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Phase noise is a crucial impairment in coherent optical communication. In this paper, a dual-polarization 16 quadrature amplitude modulation coherent optical orthogonal frequency division multiplexing (DP 16-QAM CO-OFDM) modulation system with a hybrid optical filter has been proposed for phase noise reduction. This hybrid filter is a combination of an optical designed filter and an optical Gaussian filter. Linewidth of the input laser has been varied to generate phase noise. The simulation results have been obtained in terms of bit error rate (BER). It has been seen that the use of a hybrid optical filter mitigates the adverse effect of phase noise. [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]

Visible light communication (VLC) systems leverage illumination devices, such as light-emitting diodes (LEDs), to serve a dual role as indoor high-speed communication downlinks. Though high data rates are possible using orthogonal frequency-division (OFDM) in VLC systems, the impact on the complexity and luminous efficacy of the luminaire remain among the key challenges. In this paper, square-wave spatial optical OFDM (SW-SO-OFDM) is proposed which transmits an OFDM signal using $G$ square-wave subcarriers from $G$ LED groups and allowing them to sum in space. Using a binary-level square-wave carrier signal eliminates the need for digital-to-analog conversion, non-linear pre-distortion hardware and the transmitter inverse Fourier transform, thereby greatly reducing the complexity of the transmitter in the luminaire. Further, by coordinating the binary transmissions from pairs of LED groups, SW-SO-OFDM can transmit multi-level constellations, which further improves the bandwidth efficiency. Through simulation and experiment, SW-SO-OFDM is shown to provide communication performance comparable to SO-OFDM and to significantly outperform conventional DC-biased (DCO)-OFDM at high signal-to-noise ratios, while considerably reducing the overall transmitter complexity in the luminaire.

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]

In both optical wireless and optical fiber communication, orthogonal frequency division multiplexing (OFDM) plays an important role in data transmission. In the similar context, DC-coupled optical (DCO) OFDM and asymmetrically clipped optical (ACO) OFDM are discussed. DCO-OFDM has better peak to average power ratio (PAPR) but bit error rate (BER) performance is poor; however, in case of ACO-OFDM, PAPR is poor but BER performance is good. Moreover, DCO-OFDM is more power hungry; therefore, ACO-OFDM is considered as a preferred choice. Recently, asymmetrically clipped DC-biased optical (ADO)-OFDM is proposed, which shows good PAPR and BER performance. Clipping and µ-law companding techniques are discussed for PAPR reduction. The mathematical model is developed for all the three methods for received signal while considering clipping and companding noises along with channel noise. It is found that ADO-OFDM is better choice in comparison to DCO and ACO-OFDM. However, in case of ADO-OFDM, receiver structure is more complex. [ABSTRACT FROM AUTHOR]

We present a broadband coherent orthogonal frequency-division multiplexing (OFDM) transceiver based on orthogonal sampling and low bandwidth electronic analog signal processing. Wideband superchannels, without any guardband are aggregated from low bandwidth OFDM channels in the time domain by orthogonal Nyquist sinc-pulse sequences with a rectangular bandwidth. Therefore, the method is called OFDM-Nyquist-time division multiplexing (TDM). Simulation and experimental results will be discussed for optical systems. However, with some modifications the same principle can be used for wireless or THz signals. In simulations, we show a 40 GHz bandwidth, 160 Gbps, 16-QAM, 128 OFDM x 5-Nyquist-TDM transceiver based on 4 GHz electronics for the digital-to-analog (DAC), analog-to-digital (ADC) conversion and for the digital signal processing, including Fourier transform. For the experiment, we verify the processing of a 24 GHz bandwidth, 48 Gbps QPSK, 512 OFDM x 3-Nyquist-TDM signal with a 4 GHz transmitter and receiver. Since the proposed method drastically decreases the sampling rate and bandwidth requirements for the Fourier processing, the DAC and ADC, it can be a promising alternative for future communication systems with the highest possible symbol rate.

In this paper, an adaptive iterative symbol clipping scheme for OFDM-based visible light communication systems is proposed. This scheme can avoid the disadvantages of the conventional iterative signal clipping (ISC) scheme that produces all-zero symbols due to the fixed iterative clipping times, and also avoids the information transmission rate degradation due to an adaptive symbol decomposition with serial transmission (ASDST) scheme using serial transmission. In the proposal, optical orthogonal frequency division multiplexing (O-OFDM) symbols are decomposed into multiple O-OFDM symbols that are in the linear range of the LEDs after adaptive iterative clipping according to the signal amplitude and clipping threshold, thereafter simultaneously transmitted through multiple LEDs. When the number of LEDs is large enough, clipping distortion can be significantly reduced or even completely eliminated. In addition, the all-zero symbols existing in the ISC scheme can be completely eliminated. Experimental results show that the proposed scheme can not only improve the BER performance and the optical power efficiency, but also reduce the clipping times compared with the traditional ISC scheme. Besides,our proposed scheme could achieve higher information transmission rate compared with the ASDST scheme. [ABSTRACT FROM AUTHOR]

