Optical Optical Orthogonal Frequency Division Multiplexing System Research Articles

An ADO‐OFDM with integrated precoding, companding and optimized power allocation methods for high‐speed data transmission in visible light communication

AbstractVisible light communications (VLC) has received a lot of attention in recent studies because of its benefits over radio‐frequency (RF) communications. It is a short‐range optical wireless communication system that uses light‐emitting diodes (LEDs) as transmitters. Optical orthogonal frequency division multiplexing (OFDM) is an auspicious technology for VLC high‐speed data transfer. The use of OFDM in a VLC system raises the system's peak‐to‐average power ratio (PAPR). The intrinsic non‐linearity of LED is a key concern in an asymmetrically clipped DC‐biased optical OFDM (ADO‐OFDM) system due to its high PAPR. Also, conventional ADO‐OFDM modulation scheme cannot be used for accommodating different demands of services in downlink multiple access due to the restriction of direct current biased optical OFDM (DCO‐OFDM) and asymmetrically clipped optical (ACO‐OFDM) transmission on odd and even subcarriers. To tackle this issue, a modified ADO‐OFDM (MADO‐OFDM) is proposed that adjusts the numbers of subcarriers required for the transmission of ACO‐OFDM and DCO‐OFDM adaptively based on the requests of services in downlink multiple access. Also, the proposed MADO‐OFDM is integrated with a discrete Hartley Matrix transform (DisHMT) precoder and Generalized Piecewise Linear Compander (GPLD) to provide high‐speed data transmission with less PAPR. In addition, the power is allocated to the proposed MADO‐OFDM system optimally by maximizing the channel capacity based on the Aquila optimizer algorithm. The simulation results reveal that the suggested system's PAPR is reduced by 2.4 dB and 0.8 dB, respectively, compared to conventional ADO and hybrid ADO‐OFDM. It also confirms that the suggested generalized PLC can greatly increase BER without affecting the PAPR performance.

Optimized intensity-modulated type PTS-based PAPR reduction scheme for intensity-modulated direct-detection optical OFDM systems.

High peak-to-average power ratio (PAPR) of the signal is a major drawback in optical orthogonal frequency division multiplexing (OFDM) system. In this paper, an intensity-modulated type Partial Transmit Sequences (PTS) based scheme is proposed and applied to the intensity-modulated OFDM (IMDD-OFDM) system. The proposed intensity-modulated type PTS (IM-PTS) scheme ensures that the time-domain signal output by the algorithm is real value. What's more, the complexity of the IM-PTS scheme has been reduced without much performance penalty. A simulation is performed to compare the PAPR of different signal. In the simulation, the PAPR of OFDM signal is reduced from 14.5 dB to 9.4 dB at 10-4 probability. We also compare the simulation results with another algorithm based on the PTS principle. A transmission experiment is conducted in a seven-core fiber IMDD-OFDM system at a rate of 100.8Gbit/s. The Error Vector Magnitude (EVM) of received signal is reduced from 9 to 8 at -9.4dBm received optical power. Furthermore, the experiment result shows that the reduction of complexity has little performance impact. The optimized intensity-modulated type PTS (O-IM-PTS) scheme effectively increases the tolerance of the nonlinear effect of the optical fiber and reduces the requirement for linear operating range of optical device in the transmission system. During the upgrade process of the access network, there is no need to replace the optical device in the communication system. What's more, the complexity of PTS algorithm has been reduced, which lower data processing performance requirements of the devices such as ONU and OLT. As a result, the cost of network upgrades is reduced a lot.

