Commercially available Light Emitting Diode (LED) luminaries requires both high power efficiency and low complexity from Visible Light Communication (VLC) system deployments. Asymmetrically-clipped Direct-current (DC) biased Optical-OFDM (ADO-OFDM) is a recently emerged waveform in VLC systems to achieve the optimal solution for spectral and power efficiency problems. However, in ADO-OFDM, a high DC bias is used to attain uni-polar signals which decrease the power efficiency. In this work, we conceive a novel modulation technique called Variably-biased Asymmetrically-clipped Optical OFDM (VAO-OFDM) with pre-distortion for VLC systems. In VAO-OFDM, we design a variably bias which depends on specific samples. Variably bias does not enforce any interference on the originally transmitted symbols because of its frequency components falling on the pre-defined sub-carriers. Due to the pre-distortion process, the computational-complexity of proposed VAO-OFDM is decreased considerably over the ADO-OFDM. Further, the ADO-OFDM and VAO-OFDM performance is evaluated in terms of Bit Error Rate (BER) and power efficiency using MATLAB simulations and validated on the experimental setup by the LabView software-based VLC testbed. Simulation results show that the VAO-OFDM gains SNR of 10 dB at a BER of 10(-3 )over the ADO-OFDM, and experimental results show that the BER performance of ADO-OFDM and VAO-OFDM is obtained as 1.02x10(-3) and 5.18x10(-5) at a signal bandwidth of 1 MHz. Therefore, the VAO-OFDM performance is enhanced substantially when compare to ADO-OFDM.

Layered asymmetrically-clipped optical orthogonal frequency division multiplexing (LACO-OFDM) is a modulation technique for visible light communication (VLC) that provides power and spectral efficiencies. The LACO-OFDM technique utilizes all the available subcarriers and will have a relatively higher peak-to-average power ratio (PAPR). Hence, the modulated samples are prone to distortions caused by the non-linear devices and diffused channels, resulting in reduced accuracy of delay offset estimation for timing synchronization. In this paper, we propose a method to improve the timing synchronization accuracy of LACO-OFDM modulated systems using extreme learning machines (ELM). The ELM is trained offline to identify the delay offset using the timing metric (TM) as a feature identification problem. Among all types of neural networks that are used as classifiers and non-linear estimators, the ELMs are the least complex and require significantly less computational capacity. The simulations are performed to evaluate the accuracy of delay offset estimation in the presence of unknown non-linear effects of high-powered light-emitting diodes (LEDs) and diffused channel characteristics. The results show that the probability of error (PoE) of delay offset estimation and bit error rate (BER) is significantly reduced for scenarios like different frame lengths, channel coefficients, and unknown LED parameters in contrast to conventional methods. The proposed approach attains a PoE of 10−1 at signal-to-noise ratios (SNR)s of 4 dB and 6 dB with various TM, surpassing conventional methods which only achieve this beyond 15 dB. Moreover, the PoE remains below 10−2 for SNRs exceeding 7 dB and 10 dB in the proposed method, while conventional methods require SNRs above 25 dB to achieve comparable results.

Asymmetrically-clipped optical orthogonal frequency division multiplexing (ACO-OFDM) is the optimal power-efficient modulation technique for the intensity-modulated (IM) and the direct-detection (DD) based visible light communication (VLC) systems. In ACO-OFDM, the non-linear distortions of light-emitting diodes (LEDs) and diffused channels will affect the estimation of delay offset from the received data frame. These distortions corrupt the timing metric (TM) by generating several peaks around the position of the actual delay, thus affecting the delay offset estimation. In this paper, we propose a timing synchronization method for ACO-OFDM using an extreme learning machine (ELM) based delay offset estimator by considering the identification of delay offset position as a feature extraction problem. Then, the simulations are carried out considering the real-time effects of high-power LED distortions and diffused channel characteristics. The simulation results show a reduction in the probability of error (PoE) of delay offset estimation and bit error rate (BER) for the received signal of different frame lengths, diffused channels, and randomly varied LED non-linear conditions compared to the conventional methods.

