Asymmetrically Clipped Optical OFDM Research Articles

Enhanced asymmetrically clipped DC biased optical OFDM for intensity-modulated direct-detection systems

In this paper, we propose an EADO-OFDM (Enhanced Asymmetrically Clipped DC Biased Optical Orthogonal Frequency Division Multiplexing) method for IM/DD (Intensity-Modulated DirectDetection) optical systems, in which the AV-DCO-OFDM (Absolute Valued DC Biased Optical OFDM) symbols on the even subcarriers and ACO-OFDM (Asymmetrically Clipped Optical OFDM) symbols on the odd subcarriers are combined for simultaneous transmission. Moreover, we discuss the PDF (Probability Density Function) and electrical SNR (Signal to Noise Ratio) of the symbols, which are utilized to estimate the BER (Bit Error Ratio) performance and overall performance of EADO-OFDM. The Monte Carlo simulation results have validated the theoretical analysis and have also confirmed the EADO-OFDM is attractive considering the following benefits. Firstly, EADO-OFDM is more energy effcient compared to the power-effcient DCO-OFDM (DC Biased Optical OFDM), since the required DC bias is smaller when appropriate constellation size combinations are chosen. In addition, EADO-OFDM performs better than the conventional ADO-OFDM (Asymmetrically Clipped DC Biased Optical OFDM), because the absolute value operation causes no clipping distortion.

PAPR reduction for Flip-OFDM in IM/DD systems via PTS technique

ABSTRACTOptical wireless communication (OWC) has recently gained much attention for addressing the increasing demand in the global broadband access market. Flip-orthogonal frequency division multiplexing (Flip-OFDM) is a type of OFDM that can be implemented in indoor OWC. Due to the ability of Flip-OFDM to keep the same bit error rate performance in asymmetrically clipped optical OFDM (ACO-OFDM) while saving 50% of the receiver hardware complexity, it can be used as an alternative to ACO-OFDM. However, the main drawback that can reduce the performance of the Flip-OFDM is the high peak-to-average power ratio (PAPR). So, in this article, partial transmit sequence (PTS) is applied to reduce the PAPR of Flip-OFDM over ceiling bounce model. Simulation results show that the proposed PTS-based Flip-OFDM achieves significantly lower PAPR values compared to Flip-OFDM.

PAPR Reduction for Hybrid ACO-OFDM Aided IM/DD Optical Wireless Vehicular Communications

Hybrid asymmetrically clipped optical orthogonal frequency division multiplexing (HACO-OFDM) improves the spectral efficiency compared to asymmetrically clipped optical OFDM (ACO-OFDM) and pulse-amplitude-modulated discrete multitone (PAM-DMT) modulation, while retaining the advantage of high power efficiency. HACO-OFDM has found favor in numerous applications, but its peak-to-average-power ratio (PAPR) has remained a concern in optical wireless communications, since the family of existing PAPR reduction methods cannot be directly invoked for the superimposed HACO-OFDM signals. Hence, we analyze the characteristics of HACO-OFDM signals and develop a specific PAPR reduction technique relying on tone injection. Our numerical results show that the proposed method achieves significantly improved PAPR statistics compared to the conventional methods, leading to a significant bit error ratio (BER) reduction.

Subcarrier Pairwise Coding for Short-Haul L/E-ACO-OFDM

Dispersion-induced power fading in intensity modulated direct-detection systems degrades the higher frequency subcarriers’ signal qualities. In layered/enhanced asymmetrically clipped optical OFDM (ACO-OFDM), which is a spectrally efficient version of ACO-OFDM, the error propagation in the iterative decoding can further degrade the performance, especially when a low bias current is used for the directly modulated laser. In this letter, we propose using pairwise coding within each layer to improve the bit error rate. We transmitted ~5-GBb 16-QAM OFDM signals over a 19.8-km single mode fiber, and found a 3.6-dB improvement in the $Q^{2}$ -factor.

Enhanced subcarrier-index modulation-based asymmetrically clipped optical OFDM using even subcarriers

In this paper, we propose a subcarrier-index modulation-based asymmetrically clipped optical orthogonal frequency division multiplexing (SACO-OFDM) scheme for optical wireless communication (OWC) systems, which benefits from the subcarrier-index modulation (SIM) and asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) techniques. SACO-OFDM conveys additional information via the subcarrier indexing, and the error rate of the bit transmitted by the subcarrier indexing is much lower than that of the conventional M-ary modulation scheme. On the other hand, as the signal constellation in M-ary modulation is relieved, SACO-OFDM has simple transceiver structure and low detection complexity. Moreover, considering the spectral, an enhanced SACO-OFDM (ESACO-OFDM) using even subcarriers is proposed. In this technique, the odd subcarriers are activated for SACO-OFDM, and the imaginary part of even subcarriers are activated for pulse-amplitude-modulated discrete multitone (PAM-DMT). Clearly, ESACO-OFDM achieves better spectral efficiency than the conventional optical OFDM, since all subcarriers are used for data transmission. Simulation results verify the significant bit error rate (BER) and peak-to-average power ratio (PAPR) improvement by the proposed ESACO-OFDM, especially for the medium-to-high signal-to-noise ratio (SNR) regime.

