processor would generate a stream of numbers.
These would be sent to a digital-to-analog converter connected
to a radio antenna.
Current digital electronics are too slow to receive
typical radio signals that range from 10 kHz to 2 GHz
Problem solved by using a mixer and a reference
oscillator to heterodyne the radio signal to a lower
Digital IQ modulator used.
Real analog-to-digital converters lack the
discrimination to pick up sub-microvolt, nanowatt
A low noise amplifier must precede the conversion
Typical Components of SDR
Analog Radio Frequency (RF) receiver/transmitter in the 200 MHz to
High-speed A/D and D/A converters to digitize a wide portion of the
spectrum at 25 to 210 Msamples/sec.
High-speed front-end signal processing including Digital Down
Conversion (DDC) consisting of one or more chains of mix + filter +
decimate or up conversion.
Spread spectrum and ultra wideband techniques allow several
transmitters to transmit in the same place on the same frequency
with very little interference
PC equipped with sound card
Architectures of SDR
CFR: Crest factor
LNA: Low noise
RF Front End
The principle of operation depends on the use of heterodyning or frequency
The signal from the antenna is filtered sufficiently at least to reject
the image frequency and possibly amplified.
A local oscillator in the receiver produces a sine wave which mixes with that
signal, shifting it to a specific intermediate frequency (IF), usually a lower
The IF signal is itself filtered and amplified and possibly processed in
DIGITAL IQ modulator
Two carriers of same frequency but 90 deg out of phase are used, which are
combined at transmission.
Message too is modified to consist of two separate signals 90 deg phase
original 90 deg phase shifted version
ADC & DAC
ADC- Sampling ( Nyquist theorem)
Flash ADC is the fastest of all.
DAC- weighted resistor
R-2R ladder V(out)= V( ref)* (D/2^N)
The main problem in both directions is the difficulty of conversion between the
digital and the analog domains at a high enough rate and a high enough accuracy
DDC- Digital Down Conversion
Digital radio receivers often have fast ADC converters delivering vast
amounts of data; but in many cases, the signal of interest represents a small
proportion of that bandwidth. A DDC allows the rest of that data to be
discarded. When performed in a field programmable gate array (FPGA),
simple digital down conversion is broken up into three distinct steps:
frequency shifting, filtering, and decimation
DUC-Digital Up Conversion
Digital radio transmitters use DAC, A DUC is used to generate an IF signal
and increase the sampling rate. The DUC process is the exact inverse of the
DDC process. Instead of down conversion and decimation, a DUC uses
interpolation and up conversion.
Interpolation, or up sampling, translates a low sample rate modulated
signal into a much higher sample rate signal that is ready for up conversion.
This step, often performed in software, can multiply the overall waveform
size by any factor.
Finally, the modulated, interpolated data mixes with a carrier that
upconverts the baseband signal to the required carrier frequency.
Crest factor REDUCTION (CFR)
Crest factor is a measure of a waveform, such as alternating current or
sound, showing the ratio of peak values to the average value. In other
words, crest factor indicates how extreme the peaks are in a waveform
modulation techniques that have smaller crest factors usually transmit more
bits per second than modulation techniques that have higher crest factors.
Crest factor reduction (CFR) reduces the output peak-to-average ratio by
clipping . We can operate closer to the amplifier compression
point, therefore it is more efficient.
DIGITAL PREDISTORTION (DPD)
DPD is an active linearisation technique
used to compensation for amplifier’s non-
Allows the signal to operate close to or
even below P sat.
Correction signal is injected at PA’s input
in order to reduce the overall distortion at
SDR system uses a generic hardware platform with programmable modules
(DSPs, FPGAs, microprocessors) and analog RF modules
FPGAs (Field Programmable Gate Arrays) are amazing devices that now
allow the average person to create their very own digital circuits.
It is an IC that could contain million of logic gates that can be electrically
configured to perform a certain task using HDL ( Hardware Description
More flexible than microcontroller.
Digital Signal Processing (DSP) software applications
employ the math of Fourier Transforms..
FT describes which frequencies are present in the
An open architecture
Allows third party waveform/component development
Standardised procedure for Loading and Control of
Should be one relying on proven technologies – shorter