Intensity Modulation With Direct Detection Research Articles

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.

Performance analysis for mixed [formula omitted] shadowed and exponentiated Weibull distributed dual-hop system with multiuser diversity in C-RAN

This paper investigates the performance of mixed radio-frequency/free-space optical (RF/FSO) system with multiuser diversity over mixed fading channels suitable for the Cloud Radio Access Network (C-RAN). Specifically, the considered system consists of K users, a decode-and-forward (DF) relay and a destination. The relay receives the data from the user with best channel conditions and forwards it to the destination. Furthermore, supposing that the RF link experiences κ−μ shadowed fading which combines small-scale multipath fading with line-of-sight shadowing, whereas the FSO link is subject to Exponentiated Weibull (EW) distributed turbulence. We first derived a novel probability density function (PDF) expression for the κ−μ shadowed fading so that the κ, m and can be equal to the arbitrary positive real value. Then, analytical expressions for the end-to-end outage probability (OP) and average bit error rate (ABER) of the overall network are derived assuming an intensity modulation with direct detection (IM/DD) scheme. The closed-form asymptotic expressions at high signal-to-noise (SNR) regimes are also presented. Capitalizing on the asymptotic analysis, we can obtain the diversity gain and discuss the bottlenecks in the mixed RF/FSO systems. Additionally, the impact of several parameters, including the number of users, atmospheric turbulence conditions and pointing errors on the overall system are studied. The simulation results are provided to prove the feasibility of deployment of the system in real-life scenarios.

Mitigation of Pattern-Dependent Effect in SOA at O-Band by Using DSP

Semiconductor optical amplifier (SOA) is regarded as a promising candidate to amplify the optical signal in next-generation access network, especially for O-band signal transmission. However, the SOA-based system suffers from the pattern-dependent effect due to its gain saturation. Compared to digital signal processing (DSP) at receiver, DSP in the transmitter can reduce implementation complexity, which is attractive for the multi-user system. In this paper, we propose three methods to mitigate the pattern-dependent effect in SOA and compare their performance based on: 1) look-up-table (LUT) considering pattern-dependent effect, 2) pre-distortion without considering pattern-dependent effect, 3) probabilistic shaping (PS). The performance of three mitigation methods is investigated in an intensity modulation with direct detection (IM/DD) 4-ary pulse-amplitude-modulation (PAM-4) transmission system at O-band. Our research results show that all these schemes can improve the receiver dynamic range of PAM-4 signals at 50 Gbit/s and 100 Gbit/s. Moreover, PS and LUT can also improve the receiver dynamic range of 75-Gbit/s PAM-8 signals.

A new relay based architecture in hybrid RF/FSO system

In this paper, a novel architecture in mixed Radio-Frequency (RF)/Free-Space-Optical (FSO) communication systems is presented. RF and FSO links based on Amplify-and-Forward (AF) relay with variable gain, are serially connected. In the proposed method, RF-FSO or FSO-RF paths are used for data transmission. The fading of RF and FSO systems are modeled by Rayleigh and Gamma–Gamma distributions, respectively. It is assumed that the Intensity Modulation with Direct Detection (IM/DD) and the heterodyne detection techniques are used in FSO links. New closed-form expressions for Outage Probability (OP) and Bit Error Rate (BER) are presented in this article. The performance of the proposed system is compared with the single RF-FSO system in different atmospheric disturbance conditions. Numerical results show that the new RF-FSO/FSO-RF system has a significant improvement over the single RF-FSO system. For SNR=40 dB, the outage probability for our proposed system and RF-FSO are obtained, 2.3×10−6, and 3×10−3, respectively.

IM/DD Beyond Bandwidth Limitation for Data Center Optical Interconnects

Intensity modulation with direct detection (IM/DD) provides a low-cost solution for data center optical interconnects. To achieve higher data rate, Tomlinson–Harashima precoding (THP) is adopted to increase the operating symbol rate beyond the Nyquist rate defined by the system bandwidth. By applying THP, the signal is precoded by a feedback filter at the transmitter for the mitigation of the intersymbol interference caused by the faster-than-Nyquist signaling and the spectral power fading induced by the interaction of chromatic dispersion and direct detection. In this paper, high-performance THP-based IM/DD transmission is demonstrated, increasing the achievable data rate using a limited system bandwidth. In the experimental demonstration, we achieve a net bit rate of 185 Gb/s for up to 2-km fiber link, transmitting 74 GBd precoded eight-level pulse amplitude modulation (PAM-8) using an IM/DD system with 33 GHz brick-wall electrical bandwidth limitation.