Transformer-Based Long Distance Fiber Channel Modeling for Optical OFDM Systems

The fiber channel model plays an essential role in the simulation and design of optical fiber communication systems. However, it is difficult for conventional model-driven modeling to balance accuracy and efficiency, especially in optical orthogonal frequency division multiplexing (OFDM) systems with complex and long-haul transmission. We introduce the simplified Transformer into optical OFDM systems and combine it with the feature decoupled distributed (FDD) scheme for fast and accurate fiber channel modeling. Unlike the popular Transformer architectures, we remove the Decoder part and cancel the self-attention with quadratic complexity, significantly reducing the computational cost. The modeling performance is investigated from the nonlinear fitting capability, accuracy, and generalization ability. The transmission distance ranges from 80 km to 1600 km. The highly matched four-wave mixing (FWM) power, low error vector magnitudes (EVMs), and similar signal-noise ratios (SNRs) demonstrate the high precision and robustness of the model. Furthermore, the modeling is studied under different transmission rates and is proved to be reliable over a wide transmission bandwidth. Compared to the bidirectional long short-term memory (Bi-LSTM), the Transformer performs better in accuracy and has lower computational and memory costs. For modeling under the same conditions, the required running time of the Transformer is about 60% of Bi-LSTM, less than 1% of the split-step Fourier method (SSFM). The Transformer-based method achieves high precision modeling of the fiber channel in the long-distance and high-rate optical OFDM system and makes a significant breakthrough in complexity.

Coherent Optical OFDM With Index Modulation for Long-Haul Transmission in Optical Communication Systems

In this paper, it is proposed to enhance coherent optical orthogonal frequency-division multiplexing (OFDM) system by using index modulation in terms of bit error rate (BER) and spectral efficiency for different levels of the fiber nonlinearity. In the coherent optical OFDM with index modulation system (CO-OFDM-IM), the information is conveyed not only through activated subcarriers but also by indices of the active subcarrier that are dynamically updated at the transmitted symbol rates. Unlike the conventional coherent optical OFDM (CO-OFDM) system, adjusting the number of the active subcarriers in the CO-OFDM-IM system provides improved BER performance and reduction the signal peak-to-average-ratios, when the fiber nonlinearity becomes significant. The simulation results illustrate that the proposed CO-OFDM-IM system has better BER performance when the proposed CO-OFDM-IM system and the CO-OFDM system have similar spectral efficiency. Also, the CO-OFDM-IM system can provide better spectral efficiency than the CO-OFDM system at the low-to-medium order modulation such as BPSK, QPSK, and 16QAM.

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.

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.

Polar coded optical OFDM system with chaotic encryption for physical-layer security

Abstract The security of the transmission system based on optical orthogonal frequency division multiplexing (OFDM) has been an important research direction recently. Polar code is also a key technique for reducing bit error rate (BER) as a promising forward error correction (FEC) scheme. In the paper, a polar coded optical OFDM system using chaotic encryption is proposed. In the proposed system, a two-dimensional Henon map is utilized to generate chaotic sequences to encrypt subcarriers data for enhancing physical layer security. Meanwhile, polar code can improve the reliability and performance of the system. The simulation results show that the polar code can achieve ∼ 7.4 dB gain at the BER of 1 0 − 3 compared to the conventional case after 60 km standard single mode fiber (SSMF) transmission, and chaotic encryption can effectively protect the signal from illegal attack. Hence, the proposed method is capable of enhancing the physical layer security and simultaneously achieves lower BER.

A new precoding scheme for spectral efficient optical OFDM systems

Achieving high spectral efficiency is the key requirement of 5G and optical wireless communication systems and has recently attracted much attention, aiming to satisfy the ever increasing demand for high data rates in communications systems. In this paper, we propose a new precoding/decoding algorithm for spectral efficient optical orthogonal frequency division multiplexing (OFDM) scheme based visible light communication (VLC) systems. The proposed coded modulated optical (CMO) based OFDM system can be applied for both single input single output (SISO) and multiple input multiple-output (MIMO) architectures. Firstly, the real OFDM time domain signal is obtained through invoking the precoding/decoding algorithm without the Hermitian symmetry. After that, the positive signal is achieved either by adding a DC-bias or by using the spatial multiplexing technique. The proposed CMO-OFDM scheme efficiently improves the spectral efficiency of the VLC system as it does not require the Hermitian symmetry constraint to yield real signals. A comparison of the performance improvement of the proposed scheme with other OFDM approaches is also presented in this work. Simulation results show that the proposed CMO-OFDM scheme can not only enhance the spectral efficiency of OFDM-based VLC systems but also improve bit error rate (BER) performance compared with other optical OFDM schemes.

