A CubeSat platform has become a popular choice due to inexpensive commercial off-the-shelf (COTS) components and low launch cost. However, it requires more power-efficient and higher-data rate downlink capability for space applications related to remote sensing. In addition, the platform is limited by the size, weight and power (SWaP) constraints as well as the regulatory issue of licensing the radio frequency (RF) spectrum. The requirements and limitations have put optical communications on promising alternatives to RF communications for a CubeSat platform, owing to the power efficiency and high data rate as well as the license free spectrum. In this study, we analyzed the performance of optical downlink communications compatible with CubeSat platforms in terms of data rate, bit error rate (BER) and outage probability. Mathematical models of BER and outage probability were derived based on not only the log-normal model of atmospheric turbulence but also a transmitter with a finite extinction ratio. Given the fixed slot width, the optimal guard time and modulation orders were chosen to achieve the target data rate. And the two performance metrics, BER and outage data rate, were analyzed and discussed with respect to beam divergence angle, scintillation index and zenith angle.

1. INTRODUCTION
2. SYSTEM AND CHANNEL MODELS
    2.1 Pulse Position Modulation
    2.2 Link Equation
    2.3 Log-normal Fading Channel
3. PERFORMANCE ANALYSIS
    3.1 Signal-to-Noise Ratio
    3.2 Bit Error Rate
    3.3 Outage Probability
4. NUMERICAL RESULTS AND DISCUSSIONS
5. CONCLUSIONS
REFERENCES

• Hyung-Chul Lim(Korea Astronomy and Space Science Institute) Corresponding Author
• Sung-Yeol Yu(Korea Astronomy and Space Science Institute)
• Ki-Pyoung Sung(Korea Astronomy and Space Science Institute)
• Jong Uk Park(Korea Astronomy and Space Science Institute)
• Chul-Sung Choi(Korea Astronomy and Space Science Institute)
• Mansoo Choi(Korea Astronomy and Space Science Institute)