Maximizing atmospheric-disturbed fiber coupling efficiency with speckle-based phase retrieval and a single-pixel camera.

An approach to adaptive optics utilizing a single-pixel camera (SPC) is proposed to maximize fiber coupling efficiency at the receiver side of an optical satellite-to-ground link perturbed by atmospheric turbulence. Using a single-pixel wavefront sensor enables operation at longer optical wavelengths, such as near and far infrared, which have advantageous propagation characteristics for free space optical communication. In this approach, a focal plane intensity image of the atmospheric-disturbed wavefront is taken via an SPC using a compressed sensing technique. An iterative speckle-based phase retrieval algorithm is then applied to infer the phase distortion corrected by a deformable mirror in a feedback loop. This computational approach to inferring the phase of the wavefront overcomes the limitations of traditional Shack-Hartman-based approaches, which are difficult to implement at high speed and at the long infrared wavelengths proposed for future optical satellite communication downlinks. It has been shown that fiber coupling efficiency is increased from less than 5% to 40%-50% in medium-to-strong turbulence scenarios with the phase retrieval algorithm proposed in this work.

Layered antisymmetry-constructed clipped optical OFDM for low-complexity VLC systems.

In this paper, antisymmetry-constructed clipped optical orthogonal frequency division multiplexing (AC-OFDM) is proposed for visible light communication (VLC) systems, in which an antisymmetry property is imposed directly in time domain. AC-OFDM has nearly the same spectral efficiency and peak-to-average power ratio (PAPR) as traditional asymmetrically clipped optical OFDM (ACO-OFDM) but is less complex to implement. Layered AC-OFDM (LAC-OFDM) is then proposed as an extension to further improve spectral efficiency, where different layers of AC-OFDM signals are added in the time domain and transmitted simultaneously. Computational complexity analysis and numerical results show that LAC-OFDM has nearly the same spectral efficiency as layered asymmetrically clipped optical OFDM (LACO-OFDM) and enhanced unipolar OFDM (eU-OFDM) but is less complex. Specifically, LAC-OFDM requires less than half the multiplication and addition operations compared to the comparable LACO-OFDM scheme. Additionally, a pairwise iterative receiver for LAC-OFDM is proposed and its computational complexity is analysed. Monte Carlo simulation results show that LAC-OFDM requires nearly the same optical signal-to-noise ratio (OSNR) to achieve the same BER performance as LACO- and eU-OFDM.

Impact of angular pointing error on BER performance of underwater optical wireless links.

Even in clear ocean water, underwater optical wireless communication (UOWC) is impaired not only by absorption and scattering, but also by oceanic turbulence and dynamic pointing errors which result in a fading channel, degrading the bit error rate (BER) performance. In this paper, for the first time, we quantify analytically the trade-off between geometric loss and misalignment in underwater scattering channels. A novel geometric loss model is developed which is used to compute the average BER in the presence of absorption and scattering over salinity-induced oceanic turbulence channels. Our findings suggest that UOWC systems are less sensitive to angular pointing errors due to jitter since scattering is able to alleviate such a fading effect at the expense of a higher attenuation due to geometric spread. Monte Carlo simulation results are further included to verify the developed BER expression which is valid over a wide range of signal-to-noise-ratio (SNR). Finally, the impact of inter-symbol interference (ISI) is also quantified by measuring the optical power penalty.

Optical OFDM for SiPM-Based Underwater Optical Wireless Communication Links.

Underwater optical wireless systems have dual requirements of high data rates and long ranges in harsh scattering and attenuation conditions. In this paper, we investigate the advantages and limitations of optical orthogonal frequency-division multiplexing (O-OFDM) signaling when a silicon photo-multiplier (SiPM) is used at the receiver in order to ensure high sensitivity. Considering a light-emitting diode (LED) transmitter and taking into account the limited dynamic range imposed by the transmitter and the SiPM receiver, we study the performance of three popular O-OFDM schemes, i.e., DC-biased, asymmetrically-clipped, and layered asymmetrically-clipped O-OFDM (DCO-, ACO-, and LACO-OFDM, respectively). We consider a constraint on transmit electrical power PTxe and take into account the required DC bias for the three considered schemes in practice, showing the undeniable advantage of ACO- and LACO-OFDM in terms of energy efficiency. For instance, for the considered SiPM and LED components, a spectral efficiency of ∼1 bps/Hz with a data rate of 20 Mbps, a link range of 70 m, and a target bit-error-rate (BER) of 10-3, ACO and LACO allow a reduction of about 10 and 6 mW, respectively, in the required PTxe, compared to DCO-OFDM. Meanwhile, we show that when relaxing the PTxe constraint, DCO-OFDM offers the largest operational link range within which a target BER can be achieved. For instance, for a target BER of 10-3 and a data rate of 20 Mbps, and considering PTxe of 185, 80, and 50 mW for DCO-, LACO-, and ACO-OFDM, respectively, the corresponding intervals of operational link range are about 81, 74.3, and 73.8 m. Lastly, we show that LACO-OFDM makes a good compromise between energy efficiency and operational range flexibility, although requiring a higher computational complexity and imposing a longer latency at the receiver.

