Thesis Defense - Hatef Nouri (PHDEE)

Hatef Nouri – Ph.D. Electrical & Electronics Eng.

Prof. Dr. Murat Uysal– Advisor

Date: 12.05.2022

Time: 14:30

Location: AB4-B226

“Adaptive MIMO Free Space Optical Communication Systems”

Prof. Dr. Murat Uysal, Özyeğin University

Assoc. Prof. Dr. Cenk Demiroğlu, Özyeğin University

Asst. Prof. Dr. Kadir Durak, Özyeğin University

Assoc. Prof. Dr. Tunçer Baykaş, Kadir Has University

Assoc. Prof. Dr. Tansal Güçlüoğlu, Yildiz Technical University

Abstract:

Free space optical (FSO) enjoys the high data rate of the 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 the 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 is then 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 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 the adaptive system.

Bio:

Hatef Nouri received the B.Sc. (Hons.) degree in Electrical and Electronic Engineering from the University of Tehran, Tehran, Iran, in 2010, and the M.Sc. degree in Electrical and Electronic Engineering from Ozyegin University, Istanbul, Turkey, where he is currently pursuing his Ph.D. degree in Electrical and Electronic Engineering. He has been a teaching and research assistant with the Communication Theory and Technologies Research Group since 2010. He is specializing in telecommunications with a focus on the physical layer aspects of wireless communication. His research interests include information theory, cooperative communications, optical wireless communications, and underwater acoustic wireless communications. During his M.Sc., he studied the information theoretical performance analysis of underwater acoustic communication systems. He has served as a Reviewer of the IEEE Vehicular Technology Conference in 2012, the IEEE Globecom 2013, the IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS of the 2014 Special Issue on Optical Wireless Communication, the IEEE COMMUNICATIONS LETTERS in 2016, the IEEE BlackSeaCom 2017, the IEEE International Symposium on Information Theory (ISIT) 2017, the IEEE Transactions on Information theory in 2017, the IEEE Transactions on Communications 2017-2020, and the IEEE Transactions on Wireless Communications 2017-2020. His distinctions include the rank of 178th among 400 000 students in Iran’s nationwide university entrance exam in 2006, the Faculty of Engineering Scholarship as an Exceptional Student from 2006 to 2010, and the Full Entrance Scholarship for the M.Sc. and Ph.D. degrees at Ozyegin University.