OFDM-based Visible Light Communication Systems Research Articles

Joint PAPR reduction with channel estimation for OFDM-based VLC systems

Abstract This paper presents a new combination of channel estimation (CE) methods and peak-to-average power ratio (PAPR) reduction techniques for orthogonal frequency division multiplexing (OFDM)-based visible light communication (VLC) systems. We first propose iterative clipping and filtering to minimize the PAPR of optical OFDM signals. Then, we introduce a computationally efficient, high-performance channel estimation approach using block-type pilots with modified clipped signals. The proposed method integrates clipping with three distinct CE techniques, resulting in both significant PAPR reduction and enhanced bit error rate (BER) performance. Among the CE methods evaluated, the expectation-maximization (EM) technique provides better BER results compared to other approaches. Simulation results confirm that the combined use of clipping and advanced CE methods leads to notable improvements in PAPR reduction and BER performance for OFDM-based VLC systems.

Analyze the impact of nonlinear distortions on OFDM-based visible light communication systems

Visible light communication (VLC) has gained attraction for its use in high-speed wireless connectivity leveraging LED lighting elements. Orthogonal Frequency Division Multiplexing (OFDM) is an attractive modulation scheme due to its spectral efficiency and resilience to multipath distortion. However, the nonlinear electro-optic transfer characteristics of optical components introduce signal clipping and quantization noise which corrupt OFDM signals. This paper provides an in-depth analysis of clipping and quantization noise to quantify the impact of LED nonlinearities on OFDM-based VLC system performance. Detailed mathematical models are derived for clipping distortions caused by LED optical power saturation and quantization errors from ADC/DAC finite precision in the modulator and driver circuitry. This analysis is quantified through simulations of the degradations in terms of error vector magnitude (EVM), signal-to-noise ratio (SNR) loss, and bit error rate (BER) under varying clipping ratios and quantization bit-depths. The 16-QAM OFDM transmission shows that the LED driver should possess at least 12 dB signal linear dynamic range and 3-bit quantization to restrict SNR penalty within 3 dB. Adaptive tone mapping and digital pre-distortion techniques are examined to compensate for intensity distortions enabling high-speed OFDM transmission over VLC links.

Increasing the power and spectral efficiencies of an OFDM-based VLC system through multi-objective optimization.

The nonlinearity induced by light-emitting diodes in visible light communication (VLC) systems presents a challenge to the parametrization of orthogonal frequency division multiplexing (OFDM). The goal of the multi-objective optimization problem presented in this study is to maximize the transmitted power (superimposed LED bias-current and signal amplification) for both conventional and constant envelope (CE) OFDM while also maximizing spectral efficiency. The bit error rate (BER) metric is used to evaluate the optimization using the non-dominated sorting genetic algorithm II. Simulation results show that for a BER of 1×10-3, the signal-to-noise ratio (SNR) required decreases with the guard band due to intermodulation distortions. In contrast to SNR values of approximately 13 and 25dB achieved by traditional OFDM-based systems, the VLC system with CE signals achieves a guard band of 6% of the signal bandwidth with required SNR values of approximately 10.8 and 24dB for 4-quadrature amplitude modulation (QAM) and 16-QAM modulation orders, respectively.

A reduced PAPR hybrid OFDM visible light communication system

Orthogonal frequency division multiplexing (OFDM) is used by visible light communication (VLC) systems. Its main goal is to lessen inter-symbol interference's negative impacts. As a result, a lot of bandwidth and a high data rate are transmitted. Using OFDM-based VLC systems, nonetheless, have a high peak-to-average power ratio (PAPR). This research presents a new VLC system for indoor applications that reduces the PAPR while maintaining an acceptable bit error rate (BER). To achieve this, a hybrid pre-coding technique-based PAPR reduction solution is proposed; Vander-monde like matrix (VLM) combined with Gaussian matrix (GM). This technique takes advantage of both VLM and GM as a two stages for the signal to simultaneously reduce the PAPR and save the BER from damage. According to the results, the proposed solution (VLM in conjunction with GM) lowers the PAPR to 9.5 dB. Our system performs better when compared to other systems like discrete cosine transform (DCT), GM, VLM, DCT&GM, and original OFDM by 27%, 25%, 11%, 20%, and 34%, respectively.

Throughput improvement and PAPR reduction for OFDM-based VLC systems using an integrated STC-IMADJS technique

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.

