Effect Of Pointing Errors Research Articles

Novel space-time trellis codes for free-space optical communications using transmit laser selection.

In this paper, the deployment of novel space-time trellis codes (STTCs) with transmit laser selection (TLS) for free-space optical (FSO) communication systems using intensity modulation and direct detection (IM/DD) over atmospheric turbulence and misalignment fading channels is presented. Combining TLS and STTC with rate 1 bit/(s · Hz), a new code design criterion based on the use of the largest order statistics is here proposed for multiple-input/single-output (MISO) FSO systems in order to improve the diversity order gain by properly chosing the transmit lasers out of the available L lasers. Based on a pairwise error probability (PEP) analysis, closed-form asymptotic bit error-rate (BER) expressions in the range from low to high signal-to-noise ratio (SNR) are derived when the irradiance of the transmitted optical beam is susceptible to moderate-to-strong turbulence conditions, following a gamma-gamma (GG) distribution, and pointing error effects, following a misalignment fading model where the effect of beam width, detector size and jitter variance is considered. Obtained results show diversity orders of 2L and 3L when simple two-state and four-state STTCs are considered, respectively. Simulation results are further demonstrated to confirm the analytical results.

On the capacity of MISO FSO systems over gamma-gamma and misalignment fading channels.

In this work, the ergodic capacity performance for multiple-input/single-output (MISO) free-space optical (FSO) communications system with equal gain combining (EGC) reception is analyzed over gamma-gamma and misalignment fading channels, which are modeled as statistically independent, but not necessarily identically distributed (i.n.i.d.). Novel and analytical closed-form ergodic capacity expression is obtained in terms of H-Fox function by using the well-known inequality between arithmetic and geometric mean of positive random variables (RV) in order to obtain an approximate closed-form expression of the distribution of the sum of M gamma-gamma with pointing errors variates. In addition, we present an asymptotic ergodic capacity expression at high signal-to-noise ratio (SNR) for the ergodic capacity of MISO FSO systems. It can be concluded that the use of MISO technique can significantly reduce the effect of the atmospheric turbulence as well as pointing errors and, hence, provide significant capacity gain over the direct path link (DL). The impact of pointing errors on the MISO FSO system is also analyzed, which only depends on the number of laser sources and pointing error parameters. Moreover, it can be also concluded that the ergodic capacity performance is dramatically reduced as a consequence of the severity of pointing error effects. Simulation results are further demonstrated to confirm the analytical results.

Performance Analysis of Multihop Free Space Optical Communication System with Pointing Errors

AbstractIn this paper, we have derived closed form expressions for the outage probability and average capacity of multihop free space optical communication system, employing either decode and forward relaying or amplify and forward relaying. The atmospheric channel is characterized by an aggregate channel model, which includes atmospheric attenuation, atmospheric turbulence and pointing errors. Gamma-Gamma channel model is used to model the turbulence of the atmosphere, and pointing error effects are considered by jitter standard deviation parameter. The results obtained from the analytical expressions are verified by Monte Carlo simulation. The performance of the multihop free space optical communication system improves with hop count for both relaying techniques, for same link length. The pointing error deteriorates the performance of the system under consideration, and the performance of a decode and forward relaying is better than the amplify and forward relaying, in terms of both average capacity and outage probability.

Outage performance of DF relay-assisted FSO communications using time-diversity

Outage probability as a performance measure for a decode-and-forward (DF) relaying scheme based on repetition coding with the relay is presented in the context of free-space optical (FSO) communication systems with intensity modulation and direct detection over atmospheric turbulence and misalignment fading channels. Novel closed-form asymptotic expressions for the outage probability are derived for a three-way FSO communication setup when the irradiance of the transmitted optical beam is susceptible to moderate-to-strong turbulence conditions, following a gamma-gamma distribution, and pointing error effects, following a misalignment fading model where the effect of beam width, detector size, and jitter variance is considered. Fully exploiting the potential time-diversity available in the relay turbulent channel, a greater robustness to the relay location and pointing errors is achieved, maintaining a high diversity gain regardless of the source-destination link distance and remarkably superior to that of the two-transmitter case as well as the conventional DF scheme. Simulation results are further demonstrated to confirm the accuracy and usefulness of the derived results.

