Gamma-gamma Atmospheric Turbulence Channels Research Articles

On the ASER performance of amplify-and-forward relaying MIMO/FSO systems using SC-QAM signals over log-normal and gamma-gamma atmospheric turbulence channels and pointing error impairments

ABSTRACT In this work, we analyse pointing error effects on the performance of amplify-and-forward (AF) relaying multiple-input multiple-output (MIMO) free-space optical (FSO) communication system employing sub-carrier quadrature amplitude modulation (SC-QAM) signal over log-normal and gamma-gamma distributed atmospheric turbulence channels. We study the pointing error effects by taking into account the influence of beam-width, aperture size and jitter variance on the average symbol error rate (ASER), which is derived in closed-form expressions of MIMO/FSO and SISO/FSO systems. In addition, the number of relaying stations is taken into account in the statistical model of the combined channel including atmospheric loss, atmospheric turbulence and pointing error impairments. The numerical results show that by combining AF relaying stations and MIMO/FSO configurations, the link length can be extended because the transmitted power is reduced according to the amplifier gain. Moreover, the performance of AF relaying MIMO/FSO systems is better than that of AF relaying SISO/FSO systems at the same link length.

SER Analysis of MPPM-Coded MIMO-FSO System over Uncorrelated and Correlated Gamma-Gamma Atmospheric Turbulence Channels

The performance of multiple-input multiple-output free space optical (MIMO-FSO) communication systems, that adopt multipulse pulse position modulation (MPPM) techniques, is analyzed. Both exact and approximate symbol-error rates (SERs) are derived for both cases of uncorrelated and correlated channels. The effects of background noise, receiver shot-noise, and atmospheric turbulence are taken into consideration in our analysis. The random fluctuations of the received optical irradiance, produced by the atmospheric turbulence, is modeled by the widely used gamma-gamma statistical distribution. Uncorrelated MIMO channels are modeled by the α-μ distribution. A closed-form expression for the probability density function of the optical received irradiance is derived for the case of correlated MIMO channels. Using our analytical expressions, the degradation of the system performance with the increment of the correlation coefficients between MIMO channels is corroborated.

A Closed-form Expression for BER of FSO Links over Gamma-Gamma Atmospheric Turbulence Channels with Pointing Errors

FSO communication systems links is vulnerable due to degrading effects of atmospheric turbulence and point errors. We investigate the error rate performance of Free-Space Optical (FSO) links over gamma-gamma turbulence fading channels in the presence of pointing error. Assuming Intensity-Modulation/Direct Detection (IM/DD) with On-Off Keying (OOK), a novel closed-form expression for BER of FSO is derived. Another work is a study of how the BER are affected by the atmospheric turbulence and other parameters such as the normalized beamwidth, the average transmitted optical power, the normalized jitter. Numerical examples are further provided to verify the derived analytical expressions. The results show that optimizing the beamwidth can achieve the minimum BER for a given average transmitted optical power.

Low SNR Capacity of FSO Links over Gamma-Gamma Atmospheric Turbulence Channels

In this paper, we study the ergodic capacity of free space optical communication systems over Gamma-Gamma atmospheric turbulence fading channels with perfect channel state information at both the transmitter and the receiver. In our framework, we mainly focus on the low signal-to-noise ratio range and show that the ergodic capacity scales proportionally to SNR log4(1/SNR). We show also that one-bit CSI feedback at the transmitter is enough to achieve this capacity using an on-off power control scheme.

Bit detect and forward relaying for FSO links using equal gain combining over gamma-gamma atmospheric turbulence channels with pointing errors

