Design and implementation of qpsk modulator using digital subcarrier


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Design and implementation of qpsk modulator using digital subcarrier

  1. 1. TECHNICAL REPORT ON DESIGN AND IMPLEMENTATION OF QPSK MODULATOR USING DIGITAL SUBCARRIER Submitted in partial fulfillment of requirements for the award of the degree of M.Tech(VLSI) In Electronics & Communication Engineering GONGADI NAGARAJU 126H1D5705DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING 1
  2. 2. CertificateThis is to certify that technical seminar entitled DESIGN ANDIMPLEMENTATION OF QPSK MODULATOR USING DIGITALSUBCARRIER had done by G.Nagaraju bearing roll no 126H1D5705 in thedepartment of ECE of NRI institute of Technology in partial fulfilment of M.Tech Isem for the academic year 2012-2013 Head of the Department P Sivarama Prasad Associate Professor Department of ECE 2
  3. 3. INDEX ABSTRACT1.0 Introduction 1.1 Analysis of modulation 1.2 Purpose of Modulation 1.2.1 Ease of radiation 1.2.2 Simultaneous Transmission of Several signals 1.3 Analogue Modulation Methods 1.4 Digital Modulation Methods2.0 Analysis of Quadrature Phase Shift Keying 2.1 Phase shift keying 2.2 QPSK Modulation3.0 Block Diagram Of Digitally Implemented QPSK Modulator 3.1 LVDS Data with Line Receiver 3.2 FPGA 3.3 DAC 3.4 Smoothing Filter 3.5 Bipolar Converter 3.6 Sub Carrier Generation Using FPGA4.0 Simulation Results 4.1 Specifications 4.2 Flow diagram of MATLAB Simulation 4.3 MATLAB Simulation Results5.0 Hardware Realization6.0 Acknowledgement7.0 Conclusion8.0 References 3
  4. 4. ABSTRACTThe digitally implemented QPSK modulator is developed for satellite communicationfor future satellite missions. As we know that for space application power andbandwidth are most important parameters.The size of PCB and component count arealso important parameters. To reduce these all parameters we design new approach.The new approach also minimizes the component count and hence reduces the PCBsize. In this modulator summation, orthogonal sub-carrier generation and mixing ofsubcarrier with data are all digitally implemented inside the FPGA. Earlier, each ofthese functions were being implemented by external analog components which arefrequency and temperature sensitive. So the components count was also more andleading to bigger PCB size. Therefore we design this new concept to remove alllimitations of today’s modulator circuits which are used in on board processingsystem. Digital approach for implementation of QPSK Modulation is attempted here.Realization of hardware blocks like CCITT V.35 Scrambler, Differential Encoder, ½rate Convolutional Encoder, Sine and Cosine subcarrier generation, assignment ofsymbols with respect to I and Q data, their additions to obtain QPSK stream aredigitally implemented inside an FPGA using suitable high frequency sampler.Exhaustive simulation and characterization are done to freeze design parameters.Only at the last end, the digital QPSK stream is converted to analog using 10 bitDigital to Analog Converter. Finally, the modulated signal obtained from D/AConverter is suitably translated to required carrier frequency. 4
  5. 5. 1.0 INTRODUCTIONIn satellite communication , modulation is necessary to transfer the informationbetween satellite and the earth. There are basically two types of modulation technique:ANALOG MODULATION and DIGITAL MODULATION. From these twomodulation technique digital modulation technique have several advantages overanalog modulation technique like less interference, greater fidelity, robust, high S/Nratio etc. So in satellite communication generally digital modulation is used .In digitalmodulation technique, there are several types of modulation techniques are presentlike ASK(Analog shift Keying), FSK(Frequency Shift Keying) ,PSK(Phase ShiftKeying) etc. All these have own advantages and disadvantages. In satellitecommunication power, bandwidth and size of circuit are very important parameter.These parameter must be maintained for on board processing system in satellitecommunication. Now in satellite communication power is severally limited. For thesatellite communication FSK is not generally used as it would require very highbandwidth to modulate our data resulting in very low bandwidth efficiency. Higherconstellation QAM also cant be used in satellite communication as that would requirevery high C/N ratio. The choice is between QPSK and BPSK. The advantage ofBPSK is that it requires the lowest C/N ratio. The drawback is that the data rateachieved using BPSK is very low. QPSK is basically two BPSK links operating onthe same radio channel with their carriers in phase quadrature. Therefore the BER of aQPSK remains the same as BPSK. At the same time