Journals Articles

    2019


  1. Kostas Yiannopoulos, Nikos C. Sagias, and Anthony C. Boucouvalas, “On the photon counting error probability and its application in optical wireless communications,” Physical Communication, vol. 39, Oct. 2019.
    Download   |   DOI: 10.1016/j.phycom.2019.100756   |   Abstract   |  

    We present an efficient approach for the probability of error (PER) of photon counting statistics in a pre-amplified optical receiver. Our approach enables a fast and highly accurate calculation of the instantaneous PER, as well as the average PER in optical wireless communications. By utilizing the proposed approximation, we assess the performance of an equal gain combiner (EGC) and a selection combiner (SC) in negative-exponential and γ-γ fading. The results show that the EGC provides a significant link gain under all fading conditions, while the SC is only viable in strong turbulence.

    @ARTICLE{J:Sagias_1_2019,
    AUTHOR = {Kostas Yiannopoulos and Nikos C. Sagias and Anthony C. Boucouvalas},
    TITLE = {On the photon counting error probability and its application in optical wireless communications},
    JOURNAL = {Physical Communication},
    YEAR = {2019},
    month = {Oct.},
    volume = {39},
    doi = {10.1016/j.phycom.2019.100756}
    };

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  2. Georgia P. Karatza, Kostas Peppas, Nikos C. Sagias, and George V. Tsoulos, “Unified ergodic capacity expressions for AF dual-hop systems with hardware impairments,” IEEE Communications Letters, vol. 23, no. 6, pp. 1057-1060, June 2019.
    Download   |   DOI: 10.1109/LCOMM.2019.2908902   |   Abstract   |  

    This letter presents novel analytical expressions for the ergodic capacity (EC) of dual-hop amplify-and-forward (AF) relaying systems subject to hardware impairments and operating over generalized fading channels. Our expressions are quite generic, providing that the moments-generating function (MGF) of the signal-to-noise ratio (SNR) or the MGF of the inverse SNR of each hop is readily available. The proposed analytical framework is further applied to assess the EC performance of two system setups: i) A variable-gain relaying system equipped with multiple antennas at the relay and the destination, assuming an interference-limited relay and a noise-limited destination. ii) A multi-source multi-destination AF system equipped with variable- or fixed-gain relays. The correctness of the proposed mathematical analysis is verified by Monte Carlo simulations results.

    @ARTICLE{J:Sagias_2_2019,
    AUTHOR = {Georgia P. Karatza and Kostas Peppas and Nikos C. Sagias and George V. Tsoulos},
    TITLE = {Unified ergodic capacity expressions for AF dual-hop systems with hardware impairments},
    JOURNAL = {IEEE Communications Letters},
    YEAR = {2019},
    month = {June},
    volume = {23},
    number = {6},
    pages = {1057-1060},
    doi = {10.1109/LCOMM.2019.2908902}
    };

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  3. Anthony C. Boucouvalas, Nikos C. Sagias, and Kostas Yiannopoulos, “Accurate evaluation of the average probability of error of pulse position modulation in amplified optical wireless communications under turbulence,” Applied Sciences, vol. 9, no. 4, pp. 749, Feb. 2019.
    Download   |   DOI: 10.3390/app9040749   |   Abstract   |  

    We present exact and approximate results on the average probability of error (PER) for pulse position modulation (PPM) in pre-amplified optical wireless communication systems with diversity. The approximate results are obtained by combining a new mathematical formula that we derive for binary PPM and an existing formula that associates higher-order PPM PERs with their binary PPM counterpart. The approximate results are compared with the exact in weak, moderate, and strong turbulence, and it is demonstrated that they are in good agreement under all fading conditions. Moreover, the accuracy of the approximation improves with the optical signal-to-noise ratio and the number of diversity branches that are used, which correspond to implementation scenarios that are typically anticipated in practice.

    @ARTICLE{J:Sagias_3_2019,
    AUTHOR = {Anthony C. Boucouvalas and Nikos C. Sagias and Kostas Yiannopoulos},
    TITLE = {Accurate evaluation of the average probability of error of pulse position modulation in amplified optical wireless communications under turbulence},
    JOURNAL = {Applied Sciences},
    YEAR = {2019},
    month = {Feb.},
    volume = {9},
    number = {4},
    pages = {749},
    doi = {10.3390/app9040749}
    };

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  4. 2018


  5. Panagiotis Koutsandrias, George V. Tsoulos, Georgia E. Athanasiadou, Dimitra A. Zarbouti, and Nikos C. Sagias, “Nanosatellites,” Pelopas, vol. 2, no. 2, pp. 7-29, Dec. 2018.
    Download   |     |   Abstract   |  

    The introduction of nanosatellites brought a revolution in space flight as it was considered until the late 90’s. The development of a spacecraft capable of operating inside the space environment required long term and resource exhausting projects occupying field experts into custom design endeavors. Traditional satellite missions also require strenuous component testing and verification sessions so as to assure every segment is space qualified in order to minimize the probability of possible failures in such high risk ventures. Nanosatellites are classified as satellites of mass below 10 kilograms. While traditional applications such as broadcasting and real-time telecommunications over large regions may not be feasible in their full extent at the moment, numerous applications exist and can be realized at a fraction of the cost of traditional satellites. Applications such as measurements in the environment of earth and space, imaging, telecommunications and technology verification can still be supported with great success. Additionally, nanosatellites are of high educational interest since students can participate in a real space mission in every phase of the project, namely system requirements phase, design phase, assembly phase, testing phase and in-orbit operations phase leading eventually into mission success. A solid breakthrough in small satellite development was brought by the CubeSat standard in 1998. The CubeSat’s fundamental form (1U) is a cube with edges of 100 millimeters and limited to 1 kilogram of total mass. Hundreds of CubeSat missions have been completed since the introduction of the standard, varying among multiple unit sizes, all the way up to 12U units. Numerous universities, corporations as well as space organizations, both civilian and military, have already engaged in CubeSat missions supporting the potential of this satellite class. In addition, a promising industry has been developed, providing commercial-off-the-shelf components compatible with the standard, minimizing both cost and development time for a nanosatellite mission. This article presents an overview of nanosatellites, a list of CubeSat missions launched between March 2013 and November 2014 that is indicative of the trends in nanosatellite objectives, applications and prospective stakeholders, and two high-level proposals for starting points for a future CubeSat project, that also illustrate the cost benefits from the nanosatellite class on jump starting a space mission.

    @ARTICLE{J:Sagias_4_2018,
    AUTHOR = {Panagiotis Koutsandrias and George V. Tsoulos and Georgia E. Athanasiadou and Dimitra A. Zarbouti and Nikos C. Sagias},
    TITLE = {Nanosatellites},
    JOURNAL = {Pelopas},
    YEAR = {2018},
    month = {Dec.},
    volume = {2},
    number = {2},
    pages = {7-29}
    };

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  6. Georgia P. Karatza, Kostas Peppas, and Nikos C. Sagias, “Effective capacity of multisource multidestination cooperative systems under cochannel interference,” IEEE Transactions on Vehicular Technology, vol. 67, no. 9, pp. 8411-8421, Sept. 2018.
    Download   |   DOI: 10.1109/TVT.2018.2848098   |   Abstract   |  

    We introduce a novel analytical framework for the end-to-end (e2e) maximum throughput under delay constraints, namely effective capacity (EC), for multisource and multi-destination amplify-and-forward cooperative networks. The network operates in the presence of Rayleigh fading and employs frequency-division duplex nodes having the ability to simultaneously transmit as sources and receive as relays. Cochannel interference and noise are present at the relay nodes, whereas the destination nodes are noise limited. A linear precoding technique is applied during reception to combine the input signals. As precoding, zero forcing (ZF), maximal-ratio combining (MRC), and minimum mean-squared error (MMSE) are studied. Each relay forwards the received signal to the destination by employing the maximum-ratio transmission (MRT) scheme. Both exact analytical expressions and tight high signal-to-noise ratio bounds of e2e EC are obtained for the ZF/MRT scheme while the optimal power allocation problem maximizing the e2e EC is also addressed. For the MRC/MRT and MMSE/MRT schemes, we derive approximate, yet highly accurate EC analytical expressions, as well as asymptotically tight closed-form expressions. Selected numerical and simulation results show that MMSE/MRT always yields the best performance followed by the ZF/MRT and MRC/MRT schemes. Moreover, it is shown that as the number of relay nodes increases, the ZF/MRT and MMSE/MRT schemes achieve almost identical and always better e2e EC performance than the MRC/MRT one.

    @ARTICLE{J:Sagias_5_2018,
    AUTHOR = {Georgia P. Karatza and Kostas Peppas and Nikos C. Sagias},
    TITLE = {Effective capacity of multisource multidestination cooperative systems under cochannel interference},
    JOURNAL = {IEEE Transactions on Vehicular Technology},
    YEAR = {2018},
    month = {Sept.},
    volume = {67},
    number = {9},
    pages = {8411-8421},
    doi = {10.1109/TVT.2018.2848098}
    };

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  7. Ioannis D. Moscholios, Vassilios G. Vassilakis, Nikos C. Sagias, and Michael D. Logothetis, “On channel sharing policies in LEO mobile satellite systems,” IEEE Transactions on Aerospace and Electronic Systems, vol. 54, no. 4, pp. 1628-1640, Aug. 2018.
    Download   |   DOI: 10.1109/TAES.2018.2798318   |   Abstract   |  

    We consider a low earth orbit (LEO) mobile satellite system with "satellite-fixed" cells that accommodates new and handover calls of different service-classes. We provide an analytical framework for the efficient calculation of call blocking and handover failure probabilities under two channel sharing policies, namely the fixed channel reservation and the threshold call admission policies. Simulation results verify the accuracy of the proposed formulas. Furthermore, we discuss the applicability of the policies in software-defined LEO satellites.

    @ARTICLE{J:Sagias_6_2018,
    AUTHOR = {Ioannis D. Moscholios and Vassilios G. Vassilakis and Nikos C. Sagias and Michael D. Logothetis},
    TITLE = {On channel sharing policies in LEO mobile satellite systems},
    JOURNAL = {IEEE Transactions on Aerospace and Electronic Systems},
    YEAR = {2018},
    month = {Aug.},
    volume = {54},
    number = {4},
    pages = {1628-1640},
    doi = {10.1109/TAES.2018.2798318}
    };

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  8. Kostas Yiannopoulos, Nikos C. Sagias, Anthony C. Boucouvalas, and Kostas Peppas, “Optimal combining for optical wireless systems with amplification: The χ2 noise regime,” IEEE Photonics Technology Letters, vol. 30, no. 1, pp. 119-122, Jan. 2018.
    Download   |   DOI: 10.1109/LPT.2017.2777908   |   Abstract   |  

    We present novel analytical results on the optimal combiner (OC) for pre-amplified optical wireless receivers under chi-squared noise. The results show that the OC architecture that minimizes the average bit-error rate (BER) is determined by the level of the signal-spontaneous beating noise. Moreover, the OC provides increased gain to the diversity branches with the higher energies, up to the point where the beating noise becomes detrimental to the BER. The performance of the OC is simulated for practical receiver arrangements and it is shown that it performs better than the maximal-ratio and equal-gain combiners. The performance improvement amounts to an energy gain of less than 1 dB for implementations that utilize narrow optical filters and a modest number of diversity branches.

    @ARTICLE{J:Sagias_7_2018,
    AUTHOR = {Kostas Yiannopoulos and Nikos C. Sagias and Anthony C. Boucouvalas and Kostas Peppas},
    TITLE = {Optimal combining for optical wireless systems with amplification: The χ2 noise regime},
    JOURNAL = {IEEE Photonics Technology Letters},
    YEAR = {2018},
    month = {Jan.},
    volume = {30},
    number = {1},
    pages = {119-122},
    doi = {10.1109/LPT.2017.2777908}
    };

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  9. Ioannis D. Moscholios, Vassilios G. Vassilakis, Panagiotis G. Sarigiannidis, Nikos C. Sagias, and Michael D. Logothetis, “An analytical framework in LEO mobile satellite systems servicing batched Poisson traffic,” IET Communications, vol. 12, no. 1, pp. 18-25, Jan. 2018.
    Download   |   DOI: 10.1049/iet-com.2017.0220   |   Abstract   |  

    We consider a low earth orbit (LEO) mobile satellite system (MSS) that accepts new and handover calls of multirate service-classes. New calls arrive in the system as batches, following the batched Poisson process. A batch has a generally distributed number of calls. Each call is treated separately from the others and its acceptance is decided according to the availability of the requested number of channels. Handover calls follow also a batched Poisson process. All calls compete for the available channels under the complete sharing policy. By considering the LEO-MSS as a multirate loss system with "satellite-fixed" cells, it can be analyzed via a multidimensional Markov chain, which yields to a product form solution (PFS) for the steady state distribution. Based on the PFS, we propose a recursive and yet efficient formula for the determination of the channel occupancy distribution, and consequently, for the calculation of various performance measures including call blocking and handover failure probabilities. The latter are much higher compared to the corresponding probabilities in the case of the classical (and less bursty) Poisson process. Simulation results verify the accuracy of the proposed formulas. Furthermore, we discuss the applicability of the proposed model in software-defined LEO-MSS.