Generalized channel estimation and data detection for MIMO multiplexing FSO parallel channels over limited space

In this paper, to increase the data rate of free-space optical (FSO) links, we consider multiple parallel FSO links arranged over a specified space (transceivers arranged over two rings with specified radius). Due to its ease of implementation, stability, and low cost, intensity modulation with direct detection (IM/DD) based on On–Off-Keying (OOK) is widely employed in practical terrestrial FSO links. For long-haul FSO links, avalanche photo diodes (APDs) are commonly used, which provide an internal gain in photo-detection, allowing larger transmission ranges, as compared with PIN photo-detector counterparts. Over the considered general case, firstly, we propose maximum likelihood (ML)-based detection methods when the receivers are aware of the instantaneous channel fading coefficients (CSI). Secondly, we propose an efficient OOK data detection method for a more practical case when the instantaneous CSI is not available at the receiver. For this state, initially, the channel coefficients are estimated by employing some pilot symbols and then, we use the channel estimated coefficients to detect OOK signals. For a specified space, due to crosstalk between parallel channels and pointing errors, performance of proposed data detection methods depends on the optimum value for beam waist. By providing Monte-Carlo simulation results, the optimum value of beam waist is obtained for various channel conditions. To evaluate the BER performance, we also provide analytical results and derive an analytical expression for the considered MIMO multiplexing scheme. The validity of our analytical results are confirmed by Monte-Carlo simulations.

On performance analysis of hybrid FSO/RF systems

In this work, the author investigate the performance of the hybrid free-space optical/radio frequency (FSO/RF) communication systems where RF links are subject to Nakagami-m distribution fading effects. The author derive the statistical characteristics of the hybrid FSO/RF systems in closed-form in terms of Meijer's G-function and the extended generalised bivariate Meijer's G-function. Then, the author rely on these new results to present the performance analysis of the selection combining diversity based on hybrid FSO/RF systems. Also, the author consider a Gamma–Gamma turbulence distribution under the impact of pointing errors and subject to both types of detection techniques (i.e. heterodyne detection and intensity modulation with direct detection (IM/DD)) in the FSO link. Computer-based Monte–Carlo simulations are proved the accuracy of the new analytical results.

A framework on the performance analysis of relay-assisted FSO transmission systems

In this work, we tend to analyze the performance of dual-hop relayed free-space optical (FSO) transmission systems. To do this, the FSO links are modeled as the Double Generalized Gamma (DGG) distribution with generalized pointing error impairment and under both heterodyne detection (HD) and intensity modulation with direct detection (IM/DD) methods. Using amplify-and-forward (AF) fixed-gain relaying as well as channel state information (CSI)-assisted AF relaying along with decode-and-forward (DF) relaying, we derive closed-form expressions for the outage probability (OP), the average bit error rate (BER) and the ergodic capacity (EC) in terms of the bivariate Fox-H function. Additionally, we analyze high signal-to-noise-ratio (SNR) behavior of the OP and formerly we derive the diversity order of the considered system. To asses the accuracy of the obtained results, numerical and Monte-Carlo simulation results are provided. Based on the high SNR analysis, we further provide a diversity gain analysis and discuss the bottlenecks in the dual-hop FSO systems.

Multiuser diversity for mixed RF/FSO cooperative relaying in the presence of interference

This paper presents the performance of multiuser diversity over mixed radio frequency / free space optical (RF/FSO) relay network in presence of co-channel interference at relay and destination nodes. The multiuser RF link between source-to-relay experiences Nakagami-m fading, while the FSO link between relay-to-destination is assumed to be affected by α−μ distributed atmospheric turbulence. Moreover, the amplitudes of the interfering signals at relay and destination nodes are assumed to follow independent Nakagami-m fading distribution. We consider both forms of optical demodulation schemes i.e., coherent detection and intensity modulation with direct detection (IM/DD) demodulation. Novel expression of the end-to-end cumulative distribution function (CDF) for the proposed mixed RF/FSO system has been derived which is further utilized to obtain the closed-form expression for outage probability and bit-error-rate of the overall system. Additionally, we present asymptotic expressions of outage probability and bit-error-rate to yield quick insights of the system performance. Finally, the numerical results are provided to examine the effect of various system parameters which are also supported by Monte-Carlo simulations.

