Conferences Articles

    2019


  1. P. I. Panagoulias, Ioannis D. Moscholios, Panagiotis G. Sarigiannidis, and Michael D. Logothetis, “Performance metrics in OFDM wireless networks with restricted accessibility,” Panhellenic Conference on Electronics and Telecommunications (PACET), Nov. 2019, Volos, Greece.
    Download   |     |   Abstract   |  

    In this paper we study the downlink of an Orthogonal Frequency Division Multiplexing (OFDM) based cell that accommodates calls from different service-classes with different resource requirements. We assume that calls arrive in the system according to a Poisson process while the call admission is based on restricted accessibility. In a restricted accessibility system, a new call may be blocked even if available resources do exist at the time of its arrival. To determine, via recursive formulas, the most important performance metrics, i.e., Call Blocking Probabilities (CBP) and resource utilization in this system, we model the system as a multirate loss model. The accuracy of the proposed formulas is verified via simulation and found to be quite satisfactory.

    @INPROCEEDINGS{C:Sagias_1_2019,
    AUTHOR = {P. I. Panagoulias and Ioannis D. Moscholios and Panagiotis G. Sarigiannidis and Michael D. Logothetis},
    TITLE = {Performance metrics in OFDM wireless networks with restricted accessibility},
    BOOKTITLE = {Panhellenic Conference on Electronics and Telecommunications (PACET)},
    YEAR = {2019},
    month = {Nov.},
    address = {Volos, Greece}
    };

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  2. 2018


  3. Michael C. Batistatos, Georgia E. Athanasiadou, Dimitra A. Zarbouti, George V. Tsoulos, and Nikos C. Sagias, “LTE ground-to-air measurements for UAV-assisted cellular networks,” 12th European Conference on Antennas and Propagation (EuCAP18), Apr. 2018, London, United Kingdom.
    Download   |   DOI: 10.1049/cp.2018.1160   |   Abstract   |  

    The concept of UAV-assisted cellular networks is quite new and has attracted the interest of researchers, and the telecommunications industry. The UAVs act as flying relays offering telecommunication services to underserved areas as well as providing an alternative data gateway in cases of overloaded base stations due to flash crowd situations. This paper provides representative field trial results from our two year measurement campaign with quadcopters, for different operational scenarios. The measured LTE signal strength and signal-to-interference-plus-noise ratio (SINR) in the air are studied in two different cases: a) an underserved rural area and b) the urban center of a town. In the first scenario high SINR values (>10 dB) are measured at certain altitudes allowing the extension of the network coverage. In the urban area, as the UAV ascends additional cells are detected affecting the SINR values, but also providing the means for macrodiversity.

    @INPROCEEDINGS{C:Sagias_2_2018,
    AUTHOR = {Michael C. Batistatos and Georgia E. Athanasiadou and Dimitra A. Zarbouti and George V. Tsoulos and Nikos C. Sagias},
    TITLE = {LTE ground-to-air measurements for UAV-assisted cellular networks},
    BOOKTITLE = {12th European Conference on Antennas and Propagation (EuCAP18)},
    YEAR = {2018},
    month = {Apr.},
    address = {London, United Kingdom},
    doi = {10.1049/cp.2018.1160}
    };

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  4. 2017


  5. Ioannis D. Moscholios, Vassilios G. Vassilakis, Nikos C. Sagias, Panagiotis G. Sarigiannidis, and Michael D. Logothetis, “On the fixed channel reservation policy in LEO mobile satellite systems,” IEICE Information and Communication Technology Forum (ICTF), July 2017, Poznan, Poland.
    Download   |     |   Abstract   |  

    We consider a Low Earth Orbit (LEO) Mobile Satellite System (MSS) with “satellite-fixed” cells that accommodates new and handover Poisson arriving calls of different service-classes. By modelling the LEO-MSS as a multirate loss system, we provide an analytical framework for the recursive calculation of call blocking and handover failure probabilities under the Fixed Channel Reservation (FCR) policy. In the FCR policy, an integer number of channels is reserved in order to benefit calls of high channel requirements. The loss model under the FCR policy does not have a Product Form Solution (PFS) for the steady state probabilities. However, we show that the channel occupancy distribution can be calculated via an approximate but recursive formula. Simulation results verify the accuracy of the proposed formulas.

    @INPROCEEDINGS{C:Sagias_3_2017,
    AUTHOR = {Ioannis D. Moscholios and Vassilios G. Vassilakis and Nikos C. Sagias and Panagiotis G. Sarigiannidis and Michael D. Logothetis},
    TITLE = {On the fixed channel reservation policy in LEO mobile satellite systems},
    BOOKTITLE = {IEICE Information and Communication Technology Forum (ICTF)},
    YEAR = {2017},
    month = {July},
    address = {Poznan, Poland}
    };

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  6. George N. Solidakis, Fanourios M. Tsokas, Michael C. Batistatos, Nikos C. Sagias, George V. Tsoulos, Dimitra A. Zarbouti, and Georgia E. Athanasiadou, “An Arduino-based subsystem for controlling UAVs through GSM,” 6th International Conference on Circuits and Systems Technologies (MOCAST), May 2017, Thessaloniki, Greece.
    Download   |   DOI: 10.1109/MOCAST.2017.7937656   |   Abstract   |  

    Long distance and beyond sight of view communication between a ground control station and an Unmanned Aerial Vehicle (UAV) is mainly achieved using high-gain antennas, antenna trackers or satellites. These implementations demand expensive, heavy and not easily deployed equipment, while in many cases cannot provide a sufficient communication range. The current work proposes and demonstrates a lightweight and low-cost Arduino-based telecommunication subsystem that is capable of sending control commands to the UAV based on GSM or GPRS the most widely deployed cellular networks standard. During field trials control commands successfully transferred from a mobile phone as well as a laptop to the UAV autopilot, with an average time of 2.6 sec and 0.5 sec respectively. The proposed subsystem can be embedded independently or coexist with other control systems, contributing to a ubiquitous UAVs management system.

    @INPROCEEDINGS{C:Sagias_4_2017,
    AUTHOR = {George N. Solidakis and Fanourios M. Tsokas and Michael C. Batistatos and Nikos C. Sagias and George V. Tsoulos and Dimitra A. Zarbouti and Georgia E. Athanasiadou},
    TITLE = {An Arduino-based subsystem for controlling UAVs through GSM},
    BOOKTITLE = {6th International Conference on Circuits and Systems Technologies (MOCAST)},
    YEAR = {2017},
    month = {May},
    address = {Thessaloniki, Greece},
    doi = {10.1109/MOCAST.2017.7937656}
    };

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  7. 2016


  8. Panagiotis K. Argyrakis, Athanasios Ganas, and Nikos C. Sagias, “The NOANET GSAC (Geodesy Seamless Archive Centers) tool for GNSS data dissemination in SE Europe,” 35rd General Assembly of the European Seismological Commission, Sept. 2016, Trieste, Italy.
    Download   |     |   Abstract   |  

    Since 2013, The National Observatory of Athens (NOA) participates in a USA-EU effort to unify access to GNSS data and metadata. NOA deployed a web service created by UNAVCO along with several partners, the US-based geodesy data centers GSAC. GSAC consists of a Mysql Database which is updated and synchronized through smart Python programs with the main data repository and a full implementation JAVA code, which handles the queries from several users and outputs data in desired format (JSON, HTML, CSV, FTP WGET links etc.). The NOANET GSAC offers to the user 365/24/7 stability and capacity and hosts several GNSS data repositories like (NOANET, HEMUS-NET, PPGNet, CRL, NIEP, non permanent networks, etc.

    @INPROCEEDINGS{C:Sagias_5_2016,
    AUTHOR = {Panagiotis K. Argyrakis and Athanasios Ganas and Nikos C. Sagias},
    TITLE = {The NOANET GSAC (Geodesy Seamless Archive Centers) tool for GNSS data dissemination in SE Europe},
    BOOKTITLE = {35rd General Assembly of the European Seismological Commission},
    YEAR = {2016},
    month = {Sept.},
    address = {Trieste, Italy}
    };

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  9. Nikos C. Sagias, Anthony C. Boucouvalas, Kostas Yiannopoulos, Murat Uysal, and Zabih (Fary) Ghassemlooy, “Optimal combiners in pre-amplified optical wireless systems: Operation under strong atmospheric turbulence,” IEICE Information and Communication Technology Forum 2016, July 2016, Patras, Greece.
    Download   |   DOI: 10.34385/proc.24.A5-2   |   Abstract   |  

    In this paper we analytically investigate optimal combiners in pre-amplified diversity receivers that operate under strong atmospheric turbulence. We first demonstrate that the combiner operation is strongly affected by the existence of a signal-dependent noise component, which manifests at the optical detector output as the outcome of the beating process between the amplifier spontaneous noise and the signal itself. Due to the signal-dependent nature of noise, the optimal combiner acts as a hybrid between the well-known equal gain and maximal ratio combiner architectures. Having established the optimal design, we then proceed to assess the proposed combiner performance in the negative-exponential fading environment and demonstrate that it achieves an additional link gain of several dB in comparison with selection and equal gain combiners.

