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An operation scheme using electrical peaking and optical peaking to engineer the modulation band of a Si microring modulator is presented. By incorporating an inductor design at the metal traces of a Si microring modulator, the driving signal can be magnified near the peaking frequency. Although adjusting the wavelength detuning of a ring modulator also introduces optical peaking to extend the 3-dB roll-off frequency, using inductive peaking has no detrimental effect on the low-frequency response. By exploiting both effects, the modulation band can be tailored with more degrees of freedom. We accomplish a Si microring modulator design with a wide and flat transmission band over 95 GHz, which is potentially applied for a non-return-to-zero (NRZ) data transmission over 120 Gbit/s without extra signal post-compensation.

Wireless intensity-modulated optical channels feature some very specific differences compared to wireless radio channels. These properties have a strong impact on the performance of digital modulation techniques. This paper discusses basics to consider when designing optical intensity modulation schemes. We introduce an error event probability expression for such modulation schemes that is generally applicable for all non-negative waveforms. We indicate the importance of amplitude fluctuations and the influence of pulse shaping on the power efficiency in the optical domain. In numerical examples, we apply the error event probability expression to modifications of orthogonal frequency-division multiplexing and to Hadamard coded modulation. [ABSTRACT FROM AUTHOR]

The scheme of combining relay and optical code division multiple access (OCDMA) is one of the effective methods to improve the reliability and security of free space optical (FSO) communication system. We experimentally investigate a 10 Gb/s decode-and-forward (DF) relaying FSO-CDMA system and a relaying amplify-and-forward (AF) FSO-CDMA system, respectively. Bit error rates (BERs) of DF and AF relaying FSO-CDMA are measured and compared under different turbulence conditions. The experimental results show that the BER of DF relaying FSO-CDMA is one to two orders of magnitude lower than that of AF relaying FSO-CDMA when the receiving power is −5.6 dBm. However, the performance gap is decreasing with the increase of receiving power, especially when there is an interfering user. At the receiving power −2.8 dBm, two-user AF and DF relaying FSO-CDMA systems have almost the same BER. Under both weak and moderate turbulence conditions, at the receiving power −2.8 dBm, the measured BER of the eavesdropper exceeds 0.1, which is larger than the BER threshold of soft-decision forward error correction code. Hence, physical layer security of FSO relaying system can be achieved by optical coding. [ABSTRACT FROM AUTHOR]

Multiple-input-multiple-output (MIMO) optical wireless communication systems are potential candidates for high-speed data transmission in indoor environments. In this work, we present an experimental evaluation of two different MIMO systems, i.e., conventional-MIMO (c-MIMO) and imaging-MIMO (i-MIMO) in terms of design considerations. Both systems use the same transmitter configuration featuring a one-dimensional (light-emitting diode) LED transmitting array. On the receiver side, the c-MIMO system employs a similar one-dimensional receiving avalanche photodetector (APD)-based array, whereas the i-MIMO system uses a customized imaging camera-based receiver design. To assess the performance of the two systems, the channel matrix and the bit-error rate are calculated based on the experimental data. At distances shorter than 50 m, the proposed c-MIMO system demonstrates a good transmission performance, particularly when the maximum likelihood detection (MLD) technique is applied. Whereas, at long distances ranging from 40 m to 65 m, the proposed i-MIMO system can ensure an error-free transmission due to the efficient spatial separation between the emitted optical beams. Furthermore, the experimental results confirm that our systems have the potential for high data rates over longer distances in indoor environments. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate an integrated passive optical network monitoring system for fault detection and localization. Fault detection technology is based on the recognition of reflection spectrum generated by each single fiber Bragg grating, which can support large-capacity user fiber fault monitoring. Fault localization method uses two different total received powers measured by an optical power meter to solve the established equation, and it shows a good accuracy compared with the OTDR measurement results. Fault detection and localization are carried out separately but share the same transmitter module. The introduction of the additional module ensures that the troubleshooting does not affect other normal user data communication. We also investigate the dynamic range requirements of the fault detection technology and the measurement error of the fault localization method. The calculation results show the required dynamic range for the recognition device can be maintained within a small range and the use of high-accuracy optical power meter can reduce the measurement error. The proposed monitoring system can simultaneously provide a large-capacity fault detection technology and high-accuracy fault localization method in a cost-sensitive passive optical network market. [ABSTRACT FROM AUTHOR]

Future non-terrestrial networks aim to achieve a throughput of Terabits/s. Therefore, free-space optical (FSO) communications have been adopted as a candidate solution due to their ability to achieve an extremely high data rate. Nonetheless, FSO communications are sensitive to the adverse effects of beam scintillation, beam-wander-induced pointing errors, free-space loss, and weather conditions. Space-air-ground (SAG) FSO transmission and hybrid single-hop (SH) FSO/radio frequency (RF) transmission are promising solutions to improve the performance of FSO links and can be integrated into a satellite communication (Satcom) system. In this work, we carry out a thorough capacity analysis of the resulting integrated SAG-FSO/SH-FSO/RF Satcom system, where Gamma-Gamma and Rician distributions are used to characterize FSO and RF links, respectively. The exact analytical expressions are derived and validated by Monte-Carlo simulations. We also obtain asymptotic expressions for the ergodic capacity in the high signal-to-noise ratio region. The numerical results highlight the significant potential of the integrated SAG-FSO/SH-FSO/RF Satcom system over existing solutions. We also show that the integrated Satcom system with intensity modulation and direct detection can achieve a capacity gain over that with a heterodyne detection technique over all satellite zenith angles. [ABSTRACT FROM AUTHOR]

This paper investigates the error bounds for position estimation in downlink visible light communication (VLC) systems. More precisely, we use Cramér-Rao lower bounds (CRLBs) to study the effect of signal-dependent shot noise (SDSN) on the error performance in visible light positioning. We consider synchronous, quasi-synchronous, and asynchronous visible light positioning (VLP) systems. The system performance is assessed based on the position estimation, and uses information from parameters relating to the time of arrival (TOA), time difference of arrival (TDOA), and received signal strength (RSS). The results demonstrate that SDSN has a negative impact on the error bounds in all considered scenarios. Moreover, the level of degradation observed is not uniform among all cases at higher SDSN levels. [ABSTRACT FROM AUTHOR]

