Phase Rotation Based Precoding for MISO DCO-OFDM LiFi with Highly Correlated Channels

Indoor lighting is achieved by multiple light emitting diodes (LEDs) working together, which forms a multi-input single-output (MISO) Light Fidelity (LiFi) system. Due to small separation, the channels between LEDs are highly correlated, causing performance degradation. This is the first work to apply direct-current-biased optical orthogonal frequency division multiplexing (DCO-OFDM) to reduce the adverse effects of high channel correlations in terms of phase for MISO LiFi systems. We propose an optimal phase rotation (PR) based precoding to achieve the reduction of high channel correlation effects for spatial multiplexing (SMP) and spatial modulation (SM) transmissions, respectively. The optimally rotated phase angles are obtained by maximizing minimum Euclidean distances between all candidate signals. The transmission power is not affected by the proposed PR based precoding, while in the previous works, transmitted signals of some LEDs are allocated less power. The proposed PR based precoding is designed offline, applicable to arbitrary multiple transmitted signals with any $M -$ary quadrature amplitude modulation $(M -$QAM). Simulation results show that the proposed approach provides bit error rate (BER) performance better than state-of-the-art methods. Analytical results are also derived to provide BER performance close to numerical results.