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End-to-end systems have been introduced to address the issue of independent signal processing module design in traditional communication systems, which prevents achieving global system optimization. However, research on indoor end-to-end Visible Light Communication (VLC) systems remains limited, especially regarding the channel modeling of high-speed, high-capacity Direct Current-biased Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM) systems. This paper proposes three channel modeling methods for end-to-end DCO-OFDM VLC systems. The accuracy of the proposed methods is demonstrated through R-Square model fitting performance and data distribution analysis. The effectiveness of the proposed channel modeling methods is further validated by comparing the bit error rate (BER) performance of traditional receivers and existing deep learning (DL)-based receivers. The results show that the proposed methods can effectively mitigate both linear and nonlinear distortions. By employing these channel modeling methods, communication systems can reduce the spectral occupancy of pilot signals, thereby significantly lowering the complexity of traditional channel estimation methods. Thus, these methods are suitable for use in end-to-end VLC communication systems. [ABSTRACT FROM AUTHOR]

This paper presents an analytical study of signal clipping that leads to the noise/distortion in the waveform of DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM)-based visible light communication (VLC) systems. The pilot-assisted (PA) technique is used to reduce the high peak-to-average power ratio (PAPR) of the time-domain waveform of the DCO-OFDM system. The bit error rate (BER) performance of the PA DCO-OFDM system is investigated analytically at three different clipping levels as well as without any clipping. The analytical BER performance is verified through simulation and then compared to that of the conventional DCO-OFDM without PAPR reduction at the selected clipping levels. The PA DCO-OFDM system shows improved BER performance at all three clipping levels. [ABSTRACT FROM AUTHOR]

In recent years, the lifting wavelet transform (LWT) has been used in image and signal processing applications to overcome many of the shortcomings of the classical discrete wavelet transform (DWT). The LWT is a simplified and most efficient version of the classical discrete wavelet transform. To address the issue of energy inefficiency in the direct current-biased optical OFDM (DCO-OFDM) waveform, the literature proposes the use of the non-direct current-biased optical OFDM (NDC-O-OFDM) waveform, which does not require the addition of DC bias. Firstly, this paper proposes improving the performance of the NDC-O-OFDM waveform by combining it with the LWT using the lazy wavelet (LLWT). However, the literature proposes the use of the generalized LED index modulation OFDM (GLIM-OFDM) waveform as a solution to the spectrum inefficiency problem of the NDC-O-OFDM waveform. As a second contribution, this paper presents the combination of the GLIM-OFDM waveform with the LLWT transform to enhance the performance of the GLIM-OFDM waveform. Numerical simulations were performed to evaluate the performance of the proposed LLWT-NDC-O-OFDM and LLWT-GLIM-OFDM waveforms in terms of bit error rate (BER) in 2 × 2 and 4 × 4 optical MIMO channel environments. The results show that the proposed waveforms provide a gain of approximately 6.5 dB over both the NDC-O-OFDM and GLIM-OFDM waveforms, and a gain of 13.5 dB over the DCO-OFDM waveform, while maintaining the same BER requirement of 1E-4 and modulation order of BPSK in both 2 × 2 and 4 × 4 optical MIMO channel scenarios. Thus, despite some increase in computational complexity due to LWT implementation, a spectrum and energy efficient high performance optical OFDM waveform is achieved with the proposed LLWT transform in this work. [ABSTRACT FROM AUTHOR]

End-to-end systems have been introduced to address the issue of independent signal processing module design in traditional communication systems, which prevents achieving global system optimization. However, research on indoor end-to-end Visible Light Communication (VLC) systems remains limited, especially regarding the channel modeling of high-speed, high-capacity Direct Current-biased Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM) systems. This paper proposes three channel modeling methods for end-to-end DCO-OFDM VLC systems. The accuracy of the proposed methods is demonstrated through R-Square model fitting performance and data distribution analysis. The effectiveness of the proposed channel modeling methods is further validated by comparing the bit error rate (BER) performance of traditional receivers and existing deep learning (DL)-based receivers. The results show that the proposed methods can effectively mitigate both linear and nonlinear distortions. By employing these channel modeling methods, communication systems can reduce the spectral occupancy of pilot signals, thereby significantly lowering the complexity of traditional channel estimation methods. Thus, these methods are suitable for use in end-to-end VLC communication systems.

This paper presents an analytical study of signal clipping that leads to the noise/distortion in the waveform of DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM)-based visible light communication (VLC) systems. The pilot-assisted (PA) technique is used to reduce the high peak-to-average power ratio (PAPR) of the time-domain waveform of the DCO-OFDM system. The bit error rate (BER) performance of the PA DCO-OFDM system is investigated analytically at three different clipping levels as well as without any clipping. The analytical BER performance is verified through simulation and then compared to that of the conventional DCO-OFDM without PAPR reduction at the selected clipping levels. The PA DCO-OFDM system shows improved BER performance at all three clipping levels.

