ISSN: 2277 – 9043                              International Journal of Advanced Research in Computer Science and Electron...
ISSN: 2277 – 9043                             International Journal of Advanced Research in Computer Science and Electroni...
ISSN: 2277 – 9043                               International Journal of Advanced Research in Computer Science and Electro...
ISSN: 2277 – 9043                                                         International Journal of Advanced Research in Co...
ISSN: 2277 – 9043                                    International Journal of Advanced Research in Computer Science and El...
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  1. 1. ISSN: 2277 – 9043 International Journal of Advanced Research in Computer Science and Electronics Engineering Volume 1, Issue 6, August 2012 Channel Estimation using OFDM for 4G 1. Sumit Kumar Gupta 2. Ajay kumar VishwakarmaAbstract— Research and development are taking place all over The higher data rate of 3G systems will be able to support athe world to define next generation of wireless broadband widerange of applications including Internet access, voicemultimedia communication systems.Wireless Broadband communications and mobile videophones. In addition to this,Technologies allow simultaneous delivery of voice, data and a large number of new applications will emerge to utilize thevideo over fixed or mobile platforms. WiFi, WiMAX, 3G, UWBare some of the emerging broadband technologies. WiMAX is permanent network connectivity, such as wireless appliances,standards-based (IEEE 802.16) wireless technology that provides notebooks with built in mobile phones, remote logging,high-throughput broadband connections over long distances. The wireless web cameras, car navigation systems, and so forth. Inpaper is divided into two parts channel estimation and channel fact most of these applications will not be limited by the datatracking. In the channel estimation part, estimation of the rate provided by 3G systems, but by the cost of thechannel transfer function in the preamble using MLS method is serviceeasier or more enjoyable. Applications are virtuallydown. The various effect of varying channel length and Doppler unlimited as long as the devices have the capabilities tofrequency on different modulation technique is seen. Study of the provide such type of services. Research has just recentlyeffects of channel. At high SN, both methods reach an error floor begun on the development of 4th generation (4G) mobiledue to the residual error produced by the ICI. This paperdevelop robust and low complexity channel estimation communication systems.Ultimately 4G networks shouldalgorithms and compare different modulation technique for encompass broadband wireless services, such as HighOFDMA reuse1 systems, which are in compatible with the IEEE Definition Television (HDTV) (4 - 20 Mbps) and computer802.16e standard. network applications (1 - 100 Mbps). This will allow 4G networks to replace many of the functions of WLAN systems. However, to cover this application, cost of service must beKeywords—Wireless, OFDM, 4G, Modulation, channel reduced significantly from 3G networks. The spectralestimation, channel width, Doppler frequency. efficiency of 3G networks is too low to support high data rate services at low cost. As a consequence one of the main I. INTRODUCTION focuses of 4G systems will be to significantly improve the spectral efficiency Supporting the high data rates with Now-a 3rd generation mobile systems such as the Universal sufficient robustness to radio channel impairments, requireMobile Telecommunications System (UMTS) are introduced. careful choosing of modulation technique. OFDM is a robustThese systems are striving to provide higher data rates than efficient modulation scheme for broadband communications.current 2G systems such as the Global System for Mobile It combats multipath fading and narrow band interferencecommunications (GSM) and IS-95. Second generation efficiently. OFDMA, multiple access scheme based onsystems are mainly targeted at providing voice services, while OFDM, has lots of flexibility and when coupled with feedback3rd generation systems will shift to more data oriented information it can achieve high data rates efficiently. Theservices such as Internet access. Third generation systems use wireless MAN-OFDMA is one of the air interface standard forWide-band Code Division Multiple Access (WCDMA) as the NLOS communication.carrier modulation scheme.