Currently, the atmospheric wireless optical communication generally uses intensity modulation/direct
detection (IM/DD) systems. There are typical modulation schemes such as on-off keying (OOK), pulse position
modulation (PPM), differential pulse position modulation (DPPM), differential pulse interval modulation (DPIM)
and dualheader-pulse interval modulation (DH-PIM) for atmospheric wireless optical communications. For the
high packet error rate and low information transmission rate of wireless optical communication intensity
modulations presently, which is caused by atmospheric turbulence and scintillation on the laser beam, a new
modulation scheme--Polarization Shift Keying is proposed. The modulation structures, bandwidth requirement,
transmission capacity, power requirement and packet error rate performance over weak turbulence are compared
between several intensity modulation schemes (including OOK, DPPM, DPIM, DH-PIM) and PolSK
modulation. Theoretical analysis and simulation results show that OOK offers the highest bandwidth efficiency
and transmission capacity. However, the average transmission power and bit error rate is both high. The
reliability of DPPM and DH-PIM is better, while the bandwidth efficiency is low. CPolSK offers the highest
bandwidth efficiency and the lowest bit error rate and packet error rate, which are very important in the
atmospheric wireless optical communication. CPolSK has stronger anti-interference ability compared to
intensity modulation scheme, because the polarization seems to be the most stable characteristic of a laser beam
while propagating through the atmosphere. Hence, CPolSK is superior in the atmospheric laser communication.
KEYWORDS: Polarization, Atmospheric propagation, Light sources, Atmospheric turbulence, Turbulence, Global system for mobile communications, Telecommunications, Free space optical communications, Beam propagation method, Laser beam propagation
Polarized laser has been widely used in free space optical communication, laser radar, and laser ranging system
because of its advantages of good performance in recent years. The changes of laser polarization properties in the process
of transmission in atmospheric turbulence have a certain impact on the system performance. The paper research on the
rule of polarization properties changes of Gauss Schell model beam in turbulent conditions. And analysis the main
factors to affect the polarization properties by numerical simulation using MATLAB software tools. The factors mainly
including: initial polarization, coherence coefficient, spot size and the intensity of the atmospheric turbulent. The
simulation results show that, the degree of polarization will converge to the initial polarization when the beam
propagation in turbulent conditions. The degrees of polarization change to different value when initial polarization of
beam is different in a short distance. And, the degrees of polarization converge to the initial polarization after long
distance. Beam coherence coefficient bigger, the degree of polarization and change range increases bigger. The change
of polarization more slowly for spot size is bigger. The change of polarization change is faster for longer wavelength.
The conclusion of the study indicated that the light source parameters effect the changes of polarization properties under
turbulent conditions. The research provides theory basis for the polarization properties of the laser propagation, and it
will plays a significant role in optical communication and target recognition.
In order to reduce the impacts of atmospheric turbulence and background light etc. factors to atmospheric laser communication system performance, the atmospheric laser communication system using circular polarization modulation technology is adopted and researched. This system uses polarization shift keying modulation (PloSK), which is a new standard digital modulation technique in optical communication field. In this modulation, two rotation states of the circle polarization light (left handed and right handed) representation logic signal ' 0 ' and ' 1 ', are used to information loaded and data transmission. In the receiver, the modulation optical signal is detected with dual differential probe method. Under the OptiSystem system simulation environment, several direct detection system model based on OOK intensity modulation, single rode circular polarization modulation and circular polarization modulation with balanced detection is constructed, and compares and analysis of the various communication system performance. The results show that: at the same parameter conditions, bit error rate of CPolSK system with balanced detection lower about two orders of magnitude than the OOK system and single rode CPolSK system, the eye diagram and the waveform chart are also significantly better than OOK system’s. It can be seen, based on circular polarization shift keying (CPolSK) laser communication system with dual differential detection is superior on anti-interference of atmospheric interference, and reducing error rate, and will be easy to implement.
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