An unsuitable alignment between transmitter and receiver together with fluctuations in the irradiance of the transmitted optical beam due to the atmospheric turbulence can severely degrade the performance of free-space optical (FSO) systems. In this paper, cooperative FSO communications with decode-and-forward (DF) relaying and equal gain combining (EGC) reception over atmospheric turbulence and misalignment fading channels is analyzed in order to mitigate these impairments. Novel closed-form asymptotic bit error-rate (BER) expressions are derived for a 3-way FSO communication setup when the irradiance of the transmitted optical beam is susceptible to either a wide range of turbulence conditions (weak to strong), following a gamma-gamma distribution of parameters α and β, or pointing errors, following a misalignment fading model where the effect of beam width, detector size and jitter variance is considered. Obtained results provide significant insight into the impact of various system and channel parameters, showing that the diversity order is independent of the pointing error when the equivalent beam radius at the receiver is at least 2β1/2 times the value of the pointing error displacement standard deviation at the receiver. It is contrasted that the available diversity order is strongly dependent on the relay location, achieving greater diversity gains when the diversity order is determined by βAC + βBC, where βAC and βBC are parameters corresponding to the turbulence of the source-destination and relay-destination links. Simulation results are further demonstrated to confirm the accuracy and usefulness of the derived results.

Asymptotic error-rate analysis of FSO links using transmit laser selection over gamma-gamma atmospheric turbulence channels with pointing errors

Since free-space optical (FSO) systems are usually installed on high buildings and building sway may cause vibrations in the transmitted beam, an unsuitable alignment between transmitter and receiver together with fluctuations in the irradiance of the transmitted optical beam due to the atmospheric turbulence can severely degrade the performance of optical wireless communication systems. In this paper, asymptotic bit error-rate (BER) performance for FSO communication systems using transmit laser selection over atmospheric turbulence channels with pointing errors is analyzed. Novel closed-form asymptotic expressions are derived when the irradiance of the transmitted optical beam is susceptible to either a wide range of turbulence conditions (weak to strong), following a gamma-gamma distribution of parameters α and β, or pointing errors, following a misalignment fading model where the effect of beam width, detector size and jitter variance is considered. Obtained results provide significant insight into the impact of various system and channel parameters, showing that the diversity order is independent of the pointing error when the equivalent beam radius at the receiver is at least 2(min{α,β})(1/2) times the value of the pointing error displacement standard deviation at the receiver. Moreover, since proper FSO transmission requires transmitters with accurate control of their beamwidth, asymptotic expressions are used to find the optimum beamwidth that minimizes the BER at different turbulence conditions. Simulation results are further demonstrated to confirm the accuracy and usefulness of the derived results, showing that asymptotic expressions here obtained lead to simple bounds on the bit error probability that get tighter over a wider range of signal-to-noise ratio (SNR) as the turbulence strength increases.

On the Capacity of FSO Links over Gamma-Gamma Atmospheric Turbulence Channels Using OOK Signaling

A new upper bound on the capacity of power- and bandwidth-constrained optical wireless links over gamma-gamma atmospheric turbulence channels with intensity modulation and direct detection is derived when on-off keying (OOK) formats are used. In this free-space optical (FSO) scenario, unlike previous capacity bounds derived from the classic capacity of the well-known additive white Gaussian noise (AWGN) channel with uniform input distribution, a new closed-form upper bound on the capacity is found by bounding the mutual information subject to an average optical power constraint and not only to an average electrical power constraint, showing the fact that the input distribution that maximizes the mutual information varies with the turbulence strength and the signal-to-noise ratio (SNR). Additionally, it is shown that an increase of the peak-to-average optical power ratio (PAOPR) provides higher capacity values. Simulation results for the mutual information are further demonstrated to confirm the analytical results under several turbulence conditions.

Performance of heterodyne wireless optical communication systems over gamma-gamma atmospheric turbulence channels

Closed-form expressions for the outage probability and the average bit error rate of wireless optical heterodyne communication systems over strong atmospheric turbulence are derived. The turbulence-induced fading is modelled as a multiplicative random process following the gamma-gamma distribution.

Error rate performance analysis of coded free-space optical links over gamma-gamma atmospheric turbulence channels

Error control coding can be used over free-space optical (FSO) links to mitigate turbulence-induced fading. In this paper, we derive error performance bounds for coded FSO communication systems operating over atmospheric turbulence channels, considering the recently introduced gamma-gamma turbulence model. We derive a pairwise error probability (PEP) expression and then apply the transfer function technique in conjunction with the derived PEP to obtain upper bounds on the bit error rate. Simulation results are further demonstrated to confirm the analytical results