Adaptive Modulation Technique with Cfo Correction for Ofdm Systems

A unique feature of the OFDM communication scheme is that, due to the IFFT at the transmitter and the FFT at the receiver, the frequency selective fading channel is converted into flat fading channels. However the OFDM approach can suffer from CFO (Leke and Cioffi 1998). The effects of CFO has been analyzed quite extensively in Beek et al (1997) and Jungwon Lee et al (2006). Carrier Frequency error in the transmission due to synchronization errors and Doppler shift results in loss of orthogonality between the subcarriers. There are two deleterious effects caused by CFO, one is degradation of SNR and the second is introducing ICI (Ma et al 2003). Due to this SNR degradation, the performance of BER is poor. The orthogonality among OFDM subcarriers are destroyed because of CFO. It causes the leakage of FFT, and eventually leads to common phase error (CPE) and ICI among subcarriers. Also a CFO of only 1 2 % of the subcarrier spacing results in the effective SNR being limited to 20 dB (Santhanam and Tellambura 2001, Martin and Hermann 2007).