Comparison of Bit-Loading DMT and Pre-Equalized DFT-Spread DMT for 2-km Optical Interconnect System

In this paper, we experimentally compared the performance of bit-loading discrete multi-tone (DMT) and pre-equalized discrete-Fourier-transform spread (DFT-Spread) DMT in intensity modulation with direct detection (IM/DD) system for 2-km optical interconnect for the first time. Pre-equalization is applied to compensate the high-frequency power fading in wideband communication system. However, it remains to be verified whether zero-forcing (ZF) channel estimation based complete pre-equalization can achieve the best performance for system with serious high frequency fading. In an IM/DD system with 120 Gbit/s data rate, we demonstrated that the bit error rate of DFT-Spread DMT signal with optimized quadratic curve fitting based partial pre-equalization outperforms ZF based complete pre-equalization after 2-km standard single mode fiber (SSMF) transmission. What's more, we experimentally compared the performance of pre-equalized DFT-Spread DMT and Chow–Cioffi–Bingham (CCB) algorithm-based bit loading DMT. The results show that the receiver sensitivity of pre-equalized DFT-Spread DMT is improved by 2.5 dB compared with CCB algorithm-based bit-loading DMT at hard decision forward error correction threshold of 3.8 × 10−3 after 2-km SSMF transmission.

Effect of input impedance mismatch on the large-signal noise figure of microwave photonic links

In this paper the effect of mismatch and loss of the input lossy impedance matching network on the noise figure (NF) of microwave photonic links (MWPLs) operating under large radio frequency (RF) signal modulation is theoretically investigated. An intensity-modulation with direct-detection (IMDD) MWPL in which the external modulator is a Mach-Zehnder modulator (MZM) is studied here. The nonlinear input-output relationship of the MZM under large RF signal modulation can lead this link to operate in the nonlinear large-signal regime. The main goal of this paper is to investigate and find the input impedance mismatch conditions for minimizing large-signal NF of MWPLs. To the best of our knowledge, this is the first study on this subject. It is found that large-signal NF of an IMDD MWPL depends on the input impedance mismatch factor and input applied RF power. It is shown that for M≃0.39 the NF can be minimized that is approximately 0.95dB lower compared to the NF at the perfect match (M=1). Regarding a generic optoelectronic oscillator (OEO) containing a MWPL at saturation, the value of this analytical approach is to study the effect of such large-signal NF on the OEO phase noise performance.

Performance evaluation of an IMDD optical OFDM-CDMA system.

In this paper, we propose a modulation technique for passive optical networks that harnesses two-dimensional prime hop system optical code division multiplexing access (OCDMA) and optical orthogonal frequency-division multiplexing (OFDM) for intensity modulation with direct-detection (IMDD) to enhance users' signal capacity in a cost-effective manner. The theoretical analysis is built from an analytical formula that takes into account both multiple-access interference and photodetector noise. Results show that OFDM-OCDMA with multiple users has similar performance to single-user conventional OOFDM for low transmitted powers.

Performance analysis of EM-based blind detection for ON–OFF keying modulation over atmospheric optical channels

In the free-space optical (FSO) links, atmospheric turbulence lead to scintillation in the received signal. Due to its ease of implementation, intensity modulation with direct detection (IM/DD) based on ON–OFF keying (OOK) is a popular signaling scheme in these systems. Over turbulence channel, to detect OOK symbols in a blind way, i.e., without sending pilot symbols, an expectation–maximization (EM)-based detection method was recently proposed in the literature related to free-space optical (FSO) communication. However, the performance of EM-based detection methods severely depends on the length of the observation interval (Ls). To choose the optimum values of Ls at target bit error rates (BER)s of FSO communications which are commonly lower than 10−9, Monte-Carlo simulations would be very cumbersome and require a very long processing time. To facilitate performance evaluation, in this letter we derive the analytic expressions for BER and outage probability. Numerical results validate the accuracy of our derived analytic expressions. Our results may serve to evaluate the optimum value for Ls without resorting to time-consuming Monte-Carlo simulations.

