Universal linear precoding for widely-linear equalization in OFDM systems

In orthogonal-frequency-division-multiplexing (OFDM) systems, finite-impulse response (FIR) linear precoders allow for perfect or zero-forcing (ZF) equalization of FIR channels (in the absence of noise). Recently, it has been shown that the non- circular or improper nature of some symbol constellations offers an intrinsic source of redundancy, which can be exploited to design efficient FIR widely-linear (WL) receiving structures for OFDM systems operating in the presence of narrowband interference (NBI). With regard to both cyclic-prefixed and zero-padded transmission techniques, we show that, with appropriately designed precoders, it is possible to synthesize in both cases WL-ZF universal equalizers, which guarantee perfect symbol recovery for any FIR channel and, at the same time, exhibit satisfactory NBI suppression capabilities.