Route diversity analyses for free-space optical wireless links within turbulent scenarios

Free-Space Optical (FSO) communications link performance is highly affected when propagating through the time-spatially variable turbulent environment. In order to improve signal reception, several mitigation techniques have been proposed and analytically investigated. This paper presents experimental results for the route diversity technique evaluations for a specific case when several diversity links intersects a common turbulent area and concurrently each passing regions with different turbulence flows

On Parameter Estimation for Bandlimited Optical Intensity Channels

Parameter estimation is of paramount importance in every digital receiver. This is not only true for radio, but also for optical links; otherwise, subsequent processing stages, like detector units or error correction schemes, could not be operated reliably. However, for a bandlimited optical intensity channel, the problem of parameter estimation is strongly related to non-negative pulse shapes satisfying also the Nyquist criterion to keep the detection process as simple as possible. To the best of the author’s knowledge, it is the first time that both topics—parameter estimation on the one hand and bandlimited intensity modulation on the other—are jointly investigated. Since symbol timing and signal amplitude are the parameters of interest in this case, the corresponding Cramer⁻Rao lower bounds are derived as the theoretical limit of the jitter variance generated by the related estimator algorithms. In this context, a maximum likelihood solution is developed for the recovery of both timing and amplitude. Since this approach requires a receiver matched filter destroying the Nyquist criterion of the non-negative pulse shape, we compare it to a flat receiver filter preserving the required orthogonality property. It turned out that the jitter performance of the matched filter method is close to the Cramer⁻Rao lower bound in the medium-to-low SNR range, but due to inter-symbol interference effects an error floor emerges at higher SNR values. The flat filter solution avoids this drawback, although the price to be paid is a larger noise level at the filter output, so that a somewhat increased jitter variance is observed

Non-Data-Aided SNR Estimation for Bandlimited Optical Intensity Channels

Powerful and reliable estimation of transmission parameters is an indispensable task in each receiver unit—not only for radio frequency, but also for optical wireless communication systems. In this context, the signal-to-noise ratio (SNR) plays an eminent role, especially for adaptive scenarios. Assuming a bandlimited optical intensity channel, which requires a unipolar waveform design, an algorithm for SNR estimation is developed in this paper, which requires no knowledge of the transmitted data. This non-data-aided approach benefits to a great extent from the fact that very long observation windows of payload symbols might be used for the estimation process to increase the accuracy of the result; this is in striking contrast to a data-aided approach based on pilot symbols reducing the spectral efficiency of a communication link. Since maximum likelihood, moment-based or decision-directed algorithms are not considered for complexity and performance reasons, an expectation-maximization solution is introduced whose error performance is close to the Cramer-Rao lower bound as the theoretical limit, which has been derived as well

Subcarrier PSK Performance in Terrestrial FSO Links Impaired by Gamma-Gamma Fading, Pointing Errors, and Phase Noise

For terrestrial free-space optical (FSO) communication systems, subcarrier intensity modulation represents an attractive alternative to on-off keying or pulse-position modulation, which is mainly because of the larger spectral efficiency. However, some degradation of the error performance must be taken into account due to nonperfect synchronization of carrier frequency and phase. In a recently published paper, the average bit error probability has been analyzed for M-ary phase-shift keying in the presence of phase noise for a terrestrial FSO link impaired by lognormal fading. In the this paper, we are extending this study to a gamma-gamma model, which is usually applied in case of moderate-to-strong scintillation effects. On top of that, pointing errors, caused by a misalignment between transmitter and receiver of the FSO link, are considered as well. Since a closed-form solution is not available under general conditions and because numerical methods are time-consuming, suffering in part also from serious convergence and stability problems, we provide approximate closed-form expressions, which are accurate enough over a wide signal-to-noise ratio range. © 2017 IEEE

Simo subcarrier psk fso links with phase noise and non-zero boresight pointing errors over turbulence channels

Terrestrial free-space optical (FSO) communication systems with subcarrier intensity modulation have experienced a particular research attention in the recent past. However, their performance strongly degrades in the presence of atmospheric turbulence, pointing errors, and phase noise impairments. In order to overcome these limitations, the authors consider a receiver diversity scheme of a typical subcarrier phase-shift keying (PSK) system and investigate the performance by means of the average symbol error probability (ASEP). They assume a wide range of turbulence conditions, non-zero boresight pointing errors, and phase noise strengths described through the gamma-gamma, Beckmann, and Tikhonov distributions, respectively. Novel approximate ASEP expressions are derived for single-input single-output and single-input multiple-output (SIMO) configurations. Appropriate numerical results are depicted and validated by Monte Carlo simulations. © The Institution of Engineering and Technology 2019

Exact Analysis of Different Detector Algorithms for NDA Carrier Phase Recovery of 16-APSK Signals

In the new digital video broadcasting standard for satellite communications (DVB-S2), 16-ary amplitude-phase shift keying (APSK) is recommended as modulation scheme. For carrier phase recovery with feedback loops, deci-sion-directed (DD) and non-data aided (NDA) detector algo-rithms are basically applicable to 16-APSK schemes as well. In a recent paper, however, a hybrid NDA/DD solution has been suggested although not analyzed in detail. Motivated by this fact, the latter is investigated in terms of detector characteris-tic (S-curve) and jitter variance as the main figures of merit in this respect. For comparison purposes, results for power-law and monomial-based Viterbi and Viterbi trackers, as classical NDA solutions, are also included

Extended Gardner Detector for Improved Symbol-Timing Recovery of M-PSK Signals

In a recent paper, the modified Gardner detector has been proposed for advanced symbol-timing recovery of M-ary phase-shift keying signals. Compared with the original algorithm, the self-noise jitter can be considerably reduced for highly bandlimited systems and smaller values of . In this letter, a completely different approach is investigated, which improves the self-noise performance for larger values of M, as well