    @INPROCEEDINGS{C:Sagias_6_2016,
    AUTHOR = {Nikos C. Sagias and Anthony C. Boucouvalas and Kostas Yiannopoulos and Murat Uysal and Zabih (Fary) Ghassemlooy},
    TITLE = {Optimal combiners in pre-amplified optical wireless systems: Operation under strong atmospheric turbulence},
    BOOKTITLE = {IEICE Information and Communication Technology Forum 2016},
    YEAR = {2016},
    month = {July},
    address = {Patras, Greece},
    doi = {10.34385/proc.24.A5-2}
    };

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  10. Kostas Yiannopoulos, Nikos C. Sagias, Anthony C. Boucouvalas, Murat Uysal, and Zabih (Fary) Ghassemlooy, “Optimal combiners in optical wireless systems with spatial diversity and pre-amplification,” IEEE International Conference on Communications (ICC16) Workshop, May 2016, pp. 399-404, Kuala Lumpur, Malaysia.
    Download   |   DOI: 10.1109/ICCW.2016.7503820   |   Abstract   |  

    We present analytical results on the operation of a pre-amplified optical wireless communication (OWC) system that also takes advantage of spatial diversity to improve its outage probability and average bit-error-rate (BER) performance against turbulence induced fading. The paper focuses on the design of an optimal electronic combiner that minimizes the instantaneous BER for any given channel state by properly adjusting the gains that are provided to each receiver output after detection. We demonstrate that the optimal combiner performs in a similar manner to an equal gain combiner for a limited number of receiving elements in moderate turbulence, while its potential can be harvested in systems with high diversity factors or under the strong-to-saturated turbulence regime.

    @INPROCEEDINGS{C:Sagias_7_2016,
    AUTHOR = {Kostas Yiannopoulos and Nikos C. Sagias and Anthony C. Boucouvalas and Murat Uysal and Zabih (Fary) Ghassemlooy},
    TITLE = {Optimal combiners in optical wireless systems with spatial diversity and pre-amplification},
    BOOKTITLE = {IEEE International Conference on Communications (ICC16) Workshop},
    YEAR = {2016},
    month = {May},
    pages = {399-404},
    address = {Kuala Lumpur, Malaysia},
    doi = {10.1109/ICCW.2016.7503820}
    };

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  11. 2015


  12. Kostas Yiannopoulos, Nikos C. Sagias, and Anthony C. Boucouvalas, “Semiconductor optical amplifier-assisted optical wireless links: The effect of noise and turbulence,” IEEE International Conference on Communications (ICC15), June 2015, pp. 5913-5918, London, United Kingdom.
    Download   |     |   Abstract   |  

    We present an analytical framework for estimating the benefits that arise from the utilization of semiconductor optical amplifiers (SOA)s at fade impaired outdoor optical wireless systems. We first investigate the impact of fading on both the signal optical power and the amplified spontaneous emission, and derive analytical relations that accurately associate the system bit-error-rate (BER) with the channel state. We then utilize the analytical relations to assess the performance of the amplified optical signal under moderate-to-strong fading and demonstrate that the sensitivity improvement, which is expected by the amplification process, improves the first- and second-order signal-to-noise ratio statistics at the electronic receiver. Our results show that the average BER and average fade duration can all be drastically reduced by the incorporation of the SOA, provided that the link length remains within reasonable limits that are dictated by the channel statistics.

    @INPROCEEDINGS{C:Sagias_8_2015,
    AUTHOR = {Kostas Yiannopoulos and Nikos C. Sagias and Anthony C. Boucouvalas},
    TITLE = {Semiconductor optical amplifier-assisted optical wireless links: The effect of noise and turbulence},
    BOOKTITLE = {IEEE International Conference on Communications (ICC15)},
    YEAR = {2015},
    month = {June},
    pages = {5913-5918},
    address = {London, United Kingdom}
    };

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  13. 2014


  14. Athanasios Ganas, Panagiotis K. Argyrakis, and Nikos C. Sagias, “Progress towards development of the NOANET GNSS early warning web platform: Preliminary results,” International Workshop on Mega Earthquakes and Tsunamis in Subduction Zones: Forecasting Approaches and Implications for Hazard Assessment (METSZ), Oct. 2014, Rhodes, Greece.

    @INPROCEEDINGS{C:Sagias_9_2014,
    AUTHOR = {Athanasios Ganas and Panagiotis K. Argyrakis and Nikos C. Sagias},
    TITLE = {Progress towards development of the NOANET GNSS early warning web platform: Preliminary results},
    BOOKTITLE = {International Workshop on Mega Earthquakes and Tsunamis in Subduction Zones: Forecasting Approaches and Implications for Hazard Assessment (METSZ)},
    YEAR = {2014},
    month = {Oct.},
    address = {Rhodes, Greece}
    };

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  15. Kostas Yiannopoulos, Nikos C. Sagias, and Anthony C. Boucouvalas, “Bit-error-rate performance of semiconductor optical amplifiers in negative exponential fading,” IEEE 19th International Workshop on Computer Aided Modeling and Design of Communication Links and Networks (CAMAD14), Sept. 2014, pp. 168-172, Athens, Greece.
    Download   |   DOI: 10.1109/CAMAD.2014.7033228   |   Abstract   |  

    We present analytical relations for estimating the bit-error-rate performance of semiconductor optical amplifier assisted outdoor optical wireless systems in the presence of negative-exponential fading. The relations are derived by associating the optical signal and noise powers with the channel state and are utilized to assess the system performance in terms of first and order statistics. We demonstrate that both first order (average bit-error-rate and outage probability) and second order (average fade duration) statistics are improved by a significant factor, since the deployment of the amplifier leads to a considerably reduced receiver sensitivity.

    @INPROCEEDINGS{C:Sagias_10_2014,
    AUTHOR = {Kostas Yiannopoulos and Nikos C. Sagias and Anthony C. Boucouvalas},
    TITLE = {Bit-error-rate performance of semiconductor optical amplifiers in negative exponential fading},
    BOOKTITLE = {IEEE 19th International Workshop on Computer Aided Modeling and Design of Communication Links and Networks (CAMAD14)},
    YEAR = {2014},
    month = {Sept.},
    pages = {168-172},
    address = {Athens, Greece},
    doi = {10.1109/CAMAD.2014.7033228}
    };

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  16. Evangelos A. Kosmatos, Nikolaos D. Tselikas, and Nikos C. Sagias, “Minimizing packet loss in high-mobility radio-over-fiber networks,” 9th IEEE/IET International Symposium on Communication Systems, Networks and Digital Signal Processing (CSNDSP14), July 2014, pp. 117-121, Manchester, United Kingdom.
    Download   |   DOI: 10.1109/CSNDSP.2014.6923809   |   Abstract   |  

    The paper analyzes two handoff algorithms, i.e., the traditional handoff (THO) and the moving extended cell (MEC), respectively, both used in Radio-over-Fiber (RoF) networks at 60 GHz. The mathematical analysis and the corresponding simulation study are also presented, showing that MEC is a strong candidate handoff algorithm for packet loss minimization in high-mobility RoF networks at 60 GHz.

    @INPROCEEDINGS{C:Sagias_11_2014,
    AUTHOR = {Evangelos A. Kosmatos and Nikolaos D. Tselikas and Nikos C. Sagias},
    TITLE = {Minimizing packet loss in high-mobility radio-over-fiber networks},
    BOOKTITLE = {9th IEEE/IET International Symposium on Communication Systems, Networks and Digital Signal Processing (CSNDSP14)},
    YEAR = {2014},
    month = {July},
    pages = {117-121},
    address = {Manchester, United Kingdom},
    doi = {10.1109/CSNDSP.2014.6923809}
    };

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  17. Kostas Yiannopoulos, Nikos C. Sagias, and Anthony C. Boucouvalas, “Equalization of negative-exponential fading in saturated semiconductor optical amplifiers,” 9th IEEE/IET International Symposium on Communication Systems, Networks and Digital Signal Processing (CSNDSP14), July 2014, pp. 894-898, Manchester, United Kingdom.
    Download   |   DOI: 10.1109/CSNDSP.2014.6923955   |   Abstract   |  

    In this paper, we discuss the mitigation of negative-exponential fading in Optical Wireless Communication (OWC) systems. The mitigation technique involves the utilization of a semiconductor optical amplifier (SOA) whose gain saturates in normal link operation, but which also provides a power-dependent varying gain when the link experiences a fade. The unbalanced SOA operation serves towards the equalization of the signal power at its output and as a result the link fades become less severe and of reduced duration. Our analytical results predict that the fade probability can be reduced by over 90% under negative-exponential fading conditions, while the scintillation index at the SOA output is also reduced by 75% if an optimal level of received power, and therefore gain saturation, is observed. Finally, our results show that the average duration of fades can also be reduced by a significant percentage of 68% for the same level of gain saturation in the SOA.