This paper investigates optical orthogonal frequency division multiplexing (O-OFDM) systems with index modulation (IM) based on discrete Hartley transform (DHT) for visible light communications (VLC). An interesting trade-off between spectral efficiency (SE), energy efficiency, peak-to-average power ratio (PAPR) and bit error rate (BER) performance can be made by using the IM technique. And the DHT-based O-OFDM-IM systems can achieve higher SE compared with the DFT (discrete Fourier transform)-based counterparts. Employing DHT instead of DFT enables the removal of the Hermitian symmetry requirement, which contributes to transmitting more index bits and results in SE improvement. To acquire the same SE, the proposed systems can significantly lower the constellation order leading to better BER performance by contrast with the DFT-based counterparts. We employ DC (direct current) biasing and AC (Asymmetrically clipped) techniques to investigate the O-OFDM-IM systems in this paper. Simulation results indicate that SNR gains can be obtained by the DHT-based DCO/ACO-IM in comparison to the DFT-based counterparts. We also propose a novel reduced-complexity ML (maximum likelihood) detector specifically applicable to VLC. The new detector exhibits the same BER performance as the ML detector and performs better than the LLR (log-likelihood ratio) detector with reduced computational complexity.

We propose a novel design of orthogonal frequency division multiplexing-mode division multiplexing (OFDM-MDM) based radio over free space optical (RoFSO) transceiver under the dust effect. Two separate 20 Gbps-40 GHz 4-QAM data bearing optical beams are transported over two distinctive Hermite-Gaussian modes; HG00 and HG01, of a single-frequency laser to realize a single-channel 40 Gbps-80 GHz 5G communication link. The link performance and availability are investigated under the impact of clear climate and under varied dust conditions, including very light dust, light dust, moderate dust, and dense dust using signal-to-noise ratio (SNR) and total power as the key evaluation parameters. Further, we have determined the maximum reachable link distance within acceptable performance criteria, i.e., SNR 20 dB for each desert weather conditions. A higher data rate with tolerable SNR and power requirements for 5G applications has been verified.

This paper proposes a differential index modulation (DIM) scheme to address the complex channel estimation challenges in optical orthogonal frequency division multiplexing with index modulation (OOFDM-IM). The main idea of the DIM scheme is to design a time-frequency dispersive matrix with unitary characteristics and perform index mapping based on the Lemer code principles. By applying differential operations, the DIM scheme enables decoding at the receiver without requiring channel estimation. The paper provides a detailed explanation of the design principles and theoretical bit error rate (BER) of the proposed scheme. Simulations based on the exponential Weibull atmospheric turbulence channel model are conducted to compare the DIM scheme with the existing OOFDM-IM scheme. Additionally, a carefully designed proof-of-concept experiment is performed to further validate the scheme's effectiveness and feasibility. Both simulation and experimental results demonstrate that, compared to OOFDM-IM, the proposed scheme can entirely avoid complex channel estimation with a maximum signal-to-noise ratio (SNR) loss of no more than 4 dB, even under various turbulence intensities and higher-order modulation scenarios. This provides a valuable reference for OOFDM-IM in complex environments or where channel estimation is challenging. • A novel OOFDM-DIM scheme for channel estimation free is proposed. • A theoretical performance based on EW channels has been derived. • Actual proof-of-concept experiments were designed. [ABSTRACT FROM AUTHOR]

This paper investigates optical orthogonal frequency division multiplexing (O-OFDM) systems with index modulation (IM) based on discrete Hartley transform (DHT) for visible light communications (VLC). An interesting trade-off between spectral efficiency (SE), energy efficiency, peak-to-average power ratio (PAPR) and bit error rate (BER) performance can be made by using the IM technique. And the DHT-based O-OFDM-IM systems can achieve higher SE compared with the DFT (discrete Fourier transform)-based counterparts. Employing DHT instead of DFT enables the removal of the Hermitian symmetry requirement, which contributes to transmitting more index bits and results in SE improvement. To acquire the same SE, the proposed systems can significantly lower the constellation order leading to better BER performance by contrast with the DFT-based counterparts. We employ DC (direct current) biasing and AC (Asymmetrically clipped) techniques to investigate the O-OFDM-IM systems in this paper. Simulation results indicate that SNR gains can be obtained by the DHT-based DCO/ACO-IM in comparison to the DFT-based counterparts. We also propose a novel reduced-complexity ML (maximum likelihood) detector specifically applicable to VLC. The new detector exhibits the same BER performance as the ML detector and performs better than the LLR (log-likelihood ratio) detector with reduced computational complexity. [ABSTRACT FROM AUTHOR]

