Research On 4-Level Polarization Shift Keying Modulation Scheme In Free Space Communication

Nowadays, with the development of science and technology, people’s demanding of information is increasing heavily. Especially in recent years, to meet the requirement of the high-speed communication, optical communication has become a hot spot in communication engineering field. Among all the optical communication technologies, FSO(Free Space Optical Communication) has its unique advantage. Comparing with the traditional microwave communication, FSO does not occupy the frequency resource, has better data security, and could transmit more information with smaller equipment size and lower power dissipation. Its transmission nodes could also be quickly set and canceled with better movement feasibility and cheaper price, which is more adaptable than fiber communication technology. FSO technology is widely used in “the last mile communication”, mobile base stations and switches and so on.In this thesis, we mainly focus on the 4Pol SK(4-Level Polarization Shift Keying) scheme. We have analyzed its superiority compared with other modulation schemes and got a group of polarization states with best performance in modulation. The polarization axis rotation problem in 4Pol SK scheme has also been discussed.Firstly, we have done the derivation of the receiving rules of the polarization beam splitter and built the simulation system. We use the Optisystem software to simulate the 4Pol SK system and analyze the data with Matlab. Based on the analytical results, we have found the best polarization states, with which we could get equal interval electrical signals in the receiving end.Then we have studied its performance with different noise power and got the relationship between the BER and transmitting SNR. Comparing with 4ASK(4-level Amplitude Shift Keying) system of same parameters, 4Pol SK scheme has both better BER performance in the transmitting end and lower miss judgment possibility in the receiving end.We have also studied the polarization axis rotation problem in the transmission of 4Pol SK signals. By simulation and derivation, we have found that when we use a couple of 1/4 wave plates to transform the polarization states of the signals in both transmitting end and receiving end, the 4Pol SK system would not be influenced by the polarization axis rotation. On the contrary, the receiving signal will shrink without the help of the 1/4 wave plates. The analysis shows that the 1/4 wave plates could help to keep the system from the influence of the polarization axis rotation problem.In conclusion, 4Pol SK is a kind of superior modulation scheme with lower BER, higher information rate and adaptability of polarization axis rotation. It has wide application prospect in high-speed communication.