GaN micro-LEDs have been spotlighted as promising candidates for high-speed optical communication, which may become important components in 6G communication blueprint. To achieve a superior network covering sea, land and air for 6G communication, we demonstrated cross-medium communication based on an 80 $\mu$ m double-sided emission micro-LED array. The high enough light output power radiated from both sides of the green micro-LED enabled interference-free signal transmission in different media. Data rates of 1.55 Gbps over the 3 m free-space link and 1.11 Gbps over the 3.4 m underwater link could be achieved simultaneously. Furthermore, parallel-connected micro-LEDs were utilized to demonstrate the feasibility of pattern display and communication, and expected to achieve longer communication distances in media such as underwater channels. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate an online PON monitoring scheme that can simultaneously achieve the objectives of fault detection and localization. Fault detection and localization share the same transmitter module and perform in sequence. Based on a simple algorithm, one or several fiber Bragg gratings with different central reflection wavelengths are allocated to each end-user for fault detection. A large number of wavelength combinations are generated by several fiber Bragg gratings with different center wavelengths, and are finally identified in the time domain, which is very suitable for fault detection of large-capacity and high-density PON. An additional module is added before the power splitter/combiner to ensure the online troubleshooting. The model equation is established to locate the fault using two unmodulated broadband light sources. The calculated results of the fault localization method show a good agreement with the OTDR measurement results. The proposed scheme provides an integrated online troubleshooting solution for the detection and localization of PON link faults. [ABSTRACT FROM AUTHOR]

Visible light communications achieve wireless communications using light sources such as lighting fixtures and electric signboards. The light sources need to blink at high speed to avoid flickering. However, the blinking frequency is much higher than the frame rate of the general-purpose cameras that are integrated into consumer devices such as smartphones. This paper proposes a method where the principle of the I/Q demodulator as employed in radio frequency communications is used to estimate the phase and amplitude of modulated light blinking at a sufficiently higher speed than the frame rate. This is achieved by using two cameras with slightly different exposure start timing and estimating the phase and amplitude from the pixel values of the light-emitting diode (LED) image in one frame for each camera. In contrast to the conventional method with a very short exposure time, the exposure time can be increased, resulting in higher efficiency of light capture. Additionally, we propose another configuration employing one camera and two LEDs. An experiment to verify that communication is possible with the proposed method, which achieved 200 bits/s with 60 frames per second cameras. We also confirmed the proposed methods have similar signal-to-noise ratio to bit error ratio performance. [ABSTRACT FROM AUTHOR]

A fiber nonlinearity compensation scheme based on a complex-valued dimension-reduced neural network is proposed. The proposed scheme performs all calculations in complex values and employs a dimension-reduced triplet feature vector to reduce the size of the input layer. Simulation and experiment results show that the proposed neural network needed only 20% of computational complexity to reach the saturated performance gain of the real-valued triplet-input neural network, and had a similar saturated gain to the one-step-per-span digital backpropagation. In addition, the proposed scheme was 1.7 dB more robust to the noise from training data and required less bit precision for quantizing trained weights, compared with the real-valued triplet-input neural network. [ABSTRACT FROM AUTHOR]

In this work we experimentally demonstrate the compensation of nonlinearities introduced by a semiconductor optical amplifier (SOA) operating as a power booster in a field-modulated direct detection fiber-optic system transmitter. We show that a combination of digital pre-compensation to deal with SOA nonlinearities, and digital post-compensation after Kramers-Kronig receiver for mitigating the impacts of chromatic dispersion and signal-signal beat interference (SSBI) is an effective scheme to gain ∼3dB in ROSNR in presence of SOA nonlinearity at 3dB of gain compression. [ABSTRACT FROM AUTHOR]

We introduce two-dimensional space plus time (2D+1) structure and numerically investigate it using a developed multilayer simulation framework, for the first time. The new structure is consisting of crossed grating with time-varying permittivity which is inspired from 1D+1. In this regard, we extend Fourier Modal Method (FMM) in a general approach for spacetime multilayer states. Our proposed framework is fast, robust, and powerful compared to various finite difference methods. We use the scattering matrix technique to develop the proposed spacetime simulation method for multilayer structures using a non-uniform stack of layers. The method is perfectly suitable to investigate the spatiotemporal effects of surfaces/metasurfaces which covers both the transverse electric and magnetic (TE & TM) polarizations. The results show more freedom to control the optical outcomes of the multilayer considering two spatial periodicities in addition to the modulation frequency to reach nonreciprocity as one of the main consequences of the proposed structure. Moreover, we investigate the condition and limitation of breaking the Lorentz rule for spacetime structures. 2D+1 structure is more controllable than the 1D+1 due to its greater ability to adjust spatial manipulation in addition to temporal variations to reach nonreciprocity applications, digital coding, beam steering, etc. [ABSTRACT FROM AUTHOR]

The non-line-of-sight (NLOS) underwater communication can offer a viable route in signal propagation and coverage, thus mitigating the pointing, acquisition, and tracking difficulties in line-of-sight optical communication. However, implementing the NLOS link is non-trivial. While the NLOS technique relies on light scattering, i.e., channel turbulence can facilitate NLOS communication, the associated path-loss (PL) can be significant. Signal fading can degrade link robustness, which arises due to ocean water temperature and salinity fluctuation and gradients. To evaluate the robustness of NLOS in natural waters, we systematically measure the link metrics, such as the bit error ratio, PL, and signal-to-noise ratio (SNR), of water bodies of uniform and nonuniform vertical salinity ranging from 30–40‰ (part-per-thousand). We found that salinity-induced turbulence can establish NLOS communication with PL reduction of 2.35 dB/m and SNR increase by 32.5 $\%$ for dynamic water. Furthermore, a strong correlation was obtained between the strength of signal fluctuations and the received SNR. Finally, we obtained a Gaussian distribution of the statistical scintillation behavior. These results demonstrated the benefit of using the NLOS regime for underwater wireless sensor networks for aiding designers and engineers. [ABSTRACT FROM AUTHOR]

