### Transcription

International Journal of Computer Applications (0975 – 8887)Volume 44– No.22, April 2012Implementation of Digital Video Broadcasting-Terrestrial(DVB-T) using Orthogonal Frequency DivisionMultiplexing (OFDM) on Physical Media DependentSub layerSudipta GhoshAnkit BassStudent, SECELovely Professional UniversityJalandhar, INDIAStudent, SECELovely Professional UniversityJalandhar, INDIAABSTRACTOrthogonal frequency division multiplexing (OFDM) is aspecial case of multicarrier transmission, where a single datastream is transmitted over a number of lower rate subcarriers.Orthogonal frequency division multiplexing (OFDM) hasbeen chosen as modulation technique for different applicationwireless communications. OFDM can provide large data rateswith sufficient robustness to radio channel impairments. Thepurpose of this paper is to provide a MATLAB simulation ofthe basic processing involved in the generation and receptionof an OFDM signal in a physical channel and to provide adescription of each of the steps involved. For this purpose, weshall use one of the proposed OFDM signals of the DigitalVideo Broadcasting (DVB) standard for the European digitaltelevision service i.e. Digital Video Broadcast-Terrestrial(DVB-T).General TermsFast fourier transform (FFT), Inverse fourier Transform(IFFT), Pulse Shaping, Filters.KeywordsOrthogonal Frequency Division Multiplexing (OFDM),Digital Video Broadcasting-Terrestrial (DVB-T)1. INTRODUCTIONOrthogonal frequency-division multiplexing (OFDM) is themodulation technique for European standards applicationssuch as the Digital Audio Broadcasting (DAB) and theDigital Video Broadcasting (DVB) systems. As such it hasreceived much attention and has been proposed for manyother applications, including local area networks andpersonal communication systems. OFDM is a type ofmultichannel modulation that divides a given channel intomany parallel subchannels or subcarriers, so that multiplesymbols are sent in parallel. Earlier overviews of OFDM canbe found in. The type of OFDM that we will describe in thisarticle uses the discrete Fourier transform (DFT) with acyclicprefix. DFT (implemented with a fast Fourier transform(FFT)) and the cyclic prefix have made OFDM both practicaland attractive to the radio link designer. A similarmultichannel modulation scheme, discrete multitone (DMT)modulation, has been developed for static channels such asthe digital subscriber loop. DMT also uses DFTs and thecyclic prefix but has the additional feature of bit-loadingwhich is generally not used in OFDM.OFDM also has some drawbacks. Because OFDM divides agiven spectral allotment into many narrow subcarriers eachwith inherently small carrier spacing, it is sensitive to carrierfrequency errors. Furthermore, to preserve the orthogonalitybetween subcarriers, the amplifiers need to be linear. OFDMsystems also have a high peak-to-average power ratio orcrest-factor, which may require a large amplifier power backoff and a large number of bits in the analog-to-digital (A/D)and digital-to-analog (D/A) designs. All these requirementscan put a high demand on the transmitter and receiver design.2. ORTHOGONAL FREQUENCYDIVISION MULTIPLEXING (OFDM)OFDM is a multi-carrier modulation technique where datasymbols modulate a sub-carrier which is taken fromorthogonally separated sub-carriers with a separation of „fk‟within each sub-carrier. Here, the spectra of sub-carrier areoverlapping; but the sub-carrier signals are mutuallyorthogonal, which is utilizing the bandwidth very efficiently.To maintain the orthogonality, the minimum separationbetween the sub-carriers should be „fK‟ to avoid ICI (InterCarrier Interference).By choosing the sub-carrier spacingproperly in relation to the channel coherence bandwidth.OFDM can be used to convert a frequency selective channelinto a parallel collection of frequency flat sub-channels.Techniques that are appropriate for flat fading channels canthen be applied in a straight forward fashion.3. SIGNAL MODELA communication system with multi-carrier modulationtransmits NC complex-valued source symbol SN , N 0,. ,N C - 1, in parallel on NC sub-carriers. The sourcesymbols may, for instance, be obtained after source andchannel coding, interleaving, and symbol mapping [2].The source symbol duration TS of the serial datasymbols results after serial- to-parallel conversion in theOFDM symbol duration.(1)Ts NcTdThe principle of OFDM is to modulate the NC sub-streams onsub-carriers with a spacing of(2)Fs 1/ Tsin order to achieve orthogonality between the signals on theNc sub-carriers, presuming a rectangular pulse shaping. TheNc parallel modulated source symbols Sn, n 0, . . . , NC 1,are referred to as an OFDM symbol [8]. The complex20

