Phase Rotation Based Precoding for MISO OWC Systems With Highly Correlated Channels

We consider a multiple-input single-output (MISO) optical wireless communications (OWC) system with highly correlated channels, causing bit error rate (BER) performance degradation. Because of intensity modulation and direct detection (IM/DD), the transmitted signals are real-valued and non-negative, which limits the utilization of precoding in phase domain. We employ direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) modulation to design a group of phase rotation (PR) factors in frequency domain for OWC systems, robust against high channel correlations. The proposed PR-based precoding has a number of advantages over the power factor-based design in the literature: i) no change of transmission power on each LED; ii) no signal-to-noise ratio (SNR) and no BER degradation on transmitted signals of all LEDs. We formulate an optimization problem to obtain the optimal PR factors by maximizing the minimum pairwise Euclidean distances between all candidate signals. The optimization problem is non-convex, requiring multi-dimensional exhaustive searches. In order to reduce the complexity, we propose three low-complexity PR-based precoding approaches which provide BER performances better than the power factor-based designs in the literature, and are close to their own analytical results derived, respectively. The proposed approaches are validated via a built testbed, producing comparable performance between measured and simulated BERs.