Combined transmission of PPM and WSK modulated optical signal over free space optical link enabling physical layer security

Abstract

This work proposes a novel technique for transmission of data from two different sources by using a combination of wavelength shift keying (WSK) and pulse position modulation (PPM) over a free space optical (FSO) link. It has been shown that the proposed technique has potential of improving the physical layer security of FSO links, which is a growing concern due to considerable interest in the employment of FSO for high data rate applications. The proposed technique transmits at a combined data rate of 20 Gbps over a single pulse per bit slot. The wavelength and position in time of the pulse is varied according to the combination of bits received from the two different data sources. Gamma–Gamma channel model is used to simulate the FSO link whose length has been varied from 0.5 km to 3 km to observe the bit error rate (BER) performance. The BER has also been observed for different time delays induced over the optical pulse for implementing the PPM technique. It has been shown that the proposed technique can work under different time delays, hence making it difficult for an eavesdropper to sample the optical pulses at the correct instant for information extraction. Furthermore, this scheme allows us to perform balanced detection, resulting in wider eye opening at the receiver.