    @INPROCEEDINGS{C:Sagias_12_2014,
    AUTHOR = {Kostas Yiannopoulos and Nikos C. Sagias and Anthony C. Boucouvalas},
    TITLE = {Equalization of negative-exponential fading in saturated semiconductor optical amplifiers},
    BOOKTITLE = {9th IEEE/IET International Symposium on Communication Systems, Networks and Digital Signal Processing (CSNDSP14)},
    YEAR = {2014},
    month = {July},
    pages = {894-898},
    address = {Manchester, United Kingdom},
    doi = {10.1109/CSNDSP.2014.6923955}
    };

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  18. Kostas Yiannopoulos, Nikos C. Sagias, and Anthony C. Boucouvalas, “SOA nonlinear amplification: A promising fade mitigation technique for optical wireless systems,” IEEE Wireless Communications and Networking Conference (WCNC14), Apr. 2014, pp. 287-292, Istanbul, Turkey. (PHY and Fundamentals best paper award)
    Download   |   DOI: 10.1109/WCNC.2014.6951982   |   Abstract   |  

    We present a novel fade mitigation technique that is applicable on outdoor optical wireless systems. Our key idea is to utilize the nonlinear power-dependent gain properties of a semiconductor optical amplifies (SOA) to provide unbalanced amplification between faded and non-faded instances of the optical wireless signal. We analytically demonstrate that this power equalization process smoothes out fade-induced power fluctuations and drastically reduces the probability of the system being in a fade state. In medium to strong turbulence governed by gamma-gamma statistics, our results predict that the fade probability can be reduced by over 80% when the SOA is introduced at the optical wireless receiver. We also show that the duration of remaining fades is reduced by a sizeable percentage, and a percentile reduction of the average fade duration of over 85% can be achieved at the SOA output.

    @INPROCEEDINGS{C:Sagias_13_2014,
    AUTHOR = {Kostas Yiannopoulos and Nikos C. Sagias and Anthony C. Boucouvalas},
    TITLE = {SOA nonlinear amplification: A promising fade mitigation technique for optical wireless systems},
    BOOKTITLE = {IEEE Wireless Communications and Networking Conference (WCNC14)},
    YEAR = {2014},
    month = {Apr.},
    pages = {287-292},
    address = {Istanbul, Turkey},
    doi = {10.1109/WCNC.2014.6951982}
    };

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  19. Nikos C. Sagias, Ranjan K. Mallik, and Nikolaos D. Tselikas, “Asymptotic analysis for dual-hop communication networks with PSK and imperfect CSI,” IEEE Wireless Communications and Networking Conference (WCNC14), Apr. 2014, pp. 869-874, Istanbul, Turkey.
    Download   |   DOI: 10.1109/WCNC.2014.6952202   |   Abstract   |  

    We develop an analytical framework for the end-to-end (e2e) asymptotic performance of pilot-symbol assisted Mary phase-shift keying (M-PSK) dual-hop relaying communication networks. The relays use the selective-decode-and-forward protocol and are equipped with multiple receive antennas. Channel estimation per antenna branch is done based on the least-squares estimation technique by means of pilot symbols. Also, maximal-ratio combining and coherent detection are performed at the receiving end. Simple approximate average symbol error probability (ASEP) expressions are obtained for high signal-to-noise ratio (SNR) when M≥2. Our analysis is generic enough to account for any frequency-flat, time-selective, and/or arbitrarily correlated fading channel model per hop. As a case study, we provide e2e M-PSK ASEP expressions considering arbitrarily correlated Nakagami fading channels. Moreover, the optimal power allocation is studied, while the cooperation-gain and diversity-order are extracted. Numerical results are finally presented to verify the accuracy of our asymptotic expressions.

    @INPROCEEDINGS{C:Sagias_14_2014,
    AUTHOR = {Nikos C. Sagias and Ranjan K. Mallik and Nikolaos D. Tselikas},
    TITLE = {Asymptotic analysis for dual-hop communication networks with PSK and imperfect CSI},
    BOOKTITLE = {IEEE Wireless Communications and Networking Conference (WCNC14)},
    YEAR = {2014},
    month = {Apr.},
    pages = {869-874},
    address = {Istanbul, Turkey},
    doi = {10.1109/WCNC.2014.6952202}
    };

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  20. 2013


  21. Anthony C. Boucouvalas, Nikos C. Sagias, and Kostas Yiannopoulos, “First order statistics of semiconductor optical amplifier assisted optical wireless systems under log-normal fading,” 2nd International Workshop on Optical Wireless Communications (IWOW13), Oct. 2013, pp. 142-146, Newcastle Upon Tyne, United Kingdom.
    Download   |   DOI: 10.1109/IWOW.2013.6777795   |   Abstract   |  

    In this paper, we evaluate the utilization of a semiconductor optical amplifier (SOA) at the receiver of an outdoor optical wireless system for fade mitigation purposes. Due to its instantaneous gain response, the SOA provides saturating gain to incoming pulses and partially alleviates power variations between fade impaired and non-impaired instances of the received signal. We discuss this unbalanced SOA operation and analytically derive the first order statistics of the optical signal at the SOA output, giving emphasis on the scintillation index and the fade probability. Our results show that both the scintillation index and the fade probability are improved significantly by deploying the SOA, when the system operates in the weak turbulence regime.

    @INPROCEEDINGS{C:Sagias_15_2013,
    AUTHOR = {Anthony C. Boucouvalas and Nikos C. Sagias and Kostas Yiannopoulos},
    TITLE = {First order statistics of semiconductor optical amplifier assisted optical wireless systems under log-normal fading},
    BOOKTITLE = {2nd International Workshop on Optical Wireless Communications (IWOW13)},
    YEAR = {2013},
    month = {Oct.},
    pages = {142-146},
    address = {Newcastle Upon Tyne, United Kingdom},
    doi = {10.1109/IWOW.2013.6777795}
    };

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  22. 2012


  23. Nikos C. Sagias, Athanasios Ganas, Panagiotis K. Argyrakis, Alexandra Moshou, and George Drakatos, “Geodynamic application of the PPP method using 1-s stream from NOANET stations, Greece,” GFG2: GNSS for Global Environmental Earth Observation (GEEO) and Global Earth Observation System of Systems (GEOSS), Aug. 2012, Nottingham United Kingdom.
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    @INPROCEEDINGS{C:Sagias_16_2012,
    AUTHOR = {Nikos C. Sagias and Athanasios Ganas and Panagiotis K. Argyrakis and Alexandra Moshou and George Drakatos},
    TITLE = {Geodynamic application of the PPP method using 1-s stream from NOANET stations, Greece},
    BOOKTITLE = {GFG2: GNSS for Global Environmental Earth Observation (GEEO) and Global Earth Observation System of Systems (GEOSS)},
    YEAR = {2012},
    month = {Aug.},
    address = {Nottingham United Kingdom}
    };

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  24. 2008


  25. Nikos C. Sagias, George S. Tombras, and Ranjan K. Mallik, “Error rate performance of multilevel signals with coherent detection,” IEEE Global Telecommunications Conference (GLOBECOM08), Dec. 2008, New Orleans USA.
    Download   |   DOI: 10.1109/GLOCOM.2008.ECP.228   |   Abstract   |  

    In this paper, coherent detection for multilevel correlated signaling sets in additive white Gaussian noise is addressed. From the decision rule, a general analytical expression for the symbol error probability (SEP) is derived, which is in the form of a single integral. Our new result is not only in agreement with a known equivalent expression, but it also has a much simpler analytical form. The structure of the correlation matrix under consideration of the signaling set is quite generic, including various known signaling sets as special cases. Hence the derived SEP expression is useful for analyzing the performance of various coherent modulation schemes such as multilevel phase-shift and frequency-shift keying. Specific correlation structures which minimize the SEP are also studied. Based on eigendecomposition or LU decomposition, generic methods for constructing a correlated signaling set for any correlation matrix under consideration are also provided.