Randomness evaluation of key generation based on optical OFDM system in visible light communication networks

Recently, the authors proposed a physical-layer key generation scheme for optical orthogonal frequency division multiplexing (OFDM) techniques in an indoor scenario. The key generation protocol exploits bipolar and real-valued optical OFDM samples to create confidential keys. It determines a chunk of cyclic prefix samples positioned in the space of small channel impact to generate the confidential keys during the session duration. This key-generation approach offers strong computational security to counter passive eavesdroppers and key creation on the fly, with zero bit-mismatch rates. In this Letter, the authors evaluate the randomness of the confidential keys produced from the recently proposed key-generation protocol to validate if the keys are truly random. The authors apply a statistical test suite of the National Institute of Standards and Technology to test randomness. The results demonstrate that the keys generated from the proposed key generation system pass all the tests. The results also demonstrate that the proposed key generation system provides uncorrelated keys with an approximately equal probability of bits (discrete uniform distribution) on an average, which yields a high security system against statistical attacks.

Analysis of PAPR in optical OFDM systems with grouped LEDs

The need for multiple transmit chains poses a major challenge in optical orthogonal frequency division multiplexing (OFDM) system utilising grouped light emitting diodes (LEDs) for the purpose of peak-to-average power ratio (PAPR) reduction. Thus, recent study propose the use of pilot-assisted (PA) electrical PAPR reduction technique to further reduce PAPR and limit the number of transmit chains in the system. This paper presents the application of order statistics to derive the analytical solutions of PAPR distributions in the PA optical OFDM system utilising grouped LEDs. It is shown that the results obtained via analysis are in good agreement with that of computer simulations.

Secret Key Generation Protocol for Optical OFDM Systems in Indoor VLC Networks

Although the visible light communication (VLC) channel has superior security than radio frequency channels, its broadcast character makes VLC links vulnerable to eavesdropping. Therefore, VLC networks need to support security to protect the user's data against its eavesdroppers. Keyless security approaches have been proposed to provide security for VLC in the literature. Although these approaches support good security against eavesdroppers, they require complicated implementation. Algorithms of keys generation are essential to secure wireless links. Nevertheless, the common keys generation algorithms can be very complex and costly in many setups. They use up limited resources such as bandwidth. In this paper, we enhance the confidentiality of VLC networks by suggesting a new key extraction protocol for optical orthogonal frequency division multiplexing (OFDM) schemes in an indoor environment. We introduce to extract keys from the bipolar OFDM samples produced from optical OFDM schemes. We propose to allocate a portion of cyclic prefix samples placed in the area of low channel impact for keys extraction throughout session duration. The introduced protocol extracts keys for the medium access control (MAC) level and the physical level. It supports high physical-layer security as every OFDM signal frame is encrypted by a various key. Thus, the encryption at the MAC level can be switched off beyond any danger in security. This setup can improve the throughput of the system because it decreases the processing delay in the upper layer process. Also, the presented protocol supports zero bit mismatch rates with key extraction on the fly.

Robust Key Generation From Optical OFDM Signal in Indoor VLC Networks

This letter improves the confidentiality of visible light communication links by proposing a robust key-generation mechanism for optical orthogonal frequency division multiplexing (OFDM) systems in an indoor environment. In this letter, we propose to generate keys from the bipolar OFDM samples at the output of optical OFDM systems. We suggest to assign a part of cyclic prefix samples located in the region of small channel effect for keys generation during session period. The proposed scheme generates keys for medium access control (MAC) sublayer and physical layer. It provides high computational security against passive eavesdroppers in the physical level, because each signal frame is encrypted by a different key. So, the encryption in MAC level can be turned off without any risk in security. This setting can enhance the system throughput. Also, the implemented method achieves zero bit disagreement rates with key generation on the fly.

