Automatic frequency control in packet based OFDM systems

This paper presents automatic frequency control (AFC) algorithm in packet based wireless orthogonal frequency division multiplexing (OFDM) system such as 3G-LTE [1]. We first analyze the method of general AFC using cyclic prefix (CP) in OFDM systems. The analysis will show that the compensated frequency value is unstable and related with presence of packet. This instability of frequency control is generated by the variation of accuracy of estimation value that varies with presence of packet. We show how proposed algorithm reduces the instability of frequency control. Results of simulation show that proposed algorithm reduces the instability of compensated frequency and improve tracking performance considerably. In addition, proposed algorithm has good performance in fading channel. This is because proposed algorithm can detect deep fade region and exclude it from frequency control. As a result, proposed AFC algorithm improves stability and performance of frequency control by decision of packet presence. Especially, 16-QAM and 64-QAM modulation schemes for high data rate transmission are sensitive to frequency offset, therefore proposed AFC algorithm supports to keep high data rate system performance stable.