Intensity Modulation With Direct Detection Research Articles

450 Gbps Low-cost Intensity Modulation with Direct Detection (IM/DD) Wave Length Division Multiplexing (WDM-PON) for 5G Fronthaul

450 Gbps Low-cost Intensity Modulation with Direct Detection (IM/DD) Wave Length Division Multiplexing (WDM-PON) for 5G Fronthaul

Layered hybrid PAM-DMT for IM/DD OWC systems.

Traditional pulse-amplitude-modulated discrete multitone modulation (PAM-DMT) suffers from poor overall performance of spectral and power efficiencies in optical wireless communication (OWC) systems. We propose layered hybrid PAM-DMT (LHPAM-DMT) to utilize more subcarriers to improve the performance. The real part of frequency domain signal is divided into several layers and symmetry biases are added in time domain to generate real-valued and nonnegative signals for intensity modulation with direct detection (IM/DD) OWC systems. By appropriately designing the orthogonality between the signals in lower layers and signals & added biases in higher layers, we further propose an iterative receiver to recover the transmitted information. Theoretical derivation proves that LHPAM-DMT has higher spectral efficiency than PAM-DMT and lower complexity than layered PAM-DMT. Numerical results suggest that LHPAM-DMT is more power efficient than PAM-DMT as well as direct-current (DC) biased optical OFDM (DCO-OFDM), one of the most popular schemes.

Binary Modeling and Capacity-Approaching Coding for the IM/DD Channel

The paper provides a new perspective on peak- and average-constrained Gaussian channels. Such channels model optical wireless communication (OWC) systems which employ intensity-modulation with direct detection (IM/DD). First, the paper proposes a new, capacity-preserving vector binary channel (VBC) model, consisting of dependent binary noisy bit-pipes. Then, to simplify coding over this VBC, the paper proposes coding schemes with varying levels of complexity, building on the capacity of binary-symmetric channels (BSC) and channels with state. The achievable rates are compared to capacity and capacity bounds, showing that coding for the BSC with state over the VBC achieves rates close to capacity at moderate to high signal-to-noise ratio (SNR), whereas simpler schemes achieve lower rates at lower complexity. The presented coding schemes are realizable using capacity-achieving codes for binary-input channels, such as polar codes. Numerical results are provided to validate the theoretical results and demonstrate the applicability of the proposed schemes.

Practical Considerations in Direct Detection Under Tukey Signalling

The deliberate introduction of controlled intersymbol interference (ISI) in Tukey signalling enables the recovery of signal amplitude and (in part) signal phase under direct detection, giving rise to significant data rate improvements compared to intensity modulation with direct detection (IMDD). The use of an integrate-and-dump detector makes precise waveform shaping unnecessary, thereby equipping the scheme with a high degree of robustness to nonlinear signal distortions introduced by practical modulators. Signal sequences drawn from star quadrature amplitude modulation (SQAM) formats admit an efficient trellis description that facilitates codebook design and low-complexity near maximum-likelihood sequence detection in the presence of both shot noise and thermal noise. Under the practical (though suboptimal) allocation of a 50% duty cycle between ISI-free and ISI-present signalling segments, at a symbol rate of 50 Gbaud and a launch power of -10 dBm the Tukey scheme has a maximum theoretically achievable throughput of 200 Gb/s with an (8,4)-SQAM constellation, while an IMDD scheme achieves about 145 Gb/s using PAM-8. Note that the two mentioned constellations have the same number of magnitude levels and the difference in throughput is resulting from exploiting phase information under using a complex-valued signal constellation.

