Timing and frequency synchronization for OFDMsystems

Dheerajkumar C* and Vighnesh S Bhat†Department of Electrical Engineering, Indian Institute of Technology Kanpur

Kanpur, Uttar Pradesh, India 208016Email: *dheerajc@iitk.ac.in and †vsbhat@iitk.ac.in

Abstract—Performance of an Orthogonal Frequency DivisionMultiplexing (OFDM) system is found to deteriorate becauseof timing and frequency synchronization errors (offsets) atthe receiver. TO at receiver results in capture of wrong FastTransform (FFT) window causing distortion. Frequency offset atthe receiver results in loss of orthogonality of OFDM sub carrierscausing Inter Carrier Interference (ICI). Thus the effects of theseoffsets have to be studied and compensated at the receiver. Heredifferent timing offset (TO) and frequency offset (FO) estimationtechniques are analyzed. Maximum Likelihood (ML) method forestimation of TO where, redundancy in cyclic prefix and pilotsused for channel estimation are exploited is studied. A timingsynchronization method for OFDM in the cases of continuousor burst transmissions where a training symbol is transmittedwhich aids in estimation is studied. Integer FO estimation forOFDM systems with residual TO over frequency selective fadingchannel using cross ambiguity function (CAF) based optimizationis studied. Fractional FO over mobile radio channels using costfunction based optimization is studied.

Keywords : Cross ambiguity function (CAF), Maximum like-lihood (ML), Inter Carrier Interference (ICI), Orthogonal fre-quency division multiplexing (OFDM), synchronization, Timingoffset (TO), Frequency offset (FO).

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