Pointing Errors Research Articles

Average bit error rate and channel capacity of terahertz wireless line-of-sight links with pointing errors under combined effects of turbulence and snow.

The estimation of the performance of terahertz wireless communication under the effect of various weather conditions is vital. In this work, the combined effects of cold weather conditions such as snow, as well as random effects of turbulence and pointing errors (PEs) between the transmitter and receiver, on the performance of terahertz wireless line-of-sight links have been evaluated. The exponentiated Weibull distribution has been employed to derive exact analytical closed-form expressions in the presence of combined channels. Our predictions indicate that snow can have various influences on the average bit error rate (BER) and the average channel capacity of terahertz wireless links. Dry and wet snow, based on its liquid-water content, shows different effects in terms of link parameters such as distance and frequency compared to that of rain. Random turbulence and PEs further deteriorate the link performance. It is concluded that the channel capacity is less affected than the BER under the effects of turbulence and PEs, while snow can have a remarkable effect. All of the predictions through the derived expressions are validated using Monte Carlo simulations.

Signal Intensity Estimation in Transdermal Optical Wireless Links with Stochastic Pointing Errors Effect

Transdermal optical wireless (TOW) communication links have recently gained particular research and commercial attention as a viable alternative for establishing high speed and effective implantable data transmissions, which is vital for a variety of neuroprosthetic and other medical applications. However, the development of this optical telemetry modality with medical implanted devices (IMDs) is adversely affected by skin-induced photon absorption, scattering and pointing errors effects. Thus, in this work a minimum mean-square error (MMSE) criterion is proposed for the estimation of the optical signal intensity in a typical TOW link of varying path loss and misalignment-induced fading characteristics. In this context, the stochastic nature of the transmitter–receiver misalignment has been considered and jointly modeled with transdermal path loss. Additionally, the link is assumed to employ the suitable On–Off Keying (OOK) with intensity modulation and direct detection scheme as well as a PIN photodiode at the receiver side for signal detection. Under these assumptions the results demonstrate that the stochastic amount of pointing mismatch strongly affects the received irradiance estimation.

Channel Modeling for UAV-Based Optical Wireless Links With Nonzero Boresight Pointing Errors

The channel modeling of unmanned aerial vehicle (UAV)-based free-space optical (FSO) links with nonzero boresight pointing error is the subject of this paper. In particular, utilizing log-normal turbulence model, we propose a novel closed-form statistical channel model for UAV-based FSO links that takes into account the effect of nonzero boresight pointing errors. Subsequently, utilizing Gamma-Gamma turbulence model, we propose a novel channel characterization for such links that is valid under moderate to strong turbulence conditions. The accuracy of the proposed models is verified via Monte-Carlo simulations. The proposed models are more tractable and suitable for analysis of such UAV-based FSO links.

Performance of Opportunistic Beam Selection for OWC System Under Foggy Channel with Pointing Error

Performance of optical wireless communication (OWC) in outdoor environments can be significantly degraded due to the fog and pointing errors. In this letter, we consider a multi-aperture detector which employs a low complexity opportunistic beam selection (OBS) in order to improve the performance of OWC system. We analyze the OBS scheme by deriving computable bounds on the average signal-to-noise ratio (SNR), ergodic rate, and diversity order and compare its performance by deriving exact analytical expressions of a single-aperture system. The derived bounds provide insights on the system behavior depicting significant benefit for OWC systems with small link distances which are typical in foggy environments. We also demonstrate the effect of various parameters of fog and pointing error on the OWC performance and validate the derived analytical expressions using numerical and simulation analysis. A two-aperture OBS achieves an SNR gain of approximately 4 dB and improves the outage performance by more than 20 folds for a half-km OWC link comparing to the single-aperture system without acquiring the channel state information.

AOCS General Architecture Design for SVOM Satellite

SVOM mission is a Sino-French cooperation satellite which requires high attitude accuracy and platform stability. In order to guarantee the 100s’ observation time, the Relative Pointing Error is required to up to 0.8arcsec/100sec (3 sigma). The Attitude and Orbital Control System (AOCS) is a key element for SVOM mission because of this severe pointing performance requirement. This article describes the AOCS general architecture design for SVOM satellite. First, the main requirements are presented. Second, the key hardware is introduced, including a newly designed sensor, named Fine Guidance Sensor. At last, the AOCS modes design and transfer logic are given.