the data rate is doubled. MSK isalso one type of PSK technique, but for this we require complex circuit and higherbandwidth than QPSK . Our project demands high data rate without losing much onbandwidth and power. Because of these tradeoffs we decided on using QPSKmodulation scheme for application of satellite communication. Now today in satellitecommunication for on board processing system QPSK modulator is used but in thismodulator analog components are used like local oscillator, mixer, 90° phase shifter.All these components are frequency and temperature sensitive. Also the no ofcomponents are higher so size of PCB require for this modulator is comparativelyhigh. So after few years the performances of these components are degrade. Andultimately this effects on the performance of QPSK modulator and this degrades theperformance of satellite communication. So our aim to design such a modulatorcircuit which eliminates all these limitations of today’s modulator circuit used insatellite communication. Here we design completely digital QPSK modulator. In ourcircuit we eliminate the use of local oscillator, mixer, 90° phase shifter. The functionsof all these components are performed by FPGA. Here carrier is digitally generatedusing LUT(Look Up Table). Now FPGA is digital component which is frequency andtemperature insensitive. So the performance of this modulator can not degrade as timepasses. And also the number of components are reduced and ultimately size of PCB isreduced. So all the limitations of today’s modulator which are used in satellitecommunication are eliminated. 5
  6. 6. 1.1 Analysis of Modulation Base band signals produced by various information sources are not alwayssuitable for direct transmission over a given channel. This signals are usually furthermodified to facilitate transmission. This conversion process in known as modulation.In this process the baseband signal is used to modify some parameter of a highfrequency carrier signal. A carrier is a sinusoid of high frequency, and one of its parameters - such asamplitude, frequency or phase – is varied in proportion to the base band signal.Accordingly, we have amplitude, frequency modulation or phase modulation.1.2 Purposes of Modulation1.2.1 Ease of Radiation:For efficient radiation of electromagnetic energy, the radiating antenna should be onthe order of one-tenth or more the wavelength of the signal radiated. For manybaseband signals, the wavelengths are too large for reasonable antenna dimensions.For example, the power in a speech signal is concentrated at frequencies in the rangeof 100 to 300Hz. The corresponding wave length is 100 to 3000km. This largewavelength would necessitate an impracticably huge antenna. So to avoid thisproblem, we modulate the signal with a high frequency carrier to translate the signalspectrum to the region of carrier frequencies that corresponds to a much smallerwavelength. For example, a 1 MHz carrier frequency corresponds to the wave lengthof only 300m and requires an antenna of size 30m.1.2.2 Simultaneous Transmission of Several Signals: In the case of several radio stations broadcasting audio baseband signalsdirectly, without any modification they would interfere with each other because thespectra of all the signals occupy more or less the same bandwidth. Thus it would bepossible to broadcast from only one radio or television station at a time which is awaste because the channel bandwidth may be larger than that of the signal. One wayto solve this problem is to use modulation. We can use various audio signals tomodulate different carrier frequencies, thus translating each signal to a differentfrequency range. If the various carriers are chosen sufficiently far apart in frequency,the spectra of the modulated signals will not overlap and thus will not interfere witheach other. The method of transmitting several signals simultaneously is known asFrequency Division Multiplexing. Here the bandwidth of the channel is shared byvarious signals without any overlapping. 6
  7. 7. 1.3 Analog modulation methods In analog modulation, the modulation is applied continuously in response tothe analog information signal. Common analog modulation techniques are: Angular modulation Phase modulation (PM) Frequency modulation (FM) Amplitude modulation (AM) Double-sideband modulation (DSB) Double-sideband modulation with unsuppressed carrier (DSB-WC) Double-sideband suppressed-carrier transmission (DSB-SC) Double-sideband reduced carrier transmission (DSB-RC) Single-sideband modulation (SSB)1.4 Digital Modulation Methods In digital modulation, an analog carrier signal is modulated by a digital bitstream. Digital modulation methods can be considered as digital-to-analogconversion, and the corresponding