Jaedon Park

Performance Analysis of the Asymmetric Dual-Hop Relay Transmission With Mixed RF/FSO Links

This letter reports a performance analysis of a dual-hop relay system composed of asymmetric radio-frequency and free-space optics (RF/FSO) links. This approach is based on the fact that FSO links can provide even wider bandwidths as compared to RF ones. In particular, an exact closed-form expression for the end-to-end outage probability of the RF/FSO relay links is derived. Overall, the RF/FSO links show a slightly worse performance than the RF/RF links, but the performance gap is gradually reduced as the signal-to-noise ratio (SNR) increases. Our mathematical analysis results were verified by exactly matching Monte Carlo simulation results.

Average Bit-Error Rate of the Alamouti Scheme in Gamma-Gamma Fading Channels

Multi-input multi-output (MIMO) in free-space optics communications with subcarrier intensity modulation has been a big research issue due to the diversity gain of MIMO to mitigate the signal scintillation caused by the atmospheric turbulence. In this work, we have analyzed the performance of the Alamouti scheme in FSO links. Particularly, a power series expression of the average bit-error rate (BER) of the Alamouti scheme is derived in gamma-gamma fading channels. As a result, the Alamouti scheme could achieve a high signal-to-noise ratio (SNR) gain of 37 dB in a strong turbulence regime, and also a high SNR gain of 27 dB in a moderate turbulence regime over the no diversity at the BER of 10-6.

Characteristics of piezoelectric ZnO/AlN−stacked flexible nanogenerators for energy harvesting applications

Flexible piezoelectric zinc oxide (ZnO)-based nanogenerators (NGs) using an aluminum nitride (AlN) interlayer are proposed for high-efficiency energy harvesting applications. The effects of the AlN interlayer on device performance are studied. Use of the AlN interlayer in ZnO-based vertically integrated NGs (VINGs) results in a significant improvement in terms of the magnitude of the output voltages of up to 200 times when compared with a ZnO-based VING without any AlN interlayer. The improved device energy conversion efficiency is mainly attributed to a high contact potential barrier that the AlN interlayer provides in VINGs, along with the relatively high dielectric constant and large Youngs modulus of the AlN material. In addition, the effects of AlN thickness on the electric potential and device performance of the VINGs are investigated through observation of the output voltages of ZnO-based VINGs with thickness/position-controlled AlN interlayers. Our findings in this work are expected to provide effective and useful approaches for realizing highly energy-efficient ZnO-based NGs and their extended applications, including self-power sources and sensor devices.

High-efficiency micro-energy generation based on free-carrier-modulated ZnO:N piezoelectric thin films

The free-carrier-modulated ZnO:N thin film-based flexible nanogenerators (NZTF-FNGs) are proposed and experimentally demonstrated. The suggested flexible nanogenerators (FNGs) are fabricated using N-doped ZnO thin films (NZTFs) as their piezoelectric active elements, which are deposited by a radio frequency magnetron sputtering technique with an N2O reactive gas as an in situ dopant source. Considerable numbers of N atoms are uniformly incorporated into NZTFs overall during their growth, which would enable them to significantly compensate the unintentional background free electron carriers both in the bulk and at the surface of ZnO thin films (ZTFs). This N-doping approach is found to remarkably enhance the performance of NZTF-FNGs, which shows output voltages that are almost two orders of magnitude higher than those of the conventionally grown ZnO thin film-based FNGs. This is believed to be a result of both substantial screening effect suppression in the ZTF bulk and more reliable Schottky barrier forma...

Performance Analysis of Coherent Free-Space Optical Systems With Multiple Receivers

In this letter, we investigate the performance of a coherent free-space optical (FSO) communication system with multiple receive apertures over atmospheric turbulence fading channels. In particular, we derive the average error probability and the ergodic capacity of the coherent FSO system with multiple receive apertures in series representations. To mitigate the atmospheric turbulence-induced amplitude fluctuations and phase aberrations, we use, at the receive apertures, the modal compensation technique. Numerical results confirm the derived average error probability and ergodic capacity expressions.

Impact of Pointing Errors on the Performance of Coherent Free-Space Optical Systems

In this letter, we investigate the performance of a coherent free-space optical (FSO) communications system considering geometric spread and pointing errors as well as atmospheric turbulence fading and path loss. In particular, we derive the outage probability and average error probability of the coherent FSO system, considering the combined effects of path loss, geometric spread and pointing errors, and amplitude fluctuations and phase aberrations in series representations forms. Numerical and Monte Carlo simulation results confirm the validity of the derived outage probability and average error probability expressions.

A new adaptive beamforming algorithm for MIMO smart antennas

We propose a new MIMO adaptive beamforming algorithm for smart antennas. The algorithm is based on the LMS, requiring no reference signal. Computer simulations, considering the multi-path Rayleigh fading channel, are presented to verify the performance.

Performance Analysis of Asymmetric RF/FSO Dual-Hop Relaying Systems for UAV Applications

In this paper, we analyze the performance of a dual-hop relaying system composed of asymmetric radio frequency (RF) and free-space optics (FSO) (RF/FSO) links. We consider an asymmetric amplify-and-forward (AF) relay which converts the received RF signal into an optical signal using the subcarrier intensity modulation (SIM) scheme. The RF and FSO channels are assumed to experience Rayleigh and Gamma-Gamma fading distributions, respectively. Particularly, we derive the average probability of error as well as ergodic capacity upper bound of the asymmetric RF/FSO dual-hop relaying system, in closed-forms. As a result, the asymmetric RF/FSO relaying system shows slightly worse performance in average probability of error and ergodic capacity upper bound than the RF/RF relaying system in the low SNR. Over the SNR of 20 dB, however, the asymmetric RF/FSO relaying system shows very similar performance in average probability of error and ergodic capacity upper bound to the RF/RF relaying system. The derived mathematical expressions are verified by exactly matching Monte-Carlo simulation results.

A semi-blind LMS adaptive beamforming algorithm for smart antennas in an OFDM system

This paper proposes a semi-blind LMS adaptive beamforming algorithm for antenna arrays applied to an OFDM system. The OFDM system under consideration employs the fast Fourier transform (FFT) to efficiently generate the sub-carriers. The performance of the proposed algorithm is verified by computer simulation.

Outage Probability Analysis of a Coherent FSO Amplify-and-Forward Relaying System

In this letter, we propose and analyze an amplify-and-forward (AF) relaying system for coherent free-space optics (FSOs) applications. In particular, we derive, in series expressions, the lower bound of the outage probability of the proposed system using modal compensation to mitigate the atmospheric turbulence-induced amplitude fluctuation and phase aberration. The performance of the derived lower bound of the outage probability is confirmed by the very tight Monte-Carlo simulation result. We show that the coherent FSO AF relaying system could be used for practical applications in case of Zernike compensation mode J = 20, requiring the SNR of 32.5 dB in the outage probability of 10-6. It is also noted that the AF relaying system is comparable in performance with the DF relaying system.