Visible light communication (VLC) is a promising technology with a high data rate that can supplement radio frequency communication. Although VLC systems have a natural advantage of high security due to the line-of-sight light propagation characteristic, they are still vulnerable when facing an open environment. Device fingerprinting is a technique that is widely viewed to detect transmitter impersonation attack in radio frequency (RF) based wireless systems. In this paper, we introduce the fingerprinting technique to discriminate illegal transmitting devices in VLC systems. We first investigate the hardware imperfections of the VLC transmitter, which can provide a unique device ID. Then we implement a feature separation network for transmitter fingerprinting (TF-FSN) and design a two-stage training strategy to obtain a stable classifier. Finally, we experimentally demonstrate the feasibility and performance of the proposed method. The results show that the accuracy of identification and verification is 92.65% and 98%, respectively. Moreover, our method is robust over different distances and a wide range of signal-to-noise ratios. [ABSTRACT FROM AUTHOR]

Nowadays, a high level of security is required for the transmission of critical information. Quantum Key Distribution (QKD) systems are considered the best option to protect such information. Many studies have shown the efficiency of the QKD optical fiber inspired by M-ary pulse position modulation (MPPM). Free-Space Optical (FSO) links provide an efficient and effective data transmission system. However, cumulative effects of laser beam divergence, misalignment, and turbulence-induced fading on the received irradiance in the FSO link might allow an external eavesdropper located near the authorized receiver to break the transmission under certain conditions. This paper introduces the development of an FSO system based on the MPPM and the BB84 protocol (MPPM-BB84) over the Gamma-Gamma (GG) turbulence channel with pointing errors. Time Binning is implemented using MPPM to increase system security and reduce the quantum bit error rate (QBER). The system security is investigated under photon number splitting attack and excess noise. Closed-form expressions for asymptotic expressions of the average symbol error probability (SER), raw key rate (RKR), and secret key rate (SKR) are introduced. Moreover, the Monte-Carlo simulations are then used to prove the validity of the analytical results. The optimal values for the average photon number per pulse (to achieve the maximum RKR and SKR) for each symbol length can guarantee the stability of the FSO system under different weather conditions. Smaller symbol lengths are more tolerant of detector loss. The proposed system supports linking distances from 1 km to 3 km while keeping the SKR almost constant. [ABSTRACT FROM AUTHOR]

In infrared optical wireless communications, narrow beams require the perfect calibration of the base station (BS) and user terminal (UT), which cannot support multiple moving users. This paper proposes a fiber lens based optical antenna, which employs a fiber unit array and a transmit lens to generate multiple beams, achieving full beam coverage. Specifically, we first design a fiber lens to extend the spreading angle of the emitted lights. We employ the wave optics to analyze the characteristic of the fiber lens and use the geometric optics to design the parameters of the lens. With the fiber lens, we then design the optical antenna consisting of a hexagonal fiber unit array and a transmit lens. Each fiber unit generates a beam to a specific direction and different fiber units irradiate different directions. All the generated beams can fully cover the communication area. Thus, the BS has the potential to support multi-user communication via multiple beams. Simulation results verify the performance of the designed fiber lens and the optical antenna achieving full beam coverage. [ABSTRACT FROM AUTHOR]

We experimentally demonstrated a photoelectric nonlinear compensation scheme of optical phase conjugation (OPC) with complex-valued deep neural network (CVDNN) to mitigate fiber nonlinearity in wavelength division multiplexing (WDM) 64-QAM coherent optical transmission system. The factors to affect the performance of OPC and CVDNN are comprehensively considered. OPC in WDM system is experimentally optimized to alleviate the deployment requirements of strict symmetrical distributed power and chromatic dispersion. The performance penalty caused by the simplification of the OPC is further compensated by the CVDNN. The selections of the input neurons’ number and the optimization algorithm are also considered to design a simple two-hidden-layer-structure CVDNN. The proposed method is experimentally verified and evaluated in a 12.5-GBd 4-channel WDM 64-QAM 160-km standard single-mode fiber (SSMF) transmission system with channel spacing of 50-GHz. The experimental results show that the proposed nonlinear equalizer based on the OPC with CDVNN has a strong robustness to the input signal power and wavelength, which can not only improve the Q factor of the signal by 1.5-dB, but also greatly increase the total launched signal power. [ABSTRACT FROM AUTHOR]

A split-carrier transmitter is proposed to improve loss budget for short distance 100 G-per-wavelength intra-datacentre links. By applying a low-complexity hardware modification to the transmitter, the proposed architecture reduces transmitter loss without sacrificing modulation quality. These benefits are demonstrated by implementing intensity modulated, direct-detection signals using a single-ended photodiode receiver. The architecture is evaluated using two formats: Nyquist-shaped 56 GBd PAM-4 and Nyquist-shaped double sideband 28 GBd 16-QAM. The results are compared against a conventional transmitter modulated with Nyquist-shaped 56 GBd PAM-4. Through simulation, the split-carrier transmitter is shown to achieve hard decision forward error correction ready performance after 2 km of transmission using a laser output power of just 0 dBm; a 5.2 dB improvement over the conventional transmitter.