demodulation or detection as analog-to-digitalconversion. The changes in the carrier signal are chosen from a finite number ofalternative symbols. There are four types of basic digital modulation schemes. Theseare: Amplitude-Shift Keying (ASK) Frequency-Shift Keying (FSK) Phase-Shift Keying (PSK) Quadrature amplitude modulation (QAM)The most common digital modulation techniques are: Phase-shift keying (PSK) Frequency-shift keying (FSK) (see also audio frequency-shift keying) Amplitude-shift keying (ASK) and its most common form, (OOK) Quadrature amplitude modulation(QAM) a combination of PSK and Polar modulation like QAM a combination of PSK and ASK Continuous phase modulation (CPM) Minimum-shift keying (MSK) Gaussian minimum-shift keying (GMSK) Orthogonal Frequency division multiplexing (OFDM) modulation. 7
  8. 8. 2.0 ANALYSIS OF QUADRATURE PHASE SHIFT KEYING(QPSK)2.1 Phase-Shift Keying Phase-shift keying or PSK is a digital modulation scheme that conveys data bychanging, or modulating, the phase of a reference signal or the carrier wave . Every digital modulation scheme uses a finite number of distinct signals torepresent digital data. PSK uses a finite number of phases; each assigned a uniquepattern of binary digits. Usually, each phase encodes an equal number of bits. Eachpattern of bits forms the symbol that is represented by the particular phase. Thedemodulator is designed specifically for the symbol-set used by the modulator and itdetermines the phase of the received signal and maps it back to the symbol itrepresents. Thus it recovers the original data. This requires the receiver to be able tocompare the phase of the received signal to a reference signal.Alternatively, instead of using the bit patterns to set the phase of the wave, it caninstead be used to change it by a specified amount. The demodulator then determinesthe changes in the phase of the received signal rather than the phase itself. Since thisscheme depends on the difference between successive phases, it is termed differentialphase-shift keying (DPSK). DPSK can be significantly simpler to implement thanordinary PSK since there is no need for the demodulator to have a copy of thereference signal to determine the exact phase of the received signal. In exchange, itproduces more erroneous demodulations. The exact requirements of the particularscenario under consideration determine which scheme is used. In PSK, the constellation points chosen are usually positioned with uniformangular spacing around a circle. This gives maximum phase-separation betweenadjacent points and thus the best immunity to corruption. They are positioned on acircle so that they can all be transmitted with the same energy. In this way, the moduleof the complex numbers they represent will be the same and thus so will theamplitudes needed for the cosine and sine waves. Two common examples are BinaryPhase-Shift Keying (BPSK) which uses two phases, and Quadrature Phase-ShiftKeying (QPSK) which uses four phases, although any number of phases may be used.Since the data to be conveyed are usually binary, the PSK scheme is usually designedwith the number of constellation points being a power of 2 i.e. 4, 8 or 16.2.2 QPSK Modulation Since the early days of electronics, as advances in technology were takingplace, the boundaries of both local and global communication began eroding,resulting in a world that is smaller and hence more easily accessible for the sharing ofknowledge and information. The pioneering work by Bell and Marconi formed thecornerstone of the information age exists today and paved the way for the future oftelecommunications.Traditionally, local communications was done over wires, as this presented a cost-effective way of ensuring a reliable transfer of information. For long-distancecommunications, transmission of information over radio waves was needed. Althoughthis was convenient from a hardware standpoint, radio-waves transmission raiseddoubts over the corruption of the information and was often dependent on high-power 8
  9. 9. transmitters to overcome weather conditions, large buildings, and interference fromother source of electromagnetic.Quadrature phase-shift keying (QPSK)Figure : Constellation diagram for QPSK with Gray coding. Each adjacent symbolonly differs by one bit. Digital information travels on analog carrier  Quadrature Phase-Shift Keying (QPSK) is effectively two independent BPSK systems (I-In phase and Q-Out of phase) and therefore exhibits the same performance but twice the bandwidth efficiency.  Sometimes this is known as quaternary PSK, quadriphase PSK, 4-PSK, or 4- QAM (although the root concepts of QPSK and QAM are different, the resulting modulated radio wave are exactly the same.)  QPSK uses four points on the constellation diagram, equispaced around a circle.  With 4 phases, QPSK can encode two bits per symbol to minimize the BER – sometimes misperceived as twice the BER of BPSK. 9
