Coherent Free-space Optical Systems Research Articles

Evaluation of a high-speed hybrid OAM-OFDM-MDM multiplexed coherent FSO system under desert conditions

To meet the needs of future wireless optical networks, this paper introduces a high-speed, hybrid multiplexed, coherent free-space optical (FSO) communication system that integrates an orbital angular momentum (OAM) multiplexed signal with an orthogonal frequency division multiplexing (OFDM) technique. Two independent QAM polarized beams, each carrying in-phase and quadrature (I/Q) phase 16-QAM-OFDM modulated data, are combined using mode division multiplexing (MDM) to increase the capacity of the proposed system. The reason of choosing OFDM is its capability to support higher data rate, and mitigating intersymbol interference (ISI). The signal is detected using a coherent detection-based digital signal processing (DSP) algorithm at the receiver end. The proposed hybrid FSO system is evaluated in low and heavy dust environments using bit error rate (BER), link distance, optical signal-to-noise ratio (OSNR), and received optical power performance matrices. The simulation results demonstrate the successful transmission of a 120 Gb/s single carrier over the longest link ranges of 1.5 and 0.40 km, respectively, under low and heavy dust weather environments below the signal degradation threshold value (forward error correction (FEC) limit) of BER 2.2 × 10–3 in strong turbulent conditions.

Error Probability of a Coherent M-ary PSK FSO System Influenced by Phase Noise

In this paper, we aim to develop an analytical framework for design and analysis of new generation mobile networks fronthaul/backhaul links based on the application of free-space optical (FSO) technology. Taking the receiver hardware imperfections into account, we present an efficient analytical approach in analyzing average symbol error probability (SEP) of the coherent FSO system employing M-ary phase-shift keying (PSK). Optical signal transmission is influenced by pointing errors and atmospheric turbulence. The signal intensity fluctuations caused by atmospheric turbulence are modeled by general Málaga (M) distribution, which takes into account the effect of multiple scattered components. We estimate the range of the signal-to-noise ratio at which the SEP floor appears, as well as the value of this non-removable error floor. The results illustrate that the effect of imperfect phase error compensation on the SEP is more critical under weaker turbulence conditions and for higher order modulation formats. Based on the analytical tools presented here, it is possible to estimate tolerable value of standard deviation of phase noise for the given value of SEP. This value of standard deviation is an important parameter in designing the phase-locked loop filter in the receiver.

Performance analysis of MIMO FSO systems with EGC diversity receiver over lognormal-Rician turbulence

Two approximate closed-form probability density function expressions for the sum of lognormal-Rician random variates (RVs) and the sum of square root of lognormal-Rician RVs are obtained. The proposed expressions are shown to provide high accuracy over a wide range of channel conditions according to Kolmogorov–Smirnov goodness-of-fit statistical tests results. Also, the analysis of the approximation error is presented to indicate that a higher approximation accuracy can be achieved for larger r or smaller variance σz2. To reveal the importance of the proposed approximation, new approximate closed-form expressions of the ergodic capacity, average bit error rate for the intensity-modulation direct-detection (IMDD), and coherent multiple-input multiple-output (MIMO) free-space optical (FSO) systems with equal gain combining diversity technique over lognormal-Rician turbulence channels are developed. It is observed that MIMO technology can offer a significant improvement in system performance and the performance of coherent FSO systems outperforms that of IMDD systems. The Monte Carlo simulation results are further utilized to illustrate the accuracy of the proposed approach.

Design and Performance Analysis of Probabilistically Shaped QAM Signals for Coherent FSO Systems with Gamma-Gamma Turbulence Channels

Probabilistic shaping (PS) is a promising technique to approach the Shannon limit. In this paper, we design a practical coded modulation scheme based on PS to improve the capacity of coherent free-space optical (FSO) links with quadrature amplitude modulation (QAM), where the fading channel follows the Gamma-Gamma distribution. The aim of this paper is to optimize the probability mass function (PMF) of the QAM signal points to achieve the maximum channel capacity. Due to the complexity of the objective function, the heuristic algorithm was employed to solve the optimization problem. To the best of the authors’ knowledge, the closed-form pairwise error probability (PEP) is first derived with the non-uniform signals under the turbulence channel. In addition, we measure the average symbol error rate (SER) and post-FEC bit error rate (BER) by the Monte Carlo simulation method. The numerical simulation results of both capacity and BER show that the proposed PS scheme is better than the uniform distribution. The post-FEC BER results show that the proposed PS scheme provides significant gains compared with the uniform scheme.

Asymmetric constellation transmission for a coherent free-space optical system with spatial diversity.

