Browsing by Author "Nouri, Hatef"

Now showing 1 - 13 of 13

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Adaptive free space optical communication system with multiple apertures
(2016) Nouri, Hatef; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Nouri, Hatef
In the current literature on free space optical (FSO) communication, typically open-loop designs are considered where the knowledge of channel state information is only available at the receiver. This is mostly favorable in the context of time-varying channels where the feedback implementation becomes problematic. Atmospheric turbulence channels in FSO systems exhibit quasi-static nature. The feedback information can be reliably transmitted, therefore makes possible the use of channel state information at the transmitter side. Such close-loop systems where transmission parameters are adaptively adjusted according to the channel conditions are widely used in radio frequency wireless communications. In this work, we consider adaptive transmission techniques in the context of FSO systems with multiple apertures and investigate their performance.
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Adaptive MIMO free space optical communication systems
Nouri, Hatef; Uysal, Murat; Uysal, Murat; Demiroğlu, Cenk; Durak, Kadir; Baykaş, T.; Güçlüoğlu, T.; Department of Electrical and Electronics Engineering; Nouri, Hatef
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.
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Adaptive MIMO FSO communication systems with spatial mode switching
(IEEE, 2018-08) Nouri, Hatef; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Nouri, Hatef
In this paper, we propose an adaptive multiple-input multiple-output (MIMO) free-space optical (FSO) system with dynamic adaptation between spatial modes of operation. The proposed adaptive system supports three MIMO modes. In the spatial multiplexing mode, the system transmits independent parallel data streams over multiple apertures to increase data rate. In diversity mode, the system transmits coded streams through multiple apertures to extract diversity gains. In hybrid mode, a trade-off between diversity and multiplexing gains is targeted. For each operation mode, we first derive the outage probability for the MIMO FSO system over log-normal turbulence channels. Then, we formulate the design of an adaptive FSO system to select the MIMO mode that yields the highest throughput while satisfying a predefined target outage probability. Extensive numerical results are provided to demonstrate the performance of the proposed adaptive scheme.
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Adaptive modulation for FSO IM/DD systems with multiple transmitters and receivers
(IEEE, 2023-02) Nouri, Hatef; Sait, S. M.; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Nouri, Hatef
Atmospheric turbulence is a major impairment in free space optical (FSO) communication systems. Since turbulence induced fading exhibits quasi-static characteristics, reliable feedback of channel state information is possible allowing the implementation of adaptive transmission. In this letter, 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 an adaptive algorithm where the modulation size/type is adjusted according to the channel fading conditions. We formulate the design of adaptive algorithm to maximize the spectral efficiency under peak and average power constraints while maintaining a targeted value of outage probability. We present extensive Monte Carlo simulations to demonstrate the performance of proposed adaptive scheme over turbulence channels.
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A custom-design atmospheric channel emulator for the performance evaluation of free space optical communication systems
(IEEE, 2017) Kebapci, Burak; Miramirkhani, Farshad; Nouri, Hatef; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Kebapci, Burak; Miramirkhani, Farshad; Nouri, Hatef
In this paper, we present our custom design atmospheric channel emulator for free space optical (FSO) system evaluations in a controlled environment. Our emulator is in the form of an atmospheric chamber with dimensions of 60 cm × 40 cm × 300 cm. It is equipped with adjustable heaters, coolers and fans to create the turbulence. It also houses a fog generator and droplet watering system to generate different weather conditions. Using this custom-design emulator, we experimentally investigate the performance of FSO links at different wavelengths in fog conditions.
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Diversity-multiplexing tradeoff for log-normal fading channels
(IEEE, 2016-07) Nouri, Hatef; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Nouri, Hatef
Although there has been a growing interest on the study of diversity-multiplexing tradeoff (DMT), the existing works are mostly restricted to the outcomes reported for Rayleigh, Rician, and Nakagami fading channels. In this paper, we investigate the optimal tradeoff in the presence of log-normal fading, which provides an accurate model for indoor wireless, optical wireless, and ultra-wideband channels. We consider a multiple-input multiple-output (MIMO) log-normal channel, derive the outage probability expression, and then present the asymptotical DMT expression. It is shown that the asymptotical maximum diversity gain tends to infinity with logarithm of signal-to-noise ratio (SNR). To have further insight, we normalize the DMT with that of single-input single-output case and express the relative asymptotical DMT of the MIMO log-normal channel as a function of the number of transmit and receive antennas. We further study DMT for finite SNRs and demonstrate convergence to the asymptotical case. Extensive numerical results are provided to corroborate the analytical derivation.
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Experimental investigation of pointing errors on drone-based FSO systems
(IEEE, 2020-10-05) Tosun, Anılcan; Nouri, Hatef; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Tosun, Anılcan; Nouri, Hatef
Airborne free space optical (FSO) links can be efficiently used for backhaul/fronthaul needs of future cellular networks. Unlike terrestrial FSO links where a fixed connection between two buildings is considered, airborne FSO links bring unique challenges due to the mobility of the drone. In this paper, we carry out an experimental study to demonstrate the effect of pointing errors in drone-based FSO systems. We employ a supervised learning method for tracking purpose of the FSO signal. We take power measurements and characterize the probability distribution of the received signal power. We further present comparisons between tracking and non-tracking cases and demonstrate the benefits of the tracking system.
