Enhanced Repetition Pilot-Based Timing Synchronization and Channel Estimation for DCO-OFDM OWC Systems

We design pilots that include some repetition parts to allow timing synchronization and channel estimation for direct current biased optical-orthogonal frequency division multiplexing (DCO-OFDM) optical wireless communication (OWC) systems. On one hand, we propose an enhanced repetition pilot (eRP) based timing synchronization approach, robust against low correlation between real-and-positive signals and against inter-symbol interference (ISI) caused by light emitting diode (LED)’s limited bandwidth, much more than the previous works. We define a new timing metric using the ratio of auto-correlation to power, between the first and second halves of the received pilots. Maximizing the defined timing metric allows the timing offset in the cyclic prefixed (CP) area. The phase difference between a number of adjacent subcarriers is utilized to refine timing synchronization. On the other hand, we propose an eRP based channel estimation enhancement (CEE) approach and an imaginary component minimization (ICM) based channel estimation approach, utilizing pilots the same as timing synchronization. The proposed CEE approach allows the repeated parts of pilots to be averaged for channel estimation enhancement. The proposed ICM approach is divided into LED’s limited bandwidth estimation and line-of-sight (LoS) channel estimation, exploring the correlation between the least-square (LS) estimated channel estimate information (CSI) and the trial LED frequency response. The proposed CEE approach provides low complexity, while the proposed ICM approach provides good performance. Simulation results show that, using a single DCO-OFDM symbol, the proposed structure provides bit error rate (BER) close to ideal case with perfect CSI and perfect timing synchronization.