Ergodic capacity comparison of optical wireless communications using adaptive transmissions

Area of Science:

• Optical Wireless Communications
• Free-space Optics
• Information Theory

Background:

• Optical wireless communication (OWC) systems face performance limitations due to atmospheric turbulence.
• Gamma-Gamma distribution accurately models turbulence-induced fading in OWC channels.
• Subcarrier intensity modulation (SCIM) and coherent detection are key OWC techniques.

Purpose of the Study:

• To investigate the ergodic capacity of OWC systems under Gamma-Gamma turbulence.
• To compare SCIM with direct detection against coherent systems (with/without polarization multiplexing).
• To evaluate three adaptive transmission schemes: variable-power/rate, complete channel inversion, and truncated channel inversion.

Main Methods:

• Derivation of accurate series expressions for ergodic capacity using modified Bessel function series expansion.
• Application of Mellin transformation for analyzing the Gamma-Gamma distribution.
• Asymptotic analysis at high signal-to-noise ratio (SNR) to determine capacity gains.
• Numerical simulations to validate theoretical results and assess performance under polarization control errors.

Main Results:

• Highly accurate series expressions for ergodic capacity were derived for all considered systems.
• Asymptotic analysis showed significant ergodic capacity gains with increased optical power for all schemes.
• Coherent systems with polarization multiplexing demonstrated substantial capacity improvements.
• Polarization control errors below 10° had minimal impact on polarization multiplexing system performance.

Conclusions:

• Adaptive transmission schemes effectively enhance ergodic capacity in OWC systems over Gamma-Gamma channels.
• Coherent systems, particularly with polarization multiplexing, offer superior capacity performance.
• OWC systems exhibit resilience to minor polarization control errors, ensuring reliable communication.