Adaptive Beam Divergence Control to Mitigate Scintillation Effect Caused by Pointing Error in Vertical FSO Transmissions.

Free-space optical (FSO) communication has been studied for next-generation network systems. Because an FSO system establishes point-to-point communication links, maintaining alignment among the transceivers is a critical challenge. In addition, atmospheric turbulence causes significant signal loss in FSO vertical links. Even in clear weather conditions, transmitted optical signals suffer significant scintillation losses due to random variations. Thus, the effect of atmospheric turbulence should be considered in vertical links. In this paper, we analyze the relationship between pointing error and scintillation from the aspect of beam divergence angle. Furthermore, we propose an adaptive beam that optimizes its divergence angle according to the pointing error between the communicating optical transceivers to mitigate the effect of scintillation due to pointing error. We performed a beam divergence angle optimization and compared it with adaptive beamwidth. The proposed technique was demonstrated using simulations, which revealed an enhanced signal-to-noise ratio and the mitigation of the scintillation effect. The proposed technique would be useful in minimizing the scintillation effect in vertical FSO links.

Efficient MIMO Configuration for Bi-Directional Vertical FSO Link with Multiple Beam Induced Pointing Error.

We proposed the statistical misalignment model and the power-efficient configuration of transceivers for bi-directional multi-input and multi-output (MIMO) based vertical free space optical (FSO) links. Spatial diversity based MIMO FSO systems could be used to mitigate atmospheric fading issues. However, the increased number of channels can cause additional pointing error in pointing, acquisition and tracking (PAT) systems. The statistical misalignment model for detecting misalignment error is derived from the multiple transceivers. For the bi-directional characteristics of non-terrestrial back-haul networks, transmission performance is down-leveled to the worse in the asymmetric MIMO configuration of transceivers. The symmetric structure can mitigate the effect of increased pointing error to improve transmission performance. The proposed technique can be applied to the design of power-efficient FSO systems for non-terrestrial wireless back-haul networks.

Data rate enhancement of free space optical communication using pulse positioned differential phase shift keying.

In this paper, a pulse positioned-differential phase shift keying technique is proposed to enhance the data rate in free space optical communication. Using the schematics of polarization rotation differential phase shift keying, multi-rate functionality can be achieved without using delay-line interferometers. Furthermore, the proposed novel modulation format-differential phase shift keying combined with pulse-position modulation-enables a high data rate owing to the use of an average power limited amplifier. By using the average power limited amplifier, the signal power is increased as the pulse position order increases, which enhances the bit-error-rate performance. The increased signal power can be converted to an enhanced data rate. We demonstrated that the data rate above 625 Mbps can be increased in every step, as the pulse position order increases in the pulse positioned-differential phase shift keying. The performance enhancement of the proposed technique is theoretically and experimentally demonstrated.

Ionic contrast across a lipid membrane for Debye length extension: towards an ultimate bioelectronic transducer.

Despite technological advances in biomolecule detections, evaluation of molecular interactions via potentiometric devices under ion-enriched solutions has remained a long-standing problem. To avoid severe performance degradation of bioelectronics by ionic screening effects, we cover probe surfaces of field effect transistors with a single film of the supported lipid bilayer, and realize respectable potentiometric signals from receptor-ligand bindings irrespective of ionic strength of bulky solutions by placing an ion-free water layer underneath the supported lipid bilayer. High-energy X-ray reflectometry together with the circuit analysis and molecular dynamics simulation discovered biochemical findings that effective electrical signals dominantly originated from the sub-nanoscale conformational change of lipids in the course of receptor-ligand bindings. Beyond thorough analysis on the underlying mechanism at the molecular level, the proposed supported lipid bilayer-field effect transistor platform ensures the world-record level of sensitivity in molecular detection with excellent reproducibility regardless of molecular charges and environmental ionic conditions.

Polarization-dependent SOA-based PolSK modulation for turbulence-robust FSO communication.

This paper proposes a polarization-dependent semiconductor optical amplifier (SOA)-based polarization shift keying (PolSK) modulation for turbulence-robust free-space optical (FSO) communication. Based on the nonlinear gain and polarization characteristics, the PolSK signal is modulated by the optical bit inversion using the polarization-dependent SOA and linear polarizer (LP). The capability of scintillation mitigation is analyzed under both balanced and polarization-independent SOA-based detections. The proposed technique is evaluated in experiments. The experimental results demonstrated that the bit-error-rate (BER) performance of the proposed technique was close to the conventional Mach-Zehnder modulator (MZM)-based PolSK modulation under various scintillation effects.

Dual anode single-photon avalanche diode for high-speed and low-noise Geiger-mode operation.

