Research On Free Space Optical Communication System Based On Coherent Detection

Free space optical(FSO)communication technology has been widely concerned in the field of optical communication.FSO system use optical pulse signal as the carrier of information to transmit data in the atmospheric channel.FSO technology applied in various atmospheric communication fields,such as nearly ground atmospheric optical communication,terrestrial and satellite and satellite between inter satellite optical communication.Compared with the optical fiber communication and microwave technology,FSO communication system has many obvious advantages,for instance,the rich bandwidth resource,high frequency of signal,great channel capacity,and high speed data transmission ability.However,the performance and reliability of FSO communication links severely affected by atmospheric conditions.The irregularity and randomness of atmospheric turbulence channel will cause great damage to the FSO communication system,which can bring reduction to signal-to-noise ratio and increase to bit error rate(BER),even lead to system interruption.In order to reduce the atmospheric turbulence effects on free space optical communication,many technologies has been proposed and applied in various parts of the FSO system,such as diversity receiver,channel coding,partially coherent optical transmission technology,aperture averaging technique etc..These techniques are about digital signal processing,received signal encoding,and the characteristics of weak or not related correlation between the signals to suppress channel fading effects.Traditional FSO communication system is based on the scheme of intensity modulation and direct detection(IM/DD).However,the performance of coherent system is nearly 10～25dB better than IM/DD system.It makes sensitivity get enhanced which was the important performance index in FSO communication system.Due to the prominent advantage,coherent detection technology has become a hot research area in FSO communication.In this thesis,we propose a new modulation format combined with the coherent detection technology.The hybrid modulation system can greatly reduce the influence of atmospheric channel.It has a practical guiding significance for promoting and developing FSO technology.The thesis studies the communication system which combined coherent detection techniques with new hybrid modulation technology.The main works and innovation points of this thesis can be described as the following aspects:1.According to the adverse effects of atmospheric turbulence on polarization status,and the realization of weak signal detection at receiver,the thesis put forward a CPolSK homodyne coherent detection system to deal with such problems.When the signal transmitted in atmospheric channel,CPolSK modulation use two circular polarized rotation directions to reject the change of polarization states.The polarization state of light beams is less affected by atmospheric turbulence in the free space propagation.This striking feature can increase system stability and reduce bit error rate.The performance of CPolSK system is 3dB higher than OOK system.By improving the power of local signal,the noise source is only restricted by the quantum noise at receiver.When the BER is 1.0E-9,IM/DD system needs 100photons/bit,while coherent system only needs about 10photons/bit or less.Compared with the direct detection system,coherent system greatly improves sensitivity and wavelength selectivity(the channel spacing as small as 1 GHz～10GHz).Meanwhile,the link transmission distance(1550nm wavelength can be extended to 100 km)increased significantly.So this hybrid modulation system is a potential communication system.2.Compared with traditional modulation formats,a new hybrid modulation is proposed,named PolSK-PPM-MQAM.The hybrid modulation aims at combining the advantages of a variety of modulations and improving the system performance in transmission process.This scheme makes the total number of transmission bits increased to log2N+(1+log2M)bits.It not only increases the system capacity,but also improves the anti-turbulence ability.At the same time,the results are verified by corresponding theoretical study.3.As Multiple-Input Multiple-Output technology(MIMO)can greatly enhance the performance of wireless communication system,the thesis proposed a new MIMO-CPolSK-PPM communication system,which combine MIMO technology with CPolSK-PPM hybrid modulation technology.The BER analysis is carried under the Gamma-Gamma model with medium or strong turbulence.The simulation results show that performance of new MIMO-CPolSK-PPM system is 5dB higher than OOK system and uncorrelated antennas can increase the channel capacity.The hybrid system has excellent resistance to atmospheric turbulence and it is a kind of modulation technology with practical value.4.According to the random phase disturbance polarization of an optical signal,a new type of PolSK modulation method is proposed.Two AM modulators are used to replace the traditional single phase modulator in PolSK modulation.It is good to avoid the phase change in polarization state.Based on the existing conditions of laboratory,the PolSK modulation system experimental platform was built and the back-to-back system of error free transmission is realized.Meanwhile,the experimental results of PolSK and OOK modulation system were analyzed and compared.