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Công nghệ cơ khí thường tạo ra các giả lập mô phỏng hoạt động của các đối tượng, như quy trình chế tạo thực tế theo trình tự tối ưu hóa sự thực hiện, hiệu quả kinh tế và chi phí năng lượng trước khi quyết định lựa chọn một thiết kế cụ thể. | Terrestrial Free-Space Optical communications 381 Electrical SNR dB Fig. 12. BER performance of OOK based FSO in atmospheric turbulence O12 0.2. In Fig. 13 the BER is again plotted against the normalised SNR at various levels of scintillation including when the threshold is fixed at 0.5. This is intended to show the effect of turbulence strength on the amount of SNR required to maintain a given error performance level. With a fixed threshold the BER reaches a floor at a BER that is greater than 10-4 meaning that a lower BER is not achievable at the specified low scintillation level. From this graph it can be inferred that atmospheric turbulence i causes SNR penalty for example 26 dB of SNR is needed to achieve a BER of 10-6 due to a very weak scintillation of strength Ol2 0.252 this however increases by over 20 dB as the scintillation strength increases to Ol2 0.72 and ii implies that adaptive threshold will be required to avoid a BER floor in the system performance. 382 Mobile and Wireless Communications Network layer and circuit level design Fig. 13. BER against the SNR for fixed and adaptive thresholds and various levels of scintillation. The results of this section illustrate that for the OOK modulated FSO system to perform at its best the receiver will require knowledge of both the fading strength and the noise level. This can be resolved by integrating into the system an intensity estimation network which can predict the scintillation level based on past events. The implementation of this is not trivial and as such commercial FSO designers tend to adopt the fixed threshold approach and include a sufficiently large link margin in the link budget to cater for turbulence induced fading. 5.2 Pulse Position Modulation This is an orthogonal modulation technique and a member of the pulse modulation family see Fig. 10 . The PPM modulation technique improves on the power efficiency of OOK but at the expense of an increased bandwidth requirement and greater complexity.