The feasibility of using InGaN/GaN multiple-quantum-well light-emitting diode arrays (LED arrays) as photodiodes (PDs) is investigated experimentally in addition to their light emitting function. Two discrete LED arrays are produced from one 4 × 4 LED array with a parallel-connected pixel configuration. Such compact designs are useful for light emission or detection at the transmitting/receiving terminals of optical wireless communication systems. Despite 4 × 2 LED arrays achieving a light output power of 67.4 mW at 250 mA, they exhibit an optical responsivity (detectivity) of 0.183 A/W (1.61 × 1012 cm Hz1/2W−1) under ultraviolet light illumination (λ = 380 nm) at zero bias. For 4 × 2 LED arrays, the presence of an appreciable ultraviolet light response, together with a high 3-dB bandwidth (∼8 MHz) for modulated light detection, allowed us to build a 15 Mbit/s directed optical link with these LEDs functioning as both the optical transmitter and the receiver. Finally, the unitary LED array-based optical link is capable of real-time transmission of digital audio signals (data rate = 6 Mbit/s) at a propagation distance of 100 cm in free space even though some of the constituent pixels are inactive for light detection. [ABSTRACT FROM AUTHOR]

An iterative pairwise maximum likelihood (ML) receiver is proposed for asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) in visible light communications (VLC). With the aid of the pairwise ML detection, half of the received ACO-OFDM samples are forced to zero in time domain. However, when the signal-to-noise ratio (SNR) is low, the pairwise ML detection may cause misjudgment. With the iterative pairwise ML detection, the ACO-OFDM symbols are detected in frequency domain and the SNR gain of 3 dB could be available. Simulation results show that the pairwise ML receiver without iteration provides the SNR improvement of 1.5∼2.7 dB for 7% forward error correction (FEC) limit. The proposed iterative receiver gives a SNR gain of 2.6∼2.9 dB with 7% FEC limit. [ABSTRACT FROM AUTHOR]

Matching filter (MF)-free digital filter multiplexed (DFM) single sideband (SSB) OFDM intensity modulation and direct detection (IMDD) dual-channel transmissions of 51.25Gbit/s over 25km SSMFs are experimentally demonstrated. It is shown that both transmission system impairments and digital filter characteristic variations can only lead to <1dB transmission performance degradations. Compared with the MF-free DFM-based double sideband (DSB) OFDM technique, the SSB technique has a similar receiver DSP complexity and provides almost twice the maximum signal transmission capacity. When compared with a conventional DFM technique incorporating a dedicated shaping and matching filter pair for each channel, the present SSB technique achieves a 10-fold reduction of receiver DSP complexity and 7.82Gbit/s of additional signal transmission throughput. We demonstrate these throughput and complexity advantages by modelling and measurement. [ABSTRACT FROM AUTHOR]

We investigate the monolithicintegration of RF antennas onto a silicon-based integrated microwave photonics (IMWP) chip for short-range millimeter-wave (mmW) communication. The unification of antenna with photonic integrated circuits (PICs) reduces system loss for high data rate communication by eliminating parasitic interconnects. This integration of electronics (antenna) with photonics will be a key milestone leading to increased bandwidth capability and ubiquitous wireless links for emerging applications such as 5G, Internet of Things (IoT), autonomous vehicles, high data rate point-to-point communication, and wireless sensors. Through simulation above 20 GHz, we compare the transmission of three on-chip antenna structures designed in a commercial silicon photonics (SiPh) process and consider them for both inter and intra-chip communication. Results provide insight on the transmission gain variations relative to the antenna orientation from their distinct radiation pattern. The folded monopole structure provides superior gain, smaller footprint with layout flexibility, and good transmission spectrum. The analysis supports the idea of a monolithic mmW transmitter integrated with on-chip antennas on IMWP chip. [ABSTRACT FROM AUTHOR]

In this paper, an artificial noise (AN) injection technique is incorporated in a free-space optical (FSO) communication system with the aim of enhancing the secrecy performance of the system. An intensity modulated direct detection (IM/DD) FSO link which is subjected to Malaga (M) distributed turbulence with pointing errors is considered in this paper. The performance of FSO systems is evaluated by deriving novel closed-form expressions for the secrecy outage probability (SOP), strictly positive secrecy capacity (SPSC), and throughput of the system. By formulating a constrained optimization problem, we discuss an optimal power allocation strategy for throughput maximization in the considered system. It is shown through the results that the proposed technique is very effective in improving the secrecy performance of FSO systems. [ABSTRACT FROM AUTHOR]

We propose a graph-based modulation format identification (MFI) scheme for elastic optical network (EON), which exploits the trajectory information of the adjacent received symbols to identify six commonly-used modulation formats signals. A uniform grid is constructed in the first quadrant of two-dimensional (2D) Stokes plane to capture the received symbols sequence, and then the corresponding trajectory information is converted into the adjacent matrix via graph theory. The eigenvector associated with the largest eigenvalue of the adjacent matrix is selected as the discriminated-feature of the corresponding modulation format signal. Subsequently, we employ cosine similarity algorithm to obtain the modulation format of the unknown signal by analyzing the angle between the discriminated-features of the canonical modulation formats signals and the unknown signal. Then, the effectiveness of the proposed MFI scheme is verified through 28 GBaud polarization division multiplexing (PDM)-binary phase shift keying (BPSK), PDM-quadrature phase shift keying (QPSK), PDM-8 quadrature amplitude modulation (QAM), PDM-16QAM, PDM-32QAM, and PDM-64QAM simulation systems. The simulation results show that the proposed MFI scheme achieves well performance on the required minimum optical signal-to-noise ratio (OSNR) value, the robustness to the variation of the transmission environment, residual chromatic dispersion (CD) and different group delay (DGD). Finally, the proposed MFI scheme is further verified by 20 GBaud PDM-QPSK/8QAM/-16QAM/-32QAM long-haul transmission experiments, and the results demonstrate that the proposed MFI scheme exhibits good resilience towards fiber nonlinear impairments. [ABSTRACT FROM AUTHOR]

For radio telescope array applications, the precise time and frequency reference predominantly determines the quality of the observation. In this paper, we demonstrate an integrated dissemination system which can achieve ultra-stable frequency dissemination, time synchronization and data transfer simultaneously over a single optical fiber link. The test of this integrated system was carried out on the 48-km fiber link, which consisted of 28-km buried fibers, and 20-km fiber spool in laboratory. It has been experimentally demonstrated that the frequency transfer stability is $2.6 \times {10^{ - 14}}/s$ and $3.0 \times {10^{ - 16}}/{10^4}s$. The time synchronization with 100-ps accuracy and the error bit ratio of data transfer at the level of 10-8 are also demonstrated. [ABSTRACT FROM AUTHOR]

