Aldo N. D'Andrea

Channel estimation for ultra-wideband communications

This paper deals with channel estimation in ultra-wideband communications operating in a multipath environment and in the presence of multiaccess interference. The channel parameters are the attenuations and delays incurred by the signal echoes along the propagation paths. Time-hopping modulation with binary symbols is assumed. The estimation method is based on the maximum-likelihood criterion and is applied to two different scenarios: either with known symbols (DA estimation) or with unknown symbols (NDA estimation). The effects of the estimation errors on the performance of a RAKE receiver are assessed by simulation by comparing the receiver bit-error rate with either perfect channel estimates or imperfect estimates as obtained from the proposed algorithms. The results show that the degradations are tolerable as long as the number of users is limited. They also show that the DA method has an edge over the NDA in that it can handle a larger number of users for a fixed degradation. The number of users that can be accommodated in practice is found for some values of the system parameters.

RF power amplifier linearization through amplitude and phase predistortion

This paper presents a novel technique for power amplifier linearization in digital microwave radio systems. The proposed technique is based on the use of a predistortion circuit, whose AM/AM and AM/PM responses are separately implemented as polynomial approximations of the respective responses of the ideal linearizer. The proposed scheme is shown to attain superior performance in comparison with other well-known predistortion structures, such as those based on the cancellation of third or fifth order distortion, with no substantial aggravation in implementation complexity.

Symbol timing estimation with CPM modulation

Symbol timing in continuous phase modulated signals is investigated making use of maximum likelihood estimation methods. Nondata-aided algorithms are proposed with a feedforward structure. They are suitable for digital implementation and can be employed with either full or partial response signaling. Multilevel symbol alphabets are allowed and the modulation index may be arbitrary. The performance is assessed by analysis and simulation. It turns out that full response schemes are comparatively easier to synchronize. Difficulties arise with partial response signaling especially with long frequency pulses.

Feedback frequency synchronization for OFDM applications

Maximum likelihood estimation techniques are often used to derive carrier frequency recovery schemes for digital receivers. Unfortunately, their rigorous application is generally hindered by analytical difficulties and, in consequence, some approximations must be made to achieve practical results. We concentrate on frequency recovery for orthogonal frequency division multiplexing signals, a problem already discussed in the literature. Following Daffara and Chouly (1993) we derive a feedback synchronizer based on maximum likelihood estimation methods. Making different approximations, however, we come up with a scheme that is superior in two respects: it has better accuracy and is simpler to implement. Comparisons are made with other feedback schemes based on heuristic reasoning.

A digital approach to efficient RF power amplifier linearization

This paper presents a digital algorithm for the linearization of microwave power amplifiers, based on the amplitude and phase predistortion technique. An adaptive version of the algorithm, capable to track possible variations in the amplifier parameters, is also discussed. The new algorithm is thoroughly assessed and compared to a digital predistortion scheme proposed in the literature. Our predistorter proves considerably faster than the other scheme, though approximately exhibiting the same accuracy.

Parallel amplitude and phase predistortion for RF power amplifier linearization

This paper presents a novel technique for power amplifier linearization in digital microwave radio systems. The proposed technique is based on the use of a predistortion circuit, whose AM/AM and AM/PM responses are implemented separately as polynomial approximations of the respective responses of the ideal linearizer. The proposed scheme is shown to attain superior performance in comparison with other popular predistortion structures, such as those based on the cancellation of third or fifth order distortion, with no substantial aggravation in implementation complexity. The method lends easily itself to a digital adaptive implementation.

A new carrier frequency loop for OFDM systems

We derive a decision-directed carrier frequency estimator for OFDM systems. The estimator has a closed loop structure and is based on maximum likelihood inspired methods. A simplified version of the loop is also developed that can be implemented with relatively low complexity and acceptable degradations compared with the optimum scheme. The estimation accuracy is assessed by computer simulation for AWGN and frequency-selective multipath channels. Comparisons are made with other schemes discussed in the literature. Simulation results indicate that the proposed loop has better performance.

A performance prediction model for BIC-OFDM transmissions with AMC over nonlinear fading channels

This paper presents an accurate tool yielding a reliable prediction of the link quality for the down-link of a BIC-OFDM system featuring bandlimited pulse shaping to control spectral sidelobe emission, plus AMC, and operating over a nonlinear channel. The tool enables an effective design of the communication system, with special respect to the setting of the operating point of the transmitters power amplifier. First, an analytical characterization of the nonlinear distortion effects caused by the amplifier on bandlimited BIC-OFDM signals is carried out by resorting to the extended version of the Bussgang theorem. Then, the equivalent additive noise model provided by the Bussgang-based analysis is taken into account in the derivation of a novel link quality prediction method, named BκESM. Finally, the novel BκESM tool is employed in the link design phase for choosing the best amplifier setting, aimed at maximizing the layer-3 offered traffic over a realistic wireless channel, featuring both multipath propagation and nonlinear distortions.