SOFTWARE DEFINED RADIO

PRESENTATION OVERVIEWDefinition

History

SDR advantages

Motivation toward SDR

Technical overview

Architecture

Software Overview

DEFINITION

What’s SDR

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 – Inventory

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

SDR ADVANTAGES

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 facilities

• problems in rolling-out new services/features due to wide-spread presence of legacy subscriber handsets.

TECHNICAL OVERVIEW

•IDEAL SDR

•IDEAL RECEIVER

•IDEAL TRANSMITTER

• PRACTICAL RECEIVERS

•TYPICAL COMPONENTS

IDEAL SDR

• 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 RECEIVER

• 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.

PRACTICAL RECEIVERS

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 frequency.

Digital IQ modulator used.

Real analog-to-digital converters lack the discrimination to pick up sub-microvolt, nanowatt radio signals.

A low noise amplifier must precede the conversion step.

Typical Components of SDR

Analog Radio Frequency (RF) receiver/transmitter in the 200 MHz to multi-gigahertz range.

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

DUC: Digital upconverter DDC: Digital downconverter CFR: Crest factor reduction DPD: Digital predistortion PA: Power amplifier LNA: Low noise amplifier

RF Front End

The principle of operation depends on the use of heterodyning or frequency mixing.

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 frequency.

The IF signal is itself filtered and amplified and possibly processed in additional ways

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 shifted version

original 90 deg phase shifted version

ADC & DAC

ADC- Sampling ( Nyquist theorem)

Quantisation

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-linearity

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 output.

FPGA

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 Languages)

More flexible than microcontroller.

Software Overview Digital Signal Processing (DSP) software

applications employ the math of Fourier Transforms..

FT describes which frequencies are present in the original function.

An open architecture Allows third party waveform/component development

Standardised procedure for Loading and Control of software modules

Should be one relying on proven technologies – shorter development time

THE END

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