In this paper, we establish a water-to-air (W2A) visible light communication (VLC) link using green light emitting diode (LED), and investigate the coverage characteristics of the designed W2A-VLC link under calm and wavy water conditions. By transmitting a high-frequency sinusoid signal, we extract the variation of link gain. Based on the received signals of an experimental W2A-VLC system, we obtain the real link gains at different receiver points and analyze the channel temporal properties in good link conditions (called link “on” state). It is seen that the mean channel gain under wavy water surface condition can be larger than that under calm water surface condition, especially when the transmitter and receiver are not perfectly aligned, due to the random spatial modulation effect of wavy water surface. Moreover, the “on” state duration and interval can be well fitted by a shifted exponential distribution and a shifted lognormal distribution, respectively. Bit error rate (BER) results reveal the effect of expanded link gain variation under wavy water surface condition. This work provides significant guidance for future works on the communication protocol design and related performance analysis. [ABSTRACT FROM AUTHOR]

We report 108 Gb/s Duo-Binary PAM-8 (DB-PAM-8) transmission using direct-detection aided by Volterra equalizer. The bit-error-rate (BER) performance of the DB-PAM-8 signal has been evaluated through error counting by varying the bit rate, received optical power, and reach. The experimental results suggest that up to 108, 102, and 84 Gb/s signals can be propagated in back-to-back (B2B), and over 10, and 20 km of standard single-mode fiber (SMF), respectively, below the low-density parity-check forward error correction (LDPC-FEC) threshold. Furthermore, we derive a probabilistic model of the BER for multilevel DB-PAM-M signaling that is verified using our measurements. The model of this important performance metric turned out to be accurate enough, especially at low BER values, where its use is more convenient. [ABSTRACT FROM AUTHOR]

In this article, we investigate the performance of the newly designed 4 × 4 optical wireless multiple-input-multiple-output (MIMO-OWC) system based on a linearly-shaped arrangement of transmitters and receivers, to achieve long-distance and high data rate transmission. To highlight the importance of selecting an appropriate arrangement for the optical elements, we carried on a comparative study, considering both the conventional square and the proposed horizontal linear constellation of optical modems. The performance of the bit error rate (BER) is experimentally evaluated at distances up to 65 m, using two important metrics inherent to the MIMO transmission channel matrix, i.e., the Frobenius norm and the condition number. The results show that the channel matrix shape characterized by the condition number dominates the BER if the Frobenius norm is smaller than 1. Furthermore, the proposed MIMO-OWC system employing a linearly-shaped arrangement of transmitters and receivers, shows to be more immune to the calibration error rather than the conventional square constellation optical modems. Moreover, the transmission of 100 Mbps data-rate and a BER lower than $10^{-4}$ could be successfully achieved at a 65 m communication range. [ABSTRACT FROM AUTHOR]

In this work, we analyze the effects of the presence of waveguides in a wireless on-chip optical channel, which act as obstacles for the rays composing the propagating signal. The analysis has been performed numerically by using the FDTD method, and the simulation results have been validated by the experimental characterization. This work focuses on the relationship between the received signal and the position of the obstacles on the circuit plane in a multilayered channel made of heterogeneous dielectric cladding. We show how the crossing waveguide perturbates the received power and we provide ad hoc guidelines for the best topological choices allowing to mitigate losses. Moreover, with a good agreement between numerical and experimental results, amounting to a variation of less than 1 dB, we have demonstrated that the effects of this perturbation in real chips can be predicted. The results of this analysis, as well as increasing the level of knowledge of on-chip optical wireless links, allow a better comprehension and control of unbound optical signals within the optical chip environment. [ABSTRACT FROM AUTHOR]

A novel approach to simultaneous radiofrequency self-interference cancellation (SIC) and frequency down-conversion in a unified system is proposed and demonstrated experimentally based on cascade Mach–Zehnder modulators (MZM). The local oscillator signal is injected into the MZM1, whereas the received RF signal and the reference signal are fed into MZM2 and MZM3. After photoelectrical conversion by a balanced photodetector, the desired intermediate frequency signal with SIC is obtained. The cancellation depth for the single frequency SIC with frequency down-conversion is around 38 dB, and that for a wideband signal is around 19 dB. No electrical delay and attenuation are used in the proposed system, so it can potentially reach a wider frequency band and a higher cancellation depth. [ABSTRACT FROM AUTHOR]

The misalignment of mobile underwater wireless optical communication (UWOC) systems, compounded by turbulence in underwater scenarios, is a practical problem that can be resolved through various means. This work describes a pulse-position-modulation-based (PPM-based) diffused-line-of-sight UWOC system that offers a solution to this issue. PPM is found to be power-efficient and, in terms of bit-error-ratio performance, outperforms on-off keying modulation and orthogonal frequency-division multiplexing modulation in complex dynamic underwater channels. Through indoor experiments and outdoor deployment, we validated the robustness of the proposed PPM-based mobile UWOC system. This work sheds light on the practical implementation of UWOC networks for relieving the strict pointing-acquisition-and-tracking requirements when an underwater apparatus is transmitting or receiving signals on the fly. [ABSTRACT FROM AUTHOR]

Multiple-input multiple-output visible light communications (MIMO-VLC) have been considered as an advanced technique owing to the enhancement of communication capacity and reliability. However, the performance of MIMO-VLC highly depends on channel correlation, which means that the system performs worse when the channels are more highly correlated. Some studies have developed techniques to alleviate channel correlation and improve the performance of spatial multiplexing MIMO-VLC systems. However, these works have not considered situations where all LEDs and PDs are integrated into a compact structure. In this paper, a single-user spatial multiplexing MIMO-VLC system with densely distributed LEDs (light emitting diodes) and PDs (photodiodes) is proposed. Designs of optimal signal vectors and system structures are introduced, to fulfill multiplexing gains when LEDs and PDs are set close to each other respectively. Numerical simulations are carried out and the results illustrate that the proposed system can realize multiplexing gains over the system where LEDs and PDs are distributed densely, and also outperforms the traditional repetition coding (RC) technique. [ABSTRACT FROM AUTHOR]