Channel Estimation on Individual Subcarrier Using Image Processing in Optical OFDM System

In an optical orthogonal frequency division multiplexing (OFDM) system, channel impairments, such as residual chromatic dispersion, residual synchronization error, and clock frequency offset, induce phase shifts over subcarriers. We propose a method of using image processing for phase channel estimation with negligible overhead, which is applicable to various subcarrier modulation formats. We conduct the experiment of 32-, 40-, and 48-Gbit/s direct detection optical OFDM with 16QAM, 32QAM, and 64QAM subcarrier modulation and simulation of 112-Gb/s coherent optical OFDM to verify this method and compare the results with those of the conventional method. Then, we investigate the effects of the numbers of required test phases and OFDM symbols with experimental data. We also present an approach of removing the inner part of constellation points to reduce the complexity.

Demonstration of 2.97-Gb/s video signal transmissions in DML-based IM-DDO-OFDM systems

Abstract To further investigate the feasibility of the digital signal processing (DSP) algorithms (e.g., symbol timing synchronization, channel estimation and equalization, and sampling clock frequency offset (SCFO) estimation and compensation) for real-time optical orthogonal frequency-division multiplexing (OFDM) system, 2.97-Gb/s real-time high-definition video signal parallel transmission is experimentally demonstrated in OFDM-based short-reach intensity-modulated direct-detection (IM-DD) systems. The experimental results show that, in the presence of ∼12 ppm SCFO between transmitter and receiver, the adaptively modulated OFDM signal transmission over 20 km standard single-mode fiber with an error bit rate less than 1 × 10−9 can be achieved by using only DSP-based small SCFO estimation and compensation method without utilizing forward error correction technique. To the best of our knowledge, for the first time, we successfully demonstrate that the video signal at a bit rate in excess of 1-Gb/s transmission in a simple real-valued inverse fast Fourier transform and fast Fourier transform based IM-DD optical OFDM system employing a directly modulated laser.

A PAPR reduction scheme based on a new spreading code in optical direct detection OFDM system

In this paper, we propose and experimentally demonstrate a peak-to-average power ratio (PAPR) reduction scheme based on a new spreading code in direct detection optical orthogonal frequency division multiplexing (OFDM) system. The new spreading code with low cross correlation and high auto-correlation can support $$2N+1$$2N+1 users. Thus, $$2N+1$$2N+1 users or data symbols can be transmitted over only N subcarriers. The experimental results show that, after transmission over 70 km single-mode fiber, at the bit error rate of $$10^{-3}$$10-3, with fiber launch power of 2.75 dBm, the receiver sensitivity can be improved 2.1 dB by using the proposed scheme based on new spreading code. The PAPR can be reduced about 4.6 dB, compared with the original OFDM signal at a complementary cumulative distribution function of $$10^{-4}$$10-4.

Performance Evaluation of Pilot-Assisted PAPR Reduction Technique in Optical OFDM Systems

The pilot-assisted peak-to-average power ratio (PAPR) reduction technique proposed for optical orthogonal frequency division multiplexing (OFDM) communication systems is evaluated empirically and theoretically in this letter. The PAPR reduction is achieved by rotating the phase of data symbols with P iterations of randomly generated pilot symbol sequence. The results of our hardware implementation and analysis show a close agreement to that of computer simulations. In comparison with basic OFDM, where no PAPR reduction technique is implemented, experimental PAPR reduction gain of pilot-assisted OFDM at a complementary cumulative distribution function of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> with P = 5 is ~2 dB. This gain is ~0.2 dB less than that of analytical results. The experimental results also show that the pilot-assisted technique does not cause any significant deterioration of the bit error performance.

