The military uses the electromagnetic spectrum — essential, yet invisible — to detect, deceive and disrupt the enemy while protecting friendly forces. As enemies become more capable and threats more complex, controlling the spectrum is increasingly critical. However, today’s electromagnetic (EM) spectrum is a scarce resource that is becoming increasingly congested and contested as friendly, unfriendly, and neutral entities vie for available spectrum resources at any given time, location, and frequency. Within the Department of Defense (DoD), radio frequency (RF) systems, such as communications networks and radar, must operate within this congested environment and contend with mission-compromising interference from both self-and externally generated signals.
Ultra-wideband systems can enable many capabilities in military communications, radar, electronic intelligence and electronic warfare. UWB characterizes transmission systems with instantaneous spectral occupancy in excess of 500 MHz or a fractional bandwidth of more than 20%. (The fractional bandwidth is defined as B/fc , where B := fH − fL denotes the −10 dB bandwidth and center frequency fc := (fH + fL )/2 with fH being the upper frequency of the −10 dB emission point, and fL the lower frequency of the −10 dB emission point.
The term: UltraWideBand or UWB signal has come to signify a number of synonymous terms such as: impulse, carrier-free, baseband, time domain, nonsinusoidal, orthogonal function and large-relative-bandwidth radio/radar signals.
The early applications of UWB technology were primarily radar related, driven by the promise of fine-range resolution that comes with large bandwidth.
Ultra Wide-band (UWB) radar systems transmit signals across a much wider frequency than conventional radar systems and are usually very difficult to detect. The transmitted signal is significant for its very light power spectrum, which is lower than the allowed unintentional radiated emissions for electronics. The amount of spectrum occupied by a signal transmitted by a UWB-radar (i.e., the bandwidth of the UWB signal) is at least 25 percent of the center frequency. Thus, a UWB signal centered at 2 GHz would have a minimum bandwidth of 500 MHz and the minimum bandwidth of a UWB signal centered at 4 GHz would be 1 GHz. Often the absolute bandwidth is larger than 1 GHz.
The most common technique for generating a UWB signal is to transmit pulses with very short durations (less than 1 nanosecond). The spectrum of a very narrow-width pulse has a very large frequency spectrum approaching that of white noise as the pulse becomes narrower and narrower. These very short pulses need a wider receiver bandwidth as conventional radar systems.
An example of a typical UWB radar is the pulsed noise radar. This radar is transmitting at the center frequency of 24 GHz single pulses with a duration of a few nanoseconds, and a pulse power of 4 mW. Its bandwidth is therefore 8 GHz. Whether this radar is transmitting or not cannot be determined by measurement from the outside of the radar as its transmit pulses do not differ from environmental noise.
Ultrawideband SAR radar
In terms of military use, ultra-wideband gained widespread attention for its implementation in synthetic aperture radar (SAR) technology. Due to how it retained high resolution despite its use of lower frequencies, UWB SAR was heavily researched for its object-penetration ability. Ultra-wideband is also used in “see-through-the-wall” precision radar-imaging technology, precision locating and tracking (using distance measurements between radios), and precision time-of-arrival-based localization approaches. Ultra-wideband pulse-Doppler radars have also been used to monitor vital signs of the human body, such as heart rate and respiration signals as well as human gait analysis and fall detection. The EW receivers for threat detection from unknown targets require a receiver that can operate over a wide frequency band to identify threat signals and initiate countermeasures.
Ultra-wideband (also known as UWB, ultra-wideband and ultraband) radio technology uses a very low energy level for short-range, high-bandwidth communications over a large portion of the radio spectrum. The UWB technology delivers data rates in excess of 100 Mbps up to 1 Gbps. The key advantages of the UWB systems over narrowband systems are: high data rate due to the large bandwidth, low equipment cost, low power and immunity to multipath. The UWB not only has the potential of carrying high data rate over short distance, but also it can penetrate through doors and other obstacles.
UWB technology for communications is not all about advantages. In fact, there are many challenges involved in using nanosecond-duration pulses for communications. The transmission characteristics of UWB pulses are more complicated than those of continuous narrowband sinusoids. A narrowband signal remains sinusoidal throughout the transmission channel. However, the weak and low-powered UWB pulses can be distorted significantly by the transmission link.