Visible light communication (VLC), which is seen as an alternative and complementary technology to radio frequency (RF) communication systems, has emerged as a promising new generation system. Because of its potential to combat inter-symbol interference (ISI), orthogonal frequency division multiplexing (OFDM) has been validated as the best candidate for high-speed optical wireless communications (OWC). Asymmetrically clipped optical OFDM (ACO-OFDM), DC-biased optical OFDM (DCO-OFDM), and Flip-OFDM, which are made suitable for intensity modulation/direct detection (IM/DD) systems, are unipolar OFDM schemes widely accepted for OWC in the literature. In this study, it is proposed to combine the lifting wavelet transform (LLWT), which uses the lazy wavelet, with these three optical OFDM waveforms to increase the performance of the ACO-OFDM, DCO-OFDM, and Flip-OFDM systems proposed for OWC systems. In the environment where M-level color shift keying (M-CSK) and M-level quadrature amplitude modulation (M-QAM) modulations are used, proposed waveforms are tested with computer simulations for bit error rate (BER) and peak average power ratio (PAPR) performance measures. From the obtained simulation results, it is observed that the proposed transform technique performs an approximately 6 dB SNR improvement on the waveforms investigated in this study in both modulation methods for the 1E-4 BER value. [ABSTRACT FROM AUTHOR]

The paper investigates the impact of Rayleigh-distributed statistical behavior of peak-to-average power ratio (PAPR) associated with a pre-clipped signal on the performance metrics of a direct current-biased optical orthogonal frequency division multiplexing (DCO-OFDM) system. The analytical model for the system takes into consideration a pre-clipped and dc-shifted baseband OFDM signal, driving an optical source over its linear operating range. The model employs a bias-scaling factor, which is heuristically varied over the entire range (0 to 1) to examine improvement in overall power efficiency. Further, it utilizes the cumulative distribution function (CDF) of the pre-clipped signal to get a weighted estimate of the available signal power within the clipped PAPR. The model also takes into consideration the clipping noise effects due to limited linearity of the optical source during electrical-to-optical conversion of baseband OFDM signal. Using this model, the paper aims to arrive at a realistic estimate of the system behavior in terms of bit error rate, electrical power-efficiency and spectral efficiency. Using theoretical simulation results, for a given set of operating parameters viz., signal power, PAPR, bias-scaling factor, modulation order and sub-carrier count, the paper examines the trade-offs involved in optimizing the performance metrics over appropriate dynamic range of the DCO-OFDM transmitter. [ABSTRACT FROM AUTHOR]

Nonlinear devices and channel interference can significantly impact the received signal in visible light communication (VLC). While recent research has explored receiver recovery using deep learning, existing approaches often involve replacing traditional channel estimation and equalization modules with neural network models. However, these models introduce additional data processing steps after fast Fourier transform (FFT), leading to increased complexity. To address these challenges, this study introduces a novel direct time-domain waveform equalization approach using a bidirectional gated recurrent unit (BiGRU) neural network for indoor VLC employing direct current (DC)-biased orthogonal frequency division multiplexing (DCO-OFDM). Unlike previous methods, our proposed scheme utilizes time-domain waveform data from photodiode outputs for direct balancing, harnessing the potent nonlinear processing capabilities of the BiGRU model. We first analyze the nonlinear processing capacity of the BiGRU model and subsequently compare the performance of different receiving methods on a constructed indoor visible-light communication platform. Experimental results demonstrate that the BiGRU-based approach exhibits low complexity and exceptional nonlinear channel learning capabilities. Notably, the proposed method outperforms other strategies in terms of bit error rate without the need for pilot signals. These findings validate the potential of the BiGRU-based DCO-OFDM receiving scheme as a promising solution for future VLC systems. [ABSTRACT FROM AUTHOR]

Optical-orthogonal frequency division multiplexing (O-OFDM) systems, including the direct current biased optical OFDM (DCO-OFDM) and the asymmetrically clipped optical OFDM (ACO-OFDM), are very important to achieve high data rate transmission in visible light communication (VLC). However, the DCO-OFDM and ACO-OFDM systems suffer from adjacent channel interference (ACI) caused by out of band (OOB) and a high peak to average power ratio (PAPR). In this work, a pulse-shaping signal based on a symbol-time compression (STC) scheme is presented. The proposed shaped STC-DCOOFDM and shaped STC-ACOOFDM systems are capable of reducing both the ACI and the PAPR as well. Further, the OOB, PAPR, and computing complexity are analytically investigated. The simulation results verify the analytical study of the proposed methods for OOB reduction and power savings resulting in PAPR reduction. The proposed systems are evaluated for different modulation schemes, including BPSK, QPSK and 8QAM. [ABSTRACT FROM AUTHOR]

In direct current optical orthogonal frequency division multiplexing (DCO-OFDM) systems, the high peak-to-average power ratio (PAPR) has been a significant challenge. Recently, lexicographical symbol position permutation (LSPP) using random permutations has been introduced as an efficient solution to reduce high PAPR. In this paper, we aim to evaluate the effectiveness of LSPP by comparing both adjacent and interleaved lexicographical permutation sequences with random lexicographical permutation sequences. Our findings demonstrate that random permutation yields superior PAPR reduction performance results when compared to adjacent and interleaved permutation. However, in scenarios with a limited number of sub-blocks, the use of adjacent and interleaved permutation becomes more favorable, as they can eliminate the possibility of generating identical permutation sequences, a drawback of random permutation. Additionally, we propose a novel algorithm to determine the optimal number of candidate permutation sequences that can achieve acceptable PAPR reduction performance while adhering to computational complexity constraints defined by the system requirements. [ABSTRACT FROM AUTHOR]

This paper proposes an energy-efficient asymmetrically clipped direct-current (DC) biased optical orthogonal frequency division multiplexing (EEADO-OFDM) scheme, which replaces the fixed DC bias of DC biased optical OFDM (DCO-OFDM) branch with a adaptive DC bias for optical wireless communications. In the EEADO-OFDM, an adaptive DC bias is dynamically designed according to amplitude of DCO-OFDM signal samples to enhance the power efficiency. After superimposing the obtained DC bias on the original DCO-OFDM signal, the resultant DCO-OFDM signal is non-negative and further ensure that a non-negative EEADO-OFDM signal is produced by directly combining DCO-OFDM signal with the clipped asymmetrically clipped optical OFDM (ACO-OFDM) signal. In particular, the interference caused by the obtained DC bias is proved to fall on even subcarriers only, which implies that the interference can be modulated on the partial odd subcarriers of ACO-OFDM branch. Afterward, we could use conventional ACO-OFDM receiver to demodulate the adopted odd subcarriers for the detection of interference. Simulation results show that, under the equal optical power constraints, the proposed EEADO-OFDM achieves no error floors of bit error rate compared with the ADO-OFDM, thus indicating that the clipping noise is always negligible in EEADO-OFDM.

