A new improved-performance decoding technique for Asymmetrically-Clipped Optical-OFDM

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.

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.

We propose a superposition based light emitting diode (LED) nonlinearity mitigation scheme for asymmetrically clipped optical-orthogonal frequency division multiplexing (ACO-OFDM) optical wireless communications (OWC) systems, where a non-redundant signal stream is superimposed with an ACO-OFDM source signal stream, requiring no pilot and no side information. Based on the anti-symmetry property that a non-redundant signal and its symmetric signal is equal to zero with the same absolute value, a non-redundant signal is used to reduce the large amplitude of the ACO-OFDM signal and enhance its symmetric ACO-OFDM signal that is clipped to zero due to negative signal clipping, allowing LED nonlinearity mitigation. The proposed scheme is also energy-efficient, as the large-amplitude signal reduction is equal to the symmetric signal enhancement, requiring no extra power. However, the designed non-redundant signals fall on odd subcarriers and interfere with source signals. The even subcarriers include the absolute value of source signals, which can be utilized to recover source signals. The polarity of source signals on even subcarriers can be determined by ensuring that the remaining part is higher than the clipping part of the large-amplitude signal in the process of LED nonlinearity mitigation. Simulation results show that the proposed scheme outperforms a number of state-of-the-art methods in the literature as well as ACO-OFDM systems with no LED nonlinearity mitigation in terms of bit error rate (BER).

As a promising modulation technology for optical communication, Orthogonal Frequency Division Multiplexing (OFDM) is now increasingly used and a lot of ways of improving the optical efficiency of Intensity Modulated/ Direct Detection (IM/DD) systems have been proposed. And real and non-negative OFDM signals are required in this kind of system. Asymmetrically clipped DC biased Optical OFDM (DCO-OFDM), Asymmetrically Clipped Optical OFDM (ACO-OFDM) and Asymmetrically clipped DC biased Optical OFDM (ADO-OFDM) can satisfy this requirement. In this paper, we analysis the properties and performance orderly of the above three systems and simultaneously compare their performance. The simulation results show that DCO-OFDM is efficient in terms of bandwidth while the ACO-OFDM is efficient in terms of optical power. More importantly, ADO-OFDM is both power-efficient and bandwidth-efficient. At the same time, a developmentally proposal for the future OFDM in IM/DD systems is put forward.

In this paper, we present a new form of orthogonal frequency division multiplexing (OFDM) for intensity modulated direct detection (IM/DD) systems. This new form of OFDM uses asymmetrically clipped optical OFDM (ACO-OFDM) on the odd frequency subcarriers and DC biased optical OFDM (DCO-OFDM) on the even subcarriers. By using a form of interference cancellation both the ACO-OFDM and the DCO-OFDM signals can be recovered at the receiver. It is shown that the DCO-OFDM component causes no interference on the odd frequency subcarriers so the receiver processing for the ACO-OFDM signal is identical to a conventional ACO-OFDM system. The ACO-OFDM signal causes clipping noise which falls on the even subcarriers. However, this interference can be estimated and cancelled at the receiver. The cancellation process results in a 3 dB noise penalty in the DCO-OFDM component. Simulation results are presented for an additive white Gaussian noise channel and it is shown that the new technique can achieve better optical power performance than the conventional schemes.

This paper reports on findings on the analysis of the fast Fourier transform (FFT) hybrid power line communication (PLC) - visible light communication (VLC) system (PLC- VLC) based on ACO-OFDM in impulsive noise, when the perfect nulling technique is utilized to combat noise in the system. In this work, we develop theoretical expressions that describe the error floors realized in the system due to perfect nulling. Perfect nulling refers to a noise mitigation technique that perfectly identifies impulsive noise locations in a string of data samples and clips them to zero, leading to an error floor at a high Signal to Noise (SNR) ratio. Thus, the principal contribution of this paper is to analyze and derive theoretical Bit Error Rate (BER) expressions for the error floor achieved by a BPSK OFDM signal in the FFT hybrid PLC-VLC system based on asymmetrically clipped optical - orthogonal frequency division multiplexing (ACO-OFDM), in a channel corrupted with Added White Gaussian Noise (AWGN) and impulsive noise. The derived theoretical BER results will be compared to the error floors obtained through simulations.

The main objective of this paper is to achieve high normalized spectral efficiency (in bits per OCDMA chip) in optical CDMA network transmission. We design a time-wavelength M-ary transmission scheme that uses error-control coding and trellis coded modulation (TCM), optical M-ary intensity modulation and direct detection at the receiver. We use a probabilistic wavelength-time hopping approach, in order to transmit error-control coded and TCM modulated M-ary data directly through the optical channel without using optical spreading sequences. We present simulation results showing that the proposed OCDMA scheme is robust to Poisson and thermal noise. Hundreds of active users can be supported at target BER of 10-9 and achieved spectral efficiency (1.37 bits per OCDMA chip) is almost twice larger that the best result previously reported.