The after-pulsing effect is a common problem in high-speed and low-noise single-photon detection based on single-photon avalanche diodes (SPADs). This article presents a dual anode InGaAs/InP SPAD (DA-SPAD) with two separate anode output ports that can be utilized for discriminating relatively weak avalanche signals, providing a simple and robust configuration of the SPAD-based single-photon detection system. Weak avalanche signals with amplitudes below the amplitude of the parasitic capacitive response of the SPAD were easily detected by the DA-SPAD and a simple subtraction circuit. The gated Geiger-mode performance of the DA-SPAD was also investigated. At a gating frequency of 1 GHz, the detection efficiency was 20.4% with an after-pulse probability of 3.5% at a temperature of -20 °C.

Fixed threshold on-off keying differential detection for satellite optical communications.

In this paper, on-off keying (OOK) differential detection with fixed threshold is proposed to deal with intensity fluctuation for satellite optical communications (SOCs). Since the intensity fluctuation is relatively slow compared to data rate, sequential symbols are in a quasi-stationary channel. Based on this characteristic, the proposed technique is implemented through the comparison between the difference of sequential symbols and the fixed threshold. The optimum fixed threshold is simply estimated with the knowledge of the channel model and noise power. The simulation results demonstrate that it overcomes the BER limitation of fixed threshold detection. Therefore, the proposed technique enables OOK detection with fixed threshold and real-time implementation in high-speed satellite optical communications.

Full-band TDM-OPDMA for OBI-reduced simultaneous multiple access in a single-wavelength optical access network.

Simultaneous multiple access (MA) within a single wavelength can increase the data rate and split ratio in a passive optical network while optical beat interference (OBI) becomes serious in the uplink. Previous techniques to reduce OBI were limited by their complexity and lack of extendibility; as well, bandwidth allocation among MA signals is needed for single photo diode (PD) detection. We proposed and experimentally demonstrated full-band optical pulse division multiplexing-based MA (OPDMA) in an optical access network, which can effectively reduce OBI with extendibility and fully utilize frequency resources of optical modulator without bandwidth allocation in a single-wavelength MA.

Optical signal suppression by a cascaded SOA/RSOA for wavelength reusing reflective PON upstream transmission.

An optical signal suppression technique based on a cascaded SOA and RSOA is proposed for the reflective passive optical networks (PONs) with wavelength division multiplexing (WDM). By suppressing the downstream signal of the optical carrier, the proposed reflective PON effectively reuses the downstream optical carrier for upstream signal transmission. As an experimental demonstration, we show that the proposed optical signal suppression technique is effective in terms of the signal bandwidth and bit-error-rate (BER) performance of the remodulated upstream transmission.

Novel OBI noise reduction technique by using similar-OBI estimation in optical multiple access uplink.

A novel optical beat interference (OBI) noise reduction technique for optical multiple access uplink using the same wavelength is presented. The OBI noise is estimated by similar-OBI and mitigated by subtracting estimated OBI noise using the digital signal processing technique. To derive similar-OBI, an RF clipping tone is transmitted out of signal band. We numerically and experimentally verified the validity of proposed OBI noise reduction method in orthogonal frequency division multiplexing access (OFDMA) passive optical network (PON) uplink where the OBI noise has been reduced and signal to noise ratio (SNR) has been improved from 4.4 dB to 10.6 dB.

Optical pulse division multiplexing-based OBI reduction for single wavelength uplink multiple access in IM/DD OFDMA-PON.

Orthogonal frequency division multiple access-based passive optical network (OFDMA-PON) is considered as a strong candidate for next-generation optical access network. In intensity modulation/direct detection system, OFDMA-PON downlink transmission is relatively stable, but critical issues exist in uplink multiple access. Because of different optical paths, optical beat interference (OBI) and timing offset effect are generated, which seriously disturb signal detection. We propose optical pulse division multiplexing-based OBI reduction. By considering both the spectrum broadening effect and the time domain near orthogonality, OBI could be reduced. We demonstrate that the spectral efficiency can be improved from 0.37 to 3.8 bit/s/Hz in 1-GHz signal bandwidth.

Self-reverse-biased solar panel optical receiver for simultaneous visible light communication and energy harvesting.

We propose a self-reverse-biased solar panel optical receiver for energy harvesting and visible light communication. Since the solar panel converts an optical component into an electrical component, it provides both energy harvesting and communication. The signal component can be separated from the direct current component, and these components are used for communication and energy harvesting. We employed a self-reverse-biased receiver circuit to improve the communication and energy harvesting performance. The reverse bias on the solar panel improves the responsivity and response time. The proposed system achieved 17.05 mbps discrete multitone transmission with a bit error rate of 1.1 x 10-3 and enhanced solar energy conversion efficiency.

Inter-cell interference mitigation in multi-cellular visible light communications.

