3494 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 59,NO. 12, DECEMBER2011 Transmit–Receive Duplexing Using Digital Beamforming System to Cancel Self-Interference Trevor Snow, Student Member, IEEE, Caleb Fulton, Student Member, IEEE, and William J. Chappell, Senior Member, IEEE Abstract—A near-field cancellation duplexing system is demon- strated using multiple coordinated transceivers with symmet- rically arranged antenna elements and a digital backend for baseband waveform phase and amplitude weighting controls. By adapting weights of multiple transmit elements, coupled interfer- ence at receiver elements deconstructively interferes, providing up to 50 dB of additional isolation over the coupling of a single transmitter to a receiving element. Bandwidth considerations of the array are presented. It is shown that uncorrelated transmit noise from multiple transmitters can be removed through tunable filtering or through adaptive beamforming of multiple receiving elements, both providing an additional 30–40 dB of interference reduction that is tunable over the frequency range of the system. Index Terms—Adaptive beamforming, antena arrays, communi- cation front-end, duplexers, interference between wireless systems, multifunctional systems, tunable filters. I. INTRODUCTION W ITH THE recent drive for cognitive radio, adaptive microwave systems, and multifunctional apertures that combine the operations of multiple different systems into a unified antenna aperture, there is a need for tunable wide- band front-end architectures that support these capabilities. In typical full-duplex microwave systems, a fixed-frequency duplexer separates transmit and receive circuitry with a typical isolation of 50 dB or more required for commercial devices, which is typically implemented with surface-acoustic-wave (SAW) filters or thin-film bulk acoustic resonators (FBARs) for compactness [1], [2]. While these are tenable solutions for fixed-band communications, many of these static filtering com- ponents are often required in tandem with switching networks to implement multiband cellular communication in addition to wireless local area network (WLAN) and Global Position Manuscript received July 06, 2011; revised September 27, 2011; accepted September 30, 2011. Date of publication November 23, 2011; date of current version December 14, 2011. This work was supported in part by the Naval Sur- face Warfare Center Crane and the Air Force Research Laboratory. This paper is an expanded paper from the IEEE International Microwave Symposium, Bal- timore, MD, June 5–10, 2011. T. Snow is with the Naval Surface Warfare Center Crane, Crane, IN 47522 USA (e-mail: tmsnow@purdue.edu; trevor.snow@navy.mil). C. Fulton and W. J. Chappell are with the IDEAS Microwave Lab- oratory, Department of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907 USA (e-mail: cfulton@purdue.edu; chappell@purdue.edu.) 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.2011.2172625 System (GPS) access [3]. These feed networks introduce inser- tion loss, and, as with all microwave filter technology, reduced factors are unavoidable as filter dimensions are pushed to smaller scales; this translates to more passband insertion loss and increased noise figure. To address the complexity involved with multiband operation, tunable structures are sought to reduce the number of static components and provide a level of adaptability that cannot be matched otherwise. To this end, high- piezo-tuned multiplexers have been demonstrated [4]. These cavity-based filter structures are widely tunable and offer low insertion loss. However, mechan- ically tuned filters are inherently slower than electrically tuned components. There are integrated tunable duplexers [5] that consume very little volume and are electronically tunable, yet low- values of integrated components—typically less than 50—can impact noise factor and limit peak power. One would like to operate a receiver without the previously mentioned drawbacks of various duplexing schemes while maintaining necessary transmitter isolation with simultaneous transmit and receive capabilities. Active interference cancellation methods were examined as a means to accomplish these goals. In this study, active interference cancellation methods are examined both as alternatives and supplements to these techniques. Active interference cancellation systems gen- erate a cancellation signal using a replica of the interfering signal—typically formed by coupling off of the output of the interfering source—which is injected into the receiver with phase and amplitude adjustments such that it deconstructively interferes with the undesired interfering signal [6]. These implementations require a coupler to be inserted in the receive path and a secondary amplifier to drive the cancellation signal. The coupler will impart some insertion loss on the receive chain, which can be minimized by going to lower coupling ratios (e.g., 20 dB instead of 10 dB). However, for strong coupling between receiving and transmitting antennas, the can- cellation amplifier output power required may be of the same order as the primary transmit amplifier. This merits using what would be the cancellation amplifier directly for transmitting and achieving cancellation through mutual coupling paths between antennas. We have demonstrated an array of symmetrically arranged antenna elements with digital transceivers where the wave- forms of outer transmitting elements were digitally phase- and amplitude-adjusted to cancel out at a central receiving element [7]. As opposed to the asymmetric approach in [8], wherein 0018-9480/$26.00 © 2011 IEEE