Coherent Circular Polarization Modulationfor Optical Wireless Communications through Multiple-Scattering Channel

• Main Authors: Jian-Zhi Kuo, 郭建志
• Other Authors: 吳靜雄
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/74904615931152595144

碩士 === 國立臺灣大學 === 電信工程學研究所 === 94 === Optical wireless communication (OWC) systems provide sufficiently high system bandwidth and data rate equivalent to fiber-optic network. This optical communication system makes connectivity possible between Internet provider and household subscribers to solve “The Last-mile” problem. Thus the OWC links can reduce the high installation cost of Fiber-optic networks. Such high developing potential has attracted our attention. The main difference between these two communication systems is the propagation channel, wired and wireless. The primary drawback for OWC is high dependence on adverse weather conditions. The atmospheric attenuation is mainly caused by scattering, absorption, and atmospheric turbulence, etc. The most detrimental environmental conditions concerned about are fog and haze. In this thesis, we start with the channel response of OWC systems through dense fog. Because the distributed particles diameter and the near infrared wavelength used are in the same order, we can apply Mie theory to calculate the scattering characteristics through fog particles. The group of Dr. Ishimaru computed the scattering amplitude phase function by Mie theory and solved the radiative transfer equation in frequency domain to figure out the channel response of the multiple-scattering channel model for further application. From the simulation results we can see that frequency modulated wave shows superior properties over regular wave with narrower angular spectrum and lower order scattering effect. It also preserves higher degree of polarization (DOP), which is more evident for dense fog consideration. With the particular behavior of the polarization statistics and coherence through propagation, we exploit an effective circular polarization shift keying (PolSK) scheme to realize the coherent detection in OWC systems. By taking the advantage of sufficiently high DOP and well behavior of circular polarized and frequency modulated wave, we employ a simple polarization modulation unit to generate left-handed-circularly and right-handed-circularly polarized wave. The direction of rotation is corresponding to the message bits and refers to the relative phase of two orthogonal polarized components of the optical wave. In the receiver front end an effective Stokes parameters extractor is used to extract the received Stokes parameters on the Poincare sphere. We can obtain the relative phase of the two orthogonal polarization components which is the important basis for transmitting information. Then we cover the information by binary decoding. Because we carry the information on the relative phase of the two orthogonal polarized components, adaptive polarization control unit is needless. In this thesis, we evaluate the coherent circular polarization detection scheme applied to mitigate the attenuation caused by dense fog and take account of the interference of diffuse intensity and DOP attenuation. We analyze the characteristics of receiver field-of-view (FOV), coherence parameter, and signal-to-noise ratio (SNR) to idealize the OWC system performance.