End-to-end learning systems are conceived for Orthogonal Frequency Division Multiplexing (OFDM)-aided optical Intensity Modulation paired with Direct Detection (IM/DD) communications relying on the Autoencoder (AE) architecture in deep learning. We first propose an AE-aided Layered ACO-OFDM (LACO-OFDM) scheme, termed as LACONet, for exploiting the increased bandwidth efficiency of LACO-OFDM. LACONet employs a Neural Network (NN) at the transmitter for bit-to-symbol mapping, and another NN at the receiver for recovering the data bits, which together form an AE and can be trained in an end-to-end manner for simultaneously minimizing both the BER and PAPR. Moreover, the detection architecture of LACONet is drastically simplified compared to classical LACO-OFDM, since the Fast Fourier Transform (FFT) operation is applied only once at the receiver. We further propose a generalized AE-aided optical OFDM scheme for IM/DD communications, termed as IMDD-OFDMNet, where the unipolarity of the Time Domain (TD) signal is no longer guaranteed by the Hermitian Symmetry, but rather by taking the absolute square value of the complex TD signal. As such, all available subcarriers of IMDD-OFDMNet are used for carrying useful information, hence it has a higher throughput than the LACO-based schemes. As a further benefit, its transceiver requires only a single Inverse FFT or FFT. Finally, simulation results are provided to show that our learning schemes achieve better BER and PAPR performance than their conventional counterparts. [ABSTRACT FROM AUTHOR]

In this article, the performance of a new time domain signal pre-equalization method for use with optical orthogonal frequency division multiplexing (OFDM) and a silicon photomultiplier (SiPM) based receiver is studied. A SiPM contains a large array of microcells and each microcell is able to detect single photons. Therefore, a SiPM can be used to create arguably the most sensitive optical receiver, which can detect light intensity signals by counting the number of arriving photons within each signal sampling period. However, each photon detection triggers an avalanche-and-quenching process and the related microcell becomes inactive for a recovery time of several nanoseconds. Consequently, any photons arriving during this period cannot be detected. This effect can cause a non-linear distortion of the received signal and, when the OFDM sampling period is short, also introduces interference between signal samples. In this article, a new signal pre-equalization method is specifically designed to compensate for the impact of the finite recovery time. In this method, the number of active microcells during the transmission of each OFDM signal sample is first estimated. Then, the amplitude of the time domain signal sample is pre-adjusted based on the predicted fraction of microcells that are active. Using this approach, the negative impacts of the recovery time of the microcells are significantly reduced. The results that are presented show that when this new form of pre-equalization is used the bit error rate (BER) performance of the system is improved for a wide range of irradiance levels.

In this paper, the performance of a new decision-directed signal reconstruction (DDSR) algorithm with optimal reconstruction thresholds for use with optical orthogonal frequency division multiplexing (OFDM) modulation is investigated. Clipping noise remains one of the main drawbacks in many practical optical OFDM transmission systems. It has been shown in previous studies that a time-domain based DDSR algorithm can be used to effectively reduce the clipping noise. However, in all existing works studying this DDSR algorithm, the decision threshold used to direct the time-domain signal reconstruction is fixed and equal to the level at which the transmitted signal is clipped. In this paper, it is shown that, due to the influences of the unavoidable noise at the receiver, fixing the DDSR threshold at the clipping level is not an optimal solution and can severely limit the advantages of using this algorithm. A detailed theoretical analysis is performed to investigate the key factors that influence the choices of the optimal DDSR thresholds. The presented simulation results are used to verify the accuracy of the theoretical analysis and also show that the performance of the DDSR algorithm is significantly improved by using the obtained optimal thresholds.

Visible light communication (VLC) operates on optical intensity channels that are inherently limited in bandwidth. Though Kramers-Kronig (KK) receivers have been considered as a lower complexity alternative for coherent fiber optic communication (FOC) systems, in this paper we extend this concept to bandlimited IM/DD VLC channels and propose KK optical OFDM (KKO-OFDM). In KKO-OFDM the optical power of a low-cost LED is directly modulated by the double-sideband (DSB) squared modulus of a minimum phase single-sideband (SSB) signal. This results in a real-valued, non-negative and strictly bandlimited transmit signal which is suitable for VLC channels. At the receiver, the phase of the transmitted SSB signal is reconstructed via the KK relations. The required DC bias, average electrical signal-to-noise ratio (SNR), bit error rate (BER), and capacity are analyzed with approximate closed-forms and through simulation. Numerical results show that KKO-OFDM achieves the same spectral efficiency as the existing DC biased optical OFDM (DCO-OFDM), however, realizes approximately 1 dB optical SNR gain at a BER $=10^{-4}$ while simultaneously having a small peak-to-average power ratio. [ABSTRACT FROM AUTHOR]

We propose a novel design of orthogonal frequency division multiplexing-mode division multiplexing (OFDM-MDM) based radio over free space optical (RoFSO) transceiver under the dust effect. Two separate 20 Gbps-40 GHz 4-QAM data bearing optical beams are transported over two distinctive Hermite-Gaussian modes; HG00 and HG01, of a single-frequency laser to realize a single-channel 40 Gbps-80 GHz 5G communication link. The link performance and availability are investigated under the impact of clear climate and under varied dust conditions, including very light dust, light dust, moderate dust, and dense dust using signal-to-noise ratio (SNR) and total power as the key evaluation parameters. Further, we have determined the maximum reachable link distance within acceptable performance criteria, i.e., SNR 20 dB for each desert weather conditions. A higher data rate with tolerable SNR and power requirements for 5G applications has been verified. [ABSTRACT FROM AUTHOR]

