DC-biased Optical Orthogonal Frequency Division Multiplexing Research Articles

Papr reduction through Gaussian pre-coding in DCO-OFDM systems

Visible light communication (VLC) utilizes commercially available light-emitting diodes (LEDs) for short-distance wireless data transmission, aligning with lighting standards for indoor wireless access. The inherent challenge of achieving high data rates in VLC due to the limited spectral bandwidth of white LEDs has led to the adoption of orthogonal frequency division multiplexing (OFDM). However, conventional OFDM presents issues with peak-to-average power ratio (PAPR), pushing LEDs into a nonlinear zone, resulting in energy inefficiencies and signal distortion that degrades the bit error rate (BER). The study focuses on pre-coding within DC-biased optical OFDM (DCO-OFDM) using a Gaussian matrix (GM) for PAPR reduction. The proposed Gaussian pre-coding method demonstrates superior PAPR performance compared to conventional discrete cosine transform (DCT) pre-coding method, especially when integrated with μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu$$\\end{document}-Law companding. Simulation results indicate that increasing the number of candidate GMs enhances PAPR performance, albeit with increased system complexity. The combination of DCT and Gauss pre-coding with μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu$$\\end{document}-Law companding exhibits a 2 dB improvement in PAPR, though at the expense of BER performance.

Enhancing spectral efficiency with low complexity filtered‐orthogonal frequency division multiplexing in visible light communication system

AbstractThe filtered‐orthogonal frequency division multiplexing (F‐OFDM) scheme has gained attention as a promising solution in the field of visible light communication (VLC) systems. One crucial aspect in VLC is the conversion of the complex F‐OFDM signal into a real signal that corresponds with direct detection and intensity modulation. Traditionally, achieving a real F‐OFDM signal has involved imposing Hermitian symmetry (HS) on the samples of the Inverse Fast Fourier transform (IFFT), which requires 2N‐point IFFT and obtains an N‐point FFT, thus adding complexity. In this study, a novel approach is presented and implemented, aiming to enhance spectral efficiency and reduce system complexity by generating a real F‐OFDM signal without relying on HS. This approach is then compared with HS‐free (HSF)‐OFDM, direct current biased optical OFDM, and asymmetrically clipped optical OFDM. The suggested method offers a remarkable improvement of ~50% in the required IFFT/FFT volume. Consequently, this method reduces hardware complexity and power usage compared with the traditional F‐OFDM method. Moreover, regarding error rates, the proposed method demonstrates better spectral efficiency than HSF‐OFDM.

Enhanced PAPR reduction in DCO-OFDM using multi-point constellations and DPSO optimization

DC-biased optical OFDM (DCO-OFDM) is a commonly used method of OFDM in visible light communication (VLC). Unfortunately, VLC systems that use OFDM often experience a high peak-to-average power ratio (PAPR). To address this issue, this study proposes a novel method called the multi-point constellation method (MPC) to reduce PAPR in DCO-OFDM. The MPC method involves adding extra alternative constellation points around the existing points and using the discrete particle swarm optimization (DPSO) algorithm to select the constellation points with the lowest PAPR. The proposed MPC method is also combined with selective mapping (SLM), a well-known PAPR reduction technique in the literature. Simulation results show that the proposed MPC method outperforms the SLM method in reducing PAPR in 4-QAM and 16-QAM modulations when used in combination with SLM. Furthermore, increasing the number of iterations and particles in the DPSO algorithm improves the PAPR reduction performance of the proposed method even further.

Computational Complexity Comparison between DC-biased Optical OFDM and Asymmetrically Clipped Optical OFDM in Visible Light Communication

Optical wireless systems are constrained to send real and positive values to the optical transmitter as only intensity of a signal is used to carry information. Therefore, conventional OFDM cannot be directly applied in optical systems. To combat multipath distortion, several modified OFDM systems have been studied, such as DC-biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM). In order to transmit real signal in optical environments, there is a Hermitian symmetric constraint with Discrete Fourier transform (DFT). This paper compares transceiver complexity of ACO-OFDM with that of DCO-OFDM which is used in Visible Light Communication (VLC), and it is found that for the same number of subcarriers, computational complexity is higher in ACO-OFDM.

