Optical OFDM Systems Research Articles

Proposed Different Signal Processing Tools for Efficient Optical Wireless Communications

Optical Wireless Communication (OWC) is a new trend in communication systems to achieve large bandwidth, high bit rate, high security, fast deployment, and low cost. The basic idea of the OWC is to transmit data on unguided media with light. This system requires multi-carrier modulation such as Orthogonal Frequency Division Multiplexing (OFDM). This paper studies optical OFDM performance based on Intensity Modulation with Direct Detection (IM/DD) system. This system requires a non-negativity constraint. The paper presents a framework for wireless optical OFDM system that comprises (IM/DD) with different forms, Direct Current biased Optical OFDM (DCO-OFDM), Asymmetrically Clipped Optical OFDM (ACO-OFDM), Asymmetrically DC-biased Optical OFDM (ADO-OFDM), and Flip-OFDM. It also considers channel coding as a tool for error control, channel equalization for reducing deterioration due to channel effects, and investigation of the turbulence effects. The evaluation results of the proposed framework reveal enhancement of performance. The performance of the IM/DD-OFDM system is investigated over different channel models such as AWGN, log-normal turbulence channel model, and ceiling bounce channel model. The simulation results show that the BER performance of the IM/DD-OFDM communication system is enhanced while the fading strength is decreased. The results reveal also that Hamming codes, BCH codes, and convolutional codes achieve better BER performance. Also, two algorithms of channel estimation and equalization are considered and compared. These algorithms include the Least Squares (LS) and the Minimum Mean Square Error (MMSE). The simulation results show that the MMSE algorithm gives better BER performance than the LS algorithm.

A Novel Non-Hermitian Symmetry Orthogonal Frequency Division Multiplexing System for Visible Light Communications

This paper proposes a novel non-Hermitian symmetry ${2 \times 2}$ MIMO-OFDM (Multiple Input Multiple Output-Orthogonal Frequency Division Multiplexing) system based on fast Hartley transform (FHT) with unipolar encoding (referred to as HU-OFDM) for visible light communications. The new scheme uses FHT instead of FFT (fast Fourier transform) to process the complex-valued signals reducing the computational complexity and hardware cost and employs two LEDs (light-emitting diodes) to transmit the real and imaginary parts of a complex-valued OFDM signal respectively. Unipolar encoding is applied to the transmitted signal from each LED to ensure non-negative. The restrictions of Hermitian symmetry in traditional optical OFDM systems and real constellation mapping in FHT based optical OFDM systems can be removed simultaneously in the proposed system. Compared with MIMO-ACO/DCO-OFDM system, HU-OFDM has significant performance improvement along with a tremendous decrease in hardware cost and computational complexity. Compared with other non-Hermitian symmetry MIMO-OFDM systems, HU-OFDM has significant advantages in terms of power efficiency, system design flexibility, computational complexity, or hardware cost without losing reliability.

Reliability-aware subcarrier mapping strategy of QC-LDPC encoded symbols in bandwidth-limited IM/DD OFDM systems.

We propose a simple yet effective strategy to map forward error correction (FEC) encoded symbols to subcarriers in bandwidth-limited intensity-modulation and direct-detection (IM/DD) optical orthogonal frequency division multiplexing (OFDM) systems. The design exploits the reliability diversity of OFDM subcarriers in bandwidth-limited systems and allocates FEC encoded systematic symbols and parity-check symbols to high-reliability and low-reliability subcarriers, respectively. In contrast to adaptively-loaded OFDM or systems using multiple sets of FECs with different code rates for different subcarriers, the proposed design does not induce additional complexity and avoids the round-trip delay between transceivers. We investigate the performance of the proposed design in quasi-cyclic low-density parity-check (QC-LDPC) coded OFDM system under different decoding iterations. Experimental results of 120-Gb/s signals over 2 km and 100-Gb/s signals over 5 km show that the OFDM system using the proposed mapping strategy is superior to both OFDM and discrete-Fourier-transform spread (DFT-S) OFDM using conventional mappings without considering the subcarrier diversity or using pre-equalization, regardless of the received optical power, the FEC code rate, the length of the cyclic prefix, the transmission distance, the number of decoding iterations, and the degree distribution of the QC-LDPC optimized under either an infinite or a limited number of iterations. The proposed mapping is optimal when the systematic symbols and parity-check symbols are exactly loaded to high-reliability and low-reliability subcarriers respectively. The studies enable the proposed strategy promising for bandwidth-limited IM/DD OFDM systems such as low-latency data center interconnects.