A novel low-complexity optical orthogonal frequency division multiplexing (LCO-OFDM) transmission architecture is proposed in this letter. The LCO-OFDM transmitter consists of two branches, wherein the first and second branches use odd and even subcarriers, respectively. Specifically, only complex multiplications are required in modulated symbols of each branch without Hermitian symmetry and inverse fast Fourier transform operations. Transmission signals of LCO-OFDM can be obtained by transmitting real and imaginary parts of generated complex signals in each branch over two successive orthogonal frequency division multiplexing frames and adding a direct current bias to avoid zero clipping noise. Simulation results showed the optimum performance of LCO-OFDM in terms of both peak-to-average power ratio and bit error rate. [ABSTRACT FROM AUTHOR]

Orthogonal frequency division multiplexing (OFDM) of the optical communication system is a known approach for light fidelity and the screen-to-a-camera (S2C) communication system is an innovative implementation for optical camera communication (OCC). In This paper, two dimensional (2D) asymmetrically clipping optical (ACO) OFDM for the S2C model can be obtained which develops a single screen display, consisting of several cells, for a data rate of up to 100 kb/s. This paper contains an innovative system design focused on validation of implementation and theoretical study. The source distribution of the ACO OFDM system has been implemented using LAB VIEW software. The performance of the suggested model was verified through numerical assessment techniques, as it was checked that within the transmission distance of 4.5 m and the field of view of 30 ∘ overall bit error rate (BER) of 10−6 and signal to noise ratio (SNR) 18 dB is achieved. Under the existing IEEE 802.15.7 m standard, the overall results of the S2C OFDM system are being evaluated by comparing them to the 2D OFDM OCC system to illustrate the efficiency of the proposed scheme. [ABSTRACT FROM AUTHOR]

Orthogonal frequency division multiplexing (OFDM) is a candidate modulation technique to achieve high-speed data transmission in visible light communication systems that provide both illumination and downlink data transmission. This paper proposes a method called polarity-header optical OFDM (PHO-OFDM) specially designed to account for the nonlinear response of the LED transmitters. To make the transmitted signal positive, as required for intensity modulation and direct detection systems, a signal header containing the sign information of the bipolar OFDM signal is transmitted followed by the amplitude of the signal. The binary sign information can be encoded and transmitted via a few samples using M-ary pulse-amplitude modulation. The amplitude of the OFDM signal that is larger than the maximum LED power is sent in a subsequent frame to reduce the nonlinear distortion due to peak-power clipping. Furthermore, illumination requirements are taken into account, and their effects on the system performance are discussed. This paper also considers the overhead cost of adding a cyclic prefix to minimize intersymbol interference in dispersive channels. Compared with state-of-the-art optical OFDM techniques, the proposed PHO-OFDM achieves a better BER performance and higher data throughput, and provides a larger illumination flexibility.

The orthogonal frequency division multiplexing (OFDM) waveform is a well-known implementation for light fidelity but quite a novel implementation for optical camera communication (OCC). This paper proposes the two-dimensional OFDM (2D-OFDM) OCC system, namely screen OCC, which implements a single screen luminaire, composed of multiple cells so that a data rate of up to 50 kb/s can be achieved with the 2D-OFDM waveform. This paper includes a mature system design based on theoretical analysis and implementation validation. The various technical details described include system architectures, theoretical analyses of the link budget, and detailed implemental guidance for the proposed Screen OFDM system. The reliability of the proposed system was verified through numerical performance measurements, as recorded that the overall BER of 10-5 is achievable within the communication distance of 4.5 m and the viewing angle of 30°. The overall performance of the screen OFDM is also compared with a single-carrier modulation approach, asynchronous-quick-link code within the ongoing IEEE 802.15.7m standard, to demonstrate the reliable performance of the proposing system.

Optical communication either in free space wireless mode or optical fiber mode is much preferred for high data transmission rate in the range of 100 GHz or few THz. Implementation of OFDM in fiber helps to simplify the dispersion compensation but nonlinearity in fiber material will become a major challenge. This Chapter is review and literature survey about the development and possibilities for OFDM in optical fiber communication systems. It presents the basic features of OFDM and its impact on the mitigation of optical fiber impairments. [ABSTRACT FROM AUTHOR]

Abstract Optical wireless communications (OWC) has emerged as a strong candidate for wireless communications, due to the capacity limitation in the radio frequency (RF) spectrum. Especially visible light communication (VLC) has great potential for short-range outdoor vehicular communications, as vehicle LED lights also transmit data. However, outdoor VLC channels vary fast and, experience multipath scattering and reflection resulting in time domain dispersion. Outdoor VLC links are also subjected to high levels of ambient noise, especially from the sun. Orthogonal frequency-division multiplexing (OFDM), which has proven robustness to multi path fading and noise effects in RF links can also be deployed in VLC links. In this paper, optical OFDM (O-OFDM) along with adaptive modulation scheme is investigated in VLC for vehicle to vehicle (V2V) communications. A (2×2) multiple input multiple output (MIMO) channel, with multiple polarimetric bidirectional reflections and realistic sunlight interference is considered. Two schemes of O-OFDM; direct current biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM) are investigated. Simulation results of the proposed model show increase in data rates up to 50 Mbps along with reduced bit error rate (BER) under both line of sight (LOS) and non-LOS and high noise conditions.

