In this article, we present a real-time full duplex radio system for 5G wireless networks. Full duplex radios are capable of opening new possibilities in contexts of high traffic demand where there are limited radio resources. A critical issue, however, to implementing full duplex radios, in real wireless environments, is being able to cancel self-interference. To overcome the self-interference challenge, we prototype our design on a software-defined radio (SDR) platform. This design combines a dual-polarization antenna-based analog part with a digital self-interference canceler that operates in real-time. Prototype test results confirm that the proposed full-duplex system achieves about 1.9 times higher throughput than a half-duplex system. This article concludes with a discussion of implementation challenges that remain for researchers seeking the most viable solution for full duplex communications.

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Abstract-In this article, we present a real-time threedimensional (3D) hybrid beamforming for fifth generation (5G) wireless networks. One of the key concepts in 5G cellular systems is the small cell network, which settles the high mobile traffic demand and provides uniform user-experienced data rates. The overall capacity of the small cell network can be enhanced with the enabling technology of 3D hybrid beamforming. This study validates the feasibility of the 3D hybrid beamforming, mostly for link-level performances, through the implementation of a realtime testbed using a software-defined radio (SDR) platform and fabricated antenna array. Based on the measured data, we also investigate system-level performances to verify the gain of the proposed smart small cell system over long term evolution (LTE) systems by performing system-level simulations based on a 3D ray-tracing tool.Index Terms-Small cell, three-dimensional (3D) hybrid beamforming, fifth generation (5G) communications.

This paper considers the implementation and application possibilities of a compact full duplex multiple-input multiple-output (MIMO) architecture where direct communication exists between users, e.g., device-to-device (D2D) and cellular link coexisting on the same spectrum. For the architecture of the compact full duplex radio, we combine an analog self-interference canceler based dual-polarization with high cross-polarization discrimination (XPD) and Long Term Evolution (LTE)-based persubcarrier digital self-interference canceler. While we consider the compactness and power efficiency of an analog solution, we focus on the digital canceler design with robustness to a frequency-selective channel and high compatibility with a conventional LTE system. For an over-the-air wireless experiment of full duplex testbed with a two-user-pair, we implement a full duplex MIMO physical layer (PHY), supporting 20 MHz bandwidth, on an FPGA-based software-defined radio platform. Further, we propose a novel timing synchronization method to construct a more viable full duplex MIMO link. By having the full duplex link prototype fully operating in real-time, we present the first characterization of the proposed compact full duplex MIMO performance depending on the transmit power of the full duplex node. We also show the link quality between nodes. One of the crucial insights of this work is that the full duplex operation of a user is capable of acquiring the throughput gain if the user has self-interference capability with guaranteed performance.