In this article, we review the key concepts and the progress in the design of physical-layer aspects of in-band full-duplex (IBFD) communications. One of the fundamental challenges in realizing IBFD is self-interference that can be up to 100 dB stronger than signals of interest. Thus, we start by reviewing state-of-the-art research in self-interference cancellation, addressing both model-based and emerging machine learning-based methods. Then, we turn our attention to new wireless systems with many degrees of freedom for which the traditional IBFD designs do not gracefully scale and, hence, require many innovations to enable IBFD. We provide an extensive review of basic concepts and state of the art in massive multiple-input–multiple-output IBFD. Then, we consider the mmWave band IBFD and review advanced physical-layer architectures. The above review provides the proper context to discuss IBFD innovations and new challenges for sixth-generation networks and beyond, where wireless networks are envisioned to be multifunctional, combining communications with functions such as sensing, cognitive radios, physical-layer security, and wireless power transfer. We conclude this article with a status update on the adoption of IBFD in communication standards.

Compared to traditional omnidirectional systems, in-band full-duplex (IBFD) phased arrays provide higher antenna gains and offer the ability to electronically steer beams spatially to improve communications/radar links and enable multifunction operation. While a variety of approaches have successfully incorporated self-interference cancellation (SIC) into arrays, their techniques do not scale beyond the limited sizes demonstrated. In addition to providing an overview on state-of-the-art IBFD arrays, this paper presents a novel complexity reduction method that uniquely applies infinite impulse response (IIR) filter modeling for the purpose of compensating for the dispersion effects of the array hardware. Using measured statistical performance, this process is demonstrated for a scalable array design covering 2700 to 3500 MHz, and exhibits a 10x reduction in computations for an array with 1000 elements. Additionally, the distribution of on-array SIC adaptive processing is discussed for aperture-level IBFD designs, including the use of cache-oblivious algorithms that can help facilitate real-time processing and lead to the realization of large-scale IBFD arrays.

Detailed systems engineering is required to integrate multiple self-interference cancellation (SIC) techniques for inband full-duplex (IBFD) wireless designs enabling novel applications. These SIC methods span the propagation, analog and digital domains of a typical transceiver, and must be individually analyzed before combining them to create an IBFD system. This paper discusses the various techniques within these domains as well as presents a survey of measured IBFD systems that have successfully demonstrated SIC across multiple domains. Additionally, several key research areas are outlined, and highlight the need for a resource that details both the fundamentals and state-of-the-art performance of IBFD techniques, systems and applications in order to accelerate the adoption of this technology within networking standards.

This Work-In-Progress paper details the process and lessons learned when converting a hands-on engineering mini-course to a scalable, self-paced Massively Open Online Course (MOOC). Online courseware has been part of academic and industry training and learning for decades. Learning activities in online courses strive to mimic in-person delivery by including lectures, homework assignments, software exercises and exams. While these instructional activities provide “theory and practice” for many disciplines, engineering courses often require hands-on activities with physical tools, devices and equipment. To accommodate the need for this type of learning, MIT Lincoln Laboratory's “Build A Small Radar” (BSR) course was used to explore teaching and learning strategies that support the inclusion of physical construction and real world data collection in a MOOC. These tasks are encountered across a range of engineering disciplines and the methods illustrated here are easily generalized to the learning experiences in engineering and science disciplines.

In-band full-duplex wireless communications are challenging because they require the mitigation of self-interference caused by the co-located transmitter to operate effectively. This paper presents a novel tapped delay line RF canceller architecture with multiple non-uniform pre-weighted taps to improve system isolation by cancelling both the direct antenna coupling as well as multipath effects that comprise a typical interference channel. A four-tap canceller prototype was measured over several different operating conditions, and was found to provide an average of 30 dB signal cancellation over a 30 MHz bandwidth centered at 2.45 GHz in isolated scenarios. When combined with an omni-directional high-isolation antenna, the canceller improved the overall analog isolation to 90 dB for these cases. In an indoor setting, the canceller suppressed a +30 dBm OFDM signal by 22 dB over a 20 MHz bandwidth centered at 2.45 GHz, and produced 78 dB of total analog isolation. This complete evaluation demonstrates not only the performance limitations of an optimized multitap RF canceller, but also establishes the amount of analog interference suppression that can be expected for the different environments considered.