In this letter, we present a new real-time blind signal detection technique for indoor optical communications using a supervised learning framework in an artificial neural network (ANN). We model the optical channel as a time-varying doubly-stochastic Poisson process. To compare the performance of the proposed ANN network, we present an iterative method for joint channel estimation and symbol detection using expectation-maximization (EM) and Viterbi decoding and demonstrate that the bit error rate (BER) performance of the proposed technique is better than that of the Viterbi decoding. The proposed scheme is found to be resilient to indoor channel dynamics and can achieve a good BER performance over a wide range of channel variations. [ABSTRACT FROM AUTHOR]

To solve problems of low positioning accuracy and long time-consuming in indoor visible light three-dimensional positioning system, a visible light three-dimensional positioning method based on the improved hybrid bat algorithm (IHBA) is proposed in this paper. Firstly, some beacon points are set at the beginning of the IHBA to reduce the number of iterations. Secondly, weight coefficient is defined to improve positioning accuracy when the fitness function is constructed. Thirdly, aiming at controlling the search speed reasonably, an adaptive search factor is introduced while the bat individual update formula is designed. Finally, the chaotic perturbation operation is used to avoid the algorithm falling into local optimum. In indoor simulation environment, the size of which is 5 m × 5 m × 3 m, the average positioning error of the IHBA is 1.16 cm and the positioning time is 1.85s. To verify the actual effectiveness of IHBA, a receiver is placed in the experimental environment, the size of which is 1.5 m × 1.2 m × 2 m. Through the optical power detected by the receiver, the processor calculate its coordinates with IHBA. The average positioning error of the IHBA is 3.64 cm and the positioning time is 0.89 s. The proposed algorithm has practical application value in existing indoor positioning methods. [ABSTRACT FROM AUTHOR]

This technical paper presents an optical camera communication system by using data as input collected from a sensor that continuously garners humidity and temperature information from the environment. A small light-emitting diode (LED) and an LED array are used as a transmitter and a low-speed rolling-shutter camera as a receiver. A modulation scheme standardized in IEEE 802.15.7, referred to as variable pulse-width modulation, is used to encode the data bits. The proposed scheme is flicker-free in different frequencies and applicable in both static and mobile scenarios. A neural network is designed for LED detection and improving the bit-error-rate in mobile scenarios. A new method based on the region-wise comparison and an existing method is used to remove the interference and noise generated from the neighboring light sources, respectively. The decoding procedure is performed and analyzed in Python 3.7. A data rate of 1.02 kbps is achieved using the single LED, which is further augmented to 64 kbps by using an 8 × 8 LED dot matrix. [ABSTRACT FROM AUTHOR]

We discuss an optical ray-tracing approach to minimizing path-loss in a variable link length indoor blue laser down-converted white light visible light communication (VLC) system. For a given link length, minimum path-loss is achieved by finding optimum positions of transmitter and receiver lenses relative to phosphor and detector respectively such that collection efficiency is maximized. The designed VLC system is experimentally implemented for two different optimized link lengths of 25 and 300 cm. The down-converted white light for the optimized link is found to exhibits narrow beam spread, spot-lighting type illumination profile. The white light is measured to quantify the beam profile, color rendering, illuminance and percentage of blue-content. The illumination beam profile and propagation characteristics are found to be in good agreement with optical simulations. Communication experiments with on-off modulated at 1.5 Gbps achieved BER of ∼3 × 10−3 for the optimized link, which is below the forward-error correction threshold. The above communication performance is achieved at illumination levels at the receiver as low as ∼45 and 16 lx. Comparison with previous laser-based VLC implementations shows that the path-loss optimization helps achieve gigabit-class communication at practically relevant link lengths and the lowest illuminance levels thus far reported. Such low illuminance level, narrow beam spread VLC luminaires can potentially coexist with existing lighting infrastructure for eye-safe indoor applications. [ABSTRACT FROM AUTHOR]

Optical multipath RF self-interference cancellation (SIC) based on phase modulation for full-duplex communication is proposed and demonstrated experimentally. Phase modulation is utilized to convert the RF signal into optical domain, in which the time delay tuning, amplitude tuning and phase inversion for multipath RF SIC are completed. The comprehensive theoretical model of the optical multipath RF SIC system is established, and the factors affecting SIC performance including the time delay, amplitude and phase deviations are analyzed. The experimental results verify the feasibility of the proposed scheme for full-duplex communication with the cancellation depth of 26 dB and 28 dB over 100 MHz at central frequency of 6 GHz and 10 GHz, respectively. A figure of merit of the maximum interference to signal of interest ratio is defined to characterize the SOI recovery capability of optical RF SIC system. [ABSTRACT FROM AUTHOR]

It has recently been proved that the free-space optical (FSO) communication links are susceptible to interceptions. Due to this reason, the optics community shows a special interest in studying these high-speed links in greater detail from a physical layer security (PLS) point of view. Therefore, in this paper, we propose, for the first time, enhancing the average secrecy capacity (ASC) in FSO links via multiple-input/single-output (MISO) systems. It is well-known that the fading effects in FSO channels can be significantly mitigated by exploiting spatial diversity techniques at the transmitter end. Thus, we develop a new asymptotic closed-form solution at high signal-to-noise-ratio (SNR) to accurately compute the ASC for MISO based FSO communication systems with equal gain combining (EGC) reception through generalized misalignment and atmospheric turbulence-induced fading channels. As a key feature, we investigate the impact of the eavesdropper's orientation along with its location in the pointing error model. We can conclude that the influence of the eavesdropper on recollecting radiated power is diminished considerably by increasing not only the normalized beam width at the receiver end, but also by increasing the number of laser sources. Numerical results are tested by exact Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

