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Spectrum congestion is a growing problem. It increasingly limits operational capabilities due to the increasing deployment and bandwidth of wireless communications, the use of net-centric and unmanned systems, and the need for increased flexibility in radar and communications spectrum to improve performance and to overcome sophisticated countermeasures.
Ongoing wireless revolution is fueling a voracious demand for access to the radio frequency (RF) spectrum around the world. In the civilian sector, consumer devices from smartphones to wearable fitness recorders to smart kitchen appliances are competing for bandwidth. Around 50 billion wireless devices are projected to be vying for access to mobile communications networks within the next few years and by 2030, the demand for wireless access could be 250 times what it is today. However, as the use of wireless technology proliferates, radios and communication devices often interfere with and disrupt other wireless devices.
Military spectrum requirements are also increasing exponentially. Military operations increasingly rely on access to the wireless spectrum in order to assess the tactical environment and coordinate and execute their critical missions. The demand for more and timely information at every echelon is driving an increase in DoD’s need for spectrum.“Increasingly lower echelons, including individual soldiers, require situational awareness information resulting in more spectrum-enabled network links.”
” Weapon testing stateside requires more bandwidth, as does a new generation of radars that now must detect smaller targets at longer ranges, said Frederick D. Moorefield Jr., director of spectrum policy and programs at the office of the Defense Department chief information officer. Also there is need for increased flexibility in radar and communications spectrum to improve performance and overcome sophisticated countermeasures.
However, spectrum is a finite resource and additionally DOD has to free up 500 MHz of the spectrum it has for commercial use by 2020 leading to scarcity of spectrum for DOD use.Managing this increasing demand, while combating what appears to be a looming scarcity of RF spectrum is a serious problem for our nation, both militarily and economically, says DARPA. This has resulted in complex Defense, spectrum management within and between the armed services, and any errors in the spectrum management plan may result in the denial of critical strategic and tactical links. The second is relatively easy for adversaries to target such a small part of the RF spectrum allocated exclusively to the government through jamming or electronic attack.
US DOD is now advancing new concept of spectrum sharing where disparate systems like communications, radar and electronic warfare will now be able to share the share the same spectrum because of emergence of recent technologies in radio communication. Until recently, spectrum allocation (i.e., a certain frequency band is assigned exclusively to a certain electromagnetic wave emit-ting technology like radar or wireless communications) was vital to prevent any interference among different systems. “Better spectrum management and spectrum efficiency improvements are necessary, but not sufficient — spectrum sharing required for increasing access,” said Steve Molina of DISA/DSO/Strategic Planning Division.
In 2013, the US Defense Advanced Research Projects Agency (DARPA) set up a program named Shared Spectrum Access for Radar and Communications (SSPARC) to develop spectrum-sharing technologies between military
radars and military communication systems, which is considered as an effective way of improving the spectrum utilization. The radar bands are among at the best candidates to be shared with various communication systems due to the large chunks of spectrum available at radar frequencies.
Radar and communications jointly consume most of the highly desirable spectrum below 6 GHz. DARPA’s SSPARC seeks to develop sharing technology that enables sufficient spectrum access within this desirable range for radar and communications systems and improve radar and communications joint operational capabilities to accomplish their evolving missions. DARPA opened Phase 2 of its broad agency announcement for the Shared Spectrum Access for Radar and Communications (SSPARC) program
DARPA’s new proposal is in line with DOD’s Electromagnetic Spectrum Strategy 2013, which called for ensuring the access to the congested and contested electromagnetic environment of the future, by adopting new agile and opportunistic spectrum operations, and through systems which are more efficient, flexible and adaptable and adopting new technologies capable of more efficient use of the spectrum and reduced risk of interference. It also called for pursuing access to spectrum allocated for non-federal use and spectrum sharing technologies.
Shared Spectrum Access for Radar and Communications (SSPARC) project
In general, there are two main research directions in communication and radar spectrum sharing (CRSS): 1) Radar-communication coexistence (RCC) and 2) Dual-functional Radar-Communication (DFRC) system design. By considering the coexistence of individual radar and communication systems, the first category of research aims for developing efficient interference management techniques, so that the two systems can operate without unduly interfering with each other.
On the other hand, DFRC techniques focus on designing joint systems that can simultaneously perform wireless communication and remote sensing. The joint design benefits both sensing and signalling operations, decongests the RF environment, and allows a single hardware platform for both functionalities. This type of work has been extended to numerous novel applications, including vehicular networks, indoor positioning and secrecy communications
SSPARC treats spectrum sharing as a cooperative problem. Prior work on radar/communications spectrum sharing assumes that one of the two systems ignores the other. In cooperative spectrum sharing, information is shared between the systems in near real time. The shared information enables the systems to be kept separated (i.e., noninterfering) based on how they actually use the spectrum, not based on how they might potentially use or are predicted to use the spectrum.
The solutions for spectrum sharing can be classified into three broad categories, explain Harun T Hayvaci and Bulent Tavli of TOBB University of Economics and Technology.
