Multivariate Regression Transform for Hybrid Optical OFDM in Visible Light Communications

DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) has been proposed in visible light communication (VLC) to overcome the limited modulation bandwidth of light emitting diode (LED). Due to the implementation of the inverse fast Fourier transform at the DCO-OFDM transmitter, DCO-OFDM suffers from its high peak-to-average power ratio (PAPR), which restricts its use in some VLC applications, especially where the optical power efficiency is a crucial requirement. That is because the LEDs used in VLC systems have a limited optical power-current linear range. To this end, a novel discrete Fourier transform spread-optical pulse amplitude modulation (DFTS-OPAM) signal scheme based on the single carrier-interleaved frequency division multiple access (SC-IFDMA) signal is introduced in this paper to address the high PAPR issue of OFDM. DFTS-OPAM is achieved by considering a PAM as an SC-IFDMA data symbol and duplicate the output vector of the fast Fourier transform at the SC-IFDMA transmitter side. Simulation results show that the PAPR of the proposed scheme is 7 dB lower than that of DCO-OFDM. Furthermore, this significant PAPR improvement is experimentally investigated where the practical results show that the proposed scheme can provide more 2.5 dB reduction in the average transmitted power requirement compared to DCO-OFDM and can subsequently increase the maximum achieved distance between the transmitter and the receiver up to 44%.

A direct current (DC) bias is required in DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) to enable visible light communication (VLC). However, the addition of the DC-bias leads to electrical and optical power inefficiency. An experimental demonstration of an energy efficient superposition modulation technique is presented based on augmented spectral efficiency discrete multitone (ASE-DMT) transmission. Significant energy savings of up to 14.27 dB are achieved in comparison with DCO-OFDM at low DC bias. The results highlight the potential of ASE-DMT in spectrum and energy efficient light fidelity (LiFi) systems.

The quasi-static nature of the optical wireless channel means that the channel state information (CSI) can be readily available at the transmitter and receiver prior to data transmission. This implies that electrically band-limited optical wireless communication (OWC) systems can make use of optimal channel partitioning or vector coding based multi-channel modulation (MCM) to achieve high throughput by mitigating the non-linearities arising from the optical and electrical channel. This paper proposes a pulse amplitude modulation (PAM) based DC-biased optical vector coding (DCO-VC) MCM scheme for OWC. The throughput performance of DCO-VC is evaluated and compared to the well known DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) over hybrid (line-of-sight and diffuse) and diffuse (non line-of-sight only) visible light communication (VLC) channels with additive white Gaussian noise. For the completeness of the VLC physical layer, the performance comparison is based on an uncoded and a forward error correction transmission mode using well-known convolutional codes with Viterbi decoder. The results show that the coded DCO-VC outperforms DCO-OFDM system by achieving up to 2 and 3 dB signal to noise ratio gains over hybrid and diffuse VLC channels, respectively.

Smart city planners pay more attention towards visible light communication (VLC) system to achieve seamless high speed connectivity. A light emitting diode (LED) acts as a transmitter in VLC system. However, the nonlinear characteristics and dimming nature of LED limits the performance of the VLC system by degrading the quality of transmitted signal. In this paper, we propose an adaptive-learning-architecture (ALA) based predistortion technique to estimate and compensate for LED nonlinearities as well as increase the spectral efficiency of a VLC system. A DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) signal is considered. The performance with and without predistorter is analysed assuming optical line-of-sight (LOS) channel. It is shown that degradation due to LED nonlinearity can be compensated by using ALA-based predistortion, and the overall Predistorter-LED system is able to approach near-linear performance. Further, we investigate the impact of dimming on the performance of nonlinear LED in VLC system. Simulation results based on symbol error rate (SER), amplitude distortion (AM/AM) curves and constellation plots validate the performance of our proposed technique.