A simplified and cost-effective architecture is proposed and experimentally demonstrated with a millimeter wave (MMW) signal over fiber and free-space wireless transmission. The distributed feedback (DFB) laser is operated in optimized settings after measuring its characteristics and thus the 6 Gbps 64–quadrature amplitude modulation (64-QAM) signal can be successfully transmitted after 20- and 50-km fiber and 1-meter wireless transmission with EVM of 5.98% and 6.87%, respectively. Moreover, the EVM of 10.8% over 100-km long reach is also achieved with the 32-QAM modulation format and data rate of 5 Gbps. The experimental results show that the proposed direct modulation scheme can satisfy applications in metro and rural areas. [ABSTRACT FROM AUTHOR]

In the modern wireless networks, millimeter-wave radio-frequency (RF) bands are becoming more attractive as they provide larger bandwidth and higher data rates than the today-used systems operating at frequencies below 6 GHz. In addition, according to the fact that coaxial cables exhibit extremely high attenuation for millimeter-wave RF signals, analog radio over fiber techniques (RoF) form a promising technology for delivering unaltered radio waveform to a remote antenna. This paper experimentally analyzes three types of RoF modulations, namely a directly modulated laser, an electro-absorption modulator, and a Mach-Zehnder Modulator. The primary focus is on the implementation of each RoF transmitter in an RoF system, such as those in 5G networks. The experimental study includes a detailed characterization of an RoF system with a 50-m long outdoor free-space RF channel operating in the frequency band of 25 GHz. Frequency response (S-parameters) and third-order nonlinear distortion are investigated in detail. Tests of EVM performance were conducted using an orthogonal frequency division multiplexing signal modulated with 16-quadrature amplitude modulation (16-QAM) with a long-term evolution signal. It is demonstrated that the transmitters studied can operate under a 13.5% EVM limit given for 16-QAM. Apart from the detailed system performance, the considerable power fluctuations in the 25 GHz free-space RF outdoor channel are reported. [ABSTRACT FROM AUTHOR]

This paper proposes an enhancement of semiconductor optical amplifier (SOA)-based scintillation mitigation by polarization shift on-off keying (PS-OOK) transmission in free-space optical (FSO) communication. The scintillation effect is a critical issue in a vertical FSO link. The issue of the extinction ratio (ER) degradation of the received OOK signal caused by the high dynamic gain frequency of the SOA limits the performance of the turbulence-induced scintillation effect mitigation using a gain-saturated and polarization-independent SOA. Therefore, a PS-OOK signal, which has the characteristics of orthogonal polarization of bits “1” and “0,” is transmitted at the transmitter end and a linear polarizer is deployed after the gain saturated SOA at the receiver end. Based on the polarization orthogonality of PS-OOK, the performance of the scintillation-effect suppression is significantly improved by the mitigation of the ER degradation by the blocking of the polarization of bit “0” of the received optical PS-OOK signal using the linear polarizer. The proposed scheme is experimentally verified utilizing the Mach-Zehnder modulator (MZM)-based fading simulator accommodated various turbulence channels. The experimental results illustrate that the scintillation effect is effectively suppressed by the proposed technique compared to the SOA-based conventional single-polarized OOK detection and balanced PS-OOK detection. [ABSTRACT FROM AUTHOR]

In this study, we built a single photon avalanche diode (SPAD) receiver based underwater wireless optical communication (UWOC) system. The bit error rate (BER) and signal-to-noise ratio (SNR) performance of UWOC with different distances and data transmission rates were obtained. Based on the water attenuation coefficient of 0.12 m−1, a series of neutral density (ND) filters were exploited to attenuate the light output power from the blue laser diode (LD) to simulate the long distance UWOC. The maximum estimated distances of 144 m and 117 m with corresponding BERs of 1.89 × 10−3 and 5.31 × 10−4 at data transmission rates of 500 bps and 2 Mbps were acquired in UWOC system using on-off keying (OOK) modulation scheme, respectively. Furthermore, we compared the differences between free-space and underwater channels, and a divergence angle of ∼1.02 mrad was measured experimentally at a distance of 50 m in the free space. The long UWOC distances obtained in this study partly benefit from high sensitivity SPAD, the small laser divergence angle and low light attenuation. This study provides an approach to achieve long distance UWOC using SPAD. [ABSTRACT FROM AUTHOR]

Multipath reflection degrades the performance of visible light communications (VLC) based localization systems, where it is often considered as a strong random noise. However, due to the inherent transmission features of light, the optical wireless indoor channel is static; therefore, multipath components can be modeled as deterministic functions of the transceiver location, furnishings, and room geometry. In this paper, we investigate the performance limits of fingerprinting-based localization with multipath reflection as a source of information, i.e., a fingerprinting map. Limits on the localization accuracy are determined using the Cramer-Rao lower bound (CRLB) for different numbers of photodetectors deployed in the system and received signal features captured. The tightness of the analytical CRLB is tested by comparing it to the performance of a fingerprint-based positioning algorithm that uses the nearest neighbor method. Simulation results show an achievable root mean squared positioning accuracy of 45 cm and 5 cm (for one and four photodetectors, respectively), for an empty room. We then investigate the practical limitations on localization accuracy caused by a narrow transceiver bandwidth. Numerical results show that the localization system can still achieve decimeter accuracy for system bandwidths of 200 MHz, which makes fingerprinting schemes practical for off-the-shelf infrared devices. [ABSTRACT FROM AUTHOR]

The in-band OSNR monitoring methods based on the noise-free reference optical spectrum (ROS) have attracted much attention in the field of optical performance monitoring because they are cost-effective and insensitive to chromatic dispersion and polarization mode dispersion. In this paper, we propose a new ROS based in-band OSNR monitoring method that is robust to the distortions due to non-ideal modulation, the variations of the in-line EDFA gain and the number of filtering elements traversed by the signal. Moreover, this method supports using the standard ROS retrieved by the existing machine-learning technique based optical spectrum identification technique, and thus can remove the need to measure the ROS near the transmitter, which is hard to perform, especially in flexible optical networks. These merits make the method more robust and convenient to use in practice. Extensive numerical simulations are presented to demonstrate the merits of the proposed method with respect to the existing ROS based methods. [ABSTRACT FROM AUTHOR]