  11. 11. 3.1 LVDS DATA WITH LINE RECEIVERLVDS is the abbreviation of Low Voltage Differential Signaling. It is an electricallydigital signaling standard that can run at very high speed over inexpensive twistedpair copper cables. It specifies the electrical level in detail. Here, differential signalingmeans , it transmits information as the difference between the voltages on a pair ofwires, two wire voltages are compared at line receiver. It is a dual wire system whichcan running at 180°of each other. so noise to travel at the same level, which in turncan get filtered more easily and effectively. In our project we use DS90C032 3VLVDS Quad CMOS Differential Line receiver.3.2 FPGAThis block is heart of QPSK modulator. In this approach all the main operations todevelop QPSK signal are perform by FPGA. Here our motive is to provide completedigital system and miniaturization of circuit so this can be done using FPGA. Here weprovide only two Inputs, one is our information and second is the sample clock. Weget the Digital QPSK signal from the FPGA. The internal functions of FPGA is Datascrambling, Differential coding, Convolution encoding, Carrier generation, mixing thedata with carriers and finally generate complete QPSK signal. In this project weconcentrate to use the ProASIC3E FPGA devices part no:A3PE600.This FPGA is satisfy the criteria of our application. All the features of FPGA issuitable to space requirements.3.3 DAC(DIGITAL TO ANALOG CONVERTER)As Block diagram, the output of FPGA is Digital QPSK signal. It means we get thesamples of QPSK signal. To construct analog signal from this we must use Digital toanalog converter. Here we use 10-bit current type DAC. In our project we use 10-bit,170 MSPS, AD9731 DAC.3.4 SMOOTHING FILTERDigitally modulated QPSK samples out from DAC is a stair case type. There need tobe smoothened by a smoothing filter to have a analog look of the modulated signal.The center frequency is 1.024 MHz with symbol rate 512 K bits/s, in order to preservethe main lobe while smoothing. The cutoff frequency of the filter has to be taken careof so that the main lobe is not affected but a smoothing do take place This filter isdesign using AADE filter design V4.5 tool.For our application we develop fifth order Butterworth low pass filter.3.5 BIPOLAR CONVERTERThe DAC output is current type with an offset in the -ve direction because the DACoutput is ECL type. We know that PSK or QPSK is suppressed carrier modulatedsystem. Carrier suppression is possible only if there is no DC in the data, this requires 11
  12. 12. conversion of QPSK samples to be converted into bipolar type so that there is nooverall DC so bipolar converter circuit serve this purpose.3.6 SUB CARRIER GENERATION USING FPGAAs per our main aim summation, orthogonal subcarrier generation and mixing ofsubcarrier with data are all digitally implemented inside the FPGA. Here, LVDS datais used and this data are given to he scrambler and than ½ rate FEC coder is used. Allthis blocks are present inside the FPGA. For this blocks, the modules are develop invery log language using Xilinx tool. The main part is generation of carriers meanssine and cosine signals. These signals are generated at 1.024 MHz. These signals aregenerated inside the FPGA using the quantized value of samples of both signals.There are several techniques to generate sine and cosine wave digitally. Its calledNCO (Numerically Controlled Oscillator). There are several techniques to implementNCO1. LUT Based NCO2. CORDIC Based NCO3. Xilinx ROM Based NCOAmong all these we implement LUT based NCO for our application. In this techniquea NCO consists of a lookup table made up of quantized sinusoidal samplevalues(usually implemented as a read only memory, ROM), a binary counter foraddressing the ROM, and a clock signal to drive the counter as shown in belowfigure.Successive address locations in the ROM contain the successive quantized samplevalues of the desired sinusoid signals. As the counter is clocked, each new countaddresses the next ROM location causing the appropriate digital number to appear atthe ROM output. The rate at which the counter is clocked is the sample rate of thesystem. The number of bits in the ROMs output word determines the resolution of thedesired digital sinusoid. In our application we need 9 bit output from NCO then usingtwos-complement representation would yield a numeric range of -361 to +361 for theamplitude values of our quantized sampled sinusoid signals. Many variations on thistheme can be found in the literature. The main point is that a NCO serves as a meansfor generating a digital sinusoid at a specific sample rate but with programmablefrequency. The frequency is restricted, however, to an integer multiple of the samplerate divided by the ROM word length up to a maximum of ½ of the sample rate (theNyquist constraint). The larger the ROM length (address range), the finer the 12
