Modulating Retro-reflector Research Articles

Performance enhancement of RIS-assisted MRR-UOWC systems using the spectral-power-efficient [formula omitted]QAM-MPPM

Reconfigurable Intelligent Surfaces (RISs), proposed as innovative units, possess the potential to establish reliable connections in Underwater Optical Wireless Communication (UOWC) systems, where direct transmission (DT) experiences significant weakening due to dynamic channel conditions. In this paper, we examine the performance of a UOWC system consisting of an RIS and a modulating retroreflector (MRR), referred to as the RIS-assisted MRR-UOWC system. The channel is affected by a combination of path loss, gamma–gamma (G–G) turbulence, and pointing error (PE). The considered system employs the spectral-power-efficient L-ary Quadrature Amplitude Modulation Multi-Pulse Pulse-Position Modulation (LQAM-MPPM) technique. For our system performance measures, we present upper-bound (UB) and approximate UB expressions for the average bit-error rate (BER) of LQAM-MPPM signals. These expressions are used to analyze the system’s performance. Performance metrics are established by deriving the Probability Density Function (PDF), Cumulative Distribution Function (CDF), and Moment-Generating Function (MGF) for the end-to-end signal-to-noise ratio (SNR), resulting from the sum of products of independent but not identically distributed (i.ni.d) channel coefficients. The results of our study indicate that the LQAM-MPPM scheme surpasses traditional schemes.

Wide-field-of-view auto-coupling optical antenna system for high-speed bidirectional optical wireless communications in C band.

Due to a great many superior features of infrared light communication (ILC), like high capacity and strong privacy, ILC is considered a potential candidate for serving the high demands of beyond fifth-generation/sixth-generation (B5G/6 G) communication systems. However, the terminal's limited field-of-view (FOV) induces great difficulty in establishing line-of-sight (LoS) link between the transceiver and the terminal. In this paper, we propose a wide-FOV auto-coupling optical antenna system that utilizes a wide-FOV telecentric lens to collect incident infrared beams and automatically couple them into a specific single-mode-fiber (SMF) channel of fiber array and optical switch. The performance of this optical antenna system is assessed through simulation and manual alignment operation, and validated by automatic alignment results. A coupling loss of less than 10.6 dB within a FOV of 100° for both downstream and upstream beams in C band is demonstrated by the designed system. Furthermore, we establish a bidirectional optical wireless communications (OWC) system employing this antenna and a fiber-type modulating retro-reflector (MRR) system in the terminal. Both 10-Gbps on-off keying (OOK) downstream and upstream transmissions are successfully realized with the FOV of up to 100° in C band where the measured bit-error-rate (BER) is lower than 3.8 × 10-3. To the best of our knowledge, this is a brand-new auto-coupling optical antenna system with the largest FOV in ILC automatic alignment works in terminals that have ever been reported.

Performance analysis of modulating retro-reflector optical communication system based on an Airy beam

Average Bit Error Rate (BER), outage probability and average channel capacity expressions of a Modulating Retro-Reflector (MRR) optical communication system based on an Airy beam under weak turbulence are derived. The effects of propagation distance, width parameter and exponential truncation factor of the Airy beam itself on the communication performance of the system are discussed. Meanwhile, the performance of the system based on the Gaussian beam is compared with the same turbulence condition and propagation distance. The simulation results show that under the same conditions, the system based on the Airy beam shows a lower outage probability, average BER and larger average channel capacity compared to the system based on the Gaussian beam. Moreover, the advantages of the Airy beam are more significant as the average signal-to-noise ratio (SNR) increases.

High Frequency MEMS Capacitive Mirror for Space Applications.