    @ARTICLE{J:Sagias_8_2018,
    AUTHOR = {Ioannis D. Moscholios and Vassilios G. Vassilakis and Panagiotis G. Sarigiannidis and Nikos C. Sagias and Michael D. Logothetis},
    TITLE = {An analytical framework in LEO mobile satellite systems servicing batched Poisson traffic},
    JOURNAL = {IET Communications},
    YEAR = {2018},
    month = {Jan.},
    volume = {12},
    number = {1},
    pages = {18-25},
    doi = {10.1049/iet-com.2017.0220}
    };

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  10. 2017


  11. Nikos C. Sagias and Ranjan K. Mallik, “On the statistics of the error propagation effect of binary differential phase-shift keying,” IEEE Wireless Communications Letters, vol. 6, no. 6, pp. 718-721, Dec. 2017.
    Download   |   DOI: 10.1109/LWC.2017.2735973   |   Abstract   |  

    We study a traditional problem related to binary differentially coherent phase-shift keying (DPSK) modulation, where an error in a symbol tends to cause an error in the next symbol; this phenomenon is referred to as error propagation. We derive the double bit-error rate (DBER) of two successive DPSK symbols over additive white Gaussian noise and Rician fading. Our findings show that the DBER decreases as the signal-to-noise ratio increases. A tight closed-form upper bound for the average DBER over a slowly varying Rician channel is also presented to simplify the numerical evaluation.

    @ARTICLE{J:Sagias_9_2017,
    AUTHOR = {Nikos C. Sagias and Ranjan K. Mallik},
    TITLE = {On the statistics of the error propagation effect of binary differential phase-shift keying},
    JOURNAL = {IEEE Wireless Communications Letters},
    YEAR = {2017},
    month = {Dec.},
    volume = {6},
    number = {6},
    pages = {718-721},
    doi = {10.1109/LWC.2017.2735973}
    };

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  12. Kostas Peppas, Anthony C. Boucouvalas, Zabih (Fary) Ghassemlooy, Mohhamad-Ali Khalighi, Kostas Yiannopoulos, and Nikos C. Sagias, “Semiconductor optical amplifiers for underwater optical wireless communications,” IET Optoelectronics, vol. 11, no. 1, pp. 15-19, Feb. 2017.
    Download   |   DOI: 10.1049/iet-opt.2016.0010   |   Abstract   |  

    Underwater optical wireless communications systems have recently received significant attention as an attractive solution for both research and commercial use because of their ability to provide high bandwidth communications over relatively short transmission spans along with their low operational cost. However, high absorption and scattering of optical transmission in the water limit the achievable range of underwater optical wireless links to only few meters. In this study, in an effort to increase the range and reliability of underwater optical wireless links, we propose to utilize semiconductor optical amplifiers at the receiver and discuss in a quantitative fashion the performance enhancements that can be achieved. After developing the required analytical framework, extensive numerical results are further provided to demonstrate the performance of the proposed scheme for different operating conditions. Specifically, assuming intensity modulation and direct detection (IM/DD) schemes with on-off keying (OOK) modulation, we evaluate the bit error rate of the proposed system configuration for different water types, link distances and forward error correction schemes. Performance evaluation results reveal that the use of optical amplification can significantly improve the quality of UWOC links.

    @ARTICLE{J:Sagias_10_2017,
    AUTHOR = {Kostas Peppas and Anthony C. Boucouvalas and Zabih (Fary) Ghassemlooy and Mohhamad-Ali Khalighi and Kostas Yiannopoulos and Nikos C. Sagias},
    TITLE = {Semiconductor optical amplifiers for underwater optical wireless communications},
    JOURNAL = {IET Optoelectronics},
    YEAR = {2017},
    month = {Feb.},
    volume = {11},
    number = {1},
    pages = {15-19},
    doi = {10.1049/iet-opt.2016.0010}
    };

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  13. 2016


  14. Kostas Peppas, Nikos C. Sagias, and Andreas M. Maras, “Physical layer security for multiple-antenna systems: A unified approach,” IEEE Transactions on Communications, vol. 64, no. 1, pp. 314-328, Sept. 2016.
    Download   |   DOI: 10.1109/TCOMM.2015.2495293   |   Abstract   |  

    Secrecy capacity is a fundamental information-theoretic performance metric to predict the maximum data rate of reliable communication, while the intended message is not revealed to the eavesdropper. Motivated by this consideration, in this paper a unified communication-theoretic framework for the analysis of the probability of non-zero secrecy capacity, the secrecy outage probability and the secrecy capacity of multiple-antenna systems over fading channels is proposed. Specifically, a powerful frequency-domain approach is first developed, in which the integrals involved in the evaluation of the probability of non-zero secrecy capacity and secrecy outage probability are transformed into the frequency domain, by employing Parseval’s theorem. A generic approach for the evaluation of the asymptotic secrecy outage probability at high signal-to-noise ratio (SNR) region is also introduced, thus providing useful insight as to the parameters affecting the secrecy performance. Finally, a unified numerical approach for computing the average secrecy capacity of multiple-antenna systems under arbitrary fading environments is developed. The proposed framework is general enough to accommodate any well-known multi-antenna transmission technique and fading model. Finally, the secrecy performance of several multiple-antenna system setups is assessed, in the presence of generalized fading conditions and arbitrary antenna correlation, while various numerical and computer simulation results are shown and compared to substantiate the proposed mathematical analysis.

    @ARTICLE{J:Sagias_11_2016,
    AUTHOR = {Kostas Peppas and Nikos C. Sagias and Andreas M. Maras},
    TITLE = {Physical layer security for multiple-antenna systems: A unified approach},
    JOURNAL = {IEEE Transactions on Communications},
    YEAR = {2016},
    month = {Sept.},
    volume = {64},
    number = {1},
    pages = {314-328},
    doi = {10.1109/TCOMM.2015.2495293}
    };

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  15. Nikos C. Sagias, Anthony C. Boucouvalas, Kostas Yiannopoulos, Murat Uysal, and Zabih (Fary) Ghassemlooy, “Optimal combiners in pre-amplified optical wireless systems under medium-to-strong atmospheric turbulence,” Image Processing & Communications, vol. 21, no. 1, pp. 25-34, Jan. 2016.
    Download   |   DOI: 10.1515/ipc-2016-0002   |   Abstract   |  

    In this work we analytically investigate optimal combiners for pre-amplified diversity receivers that operate under medium-to-strong atmospheric turbulence. We first demonstrate that the combiner performance is strongly affected by the existence of a signal-amplified spontaneous emission beat noise at the output of the photodetector. Due to the signal-dependent nature of noise, the optimal combiner can be classified as a hybrid one, of which performance is between the well-known equal-gain and maximal-ratio combiner architectures. Having established the optimal design, we further assess the proposed combiner performance over gamma-gamma and negative-exponential fading environments.

    @ARTICLE{J:Sagias_12_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 under medium-to-strong atmospheric turbulence},
    JOURNAL = {Image Processing & Communications},
    YEAR = {2016},
    month = {Jan.},
    volume = {21},
    number = {1},
    pages = {25-34},
    doi = {10.1515/ipc-2016-0002}
    };

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  16. 2015


  17. Nikos C. Sagias, Kostas Yiannopoulos, and Anthony C. Boucouvalas, “Semiconductor optical amplifiers in negative-exponential fading: Regenerators and pre-amplifiers,” IET Optoelectronics, vol. 9, no. 5, pp. 249-256, Oct. 2015.
    Download   |   DOI: 10.1049/iet-opt.2014.0123   |   Abstract   |  

    In this study, the authors discuss the mitigation of negative-exponential fading in optical wireless communication systems. The mitigation technique involves the utilisation of a semiconductor optical amplifier (SOA) that, depending on the link arrangement, acts either as regenerator or pre-amplifier. As a regenerator, the SOA gain saturates during normal link operation and increases when the link experiences a fade. This unbalanced SOA operation serves towards the equalisation of the signal power at its output and fades become less severe and of reduced duration. The analytical results predict that the fade probability is reduced by over 90% and the scintillation index is improved by 75% for an optimal level of the received power. Moreover, the average duration of fades is also reduced by 68% for the same power level. As a pre-amplifier, the SOA modifies the noise statistics at the receiver and provides a static sensitivity increase of at least 10 dB at 10 Gb/s, depending on the bit-error-rate (BER) target. The analytical results show that this sensitivity improvement imparts a reduction of one order of magnitude on the average BER, of at least 94% on the outage probability and of at least 78% on the average duration of fades.

    @ARTICLE{J:Sagias_13_2015,
    AUTHOR = {Nikos C. Sagias and Kostas Yiannopoulos and Anthony C. Boucouvalas},
    TITLE = {Semiconductor optical amplifiers in negative-exponential fading: Regenerators and pre-amplifiers},
    JOURNAL = {IET Optoelectronics},
    YEAR = {2015},
    month = {Oct.},
    volume = {9},
    number = {5},
    pages = { 249-256},
    doi = {10.1049/iet-opt.2014.0123}
    };

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  18. Kostas Yiannopoulos, Nikos C. Sagias, and Anthony C. Boucouvalas, “On the performance of semiconductor optical amplifier-assisted outdoor optical wireless links,” IEEE Journal on Selected Areas in Communications, vol. 33, no. 9, pp. 1869-1876, Sept. 2015.
    Download   |   DOI: 10.1109/JSAC.2015.2433052   |   Abstract   |  

    We present an analytical framework for estimating the benefits that arise from the utilization of semiconductor optical amplifiers at the receiver of 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 with the channel state. We then utilize the analytical relations to assess the performance of the amplified system under moderate-to-strong (γ–γ) fading in terms of first and second order signal statistics. Our results show that the receiver sensitivity improvement, which is imparted by the amplification process, can drastically reduce the average bit-error-rate, link outage probability and average fade duration, provided that the link length remains within certain limits that are determined by the turbulence intensity scenario under consideration.

    @ARTICLE{J:Sagias_14_2015,
    AUTHOR = {Kostas Yiannopoulos and Nikos C. Sagias and Anthony C. Boucouvalas},
    TITLE = {On the performance of semiconductor optical amplifier-assisted outdoor optical wireless links},
    JOURNAL = {IEEE Journal on Selected Areas in Communications},
    YEAR = {2015},
    month = {Sept.},
    volume = {33},
    number = {9},
    pages = {1869-1876},
    doi = {10.1109/JSAC.2015.2433052}
    };

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  19. Nikos C. Sagias, Stylianos Papaharalabos, and P. Takis Mathiopoulos, “Cooperative DVB-SH satellite broadcasting systems with rotated signal constellations,” China Communications, vol. 12, no. 6, pp. 59-72, June 2015.
    Download   |   DOI: 10.1109/CC.2015.7122481   |   Abstract   |  

    In this paper, we study the advantages of cooperation in broadcasting systems from a geosynchronous earth orbit (GEO) satellite to mobile terminals (MTs), achieved through a terrestrial complementary ground station (CGS) with fixed installment, which acts as a relay. Moreover and in the context of the digital video broadcasting-satellite- to-handheld (DVB-SH) standard, the performance improvements offered by the rotated constellations method are investigated, where prior transmission, a phase rotation of the transmitted symbols by a fixed angle is applied followed by a random component interleaver. Turbo codes with soft decision decoding and appropriate random channel interleavers are also considered. We present analytical expressions for the bit log-likelihood ratios (LLRs) that are needed for soft decision decoding at the MT turbo decoder, while the code combining technique is adapted to improve the end-to-end (E2E) performance. Then, we obtain through extensive computer simulations the average bit error probability (ABEP) of quadrature phase-shift keying (QPSK) signals received over pure land-mobile satellite (LMS) and pure CGS links for coding rates 1/3 and 6/7. Moreover, the optimal rotation angles are obtained for both links. E2E ABEP results are then presented assuming cooperation between GEO and CGS, while the power allocation issue is investigated under fixed total transmission power. Our performance evaluation results show that by using the constellation rotation technique, a performance gain can be achieved for high coding rates.

