The radio frequency (RF) spectrum environment is rapidly evolving. To achieve superiority in electromagnetic spectrum (EMS) operations in the modern era – including over adversaries gaining ground in the domain – requires higher-level performance and flexibility in critical elements. One area of priority: heightened multifunctionality and granular optimization in Active Electronically Scanned Arrays (AESAs).
DARPA’s work has long targeted foundational technological challenges and advances that provide U.S. forces a decisive edge. Acutely attuned EMS capabilities are crucial enablers for mission-critical situational awareness and tactical communications. Unlike limited, narrowband phased array systems, wideband AESAs employ hundreds or thousands of tiny antenna elements that can all transmit and receive signals.
DARPA has spent decades advancing technologies that provide enhanced situational awareness, perhaps most notably phased arrays. Having the ability to detect and monitor adversarial movement or communications provides significant advantage to U.S. military operations. For pilots in air-to-air combat, for example, advanced radar capabilities arguably give a more decisive edge than a higher maximum speed.
The current leading edge of phased arrays is the Active Electronically Scanned Array (AESA). Unlike their passive counterparts, where all the antenna elements are connected to a single transmitter and/or receiver, AESAs employ a matrix of hundreds or thousands of tiny antenna elements, each with their own transmitter and receiver. This allows you to electronically steer a beam of radio waves in different directions, instead of physically moving the antenna to point at a target.
An AESA’s ability to dynamically reconfigure radar beams and communicate across a range of frequencies is especially important in congested environments. This versatility remains key in military operations: resistance to signal jamming and interception while capable of mapping, navigating, sensing, tracking, visualizing, and creating high-bandwidth data links for cross-domain operations. However, AESAs and broader EMS advances must evolve proactively and progressively to maintain and accelerate the U.S. technological advantage.
Over the past decade, there has been increased interest in wideband AESAs with digital-at-every-element architectures. Wideband AESAs are more versatile and robust because they allow for operation at different frequencies, as opposed to narrowband arrays that are tuned for a specific use and frequency. Digital-at-every-element architectures allow AESAs to perform beamforming in the digital domain, enabling the collection of many beams simultaneously. However, the implementation of wideband, digital-at-every-element receivers in AESAs currently comes with significant trade-offs.
“Wideband, digital-at-every-element AESAs are particularly compelling for applications like advanced radar, electronic warfare (EW), and communications,” said Dr. Benjamin Griffin, a program manager in the Microsystems Technology Office (MTO). “However, high bandwidth receivers often have a limited dynamic range, leaving them vulnerable to electronic jamming. Further, digital-at-every-element exposes each element to interferers and requires filtering at the element level, leaving very little room to integrate conventional filter technologies.”
To address the challenges hampering the use of wideband AESAs in congested RF environments, DARPA launched the COmpact Front-end Filters at the ElEment-level (COFFEE) program in June 2021. COFFEE aims to develop a new class of integrable, high-frequency RF filters for next-generation wideband arrays.
The COFFEE program will focus on creating an integrable filter technology to mitigate interference and maximize performance across a challenging S-band through Ku-band (i.e., 2 GHz to 18 GHz) frequency range. These filters must not only distill signals across the expansive frequency range, but do so while physically bound within an 18 GHz half-wavelength array pitch (i.e., 69 mm2, a space smaller than a dime). This new filter technology will account for digital-at-every-element advances – impacting each of an AESA’s hundreds or thousands of tiny antenna elements – for high-frequency systems facing increasingly unacceptable tradeoffs in size, weight, performance, and interoperability.
The COFFEE filter technology will address the combination of size, performance, and reproducibility to enable protection at every element of a wideband AESA.
“Essentially, we want to build integrable filters that operate over a wide range of frequencies that are also small enough to fit behind each element of the phased array,” said Griffin. “COFFEE aims to develop filters that are on the analog front-end, making the array more robust and resistant to interference before digital processing on the back-end.”
Key to this research will be the development of filter technology that can address all microwave frequencies of a wideband AESA’s bandwidth without sacrificing performance. Further, the target filters must be physically small compared to the element area as the available space for element-level integration decreases significantly as AESA bandwidth increases. Finally, to ensure uniformity the COFFEE filters should be manufacturable with reproducible performance at each of the array elements.
A main focus of the research will be on developing a new class of resonators and integrable microwave filters that address COFFEE’s technical objectives. In addition, research studies into compact mm-wave resonators will be conducted to inform new technical approaches for potential future efforts for integrable mm-wave filters.
“DARPA has been at the forefront of creating opportunities for multifunctional AESAs, with programs such as Arrays at Commercial Timescales (ACT),” Griffin said. “COFFEE is expected to establish an integrable filter technology for Defense Department AESAs, but this work will also have implications in commercial mobile sector advances expected in the near future.”
To help solve multifaceted and mounting challenges, DARPA has selected the research teams for the COmpact Front-end Filters at the ElEment-level (COFFEE) program, which seeks to create a new class of integrable, high-frequency filters with low loss, high-power handling, and seamless uniformity. The selected teams will be led by Northrop Grumman, Raytheon, Akoustis, BAE Systems, Metamagnetics, Georgia Institute of Technology, Columbia University, Carnegie Mellon University, University of Michigan, University of Texas at Austin and University of California at Los Angeles.
BAE Systems won a $6.5 million contract from the Defense Advanced Research Projects Agency (DARPA) to provide filter technology to improve Defense Department radio frequency (RF) and microwave systems such as digital Active Electronically Scanned Arrays (AESA), the company announced in a statement.
“BAE Systems will develop filter technology that can address a broad microwave frequency range that DoD radio systems generally operate in, with element-level integration and without sacrificing performance,” the statement adds. “The company will ensure the filters are manufacturable with reproducible performance.”
“With the Defense Department-wide emphasis on electromagnetic spectrum superiority, our AESAs are tasked with a heightened demand for greater range, volume, and function. These demands are magnified by trends toward wider bandwidths and digitization at the level of the individual element,” said Dr. Benjamin Griffin, the program manager leading the COFFEE program. “There is very little room to integrate conventional filter technologies, exposing each element to the full bandwidth of potential threats. Today, there is no integrable filtering technology to meet these compounding requirements.”
The primary focus area of the program will leverage emerging microelectronics materials, integration, and design to build integrable filters, advancing new classes of miniaturized resonators as the building blocks. An additional, forward-looking focus area is oriented around millimeter-wave frequencies (demonstrating performance at 50 GHz), targeting fundamental limits of compact resonators beyond 18 GHz.
The COFFEE program, which is expected to run for 50 months across three phases – an 18-month base Phase I and two 15-month option phases – is a part of DARPA’s Electronics Resurgence Initiative (ERI) focused on advancing the U.S. semiconductor industry. The program addresses part of ERI’s focus on revolutionizing communications for the 5G era and beyond. Research will kick off in the spring of 2022.