    @INPROCEEDINGS{C:Sagias_17_2008,
    AUTHOR = {Nikos C. Sagias and George S. Tombras and Ranjan K. Mallik},
    TITLE = {Error rate performance of multilevel signals with coherent detection},
    BOOKTITLE = {IEEE Global Telecommunications Conference (GLOBECOM08)},
    YEAR = {2008},
    month = {Dec.},
    address = {New Orleans USA},
    doi = {10.1109/GLOCOM.2008.ECP.228}
    };

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  26. Nikos C. Sagias, Kostas Peppas, Fotis I. Lazarakis, Antonis A. Alexandridis, and Kostas P. Dangakis, “Error performance of triple-branch generalized selection diversity over Nakagami fading channels,” International Symposium on Wireless Pervasive Computing (ISWPC08), May 2008, pp. 722-726, Santorini Greece.
    Download   |   DOI: 10.1109/ISWPC.2008.4556304   |   Abstract   |  

    This paper deals with a trivariate Nakagami-m distribution derived from the diagonal elements of a Wishart matrix having an arbitrary covariance matrix and integer-order fading parameters. Based on that distribution, the error rate performance of triple-branch generalized selection combining (GSC) receivers is analyzed, for which, the average bit error probability for a variety of modulation schemes is analytically obtained. The performance of GSC receivers is compared to that of conventional selection and maximal-ratio diversity ones. In order to check the accuracy of the derived formulas, various performance evaluation results are presented and compared to equivalent simulation ones.

    @INPROCEEDINGS{C:Sagias_18_2008,
    AUTHOR = {Nikos C. Sagias and Kostas Peppas and Fotis I. Lazarakis and Antonis A. Alexandridis and Kostas P. Dangakis},
    TITLE = {Error performance of triple-branch generalized selection diversity over Nakagami fading channels},
    BOOKTITLE = {International Symposium on Wireless Pervasive Computing (ISWPC08)},
    YEAR = {2008},
    month = {May},
    pages = {722-726},
    address = {Santorini Greece},
    doi = {10.1109/ISWPC.2008.4556304}
    };

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  27. Zafiro G. Papadimitriou, P. Takis Mathiopoulos, Nikos C. Sagias, and Lazaros Merakos, “On the Weibull distribution with arbitrary correlation,” 3rd International Symposium on Communications, Control, and Signal Processing, (ISCCSP08), Mar. 2008, pp. 588-593, St Julians Malta. (Best paper award)
    Download   |   DOI: 10.1109/ISCCSP.2008.4537293   |   Abstract   |  

    This paper studies the performance of the quadrivariate Weibull distribution with arbitrary correlation assuming non-identical fading parameters and average powers. Novel infinite series representations for the joint probability density function (PDF), the cumulative distribution function (CDF) and the product moments are derived. Using these theoretical results, a performance analysis study in terms of outage probability, for receivers employing selection combining (SC) diversity techniques is presented. The proposed mathematical analysis is accompanied by various numerical results, with parameters of interest the fading severity and the power decay factor. Furthermore, equivalent performance evaluation results, obtained by various computer simulations have verified the accuracy of the proposed analytical method.

    @INPROCEEDINGS{C:Sagias_19_2008,
    AUTHOR = {Zafiro G. Papadimitriou and P. Takis Mathiopoulos and Nikos C. Sagias and Lazaros Merakos},
    TITLE = {On the Weibull distribution with arbitrary correlation},
    BOOKTITLE = {3rd International Symposium on Communications, Control, and Signal Processing, (ISCCSP08)},
    YEAR = {2008},
    month = {Mar.},
    pages = {588-593},
    address = {St Julians Malta},
    doi = {10.1109/ISCCSP.2008.4537293}
    };

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  28. 2007


  29. George C. Alexandropoulos, Nikos C. Sagias, Fotis I. Lazarakis, and Kostas Berberidis, “New results on SC and MRC over Nakagami-m fading channels with arbitrary correlation matrix,” IEEE Global Telecommunications Conference (GLOBECOM07), Nov. 2007, pp. 1551-1555, Washington DC USA.
    Download   |   DOI: 10.1109/GLOCOM.2007.298   |   Abstract   |  

    New results for the multichannel Nakagami-m fading model with an arbitrary correlation matrix are presented. By using an efficient tridiagonalization method based on Householder matrices, a union upper bound for the joint Nakagami-m probability density function and an infinite series representation for the moment generating function of the sum of gamma random variables are derived. Based on the proposed mathematical analysis, a tight union upper bound for the outage probability of multibranch selection diversity, as well as, exact analytical expressions for the outage and average error probability of multibranch maximal-ratio diversity receivers operating over identically distributed and arbitrarily correlated Nakagami-m fading channels are obtained. Our analysis is verified by comparisons of numerically evaluated results with extensive computer simulation ones.

    @INPROCEEDINGS{C:Sagias_20_2007,
    AUTHOR = {George C. Alexandropoulos and Nikos C. Sagias and Fotis I. Lazarakis and Kostas Berberidis},
    TITLE = {New results on SC and MRC over Nakagami-m fading channels with arbitrary correlation matrix},
    BOOKTITLE = {IEEE Global Telecommunications Conference (GLOBECOM07)},
    YEAR = {2007},
    month = {Nov.},
    pages = {1551-1555},
    address = {Washington DC USA},
    doi = {10.1109/GLOCOM.2007.298}
    };

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  30. Nikos C. Sagias, P. Takis Mathiopoulos, and Zafiro G. Papadimitriou, “Simulation of generalized gamma fading channels with finite state Markov chains,” Wireless Rural and Emergency Communications Conference (WRECOM07), Oct. 2007, Rome Italy.
    Download   |     |   Abstract   |  

    An efficient and fast simulator is developed for generating slow generalized Gamma fading channels. The level crossing rates and the cumulative distribution function of the simulation results are shown to be very close to their theoretical values. The Weibull fading channel model is examined as a special case.

    @INPROCEEDINGS{C:Sagias_21_2007,
    AUTHOR = {Nikos C. Sagias and P. Takis Mathiopoulos and Zafiro G. Papadimitriou},
    TITLE = {Simulation of generalized gamma fading channels with finite state Markov chains},
    BOOKTITLE = {Wireless Rural and Emergency Communications Conference (WRECOM07)},
    YEAR = {2007},
    month = {Oct.},
    address = {Rome Italy}
    };

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  31. George C. Alexandropoulos, Kostas Berberidis, Nikos C. Sagias, Fotis I. Lazarakis, Antonis A. Alexandridis, and Kostas P. Dangakis, “Householder-matrices based analysis of SC receivers over Rayleigh fading channels with arbitrary correlation,” IEEE 18th International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC07), Sept. 2007, Athens Greece.
    Download   |   DOI: 10.1109/PIMRC.2007.4394147   |   Abstract   |  

    In this paper, new results for the multivariate Rayleigh distribution with an arbitrary correlation matrix are presented. By using an efficient tridiagonalization method based on Householder matrices, closed-form union upper bounds for its joint probability density and cumulative density functions are derived. Based on the proposed mathematical analysis, a tight union upper bound and an analytical approximation for the outage probability of multibranch selection diversity receivers operating over identically distributed and arbitrarily correlated Rayleigh fading channels are obtained. Our analysis is verified by comparing numerically evaluated results with extensive computer simulation ones.

    @INPROCEEDINGS{C:Sagias_22_2007,
    AUTHOR = {George C. Alexandropoulos and Kostas Berberidis and Nikos C. Sagias and Fotis I. Lazarakis and Antonis A. Alexandridis and Kostas P. Dangakis},
    TITLE = {Householder-matrices based analysis of SC receivers over Rayleigh fading channels with arbitrary correlation},
    BOOKTITLE = {IEEE 18th International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC07)},
    YEAR = {2007},
    month = {Sept.},
    address = {Athens Greece},
    doi = {10.1109/PIMRC.2007.4394147}
    };

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  32. Nikos C. Sagias, Kostas Berberidis, George C. Alexandropoulos, Fotis I. Lazarakis, Christos K. Datsikas, Antonis A. Alexandridis, and Kostas P. Dangakis, “On the sum of squared correlated Rayleigh variates and applications to maximal-ratio diversity,” 18th Annual IEEE International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC07), Sept. 2007, Athens Greece.
    Download   |   DOI: 10.1109/PIMRC.2007.4394161   |   Abstract   |  

    An infinite series representation for the moment generating function of the sum of squared arbitrarily correlated Rayleigh random variables is presented. Based on the derived formula, corresponding analytical expressions for the probability density and cumulative distribution functions are extracted. As an application for the aforementioned sum, exact analytical expressions for the outage and the average error probability, as well as, the channel average spectral efficiency of multibranch maximal-ratio diversity receivers operating over identically distributed and arbitrarily correlated Rayleigh fading channels are obtained. Our analysis is verified by comparing numerically evaluated results with extensive computer simulation ones.