    @ARTICLE{J:Sagias_15_2015,
    AUTHOR = {Nikos C. Sagias and Stylianos Papaharalabos and P. Takis Mathiopoulos},
    TITLE = {Cooperative DVB-SH satellite broadcasting systems with rotated signal constellations},
    JOURNAL = {China Communications},
    YEAR = {2015},
    month = {June},
    volume = {12},
    number = {6},
    pages = {59-72},
    doi = {10.1109/CC.2015.7122481}
    };

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  20. Nikolaos D. Tselikas, Evangelos A. Kosmatos, and Nikos C. Sagias, “Packet loss optimization applying handoff algorithms in high-mobility radio-over-fiber networks A mathematical analysis,” The Mediterranean Journal of Computers and Networks, vol. 11, no. 2, pp. 390-399, 2015.
    Download   |     |   Abstract   |  

    The paper analyzes three handoff algorithms which are used in Radio-over-Fiber (RoF) networks at 60 GHz. Specifically, the Virtual Cellular Zone (VCZ) and the Moving Extended Cell (MEC), are presented, against the Traditional Handoff (THO) algorithm, which is the simplest proposed solution in such networks. The mathematical analysis regarding packet losses for all three handoff algorithms is presented and it is verified by the corresponding simulation study. The result is that both VCZ and, especially, MEC could be two strong candidate handoff algorithms for packet loss minimization in high-mobility RoF networks at 60 GHz.

    @ARTICLE{J:Sagias_16_2015,
    AUTHOR = {Nikolaos D. Tselikas and Evangelos A. Kosmatos and Nikos C. Sagias},
    TITLE = {Packet loss optimization applying handoff algorithms in high-mobility radio-over-fiber networks A mathematical analysis},
    JOURNAL = {The Mediterranean Journal of Computers and Networks},
    YEAR = {2015},
    volume = {11},
    number = {2},
    pages = {390-399}
    };

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  21. 2014


  22. Nikos C. Sagias, “Pilot-symbol assisted phase-shift keying for dual-hop relaying communication networks,” IEEE Transactions on Communications, vol. 62, no. 2, pp. 510-521, Feb. 2014.
    Download   |   DOI: 10.1109/TCOMM.2014.011814.130556   |   Abstract   |  

    We develop an analytical framework for the end-to-end (e2e) average symbol error probability (ASEP) of dual-hop relaying networks with pilot-symbol assisted M-ary phase-shift keying (M-PSK) modulation. The relays use the selective decode-and-forward protocol and are equipped with multiple receive antennas. The channels are estimated per antenna branch based on the least-squares estimation (LSE) technique by means of pilot symbols. Also, maximal-ratio combining and coherent detection are performed per receiving end. Exact e2e analytical ASEP expressions are derived for binary and quadrature phase-shift keying (BPSK and QPSK), while simple approximate expressions and bounds 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 further provide e2e ASEP expressions considering arbitrarily correlated Nakagami fading channels. For high SNR, closed-form expressions are derived, while the cooperation-gain and diversity-order are also extracted. In addition, two power allocation strategies are investigated and analytical solutions are provided. Comparisons between numerical and computer simulation results are finally presented to verify the validity of the proposed approach and the accuracy of the high-SNR approximate expressions.

    @ARTICLE{J:Sagias_17_2014,
    AUTHOR = {Nikos C. Sagias},
    TITLE = {Pilot-symbol assisted phase-shift keying for dual-hop relaying communication networks},
    JOURNAL = {IEEE Transactions on Communications},
    YEAR = {2014},
    month = {Feb.},
    volume = {62},
    number = {2},
    pages = {510-521},
    doi = {10.1109/TCOMM.2014.011814.130556}
    };

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  23. 2013


  24. Kostas Yiannopoulos, Nikos C. Sagias, and Anthony C. Boucouvalas, “Fade mitigation based on semiconductor optical amplifiers,” IEEE/OSA Journal of Lightwave Technology, vol. 31, no. 23, pp. 3621-3630, Dec. 2013.
    Download   |   DOI: 10.1109/JLT.2013.2285260   |   Abstract   |  

    We present and analyze 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 amplifier (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.

    @ARTICLE{J:Sagias_18_2013,
    AUTHOR = {Kostas Yiannopoulos and Nikos C. Sagias and Anthony C. Boucouvalas},
    TITLE = {Fade mitigation based on semiconductor optical amplifiers},
    JOURNAL = {IEEE/OSA Journal of Lightwave Technology},
    YEAR = {2013},
    month = {Dec.},
    volume = {31},
    number = {23},
    pages = {3621-3630},
    doi = {10.1109/JLT.2013.2285260}
    };

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  25. 2012


  26. Christos K. Datsikas, Kostas Peppas, Nikos C. Sagias, and George S. Tombras, “Serial relaying communications over generalized-gamma fading channels,” Wireless Communications and Mobile Computing, vol. 12, no. 13, pp. 1191-1202, Sept. 2012.
    Download   |   DOI: 10.1002/wcm.1047   |   Abstract   |  

    In this paper, a study on the end-to-end performance of multi-hop non-regenerative relaying networks over independent generalized-gamma (GG) fading channels is presented. Using an upper bound for the end-to-end signal-to-noise ratio (SNR), novel closed-form expressions for the probability density function, the moments, and the moments-generating function of the end-to-end SNR are presented. Based on these derived formulas, lower bounds for the outage and the average bit error probability (ABEP) are derived in closed form. Special attention is given to the low- and high-SNR regions having practical interest as well as to the Nakagami fading scenario. Moreover, the performance of the considered system when employing adaptive square-quadrature amplitude modulation is further analyzed in terms of the average spectral efficiency, the bit error outage, and the ABEP. Computer simulation results verify the tightness and the accuracy of the proposed bounds.

    @ARTICLE{J:Sagias_19_2012,
    AUTHOR = {Christos K. Datsikas and Kostas Peppas and Nikos C. Sagias and George S. Tombras},
    TITLE = {Serial relaying communications over generalized-gamma fading channels},
    JOURNAL = {Wireless Communications and Mobile Computing},
    YEAR = {2012},
    month = {Sept.},
    volume = {12},
    number = {13},
    pages = {1191-1202},
    doi = {10.1002/wcm.1047}
    };

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  27. 2011


  28. Ranjan K. Mallik and Nikos C. Sagias, “Distribution of inner product of complex Gaussian random vectors and its applications,” IEEE Transactions on Communications, vol. 59, no. 12, pp. 3353-3362, Dec. 2011.
    Download   |   DOI: 10.1109/TCOMM.2011.101011.110046   |   Abstract   |  

    Let X and Y be two independent Lx1 complex Gaussian random vectors distributed as CN(mX, σ2X IL) and CN((mY, σ2Y IL)), respectively, where IL denotes the LxL identity matrix. The joint characteristic function (c.f.) of the real and imaginary parts of the inner product YH X is derived in closed form, with ()H denoting the conjugate transpose. Based on this joint c.f., a unified analytical framework for the derivation of the average symbol error probability (ASEP) of a multibranch diversity reception system over flat correlated fading using M-ary phase-shift keying is developed. The receiver employs maximal-ratio combining with least squares channel estimation by means of pilot symbols. The optimal average pilot-to-noise power ratio is obtained in closed form, and the analytical framework is applied to Nakagami and Rice fading. For Nakagami fading, closed form ASEP expressions are obtained for the cases of high signal-to-noise (SNR) and binary phase-shift keying, while for Rice fading, high SNR approximate expressions are obtained in terms of a single integral under the constant correlation model and for independent and identically distributed channels. Both analytical and computer simulation results are presented and compared in order to verify the validity of the proposed analysis.

    @ARTICLE{J:Sagias_20_2011,
    AUTHOR = {Ranjan K. Mallik and Nikos C. Sagias},
    TITLE = {Distribution of inner product of complex Gaussian random vectors and its applications},
    JOURNAL = {IEEE Transactions on Communications},
    YEAR = {2011},
    month = {Dec.},
    volume = {59},
    number = {12},
    pages = {3353-3362},
    doi = {10.1109/TCOMM.2011.101011.110046}
    };

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  29. Kostas Peppas, Christos K. Datsikas, Nikos C. Sagias, and George S. Tombras, “Dual-hop MIMO relay systems over spatially correlated Nakagami-m fading channels,” IET Communications, vol. 5, no. 15, pp. 2106-2115, Oct. 2011.
    Download   |   DOI: 10.1049/iet-com.2010.0305   |   Abstract   |  

    The authors study the performance of a dual-hop plus a direct link multiple-input multiple-output (MIMO) wireless communication system using orthogonal space-time block codes. The system under consideration is based on the decode-and-forward relaying protocol and operates over spatially correlated Nakagami-m fading channels. The proposed analysis is generic enough to account for any MIMO correlation model either from measurements or having theoretical and analytical justification. Analytical expressions for the system end-to-end outage and average symbol error probability are obtained, while critical parameters of the MIMO channel are taken into consideration such as the angle of arrival, the antenna array configuration, the wavelength and non-isotropic scattering conditions. Various numerical and computer simulation results demonstrate the proposed mathematical analysis and the impact of the above parameters to the system performance.

    @ARTICLE{J:Sagias_21_2011,
    AUTHOR = {Kostas Peppas and Christos K. Datsikas and Nikos C. Sagias and George S. Tombras},
    TITLE = {Dual-hop MIMO relay systems over spatially correlated Nakagami-m fading channels},
    JOURNAL = {IET Communications},
    YEAR = {2011},
    month = {Oct.},
    volume = {5},
    number = {15},
    pages = {2106-2115},
    doi = {10.1049/iet-com.2010.0305}
    };

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  30. Petros S. Bithas, Nikos C. Sagias, and Ranjan K. Mallik, “On the sum of Kappa stochastic variates and applications to equal-gain combining,” IEEE Transactions on Communications, vol. 59, no. 9, pp. 2434-2442, Sept. 2011.
    Download   |   DOI: 10.1109/TCOMM.2011.071111.090758   |   Abstract   |  

    In this paper we study the statistics of the sum of not necessarily identically distributed kappa, that is, K, random variables (RV)s. Assuming half-integer values for the shaping parameters, novel closed-form expressions for the probability density function (PDF) of the sum of independent K RVs are obtained, while for arbitrary values of the shaping parameters, a corresponding PDF expression is derived in terms of fast converging infinite series. Furthermore, an infinite series representation for the PDF of the sum of two arbitrarily correlated K RVs is derived. The proposed analysis is employed to the performance analysis of equal-gain combining (EGC) receivers operating over composite fading/shadowing channels modeled by the K distribution. More specifically, the outage and the average bit error probabilities, as well as the average channel capacity of EGC receivers operating over such composite environment are studied. Considering different channel fading/shadowing conditions and correlation effects, various numerical performance evaluation results are presented. These results are complemented by equivalent computer simulated ones that validate the accuracy of the proposed analysis.

    @ARTICLE{J:Sagias_22_2011,
    AUTHOR = {Petros S. Bithas and Nikos C. Sagias and Ranjan K. Mallik},
    TITLE = {On the sum of Kappa stochastic variates and applications to equal-gain combining},
    JOURNAL = {IEEE Transactions on Communications},
    YEAR = {2011},
    month = {Sept.},
    volume = {59},
    number = {9},
    pages = {2434-2442},
    doi = {10.1109/TCOMM.2011.071111.090758}
    };

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  31. Nikos C. Sagias, Fotis I. Lazarakis, Antonis A. Alexandridis, Kostas P. Dangakis, and George S. Tombras, “Higher order capacity statistics of diversity receivers,” Wireless Personal Communications, vol. 56, no. 4, pp. 649-668, Feb. 2011.
    Download   |   DOI: 10.1007/s11277-009-9837-6   |   Abstract   |  

    Wireless Personal Communications, vol. 56, no. 4, pp.

