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					                                               International Journal of Advances in Science and Technology,
                                                                                          Vol. 2, No. 6, 2011


     Design of an OFDM Modulator using IEEE
                  802.11a for SDR
                           D.Lalitha Kumari 1 and S.Sreekanth2 and K.Bapayya3
                    1
                      Department of Electronics and Communication Engineering,JNTUA College of
                                         Engineering,ANANTAPUR,A.P,India
                                                lalitha_d29@yahoo.co.in
     2
       Department of Electronics and Communication Engineering,C.V.R College of Engineering,Hyderabad ,A.P,India
                                              sreekanthsuray@gmail.com
     2
       Department of Electronics and Communication Engineering,C.V.R College of Engineering,Hyderabad ,A.P,India
                                                   baapek@gmail.com

                                                     Abstract

      This paper documents the design of a SDR Platform as a research tool. In order to keep costs to a
     minimum and provide the maximum flexibility a processor-less architecture is chosen. All signal
     processing is carried out using a standard notebook computer. The platform consists of hardware
     element base band transmitter (OFDM).Software defined radio perform its processing through
     software (error correction, modulation, control over RF hardware…).The actual and next
     communication schemes tend to use OFDM systems in order to provide high baud rates and less
     inter-symbol interference. Fast converging methods to estimate the significant multipath
     components in fading wireless channels.

     I. Introduction
               One method of reducing the high infrastructure cost of a wired system is to use a fixed
     wireless radio network. The problem with this is that for rural and urban areas, large cell sizes are
     required to obtain sufficient coverage. These results caused by large signal path loss and long delay
     times in multipath signal propagation. Currently Global System for Mobile telecommunications (GSM)
     technology is being applied for fixed wireless phone systems in rural areas. However, GSM uses Time
     Division Multiple Access (TDMA), which has a high symbol rate leading to problems with multipath
     causing inter-symbol interference. Several techniques are under consideration for the next generation
     of digital phone systems, with the aim of improving cell capacity, multipath immunity, and flexibility.
     These include Code Division Multiple Access (CDMA) and Orthogonal Frequency Division
     Multiplexing (OFDM). Both these techniques could be applied for providing a fixed wireless system
     for rural areas. However, each technique has different properties, making it more suited for specific
     applications. OFDM is currently being used in several new radio broadcast systems including the
     proposal for high definition digital television, Digital Video Broadcasting (DVB) and Digital Audio
     Broadcasting (DAB).
     2.OFDM Fundamentals:
               Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier transmission
     technique, which divides the available spectrum into many carriers, each one being modulated by a low
     rate data stream. OFDM is similar to FDMA in that the multiple user access is achieved by subdividing
     the available bandwidth into multiple channels, which are then allocated to users. However, OFDM
     uses the spectrum much more efficiently by spacing the channels much closer together. This is
     achieved by making all the carriers orthogonal to one another, preventing interference between the
     closely spaced carriers.
     3. System Design Flow[1]:
     OFDM generation:
     To generate OFDM successfully the relationship between all the carriers must be carefully controlled
     to maintain the orthogonality of the carriers. For this reason, OFDM is generated by firstly choosing
     the spectrum required, based on the input data, and modulation scheme used. Each carrier to be




June Issue                                         Page 59 of 102                                      ISSN 2229 5216
                                              International Journal of Advances in Science and Technology,
                                                                                         Vol. 2, No. 6, 2011

     produced is assigned some data to transmit. The required amplitude and phase of the carrier is then
     calculated based on the modulation scheme (typically differential BPSK, QPSK, or QAM).




