Analysis of Transmit Diversity Performance Over Vertical FSO Channels With Altitude-Dependent Impairments

This paper studies the performance of transmit diversity schemes over high altitude platform-to-ground vertical free-space optical (VFSO) link under various channel impairments. Unlike conventional independent layered channel model, it considers concatenated vertical channel, modeled using <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i>* gamma-gamma distributions. Wavelength and time diversity techniques are considered and the effectiveness in mitigating atmospheric turbulence and pointing errors is studied in terms of average bit error rate (ABER). The derived closed-form ABER expressions are shown to match well with Monte-Carlo simulation of VFSO channel using correlated phase screen method. Numerical results show that under strong turbulence time diversity offers 7.5 dB gain over wavelength diversity in achieving an ABER of 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup>, whereas wavelength diversity outperforms time diversity by 10 dB where pointing error dominates.