On the performance of delta sigma modulators for DCO-OFDM based NOMA visible light communication systems

The revolution of the solid-state lighting technology and the looming radio frequency (RF) spectrum crisis enabled the rapid evolution of visible light communication (VLC) in the recent times. To furnish contemporaneous illumination and communication, VLC relies on white light emitting diodes (WLEDs). However, the limited modulation bandwidth of WLEDs poses a threat to VLC as the achievable data rates are drastically minimized. Eventually, to enhance the achievable throughput, the pre-eminent way is to apply orthogonal frequency division multiplexing (OFDM) to a VLC system. Furthermore, a VLC system can also exploit non-orthogonal multiple access (NOMA) scheme to bestow with seamless services to the multiple users. However, much similar to the RF-based OFDM system, the OFDM-VLC system also inherits one of the serious disadvantages like the high peak to average power ratio (PAPR). In addition, the rapid fluctuations of the continuous time-domain signal amplitude in DC-biased optical OFDM (DCO-OFDM) results in much complicated design of the LED driver circuitry for driving the LED. Thus, to overcome such drawbacks, we incorporate delta-sigma modulators (DSM) to the NOMA-VLC system which is making use of DCO-OFDM, and we analyze its performance over VLC channel environment. The stunning advantage imparted by the proposed DSM-based DCO-OFDM-NOMA-VLC system is that the continuous amplitude of the time-domain transmitted signal is converted into two levels for driving the LED. Additionally, with the major goal to maximize the sum throughput of the proposed multiuser VLC system by taking into consideration both the user fairness as well as the intensity constraints, we derived the optimal values for the power allocation coefficients corresponding to the fair power allocation algorithm. Furthermore, the dynamically varying fair power allocation algorithm is compared with the fixed/static power allocation algorithm. The major contribution of this work is of two-fold: firstly, the proposed DCO-OFDM-NOMA-VLC system which is making use of DSM exhibits a remarkable reduction in PAPR when compared with the conventional system without the application of DSM, where the proposed system demonstrates a significant gain of 4.78 dB in terms of PAPR reduction. Secondly, from the simulated results it can affirmed that the proposed system not only achieves enhanced sum rates and better outage performance but also imparts a better bit error ratio (BER) performance corresponding to the far user.

Enhanced Performance of Asymmetrically Clipped DC-Biased Optical OFDM Systems Using Adjacent Symbol Detection

Enhanced Performance of Asymmetrically Clipped DC-Biased Optical OFDM Systems Using Adjacent Symbol Detection

Reverse polarity optical Orthogonal frequency Division Multiplexing for High-Speed visible light communications system

The radio frequency spectrum, which has been steadily getting smaller, is under pressure from growing cellular data traffic. A more dependable communication channel is required. To setup the communication medium, VLC uses LED illumination. In order to increase network throughput, the goal of this study is to analyze and model signal conditioning techniques in the visible spectrum. VLC technology offers synchronized illumination of LED lamps as well as wireless broadband connectivity. Using the real-valued O-OFDM baseband signal, optical orthogonal frequency division multiplexing (O-OFDM) offers a viable modulation option for very-low-bit-rate (VLC) systems to modulate the optical carrier's instantaneous power to attain gigabit data rates. However, one important design difficulty preventing VLC from being commercialized is how to combine industry-favored pulse width modulation (PWM) dimming technology while retaining a dependable, broadband connection. In this paper, Reverse Polarity optical orthogonal frequency division multiplexing (RPO-OFDM) is proposed as a new signal format for combining the fast O-OFDM contact signal with the comparatively slow PWM dimming signal, as both signals contribute to the illuminative brightness of the LED. The average SNR, additional testing demonstrates that the suggested method outperforms the Asymmetrically clipped optical OFDM (ACO), Optimized Flipped Optical (OFO) and DC biased optical OFDM (DCO), and algorithms in similar operating conditions.

Layered hybrid PAM-DMT for IM/DD OWC systems.