Compared to the direct current (DC) biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) schemes, the asymmetrically clipped DC biased optical OFDM (ADO-OFDM) has demonstrated a better balance between optical power efficiency and spectrum efficiency. However, both the traditional ADO-OFDM and ADO-OFDM with iteration receiver suffer from an error floor issue when a lower DC bias is allocated to the DCO-OFDM branch. In this paper, we propose an enhanced receiving technique for ADO-OFDM that introduces an adjacent symbol detection (ASD) scheme in the DCO-OFDM branch of ADO-OFDM, named as ASD-based ADO-OFDM (ASD-ADO-OFDM). Simulation results indicate that the proposed ASD-ADO-OFDM provides a lower bit error rate performance at lower DC bias and fewer computational complexity compared with traditional ADO-OFDM with iteration receiver.

Underwater optical wireless communication (UOWC) has been a promising high data rate communication system. The performance of UOWC has numerous constraints in terms of maximum attainable distance, absorption, and scattering of the intensity, which vary depending on the quality of the water environment. This paper proposes a UOWC system based on filtered orthogonal frequency division multiplexing (F-OFDM). It has achieved large bandwidth while also addressing the issue of intersymbol interference (ISI). F-OFDM has attained lower out-of-band emission (OOB) while preserving bit error rate (BER) performance. Furthermore, to permit many users to share the entire UOWC system, non-orthogonal multiple access (NOMA) is adopted with F-OFDM, taking advantage of filtering subcarriers that reduce user interference. NOMA F-OFDM has been examined across harbor water channels under different power coefficients and modulation. Simulations have shown that F-OFDM is better than DC-biased optical OFDM (DCO-OFDM), with an OOB improvement of about 120 dB and a signal-to-noise ratio (SNR) gain of about 13 dB for BER = 10–3 using 4-QAM modulation. As well, NOMA F-OFDM has better BER performance than NOMA DCO-OFDM. [ABSTRACT FROM AUTHOR]

In recent times, there has been a surge in interest surrounding visible light communication (VLC) as a compelling adjunct to radio frequency (RF) technology. This enthusiasm stems from deployments showcasing its efficacy in high-speed data transmission, characterized by low complexity, robust security, and high reliability. Among the variants of orthogonal frequency division multiplexing (OFDM), direct current-biased optical OFDM (DCO-OFDM) stands out in the realm of VLC systems, owing to its ability to furnish rapid data provisioning. However, a significant drawback of selective mapping (SLM) based DCO-OFDM transceivers lies in its computational complexity and practical implementation challenges. To address this limitation, this paper presents a modified SLM technique termed selective phase modulation (SPM) aimed at mitigating the peak-to-average power ratio (PAPR) in the transmitter section while simplifying the data decoding process at the receiver end. The SPM method utilizes a phase modulation and demodulation process based on clusters and assisted by pilot signals, streamlining the transceiver architecture. The SPM methodology offers reduced computational complexity at the transmitter by selectively applying phase sequences to data, circumventing the necessity for side information (SI) transmission and SI estimation at the receiver with a known pilot symbol inserted in the data. This dual-functionality mechanism not only achieves complexity reduction but also facilitates channel estimation at the receiver for proper data recovery. Based on complexity analysis and simulation outcomes, the computational complexity reduction strategy outperforms the conventional selective mapping (CSLM) technique, VLC-based pilot-assisted PAPR reduction systems, and embedded coded modulation (ECM). This allows for smooth incorporation of this technology into future VLC systems. [Display omitted] • Innovative SLM Scheme : The research introduces a novel selected mapping (SLM) scheme to simplify visible light communication (VLC) systems, enhancing overall efficiency by reducing system complexity. • Clustered Symbol Approach : The method segments DCO-OFDM symbols, separates even and odd ones, and inserts pilot symbols to enhance adaptability. • Selective phase modulation (SPM) : Unlike traditional SLM, odd subcarriers are selectively multiplied, optimizing computational resources. • Dual-Purpose Technique : Inserts pilot symbols, reducing complexity and ensuring accurate channel estimation, eliminating SI transmission. [ABSTRACT FROM AUTHOR]

Visible Light Communications (VLC) are receiving increased attention in the wireless communications research community. VLC is secured, power efficient, and operates in the visible light range, thus RF communication bandwidth limitation is overcome. In this article, the authors enhance the data rate, system complexity, power efficiency, and spectrum efficiency in VLC systems. An innovative unipolar transceiver system is proposed, mathematically analyzed, and compared with other existing techniques and it demonstrates to have a very high data rate ratio (43.75%) with a good system bit energy to noise ratio ${(E}_{b}/N_{O})$ compared to other existing techniques. Development for the traditional asymmetrically and symmetrically clipping optical (ASCO-OFDM) system is also proposed, which involves combining a modified receiver with the ASCO-OFDM system traditional transmitter. The proposed receiver reduces the system complexity by O ( $N\log _{2}N$ ) with better $E_{b}/N_{O}$ than the conventional ASCO-OFDM. Detailed analysis, simulation results, and comparison of the proposed systems with the existing systems are presented beside a brief assessment of existing techniques.

