Dimming Compatible Optical OFDM for High Speed Visible Light Communications

In this letter, we present a novel hybrid intensity modulation and direct detection communication system, which integrates asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) and on-off keying (OOK) modulation schemes. First, the negative ACO-OFDM is proposed based on the conventional ACO-OFDM system to accommodate the on case with direct current in the OOK modulation, while the ACO-OFDM can match the off case in the OOK modulation. Therefore, the novel hybrid system combines the ACO-OFDM and OOK modulation schemes, while the signals can be recovered at the receiver to support the different qualities of service with high spectral efficiency and can be adapted to various receivers with different complexities. Simulation results are reported for the visible light channel and show that both the ACO-OFDM and OOK signals can be well recovered.

A new precoding scheme for spectral efficient optical OFDM systems

In this paper, fractional reverse polarity optical orthogonal frequency division multiplexing (FRPO-OFDM) is studied to enable dimming compatible visible light communications. The scheme combines a layered asymmetrically clipped optical OFDM (ACO-OFDM) sequence with an information-carrying brightness control sequence (BCS) in the form of $M$ -ary pulse position modulation. We derive the expressions of the FRPO-OFDM signal and its achievable brightness level, and develop an effective detector which can recover information from both sequences based on maximum likelihood detection. We show that when the detector is to be implemented, the use of multi-layer ACO-OFDM imposes strong periodicity on the BCS, which leads to a trade-off between spectral efficiency and brightness resolution for dimming control. It is shown that high spectral efficiency can be achieved with practical dimming requirements. Simulation results show that the extra information carried by the BCS can be decoded with extremely low bit error rate and thus has negligible impacts on the demodulation of the ACO-OFDM signal, when the system nonlinearity is not dominating.

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.

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

Adaptive least squares channel estimation for visible light communications based on tap detection

The limited modulation bandwidth and nonlinearity of light-emitting diodes (LEDs) are the important factors that constrain the high-speed transmission of visible light communication (VLC). As a LED lamp includes multiple LED chips and the superposition characteristics of light is approximately linear, a multi-LED optical orthogonal frequency division multiplexing (OFDM) system is proposed. The multiple LED chips are used to transmit multiple asymmetrically clipped optical OFDM (ACO-OFDM) and unipolar OFDM (U-OFDM) signals in parallel, while the signals can be recovered at the receiver. When adopting the same M-ary quadrature amplitude modulation (M-QAM), its spectral efficiency is higher than the conventional ACO-OFDM system. The power of the multi-LED transmission system is split equally to each branch, and the amplitude range of the signal is smaller, thus it has better anti-nonlinearity performance. At last, its advantages are verified by the simulation comparisons with other optical OFDM systems.

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.

