Min Soo Sim

Prototyping real-time full duplex radios

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, in implementing full duplex radios in real wireless environments is being able to cancel self-interference. To overcome the selfinterference challenge, we prototype our design on a software-defined radio 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.

Nonlinear Self-Interference Cancellation for Full-Duplex Radios: From Link-Level and System-Level Performance Perspectives

One of the promising technologies for Long Term Evolution is full-duplex radio, an innovation that is expected to double spectral efficiency. To realize full-duplex in practice, the main challenge is overcoming self-interference, and to do so, researchers have developed self-interference cancellation techniques. Since most wireless transceivers use power amplifiers, especially in cellular systems, researchers have revealed the importance of nonlinear self-interference cancellation. In this article, we first explore several nonlinear digital self-interference cancellation techniques. We then propose a low-complexity pre-calibration- based nonlinear digital self-interference cancellation technique. Next, we discuss the issues about reference signal allocation and the overhead of each technique. For performance evaluations, we carry out extensive measurements through a real-time prototype and link-/system-level simulations. For link-level analysis, we measure the amount of cancelled self-interference for each technique. We also evaluate system-level performance through 3D ray-tracing-based simulations. Numerical results confirm the significant performance improvement over a half-duplex system even in interference-limited indoor environments.

Compact Full Duplex MIMO Radios in D2D Underlaid Cellular Networks: From System Design to Prototype Results

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 and long-term evolution (LTE)-based per-subcarrier 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, supporting 20-MHz bandwidth, on an Field-Programmable Gate Array-based software-defined radio platform. Furthermore, 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 paper 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.

Low-Complexity Nonlinear Self-Interference Cancellation for Full-Duplex Radios

Full-duplex radio, expected to double the spectral efficiency, is one of the promising technologies for fifth generation communication. To overcome self-interference, which is a main obstacle to full-duplex radio, various self-interference cancellation (SIC) techniques have been developed. For digital SIC, the nonlinearity of a transmitter occurred by a power amplifier is a main bottleneck. The nonlinear SIC methods proposed in prior work are not feasible due to high computational complexity, or the cost of extra hardwares. In this paper, we propose the pre-calibration method that linearizes the transmitter, and applies only linear SIC. Numerical results confirm that the proposed SIC method shows 43 dB cancellation performance while the existing method shows only 18 dB in fast fading channels.