    @INPROCEEDINGS{C:Sagias_23_2007,
    AUTHOR = {Nikos C. Sagias and Kostas Berberidis and George C. Alexandropoulos and Fotis I. Lazarakis and Christos K. Datsikas and Antonis A. Alexandridis and Kostas P. Dangakis},
    TITLE = {On the sum of squared correlated Rayleigh variates and applications to maximal-ratio diversity},
    BOOKTITLE = {18th Annual IEEE International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC07)},
    YEAR = {2007},
    month = {Sept.},
    address = {Athens Greece},
    doi = {10.1109/PIMRC.2007.4394161}
    };

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  33. Nikos C. Sagias, George S. Tombras, George C. Alexandropoulos, Fotis I. Lazarakis, Christos K. Datsikas, Antonis A. Alexandridis, and Kostas P. Dangakis, “Dual-hop relaying networks over Nakagami-m fading channels,” 18th Annual IEEE International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC07), Sept. 2007, Athens Greece.
    Download   |   DOI: 10.1109/PIMRC.2007.4394814   |   Abstract   |  

    In this paper, for an L-relays dual-hop plus a direct link wireless network, in which the decode-and-forward relaying protocol is employed, closed-form expressions for the end-end outage probability are presented. Our analysis considers a Nakagami-m fading environment with either equal or distinct second hops fading parameter to average power ratios. Various numerical examples illustrate the proposed analysis.

    @INPROCEEDINGS{C:Sagias_24_2007,
    AUTHOR = {Nikos C. Sagias and George S. Tombras and George C. Alexandropoulos and Fotis I. Lazarakis and Christos K. Datsikas and Antonis A. Alexandridis and Kostas P. Dangakis},
    TITLE = {Dual-hop relaying networks over Nakagami-m fading channels},
    BOOKTITLE = {18th Annual IEEE International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC07)},
    YEAR = {2007},
    month = {Sept.},
    address = {Athens Greece},
    doi = {10.1109/PIMRC.2007.4394814}
    };

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  34. Nikos C. Sagias, P. Takis Mathiopoulos, Petros S. Bithas, Lazaros Merakos, and Zafiro G. Papadimitriou, “Triple-branch MRC diversity in Weibull fading channels,” 3rd International Workshop on Signal Design and Its Applications in Communications (IWSDA07), Sept. 2007, pp. 247-251, Chengdu, China.
    Download   |   DOI: 10.1109/IWSDA.2007.4408369   |   Abstract   |  

    In this paper a performance analysis of triple-branch maximal ratio combining (MRC) diversity receivers operating over an arbitrarily correlated Weibull fading environment is presented. For the trivariate Weibull distribution infinite series representations for the joint probability density function (PDF) and the cumulative distribution function (CDF) are derived. Moreover, a novel analytical expression for the joint moment-generating function (MGF) is presented. It is assumed that the arbitrarily correlated variates do not necessarily have identical fading parameters and average powers. These series representations are readily applicable to the performance analysis of a triple-branch MRC receiver. Furthermore, the average bit error probability (ABEP) is then derived and analytically studied. The proposed mathematical analysis is accompanied by various numerical results, with parameters of interest the fading severity, the correlation and the power decay factor.

    @INPROCEEDINGS{C:Sagias_25_2007,
    AUTHOR = {Nikos C. Sagias and P. Takis Mathiopoulos and Petros S. Bithas and Lazaros Merakos and Zafiro G. Papadimitriou},
    TITLE = {Triple-branch MRC diversity in Weibull fading channels},
    BOOKTITLE = {3rd International Workshop on Signal Design and Its Applications in Communications (IWSDA07)},
    YEAR = {2007},
    month = {Sept.},
    pages = {247-251},
    address = {Chengdu, China},
    doi = {10.1109/IWSDA.2007.4408369}
    };

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  35. Nikos C. Sagias, George K. Karagiannidis, Theodoros A. Tsiftsis, and Petros S. Bithas, “Products and ratios of two Gaussian class correlated Weibull rvs,” 12th International Conference on Applied Stochastic Models and Data Analysis (ASMDA07), June 2007, Creta Greece.
    Download   |     |   Abstract   |  

    Starting from on a recently introduced Gaussian class bivariate Weibull stochastic model, the probability density and the cumulative distribution functions of the product (Z1 Z2)c and the ratio (Z1/Z2)c, when Z1 and Z2 are correlated Weibull random variables belonging to this class (c > 0), are derived in closed form. Moreover, using the inequality between arithmetic and geometric mean, a union upper bound for the distribution of the sum of two correlated Weibull variates Z1c + Z2c is also presented. Special cases of our results are in agreement with previously published ones. The proposed analysis is useful in several scientific fields of engineering.

    @INPROCEEDINGS{C:Sagias_26_2007,
    AUTHOR = {Nikos C. Sagias and George K. Karagiannidis and Theodoros A. Tsiftsis and Petros S. Bithas},
    TITLE = {Products and ratios of two Gaussian class correlated Weibull rvs},
    BOOKTITLE = {12th International Conference on Applied Stochastic Models and Data Analysis (ASMDA07)},
    YEAR = {2007},
    month = {June},
    address = {Creta Greece}
    };

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  36. Petros S. Bithas, Nikos C. Sagias, Theodoros A. Tsiftsis, and George K. Karagiannidis, “Distributions involving correlated generalized Gamma variables,” 12th International Conference on Applied Stochastic Models and Data Analysis (ASMDA07), June 2007, Creta Greece.
    Download   |     |   Abstract   |  

    In this paper, some of the most important statistical properties concerning the product and the ratio of two correlated generalized Gamma (GG) random variables (rvs) are studied. The probability density function of both the product and the ratio of two correlated GG rvs are obtained in closed form, while the cumulative distribution function of the product is derived in terms of an infinite series. Capitalizing on the distribution of the product, a union upper bound for the distribution of the sum of two correlated GG rvs is also derived.

    @INPROCEEDINGS{C:Sagias_27_2007,
    AUTHOR = {Petros S. Bithas and Nikos C. Sagias and Theodoros A. Tsiftsis and George K. Karagiannidis},
    TITLE = {Distributions involving correlated generalized Gamma variables},
    BOOKTITLE = {12th International Conference on Applied Stochastic Models and Data Analysis (ASMDA07)},
    YEAR = {2007},
    month = {June},
    address = {Creta Greece}
    };

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  37. 2006


  38. Nikos C. Sagias, George K. Karagiannidis, Stavros A. Kotsopoulos, and Theodoros A. Tsiftsis, “Performance of MRC diversity receivers over correlated Nakagami-m fading channels,” 5th International Symposium of Communication Systems, Networks and Digital Signal Processing (CSNDSP06), July 2006, Patras Greece.
    Download   |     |   Abstract   |  

    ABSTRACT We present exact closed-form expressions for the statistics of the sum of non-identical squared Nakagami-m random variables and it is shown that it can be written as a weighted sum of Erlang distributions. The analysis includes both independent and correlated cases with distinct average powers and integer-order fading parameters. The proposed formulation significantly improves previously published results which are in the form of infinite sums or higher order derivatives. The obtained formulae can be applied on the performance analysis of maximal-ratio combining diversity receivers operating over Nakagami-m fading channels.

    @INPROCEEDINGS{C:Sagias_28_2006,
    AUTHOR = {Nikos C. Sagias and George K. Karagiannidis and Stavros A. Kotsopoulos and Theodoros A. Tsiftsis},
    TITLE = {Performance of MRC diversity receivers over correlated Nakagami-m fading channels},
    BOOKTITLE = {5th International Symposium of Communication Systems, Networks and Digital Signal Processing (CSNDSP06)},
    YEAR = {2006},
    month = {July},
    address = {Patras Greece}
    };

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  39. Nikos C. Sagias, George S. Tombras, George K. Karagiannidis, and Theodoros A. Tsiftsis, “Equal-gain combining receivers over interference-limited Nakagami-m fading with multiple cochannel interferers,” National Conference on Communications (NCC06), Jan. 2006, New Delhi India.
    Download   |     |   Abstract   |  

    This paper deals with the performance of predetection equal-gain combining (EGC) receivers operating over an interference-limited fading environment with cochannel interference (CCI). It is considered that the desired components of the received signals experience independent, but not necessarily identically distributed, Nakagami-m fading, while the interferers are subjected to independent Rayleigh fading. The analysis is not only limited to identically distributed interferers, but rather includes the case of interferers with distinct average powers. Following the coherent interference power calculation and by using a useful lower-bound for the distribution of the sum of Nakagami-m fading envelopes, novel closed-form upper-bounds for the outage and average symbol error probability for several modulation schemes are presented. Moreover, lower-bounds for the Shannon average spectral efficiency are derived in closed form. Numerical results demonstrate the effect of the number of the interferers, the number of the receiver branches, and the severity of the fading on the EGC performance. Computer simulations are also performed to verify the tightness and the correctness of the proposed mathematical analysis.