Performance analysis of novel precoding matrix techniques for optical OFDM-based visible light communication systems

Visible Light Communication (VLC) exploits the renowned modulation scheme like orthogonal frequency division multiplexing (OFDM) to accomplish high data rate transmission. However, the optical OFDM system like DC biased optical OFDM (DCO-OFDM) is vulnerable to high peak to average power ratio (PAPR) which results in detrimental clipping distortion which in turn degrades the overall system performance. The limited dynamic range of the LEDs further agitates this issue. On these grounds, PAPR reduction techniques like precoding matrix schemes have been proposed for reducing the high amount of PAPR in DCO-OFDM system. In this paper, discrete Hartley Matrix transform (DHMT) precoded discrete Hartley transform (DHT)-based DCO-OFDM system is proposed for reducing PAPR. In addition, we comparatively investigate the PAPR performance of a new precoding technique like T- transform which is a hybrid combination of Walsh Hadamard transform (WHT) and Zadoff chu transform (ZCT) with other precoding matrix techniques like discrete cosine matrix transform (DCMT), discrete Fourier matrix transform (DFMT), Walsh Hadamard Matrix transform (WHMT) and DHMT. Furthermore, this paper derives the mathematical analysis corresponding to the time-domain signal formats for the precoded DCO-OFDM system which is exploiting complex Fourier signal processing and real trigonometric transform like DHT. From the simulated results, it can be surmised that when compared with other precoding techniques, DHMT precoded DHT-based DCO-OFDM system not only achieves a significant reduction in PAPR but also achieves a better bit error rate (BER) and spectral efficiency performance. Furthermore, less computational complexity is required to implement it.

BER performance analysis of an OFDM-based visible light communication system using DHT post-coding

BER performance analysis of an OFDM-based visible light communication system using DHT post-coding

Digital Pre-Equalization for OFDM-Based VLC Systems: Centralized or Distributed?

We propose and experimentally demonstrate a distributed digital pre-equalization (DPE) technique for orthogonal frequency division multiplexing (OFDM)-based bandlimited visible light communication (VLC) systems. In the VLC system applying distributed DPE, the subcarriers are divided into two bands, where the bandwidth and the power of each band can be flexibly adjusted to maximize the achievable data rate of the system. Hence, distributed DPE exhibits much higher tolerance against light-emitting diode (LED) nonlinearity than conventional centralized DPE. Experimental results verify the superiority of distributed DPE for OFDM-based bandlimited VLC systems. More specifically, a data rate of 976.6 Mbit/s is achieved by using distributed DPE, which corresponds to an achievable rate improvement of 25% in comparison to centralized DPE.

A novel timing synchronization method for OFDM based VLC systems

With an aim to render high data rates and to overcome spectrum crisis, Visible Light Communication (VLC) is explored in which the transmitted signal is constrained to be unipolar. Timing synchronization is one of the critical issue that deteriorates the performance of VLC systems in real time. In this work, traditional synchronization methods are modified according to a realistic VLC channel for Optical Orthogonal Frequency Division Multiplexing (OOFDM) based systems. Further, a new training symbol is designed and a novel frame synchronization method is proposed to improve the estimation accuracy of timing offset. The new training symbol is designed by employing symmetry on a repetitive symbol pattern in frequency domain. Simulation results proves that the proposed frame detection is giving accurate estimate of the symbol timing compared to conventional methods, in the channel having both Line of Sight (LoS) and Non-Line of Sight (NLoS) components under DC bias with clipping distortion.

PAPR reduction by combining clipping and piecewise linear companding for OFDM-based VLC systems

A scheme called clipping-piecewise linear companding (C-PLC) was proposed in this paper to address the problem of high clipping noise and peak-to-average power ratio (PAPR) in optical orthogonal frequency division multiplexing (O-OFDM) signal for visible light communication (VLC) systems. The clipping information of the signal could be retained by adding a suffix at its end, which could reduce the clipping noise and PAPR of the VLC system, thus limiting the dynamic range of the signal effectively. In addition, the retained clipping information could be recovered at the receiver, with which the bit error rate (BER) performance could be pronouncedly improved. Furthermore, PLC could map bipolar signals into unipolar signals suitable for VLC and effectively reduce the PAPR of the signals. Simulation results showed that C-PLC system has lower PAPR than that of direct-current biased optical-OFDM and μ-OFDM schemes, and it also demonstrates an improved BER performance. This work will be of good help for the research and development of VLC system.

Combined Envelope Scaling with Modified SLM Method for PAPR Reduction in OFDM-Based VLC Systems

Abstract Orthogonal frequency-division multiplexing technique (OFDM) has been adopted widely as a modulation technique for radio frequency (RF) and optical communication systems such as visible-light communication (VLC) due to its high spectral efficiency and low-complexity implementation. VLC-OFDM is recommended in 5 G mobile communication. However, VLC-OFDM suffer from the high peak to-average power ratio (PAPR). In this paper, a modified selective mapping (MSLM) method is applied to the proposed system followed by a new envelope scaling process for further reductions in PAPR of VLC-OFDM system. Simulation results show that the proposed method reduces the PAPR by about 6.8 and 1.7 dB comparing with the original signal and the traditional SLM with a number of rotation vector U = 8, respectively.

