Piecewise Companding Transform Assisted Optical-OFDM Systems for Indoor Visible Light Communications

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.

Visible light communication (VLC) is now used to achieve high data rates. Opticalorthogonal frequency division multiplexing (O-OFDM) is a strong technique for intensity modulation and direct detection (IM-DD). Direct current biased O-OFDM (DCO-OFDM) and asymmetrically clipped O-OFDM are two modern O-OFDM systems that operate with IM-DD constraints (ACO-OFDM). Due to severe out of band interference, the DCO-OFDM and ACO-OFDM systems have a high peak to average power ratio (PAPR) and substantial adjacent channel interference (ACI) (OOB). Furthermore, as compared to bipolar OFDM systems, they obtain a greater chance of bit error. The combination of the STC scheme, symbol extension, and pulse shaping signal is presented in this paper for improving the performance of the O-OFDM system without affecting system throughput. This article also decreases the OOB, PAPR, and bit error rate (BER) of DCO-OFDM and ACO-OFDM systems. Further analytical analyses for the proposed systems, such as OOB, PAPR, and computing complexity, are obtained in this paper. The simulation results verify the analytical study of the proposed methods for OOB reduction and PAPR reduction. As compared to typical O-OFDM systems, the proposed shaped STC-DCO-OFDM scheme and the proposed shaped STC-ACO-OFDM scheme lower the OOB by about 40 dB and the PAPR by about 4 dB at the 10 -3 probability point of the complementary cumulative distribution function (CCDF). The proposed systems outperform existing O-OFDM systems under AWGN for the BPSK modulation scheme in terms of BER, and reduce the computational complexity.

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.

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.

Light fidelity (Li-Fi) systems based on the orthogonal frequency division multiplexing (OFDM) scheme have gained more attention in the communications circles, as a means to provide high spectral efficiency and ensure stricter data rate requirements for visible light communication (VLC) systems. However, common OFDM schemes employ Hermitian symmetry to obtain a real-valued signal which is necessary in intensity modulation and direct detection (IM/DD) optical systems, at the expense of doubling the required bandwidth. In this paper, a novel transmission scheme for an OFDM-based Li-Fi system is proposed to tackle the issue in question. A new approach to complex signal mapping (CSM), based on the pairing function method, has been developed for Li-Fi systems. It does not require Hermitian symmetry and, hence, saves about 50% of the required bandwidth. Unlike existing OFDM-based VLC approaches, the proposed scheme employs CSM to ensure a real and positive signal without Hermitian symmetry in order to fully utilize the bandwidth available to Li-Fi networks. Simulation results show that the proposed scheme significantly outperforms other systems in terms of spectral efficiency. The CSM-OFDM based Li-Fi system also achieves a good peak-to-average power ratio (PAPR) reduction with acceptable bit-error-rate (BER) performance, compared to conventional approaches.

In this paper, the performance of a 60 GHz radio over fiber (RoF) system with 4/16/64 quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) downstream signals is studied. Delivery of 10 Gbit/s M-ary QAM (MQAM) OFDM signals through the 20-km-long single-mode fiber (SMF) is complicated in terms of intensity modulation and direct detection (IM/DD). Using self-homodyne method, the beating of two independent light waves generating the millimeter-wave at the photodetector can be down-converted to baseband in the electrical domain. Meanwhile, three kinds of sub-carrier arrangement schemes are compared and discussed, and the simulation results show that lower peak-to-average power ratio (PAPR) can be obtained adopting the adjacent scheme. At bit error rate (BER) of 10-3, the receiver sensitivity using 4QAM-OFDM sub-carrier signal is almost enhanced by 4 dB and 9 dB compared with those of 16QAM-OFDM signal and 64QAM-OFDM signal.