Free space optics laser communication using modulating retroreflectors (MR) is a challenging application for an active mirror, due to the high frequencies (>100 kHz) required to enable sufficient data transfer. Micro Electromechanical (MEMS) mirrors are a promising option for high-frequency applications, given the very small moving mass typical of such devices. Capacitive MEMS mirrors are presented here for free space communications, based on a novel fabrication sequence that introduces a single-layer thin film aluminum mirror structure with an underlying silicon oxide sacrificial layer. The use of aluminum instead of gold as a mirror layer diminishes the heating generated by the absorption of the sun's radiation once the mirrors exit the earth's atmosphere. Thanks to the novel fabrication sequence, the presented mirror devices have a full range actuation voltage of less than 40 V, and a high operational frequency with an eigenfrequency above 2 MHz. The devices were manufactured and characterized, and their main parameters were obtained from experimental data combined with finite element analysis, thus enabling future design optimization of the reported MEMS technology. By optical characterization of the far field diffraction pattern, good mirror performance was demonstrated.

Modulating Retroreflector Based Free Space Optical Link for UAV-to-Ground Communications

Weight reduction and low power consumption are key requirements in the next generation of unmanned aerial vehicle (UAV) networks. Employing modulating retro-reflector (MRR)-based free space optical (FSO) technology is an innovative technique for UAV-to-ground communication in order to reduce the payload weight and power consumption of UAVs which leads to increased maneuverability and flight time of UAV. In this paper, we consider an MRR-based FSO system for UAV-to-ground communication. We will show that the performance of the considered system is very sensitive to tracking errors. Therefore, to assess the benefits of MRR-based UAV deployment for FSO communications, the MRR-based UAV FSO channel is characterized by taking into account tracking system errors along with UAV’s orientation fluctuations, link length, UAV’s height, optical beam divergence angle, effective area of MRR, atmospheric turbulence and optical channel loss in the double-pass channels. To enable effective performance analysis, tractable and closed-form expressions are derived for probability density function of end-to-end signal to noise ratio, outage probability and bit error rate of the considered system under both weak-to-moderate and moderate-to-strong atmospheric turbulence conditions. The accuracy of the analytical expressions is verified by extensive simulations. Analytical results are then used to study the relationship between the optimal system design and tracking system errors.

Analysis of the influence of atmospheric turbulence on modulating retro-reflector optical communication

The paper discusses the atmospheric turbulence effect on modulating retro-reflector (MRR) optical communication systems with theoretical analysis and experimental verification. The transmission model is proposed by the phase screen method. Based on this model, the scintillation indices (SI) are calculated, and the aperture averaging effect of the MRR terminal is analyzed. Furthermore, a test bed with 506 m transmission link is established. In the experiment, the SI values of the MRR link and the direct link are obtained and compared. Experimental data agree well with the numerical simulation results and verify the theoretical model. The theoretical results are useful for analyzing the influence of turbulence on MRR systems.

A Novel System of Mixed RF/FSO UAV Communication Based on MRR and RIS by Adopting Hybrid Modulation

In this paper, we propose a mixed radio frequency (RF)/free space optical (FSO) unmanned aerial vehicle (UAV) communication system, based on modulating retro-reflector (MRR) and reconfigurable intelligent surface (RIS), which adopts the hybrid L-ary pulse position modulation-binary phase shift keying-subcarrier intensity modulation (L-PPM-BPSK-SIM). More specifically, the RF channel follows Rayleigh distribution, while the FSO channel obeys Gamma–Gamma distribution that considers atmospheric turbulence and pointing error. For decode-and-forward (DF) relay, the MRR is installed on the UAV to reduce its weight, size, and power consumption. In particular, the RIS is used as user terminal along with the RF signal generator to achieve signal enhancement. Based on this, closed expressions for the outage probability, average bit error rate (BER) and average channel capacity of the end-to-end uplink and downlink are derived. Numerical results confirm that while the relay limitation is solved by MRR, RIS significantly reduces the outage probability and average BER as well as obviously increases the average channel capacity. Furthermore, the hybrid L-PPM-BPSK-SIM with average symbol length greater than eight can effectively improve the average BER performance of the system.

