Journal of Emerging Technologies in Web Intelligence Contents

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124 JOURNAL OF EMERGING TECHNOLOGIES IN WEB INTELLIGENCE, VOL. 2, NO. 2, MAY 2010In this paper, we take a stance that lies inbetween the two cases described above. The digitizationof the signal is performed inthe Intermediate frequencyband, followed by the digital down conversion. Thebaseband processing is done in a general purposecomputer. Though this setupdoes not giveus a completeesoftware control over the radio spectrum, it does offer usa great level of flexibility with the software since we canactually demodulate the signal using software. Thearchitecture used in this project is indicated in Figure 3.This helps us to explore the part of radio spectrum whichhfalls in the IF region that canbe digitized.The digital radio systemconsists of three mainfunctional blocks: RF section, IF section and basebandsection. The RF section consists of essentially analoghardware modules while IF and baseband sectionsscontain digitalhardware modules. The RFsection (alsocalledas RF front-end) is responsiblefortransmitting/reeceiving the radio frequency (RF) signalfrom the antenna via a coupler and converting the RFsignal to an intermediate frequency (IF) signal. The RFfront-end on the receive path performs RFamplificationand analog down conversion from RF to IF. On thetransmit path, RF front-end performs analog upconversion andRF power amplification.The ADC/ /DAC blocks perform analog-to-digitaconversion (on receive path) and digital-to analogconversion (ontransmit path), respectively. DDC/DUCCblocks performdigital-downconversion (on receive path)anddigital-up-conversionrespectively. DUC/DDC blocks essentially performmodem operations, i.e., modulation of the signal ontransmit path and demodulation of the signal on receiveepath.For this paper work, the RF section andthe IF sectionare handled bythe hardware, followed bythe basebandprocessing by Computer. The hardware used is known asthe Universal Software RadioPeripheral (USRP).(on transmitpath) ),III. SOFFTWARE BASEDFM RECEIVERRThe real implementation of the project starts fromhere. The hardware used is a custom built board provideddby the creators of the GNU Radio community, which isdesigned to run the free software toolkit provided bythem. The main goal isto take the FM baseddemodulationone step further to receive up to four FMstations simultaneously.What theUSRP gives to the Computer is a Digital-Down converted, complex, quadrature signal in theBaseband. Theremaining processing willl be taken careby the software after gettingg the signal from the USRPboard. Thus once the signal enters the computer, thedemodulation of the FMsignal consists of the followingsteps.(a) Signal flow from the air to the computer (from real tocomplex)(b) Getting the instantaneous frequency (from complex toreal)(c) De-emphasizer(d) AudioFIR decimation filter(e) Outputo Soundcard/FileThus each stage of this signal processing acts as a blockwith input and outputt ports. Each block receives thesignal asinput from the previous block, performs therequired signal processing and gives the output to thenext block in the chain.The software architecture is implemented in two layers.The bottom layer is implemented in C++, which satisfiesthe performance requirement of thedemodulationandsignal processing. The top layer is implemented inpython, which acts as a mask layer, interconnecting thebottom C++ layers which perform the bulk of the signalprocessing. This two layered architecture gives us theadvantageor reusing the signal processing blocks andmakes it easier to customize the signal processingflownecessary. Thus connecting or removing a processingblock from the software demodulation chain is mucheasier. The entire software layer is built and run on a freeUNIX port for windows called cygwin. Cygwin offers aUNIX platform on windows based systemIV. SIGNAL FLOW FROM THE AIR TO THE COMPUTER(FROM REAL TO COMPLEEX).Basically what the USRP does is to select the part ofthe spectrum we are interested in and decimate the digitalsequence by some factor N. The resulting signal iscomplex with I/Q two channels. Thus USRP gives out a`complex' signal, witha data rate 256k samples persecond, called `quadrature rate' – or quad rate becausethe complex signal has I/Q quadrature components(Figure. 3).Figure.3 Digital down Converter© 2010 ACADEMY PUBLISHER