              The required spectrum is then converted back to its time domain signal using an Inverse
     Fourier Transform. In most applications, an Inverse Fast Fourier Transform (IFFT) is used. The IFFT
     performs the transformation very efficiently, and provides a simple way of ensuring the carrier signals
     produced are orthogonal [1].
     i)Scrambler Module
              The scrambler randomizes the input binary data to spread the transmit signal spectrum over
     the transmit band. The purpose of the scrambler is to facilitate effective transmission of the data over
     the telephone channel and to improve the convergence of the adaptive equalization and echo
     cancellation in the receiver.
     ii) Convolution Encoder Module
              It is used for introducing forward error correction in the link by adding redundant data bits to
     the actual data to be transmitted so that the receiver can detect and correct errors without the need to
     ask the sender for additional data. This eliminates the requirement of back channel or avoids re-
     transmission of data which requires higher bandwidth or is relativelycostlier. The convolution Encoder
     has been designed with a coding rate of R= ½.
     iii) Mapper Module
     a) Quadrature Amplitude Modulation (QAM) [1]:
              This modulation scheme is also called quadrature carrier multiplexing. Infarct, this
     modulation scheme enables to DSB-SC modulated signals to occupy the same transmission BW at the
     receiver o/p. it is, therefore, known as a bandwidth-conservation scheme[1].
     b) Quadrature Phase Shift Keying (QPSK) :
     In communication systems, we have two main resources. These are:
     1.Transmission Power
     2.Channel bandwidth


                                                   Coded                           Data
                                                                     Coded
                         Data                      bits per                         bits
                                              Codi                  bits for
                         rate Modu                   sub                            per
                                               ng                   OFDM
                        (Mbits/ lation             carrier                        OFDM
                                              rate                  symbol
                         sec)                       (NBPSC                        symbol
                                                                     (NCBPS)
                                                        )                         (NDBPS)
                            6      BPSK        1/2     1                48          24
                            9      BPSK        3/4         1            48            36
                           12      QPSK        1/2         2            96            48
                           18      QPSK        3/4         2            96            72
                           24        16-       1/2         4           192            96
                                   QAM
                           36        16-       3/4         4           192           144
                                   QAM




June Issue                                         Page 60 of 102                                    ISSN 2229 5216
                                              International Journal of Advances in Science and Technology,
                                                                                         Vol. 2, No. 6, 2011

                                   1. Rate-dependent modulation parameters[1]
     If two or more bits are combined in some symbols, then the signaling rate will be reduced. Thus, the
     frequency of the carrier needed is also reduced. This reduces the transmission channel B.W. Hence,
     because of grouping of bits in symbols; the transmission channel B.W can be reduced. In QPSK two
     successive bits in the data sequence are grouped together. This reduces the bits rate or signaling rate
     and thus reduces the B.W of the channel.
     In case of BPSK, we know that when sym. Changes the level, the phase of the carrier is changed by
     180. Because, there were only two sym’s in BPSK, the phase shift occurs in 2 levels only. However,
     in QPSK, 2 successive bits are combined. Infact, this combination of two bits forms 4 distinct symbols.
     When the sym is changed to next sym, then the phase of the carrier is changed by 45 degrees.

     iV) IFFT Module[3]
     The Fourier Transform provides the means of transforming a signal defined in the time domain into
     one defined in the frequency domain. When a function is evaluated by numerical procedures, it is
     always necessary to sample it in some fashion. This means that in order to fully evaluate a Fourier
     transform with digital operations, it is necessary that the time and frequency functions be sampled in
     some form or another[3].
     The fast Fourier transform (FFT) is simply a class of special algorithms which implement the discrete
     Fourier transform with considerable savings in computational time. It must be pointed out that the FFT
     is not a different transform from the DFT, but rather just a means of computing the DFT with a
     considerable reduction in the number of calculations required.
              The idea behind the FFT is the divide and conquer approach, to break up the original N point
     sample into two (N / 2) sequences. This is because a series of smaller problems is easier to solve than
     one large one. The DFT requires (N-1)2 complex multiplications and N(N-1) complex additions as
     opposed to the FFT's approach of breaking it down into a series of 2 point samples which only require
     1 multiplication and 2 additions and the recombination of the points which is minimal.

     V.The Benefits Of REConfigurable Architechtures
     Software Defined Radio with reconfigurable RF architectures offer both a solution to the current
     challenges faced by wireless device manufacturers as well as a path to quickly creating new
     applications and functionality.
     Cognitive Radio (often defined as a radio capable of modifying its transmission characteristics to avoid
     interference) has been predicated on the development of a reconfigurable radio. SDR has long
     promised to be the foundation for cognitive radio, maybe it’s time has finally arrived.
     4.RESULTS:




                                    2.Fig: TOP LEVEL MODULE OF QAM
     Discussion: The above figure shows the RTL schematic of quadrature amplitude modulation.This is
     used in the transmitter section. The above figure is the top level module for QAM Modulator. The
     above top module shows scrambler, encoder, mapper and ifft blocks.