Hybrid scheme of precoder with μ‐law compander for PAPR reduction and nonlinearity improvement in ADO‐OFDM system

SummaryThe application of orthogonal frequency division multiplexing (OFDM) in a visible light communication (VLC) system increases the peak‐to‐average power ratio (PAPR) of the system. Owing to high PAPR, the intrinsic nonlinearity of LED is an important issue in an asymmetrically clipped DC‐biased optical OFDM (ADO‐OFDM) system. In this paper, a hybrid scheme is proposed to reduce the PAPR as well as to mitigate the nonlinearity in an ADO‐OFDM system. The proposed method is a combination of two ADO‐OFDM formats: precoded ADO‐OFDM and μ‐ADO‐OFDM. The preprocessing of the input using a predetermined matrix (PM) in the precoder relieves the need for a handshake at the receiver. The compression of the peak values and enhancement of low amplitudes of the precoded signal in the time domain yield a low PAPR of 2.4 dB only. Besides PAPR reduction, the union of precoder and compander boosts the bit error rate (BER) performance of the traditional ADO‐OFDM system for μ≤128. The combination of precoder with compander decreases the power requirements for high bit rate transmission for a reference BER of 10−3. In this work, nonlinearity is quantified in terms of error vector magnitude (EVM) in percent. The hybrid scheme shows an EVM of 17.23% for 0 dB input backoff power (IBO) much less than 66.4% EVM for a simple ADO‐OFDM system. In addition to low PAPR and mitigation of nonlinearity, the proposed method shows the improvement in power spectral density (PSD) of the system.

Digital interpolation-based SFO compensation in OCT precoding-enabled NHS-OFDM transmission systems

The non-Hermitian symmetric optical orthogonal frequency division multiplexing (NHS-OFDM) system exhibits lower hardware implementation complexity and power consumption as maintaining the same bandwidth granularity, compared to the conventional Hermitian symmetric optical OFDM (HS-OFDM) system. Also, NHS-OFDM can obtain a similar peak-to-average power ratio as well as bit error rate (BER) performance. It is expected to be a new candidate for the next-generation passive optical network. In the asynchronous NHS-OFDM system, the sampling clocks for the analog to digital converter in the receiver and the digital to analog converter in the transmitter generated from different sources and their frequencies always have a certain degree of deviation, i.e., sampling frequency offset (SFO). It may significantly deteriorate the receiver performance. Besides, the imperfect frequency response of the optical/electrical devices is another factor for the degraded BER performance. In this work, we experimentally investigate the digital interpolation (DI)-based SFO compensation in a short-reach NHS-OFDM transmission system. The orthogonal circulant matrix transform (OCT) precoding technique is employed to equalize the signal-to-noise ratio over data-carrying subcarriers and then improve the BER performance. The experimental results indicate that, with the help of the 4-th order DI, the SFO up to 200 ppm can be well compensated without power penalties in comparison with the SFO-free case. Furthermore, the receiver sensitivity of the NHS-OFDM enabled by OCT precoding can be improved by more than 2 dB at the BER of 1e-3 after 20 km standard single-mode fiber transmission.

Performance investigation of long-haul high data rate optical OFDM IM/DD system with different QAM modulations

Abstract In this paper and as a first time, we have designed and simulated 100 Gbps long haul intensity modulation and direct detection (IM/DD) optical orthogonal frequency division multiplexing (OFDM) system with high order modulation techniques by using optisystem software QAM IM/DD OFDM system was analyzed and simulated to provide high data rate for downstream signal by using dispersion compensation fiber (DCF) inside fiber link. 4-QAM OFDM system demonstrated the best BER performance compared to other simulated systems for long haul transmission distance. For comparison and investigation, important results as eye diagram, Q factor, Eb/No and BER are explained against propagation length for every simulated system.

Performance enhancement for visible light communication based ADO-OFDM

The increasing demand for bandwidth through modern applications and multimedia services has led to high-speed wireless communications. Optical wireless communications (OWC) encourages solutions that provide a higher data rate due to the large bandwidth available. In this paper, performance enhancement approaches are studied and simulated for visible light communication (VLC) as a case study. The orthogonal frequency division multiplexing (OFDM) systems are used to investigate Intensity modulation/direct detection (IM/DD) to improve the performance of VLC. The IM/DD in OWC requires positive real OFDM symbols, so there are many approaches to satisfy this requirement. This paper proposes a model of Asymmetrically Clipped DC-Biased Optical (ADO-OFDM) to use in OWC/VLC environment. The proposed system has avoided the use of the noise cancellation technology that is used in traditional ADO-OFDM. The results show that the ADO-OFDM has the best spectral efficiency than DC-biased optical (DCO-OFDM) and Asymmetrically clipped optical (ACO-OFDM). Also, it has better optical efficiency than DCO-OFDM with the equally overall bit error rate (BER) at the same signal-to-noise ratio. Hamming channel coding/decoding with different code lengths is applied in various optical OFDM schemes for BER improvements. Furthermore, we simulate and analyze these optical OFDM systems with many modulation orders.

