الفهرس | Only 14 pages are availabe for public view |
Abstract Optical wireless communications (OWC) is becoming an attractive alternative medium to optical fiber and radio frequency (RF) communications because OWC requires no spectrum licensing and interference to and from other systems unlike radio and microwave systems. In addition, the point-to-point laser signal is extremely difficult to intercept, making it ideal for covey communications. Free space optical communications offer data rates comparable to fiber optical communications at a fraction of the deployment cost while extremely narrow laser beam widths provide no limit to the number of free space optical links that may be installed in a given location. But, the performance of wireless infrared transmission systems is severely impaired by noise and interference induced by natural and artificial ambient light. In order to combat the effects of ambient light on the system performance, both optical filtering and electrical filtering are usually adopted. However, even when resorting to these techniques, the optical power penalty imposed by the interference may be very large. In particular, the interference produced by fluorescent lamps driven by electronic ballasts imposes very large optical power penalties on systems operating at data rates up to a few tens ofMbps. In this thesis, many techniques are used to overcome the penalty induced by artificial light interference and are analyzed. These techniques are used to reduce noise current or removing it by using an electrical filter or a polarizer or both of them together. We are going to display the effect of noise voltage, flicker voltage and signal to noise ratio of the systems by changing each parameter to reach the best solution for reducing effect of noise on OWe. from this thesis, we can summarize the main results in the following points: 1. Using a single polarizer with a photodetector decreased noise and flicker voltages by a factor equals to T” and improved SNR. 2. The differential method is better than using a single photodetector to reduce the effect of ambient noise current. 3. The best SNR is obtained in the differential detector with two orthogonal polarizers system and the worst one is in the system that uses a single photodetector. 4. The best photodetector is that one that has tiny bandwidth, lowest excess noise factor and, lowest current gain. 5. Using a polarizer with an electrical filter make improvement in SNR than when using each of them individually. |