Development of a new dual diffuser modulation technique to alleviate the scintillation effect in free space optical communication
Abdul Rahman, Kram
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This thesis presents new approach for modulation technique in free space optical communication due to atmospheric turbulence. The new modulation technique is based on the modification conventional intensity modulation for direct detection on off keying (CIM/DD-OOK) which capable to optimize the phase screens diffuser effect, enhance power received and improve threshold signal level. The main limitation of the CIM/ DD-OOK is suffering with the threshold detection for the occurrence of signal fluctuations and therefore the use of adaptive threshold for optimum performance of which requires a very complex process. The using of diffuser can reduced the scintillation index however it can attenuate the signal power. The development of new modulation technique consists of three major parts. The first part is developing the Gaussian atmospheric channel analysis to determine the direction of research study. The second part is the mathematical derivation development where to analyze theoretically the new modulation technique based on conventional modulation technique. The performance analysis will investigate the signal power, threshold signal level, signal to noise ratio and bit error rate. Lastly, the third part is simulating the new modulation technique by using the OptiSystem software. This can measured the capability of DDM in close in real FSO situation and at same time can validate with the theoretical part. The result shows that for the receiving power performance at acceptable BER 10-9 under strong turbulence condition, the DDM technique able to detect weak signal up to - 18dBm. Meanwhile for CIM/DD-OOK with and without diffuser are at -14dBm and 0dBm respectively with different received power approximately 4dBm and 18dBm or equivalent to 66 percent and 99 percent improvement. In a bit rate analysis, the evaluation data rate from 622Mbps up to 10Gbps. Analysis shows that when the FSO system operate under strong turbulence effect and if the power receive is set fix at - 10dBm, clearly the DDM technique shows the superior performance if compare to conventional OOK modulation technique. At 2.5Gbps, the magnitude of BER for DDM technique increase with 12 magnitudes or equivalent to 100 percent improvement and the CIM/DD-OOK with diffuser only 2 magnitudes which give the 33 percent improvement. Meanwhile for the analysis result of receiving power DDM at 3km distance propagation with consider 0dBm power transmit, wavelength 1550nm and diffuser strength at lc = 0.01, the received power is -4.59dBm compare with conventional OOK that using diffuser only -7.6dBm which equal to 3dBm different or around 100 percent improvement. In term of BER performance, the DDM can further the distance propagation with approximately 42 percent improvement. Thus the new modulation DDM technique performs better than the conventional intensity modulation for direct detection on off keying (CIM/DD-OOK) to overcome the degradation of signal fading due to strong turbulence condition.