Interference in IM/DD Optical Wireless Communication Networks

The emerging field of optical wireless communications (OWC) offers exceptional promise as a technology for next-generation wireless networks. High data rate capabilities and ultra-dense access point deployment will allow OWC to supplement traditional RF technologies and remove congestion from the crowded RF spectrum. When implementing OWC via intensity modulation with direct detection (IM/DD), peak optical power emission constrains the instantaneous optical power. Average optical power is also constrained by eye safety regulations and illumination requirements of infrared and visible light communication (VLC) systems, respectively. These constraints differ from the conventional electrical power constraint of RF and wireline systems. Accordingly, performance metrics such as signal-to-noise ratio (SNR) and signal-to-interference-plus-noise ratio (SINR) have been redefined in relation to the optical channel constraints in order to provide fair comparison across OWC implementations. In densely deployed networks, interference has a significant effect on system performance. Two key properties simplify the analysis of RF networks: 1) the relationship between electrical power and interference variance is modulation agnostic and 2) many interferers are typically assumed. The former allows SINR to be defined in terms of the channel constraint and the latter allows the aggregate disturbance from interference-plus-noise to be modeled as an additive Gaussian random component. In OWC networks, the optical power constraints relate to interference variance in a modulation specific manner, and the highly directional optical channel creates instances where a small set of interferers dominate the aggregate disturbance. In this work, we first derive bounds on the variance of OWC interference under the constraints of an OWC channel. We then evaluate the accuracy of assuming that interference follows a Gaussian distribution. Finally, we show results of a study in a testbed environment with 15 VLC-enabled LED luminaires in order to empirically evaluate OWC interference characteristics.

FSO channel estimation for OOK modulation with APD receiver over atmospheric turbulence and pointing errors

In the free-space optical (FSO) links, atmospheric turbulence and pointing errors lead to scintillation in the received signal. Due to its ease of implementation, intensity modulation with direct detection (IM/DD) based on ON–OFF-keying(OOK) is a popular signaling scheme in these systems. For long-haul FSO links, avalanche photo diodes (APDs) are commonly used, which provide an internal gain in photo-detection, allowing larger transmission ranges, as compared with PIN photo-detector (PD) counterparts. Since optimal OOK detection at the receiver requires the knowledge of the instantaneous channel fading coefficient, channel estimation is an important task that can considerably impact the link performance. In this paper, we investigate the channel estimation issue when using an APD at the receiver. Here, optimal signal detection is quite more delicate than in the case of using a PIN PD. In fact, given that APD-based receivers are usually shot-noise limited, the receiver noise will have a different distribution depending on whether the transmitted bit is ‘0’ or ‘1’, and moreover, its statistics are further affected by the scintillation. To deal with this, we first consider minimum mean-square-error (MMSE), maximum a posteriori probability (MAP) and maximum likelihood (ML) channel estimation over an observation window encompassing several consecutive received OOK symbols. Due to the high computational complexity of these methods, in a second step, we propose an ML channel estimator based on the expectation–maximization (EM) algorithm which has a low implementation complexity, making it suitable for high data-rate FSO communications. Numerical results show that for a sufficiently large observation window, by using the proposed EM channel estimator, we can achieve bit error rate performance very close to that with perfect channel state information. We also derive the Cramer–Rao lower bound (CRLB) of MSE of estimation errors and show that for a large enough observation window, this CRLB can be adequately tight.

On the Performance Analysis of Dual-Hop Mixed FSO/RF Systems

This paper presents novel results for the performance analysis of dual-hop free-space optical/radio frequency (FSO/RF) transmission systems where the FSO link is modeled by the Gamma-Gamma distribution with pointing error impairments and under both heterodyne detection and intensity modulation with direct detection (IM/DD), and the RF link experiences the generalized Nakagami- $m$ fading. Using amplify-and-forward fixed-gain relaying as well as channel-state-information(CSI)-assisted relaying, we derive closed-form expressions for the outage probability, the average bit-error rate (BER), and the ergodic capacity in terms of the bivariate H-Fox function. For a special case, we obtain simplified results for Nakagami- $m$ fading channels in the RF link. Furthermore, new asymptotic results for the outage probability and the average BER at high signal-to-noise ratio (SNR) regime are presented in terms of simple functions. Numerical and Monte-Carlo simulation results are provided to verify the accuracy of the newly proposed results, and a perfect agreement is observed.

