Jaime A. Anguita

Turbulence-induced channel crosstalk in an orbital angular momentum-multiplexed free-space optical link

A multichannel free-space optical (FSO) communication system based on orbital angular momentum (OAM)-carrying beams is studied. We numerically analyze the effects of atmospheric turbulence on the system and find that turbulence induces attenuation and crosstalk among channels. Based on a model in which the constituent channels are binary symmetric and crosstalk is a Gaussian noise source, we find optimal sets of OAM states at each turbulence condition studied and determine the aggregate capacity of the multichannel system at those conditions. OAM-multiplexed FSO systems that operate in the weak turbulence regime are found to offer good performance. We verify that the aggregate capacity decreases as the turbulence increases. A per-channel bit-error rate evaluation is presented to show the uneven effects of crosstalk on the constituent channels.

Shannon capacities and error-correction codes for optical atmospheric turbulent channels

Feature Issue on Optical Wireless Communications (OWC) The propagation of an on-off keying modulated optical signal through an optical atmospheric turbulent channel is considered. The intensity fluctuations of the signal observed at the receiver are modeled using a gamma-gamma distribution. The capacity of this channel is determined for a wide range of turbulence conditions. For a zero inner scale, the capacity decreases monotonically as the turbulence strengthens. For non-zero inner scale, the capacity is not monotonic with turbulence strength. Two error-correction schemes, based on low-density parity-check (LDPC) codes, are investigated as a means to improve the bit-error rate (BER) performance of the system. Very large coding gains--ranging from 5.5 to 14 dB, depending on the turbulence conditions--are obtained by these LDPC codes compared with Reed-Solomon error-correction codes of similar rates and lengths.

Spatial correlation and irradiance statistics in a multiple-beam terrestrial free-space optical communication link

By means of numerical simulations we analyze the statistical properties of the power fluctuations induced by the incoherent superposition of multiple transmitted laser beams in a terrestrial free-space optical communication link. The measured signals arising from different transmitted optical beams are found to be statistically correlated. This channel correlation increases with receiver aperture and propagation distance. We find a simple scaling rule for the spatial correlation coefficient in terms of the propagation distance and we are able to predict the scintillation reduction in previously reported experiments with good accuracy. We propose an approximation to the probability density function of the received power of a spatially correlated multiple-beam system in terms of the parameters of the single-channel gamma-gamma function. A bit-error-rate evaluation is also presented to demonstrate the improvement of a multibeam system over its single-beam counterpart.

Rateless Coding on Experimental Temporally Correlated FSO Channels

We present a demonstration of two error-correction coding schemes that can successfully operate on a free-space optical (FSO) communication channel subject to atmospheric turbulence. The codes (a puntured Low-density parity-check code and a Raptor code) operate by continuously adapting the information rate to accommodate the varying channel conditions. Because these coding schemes require the use of a feedback channel, we evaluate the bandwidth cost incurred. The evaluation of the codes is performed offline and uses experimental optical signals recorded from an FSO link. We analyze the temporal characteristics of the experimental channels and compare the performance of the codes for different bit rates to asses the effect of temporal correlation and imperfect channel state information.

Error-Correction Coded Orbital-Angular-Momentum Modulation for FSO Channels Affected by Turbulence

The performance of LDPC-precoded, orbital-angular-momentum (OAM) modulation is studied over a 1-km free-space laser communication link subject to OAM modal crosstalk induced by atmospheric turbulence. The multidimensional signal constellation is designed as the Cartesian product of a one-dimensional non-negative pulse-amplitude modulation and a set of orthogonal OAM modes. We evaluate the performance of this modulation scheme by first determining conditional probability density functions (PDFs) of the modal crosstalk for each symbol, resulting from the propagation in weak turbulence using a numerical propagation model. It is observed that OAM modulation is more sensitive to atmospheric turbulence as the number of dimensions increases. However, this can be efficiently mitigated by an error-correction code. The coded OAM modulation scheme provides an energy-efficient alternative to single-mode transmission, since a larger rate can be obtained per given bandwidth.

