Linearly Polarized 64-element Antenna Array for mm-Wave Mobile Backhaul Application Marko Sonkki 1 , Sami Myllymäki 2 , Nuutti Tervo 1 , Marko E. Leinonen 1 , Maciej Sobocinski 2 , Giuseppe Destino 1 , Aarno Pärssinen 1 1 Centre for Wireless Communications, University of Oulu, Oulu, Finland, email: marko.sonkki@oulu.fi 2 Microelectronics Research Unit, University of Oulu, Oulu, Finland, email: sami.myllymaki@oulu.fi Abstract—The paper presents simulated and measured results of a large millimeter-wave antenna array, designed by keeping mind the particular interests for proof of concepts in 5G demonstrations in South-Korean Winter Olympics 2018. The array consist of 16 (2x8) unit cells, each having four (2x2) linearly polarized patch elements exited with the same amplitude and phase. The desired -10 dB impedance bandwidth for the array is from 25.65 GHz to 27.50 GHz, and the proposed structure achieves lower than -30 dB mutual coupling between the unit cells. The presented simulation and measurement results show good match with each other, as well as with the specifications. The radiation pattern is measured element by element at 27 GHz, and the results are summed in post-processing to perform the array factor. Sidelobe levels are 15 dB below the maximum gain, whereas the measured maximum gain is around 20 dB as the numerical results predicted 21.5 dB. Index Terms — 5G antennas, beamforming, beam-steering, massive-MIMO, patch antenna array, phased array, sub- array, unit cell. I. INTRODUCTION One of the key drivers for the development of the next generation of communication system, i.e., the 5G, is the demand of 10-times higher communication data rates. This means up to 2 Gbps peak rate in downlink, and 1 Gbps in uplink compared to the current Long Term Evolution (LTE) systems. A promising and concrete approach is to exploit the large available spectrum available in the Ka-band (26.5-40 GHz) [1] to allow wider transmission bandwidths from 100 MHz up to 1 GHz. However, to achieve data rates at Ka-band requires the usage of large antenna arrays with adaptive beamforming capabilities against the high path-loss and sensitivity of the radio-link for dynamic obstructions. In [2], for instance, the results of a channel measurement campaign at 28 GHz shows how line-of-sight (LOS) and multipath components of the radio channel are highly affected by moving objects, e.g. cars. Millimeter waves (mm-Waves) easily employ large antenna arrays to obtain a sufficient array gain for longer communication coverage, beamforming capabilities for beam adaptation, and MIMO-OFDM (Multiple-Input and Multiple-Output – Orthogonal Frequency Division Multiplexing) with high order modulation schemes to achieve the desired data rate. The mm-Wave wireless backhaul link, which for the proposed antenna array is designed, need to meet the following requirements [3]: 1) long-coverage (a few hundreds of meters), 2), > 2 Gbps data-rate, 3) capability of MIMO transmissions, and 4) adaptive beamforming. Details of the actual system specifications with implementation architecture, system specifications, and baseband configurations for what the antenna array is designed, can be found in [4][5]. The designed antenna array will be integrated into a radio unit, which includes two identical radio cards with 16 transceivers in each. Thus one radio unit has two antenna arrays which may be in the same or in the opposite polarization depending on MIMO requirements of the radio link. The implemented mobile backhaul uses similar radio units at both ends of the link. To design a mm-Wave antenna array with large bandwidth and high gain for 5G network, enormous research effort have been proposed to use modified feeding network techniques for patch and new material [6]-[8]. Recently, a comparative review study on 5G mm-Wave array antenna has been done for outdoor mm-Wave wireless communications, considering the array gain and directivity factors [9]. Radiation pattern of four different antenna array architectures have been investigated including; 8×8 rectangle array, 64 circular array, 61 hexagonal array and 16 crisscross array. To enhance radio capacity data rate, by reducing antenna 3 dB beamwidth from 65° to 30°, it can increase the capacity of mm-Wave cellular networks operating at 28 GHz frequency roughly by three times at a distance of 220 m from the base station (BS) [10]. This means the antenna beamwidth have an impact on network performance. Mutual coupling in antenna arrays and its feeding networks have been subjects of intense research for several decades. Several reducing techniques have proposed including suppressing surface waves propagation using electromagnetic band gap (EBG) [11], slitted on the ground plan [12]. Also, a comprehensive review study has been done to study and reduce the mutual coupling impact on planar array antennas performance [13]. This paper is proposing a massive-MIMO antenna array design for Winter Olympics in Korea 2018 for the spectrum between 26.65-27.5 GHz [14]. The band has a particular interests for proof of concepts in 5G demonstrations. The