Novel adaptive hard-switching based hybrid RF-FSO system architecture using two threshold values in the presence of atmospheric turbulence and pointing error

In recent years and in the ongoing development of all-optical systems, we propose two novel hard-switching based hybrid RF-FSO system architecture in the presence of atmospheric turbulence and pointing error. In the first proposed scheme, the hybrid Relay based RF-FSO link (path 1) is paralleled with a RF link (path 2). In the second scheme, the FSO (path 1) and relay based RF-FSO (path 2) links are paralleled.In these two novel schemes, we tried to increase the lifetime of the main path using the double threshold SNR technique. In these scenarios, the hard switch selects one path as a data transmitter using two threshold values. The RF and FSO links are modeled by Rayleigh and Gamma-Gamma distributions, respectively. We derive closed-form expressions for the Outage Probability (OP). Also, the Intensity Modulation with Direct Detection (IM/DD) and the heterodyne detection techniques are used. The performance of proposed systems are presented in the different atmospheric and pointing error conditions. Numerical results show that the proposed schemes have succeeded in achieving the set goal.

Reducing peak to average power ratio (PAPR) for visible light communications (VLC) system using Walsh–Hadamard precoded discrete Hartley transforms

Abstract In visible light communication (VLC), intensity modulation with direct detection (IM/DD) based on DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) is used to transfer data at a high rate. Because OFDM signals are modulated using IM/DD, the peak of the transmitted signals is scaled up, resulting in a high peak-to-average power ratio (PAPR). In this article, the Walsh–Hadamard transform (WHT) is used as a precoder for discrete Hartley transform (DHT) to decrease the PAPR and the DC bias and achieve a good bit error rate (BER) performance in such a system. PAPR reduction results from the decrease of superposition of the transmitted signals owing to the use of WHT as a precoder. The proposed scheme and conventional DCO-OFDM indoor VLC system based on discrete Hartley transform (DHT) are compared to assure the new system’s benefits and efficiency.

A modified Volterra equalizer for compensation distortion in C-band DML-based short reach limited-bandwidth system with 80-Gb/s PAM-4 signals

In this paper, we investigate the system model of intensity modulation with direct detection (IM/DD)-based high-speed short-reach link with directly modulation laser (DML) and bandwidth-limitation optics devices. And, based on this model, we propose a modified Volterra equalizer to compensate distortion by constructing second-order feature vectors for support vector machine (SVM) trainer in this high-capacity and limited-bandwidth system. Owing to high-dimensional mapping characteristics of SVM, for our scheme, only a single classification hyperplane is needed to be trained to achieve satisfactory equalization performance for the multi-classification tasks of PAM-M, which would reduce the calculation complexity. Moreover, the 80-Gb/s PAM-4 per wavelength transmission experiment with 10-G optics in 20-km standard single mode fiber (SSMF) DML system is conducted and utilized to manifest the feasibility and effectiveness of our scheme. The results show that our scheme can reach the HD-FEC threshold and achieve outstanding performance than commonly used equalizers.

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.

Low-complexity nonlinear equalizer based on artificial neural network for 112 Gbit/s PAM-4 transmission using DML

Artificial neural network (ANN) can easily approximate a nonlinear function and has strong nonlinear representation ability. Nonlinear equalizer based on artificial neural network (ANN-NLE) has been proved to be effective in dealing with linear and nonlinear impairments in different types of short reach intensity modulation with direct detection (IM/DD) systems. However, the complexity of ANN-NLE is very high, which limits the application in low-cost direct detection systems. We propose a novel weight pruning method named dynamic pruning to reduce the complexity of ANN-NLE. We demonstrate a C-band 112 Gbit/s four-level pulse-amplitude modulation (PAM-4) system over 4 km standard single mode fiber (SSMF) transmission. The simulation results show that the dynamic pruning method and weight pruning method have similar computational complexity after pruning, and the dynamic pruning method has better performance and lower bit error rate (BER).

On the Performance of Mixed FSO-UWOC Dual-Hop Transmission Systems

In this letter, a performance study of a mixed dual-hop communication system consisting of free-space optical/underwater wireless optical communication (FSO-UWOC) links is presented. In particular, considering both heterodyne detection and intensity modulation with direct detection (IM/DD), we assume that the FSO channel suffers a Gamma-Gamma fading in the presence of pointing error impairments, while the UWOC channel experiences a mixture Exponential-Generalized Gamma (EGG) fading with pointing errors. Assuming a fixed-gain amplify-and-forward (AF) protocol, closed-form expressions for the outage probability, average bit-error-rate, and average capacity in terms of the bivariate Fox’s H-function are derived. To verify the derived analytical expressions, numerical results are provided to verify the accuracy of our results.