Efficient utilization of the spectrum, increased resilience to inter-symbol interference (ISI) and simpler channel equalization are becoming important considerations in the design of wireless communication systems including visible light communication (VLC) systems. In this regard, orthogonal frequency division multiplexing (OFDM) has become a preferred modulation technique in wireless communication systems design. However, one of its major challenges is the high peak-to-average power ratio that leads to major inefficiencies. Symbol position permutation (SPP) is a distortion-less technique that achieves substantial PAPR reduction without BER degradation. However, the existing works focus on PAPR reduction using SPP for the radio frequency (RF) OFDM and the use of this technique in optical OFDM is not properly investigated. Therefore, in this paper, we present a new PAPR reduction technique based on lexicographical permutations called lexicographical symbol position permutation (LSPP) for PAPR reduction in direct current optical OFDM (DCO-OFDM). The proposed scheme is less complex than the conventional selective mapping (CSLM) scheme since there is no multiplication of the phase sequences with the DCO OFDM symbol to generate the candidate signals. We further introduce a new way of reducing the complexity by introducing a threshold PAPR and demonstrate that the complexity in terms of inverse fast Fourier transform (IFFT) operations can be reduced substantially depending on the selected threshold and the number of candidate signals.

Seamless light fidelity (LiFi) communications in a realistic indoor environment faces a number of important challenges including the loss of line-of-sight (LOS) link due to the random orientation of mobile devices or blockage by users or furniture in the room. These effects make the frequency response of the LiFi channel highly environment-dependent. The dynamic nature of the channel is not only dependent on the geometrical configuration of the room but also on the user behavior. In this paper, we investigate the hypothesis that, deep learning (DL) based LiFi communication techniques can effectively learn the distinct features of the indoor environment and user behavior to provide superior performance compared to conventional channel estimation techniques particularly when access to real-time channel state information (CSI) is restricted. In particular, we implement DL methods in two different problems, namely, signal detection and resource allocation for orthogonal frequency division multiplexing (OFDM) LiFi systems. To effectively test this hypothesis, a realistic LiFi channel impaired by random device orientation and blockage is simulated considering different geometrical configurations and user behavior in an indoor environment, e.g. presence or lack of furniture and varying user distribution defined by a conditional hotspot model. The simulation results confirm that DL-based LiFi systems with partial CSI are able to offer close performance to the optimal signal detection and resource allocation with perfect CSI. Moreover, the DL-based techniques demonstrate substantial gain against conventional benchmark schemes that employ channel estimation algorithms such as least squares (LS) or minimum mean square error (MMSE) and this gain increases in a more realistic or complex indoor environment, e.g. room with furniture or a hotspot scenario.

The trend of high data rates in indoor environments has increased the demand for visible light communication (VLC). This paper investigates the performance of PAPR with dynamic range (DR) of LED in the existing system models of optical OFDM for indoor communication environments based on complimentary cumulative distribution function (CCDF). Four commonly used existing unipolar OFDM schemes which are DC-biased optical OFDM (DCO-OFDM), asymmetrically clipped optical OFDM (ACO-OFDM), Flip-OFDM and asymmetrically clipped DC-Biased (ADO-OFDM) Optical OFDM. These schemes have been investigated and compared with the dynamic range of LED in terms of the illumination level requirements in indoor environments for PAPR with and without dynamic range. The PAPR for different system models in optical domain has never been analyzed and compared earlier with the dynamic range of LED. The simulation and analytical results of BER are also compared to show the superiority of ADO and DCO over ACO and FLip optical OFDM schemes. The results indicate the superiority of DCO-OFDM and ADO-OFDM as the best ones among the four schemes in terms of LED's DR at different indoor illumination levels. [ABSTRACT FROM AUTHOR]

The power domain non-orthogonal multiple access (NOMA) has been considered as an outstanding candidate for 5G visible light communication networks. Due to the various salient features like robustness to multi-path fading, low latency, high compatibility with multiple input multiple output systems, etc. orthogonal frequency division multiplexing access (OFDM) is foreseen to exist in 5G systems. NOMA-based optical OFDM (O-OFDM) is expected to be a promising candidate for supporting high rate internet connectivity, especially for green indoor mobile networks. The OFDM waveform suffers from high PAPR issue. The high PAPR degrades the BER and also enhances the non-linearity in an O-OFDM system. In this paper, a hybrid technique using a blend of precoder with compander is implemented to reduce the PAPR of NOMA based DC-biased O-OFDM (DCO-OFDM) system. The proposed NOMA DCO-OFDM system exhibits a low PAPR of only 4.3 dB. Also, for a reference bit error rate of 10 - 4 , the proposed technique displays an error vector magnitude of 13.2 dB. [ABSTRACT FROM AUTHOR]

Optical wireless communications (OWC) are among the best alternative techniques for transmitting information-laden optical radiation across a free-space channel from one place to another. DC-biased optical OFDM (DCO-OFDM) is a technique that sacrifices the power efficiency to transmit unipolar OFDM signals. The primary drawback with DCO-OFDM is its clipping noise, which causes distortion and lowers the bit error rate (BER). Thus, in this paper, we show the performance of DCO with different coded techniques to improve the BER in additive white Gaussian noise (AWGN) for IM/DD systems. The experimental results show that the coded DCO-OFDM has the best performance. Furthermore, turbo coding has the best coding technique added to the DCO-OFDM system. [ABSTRACT FROM AUTHOR]

