Entropy loading in layered asymmetrically clipped optical OFDM systems for high spectrum efficiency visible light communication

Layered asymmetrically clipped optical orthogonal frequency division multiplexing (LACO-OFDM), which can offer high efficiency in both spectral and energy under the unipolar signal constraint, is a state-of-the-art multi-carrier modulation format for visible light communication (VLC) systems. However, entropy loading (EL), which can squeeze out the last few bits of the multi-carrier system, has not been demonstrated in LACO-OFDM systems yet. Unlike the <i>single-layer</i> multi-carrier system, where orthogonal subcarriers allow straightforward EL application, the <i>multi-layer</i> based LACO-OFDM complicates EL application due to non-orthogonal subcarriers caused by inter-layer interference (ILI) from residual clipping noise (RCN) in demodulation. After theoretically analyzing the impact of probabilistic constellation shaping (PCS) on RCN in LACO-OFDM, a probability-aware RCN power estimation method based on constellation trimming (CT) is proposed to accurately estimate the effective signal-to-noise-ratio (SNR) of the RCN-contaminated subcarriers. Numerical studies have been conducted to evaluate the performance of the proposed method under various channel conditions, which show that a "win-win" outcome, i.e., higher estimation accuracy with lower computational complexity, can be achieved with a proper threshold in CT. Based on this, a total power constrained EL algorithm is proposed for LACO-OFDM VLC systems, to the best of our knowledge, for the first time. The performance investigation of the proposed EL algorithm in the LACO-OFDM VLC system has been carried out via both simulation and experiment. The experimental results show that: (a) compared with the existing bit loading (BL) method which also considers the impact of RCN in LACO-OFDM, our EL method offers higher generalized mutual information (GMI) across the whole range of received optical power (ROP) under test, with an average GMI improvement of ~10% and an maximum GMI gain up to 0.2 bits/symbol; (b) setting the CT threshold to 0.5 yields GMI comparable to the case without CT while substantially reducing the computing time (e.g., by ~80% at the ROP of 28.44 µW), with time savings becoming more pronounced at higher ROP levels.