Performance analysis of modulating retro-reflector link based on orbital angular momentum coding in underwater channels

The modulating retro-reflector (MRR) communication is developed to solve the problem of insufficient terminal power in underwater optical communication (UOC) where size, weight and power are severely restricted. Here, we propose a new MRR link scheme using the optical orbital angular momentum (OAM) for such scenarios to realize high-order coding. Furthermore, we simulate the OAM MRR link in the oceanic turbulence channel and use the convolutional neural network (CNN) to classify OAM patterns. The simulation results show that the OAM MRR link can realize error-free image transmission under weak turbulence strength of Cn2=10−15K2m−2/3 within 15 m distance using 8/16-ary OAM superposed modes, and with the increasing of the distance and the turbulence strength, the performance is decreasing, where the 16-ary format has a higher coding efficiency but lower performance than the 8-ary format. An experiment of the OAM MRR link using CNN to classify 16-ary OAM superposed modes is also demonstrated, where two space light modulators (SLMs) are used to simulate the short-distance oceanic turbulence channel and modulate the OAM beam, respectively. Our experiment result shows the recognition rate of 16-ary OAM set with different types of channel conditions. Our work may provide a novel way for high-order coding MRR links in underwater channels and hopefully increase the data rate of the MRR link in the future.

Performance analysis of modulating retro‐reflector free space optical communication system over gamma‐gamma fading channels

AbstractIn this paper, we analyze the modulating retro‐reflector (MRR) channel under different correlated conditions and derived its closed‐form expressions for the probability density function, which is helpful to the design of practical MRR free space optical (FSO) systems. A series of closed‐form expressions for BER performance of several perspective modulation schemes is investigated for MRR FSO communication system over moderate to strong fading channels by use of Gamma‐Gamma (G‐G) distribution. Simulation results show that the effect of correlation between links play a significant role in FSO communication, while varies markedly under different modulation schemes, which provide guidelines for the design and optimization of satellite‐ground laser communication system.

High-speed modulating retro-reflectors with optical phase conjugation compensation

A modulating retro-reflector (MRR) with optical phase conjugation (OPC) compensation is presented and demonstrated to mitigate the turbulence-induced channel distortion for high-speed coherent free-space optical (FSO) communications. The impact of the turbulence on the beam spot, the phase fluctuation and the bit error rate (BER) is analyzed for the MRR FSO link carrying quadrature phase-shift keying (QPSK) signals with spatial or single-mode fiber (SMF)-based OPC. The mitigation of the atmospheric turbulence is verified by comparing the transmission performance of the MRR FSO links with and without OPC. The compensation using SMF-based OPC is experimentally demonstrated in a turbulence-tunable atmospheric chamber. The BER is improved by two orders of magnitude in the turbulent channel with the temperature difference of 250 °C, and the power penalty is reduced by about 2.0 dB, which agree well with the simulation results. A larger receiving aperture size will improve the compensation effect

Free Space Measurement Device Independent Quantum Key Distribution with Modulating Retro-Reflectors under Correlated Turbulent Channel.

Modulating retro-reflector (MRR), originally introduced to support laser communication, relieves most of the weight, power, and pointing requirements to the ground station. In this paper, a plug-and-play measurement device independent quantum key distribution (MDI-QKD) scheme with MRR is proposed not only to eliminate detector side channels and allow an untrusted satellite relay between two users, but also to simplify the requirements set-ups in practical flexible moving scenarios. The plug-and-play architecture compensates for the polarization drift during the transmission to provide superior performance in implementing the MDI-QKD on a free-space channel, and the MRR device is adopted to relax the requirements on both communication terminals. A double-pass correlated turbulent channel model is presented to investigate the complex and unstable channel characteristics caused by the atmospheric turbulence. Furthermore, the security of the modified MDI-QKD scheme is analyzed under some classical attacks and the simulation results indicate the feasibility under the situation that the system performance deteriorates with the increase of fading correlation coefficient and the turbulence intensity, which provides a meaningful step towards an MDI-QKD based on the moving platforms to join a dynamic quantum network with untrusted relays.

A High Speed Retro-Reflective Free Space Optics Links With UAV

In this work we report the design and implementation of a very high speed retro-reflective free space communication system between a ground station and a commercial unmanned aerial vehicle (UAV). The system uses a pixelated electro-absorption modulator (EAM) modulating retro-reflector (MRR) to establish a data link operating at 500 Mbps at a range of 560 m and a bit error rate (BER) of 7.6·10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> . The MRR provides an effective aperture of 11 mm and full field of view (FFOV) of 6.4°. To the best of our knowledge, this is the fastest demonstration of an outdoor link of this type. In this paper the design and implementation of the system is described, as well as results from experimental trials.