A D-band vector millimeter wave (mm-wave) signal generation and propagation scheme employing cascaded intensity modulator (IM) and in-phase/quadrature (I/Q) modulator is proposed and implemented. In experiment, both the IM and I/Q modulator are operating on optical-carrier-suppressing (OCS) mode to generate two pairs of dual single-sideband (SSB) signals, and the undesired optical sidebands are not eliminated by an optical filter. For each pair of dual SSB signals, one is unmodulated the other is vector-modulated, they are located on both sides of the center suppressed carrier. After optical fiber transmission, SSB signals will beat with each other at the photoelectric detection end, thus the electrical mm-wave signals are generated. We experimentally demonstrated 2-Gbaud 16QAM, 4-Gbaud QPSK, and 8-Gbaud QPSK Radio-over-Fiber (RoF) transmission at 130-GHz over 22.5 km standard single mode fiber and 1 m wireless link with a Bit-Error-Rate (BER) under ${3}{{.8}} \times {1}{{0}^{{{ - 3}}}}$. [ABSTRACT FROM AUTHOR]

The ever-growing machine-to-machine traffic in data centers has stimulated the increase of transceiver data rate from 25 Gb/s/λ to 100 Gb/s/λ and beyond. It is believed that advanced modulation formats and digital-to-analog converters (DACs) will be employed in next generation short-reach transceivers. Digital pre-emphasis techniques are widely employed in DAC-based transceivers to compensate for the high frequency roll-off due to the RF and optoelectronics components in optical transceivers. However, digital pre-emphasis essentially reduces the magnitude of the signal low frequency components for a flat frequency response, which unavoidably increases quantization error, reducing the overall signal-to-noise ratio. This trade-off between SNR and bandwidth conflicts with the high SNR requirement of advanced modulation formats such as the Nyquist-shaped pulse amplitude modulation (PAM). To mitigate the quantization error induced SNR degradation, we show that analog pre-emphasis filters can be used in conjunction with digital pre-emphasis for improved system performance. To understand the impact of the analog pre-emphasis filter on system performance, we analyze the relationship between the flatness of the system frequency response and the SNR degradation due to digital pre-emphasis, and demonstrate 1.1-dB increase of receiver sensitivities, for both 64-Gb/s and 128-Gb/s intensity-modulation direct detection (IM-DD) PAM4 signals, respectively employing a directly modulated laser (DML) and an electroabsorption modulator (EAM). [ABSTRACT FROM AUTHOR]

In this paper, we propose a novel method to generate real signal for Visible Light Communication (VLC) systems without using traditional hermitian symmetry on the data symbols obtained using M-ary Pulse Amplitude Modulation (PAM) which is named as Auxiliary PAM (Ax-PAM). We mathematically analyse this method to generate a real signal with tunable energy using auxiliary symbols at the transmitter and the corresponding receiver. Simulation results for Bit Error Rate (BER) show better performance over conventional PAM Discrete Multi-Tone (PAM-DMT) and Asymmetrically Clipped Optical Orthogonal Frequency Division Multiplexing (ACO-OFDM) even under clipping distortion and also demonstrate the tunable energy for the proposed scheme. Furthermore, the proposed scheme is implemented on a VLC test bed designed using Universal Software defined Radio Peripheral (USRP). The experimental results for estimated Signal to Noise Ratio (SNR) and achieved BER for Ax-PAM outperforms PAM-DMT and ACO-OFDM. [ABSTRACT FROM AUTHOR]

In this paper, a novel spectral-efficient coherent radio-over-fiber (RoF) link with linear digital phase demodulation is proposed and experimentally demonstrated. At the transmitter, to make an efficient use of the optical power and spectra, an intensity-modulated optical signal serving as the optical reference signal and a phase-modulated optical signal are polarization-multiplexed on a single optical carrier. At the receiver, the two optical signals are coherently detected with an optical local oscillator (OLO) and demodulated free of laser phase fluctuation through digital signal processing. Owing to simple and linear digital phase demodulation, an RF input signal is linearly demodulated from the optical phase without approximations and preconditions, which preserves the linearity of the phase-modulated RoF optical link from the transmitter end to the receiver end. The proposed scheme is experimentally verified by 25-km single-mode-fiber (SMF) transmission of two 16-QAM microwave vector signals at 2 GHz and 2.4 GHz, both with a symbol rate of 50 Msymbol/s. The transmission performance in term of error vector magnitude (EVM) is evaluated. Additionally, 25-km SMF transmission of the phase-modulated input signal with a spurious-free dynamic range (SFDR) of 112.8 dB·Hz2/3 is obtained. [ABSTRACT FROM AUTHOR]

A deep transfer learning (TL)-based comprehensive eye diagram analysis and diagnosis scheme that can output essential eye diagram parameters, estimate fiber link length, calculate Q-factor, and diagnose device imperfection-induced impairments is proposed. TL can be used to extract system information and optical signal characteristics contained in eye diagrams and apply the learned knowledge and extracted features obtained from source tasks to related target tasks. As a source task, the proposed method estimates the transmission distance of a fiber link using convolutional neural network (CNN)-based eye diagram recognition. The feature extraction layers of the CNN are transferred to six target tasks involving the recognition of cross percentage, levels “0” and “1,” eye height and width, and Q-factor. Using TL reduces the total training times for on-off keying (OOK) and pulse amplitude modulation (PAM4) formats by $>$ 95% and 60%, respectively. We also investigated six common PAM4 impairments caused by transmitter imperfection by setting the impairment category identification as source task and the impairment-degree diagnoses as target tasks. The TL methods consistently outperformed non-TL methods, with higher accuracies and significantly reduced training times. The proposed impairment diagnosis technique should be useful in impairment healing and fault correction. [ABSTRACT FROM AUTHOR]