International Journal of Computer Applications (0975 – 8887)Volume 44– No.22, April 2012envelope of an OFDM symbol with rectangular pulse shapinghas the formx(t ) 1 Nc 1j 2 fntSne Nc n 0,0 t Ts(3)The Nc sub-carrier frequencies are located atfn nTsn 0,., Nc 1,(4)The symbols Sn, n 0, . . . , NC 1, are transmitted with equalpower. The dotted curve illustrates the power densityspectrum of the first modulated sub-carrier and indicates theconstruction of the overall power density spectrum as the sumof NC individual power density spectra, each shifted by FS.For large values of NC, the power density spectrum becomesflatter in the normalized frequency range of 0.5 fTd 0.5containing the NC subchannels. Only sub-channels near theband edges contribute to the out-of-band power emission.Therefore, as NC becomes large, the power density spectrumapproaches that of single carrier modulation with idealNyquist filtering [1]. A key advantage of using OFDM is thatmulti-carrier modulation can be implemented in the discretedomain by using an IDFT, or a more computationally efficientIFFT. When sampling the complex envelope x(t) of an OFDMsymbol with rate 1/Td the samples arexv 1 Nc 1j 2 nv / NcSne Nc 0,v 0,., Nc 1that was first published in 1997 and first broadcast in the UKin 1998. This system transmits compressed digital audio,digital video and other data in an MPEG transport stream,using orthogonal frequency-division multiplexing (OFDM)modulation. In the case of DVB-T, there are two choicesfor the number of carriers known as 2K-mode or 8K-mode.These are actually 1,705 or 6,817 carriers that areapproximately 4 kHz or 1 kHz apart. DVB-T offers threedifferent modulation schemes (QPSK, 16QAM, 64QAM)[10]. DVB-T is a digital transmission system that delivers aseries of data at the symbol rate. DVB-T is an application ofOrthogonal Frequency Division (OFDM). The use of OFDMhelps the receiver to counter the effects of multipath in urbanenvironment. The effects of multipath can be counteredby using guard interval bit insertion. The length of theguard interval can be chosen as per our requirement anddemands. This also results in a trade-off between the data rateand SFN capability. The insertion of guard interval alsoeliminates the effect of ISI to a great extent. DVB-T hasbeen adopted or proposed for digital television broadcastingby many countries, using mainly VHF 7 MHz and UHF 8MHz channels whereas Taiwan, Colombia, Panama,Trinidad and Tobago and the Philippines use 6 MHzchannels [10]. The general block diagram of DVB-Ttransmitter is shown in Fig. 2.4.1 DVB-T TRANSMISSIONThe first consideration that is made is that the OFDMspectrum is centered on f c i.e., subcarrier 1 is 7.162 MHzto the left of the carrier and subcarrier 1,705 is 7.162MHz to the right. The simplest method of achieving thecentering is to use a 2N-IFFT and T/2 as the elementaryperiod. As we can see in Table 1, the OFDM symbol durationTU, is specified considering a 2,048- IFFT (N 2,048);therefore, we shall use a 54,096-IFFT. A block diagram of thegeneration of one OFDM symbol is shown in Fig. 3, wherewe have indicated the variables used in the MATLAB code.(5)40961705The normalized power spectrum of OFDM is shown in Fig. 1Transmit spectrum OFDM (based on 802.11a)-254QAMSYMBOLSAIFFTCBEDg(t)LPFS(t)-30power spectral densityUOFT-35InfoCarriersUfc-40Figure 3.Generation of OFDM symbols for DVB-T-45-50-55-10-8-6-4-202frequency, MHz46810Figure 1. Normalized power spectrum of OFDM4. DIGITAL VIDEO BROADCASTTERRESTRIAL (DVB-T)Digital Video Broadcast- Terrestrial can be abbreviated asDVB-T. it is the DVB European-based consortium standardfor the broadcast transmission of digital terrestrial television21