Traditional pulse-amplitude-modulated discrete multitone modulation (PAM-DMT) suffers from poor overall performance of spectral and power efficiencies in optical wireless communication (OWC) systems. We propose layered hybrid PAM-DMT (LHPAM-DMT) to utilize more subcarriers to improve the performance. The real part of frequency domain signal is divided into several layers and symmetry biases are added in time domain to generate real-valued and nonnegative signals for intensity modulation with direct detection (IM/DD) OWC systems. By appropriately designing the orthogonality between the signals in lower layers and signals & added biases in higher layers, we further propose an iterative receiver to recover the transmitted information. Theoretical derivation proves that LHPAM-DMT has higher spectral efficiency than PAM-DMT and lower complexity than layered PAM-DMT. Numerical results suggest that LHPAM-DMT is more power efficient than PAM-DMT as well as direct-current (DC) biased optical OFDM (DCO-OFDM), one of the most popular schemes.

Vector Coding Optical Wireless Links

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

LiNEV: Visible Light Networking for Connected Vehicles

DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) has been introduced to visible light networking framework for connected vehicles (LiNEV) systems as a modulation and multiplexing scheme. This is to overcome the light-emitting diode (LED) bandwidth limitation, as well as to reduce the inter-symbol interference caused by the multipath road fading. Due to the implementation of the inverse fast Fourier transform, DC-OFDM suffers from its large peak-to-average power ratio (PAPR), which degrades the performance in LiNEV systems, as the LEDs used in the vehicles’ headlights have a limited optical power-current linear range. To tackle this issue, discrete Fourier transform spread-optical pulse amplitude modulation (DFTS-OPAM) has been proposed as an alternative modulation scheme for LiNEV systems instead of DCO-OFDM. In this paper, we investigate the system performance of both schemes considering the light-emitting diode linear dynamic range and LED 3 dB modulation bandwidth limitations. The simulation results indicate that DCO-OFDM has a 9 dB higher PAPR value compared with DFTS-OPAM. Additionally, it is demonstrated that DCO-OFDM requires an LED with a linear range that is twice the one required by DFTS-OPAM for the same high quadrature amplitude modulation (QAM) order. Furthermore, the findings illustrate that when the signal bandwidth of both schemes significantly exceeds the LED modulation bandwidth, DCO-OFDM outperforms DFTS-OPAM, as it requires a lower signal-to-noise ratio at a high QAM order.

Channel Characteristics and Link Adaption for Visible Light Communication in an Industrial Scenario.

Visible light communication (VLC) is one of the key technologies for the sixth generation (6G) to support the connection and throughput of the Industrial Internet of Things (IIoT). Furthermore, VLC channel modeling is the foundation for designing efficient and robust VLC systems. In this paper, the ray-tracing simulation method is adopted to investigate the VLC channel in IIoT scenarios. The main contributions of this paper are divided into three aspects. Firstly, based on the simulated data, large-scale fading and multipath-related characteristics, including the channel impulse response (CIR), optical path loss (OPL), delay spread (DS), and angular spread (AS), are analyzed and modeled through the distance-dependent and statistical distribution models. The modeling results indicate that the channel characteristics under the single transmitter (TX) are proportional to the propagation distance. It is also found that the degree of time domain and spatial domain dispersion is higher than that in the typical rooms (conference room and corridor). Secondly, the density of surrounding objects and the effects of user heights on these channel characteristics are also investigated. Through the analysis, it can be observed that the denser objects can contribute to the smaller OPL and the larger RMS DS under the single TX case. Furthermore, due to the blocking effect of surrounding objects, the larger OPL and the smaller RMS DS can be observed at the receiver with a low height. Thirdly, due to the distance dependence of the channel characteristics and large time-domain dispersion, the link adaption method is further proposed to optimize the multipath interference problem. This method combines a luminary adaptive selection and delay adaption technique. Then, the performance of the link adaption method is verified from four aspects through simulation, including the signal-to-noise (SNR), the RMS DS, the CIRs, and the bit-error rate (BER) of a direct-current-biased optical orthogonal frequency division multiplexing (DCO-OFDM) system. The verification results indicate that our proposed method has a significant optimization for multipath interference.