Transmit alternate laser selection with time diversity for FSO communications.

In this paper, a new transmit alternate laser selection (TALS) scheme for FSO communication systems using intensity modulation and direct detection (IM/DD) over atmospheric turbulence and misalignment fading channels is presented when limited time diversity is available in the turbulent channel. Assuming channel state information (CSI) at the transmitter and receiver and a time diversity order (TDO) limited, we propose the transmit diversity technique based on the rotating selection of TDO out of the available L lasers corresponding to the optical paths with greater values of scintillation. Implementing repetition coding with blocks of TDO information bits, each information bit will be retransmitted TDO times using the TDO largest order statistics in an alternating way. Closed-form asymptotic bit error-rate (BER) expressions are derived when the irradiance of the transmitted optical beam is susceptible to moderate-to-strong turbulence conditions, following a gamma-gamma (GG) distribution, and pointing error effects, following a misalignment fading model where the effect of beam width, detector size and jitter variance is considered. Fully exploiting the potential time-diversity TDO available in the turbulent channel, a significant diversity gain is achieved, providing a diversity order of (2L + 1 - TDO)TDO/2.

Pointing error effects on performance of free-space optical communication systems using SC-QAM signals over atmospheric turbulence channels

In this paper, we theoretically analyze pointing error effects on performance of free-space optical (FSO) communication systems using subcarrier intensity quadrature amplitude modulation (SC-QAM) signals over atmospheric turbulence channels. Unlike previous studies, we take into account both atmospheric turbulence channels and the pointing error effect. In order to model atmospheric turbulence channels, we employ a log-normal distribution for weak-to-moderate turbulent condition and a gamma–gamma distribution for strong turbulent condition. Moreover, we study the pointing error effect by taking into account the influence of beamwidth, aperture size and jitter variance. In addition, we use a combination of these models to analyze the combined effect of atmospheric turbulence and pointing error to FSO/SC-QAM systems. Finally, we derive analytical expressions to evaluate the average symbol error rate (ASER) performance of such systems. Numerical results present the impact of pointing error on the performance of FSO/SC-QAM systems and how we use proper values of aperture size and beamwidth to improve the performance of such systems. In addition, simulation results of FSO/SC-QAM performance over strong atmospheric turbulence and pointing errors show that the closed-form expression can provide a precision for evaluating ASER of such systems.

Free-Space Optical Communication with Nonzero Boresight Pointing Errors

The performance of free-space optical (FSO) communication systems is compromised by atmospheric fading and pointing errors. The pointing errors are widely considered as a combination of two components: boresight and jitter. A statistical model is investigated for pointing errors with nonzero boresight by taking into account the laser beamwidth, detector aperture size, and jitter variance. A novel closed-form probability density function (PDF) is derived for this new nonzero boresight pointing error model. Furthermore, we obtain closed-form PDF for the composite lognormal turbulence channels and finite series approximate PDF for the composite Gamma-Gamma turbulence channels, which is suitable for terrestrial FSO applications impaired by building sway. We conduct error rate analysis of on-off keying signaling with intensity modulation and direct detection over the lognormal and Gamma-Gamma fading channels. Asymptotic error rate analysis and outage probability of such a system are also presented based on the derived composite PDFs. It is shown that the boresight can only affect the coding gain, while the diversity order is determined by the atmospheric fading effect as well as the pointing error effect.

Using a HAP Network to Transfer WiMAX OFDM Signals: Outage Probability Analysis

Transferring wireless broadband services across extremely far distances on Earth is usually implemented nowadays either by ground-based or satellite communications. Terrestrial networks require, however, heavy installation and maintenance costs, whereas satellite networks suffer from excessively high power requirements. An alternative but highly challenging solution is to employ a network consisting of high-altitude platforms (HAPs). In this paper, we propose a way of delivering WiMAX traffic using a serial multi-hop HAP network configuration. HAPs are located at specific locations in the stratosphere, pick up the traffic from the Earth region they cover, and communicate with each other using optical links. In such a configuration, we determine the WiMAX quality of service by evaluating the outage probability for the entire HAP network. The overall performance is examined by using a channel model that takes into account laser path loss and pointing error effects. The findings of the present study indicate that the consideration of specific network and channel model parameters is crucial toward the design and implementation of future multi-hop HAP networks.