Performance of Modulating Retro-Reflector FSO Communication Systems With Nonzero Boresight Pointing Error

A comprehensive investigation of the impact of nonzero boresight pointing error on the performance of modulating retro-reflector (MRR) free space optical (FSO) communication systems is the goal of this work. In particular, the end-to-end signal-to-noise ratio (SNR) statistical characteristics including probability density function (PDF), cumulative distribution function (CDF), moments as well as moment generating function (MGF) are formulated. Capitalizing on these results, the system average BER and outage probability are derived. Moreover, the asymptotic performances of the system ergodic capacity at low and high SNR values are deduced as well. The obtained expressions are verified via Monte-Carlo simulation.

Wavelength Tunable Asymmetric B-OWC System Based on Self-Injection Locking for TDM-PONs

The challenge of asymmetric bidirectional optical wireless communication is majority resulted from the power shortage of uplink transmission, since it is double the downlink transmission distance. In this research, a wavelength tunable self-injection locking (SIL) system based on free-space configuration for uplink transmission in a modulated retroreflector based asymmetric bidirectional optical wireless communication system is proposed and experimentally evaluated. The proposed works were investigated from access node (AN) to terminal user(TU) respectively. A power gain of 34.51 dB, and uplink data transmission test over 2 Gbits/s, with Q-factors beyond FEC limit, are reported.

Performance analysis of MRR FSO communication system under Gamma–Gamma​ turbulence channel with pointing error

The performance of modulating retro-reflector (MRR) free space optical (FSO) communication system has been studied before for various atmospheric turbulence conditions. However, the effect of pointing error (PE) on the MRR FSO communication system has not been studied yet. In this work, we investigate the PE impact on the MRR FSO communication system. Specifically, we obtain the probability density function (PDF) of the system in closed form expression regarding Gamma–Gamma turbulence channel and Rayleigh PE. Using this PDF, we derive the statistical characteristics closed form expressions of the received optical signal-to-noise ratio (SNR) including PDF, cumulative distribution function (CDF), moments and moment generating function (MGF). Relying on the obtained analytical results, we derive the system average bit error rate (BER), outage probability, ergodic capacity (EC) and amount of fading (AF) performances in closed forms. The closed form expressions are given in terms of Meijer’s G-function with the exception of obtaining the moments of the received optical SNR and AF in terms of elementary functions. For the given weather, PE and system conditions, we optimize the beam divergence angle to minimize the system average BER. Based on this, we suggest controlling of the beam divergence angle with the transmission distance according to the optimum beam divergence angle to improve the system BER performance. Finally we demonstrate the system performances according to the optimum beam divergence angle.

Asymmetric optical links using monolithic III-nitride diodes.

Multiple-quantum well (MQW) III-nitride diodes can both emit and detect light. In particular, a III-nitride diode can absorb shorter-wavelength photons generated from another III-nitride diode that shares an identical MQW structure because of the spectral overlap between the emission and detection spectra of the III-nitride diode, which establishes a wireless visible light communication system using two identical III-nitride diodes. Moreover, a wireless light communication system using a modulating retro-reflector (MRR) enables asymmetric optical links, which forms a two-way optical link using a single transmitter and receiver. Here, in association with an MRR, we propose, fabricate, and characterize asymmetric optical links using monolithic III-nitride diodes, where one III-nitride diode functions as a transmitter to emit light, an MRR reflects light with the encoded information, another monolithically integrated III-nitride diode serves as a receiver to absorb the reflected light to convert optical signals into electrical ones, and the encoded information is finally decoded. Advanced monolithic III-nitride asymmetric optical links can be developed toward Internet of Things (IoT) deployment based on such multifunction devices.

