This paper aims at clarifying the role of the outer loop power control for CDMA satellite systems with on-board power constraints. If the inner loop of the power control is perfect, the channel turns into a AWGN channel and there is no need of the outer loop. In satellite CDMA systems, due to the longer propagation delay with respect to a terrestrial system, the inner loop of power control is only partly able to track power variations due to fast fading. Moreover, the Rice factor, which characterizes the channel statistics, can widely vary even if the user does not move but just because of the change of the elevation angle. Because of that, a wide range of target SNIR (and larger than in typical terrestrial systems) may be necessary to get the same BER performance. Therefore, the outer loop power control turns out to be essential to minimize the dynamic of the power link margins and avoid capacity degradations induced by the systematic use of static link margins. A semi-analytical model for the capacity evaluation has been developed, which is specifically intended for the power-limited satellite-to-mobile link with multi satellite reception. We found that the capacity gain with respect to a pure SNIR-based strategy (i.e., only inner loop) can reach the 40% of the total capacity in a single reception scheme. A smaller, but still noticeable capacity gain of the order of 20 - 30% is observed in presence of satellite diversity. Therefore, any dimensioning of CDMA satellite systems should not neglect this component of the power control.

Fast closed-loop-power-control (CLPC) plays a key role in direct-sequence (DS) code division multiple access (CDMA) systems, aiming at achieving acceptable carrier-to-interference (C/I) ratios in all active links. Assuming a terrestrial mobile access, where the round-trip delay may be smaller than the channel time-coherence, fast CLPC can compensate for fading and reduce the error burstiness, as well. This paper provides analytical expressions for the bit error probability given: the CLPC algorithm specified in terms of loop delay and updating rate, the propagation power delay-profile and the terminal speed. A binary PSK DS spread spectrum radio interface, with RAKE processing of multipath or diversity, is considered. Our link-level analysis, being conditioned to a specified average value of C/I, is focused on the inverse update algorithm, which is aimed at keeping the received power at a constant level. Applications of the derived method, aimed at getting insight about CLPC performance in 3rd generation radio interfaces, is also provided and discussed.

With non-GEO constellations the primary means of counteracting the shadowing and the blockage effects is through the use of the satellite diversity, i.e. multiple satellites in the constellation are simultaneously visible to user. The probability system availability is improved by increasing the probability at least one satellite is in clear line of sight. In the selection diversity (SD) scheme the user is served by one satellite selected on the basis of the better signal to noise ratio. In CDMA systems the spatial diversity provided by the satellite diversity can be more effectively exploited by combining signal replica coming from satellites in view with the aim at increasing the system capacity. Actually, the performance of signal combining/selection diversity schemes depend on the specific propagation scenario and it is not always effective to combine signal replica. In this paper hybrid selection/combining diversity schemes are proposed and a performance comparison in terms of system availability and capacity enhancement is carried out. Furthermore, an analytical methodology of the downlink of DS-CDMA systems which takes into account the power constraints of a satellite environment is described and applied for the capacity assessment of the proposed satellite diversity schemes.

We propose a decision rule for symbol erasure insertion in a dispersive propagation environment based on Bayesian decision theory. The method is applied to frequency-hop communications with Reed-Solomon coding and error-and-erasures decoding. The proposed algorithm is based on decision feedback to eliminate statistical dependence among the envelope detectors' outputs of an M-ary frequency-shift keying noncoherent demodulator. The performance is analyzed for fading channels with different levels of frequency selectivity and synchronous multiple-access interference. It is shown that the use of memoryless decision rules results in complete inefficiency in such channel conditions, whereas the proposed rule provides large coding gains even for severe selectivity. The performance sensitivity with respect to the estimation of the power delay profile denotes high robustness of the proposed approach, provided that the selectivity is somewhat overestimated. Sensitivity results for error propagation due to decision feedback are also given.

The application of neural classifiers for providing optimal decision boundaries of a warped and clustered M-QAM constellation affected by nonlinearity is analyzed in this paper. The classifier behavior, for the specific application, has been evaluated both by the carrier to noise ratio (CNR) degradation (DeltaC/N) due to nonlinearity for a target error rate P(c)=10(-3), and more thoroughly by classical figures of merit of the pattern recognition theory such as classification confidence and generalization capability. The influence of the probability distribution of the training examples and the effects of activation functions' sharpness (namely the temperature of the net) have also been investigated. The results, obtained on a simulation basis, indicate optimal matching with respect to upper bounds evaluated with some minor simplifying hypothesis, even if the overall method's effectiveness can be adequate only for mild nonlinearity conditions.

In this paper we propose a method for symbol erasure decision in a dispersive propagation environment, based on Bayesian decision theory. The decision rule is used with Reed-Solomon coding and error-and-erasures decoding. The proposed method is based on a decision feedback approach to eliminate statistical dependence among envelope detector's outputs of an M-FSK non-coherent demodulator. The performance is analyzed for channels with different levels of selectivity and synchronous multiple access interference. It is shown that the use of memoryless decision approaches result in poor performance, whereas the proposed technique provides large coding gains even for severe selectivity. Sensitivity results on error propagation due to decision feedback are also given.

A compensation technique for nonlinearities is usually necessary in QAM digital satellite links. Analog waveform compensation has the advantage of reducing out-of-band spectral emissions with respect to end-to-end digital predistortion, in addition to its easy realization. Nevertheless, it has been often neglected in the literature because of its lack of adaptivity; we propose making the predistorter fully adaptive. Different control schemes applied to fifth-order IF predistortion with different optimization criteria have been investigated. A simulation-based analysis shows that adaptive IF compensation achieves excellent performance if compared with static predistortion of the same order. Comparisons with upper bounds related to digital predistortion with memory, moreover, denote that IF analog predistortion can be truly considered an alternative approach, especially for narrow pulse shaping.