    @ARTICLE{J:Sagias_23_2011,
    AUTHOR = {Nikos C. Sagias and Fotis I. Lazarakis and Antonis A. Alexandridis and Kostas P. Dangakis and George S. Tombras},
    TITLE = {Higher order capacity statistics of diversity receivers},
    JOURNAL = {Wireless Personal Communications},
    YEAR = {2011},
    month = {Feb.},
    volume = {56},
    number = {4},
    pages = {649-668},
    doi = {10.1007/s11277-009-9837-6}
    };

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  32. 2010


  33. Petros S. Bithas, George P. Efthymoglou, and Nikos C. Sagias, “Spectral efficiency of adaptive transmission and selection diversity on generalized fading channels,” IET Communications, vol. 4, no. 17, pp. 2058-2064, Nov. 2010.
    Download   |   DOI: 10.1049/iet-com.2009.0656   |   Abstract   |  

    The spectral efficiency of L-branch selection diversity (SD) receiver with different adaptive transmission techniques operating over generalised-Gamma (GG) fading channels is studied. Novel expressions for important adaptive transmission techniques are obtained, namely optimal power and rate adaptation, optimal rate adaptation, channel inversion with fixed rate (CIFR) and truncated CIFR. Furthermore, simplified expressions for the capacities are obtained for SD reception in Nakagami-m as well as Weibull fading channels. The derived closed-from expressions extend previously published results to the GG fading model. Numerical results are given to demonstrate the usefulness of the theoretical approach.

    @ARTICLE{J:Sagias_24_2010,
    AUTHOR = {Petros S. Bithas and George P. Efthymoglou and Nikos C. Sagias},
    TITLE = {Spectral efficiency of adaptive transmission and selection diversity on generalized fading channels},
    JOURNAL = {IET Communications},
    YEAR = {2010},
    month = {Nov.},
    volume = {4},
    number = {17},
    pages = {2058-2064},
    doi = {10.1049/iet-com.2009.0656}
    };

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  34. Nikos C. Sagias, Ranjan K. Mallik, and George S. Tombras, “Error rate performance of multilevel signals with coherent detection,” IEEE Transactions on Communications, vol. 58, no. 8, pp. 2188-2192, Aug. 2010.
    Download   |   DOI: 10.1109/TCOMM.2010.08.090262   |   Abstract   |  

    In this paper, coherent detection for multilevel correlated signaling sets in additive white Gaussian noise is addressed. The contribution is twofold. Firstly, correlation structures that minimize the symbol error probability (SEP) of M-ary frequency-shift keying as well as of arbitrarily correlated signaling sets are investigated, while secondly, a general and analytical expression for the SEP is derived in the form of a single integral. The structure of the associated correlation matrix is generic and includes various known signaling sets as special cases. Specific correlation structures that 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.

    @ARTICLE{J:Sagias_25_2010,
    AUTHOR = {Nikos C. Sagias and Ranjan K. Mallik and George S. Tombras},
    TITLE = {Error rate performance of multilevel signals with coherent detection},
    JOURNAL = {IEEE Transactions on Communications},
    YEAR = {2010},
    month = {Aug.},
    volume = {58},
    number = {8},
    pages = {2188-2192},
    doi = {10.1109/TCOMM.2010.08.090262}
    };

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  35. Christos K. Datsikas, Kostas Peppas, Nikos C. Sagias, and George S. Tombras, “Serial free-space optical relaying communications over gamma-gamma atmospheric turbulence channels,” IEEE/OSA Journal of Optical Communications and Networking, vol. 2, no. 8, pp. 576-586, Aug. 2010.
    Download   |   DOI: 10.1364/JOCN.2.000576   |   Abstract   |  

    In this paper, a study on the end-to-end performance of multihop free-space optical wireless systems over turbulence-induced fading channels, modeled by the gamma-gamma distribution, is presented. Our analysis is carried out for systems employing amplify-and-forward channel-state-information-assisted or fixed-gain relays. To assess the statistical properties of the end-to-end signal-to-noise ratio for both considered systems, we derive novel closed-form expressions for the moment-generating function, the probability density function, and the cumulative distribution function of the product of rational powers of statistically independent squared gamma-gamma random variables. These statistical results are then applied to studying the outage probability and the average bit error probability of binary modulation schemes. Also, for the case of channel-state-information-assisted relays, an accurate asymptotic performance analysis at high SNR values is presented. Numerical examples compare analytical and simulation results, verifying the correctness of the proposed mathematical analysis.

    @ARTICLE{J:Sagias_26_2010,
    AUTHOR = {Christos K. Datsikas and Kostas Peppas and Nikos C. Sagias and George S. Tombras},
    TITLE = {Serial free-space optical relaying communications over gamma-gamma atmospheric turbulence channels},
    JOURNAL = {IEEE/OSA Journal of Optical Communications and Networking},
    YEAR = {2010},
    month = {Aug.},
    volume = {2},
    number = {8},
    pages = {576-586},
    doi = {10.1364/JOCN.2.000576}
    };

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  36. George C. Alexandropoulos, P. Takis Mathiopoulos, and Nikos C. Sagias, “Switch-and-examine diversity over arbitrary correlated Nakagami-m fading channels,” IEEE Transactions on Vehicular Technology, vol. 58, no. 4, pp. 2080-2087, May 2010.
    Download   |   DOI: 10.1109/TVT.2010.2041016   |   Abstract   |  

    The performance of switch-and-examine diversity (SED) over L arbitrarily correlated and not necessarily identically distributed Nakagami-m fading channels is studied. Analytical expressions for the distribution of the SED output signal-to-noise ratio (SNR) are obtained for the constant correlation model. For the most general case of arbitrary correlation, by assuming half-integer or integer values for the fading parameter m, analytical expressions for the distribution of the output SNR with L ≤ 3 are derived. Moreover, for L>3, analytical approximations for the output SNR are presented. The derived expressions are used to study the outage and average symbol error probability of SED receivers. Performance results obtained by numerical evaluation and verified by means of computer simulations show that the performance of the receivers under consideration is degraded with increasing branch correlation. Nevertheless, SED receivers outperform uncorrelated switch-and-stay diversity receivers, even when they operate under high branch correlation.

    @ARTICLE{J:Sagias_27_2010,
    AUTHOR = {George C. Alexandropoulos and P. Takis Mathiopoulos and Nikos C. Sagias},
    TITLE = {Switch-and-examine diversity over arbitrary correlated Nakagami-m fading channels},
    JOURNAL = {IEEE Transactions on Vehicular Technology},
    YEAR = {2010},
    month = {May},
    volume = {58},
    number = {4},
    pages = {2080-2087},
    doi = {10.1109/TVT.2010.2041016}
    };

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  37. 2009


  38. Zafiro G. Papadimitriou, P. Takis Mathiopoulos, and Nikos C. Sagias, “The trivariate and quadrivariate Weibull fading distributions with arbitrary correlation and their applications to diversity reception,” IEEE Transactions on Communications, vol. 57, no. 11, pp. 3230-3234, Nov. 2009.
    Download   |   DOI: 10.1109/TCOMM.2008.11.080140   |   Abstract   |  

    The statistical characteristics of the trivariate and quadrivariate Weibull fading distribution with arbitrary correlation, non-identical fading parameters and average powers are analytically studied. Novel expressions for important joint statistics are derived using the Weibull power transformation. These expressions are used to evaluate the performance of selection combining (SC) and maximal ratio combining (MRC) diversity receivers in the presence of such fading channels.

    @ARTICLE{J:Sagias_28_2009,
    AUTHOR = {Zafiro G. Papadimitriou and P. Takis Mathiopoulos and Nikos C. Sagias},
    TITLE = {The trivariate and quadrivariate Weibull fading distributions with arbitrary correlation and their applications to diversity reception},
    JOURNAL = {IEEE Transactions on Communications},
    YEAR = {2009},
    month = {Nov.},
    volume = {57},
    number = {11},
    pages = {3230-3234},
    doi = {10.1109/TCOMM.2008.11.080140}
    };

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  39. Petros S. Bithas, Nikos C. Sagias, and P. Takis Mathiopoulos, “The bivariate generalized-K distribution and its application to diversity receivers,” IEEE Transactions on Communications, vol. 57, no. 9, pp. 2655-2662, Sept. 2009.
    Download   |   DOI: 10.1109/TCOMM.2009.09.080039   |   Abstract   |  

    The correlated bivariate generalized-K (KG) distribution, with not necessarily identical shaping and scaling parameters, is introduced and studied. This composite distribution is convenient for modeling multipath/shadowing correlated fading environments when the correlations between the signal envelopes and their powers are different. Generic infinite series expressions are derived for the probability density function (PDF), the cumulative distribution function (CDF) and the joint moments. Assuming identical shaping parameters, simpler expressions for the PDF, CDF and the characteristic function (CF) are provided, while the joint moments are derived in closed form. Furthermore, the PDFs of the product and ratio of two correlated KG random variables are obtained. Capitalizing on these theoretical expressions for the statistical characteristics of the correlated KG distribution, the performance analysis of various diversity reception techniques, such as maximal ratio combining (MRC), equal gain combining (EGC) and selection diversity (SD), over bivariate KG fading channels is presented. For the SD, the outage probability is studied, while for the MRC and EGC the average bit error probability is obtained. The proposed analysis is accompanied by numerical results,clearly demonstrating the usefulness of the theoretical approach as well as the appropriateness of the KG distribution to model multipath/shadowing fading channels.

    @ARTICLE{J:Sagias_29_2009,
    AUTHOR = {Petros S. Bithas and Nikos C. Sagias and P. Takis Mathiopoulos},
    TITLE = {The bivariate generalized-K distribution and its application to diversity receivers},
    JOURNAL = {IEEE Transactions on Communications},
    YEAR = {2009},
    month = {Sept.},
    volume = {57},
    number = {9},
    pages = {2655-2662},
    doi = {10.1109/TCOMM.2009.09.080039}
    };

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  40. Dionysia Triantafyllopoulou, Nikos Passas, Lazaros Merakos, Nikos C. Sagias, and P. Takis Mathiopoulos, “E-CLEMA: A cross-layer design for improved quality of service in mobile WiMAX networks,” Wireless Communications and Mobile Computing, vol. 9, no. 9, pp. 1274-1286, Sept. 2009.
    Download   |   DOI: 10.1002/wcm.699   |   Abstract   |  

    In this paper, an efficient cross-layer design that performs joint adaptation of the physical (PHY) and application layers of a mobile WiMAX network is proposed. The design takes into account channel state and performance information from the PHY and medium access control (MAC) layers, respectively. It uses a decision algorithm to evaluate this information, specify unfavorable conditions regarding low channel quality and increased congestion, and take measures by coordinating modulation order, transmission power, and media encoding rate, toward improved overall quality of service (QoS) offered to the user. Extensive simulation results show that the proposed design achieves considerably reduced packet loss and power consumption, combined with increased throughput as compared to a typical mobile WiMAX system.

    @ARTICLE{J:Sagias_30_2009,
    AUTHOR = {Dionysia Triantafyllopoulou and Nikos Passas and Lazaros Merakos and Nikos C. Sagias and P. Takis Mathiopoulos},
    TITLE = {E-CLEMA: A cross-layer design for improved quality of service in mobile WiMAX networks},
    JOURNAL = {Wireless Communications and Mobile Computing},
    YEAR = {2009},
    month = {Sept.},
    volume = {9},
    number = {9},
    pages = {1274-1286},
    doi = {10.1002/wcm.699}
    };

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  41. Kostas Peppas and Nikos C. Sagias, “A trivariate Nakagami-m distribution with arbitrary covariance matrix and applications to generalized selection diversity receivers,” IEEE Transactions on Communications, vol. 57, no. 7, pp. 1896-1902, July 2009.
    Download   |   DOI: 10.1109/TCOMM.2009.07.070071   |   Abstract   |  

    This paper deals with a trivariate Nakagami-m distribution derived from the diagonal elements of a Wishart matrix. For this distribution, infinite series representations for its probability density and cumulative distribution functions are derived having an arbitrary covariance matrix and integer-order fading parameters. Moreover, upper bounds on the error resulting from truncating the infinite series are obtained. Based on the derived formulas, the performance of triple-branch generalized-selection combining (GSC) receivers is analyzed. For this type of receivers, the outage and the average bit error probability for a variety of modulation schemes are analytically obtained. The performance of GSC receivers is compared to that of conventional selection and maximal-ratio diversity schemes. In order to check the accuracy and convergence of the derived formulas, various performance evaluation results are presented and compared to equivalent simulation ones.