PAPR Reduction of OFDM Signal Through DFT Precoding and GMSK Pulse Shaping in Indoor VLC

Optical orthogonal-frequency-division-multiplexing (O-OFDM) has been widely explored for visible light communication (VLC) to achieve high data rates. However, O-OFDM suffers from high peak-to-average power ratio (PAPR), which causes clipping distortion, reduces illumination to communication conversion efficiency and affects the lifetime of the light-emitting diode (LED). This paper proposes a novel modulation technique, an amalgamation of discrete Fourier transform (DFT) precoding and Gaussian minimum shift keying (GMSK) pulse shaping to reduce the PAPR of OFDM based VLC system. Furthermore, the concept of group precoding is also introduced to deal with the increased complexity of the DFT precoding system. The results show that different variants of the proposed scheme provide improved PAPR, symbol-error-rate (SER), and power-saving performance as compared to the corresponding DFT precoded O-OFDM and O-OFDM counterpart. Moreover, the analytical study of PAPR, computational complexity and spectral efficiency for the proposed system have also been investigated and reported in this paper.

Enhanced performance of indoor VLC using anti-periodic asymmetrically clipped OFDM and multiple LEDS

Visible light communication (VLC) systems make use of the available LED lamps to realize a high spatial density of high-speed optical links superposed to their natural illumination function. Orthogonal frequency division multiplexing (OFDM) has been adopted for these intensity modulation/direct-detection optical channels. However, it suffers from low power efficiency or low spectral efficiency. In this paper, we aim to improve the performance of OFDM-based indoor VLC systems. By utilizing the principle of spatial summing, we propose an anti-periodic asymmetrically clipped OFDM scheme to make efficient use of both power and spectrum. In this scheme, the OFDM subcarriers are partitioned into multiple specific groups to form independent unipolar signals. These signals are simultaneously transmitted from different LEDs that are within the same light fixture and are summed in space before being detected by a conventional OFDM receiver. The computer simulation results show that the proposed approach can achieve lower PAPR and more robustness to the clipping distortion caused by the nonlinearities of LED leading to better bit error rate performance for the same spectral efficiency than that of the conventional DCO-OFDM and ACO-OFDM schemes do under both LOS and DOW channels.

Image transmission in an OFDM VLC system using symbol scrambling and chaotic Walsh-Hadamard precoding

An image secure transmission scheme combining symbol scrambling and chaotic Wahsh-Hardmard precoding is proposed in orthogonal frequency division multiplexing (OFDM)-based visible light communication (VLC) system. In the proposed scheme, a two-dimensional (2D) logistic-sine-coupling map (LSCM) is employed to generate two chaotic sequences. One is used to scramble the 16 quadrature amplitude modulation (QAM) symbols, and the other is used to scramble the rows of standard Walsh-Hadamard transform (WHT). The simulation results show that the proposed scheme can not only reduce the peak-average-power ratio (PAPR) but also achieve the image secure transmission over OFDM-based VLC systems.

Secure Image Transmission in Orthogonal Frequency Division Multiplexing Visible Light Communication Systems

In this paper, a novel physical layer security based on two-level encryption is proposed for orthogonal frequency division multiplexing (OFDM) visible light communication (VLC) systems. In the two-level encryption scheme, the bit signal, which is generated from image data, is first scrambled by a chaotic sequence. After the encrypted bit signal is mapped into 16 QAM symbols, the real and imaginary components of the resulting symbols are encrypted again by two separate chaotic sequences, respectively. A mathematical model is established to securely transmit images in OFDM-based VLC systems based on encryption. Based on the established mathematic model, the bit error rate (BER) formula to securely transmit OFDM-VLC is derived over the dispersive visible light communication (VLC) channel. A theoretical analysis shows that the encryption does not degrade the BER performance of the systems. Additionally, the validity of the BER formula obtained is verified by computer simulation. Theoretical and simulation results both show that the encryption operation has no effect on the BER of the proposed system. However, encryption is helpful for reducing the peak-to-average power ratio (PAPR) of the image-based OFDM. The simulation results show that the security of the image-based OFDM is enhanced and the PAPR is reduced.

OFDM-based visible light communication with rotated polarity modulation aided complex color shift keying

High-rate techniques, such as optical orthogonal frequency division multiplexing (OFDM) and color shift keying (CSK), have been proposed for visible light communication (VLC). To fully exploit their advantages, in this Letter, we design a modulation scheme called rotated polarity modulation (RPM) aided complex CSK (CCSK) for OFDM-based VLC systems and derive its theoretical bit error rate and an optimal scaling factor. Analytical and simulation results show that in comparison to the existing schemes, the new RPM-CCSK-OFDM system offers an improved link performance and data rate under a modest complexity. It can also be applied to VLC systems equipped with different types of LED devices, thus enabling flexible deployments.