Visible Light Communication (VLC) has emerged as a promising alternative for indoor and vehicular wireless communication, offering several advantages over traditional radio frequency (RF) technology. With the adoption of optical-orthogonal frequency division multiplexing (O-OFDM) schemes, visible light communication (VLC) has become more robust and adaptable in indoor, outdoor, vehicular, and underwear communications. Recently, an orthogonal time frequency space (OTFS) modulation technique has evolved with better performance than OFDM. The recent finding in the context of VLC shows that the OTFS technique shows remarkable advantages over conventional OFDM techniques except for the modem design complexity. This work introduces a low-complexity direct current-biased optical OTFS (DCO-OTFS) modulation based on OFDM. This paper evaluates the proposed system’s performance through simulations, providing evidence of its bit-error-rate (BER), peak-to-average power ratio (PAPR), and complexity behavior. Comparative assessments against the DCO-OFDM system are presented to understand the advantages of the low-complex DCO-OTFS system. The findings reveal that the proposed system not only provides low computational complexity in modem design but also maintains superior error performance, with a notable 10 dB signal-to-noise ratio (SNR) gain over DCO-OFDM along with a superior PAPR, making it a commendable choice for VLC applications.

We have designed an OTFS system with an inherent OFDM modulation accomplished by applying the weighted fractional Fourier transform (WFrFT), that employs the fast Fourier transform (FFT) algorithm, unlike DFrFT, to bring down the complexity. The system’s bit-error rate (BER) is comparable to the DFrFT-OFDM-based OTFS system, having a dB gain over the standard OTFS system with the same complexity and PAPR

Asymmetrically clipped direct current (DC) biased optical orthogonal frequency division multiplexing (ADO-OFDM) is a recently emerged waveform in the visible light communication (VLC) to obtain the optimal solution for the spectral and power efficiency problems. However, in ADO-OFDM, high DC-bias is used to attain unipolar signals, which decreases the power efficiency. In this work, we conceive a novel modulation technique called variably biased asymmetrically clipped optical orthogonal frequency division multiplexing (VAO-OFDM) for VLC systems. In which the bias varies with respect to the amplitude of the specific samples, which results in power efficiency. Further, the performance of the VAO-OFDM system is evaluated using LabView software-based simulations and validated on the experimental setup by the VLC testbed using USRP hardware.

Orthogonal Time Frequency Space modulation (OTFS) has evolved as an astounding modulation technique for high-speed communication in a doubly dispersive channel. In any wireless communication system, channel estimation and equalization are essential at the receiver to recover the transmitted data. To accomplish this for the emerging OTFS based systems, a modified embedded pilot-based channel estimation technique and low complexity feedback equalization algorithm for integer Doppler shifts in the delay-Doppler domain are proposed in this paper. Our channel estimation scheme exploits embedded-pilot arrangement, and the symbol equalization relies on the Interference calculation and its mitigation iteratively. To achieve this we contemplate a prudent arrangement of symbols in the OTFS frame in such a way that the Guard symbols prevent the interference between data symbols and the pilot symbol at the receiver. Two distinct lumps of received data of the same OTFS frame will be engaged in channel estimation and data detection. An analytical expression of the theoretical Cramer Rao Lower Bound (CRLB) is derived and plotted for the proposed channel estimation scheme. The attained simulation results for Bit-Error-Rate (BER) under the proposed scheme show a significant error rate improvement over the Minimum Mean Squared Error (MMSE) equalization algorithm. Further, a lower computational complexity is also achieved in comparison with modified MMSE detection and MP detection algorithms.

Orthogonal Time Frequency Space modulation (OTFS) is a promising modulation technique expected to counter the severe Doppler effects encountered in a doubly dispersive channel. OTFS scheme is developed on the basis of Orthogonal Frequency Division Multiplexing (OFDM) systems to support communication between high-speed vehicles, whose superiority lies in the domain of the equivalent channel. OTFS converts the Time-Frequency (TF) domain channel in the OFDM system to Delay-Doppler (DD) domain channel that becomes a handy tool to overcome the difficulties faced in a frequency selective channel. This letter presents an OTFS system design that is developed on the Discrete Fractional Fourier Transform (DFrFT) based OFDM system, which is designed to perform better compared to the conventional OTFS system with the same design complexity. The simulation results evidentially show 1 dB gain in power at a Bit-Error-Rate (BER) of 10−5 and a significant 3 dB decrement in the PAPR when a high power pilot with a 25 dB is inserted in the OTFS data frame.