    @ARTICLE{J:Sagias_31_2009,
    AUTHOR = {Kostas Peppas and Nikos C. Sagias},
    TITLE = {A trivariate Nakagami-m distribution with arbitrary covariance matrix and applications to generalized selection diversity receivers},
    JOURNAL = {IEEE Transactions on Communications},
    YEAR = {2009},
    month = {July},
    volume = {57},
    number = {7},
    pages = {1896-1902},
    doi = {10.1109/TCOMM.2009.07.070071}
    };

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  42. George C. Alexandropoulos, Nikos C. Sagias, Fotis I. Lazarakis, and Kostas Berberidis, “New results for the multivariate Nakagami-m fading model with arbitrary correlation matrix and applications,” IEEE Transactions on Wireless Communications, vol. 8, no. 1, pp. 245-255, Jan. 2009.
    Download   |   DOI: 10.1109/T-WC.2009.070521   |   Abstract   |  

    New results for the multichannel Nakagami-m fading model with an arbitrary correlation matrix are presented in this paper. By using an efficient tridiagonalization method based on Householder matrices, the inverse of the Gaussian correlation matrix is transformed to tridiagonal, managing to derive a closed-form union upper bound for the joint Nakagami-m probability density function and an exact analytical expression for the moment generating function of the sum of identically distributed gamma random variables. Our analysis considers an arbitrary correlation structure, which includes as special cases the exponential, constant, circular, and linear correlation ones. Based on the proposed mathematical analysis, we obtain a tight union upper bound for the outage probability of multibranch selection diversity receivers as well as exact analytical expressions for the outage and the average error probability of multibranch maximal-ratio diversity receivers. Our analysis is verified by comparing numerically evaluated with extensive computer simulation performance evaluation results, showing the usefulness of the proposed approach.

    @ARTICLE{J:Sagias_32_2009,
    AUTHOR = {George C. Alexandropoulos and Nikos C. Sagias and Fotis I. Lazarakis and Kostas Berberidis},
    TITLE = {New results for the multivariate Nakagami-m fading model with arbitrary correlation matrix and applications},
    JOURNAL = {IEEE Transactions on Wireless Communications},
    YEAR = {2009},
    month = {Jan.},
    volume = {8},
    number = {1},
    pages = {245-255},
    doi = {10.1109/T-WC.2009.070521}
    };

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  43. 2008


  44. Petros S. Bithas, Nikos C. Sagias, P. Takis Mathiopoulos, Stavros A. Kotsopoulos, and Andreas M. Maras, “On the correlated K-distribution with arbitrary fading parameters,” IEEE Signal Processing Letters, vol. 15, no. 2008, pp. 541-544, Dec. 2008.
    Download   |   DOI: 10.1109/LSP.2008.925737   |   Abstract   |  

    The correlated bivariate K-distribution with arbitrary and not necessarily identical parameters is introduced and analyzed. Novel infinite series expressions for the joint probability density function and moments are derived for the general case where the associated bivariate distributions, i.e., Rayleigh and gamma, are both arbitrary correlated. These expressions generalize previously known analytical results obtained for identical parameter cases. Furthermore, considering independent gamma distributions, the cumulative distribution and characteristic functions are analytically obtained. Although the derived expressions can be used in a wide range of applications, this letter focuses on the performance analysis of dual branch diversity receivers. Specifically, the outage performance of dual selection diversity receivers operating over correlated K fading/shadowing channels is analytically evaluated. Moreover, for low normalized outage threshold values, closed-form expressions are obtained.

    @ARTICLE{J:Sagias_33_2008,
    AUTHOR = {Petros S. Bithas and Nikos C. Sagias and P. Takis Mathiopoulos and Stavros A. Kotsopoulos and Andreas M. Maras},
    TITLE = {On the correlated K-distribution with arbitrary fading parameters},
    JOURNAL = {IEEE Signal Processing Letters},
    YEAR = {2008},
    month = {Dec.},
    volume = {15},
    number = {2008},
    pages = {541-544},
    doi = {10.1109/LSP.2008.925737}
    };

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  45. Petros S. Bithas, Nikos C. Sagias, and Theodoros A. Tsiftsis, “Performance analysis of dual-diversity receivers over correlated generalised gamma fading channels,” IET Communications, vol. 2, no. 1, pp. 174-178, Jan. 2008.
    Download   |   DOI: 10.1049/iet-com:20070207   |   Abstract   |  

    The performance of dual-branch equal-gain combining (EGC) and maximal-ratio combining receivers operating over a composite correlated fading environment, modelled by the generalised Gamma (GG) distribution, is analysed. The moments of the output signal-to-noise ratio are derived in closed form for both types of receivers, and by employing the Pade´ approximants method, the average bit error probability is studied for a great variety of modulation schemes. Furthermore, based on the statistic of the product of two correlated GG random variables, a tight union upper bound for the outage probability of the EGC is obtained, whereas for the special case of Weibull fading a simpler bound is derived in closed form. The proposed mathematical analysis is complemented by various, numerically evaluated performance results, whereas simulations verify the correctness of the proposed analysis.

    @ARTICLE{J:Sagias_34_2008,
    AUTHOR = {Petros S. Bithas and Nikos C. Sagias and Theodoros A. Tsiftsis},
    TITLE = {Performance analysis of dual-diversity receivers over correlated generalised gamma fading channels},
    JOURNAL = {IET Communications},
    YEAR = {2008},
    month = {Jan.},
    volume = {2},
    number = {1},
    pages = {174-178},
    doi = {10.1049/iet-com:20070207}
    };

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  46. Christos K. Datsikas, Nikos C. Sagias, Fotis I. Lazarakis, and George S. Tombras, “Outage analysis of decode-and-forward relaying over Nakagami-m fading channels,” IEEE Signal Processing Letters, vol. 15, no. 1, pp. 41-44, Jan. 2008.
    Download   |   DOI: 10.1109/LSP.2007.910317   |   Abstract   |  

    In this letter, closed-form outage probability expressions are presented for an L-relays dual-hop plus a direct link network, in which the decode-and-forward relaying protocol is employed. Our analysis significantly extends previous results on Rayleigh fading, considering a Nakagami-m fading environment with either equal or distinct second hops fading parameters to average power ratios. Various numerical examples illustrate the proposed analysis.

    @ARTICLE{J:Sagias_35_2008,
    AUTHOR = {Christos K. Datsikas and Nikos C. Sagias and Fotis I. Lazarakis and George S. Tombras},
    TITLE = {Outage analysis of decode-and-forward relaying over Nakagami-m fading channels},
    JOURNAL = {IEEE Signal Processing Letters},
    YEAR = {2008},
    month = {Jan.},
    volume = {15},
    number = {1},
    pages = {41-44},
    doi = {10.1109/LSP.2007.910317}
    };

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  47. 2007


  48. Petros S. Bithas, Nikos C. Sagias, and P. Takis Mathiopoulos, “GSC diversity receivers over generalized-gamma fading channels,” IEEE Communications Letters, vol. 11, no. 12, pp. 964-966, Dec. 2007.
    Download   |   DOI: 10.1109/LCOMM.2007.071095   |   Abstract   |  

    A detailed performance analysis of generalized-selection combining GSC(2, L) receivers operating over generalized-Gamma fading channels is presented. For this class of receivers, a novel closed-form expression for the moments output signal-to-noise ratio is derived. Furthermore, infinite series representations for the moments-generating and the cumulative distribution functions are obtained. The proposed mathematical analysis is accompanied by various performance evaluation results. These theoretical results are complemented by equivalent computer simulated results, which validate the accuracy of the proposed analysis.

    @ARTICLE{J:Sagias_36_2007,
    AUTHOR = {Petros S. Bithas and Nikos C. Sagias and P. Takis Mathiopoulos},
    TITLE = {GSC diversity receivers over generalized-gamma fading channels},
    JOURNAL = {IEEE Communications Letters},
    YEAR = {2007},
    month = {Dec.},
    volume = {11},
    number = {12},
    pages = {964-966},
    doi = {10.1109/LCOMM.2007.071095}
    };

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  49. Nikos C. Sagias and Kostas Peppas, “Error rate analysis of threshold-based hybrid selection/maximal-ratio diversity over correlated Nakagami-m fading channels,” IEEE Communications Letters, vol. 11, no. 12, pp. 922-924, Dec. 2007.
    Download   |   DOI: 10.1109/LCOMM.2007.071432   |   Abstract   |  

    An exact performance analysis of triple-branch threshold-based hybrid selection/maximal-ratio combining (THS/MRC) receivers over correlated Nakagami-m fading channels is presented. Our analysis is valid for integer-order fading parameters and an arbitrary covariance matrix. Following the moment-generating function-based approach, the error rate performance of T-HS/MRC receivers for various modulation formats is analytically obtained. Various performance evaluation results are also presented and compared to equivalent simulation ones.

    @ARTICLE{J:Sagias_37_2007,
    AUTHOR = {Nikos C. Sagias and Kostas Peppas},
    TITLE = {Error rate analysis of threshold-based hybrid selection/maximal-ratio diversity over correlated Nakagami-m fading channels},
    JOURNAL = {IEEE Communications Letters},
    YEAR = {2007},
    month = {Dec.},
    volume = {11},
    number = {12},
    pages = {922-924},
    doi = {10.1109/LCOMM.2007.071432}
    };

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  50. George C. Alexandropoulos, Nikos C. Sagias, and Kostas Berberidis, “On the multivariate Weibull fading model with arbitrary correlation matrix,” IEEE Antennas and Wireless Propagation Letters, vol. 6, pp. 93-95, Dec. 2007.
    Download   |   DOI: 10.1109/LAWP.2007.893093   |   Abstract   |  

    An efficient approximation for the multivariate Weibull distribution with arbitrary correlations is presented. By approximating the Gaussian correlation matrix with a Green’s matrix, a useful analytical expression for the joint distribution of the fading envelopes is derived. As an application, the outage performance of multibranch receivers operating over arbitrarily correlated Weibull fading channels is studied. Numerical and computer simulation results are presented and compared to illustrate the accuracy of the proposed approximation.

    @ARTICLE{J:Sagias_38_2007,
    AUTHOR = {George C. Alexandropoulos and Nikos C. Sagias and Kostas Berberidis},
    TITLE = {On the multivariate Weibull fading model with arbitrary correlation matrix},
    JOURNAL = {IEEE Antennas and Wireless Propagation Letters},
    YEAR = {2007},
    month = {Dec.},
    volume = {6},
    pages = {93-95},
    doi = {10.1109/LAWP.2007.893093}
    };

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  51. Nikos C. Sagias and George K. Karagiannidis, “Comments on "Average LCR and AFD for SC diversity over correlated Weibull fading channels",” Wireless Personal Communications, vol. 43, no. 2, pp. 699-701, Oct. 2007.
    Download   |   DOI: 10.1007/s11277-007-9274-3   |   Abstract   |  

    In a recent paper, two formulae for the average level crossing rate and fade duration at the output of dual-branch selection diversity receivers have been derived. In this short communication, some previously published works including results being in a more general setting are reviewed and compared.