Performance enhancement of the average spectral efficiency using an aperture averaging and spatial-coherence diversity based on the modified-PPM modulation for MISO FSO links

The performance evaluation and enhancement of the proposed model for a free-space optical (FSO) communication links using the modified pulse-position modulation (MPPM) and spatial pulse-position modulation (SPPM) is analyzed in this paper. The average spectral efficiency (ASE) is investigated for the FSO communication using MPPM, SPPM, and spatial-coherence diversity in a coherent optical wireless communication (coherent OWC) systems. We proposed the hybrid MPPM/SPPM as a new modulation to improve the ASE performance of pulse-position modulation. To reduce the atmospheric turbulence (AT) effects and scintillation, the use of an aperture averaging (AA) technique has been proposed in our work. Here, we analyze the pointing error (PE) effects on the performance of M−ary MPPM based on a multiple-input single-output (MISO) of the repetition codes (RCs) and FSO systems using SPPM. The enhancement in the ASE can be accomplished with an increase in the receiver aperture and decreasing the beam radius in the presence of PEs. The ASE performances of 53 bits/s/Hz and 37 bits/s/Hz are achieved with the average transmitted optical power of 10 dBm for an aperture diameter of 10 cm over the effect of without and with PEs for coherent OWC technique, respectively. The simulation results obtained for the bit-error-rate (BER) and ASE against various AT conditions are compared with the diversity techniques and proved that both of the hybrid SPPM and MPPM/MISO−RCs based on the AA and spatial diversity offers better performance in the average channel capacity (ACC). Numerical results show that the ASE performance can be improved using the spatial-coherence diversity in coherent OWC technique. The spatial-coherence diversity in a coherent OWC system significantly outperforms on-off keying and MPPM system even in the presence of PEs under strong turbulence. Efficient estimation of the signal-to-noise ratio also allows accurate quantification of the BER and ACC for adequate quality of service and network link resource allocation.

위상천이기 분해능에 따른 위상 배열 안테나의 최대 빔 조향 오차에 관한 연구

디지털 위상천이기를 사용하는 위상배열안테나 시스템에 있어, 위상천이기의 분해능에 따른 양자화 위상천이 오차에 의해 빔 조향 오차가 발생한다. 양자화 위상천이 오차에 대응한 빔 조향 오차를 분석할 수 있는 간소화된 수식을 제시하였다. 제시된 수식은 선형 배열과 평면 배열안테나에 대하여 유도하였으며, 다양한 양자화 위상천이 오차를 갖는 위상배열안테나들의 모의실험을 통해 검증하였다. 그리고 검증된 수식을 이용하여, 위상천이기 분해능과 배열구성에 따른 빔 조향 오차를 참조할 수 있는 지표를 제시하였다.

Real-Time Estimation of Wind-Induced Pointing Error via Accelerations Measurement for Large Reflector Antenna

The online monitoring of pointing error (PE) caused by the main reflector (MR) deformation under wind disturbance has become a knotty problem for large reflector antennas. This paper presents a real-time estimation algorithm for monitoring wind-induced PE by optimized acceleration measurement points. First, based on an unbiased minimum-variance input-state estimation theory via accelerations measurement, we set up the PE observation model which illustrates the relation between the structural vibration state and PE. Moreover, compared with the frequency domain integration approach, the non-integral observation model without introducing a low-frequency-cut filter can sense pseudo-static and low-frequency PE. Second, a sensor placement method considering sensor constraint spacing (SPMCSCS) is proposed to achieve a high-quality monitoring effect with low information redundancy. In the method, the constraint spacing between sensors is determined by a novel scan recognition algorithm in which regional division, and regional correlation analysis of the modal shape are carried out successively. Finally, the simulation implementation of a 7.3-m antenna under a wind disturbance is used to illustrate the algorithm.

