Robust and Low-Complexity Timing Synchronization for DCO-OFDM LiFi Systems

Light fidelity (LiFi), using light devices like light emitting diodes (LEDs) and visible light spectrum between 400 THz and 800 THz, provides a new layer of wireless connectivity within existing heterogeneous radio frequency (RF) wireless networks. Link data rates of 10 Gbps from a single transmitter have been demonstrated under ideal lab conditions. Synchronization is one of these issues usually assumed to be ideal. However, in a practical deployment, this is no longer a valid assumption. Therefore, we propose for the first time a low-complexity maximum likelihood (ML) based timing synchronization process that includes frame detection and sampling clock synchronization for direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) LiFi systems. The proposed timing synchronization structure can reduce the highcomplexity two-dimensional search to two low-complexity onedimensional searches for frame detection and sampling clock synchronization. By employing a single training block, frame detection can be realized, and then sampling clock offset (SCO) and channels can be estimated jointly. We propose a number of three frame detection approaches, robust against the combined effects of both SCO and the low-pass characteristic of LEDs. Furthermore, we derive the Cramér-Rao lower bounds (CRBs) of SCO and channel estimations, respectively. In order to minimize the CRBs and improve synchronization performance, a single training block is designed based on the optimization of training sequences, the selection of training length and the selection of DC bias. Therefore, the designed training block allows us to analyze the trade-offs between estimation accuracy, spectral efficiency, energy efficiency, and complexity. The proposed timing synchronization mechanism demonstrates low-complexity and robustness benefits, and provides performance significantly better than existing methods.