Flip-OFDM for Unipolar Communication Systems

Abstract

Abstract-Unipolar communications systems can transmit information using only real and positive signals. This includes a variety of physical channels ranging from optical (fiber or freespace), to RF wireless using amplitude modulation with noncoherent reception, to baseband single wire communications. Unipolar OFDM techniques can efficiently compensate frequency selective channel distortion in unipolar communication systems. One of the leading example 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. In particular, Flip-OFDM offers 50% saving in hardware complexity at the receiver over ACO-OFDM. 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.

OFDM-Based Visible Light Communications

“…The concepts of U-OFDM and Flip-OFDM have been developed in parallel completely independently from each other and broadly within the same time period. It is interesting to note that all four approaches -ACO-OFDM, PAM-DMT, Flip-OFDM and U-OFDM -achieve the same performance in both a linear and a non-linear additive white Gaussian noise (AWGN) channel as will be illustrated later in the context of this work [3][4][5][6][7][8]. For an equivalent M-QAM/M-PAM modulation order, the spectral efficiency of each of these four methods is halved in comparison to DCO-OFDM.…”

“…The scheme presented in this chapter, unipolar orthogonal frequency division multiplexing (U-OFDM), is inspired by the concept of subcarrier-index modulation orthogonal frequency division multiplexing (SIM-OFDM) and aims to close the 3 dB gap between OFDM and ACO-OFDM for bipolar signals, whilst generating a unipolar signal, which does not require biasing. It should be noted that the concepts presented for U-OFDM in the first part of this chapter have also been introduced by another research group as Flip-OFDM in [5] and in [6]. The concepts of U-OFDM and Flip-OFDM have been developed in parallel completely independently from each other and broadly within the same time period.…”

“…Thus, the premise is that a larger constellation can be used in ACO-OFDM, PAM-DMT and U-OFDM in order to compensate for the loss in spectral efficiency from the modulation technique, but the energy efficiency can still be better than in DCO-OFDM. Improved decoders which are equivalent in performance have been developed for ACO-OFDM [9], U-OFDM [7] and Flip-OFDM [6]. Even though, to the best of the authors' knowledge, such an improved decoder is not presented in the published research for PAM-DMT, it would be straightforward to design.…”

Index Modulation-Aided OFDM for Visible Light Communications

“…Since intensity modulation with direct detection is utilized in VLC for low-cost implementation, the signals modulated on the light-emitting diodes (LEDs) should be real-valued and nonnegative [2]. Therefore, various optical OFDM schemes have been proposed to satisfy these constraints, namely DC-biased optical OFDM (DCO-OFDM) [3], asymmetrically clipped optical OFDM (ACO-OFDM) [4], pulse-amplitude-modulated discrete multi-tone (PAM-DMT) [5], unipolar OFDM (U-OFDM) [6], and Flip OFDM [7]. In all these optical OFDM schemes, Hermitian symmetry is utilized on the OFDM subcarriers before inverse fast Fourier transform (IFFT), so that the time-domain signals are real-valued.…”

“…Alternatively, special arrangements on the signals in the time or frequency domain are used in Refs. [3][4][5][6][7] without the need of DC bias, which generate non-negative signals at the cost of spectral efficiency loss. In order to overcome the spectral efficiency loss while maintaining high-energy efficiency, several hybrid schemes are proposed [8][9][10][11].…”