Nonlinear devices and channel interference can significantly impact the received signal in visible light communication (VLC). While recent research has explored receiver recovery using deep learning, existing approaches often involve replacing traditional channel estimation and equalization modules with neural network models. However, these models introduce additional data processing steps after fast Fourier transform (FFT), leading to increased complexity. To address these challenges, this study introduces a novel direct time-domain waveform equalization approach using a bidirectional gated recurrent unit (BiGRU) neural network for indoor VLC employing direct current (DC)-biased orthogonal frequency division multiplexing (DCO-OFDM). Unlike previous methods, our proposed scheme utilizes time-domain waveform data from photodiode outputs for direct balancing, harnessing the potent nonlinear processing capabilities of the BiGRU model. We first analyze the nonlinear processing capacity of the BiGRU model and subsequently compare the performance of different receiving methods on a constructed indoor visible-light communication platform. Experimental results demonstrate that the BiGRU-based approach exhibits low complexity and exceptional nonlinear channel learning capabilities. Notably, the proposed method outperforms other strategies in terms of bit error rate without the need for pilot signals. These findings validate the potential of the BiGRU-based DCO-OFDM receiving scheme as a promising solution for future VLC systems.

Vehicular visible light communication (V-VLC) aims to provide secure complementary vehicle-to-everything-communications (V2X) to increase road safety and traffic efficiency. V-VLC provides directional transmissions, mainly enabling line-of-sight (LoS) communications. However, reflections due to nearby objects enable non-line-of-sight (NLoS) transmissions, extending the usage scenarios beyond LoS. In this paper, we propose wide-band measurement based NLoS channel characterization, and evaluate the performance of direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) V-VLC scheme for NLoS channel. We propose a distance based NLoS V-VLC channel path loss model considering reflection surface characteristics and NLoS V-VLC channel impulse response (CIR) incorporating the temporal broadening effect due to vehicle reflections through weighted double gamma function. The proposed path loss model yields higher accuracy up to 14 dB when compared to single order reflection model whereas CIR model estimates the full width at half maximum up to 2 ns accuracy. We further demonstrate that the target bit-error-rate of $10^{-3}$ can be achieved up to 7.86 m, 9.79 m, and 17.62 m distances for black, orange and white vehicle reflection induced measured NLoS V-VLC channels for DCO-OFDM transmissions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Optical wireless communication (OWC) systems suffer inter-symbol interference (ISI) in time domain due to light emitting diode (LED)’s limited bandwidth at transmitter. As intensity modulation and direct detection (IM/DD) is used, the transmitted signals are unipolar and real-valued rather than bipolar and complex-valued as in radio frequency (RF). Thus, it is challenging to employ the RF-based timing offset estimation for OWC. Also, there is no carrier frequency offset (CFO) like in RF. It is possible to estimate timing offset in frequency domain for two reasons. First, the effect of ISI in time domain is mitigated in frequency domain with correct timing offset estimation, robust against LED’s limited bandwidth. Second, the correlation between frequency-domain signals are high, as signals are bipolar and complex-valued in frequency domain rather than unipolar and real-valued in time domain. Therefore, we propose an effective frequency-domain timing offset estimation approach for direct current biased optical-orthogonal frequency division multiplexing (DCO-OFDM) OWC systems, where a small number of pilots are applied on few DCO-OFDM subcarriers, leading to very low spectral overhead. The property of no CFO is utilized to formulate an error function, by exploring the correlation between received signals and pilots. We propose a squared error function (SEF)-based timing offset estimation approach, conducted by maximizing the ratio of two successive variances of squared errors. Simulation results show that, with a very low training overhead of 0.48%, the proposed SEF-based approach provides a bit error ratio (BER) performance close to ideal case with perfect timing offset estimation, perfect channel state information and zero-forcing (ZF) equalization. [ABSTRACT FROM AUTHOR]

Optical Wireless Communication (OWC) is a new trend in communication systems to achieve large bandwidth, high bit rate, high security, fast deployment, and low cost. The basic idea of the OWC is to transmit data on unguided media with light. This system requires multi-carrier modulation such as Orthogonal Frequency Division Multiplexing (OFDM). This paper studies optical OFDM performance based on Intensity Modulation with Direct Detection (IM/DD) system. This system requires a non-negativity constraint. The paper presents a framework for wireless optical OFDM system that comprises (IM/DD) with different forms, Direct Current biased Optical OFDM (DCO-OFDM), Asymmetrically Clipped Optical OFDM (ACO-OFDM), Asymmetrically DC-biased Optical OFDM (ADO-OFDM), and Flip-OFDM. It also considers channel coding as a tool for error control, channel equalization for reducing deterioration due to channel effects, and investigation of the turbulence effects. The evaluation results of the proposed framework reveal enhancement of performance. The performance of the IM/DD-OFDM system is investigated over different channel models such as AWGN, log-normal turbulence channel model, and ceiling bounce channel model. The simulation results show that the BER performance of the IM/DD-OFDM communication system is enhanced while the fading strength is decreased. The results reveal also that Hamming codes, BCH codes, and convolutional codes achieve better BER performance. Also, two algorithms of channel estimation and equalization are considered and compared. These algorithms include the Least Squares (LS) and the Minimum Mean Square Error (MMSE). The simulation results show that the MMSE algorithm gives better BER performance than the LS algorithm. [ABSTRACT FROM AUTHOR]