Asymmetrically Clipped Absolute Value Optical OFDM for Intensity-Modulated Direct-Detection Systems

Orthogonal frequency division multiplexing (OFDM) is attracting increasing attention in optical communication systems, thanks to its inherent benefits such as high spectral efficiency and resistance to frequency-selective channels. In this paper, a novel energy and spectrally efficient scheme called asymmetrically clipped absolute value optical OFDM (AAO-OFDM) is proposed for intensity-modulated direct-detection systems. In AAO-OFDM, absolute value optical OFDM (AVO-OFDM) signals on the even subcarriers and asymmetrically clipped optical OFDM (ACO-OFDM) signals on the odd subcarriers are combined for simultaneous transmission, which employs all the subcarriers requiring no dc biases. For AVO-OFDM scheme, the frequency symbols are first modulated on the even subcarriers, which are then fed into an inverse fast Fourier transform block. Afterward, the absolute values of the bipolar time-domain signals are taken to guarantee non-negativity, while their signs are mapped to the complex-valued symbols and modulated on the odd subcarriers. Since there remain unused odd subcarriers, other useful symbols can be modulated on them, which leads to the conventional ACO-OFDM scheme. At the receiver, the ACO-OFDM symbols on the odd subcarriers are demodulated first, which are reconstructed and removed from the received signals. Afterward, the remaining signals are utilized to detect the AVO-OFDM symbols with the aid of the demodulated sign symbols on the odd subcarriers. Theoretical analysis and simulation results show that AAO-OFDM has lower peak-to-average power ratio than other optical OFDM schemes, which makes it less sensitive to the nonlinearity of the optical devices. Furthermore, it achieves better bit error rate performance compared to its counterparts for the same spectral efficiency.

A performance improvement and cost-efficient ACO-OFDM scheme for visible light communications

In this paper, we propose a performance improvement and cost-efficient discrete Hartley transform (DHT)-based asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) scheme for visible light communications (VLC). The simple one-dimensional modulation constellation and simplified encoding structure reduce the complexity of system considerably. The DHT-spreading technique is employed to reduce peak-to-average power ratio (PAPR) of ACO-OFDM signals. Moreover, the intra-symbol frequency-domain averaging (ISFA) technique is used to increase the accuracy of channel estimation by removing the effect of ambient noise in the VLC channel effectively. To verify the feasibility of the proposed scheme, we study its performance via simulation. This scheme reduces the requirement to the resolution of DAC and increases the tolerance to the nonlinear characteristics of LED, both of which are cost-efficient. At forward error correction (FEC) limit (BER=1×10−3), simulation results illustrate that compared with DHT-based ACO-OFDM without the ISFA technique, our scheme has 3.2 dB and 2.7 dB improvement of the required Eb∕N0 when BPSK and 4-PAM are modulated, respectively.

Assessment of the DC Bias to Mitigate the Clipping Noise in DCO-OFDM, ACO-OFDM; and Non-linear Distortion of DFB Laser Transmitted through Dispersive Single Mode Fibers in IM/DD Systems

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.

Experimental Layered/Enhanced ACO-OFDM Short-Haul Optical Fiber Link

Asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) is theoretically more optically power efficient than dc-biased OFDM (DCO-OFDM); unfortunately, its spectral efficiency is halved to accommodate the clipping distortion. Recently, layered/enhanced ACO-OFDM has been developed to mostly regain the lost spectral efficiency, and theoretically compared with other modulation formats. In this letter, we experimentally demonstrate L/E-ACO-OFDM for the first time over a fiber link. We transmit over a 19.8-km single mode fiber at 4.375 Gb/s, and show a 2-dB improvement in signal quality for the same laser bias current and received optical power over DCO-OFDM.

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

Visible light communication (VLC) systems using orthogonal frequency division multiplexing (OFDM) are attracting increasing interests due to its inherent benefits such as high spectral efficiency, resistance to frequency-selective channels and so on. In this paper, a novel iterative receiver is proposed for asymmetrically clipped DC biased optical OFDM (ADO-OFDM), where asymmetrically clipped optical OFDM (ACO-OFDM) and DC biased OFDM (DCO-OFDM) signals are transmitted simultaneously. In our proposed iterative receiver, ACO-OFDM and DCO-OFDM time-domain signals are distinguished firstly. Then pairwise clipping, negative clipping and pairwise averaging are utilized in the iterative receiver to reduce the effect of noise and interference. In addition, an optimal solution to the optical power allocation factor for ACO-OFDM and DCO-OFDM signals is derived. Furthermore, to reduce the computational complexity, an approximation of the optimal solution is obtained. Both theoretical analysis and simulation results indicate that the approximate solution is near-optimal, and only a few detection iterations are required for the iterative receiver.