    @ARTICLE{J:Sagias_39_2007,
    AUTHOR = {Nikos C. Sagias and George K. Karagiannidis},
    TITLE = {Comments on "Average LCR and AFD for SC diversity over correlated Weibull fading channels"},
    JOURNAL = {Wireless Personal Communications},
    YEAR = {2007},
    month = {Oct.},
    volume = {43},
    number = {2},
    pages = {699-701},
    doi = {10.1007/s11277-007-9274-3}
    };

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  52. George K. Karagiannidis, Nikos C. Sagias, and P. Takis Mathiopoulos, “N*Nakagami: A novel stochastic model for cascaded fading channels,” IEEE Transactions on Communications, vol. 55, no. 8, pp. 1453-1458, Aug. 2007.
    Download   |   DOI: 10.1109/TCOMM.2007.902497   |   Abstract   |  

    A generic and novel distribution, referred to as N∗Nakagami, constructed as the product of N statistically independent, but not necessarily identically distributed, Nakagami-m random variables (RVs), is introduced and analyzed. The proposed distribution turns out to be a very convenient tool for modelling cascaded Nakagami-m fading channels and analyzing the performance of digital communications systems operating over such channels. The moments-generating, probability density, cumulative distribution, and moments functions of the N∗Nakagami distribution are developed in closed form using the Meijer’s Gfunction. Using these formulas, generic closed-form expressions for the outage probability, amount of fading, and average error probabilities for several binary and multilevel modulation signals of digital communication systems operating over the N∗Nakagami fading and the additive white Gaussian noise channel are presented. Complementary numerical and computer simulation performance evaluation results verify the correctness of the proposed formulation. The suitability of the N∗Nakagami fading distribution to approximate the lognormal distribution is also being investigated. Using Kolmogorov–Smirnov tests, the rate of convergence of the central limit theorem as pertaining to the multiplication of Nakagami-m RVs is quantified.

    @ARTICLE{J:Sagias_40_2007,
    AUTHOR = {George K. Karagiannidis and Nikos C. Sagias and P. Takis Mathiopoulos},
    TITLE = {N*Nakagami: A novel stochastic model for cascaded fading channels},
    JOURNAL = {IEEE Transactions on Communications},
    YEAR = {2007},
    month = {Aug.},
    volume = {55},
    number = {8},
    pages = {1453-1458},
    doi = {10.1109/TCOMM.2007.902497}
    };

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  53. Petros S. Bithas, Nikos C. Sagias, and P. Takis Mathiopoulos, “Dual diversity over correlated Ricean fading channels,” Journal of Communications and Networks, vol. 9, no. 1, pp. 67-74, Mar. 2007.
    Download   |   DOI: 10.1109/JCN.2007.6182815   |   Abstract   |  

    The performance of dual diversity receivers operating over correlated Ricean fading channels is analyzed. Using a previously derived rapidly converging infinite series representation for the bivariate Ricean probability density function, analytical expressions for the statistics of dual-branch selection combining, maximal-ratio combining, and equal-gain combining output signal-to-noise ratio (SNR) are derived. These expressions are employed to obtain novel analytical formulae for the average output SNR, amount of fading, average bit error probability, and outage probability. The proposed mathematical analysis is used to study various novel performance evaluation results with parameters of interest the fading severity, average input SNRs, and the correlation coefficient. The series convergence rate is also examined verifying the fast convergence of the analytical expressions. The accuracy of most of the theoretical performance evaluation results are validated by means of computer simulations.

    @ARTICLE{J:Sagias_41_2007,
    AUTHOR = {Petros S. Bithas and Nikos C. Sagias and P. Takis Mathiopoulos},
    TITLE = {Dual diversity over correlated Ricean fading channels},
    JOURNAL = {Journal of Communications and Networks},
    YEAR = {2007},
    month = {Mar.},
    volume = {9},
    number = {1},
    pages = {67-74},
    doi = {10.1109/JCN.2007.6182815}
    };

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  54. Nikos C. Sagias, “Closed-form analysis of equal-gain diversity in wireless radio networks,” IEEE Transactions on Vehicular Technology, vol. 56, no. 1, pp. 173-182, Jan. 2007.
    Download   |   DOI: 10.1109/TVT.2006.883803   |   Abstract   |  

    This paper deals with the performance of predetection equal-gain combining (EGC) receivers operating over multipath fading plus cochannel interference (CCI) and additive white Gaussian noise channels. The desired components of the received signals are considered to experience independent but not necessarily identically distributed Nakagami-m fading, while the interferers are subject to independent Rayleigh fading. The analysis is not only limited to equal average fading power interferers, but the case of interferers with distinct average powers is also examined. By following the coherent interference power calculation, novel closed-form expressions for the moments of the EGC output signal-to-interference-plus-noise ratio (SINR) are derived, which are being used to study the performance of the average output SINR. Furthermore, by assuming an interference-limited fading scenario, novel closed-form union performance bounds are derived. More specifically, tight upper bounds for the outage and average symbol error probability for several constant envelope modulation schemes, and lower bounds for the Shannon average spectral efficiency, are provided. Numerical results demonstrate the effect of the number of interferers, the number of the receiver branches, and the severity of fading on the EGC receiver performance. Computer simulations have been also performed to verify the tightness of the proposed bounds and the correctness of the mathematical analysis. It is shown that the performance of cellular radio systems in the uplink is degraded mainly from the first-tier CCI of the adjacent cells.

    @ARTICLE{J:Sagias_42_2007,
    AUTHOR = {Nikos C. Sagias},
    TITLE = {Closed-form analysis of equal-gain diversity in wireless radio networks},
    JOURNAL = {IEEE Transactions on Vehicular Technology},
    YEAR = {2007},
    month = {Jan.},
    volume = {56},
    number = {1},
    pages = {173-182},
    doi = {10.1109/TVT.2006.883803}
    };

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  55. Nikos C. Sagias and George S. Tombras, “On the cascaded Weibull fading channel model,” Journal of the Franklin Institute, vol. 344, no. 1, pp. 1-11, Jan. 2007.
    Download   |   DOI: 10.1016/j.jfranklin.2006.07.004   |   Abstract   |  

    A new stochastic fading channel model called cascaded Weibull fading is introduced and the associated capacity is derived in closed form. This model is generated by the product of independent, but not necessarily identically distributed, Weibull random variables (RVs). By quantifying the convergence rate of the central limit theorem as pertaining to the multiplication of Weibull distributed RVs, the statistical basis of the lognormal distribution is investigated. By performing Kolmogorov–Smirnov tests, the null hypothesis for this product to be approximated by the lognormal distribution is studied. Another null hypothesis is also examined for this product to be approximated by a Weibull distribution with properly adjusted statistical parameters.

    @ARTICLE{J:Sagias_43_2007,
    AUTHOR = {Nikos C. Sagias and George S. Tombras},
    TITLE = {On the cascaded Weibull fading channel model},
    JOURNAL = {Journal of the Franklin Institute},
    YEAR = {2007},
    month = {Jan.},
    volume = {344},
    number = {1},
    pages = {1-11},
    doi = {10.1016/j.jfranklin.2006.07.004}
    };

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  56. 2006


  57. Nikos C. Sagias, George K. Karagiannidis, P. Takis Mathiopoulos, and Theodoros A. Tsiftsis, “On the performance analysis of equal-gain diversity receivers over generalized Gamma fading channels,” IEEE Transactions on Wireless Communications, vol. 5, no. 10, pp. 2967-2975, Oct. 2006.
    Download   |   DOI: 10.1109/TWC.2006.05301   |   Abstract   |  

    A versatile envelope distribution which generalizes many commonly used models for multipath and shadow fading is the so-called generalized Gamma (GG) distribution. By considering the product of N GG random variables (RV)s, novel expressions for its moments-generating, probability density, and cumulative distribution functions are obtained in closed form. These expressions are used to derive a 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, first the moments of the signal-to-noise (SNR) at the output, including average SNR and amount of fading, are obtained in closed form. Furthermore, novel union upper bounds for the outage and the average bit error probability are derived and evaluated in terms of Meijer’s G-functions. The tightness of the proposed bounds is verified by performing comparisons between numerical evaluation and computer simulations results.

    @ARTICLE{J:Sagias_44_2006,
    AUTHOR = {Nikos C. Sagias and George K. Karagiannidis and P. Takis Mathiopoulos and Theodoros A. Tsiftsis},
    TITLE = {On the performance analysis of equal-gain diversity receivers over generalized Gamma fading channels},
    JOURNAL = {IEEE Transactions on Wireless Communications},
    YEAR = {2006},
    month = {Oct.},
    volume = {5},
    number = {10},
    pages = {2967-2975},
    doi = {10.1109/TWC.2006.05301}
    };

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  58. George K. Karagiannidis, Nikos C. Sagias, and Theodoros A. Tsiftsis, “Closed-form statistics for the sum of squared Nakagami-m variates and its applications,” IEEE Transactions on Communications, vol. 54, no. 8, pp. 1353-1359, Aug. 2006.
    Download   |   DOI: 10.1109/TCOMM.2006.878812   |   Abstract   |  

    We present closed-form expressions for the probability density function (PDF) and the cumulative distribution function (CDF) of the sum of non-identical squared Nakagami-m random variables (RVs) with integer-order fading parameters. As it is shown, they can be written as a weighted sum of Erlang PDFs and CDFs, respectively, while the analysis includes both independent and correlated sums of RVs. The proposed formulation significantly improves previously published results, which are either in the form of infinite sums or higher order derivatives of the fading parameter m. The obtained formulas can be applied to the performance analysis of diversity combining receivers operating over Nakagami-m fading channels.

    @ARTICLE{J:Sagias_45_2006,
    AUTHOR = {George K. Karagiannidis and Nikos C. Sagias and Theodoros A. Tsiftsis},
    TITLE = {Closed-form statistics for the sum of squared Nakagami-m variates and its applications},
    JOURNAL = {IEEE Transactions on Communications},
    YEAR = {2006},
    month = {Aug.},
    volume = {54},
    number = {8},
    pages = {1353-1359},
    doi = {10.1109/TCOMM.2006.878812}
    };

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  59. Petros S. Bithas, Nikos C. Sagias, P. Takis Mathiopoulos, George K. Karagiannidis, and Athanasios A. Rontogiannis, “On the performance analysis of digital communications over generalized-K fading channels,” IEEE Communications Letters, vol. 10, no. 5, pp. 353-355, May 2006.
    Download   |   DOI: 10.1109/LCOMM.2006.05030   |   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.

    @ARTICLE{J:Sagias_46_2006,
    AUTHOR = {Petros S. Bithas and Nikos C. Sagias and P. Takis Mathiopoulos and George K. Karagiannidis and Athanasios A. Rontogiannis},
    TITLE = {On the performance analysis of digital communications over generalized-K fading channels},
    JOURNAL = {IEEE Communications Letters},
    YEAR = {2006},
    month = {May},
    volume = {10},
    number = {5},
    pages = {353-355},
    doi = {10.1109/LCOMM.2006.05030}
    };

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  60. Nikos C. Sagias, “Capacity of dual-branch selection diversity receivers in correlative Weibull fading,” European Transactions on Telecommunications, vol. 16, no. 1, pp. 37-43, Jan. 2006.
    Download   |   DOI: 10.1002/ett.1082   |   Abstract   |  

    An analysis for the Shannon channel capacity of dual-branch selection diversity receivers operating over correlative Weibull fading is presented under three adaptive policies: constant power with optimal rate adaptation, optimal power and rate adaptation and channel inversion with fixed rate. In this context, useful formulae for the average channel capacity with not necessarily identical fading statistics and arbitrary parameters for both the correlation coefficient and fading severity are derived in closed form. The analysis also includes the performance of single-branch receivers where special cases of the expressions agree with known results. Illustrative numerical examples are also presented, demonstrating the effects of the correlation coefficient, fading severity and signal-to-noise ratio unbalance on the receiver’s performance.

    @ARTICLE{J:Sagias_47_2006,
    AUTHOR = {Nikos C. Sagias},
    TITLE = {Capacity of dual-branch selection diversity receivers in correlative Weibull fading},
    JOURNAL = {European Transactions on Telecommunications},
    YEAR = {2006},
    month = {Jan.},
    volume = {16},
    number = {1},
    pages = {37-43},
    doi = {10.1002/ett.1082}
    };

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  61. 2005


  62. Nikos C. Sagias, Dimitris A. Zogas, and George K. Karagiannidis, “Selection diversity receivers over nonidentical Weibull fading channels,” IEEE Transactions on Vehicular Technology, vol. 54, no. 6, pp. 2146-2151, Dec. 2005.
    Download   |   DOI: 10.1109/TVT.2005.853452   |   Abstract   |  

    The performance of selection combining (SC) receivers operating over independent, but not necessarily identically distributed,Weibull fading channels is studied. A novel closed form expression for the moments of the SC output signal-to-noise ratio (SNR)is derived, which is used to study the corresponding average output SNR and amount of fading. Second-order statistical parameters such as the average level crossing rate and average fade duration at the output of the SC are also obtained in closed form. Moreover, the average symbol error probability for several coherent and noncoherent modulations schemes as well as the Shannon capacity are extracted in terms of the tabulated Meijer’s G-function. Simulations are also performed to validate the proposed formulation.

