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  • 1. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME469OFDM BASED WIRELESS LAN TRANSMITTERMs. S.D. Giri and Prof. Ms A. R. Salunke (IEEE Member)Dept. of Electronics & Telecommunication, Dr. B. A. M. UniversityJawaharlal Nehru Engineering College, AurangabadABSTRACTOFDM (orthogonal frequency-division multiplexing) signals used in 802.11a and802.11g wireless LAN systems include provisions for equalization in the receiver to correctfor some types of system and channel impairments. High data rate wireless communicationhas improved by a factor of minimum four while migrating from one generation to nextgeneration. And now we have entered into an era, where fast, accurate and secured datatransmission is of maximum value. The Orthogonal Frequency Division Multiplexing(OFDM) put its need in the communication for the effective bandwidth utilization with largedata transmission. In this paper, we present a proposed OFDM based wireless LANarchitecture and its implementation using Verilog for an efficient physical layerimplementation of an OFDM technique. Our architecture improves resources utilization,compared with the other architecture.Keywords: W-LAN, OFDM, IEEE 802.11a.I. INTRODUCTIONThe main goals in developing next-generation wireless communication systems areincreasing the link throughput (bit rate) and the network capacity. Important improvements inthroughput can be achieved when multiple antennas are applied at both the transmitter andreceiver side, especially in a rich scattering environment. This has been shown for wirelesscommunication. With the rapid growth of digital communication in recent years, the need forhigh- speed data transmission has been increased [1].The major challenges in future wireless communication system design are increasedspectral efficiency and improved link reliability. Wireless networking shares severalimportant advantages, no matter how the protocols are designed, or even what type of datathey carry. The most obvious advantage of wireless networking is mobility. Wirelessnetworks typically have a great deal of flexibility, which can translate into rapidINTERNATIONAL JOURNAL OF ELECTRONICS ANDCOMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)ISSN 0976 – 6464(Print)ISSN 0976 – 6472(Online)Volume 4, Issue 2, March – April, 2013, pp. 469-476© IAEME: www.iaeme.com/ijecet.aspJournal Impact Factor (2013): 5.8896 (Calculated by GISI)www.jifactor.comIJECET© I A E M E
  • 2. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME470development. Wireless networks use a number of base stations to connect users to existingnetworks. The upcoming standard 802.11n WLAN, however, can achieve 250 Mbits/s byvirtue of Multiple Input Multiple Output OFDM (MIMO-OFDM) technology. Wirelesscommunication is obviously less reliable than wired communication. For example, the IEEEstandard 802.11a. has various communication modes with possible data rates of 6, 9, 12, 18,24, 36, 48, and 54 Mbits/s [2].II. BACKGROUNDA. Introduction of OFDMOFDM was first developed in the 1960s, only in recent years, it has been recognizedas an outstanding method for high-speed cellular data communication where itsimplementation relies on very high-speed digital signal processing. This method has onlyrecently become available with reasonable prices versus performance of hardwareimplementation.Frequency Division Multiplexing (FDM) transmits multiple signals simultaneouslyover a single path. Each signal has a unique frequency range (carrier). Orthogonal FDM(OFDM) is a special case of FDM where a single data stream is distributed over several lowerrate sub-carriers. In other words, one signal is transmitted by multiple carriers. Sub-carriersare separated by given frequency ranges, to avoid cross-carrier interference. The benefit oforthogonality is that it gives a high spectral density. A guard band interval is employed toavoid the Inter-Symbol Interference (ISI) problemB. Orthogonal Frequency Division MultiplexingFrequency Division Multiplexing (FDM) transmits multiple signals simultaneouslyover a single path. Each signal has a unique frequency range (carrier). Orthogonal FDM(OFDM) is a special case of FDM where a single data stream is distributed over several lowerrate sub-carriers. In other words, one signal is transmitted by multiple carriers. Sub-carriersare separated by the given frequency ranges, to avoid cross-carrier interference. The benefitsof orthogonally is that it gives a high spectral density (maximizing channel usage). A guardband intervals is employed to OFDM (Orthogonal Frequency Division Multiplexing) is amethod of using many carriers waves instead of only one, and using each carrier wave foronly part of the message. OFDM is also called multicarrier modulation (MCM) or DiscreteMulti-Tone (DMT).OFDM is not really a modulation scheme since it does not conflict with othermodulation schemes. It is more a coding scheme or a transport scheme. OrthogonalFrequency Division is where the spacing between carriers is equal to the speed(bit rate) of themessage in earlier multiplexing literature, a multiplexer was primarily used to allow manyusers to share a communication medium like a phone trunk between two telephone centraloffices[3].A more detailed understanding of Orthogonal arises when we observe that thebandwidth of a modulated carrier has a so-called sinc shape (sinx/x) with nulls spaced by thebit rate. In OFDM, the carriers are spaced at the bit rate, so that the carriers fit in the nulls ofthe other carrier. Another view of Orthogonal is that each carrier has an integer number ofsinc wave cycles in one-bit period.