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 article 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 article, 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. [ABSTRACT FROM AUTHOR]

Optical orthogonal frequency division multiplexing (O-OFDM) schemes are variations of OFDM schemes that produce non-negative signals. Asymmetrically-clipped O-OFDM (ACO-OFDM) is a single-layer O-OFDM scheme, whose spectral efficiency can be enhanced by adopting multiple ACO-OFDM layers or a combination of ACO-OFDM and other O-OFDM schemes. However, since symbol detection in such enhanced ACO-OFDM (eACO-OFDM) is done iteratively, erroneous detection leads to residual clipping noise (RCN) which can degrade performance in practice. Thus, it is necessary to develop an accurate model for RCN in designing RCN-aware eACO-OFDM schemes. To this end, this paper provides a mathematical analysis of RCN leading to an accurate model of RCN power. The obtained model is used to analyse the performance of various eACO-OFDM schemes. It is shown that the model provides an accurate evaluation of symbol error rate (SER), which would be underestimated if RCN is ignored. Moreover, the model is shown to be useful for designing an RCN-aware resource allocation that increases the robustness of the system in terms of meeting a target SER, compared to an RCN-unaware design. [ABSTRACT FROM AUTHOR]

Efficient frame synchronization is essential for data recovery in communication systems. In this study, a single pilot sequence is used to achieve both frame synchronization and peak-to-average power ratio (PAPR) reduction. The two systems considered are direct-current biased optical orthogonal frequency division multiplexing (DCO-OFDM) and asymmetrically clipped O-OFDM (ACO-OFDM). The pilot symbol is allocated to odd indexed subcarriers only. Thus, the synchronization algorithm leverages the mirror symmetric property of the pilot symbol within a frame to detect the start of the pilot signal at the receiver. This scheme has low complexity and gives precise frame synchronization at signal-to-noise ratios as low as 4 dB in an indoor visible light communication (VLC) channel. [ABSTRACT FROM AUTHOR]

Laser phase noise tolerance in direct detection optical orthogonal frequency division multiplexing (OFDM) transmission system is experimentally demonstrated using laser linewidth emulator. With this linewidth emulator, the linewidth of the laser can be broadened independently without inducing other laser's noise characteristics such as relative intensity noise and chirping. The results show the effect of phase-to-intensity noise conversion due to the laser's phase noise and chromatic dispersion. This induces noise pedestals not only onto the optical carrier, but also to each of the subcarriers, and thus produces intersubcarriers interference. Measurements show 6 dB Q-degradation is suffered by the system when 10 MHz linewidth is used as compared to using narrowest emulator linewidth of 256 kHz. Transmission with the linewidth emulator set to 20 MHz is also shown where it can still be tolerated by 400 km of fiber at the Q of 9.606 dB with respect to $1.6\,\times \,10^{{\rm - 3}}$ bit error rate. Simulation of 16 quadratic-amplitude modulation (QAM) (∼128 Gb/s) and 4 QAM (∼100 Gb/s) using VPItransmissionMaker are also presented to demonstrate the effect of carrier's phase-to-intensity noise conversion when a high data rate and wide OFDM signal bandwidth are used with the linewidth emulator as the system's laser source. The results show that a phase rotation term is insignificant for lower M-size QAM with fiber length less than 500 km. This presents the high effectiveness and reliability of the laser linewidth emulator to discretely determine an independent phase noise tolerance from the other laser's noise characteristics in the optical OFDM transmission system.

In visible light communications (VLCs) relying on intensity-modulation and direct detection (IM/DD), the conversion from electrical signals to optical signals and the limited dynamic range of the light-emitting diodes (LEDs) constitute the fundamental impediments in the way of high-integrity communications, especially when orthogonal frequency-division multiplexing (OFDM) is employed. In IM/DD VLCs, only real-valued positive signals are used for signal transmission. However, the Fourier transform of OFDM systems is operated in the complex domain. In order to meet the requirements of the IM/DD VLCs, the complex-to-real conversion is achieved at the cost of reducing the bandwidth efficiency. Moreover, OFDM signals experience a high peak-to-average power ratio; hence, typically clipping is used for confining the positive-valued signals within the LED's dynamic range. However, hard clipping leads to the loss of orthogonality for optical OFDM (O-OFDM) signals, generating inter-carrier interference. As a result, the performance of the clipping-based O-OFDM systems may be severely degraded. In this paper, the concept of piecewise companding transform (CT) is introduced into the O-OFDM system advocated, forming the CTO-OFDM arrangement. We first investigate the general principles and design criteria of the piecewise CTO-OFDM. Based on our studies, three types of piecewise companders, namely, the constant probability sub-distribution function, linear PsDF (LPsDF), and the non-LPsDF-based CT, are designed. Furthermore, we investigate the nonlinear effect of hard clipping and of our CT on O-OFDM systems in the context of different scenarios by both analytical and simulation techniques. Our investigations show that the CTO-OFDM constitutes a promising signaling scheme conceived for VLCs, which exhibits a high bandwidth efficiency, high flexibility, high reliability, as well as a high data-rate, despite experiencing nonlinear distortions.