    @ARTICLE{J:Sagias_48_2005,
    AUTHOR = {Nikos C. Sagias and Dimitris A. Zogas and George K. Karagiannidis},
    TITLE = {Selection diversity receivers over nonidentical Weibull fading channels},
    JOURNAL = {IEEE Transactions on Vehicular Technology},
    YEAR = {2005},
    month = {Dec.},
    volume = {54},
    number = {6},
    pages = {2146-2151},
    doi = {10.1109/TVT.2005.853452}
    };

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  63. Petros S. Bithas, George K. Karagiannidis, Nikos C. Sagias, P. Takis Mathiopoulos, Stavros A. Kotsopoulos, and Giovanni E. Corazza, “Performance analysis of a class of GSC receivers over non-identical Weibull fading channels,” IEEE Transactions on Vehicular Technology, vol. 54, no. 6, pp. 1963-1970, Dec. 2005.
    Download   |   DOI: 10.1109/TVT.2005.858194   |   Abstract   |  

    The performance of a class of generalized-selection combining (GSC) receivers operating over independent but nonidentically distributed Weibull fading channels is studied.We consider the case where the two branches with the largest instantaneous signal-to-noise ratio (SNR), from a total of L available, GSC(2,L) are selected. By introducing a novel property for the product of moments of ordered Weibull random variables, convenient closed form expressions for the moments of the GSC(2,L) output SNR are derived. Using these expressions, important performance criteria, such as average output SNR and amount of fading, are obtained in closed form. Furthermore, employing the Pade approximants theory and the moment-generating function approach, outage and bit-error rate performance are studied. An attempt is also made to identify the equivalency between the Weibull and the Rice fading channel, which is typically used to model the mobile satellite channel. We present various numerical performance evaluation results for different modulation formats and channel conditions. These results are complemented by equivalent computer simulated results which validate the accuracy of the proposed analysis.

    @ARTICLE{J:Sagias_49_2005,
    AUTHOR = {Petros S. Bithas and George K. Karagiannidis and Nikos C. Sagias and P. Takis Mathiopoulos and Stavros A. Kotsopoulos and Giovanni E. Corazza},
    TITLE = {Performance analysis of a class of GSC receivers over non-identical Weibull fading channels},
    JOURNAL = {IEEE Transactions on Vehicular Technology},
    YEAR = {2005},
    month = {Dec.},
    volume = {54},
    number = {6},
    pages = {1963-1970},
    doi = {10.1109/TVT.2005.858194}
    };

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  64. Dimitris A. Zogas, Nikos C. Sagias, George S. Tombras, and George K. Karagiannidis, “Average output SNR of equal-gain diversity receivers over correlative Weibull fading channels,” European Transactions on Telecommunications, vol. 16, no. 6, pp. 521-525, Nov. 2005.
    Download   |   DOI: 10.1002/ett.1055   |   Abstract   |  

    In this letter, capitalising on a recently presented expression for the joint probability density function of the Weibull fading statistical model, a novel closed-form expression is derived for the average output signal-to-noise ratio (SNR) of an L-branch receiver employing equal-gain combining (EGC). The obtained expression includes the case of correlative fading with arbitrary correlation among the input branches, as well as non-identical fading statistics. Numerical results depict the effects of the fading severity, the fading correlation and the number of diversity branches, on EGC performance. An interesting outcome is that the average output SNR is not a clear and meaningful measure for the EGC receivers performance, when these operate in correlative fading channels.

    @ARTICLE{J:Sagias_50_2005,
    AUTHOR = {Dimitris A. Zogas and Nikos C. Sagias and George S. Tombras and George K. Karagiannidis},
    TITLE = {Average output SNR of equal-gain diversity receivers over correlative Weibull fading channels},
    JOURNAL = {European Transactions on Telecommunications},
    YEAR = {2005},
    month = {Nov.},
    volume = {16},
    number = {6},
    pages = {521-525},
    doi = {10.1002/ett.1055}
    };

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  65. Nikos C. Sagias and P. Takis Mathiopoulos, “Switched diversity receivers over generalized Gamma fading channels,” IEEE Communications Letters, vol. 9, no. 10, pp. 871-873, Oct. 2005.
    Download   |   DOI: 10.1109/LCOMM.2005.10026   |   Abstract   |  

    A versatile envelope distribution which generalizes several commonly used fading models is the generalized Gamma (GG) distribution. This letter deals with the performance analysis of switch and stay combining (SSC) receivers operating over not necessarily identical GG fading channels. For these receivers, novel analytical expressions for the moments of the output signal-to-noise ratio (SNR) (including average SNR and amount of fading), outage probability, average bit error probability (ABEP), and Shannon average spectral efficiency (ASE) are derived. Moreover, closed-form expressions are obtained for the optimal average SNR, ABEP, and ASE switching thresholds. Special cases of the derived expressions agree with known results.

    @ARTICLE{J:Sagias_51_2005,
    AUTHOR = {Nikos C. Sagias and P. Takis Mathiopoulos},
    TITLE = {Switched diversity receivers over generalized Gamma fading channels},
    JOURNAL = {IEEE Communications Letters},
    YEAR = {2005},
    month = {Oct.},
    volume = {9},
    number = {10},
    pages = {871-873},
    doi = {10.1109/LCOMM.2005.10026}
    };

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  66. Nikos C. Sagias and George K. Karagiannidis, “Gaussian class multivariate Weibull distributions: Theory and applications in fading channels,” IEEE Transactions on Information Theory, vol. 51, no. 10, pp. 3608-3619, Oct. 2005.
    Download   |   DOI: 10.1109/TIT.2005.855598   |   Abstract   |  

    Ascertaining on the suitability of the Weibull distribution to model fading channels, a theoretical framework of a class of multivariate Weibull distributions, originated from Gaussian random processes, is introduced and analyzed. Novel analytical expressions for the joint probability density function (pdf), moment generating function (mgf), and cumulative distribution function (cdf) are derived for the bivariate distribution of this class with not necessarily identical fading parameters and average powers. Two specific distributions with arbitrary number of correlated variates are considered and studied: with exponential and with constant correlation where their pdfs are introduced. Both cases assume equal average fading powers, but not necessarily identical fading parameters. For the multivariate Weibull distribution with exponential correlation, useful corresponding formulas, as for the bivariate case, are derived. The presented theoretical results are applied to analyze the performance of several diversity receivers employed with selection, equal-gain, and maximal-ratio combining (MRC) techniques operating over correlated Weibull fading channels. For these diversity receivers, several useful performance criteria such as the moments of the output signal-to-noise ratio (SNR) (including average output SNR and amount of fading) and outage probability are analytically derived. Moreover, the average symbol error probability for several coherent and noncoherent modulation schemes is studied using the mgf approach. The proposed mathematical analysis is complemented by various evaluation results, showing the effects of the fading severity as well as the fading correlation on the diversity receivers performance.

    @ARTICLE{J:Sagias_52_2005,
    AUTHOR = {Nikos C. Sagias and George K. Karagiannidis},
    TITLE = {Gaussian class multivariate Weibull distributions: Theory and applications in fading channels},
    JOURNAL = {IEEE Transactions on Information Theory},
    YEAR = {2005},
    month = {Oct.},
    volume = {51},
    number = {10},
    pages = {3608-3619},
    doi = {10.1109/TIT.2005.855598}
    };

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  67. Nikos C. Sagias, George K. Karagiannidis, Dimitris A. Zogas, George S. Tombras, and Stavros A. Kotsopoulos, “Average output SINR of equal gain diversity in Nakagami fading with cochannel interference,” IEEE Transactions on Wireless Communications, vol. 4, no. 4, pp. 1407-1411, July 2005.
    Download   |   DOI: 10.1109/TWC.2005.850384   |   Abstract   |  

    In mobile radio systems, antenna diversity is used to combat fading and reduce the impact of cochannel interference. In this paper, we analyze the performance of L-branch equal-gain combining receivers over correlated nonidentically distributed Nakagami-m fading channels, in the presence of multiple identical cochannel interferers and additive white Gaussian noise. The performance criterion considered is the average output signal-to-interference-plus-noise ratio, which is obtained in closed form for both independent and correlative fading. Due to the simple form of the derived expressions, they readily allow numerical evaluation for cases of practical interest.

    @ARTICLE{J:Sagias_53_2005,
    AUTHOR = {Nikos C. Sagias and George K. Karagiannidis and Dimitris A. Zogas and George S. Tombras and Stavros A. Kotsopoulos},
    TITLE = {Average output SINR of equal gain diversity in Nakagami fading with cochannel interference},
    JOURNAL = {IEEE Transactions on Wireless Communications},
    YEAR = {2005},
    month = {July},
    volume = {4},
    number = {4},
    pages = {1407-1411},
    doi = {10.1109/TWC.2005.850384}
    };

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  68. George K. Karagiannidis, Theodoros A. Tsiftsis, and Nikos C. Sagias, “A closed-form upper-bound for the distribution of the weighted sum of Rayleigh variates,” IEEE Communications Letters, vol. 9, no. 7, pp. 589-591, July 2005.
    Download   |   DOI: 10.1109/LCOMM.2005.07011   |   Abstract   |  

    The problem of finding the distribution of the sum of more than two Rayleigh fading envelopes has never been solved in terms of tabulated functions. In this letter, we present a closed-form union upper-bound for the cumulative distribution function of the weighted sum of N independent Rayleigh fading envelopes. Computer simulation results verify the tightness of the proposed bound for several values of N. The proposed bound can be efficiently applied in various wireless applications, such as satellite communications, equal-gain receivers, and radars.

    @ARTICLE{J:Sagias_54_2005,
    AUTHOR = {George K. Karagiannidis and Theodoros A. Tsiftsis and Nikos C. Sagias},
    TITLE = {A closed-form upper-bound for the distribution of the weighted sum of Rayleigh variates},
    JOURNAL = {IEEE Communications Letters},
    YEAR = {2005},
    month = {July},
    volume = {9},
    number = {7},
    pages = {589-591},
    doi = {10.1109/LCOMM.2005.07011}
    };

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  69. Nikos C. Sagias and George K. Karagiannidis, “Effects of carrier phase error on EGC receivers in correlated Nakagami-m fading,” IEEE Communications Letters, vol. 9, no. 7, pp. 580-582, July 2005.
    Download   |   DOI: 10.1109/LCOMM.2005.07006   |   Abstract   |  

    The effects of incoherently combining on dual-branch equal-gain combining (EGC) receivers in the presence of correlated, but not necessarily identical, Nakagami-m fading and additive white Gaussian noise are studied. Novel closed-form expressions for the moments of the output signal-to-noise ratio (SNR) are derived. Based on these expressions, the average output SNR and the amount of fading are obtained in closed-form. Moreover, the outage and the average bit error probability for binary and quadrature phase-shift keying are also studied using the moments-based approach. Numerical and computer simulation results clearly depict the effect of the carrier phase error, correlation coefficient, and fading severity on the EGC performance. An interesting finding is that higher values of the correlation coefficient results to lower irreducible error floors.

    @ARTICLE{J:Sagias_55_2005,
    AUTHOR = {Nikos C. Sagias and George K. Karagiannidis},
    TITLE = {Effects of carrier phase error on EGC receivers in correlated Nakagami-m fading},
    JOURNAL = {IEEE Communications Letters},
    YEAR = {2005},
    month = {July},
    volume = {9},
    number = {7},
    pages = {580-582},
    doi = {10.1109/LCOMM.2005.07006}
    };

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  70. George K. Karagiannidis, Dimitris A. Zogas, Nikos C. Sagias, Stavros A. Kotsopoulos, and George S. Tombras, “Equal-gain and maximal-ratio combining over Weibull fading channels,” IEEE Transactions on Wireless Communications, vol. 4, no. 3, pp. 841-846, May 2005.
    Download   |   DOI: 10.1109/TWC.2005.846982   |   Abstract   |  

    We study the performance of L-branch equal-gain combining (EGC) and maximal-ratio combining (MRC) receivers operating over nonidentical Weibull fading channels. Closed-form expressions are derived for the moments of the signal-to-noise ratio (SNR) at the output of the combiner and significant performance criteria, for both independent and correlative fading, such as average output SNR, amount of fading and spectral efficiency at the low power regime, are studied. We also evaluate the outage and the average symbol error probability (ASEP) for several coherent and noncoherent modulation schemes, using a closed-form expression for the moment-generating function (mgf) of the output SNR for MRC receivers and the Padé approximation to the mgf for EGC receivers. The ASEP of dual-branch EGC and MRC receivers is also obtained in correlative fading. The proposed mathematical analysis is complimented by various numerical results, which point out the effects of fading severity and correlation on the overall system performance. Computer simulations are also performed to verify the validity and the accuracy of the proposed theoretical approach.