  • 3. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME471III. WIRELESS LANIn just the past few years, wireless LANs have come to occupy a significant niche inthe local area network market. Increasingly, organizations are finding that wireless LANs arean indispensable adjust to traditional wired LANs, to satisfy requirements for mobility,relocation, ad hoc networking, and coverage of locations difficult to wire.IEEE 802.11aOne of today’s biggest high-tech controversies is how best to encourage and delivertechnology innovations. Everyone agrees that standards play an important role in ensuringinteroperability and cost reduction (through volume production of key components). Noteveryone agrees, however, about what makes some standards more successful than others.The IEEE 802.11a standard defines an OFDM-based wireless LAN that supports raw datarates between 6 and 54 Mbps in the 5 GHz band. The block structure of the transmitter isshown in Fig. Multiple data rates are achieved by employing two puncturing patterns (of rate2/3 and 3/4) in addition to the basic convolutional code of rate 1/2, as well as by using fourdifferent modulation schemes (BPSK, QPSK, 16-QAM, 64-QAM). Of the twelve possiblecombinations, only eight are defined by the standard. In particular there is difference inopinion regarding whether standards drive innovation or innovation drives standards. TheSignal Processing Designer IEEE 802.11a models are based on the High-speed PhysicalLayer standard defined over the 5 GHz band [4].IV. AN OFDM BASED WIRELESS LAN ARCHITECTURE:In wireless communication when the data is transmitted in air the different types ofprecaution you need to take care of in order to protect it from following types error andnoises, which may corrupt or lose your data in a deep fad.Fig.1 OFDM based TransmitterTime and Frequency Recovery: This kernel is composed of packet detection, timesynchronization and frequency correction. Packet detection roughly identifies transmittedsignals & time synchronization exactly distinguishes transmitted signals from noise signal.Frequency correction estimates frequency error in the original transmitted signals, andrecovers the original frequency.Channel Estimation: An OFDM carrier signal is the sum of a number of orthogonal sub-carriers. Each sub-carrier is located at a particular frequency, Wireless channels affect onthese sub-carriers to varying degrees. In the ideal channel case, this gain is 1 for all sub-
  • 4. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME472carriers. However, for the real channel environment, the channel estimation block estimatesthe amount of gain change, and recovers the original gain for each sub-carrier [5]. Generalfrom of global transmitter block is shown in fig.2.Fig.2 Global TransmitterChannel Coding System: Channel coding is often used in digital communication systemsto protect the digital information from noise and interference and reduce the number of biterrors. Channel Coding is mostly accomplished by selectively introducing redundant bits intothe transmitted information stream. These additional bits will allow detection and correctionof bit errors in the received data stream and provide more reliable information transmission.The cost of using channel coding to protect the information is a reduction in data rate or anexpansion in bandwidth [6].With the large amount of applications today, that use digital communication to storeand retrieve data, this information needs to sent and received with minimal errors. Despite thecommon perception that digital transmission yields no errors, this is not often the case asmany digital communications contain errors due to noise. This can be combated using errorcorrection codes.Coding techniques have long been used for error correction to decrease the bit errorrate (BER) in data transmission systems. This decrease in BER is