Research On Carrier Synchronization In OFDM Systems

Orthogonal Frequency Division Multiplexing (OFDM) is a multi carrier modulation technology with high spectrum efficiency and robust transmission against multipath interference in wireless communications domain, has a wide range of application and will become the core technology of the next generation mobile communications. However all the advantage of OFDM can only be reflected when the orthogonality between sub channels is maintained. Once the orthogonality is destroyed, the performance of the system will deteriorate severely. But OFDM has its own shortcomings. It is very sensitive to Carrier Frequency Offset (CFO) which can destroy the orthogonality between sub carriers and then deteriorate the performance of OFDM system. Hence the research on carrier frequency offset has become crucial for OFDM.This paper mainly studies OFDM carrier synchronization problem, aiming at in depth analysis , research and comparison of classic carrier frequency offset algorithms. Firstly the basic principle of OFDM is introduced and then three synchronization error models, i.e., frame synchronization errors, sample synchronization errors and CFOs are talked about with emphasis on their effects on OFDM systems. The carrier synchronization algorithms are divided into two kinds: Non Data Aided (NDA) and Data Aided (DA). For NDA maximum likelihood Estimation (MLE) algorithm based Cycle Prefix (CP) algorithm is described with computer simulation and some improvement is given. For DA the nonlinear least square estimation algorithm is proposed based on comb type pilots. Also discussed are the famous P.H.Moose and Schmidl & Cox algorithms both based on training sequences. Finally the carrier synchronous algorithm related to IEEE802.16a standards is analyzed based on training sequences. Both advantages and disadvantages of methods presented are compared, and applicable situations are also depicted.