We provide an interpretation for a recently proposed pre-distorted asymmetrically clipped DC-biased optical orthogonal frequency division multiplexing (ADO-OFDM) system. Through our derivation, we find that the predistorted ADO-OFDM system can be regarded as a direct current biased optical OFDM (DCO-OFDM) system in which the odd-index and the even-index subcarriers are based on different power assignments. For multipath channels that have no channel side information (CSI), the mechanism for power allocation and adaptive modulation is impractical. Using state-of-the-art polar coding that includes an interleaver is appropriate to combat such a situation. The performance comparison between an interleaved polar coded ADO-OFDM system both with and without pre-distortion are simulated and concluded.

In direct-detection OFDM systems, the nonlinear effects caused by optical modulation and fiber transmission can degrade the system performance severely. In this study, we propose a new nonlinear companding transform to improve the performance of direct detection optical OFDM transmission systems. The demonstration is realized by Monte-Carlo simulation of the intensity modulation and direct detection DCOOFDM optical transmission system at 40 Gbps over a 80 km of standard single mode fiber link. The influence of the companding parameters on the performance of system in different nonlinear transmission conditions has been investigated via simulation.

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. [ABSTRACT FROM AUTHOR]

In recent years, relaying technique and non-orthogonal multiple access (NOMA) are introduced to visible light communication (VLC) system to improve the system performance. Relay-assisted VLC system can boost link reliability and extend effective communication coverage compared with the traditional VLC system. Besides, the adoption of NOMA can significantly improve the system throughput, enhance the spectral efficiency and support massive connectivity. A novel relay-assisted VLC system based on NOMA is proposed in this paper. All the source-to-relay and relay-to-destination links in the proposed system use DCO-OFDM scheme to transmit information. The relay terminal serves two destination terminals at the same time. The signals in these two relay-to-destination links are superposed in the power domain based on NOMA. The total throughput with different parameters such as power allocation factors and DC bias is modeled and deduced in this paper. By simulations, the optimal values of DC bias and power allocation factors that maximize the system throughput are obtained. Moreover, the simulation results also show superiority of the NOMA-based VLC system over the OMA-based VLC system. [ABSTRACT FROM AUTHOR]

In this article, different forms of optical orthogonal frequency division multiplexing (OFDM) were observed which were suitable for optical camera communication (OCC) systems. This research aims to establish the bit error rate (BER) versus signal-to-noise ratio (SNR) of the OCC system. This research will focus on OCC systems and the design that produces the noise of the clipping but will gain SNR as a whole if an optimum clipping factor is chosen. The BER versus SNR analysis was investigated for the different clipping factors 0.7, 1.4, and 2.6. The BER performance of the asymmetrically clipped optical OFDM (ACO-OFDM) was also compared with the direct current optical OFDM (DCO-OFDM) to show the suitable effectiveness of the proposed approach. ACO-OFDM was considered to be better due to lower bit loading, but DCO-OFDM was efficient for higher SNR values. This was because the DC bias used was inefficient in terms of optical capacity, while ACO-OFDM used only half of the subcarriers to transmit the information. Moreover, ACO-OFDM two-dimensional half-subcarriers of mapping rule would introduce the clipping noise to its unused 2D subcarriers, although further data can be provided by the 2D DCO-OFDM mapping rule. [ABSTRACT FROM AUTHOR]

Visible Light Communication (VLC) becomes a promising technology in the Optical Wireless Communication OWC field. VLC has been developed based on Optical Orthogonal Frequency Division Multiplexing (O-OFDM) in recent years. The Bit Error Rate (BER) performance for various O-OFDM is studied in this paper. The non-linearity characteristic of Light-Emitting Diode (LED), modulation constellation size, O-OFDM scheme, and channel types impacts the BER performance of VLC systems significantly. The proposed model utilizes the conVolutional encoder of 2/3 rate and seven memory cells in the transmitter with O-OFDM for BER enhancement. At the receiver, the Viterbi algorithm system is employed with hard or soft-decision decoding. The proposed system is evaluated in Line-of-Sight and Non-Line-of-Sight (NLOS) channels. In the NLOS model, Zero Forcing Equalizer (ZFE) with channel estimation is added at the receiver path for equalizing the multipath diffused components. This study proves the capability of the proposed system in mitigating the LED non-linearity and reducing BER. The simulation results show the Signal-to-Noise Ratio (SNR) enhancement of the exanimated O-OFDM-based softdecision between 7 to 10 dB for the BER 10-5 in the LOS model, and for the NLOS model, the BER can be improved from 0.02 up to 10-5 at SNR 45dB. [ABSTRACT FROM AUTHOR]

Penggunaan sistem komunikasi cahaya tampak sebagai metode komunikasi alternatif selain RF didorong oleh beberapa faktor, salah satunya adalah penghematan energi karena sistem ini menggunakan satu devais yang sama untuk pencahayaan ruangan sekaligus komunikasi data yaitu LED. Penelitian ini bertujuan untuk memodelkan sistem komunikasi cahaya tampak berbasis multi-carrier modulation (yakni DCO-OFDM) dengan bantuan perangkat lunak MATLAB. Pada penelitian ini, sistem dimodelkan atas dua bagian yakni blok Digital Signal Processing (DSP) dan kanal optik. Blok DSP terdiri atas FFT, convolutional encoder, modulator-demodulator, interleaver-deinterleaver, dan synchronizer. Untuk error control coding (ECC) digunakan algoritma Viterbi decoder. Hasil penelitian menunjukkan bahwa frame DCO-OFDM telah berhasil didesain dan berdasarkan perhitungan matematis melalui pendekatan MATLAB, sistem VLC dapat bekerja dengan baik pada ruangan 5m x 5m x 3m, yakni BER = 0 untuk nilai minimum SNR = 10 dB, sementara nilai SNR tertinggi yang dapat dicapai adalah ~30 dB.

