Expanded mission areas and the implementation of additional data routing resulting from future warfighting capabilities place more demand on data distribution services in the form of higher data bandwidths and reduced latencies. These demands require improvements in Radio Frequency (RF) spectrum utilization and advances in antenna technologies. Digital array antenna technology promises to enable these improvements by dramatically increasing operational flexibility.
Today’s critical DoD applications such as radar, communications and electronic warfare use antenna arrays to provide unique capabilities, such as multiple beam forming and electronic steering. Phased radio frequency (RF) arrays use numerous small antennas to steer RF beams without mechanical movement. Their lack of moving parts reduces maintenance requirements and their advanced electromagnetic capabilities, such as the ability to look in multiple directions at once, are extremely useful in the field. They also provide with increased range and power, agility and sensitivity, reliability and multi-function capability. The development of much of the enabling advanced electronics pushed phased array technology up through X- and Ku-band.
Military has increasing interest in making broader use of the millimeter wave frequency band for communications on small mobile platforms where narrow antenna beams from small radiating apertures provide enhanced communication security. For example DARPA’s 100G program is developing the technologies and system concepts to project fiber-optic-class 100 Gb/s capacity via airborne data links anywhere within the area of responsibility (AOR). The goal is to create a 100 Gb/s data link that achieves a range greater than 200 kilometers between airborne assets and a range greater than 100 kilometers between an high-altitude long-endurance aerial platforms (at 60,000 feet) and the ground.
Phased-arrays operating at millimeter wave–or very high frequencies–are already an active area of research by the emerging 5G cellular market. Commercial applications are primarily solving the “last mile” problem, where consumers are demanding more bandwidth for high-throughput applications over relatively short ranges at predetermined frequencies and with minimal obstacles to user discovery. DoD platforms create far more complex communications environments than commercial environments. Often separated by tens or even hundreds of nautical miles, today’s military platforms are moving in three dimensions with unknown orientations. This environment is creating unique beamforming challenges that can’t easily be solved by applying current communications approaches.
“Imagine two aircraft both traveling at high speed and moving relative to one another,” said DARPA program manager Timothy Hancock. “They have to find each other in space to communicate with directional antenna beams, creating a very difficult challenge that can’t be solved with the phased-array solutions emerging in the commercial marketplace.” To expand the use of millimeter wave phased-arrays and make them broadly applicable across DoD systems, many technical challenges must be addressed, including wideband frequency coverage, precision beam pointing, user discover and mesh networking.
DARPA launched the Millimeter-Wave Digital Arrays (MIDAS) program in Jan 2018 that aims to develop element-level digital phased-array technology that will enable next generation DoD millimeter wave systems. Advances in element-level digital beamforming in phased-array designs is enabling new multi-beam communications schemes—or the use of several beams receiving and transmitting in multiple directions simultaneously—to help significantly reduce node discovery time and improve network throughput. “While critical to the next generation of phased-arrays, today’s digital beamforming is limited to lower frequencies, making the resulting arrays too large for use on small mobile platforms,” said Hancock.
To reduce the size of the arrays, advances in millimeter wave technology will help push the frequency of operation to higher bands, bringing the capabilities of directional antennas to small mobile platforms. “Through MIDAS, we are seeking proposals that combine advances in millimeter wave and digital beamforming technologies to create radios that will deliver secure communications for our military,” said Hancock.
There are many DoD applications that would benefit from millimeter wave phased arrays. These include a wide variety of short-range, high-data rate and long-range, low-data rate communication links for air-to-air and air-to-ground scenarios. The ability to support legacy commercial and military satellite communication bands to geosynchronous satellites is also important. Additionally, there is growing interest in the use of Ka- through V-band for low-earth orbit satellite constellations to provide connectivity to ground users.
Today’s millimeter wave systems, however, are not user friendly and are designed to be platform specific, lacking interoperability and are thus reserved for only the most complex platforms.
The objective of the MIDAS program is to explore the extent to which multi-beam systems can be employed at millimeter wave over extremely wide ranges of frequencies, which necessitates digitization within the array itself. A reduction in size and power of digital transceivers at millimeter wave is expected to and will likely involve innovative sampling and frequency conversion schemes to meet the linearity requirements.
To help solve the adaptive beamforming problem and ensure wide application of the resulting solutions, MIDAS seeks to create a common digital array tile that will enable multi-beam directional communications. Research efforts will focus on reducing the size and power of digital millimeter wave transceivers, enabling phased-array technology for mobile platforms and elevating mobile communications to the less crowded millimeter wave frequencies.
