Study On The Circle Polarization Shift Keying And The Theory Of Polarization Propagation In Wireless Optical Communication

With the progress of the technology and the economy, the communication network meets increasingly high demands. The communication networks with high-data-rate capacity, excellent security and high flexibility can help people feel more comfortable in work and daily life. Also they are important for the national defense construction and for the emergency service and disaster relief. Because of the high cost, the long building period, the restriction of bandwith and other disadvantages, the fiber communication technology and the microwave communication technology are falling behind of the increasing demands in networks. Consequently, some new communication technologies are required to meet this case.Free space optical (FSO) technology is the combination of the fiber communication and the microwave communication techniques. FSO technology has many advantages such as potential high-data-rate capacity, excellent security, fast link installation and particularly wide bandwidth on unregulated spectrum. It can be used in satellite communication, the-last-mile-access, fast communication link installation and so on. FSO technology is the beneficial complement of the traditional communication techniques. However, in FSO systems the signals transmit directly through the air, the received signal field is affected inevitably by the atmospheric turbulence and experience random amplitude and phase fluctuations, also known as scintillation in optical communication terminology. The scintillation severely degrades the system performance and restricts the application domains of the FSO technology.In order to improve the reliability of the FSO system, a new polarization modulation scheme is proposed in this dissertation. The variation of the polarization properties in propagation is deeply studied. The author tries to provide a FSO system scheme with low cost, high reliability and high flexibility by using this new modulation. The main contents of this dissertation are shown as following. Firstly, based on the analyses of the atmosphereric channel and the current modulation technologies, the circle polarization shift keying (CPolSK) modulation scheme is proposed. In this scheme, the two rotation states of circle polarization are used to express the two data states of the digital signal. Considering the polarization properties of laser beams can keep unchanged through long distance propagation in the atmosphere, the modulation has higher reliability than other schemes. Meanwhile, because the circle polarization is rotational symmetrc, the CPolSK system can work extremely well even on the moving objects, such as the airplanes, the balloons. Secondly, the performances of the power efficiency, the bandwidth efficiency and the bit error rate are compared between CPolSK modulation and several intensity modulations (including OOK, PPM, DPPM and DPIM). The results show that the CPolSK modulation has the lowest power efficiency, the highest bandwidth efficiency and the lowest bit error rate. Considering the current situation of the laser technology, the relatively high transmission power will not be a drawback of the CPolSK. The high data rate and the low bit error rate are very important in the wireless communication. Consequently, the CPolSK is an excellent modulation scheme.Thirdly, the CPolSK systems are simulated in the OptiSystem software. The results show that the CPolSK system has a two magnitudes lower BER than the OOK system. The eye diagrams and the waveform graphs of CPolSK systems are also better than ones of the OOK system.Fourtyly, based on the unified of coherence and polarization, the polarization variances of the Gaussian-Schell model (GSM) beams are calculated in the free space and in the turbulence. The polarization parameters change identically and tend to the same certain values for the two channels in short distance. If the beam propagates further, the polarization parameters will keep the values in free space. However, for the atmosphere channels, they will return the initial values of the source plane in the far zone.Fifthly, based on the expressions of the polarization degree and the polarization state, the condition of polarization invariance is derived. Three formulas are obtained and the first two denote the linear polarized GSM beams. The third formula is more useful because it can express the beams with arbitrary polarization state. The beams satisfying any of the three formulas can keep the polarization properties unchanged both in free space and in the turbulence.At last, the three-dimensional polarization degrees are calculated for the partially polarized non-paraxial GSM beams. The 3×3 cross-spectral density matrix is used to describe the vector light field. The Rayleigh-Sommerfeld diffraction integrals are used to calculate the propagation of the beams. The results show that the three-dimensional polarization degrees vary identically to the two-dimensional polarization degrees in the paraxial region. In the areas far from the beam axis, the three-dimensional polarization degrees tend to the certain value. This value is independent of the propagation distance and the spectral coherent widths.