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Introduction
In the rapidly evolving landscape of the electromagnetic spectrum (EMS), DARPA’s COmpact Front-end Filters at the ElEment-level (COFFEE) program emerges as a pioneering initiative. Aimed at enhancing the capabilities of Active Electronically Scanned Arrays (AESAs), COFFEE addresses the critical need for heightened performance and adaptability in the modern EMS environment.
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.
The Evolving EMS Landscape and DARPA’s Mission
The electromagnetic spectrum (EMS) environment is undergoing rapid changes, demanding superior performance and flexibility for electromagnetic spectrum (EMS) operations. DARPA has consistently targeted foundational technological challenges, and with COFFEE, the focus is on achieving a decisive edge in AESA technology, a critical component of modern military operations.
Active Electronically Scanned Arrays (AESAs) play a pivotal role in providing enhanced situational awareness and tactical communications. Unlike traditional phased array systems, AESAs employ a matrix of tiny antenna elements, enabling dynamic signal transmission and reception. The current pinnacle of phased arrays, AESAs offer electronic beam steering and communication across various frequencies, providing a substantial advantage in military operations.
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.
Challenges and the Need for Evolution
While AESAs offer versatility, they face challenges in congested RF environments, such as susceptibility to electronic jamming and interference. DARPA recognizes the need for evolution in AESA technology to maintain and accelerate the technological advantage of the United States.
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.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.
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.”
COFFEE Program Overview
In response to the challenges faced by AESAs, DARPA initiated the COFFEE program in June 2021. This program aims to develop a new class of integrable, high-frequency RF filters specifically designed for next-generation wideband arrays. The primary focus is on mitigating interference and optimizing performance across a demanding frequency range (S-band through Ku-band).
Objectives and Technical Focus of COFFEE
COFFEE focuses on developing filter technology that can operate across a broad frequency range while being physically compact enough for element-level integration. This technology is envisioned to enhance the resilience of each element of a wideband AESA, making it more robust against interference before digital processing on the back-end.
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.”
Research Approach and Key Technologies
The research under COFFEE will concentrate on developing a new class of resonators and integrable microwave filters. This includes the exploration of compact mm-wave resonators, paving the way for potential future efforts in integrable mm-wave filters.
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.”
Broader Implications and Future Prospects
The potential benefits of COFFEE:
- Enhanced situational awareness: By filtering out unwanted signals, COFFEE will improve the accuracy and range of AESA radars, providing commanders with a clearer picture of the battlefield.
- Increased communication security: Advanced filtering will safeguard military communications from interception, ensuring secure information exchange during critical operations.
- Improved radar performance: Filtering out clutter and interference will allow AESAs to detect and track targets more effectively, even in complex environments. COFFEE is expected to revolutionize radar technology by enabling higher power, longer range, and increased sensitivity.
COFFEE’s research not only aims to establish integrable filter technology for Defense Department AESAs but also holds implications for advancements in the commercial mobile sector. Potential applications extend beyond radar to include high-power communications, electronic warfare systems, and medical imaging devices. The program is anticipated to create new jobs and boost the U.S. electronics industry by driving the development of cutting-edge technologies. By staying at the forefront of creating opportunities for multifunctional AESAs, DARPA anticipates maintaining technological leadership.
Collaborative Efforts and Timeline
COFFEE has enlisted the expertise of leading organizations and research institutions, including Northrop Grumman, Raytheon, Akoustis, BAE Systems, and various universities. With a timeline spanning 50 months across three phases, COFFEE represents a significant stride in DARPA’s Electronics Resurgence Initiative (ERI), focusing on revolutionizing communications for the 5G era and beyond.
Awards
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.
Key advancements:
- Transistor design breakthroughs: Raytheon has achieved a 75 W/mm power density for gallium nitride (GaN) transistors, exceeding the program’s initial target of 50 W/mm.
- Heat management innovation: MIT researchers have demonstrated a new thermal management method using diamond substrates, reducing transistor thermal resistance by 20%.
- Prototype development underway: Several contractors are building early prototypes of high-power RF amplifiers based on these new technologies.
Akoustis Awarded Phase 2 For DARPA COFFEE Program
Akoustis Technologies, Inc., an integrated device manufacturer (IDM) of patented bulk acoustic wave (BAW) high-band RF filters for mobile and other wireless applications, announced that it has successfully completed Phase 1 of its contract with the Defense Advanced Research Projects Agency (DARPA) to pursue new materials and device manufacturing methods that can scale its XBAW® technology to 18 GHz, and was awarded a new multi-year, multi-million dollar contract for Phase 2 of the DARPA COmpact Front-end Filters at the ElEment-level (COFFEE) program.
In DARPA’s COFFEE program, Akoustis developed a novel technology (“XP3F”) to overcome trade-offs inherent in traditional BAW frequency scaling approaches. Akoustis plans to introduce XP3F filters and resonators that fully exploit the entire electromechanical coupling capability of the underlying piezoelectric material.
Jeff Shealy, founder and CEO of Akoustis, stated, “The first phase of the COFFEE program has been a resounding success for Akoustis as we have been able to fully leverage the inherent advantages of single crystal materials to develop a superior technology over purely poly-crystal based technologies.” Mr. Shealy continued, “Through DARPA COFFEE, we successfully introduced a new method of BAW overtone operation that can maintain high Q-factor while mitigating the decrease in electromechanical coupling. Combining our existing, patented XBAW® technology and the XP3F overtone technology developed in Phase 1 of COFFEE, we expect to advance the state-of the-art in BAW RF filter technology from X-band through 18 GHz—specifically in terms of power handling, Q-factor, and electro-mechanical coupling relative to incumbent BAW technologies in the market today.”
Program milestones and funding:
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.
- Phase 2 nearing completion: The current phase focuses on transistor design and fabrication and is nearing its conclusion.
- Transition to Phase 3 expected in early 2024: This phase will focus on integration and prototyping of advanced filter systems.
- Significant funding allocated: The program has received approximately $60 million for Phase 2.
Conclusion
As COFFEE commences its research in the spring of 2022, it marks the beginning of a new era in filtering technology. With its far-reaching implications, COFFEE is poised to contribute to the evolution of AESA technology, ensuring that the United States maintains its edge in the dynamic landscape of the electromagnetic spectrum.
