S. De Fina

Analysis of different optimization criteria for IF predistortion in digital radio links with nonlinear amplifiers

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.

Error probability analysis for CDMA systems with closed-loop 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.

Application of neural waveform predistortion to experimental TWT data

An evaluation is made of the predistorters achievable performance using HPA (high-power amplifier) models matched to experimental TWT data. How the neural net capability for inverse modeling, and then as predistorter, is related to the various TWTs that must be fitted is discussed. A supervised neural net is used with one internal neuron and no more than 10 internal unit, and the backpropagation algorithm for the learning process. The results related to TWT data obtained confirm the performances achievable with the generic TWT model: an average gain of 3 dB for the 64-QAM and an average gain of 5.5 dB for the 256-QAM systems, with respect to a baseband predistorter.<<ETX>>

On the generation of side information for frequency hopping multiple access in Rice fading

In the framework of wideband wireless local area networks (WWLAN), spread spectrum techniques are a viable solution for uncoordinated multi-user communications. Here, frequency hopping is considered for its less stringent power control requirements with respect to direct sequence. In particular, the generation of side information to allow errors-and-erasures decoding of Reed-Solomon codes in frequency hopping multiple access communications is addressed. Symbols are erased following a Bayesian decision rule (BDR). New expressions are given for the erasure decision function in Rice fading conditions for both synchronous and asynchronous hopping. The performance sensitivity with respect to the Rice factor is studied, and compared to that of the ratio-threshold test (RTT). It is found that while BDR as more robust than RTT for the synchronous case, the opposite seems to hold in the asynchronous case.

Bayesian methods for erasure insertion in frequency-hopping multiple access with Rice selective fading

A Bayesian decision rule for erasure insertion is derived, assuming frequency-hop multiple-access communications in Rician selective fading. The proposed method is based on a channel-identification approach together with decision feedback. Given the propagation environment under study, which can be deemed typical for broadband mobile systems, the Rice factor, in addition to the channel power delay profile, needs to be estimated by the receiver. Significant performance gain, compared to the simple ratio-threshold test (RTT), can be achieved with the proposed method at the expense of greater complexity, mainly due to the channel-identification requirement. Nevertheless, it is shown that this Bayesian decision rule exhibits high robustness in respect of the joint estimation of the Rice factor and power-delay profile, thus proving its actual applicability in real receivers employing erasure insertion.

Spectral and power efficiency of the UMTS satellite component

This paper provides an analysis framework for the design of the UMTS (Universal Mobile Telecommunication System) satellite component, focusing on resources optimization issues for a wideband CDMA (code division multiple access) scheme. Power efficiency is here considered in addition to spectral efficiency, giving insight about the systems which are expected to be interference limited or noise limited and about the achievable capacities. Numerical results related to the voice service are shown for different orbits and frequency bands. The effect of shadowing and the use of dual-satellite diversity have been included in the analysis.

Bayesian approach for the generation of side information in frequency-hop communications with selective fading and multiple access interference

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 detectors 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.

Optimization of an adaptive cubic predistorter for multilevel quadrature amplitude modulation

The performance of an optimized waveform predistorter to compensate for the nonlinear distortions in M-QAM (quadrature amplitude modulation) digital radio links is analyzed. An optimization technique for cubic predistorters is proposed. The optimization is carried out by varying the predistorter characteristic against the working point of the power amplifier, whereas conventional cubic distorters are aimed at approximating the HPA (high power amplifier) characteristic only in a fixed point (the origin or the saturation point). The receiver is supposed to provide amplitude and phase control. Simulation results show that the proposed solution performs much better than not only the conventional cubic predistorter but also the baseband digital predistorter.<<ETX>>

The use of a Neural Net for Copeing with Nonlinear Distortions

In this paper a predistorter realized with a neural net is proposed. The predistortion acts on the whole input waveform : it provides a continuous time prewarping where convenctional base band predistortion acts only on the discrete time input sequence. The overall linearity of the transmitter is so recovered and the nonlinear ISI is removed at sampling point The work is a development of a previous one [10] in which a neural net was employed to realize a base-band memoryless predistortion. The results obtained for 16, 64 and 256 QAM modulation systems show excellent performance if compared with base-band memoryless predistortion expecially if small roll-off factors are employed. The major improvement, however, is represented by the extreme simplicity of the proposed solution compared with convenctional predistorters with memory. In this work we have assumed some simplification concerning the neural net architecture; future works, however, will be aimed to removing these reductive assumptions.

Cyclic equalization for nonlinear modulations

The paper is focused on the application of cyclic equalization to the received signal sampled prior to demodulation. This technique, particularly, has been proposed for nonlinear modulations, namely FSK, where the nonlinear dependence of the received signal on the modulating sequence precludes the use of conventional equalization, being controlled through data observation. In cyclic equalization, in fact, the particular structure of the training sequence makes it possible to employ a straightforward control of the signal distortion by means of frequency observation. The technique can be particularly suitable for frequency-hopping transmissions since it can provide fast equalizer set up each time a new radio channel is engaged. Simulation analysis, carried out for FSK and linear modulations over the HF radio channel, yields to consider this proposal an attractive solution for all those systems in which the only alternative to compensate for the effects of channel distortion is the complex MLSE.