| Summary: | 碩士 === 國立虎尾科技大學 === 電機工程系碩士班 === 105 === In this paper, we propose a hybrid orthogonal frequency division multiplexing/optical code-division multiple-access multiplexing (OFDM/OCDMA) via polarization multiplexing technique and multi-input multi-output (MIMO) technologies in free space optics (FSO) transmission to improve the accuracy of the transmission signal of log normal channel. This system has the advantages of stable transmission rate, data security and low transmission cost. We adopt quasi-orthogonal Walsh-Hadamard codes (WHC) that with fixed cross correlation value as the codeword for each user. With the property, the multiple access interference (MAI) can be efficiently eliminated by using the balanced detection scheme at the receive end. Besides, we adopt MIMO scheme of the space-time black coding (STBC) technology in wireless optical channel of various environments to maintain the signal energy transmission during multipath degradation.
In simulation scenario, we use the commercial optical communication software Opitsysytem 14.0 and Matlab to collect the data and perform simulation to confirm the feasibility of the proposed architecture. Atmospheric turbulence is one of the major challenges in wireless optical systems. The use of multiple receivers and multiple transmitters provides an effective approach to mitigate the fading effect caused by the atmospheric turbulence. In the case of multi-user scenaior, we use log-normal channel as our free space optical channel and tune the variables as clear, rain weathers to confirm the proposed MIMO OFDM-OCDMA performance. We derivative the mathematical model of SISO (single input single output) and MIMO channel conditions and use Matlab to calculate and compare the signal quality between different transmission architectures.
In fading channel conditions, the simulation results, we can find that the system performance can be improved by using MIMO scheme in FSO communications.
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