Modern handheld mobile devices are equipped with a complementary metal-oxide-semiconductor (CMOS) image sensor. These image sensors are capable of perceiving intensity changes in optically modulated signals, driving the rapid development of optical camera communication (OCC). In this study, direct-current offset orthogonal frequency division multiplexing (DCO-OFDM) was developed for a rolling shutter OCC system in order to improve the data transmission rates of OCC systems. Two DCO-OFDM OCC signal detection algorithms, namely the parabolic column vector selection (P-CVS) algorithm and the decomposition of extremely low frequency component (D-ELFC) algorithm, were proposed, along with a frequency-domain downsampling scheme, to eliminate the influence from both the blooming effect and shot noise. The experimental results, obtained after using a self-built DCO-OFDM OCC testbed with a general commercial smartphone camera, demonstrated that the D-ELFC algorithm outperformed in the bit error rate (BER) in comparison to the P-CVS algorithm and achieved a data transmission rate >22 kbps under the conditions of the BER being below the 20% forward error correction (FEC) limit and the illuminance being higher than 510 lux.

This paper proposes a novel peak-to-average power ratio (PAPR) reduction scheme, which has several advantages, such as low-complexity candidate construction and no side information (SI) transmission in direct current-biased optical orthogonal frequency division multiplexing (DCO-OFDM) visible light communication (VLC) systems. In the proposed scheme, the candidates can be generated by linearly combining time-domain even and odd sequences with cyclic shift and phase rotation. Even and odd sequences are obtained by adopting an inverse fast Fourier transform on the even and odd parts of a Hermitian symmetry encoded signal, respectively. Furthermore, a deterministic and efficient selection method of cyclic shift and phase rotation is derived to improve PAPR reduction performance and ensure that each candidate is unique, so that the receiver can recover the transmitted candidate signal without the use of SI. Simulation results show that the proposed scheme can achieve comparable PAPR reduction performance with the conventional selected mapping (CSLM) scheme with random and chaotic sequences, but at a considerably low computational complexity. Moreover, the proposed blind SI detection scheme provides almost the same bit error rate (BER) performance as that of the CSLM scheme with a perfect SI detection and symmetric selected mapping for a DCO-OFDM VLC system with 1024 subcarriers and 16-QAM. In contrast to other related time-domain sequences that combine methods used for DCO-OFDM systems, the proposed scheme improves BER performance with a slight increase in computational complexity and PAPR.

Most studies on optical wireless communications (OWC) have neglected the effect of random orientation in their performance analysis due to the lack of a proper model for the random orientation. Our recent empirical-based research illustrates that the random orientation follows a Laplace distribution for the static user equipment (UE). In this paper, we analyze the device orientation and assess its importance on system performance. The reliability of the OWC channel highly depends on the availability and alignment of line-of-sight (LOS) links. In this paper, the effect of receiver orientation, including both polar and azimuth angles on the LOS channel gain are analyzed. The probability of establishing the LOS link is investigated and the probability density function (PDF) of signal-to-noise ratio (SNR) for a randomly oriented device is derived. By means of the PDF of SNR, the bit-error ratio (BER) of DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) in additive white Gaussian noise (AWGN) channels is evaluated. A closed-form approximation for the BER of UE with random orientation is presented which shows a good match with Monte-Carlo simulation results. Furthermore, the impact of UE's random motion on the BER performance has been assessed. Finally, the effect of random orientation on the average signal-to-interference-plus-noise ratio (SINR) in a multiple access points (AP) scenario is investigated.

Due to seawater scattering characteristics, Inter-Symbol Interference (ISI) will occur in underwater transmission, especially in poor transmission environment. In order to solve this problem, pre-equalization technology is applied to underwater optical communication system based on Direct Current Offset Orthogonal Frequency Division Multiplexing (DCO-OFDM) modulation, and a pre-equalization method based on prior information is proposed. The performance of Turbid Harbor seawater channel is compared with that of traditional equalization algorithm. The simulation results show that the ISI in underwater optical communication system can be effectively reduced, as well as the Bit Error Rate (BER). The reliability of channel estimation can be improved under the condition of poor Signal Noise Ratio (SNR). Compared with the Least Squares (LS) post-equalization, LS pre-equalization and Mini-mental State Examination (MMSE) algorithms, the BER can be reduced by~3.56, ~3.00 and~1.85 dB, respectively.