[1] This modulation scheme has ahigh multipath tolerance, flexible data rate, and allows agreater cellular spectral efficiency than 2G systems. Third II. CHANNEL ESTIMATION: PRINCIPLESgeneration systems will provide a significantly higher datarate (64 kbps – 2 Mbps) than second generation systems (9.6 – The channel is the medium through which the signal travels14.4kbps). from the transmitter to the receiver. Unlike wired channels that are stationary and deterministic, wireless channels are extremely random in nature. Some of the features of wireless Sumit Kumar Gupta, Asst .Professor ,Dept. Of Electronics communication like mobility, place fundamental limitations& communication, Bansal institute of engineering & on the performance in a wireless system. The transmissiontechnology (BIET),Lucknow, India, 9452052155) path between the transmitter and receiver can vary from line- of-sight (LoS) to one that is severely obstructed by buildings, Ajay kumar Vishwakarma,Sr.Lecturer Dept. Of terrain and foliage. Efficient channel estimation strategies areElectronics & communication, Bansal institute of engineering required for coherent detection and decoding. Channel& technology BIET,Lucknow, India,9839039699 estimates are used in diversity techniques like Maximal Ratio combining. In opportunistic communication systems, the channel estimate is feedback to the transmitter. The transmitter uses the channel knowledge to exploit Multi User 41 All Rights Reserved © 2012 IJARCSEE
  2. 2. ISSN: 2277 – 9043 International Journal of Advanced Research in Computer Science and Electronics Engineering Volume 1, Issue 6, August 2012Diversity (MUD). Therefore, the channel estimator is IV. ORTHOGONAL FREQUENCY DIVISIONinevitable in any wireless communication system.[4] MULTIPLEXING (OFDM)A channel model on the other hand can be thought of as amathematical representation of the transfer characteristics of In the scenario of high bit rate digital communications overthis physical medium. This model could be based on some wireless channels, one often has to deal with transmissionknown underlying physical phenomenon or it could be formed channels that exhibit a phenomenon known as fading or aby fitting the best mathematical / statistical model on the highly time-varying frequency response over the signalobserved channel behavior. Most channel models are bandwidth. This fading occurs over wireless transmissionformulated by observing the characteristics of the received media primarily because the signal in such media issignals for each specific environment. Different mathematical transmitted across multiple paths of varying attenuation andmodels that explain the received signal are then fit over the delay spread. In such a system, with the use of single carrieraccumulated data. Usually the one that best explains the modulation systems, channel equalization turns out to be abehavior of the received signal is used to model the given very difficult task, especially in the presence of time-varyingphysical channel. channels, and the discrepancies in the SNR at differentChannel estimation is simply defined as the process of frequencies lead to a low spectral efficiency, or even to acharacterizing the effect of the physical channel on the input reduction in the useful bandwidth. The idea underlying thesequence. If the channel is assumed to be linear, the channel adoption of multicarrier modulation systems is that of dividingestimate is simply the estimate of the impulse response of the the available channel band into a very high number of sub-system. It must be stressed once more that channel estimation bands (sub channels) , each one so small that the channelis only a mathematical representation of what is truly frequency response can be assumed to be constant within ahappening. single sub-band . The overall information stream is thereby partitioned into corresponding sub streams, each one of them being transmitted over a different sub-channel. By dividing III. NEED FOR CHANNEL ESTIMATION the available bandwidth into several narrowband sub-channels each of which undergoes frequency flat fading, high-rate data Channel estimation algorithms allow the receiver to is sent through low-rate sub channels in parallel. Also, a