accomplished by addingredundant data bits to the transmitted data bits and, in some cases, scrambling the order of theoriginal data bits. There are many types of coding techniques used to correct different error.Convolutional encoding method, a good technique used for correcting errors that occurduring data transmission.The overall purpose of the wireless communication is to transfer information frompoint in space and time, called the source, to another point the user called destination. Theinformation source and the destination point are usually separated in space. The channelprovide the electrical connection between the information source and the user.Interleaver: Interleaving combats signal fading. The block size corresponds to the number ofbits in a single OFDM symbol. Interleaving enhance the quality of digital transmission overthe radio fading channel. This is usually accomplished by scrambling successive symbols ofthe transmitted sequence into different time slots. A channel is considered fully interleavedwhen consecutive symbols of the received sequence appear to be independent i.e. not affectedby the same error burst. Interleaving improves the performance of digital radio system at thecost of increasing memory space requirement, system complexity and time delay [7].QAM Mapping: The encoded output of Convolutional encoder is fed to modulator ormapper for modulation. According to IEEE 802.11a the OFDM subcarriers shall bemodulated by using BPSK, QPSK, 16-QAM, or 64-QAM modulation, depending on theRATE requested. The conversion shall be performed according to Gray-coded constellationmappings, illustrated in Fig.3. The output values, d, are formed by multiplying the resulting(I+ j Q) value by a normalization factor KMOD.d = (I + j Q) x KMOD
  • 5. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, MarchFig.3 constellation diagram for 16The normalization factor, KMOD, depends on the base modulation modenormalization factor is to achieve the same average power for all mappings. Theshows the modulation-dependentTable 1 ModulationModulationBPSKQPSK16-QAM64-QAMFor 16-QAM, d = (I + j Q) x 1/√10The correct detection of a signal depends upon a separation between the signal pointsignal space. In case of PSK al points lie on the circumference of circle. This is because PSKsignal has constant amplitude throughoutIFFT: IFFT performs the reverse process, transforming a spectrum (amplitude and phase ofeach component) into a time domain signal. An IFFT converts a number of complex datapoints, of length which is a power of 2, into the time domain signal of the same numberpoints. Each data point in frequency spectrum used for an FFT or IFFT is calledorthogonal carriers required for the OFDM signal can be easily generated by setting theamplitude and phase of each bin, then performing the IFFT. IFFT reduces tcomplex multiplications from Ncomputation time. How IFFT works in transAt the transmitter side, an OFDM system treats the sourcein the frequency domain. These symbols are feed to an IFFT block which brings the signalinto the time domain. If the N numbers of subcarriersInternational Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN6472(Online) Volume 4, Issue 2, March – April (2013), © IA473constellation diagram for 16-QAM.The normalization factor, KMOD, depends on the base modulation mode. The purpose of thenormalization factor is to achieve the same average power for all mappings. Thedependent normalization factor KMOD.Modulation-dependent normalization factor KMOD.Modulation KMOD11/√2QAM 1/√10QAM 1/√42√10The correct detection of a signal depends upon a separation between the signal pointsignal space. In case of PSK al points lie on the circumference of circle. This is because PSKthroughout [8].IFFT performs the reverse process, transforming a spectrum (amplitude and phase ofeach component) into a time domain signal. An IFFT