Coherent optical orthogonal frequency division multiplexing (CO-OFDM) is highly tolerant against channel impairments such as chromatic dispersion (CD) and polarization-mode dispersion (PMD) because of the added cyclic prefix (CP). To make CO-OFDM work, the signal must be preprocessed before the received one can be demodulated; additionally, CD and PMD compensation must be don after the received signal has been demodulated. Two important issues for OFDM receiver preprocessing are treated. The first issue is synchronization. It is the alignment of the starting point of the OFDM symbols to acquire the fast Fourier transform (FFT) window. In case of mismatch, intersymbol interference (ISI) and intercarrier interference (ICI) will occur. A simple and powerful method to detect the starting point of OFDM frames, and to compensate for IQ swapping and inversion is proposed. The second issue is carrier frequency offset (CFO) correction. CFO leads directly to ICI. Two steps RF-pilot based method is proposed. Only one pilot tone is needed.Next, for channel compensation, several pilots are uniformly inserted into each OFDM symbol. The proposed method can continuously track the effect of CD symbol-by-symbol, and it outperforms the common phase error method (CPE) method. It is performed in the frequency domain after the FFT unit to recover the information.For long-haul communication, the accumulated CD effect becomes very large. To neglect ISI due to CD, the CP length must be large as well. However, these additional samples reduce the effective data rate, and the laser phase noise (PN) becomes a serious problem. Therefore, reduction or neglecting of CP for OFDM symbol is very attractive for our investigation. To increase the ISI tolerance, this thesis investigates the predistortion of OFDM symbols by the windowing method where a half and a full window are considered. All of them are done in the time domain.