Trellis Shaping for Fiber Nonlinearity Mitigation in Coherent Optical OFDM Systems

In this article, trellis shaping technique is numerically and experimentally investigated to improve the performance of optical orthogonal frequency division multiplexing (OFDM) signal. Different from other constellation shaping techniques, trellis shaping can be designed to reduce the correlation value of OFDM data sequence and average power simultaneously in one OFDM symbol. It means that optical OFDM signals generated by trellis shaping can improve the performance under both linear and nonlinear channels. For comparison with trellis shaping technique, we also consider optimal Maxwell–Boltzmann (MB) shaping and uniform signaling techniques at spectral efficiency (SE) of 10 and 14 bits/4D-sym. Numerical simulations show that trellis shaping can provide certain shaping gain over uniform signaling under linear additive white Gaussian noise (AWGN) channel. The fiber transmission results indicate that OFDM signals with trellis shaping have higher effective signal-to-noise ratio (SNR) than those with MB shaping and uniform signaling. It means that trellis shaping technique has the ability to mitigate the fiber nonlinear noise. Considering both linear and nonlinear noise in the fiber link, trellis shaping can extend the transmission distance by $\sim$ 110 km and $\sim$ 200 km over MB shaping and uniform signaling at SE of 10 bits/4D-sym. At SE of 14 bits/4D-sym, trellis shaping provides similar performance as MB shaping but $\sim$ 120-km extension reach in comparison with uniform signaling. The performances of OFDM signals with trellis shaping, MB shaping and uniform signaling are also experimentally investigated in a four-channel wavelength division multiplexing (WDM) fiber transmission systems. The experimental results show that trellis shaping can also mitigate the nonlinear distortions induced by the clipping effects at the transmitter side. Compared with MB shaping, trellis shaping extends the transmission reach by $\sim$ 240 km and $\sim$ 90 km at SE of 10 and 14 bits/4D-sym, respectively.

Signal Pre-Equalization in a Silicon Photomultiplier-Based Optical OFDM System

In this article, the performance of a new time domain signal pre-equalization method for use with optical orthogonal frequency division multiplexing (OFDM) and a silicon photomultiplier (SiPM) based receiver is studied. A SiPM contains a large array of microcells and each microcell is able to detect single photons. Therefore, a SiPM can be used to create arguably the most sensitive optical receiver, which can detect light intensity signals by counting the number of arriving photons within each signal sampling period. However, each photon detection triggers an avalanche-and-quenching process and the related microcell becomes inactive for a recovery time of several nanoseconds. Consequently, any photons arriving during this period cannot be detected. This effect can cause a non-linear distortion of the received signal and, when the OFDM sampling period is short, also introduces interference between signal samples. In this article, a new signal pre-equalization method is specifically designed to compensate for the impact of the finite recovery time. In this method, the number of active microcells during the transmission of each OFDM signal sample is first estimated. Then, the amplitude of the time domain signal sample is pre-adjusted based on the predicted fraction of microcells that are active. Using this approach, the negative impacts of the recovery time of the microcells are significantly reduced. The results that are presented show that when this new form of pre-equalization is used the bit error rate (BER) performance of the system is improved for a wide range of irradiance levels.

Polarization Multiplexed Optical OFDM System With a Beat Interference Cancellation Receiver

A novel beat interference cancellation receiver (BICR) is proposed to cancel the signal-signal beat interference (SSBI) for polarization division multiplexed (PDM) optical orthogonal frequency division multiplexing (OOFDM) system, where two single sideband (SSB) OOFDM signals are polarization multiplexed along with their offset optical carriers (OCs) located at opposite sides. At receiver, the PDM SSB-OOFDM signal is equally power split into four beams with one or two OCs filtered out, and then fed into two balanced photodiode pairs for recovering the RF-OFDM signals with SSBI cancellation. The PDM SSB-OOFDM system is insensitive to fiber polarization mode dispersion since the proposed BICR is implemented without polarization-sensitive devices. Moreover, the spectral efficiency is greatly improved because of PDM and the small guard band since the SSBIs are eliminated. The proposed BICR scheme is validated with a simulation link of 112 Gbps 16-QAM PDM SSB-OOFDM signals. After 120km SSMF transmission, the EVM keeps below forward error correction limit of 16.3%.