  13. 13. frequency resolution. The more bits in the ROM output word,the finer the amplituderesolution. 4.0 SIMULATION RESULTS4.1 SPECIFICATIONModulation : QPSKCarrier Frequency : 70 MHzData Rate : 512 KbpsSymbol Rate : 256 KbpsSub carrier frequency : 1.024 MHzSub carrier quantization : 10 bitsAmplitude imbalance : 0.4 dB(max)Phase imbalance : within 3°Scrambler : CCITT V.35Input signal Interface : LVDSPhase Ambiguity Removal : By differential coding 13
  14. 14. 4.2 FLOW DIAGRAM OF MATLAB SIMULATIONThe flow diagram gives the steps to perform the simulation of digitally implementedQPSK modulator in MATLAB. 14
  15. 15. 4.3 MATLAB SIMULATION RESULTSAs per flow diagram COSINE and SINE carriers for I and Q data respectively aregenerated. Here the sub carriers with frequency 1.024 MHz and 24 samples per cycleare generated. There are 4 cycles per symbols are necessary to modulate incomingdata. These carriers are first multiplex with the I and Q data and than added togetherin single FPGA. In following simulation results the subcarriers per symbol, the finalQPSK modulated signal and its frequency spectrum are given The subcarriergeneration per symbol is given in below figure:.fig: Subcarrier generation of carrier per symbolNow, This carrier is ready to modulate. The data 1100 is given for modulation. Thisdata is first converted into parallel form and individual data is multiply with eithercosine or sine carrier like BPSK. After that these individual modulated data is addedtogether and QPSK modulated signal is generated The simulation result for timedomain representation of QPSK signal is given in below figure: 15
  16. 16. Frequency Domain Representation for QPSK signal is:fig: Frequency domain representation of QPSK signal 16
  17. 17. 5.0 HARDWARE REALIZATIONIn hardware realization we use Actel kit with ProASIC3000 FPGA. In this kit we loadour program which is written in verilog. The Actel Kit is shown below:Figure:7 Actel ProASIC 3000 kitThe output of kit is shown in logic analyzer. Here in Actel kit Scrambler, Differentialencoder,Convolutional coder and subcarrier generator are implemented. The setup ofthis implementation withresults are shown below. And as per output which is shownin figure there is no phase and amplitude imbalance between sin and cosinesubcarriers. 17
  18. 18. Figure:8 Set up of digitally implemented QPSK modulator 6.0 ACKNOWLEDGEMENTThe work presented here is carried out at Space Application Center(SAC),ISRO,Ahmadabad. I am highly indebted to my guide Mr. Kishorilal Sah, SCI/ENGR ofOSPD, SAC, Ahmadabad for assigning me this project, for timely guidance. I wouldlike to express my gratitude towards the Head of the Department Mr.T.V.S.Ram,SCI/ENGR, SG, Head of OSPD, SAC, Ahmadabad for his kind encouragement andcooperation and for his moral Support. My thanks and appreciations also go to thepeople of my Division OSPD at SAC who have willingly helped me out with theirabilities. 7.0 CONCLUSIONHere, we design digitally QPSK modulator. In this modulator , analog componentslike local oscillator and mixer are completely eliminated which are frequency andtemperature sensitive. Here all the functions are performed by single FPGA. So thelimitations of modulator are completely removed for satellite communication. For thesatellite communication PCB size is also important parameter and using this newapproach number of component count is less and ultimately size of PCB is becomesmall. 18
  19. 19. 8.0 REFRENCES1. Asif Iqbal Ahmed,Sayed Hafizur Rahman and Otmane Ait Mohamed, FPGA Implementation and Performance Evaluation of a Digital Carrier Synchronizer using Different Numerically Controlled Oscillators, IEEE,2007.2. Di Xie,Shulin Tian,Ke Liu,Design and implementation of DDS based Digital Amplitude Modulation,IEEE,2009.3. Teena Sakla,Divya Jain,Sandhya Gautam, IMPLEMENTATION OF DIGITAL QPSK MODULATOR BY USING VHDL /MATLAB.International Journal of Engineering Science and Technology,Vol. 2(9), 2010.4. Ken Gentile, Fundamentals of Digital Quadrature Modulation,, 2003.5. Xilinx logicore, Sine/Cosine Look-Up Table v5.0,Product specification, DS275 April 28, 2005.6. IP Data Sheet of Block Convolutional Encoder, September 2004.7. Data Sheet of DS26F32MQML Quad Differential Line Receivers ,March 2006.8. Data Sheet of AD9731 10-Bit, 170 MSPS, DAC. 19
  20. 20. 9. Data Sheet of ProASIC3E Flash Family FPGAs with Optional Soft ARM Support, Actel v1.0. 20