Performance Comparison of OOK and PPM Modulation Schemes in FSO Links Under Various Atmospheric Conditions

Free Space Optics (FSO) technologies are seen today as a very promising and relatively economical alternative to satisfy the growing needs for bandwidth and transmission rate while guaranteeing a high level of security of the transmission link. They are also compatible with OCDMA (Optical Code Division Multiple Access) systems perceived as potential candidates to guarantee multiple access to multiple users for the achievement of a future all-optical communication network. However, those technologies are still very dependent on atmospheric conditions which can induce a strong scintillation at the receptor causing a loss of information or even a total rupture of the transmission link, hence the interest in choosing the appropriate type of modulation to be applied. In this work, we simulated the effect of atmospheric turbulence on an FSO-OCDMA link. We estimated the BER (Bit Error Rate) and the Quality Factor as a function of the SNR (Signal to Noise Ratio) for two types of modulation schemes widely used in FSO transmissions: OOK (On–Off Keying) and PPM (Pulse Position Modulation), and considering two different OCDMA codes with are Walsh–Hadamard code and Gold code. The study was conducted by simulating various atmospheric conditions adopting the Gamma-Gamma model for scintillation and using a Monte Carlo algorithm. Our simulations show that the PPM modulation scheme is more resistant to the effects of scintillation. This makes it more suitable for use in a channel subject to severe atmospheric turbulence.