Conventional non-imaging receiver for multiple-input-multiple-output (MIMO) visible light communications (VLC) systems always suffer from the problem of ill-conditioned channel matrix due to the high correlation among sub-channels. So in this paper, a deflected field-of-views (FOVs) receiver for MIMO VLC system is proposed to reduce the correlation and also have a compact planar structure. The receiver has a diaphragm with several apertures to cover the planar array of photodetectors (PDs), so each PD will own different deflected FOV. Considering the diversity of sub-channels, the precoding method and water-filling algorithm are applied at the transmitters to achieve higher data rate or lower power consumption. In the simulation, the optimal structure parameter of the receiver is demonstrated, and the BER performance of the receiver is also verified, where DC-biased Optical orthogonal frequency division multiplexing (DCO-OFDM) is applied. The results show that the precoding method can significantly improve the system performance and is therefore, perfectly suitable for the deflected FOVs receiver in MIMO-VLC systems.

Orthogonal frequency-division multiplexing (OFDM) is a practical technology in visible light communication (VLC) for high-speed transmissions. However, one of its operational limitations is the peak-to-average power ratio (PAPR) of the transmitted signal. In this paper, we analyze the PAPR distributions of four VLC OFDM schemes, namely DC-biased optical OFDM (DCO-OFDM), asymmetrically clipped optical OFDM (ACO-OFDM), pulse amplitude modulated discrete multitone (PAM-DMT), and Flip-OFDM. Both lower and upper clippings are considered. We analytically derive the complementary cumulative distribution functions (CCDFs) of the PAPRs of the clipped VLC OFDM signals, and investigate the impact of lower and upper clippings on PAPR distributions. Our analytical results, as verified by numerical simulations, provide useful insights and guidelines for VLC OFDM system designs.

In this paper, we investigate physical-layer security for the indoor single-input single-output (SISO) DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) visible light communication (VLC) wiretap systems. To increase system security, we propose a precoding scheme using time-domain artificial noise (AN) to exploit the degrees of freedom (DOFs) provided by the cyclic prefix (CP) of OFDM. We further propose a convex optimization method to restrict the increase of peak to average power ratio (PAPR) introduced by AN precoding and then maximize the secrecy rates. Numerical results suggest that the proposed scheme can significantly boost the secrecy performance and alleviate the impact of PAPR increment.

Visible light communication (VLC) is a technology that is currently being employed to achieve high data rates. Orthogonal frequency division multiplexing (OFDM) is a powerful scheme for intensity modulation and direct detection (IM-DD) that is becoming increasingly important. OFDM systems based on VLC are accomplished through the implementation of IM-DD constraints, including direct current biased optical OFDM (DCO-OFDM), asymmetrically clipped optical OFDM (ACO-OFDM), and asymmetrically clipped DC-biased (ADO-OFDM), among others. The OFDM technique based on VLC is inefficient in terms of spectral efficiency and has a high peak to average power ratio (PAPR). In this article, the symbol time compression-image adjust (STC-IMADJS) technique will be proposed for throughput maximization and PAPR reduction in the DCO-OFDM, ACO-OFDM, and ADO-OFDM systems. Furthermore, the STC technique enables the simultaneous transmission of two sub-carriers through Walsh spreading codes without inference between them. As a result, the OFDM symbol has been reduced by 50%. The IMADJS strategy reduces the high PAPR of transmitted signals by compressing large signals and expanding small signals, as long as the average power level remains constant after compression. Therefore, the proposed STC-IMADJS technique doubles the throughput as well as significantly reduces the PAPR for OFDM systems based on VLC. The simulation results are performed using Matlab-2021a for 1024 sub-carriers. As a result, the simulation results show that the proposed technique reduces the PAPR by 3.7 dB (86%), 8.16 dB (93%), and 1.81 dB (20%) for DCO-OFDM, ACO-OFDM, and ADO-OFDM systems respectively. Furthermore, the technique has a significantly lower computational complexity than conventional OFDM systems based on VLC. Finally, the measures of performance through this article are bit error rate and complementary cumulative distribution function.