Spatial diversity is widely recognized as an effective technique for free-space optical (FSO) communication to combat channel fading. Most existing works on spatial diversity in FSO systems focus on intensity modulation and direct detection, while fewer mention coherent optical detection, which excels in optical receiver responsivity. Spatial diversity enables the FSO coherent system to demodulate the signal components of in-phase (I) and quadrature (Q) separately and thus simplify the coherent detector. More importantly, due to the differential channel conditions between I and Q components, we are offered a chance to realize an asymmetric quadrature amplitude modulation (QAM) constellation that can utilize the channel capacity of both I and Q components more sufficiently. Performance results demonstrate that the simplified structure and improved spectrum efficiency can be realized in the asymmetric QAM transmission system with an acceptable bit-error-rate loss.

G‐MGF based performance analysis of coherent FSO systems with multiple receivers

This letter investigates the performance of coherent free-space optical systems with multiple receivers especially through the generalised moment-generating function based approach. Specifically, we first derive a closed-form expression of the generalised moment-generating function of the signal-to-noise ratio for the coherent free-space optical systems. The derived generalised moment-generating function formula is further utilised to effectively analyse the several performance metrics such as amount of fading and ergodic capacity. The accuracy of our theoretical analyses are corroborated via some numerical results.

Free-Space Optical Systems over Correlated Atmospheric Fading Channels: Spatial Diversity or Multihop Relaying?

Both spatial diversity and multihop relaying are considered to be effective methods for mitigating the impact of atmospheric turbulence-induced fading on the performance of free-space optical (FSO) systems. Multihop relaying can significantly reduce the impact of fading by relaying the information over a number of shorter hops. However, it is not feasible or economical to deploy relays in many practical scenarios. Spatial diversity could substantially reduce the fading variance by introducing additional degrees of freedom in the spatial domain. Nevertheless, its superiority is diminished when the fading sub-channels are correlated. In this paper, our aim is to study the fundamental performance limits of spatial diversity suffering from correlated Gamma-Gamma (G-G) fading channels in multihop coherent FSO systems. For the performance analysis, we propose to approximate the sum of correlated G-G random variables (RVs) as a G-G RV, which is then verified by the Kolmogorov-Smirnov (KS) goodness-of-fit statistical test. Performance metrics, including the outage probability and the ergodic capacity, are newly derived in closed-form expressions and thoroughly investigated. Monte-Carlo (M-C) simulations are also performed to validate the analytical results.

Comments on “Performance Analysis of Coherent Free-Space Optical Systems With Multiple Receivers”

In this letter, we point out that a much simpler ergodic capacity formula for a coherent free-space optical system employing multiple receive apertures over atmospheric turbulence-induced fading channels could be derived, in a more useful and elegant approach to Meijer’s G-function, than the one provided in the above letter. Furthermore, we also derive a concise asymptotic ergodic capacity expression, which offers more explicit insights into the impact of system and channel parameters on the ergodic capacity in the high signal-to-noise ratio regime.

BER Analysis of Coherent Free Space Optical Communication Systems with Holographic Modal Wavefront Sensor

Degradation of bit-error-rate (BER), caused by atmospheric turbulence, seriously hinders the performance of coherent Free Space Optical (FSO) communication systems. An adaptive optics system proves to be effective in suppressing the atmospheric turbulence. The holographic modal wavefront sensor (HMWFS) proposed in our previous work, noted for its fast detecting rates and insensitivity to beam scintillation, is applied to the coherent FSO communication systems. In this paper, based on our previous work, we first introduce the principle of the HMWFS in brief and give the BER of the coherent FSO with homodyne detection in theory, and then analyze the improvement of BER for a coherent FSO system based on our previous simulation works. The results show that the wavefront sensor we propose is better for weak atmospheric turbulence. The most obvious advantages of HMWFS are fast detecting rates and insensitivity to beam scintillation.

Slant-path coherent free space optical communications over the maritime and terrestrial atmospheres with the use of adaptive optics for beam wavefront correction.

As a continuation of our previous work [Appl. Opt.54, 1453 (2015)APOPAI0003-693510.1364/AO.54.001453] in which we have studied the performance of coherent free space optical (FSO) communication systems operating over a horizontal path, in this paper we study the coherent FSO system operating over a general slant path. We evaluated system bit-error-rate (BER) in the case when the quadrature phase-shift keying (QPSK) modulation format is applied and when an adaptive optics (AO) system is employed to mitigate the air turbulence effects for both maritime and terrestrial air transmission scenarios. We adopted a multiple-layer scheme to efficiently model the FSO slant-path links. The atmospheric channel fading was characterized by the wavefront phase distortions and the log-amplitude fluctuations. We derived analytical expressions to characterize log-amplitude fluctuations of air turbulence by asserting the aperture averaging within the frame of the multiple-layer model. The obtained results showed that use of AO enabled improvement of system performance for both uplinks and downlinks, and also revealed that it is more beneficial for the FSO downlinks. Also, AO employment brought larger enhancements in BER performance for the maritime slant-path FSO links than for the terrestrial ones, with an additional striking increase in performance when the AO correction is combined with the aperture averaging.

