Required SNR improvement for downlink optical communication via channel coding and aperture diversity by utilising PPM

Abstract

In this study, it is aimed to investigate the required signal-to-noise ratio (SNR) that provides a sufficient bit error rate (BER) according to the zenith angle between the ground station and satellite when pulse position modulation is utilised. Many parameters such as scintillation index, link distance, wavelength, Rytov variance, Fried's parameter, aperture diameter of the receiver and zenith angle have been considered to achieve realistic results. Due to the restrictions on the system, some parameters are specified as deterministic; the obtained results suggest that the required SNR value changes in a wide range from small to large values of the zenith angle to achieve sufficient BER. Therefore, either SNR adjustment as a function of the zenith angle is provided or the interval of the zenith angles to establish the link is determined previously in order to avoid the high link margin. It is shown that, instead of the second choice, aperture diversity is utilised at the receiver to establish communication even for large zenith angles with a reasonable value of the SNR. Besides aperture diversity, channel coding with error correction codes further improves the performance.