I. Introduction
The requirement of transmitting high data rates over doubly selective channels (DSC) while maintaining reasonable spectral efficiency forces single carrier (SC) and orthogonal frequency division multiplexing (OFDM) systems to work under high interference conditions. In the case of SC wideband radio communications, the delay spread is so large that the inter-symbol interference (ISI) covers several symbols. In the case of OFDM based systems, Doppler spread introduces inter-carrier interference (ICI), which usually covers several carriers. Restricting the analysis to the case of block-type transmissions, either ICI in OFDM or ISI in SC become equivalent problems, so similar techniques have been developed in order to combat them. However, overall interference introduced by the DSC in the data symbols is commonly less severe in the OFDM system, as it exhibits rapid decay of interference contribution with respect to the symbol distance in SC systems; therefore, this work is focused on the operation of OFDM systems in DSC. In this context, there are two main approaches for dealing with transmission over DSC: a) Transmitting the traditional OFDM signals while increasing the complexity in the receiver (Rx) and b) Devising novel systems specifically designed to operate over DSC.