    @INPROCEEDINGS{C:Sagias_29_2006,
    AUTHOR = {Nikos C. Sagias and George S. Tombras and George K. Karagiannidis and Theodoros A. Tsiftsis},
    TITLE = {Equal-gain combining receivers over interference-limited Nakagami-m fading with multiple cochannel interferers},
    BOOKTITLE = {National Conference on Communications (NCC06)},
    YEAR = {2006},
    month = {Jan.},
    address = {New Delhi India}
    };

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  40. Nikos C. Sagias, George K. Karagiannidis, Theodoros A. Tsiftsis, and Harilaos G. Sandalidis, “Multihop free-space optical communications over strong turbulence channels,” IEEE International Conference on Communications (ICC06), vol. 6, Jan. 2006, pp. 2755-2759, Istanbul Turkey.
    Download   |   DOI: 10.1109/ICC.2006.255196   |   Abstract   |  

    In this paper, we study the performance of multihop free-space optical (FSO) wireless systems over turbulence-induced fading channels. The analysis is carried out for systems employing amplify-and-forward (AF) or decode-and-forward (DF) relays and for turbulence channels which can be modeled by the Gamma-Gamma distribution. An exact analytical expression for the end-to-end outage probability of AF systems is obtained, while a closed-form expression of DF systems is derived. The average bit-error probability of a dual-hop FSO system employing a DF relay is studied as a special case. Numerical examples are also presented to illustrate the proposed analysis and to further investigate the effects of the turbulence severity on the multihop FSO systems' performance.

    @INPROCEEDINGS{C:Sagias_30_2006,
    AUTHOR = {Nikos C. Sagias and George K. Karagiannidis and Theodoros A. Tsiftsis and Harilaos G. Sandalidis},
    TITLE = {Multihop free-space optical communications over strong turbulence channels},
    BOOKTITLE = {IEEE International Conference on Communications (ICC06)},
    YEAR = {2006},
    month = {Jan.},
    volume = {6},
    pages = {2755-2759},
    address = {Istanbul Turkey},
    doi = {10.1109/ICC.2006.255196}
    };

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  41. Nikos C. Sagias, P. Takis Mathiopoulos, Petros S. Bithas, Lazaros Merakos, and Zafiro G. Papadimitriou, “The trivariate Weibull distribution with arbitrary correlation,” International Workshop on Satellite and Space Communications (IWSSC06), Jan. 2006, pp. 249-253, Madrid Spain.
    Download   |   DOI: 10.1109/IWSSC.2006.256036   |   Abstract   |  

    In this paper an analytical framework for analyzing the arbitrarily correlated trivariate Weibull distribution is introduced. For this distribution infinite series representations for the joint probability density function, the cumulative distribution function (CDF) and the moments are derived. Two special correlation cases of the distribution are studied: the exponential and the constant. These series representations are readily applicable to the performance analysis of a 3-branch selection combining (SC) receiver operating in a Weibull correlation fading environment and the outage probability is derived. The proposed mathematical analysis is complemented by various numerical results, showing the effects of fading severity, correlation and the power decay factor

    @INPROCEEDINGS{C:Sagias_31_2006,
    AUTHOR = {Nikos C. Sagias and P. Takis Mathiopoulos and Petros S. Bithas and Lazaros Merakos and Zafiro G. Papadimitriou},
    TITLE = {The trivariate Weibull distribution with arbitrary correlation},
    BOOKTITLE = {International Workshop on Satellite and Space Communications (IWSSC06)},
    YEAR = {2006},
    month = {Jan.},
    pages = {249-253},
    address = {Madrid Spain},
    doi = {10.1109/IWSSC.2006.256036}
    };

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  42. 2005


  43. Petros S. Bithas, George K. Karagiannidis, Nikos C. Sagias, P. Takis Mathiopoulos, and Dimitris A. Zogas, “Dual-branch diversity receivers in correlative Rice fading,” IEEE Vehicular Technology Conference (VTC05), vol. 4, Sept. 2005, pp. 2642-2646, Dallas Texas USA.
    Download   |   DOI: 10.1109/VETECF.2005.1559028   |   Abstract   |  

    In this paper the performance of dual-branch diversity receivers operating over correlated Rician fading channels is analyzed and novel performance evaluation results are presented. By representing the bivariate Rician probability density function of the signal-to-noise ratio (SNR) as a rapidly converging infinite sum, useful analytical expressions for the performance of dual-branch selection combining and maximal ratio combining receivers are derived. Capitalizing on these infinite series representations, novel analytical formulae for the average bit-error and outage probability are obtained. The proposed analysis is used to obtain various novel performance evaluation results having as parameters of interest fading severity, average SNR and Rician correlation coefficient. The series convergence rate is also studied verifying the fast convergence of the analytical expressions. The accuracy of most of the theoretical results have been verified by means of computer simulation.

    @INPROCEEDINGS{C:Sagias_32_2005,
    AUTHOR = {Petros S. Bithas and George K. Karagiannidis and Nikos C. Sagias and P. Takis Mathiopoulos and Dimitris A. Zogas},
    TITLE = {Dual-branch diversity receivers in correlative Rice fading},
    BOOKTITLE = {IEEE Vehicular Technology Conference (VTC05)},
    YEAR = {2005},
    month = {Sept.},
    volume = {4},
    pages = {2642-2646},
    address = {Dallas Texas USA},
    doi = {10.1109/VETECF.2005.1559028}
    };

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  44. Nikos C. Sagias, George K. Karagiannidis, Petros S. Bithas, and P. Takis Mathiopoulos, “On the correlated Weibull fading model and its applications,” IEEE Vehicular Technology Conference (VTC05), vol. 4, Sept. 2005, pp. 2149-2153, Dallas Texas USA.
    Download   |   DOI: 10.1109/VETECF.2005.1558500   |   Abstract   |  

    Ascertaining on the suitability of the Weibull model to describe fading channels, a theoretical framework for the multivariate Weibull distribution, generated from correlated Gaussian elements, is presented. Concerning the bivariate Weibull model with arbitrary average fading powers, novel closed-form expressions for the joint probability density function (pdf), moments-generating function, cumulative distribution function (cdf), product moments, and the correlation coefficient are presented. Moreover, useful analytical formulae for the pdf and cdf of the multivariate Weibull distribution, with identical average fading powers and exponential correlation, are obtained. The derived theoretical results are applied to analytically evaluate the outage probability of selection diversity receivers, operating over correlated Weibull fading channels.

    @INPROCEEDINGS{C:Sagias_33_2005,
    AUTHOR = {Nikos C. Sagias and George K. Karagiannidis and Petros S. Bithas and P. Takis Mathiopoulos},
    TITLE = {On the correlated Weibull fading model and its applications},
    BOOKTITLE = {IEEE Vehicular Technology Conference (VTC05)},
    YEAR = {2005},
    month = {Sept.},
    volume = {4},
    pages = {2149-2153},
    address = {Dallas Texas USA},
    doi = {10.1109/VETECF.2005.1558500}
    };

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  45. George K. Karagiannidis, Theodoros A. Tsiftsis, Ranjan K. Mallik, Nikos C. Sagias, and Stavros A. Kotsopoulos, “Closed-form bounds for multihop relayed communications in Nakagami-m fading,” IEEE International Conference on Communications (ICC05), vol. 4, May 2005, pp. 2362-2366, Seoul Korea.
    Download   |   DOI: 10.1109/ICC.2005.1494758   |   Abstract   |  

    We present closed-form bounds for the performance of multihop transmissions with non-regenerative relays over Nakagami-m fading channels. The end-to-end signal-to-noise ratio (SNR) is formulated and upper bounded by using the inequality between harmonic and geometric mean of positive random variables (RVs). Novel closed-form expressions are derived for the moment generating function, the probability density function, and the cumulative distribution function of the product of arbitrary powers of statistically independent Gamma RVs in terms of the Meijer's G-function. Using these theoretical results, closed-form lower bounds are obtained for the outage and average bit error probability of phase or frequency modulated signallings, while simple asymptotic expressions are also given for the bounds at high SNRs. Numerical results are compared to computer simulations, to show the tightness of the proposed bounds.

    @INPROCEEDINGS{C:Sagias_34_2005,
    AUTHOR = {George K. Karagiannidis and Theodoros A. Tsiftsis and Ranjan K. Mallik and Nikos C. Sagias and Stavros A. Kotsopoulos},
    TITLE = {Closed-form bounds for multihop relayed communications in Nakagami-m fading},
    BOOKTITLE = {IEEE International Conference on Communications (ICC05)},
    YEAR = {2005},
    month = {May},
    volume = {4},
    pages = {2362-2366},
    address = {Seoul Korea},
    doi = {10.1109/ICC.2005.1494758}
    };

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  46. Petros S. Bithas, Nikos C. Sagias, P. Takis Mathiopoulos, George K. Karagiannidis, and Athanasios A. Rontogiannis, “Digital communications over generalized-K fading channels,” IEEE International Symposium on Wireless Communication Systems (ISWCS05), Jan. 2005, pp. 684-687, Sienna Italy.
    Download   |   DOI: 10.1109/LCOMM.2006.1633320   |   Abstract   |  

    The performance of digital communication systems over generalized-K (KG) fading channels is analyzed and evaluated. Novel closed form expressions for the SNR statistics, the average Shannon's channel capacity and the bit error rate (BER) are derived. These expressions are used to study important performance criteria such as the outage performance, the average capacity and the BER for a great variety of modulation formats in KG fading channels. The proposed mathematical analysis is accompanied with various performance evaluation results, which demonstrate the usefulness of the proposed approach.

