Augmenting the spectral efficiency of enhanced PAM-DMT-based optical wireless communications.

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.

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.

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%.

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.

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.

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.

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

In this paper, a multivariate regression transform (MRT) is proposed for pulse-amplitude-modulated-based hybrid optical orthogonal frequency division multiplexing (PHO-OFDM) visible light communications (VLC). PHO-OFDM is a hybrid modulation scheme that combines pulse amplitude modulated discrete multitone (PAM-DMT) and DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) for simultaneous transmission. The proposed method reduces the peak-to-average power ratio of PHO-OFDM to improve its system performance. Complementary cumulative distribution function simulation results show that the proposed method can effectively mitigate the PAPR of PHO-OFDM under different constellation combinations.

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.

This paper analyzes the performance of indoor orthogonal frequency division multiplexing (OFDM) optical wireless communication systems in the presence of light emitting diode (LED) nonlinear distortions. There are several forms of optical OFDM using intensity modulation [7th Int. Symp. on Communication Systems Networks and Digital Signal Processing (CSNDSP), 2010, pp. 566-570]. In this paper, DC-biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM) are considered. ACO-OFDM produces a half-wave symmetry time signal at the output of the OFDM modulator by special assignment of subcarriers, thus allowing signal clipping at the zero level and avoiding the need for DC bias at the expense of data rate reduction. DCO-OFDM assigns data to all possible subcarriers to increase the data rate. However, half-wave symmetry signals cannot be achieved and a high DC bias is needed to convert the bipolar signal to a unipolar signal before modulating the LED intensity. This paper considers a practical LED model and studies the performance of both systems in terms of average electrical OFDM signal power versus bit error ratio in the presence of an additive white Gaussian noise (AWGN) channel. In addition, DC power consumption and the transmitted optical power for the two systems are compared. The analytical results are validated through Monte Carlo simulations and the obtained results demonstrate close match. It is shown that LED clipping has significant impact on the performance of both systems and an optimum system design should take into account the OFDM signal power, DC-bias point, and LED dynamic range.

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.

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.

In this paper, we present a new form of orthogonal frequency division multiplexing (OFDM) for intensity modulated direct detection (IM/DD) systems. This new form of OFDM uses asymmetrically clipped optical OFDM (ACO-OFDM) on the odd frequency subcarriers and DC biased optical OFDM (DCO-OFDM) on the even subcarriers. By using a form of interference cancellation both the ACO-OFDM and the DCO-OFDM signals can be recovered at the receiver. It is shown that the DCO-OFDM component causes no interference on the odd frequency subcarriers so the receiver processing for the ACO-OFDM signal is identical to a conventional ACO-OFDM system. The ACO-OFDM signal causes clipping noise which falls on the even subcarriers. However, this interference can be estimated and cancelled at the receiver. The cancellation process results in a 3 dB noise penalty in the DCO-OFDM component. Simulation results are presented for an additive white Gaussian noise channel and it is shown that the new technique can achieve better optical power performance than the conventional schemes.

In this paper, three forms of orthogonal frequency division multiplexing (OFDM) designed for intensity modulated/direct detection (IM/DD) optical systems are compared. These are asymmetrically clipped optical OFDM (ACO-OFDM), DC biased optical OFDM (DCO-OFDM) and asymmetrically clipped DC biased optical OFDM (ADO-OFDM). ADO-OFDM is a new technique that combines aspects of ACO-OFDM and DCO-OFDM by simultaneously transmitting ACO-OFDM on the odd subcarriers and DCO-OFDM on the even subcarriers. The odd subcarriers are demodulated as in a conventional ACO-OFDM receiver and the even subcarriers are demodulated using a form of interference cancellation. ADO-OFDM is shown to be more optically power efficient than conventional ACO-OFDM and DCO-OFDM, for some bit rate/normalized bandwidths. It is also shown that by varying the proportion of optical power on the ACO-OFDM component, the DC bias level of DCO-OFDM and the constellations sent on the odd and even subcarriers, the optical power efficiency of ADO-OFDM can be changed.

As a promising modulation technology for optical communication, Orthogonal Frequency Division Multiplexing (OFDM) is now increasingly used and a lot of ways of improving the optical efficiency of Intensity Modulated/ Direct Detection (IM/DD) systems have been proposed. And real and non-negative OFDM signals are required in this kind of system. Asymmetrically clipped DC biased Optical OFDM (DCO-OFDM), Asymmetrically Clipped Optical OFDM (ACO-OFDM) and Asymmetrically clipped DC biased Optical OFDM (ADO-OFDM) can satisfy this requirement. In this paper, we analysis the properties and performance orderly of the above three systems and simultaneously compare their performance. The simulation results show that DCO-OFDM is efficient in terms of bandwidth while the ACO-OFDM is efficient in terms of optical power. More importantly, ADO-OFDM is both power-efficient and bandwidth-efficient. At the same time, a developmentally proposal for the future OFDM in IM/DD systems is put forward.