Visible light communications (VLC), a short-range optical wireless communication scheme using light-emitting diodes (LEDs) as transmitters, has been proposed to support multiple users, predominantly in indoor environments. This paper explores using orthogonal frequency division multiple access (OFDMA) for VLC due to its high spectrum utilization efficiency and potentially high transmission data rate. The proposed optical OFDMA uses adjacent subcarriers as a set to support each user. Doing so allows some users to sample at a lower sampling rate and employ a smaller fast Fourier transform (FFT) to lower the computational complexity and hardware cost. A multi-cell structure is naturally obtained in VLC since more than one LED lamp is usually installed in an indoor environment. In this paper, users located in different cells can reuse the same subcarrier sets, potentially causing multiple access interference. Analysis and simulation results are used to predict the subcarrier-set reuse probability over the entire indoor space. When the OFDM modulation index is optimized considering the peak-power-limit of the LEDs, the average transmission data rate is twice that of using OFDM that does not reuse the spectral resources, for the scenarios tested.

Optical wireless communication is an energy-efficient and cost-effective solution for high-speed and highly-secure wireless connections. In this paper, we compare, discuss, and analyze three popular optical orthogonal frequency division multiplexing (OFDM) techniques, such as DC-biased optical OFDM (DCO-OFDM), asymmetrically-clipped optical OFDM (ACO-OFDM), and unipolar OFDM (U-OFDM), for underwater optical wireless communication systems. The peak power constraint, bandwidth limit of the light source, turbulence fading underwater channel, and the channel estimation error are taken into account. To maximize the achievable data propagation distance, we propose to optimize the modulation index that controls the signal magnitude, and a bitloading algorithm is applied. This optimization process trades off the clipping distortion caused by the peak power constraint and the signal to noise ratio (SNR). The SNR and clipping effects of the three compared OFDM techniques are modeled in this paper. From the numerical results, DCO-OFDM outperforms ACO- and U-OFDM when the transmitted bit rate is high compared to the channel bandwidth. Otherwise, U-OFDM can provide a longer propagation distance or requires less transmitted power.

Visible light communications (VLC), a new optical wireless communication technology that uses illumination light-emitting diodes (LEDs) as transmitters, requires a modulation scheme that is well suited to these devices' nonlinear response. Optical orthogonal frequency division multiplexing (OFDM) is a promising technique to provide high-speed data transmission for VLC. However, the peak transmitted power limitation and nonnegative transmitted signal constraint of the lighting sources can result in nonlinear signal distortion from clipping. In this paper, we propose a novel optical OFDM scheme for VLC systems called clipping-enhanced optical OFDM (CEO-OFDM) that transmits via extra time slots the information clipped by the peak power constraint. CEO-OFDM sacrifices bandwidth to allow a higher modulation index to improve the signal to noise ratio and reduce the clipping distortion caused by the peak power limitation. From analytical and numerical results, the proposed CEO-OFDM provides better bit error rate performance and higher data rate than DC-biased optical OFDM (DCO-OFDM), unipolar OFDM (U-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM). Furthermore, CEO-OFDM can provide a better illumination performance that supports light dimming.

Underwater integrated sensing and communication (UISAC) systems employ a unified signal waveform to facilitate both data exchange and sense. However, communication-centric UISAC systems face challenges due to the stochastic nature of communication signals used for sensing, leading to diminished sensing performance. Meanwhile, sonar systems, in comparison to radar systems, operate in a complex noise environment with time-varying and non-Gaussian properties, further deteriorating sensing capabilities. To address these challenges, we propose an adaptive algorithm using orthogonal time-frequency space (OTFS) signals for underwater target sensing. Our approach incorporates an online Gaussian-mixture model (GMM) to track time-varying background noise and utilizes a weighted Mahalanobis distance as a statistical measure to differentiate between reflected OTFS signals and ambient noise rather than relying solely on waveform. Comparative evaluations with existing energy detectors in UISAC system show that our strategy improves sensing accuracy, enhancing performance even in the presence of complex oceanic noise.

Light-emitting diode (LED)-based communications, such as visible light communications and infrared communications, are candidate techniques to provide short-range and high-speed data transmission. In this letter, M-ary pulse amplitude modulation (M-PAM), used as a high bandwidth efficiency scheme, is compared with three well-known optical orthogonal frequency-division multiplexing (OFDM) schemes. Considering the bandwidth limit and constrained peak transmitted power characteristics of LEDs, a bit loading algorithm and a single-tap equalizer with an optimized modulation index are used for the optical OFDM schemes tested. To reduce the inter-symbol interference caused by the bandlimited channel, an optimized pulse shape and a minimum mean squared error equalizer are applied to the M-PAM system. From numerical results, M-PAM can provide a substantially higher data rate than OFDM for bandlimited channels. When the channel bandwidth is ample compared with the symbol rate, optical OFDM outperforms M-PAM.