Uyarlanabilir MIMO serbest uzay optik haberleşme sistemleri

Free space optical (FSO) enjoys the high data rate of optical spectrum and have also the flexibility of RF links. FSO systems provide many advantages to the line of sight wireless communication technology. With the recent increasing interest on this promising technology, there is a need for a comprehensive understanding of system limitations which is mainly due to atmospheric conditions. In the first part of this research, we use our custom design atmospheric channel emulator for FSO system evaluations in a controlled environment and experimentally investigate the performance of FSO links. Specifically, we investigate the geometric loss, absorption loss, different weather conditions (like foggy and rainy), different beam shapes, and atmospheric turbulence using the atmospheric chamber. Atmospheric turbulence is a significant impairment in FSO channels which results in random fluctuations in the received signal level. By generating a desired level of atmospheric turbulence in the chamber, we investigate effect of wavelength and aperture averaging on the performance of FSO systems. Aperture averaging extracts inherent receive diversity gains and can be used as an effective fading mitigation technique. Furthermore, multiple apertures systems are also adopted in practical FSO systems to mitigate the turbulence induced fading effects and offer dramatic performance improvements in terms of link reliability (via diversity gain) and data rates (via multiplexing gain). On the other hand, the turbulence induced fading is characterized as very slow-varying, hence reliable feedback would be possible and adaptive transmission can be implemented in practical FSO systems and brings a noticeable performance improvement. Although the literature for adaptive transmission of RF systems is mature, it has been recently applied to SISO FSO systems and its direct application to MIMO FSO systems is challenging. Aiming to fill research gaps in this growing field, this work develops a framework for practical FSO systems with adaptive MIMO architectures. A MIMO system over a frequency-flat, log-normal or Gamma-Gamma slow-fading channel is considered in our work. In MIMO FSO systems, the space-time transmission strategy can also be adjusted, introducing a new dimension for adaptation. This means that practical MIMO link adaptation algorithms must also provide a dynamic adaptation between diversity and multiplexing modes of operation which needs a fundamental understanding of diversity-multiplexing tradeoff (DMT) under log-normal fading channels. Although there has been a growing interest on the study of DMT, the existing works are mostly restricted to the outcomes reported for Rayleigh, Rician, and Nakagami fading channels. In the next part of this research, we investigate the optimal tradeoff in the presence of log-normal fading channels. We derive the outage probability expression, and then present the asymptotical DMT expression. We further investigate DMT for finite SNRs and demonstrate convergence to the asymptotical case. Next, we suggest a framework for practical MIMO FSO system with adaptive architectures and shows how to use this framework to increase either link reliability (via diversity gain) and or data rates (via multiplexing gain). To illustrate our approach, we consider three MIMO transmission mapping matrices which includes: Matrix A (multiplexing), employing only spatial multiplexing; Matrix B (diversity), exploiting only diversity; and Matrix C (hybrid), combining diversity and spatial multiplexing. We first obtain expressions for the outage capacity of these matrices as the metric to maximize the rate of system for a fix target outage probability. Limiting the adaptation modes to a small subset is the key of adaptive strategy. The spatial adaptation can be combined with conventional adaptive modulation and coding (AMC) to give the optimal system performance. Particularly, we consider multiple-input single-output (MISO) and single-input multiple-output (SIMO) FSO systems with pulse position modulation (PPM) and pulse amplitude modulation (PAM). We propose three adaptive algorithms where the modulation size and/or transmit power are adjusted according to the channel conditions. We formulate the design of adaptive algorithms to maximize the spectral efficiency under peak and average power constraints while maintaining a targeted value of outage probability. In conclusion, this work propose a promising progress to overcome the main impairments (fog attenuations and turbulence induced fading) of the FSO links in four ways: 1) by examining the channel and proposing novel models and characteristics for atmospheric attenuations 2) by taking advantage of aperture averaging and wavelength dependency trade-off 3) by investigating and proposing spatial adaptation in MIMO FSO links and 4) by employing adaptive modulation and power control scenarios and approving the promising performance of adaptive system.