코히런트 무선 광통신 시스템에서 펄스 재밍으로 인한 비트오류율 분석

In this letter, we investigate the performance of coherent free-space optical(FSO) systems under pulsed noise jamming conditions. In particular, we derive the average bit error probability of the coherent FSO systems with the pulsed noise jamming in a closed-form. Also, we derive the optimal fraction of symbol time of the jammer. We confirm the derived average error probability expressions by the exactly matching Monte-Carlo simulation results.

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.

Performance analysis of coherent FSO system with SSC receiver

This paper analyzes the performance of coherent free-space optical (FSO) system employing the switch-and-stay (SSC) dual diversity receiver. The intensity fluctuations of the optical signal are modeled by Gamma-Gamma distribution, being caused by atmospheric turbulence. In addition, pointing errors are taken into account. Novel analytical expressions for the outage probability are derived. The general scenario of unbalanced average signal-to-noise ratios (SNRs) of independent and identically distributed SSC branches is considered, which is further simplified to the balanced SNR case. The effects of various system and channel parameters are investigated and discussed.

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.

Outage and Error Rate Analysis of Network-Coded Coherent TWR-FSO Systems

This letter investigates a two-way relay assisted coherent free space optical system when the optical beam is subjected to gamma-gamma atmospheric turbulence and misalignment errors (pointing errors). A half-duplex relaying system consisting of two terminal nodes and a bidirectional relay is considered. The relay is based on decode-and-forward protocol and employs linear network coding to establish communication between the two terminal nodes in two time slots. Coherent detection with M-ary phase shift keying is employed. Closed form expressions for system outage and error performance for both binary and nonbinary cases are derived in terms of the well known Meijer's G-function and are mathematically analyzed.

MDPSK-Based Nonequalization OFDM for Coherent Free-Space Optical Communication

In this letter, we apply a nonequalization orthogonal frequency-division multiplexing (NE-OFDM) technique in optical turbulence channel for coherent free-space optical (FSO) systems, utilizing multilevel differential phase-shift keying (MDPSK). We adopt the modified Rician distribution as the optical turbulence channel for coherent FSO systems, where both amplitude fading and phase distortion are taken into consideration. The differential encoding process of MDPSK-based NE-OFDM can be performed either in the frequency domain (FD) or in the time domain (TD). Our simulation results show that NE-OFDM using FD-MDPSK achieves nearly the same bit-error-rate (BER) performance as that using TD-MDPSK, but with a relatively lower hardware complexity and a higher spectral efficiency. It is also revealed that FD-MDPSK-based NE-OFDM attains a comparable BER performance with the conventional OFDM with pilot tones or training sequences assisted channel estimation and equalization. Hence, FD-MDPSK-based NE-OFDM is promising for coherent FSO communication.

Information Theoretic Analysis of Hybrid-ARQ Protocols in Coherent Free-Space Optical Systems

Automatic retransmission request (ARQ) is a feedback-based data link layer technique which enhances the reliability of communication in fading channels. In this paper, we investigate the performance of hybrid-ARQ (H-ARQ) techniques in coherent free-space optical (FSO) communication systems over atmospheric turbulence-induced fading channels. Under the assumption of a Gamma-Gamma statistical fading channel model, we derive outage probability and throughput expressions for three H-ARQ protocols. We further characterize the outage performance at high values of signal-to-noise-ratio (SNR) through diversity and coding gains. Our results provide insight into the performance mechanisms of different H-ARQ protocols in coherent FSO systems and demonstrate that significant performance gains can be achieved through the deployment of H-ARQ particularly in the strong turbulence regime.

Diversity–Multiplexing Trade-Off in Coherent Free-Space Optical Systems With Multiple Receivers

In this paper, from an information theory point of view, we investigate the performance of a coherent free-space optical (FSO) communication system with multiple receive apertures over atmospheric turbulence channels. Our study builds on a recently introduced statistical model that characterizes the combined effects of turbulence-induced wavefront distortion and amplitude fluctuation in coherent receivers with phase compensation. We investigate the link reliability as quantified by “diversity gain” and the relationship between the link reliability and the spectral efficiency as quantified by “diversity-multiplexing trade-off (DMT).” Our results provide insight into the performance mechanisms of coherent FSO systems and demonstrate significant performance gains that can be obtained through the deployment of multiple receive apertures and phase compensation techniques.

Exact error rate analysis of equal gain and selection diversity for coherent free-space optical systems on strong turbulence channels

Exact error rate performances are studied for coherent free-space optical communication systems under strong turbulence with diversity reception. Equal gain and selection diversity are considered as practical schemes to mitigate turbulence. The exact bit-error rate for binary phase-shift keying and outage probability are developed for equal gain diversity. Analytical expressions are obtained for the bit-error rate of differential phase-shift keying and asynchronous frequency-shift keying, as well as for outage probability using selection diversity. Furthermore, we provide the closed-form expressions of diversity order and coding gain with both diversity receptions. The analytical results are verified by computer simulations and are suitable for rapid error rates calculation.