Time synchronization is a major challenge and a rich area of study in UWB communications systems. As with any other wireless communications system, time synchronization between the receiver and the transmitter is a must for UWB transmitter/receiver pairs. However, sampling and synchronizing nanosecond pulses place a major limitation on the design of UWB systems. In order to sample these narrow pulses, very fast (on the order of gigahertz) analog-to-digital converters (ADCs) are needed. Moreover, the strict power limitations and short pulse duration make the performance of UWB systems highly sensitive to timing errors such as jitter and drift.
The operation of the UWB is based on two conditions: (1) This device may not cause harmful interference, and (2) this device must accept any interference received, including interference that may cause undesirable operation. A desire to support wideband EM spectrum operations also adds to the burden, as current approaches to mitigating wideband receiver interference are sub-optimal and force compromises around signal sensitivity, bandwidth usage, and system performance. Further, in the case of self-interference, traditional mitigation approaches such as antenna isolation alone are often not sufficient for protecting wideband receivers.
“Protecting our wideband digital radios from interference and jamming in the unpredictable EM environment is critical to our defense capabilities, and has prompted the exploration of wideband tunable circuit architectures to support cognitive radio technology,” said DARPA program manager, Dr. Timothy Hancock. “Unlike narrowband radios that rely on switching between pre-planned filtering and narrowband signal cancellation, today’s wideband radios lack the RF front-ends that could help mitigate harmful signals before they reach the sensitive receiver electronics.”
Wideband Adaptive RF Protection (WARP) program
The UWB radios can be implemented either as multiband OFDM (MB-OFDM) or directs sequence impulse radio (DS-IR). The IR system is relying on a very short duration of the pulses with several Gigahertz bandwidth. The main challenge facing with IR system, is the existence of the neighbor narrowband systems. Since IR receiver/transmitter systems are based on the short pulses, each narrowband signal with the same band from another system can fall on the IR fundamental band and disrupt the signal. A solution to this problem is to use a notch filter, however not only the design of a precise narrowband notch filter is very challenging but also the notch filter can simply disturb the useful signal.
Over the last decade, wideband analog-to-digital converter (ADC) technology has improved in both bandwidth and resolution to a point that wideband RF sampling receivers are now a reality. A/D converter technology has achieved greater than 10 GHz of instantaneous bandwidth with 8-10 effective number of bits (ENOB). However, wideband ADCs typically poses two challenges. Wideband A/D converters typically have a relatively small available input voltage swing and reduced spur-free dynamic range when compared to their narrowband counterparts; and are typically exposed to more signals simultaneously due to the wide bandwidth.
Despite the advantages associated with more bandwidth, the dynamic range limitation can prevent the use of wideband receivers in multi-function applications that support wideband electromagnetic spectrum operations (EMSO). The Wideband Adaptive RF Protection (WARP) program seeks to enhance protections for wideband receivers operating in congested and contested EM environments, ultimately enabling the use of wideband software-defined radios in contested and congested spectral environments.
Today, receivers are protected from external interference through static filtering, automatic gain control, or signal limiters. Yet static filtering only uses a fraction of the digital receiver bandwidth, which gives good sensitivity but does not take advantage of available receiver bandwidth. Automatic gain control, meanwhile, capitalizes on system bandwidth, but decreases sensitivity to small signals. At the same time, signal limiters can cause cross-modulation distortion and may decrease the overall sensitivity of the system. Tunable filters sometimes are a solution, but rarely can tune over achievable bandwidth.
Instead, the WARP program seeks is to develop wideband, adaptive filters and analog signal cancellers that selectively attenuate – or cancel – externally generated interference signals (from adversarial jamming, for instance) and self-generated interference signals (like those created by a radio’s own transmitter) to protect wideband digital radios from saturation. Saturation occurs when the power level of a received signal exceeds the receiver’s dynamic range – or the range of weak to strong signals it can handle. When exposed to interference or jamming, the target WARP components will sense and adapt to the EM environment through the intelligent control of adaptive hardware.
To address external interference, WARP will explore the development of wideband tunable filters that can continuously sense the EM environment and adapt to maintain the receiver’s dynamic range without decreasing signal sensitivity or bandwidth. The research will look at innovative filter architectures supported by state-of-the-art components and packaging to achieve the program’s target metrics.