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Experimental investigation on the effect of wavelength on aperture averaging in FSO communications
(The Optical Society, 2020-06-01) Nouri, Hatef; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Nouri, Hatef
Atmospheric turbulence is a major impairment in free-space optical (FSO) communication systems. Based on the fact that the size of receiver aperture is much larger than operation wavelength, aperture averaging extracts inherent receiver diversity gains and can be used as an effective fading mitigation technique. In this Letter, we consider the three most common wavelengths used in telecommunications, namely 1550, 1310, and 850 nm, and present a comparative performance evaluation of aperture averaging under the same emulated atmospheric conditions. Our emulator is in the form of an atmospheric chamber equipped with adjustable heaters, coolers, and fans to create the desired level of turbulence. Our results demonstrate that by changing the wavelength and/or aperture size, a strong turbulence condition with Gamma-Gamma statistics can turn into a weak turbulence condition with log-normal statistics. Such a phenomenon can be observed even for relatively small aperture sizes if the wavelength is sufficiently high.
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Information theoretic analysis and optimization of underwater acoustic communication systems
(IEEE, 2013) Nouri, Hatef; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Nouri, Hatef
In this paper, we investigate the information theoretical limits of underwater acoustic channel with intersymbol interference. Under the assumptions of sparse and frequency-selective Rician fading channel and non-white correlated Gaussian ambient noise, we derive an expression for underwater channel capacity which we later use to optimize the carrier frequency and input signaling. We further demonstrate the effect of several system and environmental parameters such as distance, temperature, depth, etc. on the capacity.
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Information theoretical performance analysis and optimisation of cooperative underwater acoustic communication systems
(IEEE, 2016-05) Nouri, Hatef; Uysal, Murat; Panayirci, E.; Electrical & Electronics Engineering; UYSAL, Murat; Nouri, Hatef
In this study, the authors investigate the information theoretical limits on the performance of single-carrier cooperative underwater acoustic communication systems in the presence of intersymbol interference. The authors assume decode-and-forward relaying and consider orthogonal half-duplex cooperation. Under the assumptions of sparse and frequency-selective Rician fading channel and non-white correlated Gaussian ambient noise, the authors derive expressions for bounds on the individual and achievable rates for both cases where channel state information is available at the transmitter or not. Using these expressions, the authors optimise input signalling and relay location to maximise average achievable rates.
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Information theoretical performance analysis and optimization of underwater acoustic communication systems
(2013-08) Nouri, Hatef; Uysal, Murat; Uysal, Murat; Ercan, Ali Özer; İlhan, H.; Department of Electrical and Electronics Engineering; Nouri, Hatef
Underwater wireless communication is a rapidly growing field of research and engineering as its applications, which were once exclusively military, are extending into commercial fields. The need for underwater wireless communications exists in a wide range of applications including offshore oil field exploration/monitoring, oceanographic data collection, maritime archaeology, seismic observation, environmental monitoring, disaster preventing, port and border security among many others. Although capacity calculations for terrestrial radio-frequency channels have been extensively studied, the literature on the capacity of underwater acoustic (UWA) channels is sporadic with many remaining open questions. Aiming to fill research gaps in this growing field, this thesis makes several contributions to the information theoretical performance analysis of point-to-point and relay-assisted UWA systems. A single-carrier communication architecture and sparse Rician frequency-selective UWA channel with intersymbol interference (ISI) is considered in our work. We assume non-white Gaussian distribution to model the ambient noise and consider Francois-Garrison path loss formula to take into account the effects of environmental parameters such as temperature, salinity, pressure as well as system parameters such as distance and frequency. We develop an equivalent channel model for UWA channel with ISI under consideration and show that the capacity of the equivalent channel converges to that of the operating channel in the limit of infinite block length. Using these results, we first obtain a capacity expression for the UWA channel and demonstrate the dependency on channel parameters such as the number and location of significant taps and power delay profile, and environmental parameters such as temperature, salinity, and pressure. Then, we use this expression to determine the optimal carrier frequency, input signaling, and bandwidth. A closed-form formula for the optimum carrier frequency is further obtained. In the second part of the thesis, we extend our results to cooperative UWA systems and obtain achievable rates of single-carrier cooperative UWA systems with orthogonal decode-and-forward (DF) relaying. We take into account the effect of relay geometry in the derivations of achievable rates, and use the derived expressions to optimize the location of the relay.
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Information theoretical performance limits of single-carrier underwater acoustic systems
(IEEE, 2014) Nouri, Hatef; Uysal, Murat; Panayırcı, E.; Senol, H.; Electrical & Electronics Engineering; UYSAL, Murat; Nouri, Hatef
In this study, the authors investigate the information theoretical limits on the performance of point-to-point single-carrier acoustic systems over frequency-selective underwater channels with intersymbol interference. Under the assumptions of sparse and frequency-selective Rician fading channel and non-white correlated Gaussian ambient noise, the authors derive an expression for channel capacity and demonstrate the dependency on channel parameters such as the number, location and power delay profile of significant taps, as well as environmental parameters such as distance, temperature, salinity, pressure and depth. Then, the authors use this expression to determine the optimal carrier frequency, input signalling and bandwidth for capacity maximisation.
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Optical wireless communications – an emerging technology
(IEEE, 2014) Uysal, Murat; Nouri, Hatef; Electrical & Electronics Engineering; UYSAL, Murat; Nouri, Hatef
Optical wireless communication (OWC) enables wireless connectivity using infrared, visible or ultraviolet bands. With its powerful features such as high bandwidth, low cost and operation in an unregulated spectrum, OWC can be, in some applications, a powerful alternative to and, in others, complementary to the existing wireless technologies. It is one of the most promising current areas of research with significant potentials for high-impact results which will considerably change the wireless market mostly dominated by the radio-frequency (RF) technologies. In this paper, we provide an overview of OWC highlighting the advantages and wide range of application areas of this emerging technology.