Compensation of nonlinear distortion in coherent optical OFDM systems using a MIMO deep neural network-based equalizer.

A novel nonlinear equalizer based on a multiple-input multiple-output (MIMO) deep neural network (DNN) is proposed and experimentally demonstrated for compensation of inter-subcarrier nonlinearities in a 40 Gb/s coherent optical orthogonal frequency division multiplexing system. Experimental results reveal that MIMO-DNN can extend the power margin by 4 dB at 2000 km of standard single-mode fiber transmission when compared to linear compensation or conventional single-input single-output DNN. It is also found that MIMO-DNN outperforms digital back propagation by increasing up to 1 dB the effectiveQ-factor and reducing by a factor of three the computational cost.

Compressive sensing for PAPR reduction of DC-biased optical OFDM signals with exploiting joint sparsity for signal reconstruction

Compressive sensing (CS) is an attractive technique to mitigate the high peak-to-average power ratio (PAPR) in optical orthogonal frequency division multiplexing (OFDM) systems. The lower computational complexity, easy implementation, and lack of side information requirement make signal compression an appropriate approach for PAPR reduction in both original and optical OFDM systems. We propose the CS approach for compressing the time-domain DC-biased optical OFDM (DCO-OFDM) signals at the transmitter side and multiple measurement vectors (MMV) based on orthogonal matching pursuit (OMP) algorithm to reconstruct the original signals at the receiver end. Simulations are conducted for QPSK and 16-QAM modulated symbols, and the results indicate that the PAPR of compressed signals is significantly reduced compared with the PAPR of the original DCO-OFDM signals. Furthermore, using the MMV-OMP algorithm and only about 1 dB additional signal-to-noise ratio, the original symbols are reconstructed at the receiver end without deteriorating the bit error rate performance.

A Simple Nonlinear Companding Transfrom for Nonlinear Compensation of Direct-Detection Optical OFDM Systems

In direct-detection OFDM systems, the nonlinear effects caused by optical modulation and fiber transmission can degrade the system performance severely. In this study, we propose a new nonlinear companding transform to improve the performance of direct detection optical OFDM transmission systems. The demonstration is realized by Monte-Carlo simulation of the intensity modulation and direct-detection DCO-OFDM optical transmission system at 40 Gbps over a 80 km of standard single mode fiber link. The influence of the companding parameters on the performance of system in different nonlinear transmission conditions has been investigated via simulation.

Peak to Average Power Ratio Reduction for Coherent Optical OFDM Systems Using Phase Rotation of The Sub-Carriers

Peak to Average Power Ratio Reduction for Coherent Optical OFDM Systems Using Phase Rotation of The Sub-Carriers

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

Orthogonal frequency division multiplexing (OFDM) is a practical approach for indoor optical communications because of its high rate data transmission and high power efficiency. The high peak-to-average power ratio (PAPR) is the main problem of optical OFDM signals which, degrades the system performance due to the LED nonlinearity. Partial transmit sequence (PTS) is the most popular PAPR reduction technique because of its distortionless characteristics and high performance. But, PTS has a high computational complexity, because it requires an exhaustive search to obtain the optimal phase factors. To reduce the search complexity of the PTS, we introduce a new swarm intelligence algorithm, called improved flower pollination (IFP) algorithm. The IFP algorithm is a novel optimization technique derived from the pollination behavior of flowers. The proposed IFP–PTS approach is applied in the asymmetrically clipped optical OFDM (ACO-OFDM) system and evaluated using several PAPR reduction benchmarks and compared with different state-of-the-art approaches in terms of PAPR reduction performance and computational complexity. The results justify the advantages of the IFP–PTS method compared with its counterparts.

Experimental Investigation of Intensity Modulator/Direct Detection (IM/DD) Optical OFDM System with Fiber Bragg Grating (FBG)

AbstractTo improve the performance of optical fiber transmission and to compensate the fiber chromatic dispersion (CD), we propose to use fiber Bragg grating (FBG) in intensity modulator/direct detection IM/DD optical orthogonal frequency division multiplexing (OOFDM) system, and experimentally demonstrate 2.5-Gbit/s QPSK-OFDM transmission over 200 km SMF-28. FBG used before the detection as a chromatic dispersion compensation module, reducing the beating noise between ASE noise and OFDM signal. By using the FBG in IM/DD OOFDM system, our experimental results show that the receiver sensitivity was improved about 2 dB at a bit error rate (BER) of $1 \times {10^{ - 3}}$ for 2.5 Gbit/s QPSK-OFDM signals after 200 km SMF-transmission compared to regular system without FBG.