Visible light communication (VLC) is now used to achieve high data rates. Opticalorthogonal frequency division multiplexing (O-OFDM) is a strong technique for intensity modulation and direct detection (IM-DD). Direct current biased O-OFDM (DCO-OFDM) and asymmetrically clipped O-OFDM are two modern O-OFDM systems that operate with IM-DD constraints (ACO-OFDM). Due to severe out of band interference, the DCO-OFDM and ACO-OFDM systems have a high peak to average power ratio (PAPR) and substantial adjacent channel interference (ACI) (OOB). Furthermore, as compared to bipolar OFDM systems, they obtain a greater chance of bit error. The combination of the STC scheme, symbol extension, and pulse shaping signal is presented in this paper for improving the performance of the O-OFDM system without affecting system throughput. This article also decreases the OOB, PAPR, and bit error rate (BER) of DCO-OFDM and ACO-OFDM systems. Further analytical analyses for the proposed systems, such as OOB, PAPR, and computing complexity, are obtained in this paper. The simulation results verify the analytical study of the proposed methods for OOB reduction and PAPR reduction. As compared to typical O-OFDM systems, the proposed shaped STC-DCO-OFDM scheme and the proposed shaped STC-ACO-OFDM scheme lower the OOB by about 40 dB and the PAPR by about 4 dB at the 10 -3 probability point of the complementary cumulative distribution function (CCDF). The proposed systems outperform existing O-OFDM systems under AWGN for the BPSK modulation scheme in terms of BER, and reduce the computational complexity.

Visible light communication (VLC) is a promising and viable optical wireless communication (OWC) system that utilizes light-emitting diodes (LEDs) to achieve high data rates, energy efficiency, and resistance to electromagnetic interference (EMI). Direct-current biased optical orthogonal frequency division multiplexing (DCO-OFDM) is a widely used scheme in VLC due to its high spectral efficiency. However, it suffers from significant challenges related to its elevated peak-to-average power ratio (PAPR), which causes nonlinear distortion and signal clipping due to LED limitations. To address this issue, this paper proposes a combination of Vandermonde-like matrix (VLM) precoding and nonlinear companding transform (NCT) techniques (specifically A-law and μ-law) over DCO-OFDM-based VLC. The VLM precoding minimizes the autocorrelation of the OFDM signal, hence reducing its PAPR, and NCT compresses the dynamic range of the signal. Simulation results indicated that the hybrid techniques achieved a PAPR reduction of around 3.5 dB (by 76 %); this is in the best case reduction compared to the conventional DCO-OFDM while maintaining an acceptable bit error rate (BER), moderately higher signal-to-noise ratio (SNR) is required.

The energy efficiency of pulse-amplitude-modulated discrete multitone modulation (PAM-DMT) decreases as the modulation order of M-PAM modulation increases. Enhanced PAM-DMT (ePAM-DMT) was proposed as a solution to the reduced energy efficiency of PAM-DMT. This was achieved by allowing multiple streams of PAM-DMT to be superimposed and successively demodulated at the receiver side. In order to maintain a distortion-free unipolar ePAM-DMT system, the multiple time-domain PAM-DMT streams are required to be aligned. However, aligning the antisymmetry in ePAM-DMT is complex and results in efficiency losses. In this paper, a novel simplified method to apply the superposition modulation on M-PAM modulated discrete multitone (DMT) is introduced. Contrary to ePAM-DMT, the signal generation of the proposed system, termed augmented spectral efficiency discrete multitone (ASE-DMT), occurs in the frequency domain. This results in an improved spectral and energy efficiency. The analytical bit error rate (BER) performance bound of the proposed system is derived and compared with Monte-Carlo simulations. The system performance is shown to offer significant electrical and optical energy savings compared with ePAM-DMT and DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM).