June Issue                                        Page 61 of 102                                    ISSN 2229 5216
                                              International Journal of Advances in Science and Technology,
                                                                                         Vol. 2, No. 6, 2011




                                   3.Fig: SIMULATION RESULTS OF QAM
     Discussion: The above figure shows the simulation results of quadrature amplitude modulation. This is
     used in the transmitter section. In the above figure clk, reset, datain are the inputs and dout, encoder
     dout, mapper dout,out0 to out8 are the output of the circuit.when Rst is low, the Linear Feedback Shift
     Register (LFSR) is loaded with seed 1011101 (5D). At every rising edge of the clock, scrambling is
     performed provided Scrambler Enable and Reset are high. Encoding is done provided Encoder Enable
     is high. For a single bit input, a two-bit output is obtained as the coding rate R=1/2. For mapper
     encoder datain, mapper enable are the inputs and mapper data is the output of the circuit.




                                   4.Fig: TOP LEVEL MODULE OF QPSK
     Discussion: The above figure shows the RTL schematic of quadrature phase shift keying
     modulation.This is used in the transmitter section. The above figure is the top level module for QPSK
     Modulator. The above top module shows clock distributor, input sampler, mapper, serial to parallel
     converter and ifft blocks.




                                   5.Fig: SIMULATION RESULTS OF QPSK
     Discussion: The above figure shows the simulation results of quadrature phase shift keying. This is
     used in the transmitter section. In the above figure clk, reset, datain are the inputs and dout, encoder
     dout, mapper dout,out0 to out8 are the output of the circuit. Above simulation having master_clk, reset
     are the inputs and generate enable_divide_2, enable_divide_16 are the outputs of the clock distributor.
     After every two clock pulses enable_divide_2 goes high and after every eight clock pulses
     enable_divide_16 goes high and IQ_data[0], IQ_data[1], are the outputs of the input sampler. For
     mapper clk, rst,enable,IQ_data(1:0) are the inputs and imaginary_output(1:0), real_output(1:0), are the
     outputs of the mapper. ifft having eight input values and eight output values.




June Issue                                        Page 62 of 102                                    ISSN 2229 5216
                                             International Journal of Advances in Science and Technology,
                                                                                        Vol. 2, No. 6, 2011


     5.CONCLUSION
               As mentioned in the objectives, a base band OFDM modulator is successfully designed. The
     output from each module is tested using appropriate software to ensure the correctness of the output
     result. In the modulator block diagram, there are four blocks which has scrambler, convolution
     encoder, mapper (modulation), IFFT and serial to parallel block . In the case of Map per module,QAM,
     QPSK modulation scheme has been used. A 8-point IFFT algorithm with single precision has been
     designed. Each of these blocks was tested using Xilinx ISE9.2i software during design process. The
     waveform results for these modules have been provided and the operation of these modules has also
     been explained.


