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A novel analytical framework for the accurate and efficient evaluation of the performance of channel resource management strategies for low Earth orbit mobile satellite systems (LEO-MSSs) supporting multiparty traffic is presented. By considering a fixed channel reservation (FCR) scheme as a benchmark, an efficient and accurate analytical approach is developed for obtaining the performance of multiparty traffic under various quality-of-service (QoS) performance measure criteria. The proposed approach is based on a Markovian queuing model, and its correctness and accuracy have been verified by means of computer simulations. To improve the overall performance of LEO-MSS, two novel resource management techniques are introduced and analyzed. The first one is an efficient adaptive channel reservation (ACR) scheme, which allows priority to be given to handover requests that are generated by multiparty traffic. The second one is a new call queuing (NCQ) policy, which efficiently reduces the new call blocking probability with little impact on other system performance measures, such as call dropping probability and unsuccessful call probability. Various performance results show that when ACR is used in conjunction with NCQ, extremely low blocking and handover failure probabilities can be achieved for multiparty traffic. Index Terms--Channel resource management, fixed channel reservation (FCR), handover, low Earth orbit (LEO), mobile satellite systems (MSSs), multiparty.

The exact symbol error rates (SERs) of several M-ary modulation schemes are obtained in a unified manner for mobile radio receivers employing dual-branch coherent equalgain combining (EGC), where the received signals on each branch follow either similar or different fading distributions. Specifically, the cases considered are Nakagami-m, Rice, and Hoyt fading on both branches, but with not necessarily identical parameters, and mixed Nakagami-m/Rice and Nakagami-m/Hoyt fading; only the integer or half-integer values of the Nakagami-m parameter are considered. While the previous exact expressions for the SERs of general M-ary modulations with L-branch EGC contained double integrals, the error probability expressions derived in this paper for the case of L = 2 contain only a single integral. In addition, the derived probability distributions also allow the direct obtaining of other useful performance measures, such as the exact level crossing rates, average fade durations, and outage probabilities. Index Terms--Equal-gain combining (EGC), Hoyt fading, Nakagami fading, Rician fading.

Broadband satellite services to fixed terminals are currently offered in the forward link by the 2nd Generation (2G) Digital Video Broadcasting Satellite (DVB-S2) standard. For this standard the use of powerful Low-Density Parity-Check (LDPC) error correcting codes has been adopted performing within approximately 1 dB from the Shannon capacity limit. This paper studies and compares for the first time in a systematic manner different approximation methods used in check node update computation of DVB-S2 LDPC decoding with the aim of reducing computational complexity. Various performance evaluation resuits are presented for a wide range of DVB-S2 parameters, such as LDPC codeword size, coding rate, modulation format and including several decoding algorithms. It is shown that the proposed check node update approximations have a robust behavior, i.e. the resulting performance is quite independent of the DVB-S2 modulation and coding parameters. It is further shown that these approximations perform very close to the optimal Sum-Product Algorithm (SPA) in degradation, which is less than 0.2 dB. Despite this small degradation, the reduction in computational complexity compared to the optimal SPA is significant and can be as high as 40% in computational time savings. Index Terms--Broadcasting, iterative decoding, Low-Density Parity-Check (LDPC) codes.

In this study, a new method for digitizing a combination of different analog signals occupying significantly different bandwidths and having a very high dynamic range is proposed and analyzed. Since it is based upon signal-prediction/cancellation principles, it is referred to as adaptive prediction and cancellation digitization (APCD) method and is applied to various families of signals simultaneously received by a multistandard software radio (SWR) base-station receiver. It is shown theoretically and by means of computer simulations that the APCD method can effectively reduce the high dynamic range of the signals before digitization takes place. Hence, the stringent analog-to-digital-converter (ADC) resolution requirements imposed by the operation of such SWR base-station receivers can be significant relaxed. The signal dynamic-range reduction is achieved by applying appropriate signal processing techniques, e.g., autoregressive (AR) and periodic autoregressive (PAR) prediction. Such techniques allow accurate prediction and subsequent cancellation of high-power narrowband signals present among the composite received analog signal. As these signals usually have cyclostationary statistical characteristics, analysis and performance evaluation of AR and PAR predictors, when used to predict cyclostationary signals, were presented. A new adaptive algorithm for implementing the PAR predictor is also proposed, and its validity is justified by theoretical analysis as weil as by various performance evaluation results obtained by means of computer simulations. Index Terms--Analog-digital conversion, cyclostationary signal, multistandard base-station receiver, signal prediction, software radio (SWR).

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. Index Terms--Amount of fading ([A.sub.F]), bit-error rate (BER), generalized-selection combining (GSC), land-mobile satellite, moment-generating function (MGF), ordered statistics, outage probability, Pade approximants, Weibull fading channels.

Using results from the theory of multidimensional Gaussian distributions, we provide closed-form or infinite series expressions for the envelope autocorrelatinn function of the output of a maximal-ratio combiner, for Rayleigh, Nakagami, and Rician fading. These expressions generalize known analytical results obtained for the case of no diversity. The power spectrum of the combined envelope is also obtained for Rayleigh and Nakagami fading. The validity of the analytical approach is verified using computer simulation results obtained with a land-mobile fading emulator. It is observed that, for the land-mobile radio channel, the use of diversity slightly increases the time correlation between the receiver inputs, but in a nonuniform manner across the range of time lags. Index Terms--Envelope correlation, maximal-ratio combining (MRC), Nakagami fading, Rician fading.