High-Speed Imaging Receiver Design for 6G Optical Wireless Communications: A Rate-FOV Trade-Off

The design of a compact high-speed and wide field of view (FOV) receiver is challenging due to the presence of two well-known trade-offs. The first one is the area-bandwidth trade-off of photodetectors (PDs) and the second one is the gain-FOV trade-off due to the use of optics. The combined effects of these two trade-offs imply that the achievable data rate of an imaging optical receiver is limited by its FOV, i.e., a rate-FOV trade-off. In this paper, we propose an imaging receiver design in the form of an array of (PD) arrays. To control the area-bandwidth trade-off, small PDs are used in an array of arrays structure instead of a single large PD. Moreover, to achieve a reasonable receiver FOV, we use an array of focusing lenses that focus the light individually on each inner PD array. The proposed array of arrays structure provides an effective method to control both gain-FOV trade-off (via an array of lenses) and area-bandwidth trade-off (via arrays of small PDs). We first derive a tractable analytical model for the signal-to-noise ratio (SNR) of an array of PDs that is equipped with a focusing lens assuming maximum ratio combining (MRC). Then, we extend the model to the proposed array of arrays structure and the accuracy of the analytical model is verified based on several Optic Studio-based simulations. Next, we formulate an optimization problem to maximize the achievable data rate of the imaging receiver subject to a minimum required FOV. The optimization problem is solved for two commonly used modulation techniques, namely, on-off keying (OOK) and direct current (DC) biased optical orthogonal frequency division multiplexing (DCO-OFDM) with variable rate quadrature amplitude modulation (QAM). Our results show the limits of high speed wide-FOV imaging receivers that can support mobility. For example, it is demonstrated that a data rate of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 24$ </tex-math></inline-formula> Gbps with a FOV of 15° is achievable using OOK with a total receiver size of 2 cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\!\times \!\,\,2$ </tex-math></inline-formula> cm.

Design of IM-DD Optical Communication using Matlab

A modulation scheme where the intensity of an optical source is modulated by the RF or mm-wave signal. Demodulation is achieved through direct detection of the optical carrier and conversion using a photo detector is called IM-DD Systems. In these systems the multi-layer modulation (MLM) is used to design both asymmetric clipped optical OFDM (ACO-OFDM) and dc-biased optical OFDM (DCO-OFDM) as a function. In this systems the DCO OFDM is more spectrally efficient than non-DC biased systems because this have better tradeoff between spectral and power efficiency. In the direct detection system, the detection process is non-linear due to the photo current proportional to absolute square of electric field intensity due to this the signal is slow. The equalizer is comprised of one hidden layer which leads to super-fast of the signal .The channel capacity for this systems can be measured by the transmitting of the signals. This IM/DD is a cost-effective optical communication which finds wide applications in fiber communication, free-space optical communication, and indoor visible light communication. In these systems when the bandwidth of signal exceeds the modulation bandwidth of LED, multi path effect occurs. Therefore, we use ACO-OFDM to avoid this multi path distortions. OFDM is a suitable efficient cost-effective solution for GPON deployment with benefit of use of low-cost laser bandwidth in comparison with DCO-OFDM. At the same BER performance, in comparison with D-C ACO-OFDM at high bit rates transmission, DCO-OFDM promises to deliver higher throughput.

Reducing peak to average power ratio (PAPR) for visible light communications (VLC) system using Walsh–Hadamard precoded discrete Hartley transforms

Abstract In visible light communication (VLC), intensity modulation with direct detection (IM/DD) based on DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) is used to transfer data at a high rate. Because OFDM signals are modulated using IM/DD, the peak of the transmitted signals is scaled up, resulting in a high peak-to-average power ratio (PAPR). In this article, the Walsh–Hadamard transform (WHT) is used as a precoder for discrete Hartley transform (DHT) to decrease the PAPR and the DC bias and achieve a good bit error rate (BER) performance in such a system. PAPR reduction results from the decrease of superposition of the transmitted signals owing to the use of WHT as a precoder. The proposed scheme and conventional DCO-OFDM indoor VLC system based on discrete Hartley transform (DHT) are compared to assure the new system’s benefits and efficiency.

Blind channel estimation for DCO-OFDM based vehicular visible light communication