Hybrid two-level MPPM-MDPSK modulation for high-speed optical communication networks.

A hybrid optical modulation approach is described, which layers a continuous wave $M$M-ary differential phase-shift keying ($M{\rm DPSK}$MDPSK) and a two-level ($2L$2L) multipulse pulse-position modulation (MPPM) intensity-modulated signal for improved spectral efficiency. These $2L$2L techniques are a generalization of earlier hybrid MPPM-$M{\rm DPSK}$MDPSK techniques and have the added advantage of reducing transmitter and detector complexities over previous hybrid modulation approaches. The spectral and power efficiencies for the proposed $2L$2L-MPPM-$M{\rm DPSK}$MDPSK modulation techniques are formulated and shown to have the highest spectral efficiency in comparison to other hybrid techniques with lower implementation complexity. The performance of the proposed $2L$2L hybrid techniques is quantified over free-space optical (FSO) networks as well as fiber networks and verified using Monte Carlo simulation. For FSO channels, the proposed $2L$2L-MPPM-$M{\rm DPSK}$MDPSK technique outperforms the traditional MPPM-$M{\rm DPSK}$MDPSK scheme by approximately 2 dB at a bit-error rate (BER) of ${10^{-4}}$10-4 and a spectral efficiency of 2.5 bit/s/Hz. Similarly, in optical fiber, the proposed scheme relaxes the impact of nonlinearity in comparison to traditional MPPM-$M{\rm DPSK}$MDPSK. Specifically, at a ${\rm BER}{=10^{-3}}$BER=10-3, the $2L$2L-MPPM-$M{\rm DPSK}$MDPSK technique outreaches the MPPM-$M{\rm DPSK}$MDPSK by 2000 km at a spectral efficiency of 2.5 bit/s/Hz and an average transmit power of $-{3}\,\,{\rm dBm}$-3dBm.

Simulation of atmospheric turbulence for optical systems with extended sources.

In this paper, the method of random wave vectors for simulation of atmospheric turbulence is extended to 2D×2D space to provide spatial degrees of freedom at both input and output planes. The modified technique can thus simultaneously simulate the turbulence-induced log-amplitude and phase distortions for optical systems with extended sources either implemented as a single large aperture or multiple apertures. The reliability of our simulation technique is validated in different conditions and its application is briefly investigated in a multibeam free-space optical communication scenario.

Impact of finite receiver-aperture size in a non-line-of-sight single-scatter propagation model.

In this paper, a single-scatter propagation model is developed that expands the classical model by considering a finite receiver-aperture size for non-line-of-sight communication. The expanded model overcomes some of the difficulties with the classical model, most notably, inaccuracies in scenarios with short range and low elevation angle where significant scattering takes place near the receiver. The developed model does not approximate the receiver aperture as a point, but uses its dimensions for both field-of-view and solid-angle computations. To verify the model, a Monte Carlo simulation of photon transport in a turbid medium is applied. Simulation results for temporal responses and path losses are presented at a wavelength of 260 nm that lies in the solar-blind ultraviolet region.

Non-line-of-sight single-scatter propagation model for noncoplanar geometries.

In this paper, a geometrical propagation model is developed that generalizes the classical single-scatter model under the assumption of first-order scattering and non-line-of-sight (NLOS) communication. The generalized model considers the case of a noncoplanar geometry, where it overcomes the restriction that the transmitter and the receiver cone axes lie in the same plane. To verify the model, a Monte Carlo (MC) radiative transfer model based on a photon transport algorithm is constructed. Numerical examples for a wavelength of 266 nm are illustrated, which corresponds to a solar-blind NLOS UV communication system. A comparison of the temporal responses of the generalized model and the MC simulation results shows close agreement. Path loss and delay spread are also shown for different pointing directions.