كاهش توان گلبرگهاي فرعي در سيستم هاي راديويي هوشمند مبتني بر OFdM

چكيده انگليسي :

Sidelobe Suppression in OFDM based Cognitive Radio systems Somayeh Mahmoodi smahmoodi@ec iut ac ir 1388 02 08 Department of Electrical and Computer Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language FarsiSupervisor Mohamad Javad Omidi omidi@cc iut ac irAbstractAs wireless applications are ever increasing because of spectrum scarcity it is not possible to assignspectrum to each application statistically Federal Communications Commission FCC measurements revealextensive portions of allocated spectrum in different time and frequencies are not used sufficiently Thuslicensed spectrum is not utilized optimally In order to deal with these two problems the concept of dynamicspectrum access DSA has been proposed Cognitive radios offer wireless transceivers enabling DSA Indynamic spectrum access the secondary user decides on whether or not a particular licensed frequency bandis currently used and transmits data in the band if it is not being used by primary user While secondary useris renting the band it must insure that performance of primary user as well as secondary user is not affected Therefore coexistence of the primary and the secondary users is an important criterion that makes DSA afeasible solution for efficient spectrum usage This dissertation investigates physical layer techniques toenhance a cognitive radio performance Orthogonal frequency division multiplexing OFDM has proven to be the prime candidate for cognitiveradio systems as it divides spectrum into many subcarriers Using OFDM it is possible to partition broadsections of spectrum into a lot of channels and assign each channel to a distinct user This thesis investigates an important problem concerning the coexistence of the primary and the secondaryusers The problem is that the interference level imposed on primary user by neighboring secondary usermust be kept within a determined limited range As OFDM uses sinc type pulses in representing thesymbols transmitted over all the subcarriers during one time instant the large sidelobes that occur canpotentially interfere with the signal transmissions of the adjacent primary systems or with the transmissionsof other secondary users These high level sidelobes can also cause inter carrier interference ICI Thus thefundamental goal of this thesis is to develop a technique which suppresses the interference caused by thesecondary user while not significantly having influence on the system performance of the secondary user High spectral efficiency of MSK provides great reduction in out of band radiation and the same idea is usedto decrease inter carrier interference and consequently out of band interference to primary user in OFDMbased cognitive radio systems This thesis uses OFDM MSK for this objective OFDM MSK is presented inorder to reduce system sensitivity against phase noise and carrier frequency offset in OFDM systems OFDM MSK exploits MSK to transmit information on each of subcarriers Using OFDM MSK out of bandpower is significantly reduced This thesis studies some present methods used for out of band radiationreduction It presents a novel new structure for OFDM MSK as well For its implementation we employ adifferent perspective of MSK as a kind of OQPSK Such a viewpoint in addition to having MSK propertiesresults in a simpler implementation Advantages and drawbacks of MSK OFDM are studied In order toreduce system complexity appropriate transmitter and receiver structures are offered in which FFT IFFT isemployed Key Wordscognitive radio systems OFDM sidelobe level out of band interference ICI

محمودي، سميه

كاهش توان گلبرگهاي فرعي در سيستم هاي راديويي هوشمند مبتني بر OFdM

تداخل خارج ا ز باند , ICI , MSK