cyclicapproximate the impulse response of the channel and explain prefix is used to make sub-channels orthogonal to one anotherthe behavior of the channel. This knowledge of the channels so that inter carrier interference (ICI) and inter symbolbehavior is well-utilized in modern radio communications. interference (ISI) are avoided. With these advantages OFDMAdaptive channel equalizers utilize channel estimates to has been adopted as the standard transmission technology forovercome the effects of inter symbol interference. Diversity digital audio broadcasting (DAB) and terrestrial digital videotechniques (for e.g. the IS-95 Rake receiver) utilize the broadcasting (DVB-T) in Europe, and recently for wirelesschannel estimate to implement a matched filter such that the local area networks (WLAN) such as HIPERLAN/2 IEEEreceiver is optimally matched to the received signal instead of 802.11a and IEEE 802.16. It is also one of the promisingthe transmitted one. Maximum likelihood detectors utilize techniques for the fourth generation (4G) cellular system.channel estimates to minimize the error probability. One of Although the theoretical basis for development of OFDM isthe most important benefits of channel estimation is that it over 40 years old, it has not been implemented successfullyallows the implementation of coherent demodulation.Coherent until recently owing to high amount of computationaldemodulation requires the knowledge the phase of the signal. complexity and memory requirements of the receiver for suchThis can be accomplished by using channel estimation a system.techniques. The channel estimation in OFDMA can be widelyclassified into two categories.[7] 1. Pilot Based channel Estimation: known symbol calledpilots are transmitted. 2. Blind Channel Estimation Methods: No pilots arerequired. It uses some underlying mathematical properties of data sent. The Blindchannel estimation methods are computationally complex and (a) Time domain, The number of cycleshard to implement. The Pilot based channel estimationmethods are easy to implement, but they reduce bandwidth (b) Frequency domain, the peak of thein Symbol differ by oneefficiency. The Pilot based methods are more popular for adjacent sinc comes at the null of other sincsubnowadays. IEEE802.16e standards supports the Pilot based carriers waveforms.channel estimation Fig 1. Orthogonality of subcarriers 42 All Rights Reserved © 2012 IJARCSEE
  3. 3. ISSN: 2277 – 9043 International Journal of Advanced Research in Computer Science and Electronics Engineering Volume 1, Issue 6, August 2012 V. SYSTEM MODEL higher.Variation in QPSK amplitude is not much. Hence carrier power almost remains constant.We derive a linear system model which relates the transmitted iv) QAM:-OFDM symbol (in frequency domain), [ ] 1 2 1 , ,... ,........ − = For same bit error the BW required by QAM is reducedk N x x x x x and the received OFDM symbol [ ] 1 2 1 , ,.... to half as compared to BPSK. Less bit error probability.,......... − = k N y y y y y . Assuming that most of the energy inthe impulse response is concentrated in L taps, the channel VI. SIMULATIONS PARAMETERSvector is given by [ ] 1 2 , 1 , ,..... 0,..0 L NX h = h h h .Assume that channel remains constant over one OFDM A system level simulator, developed in MATLAB, for singlesymbol and L is less than the cyclic prefix.[4] cellular, single user environment has been used. The Simulation parameters are shown in the table below Fig 2. OFDM System Model VI. MODULATON SCHEMESIt is defined as the process by which some characteristic of asignal called carrier is varied in accordance with modulatingsignal.Used modulation technique arei). Binary Phase Shift Keying (BPSK)ii).Differential Phase Shift Keying (DPSK)iii).Quadrature Phase Shift Keying (QPSK)iv).Quadrature Amplitude Modulation (QAM)i) BPSK:- Table 1.Less Probability of error.It need a complicated synchronizingckt at the receiver for generation of local carrier. Ambiguity in The fading channel is implemented with the Jakes Methodthe output signal because use square law device in receiver (Sum of Sinusoids method). The fading channel model, withside. power delay profile corresponding to PED B, 1 is used. The channel parameters are listed below. Only sample spacedii) DPSK:- channels are considered. The non-integer samples of the PDPDPSK does not need carrier at it receiver. This means that the are mapped to the nearest sample position. The pilots arecomplicated circuitry for generation of local carrier is QPSK modulated and they have unit energy.avoided. The BW requirement of DPSK is reduced comparedto that of BPSK.The probability or error rate of DPSK ishigher than that of BPSK. Noise interference in DPSK hasmore.iii). QPSK:- For the same bit error rate, the BW required by QPSK isreduced to half as compared to BPSK. Because of reducedBW, the information transmission rate of QPSK is 43 All Rights Reserved © 2012 IJARCSEE