    @ARTICLE{J:Sagias_56_2005,
    AUTHOR = {George K. Karagiannidis and Dimitris A. Zogas and Nikos C. Sagias and Stavros A. Kotsopoulos and George S. Tombras},
    TITLE = {Equal-gain and maximal-ratio combining over Weibull fading channels},
    JOURNAL = {IEEE Transactions on Wireless Communications},
    YEAR = {2005},
    month = {May},
    volume = {4},
    number = {3},
    pages = {841-846},
    doi = {10.1109/TWC.2005.846982}
    };

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  71. George K. Karagiannidis, Nikos C. Sagias, and Dimitris A. Zogas, “Error analysis of M-QAM with equal-gain diversity over generalized fading channels,” IET Proceedings on Communications, vol. 152, no. 1, pp. 69-74, Feb. 2005.
    Download   |   DOI: 10.1049/ip-com:20041184   |   Abstract   |  

    M-ary quadrature amplitude modulation (M-QAM) is a spectrally efficient modulation scheme, used for video transmission applications in third and forthcoming generations of wireless networks. In the paper, the authors present a unified framework for the error performance of MQAM, employing L-branch equal-gain combining over generalised fading channels, such as Nakagami-m, Rice, Hoyt or Weibull. For each channel model an exact closed-form expression is derived for the moments of the EGC output signal-to-noise ratio (SNR). Using these expressions, the symbol error performance of M-QAM is studied, with the aid of the moment-generating function approach and the Pade approximants theory. The proposed mathematical analysis is illustrated by selected numerical results, pointing out the effect of the input SNRs unbalancing, as well as the fading severity on the system’s performance. Simulations are also performed to check the validity and the accuracy of the proposed analysis.

    @ARTICLE{J:Sagias_57_2005,
    AUTHOR = {George K. Karagiannidis and Nikos C. Sagias and Dimitris A. Zogas},
    TITLE = {Error analysis of M-QAM with equal-gain diversity over generalized fading channels},
    JOURNAL = {IET Proceedings on Communications},
    YEAR = {2005},
    month = {Feb.},
    volume = {152},
    number = {1},
    pages = {69-74},
    doi = {10.1049/ip-com:20041184}
    };

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  72. Nikos C. Sagias, George S. Tombras, and George K. Karagiannidis, “New results for the Shannon channel capacity in generalized fading channels,” IEEE Communications Letters, vol. 9, no. 2, pp. 97-99, Feb. 2005.
    Download   |   DOI: 10.1109/LCOMM.2004.831319   |   Abstract   |  

    Novel, closed-form expressions for the average Shannon capacity of single-branch receivers, operating over generalized fading channels (Nakagami-m, Rice and Weibull), are derived. As an application, the optimum switching threshold for maximizing the data transmission rate of switched and stay combining receivers is obtained and several numerical results are presented.

    @ARTICLE{J:Sagias_58_2005,
    AUTHOR = {Nikos C. Sagias and George S. Tombras and George K. Karagiannidis},
    TITLE = {New results for the Shannon channel capacity in generalized fading channels},
    JOURNAL = {IEEE Communications Letters},
    YEAR = {2005},
    month = {Feb.},
    volume = {9},
    number = {2},
    pages = {97-99},
    doi = {10.1109/LCOMM.2004.831319}
    };

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  73. Nikos C. Sagias, Panagiotis Varzakas, George S. Tombras, and George K. Karagiannidis, “Spectral efficiency for selection combining RAKE receivers over Weibull fading channels,” Journal of the Franklin Institute, vol. 342, no. 1, pp. 7-13, Jan. 2005.
    Download   |   DOI: 10.1016/j.jfranklin.2004.07.007   |   Abstract   |  

    Novel closed-form expressions for the probability density function and the average output signal-to-noise ratio at the output of a selection combiner in Weibull fading are derived. Using these expressions, the spectral efficiency of a direct sequence code division multiple access system is analytically obtained and performance evaluation results are presented.

    @ARTICLE{J:Sagias_59_2005,
    AUTHOR = {Nikos C. Sagias and Panagiotis Varzakas and George S. Tombras and George K. Karagiannidis},
    TITLE = {Spectral efficiency for selection combining RAKE receivers over Weibull fading channels},
    JOURNAL = {Journal of the Franklin Institute},
    YEAR = {2005},
    month = {Jan.},
    volume = {342},
    number = {1},
    pages = {7-13},
    doi = {10.1016/j.jfranklin.2004.07.007}
    };

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  74. 2004


  75. Nikos C. Sagias, Panagiotis Varzakas, George S. Tombras, and George K. Karagiannidis, “Average channel capacity for generalized selection combining RAKE receivers,” European Transactions on Telecommunications, vol. 15, no. 5, pp. 497-500, Sept. 2004.
    Download   |   DOI: 10.1002/ett.1002   |   Abstract   |  

    A novel closed-form expression for the achievable average channel capacity of a generalized-selection combining RAKE receiver in Rayleigh fading, is derived. Performance comparison for the capacity achieved with maximal-ratio combining and RAKE receivers is also presented. The expression derived, fully conforms to the upper bound of the Shannon–Hartley theorem.

    @ARTICLE{J:Sagias_60_2004,
    AUTHOR = {Nikos C. Sagias and Panagiotis Varzakas and George S. Tombras and George K. Karagiannidis},
    TITLE = {Average channel capacity for generalized selection combining RAKE receivers},
    JOURNAL = {European Transactions on Telecommunications},
    YEAR = {2004},
    month = {Sept.},
    volume = {15},
    number = {5},
    pages = {497-500},
    doi = {10.1002/ett.1002}
    };

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  76. Nikos C. Sagias, George K. Karagiannidis, Dimitris A. Zogas, P. Takis Mathiopoulos, and George S. Tombras, “Performance analysis of dual selection diversity over correlated Weibull fading channels,” IEEE Transactions on Communications, vol. 52, no. 7, pp. 1063-1067, July 2004.
    Download   |   DOI: 10.1109/TCOMM.2004.831362   |   Abstract   |  

    Ascertaining the importance of the dual selection combining (SC) receivers and the suitability of the Weibull model to describe mobile fading channels, we study the performance of a dual SC receiver over correlated Weibull fading channels with arbitrary parameters. Exact closed-form expressions are derived for the probability density function, the cumulative distribution function, and the moments of the output signal-to-noise ratio (SNR). Important performance criteria, such as average output SNR, amount of fading, outage probability, and average bit-error probability for several modulation schemes are studied. Furthermore, for these performance criteria, novel closed-form analytical expressions are derived. The proposed analysis is complemented by various performance evaluation results, including the effects of the input SNR's unbalancing, fading severity, and fading correlation on the overall system's performance. Computer simulation results have verified the validity and accuracy of the proposed analysis.

    @ARTICLE{J:Sagias_61_2004,
    AUTHOR = {Nikos C. Sagias and George K. Karagiannidis and Dimitris A. Zogas and P. Takis Mathiopoulos and George S. Tombras},
    TITLE = {Performance analysis of dual selection diversity over correlated Weibull fading channels},
    JOURNAL = {IEEE Transactions on Communications},
    YEAR = {2004},
    month = {July},
    volume = {52},
    number = {7},
    pages = {1063-1067},
    doi = {10.1109/TCOMM.2004.831362}
    };

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  77. Nikos C. Sagias, Dimitris A. Zogas, George K. Karagiannidis, and George S. Tombras, “Channel capacity and second order statistics in Weibull fading,” IEEE Communications Letters, vol. 8, no. 6, pp. 377-379, June 2004.
    Download   |   DOI: 10.1109/LCOMM.2004.831319   |   Abstract   |  

    The second-order statistics and the channel capacity of the Weibull fading channel are studied. Exact closed-form expressions are derived for the average level crossing rate, the average fade duration, as well as the average Shannon's channel capacity of the Weibull fading process. Numerical results are presented to illustrate the proposed mathematical analysis and to examine the effects of the fading severity on the concerned quantities.

    @ARTICLE{J:Sagias_62_2004,
    AUTHOR = {Nikos C. Sagias and Dimitris A. Zogas and George K. Karagiannidis and George S. Tombras},
    TITLE = {Channel capacity and second order statistics in Weibull fading},
    JOURNAL = {IEEE Communications Letters},
    YEAR = {2004},
    month = {June},
    volume = {8},
    number = {6},
    pages = {377-379},
    doi = {10.1109/LCOMM.2004.831319}
    };

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  78. Nikos C. Sagias, George K. Karagiannidis, and George S. Tombras, “Error-rate analysis of switched diversity receivers in Weibull fading,” Electronics Letters, vol. 40, no. 11, pp. 681-682, May 2004.
    Download   |   DOI: 10.1049/el:20040479   |   Abstract   |  

    Novel expressions for the average symbol error probability (ASEP) in switched and stay combining receivers operating over Weibull fading channels have been derived. Closed-form expressions for the optimum switching threshold in the sense of maximum ASEP have also been obtained. The single-branch receiver performance in the Weibull channel is investigated as a special case.

    @ARTICLE{J:Sagias_63_2004,
    AUTHOR = {Nikos C. Sagias and George K. Karagiannidis and George S. Tombras},
    TITLE = {Error-rate analysis of switched diversity receivers in Weibull fading},
    JOURNAL = {Electronics Letters},
    YEAR = {2004},
    month = {May},
    volume = {40},
    number = {11},
    pages = {681-682},
    doi = {10.1049/el:20040479}
    };

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  79. 2003


  80. Nikos C. Sagias, P. Takis Mathiopoulos, and George S. Tombras, “Selection diversity receivers in Weibull fading: Outage probability and average signal-to-noise ratio,” Electronics Letters, vol. 39, no. 25, pp. 1859-1860, Dec. 2003.
    Download   |   DOI: 10.1049/el:20031189   |   Abstract   |  

    Novel expressions for the probability and the cumulative density function of the signal-to-noise ratio (SNR) at the output of an L-branch selection combining receiver, operating in Weibull fading, are derived. Capitalising on these expressions, the outage probability and the average output SNR are obtained in closed-form.

    @ARTICLE{J:Sagias_64_2003,
    AUTHOR = {Nikos C. Sagias and P. Takis Mathiopoulos and George S. Tombras},
    TITLE = {Selection diversity receivers in Weibull fading: Outage probability and average signal-to-noise ratio},
    JOURNAL = {Electronics Letters},
    YEAR = {2003},
    month = {Dec.},
    volume = {39},
    number = {25},
    pages = {1859-1860},
    doi = {10.1049/el:20031189}
    };

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  81. Nikos C. Sagias, Dimitris A. Zogas, George K. Karagiannidis, and George S. Tombras, “Performance analysis of switched diversity in Weibull fading,” Electronics Letters, vol. 39, no. 20, pp. 1472-1474, Oct. 2003.
    Download   |   DOI: 10.1049/el:20030956   |   Abstract   |  

    A novel approach to the performance analysis of switched-and-stay combining diversity receivers over independent Weibull fading channels is presented. Closed-form expressions are extracted for important performance parameters, such as the average output signal-to-noise ratio, the amount of fading, the outage probability, and the switching rate.

    @ARTICLE{J:Sagias_65_2003,
    AUTHOR = {Nikos C. Sagias and Dimitris A. Zogas and George K. Karagiannidis and George S. Tombras},
    TITLE = {Performance analysis of switched diversity in Weibull fading},
    JOURNAL = {Electronics Letters},
    YEAR = {2003},
    month = {Oct.},
    volume = {39},
    number = {20},
    pages = {1472-1474},
    doi = {10.1049/el:20030956}
    };

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Journal Papers Distribution

The following pie presents the distribution of my journal papers categorized as IEEE, IET and others.