Research staff:María Jesús Cañavate Sánchez and Prof George Goussetis
External collaborators: Prof Savvas Kosmopoulos
Funding bodies: Heriot-Watt University and Space Engineering
Project Description: Increasingly complex coding and modulation schemes are being applied as means to enhance spectral efficiency. Coupled with the requirement for low-cost and environmentally friendly communications, the aspect of signal amplification becomes entangled with the actual spectral content of the system. It is now widely recognised that power amplification efficiency can only be optimised when the spectral characteristics of the signal to be processed are considered. To that end, fast and efficient methods to model signal distortion in non-linear circuits has emerged as a key priority. The aim of this project is to investigate methods and techniques for efficient modelling of signal distortion in PAs and focus on the development, improvement and experimental verification of one of the state-of-the-art methodologies, the Bessel-Fourier power amplifier, PA, behavioural model.
The relevance of the Bessel-Fourier PA behavioural model relies on the possibility of estimating the power, phase and frequency of every intermodulation product at the output of the amplifier independently, which is useful for the control of aliasing. Furthermore, as it is based on a Bessel series, the accuracy of the method can be improved by increasing the number of terms. However, the accuracy of the model decreases drastically when trying to estimate the nonlinear behavior of the amplifier at the highest power levels of its AM/AM and AM/PM measurements. This is a significant issue, not only for multicarrier communication systems, but for RADAR applications. Therefore, an extrapolation method has been proposed to improve the accuracy of the model when the amplifier is operating in full saturation, which means that the behavior of the PA when its maximum input power level (no damage) is applied will be characterized. Additionally, an efficient way to obtain the coefficients of the model has been studied and experimental validations in which several number of carriers with different power levels are applied at the input of the amplifier has been obtained for the first time in the literature (Figures 1 and 2).
Figure 1: Comparison between measured and computed frequency spectrum at the output of ZJL-4HG+ when 10 carriers with equal power levels are applied at the input.
Figure 2: Comparison between measured and computed frequency spectrum at the output of ZJL-4HG+ when 5 carriers with unequal power levels are applied at the input.
As the presented PA behavioural model has been developed for only angle modulated signals, future modifications must be applied to consider amplitude modulation as well (i.e. for LTE signals). Finally, the model will be tested against commercial programs.