Non-Adaptive Decision Thresholds for Gain Saturated FSO Links Limited by Weak to Strong Turbulence and Pointing Errors

In free space optical (FSO) communication systems limited by atmospheric turbulence, the use of non-adaptive decision thresholds to determine the transmitted bits results in bit error rate (BER) floors at high BER values in all turbulence regimes. Practically implementing an adaptive decision threshold that can properly track the fluctuations due to atmospheric turbulence is challenging, therefore, devising ways of optimising the non-adaptive decision threshold used by FSO designers is necessary. In this paper, the investigation of gain saturated pre-amplified FSO communication systems using non-adaptive decision thresholds in the presence of atmospheric turbulence, pointing errors (PEs), geometric spread (GS) and amplified spontaneous emission noise is carried out by applying analytical methods and Monte Carlo (MC) simulation techniques. System performance is carried out for various turbulence regimes, normalised beam widths, normalised PE standard deviations and small signal gains using fixed gain and gain saturated optical amplifiers (OAs). Results obtained show that in the presence of atmospheric turbulence, PE and GS, optimal BER performances are obtained with OA input powers higher than the internal saturation power of the OA. Also, by using high gain OAs and varying the decision threshold level, acceptable BER performances can be obtained in strong turbulence regimes with a non-adaptive decision threshold.

Temporal Discrimination Thresholds and Proprioceptive Performance: Impact of Age and Nerve Conduction.

BackgroundIncreasing attention is payed to the contribution of somatosensory processing in motor control. In particular, temporal somatosensory discrimination has been found to be altered differentially in common movement disorders. To date, there have only been speculations as to how impaired temporal discrimination and clinical motor signs may relate to each other. Prior to disentangling this relationship, potential confounders of temporal discrimination, in particular age and peripheral nerve conduction, should be assessed, and a quantifiable measure of proprioceptive performance should be established.ObjectiveTo assess the influence of age and polyneuropathy (PNP) on somatosensory temporal discrimination threshold (STDT), temporal discrimination movement threshold (TDMT), and behavioral measures of proprioception of upper and lower limbs.MethodsSTDT and TDMT were assessed in 79 subjects (54 healthy, 25 with PNP; age 30–79 years). STDT was tested with surface electrodes over the thenar or dorsal foot region. TDMT was probed with needle electrodes in flexor carpi radialis (FCR) and tibialis anterior (TA) muscle. Goniometer-based devices were used to assess limb proprioception during (i) active pointing to LED markers, (ii) active movements in response to variable visual cues, and (iii) estimation of limb position following passive movements. Pointing (or estimation) error was taken as a measure of proprioceptive performance.ResultsIn healthy subjects, higher age was associated with higher STDT and TDMT at upper and lower extremities, while age did not correlate with proprioceptive performance. Patients with PNP showed higher STDT and TDMT values and decreased proprioceptive performance in active pointing tasks compared to matched healthy subjects. As an additional finding, there was a significant correlation between performance in active pointing tasks and temporal discrimination thresholds.ConclusionGiven their notable impact on measures of temporal discrimination, age and peripheral nerve conduction need to be accounted for if STDT and TDMT are applied in patients with movement disorders. As a side observation, the correlation between measures of proprioception and temporal discrimination may prompt further studies on the presumptive link between these two domains.

Alleviation of Jamming in Free Space Optical Communication over Gamma-Gamma Channel with Pointing Errors

In this paper, the groundwork on jamming effect and its alleviation mechanisms in free space optical (FSO) communication are studied over Gamma-Gamma (GG) fading channels along with the pointing error (PE) effects. A closed-form expression of the bit error rate (BER) is evaluated analytically for a single-input single-output (SISO) FSO system in the presence of jammer. The worst case jamming condition for maximization of error is calculated numerically. The jammer channel is considered to be GG distributed. Being the most dominating noise, the jamming signal is acted as the only noise source in the considered FSO set-up. Therefore, this study is performed over additive GG noise channel. The error performances are investigated for different atmospheric turbulence (AT) regimes (weak to strong) and for different PE parameters of jamming noise. Moreover, to combat the jamming effect, a multiple-input single-output (MISO) FSO system is considered. A closed-form expression of the BER of the multiple transmit channels over the GG noise is analytically calculated. The analytical results for both SISO and MISO FSO systems are verified with the help of the simulation results obtained by MATLAB software. It is established that MISO FSO performs better than SISO FSO system in terms of BER performance under the influence of jamming noise. Furthermore, many important observations are made upon the BER performances for different AT and PE parameters for SISO and MISO FSO systems.