190Gbaud OOK signal generation with a sub-sampling rate of 0.63 using silicon photonic dual-drive Mach–Zehnder modulator

Intensity modulation with direct detection (IM/DD) has been served as a low-cost technology of data center optical interconnects. To achieve higher data rate, high-order partial response (PR) signaling is utilized to narrow the signal bandwidth below the Nyquist frequency of digital-to-analog converter (DAC) sampling rate. In this work, we experimentally demonstrate ultra-high speed optical interconnects based on a conventional silicon photonic traveling-wave Mach–Zehnder modulator (MZM) with a 3-dB bandwidth of 28.5 GHz. By applying PR narrowing and digital anti-aliasing brick-wall filter that serves to avoid spectral overlap, up to 190Gbaud OOK signal is generated using a 120GSa/s DAC with a sampling ratio as low as 0.63. Furthermore, a 180Gbaud OOK signal is generated and transmitted over 500 m standard single-mode fiber (SSMF) below 20% hard-decision forward error-correction (HD-FEC) threshold of 1.5 × 10−2.

Impact of Correlation and Pointing Error on Secure Outage Performance Over Arbitrary Correlated Nakagami-$m$ and $\mathcal {M}$-Turbulent Fading Mixed RF-FSO Channel

In Radio Frequency (RF)-Free Space Optical (FSO) mixed links, secrecy capacity (SC) can be improved by exploiting spatial diversity (i.e., antenna diversity) in the RF path. In addition to atmospheric turbulence and point error of the FSO link, antenna correlation in the RF link can significantly deteriorate the secrecy performance. In this paper, a secrecy rate of wiretap channels with a single source, relay, destination, and eavesdropper is analyzed under practical environments with the aforementioned impairments. The RF hop (source-to-relay) and the FSO hop (relay-to-destination) are modeled utilizing arbitrarily correlated Nakagami- $m$ and Malaga ( $\mathcal {M}$ ) distributions, respectively. The correlated signal branches of the RF hop are combined at the relay exploiting equal gain combining reception technique. We assume that the eavesdropper is capable of wiretapping via RF and FSO links separately. We derive novel closed-form expressions for secrecy outage probability (SOP) and strictly positive secrecy capacity (SPSC) considering heterodyne detection (HD) and intensity modulation with direct detection (IM/DD) techniques in order to examine the impact of atmospheric scintillation, pointing error, fading, and correlation on the system's secrecy performance. It is shown that the HD technique exhibits a better performance than an IM/DD technique. In addition, similar to the pointing error and turbulent fading, the correlation imposes a detrimental impact on SC. Finally, Monte-Carlo simulation results are provided for validation of the derived expressions.

Secure hybrid satellite-UWOC cooperative relaying system under malicious unmanned aerial vehicle eavesdropper threat

In this paper, the secrecy performance of a hybrid satellite-Underwater Optical Communication (UWOC) system in the presence of an Unmanned Aerial Vehicle (UAV) eavesdropper is investigated. The satellite and eavesdropper Radio Frequency (RF) links are respectively subjected to Shadowed-Rician and Nakagami-m fading distributions whereas the UWOC link experiences mixed Exponential-Gamma distribution under difference detection schemes such as heterodyne detection and Intensity Modulation with Direct Detection (IM/DD). Specifically, the equivalent Cumulative Distribution Function (CDF) closed-form expression of the concerned system is then obtained. Based on this, the analytical closed-form expressions of the system Connection Outage Probability (COP), Secrecy Outage Probability (SOP), Strictly Positive Secrecy Capacity (SPSC) and Average Secrecy Capacity (ASC) are derived. By our findings, it is found that the satellite shadowing effect, air bubbles level and temperature gradients and eavesdropper distance significantly have impact on the system secrecy performance. The result also illustrated that the heterodyne detection outperforms the IM/DD under the system and channel conditions. Finally, the accuracy of the analytical expressions is justified through Monte-Carlo simulations.

