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122 JOURNAL OF EMERGING TECHNOLOGIES IN WEB INTELLIGENCE, VOL. 2, NO. 2, MAY 2010SDR and Error Correction using ConvolutionEncoding with Viterbi DecodingShriram K. VasudevanVIT/Embedded Systems, Vellore, IndiaEmail: shriramkv@rocketmail.comSiva Janakiraman and Subashri VasudevanSASTRA/Communication, Tanjore, IndiaSASTRA/Computer Science, Tanjore, IndiaEmail: sivajanakiraman@gmail.com,vrsuba@gmail.comAbstract— The aim of the paper is to build asoftware based radio capable of demodulating FrequencyModulated signals with the ability to receive upto fourstations simultaneously. In addition convolution encodingalong with Viterbi decoding is also implemented to aid inerror correction capability of digital transmissionsystems. The initial phase is aimed at software baseddemodulation of multiple channels of FrequencyModulated signals. The second phase is directed towardsthe Error correction using convolution encoding withViterbi decoding for streaming data encountered indigital broadcast systems.Software Radio is a way of designing softwarevery close to the antenna. The basic feature of softwareradio is that software will define the transmittedwaveforms, and software will demodulate the receivedwaveforms. This paper is developed with the help of freesoftware based radio toolkit given by a community namedGNU radio. Channel coding schemes need to be usedextensively in the case of transmission of digital data overthe air or through any other medium. These codingschemes also called FEC (Forward Error Correction)are used in cases where the re-transmission of the data isnot feasible or possible. The channel coding schemescomes to the rescue in such cases where redundant dataare sent over the transmission medium along with themessage. In the receiver side, this channel coded signal isdecoded to get back the original data even if the channelcoded signal undergoes some interference from the noisein the transmission medium. Though the channel codinghas a downside of requirement to transmit additionaldata over the transmission medium, the advantagesoffered by error detection and correction mechanisms tothe receiver outweighs the disadvantage of additionaldata transfer rate and bandwidth requirements.Index Terms—SDR, Viterbi Decoder, Convolution EncoderI. INTRODUCTIONSoftware-Defined Radio (SDR) is an evolvingtechnology that has received enormous recognition. In thepast few years, analog radio systems are replaced bydigital radio systems for various radio applications incivilian, military and commercial spaces. In addition tothis, programmable hardware modules are playing a vitalrole in digital radio systems at different functional levels.SDR totally aims to take benefits of these programmablehardware modules to get open-architecture based radiosystem software.SDR technology supports implementation of some ofthe functional modules in a radio system such asmodulation/demodulation, signal generation, coding andlink-layer protocols in software. This assists in gettingreconfigurable software radio systems where dynamicselection of parameters for each of the above-mentionedfunctional modules is possible. A Complete hardwarebased system has many limitations.Before getting into what software radio does, it is goodto review the design of a traditional analog, hardwarebasedradio (Figure.1). In wireless communications,information is encoded into radio waves. These arecollected (or transmitted) from (to) the air by the antenna.The received signal is then passed to a series ofcomponents that extract the useful information andconvert it into the output of the radio. The basic design isthe same whether the radio signal is destined for a cellphone, microwave repeater, or AM/FM car radio.Traditional radios are based on the super heterodyne(superhet) receiver circuit.Manuscript received August 2, 2009; revised August 20, 2009;© 2010 ACADEMY PUBLISHERdoi:10.4304/jetwi.2.2.122-130
JOURNAL OF EMERGING TECHNOLOGIES IN WEB INTELLIGENCE, VOL. 2, NO. 2, MAY 2010 123key reasons why software radios will not bedeployed first in end-user devices suchas cell phones, butrather inbase stations which can take advantage ofexternal power sources.Figure 1. Generic FM receiverIn the superhet receiver, the incomingsignal, whichhis Radio Frequency (RF), is first down-converted to alower intermediate frequency(IF). The IF is then filteredfor noise andamplified before being demodulated toproduce the baseband signal that represents the desireddinformation. This analog baseband signal may be passeddirectly to further downstage processing, or it may firstbe digitized and subjected toadditional signal processing.There are several important reasons for down convertingto a lower and standardized IF:(a) It is easierand hence less expensive, to build filtersand amplifiers – especially linear amplifiers – for a lowerfrequency signal.(b) Use of a common IF enables standardization in thedesign of radiocomponents.Traditionaldesignsfor implementingthe superheterodyne receiver architecture were optimized forspecific frequencies and applications and each of thestages were implemented inhardware that was closelycoupled. This was due largely to the difficulties inherentin and limitations in the state of the art inthe design ofanalog signal processing components. Analog processingis much more complicated than digital processing, whichhis one of the key reasons why the transition to digitalsignals is so important.In this paper chapter 2 deals about theperformance and power limitations, Chapter 3 talks aboutarchitecture ofSDR. Chapter4 is on signalflow, Chapter5 is dealing with how to derive instantaneous frequency.Chapter 6 talksabout De Emphasizer. Chapter 7 explainsabout multi channel FM reception. Chapter 8 will givedetails about the Forward Error Correction.Chapter 9 and10 speaks on implementation and results. Chapter 11 isconcluding thepaper followed by references.II. PERFORMANCE ANDPOWER LIMITATIONSThe change from hardware to software radioshas faced lot of problems. First, performance generallycomes down in the shift from dedicated to general-purpose hardware.Secondly, the transition from hardware tosoftware processing results in a substantial increase incomputation which ultimately results in increased powerrequirements. This reduces battery life and is one of theIII. ARCHITECTUREWe now take a view at the overview of a basicconventional digital radio system and then look athowSDR technology can be used toimplement radiofunctions in software. This will be followed by thesoftware architecture of SDR. Thedefinition of thesoftware radio system is not absolute and depends onwhere theAnalog to digital conversion is performed. Thequestion of where theA/D conversion is performeddetermines what radiofunctions can be movedintosoftware and what types of hardware are required. At oneextreme, we consider calling it software radio if softwareis used atany stage within the radio.This case typicallyinvolves the digitization at the baseband for signalprocessing. In this casethe actual demodulation processis performed before theconversion todigital format. Thissetup is more of a digital radio than software radio. Whatprocess can be done using software isvery much limited.Since theactual demodulation itself takes placewithhardware, it cannot beused for demodulation of otherkinds ofsignal and hence is very limited in itsfunctionality.At the other extreme, we might choose to use softwareradio forcases in which A/D conversion is performedright at the antennaa with all radio functionalityimplemented in software running on general-purposehardware. In this case the Digitizationof the signaltakesplace right at the antenna and thus gives us the completeflexibility. This setup requires the use of a veryhighspeed andwideband ADC, not economically feasible atthis point in time. Figure 2 shows the basic softwareradio architecture.Figure.2 Basic Software Radio Architecturee© 2010 ACADEMY PUBLISHER
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