The ideal wideband receiver would adapt to EW jamming or blocking to maintain dynamic range without decreasing sensitivity and bandwidth. the WARP project seeks to develop adaptive filters to reconfigure their frequency response automatically to include pass/stop bands with bandwidth and center frequency tuning and attenuate large signals selectively while passing small or desired signals. “With the WARP filters, the goal is to reduce the effect of large signals without attenuating smaller signals. By attenuating the large signals, a wideband RF system is better able to listen to both weak and strong signals over a wide bandwidth,” noted Hancock.
WARP will also address self-generated inference with the development of adaptive, analog signal cancellers. “Sometimes a system’s own transmitter is the biggest interferer to the receiver. To avoid this issue, transmitting and receiving at different frequencies has traditionally been commonplace, aided by the use of a frequency duplexer to keep the two bands separate. However, for defense systems there are a number of benefits to transmitting and receiving on the same frequency – such as doubling spectrum efficiency and increasing network throughput. This concept is referred to as same-frequency simultaneous transmit and receive (STAR),” said Hancock.
The use of same-frequency STAR has been limited due to few available means of ensuring the transmitter leakage does not interfere with the receiver. To combat this, WARP will explore analog cancellers that will reduce the transmit leakage before the wideband digital receiver, such that any residual leakage will be sampled and further cancelled in the digital domain.
To accomplish its goals, WARP is focused on two key technical areas. In technical area 1, the focus is on mitigating external interference in the 2-18 GHz band with tunable filtering that can adapt to the signals present in the spectrum. This will be accomplished by first targeting new filter architectures and associated underlying technology to achieve a 2:1 and 3:1 tuning ratio in Phase 1 and Phase 2 of the program, respectively, before culminating in full-band coverage by the end of the program.
In technical area 2, the focus is on mitigating self-interference from a co-located transmitter in the 0.1-6 GHz band to enable same-frequency simultaneous transmit and receive (STAR). To achieve wideband operation, this will require the canceller to have a time-bandwidth product of approximately 10 by the end of the program, well beyond the state-of-the-art in simple resonant RF canceller circuits.
In these new approaches to wideband reconfigurable filtering and signal cancellation, it is expected that the number of tuning inputs could be large. To manage this tuning challenge, it is expected that the hardware will adapt to the environment in real-time through embedded sensing and control. If successful, by the end of the program, when exposed to external or self-interference, WARP filters and cancellers will autonomously adapt to the spectral environment to protect a wideband RF receiver.
“Through WARP’s technological developments, our ability to reduce critical interference issues and protect wideband radios will significantly improve. Further, if successful, these technologies will enable the use of software-defined radios (SDRs) in congested and dynamic spectral environments – something that is limited today,” concluded Hancock.
U.S. Air Force Research Laboratory at Wright-Patterson Air Force Base, Ohio., announced a $7.2 million contract to L3Harris in Feb 2021 for the Wideband Adaptive RF Protection (WARP) project.
DARPA, awarded two contracts to BAE Systems in April 2021 totaling $5 million under the Wideband Adaptive RF Protection (WARP) program which is designed to develop wideband adaptive filtering and signal cancellation architectures to safeguard emerging wideband receivers against both external and self-interference.
Within the Department of Defense, radio frequency (RF) systems must operate within an increasingly crowded electromagnetic spectrum and contend with mission-compromising interference from friendly and hostile sources. “The ability to control signal strength across the electromagnetic spectrum is critical to the robust operation of wideband RF electronics,” said Chris Rappa, product line director at BAE Systems’ FAST Labs™ research and development organization. “WARP signal filters and cancellers will sense and adapt to the electromagnetic environment through the intelligent control of adaptive hardware.”
The technical areas of the program focus on enhancing electronic warfare technology to improve adaptive control of electromagnetic spectrum – enabling allied forces to freely operate while denying that advantage to adversaries. Specifically, Technical Area 1 is focused on mitigating external interference and Technical Area 2 is focused on mitigating self-interference from co-located transmitters to enable same-frequency simultaneous transmit and receive, also known as STAR.
The WARP awards add to the advanced defense electronics and electronic warfare research and development portfolio and are based on many years of investment on various programs including T-MUSIC, CONverged Collaborative Elements for RF Task Operations (CONCERTO) and Radio Frequency Field Programmable Gate Arrays (RF-FPGA).