Compressed sensing-based channel estimation for ACO-OFDM visible light communications in 5G systems

In this paper, we propose a compressive sensing (CS)-based channel estimation technique for asymmetrically clipped optical-orthogonal frequency division multiplexing (ACO-OFDM) visible light communications (VLC) in 5G systems. We proposed a modified version of sparsity adaptive matching pursuit (SAMP) algorithm which is named as self-aware step size sparsity adaptive matching pursuit (SS-SAMP) algorithm. It utilizes the built-in features of SAMP and with additional ability to select step size according to the present situation, hence term self-aware, can provide better accuracy and low computational cost. It also does not require any prior knowledge of the sparsity of the signal which makes it self-sufficient. CS-based algorithms such as orthogonal matching pursuit (OMP), SAMP, and our proposed SS-SAMP were implemented on ACO-OFDM VLC. The paper is supported by simulation results which demonstrate performance of proposed scheme in terms of bit error rate (BER), mean square error (MSE), computational complexity, and key VLC parameter (LED nonlinearity, shot noise, thermal noise, channel response, and peak-to-average power ratio (PAPR). It is shown that the SS-SAMP is a good candidate for ACO-OFDM-based VLC that are mobile and have limited processing power, based on its performance and computational complexity.

PAPR analysis for OFDM visible light communication.

Orthogonal frequency-division multiplexing (OFDM) is a practical technology in visible light communication (VLC) for high-speed transmissions. However, one of its operational limitations is the peak-to-average power ratio (PAPR) of the transmitted signal. In this paper, we analyze the PAPR distributions of four VLC OFDM schemes, namely DC-biased optical OFDM (DCO-OFDM), asymmetrically clipped optical OFDM (ACO-OFDM), pulse amplitude modulated discrete multitone (PAM-DMT), and Flip-OFDM. Both lower and upper clippings are considered. We analytically derive the complementary cumulative distribution functions (CCDFs) of the PAPRs of the clipped VLC OFDM signals, and investigate the impact of lower and upper clippings on PAPR distributions. Our analytical results, as verified by numerical simulations, provide useful insights and guidelines for VLC OFDM system designs.

PAPR reduction scheme for ACO-OFDM based visible light communication systems

In this paper, we proposed a novel peak to average power ratio (PAPR) reduction scheme for the asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) visible light communications system. We implement the Toeplitz matrix based Gaussian blur method to reduce the high PAPR of ACO-OFDM at the transmitter and use the orthogonal matching pursuit algorithm to recover the original ACO-OFDM frame at the receiver. Simulation results show that for the 256-subcarrier ACO-OFDM system a ~6dB improvement in PAPR is achieved compared with the original ACO-OFDM in term of the complementary cumulative distribution function, while maintaining a competitive bit-error rate performance compared with the ideal ACO-OFDM lower bound. We also demonstrated the optimal parameter C of 2 for the recovery algorithm based on the tradeoff between the data rate and recovery accuracy. The recovery results show that using the proposed scheme the ACO-OFDM can faithfully be reconstructed judging by the very low value for the reconstruct error of 0.06.

Novel Visible Light Communication Approach Based on Hybrid OOK and ACO-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.

Performance analysis of modified Asymmetrically-Clipped Optical Orthogonal Frequency-Division Multiplexing systems

A modification to the Asymmetrically-Clipped Optical Orthogonal Frequency-Division Multiplexing (ACO-OFDM) technique is proposed through unipolar encoding. A performance analysis of the Bit Error Rate (BER) is developed and Monte Carlo simulations are carried out to verify the analysis. Results are compared to that of the corresponding ACO-OFDM system under the same bit energy and transmission rate; an improvement of 1dB is obtained at a BER of 10−4 . In addition, the performance of the proposed system in the presence of atmospheric turbulence is investigated using single-input multiple-output (SIMO) configuration and its performance under that environment is compared to that of ACO-OFDM. Energy improvements of 4dB and 2.2dB are obtained at a BER of 10−4 for SIMO systems of 1 and 2 photodetectors at the receiver for the case of strong turbulence, respectively.