converts a number of complex datapoints, of length which is a power of 2, into the time domain signal of the same numberpoints. Each data point in frequency spectrum used for an FFT or IFFT is calledorthogonal carriers required for the OFDM signal can be easily generated by setting theamplitude and phase of each bin, then performing the IFFT. IFFT reduces the number ofcomplex multiplications from N2to (N/2) log2N as required in IDFT. It reduces thecomputation time. How IFFT works in transmitter side is shown in fig.4.Fig.4 workings of IFFTAt the transmitter side, an OFDM system treats the source symbols as though they areThese symbols are feed to an IFFT block which brings the signalIf the N numbers of subcarriers are chosen for the system, tInternational Journal of Electronics and Communication Engineering & Technology (IJECET), ISSNApril (2013), © IAEME. The purpose of thenormalization factor is to achieve the same average power for all mappings. The table 1The correct detection of a signal depends upon a separation between the signal point in thesignal space. In case of PSK al points lie on the circumference of circle. This is because PSKIFFT performs the reverse process, transforming a spectrum (amplitude and phase ofeach component) into a time domain signal. An IFFT converts a number of complex datapoints, of length which is a power of 2, into the time domain signal of the same number ofpoints. Each data point in frequency spectrum used for an FFT or IFFT is called a bin. Theorthogonal carriers required for the OFDM signal can be easily generated by setting thehe number ofN as required in IDFT. It reduces thesymbols as though they areThese symbols are feed to an IFFT block which brings the signalare chosen for the system, the basic
  • 6. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME474functions for the IFFT are N orthogonal sinusoids of distinct frequency and IFFT receive Nsymbols at a time. Each of N complex valued input symbols determines both the amplitudeand phase of the sinusoid for that subcarrier. The output of the IFFT is the summation of allN sinusoids and makes up a single OFDM symbol.V. IMPLEMENTATION OF OFDM BASED LAN TRANSMITTER USINGVERILOGFollowing are the different tools used for implementation of LAN transmitter.1) Model simulation - For model testing and function simulationa) Verilog HDLb) VHDL2) Active HDLActive HDL OverviewActive-HDL is an integrated environment designed for development of VHDL,Verilog, EDIF and mixed VHDL – Verilog – EDIF designs. It comprises three differentdesign entry tools, VHDL’93 compiler, Verilog compiler, single simulation kernel, severaldebugging tools, graphical and textual simulation output viewers, and auxiliary utilitiesdesigned for easy management of resource files, designs, and libraries [9].Standards Supported1 .VHDL: The VHDL simulator implemented in Active-HDL supports the IEEE Std.1076-1993 standard.2 Verilog: The Verilog simulator implemented in Active-HDL supports the IEEE Std.1364-1995 standard. Both PLI (Programming Language Interface) and VCD (ValueChange Dump) are also supported to Active-HDL.Wireless Standards for Wireless LANsThe new uses, as well as the growing number of convectional WLAN users,increasingly combine to strain existing Wi-Fi networks. Fortunately, a solution is close athand. The industry has come to an agreement on the components that will make up 802.11n,a new WLAN standard that promises both higher data rates and increased reliability, and theIEEE standards-setting body is ironing out the final details. Tough the specification is notexpected to be finalized yet, the draft is proving to be reasonably stable as it progressesthrough the formal IEEE review process [10].The table 2 shows a brief list of all available LAN standards and their specificationsTable 2 LAN Standards and their specifications.802.11a 802.11b 802.11g 802.11nStandard Approved July 1999 July 2003 June 2003 Not yet ratifiedMaximum Data Rate 54 Mbps 11 Mbps 54 Mbps 600 MbpsModulation OFDM DSSS or CCK DSSS or CCK orOFDMDSSS or CCK orOFDMRF Band 5 GHz 2.4 GHz 2.4 GHz 2.4 or 5 GHzNumber of Streams 1 1 1 1,2,3 or 4Channel Width 20 MHz 20 MHz 20 MHz 20 or 40 MHz