In the first category, radar system as taken as the primary user and the objective is the maximize the performance of the communication system utilizing radar spectrum as a secondary user (i.e., radar performance should not be deteriorated by the communication system). In this category radar system is not affected by the shared use of the spectrum and the burden of ensuring this constraint is on the communication system, entirely.
In the second category, solutions are proposed to mitigate the interference caused by the communication system on the radar. Although it is assumed the communication system is operating cognitively, the proposed solutions are developed by assuming the interference mitigation responsibility is on radar itself without any level of explicit cooperation among the radar and the communication system.
The third category is the most sophisticated category and it, potentially, brings the highest gains for both radar and communication systems operating in the same frequency band. In this category, both radar and communication systems cooperatively alleviate the effects of interference to each other which necessitates joint design of both systems for interference mitigation.
DARPA experts also want to find ways to protect radar and communications systems from electronic warfare jamming and cyber-attacks with techniques like minimization, encryption, obfuscation, and short-time validity of shared information.
Shared Spectrum Access for Radar and Communications (SSPARC) project
The SSPARC program seeks to support two types of spectrum sharing.
- Spectrum sharing between military radars and military communications systems (“military/military sharing”)increases both capabilities simultaneously when operating in congested and contested spectral environments.
- Spectrum sharing between military radars and commercial communications systems (“military/commercial sharing”)preserves radar capability while meeting national and international needs for increased commercial communications spectrum, without incurring the high cost of relocating radars to new frequency bands.
Although SSPARC technology is expected to be widely applicable, the research focus of the program is on the following spectrum sharing challenge.
• S-band, 2 GHz – 4 GHz
• Radar
Ground or naval-surface
Electronically steered phased array
Multifunction – combines air surveillance, air tracking, non-cooperative target identification, and optionally, weather monitoring
• Communications system
Ground or naval-surface
Military system type: MANET
Commercial system type: Small-cell broadband
Approaches of Interest
Two forms of codesign are of interest in this phase.
1. An integrated system that performs radar and communications functions in a single spectrum allocation.
2. Separate radar and communications systems that optimize performance when given a common allocation or overlapping allocations.
The program has determined that the following scenarios are relevant and appropriate.
- Multistatic/distributed radar: Multistatic and distributed radars can require high data rates between the radar nodes. In some operational contexts, it is difficult to get the spectrum allocations required for the necessary communications links. Combining the communications links and the radar enables deployment of the overall system, and provides additional data capacity that meets other communication needs of the platforms hosting the radar nodes. The nodes can be assumed to be in line of sight of each other.
- Airborne Long Range Radar:The mission effectiveness of current airborne long-range radar platforms can be limited by constraints on the range, capacity, and coverage of the communications links they carry. Adding new communications links into the radar as part of a radar upgrade can improve communications to other air and surface platforms. The goal is to improve platform effectiveness for missions such as integrated fire control, or airborne command and control.
- Intelligence, Reconnaissance and Surveillance (ISR) In Contested Airspace:In contested environments, current airborne ISR concepts envision collaboration between a stand-off high-value platform and multiple attritable stand-in unmanned platforms. High rate reachback links from the stand-in platforms can require high transmit power and/or high gain antennas. Combining communications and radar may reduce the Size Weight and Power (SWAP) of the stand-in electronics and aperture, thus enabling use of smaller stand-in platforms. Reducing stand-in platform size is valuable because it reduces cost, which enables deploying more aircraft. Reducing platform size also contributes to reducing the radar cross section. Both effects increase mission survivability.
- Protected Surface Radar:Monostatic surface-level radars performing air surveillance and tracking face threats from proliferated jamming technology. Augmenting the radar receiver with sensing performed by the communications nodes can improve Electronic Protection and overall mission success. At the same time, use of the radar transmitter/aperture or use of radar spectrum for communications can improve communications range, capacity and coverage
The second phase of the SSPARC program will build on technologies developed over the past 18 months by four companies during the program’s first phase, which aimed at improving radar and communications capabilities while avoiding cross-platform interference.
The four organizations involved in SSPARC phase one are Michigan Technological University (Michigan Tech) in Houghton, Mich.; SAZE Technologies LLC in Silver Spring, Md.; the Lockheed Martin Corp. Advanced Technology Laboratories in Cherry Hill, N.J.; and Science Applications International Corp. (SAIC) in McLean, Va.
Projects under Phase 1 of SSPARC studied two radars, the AN/SPY-1 Aegis radar and the AN/TPS-80 Ground/Air Task Oriented Radar (G/ATOR), as well as several communications systems, including the Joint Tactical Radio System (JTRS) Wideband Networking Waveform (WNW), Harris Adaptive Networking Wideband Waveform (ANW2), and Long-Term Evolution (LTE), a fourth-generation commercial wireless communications standard.