WiMAX on FSO: Outage Probability Analysis

The transmission of multiple wireless signals over optical links has attained a great research interest nowadays. In case where optical fibers are difficult to be deployed, or installation cost is prohibited, optical wireless systems provide an efficient alternative means. In this paper, we consider the WiMAX (IEEE802.16) standard and construct a simple but adequate scenario to investigate radio signal transmission over terrestrial optical wireless channels. An appropriate system architecture is adopted and a channel model, which entails some of the most critical impairments of the optical channel, i.e., attenuation, turbulence, pointing error effects, as well as of the RF channel, i.e., path loss, shadowing, and Rayleigh fading, is taken into account. The overall link budget and a closed-form of the outage probability of the system are deduced. Several analytical results are depicted using a realistic set of parameter values, to lend a helpful insight to the performance of the proposed architecture.

Outage performance of MIMO FSO links over strong turbulence and misalignment fading channels

Atmospheric turbulence produces fluctuations in the irradiance of the transmitted optical beam, which is known as atmospheric scintillation, severely degrading the performance over free-space optical (FSO) links. Additionally, since FSO systems are usually installed on high buildings, building sway causes vibrations in the transmitted beam, leading to an unsuitable alignment between transmitter and receiver and, hence, a greater deterioration in performance. In this paper, the outage probability as a performance measure for multiple-input/multiple-output (MIMO) FSO communication systems with intensity modulation and direct detection (IM/DD) over strong atmospheric turbulence channels with pointing errors is analyzed. Novel closed-form expressions for the outage probability as well as their corresponding asymptotic expressions are presented when the irradiance of the transmitted optical beam is susceptible to either strong turbulence conditions, following a negative exponential distribution, and pointing error effects, following a misalignment fading model where the effect of beam width, detector size and jitter variance is considered. Obtained results show that the diversity order is independent of the pointing error when the equivalent beam radius at the receiver is at least twice the value of the pointing error displacement standard deviation at the receiver. Simulation results are further demonstrated to confirm the analytical results. Additionally, since proper FSO transmission requires transmitters with accurate control of their beamwidth, asymptotic expressions here obtained for different diversity techniques are used to find the optimum beamwidth that minimizes the outage performance.

Coded Free-Space Optical Links over Strong Turbulence and Misalignment Fading Channels

The performance of optical wireless systems deteriorates to a large extent from the presence of turbulence and pointing error effects. To meet the typical bit error rate (BER) targets for reliable communications within the practical ranges of signal-to-noise ratio, error control coding schemes are often proposed. This paper investigates the error performance for convolutional coded on-off keying free-space optical systems through symbol by symbol interleaved channels characterized by strong turbulence and/or pointing error effects. We consider several channel types and derive exact analytical expressions for the pairwise error probability. These expressions are applied to obtain upper bounds on the BER performance using the transfer function technique.

Pointing Error Effects on Free-Space Optical Communication Links in the Presence of Atmospheric Turbulence

The effects of pointing errors on the performance of a free-space optical laser communication system are studied. The log-normal and gamma-gamma irradiance probability density function models have been considered to include the effects of optical turbulence. An expression for the mean intensity under pointing fluctuations is derived. Results for ergodic and outage channel capacities are presented. Since the irradiance statistics vary with pointing fluctuations, a wave-optics-based approach is proposed to evaluate the capacity expressions. Although, in general, turbulence degrades communications link performance, it can reduce sensitivity to pointing errors by increasing the effective beam radius. Finally, we present analytical expressions for evaluating the bit-error-rate performance of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</i> keying system in the presence of pointing errors, and provide error performance results.

Optimization Models for Misalignment Fading Mitigation in Optical Wireless Links

We consider a free-space optical (FSO) channel model affected by misalignment fading (pointing error) effects. Assuming intensity modulation/direct detection (IM/DD) with on-off keying (OOK), new closed form expressions for the bit-error rate (BER) and the outage probability are presented. Furthermore, four optimization models are formulated and solved taking into account various metrics such as the beamwidth, the electrical signal-to-noise ratio, the normalized jitter, the BER, and the outage probability. The results obtained can be a useful outcome for FSO system designers in order to limit pointing error effects and achieve, thus, an optimum performance.