This paper presents a novel theoretical characterization of the pilot-assisted (PA) technique proposed for peak-to-average-power-ratio (PAPR) reduction in optical orthogonal frequency division multiplexing (O-OFDM). The two systems considered are direct-current biased O-OFDM (DCO-OFDM) and asymmetrically clipped O-OFDM (ACO-OFDM) in optical wireless communications. The DCO-OFDM and ACO-OFDM time-domain signals approach Gaussian and half-Gaussian distributions, respectively. The PA technique uses P iterations of a pilot sequence to rotate the phase of U data symbols within a PA O-OFDM frame and select the frame with the least PAPR. Thus, we utilize order statistics to characterize the PAPR distributions of the PA DCO-OFDM and ACO-OFDM system. The PA technique results in higher reduction in PAPR for high P but at the expense of increased complexity. In the theoretical framework developed, we are able to determine P that gives reasonable PAPR reduction gain. The theoretical analysis of PAPR reduction effects on the average optical and electrical signal power is studied. Results show that the PA technique is capable of reducing the optical energy per bit to noise power spectral density Eb(opt)/N0 ratio required to meet target bit-error-rate in an additive white Gaussian noise channel. Comparisons of the analytical results of PA O-OFDM signal with that of computer simulations show very good agreement.

Visible light communications (VLCs) have received extensive attention in the research community thanks to their advantages of high bandwidth, low cost, robustness to electromagnetic interference, operation in an unregulated spectrum, and high degree of spatial confinement in indoor scenarios. One of the main limitations for high data-rate transmission in VLC systems is the limited modulation bandwidth of commercial light emitting diodes. To circumvent this limitation, spectrally efficient modulation schemes should be used. Optical orthogonal frequency-division multiplexing (O-OFDM)-based schemes have become very popular and several proofs of concept have shown their ability to attain over gigabits per second transmission rates. We consider here the use of pulse amplitude modulation and carrier-less amplitude and phase modulation schemes together with frequency-domain equalization (FDE) at the receiver as interesting alternatives to O-OFDM. We show the advantages of the former schemes in terms of the peak-to-average-power ratio, and demonstrate through numerical results the merits of FDE-based signaling in attaining high data rates.

Orthogonal frequency division multiplexing (OFDM) is a candidate modulation technique to achieve high-speed data transmission in visible light communication systems that provide both illumination and downlink data transmission. This paper proposes a method called polarity-header optical OFDM (PHO-OFDM) specially designed to account for the nonlinear response of the LED transmitters. To make the transmitted signal positive, as required for intensity modulation and direct detection systems, a signal header containing the sign information of the bipolar OFDM signal is transmitted followed by the amplitude of the signal. The binary sign information can be encoded and transmitted via a few samples using $M$ -ary pulse-amplitude modulation. The amplitude of the OFDM signal that is larger than the maximum LED power is sent in a subsequent frame to reduce the nonlinear distortion due to peak-power clipping. Furthermore, illumination requirements are taken into account, and their effects on the system performance are discussed. This paper also considers the overhead cost of adding a cyclic prefix to minimize intersymbol interference in dispersive channels. Compared with state-of-the-art optical OFDM techniques, the proposed PHO-OFDM achieves a better BER performance and higher data throughput, and provides a larger illumination flexibility. [ABSTRACT FROM AUTHOR]

Visible light communications (VLC), a new optical wireless communication technology that uses illumination light-emitting diodes 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, unipolar OFDM, and asymmetrically clipped optical OFDM. Furthermore, CEO-OFDM can provide a better illumination performance that supports light dimming. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate a capacity-approaching scheme via joint probabilistic shaping (PS) and precoding in an optical wireless communication (OWC) system. Different from the bit-and-power- and entropy-loading schemes, the proposed scheme only requires one modulation format with one single-PS distribution while offering the ability of maximizing the system’s throughput. Moreover, we show that the proposed scheme also exhibits superior spectral efficiency (SE) and generalized mutual information (GMI) performances. As a result, the implementation complexity is significantly reduced without affecting the capacity-approaching capability. We show that for a 36-Gbit/s OWC transmission, ~5 dB improvement in the received optical power (ROP) sensitivity can be achieved by using the proposed scheme. In addition, a maximized 48-Gbit/s transmission over 2-m OWC channel is successfully demonstrated with a BER lower than $3.8\times 10^{-3}$ , which corresponds to more than 33% capacity enhancement. [ABSTRACT FROM AUTHOR]