     References
     [1] IEEE Standard 802.11a-1999(Supplement to IEEE Std 802.11-1999), Part 11: Wireless LAN
     Medium Access Control (MAC) and Physical Layer (PHY) specifications, IEEE, September 1999.
     [2] Alvin M. Despain, “Very Fast Fourier Transform Algorithms Hardware for Implementation”, IEEE
     transactions on computers, Vol. c-28 No 5, May 1979.
     [3] Proakis, “Digital Signal Processing” Third Edition, PHI Publications, 2000.
     [4] Michael D. Ciletti, “Starter's Guide to Verilog 2001”. [9] Michael D. Ciletti, “Starter's Guide to
     Verilog 2001”.
     [5]http://www.cs.vu.nl/~costa/cn_slides/ofdm.pdf
     [6]http://rfdesign.com/mag/radio_principles_ofdm
     [7]http://www.complextoreal.com/chapters/ofdm2.pdf.
     [8]http://www.cs.vu.nl/~costa/cn_slides/ofdm.pdf [9]http://rfdesign.com/mag/radio_principles_ofdm
     [10] Van Nee, Richard and Ramjee Prasad ,"OFDM for multimedia wireless communications”.
     [11]Timothy M. Schmidl and Donald C. Cox, “Robust Frequency and Timing Synchronization for
     OFDM”, IEEE TRANSACTIONSONCOMMUNICATIONS, VOL. 45, NO. 12, DECEMBER 1997.
     [12] Ahmad R.S. Bahai, Manoneet Singh, Andrea J. Goldsmith, and Burton R. Saltzberg, “A New
     Approach for Evaluating Clipping Distortion in Multicarrier Systems”, IEEE JOURNAL ON
     SELECTEDAREASINCOMMUNICATIONS, VOL. 20, NO. 5, MAY 2002.
     [13] ISO/IEC 8802-11 ANSI/IEEE Standard 802.11-1999, Part 11: Wireless LAN Medium Access
     Control (MAC) and Physical Layer (PHY) specifications, IEEE, 20th August 1999.
     [14] E. Grass, K. Tittelbach, U. Jagdhold, A. Troya, G. Lippert, O. Krueger, J. Lehmann, K.
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     48_57, Dec. 2001.
     [15] K. Maharatna, E. Grass, and U. Jagdhold, _A novel 64-point FFT/IFFT processor for IEEE
     802.11a tandard,_ in Proc. ICASSP_03, vol. II, pp. II-321_II-324.
     [16] Schmidl,T.M., and Cox, D.C.,_ Robust Frequency and timing Synchronization on OFDM_ , IEEE
     Trans. on Comm., Vol. 45, No. 12, pp. 1613-1621, Dec. 1997.
     [17] Classen, F, Meyr, H,_Frequency synchronization algorithms for OFDM systems suitable for
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     Technology Conference (VTC), IEEE, 1994. pp.1655-9.
     [18] Tufvesson, F, Maseng, T,_ Pilot assisted channel estimation for OFDM in mobile cellular
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     7 May 1997, vol.3 pp.1639-43,1997.
     [19] Erik G. Larsson, Guoqing Liu, Jian Li, and Georgios B. Giannakis, Fellow, IEEE _Joint Symbol
     Timing and Channel Estimation for OFDM Based WLANs_IEEE COMMUNICATIONS LETTERS,
     VOL. 5, NO. 8, AUGUST 2001
     [20] WLAN MAC and PHY Specifications: High-speed Physical Layer in the5 GHz Band_, IEEE Std
     802.11a Supplement to IEEE Std Part 11, Sept.1999.




June Issue                                       Page 63 of 102                                   ISSN 2229 5216
                              International Journal of Advances in Science and Technology,
                                                                         Vol. 2, No. 6, 2011


               Mrs. D. Lalitha Kumari was born in 1982.She received her B.tech in ECE from
               Jawaharlal Nehru Technological University, Hyderabad in 2003.She has completed
               her Master of technology in Digital Systems And Computer Electronics from RGM
               Nandyal Hyderabad in 2008.She is currently working as a Assistant professor in
               JNTUA College of Engineering, Anantapur. Her research interest includes video
               signal processing and digital image processing.




             Mr. S. Sreekanth was born in 1984.He received his B.tech in ECE from Jawaharlal
             Nehru Technological University ,Hyderabad in 2006.He has completed his Master of
             Engineering in Systems and Signal Processing from University college of engineering,
             OU, Hyderabad in 2009.He is currently working as a Assistant professor inC.V.R
             COLLEGE OF ENGINEERING,HYDERABAD.His research interest includes video
             signal processing and digital image processing.




              Mr. K.Bapayya was born in 1983 .He received his B.tech in ECE from Jawaharlal
              Nehru Technological University ,Hyderabad in 2005.He has completed his Master of
             Technology from Gudlavalleru engineering colleege, Gudlavalleru in 2007. He is
             currently working as a Assistant professor in C.V.R COLLEGE OF ENGINEERING,
             HYDERABAD. His research interest includes communications ,VLSI ,Image
             processing.




June Issue                         Page 64 of 102                                      ISSN 2229 5216

				
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