Leidos wins R&D DARPA contract in support of Shared Spectrum Access for Radar and Communications program
Defense Advanced Research Projects Agency (DARPA)’s officials selected Leidos for a research and development contract in support of Phase 2 for the Shared Spectrum Access for Radar and Communications (SSPARC) program. During Phase 1, using a high-fidelity end-to-end simulation, the Leidos team demonstrated that the minimum distance from military radar that commercial wireless may operate can be reduced by 50 times, officials say.
During Phase 2, engineers will perform lab testing as well as a field demonstration to show the technology behind using real-time software with physical radar and communications systems. The algorithms developed can increase radio frequency spectrum availability for both radar and communications systems, if engineers are successful.
Leidos has partnered with Federated Wireless for commercial wireless-radar spectrum sharing.
Federated Wireless said it aims to demonstrate the company’s Spectrum Access Service platform in conjunction with an integrated commercial LTE infrastructure through the SSPARC project.
Federated Wireless’ technology is a dynamic three-tiered Spectrum Access System (SAS), which will enable carriers and other industry participants to unlock the value of licensed shared spectrum. This approach provides for allocation and management of spectrum resources in real time. As a result, wireless networks will be enhanced, spectrum capacity will be increased, and the end user will get the high quality of experience they demand.
The SAS technology uses a neural network of radio sensors intended to help organizations access and manage licensed spectrum.
After SSPARC phase two is finished, DARPA officials plan to launch an 18-month third phase that will include extensive field testing with radar and communications users. Tests will assess mitigation of nuisance jamming and problems caused by malfunctioning communications devices.
Spectrum Sharing Radar
Researchers Deborah Cohen, Kumar Vijay Mishra, and Yonina C. Elda from israel presented a spectrum sharing technology enabling interference-free operation of a surveillance radar and communication transmissions over a common spectrum. A cognitive radio receiver senses the spectrum using low sampling and processing rates. The radar is a cognitive system that employs a Xampling-based receiver and transmits in several narrow bands.
“From L-band (1-2 GHz) onward, the radars begin to witness spectral intrusion from LTE.
L-band (1-2 GHz):
This band is primarily used for long-range air-surveillance radars, such as Air Traffic Control (ATC) radar, which transmits high-power pulses with broad bandwidth. The same band, however, is also used by 5G NR and FDD-LTE cellular systems as well as the Global Navigation Satellite System (GNSS) both in their downlink (DL) and uplink (UL)
An example is the Air Route Surveillance Radar (ARSR) used by Federal Aviation Administration (FAA) sharing frequencies with WiMAX (Wireless Interoperability Microwave Access) devices. Military radio services such as the Joint Tactical Information Distribution System (JTIDS) in the 969-1206 MHz band are also known to interfere with L-band radars .
S-band (2-4 GHz):
This band is typically used for airborne early warning radars at considerably higher transmit power. Some long-range weather radars also operate in this band due to moderate weather effects in heavy precipitation. Communication systems present in this band include 802.11b/g/n/ax/y WLAN networks, 3.5 GHz TDD-LTE and 5G NR.
However, a majority of LTE waveforms, e.g. 802.11b/g/n (2.4 GHz) WCDMA (Wide-band Code Division Multiplexing Access), WiMAX LTE, LTE GSM (Global System for Mobile comm), EDGE (Enhanced Data rates for GSM Evolution), coexist within the S-band (2-4 GHz). Therefore, most of the spectrum sharing studies are concerned with S-band radars.”
C-band (4-8 GHz):
This band is very sensitive to weather patterns. Therefore, it is assigned to most types of weather radars for locating light/medium rain. On the same band operate radars used for battlefield/ground surveillance and vessel traffic service (VTS). Wireless systems in this band mainly include WLAN networks, such as 802.11a/h/j/n/p/ac/ax
Spectral coexistence systems for C-band (4-8 GHz) are gradually gaining at traction due to the latest 5 GHz band allocation to 802.11a/ac VHT (Very High Throughput) wireless LAN (WLAN) technology.
MmWave band (30-300 GHz)
Typically, communication systems operated close to 30GHz (e.g. 28GHz) are also referred as mmWave systems.: This band is conventionally used by automotive radars for collision detection and avoidance, as well as by high-resolution imaging radars . However, it is bound to become busier, as there is a huge interest raised by the wireless community concerning mmWave communications, which are soon to be finalized as part of the 5G NR standard. Currently, the mmWave band is also exploited by the 802.11ad/ay WLAN protocols
“A variety of system architectures have been proposed for spectrum sharing radars. Most put emphasis on optimizing the performance of either radar or comm while ignoring the performance of the other. The radar-centric architectures usually assume fixed interference levels from comm and design the system for high probability of detection (Pd). Similarly, the comm-centric systems attempt to improve performance metrics like the error vector magnitude (EVM) and bit/symbol error rate (BER/SER) for
interference from radar.”
“Our model is that of a “friendly” spectral coexistence where an active cooperation between radar and comm is required, as also envisaged by the SSPARC program. This is different than the spectrum sharing techniques where the two systems operate independently of each other and attempt to minimize interference in their respective spectra.”
