Conferences Articles

We present analytical results on the average bit error probability (ABEP) of an optically pre-amplified pulse-position-modulation (PPM) receiver that operates under fading and pointing errors. The analytical description enables the efficient calculation of the ABEP in Malaga-M, γ - γ and negative-exponential fading for a wide range of modulation orders and noise modes. As expected, a significant power budget gain is initially obtained by increasing the PPM modulation order, however a further improvement requires the optimization of the received beam waist. Finally, the noise modes that enter the receiver negatively affect its operation irrespective of fading and pointing errors.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The performance of digital communication systems over generalized-K (K_G) 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 K_G fading channels. The proposed mathematical analysis is accompanied with various performance evaluation results, which demonstrate the usefulness of the proposed approach.

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

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

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

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

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

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

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

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

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