The primary goal of the program is to develop and demonstrate a tile building block sub-array (>16 elements) that supports scaling to large arrays (100’s-10,000+) in the 18-50 GHz band and does not eliminate spatial degrees of freedom within the sub-array. It is expected that this will be enabling hardware for multi-function, multi-beam phased array applications and emerging massive multiple-input-multiple-output (MIMO) techniques in communication and sensing.
To accomplish its goals, MIDAS is focused on two key technical areas. The first is the development of the silicon chips to form the core transceiver for the array tile. The second area is focused on the development of wide-band antennas, transmit/receive (T/R) components, and the overall integration of the system that will enable the technology to be used across multiple applications, including line-of-sight communications between tactical platforms as well as current and emerging satellite communications.
A reduction in the size and power of digital transceivers at millimeter wave is core to this goal and will need to be researched and developed within this program. Areas of expected research will be innovative sampling and frequency conversion schemes with high linearity for receive and transmit, distributed LO/clock generation and synchronization for each element, wideband/efficient transmit/receive amplifiers, radiating apertures and novel manufacturing to realize the integration and packaging all of these components into a scalable tile building block.
Achieving both of these goals will result in a 16-element tile building block that can be used to form larger apertures. In the third phase of the program, the focus will be on the creation of a 256-element array that is composed of the MIDAS tile building blocks and is capable of demonstrating digital multi-beam functionality over the full band of interest.
Program Awards by DARPA for high frequency digital communications research
Raytheon has been awarded an $11.5 million contract for development work on the Millimeter-Wave Digital Arrays program, an effort by the Defense Advanced Research Projects Activity program to enhance military communication security. The contract, announced in Nov 2018 by the Department of Defense, will see Raytheon focus on digital tile architecture and scalable apertures with transmit and receive components for the MIDAS program.
CoSMIC lab wins an ~$1M award under the DARPA MIDAS program in collaboration with Oregon State University. The DARPA MIDAS program “aims to develop element-level digital phased-array technology that will enable next generation DoD millimeter wave systems. To help solve the adaptive beamforming problem and ensure wide application of the resulting solutions, MIDAS seeks to create a common digital array tile that will enable multi-beam directional communications. Research efforts will focus on reducing the size and power of digital millimeter wave transceivers enabling phased-array technology for mobile platforms and elevating mobile communications to the less crowded millimeter wave frequencies.”
Navy Developing a digital, C-Band Transmit (Tx) and Receive (Rx) array antenna that transmits and receives multiple spatially and spectrally diverse narrowband signals.
Digital arrays are not off the shelf available; but rather, industry contractors develop digital arrays in response to acquisition efforts. The commercial development of multi-beam 5G networks will focus on small picocells. Lower power levels and reduced linearity challenge leave a significant gap preventing commercial technology from being useful in Navy applications. Defense Advanced Research Projects Agency (DARPA) efforts have made digital arrays a more off the shelf technology. Notable among these is the Arrays at Commercial Time Scales (ACT) and Millimeter-wave Digital Arrays (MIDAS) program. These programs focus on the transceiver and beamforming functionality of the array as opposed to the aperture. However, this technology is still not off the shelf and integration work would be required to meet the digital array needs even using this technology. The Navy must overcome some technology risks with a critical one being the development of digital array technology that can operate at the necessary bandwidths and frequencies while in complex RF environments.
The Navy seeks to expand and refine the battlespace by improving and expanding tactical network functionality. Increased data throughput is needed to enable the flow of more data and support of new mission areas. Decreased latency is needed to enable new and compressed kill chains against advancing threats as well as larger networks. Increased network throughput and decreased latency will be attained by developing 4-channel Transmit (Tx) and Receive (Rx) capability for digital communications arrays. The level of improvement in the fielded system will depend on the topology, size, and operation of the network. For large, half-duplex (i.e., cannot transmit and receive simultaneously) networks of four-beam nodes having all nodes connected along a line, the level of throughput improvement will approach a factor of 2. For large, half-duplex networks of four-beam nodes having topologies where all the nodes are connected to each other, the throughput improvement will approach a factor of 4. For other networks, the improvement will be somewhere in between. Of course, the fielded system may have a different number of beams per node. Four was chosen based on engineering judgement as a compromise between complexity, technical challenge, and capability improvement.