In this paper, we comprehensively investigate the achievable rates of selected band-limited intensity modulation schemes, which are important for optical wireless communication applications, while accounting for the specific nature of their signal construction (non-negative, real, and baseband), and imposing identical bandwidth and average optical power constraints. Furthermore, we identify/devise methods to effectively trade between these parameters. Three variants of orthogonal frequency division multiplexing (OFDM), namely, asymmetrically clipped optical OFDM (ACO-OFDM), spectrally and energy efficient OFDM (SEE-OFDM), and dc-biased optical OFDM (DCO-OFDM), and single-carrier pulse amplitude modulation are studied. The clipping noise in ACO-OFDM and SEE-OFDM is found to consume a large excess bandwidth. The detrimental effects of this excess bandwidth on the achievable rate are evaluated. For SEE-OFDM, the problem of optimal power allocation among its components is formulated and solved using the Karush–Kuhn–Tucker method. For DCO-OFDM, the clipping noise is modeled and incorporated in the analysis. Among the existing schemes, DCO-OFDM yields the best overall performance, due to its compact spectrum. In order to improve the achievable rate, we propose and analyze two improved distortionless variants, filtered ACO-OFDM and filtered SEE-OFDM (FSEE-OFDM), which yield better spectral efficiency than ACO-OFDM and SEE-OFDM, respectively. FSEE-OFDM, being the most spectrally efficient, outperforms all schemes. [ABSTRACT FROM AUTHOR]

The massive deployment of light-emitting diode (LED) lightning infrastructure has opened the opportunity to reuse it as visible light communication (VLC) to leverage the current RF spectrum crisis in indoor scenarios. One of the main problems in VLC is the limited dynamic range of LEDs and their nonlinear response, which may lead to a severe degradation in the communication, and more specifically in the bit error rate (BER). This is aggravated by the extensive usage of multicarrier multiplexing, based on optical orthogonal frequency division multiplexing (O-OFDM), characterized by a high peak-to-average power ratio. Here, we present a chaos-based coded modulation (CCM) setup specifically adapted to the LED nonlinearities. It replaces the usual modulation, while keeping the multicarrier O-OFDM structure unchanged. First, we obtain a semi-analytical bound for the bit error probability, taking into account the LED nonlinear response. The bound results particularly tight for the range of signal-to-noise ratio of interest. Then, we propose a method to design the modulator based on optimization techniques. The objective function is the semi-analytical bound, and the optimization is applied to a parameterization of the CCM conjugation function. This appropriately shapes the chaotic waveform and leads to BER improvements that outperform classical counterparts under ideal predistortion. [ABSTRACT FROM AUTHOR]

In optical communication systems, orthogonal frequency division multiplexing (OFDM) is widely used to combat inter-symbol interference (ISI) caused by multipath propagation. Optical systems which use intensity modulation and direct detection (IM/DD) can only transmit real valued symbols, but the inverse discrete Fourier transform (IDFT) or its computationally efficient form inverse-fast Fourier transform (IFFT) required for the OFDM waveform construction produces complex values. Hermitian symmetry is often used to obtain real valued symbols. For this purpose, some trigonometric transformations such as discrete cosine transform (DCT) are also used; however, these transformations can eliminate the ISI only under certain conditions. In this paper, we propose a completely different method for the construction of OFDM waveform with IFFT to obtain real valued symbols by combining the real and imaginary parts (CRIP) of IFFT output electrically (E-CRIP) or optically (O-CRIP). Analytical analysis and simulation works are presented to show that compared to the Hermitian symmetric system, the proposed method slightly increases the spectral efficiency, eliminates ISI, significantly reduces the amount of needed calculation, and does not affect the error performance. In addition, the O-CRIP method is less affected by clipping noise that may occur due to the imperfections of the transmitter front-ends. [ABSTRACT FROM AUTHOR]

Underwater optical wireless systems have dual requirements of high data rates and long ranges in harsh scattering and attenuation conditions. In this paper, we investigate the advantages and limitations of optical orthogonal frequency-division multiplexing (O-OFDM) signaling when a silicon photo-multiplier (SiPM) is used at the receiver in order to ensure high sensitivity. Considering a light-emitting diode (LED) transmitter and taking into account the limited dynamic range imposed by the transmitter and the SiPM receiver, we study the performance of three popular O-OFDM schemes, i.e., DC-biased, asymmetrically-clipped, and layered asymmetrically-clipped O-OFDM (DCO-, ACO-, and LACO-OFDM, respectively). We consider a constraint on transmit electrical power PTxe and take into account the required DC bias for the three considered schemes in practice, showing the undeniable advantage of ACO- and LACO-OFDM in terms of energy efficiency. For instance, for the considered SiPM and LED components, a spectral efficiency of ∼1 bps/Hz with a data rate of 20 Mbps, a link range of 70 m, and a target bit-error-rate (BER) of 10−3, ACO and LACO allow a reduction of about 10 and 6 mW, respectively, in the required PTxe, compared to DCO-OFDM. Meanwhile, we show that when relaxing the PTxe constraint, DCO-OFDM offers the largest operational link range within which a target BER can be achieved. For instance, for a target BER of 10−3 and a data rate of 20 Mbps, and considering PTxe of 185, 80, and 50 mW for DCO-, LACO-, and ACO-OFDM, respectively, the corresponding intervals of operational link range are about 81, 74.3, and 73.8 m. Lastly, we show that LACO-OFDM makes a good compromise between energy efficiency and operational range flexibility, although requiring a higher computational complexity and imposing a longer latency at the receiver.

