مدولاسيون چند حاملي و طيف گسترده چربي بر مبناي تبديل فوريه كسري

چكيده انگليسي :

Multi Carrier Modulation and Chirp Spread Spectrum based on Fractional Fourier Transform Hamid Aminoroaya h aminoroaya@ec iut ac ir Date of Submission 2011 5 2 Department of Electrical and Computer Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language PersianSupervisor Mohammad Javad Omidi omidi@cc iut ac irAbstract Nowadays the demand for high rate data transmission in wireless environment increases rapidly Oneof the best techniques in this environment is multi carrier modulations MCM The orthogonal frequencydivision multiplexing OFDM is one of the most important classes of MCM with equally spacedsubcarriers and overlapping spectra The bases in this modulation are cosine signal and can be easilyimplemented by using discrete Fourier transform So it has been implemented in physical layers of manystandards and applications However when the channel is doubly selective time frequency selective as itusually happens in the rapidly fading wireless channel inter carrier interference ICI degrades an OFDMsystem performance Since the channel frequency response is changing rapidly the optimal methodologyshould be able to process non stationary signals In other words the sub carrier frequencies should be timevarying We introduce the use of a multicarrier system that employs orthogonal signal bases of the chirptype Such bases are related to the Fractional Fourier Transform FrFT Therefore the Fractional FourierTransform is considered as a powerful tool for processing the signals with time varying spectrum Inaddition its relationship with other transforms and methods of computing discrete FrFT DFrFT isexplained in this project Then multi carrier modulation based on Fractional Fourier Transform MC FrFT is presented In this system the traditional exponential subcarrier signal bases are replaced by the chirpsubcarrier signal bases and FFT IFFT block in OFDM is replaced by DFRFT IDFRFT Its performance iscompared with OFDM on peak to average power ratio PAPR inter carrier interference ICI equalizeretc The bases in MC FrFT are chirp signals that sweep a frequency interval in specific time interval It hasshown that out of band components in MC FrFT are more than OFDM Therefore the idea is that thismethod can be used in spread spectrum communication Spread spectrum SS is a means of transmission inwhich the signal occupies a bandwidth in excess of the minimum necessity to send the information Chirpspread spectrum CSS is one way to increase the frequency spectra that multiplies signal with chirp Several techniques of chirp spread spectrum and IEEE 802 15 4a standard with CSS physical layer areexplained in this thesis The main limitation of CSS techniques is multiple access interference MAI Multiple access was designed for simultaneously accessing the common channel by many users The MAIoccurs because of high cross correlation between two user chirps and this limitation can be waived by usingFrFT In this thesis we have proposed the chirp spread spectrum system that uses FrFT in both transmitterand receiver This method has less complexity easier implementation and more flexibility Additive whiteGaussian noise AWGN is considered as a channel noise for evaluation of performance Bit error rate BER is taken as a measurement of performance with different signal to noise ratio and the results of thenovel system are presented and discussed Keywords Multi Carrier Modulation Fractional Fourier Transform FrFT Orthogonal FrequencyDivision Multiplexing OFDM Inter Carrier Interference ICI Chirp Spread Spectrum CSS IEEE802 15 4a standard

امين الرعايا، حميد

مدولاسيون چند حاملي و طيف گسترده چربي بر مبناي تبديل فوريه كسري

مدولاسيون چند حاملي , تداخل بين حاملي , استاندارد IEEE802.15.4a , مدولاسيون تقسيم فركانسي متعامد