    @INPROCEEDINGS{C:Sagias_35_2005,
    AUTHOR = {Petros S. Bithas and Nikos C. Sagias and P. Takis Mathiopoulos and George K. Karagiannidis and Athanasios A. Rontogiannis},
    TITLE = {Digital communications over generalized-K fading channels},
    BOOKTITLE = {IEEE International Symposium on Wireless Communication Systems (ISWCS05)},
    YEAR = {2005},
    month = {Jan.},
    pages = {684-687},
    address = {Sienna Italy},
    doi = {10.1109/LCOMM.2006.1633320}
    };

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  47. George K. Karagiannidis, Nikos C. Sagias, and P. Takis Mathiopoulos, “The N*Nakagami fading channel model,” IEEE International Symposium on Wireless Communication Systems (ISWCS05), Jan. 2005, pp. 185-189, Sienna Italy.
    Download   |   DOI: 10.1109/ISWCS.2005.1547683   |   Abstract   |  

    A generic distribution, referred to as N*Nakagami, constructed as the product of N statistically independent, but not necessarily identically distributed, Nakagami-m random variables (RV)s, is introduced and analyzed. The proposed distribution turns out to be an extremely convenient tool for analyzing the performance of digital communication systems over generalized fading channels. The main result contributions of the paper are two-fold. Firstly, the moments-generating function (MGF), probability density function, cumulative distribution function (CDF), and moments of the N*Nakagami distribution are derived in closed-form. Using these formulae, generic closed-form expressions for the outage probability, amount of fading, and average symbol error probability for several binary and multilevel modulation signals of digital communication systems operating over the N*Nakagami fading channel model are presented. Various numerical and computer simulation results verify the correctness of the proposed formulation. Secondly, the suitability of the N*Nakagami fading distribution to approximate the lognormal distribution is being investigated. Using Kolmogorov-Smirnov tests, the rate of convergence of the central limit theorem as pertaining to the multiplication of RVs is quantified.

    @INPROCEEDINGS{C:Sagias_36_2005,
    AUTHOR = {George K. Karagiannidis and Nikos C. Sagias and P. Takis Mathiopoulos},
    TITLE = {The N*Nakagami fading channel model},
    BOOKTITLE = {IEEE International Symposium on Wireless Communication Systems (ISWCS05)},
    YEAR = {2005},
    month = {Jan.},
    pages = {185-189},
    address = {Sienna Italy},
    doi = {10.1109/ISWCS.2005.1547683}
    };

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  48. Nikos C. Sagias, George K. Karagiannidis, P. Takis Mathiopoulos, and Petros S. Bithas, “On the distribution of the sum of generalized Gamma variates and applications to satellite digital communications,” IEEE International Symposium on Wireless Communication Systems (ISWCS05), Jan. 2005, pp. 785-789, Sienna Italy.
    Download   |   DOI: 0.1109/ISWCS.2005.1547816   |   Abstract   |  

    A versatile envelope distribution which may be useful for both terrestrial and satellite fading channel modelling is the generalized Gamma (GG) distribution. By considering the product of N GG random variables (RV)s, useful expressions for its moments-generating and cumulative distribution functions are obtained in closed-form. These expressions are used to derive closed-form union upper-bound for the distribution of the sum of GG distributed RVs. The proposed bound turns out to be an extremely convenient analytical tool for studying the performance of N-branch equal-gain combining receivers operating over GG fading channels. For such receivers, novel union upper-bounds for the outage and the average bit error probability are derived and evaluated in terms of the Meijer's G-function. The tightness of the proposed bounds is verified by performing comparisons between numerical evaluation and computer simulations results.

    @INPROCEEDINGS{C:Sagias_37_2005,
    AUTHOR = {Nikos C. Sagias and George K. Karagiannidis and P. Takis Mathiopoulos and Petros S. Bithas},
    TITLE = {On the distribution of the sum of generalized Gamma variates and applications to satellite digital communications},
    BOOKTITLE = {IEEE International Symposium on Wireless Communication Systems (ISWCS05)},
    YEAR = {2005},
    month = {Jan.},
    pages = {785-789},
    address = {Sienna Italy},
    doi = {0.1109/ISWCS.2005.1547816}
    };

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    unavailable
  49. 2004


  50. George K. Karagiannidis, Dimitris A. Zogas, Nikos C. Sagias, Theodoros A. Tsiftsis, and P. Takis Mathiopoulos, “Multihop communications with fixed-gain relays over generalized fading channels,” IEEE Global Telecommunications Conference (GLOBECOM04), vol. 1, Dec. 2004, pp. 36-40, Dallas Texas USA.
    Download   |   DOI: 10.1109/GLOCOM.2004.1377908   |   Abstract   |  

    Efficient performance bounds for multihop wireless communications systems with non-regenerative fixed-gain relays operating over non-identical generalized fading channels, are presented. More specifically, the end-to-end signal-to-noise ratio (SNR) is formulated and upper bounded by using the well-known inequality between harmonic and geometric mean of positive random variables. Based on this bound, the moments of the end-to-end SNR for Rayleigh, Nakagami-m, and Rice fading channels, are obtained in simple closed-forms. Furthermore, the outage performance and the average error probability for coherent and non-coherent modulation schemes are also studied using the moment-generating function (MGF) approach. The proposed method for the evaluation of the MGF is based on the Pade approximants theory. Moreover, new expressions are derived for the gain of previously proposed "semi-blind" relays. These expressions are used in numerical and computer simulations examples, to verify the accuracy and to show the tightness of the proposed bounds.

    @INPROCEEDINGS{C:Sagias_38_2004,
    AUTHOR = {George K. Karagiannidis and Dimitris A. Zogas and Nikos C. Sagias and Theodoros A. Tsiftsis and P. Takis Mathiopoulos},
    TITLE = {Multihop communications with fixed-gain relays over generalized fading channels},
    BOOKTITLE = {IEEE Global Telecommunications Conference (GLOBECOM04)},
    YEAR = {2004},
    month = {Dec.},
    volume = {1},
    pages = {36-40},
    address = {Dallas Texas USA},
    doi = {10.1109/GLOCOM.2004.1377908}
    };

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  51. Nikos C. Sagias, George K. Karagiannidis, Dimitris A. Zogas, and P. Takis Mathiopoulos, “Selection diversity for wireless communications with non-identical Weibull statistics,” IEEE Global Telecommunications Conference (GLOBECOM04), vol. 6, Dec. 2004, pp. 3690-3694, Dallas Texas USA.
    Download   |   DOI: 10.1109/GLOCOM.2004.1379058   |   Abstract   |  

    Motivated by the suitability of the Weibull distribution to model multipath fading channels, the performance of L-branch selection combining (SC) receivers, with non-identical statistics, is presented. Deriving a useful expression for the probability density function of the SC output signal-to-noise ratio (SNR), important performance metrics are studied. More specifically, a closed-form expression is derived for the moments of the combiner output SNR, which is used to study the average output SNR and the amount of fading. The average symbol error probability for several coherent and non-coherent, binary and multilevel modulation schemes is also obtained in closed-form.

    @INPROCEEDINGS{C:Sagias_39_2004,
    AUTHOR = {Nikos C. Sagias and George K. Karagiannidis and Dimitris A. Zogas and P. Takis Mathiopoulos},
    TITLE = {Selection diversity for wireless communications with non-identical Weibull statistics},
    BOOKTITLE = {IEEE Global Telecommunications Conference (GLOBECOM04)},
    YEAR = {2004},
    month = {Dec.},
    volume = {6},
    pages = {3690-3694},
    address = {Dallas Texas USA},
    doi = {10.1109/GLOCOM.2004.1379058}
    };

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  52. Nikos C. Sagias, George K. Karagiannidis, Dimitris A. Zogas, P. Takis Mathiopoulos, George S. Tombras, and Fotini-Niovi Pavlidou, “Second order statistics and spectral efficiency for selection diversity receivers in Weibull fading,” IEEE International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC04), vol. 3, Sept. 2004, pp. 2140-2145, Barcelona Spain.
    Download   |   DOI: 10.1109/PIMRC.2004.1368376   |   Abstract   |  

    Motivated by the suitability of the Weibull distribution to model multipath fading channels, the second order statistics and the spectral efficiency (SE) of L-branch SC receivers are studied. Deriving a novel closed-form expression for the probability density function (pdf) of the SC output SNR, the average level crossing rate (LCR), the average fade duration (AFD) and the Shannon's average SE, at the output of the SC, are derived in closed-forms. Our results are sufficiently general to handle arbitrary fading parameter and dissimilar branch powers.

