Gonçalo Tavares

Improved Cramer-Rao lower bounds for phase and frequency estimation with M-PSK signals

We derive new Cramer-Rao lower bounds (CRBs) for the estimation of carrier phase and frequency offset from an unmodulated carrier or from a M-PSK data signal. The new CRBs are obtained formally from the exact likelihood function for the carrier phase and frequency offset, build from a block of received phase samples, and are applicable for a general alphabet size M. The bounds are compared with other previously obtained bounds (which also take into account the effect of random phase modulation) and with the performance of some popular feedforward algorithms for carrier phase and frequency offset estimation.

Sequence Design for Data-Aided Estimation of Synchronization Parameters

In this letter, we propose a systematic technique for optimum data-sequence design for data-aided (DA) estimation of synchronization parameters. It is assumed that the system operates in burst mode, transmitting a linearly modulated information signal over an additive white Gaussian noise channel. The method is based on the minimization of the corresponding Crameacuter-Rao lower bound subject to a power constraint and is applied in four practical receiver synchronization contexts: estimation of the symbol timing with known carrier frequency offset and either known or unknown carrier phase, joint estimation of the symbol timing and carrier phase with known carrier frequency offset, and joint estimation of the carrier phase and carrier frequency offset with known symbol timing

A fading channel simulator based on a modified Karhunen-Loève expansion

This paper presents a Rayleigh fading channel simulator based on a modified Karhunen-Loeve expansion of a Gaussian complex fading process. The method is similar to the well-known IFFT method but with a different frequency mask. The simulation accuracy is assessed by the computation of power margins (for the autocorrelation) and Kullback-Leibler divergence (for the envelope probability density function) of the simulated fading process. These results demonstrate a good agreement with the theory and a slight improvement relative to the IFFT method.

A reconfigurable calibration method for current-steering DACs

A new calibration method for current steering DACs based on dynamically rearranging the switching sequence of the MSB current sources, also called switching-sequence post-adjustment (SSPA) is presented. This calibration method conducts a search for the optimal switching sequence that maximizes any desired performance metric instead of generating it based on ad-hoc or heuristic arguments. An add-on to the calibration method based on the correction of the output current is also proposed. The proposed calibration method improves the INL up to 9 times and the ENOB up to 1.1 bit when compared to a state-of-the-art SSPA calibration method.

Low frequency PWM modulation for high efficiency Class-D audio driving

This paper is focused on the necessity of developing a low frequency modulation system to convert a PCM audio signal into a binary signal with suitable properties to drive Class-D audio amplifiers. Several modulation techniques are being used to overcome the inherent distortion of PWM techniques particularly its need for a high sampling frequency and correspondingly high output switching rates with negative impact on efficiency. A full-digital click modulation system is presented along with its implementation constraints and advantages when compared with other modulation solutions. The simulation results presented suggest that this modulation can be very competitive, especially when the efficiency of the amplification system is a key target.

Live demonstration: A pulsar signal receiver system for navigation

Pulsars emit broadband electromagnetic radiation, with an individual signature (period and shape), ranging from radio to X-ray and gamma-ray frequencies. For the purpose of navigation the pulsars of interest are fast rotating neutron stars that emit short stable periodic pulses, having periods between milliseconds and seconds. By detecting a set of known pulsars, it is possible to use them as beacons for navigation in celestial coordinates, being seen as the only truly navigation solution for deep-space navigation [1]. In this demo a proof of concept is presented for an innovative receiver architecture, overcoming, some of the issues presented in the paper entitled “Radio Pulsar Receiver Systems for Space Navigation” [2]. Furthermore, a fake pulsar generation board was developed and also an acquisition board, for both, simulate a pulsar signal and to process the data from the receiver and acquire the pulsar signal from noise.

On the Statistics of the IEEE 802.11n/ac Fading Channel Models

In this letter, we present closed-form expressions for the complex autocorrelation function (ACF) and for the envelope level crossing rate (LCR) and average fade duration (AFD) parameters pertaining to the fading channel models proposed in the IEEE 802.11n/ac standards, which specify a bell-shaped Doppler power spectral density (PSD), truncated to a maximum cutoff frequency fmax. It is shown that the ACF is not very sensitive to fmax and that the dependence of the LCR and AFD on this parameter may be exploited to ease the implementation of practical channel simulators for this standard. A new series expansion for the LCR of general Rice-distributed envelopes, which avoids numerical integration, together with a useful upper bound on the precision of the truncated series is also presented.

A Click Modulation audio player

This paper presents a technical solution for a audio player based on Click Modulation, capable of generating a PWM output out of a pre generated audio file. This modulation allows a spurious-free baseband with very low PWM switching rates. The player is based on a Texas Instrument (TI) C2000 32-bit microcontroller, that provides high resolution PWM generation with low system clock and very competitive price. Off-line Click Modulation and typical PWM modulation solutions (NPWM and USPWM) are also reported. In addition, overview of two analytical approaches for PWM spectrum determination and its results are used to help the different process used to generate the tested signals. Also a numerical approach is used for PWM spectrum determination. The experimental results of the off-line modulations used validate the theoretical and the numerical models used.

A simple method for the simulation of autocorrelated and arbitrarily distributed envelope fading processes

We propose an efficient and accurate method for imposing a desired autocorrelation on random sequences with arbitrarily distributed envelope. In particular, the method is applied to the simulation of Nakagami-m and Weibull fading channels, with arbitrary values of the fading parameter. The proposed method is based on the unidimensional Iman-Conover method. This method is computationally unexpensive and the experimental results demonstrate the statistical quality of the samples generated for both these fading channels.