International Journal of Computer Applications (0975 – 8887)Volume 44– No.22, April erFrameAdaptationTPS andPilotsignalTransmittedSignalDAC andfront endOFDMGuardintervalinsertionFigure.2 Block diagram of Digital Video Broadcasting Terrestrial (DVB-T)The elementary time period for a base band signal is taken as T.Here we consider a simple integer relation RS 40/T. This integer100relation gives us a frequency close to 90 MHZ. Now we design the50transmitter, and for that the steps undertaken have been shown inthe Fig.3. At first we add 4,096-1,705 2,391 zeros to the signal0info at (A) to achieve over- sampling, and to center the spectrum.In Fig. 4 and Fig. 5, we observe the result of this operation and that-50the signal “carriers” uses T/2 as its time period. We can also notice-100that “carriers” is a discrete time baseband signal. The first step is00.20.40.60.811.2to produce a continuous-time signal and to apply a filter g(t), to-6x 10the complex signal “carriers”. The impulse response, or pulse50shape, of g(t) is shown in Fig. 6. The output of this transmit filter isshown in Fig. 7 in the time-domain and in Fig. 8 in the frequency0domain. The frequency response of Fig. 8 is periodic as required ofthe frequency response of a discrete-time system , and thebandwidth of the spectrum shown in this figure is given by Rs.-50U(t).s period is 2/T, and we have (2/T 18.286)-7.61 10.675 MHzof transition bandwidth for the reconstruction filter. If we were to-10000.20.40.60.811.2use an N-IFFT, we would only have (1/T 9.143)-7.61 1.533 MHz-6of transition bandwidth; therefore, we would require a very sharpx 10roll-off, hence high complexity, in the reconstruction filter to avoidaliasing. The proposed reconstruction or D/A filter response isFigure 4. Time response of signal carriersshown in Fig. 9. It is a Butterworth filter of order 13 and cut-offfrequency of approximately 1/T [9].22

International Journal of Computer Applications (0975 – 8887)Volume 44– No.22, April 82x 10Welch Power Spectral Density Estimate-20-40-602.533.54Welch Power Spectral Density Estimate0-50-100-150050100150200Frequency (MHz)-80-100287Power/frequency (dB/Hz)0x 10Power/frequency (dB/Hz)0024681012Frequency (MHz)1416250300350Figure.8 Frequency response of signal U18100Figure.5 Frequency response of signal 7000.400.20.40.60.811.21.41.61.828x 100.30.2Figure.9 D/A filter response0.1001234567-8x 10100Figure.6 Pulse shape g(t)500The filter‟s output is shown in Fig. 10 and Fig. 11. The firstthing to notice is the delay of approximately 2*10 -7 producedby the filtering process. Besides this delay, the filteringperforms as expected since we are left with only the basebandspectrum. We must recall that subcarriers 853 to 1705 arelocated at the right of 0 Hz, and the subcarriers 1 to 852 are tothe left of 4 Hz. The next step is to perform the quadratureamplitude modulation(QAM) of the signal named UOFT inFig. 3. In this modulation, an in phase signal and a quadraturesignal are modulated.-50-1002468101214-7x 10500-50-1002468101214-7x 10Figure.10 Time response of the filter output (UOFT)10050060-50-1004000.20.40.60.811.2-6x 1020500000.511.522.533.548x 10-50-10000.20.40.60.811.2-6x 10Figure.7 Time response of signal UPower/frequency (dB/Hz)Welch Power Spectral Density Estimate0-50-100-150050100150200Frequency (MHz)250300350Figure.11 Frequency response of the filter output23

International Journal of Computer Applications (0975 – 8887)Volume 44– No.22, April 20124.2 DVB-T RECEPTION806040200-20-40-60-802468101214-7The design of an OFDM receiver is open. For example, thefrequency sensitivity drawback is mainly a transmission channelprediction issue ,something that is done at the receiver ;therefore,we shall only present a basic receiver structure in this report. OFDMis very sensitive to timing and frequency offsets[7]. Even in thisideal simulation environment, we have to consider the delayproduced by the filtering operation. For our simulation, the delayproduced by the reconstruction and demodulation filters is aboutTD 64/RS. This delay is enough to impede the reception, and it isthe cause of the slight differences we can see between thetransmitted and received signals. With the delay taken care of,the rest of the reception process is straightforward. As in thetransmission case, we specified the names of the simulationvariables and the output processes in the reception description ofFig.14 shown below.x 10Figure.12 Time response of the ouput signal s(t)30204096FS 2/T100LPFr tilde00.511.522.533.54FFTr infor datainfoh8x 10Power/frequency (dB/Hz)Welch Power Spectral Density Estimate4QAMSlicer0Figure 14: OFDM reception-50The results of this simulation are shown in Fig.15 to Fig.22.-100-15050020406080100 120Frequency (MHz)1401601800Figure.13 Frequency response of the output signal s(t)-5000.20.40.60.811.2-6x 1015010050Table.1 Parameter values used in simulation0-50Allowed guard intervalDuration of TuDuration of guard intervalSymbol Duration-1002K mode7/64µs170501704224µs4464 Hz7.61 MHz¼1/8 1/161/32224µs56µs 28 µs 14 µs 7 µs280µs 252µs 238µs 231 µs00.20.40.60.811.2-6x 10Figure.15 Time response of signal r tilde302010000.511.522.533.548x 10Welch Power Spectral Density EstimatePower/frequency (dB/Hz)PARAMETERElementary period TNumber of carriers KValue of KminValue of KmaxDuartion TuCarrier spacing 1/TuSpacing between kmin andKmax(K-1)/Tu0-50-100-150050100150200Frequency (MHz)250300350Figure.16 Frequency response signal of r tilde24a hat