• Core Innovation & Objective • Developed a novel, integrated system architecture combining a modified USRP X310 software-defined radio platform with a custom-designed high-power LED optical transceiver. • Achieved high-speed, low-latency, and highly reliable audio-video transmission over Visible Light Communication (VLC) for intelligent transportation and vehicle-to-vehicle (V2V) applications, particularly in electromagnetically sensitive environments. • Key Technical Breakthroughs • LED Bandwidth Extension: Designed an innovative active pre-equalization circuit (based on a 4th-order high-pass RC bypass and op-amps) that dramatically increased the 3dB modulation bandwidth of a commercial 15W LED from 3 MHz to 120 MHz. • High-Speed Modulation: Successfully implemented 64QAM-OFDM modulation/demodulation using GNU Radio on the USRP, enabling spectrally efficient data transmission. • Advanced Optical Design: Custom-designed optical lenses (transmitter and receiver) and a high-power Bias-T circuit for efficient AC/DC coupling, ensuring the LED operates in its linear region and maximizing optical power transfer. • Stable Link Performance: Incorporated an Automatic Gain Control (AGC) circuit at the receiver, stabilizing the output voltage between 1.2V - 2.0V and maintaining a modulation signal-to-noise ratio (SNR) > 23 dB over varying distances. • Outstanding System Performance • The experimentally demonstrated performance metrics are exceptional for a VLC system: • Peak Data Rate: 500 Mbps • Communication Distance: 0 - 110 meters • Bit Error Rate (BER): As low as 1.74 × 10⁻⁵ (far below the 1 × 10⁻⁴ target) • End-to-End Latency: < 0.1 s • Beam Angle: 11.5° (Transmitter) • Field of View (FOV): ≤ 18° (Receiver) • System Integration and Design • Hardware-Software Co-Design: Seamless integration of the USRP X310 (handling digital baseband processing) with the custom analog optical front-end. • Compact and Practical: The system is designed for external mounting on vehicles, offering a practical solution for real-world deployment. • Simulation-Guided Design: Used LightTools for optical design and Optisystem for communication link simulation to validate the design before implementation. • Significance and Application Value • Solves Critical Challenges: Provides a high-speed, secure, and EMI-free communication alternative to traditional RF in environments like military operations, petroleum facilities, and intelligent transportation systems. • 6G Potential: Serves as a practical reference for the development of VLC as a complementary technology in 6G networks. • Demonstrates Feasibility: Successfully proves the viability of using commercial LEDs and USRP platforms for building high-performance, medium-to-long-range optical communication systems. • This work represents a significant step forward in practical high-speed VLC system design, combining theoretical innovation with robust engineering to achieve state-of-the-art performance. Existing vehicular VLC systems suffer from limited bandwidth, short transmission distances, low data rates, and support only a narrow range of communication services. To address these issues, this paper proposes a vehicle-mounted high-speed LED visible light communication (VLC) system based on Universal Software Radio Peripheral (USRP). Leveraging 64QAM-OFDM digital modulation, an improved USRP X310 hardware platform, and a PC, we establish a novel architecture comprising digital baseband and optical transceiver modules. The baseband module eliminates traditional spectrum-shifting circuits and antennas, retaining only FPGA and ADC/DAC units. Audio/video files undergo 64QAM-OFDM modulation directly on the PC, with modulated signals output via DAC to a custom-designed wide-bandwidth LED transceiver. A broadband hardware equalization network based on a fourth-order RC bypass circuit was designed expands the 3dB bandwidth of high-power LEDs from 3 MHz to 120 MHz. Combined with automatic gain control (AGC), this achieves an optical transceiver with 12–15 W output power, >23 dB optical modulation signal-to-noise ratio (SNR), and stable receiver output voltage (1.2–2.0 V). The resulting compact system delivers low latency, high data rates, and high reliability. Under aligned optical path conditions, the system achieves >1 Mbit/s at 110 m (BER 0.49 × 10⁻⁵, delay 0.009-0.05s) and a peak rate of 500 Mbit/s at 10 m (BER 7.4 × 10⁻⁵), with an 11.5° transmit angle and a receiver field of view (FOV) of less than 18°. This work demonstrates the feasibility of high-power LED-based 64QAM-OFDM communication using USRP X310 for medium- to long-range vehicular links, providing a reference for intelligent transportation systems.

Free space optical communication (FSO) is widely deployed to transmit high data rates for rapid communication traffic increase. Asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) modulation is a very efficient FSO communication technique in terms of transmitted optical power. However, its performance is limited by atmospheric turbulence. When the channel includes strong turbulence or is non-deterministic, the bit error rate (BER) increases. To reach optimal performance, the ACO-OFDM decoder needs to know accurate channel state information (CSI). We propose novel detection using different deep learning (DL) algorithms. Our DL models are compared with minimum mean square error (MMSE) detection methods in different turbulent channels and improve performance especially for non-stationary and non-deterministic channels. Our models yield performance very close to that of the MMSE estimator when the channel is characterized by weak or strong turbulence and is stationary. However, when the channel is non-stationary and variable, our DL model succeeds in improving the performance of the system and decreasing the signal to noise ratio (SNR) by more than 8 dB compared to that of the MMSE estimator, and it succeeds in recovering the received data without needing to know accurate CSI. Our DL decoders also show notable speed and energy efficiency improvement.