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.

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.

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

Adaptively modulated optical orthogonal frequency-division multiplexing (AMOOFDM) using non-linear signal compression (NSC) (AMOOFDM-NSC) is proposed and numerically explored, for the first time, in intensity modulation and direct detection (IMDD) single-mode fibre transmission systems for cost-sensitive optical access networks. AMOOFDM-NSC utilises a simple cosine operation to compress digital electrical OFDM signals prior to performing digital-to-analogue conversion and optical intensity modulation in the transmitter. Compared with conventional AMOOFDM, AMOOFDM-NSC can not only significantly lower requirements on minimum quantisation bits and sampling rates of digital-to-analogue/analogue-to-digital convertors, but also simultaneously doubles the signal transmission capacity for quantisation bits of < 4. In addition, AMOOFDM-NSC also shows excellent system flexibility and has good performance robustness to variations in system characteristics. AMOOFDM-NSC may have great potential for providing a cost-effective technical solution for practical implementations in cost-sensitive access networks.

A novel phased array receive antenna steering system is introduced. The core of this system is an optical ring resonator-hased broadband, continuously tunable optical beam forming network (OBFN). In the proposed system architecture, filter-based optical single-sideband suppressed-carrier modulation and balanced coherent optical detection are used. Such architecture has significant advantages over a straightforward architecture using optical double-sideband modulation and direct optical detection, namely relaxed bandwidth requirements on the optical modulators and detectors, reduced complexity of the OBFN chip, and enhanced dynamic range. Initial measurements on an actual 1 times 8 OBFN chip and an optical sideband filter chip arc presented. Both are realized in CMOS-compatible planar optical waveguide technology.

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.

Spectral efficient frequency division multiplexing (SEFDM) can improve the spectral efficiency for next-generation optical and wireless communications. In this work, we apply SEFDM in beyond 100-Gb/s optical intensity modulation and direct detection transmissions and propose a low-complexity logarithmic-maximum-a-posteriori (log-MAP) Viterbi decoding algorithm to achieve the maximum likelihood (ML) detection. We evaluate the likelihood of detections using a posteriori probability instead of Euclidean distance by taking both noise and inter-carrier interference into consideration. In order to balance the performance and complexity, we then employ Viterbi decoding principle to retain only certain paths with ML detections (a.k.a., the surviving paths) while discarding the others during the decoding procedure. Results show that the proposed log-MAP Viterbi decoding scheme achieves optimal performance due to the precise likelihood evaluation, which guarantees the retention of the global ML detection. By using the proposed decoding scheme, the data rate of SEFDM signals can reach 150-Gb/s in a 2-km standard single mode fiber transmission, using only 28-GHz bandwidth and 16-QAM modulation. Experimental results show that the 16-QAM modulated SEFDM signal with a bandwidth compression factor of 0.8 outperforms 32-QAM modulated OFDM, while both signals have the same bandwidth (28-GHz) and data rate (140-Gb/s), which demonstrate the superiority of SEFDM in optical short reach applications.

An analytical investigation on the impact of the DC bias on the clipping noise and the laser non-linearity in Asymmetrically Clipped Optical Orthogonal Frequency Division Multiplexing (ACO-OFDM) and Direct Current--Biased Optical OFDM (DCO-OFDM) is performed. Clipping noise in the OFDM system is introduced either by transmitter non-linearity or due to the low power sensitivity of the receiver. However the clipping noise in the DCO-OFDM is also dependent upon the DC bias applied to make the bipolar OFDM signal unipolar. The addition of this extra DC bias increases the average signal power of the DCO-OFDM signal. The non-linearity of the DFB laser increases with the optical modulation index, which gives rise to intermodulation and harmonic distortions at the receiver. The OFDM signal when transmitted through the Single Mode Fiber (SMF) suffers from group--velocity dispersion (GVD) which limits its performance in the Passive Optical Networks (PON). In this paper, an assessment of the DC bias to reduce the clipping noise in the two systems under consideration is performed. A practical laser model is considered and analysis is performed to estimate the average laser drive power to maintain its non-linearities within a tolerable region for both the OFDM systems. A simulation is performed to transmit 5 and 8 Gb/s (ACO and DCO) OFDM signal over 120 km SMF where the effect of GVD is taken into consideration. An optimum OFDM drive signal power is calculated to maintain the laser transmitter in its linear region such that the effect of second order harmonic distortions (HD2) and third order intermodulation distortion (IMD3) is low. The SNR requirement of DCO-OFDM being higher than the ACO-OFDM leads to more clipping noise, HD2 and IMD3.