Cancelamento das distorções não lineares em sistemas ofdm cooperativos amplifica-e-encaminha

Abstract

The integration of different technologies is a key approach for wireless communication systems of the next generations. The challenge of optimizing existing resources and increasing the quality of services gives rise to hybrid systems. Considering these aspects, this research integrates two major strategies, OFDM and cooperative communication, to enhance the performance of systems and increase the quality of service. However, an important feature of OFDM systems is the sensitivity to nonlinear PAs (Power Amplifiers). These devices may damage the transmitted signal by inserting distortions in the signal waveform, thereby compromising detection at the receiver. In this context, the objective of this dissertation is to propose iterative techniques for cancellation of nonlinear distortions in AF (Amplify-and-Forward) cooperative OFDM systems. The proposed techniques estimate and cancel nonlinear distortions caused by the use of PAs at source and relay. Initially, we propose a technique that does not consider the use of the direct channel between source and destination (SD). The system model is composed by a source (S) equipped with a nonlinear PA, a relay (R) with a nonlinear PA and AF (Amplify-and-Forward) protocol and a destination (D). The first technique proposed for environments without exploitation of cooperative diversity is called Double Cancellation Iterative with Hard Detection (Double-CIDH) and has great ability to estimate and cancel the nonlinear distortions of the source and relay. Indeed, the estimates of nonlinear source distortions are used in the detection and cancellation of relay nonlinear distortions. For the proposal of the second algorithm, the direct link (Source-Destination - SD) and the cooperative link (Source-Relay-Destination - SRD) were considered. This new algorithm named Double-CIDH with Cooperative Diversity (Duplo-CIDH-DC) has a greater degree of sophistication, as it presents in its structure blocks that will pre-whitening the incoming signals and then combine them through the maximum combining method Maximal Ratio Combining (MRC). We consider that the receiver has full knowledge about the state of the channels. The proposed techniques were evaluated in terms of the Symbol Error Rate (SER) through numerical simulations. The obtained results allow us to verify that both proposed techniques have performed well the task of cancelling the nonlinear distortions, providing results close to the case with linear PAs.