Inter-cell interference hinders multi-cellular optical wireless communication to support various applications. We proposed and experimentally demonstrated a multicarrier-based cell partitioning scheme, combined with frequency reuse, which could be effective in optical communications although it is inefficient in RF wireless communications. For multicarrier-based cell partitioning, Orthogonal frequency division multiplexing-based multiple access (OFDMA) was employed to accommodate multi-cellular optical wireless communications without a large guard band between adjacent cells and without additional RF components. Moreover, we employed filter bank-based multicarrier (FBMC) to mitigate inter-cell interference generated in OFDMA-based cell partitioning due to asynchronous signals originated from RF path difference. By using FBMC-based cell partitioning, inter-cell interference could be effectively mitigated as well as capacity and spectral efficiency were improved about 1.5 times compared to those of OFDMA. Because no cyclic prefix (CP) is required in FBMC, the improvement factor could be increased if there is a large RF path difference between lighting cells. Moreover, it could be a stronger solution when many neighboring cells exist causing large interference. The proposed multicarrier-based cell partitioning combined with FBMC will effectively support visible light communication (VLC)-based localization-based services (LBS) and indoor positioning system by transparently providing trilateration-based positioning method.

Mitigation of timing offset effect in IM/DD based OFDMA-PON uplink multiple access.

In orthogonal frequency division multiple access based passive optical network (OFDMA-PON) uplink, synchronization between optical network units (ONUs) is very important to maintain orthogonality. The synchronization among uplink signals is considered as one of the main challenges in OFDMA-PON due to optical path difference. In this paper, the performance degradation according to timing offset between ONUs is experimentally analyzed. And we propose and demonstrate timing offset effect reduction in asynchronous multiple access by using CP extension and filter bank based multicarrier (FBMC) system in intensity modulation/direct detection (IM/DD) based OFDMA-PON uplink transmission.

Optical beat interference noise reduction by using out-of-band RF clipping tone signal in remotely fed OFDMA-PON link.

A novel technique for mitigating the optical beat interference (OBI) noise in an optical orthogonal frequency division multiple access passive optical network (OFDMA-PON) uplink transmission is presented. By using an out of signal band RF clipping tone to the optical seed carrier, the OBI noise has been reduced and the resulting throughput and spectral efficiency has been improved. As an experimental verification, we demonstrate that the spectral efficiency of 23 km and 50 km have been doubled in the OFDMA-PON uplink transmission.

Colorless WDM-PON based on a Fabry-Pérot laser diode and reflective semiconductor optical amplifiers for simultaneous transmission of bidirectional gigabit baseband signals and broadcasting signal.

A novel WDM-PON system delivering bidirectional baseband data and broadcasting data is proposed and demonstrated. A subcarrier multiplexing signal is broadcasted to all users by modulating a broadband optical source based on a Fabry-Pérot laser diode. Reflective semiconductor optical amplifiers are used as colorless modulators for the baseband data at both optical line terminal and remote optical network units. Transmission performance including bit error rate of bidirectional gigabit data and error vector magnitude of broadcasting data of many optical channels is investigated. Additionally, the data rate for the broadcasting signal was improved by using an external modulator.

Gigabit radio-over-fiber link for converged baseband and millimeter-wave band signal transmission using cascaded injection-locked Fabry-Pérot laser diodes.

A novel scheme, for both baseband and millimeter-wave band gigabit data transmission in radio-over-fiber system, is proposed and experimentally demonstrated by using cascaded injection-locked Fabry- Pérot laser diodes. It was able to improve suppression ratio of carrier suppressed signal using the cascaded injection-locking. The suppression ratio improvement of the optical carrier suppressed signal of 20 dB was verified. Applying this mechanism, 60-GHz millimeter-wave carrier of enhanced signal quality could be accomplished. Its peak power and phase noise were obtained as -40 dBm and -103.5 dBm/Hz respectively, which was suitable for 60-GHz data transmission. In addition, a successful bidirectional transmission of 1.25-Gbps wired and wireless data was achieved by adopting remodulation technique using a gain-saturated reflective semiconductor optical amplifier for uplink.

Performance analysis of a wavelength-locked Fabry-Perot laser diode by light injection of an external spectrally sliced Fabry-Perot laser diode.

We have analyzed and experimentally demonstrated the performance of a wavelength-locked Fabry-Perot laser diode (FP-LD) with a spectrally sliced FP-LD as an external-injection optical source. A high side-mode suppression ratio exceeding 30 dB was obtained, and the noise characteristic of the wavelength-locked FP-LD was analyzed in terms of both mode partition noise and interferometric noise. The measured relative intensity noise had a minimum value of -140 dB/Hz. We also investigated the transmission characteristics such as the Q factor and extinction ratio for a 622 Mbits/s and 2.5 Gbits/s baseband digital signal.