Flip-OFDM for optical wireless communications

Abstract

Abstract-We consider two uniploar OFDM techniques for optical wireless communications: asymmetric clipped optical OFDM (ACO-OFDM) and Flip-OFDM. Both techniques can be used to compensate multipath distortion effects in optical wireless channels. However, ACO-OFDM has been widely studied in the literature, while the performance of Flip-OFDM has never been investigated. In this paper, we conduct the performance analysis of Flip-OFDM and propose additional modification to the original scheme in order to compare the performance of both techniques. Finally, it is shown by simulation that both techniques have the same performance but different hardware complexities. In particular, for slow fading channels, Flip-OFDM offers 50% saving in hardware complexity over ACO-OFDM at the receiver.

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.…”

“…Then U-OFDM has double the initial latency to decode a single information frame because two consecutive frames need to be received in order for the demodulation process to begin. The authors in [5] have made the case that Flip-OFDM/U-OFDM is computationally more efficient than ACO-OFDM as it requires a smaller FFT size for decoding the same information per frame. This is true for the case when the two U-OFDM/Flip-OFDM frames have a combined length equal to one ACO-OFDM frame.…”

“…This operation can also be described as the act of taking all negative samples from the original bipolar frame, replacing them with zeros and then flipping their values and transmitting them in a second frame where the positions of the original positive samples are occupied by zeros. This description of the signal generation process has given the term Flip-OFDM, used by the authors in [5]. Alternatively, the same modulation procedure can be expressed as the act of transmitting two copies of the original bipolar frame one after the other.…”

“…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.…”

Efficient OFDM transceiver design for VLC sys-tems in fading channel

“…However OFDM symbols are bipolar. Since the intensity modulation with direct detection (IM/DD) is used in VLC systems, the transmitting signals must be real and nonnegative [1] - [2].Hence the OFDM symbols should be converted to unipolar format before applying to a VLC system. Several techniques are available to convert bipolar OFDM to unipolar.…”