Linearization of Direct Detection Optical Channels Using Self-Coherent Subsystems

Intensity modulation with direct detection (IM-DD) dominates the commercial short-reach optical communications. However, when upgrading the data-rate distance product to 1000 Gb/s·km per wavelength and beyond, IM-DD faces severe performance barrier. Although a linear mapping exists between the intensity of transmitter and receiver in IM-DD back-to-back system, it becomes nonlinear under fiber channel impairments; for instance, the dominant chromatic dispersion. Coherent detection overcomes this fundamental obstacle by recovering a linear replica of the optical field instead of intensity. The local oscillator provides a reference carrier which mixes with the received signal, from which both intensity and phase of the signal can be recovered. Borrowing the idea from coherent detection, in DD system, a carrier can be sent along with the signal so that the receiver utilizes the reference carrier originated from transmitter; namely, the receiver conducts self-coherent (SCOH) detection. In this paper, we review a variety of optical SCOH subsystems, and reveal how they realize linear channels, while maintaining the simple and low-cost DD. SCOH can readily reach a data-rate distance product beyond 10000 Gb/s·km, making it suitable for the future high-speed short- and medium-reach applications, such as the data center interconnect, passive access network, and metropolitan area network.

An optimal scaling scheme for DCO-OFDM based visible light communications

DC-biased optical orthogonal frequency-division multiplexing (DCO-OFDM) is widely used in visible light communication (VLC) systems to provide high data rate transmission. As intensity modulation with direct detection (IM/DD) is employed to modulate the OFDM signal, scale up the amplitude of the signal can increase the effective transmitted electrical power whereas more signals are likely to be clipped due to the limited dynamic range of LEDs, resulting in severe clipping distortion. Thus, it is crucial to scale the signal to find a tradeoff between the effective electrical power and the clipping distortion. In this paper, an optimal scaling scheme is proposed to maximize the received signal-to-noise-plus-distortion ratio (SNDR) with the constraint of the radiated optical power in a practical scenario where DC bias is fixed for a desired dimming level. Simulation results show that the system with the optimal scaling factor outperforms that with fixed scaling factor under different equivalent noise power in terms of the bit error ratio (BER) performance.

Performance comparison of analogue inter‐satellite microwave photonics link using intensity modulation with direct detection and phase modulation with interferometric detection

The performance of optical inter-satellite communications using microwave photonics technologies employing intensity modulation with direct detection (IMDD) and phase modulation (PM) with interferometric detection are investigated and demonstrated with a numerical example. The theoretical model considering the saturated gain and amplifier spontaneous emission noise of optical booster amplifier and optical preamplifier is constructed for both architectures. Furthermore, the authors compare the performance of PM with interferometric detection to optimum biased IMDD in terms of radio frequency (RF) gain, RF noise figure and compression dynamic range (CDR). The numerical results show that the PM with interferometric detection can achieve better RF gain, RF noise figure and CDR than the optimum biased IMDD link as the laser output power is lower than 14.34, 12.45 and 4.2 dBm, respectively.

Combined discrete Fourier transform precoding and clipping using direct detection optical OFDM

We propose and demonstrate experimentally the bit error rate (BER) performance of intensity modulation with direct detection (IM/DD) optical orthogonal frequency division multiplexing (OFDM) system by combining discrete Fourier transform (DFT) precoding with clipping. The experimental results show that the received sensitivity at a BER of 10−4 for a 12Gs/s DFT-precoded and clipped OFDM signal and after 100-km standard single-mode fiber transmission has been improved by 3.5dB when compared with the original OFDM systems in the SMF link, and by 2.5dB when compared with compared with the DFT-precoded OFDM signals. When compared with the DFT-precoded QPSK OFDM signal, the DFT-precoded and clipped OFDM QPSK signal can achieve approximately 4.5dB of peak-to-average power ratio (PAPR) reduction at a complementary cumulative distribution functional value of 10−3.