  Unipolar communications systems can transmit information using only real and
positive signals. This includes a variety of physical channels ranging from
optical (fiber or free-space), to RF wireless using amplitude modulation with
non-coherent reception, to baseband single wire communications. Unipolar OFDM
techniques enable to efficiently compensate frequency selective distortion in
the unipolar communication systems. One of the leading examples of unipolar
OFDM is asymmetric clipped optical OFDM (ACO-OFDM) originally proposed for
optical communications. Flip-OFDM is an alternative approach that was proposed
in a patent, but its performance and full potentials have never been
investigated in the literature. In this paper, we first compare Flip-OFDM and
ACO-OFDM, and show that both techniques have the same performance but different
complexities (Flip-OFDM offers 50% saving). We then propose a new detection
scheme, which enables to reduce the noise at the Flip-OFDM receiver by almost
3dB. The analytical performance of the noise filtering schemes is supported by
the simulation results.