JOURNAL OF EMERGING TECHNOLOGIES IN WEB INTELLIGENCE, VOL. 2, NO. 2, MAY 2010 125V. GETTING THE INSTANTANEOUS FREQUENCY (FROMCOMPLEX TO REAL).For FM, the instantaneousfrequency of thetransmitted waveform is varied as a function of the inputsignal. The instantaneous frequency at anytime is givenby the following formula:f(t) = k * m(t) + f cm(t) is the input signal, k isa constant that controls thefrequency sensitivity and f c is the frequency of the carrier(for example, 100.1MHz). So to recover m(t), two stepssare needed. First we need toremove the carrier f c , thenwe're left with a baseband signal that has aninstantaneousfrequency proportional tothe originalmessage m(t) ). The secondstep is to compute theinstantaneous frequency of the baseband signal.Removing the carrier is taken care by the USRP, viathe digital down converter (DDC). The resulting signalcoming into the Computer has already become abaseband signal and the remaining task is to calculate itsinstantaneousfrequency. If we integrate frequency, weget phase, or angle. Conversely, differentiating phasewith respect totime gives frequency. These are the keyinsights we use to build the receiver. The angle betweentwosubsequentsamples can be determined bymultiplying one by the complex conjugate of the otherand then takingthe arc tangent of the product.Thus Arctangent of theproduct gives the phasedifference between adjacentsamples, if we divide it bythe sample interval or multiply the data rate, we get theradian frequency, which gives the instantaneoussfrequency (f) if further divided by 2. This process ofrecoveringtheinstantaneousfrequency from thequadrature signal from the USRP is called the“Quadrature Demodulation”(Figure. 4).Figure.4 Quadrature Demodulationfrequency end usually becomes unbearable. Tocircumvent this effect, `pre-emphasis' and `de-emphasis'were introduced into the FM system. At the transmitter,proper pre-emphasis circuits are used to manuallyamplify the high frequency components, and the converseoperations are done at the receiver to recover the originalpower distribution of the signal. As a result, the SNR isimprovedeffectively. Inthe analog world, a simple firstorder RLC circuit usually suffices for pre-emphasis andde-emphasis. In the case of the Software defined Radio, afirst order IIR filter is implemented to get the magnitudeof amplified, high frequency signal down tothemagnitude of normal lower frequency signal, therebyimproving the SNR.Let us now see about Audio FIR decimation filter:After passing the de-emphasizer, of 256kHz.It is a baseband signal, containingall the frequency components of a FM station. Theitis a real signal witha data ratebandwidth of a FM station is usually around 2 * 100kHz.A sample rate of 256kHz is suitable for the 200kHzbandwidth, without losing any spectrum informationnFigure.5 Typical FM BandThe FMsignal band is spread out with various band ofinformation (Figure. 5) ). From 0 to about 16 kHz is theleft plus right (L + R) audio. The peak at 19 kHz is thestereo pilot tone. The left minus right (L - R) stereoinformation is centeredat 2x the pilot (38kHz) and isAM-modulated on top of the FM. Additional sub carriersare sometimes found in the region of 57kHz - 96kHz.Thus for the reception of a mono FMstation, a LowpassFIR filterwith a pass band frequency of 15Khz and atransitionn band with 1Khz should suffice. Thus, once thesignal comes out of theFIR decimation filter block, it isnow ready to be played into the soundcard.Output toSoundcard/File:VI. DE-EMPLASIZERIt has beentheoreticallyproved that, in FM detector,the power of the output noise increases with thefrequency quadratically. However, for most practicalsignals, such as human voiceand music, the power of thesignal decreases significantlyas frequencyincreases. Asa result, the “Signal to Noise Ratio” (SNR) at the highOnce the signal passes through the FIR Decimationfilter, it is ready to be played bythe soundcard oralternatively stored in a File. The signal from theFIRdecimation filter is ina raw bit format and can beconvertedto other formats using any suitablecompression algorithms.© 2010 ACADEMY PUBLISHER
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