We designed a multi-channel Nakagami fading simulator by modeling the received combined signal-to-noise ratio as a finite-state Markov chain, following a previously proposed approach. Our model generates directly the error process at the output of a diversity receiver and can emulate selection, maximal-ratio, and equal-gain combining. As the order of diversity increases, the savings in computational complexity improve linearly with respect to a traditional waveform simulator. The level crossing rates of the simulated envelope are shown to be very close to their theoretical values. The simulator's performance is also evaluated in terms of the accuracy of the obtained bit error rates, for both uncoded and coded systems. The simulator speeds up the performance evaluation of high-rate communication links where a high number of samples is needed. Index Terms--Diversity methods, fading channels, level-crossing problems, radio broadcast receivers, simulation software.

This paper deals with the process of constructive and destructive addition of multipath signals in a Rayleigh fading channel. It derives closed formulas associating the number of multipaths with the Rayleigh statistics of the channel and it displays the accuracy of these formulas by means of simulation. The primary conclusion is that multipaths add constructively with high probability, thus the more the multipaths the larger the mean power level experienced at the receiver. In this context, multipath propagation exhibits a beneficial property that could be exploited for designing improved performance receivers. Index terms - Multipath combining, Multipath fading, Rayleigh fading, Mobile channels.

The performance of differentially encoded quadrature phase-shift keying (DQPSK) systems employing nonredundant error correction (NEC) receivers with single- and double-error correction capability is analyzed and evaluated for the aeronautical satellite channel. The NEC is an attractive coding technique which employs differential detectors with more than one symbol delay elements and which does not introduce any redundancy as other coding schemes do. As typical for aeronautical satellite communications, a Rician fading channel with Gaussian power spectrum has been considered. Unlike the additive, uncorrelated from symbol to symbol interference such as additive white Gaussian noise (AWGN) or static cochannel interference (CCI) which has been investigated in the past, analysis of the performance in a fading channel is much more difficult. The difficulty arises from the multiplicative and correlative nature of the fading interference. Bit error rate (BER) performance evaluation results have been obtained by means of computer simulation for various channel conditions, including different values of the K-factor and the fading [B.sub.D]T. These results have indicated thai considerable performance gains as compared with conventional differentially detected systems are achieved for high values of K and for very fast fading. Both of these conditions are encountered in typical aeronautical communication systems. Wherever possible, heuristic explanations of the trend of the obtained BER performance evaluation results are also given.

In this paper, we derive the optimal sequence estimator for digital signals received over Lambda different channels. Each of these channels corrupts the transmitted signal by a mixture of additive white Gaussian noise (AWGN) and frequency-nonselective, correlated, fast Rician fading. By analysis we show that for the lth (1 is less than or equal to l is less than or equal to Lambda) diversity channel, the basic hardware structure of the optimal receiver consists of a combination of envelope, multiple differential, and coherent detectors. This is the same receiver structure recently derived by the authors for the same channel conditions, but with a nondiversity receiver. Nevertheless, the metric expression of the optimal sequence estimator required to decode the received digital signals is different and depends upon the number of diversity branches, the ratio of the fading-to-Gaussian noise powers, and the transmitted sequence. In order to reduce the overall implementation complexity, we also have proposed and evaluated suboptimal, e.g., having a small number of differential detectors and equal combining diversity structures, versions of the optimal receivers. Two modulation schemes were chosen in order to evaluate the overall performance of the proposed reduced-complexity diversity receivers; namely, the Pi/4-shift 8-DQAM (differential quadrature amplitude modulation) and the well-known 8-DPSK (differential phase shift keying). The obtained bit-error-rate (BER) performance evaluation results clearly indicated significant performance improvements, including substantial error-floor reduction, as compared to conventional differentially detected Pi/4-shift differential quadrature phase shift keying (DQPSK) systems. Most of the improvements are obtained using diversity of order two (Lambda = 2). By means of computer simulation, the effect of correlation between the fading processes on the Lambda diversity channels was also investigated. It was found that the performance improvement obtained when using diversity greatly offsets the degradation caused by any practical level of correlation between diversity channels. BER performance results further demonstrated that the sequence estimation algorithm proposed is quite insensitive to significantly varying channel conditions, i.e., variations in the fading model spectral shape and parameters. This important quality renders it particularly suitable when considering a practical system implementation.

The end-to-end performance of dual-hop cooperative diversity systems equipped with nonregenerative relays and a selection combining receiver at the destination terminal over independent and nonidentical Nakagami- fading channels is studied. Closed-form expressions for the cumulative distribution function and the probability density function of the end-to-end signal-to-noise ratio ( ) are presented, while analytical formulae are derived for the moments and the moment generating function. Using these statistical results, closed-form expressions for the outage probability are presented for both channel state information and fixed gain relays. Furthermore, for the case of fixed gain relay, the average end-to-end , the amount of fading, and the average bit error rate can be numerically evaluated. The proposed mathematical analysis is complemented by numerical examples, including the effects on the overall performance of the s unbalancing as well as the fading severity.