Orthogonal frequency division multiplexing (OFDM) has been considered for\nvisible light communication (VLC) thanks to its ability to boost data rates as\nwell as its robustness against frequency-selective fading channels. A major\ndisadvantage of OFDM is the large dynamic range of its time-domain waveforms,\nmaking OFDM vulnerable to nonlinearity of light emitting diodes (LEDs). DC\nbiased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM\n(ACO-OFDM) are two popular OFDM techniques developed for the VLC. In this\npaper, we will analyze the performance of the DCO-OFDM and ACO-OFDM signals in\nterms of error vector magnitude (EVM), signal-to-distortion ratio (SDR), and\nachievable data rates under both average optical power and dynamic optical\npower constraints. EVM is a commonly used metric to characterize distortions.\nWe will describe an approach to numerically calculate the EVM for DCO-OFDM and\nACO-OFDM. We will derive the optimum biasing ratio in the sense of minimizing\nEVM for DCO-OFDM. Additionally, we will formulate the EVM minimization problem\nas a convex linear optimization problem and obtain an EVM lower bound against\nwhich to compare the DCO-OFDM and ACO-OFDM techniques. We will prove that the\nACO-OFDM can achieve the lower bound. Average optical power and dynamic optical\npower are two main constraints in VLC. We will derive the achievable data rates\nunder these two constraints for both additive white Gaussian noise (AWGN)\nchannel and frequency-selective channel. We will compare the performance of\nDCO-OFDM and ACO-OFDM under different power constraint scenarios.\n

The dual property of LEDs to facilitate illumination and Communication concurrently has driven Visible Light Communication (VLC) as an emerging technology and a promising alternative to Radio Frequency (RF) Communication. The primary objective of VLC is to increase the data rates and to have better performance of the networks especially for indoor applications. In this juncture, for the purpose of increasing the Information Spectral Density (ISD), Fast Optical Orthogonal Frequency Division Multiplexing (FOOFDM) can be exploited. In this work, the performance of FOOFDM is evaluated by employing various channel estimation algorithms and Probability of Error expression is derived for the developed system models. In addition, investigations on channel estimation in DC Biased Optical OFDM (DCO-OFDM) is carried out where, the result analysis emphasize that, FOOFDM is superior in terms of both power efficiency and spectral efficiency when compared to DCO-OFDM. Employing a real-valued transform and its simple one dimensional modulation, makes FOOOFM to exhibit lower computational complexity when compared with hermitian symmetry imposed optical OFDM (DCO-OFDM).

Visible light communication (VLC) has emerged as a viable complement to traditional radio frequency (RF) based systems and as an enabler for high data rate communications for beyond-5G (B5G) indoor communication systems. In particular, the emergence of new B5G-based applications with quality of service (QoS) requirements and massive connectivity has recently led to research on the required service-levels and the development of improved physical (PHY) layer methods. As part of recent VLC standards development activities, the IEEE has formed the 802.11bb “Light Communications (LC) for Wireless Local Area Networking” standardization group. This paper investigates the network requirements of 5G indoor services such as virtual reality (VR) and high-definition (HD) video for residential environments using VLC. In this paper, we consider such typical VLC scenarios with additional impairments such as light-emitting diode (LED) nonlinearity and imperfect channel feedback, and propose hyperparameter-free mitigation techniques using Reproducing Kernel Hilbert Space (RKHS) methods. In this context, we also propose using a direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM)-based adaptive VLC transmission method that uses precomputed bit error rate (BER) expressions for these RKHS-based detection methods and performs adaptive BER-based modulation-order switching. Simulations of channel impulse responses (CIRs) show that the adaptive transmission method provides significantly improved error rate performance, which makes it promising for high data rate VLC-based 5G indoor services.

In this paper, several Orthogonal frequency division multiplexing (OFDM)-based Visible light Communications (VLC) techniques designed for intensity modulation/direct detection (IM/DD) are evaluated and compared for various optical channels. Evaluations are done for line-of-sight (LOS) and no-line-of-sight (NLOS) channels. The techniques analyzed are asymmetrically clipped optical OFDM (ACO-OFDM), direct-current (DC) biased optical OFDM (DCO-OFDM), and asymmetrically clipped DC biased optical OFDM (ADO-OFDM). These schemes are evaluated and compared for different M-QAM modulations in terms of bit error rate (BER). It is shown that ACO-OFDM, in general terms, presents a better performance compared to the conventional DCO-OFDM and ADO-OFDM schemes.