Prefixed-threshold real-time selection for correlated turbulent channel model for quantum key distribution with modulating retro-reflectors

Free-space quantum key distribution (QKD) with modulating retro-reflectors (MRR) is adopted to simplify the pointing requirements in practical experiment, which has the potential for mobile and flexible QKD platforms. However, this protocol suffers double-pass weak turbulence that may affect the performance of the QKD experiment in free-space links. In this paper, the free-space QKD with MRR under double-pass correlated turbulent channel is analyzed to examine the relationship between system performance and two parameters of the fading correlation coefficient and the turbulence intensity. The threshold post-selection method (P-RTS) is extended to MRR-QKD scheme under correlated turbulent channel and the simulations show that the P-RTS method is effective to alleviate the effect of atmospheric turbulence and suitable for the situation of double-pass correlated turbulent channel to make the protocol more practical in actual environment.

Modulated Retro-Reflector-Based Physical-Layer Network Coding for Space Optical Communications

We propose a Modulated Retro-Reflector (MRR) based Physical-Layer Network Coding (PNC) framework for space two-way relay (TWR) optical communications. MRR PNC simplifies the end node's design by replacing pointing, acquisition and tracking mechanisms which require large size, weight, and power (SWaP) consumption. Each end node modulates its data on top of the interrogating beam from the relay node so that the two end nodes' frequencies are synchronized and there is no carrier frequency offset between the two end nodes and the relay. However, the transmitted signal based on the interrogating beam and twice the channel fading leads to noise aggregation. We analyze the impact of this noise aggregation by deriving the bit error rate (BER) for MRR PNC and performing simulations in Gamma-Gamma fading channels. The simulation results show that MRR PNC has a 3 dB optical signal noise ratio penalty compared with MRR “point-to-point” systems. The overall system throughput of MRR PNC is 99% higher than the traditional non-network-coded scheme in the ≥6 dB (for BPSK, ≥9 dB for QPSK) signal noise ratio regime. We propose using low-density parity-check (LDPC) codes to mitigate the problem of the degradation of the BER caused by the noise aggregation in MRR PNC. Our simulation results show that a rate 1/2 LDPC code can achieve 2 dB coding gain on BER 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . We show that the asynchrony penalty is 3 dB on BER 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> compared with a synchronous system. The implication is that when channel coding is used, the system can still work. Our findings show that MRR PNC can double the system throughput while keeping low values of SWaP, is feasible for space TWR optical communications.

Experimental Channel Statistics of Drone-to-Ground Retro-Reflected FSO Links With Fine-Tracking Systems

The utilization of drones as flying base stations (BSs) in the sixth-generation (6G) cellular networks has attracted much attention recently. In this context, free-space optical (FSO) systems could be deployed to provide high-capacity fronthaul/backhaul links between drones and ground BSs. Particularly, a drone-to-ground FSO communication link can be established by equipping the drone with a modulating retro-reflector (MRR) to modulate the incoming optical beam and reflect the modulated beam back along the same path. This helps to alleviate stringent pointing requirements from the drone while satisfying the limited size, weight, and power (SWaP) consumption requirements. Nevertheless, the underlying physical channel effects of this aerial retro-reflected FSO system have never been experimentally explored in the literature. In this paper, we report, for the first time, the experimental channel statistics of a drone-to-ground retro-reflected FSO link, offering practical insights into the angle-of-arrival (AoA) fluctuations at the receiver, the channel coherence time, the probability of fade, the level crossing rate, the average fade duration, and the time-frequency channel characteristics. Our results are expected to serve as practical sources of reference for the theoretical performance analyses and engineering designs of drone-based retro-reflected FSO systems.

Wavefront Compensation With the Micro Corner-Cube Reflector Array in Modulating Retroreflector Free-Space Optical Channels

In this article, the micro corner-cube reflector array (MCCRA) is considered as the optical retroreflector in the modulating retroreflector (MRR) free-space optical (FSO) channel and its wavefront compensation accompanying with the pseudo phase conjugate optical return wave in the double-pass propagation is investigated by the theoretical analysis and wave-optics (WO) simulation. By WO simulations, we show the impacts of the shape and size of the retroreflector cell, the whole aperture size of the MCCRA, the turbulence condition on the beam spot, the Strehl ratio and the scintillation index of the return wave. Also, we investigate the limitations caused by the phase errors of the MCCRA and the oblique incidence of the optical beam. Finally, the wavefront compensation is demonstrated by the experiments conducted in a laboratory atmospheric chamber.