Coherent Multimode OAM Superpositions for Multidimensional Modulation

The generation, propagation, and detection of high-quality and coherently superimposed optical vortices, carrying two or more orbital angular momentum (OAM) states, is experimentally demonstrated using an optical arrangement based on spatial light modulators. We compare our results with numerical simulations and show that, in the context of turbulence-free wireless optical communication (indoor or satellite), individual OAM state identification at the receiver of an OAM-modulated system can be achieved with good precision, to accommodate for high-dimensional OAM modulation architectures. We apply our results to the simulation of a communication system using low-density parity-check-coded modulation that considers optimal signal constellation design in a channel that includes OAM crosstalk induced by realistic (imperfect) detection.

Information Theoretic Limits for Free-Space Optical Channels With and Without Memory

The availability of Channel State Information (CSI) and the effects of channel memory on the capacities and the achievable rates of free-space optical communication channels are investigated. For memoryless channels, the capacities and achievable rates are computed and compared for both uniform and ldquopositiverdquo Gaussian inputs subject to different assumptions on the CSI availability. For the strong turbulence regime, it is shown that the knowledge of CSI both at the transmitter and the receiver increases the achievable rates for low-to-moderate Signal-to-Noise Ratios (SNRs) in comparison to the cases for which the CSI is known only at the receiver. For the weak turbulence regime however, the availability of CSI at both ends of the link does not provide any improvement over a system with CSI known at the receiver alone, and we find that a simple channel inversion technique suffices. In addition, for low SNRs, Pulse Amplitude Modulation (PAM) with M ges 4 levels outperforms Gaussian-distributed inputs regardless of the knowledge of CSI at the transmitter. For high SNRs, a Gaussian distribution gives superior results, implying the need for new, more efficient positive signal constellations. For channels with memory and without knowledge of CSI, a change in the channel quasifrequency has negligible effects on the capacity for any turbulence regime.

Experimental evaluation of transmitter and receiver diversity in a terrestrial FSO link

We experimentally measure and characterize the channel fluctuations in a terrestrial laser communication link using spatial diversity. We measure the signals arising from the propagation of laser beams in a system that comprises two transmit and two receive apertures under various design settings. We find that the spatial channels are statistically correlated. This spatial correlation increases with receiver aperture size and with increasing proximity between beams and/or receiver apertures. By quantifying the cross-covariance between pairs of simultaneously recorded time functions, we map the spatial channel correlation of the optical wireless link as a function of beam separation at the transmitter, receiver aperture, and receiver optics separation. We discuss the effects of transverse wind velocity and establish criteria for the design of effective multiple-input multiple-output free-space optical links affected by atmospheric turbulence.

Orbital-angular-momentum crosstalk and temporal fading in a terrestrial laser link using single-mode fiber coupling.

Using a mobile experimental testbed, we perform a series of measurements on the detection of laser beams carrying orbital angular momentum (OAM) to evaluate turbulent channel distortions and crosstalk among receive states in an 84-m roofed optical link. We find that a receiver assembly using single-mode fiber coupling serves as a good signal selector in terms of crosstalk rejection. From the recorded temporal channel waveforms, we estimate average crosstalk profiles and propose an appropriate probability density function for the fluctuations of the detected OAM signal. Further measurements of OAM crosstalk are described for a horizontal 400-m link established over our campus.

LDPC-Coded MIMO Optical Communication Over the Atmospheric Turbulence Channel

We describe a multiple optical sources - multiple detectors scheme, based on either repetition MIMO or space-time coding and low-density parity-check (LDPC) codes. The proposed scheme is able to operate under strong atmospheric turbulence and provides excellent coding gains. The LDPC codes are designed using the concept of pairwise-balanced designs. Bit-error rates and achievable information rates are reported assuming non-ideal photodetection. To improve the spectral efficiency we employ the concept of bit-interleaved LDPC-coded modulation based on pulse-amplitude modulation.