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.

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.

Abstract-Self-heterodyne OFDM (self-het OFDM) is known to provide complete immunity against frequency-offset and phase noise, with a much lower RF frontend complexity, when compared to conventional OFDM techniques. Self-het OFDM is considered to be a promising physical layer technology for millimeter-wave RF communications, where the implementation of low complexity stable oscillators is technically difficult. Although self-het OFDM has great potential, it has only been studied for additive white Gaussian noise and two-ray channel models. In this paper, we analyze the performance of self-het OFDM for general frequency selective channels and show that the standard self-het OFDM undergoes an outage if the RF carrier is affected by deep fading. In order to avoid this, we introduce a new technique called smart carrier positioning. We show both analytically and by simulation that the smart carrier positioning can improve the diversity order and the performance of standard self-het OFDM by approximately 4dB at bit error rate of 10 −2 . In addition, we investigate the optimum power allocation between the carrier and the OFDM subcarriers under frequency selective conditions.

Self-heterodyne OFDM (self-het OFDM) is a promising physical layer technique for millimeter-wave and terahertz RF communication due to its simple RF frontend and complete immunity against frequency-offset and phase noise. In this paper, we proposed a multiple-input multiple-output (MIMO) self-het OFDM with the adaption of smart carrier position (SCP) technique. At the transmitter, a space-time block code (STBC) is used to produce the coded information symbols to be transmitted on each antenna over the self-het OFDM subcarriers. At the receiver, a simple non-linear detection is adopted at each receive antenna. The achievable diversity order of such setting is analysed, and it is found that with the adaptation of SCP technique, the diversity loss in comparison to the conventional coherent MIMO-OFDM can be compensated. We further derive analytically both lower and upper bounds on the diversity order of the proposed scheme and simulations will be shown to illustrate the performance of such combination.

In this paper, we present a subcarrier pairing scheme to improve the overall error performance of selfheterodyne (self-het) OFDM communications. The proposed pairing scheme exploits the average signal-to-interference-to noise ratios (SINRs) imbalance experienced among self-het OFDM subcarriers. At the transmitter, two simple operations, symbol constellation rotation and component interleaving, are performed before pairing the good and the bad OFDM subcarriers, and maximum likelihood detection is used at the receiver to decode the information. The simulation results show that the proposed pairing scheme improves the system performance by 2.5 dB and 0.6 dB for Rayleigh fading and AWGN channels at bit error rate (BER) of 10 −3 , respectively, without any coding overhead. In addition, we show that, in the presence of phase noise, self-het OFDM using the proposed pairing scheme outperforms the conventional OFDM schemes with superheterodyne receiver structures.

In this paper, we consider two enhancements for self-heterodyne OFDM (Self-Het OFDM): subcarrier pairing and smart carrier positioning (SCP). We analyze their performance over a practical 60 GHz indoor RF channel. This channel deviates from the standard AWGN and Rayleigh fading models for Line of Sight (LoS) and Non LoS. We show that for NLoS channels they can jointly improve the diversity order and performance by 7.7 dB at BER of 10 −2 , when compared to the standard self-het OFDM. Similar improvements are observed for channels with strong LoS components.

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