Friday, December 20, 2013

CFO estimation in OFDM

Frequency error in OFDM systems is often called carrier frequency offset (CFO).

CFO can be caused by frequency differences between the transmitter and receiver oscillators, Doppler shift of mobile channels, or oscillator instabilities.

All sub-carriers are affected by the same amount of CFO. CFO is classified into two categories:

fractional sub-carrier spacing CFO, and integer sub-carrier spacing CFO.

Fractional CFO introduces inter-carrier interference (ICI) between sub-carriers. It destroys the orthogonality of sub-carriers and results in bit error rate (BER) degradation.

Integer CFO does not introduce ICI between sub-carriers, but does introduce a cyclic shift of data sub-carriers and a phase change proportional to OFDM symbol number.

Therefore, it’s necessary to correct this CFO at the OFDM receiver.

There are several techniques to estimate and compensate for these errors using time-domain or frequency-domain approaches, sometimes called pre-FFT and post-FFT synchronization, respectively.

More details here

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