# Tolerance-Aided Interference Degradation for Optical OFDM in Power-Constrained Systems
###### Abstract  
Optical wireless communications (OWCs) are power-efficient for providing short-range and high-speed data transmission. In this paper, we propose an interference degradation algorithm for power-constrained OWC systems when using orthogonal frequency division multiplexing (OFDM). In this algorithm, we introduce a decision tolerance at the receiver that can be optimally designed to reduce interference caused by peak power clipping distortion and decrease additive noise collected by the photodetectors. Although the recently proposed clipping-enhanced optical OFDM (CEO-OFDM) can transmit clipped information through extra signal frames, reconstructing the received signal introduces more noise. Using the proposed decision tolerance, we can determine whether the signals in the extra frame are required for data reconstruction. By optimally choosing the decision tolerance, the bit error rate (BER) performance is enhanced compared to those of DCO, ACO, and CEO-OFDM. Additionally, the proposed algorithm offers a wider dynamic modulation index range than DCO, ACO, and CEO-OFDM at the same BER. Using the tested parameters, the proposed algorithm with 64-QAM achieves a similar best BER performance as DCO, ACO, and CEO-OFDM using 32-QAM. Therefore, the proposed algorithm achieves a higher data rate than those of the other compared techniques. optical wireless communications; power-constrained systems; OFDM; clipping distortion 20231002032  
ACO-OFDM sacrifices half of its bandwidth efficiency. Alternatively, unipolar OFDM (U-OFDM), or Flip-OFDM, transmits positive and negative signals in separate adjacent frames. Nevertheless, U-OFDM utilizes an extra signal frame in each OFDM symbol, leading to a spectrum efficiency that is only half that of DCO-OFDM [6]. Given the inherent peak power constraint associated with light sources, it becomes imperative to account for the presence of clipping distortion, which can exert a pronounced adverse effect on system performance. To address this challenge, contemporary research has introduced a novel solution known as clipping-enhanced optical OFDM (CEO-OFDM), tailored specifically for power-constrained OWC systems [7]. CEO-OFDM employs an ingenious approach, whereby the clipped portion of the signal is transmitted via extra signal frames. This strategic utilization of the clipped portion effectively mitigates the nonlinear distortion stemming from peak power constraints. Derived from both analytical assessments and simulation experiments, CEO-OFDM2 exhibits superior performance, achieving a lower biterror rate (BER) or a higher data transmission rate, compared to existing state-of-the-art OFDM schemes. Nevertheless, it is important to acknowledge that the incorporation of extra signal frames necessitates a wider bandwidth, thereby introducing additional noise components at the receiver. Furthermore, the reconstruction of the intended data involves the amalgamation of multiple signal frames, a process that compounds the noise from each frame, ultimately amplifying the cumulative effective noise. The current research focus in the field of optical OFDM revolves around addressing the issues related to clipping distortions caused by the nonlinear behavior of light sources and improving receiver design. Recent studies have explored various strategies to tackle these challenges. One approach involves optimizing power allocation for individual subcarriers in DCO- and ACO-OFDM systems, with the goal of maximizing the signal-to-noise ratio (SNR) while considering clipping distortions [8]. Furthermore, researchers have introduced variations of DCO- and ACO-OFDM, such as adaptively biased OFDM and superposition-based nonlinearity mitigation for ACO-OFDM, to mitigate clipping distortions [9; 10]. To address receiver-related challenges, novel methods have been developed to combat clipping distortions. Unlike pre-processing techniques applied at the transmitter, these algorithms are designed specifically for the receiver.