Semi-Blind Joint Synchronization and Channel Estimation for DCO-OFDM OWC Systems Using DC Bias

Direct current (DC) is added into intensity modulation and direct detection (IM/DD) signals in time domain to make most negative signals be positive in DC biased-optical orthogonal frequency division multiplexing (DCO-OFDM) systems for optical wireless communication (OWC). To the best of the authors' knowledge, this is the first work to utilize DC inherent in DCO-OFDM signals to allow semi-blind joint synchronization and channel estimation for DCO-OFDM OWC systems. We propose a novel semi-blind DC-based joint estimation (SDJE) approach, where sampling time offset (STO) as one of timing synchronizations and channels can be jointly estimated based on DC inherent in DCO-OFDM signals. The proposed SDJE approach is spectral-efficient, requiring no pilot. This is different from state-of-the-art works that employ a number of pilots to perform synchronization and channel estimation. We formulate a cost function by exploring the difference between the received signals and DC signals. The joint STO and channel estimations can be conducted by minimizing the formulated cost function. Furthermore, we propose a dimensionality reduction approach, and design an offline database to reduce complexity. Simulation results show that the proposed SDJE approach provides bit error rate (BER) performance close to the ideal case with no STO and perfect channel state information (CSI).