On the Performance of Quantum Key Distribution FSO Systems Under a Generalized Pointing Error Model

In this letter, the performance of a quantum key distribution (QKD) free-space optical (FSO) system is analyzed while taking a generalized pointing error model into account. More specifically, closed-form expressions for the average received powers at both the legitimate receiver and eavesdropper are derived. In addition, their corresponding asymptotic results valid in the high telescope gain regime are also presented. To capture the secure performance, we also investigate the ratio of received powers at the eavesdropper and at the legitimate receiver. Further, in some special cases, we find the optimal telescope gains for the received powers at both the legitimate receiver and eavesdropper, as well as the power ratio, which is important and useful for a secure QKD FSO system design. Finally, some selected numerical results are presented to illustrate the mathematical formalism and validate the accuracy of the derived analytical expressions.

Performance analysis of FSO coherent BPSK systems over Rician turbulence channel with pointing errors.

We investigate the performance of coherent free-space optical (FSO) system in terms of bit error rate (BER) evaluation by adopting the modified Rician distribution based coherent channel model which allows taking into consideration the composite effects of both Rician turbulence, including amplitude fluctuation and optical phase distortion, and pointing errors (PEs). By expanding the Rician distribution, a mathematically traceable expression of the probability density function (PDF) for the composite channel is derived in the form of the Meijer-G function. Based on the composite channel PDF, the exact BER expression is obtained, allowing the analysis of BER performance for single-input single-output (SISO) links. This analysis is extended to single-input multi-output (SIMO) links with maximal ratio combining (MRC). With the help of the moment generating function (MGF), the exact BER expression can be simplified into a single integral, facilitating the analysis with high accuracy and reducing calculation complexity. Engineering insights including high-SNR approximated channel PDF, asymptotic BER expression, coding and diversity gains, are investigated and cross-validated for both SISO and SIMO links. Through both analytical and numerical verifications, the impairment due to PEs as well as the effect of modal compensation on the BER performance are discussed in detail, unveiling the fact that their inner relations should be taken into account for optimization. These verify the effectiveness of our models for both SISO and SIMO links with a wide range of different conditions and can be feasibly applied for different types of coherent FSO links.

Performance of SIMO FSO Links over Mixture Composite Irradiance Channels

Free space optics (FSO) technology has demonstrated an increasingly scientific and commercial interest over the past few years. However, due to signal propagation in the atmosphere, the operation depends strongly on the atmospheric conditions and some random impairments, including turbulence and pointing error (PE) effects. In the present study, a single-input multiple-output FSO system with wavelength, spatial, or time diversity over the turbulence and non-zero boresight PE effects is thoroughly investigated. A versatile mixture composite model which accurately describes both impairments is employed for the performance evaluation. Novel mathematical expressions of the outage probability and the average bit-error rate assuming intensity modulation/direct detection and optimal combining at the reception are provided.

Outage Capacity of Rate-Adaptive Relaying for FSO Links With Nonzero Boresight Pointing Errors

An outage capacity analysis of a three-way free-space optical (FSO) setup based on the optical path selection, either source-destination (S-D) or source-relay-destination (S-R-D), with a greater value of channel gain along with the use of repetition coding (RC) is analyzed over atmospheric turbulence channels with nonzero boresight pointing errors. The developed expression, which is validated through Monte Carlo simulation, is used to carefully analyze the performance and design of rate-adaptive relaying FSO systems from an information theory point of view. It is highlighted that a greater outage diversity is achieved by the obtained results without investing in extra hardware at the expense of an information rate reduction.

Enhancing spectral efficiency of FSO system using adaptive SIM/M‐PSK and SIMO in the presence of atmospheric turbulence and pointing errors

SummaryThis paper proposes an adaptive transmission modulation (ATM) technique for free‐space optical (FSO) links over gamma‐gamma turbulence channels.The ATM technique provides efficient utilization of the FSO channel capacity for improving spectral efficiency, by adapting the order of the phase‐shift keying modulation scheme, according to the channel conditions and the required bit error rate (BER). To overcome the channel degradation resulting from the turbulence effects as well as the pointing errors (PEs), single‐input multiple‐output (SIMO) system with maximal ratio combining (MRC) is proposed. Exact closed‐form expressions of BER and upper bound of the capacity are derived and verified by Monte Carlo simulations. The numerical results show that the proposed adaptive technique improves the spectral efficiency (SE) five times higher than the nonadaptive technique at the same BER threshold (10−3).This improvement is achieved at signal‐to‐noise ratio (SNR) equals 27 and 42 dB in the case of atmospheric turbulence without and with PE, respectively. Furthermore, this SE could be obtained while the SNR = 30 dB by using (1 × 4) SIMO scheme with MRC and PE and having the same transmitting optical power.