The integration of sensing and communication (ISAC) based on optical fibers has been a key enabling technique to achieve high-speed communication while ubiquitous sensing. In this paper, to ensure the coexistence of communication signal and sensing signal in low-cost optical access networks i.e., intensity modulation/direct detection (IM/DD) based short-reach optical transmission systems, a common bias-digital subcarrier multiplexing (CB-DSCM) scheme is proposed for ISAC signal generation. To assure distributed fiber acoustic sensing (DAS) signal having high spatial resolution and longer detection distance, linear frequency modulation (LFM) pulse compression technique, biased at the null point of the modulator, is used for sensing signal generation at the transmitter, and coherent detection is employed at the receiver. Then, due to the bias voltage at null point, asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) signal is utilized to carry communication signal in the IM/DD system. Therefore, both signals can be simultaneously transmitted using the already deployed modulator, enabling integrated sensing and communication in short-reach optical transmission systems. Experimental results demonstrate that, the proposed CB-DSCM scheme can achieve a net bit rate of 3.01 Gb/s with a bit error rate (BER) as low as 6 × 10-4 over a 19.8 km fiber-optic transmission link, while it can successfully reconstruct various vibration signals with a spatial resolution of 1 m in short-reach optical transmission systems.

This letter proposes a novel receiver for asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) system in visible light communication to enhance its performance. In ACO-OFDM, the even subcarriers are distorted by the clipping noise and are simply discarded in the conventional receiver. In this letter we show that the signals on the even subcarriers can be viewed as data-dependent precoded OFDM. A novel receiver based on minimum mean square error criteria is then proposed to exploit the even subcarriers to collect additional diversity and coding gains, leading to improved bit error rate performance. Simulations show that the proposed receiver provides significant signal-to-noise ratio gain (up to 10 dB) over the conventional receiver, especially in non-line-of-sight channels. Moreover, the proposed receiver is also superior to some existing advanced ones.

In the recent times, Visible Light Communication (VLC) has evolved as a new communication standard due to its stunning notability to render ubiquitous connectivity by emerging as a prominent revolutionary to radio frequency (RF) based wireless communications. However, an orthogonal frequency division multiplexing (OFDM) based VLC system is susceptible to timing errors. Therefore, this work addresses the synchronization aspects in asymmetrically clipped optical OFDM (ACO-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, this optical OFDM system takes the advantages of real signal processing transformation technique like DST. Thus, this technique reinforces the transmission of double the data symbols in comparison to conventional optical OFDM system which is based on standard Fourier signal processing. Besides, this work derives the elaborate mathematical expressions illustrating the deleterious effects of unreliable timing errors in DST-based ACO-OFDM system. In addition, this paper interprets as well as compares the performance of timing synchronization algorithms exploiting the periodicity property of the optical OFDM symbol with the training symbol based STO estimation algorithm in DST-based ACO-OFDM system. Moreover, this work validates the derived Cramer Rao Lower Bound (CRLB) and the simulated results infer that derived CRLB pertaining to the timing errors contributes the minimum mean square error (MSE) when compared with other STO estimation algorithms.

Data transmission between the downhole and surface called the downhole telemetry system is a critical part of monitoring and extraction processes in the oil and gas industry. Numerous real-time data transmission techniques for downhole telemetry systems have been implemented, however, growing demands of higher bandwidth and data rate for downhole communications made it necessary to find a new, reliable, and low-cost communication system. In this area, visible light communications (VLC) with a huge available spectrum providing high bandwidth and data rate are indispensable opportunities for downhole telemetry systems. This paper addresses the issues related to the VLC usage in downhole gas pipeline telemetry/monitoring systems. To emulate real field conditions, a carbon steel coated and carbon dioxide-filled gas pipeline is created with a circulating pump and a pressure gauge, and the VLC setup is established. The performance of a VLC system employing an asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) modulation technique with 128-FFT and guard band is practically tested by taking channel estimation and synchronization into account. Moreover, the channel impulse response of the carbon dioxide-filled gas pipeline is presented along with a proposed path-loss model. The BER against SNR performance of the proposed downhole telemetry system is investigated for carbon dioxide gas at atmospheric pressure. Furthermore, the effect of the circulating pump and different colors of led on system performance is presented. Measurement results of the free-space air medium are also included as a benchmark.

Iterative receiver for ADO-OFDM with near-optimal optical power allocation