    @INPROCEEDINGS{C:Sagias_40_2004,
    AUTHOR = {Nikos C. Sagias and George K. Karagiannidis and Dimitris A. Zogas and P. Takis Mathiopoulos and George S. Tombras and Fotini-Niovi Pavlidou},
    TITLE = {Second order statistics and spectral efficiency for selection diversity receivers in Weibull fading},
    BOOKTITLE = {IEEE International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC04)},
    YEAR = {2004},
    month = {Sept.},
    volume = {3},
    pages = {2140-2145},
    address = {Barcelona Spain},
    doi = {10.1109/PIMRC.2004.1368376}
    };

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  53. Dimitris A. Zogas, George K. Karagiannidis, Nikos C. Sagias, Theodoros A. Tsiftsis, P. Takis Mathiopoulos, and Stavros A. Kotsopoulos, “Dual hop wireless communications over Nakagami fading,” IEEE Vehicular Technology Conference (VTC04), vol. 4, May 2004, pp. 2200-2204, Milan Italy.
    Download   |   DOI: 10.1109/VETECS.2004.1390664   |   Abstract   |  

    We study the performance of dual-hop wireless communications systems with non-regenerative relays over Nakagami-m fading channels. Deriving tight upper bounds for the end-to-end SNR, important performance criteria such as average end-to-end SNR and error probability are studied. Numerical and computer simulations results are presented to verify the accuracy of the proposed mathematical analysis and to compare our results with previously published ones.

    @INPROCEEDINGS{C:Sagias_41_2004,
    AUTHOR = {Dimitris A. Zogas and George K. Karagiannidis and Nikos C. Sagias and Theodoros A. Tsiftsis and P. Takis Mathiopoulos and Stavros A. Kotsopoulos},
    TITLE = {Dual hop wireless communications over Nakagami fading},
    BOOKTITLE = {IEEE Vehicular Technology Conference (VTC04)},
    YEAR = {2004},
    month = {May},
    volume = {4},
    pages = {2200-2204},
    address = {Milan Italy},
    doi = {10.1109/VETECS.2004.1390664}
    };

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  54. Nikos C. Sagias, George K. Karagiannidis, Dimitris A. Zogas, P. Takis Mathiopoulos, Stavros A. Kotsopoulos, and George S. Tombras, “Performance of diversity receivers over non-identical Weibull fading channels,” IEEE Vehicular Technology Conference (VTC04), vol. 1, May 2004, pp. 480-484, Milan Italy.
    Download   |   DOI: 10.1109/VETECS.2004.1387999   |   Abstract   |  

    The performance of L-branch equal-gain combining (EGC) and maximal-ratio combining (MRC) receivers operating over correlative, non-identical Weibull fading channels is studied. Closed-form expressions are derived for the moments of the signal-to-noise ratio (SNR) at the output of the combiner and important performance criteria, such as average output SNR, amount of fading (AoF) and spectral efficiency (SE) at the low power regime, are extracted. We also evaluate the outage and average symbol error probability (ASEP) for several coherent and non-coherent modulation schemes, using a closed-form expression for the moment generating function (mgf) of the output SNR for MRC receivers and the Pade approximation to the mgf for EGC receivers. The proposed mathematical analysis is complimented by various numerical and computer simulations results, which point out the effects of fading severity and correlation on the overall system performance.

    @INPROCEEDINGS{C:Sagias_42_2004,
    AUTHOR = {Nikos C. Sagias and George K. Karagiannidis and Dimitris A. Zogas and P. Takis Mathiopoulos and Stavros A. Kotsopoulos and George S. Tombras},
    TITLE = {Performance of diversity receivers over non-identical Weibull fading channels},
    BOOKTITLE = {IEEE Vehicular Technology Conference (VTC04)},
    YEAR = {2004},
    month = {May},
    volume = {1},
    pages = {480-484},
    address = {Milan Italy},
    doi = {10.1109/VETECS.2004.1387999}
    };

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  55. Dimitris A. Zogas, George K. Karagiannidis, Nikos C. Sagias, P. Takis Mathiopoulos, Stavros A. Kotsopoulos, and George S. Tombras, “Dual selection diversity over correlated Weibull fading channels,” IEEE International Conference on Communications (ICC04), vol. 6, Jan. 2004, pp. 3384-3388, Paris, France.
    Download   |   DOI: 10.1109/ICC.2004.1313172   |   Abstract   |  

    The performance of dual-branch SC receivers over correlated Weibull fading channels is studied in this paper. Exact closed-form expressions are derived for the probability/cumulative density functions and the moments of the output signal-to-noise ratio (SNR). Important performance criteria, such as average output SNR, amount of fading (AoF), outage probability and average bit error probability (ABEP) for several modulation schemes, are studied and novel closed-form analytical expressions are derived. The proposed analysis is complemented by various performance evaluation results, including the effects of input SNRs unbalancing, fading severity and fading correlation on the overall system performance. Computer simulations results verify the validity and the accuracy of the proposed analysis.

    @INPROCEEDINGS{C:Sagias_43_2004,
    AUTHOR = {Dimitris A. Zogas and George K. Karagiannidis and Nikos C. Sagias and P. Takis Mathiopoulos and Stavros A. Kotsopoulos and George S. Tombras},
    TITLE = {Dual selection diversity over correlated Weibull fading channels},
    BOOKTITLE = {IEEE International Conference on Communications (ICC04)},
    YEAR = {2004},
    month = {Jan.},
    volume = {6},
    pages = {3384-3388},
    address = {Paris, France},
    doi = {10.1109/ICC.2004.1313172}
    };

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  56. 2001


  57. Nikos C. Sagias, Apostolos Papathanassiou, P. Takis Mathiopoulos, and George S. Tombras, “Burst timing synchronization for OFDM-based wideband mobile LEO and MEO satellite systems,” 7th International Workshop on DSP Techniques for Space Communications, Oct. 2001, Sesimbra Portugal.
    Download   |     |   Abstract   |  

    A burst timing offset estimation method is examined in fast time varying frequency selective fading channels for broadband orthogonal frequency division multiplexing (OFDM)–based systems. The estimation is obtained in the presence of Gaussian noise and Rayleigh or Rice multipath fading. Two formats for the OFDM training symbol and the corresponding timing metrics are proposed and compared with respect to their estimation performance and improvement is succeeded. The performance is evaluated by computer simulation in terms of estimation variance. The synchronization in the OFDM system is examined by setting as parameters the number of subcarriers, the signal–to–noise ratio (SNR) and the high Doppler shift present in LEO/MEO satellite communications. II. INTRODUCTION The utilization of satellite networks based on low earth orbit (LEO) and medium earth orbit (MEO) satellites constitutes one of the most promising technological solutions to attain reliable and economically efficient mobile and personal telecommunications on a global scale. LEO and MEO satellites, while maintaining the important characteristics of geostationary earth orbit (GEO) satellites, such as wide coverage area and direct radio path, feature a number of additional advantages that are essential to the provision of mobile multimedia services. These advantages of LEO and MEO satellites, which originate from their lower altitude as compared to GEO satellites, include the short propagation delay, the low propagation loss and the high elevation angle at high latitudes. A direct consequence of these advantages is that the tight delay requirements of multimedia applications are more easily met. In addition, low-power hand-held terminals are less demanding and more cost effective. Therefore, LEO and MEO satellite based communications systems are foreseen to play a significant role as they can naturally provide cost-effective access to the global multimedia infrastructure in cases where cable network cannot be established, e.g., in aeronautical and navigational applications and in areas where their terrestrial counterparts require high deployment cost and, in general, inefficient utilization [9]. In LEO and MEO satellite based mobile wideband communications systems, except for using code division multiple access (CDMA), which would constitute a natural extension of the terrestrial air interface employed in third generation (3G) terrestrial mobile communications systems, the use of OFDM has recently attracted considerable attention as an alternative to CDMA. However, the use of OFDM as a candidate modulation technique for LEO and MEO satellite communications systems has not yet been thoroughly investigated.

    @INPROCEEDINGS{C:Sagias_44_2001,
    AUTHOR = {Nikos C. Sagias and Apostolos Papathanassiou and P. Takis Mathiopoulos and George S. Tombras},
    TITLE = {Burst timing synchronization for OFDM-based wideband mobile LEO and MEO satellite systems},
    BOOKTITLE = {7th International Workshop on DSP Techniques for Space Communications},
    YEAR = {2001},
    month = {Oct.},
    address = {Sesimbra Portugal}
    };

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Conference Papers Distribution

The following pie presents the distribution of my papers in the most prestigious IEEE conferences.