1536-1276 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TWC.2016.2594176, IEEE Transactions on Wireless Communications 1 Self-Coherent OFDM with Undersampling Down-conversion for Wireless Communications Qianyu Jin, Student Member, IEEE and Yi Hong, Senior Member, IEEE Abstract—In this paper, we introduce self-coherent orthogonal frequency-division multiplexing (OFDM), a well-known non- coherent technique in optical communications, for wireless radio frequency (RF) communications. Self-coherent OFDM provides complete immunity against phase noise (PN) using a non- coherent receiver and a significantly higher spectral efficiency than self-heterodyne (self-het) OFDM, which utilizes at most 50% of the available spectrum for communications. We present the performance analysis of self-coherent OFDM over additive white Gaussian noise (AWGN) and frequency selective fading channels, and show by simulations that self-coherent OFDM provides both higher spectral efficiency and better bit error rate (BER) performance than self-het OFDM. Considering that filter realization in high frequency bands is challenging, we adopt a undersampling down-conversion technique in conjunction with self-coherent OFDM. We show that with the self-coherent demodulation, the additional PN introduced by undersampling down-conversion can be significantly reduced. We compare ana- lytically the system performance of self-coherent OFDM using undersampling down-conversion with two other conventional OFDM systems: one with super-heterodyne receiver and the other with undersampling down-conversion. We show theoretically and by simulations that both in AWGN and frequency selective fading channels, self-coherent OFDM with undersampling down- conversion outperforms the two conventional OFDM systems even when intercarrier interference (ICI) compensation schemes are applied. Keywords—non-coherent, OFDM, self-coherent, phase noise, multipath fading, low-complexity receivers, undersampling down- conversion. I. I NTRODUCTION Orthogonal frequency division multiplexing (OFDM) is a popular technique that brings the advantages of high spec- tral efficiency, robustness to intersymbol interference, simple channel equalization and efficient implementation using Fast Fourier Transform (FFT). It is known that the intercarrier inter- ference (ICI) is a major problem in conventional OFDM due to Doppler frequency drift, phase offset, local oscillator frequency drift, and sampling clock offset [1]. For very high frequencies, used in millimeter-wave and terahertz communications, high level residual phase noise (PN) due to instabilities of oscillators and mixers of the coherent reception can be severe [2]. Hence, new OFDM techniques that are robust to ICI and equipped with low complexity transceiver structures are needed to implement millimeter-wave wireless communications [3]. Qianyu Jin and Yi Hong are with Department of Electrical and Computer Systems Engineering (ECSE), Faculty of Engineering, Monash University, Clayton, VIC 3800, Australia (e-mail: {qianyu.jin, yi.hong}@monash.edu). This work is supported by the Australian Research Council Discovery Projects with ARC DP130100336 and ARC DP160100528. Self-heterodyne (self-het) OFDM was proposed by Shoji et al. in [4] to cope with high level oscillator instabilities in 60 GHz wireless communications using quadrature amplitude modulation (QAM)/OFDM signalling. In a self-het OFDM sys- tem, the local radio frequency (RF) carrier is transmitted with the information subcarriers. This guarantees that the carrier phase is perfectly synchronous with the subcarriers and it pro- vides complete immunity against PN. Moreover, a square-law circuitry (self-mixing) is used, instead of a super-heterodyne structure, to down-convert the RF signal. Thus the processes of local carrier generation, carrier frequency correction and carrier phase recovery can be omitted. This greatly reduces the complexity of the self-het OFDM transceivers compared to conventional OFDMs using super-heterodyne receivers. There have been a number of research developments on self-het OFDM for additive white Gaussian noise (AWGN) and two- ray channels [5]. Very recently, Fernando et al extended the self-het OFDM system to frequency selective fading channels and applied coding techniques to further improve the system performance [6], [7]. However, the disadvantage is that self-het OFDM uses at most 50% of the available spectrum. To improve spectral efficiency, while maintaining the sim- plicity of the RF front-end receiver and PN immunity, we consider self-coherent OFDM, a popular technique in optical communications, originally proposed by Tetsuya Miyazaki in [8]. Self-coherent OFDM jointly transmits a carrier and information subcarriers separated by a guard band, much smaller than self-het OFDM, to ensure phase synchronization between transmitter and receiver. In optical communications, the homodyne detection with a polarization-modulation tech- nique was used to generate a pilot carrier at the transmitter and a pilot-carrier combining module at the receiver. Recent research on self-coherent OFDM for optical communications is summarized in [9]. In this paper, we adapt self-coherent OFDM to wireless communications over AWGN and frequency selective fading channels. In particular, at the receiver of self-coherent OFDM, the carrier and information subcarriers are separated using a low pass filter (LPF) and a high pass filter (HPF), respectively, and are further processed by two square-law devices and a LPF to down-convert the RF signal. For very high frequency bands such as millimeter-wave RF bands, the realization of such LPF and HPF can be challenging. We thus introduce the undersampling down-conversion technique to self-coherent OFDM. After undersampling at the receiver, the received signal is down-converted to a lower frequency band, where filters can be more easily implemented to demodulate the in- formation subcarriers. In the undersampling down-conversion, PN is additionally introduced to the system by the instability