Bradley T. Perry

Multitap RF Canceller for In-Band Full-Duplex Wireless Communications

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.

The MIT IAP radar course: Build a small radar system capable of sensing range, Doppler, and synthetic aperture (SAR) imaging

MIT Lincoln Laboratory sponsored a radar short course at MIT campus during the January 2011 Independent Activities Period (IAP). The objective of this course was to generate student interest in applied electromagnetics, antennas, radio frequency (RF) electronics, analog circuits, and signal processing by building a short-range radar sensor and using it in a series of field tests. Lectures on the fundamentals of radar, modular RF design, antennas, pulse compression and synthetic aperture radar (SAR) imaging were presented. Teams of three students built a radar system from a kit. This kit was developed by the authors and uses a frequency modulated continuous wave (FMCW) architecture. To save costs, empty metal coffee cans are used for antennas, components are mounted on a wood block, the system uses only six coaxial microwave parts, analog circuitry on a solderless breadboard, and runs on eight AA batteries. Analog data is acquired by the audio input port on a laptop computer. The total cost of each kit was

Adaptive RF canceller for transmit-receive isolation improvement

360 which made this radar technology accessible to students. Of the nine student groups, all succeeded in building their radar, acquiring Doppler vs. time and range vs. time plots, seven succeeded in acquiring SAR imagery, and some groups improved the radar system. By presenting these difficult topics at a high level while at the same time making a radar kit and performing field experiments, students became self motivated to explore these topics and much interest in radar design was generated.

Wideband antenna array for Simultaneous Transmit and Receive (STAR) applications

For effective operation, Simultaneous Transmit And Receive (STAR) systems require high isolation between the transmitted signals and the receiver input, the absence of which can lead to the saturation of a receivers front end. This paper presents an adaptive RF canceller used to improve isolation. The canceller is configured as an RF tapped delay line with four taps, each with independent amplitude and phase weights that are tuned by a Dithered Linear Search algorithm. This canceller produces 30 dB of signal cancellation over a 30 MHz bandwidth centered at 2.45 GHz in an isolated environment. When combined with a high-isolation antenna, an overall STAR system isolation of 90 dB is achieved, while also maintaining omni-directional transmit and receive antenna patterns.

Simultaneous Transmit and Receive (STAR) system architecture using multiple analog cancellation layers

A wideband antenna array for Simultaneous Transmit and Receive (STAR) applications is presented. The design is comprised of a ring array of TEM horns, and a monocone at the arrays center. When the array is phased with the first order circular mode, it is isolated from the monocone. Thus, the array may be used in reception while the monocone is used in transmission, or vice versa. The array and monocone both produce quasi-omnidirectional patterns in the azimuthal planes. Simulations suggest that the design operates across an 8.4 : 1 bandwidth. This wide bandwidth is possible through the use of a novel capacitive feed employed in the TEM horn array.

Low cost phased array radar for applications in engineering education

Simultaneous Transmit and Receive operation requires a high amount of transmit-to-receive isolation in order to avoid self-interference. This isolation is best achieved by utilizing multiple cancellation techniques. The combination of adaptive multiple-input multiple-output spatial cancellation with a high-isolation antenna and RF canceller produces a novel system architecture that focuses on cancellation in the analog domain before the receivers low-noise amplifier. A prototype of this system has been implemented on a moving vehicle, and measurements have proven that this design is capable of providing more than 90 dB of total isolation in realistic multipath environments over a 30 MHz bandwidth centered at 2.45 GHz.

Single antenna in-band full-duplex isolation-improvement techniques

Hands-on instruction in engineering education is beneficial to the development of a workforce that understands the complexity of building radar systems. Unfortunately, building phased array systems tends to be too costly to allow student access to the hardware necessary for developing these skills. This paper presents a low cost phased array based on a time-domain multiplexed, multiple-input, multiple-output (TDM-MIMO) approach that has been built for education. This array has been utilized in several free courses held at the Massachusetts Institute of Technology during the Independent Activity Period (IAP) between semesters. Students have built, tested, and taken home a number of these radars and continue to operate these on their own, either for recreation or as part of their undergraduate research activities.

Simultaneous transmit and receive antenna isolation improvement in scattering environments

Many in-band full-duplex wireless systems transmit and receive on a single antenna to minimize redundancy and maintain compact form factors. For effective operation, all of these systems need to maximize transmit-to-receive isolation, which is limited by non-ideal antenna matching and non-zero circulator leakage. Several isolation-improvement techniques are investigated in this paper, and illustrate how RF components can be used to minimize the consequential self-interference of these systems. Two unique cancellation schemes were validated, and the isolation of a single-antenna transceiver was measured to improve by 15 and 33 dB over the 100 MHz bandwidth centered at 2.45 GHz.

Simultaneous transmit and receive (STAR) mobile testbed

Simultaneous transmit and receive (STAR) systems require high isolation between the transmitter and receiver to avoid self-interference. Antenna isolation degradation stems from errors in the physical construction and beamformer design, as well as reflections from scattering objects in the environment. An RF canceller subsystem can be inserted at the antenna feeds to improve the isolation in the presence of reflecting objects by 30 dB over 30 MHz centered at 2.45 GHz. This results in 90 dB of effective antenna isolation when paired with a high-isolation antenna that exhibits omni-directional radiation patterns, signifying that STAR systems can be practically deployed.

Simultaneous transmit and receive with digital phased arrays

Simultaneous Transmit and Receive (STAR) systems typically utilize multiple cancellation layers to improve system isolation and avoid self-interference. The design of these different layers must be evaluated both individually and as a whole to determine their effectiveness in various environments. A flexible and reusable mobile testbed was constructed to aid in the development and assessment of these different STAR technologies for both stationary and non-stationary applications. The usefulness of this platform was confirmed during the integration of an example STAR system that measured greater than 100 dB of total system isolation over a 30 MHz bandwidth centered at 2.45 GHz.