The increasing demand for bandwidth through modern applications and multimedia services has led to high-speed wireless communications. Optical wireless communications (OWC) encourages solutions that provide a higher data rate due to the large bandwidth available. In this paper, performance enhancement approaches are studied and simulated for visible light communication (VLC) as a case study. The orthogonal frequency division multiplexing (OFDM) systems are used to investigate Intensity modulation/direct detection (IM/DD) to improve the performance of VLC. The IM/DD in OWC requires positive real OFDM symbols, so there are many approaches to satisfy this requirement. This paper proposes a model of Asymmetrically Clipped DC-Biased Optical (ADO-OFDM) to use in OWC/VLC environment. The proposed system has avoided the use of the noise cancellation technology that is used in traditional ADO-OFDM. The results show that the ADO-OFDM has the best spectral efficiency than DC-biased optical (DCO-OFDM) and Asymmetrically clipped optical (ACO-OFDM). Also, it has better optical efficiency than DCO-OFDM with the equally overall bit error rate (BER) at the same signal-to-noise ratio. Hamming channel coding/decoding with different code lengths is applied in various optical OFDM schemes for BER improvements. Furthermore, we simulate and analyze these optical OFDM systems with many modulation orders. [ABSTRACT FROM AUTHOR]

The research in underwater communication system is focused towards wireless networks in view of recent developments in wireless technology and increasing need for deep sea data mining. The underwater wireless optical communication (UWOC) have more advantages for short-range wireless links due to very high bandwidth and data rate than acoustic communication. Although UWOC offers high capacity links at low latencies, they suffer from limited communication range due to various distinctive characteristics of underwater channel. A major degrading effect associated with the underwater channel is turbulence-induced fading. To mitigate this fading and to extend the viable communication range, spatial multipath diversity techniques can be adopted over UWOC links. The proposed work evaluates the performance of a multicarrier modulation scheme, i.e., MIMO based DC biased Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM) for underwater wireless optical channel. The simulation of MIMO-DC biased optical OFDM technique employing spatial diversity in underwater optical communication is a novel approach towards the determination of an optimal modulation technique for underwater optical wireless channels. The performance is evaluated based on the various parameters such as Signal to Interference plus Noise Ratio (SINR), Throughput and Bit Error Rate (BER). A simple experiment is conducted to demonstrate the BER performance assessment for different link distance of 0.45 m and 1.5 m with different transmitter and receiver configurations (SIMO and MIMO) for both clear water and coastal water using MIMO DCO OFDM technique. [ABSTRACT FROM AUTHOR]

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In this paper, we present a unified resource allocation and mobility management algorithm based on particle swarm optimization (PSO) for indoor visible light communication (VLC) networks. We consider a VLC network where multiple LEDs serve as access points (APs). A centralized controller collects channel state information, quality of service requirements of the users, and the overload status of the APs. Based on the available information, in each time interval, the controller decides which user is served by which AP and assigns subcarriers to the users with the objective of maximizing both the system throughput and user satisfaction. We formulate the resource allocation problem as a constrained nonlinear integer programming problem and solve it using meta-heuristic PSO. Through an extensive simulation study, the superiority of the proposed algorithm in terms of system throughput and user satisfaction over round robin, best channel quality information, and genetic algorithms is demonstrated.

Side-effect modulation (SEM) has the potential to be a significant source of interference in future visible light communication (VLC) systems. SEM is a variation in the intensity of the light emitted by a luminaire and is usually a side effect caused by the power supply used to drive the luminaires. For LED luminaires powered by a switched mode power supply, the SEM can be at much higher frequencies than that emitted by conventional incandescent or fluorescent lighting. It has been shown that the SEM caused by commercially available LED luminaires is often periodic and of low power. In this paper, we investigate the impact of typical forms of SEM on the performance of optical OFDM VLC systems; both ACO-OFDM and DCO-OFDM are considered. Our results show that even low levels of SEM power can significantly degrade the bit-error-rate performance. To solve this problem, an SEM mitigation scheme is described. The mitigation scheme is decision-directed and is based on estimating and subtracting the fundamental component of the SEM from the received signal. We describe two forms of the algorithm; one uses blind estimation, while the other uses pilot-assisted estimation based on a training sequence. Decision errors, resulting in decision noise, limit the performance of the blind estimator even when estimation is based on very long signals. However, the pilot system can achieve more accurate estimations, and thus a better performance. Results are first presented for typical SEM waveforms for the case where the fundamental frequency of the SEM is known. The algorithms are then extended to include a frequency estimation step and the mitigation algorithm is shown also to be effective in this case.

Summary: Recently, visible light communications (VLCs) based on the orthogonal frequency‐division multiplexing (OFDM) technique play an important role in the field of wireless communications. Researches in that field found techniques to improve performance and reduce the system complexity. This paper proposes a system named asymmetric symmetric subcarriers enhanced optical (ASSEO)‐OFDM applied to VLC based on OFDM technique. The proposed system enhances the transmitted data rate by a factor of 1/8 compared with other existing systems. Keeping the same transmitted data rate as the comparable techniques, simulation results show an enhancement of the Eb/No for the proposed system by at least 0.65 dB than asymmetrically and symmetrically clipping optical (ASCO)‐OFDM, by at least 2 dB than asymmetrically clipped optical (ACO)‐OFDM and FLIP‐OFDM, and by at least 0.9 dB than DC‐biased optical (DCO)‐OFDM system at bit error rate (BER) of 10−4. An analytical description of the ASSEO proposed system is also introduced in this paper besides a comparative study with other existing systems. [ABSTRACT FROM AUTHOR]

In direct-detection OFDM systems, the nonlinear effects caused by optical modulation and fiber transmission can degrade the system performance severely. In this study, we propose a new nonlinear companding transform to improve the performance of direct detection optical OFDM transmission systems. The demonstration is realized by Monte-Carlo simulation of the intensity modulation and direct detection DCOOFDM optical transmission system at 40 Gbps over a 80 km of standard single mode fiber link. The influence of the companding parameters on the performance of system in different nonlinear transmission conditions has been investigated via simulation. [ABSTRACT FROM AUTHOR]