بررسي سيستم هاي مخابرات نوري فضاي آزاد و ارائه روش هاي جديد مدولاسيون

چكيده انگليسي :

Investigation of the Free Space Optical Communication Systems and presentation new Modulation Schemes Monire Faridzadeh m faridzadeh@ec iut ac ir Date of Submission Department of Electrical and Computer Engineering Isfahan University of Technology Isfahan Iran Degree M Sc Language FarsiSupervisor Asghar Gholami Mohamad Reza Ashori gholami ashori@cc iut ac irAbstract Ever increasing demand for more bandwidth has compelled to search in optical communication But asignificant attention has been given in last decades to terrestrial FSO communications FSO communicationis a line of sight technology based on transmitting data laden light through the atmosphere In addition toproviding high bandwidth FSO system benefits from the license free operation light weight low installationcost and immunity to electromagnetic interference The main challeng in this technology results from itschannel The atmospheric channel is not stationary and changes with time of the day seasons and weatherconditions The optical beam traversing the atmosphere experiences absorption and scattering results bymolecular and aerosol constituent Unlike RF systems FSO is less affected by snow and rain but heavilyinfluenced by fog which limits the link range In the dense fog condition availability of the link islimited Though the attenuation in clear atmosphere condition is very low the performance of FSOcommunication systems suffers severely from the turbulence The turbulence is the results of wind andtemperature gradients that create air pockets with varying refractive index The traversing light beamexperiences random fluctuation in its intensity scintillation and phase leading to degradation inperformance In order to describe the atmospheric turbulence some models have been suggested aslognormal and negative exponential which are simulated in this work The optical carrier can be modulated inits frequency phase intensity and state of polarization The intensity modulation is mostly preferred due tosimplicity in implementation and cost There are many different modulation schemes which are suitable forFSO communication but much attention allocated to OOK modulation for its simplicity But due toatmospheric turbulence it needs an adaptive threshold which imposes high complexity to receiver PPM is anorthogonal digital pulse modulationadopted in optical communications due to its high power efficiency but incost of increasing requirement bandwidth and complexity To address these challenges the SIM is proppsedwhich needs no adaptive threshold and its requiremet bandwidth is less than PPM but its consuming power ishigh In this work two new hybrid PPM BPSK SIM and BPSK SIM PPM schemes are proposed whichcombines the advantage of the PPM and BPSK SIM The theoretical analysis of proposed scheme is carriedand is verified using computer simulations The performance of the PPM BPSK SIM is compared withBPSK SIM and PPM schemes which show good performance compared to BPSK SIM and PPM in strongturbulence and its improvement in compare with them becomes more by increasing the turbulence strength In order to increase the baudrate multiple subcarriers can be used In this work a two subcarriers system isinvestigated in three symbol combination methods via simulation and theoretical analysis and compared toBPSK SIM with multiple qsubcarriers The performance of BPSK SIM PPM is investigated via simulatinwhich shows good performance in copmarision to BPSK SIM Keywords Free Space optical communication Pulse Position Modulation Subcarrier IntensityModulation

فريدزاده، منيره

بررسي سيستم هاي مخابرات نوري فضاي آزاد و ارائه روش هاي جديد مدولاسيون

مدولاسيون موقعيت پالس , مدولاسيون شدت زير حامل