Performance Analysis of Dual-Hop Wireless Systems Over Mixed FSO/RF Fading Channel

This paper investigates the performance of a dual-hop mixed free-space optical (FSO)/radio frequency (RF) system. To this end, the FSO link is subject to the Gamma-Gamma distribution effects that account for pointing error impairments and under both heterodyne detection and intensity modulation with direct detection (IM/DD) methods, while the RF link undergoes the α- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</i> distribution fading. For amplify-and-forward (AF) and decode-and-forward (DF) relaying, we derived the closed-form expressions of various important performance metrics by using bivariate Fox's H-functions, such as the average bit-error rate (BER), end-to-end outage probability (OP), effective capacity, and ergodic capacity. We unveil the effects of system and channel parameters on performance. Furthermore, we provide an asymptotical analysis to discuss significant diversity gains. Note that our results include previous works as special cases, which provides a framework. Finally, our derived expressions have confirmed the correctness via exactly matching Monte Carlo simulation results.

Applications of Meijer’s Factorization Theorems in Performance Analyses of All-Optical Multi-Hop FSO Systems

The use of bivariate Fox H-functions (BFHFs) in performance analyses of wireless communication systems has gained considerable attention in past few decades. However, the non-existence of robust built-in routines for evaluating such functions in standard computing systems poses numerous challenges in numerical experiments and simulations. Motivated by the apparent need to circumvent these difficulties in performance analyses of cooperative wireless communications, this work presents an alternative method for obtaining the exact, approximate and asymptotic BFHF-free cumulative distribution function (CDF) of the end-to-end (e2e) signal-to-noise ratio (SNR) of multi-hop amplify-and-forward (AF) relaying wireless communication systems. As an illustration, the e2e performance analysis of an all-optical dual-hop free-space optical (FSO) transmission system over Gamma-Gamma turbulence in the presence of pointing errors is revisited. Specifically, new mathematical formulae for the statistical characteristics of the e2e SNR for systems with AF fixed-gain relaying as well as channel-state-information(CSI)-assisted using heterodyne detection (HD) or intensity modulation with direct detection (IM/DD) are derived in terms of mathematically malleable and uniformly convergent infinite series of weighted Meijer G-functions. The usefulness of the derived CDFs is illustrated through derivation of traditional system performance metrics. The accuracy of the derived analytical formulae is verified via Monte Carlo simulations in MATLAB®. Finally, based on results observed in this paper, useful expansions of common BFHFs in terms of easily computable univariate hypergeometric functions are proposed.

Empirical Study of the Underwater Turbulence Effect on Non-Coherent Light

This letter presents an experimental study comparing the relative impact of turbulence induced scattering on coherent and non-coherent light propagating through water. It is shown that the scintillation index increases with increasing temperature inhomogeneity in the underwater channel. Our results indicate that a light beam from a non-coherent source has a greater resilience to temperature inhomogeneity induced turbulence effect in an underwater channel. These results will help researchers in simulating realistic channel conditions when modelling a light emitting diode (LED) based intensity modulation with direct detection (IM/DD) underwater optical wireless communication (UOWC) link.

On the Performance Analysis of Higher Order QAM Schemes Over Mixed RF/FSO Systems

In this work, the performance of a dual-hop variable gain amplify-and-forward (AF) mixed radio frequency (RF)/free space optics (FSO) system is analyzed in details. As variable gain AF relay network is considered, processing delay results in outdated channel state information (CSI) during amplification at the relay. For general applicability, the RF link is modeled with the generalized Nakagami-m fading. The FSO link is modeled with the Gamma-Gamma distribution which is affected with the atmospheric turbulence and pointing error impairments, and both the intensity modulation with direct detection (IM/DD) and heterodyne detection are employed at the FSO receiver. In this context, analytical expressions of outage probability and ergodic capacity are derived in terms of Meijer-G function and extended generalized bivariate Meijer-G function. For diversity order, asymptotic outage probability expression is also derived. Further, cumulative distribution function based generalized average symbol error rate expressions for various quadrature amplitude modulation (QAM) schemes such as hexagonal QAM, rectangular QAM, and cross QAM are derived in terms of Meijer-G function. Furthermore, a detailed comparative study of various modulation schemes is presented and the impact of pointing error, atmospheric turbulence, outdated CSI, and Nakagami-m parameter are highlighted on the system performance. Finally, all the analytical results are verified through Monte-Carlo simulations.