SIMO Subcarrier PSK FSO Links with Phase Noise and Nonzero Boresight Pointing Errors over Turbulence Channels

SIMO Subcarrier PSK FSO Links with Phase Noise and Nonzero Boresight Pointing Errors over Turbulence Channels

Exact and Asymptotic Analysis of Partial Relay Selection for Cognitive RF-FSO Systems With Non-Zero Boresight Pointing Errors

In this paper, we investigate the performance of asymmetric radio frequency (RF) and free space optical (FSO) dual-hop cooperative relay network along with a direct RF link between source and destination. The FSO link experiences double generalized Gamma turbulence in the presence of the generalized non-zero boresight pointing errors, and RF links experience non-identically distributed Nakagami-m fading. Moreover, considering both cognitive radio and non-cognitive scenarios, a partial relay selection (PRS) strategy has been employed. Furthermore, both relay selection and underlay power restriction are governed with the outdated channel state information (CSI). We also assume both heterodyne and intensity modulation/direct detection methods in the FSO receiver. Under the assumption of amplify-and-forward relaying and PRS, we derive closed-form expressions for outage probability (OP) of both scenarios, while bit-error probability and ergodic capacity of the non-cognitive scenario are also obtained. The asymptotic expressions of OP and diversity order of this network are derived for both perfect CSI and outdated CSI cases. It is demonstrated that the diversity order is a function of the fading severity of the RF links, turbulence parameters of the FSO link, and pointing error, regardless of interference channel parameter of the primary user.

Moment-Based Approach for Statistical and Simulative Analysis of Turbulent Atmospheric Channels in FSO Communication

The free space optical (FSO) or wireless optical communication networks involve atmosphere as the channel, which at times can be turbulent in nature, thus degrading the system performance. These turbulent channels can be analyzed by introducing various channel models and modulation techniques. Various researchers have analyzed the performance by presenting different approaches for each type of channel model. In this paper, we present a unified approach for the performance analysis of the Log-normal, Nakagami-n (Rician), and Rayleigh statistical models by deriving the exact closed-form expressions for average bit error probability and the outage probability of each model. It is achieved by using the alternative form of Q-function and Gauss-Hermite Quadrature approximation to the moment-generating function of the output signal-to-noise ratio (SNR). Furthermore, definite expressions for each type of modulation technique are also derived to describe the channel fading in an FSO communication system. The system of interest includes the M-ary phase shift keying and M-ary quadrature amplitude modulation techniques. Moreover, the average output SNR, the average received irradiance, and the amount of fading (fading strength) are also expressed in simple closed forms for above-mentioned turbulent channels. In addition to this, the performance of the FSO system in the presence of atmospheric turbulence and pointing errors is also investigated. The statistical model is examined for zero and nonzero boresight point errors modeled by Rayleigh and Rician distributions in a Log-normal fading channel. The composite Log-normal PDFs along with asymptotic error rate and outage probability are also presented for such system. The effect of Tikhonov distributed phase noise arise due to BPSK modulation on BER performance is evaluated on three turbulent channels by deriving its closed-form expression using a moment-based approach. The mathematical analysis is verified by comparing the numerical results with Monte-Carlo simulated values and graphs.

Mixed Topology of DF Relayed Terrestrial Optical Wireless Links with Generalized Pointing Errors over Turbulence Channels

This study investigated the outage performance of a terrestrial FSO communication system that uses mixed series and parallel decode-and-forward (DF) relay-assisted (i.e., cooperative diversity) configurations, taking into account the influence of both atmospheric turbulence and pointing error effects. Turbulence-induced optical signal fading is modeled by gamma-gamma or the negative exponential distribution for weak to strong and saturated turbulence conditions, respectively. Additionally, weak to strong non-zero boresight misalignment-induced optical signal fading is modeled by the generalized Beckmann distribution. Under these conditions, an outage analysis of the examined FSO system is performed, in terms of both outage probability and mean outage duration metrics. Thus, fairly accurate closed-form mathematical expressions for both performance metrics are derived, while their corresponding analytical results demonstrate concrete performance and availability improvements for the total FSO system, especially when the number of the connected in parallel DF relays increases. Moreover, the obtained results are verified through the corresponding simulation results.