New hybrid peak-to-average power ratio reduction technique based on carrier interferometry codes and companding technique for optical direct-detection orthogonal frequency division multiplexing system

In direct-detection optical orthogonal frequency division multiplexing (OFDM) systems, the high peak-to-average power ratio (PAPR) will cause nonlinear effects in both electrical and optical devices and optical fiber transmission when the nonlinear amplifiers are employed. A new hybrid technique based on carrier interferometry codes and companding transform has been proposed and experimentally demonstrated to reduce the high PAPR in an optical direct-detection optical OFDM system. The proposed technique is then experimentally demonstrated and the results show the effectiveness of the new method. The PAPR of the hybrid signal has been reduced by about 5.7 dB when compared to the regular system at a complementary cumulative distribution function of 10 −4 . At a bit error rate of 10 −4 , after transmission over 100-km single-mode fiber with a μ of 2, the receiver sensitivity is improved by 3.7, 4.2, and 5 dB with launch powers of 3, 6, and 9 dBm, respectively.

Channel Equalization in Optical OFDM Systems Using Independent Component Analysis

We study the channel equalizer (CE) based on independent component analysis (ICA) for optical orthogonal-frequency-division-multiplexing (OFDM) systems without using training symbols (TSs). We begin by studying mathematical ICA models of optical OFDM systems and considering both the direct-detection optical OFDM (DDO-OFDM) and polarization division multiplexing coherent-detection optical OFDM (PDM-CO-OFDM) systems. One purpose of this paper is to provide a comprehensive study of ICA-based CE in both DDO-OFDM and CO-OFDM systems. Next, we propose a channel matrix initialization method to solve the permutation indeterminacy and complex uncertain scaling problems in ICA for both DDO-OFDM and PDM-CO-OFDM systems. Several algorithms are then investigated for ICA-based CEs including maximization of negentropy (MN), maximum likelihood (ML), minimization of mutual information (MMI), and fast ICA. It is found that the ICA-based CEs using MN, ML, and MMI can successfully recover the OFDM signal. Through both simulation and experiment, we show that in comparison to conventional TSs-based CEs with and without inter-symbol frequency-domain averaging (ISFA) and adaptive decision-directed CE, ICA-based CEs can provide slightly better or similar performances for both SP and DP optical OFDM systems with QPSK and 16-QAM modulations. Specifically, apart from higher spectral efficiency, the ICA-based CEs show significant better chromatic dispersion and polarization mode dispersion (PMD) tolerances than TSs-based CEs with ISFA in PDM-CO-OFDM systems over long-haul transmission.

Study on the improved peak-to-average-power ratio reduction technology based on the partial transmission sequences method in optical orthogonal frequency division multiplexing systems

According to the defect of the high peak-to-average-power ratio (PAPR) in optical orthogonal frequency division multiplexing (OFDM) systems, the PAPR reduction technology based on the partial transmission sequences (PTS) method has been deeply studied. An improved enhanced-iterative-algorithm-PTS (EIA-PTS) technology is proposed. The proposed EIA-PTS technology, compared with the original PTS (O-PTS), can reduce the computational complexity. The simulation analysis shows that the computational complexity of the O-PTS method grows exponentially with an increase in the number of both subblocks and phase factors, while the computational complexity of the EIA-PTS technology basically remains stable and is lower than that of the O-PTS method. On the basis of the proposed EIA-PTS technology, an improved EIA-PTS-Clipping combined PAPR reduction technology that combines EIA-PTS technology with clipping technology is proposed. The simulation result shows the proposed EIA-PTS-Clipping combined PAPR reduction technology, compared with the previous proposed EIA-PTS technology, can further improve the PAPR reduction performance and has a higher application value because it can have a better tradeoff between the bit error rate performance and PAPR reduction effect for optical OFDM systems.