Blind channel estimation (CE) methods for OFDM based RF communication provide high-rate transmission by eliminating pilot overhead in conventional pilot-based methods, at the cost of lower accuracy. However, there is no work on the blind CE for OFDM based visible light communication (VLC) systems. In this paper, we propose a novel blind CE method for vehicular VLC with the goal of improving CE accuracy based on the exploitation of the channel statistics derived, by utilizing extensive amount of data collected for different communication angles, distances, and ambient light conditions. First, the normalized channel frequency response (CFR) of the V2LC channel is demonstrated to be invariant of inter-vehicular distance, relative transmitter/receiver zenith angle and ambient light. Then, this channel characteristic is exploited in the blind CE to improve its accuracy with two-step estimation of normalization factor. Extensive simulations at different vehicle speeds show that the proposed method outperforms the pilot-based and superimposed training-based CE methods in terms of spectral efficiency both for all modulation schemes and at all relative speeds. The proposed blind channel estimation (CE) method provides 9.77% increase in the spectral efficiency, compared to the second best method, superimposed training-based CE, at 20 dB signal-to-noise ratio (SNR) and 160 km/h relative speed, for 64-Quadrature Amplitude Modulation (QAM) Direct Current-Biased Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM). Moreover, the real-time performance of the proposed blind CE is demonstrated for a realistic vehicle mobility scenario extracted from SUMO.

Trade-offs in the performance of a Free Space Optical communication system employing DCO-OFDM transmission with a PAPR-dependent DC-bias

Free Space Optical (FSO) communication technology has gained popularity because of its high bandwidth, which supports high-speed data transmission. Despite its merits over Radio Frequency technology, the FSO system performance is restricted by the combined impact of beam divergence, beam scintillation, and beam-wander effects triggered by atmospheric turbulence. A Gamma–Gamma probability density function (PDF) channel model is employed to model the irradiance fluctuations due to turbulence. In this paper, we evaluate the performance metrics of a FSO link employing the Direct Current-biased Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM) modulation technique for different turbulent conditions. In our paper, we determine the DC-bias, which is proportional to peak-to-average-power ratio (PAPR) and a preclipping factor of nclip (<1) in order to reduce the transmitted optical power of the DCO-OFDM signal. We evaluate the average bit error rate (BER) and outage probability of the FSO link by considering the impact of system design parameters related to the OFDM signal, laser beam, and atmospheric turbulence on the received signal. Numerical simulation results show that, depending on the chosen values of PAPR and preclipping, an FSO link can experience either a power saving or a power penalty, for meeting the BER requirement. It is further demonstrated that trade-offs in performance at various levels exist between data transmission rate (QAM size), system power efficiency (preclipping), signal-PAPR and link-span to ensure target signal quality (BER and Outage). Under turbulence, the link with a longer span not only incurs a power penalty but also suffers from a reduced operating range by 3–4 dB.

Measurement Based Non-Line-of-Sight Vehicular Visible Light Communication Channel Characterization

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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$10^{-3}$</tex-math></inline-formula> 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.

Impact of STO and STO-estimation in DST-DCO-OFDM-based visible light communication systems

This work addresses the timing synchronization aspects in a DC-biased optical OFDM (DCO-OFDM) system which is exploiting real trigonometric transform like discrete sine transform (DST). The striking notability is that without the necessity of Hermitian Symmetry criteria, the optical OFDM system takes the advantage of real signal processing transformation techniques like DST. Furthermore, this paper compares the performance of timing synchronization algorithms exploiting the periodicity property of the DST-DCO-OFDM symbol with the training symbol-based STO estimation algorithm. Among the STO estimation algorithms, the training symbol-based STO estimation technique imparts a better mean square error when compared with that of the periodicity-based STO estimation algorithms. Moreover, this work validates the derived Cramer Rao Lower Bound (CRLB) for the timing errors.

Spectral and Energy Efficiency of DCO-OFDM in Visible Light Communication Systems With Finite-Alphabet Inputs

The bound of the information transmission rate of direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) for visible light communication (VLC) with finite-alphabet inputs is yet unknown, where the corresponding spectral efficiency (SE) and energy efficiency (EE) stems out as the open research problems. In this paper, we derive the exact achievable rate of the DCO-OFDM system with finite-alphabet inputs for the first time. Furthermore, we investigate SE maximization problems of the DCO-OFDM system subject to both electrical and optical power constraints. By exploiting the relationship between the mutual information and the minimum mean-squared error, we propose a multi-level mercury-water-filling power allocation scheme to achieve the maximum SE. Moreover, the EE maximization problems of the DCO-OFDM system are studied, and the Dinkelbach-type power allocation scheme is developed for the maximum EE. Numerical results verify the effectiveness of the proposed theories and power allocation schemes.

Robust Frequency-Domain Timing Offset Estimation for DCO-OFDM Systems

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.