PAPR reduction of asymmetrically clipped optical OFDM signals: Optimizing PTS technique using improved flower pollination algorithm

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

Detailed investigations of dynamic negative power penalty characteristics of optical orthogonal frequency division multiplexing (OOFDM) signal transmissions are undertaken, for the first time, in directly modulated distributed feedback (DFB) laser (DML)-based intensity modulation and direct detection (IMDD) systems incorporating negative dispersion fibers such as MetroCor fibers. Excellent agreements between numerical simulations and real-time experimental measurements are obtained over a wide diversity of transmission conditions of the aforementioned systems. The physical mechanism underpinning the occurrence of negative power penalties is the reduction in subcarrier intermixing impairment due to the compensation between DML positive frequency chirps and MetroCor negative chromatic dispersions. It is also shown that the negative power penalty is independent of both cyclic prefix and signal modulation format and, more importantly, controllable when adaptive modulation and/or appropriate adjustments of DML operating conditions are applied.

This paper analyzes the performance of indoor orthogonal frequency division multiplexing (OFDM) optical wireless communication systems in the presence of light emitting diode (LED) nonlinear distortions. There are several forms of optical OFDM using intensity modulation [7th Int. Symp. on Communication Systems Networks and Digital Signal Processing (CSNDSP), 2010, pp. 566-570]. In this paper, DC-biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM) are considered. ACO-OFDM produces a half-wave symmetry time signal at the output of the OFDM modulator by special assignment of subcarriers, thus allowing signal clipping at the zero level and avoiding the need for DC bias at the expense of data rate reduction. DCO-OFDM assigns data to all possible subcarriers to increase the data rate. However, half-wave symmetry signals cannot be achieved and a high DC bias is needed to convert the bipolar signal to a unipolar signal before modulating the LED intensity. This paper considers a practical LED model and studies the performance of both systems in terms of average electrical OFDM signal power versus bit error ratio in the presence of an additive white Gaussian noise (AWGN) channel. In addition, DC power consumption and the transmitted optical power for the two systems are compared. The analytical results are validated through Monte Carlo simulations and the obtained results demonstrate close match. It is shown that LED clipping has significant impact on the performance of both systems and an optimum system design should take into account the OFDM signal power, DC-bias point, and LED dynamic range.

A new precoding scheme for spectral efficient optical OFDM systems

Spectral efficient frequency division multiplexing (SEFDM) can offer a higher spectral efficiency (SE) than orthogonal frequency division multiplexing (OFDM). In this work, we propose a diversity technique based on SEFDM for beyond 100-Gb/s optical intensity modulation and direct detection (IM/DD) long reach (LR) applications. We mathematically demonstrate that the self-created inter-carrier interference of SEFDM signals can be reused to achieve a diversity gain on each sub-carrier and, in turn, improve the tolerance to power fading induced by chromatic dispersion (CD) in IM/DD LR links. Based on the proposed diversity technique, we further demonstrated a 112-Gb/s SEFDM transmission over 80-km standard single-mode fiber, using only 28-GHz bandwidth and modulation format of up to 16-QAM. Experimental results show that SEFDM with the proposed diversity technique performs robust against CD effects and outperforms the conventional OFDM with adaptive bit and power loading of the same bandwidth and data rate, which validates the superiority of the proposed SEFDM in optical IM/DD LR transmissions.

In this paper, we propose a novel method to reduce the high peak-to-average power ratio (PAPR) of the optical orthogonal frequency division multiplexing (OFDM) signal. The method is based on the technique of the Hadamard transform combined with a companding transform. We experimentally demonstrate that, in an optical intensity-modulation direct-detection OFDM transmission system, the hybrid method has better performance in reducing the PAPR compared with the case of applying only the Hadamard transform or only a companding transform.

High Peak-to-Average Power Ratio (PAPR) of the Orthogonal Frequency Division Multiplexing (OFDM) signal is one of the limitations to the transmission performance of optical OFDM system. In this paper, we propose a novel hybrid PAPR reduction technique called Hadamard Transform Combined with Partial transmit sequence (HTCP). The proposed HTCP scheme combines the merits of two complementary techniques, i.e., Hadamard transform and Partial Transmit Sequence (PTS), to improve the performance of the optical OFDM system in terms of PAPR and Bit Error Rate (BER). Simulation results show that the HTCP scheme has better performance with regard to PAPR and BER compared with the case of applying only the Hadamard transform or PTS technique.

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