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

Abstract

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 2β 1/2 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 β AC + β BC , where β AC and β BC 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. site," URL http://functions.wolfram.com. 36. V. S. Adamchik and O. I. Marichev, "The algorithm for calculating integrals of hypergeometric type functions and its realization in REDUCE system," in Proc. Int. Conf. on Symbolic and Algebraic Computation, 212-224 (Tokyo, Japan, 1990).

New Results in DF Relaying Schemes Using Time Diversity for Free-Space Optical Links

“…Simulation results are further demonstrated to confirm the accuracy and usefulness of the derived results. The superiority of the BDF relaying scheme using time diversity, compared with the cooperative protocol in [19], is corroborated by the obtained results since a greater robustness is provided not only to the pointing errors but also to the relay location, presenting a similar performance regardless of the source-destination link distance.…”

“…This is a well known technique employed in radio-frequency (RF) systems, wherein more attention has been paid to the concept of user cooperation as a new form of diversity for future wireless communication systems [12]. Recently, several works have investigated the adoption of this technique in the context of FSO systems [13][14][15][16][17][18][19], being recognized as a very promising solution for future ad-hoc optical wireless systems. In [14,15] a 3-way FSO communication setup is proposed to implement a cooperative protocol in order to improve spatial diversity without much increase in hardware, being evaluated the error-rate performance by using the photon-count method as well as the outage performance for both amplify-and-forward (AF) and decode-and-forward (DF) strategies.…”

“…In [14,15] a 3-way FSO communication setup is proposed to implement a cooperative protocol in order to improve spatial diversity without much increase in hardware, being evaluated the error-rate performance by using the photon-count method as well as the outage performance for both amplify-and-forward (AF) and decode-and-forward (DF) strategies. In [19], following the bit-detect-and-forward (BDF) cooperative protocol presented in [14], the analysis is extended to FSO communication systems using IM/DD over atmospheric turbulence and misalignment fading channels, considering cooperative communications with BDF relaying and equal gain combining (EGC) reception. In contrast to the BDF strategy considered in [14], it is assumed in [19] in addition to the analysis of the impact of pointing errors that all the bits detected at the relay are always resended regardless of these bits are detected correctly or incorrectly.…”

“…In [19], following the bit-detect-and-forward (BDF) cooperative protocol presented in [14], the analysis is extended to FSO communication systems using IM/DD over atmospheric turbulence and misalignment fading channels, considering cooperative communications with BDF relaying and equal gain combining (EGC) reception. In contrast to the BDF strategy considered in [14], it is assumed in [19] in addition to the analysis of the impact of pointing errors that all the bits detected at the relay are always resended regardless of these bits are detected correctly or incorrectly. Next, bits received directly from the source and from the relay are detected at destination node following an EGC technique.…”

“…Next, bits received directly from the source and from the relay are detected at destination node following an EGC technique. In [20], a novel BDF relaying scheme based on repetition coding with the relay and EGC is proposed, improving the robustness to impairments proper to these systems such as unsuitable alignment between transmitter and receiver as well as fluctuations in the irradiance of the transmitted optical beam due to the atmospheric turbulence, compared to the BDF relaying scheme analyzed in [19].…”