Optical Back-Propagation for Nonlinear Compensation in OFDM-Based Long Range-Passive Optical Networks

In direct-detection optical OFDM system, the nonlinear impairment is the key factor that limits the system performance. The back-propagation techniques in digital and optical domains have been proposed to compensate the nonlinear effects, however they can be unsuitable for long-range passive optical networks (LR-PONs) due to their implementation at receiver. In this study, we propose an optical back propagation (OBP) approach for compensation of the nonlinear and dispersion distortions in direct-detection optical OFDM system. The proposed OBP using split-step Fourier method is implemented at transmitter that is suitable for high-rate OFDM-based LR-PONs applications. In this OBP, the fiber Bragg grating (FBG) is used as a step for dispersion compensation and the high-nonlinear fiber (HNLF) with a short length is used as a step for nonlinear compensation. The performance improvement based on our proposed approach has been demonstrated via Monte-Carlo simulations of the 100 Gbit/s direct-detection optical OFDM system with 80 km of standard single mode fiber link. The influence of optical conjugation process and launching conditions has been investigated. The obtained results show that the proposed OBP can improve remarkably the performance of system with the launched power range from -2 dBm to 6 dBm.

Research and Design of LED Light Suppression Flicker Technology in Visible Light Communication Technology

“Lighting communication integration” is a key technology research for the future application of visible light communication(VLC). However, the bandwidth characteristics, dynamic modulation range and linear modulation performance of existing white LED still do not meet the requirements of high-speed communication. After adding the dimming function, the naked eye It is noticeable that flicker is felt. In order to solve the problem of LED flicker caused by high-speed communication, this paper proposes a “lighting and communication integration” technology scheme based on visible light communication(VLC), which studies from the aspects of suppressing flickering VLC scheme, LED communication dimming, and high-efficiency modulation driving circuit design. A PWM dimming control technology based on optical OFDM system is used for VLC and LED dimming scheme. Finally, the technology is applied to the typical application scenario of the power grid—cable tunnel, which realizes the two-way real-time voice communication of cable tunnel inspection personnel. And accurate positioning to ensure the personal safety of the inspectors and the safety of the grid production.

Modeling and Analysis of the Receiver Performance in External OFDM-RoF Network Using QAM Modulation

The use of optical-OFDM systems is mainly restricted by their limited dynamic range as a result of a high peak to average power ratio (PAPR) and nonlinear distortion (NLD). Radio-over-Fiber (RoF) systems are mainly deployed for light modulation and mm-wave signals transmission over fiber links. This paper evaluated the performance of RoF links with respect to their external modulation in a bid to highlight the challenges attributable to the various optical system components. The Mach-Zehnder Modulator (MZM) have been used in external modulation; it is a tested modulation scheme for quadrature amplitude modulation QAM (a vector modulation format) where signal subcarriers are generated using an OFDM scheme. Simulation results based on Optisystem 13 software exposes that the receiver performance of the optimum system for achieving has a better Bit Error Rate (BER) and three longest distance as 10, 50, 100 km for 16 QAM-OFDM-RoF system are reached by using optimum receiver launch power.

Low complexity DCO-FBMC visible light communication system

Filter Bank Multicarrier (FBMC) is a new waveform candidate in the visible light communication system (VLC). FBMC is a distinctive kind of multi-carrier modulation that can be regarded as an alternative to orthogonal frequency Division Multicarrier (OFDM) with CP (cyclic prefix). DCO-FBMC (DC-bias optical FBMC) has recently been used in VLC, because it overcomes all defects in the optical-OFDM system and has high spectral efficiency. but at the same time the traditional DCO-FBMC suffers from high complexity due to the use of Hermitian Symmetry for real signal, by using 2N-point subcarrier IFFT (Inverse Fast Fourier Transformer) in the modulator, and the output is N-point subcarrier FFT (Fast Fourier Transform) in the demodulator. In this paper, for the first time, the possibility of minimizing complexity and generating a real signal without the use of Hermitian Symmetry or any other technique has been verified. The proposed technology provides 50% of the size of the IFFT / FFT and this results in a significant reduction in power consumption and occupied chip area.