On the Effect of Interference and Misalignment Error in Mixed RF/FSO Systems Over Generalized Fading Channels

In this paper, the end-to-end performance of a mixed radio frequency (RF)/free space optical (FSO) affected by co-channel interference (CCI), is studied. We consider that the RF link experiences $\eta -\mu $ fading and the FSO link is subjected to atmospheric turbulence, which is modeled by the $\alpha -\mu $ distribution. Also, the statistics of the FSO link is presented for the case of zero and non-zero boresight pointing errors. Furthermore, we assume intensity modulation with direct detection (IM/DD) and coherent demodulation. In particular, we present a closed-form expression for the probability density function of the FSO link, which is then used to obtain a closed-form and an asymptotic expression for the outage probability. In order to quantify the system performance, we utilize this asymptotic result to yield the system’s coding gain and diversity order. Moreover, we have presented analytical expressions for the average bit error rate (BER) and ergodic capacity for the system design. In order to gain more insights, high signal-to-noise ratio (SNR) approximations expressions for the BER and ergodic capacity, are also derived. Finally, the analytical results presented in the paper are validated through computer simulations.

Secrecy Performance Analysis of Mixed Hyper-Gamma and Gamma-Gamma Cooperative Relaying System

In this paper, we investigate a secure dual-hop radio frequency-free space optical (RF-FSO) mixed variable gain relaying framework in the presence of a single eavesdropper. The RF and FSO links are modeled with hyper Gamma (HG) and Gamma-Gamma (ΓΓ) fading channels, respectively. We assume that the eavesdropper utilizes another HG fading channel to wiretap the transmitted confidential data from the RF link. Our key concern is to defend this information against passive eavesdropping. We carry out the secrecy measurements by deriving closed-form mathematical expressions of average secrecy capacity (ASC), secure outage probability (SOP), and strictly positive secrecy capacity (SPSC), all in terms of Meijer's G function. Capitalizing on the derived expressions, we analyze the impacts of atmospheric turbulence and pointing errors on the secrecy capacity and outage performance of the proposed scenario. For gaining more insights, we also analyze the asymptotic outage behaviour for high signal-to-noise ratio. Two detection techniques i.e. heterodyne (HD) and the intensity modulation with direct detection (IM/DD) are taken into consideration and our results demonstrate that HD technique notably outperforms the IM/DD scheme. The supremacy and novelty of the model is demonstrated via utilizing generic properties of the HG fading channel. Finally, we provide a justification of the derived expressions via Monte-Carlo simulations.

Spectrum-Efficient Triple-Layer Hybrid Optical OFDM for IM/DD-Based Optical Wireless Communications

In this paper, a triple-layer hybrid optical orthogonal frequency division multiplexing (THO-OFDM) for intensity modulation with direct detection (IM/DD) systems with a high spectral efficiency is proposed. We combine $N$ -point asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM), $N$ /2-point ACO-OFDM, and $N$ /2-point pulse amplitude modulated discrete multitoned (PAM-DMT) in a single frame for simultaneous transmission. The time- and frequency-domain demodulation methods are introduced by fully exploiting the special structure of the proposed THO-OFDM. Theoretical analysis show that, the proposed THO-OFDM can reach the spectral efficiency limit of the conventional layered ACO-OFDM (LACO-OFDM). Simulation results demonstrate that, the time-domain receiver offers improved bit error rate (BER) performance compared with the frequency-domain with ~40% reduced computation complexity when using 512 subcarriers. Furthermore, we show a 3 dB improvement in the peak-to-average power ratio (PAPR) compared with LACO-OFDM for the same three layers.