تخمين كانال و حذف تداخل بين حاملي در كانال هاي دوگانه انتخابي با استفاده از مدل بسط پايه اي براي سيستم هاي OFDM

چكيده انگليسي :

Channel Estimation and ICI Mitigation in Doubly Selective Fading Channels using BEM for OFDM Systems Paria Rezaeinia p rezaeinia@ec iut ac ir Date of Submission 2012 02 21 Department of Electrical and Computer Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language Farsi Supervisor A M Doosthosseini alimdh@cc iut ac ir Abstract Rapid growth of digital communication applications in recent years has increased the demand for highdata rates Increasing data rate in single carrier modulations reduces the symbol period and increases intersymbol interference ISI due to the spread of channel impulse response Multicarrier modulations areintroduced to overcome this problem by dividing the frequency band into N adjacent sub channels appropriate for being modeled as flat fading Therefore in multicarrier modulation using a one tap equalizerin each sub channel is sufficient to mitigate the impairments of a wireless channel Orthogonal frequencydivision multiplexing OFDM is a kind of multicarrier modulation with orthogonal subcarriers One of themain problems in these systems is their sensivity to frequency offset which destroys the orthogonality of thesubcarriers and therefore reduces the efficiency of the whole system Doppler shift in wireless channels andfrequency offset of local oscillators are the major sources of frequency displacement in OFDM systems We consider OFDM systems with inter carrier interference ICI due to time variations of wirelesschannel The purpose of this thesis is to deploy a proper model and estimate the channel at the receiver usingComb type pilots and mitigate ICI based on this estimation The frequency domain channel matrix is dividedinto two separate matrices The first one which determines the desired effect of the channel contains thediagonal elements of the channel matrix and the second one determines the ICI and contains off diagonalelements Polynomial basis expansion model P BEM and complex exponential basis expansion model CE BEM are used to model the time variations of the channel We show that in CE BEM we can improve thebit error rate BER for high Doppler frequencies without increasing the number of parameters of the system But in P BEM increasing the number of basis functions leads to the building up of the parameters andcomputational complexity After estimation of the channel we use an iterative method to cancel the ICI Inthe equalization stage we calculate an estimation of the transmitted signal based on the received signal and aone tap MMSE equalizer for each tap The weights of the equalizer taps are estimated using diagonalelements of the channel matrix and noise power The next step is to cancel the effect of ICI so we calculatethe ICI term based on the off diagonal elemnts of channel matrix and substract this term from the receivedsignal The received signal vector calculated in this step is used for the next iteration The number ofiterations depends on the desired BER The time domain equivalent of the ICI component of channel matrixis a matrix with L 1 non zero elements in each row where L is the number of taps Thus to reduce theamount of calculation this iterative ICI cancellation is done in time domain The order of calculations infrequency domain is N N and in time domain it is reduced to L 1 N Simulation results show that CE BEM is more efficient than P BEM especially for high mobilityenvironments On the other hand increasing the number of basis functions in CE BEM increases the accuracyof the model and results in better performance of the algorithm and a lower BER Key Words Doubly selective fading channels OFDM Basis expansion model Inter carrier interference ICI Equalization

رضايي نيا، پريا

تخمين كانال و حذف تداخل بين حاملي در كانال هاي دوگانه انتخابي با استفاده از مدل بسط پايه اي براي سيستم هاي OFDM

تداخل بين حاملي ICI , تداخل بين سمبلي ISI , BEM