Efficient frame synchronization is essential for data recovery in communication systems. In this study, a single pilot sequence is used to achieve both frame synchronization and peak-to-average power ratio (PAPR) reduction. The two systems considered are direct-current biased optical orthogonal frequency division multiplexing (DCO-OFDM) and asymmetrically clipped O-OFDM (ACO-OFDM). The pilot symbol is allocated to odd indexed subcarriers only. Thus, the synchronization algorithm leverages the mirror symmetric property of the pilot symbol within a frame to detect the start of the pilot signal at the receiver. This scheme has low complexity and gives precise frame synchronization at signal-to-noise ratios as low as 4 dB in an indoor visible light communication (VLC) channel.

Digital coherent superposition (DCS) for optical orthogonal frequency-division multiplexing (OFDM) subcarrier pairs with Hermitian symmetry can simultaneously reduce the inter-carrier interference (ICI) from laser phase noise and cancel the fiber nonlinearity to the first order. In this paper, we study an interesting scenario, where we upgrade the conventional OFDM with 4-quadrature amplitude modulation (QAM) to DCS-OFDM with 16-QAM, which have equal spectrum efficiency and require no change on hardware. We conduct a simulation to compare the tolerance to the laser phase noise and fiber nonlinearity, which are the main concerns for such an upgrading. First, the 16-QAM DCS-OFDM relaxes the laser linewidth requirement and performs better with a smaller fast Fourier transform size. Second, the nonlinear tolerance of 16-QAM DCS-OFDM is better when the nonlinear effects appear. The nonlinear tolerance of DCS-OFDM depends on the dispersion compensation scheme. The maximum SNR improvement is 3.4 dB with full post-dispersion compensation, and it can increase to 6.6 dB with symmetrical dispersion compensation at the launch power of 4 dBm. Finally, we verify that the maximum SNR improvement of 16-QAM DCS-OFDM with full post-dispersion compensation is 3.7 dB at the launch power of 6.2 dBm in the experiment.

Radio-over-fiber (RoF) systems comprise light modulation and transmission of millimeter-wave signals over fiber links. The aim of this study is to investigate the performance of external and direct intensity modulation in RoF links and to analyze the drawbacks induced by different components of the optical system. In external modulation, the Mach–Zehnder modulator (MZM) is used, whereas the vertical-cavity surface-emitting laser (VCSEL) is utilized in direct modulation. Both modulation schemes are tested for a vector modulation format, i.e., the quadrature amplitude modulation (QAM), where an orthogonal frequency-division multiplexing (OFDM) scheme is used to generate signal subcarriers. The simulations are carried out with the same values of common global parameters for both schemes of intensity modulation. Although VCSEL is a promising device for future RoF systems, the external modulation shows a more robust performance compared with that of VCSEL when implemented with the OFDM modulation technique.

In this study, we aim to reduce the peak-to-average power ratio (PAPR) for indoor visible light communication (VLC) orthogonal frequency division multiplexing (OFDM) systems and improve the system performance. By utilizing the spatial multiplexing (SMP) technique, we propose a subcarrier spatial grouping (SSG) scheme to address the high PAPR and LED restricted linear range problems. In this scheme, the high-PAPR wideband OFDM signals are partitioned into several low-PAPR narrowband independent signals that are simultaneously transmitted from multiple LED light sources. At the receiver, zero forcing (ZF) is used to reverse the impairments of the channel transform. The computer simulation results demonstrate that the proposed approach achieves lower PAPR and more robustness to LED nonlinearities leading to better bit error rate performance for the same spectral efficiency than that of the conventional DCO-OFDM scheme does in the whole signal-to-noise ratios range. [ABSTRACT FROM AUTHOR]

Optical wireless communication (OWC) is an age-long technology, which is based on optical data transmission through free space, and it can be implemented in both indoor and outdoor applications. Asymmetrically clipped DC-biased optical orthogonal frequency division multiplexing (ADO-OFDM) is a modulation scheme that can be utilized in indoor OWC systems. It is based on transmitting DC-biased optical OFDM (DCO-OFDM) for even-frequency subcarriers and asymmetrically clipped optical OFDM (ACO-OFDM) for odd-frequency subcarriers. The ADO-OFDM exhibits better optical power performance than those of the conventional ACO-OFDM and DCO-OFDM. The main disadvantage of ADO-OFDM is the clipping noise, which mainly affects the even subcarriers. So, in this paper, convolutional and turbo coding techniques are investigated to improve the bit error rate performance of the ADO-OFDM over Additive white Gaussian noise and diffuse channels. Simulation results show that the proposed coded ADO-OFDM using convolutional and turbo coding techniques achieves significantly lower BERs compared to that of the uncoded ADO-OFDM. In addition, turbo-coded ADO-OFDM gives the best BER performance. [ABSTRACT FROM AUTHOR]