Encrypted image transmission in OFDM-based VLC systems using symbol scrambling and chaotic DFT precoding

Abstract In this paper, a joint symbol-scrambling and chaotic discrete Fourier transform (DFT) precoding scheme is proposed for orthogonal frequency-division multiplexing (OFDM) visible light communication (VLC) systems for encrypted image data. In the proposed scheme, the original image data is first divided into sub-blocks and then an inter-block scrambling operation is applied to the resulting image data. Secondly, the real and imaginary parts of the transmitted QAM symbols are encrypted using two separate chaotic sequences generated from different chaotic maps. Thirdly, the resulting encrypted QAM signals are transformed into new signals using a chaotic DFT-precoding matrix in order to further improve the security and bit error rate (BER) performance of the OFDM signals. The mathematical model of our proposed encrypted OFDM-based VLC system is established and the BER formula is derived. The obtained BER formula is confirmed by our simulation results. Our simulation results also show that both symbol scrambling and image-block scrambling have no effect on the BER of the proposed DFT-precoded OFDM-based VLC system. The proposed scheme not only improves the quality of the transmitted image but also protects the transmitted image data from eavesdroppers.

On the study of the relation between linear/nonlinear PAPR reduction and transmission performance for OFDM-based VLC systems.

The relation between the peak-to-average-power-ratio (PAPR) reduction schemes and the transmission performance of the orthogonal frequency division multiplexing (OFDM) based visible light communication (VLC) system is experimentally investigated. The linear selective mapping (SLM) scheme and the nonlinear logarithmic companding scheme are optimized by considering both PAPR reduction and bit error rate (BER) performance. It is demonstrated that the logarithmic companding scheme, albeit providing larger PAPR reduction when compared to the SLM scheme, may result in worse BER performance due to the additional noise induced in its expanding process. Both numerical and experimental investigations show that, at the expense of increased complexity and reduced efficiency, the VLC system using the linear SLM scheme exhibits better BER performance compared to the system using the nonlinear logarithmic companding scheme. We show that for a 400-Mb/s transmission at a distance of 1 m, the BER can be reduced from 2.02 × 10-3 to 2.51 × 10-4 by using logarithmic companding scheme. By using the linear SLM scheme, the BER can be further reduced to 1.77 × 10-6.

A new precoding scheme for spectral efficient optical OFDM systems

Achieving high spectral efficiency is the key requirement of 5G and optical wireless communication systems and has recently attracted much attention, aiming to satisfy the ever increasing demand for high data rates in communications systems. In this paper, we propose a new precoding/decoding algorithm for spectral efficient optical orthogonal frequency division multiplexing (OFDM) scheme based visible light communication (VLC) systems. The proposed coded modulated optical (CMO) based OFDM system can be applied for both single input single output (SISO) and multiple input multiple-output (MIMO) architectures. Firstly, the real OFDM time domain signal is obtained through invoking the precoding/decoding algorithm without the Hermitian symmetry. After that, the positive signal is achieved either by adding a DC-bias or by using the spatial multiplexing technique. The proposed CMO-OFDM scheme efficiently improves the spectral efficiency of the VLC system as it does not require the Hermitian symmetry constraint to yield real signals. A comparison of the performance improvement of the proposed scheme with other OFDM approaches is also presented in this work. Simulation results show that the proposed CMO-OFDM scheme can not only enhance the spectral efficiency of OFDM-based VLC systems but also improve bit error rate (BER) performance compared with other optical OFDM schemes.

An Overview of OFDM-Based Visible Light Communication Systems From the Perspective of Energy Efficiency Versus Spectral Efficiency

In this paper, the energy efficiency and the spectral efficiency for orthogonal frequency division multiplexing (OFDM)-based visible light communication schemes are studied, which is crucial for practical application with limited energy resources. The conventional schemes, including asymmetrically clipped optical OFDM (ACO-OFDM), pulse-amplitude-modulated discrete multitone, and direct current biased optical OFDM, are compared in terms of energy efficiency and spectral efficiency relationship. The influence of power allocation for asymmetrically clipped dc biased optical OFDM and hybrid ACO-OFDM is also investigated. The energy efficiency and spectral efficiency of layered ACO-OFDM with a variable layer number are calculated and their relationship is also formulated. These conventional and hybrid modulation schemes are analyzed and compared through computer simulations, which should be considered in practice according to the requirements of illumination and transmission.