This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES 1 Spatially Distributed Multi-Input Interferometric Receiver for 5G Wireless Systems and Beyond Bilel Mnasri , Student Member, IEEE, Tarek Djerafi, Member, IEEE, Serioja Ovidiu Tatu , Senior Member, IEEE, and Ke Wu , Fellow, IEEE Abstract—This paper presents a spatially distributed multi- input direct conversion receiver architecture based on inter- ferometric correlations of wave signals for fifth generation systems and beyond. The proposed receiver scheme inherits the advantages of conventional six-port receivers. The spatially distributed multi-input architecture is based on the use of a set of equally spaced four antenna elements instead of using a six-port junction composed of four hybrid couplers or other alternative circuit-based topologies. In this paper, the mathematical mod- eling of the proposed receiver is derived and presented. Then, an experimental test bench that is fabricated to operate around 5 GHz is introduced in order to validate the proposed scheme and its theoretical results. Various modulations including BPSK, QPSK, QAM-16, and QAM-32 have been successfully studied and demonstrated by this receiver at a relatively low date rate of 1 mega symbol per second with a maximum error vector magnitude of 10.3%. The maximum bit rate is fundamentally limited by the speed of power detectors, which is linked to their high rise time in our experiments. It is anticipated that multiple hundred megabits and gigabits per second can be achieved with the proposed direct conversion receiver if higher speed power detectors are used. Index Terms— Fifth generation (5G) systems, direct conversion receiver, quadrature amplitude modulation, six-port junction, spatially distributed multi-input interferometer. I. I NTRODUCTION T HE exponential growth of data traffic that is facing today’s 3G and 4G operators has urged the international regulatory agencies like Federal Communications Commission and the International Telecommunications Union to create special focus groups such as IMT-2020 in order to estab- lish the technical recommendations and general guidelines to be adopted through the fifth generation (5G) of wireless systems [1]. Manuscript received July 20, 2018; revised September 28, 2018; accepted October 6, 2018. This work was supported in part by the Natural Sciences and Engineering Research Council of Canada and in part by the NSERC–Huawei Industrial Research Chair in Future Wireless Technologies. (Corresponding author: Bilel Mnasri.) B. Mnasri is with the Poly-Grames Research Center, Ecole Polytechnique de Montréal, University of Montreal, Montreal, QC H3T 1J4, Canada (e-mail: bilel.mnasri@polymtl.ca). T. Djerafi and S. O. Tatu are with the Institut National de la Recherche Scientifique, Montreal, QC H5A 1K6, Canada. K. Wu is with the Faculty of Electrical Engineering and Computer Science, Ningbo University, Ningbo 315211, China, on leave from the Department of Electrical Engineering, Poly-Grames Research Center, Ecole Polytech- nique de Montréal, University of Montreal, Montreal, QC H3T 1J4, Canada (e-mail:ke.wu@polymtl.ca). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMTT.2018.2876260 In fact, emerging wireless communication standards are required to support the transmission of data rates on the order of multiple gigabits per second with a latency of less than 1 ms [2]. In order to accommodate the challenging requirements and technical specifications for better performances in 5G systems, the whole network architecture has been redrawn. Indeed, the concept of ultradense networks has been introduced, which stands for the deployment of multiple base stations and access points within the same area to get as close as possible to the end user [3]. Moreover, the whole backhauling system that enables fast and point-to-point communications between access nodes must be deployed wirelessly to avoid the well-known prohibited cost of fiber optics interconnectivities [4]. In addition, the energy consumption, the digital signal processing requirements, and the cost per unit access point within the wireless backhauling system must be decreased as much as possible to enable large-scale deployment and easy penetration of the emerging technology into the market. Consequently, novel RF front ends must be revisited, studied, and validated in order to accommodate the aforementioned strict requirements concern- ing emerging 5G systems and beyond, including point-to- point communications between access points or fixed last mile entities of the backhauling network. Many RF front ends for backhauling and point-to-point communications purposes have been introduced and studied in the literature. In [5], a novel front end with high directivity and self-beam/null steering was presented, which operates at 2.4 GHz, enabling point-to-point communications while focusing on reducing the probability of intrusion and unwanted interception. Another low-cost smart antenna receiver sub- system operating over E-Band was introduced in [6]. This front-end is based on a beam-switching scheme, implemented through an SIW Butler matrix, which alleviates the problem of possible mismatch between highly directive 60-GHz antennas and, hence, presents an excellent candidate for short-range point-to-point communications. A highly efficient digital baseband receiver was also pre- sented in [7]. The proposed receiver is mainly based on an analog symbol timing recovery as well as digital carrier recovery. This prototype can provide data rates of about 5 Gb/s using 16-QAM in E-band with an optimal bit error rate (BER) of about 10 -10 , but the heterodyne nature of this receiver as well as the complexity of its architecture would increase its cost and energy consumption. Six-port direct conversion receivers have shown promising capabilities since their first introduction in 1994 [8]. In fact, 0018-9480 © 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. 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