  • 7. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME475VI. CONCLUSIONSThe different techniques available for above discussed system of transmitter willdecide the overall performance and specification of the system. Channel encoding isindispensable in wireless communication system. It is used for error correction, by addingredundant bits to the original. It is observed through some practical experiments that thistransmitter system increases the row data rate and hence, it increases the bandwidthrequirement. We will also like to make one comment that this increased bandwidthrequirement can be solved by using the concept of Orthogonal Frequency DivisionMultiplexing (OFDM) and by using the concept of parallel processing the data rate can beincreased. The dynamic reconfigurable system discussed above can achieve flexibility withregard to changing data rates, increasing range, and increasing diversity, while offeringefficient resource utilizationREFERENCES[1] Multi Band OFDM Physical Layer Proposal for IEEE 802.15.3a. September 2004, MBOASiG.[2] “Digital communication: Fundamentals and applications” by Bernard Sklar, PearsonEducation Publication, fourth edition, year 2003, pp. 461-466.[3] C.Snow, L.Lampe, andR.Schober, “Performance Ana-lysis of Multi band OFDM for UWBCommunication”, IEEE Intl. Conf. on Communication (ICC), VOL.4, Seoul, May 2005.[4] “802.11 –Wireless networks: The definitive guide”, by Matthew S. Gast, SHROFFPublication, second edition, July 2004, pp.198-213.[5] Lee, and L.H.C., “New rate-compatible punctured convolutional codes for ViterbitDecoding”, IEEE Transactions on communications volume 42 Issue: 12, Dec.[6] Zou Decai Nat. Time Service Center, Chinese Acad. Of Sci., Lintong, Lu Xiaochum; WuHaitao; Xu Jinsong, “Implementation of convolutional code based on FPGAIn OFDM-UWB system” Industrial Electronics and Application, 2008. ICIEA 2008, Pp 1119-1122.[7] Davinder Pal Sharms, Jasvir Singh “DSP Based Implementation of Scrambler”, SignalProcessing-An International Journal, Vol.4: issue 2.[8] Szczecinski,Leszek L. Inst. Nat. de la recherché scientifique, “rectangle QAM arbitraryconstellation mapping.” IEEE Transactions on communications.[9] “VHDL Programming by example” By Douglas L. Perry.[10] IEEE 802.11 wireless LAN Medium access control (MAC) and Physical layer (PHY)Specifications. IEEE-5A 5 April2012.[11] Sandeep Bidwai, Saylee S. Bidwai, Dr. S.P. Patil and Sunita S. Shinde, “Implementation &Performance Analysis of Cordic in OFDM Based WLAN System using VHDL”, Internationaljournal of Electronics and Communication Engineering & Technology (IJECET), Volume 3,Issue 3, 2012, pp. 103 - 111, ISSN Print: 0976- 6464, ISSN Online: 0976 –6472.[12] T.Priyadarsini, B.Arunkumar, K.Sathish and V.Karthika, “Traffic Information Disseminationin Vanet using IEEE-802.11”, International Journal of Electronics and CommunicationEngineering & Technology (IJECET), Volume 4, Issue 1, 2013, pp. 294 - 303, ISSN Print:0976- 6464, ISSN Online: 0976 –6472.[13] Rambabu.A.Vatti and Dr.A.N.Gaikwad, “Throughput Improvement of IEEE 802.15.4 BasedMedical Ad-Hoc Sensor Networks Using Clear Channel Assessment”, International journal ofElectronics and Communication Engineering & Technology (IJECET), Volume 3, Issue 3, 2012,pp. 43 - 48, ISSN Print: 0976- 6464, ISSN Online: 0976 –6472.
  • 8. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME476AUTHORSMs. Sunita D. Giri, a student of ME (EC) at JNEC, Aurangabad,Maharashtra.Prof. Aarti R. Salunke, Department of ECT, MGM’s Jawaharlal NehruEngineering College, N-6, CIDCo Aurangabad, Maharashtra.