Widely linear (WL) receivers are able to fulfill single antenna interference cancellation (SAIC) of one rectilinear (R) or quasi-rectilinear (QR) co-channel interference (CCI), a function which is operational in GSM handsets in particular. However, SAIC requires enhancements for QR signals, for both VAMOS standard, an evolution of GSM/EDGE standard, and FBMC-OQAM networks, which are candidate for 5G mobile networks. In this context, the purpose of this paper is twofold. The first one is to show that, contrary to what is accepted as true in the literature, QR signals are less efficient than R ones for conventional WL filtering in the presence of CCI. From this result and for QR signals, the second purpose is to propose a SAIC/MAIC enhancement, based on WL frequency shifted (FRESH) filtering, making QR signals almost equivalent to R ones