The Navy needs a digital communications array to realize simultaneous, multichannel Tx and Rx capability. The digital communications array is a key enabler for higher data throughputs and reduced latency needed to engage evolving threats and enabling significant improvement in utilization of spectrum. This must be done while pushing the boundaries of signal integrity, dynamic range, isolation of signals and resistance to interference to maximize link performance. No technology currently meets all these requirements.
An innovative digital antenna subarray architecture is sought to attain the previously stated requirements. More specific antenna system goals include a 1 x 4 linear configuration and element level signal generation and digitization. Beam steering in azimuth should be ±60º. The subarray should transmit and receive 4 simultaneous beams in half duplex mode. The operational bandwidth is C-band (4 GHz to 8 GHz). Compared to the operational bandwidth, the instantaneous bandwidth is relatively narrow. The element level Equivalent Isotropic Radiated Power (EIRP) should be 0 dBW over the scan volume. The output Error Vector Magnitude (EVM) should be less than 3%. The antenna should be able to receive an incident signal with incident power density measured at the free-space-to-antenna interface ranging from -134 dBW to -53 dBW and output a digital signal with 20 dB signal to interference plus noise ratio. The goal for the spur free dynamic range is 80 dB. 32 dBm is the goal for the input third order intercept. The polarization should be selectable, with four options. These options should be horizontal, vertical, right hand circular and left hand circular. The polarization loss factor should be less than 0.25 dB. The antenna will be capable of null steering with a null depth goal of 80 dB relative to the mainlobe.
The subarray must be capable of processing 4 narrowband signals located arbitrarily within a contiguous operational bandwidth within C-band. The design should permit any two 1 x 4 subarrays to be connected in any configuration and beam-steered. A two-dimensional array must be capable of having its beam steered in both dimensions. The design should permit connecting 1 x 4 or 4 x 1 subarrays into a contiguous rectangular array of arbitrary size. For example, three 1 x 4 subarrays must be able to be configured to form a 1 x 12 and then reconfigured to form a 3 x 4; without re-flashing firmware. Moreover, both configurations must demonstrate vertical, horizontal, right-hand circular, and left-hand circular polarizations while attaining 0.25 dB of polarization loss factor for each of these four polarizations. The design should include built-in testing to indicate failures that occur. The interface to the digital array on the transceiver side will use a standard format to send digits of data, such as Ethernet. Beam steering commands sent to the array will contain azimuth and elevation angles relative to the array face, frequency and Tx or Rx identification.
Testing, evaluation, and demonstration should include configuring and measuring antenna patterns for a 1 x 12 and 3 x 4 array using the same three (3) 1 x 4 subarrays. Moreover, vertical, horizontal, right-hand circular, and left-hand circular polarizations should be demonstrated. Validation of the prototype will be through comparison of model predictions to measured performance. The location for the demonstration may occur at the small business’s facility or at a Government-identified location.
Work produced in Phase II may become classified. Note: The prospective contractor(s) must be U.S. Owned and Operated with no Foreign Influence as defined by DOD 5220.22-M, National Industrial Security Program Operating Manual, unless acceptable mitigating procedures can and have been be implemented and approved by the Defense Security Service (DSS). The selected contractor and/or subcontractor must be able to acquire and maintain a secret level facility and Personnel Security Clearances, in order to perform on advanced phases of this contract as set forth by DSS and NAVSEA in order to gain access to classified information pertaining to the national defense of the United States and its allies; this will be an inherent requirement. The selected company will be required to safeguard classified material IAW DoD 5220.22-M during the advance phases of this contract.
PHASE I: Define and develop a concept for a digital C-Band Tx and Rx array antenna. Demonstrate that the concept can feasibly meet the Navy requirements as provided in the Description. Establish feasibility by a combination of initial analysis and modeling. The Phase I Option, if exercised, will include the initial design specifications and capabilities description to build a prototype in Phase II.
PHASE II: Develop and deliver a prototype digital C-Band Tx and Rx array antenna that demonstrates the performance parameters outlined in the Description. Conduct prototype testing, evaluation, and demonstration (at the small business’s facility or at a Government-identified location). Provide an interface control document guide for developing the signal and control interface for the array. Include configuring and measuring antenna patterns for a 1 x 12 and 3 x 4 array using the same three (3) 1 x 4 subarrays in the demonstration plus vertical, horizontal, right-hand circular, and left-hand circular polarizations. Validate the prototype through comparison of model predictions to measured performance.