International Journal of Computer Applications (0975 – 8887)Volume 44– No.22, April 2012ahat 4-QAM1.55010.500-5000.20.40.60.81-0.51.2-6x 10-1150-1.5-1.5100-1-0.500.511.550Figure.22 a hat constellation0-50-10000.20.40.60.815. CONCLUION1.2-6x 10The transmission and the reception model have beendiscussed in details along with their Welch power spectraldensity estimation. For each step their time domain signal andalso their frequency domain signal has been plotted. For thepower spectral density estimation we used the Welch methodrather than the Bartlett method as it cuts down on the noisefactor. This whole paper is based on the simulation resultsobtained from the simulation of DVB-T signal using the 2Kmode and the parameters specified for the same. Furtherstudies are being done in order to compare the obtained resultswith the 8K mode of DVB-T.Figure.17 Time response of signal r info604020000.511.522.533.548x 10Power/frequency (dB/Hz)Welch Power Spectral Density Estimate0-50-100-1506. ACKNOWLEDGEMENT050100150200Frequency (MHz)250300350We would like to thank Mr. Chandika Mohan Babu, ourguide, and Mrs. Vinit Dhaliwal, our COD, and our teacherMs. Ria Kalra for helping us in doing this research paper.Without their help this would not have been a success.Figure.18 Frequency response signal r info507. REFERENCES[1]0-5000.20.40.60.811.2-6x 10150100500[2]-50-10000.20.40.60.811.2-6x 10Figure.19 Time response of signal r data[3]1.510.50[4]00.20.40.60.811.21.41.61.827x 10Power/frequency (dB/Hz)Welch Power Spectral Density Estimate-20-40[5]-60-80-1000246810Frequency (MHz)12141618Figure.20 Frequency response of signal r data[6][7]info-h Received 10]-1-1-0.500.5A. Yiwleak and C. Pirak, “Intercarrier InterferenceCancellation Using Complex Conjugate Technique forAlamouti-Coded MIMO-OFDM Systems” In ineering/ElectronicsComputer Telecommunicationsand Information Technology no. 5, pp-1168-1172,(Chaing Mai) 2010.C. Yuen, Y. Wu, and S. Sun, “Comparative study ofopen-loop transmit diversity schemes for four transmitantennas in coded OFDM systems,” In Proceeding ofIEEE conference on Vehicular Technology, pp. 482–485,(Baltimore, MD)September 2004A. Boariu and D.M. Ionescu “A class of nonorthogonalrate-one space time block codes with controlledinterference,” IEEE Transactions on WirelessCommunications, Vol. 2, Issue 2, pp. 270–395, March2003V. Tarokh, H. Jafarkhani and A. R. Calderbank, “Space–time block codes from orthogonal designs”, IEEETransactions on Information Theory, Vol. 45, pp. 1456–1467, July 1999J. Kim and I. Lee, “Space–time coded OFDM systemswith four transmit antennas,” In Proceeding of IEEEconference on Vehicular Technology, vol. 2, pp. 2434–2438, September 2004.Orthogonal frequency division multiplexing for highspeed optical transmission, by Ivan B. Djordjevic et alA Novel Construction Technique For Designing OfVideo Application Using Wirless 4G, by M.Suman et al.Comparison of OFDM, SC-FDMA and MC-CDMA asAccessTechniques for Mobile Communication, byZohaib Shaikh et .edu/research/labs/sarl/tutorials/OFDM/Tutorial 21 info h constellation25

4. DIGITAL VIDEO BROADCAST- TERRESTRIAL (DVB-T) Digital Video Broadcast- Terrestrial can be abbreviated as DVB-T. it is the DVB European-based consortium standard for the broadcast transmission of digital terrestrial television that was first