Dimmable Visible Light Communications Based on Multilayer ACO-OFDM

This paper proposes a dimmable scheme for a visible light communication (VLC) system based on multilayer asymmetrically clipped optical orthogonal frequency-division multiplexing (ACO-OFDM), which is able to support a wide dimming range for different illumination requirements. In the proposed scheme, multiple layers of ACO-OFDM occupying different subcarriers are combined so that almost all of the subcarriers can be used for data transmission. The polarities of different layers of ACO-OFDM are varied to obtain flexible time-domain waveform, which can fully exploit the dynamic range of light-emitting diodes (LEDs) and achieve better performance. The scaling factor and modulation order for each layer, as well as the dc bias, are optimized for different dimming requirements to achieve improved spectral efficiency. Simulation results demonstrate that the proposed scheme can support communication over a wide dimming range and achieve higher spectral efficiency, compared with existing methods under different dimming requirements.

Experimental Demonstration of Diversity Combining for Asymmetrically Clipped Optical OFDM

Experimental results demonstrating the use of diversity combining in an optical wireless system using asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) are presented. A standard white lighting LED of the type often used in visible light communications (VLC) is used as the transmitter. The experiments show that diversity combining gives an improvement of 2–3 dB, and that uncorrected DC offset in the receiver can cause diversity combining to fail, but that DC correction methods are effective. It is shown that diversity combining changes the frequency distribution of noise received on the even subcarriers.

Energy Efficient Visible Light Communications Relying on Amorphous Cells

In this paper, we design an energy efficient indoor visible light communications (VLC) system from a radically new perspective based on an amorphous user-to-network association structure. Explicitly, this intriguing problem is approached from three inter-linked perspectives, considering the cell formation, link-level transmission and system-level optimisation, critically appraising the related optical constraints. To elaborate, apart from proposing hitherto unexplored amorphous cells (A-Cells), we employ a powerful amalgam of asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) and transmitter pre-coding aided multi-input single-output (MISO) transmission. As far as the overall system-level optimisation is concerned, we propose a low-complexity solution dispensing with the classic Dinkelbach's algorithmic structure. Our numerical study compares a range of different cell formation strategies and investigates diverse design aspects of the proposed A-Cells. Specifically, our results show that the A-Cells proposed are capable of achieving a much higher energy efficiency per user compared to that of the conventional cell formation for a range of practical field of views (FoVs) angles.

Comparisons of spectrally-enhanced asymmetrically-clipped optical OFDM systems.

Asymmetrically clipped optical orthogonal frequency-division multiplexing (ACO-OFDM) is a technique that sacrifices spectral efficiency in order to transmit an orthogonally frequency-division multiplexed signal over a unipolar channel, such as a directly modulated direct-detection fiber or free-space channel. Several methods have been proposed to regain this spectral efficiency, including: asymmetrically clipped DC-biased optical OFDM (ADO-OFDM), enhanced U-OFDM (EU-OFDM), spectral and energy efficient OFDM (SEE-OFDM), Hybrid-ACO-OFDM and Layered-ACO-OFDM. This paper presents simulations up to high-order constellation sizes to show that Layered-ACO-OFDM offers the highest receiver sensitivity for a given optical power at spectral efficiencies above 3 bit/s/Hz. For comparison purposes, white Gaussian noise is added at the receiver, component nonlinearities are not considered, and the fiber is considered to be linear and dispersion-less. The simulations show that LACO-OFDM has a 7-dB sensitivity advantage over DC-biased OFDM (DCO-OFDM) for 1024-QAM at 87.5% of DCO-OFDM's spectral efficiency, at the same bit rate and optical power. This is approximately equivalent to a 4.4-dB advantage at the same spectral efficiency of 87.7% if 896-QAM were to be used for DCO-OFDM.

Asymmetrically Reconstructed Optical OFDM for Visible Light Communications

In conventional optical orthogonal frequency-division multiplexing (OOFDM) systems, such as pulse-amplitude-modulated discrete multitone (PAM-DMT) and asymmetrically clipped optical OFDM (ACO-OFDM), half of the time-domain transmitted signals carry meaningful information, while the other half are set at zero after asymmetric clipping. By specifically exploiting the asymmetric structure of OOFDM such as PAM-DMT and ACO-OFDM, we present a new framework for implementing OOFDM with enhanced performance in terms of both bit error rate (BER) and peak-to-average power ratio (PAPR). The proposed scheme introduces asymmetric signal reconstruction, instead of direct clipping, in the time domain before transmission. The designed asymmetric reconstruction refines the time-domain statistics of the OOFDM signals by constructively exploiting zero-padded signal positions in conventional OOFDMs. This way, the PAPR of the reconstructed OOFDM signals is effectively reduced. For receivers, to recover the received signals precisely, we further develop an optimal maximum a posteriori detection method, along with an efficient simplified method. Simulation results validate that the proposed scheme enhances the BER performance compared with conventional OOFDMs under both an ideal and a dispersive visible light communication (VLC) channel via our prototype. Specifically, under an ideal additive white Gaussian noise channel, an approximately 4-dB gain is achieved at the BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> , along with a 2.5-dB lower PAPR constraint. While under the real dispersive channel, our test shows about 6-dB gain in BER performance.