On behalf of the IEEE joint VT/COM/IT Sweden Chapter Board, we are delighted to invite you to an IEEE Vehicular Technology Society Virtual Distinguished Lecture (VDL) by Prof. Rui Dinis, Technical University of Lisbon, Portugal.
Title: Nonlinear effects in digital communications: analytical evaluation of their effects and how to use them to improve the performance
Time and date: May 11, 2022, 10:15-11:30 CET
It is widely accepted that nonlinear effects is something that should be avoided in digital communications. There are two main reasons for this. The first one is that the theoretical analysis of the impact of a given nonlinear device is not simple. The second, and more important, is the nonlinear devices can lead to significant spectral widening effects and/or performance degradation. Since the signals associated to widely employed techniques like OFDM (Orthogonal Frequency Division Multiplexing) and/or MIMO (Multi-Input, Multi-Output) schemes can have very large envelope fluctuations and PAPR (Peak-to-Average Power Rate), they are prone to nonlinear distortion effects like the ones associated to quantizers and power amplifiers. For these reasons, there has been a huge effort in the design of quasi-linear amplifiers, as well as techniques to reduce the PAPR of digital signals.
It was recently shown that strong nonlinear distortion effects do not necessarily mean performance degradation. This is due to the fact that the nonlinear distortion component has some information on the transmitted signals, which can be employed to improve the performance. In fact, the optimum maximum likelihood (ML) performance of nonlinear OFDM schemes can even be better than the performance of the corresponding linear. However, the complexity of optimum ML receivers is prohibitively high, even for a moderate number of subcarriers, which lead to the development of practical, sub-optimum receivers able to achieve the optimum performance of nonlinear OFDM.
In this talk we start by making an overview on the common nonlinear characteristics that arise in digital communications, as well as techniques for studying analytically the impact of nonlinear effects on given signals, with emphasis on the signals associated OFDM techniques and MIMO schemes. Then we present some key results on the impact of nonlinear operations in the spectral occupation and performance. Next, we study the optimum performance of nonlinear OFDM and MIMO schemes, showing the remarkable result that nonlinear techniques can have much better performance than the corresponding linear ones. In fact, a nonlinear device at the transmitter can be regarded as a kind of channel coding scheme if it is properly designed. Finally, we present some practical receivers able to harvest these potential nonlinear performance gains.