  4. 4. ISSN: 2277 – 9043 International Journal of Advanced Research in Computer Science and Electronics Engineering Volume 1, Issue 6, August 2012 10 PSK QPSK 0 QAM Normalized MeanSquareError -10 -20 -30 -40 -50 -60 5 10 15 20 25 30 35 40 Table 2. Symbol Index VII. RESULTS:- Fig-4 Channel Length=6 and Doppler Frequency 40 Hz Parameter Modulation Technique 10 PSK Doppler PSK QPSK QAM 0 QPSK QAM frequency NMSE NMSE NMSE Normalized MeanSquareError Hz -10 in dB in dB in dB -20 20 -40 -42 -42 -30 40 -46 -41 -47 -40 60 -41 -38 -49 -50 70 -45 -38 -49 -60 5 10 15 20 25 30 35 40 Symbol Index Table 3.For fixed channel length and varying Doppler frequency 10 PSK 10 PSK QPSK 0 QPSK QAM 0 QAM Normalized MeanSquareError -10 Normalized MeanSquareError -10 -20 -20 -30 -30 -40 -40 -50 -50 -60 -60 5 10 15 20 25 30 35 40 5 10 15 20 25 30 35 40 Symbol Index Symbol Index Fig -5 Fig-3 Channel Length=6 and Doppler Frequency 60 HzChannel length=6 and Doppler Frequency=20 Hz Fig-6 Channel length=6 and Doppler Frequency=70 Hz 44 All Rights Reserved © 2012 IJARCSEE
  5. 5. ISSN: 2277 – 9043 International Journal of Advanced Research in Computer Science and Electronics Engineering Volume 1, Issue 6, August 2012 VIII. CONCLUSIONThe pilot based channel estimation and tracking algorithmsare investigated for reuse1 OFDMA system, which is incompatible with IEEE 802.16e standard. We studied andsimulated MSL method for sample spaced channels. The FFTbased method is affected by the ―smearing effect‖ due to thepulse shaping in the OFDMA symbol because of this it’sperformance curve reaches an error floor at high SNR. MLSmethod is always superior to the other. We studied theperformance of MLS method under the channel varying insidean OFDMA symbol. This introduces ICI and causes the MLSmethod to reach an error floor at high SNR.I have seen the effect of varying Doppler frequency ondifferent modulation technique. We get that QAM is better Sumit Kumar Gupta, B.Tech, M.Tech (Digital Communication) from MANIT Bhopal(M.P). I have five year teaching experience.than other modulation technique for varying Dopplerfrequency.REFERENCES:-[1] Panayiotis Kolios, Vasilis Friderikos, Katerina Papadaki, ‖FutureWireless Mobile Networks: Energy Consumption and Resource Utilization inMechnical Relaying‖, IEEE Vehicular Technology Magazine, March 2011.[2] S.M. Riazul Islam and Kyung Sup Kwak,―Winner-Hopf InterpolationAided Kalman Filter-Based Channel Estimation for MB-OFDM UWBSystems in Time Varying Dispersive Fading Channel‖, ICACT, Feb.7-10,2011[3] Hussain Hijazi, Eric Pierre Simon, Martine Lienard, Laurent Ros,―Channel Estimation for MIMO-OFDM Systems in Fast Time-VaryingEnvironments‖, 4th ISCCSP 2010, Limassol, Cyprus, 3-5 March 2010[4] Darryn Lowe and Xiaojing Huang ―Complementary ChannelEstimation and Synchronization for OFDM‖ The 2nd InternationalConference on Wireless.IEEE 2007[5] Yoshida Honmachi Sakyo-ku, ― Uplink Channel Estimation for OFDMASystem‖ Kazunori Hayashi and Hideaki Sakai Graduate School ofInformatics, Kyoto University JAPAN IEEE ICASSP 2007[6] IEEE Std. 802.16e, ―Air interface for fixed and mobile broadband wirelessaccess systems amendment for physicaland medium access control layers forcombined fixedand mobile operation in licensed bands,‖ IEEE, 2006.[7] A. Mobasher and A. K. Khandani "Integer-Based Constellation ShapingMethod for PAPR Reduction in OFDM Systems," IEEE Transactions onCommunications, vol. 54, Issue 1, Jan. 2006, pp. 119-127..[8] A. Aassie-Ali, 0. Aly and A.S. Omar, "High resolution WLAN indoorchannel parameter estimation and measurements for communication andpositioning applications at 2.4, 5.2 and 5.8 GHz," in Proc. IEEE Radio andWireless Symposium, RWS-2006 (San Diego) 2006. 45 All Rights Reserved © 2012 IJARCSEE

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