Millimeter-wave (mm-wave) technology is playing a central role in next generation radar and wireless communications. With the already massive demand for spectrum space in the 2.4/5 GHz bands, the Ka-Band and beyond provide expansive spectrum availability and flexibility. Already implemented in automotive and many security applications, mm-wave imaging radar enables self-driving and nearly autonomous automotive capabilities, and is also leveraged to a high degree in airport security for the detection of concealed objects. From ground- and air-based armored target imaging scenarios to the detection of objects concealed under clothing, mm-wave radar can enable a great deal of situational awareness.
Millimeter-wave generally corresponds to the 30 to 300 GHz frequency range with wavelengths ranging from 1 cm to 1 mm where the range from 1 mm to 0.3 mm is generally referred to as sub-millimeter wave; the specific frequency bands can be shown in Table 1. Typically, radar is associated with the RF range with equipment such as massive parabolic antennas to high powered klystrons; these designs generally serve specific high powered, long range applications. The mm-wave band has more recently become advantageous in radar particularly for shorter ranged applications with tight size and weight tolerances.
As the transmit frequency increases the size of frequency-sensitive elements decreases, offering an obvious advantage of small size. A long operating range is also not needed for mm-wave components, so the high transmit power that is used in massive antenna structures is normally not an obstacle. Along with the advancements in Active Electronically Scanned Arrays (AESA), many mm-wave radar platforms can be built as a System on Chip (SoC), or into small form factors in multi-module chip systems. Another advantage is the ability to function in low visibility conditions such as rain, smoke, and fog along with nearly negligible propagation losses due to atmospheric oxygen (in W-Band). These allow for high precision targeting in military applications, a high operation in automotive applications, and potential viability even in medical applications.
Long Range Radar in Automotive Applications
Currently there are many antennas implemented in a car design for the great variety of subsystems in modern vehicles including: AM/FM radio (VLF-Band), satellite TV (X-Band), Traffic Message Channel (TMC) through satellite radio (S-Band), Digital Audio Broadcasting (DAB) (L-Band), GPS (L-Band), Tire Pressure Monitoring System (TPMS) (UHF-Band), Remote Keyless Entry (RKE) (UHF-Band), Remote Start Engine (RSE) (UHF-Band), Dedicated Short Range Communications (DSRC) (C-Band), and automotive radar (W-Band). As shown in Figure 1, adaptive cruise control (ACC) is becoming increasingly popular, with new models coming out every year integrating this feature. Automotive radar is a fundamental component of autonomous vehicle capabilities so it is important to reliably fabricate an automotive radar system to extremely small form factors or even to potentially fit to a radar system on chip (SoC) design.
An evident advantage of mm-wave radar is that the antenna can be fabricated small and there is no need for a long operating range, making them ideal for short-range discrete tracking. In automotive applications, the small antenna can fit stringent dimensional and aesthetic requirements as car manufacturers are very reluctant to alter the shape of a vehicle to accommodate any electronic apparatus. Most automobiles have adopted the 24 GHz and 77-79 GHz systems for automotive radar as it is a good compromise between compactness and cost where the 77 GHz band is leveraged for Long Range Radar (LRR) up to 150m in front of the vehicle and the 24 GHz band for Short Range Radar (SRR) with a 30m range. The LRR system operates with antenna arrays that contain multiple antenna elements, beamforming networks, and a receiver/transmitter.
The antenna elements can range from a simple dipole to a printed patch design. The LRR antenna arrays can leverage either monostatic or bistatic beamforming techniques for approximately ±10 degrees of total azimuthal coverage. Some LRR technologies such as Delphi’s and Fuitsu Ten’s mechanical radar leverage fixed beam directional arrays with mechanical azimuthal steering with a dielectric lens, while some recent radars employ Active Electronically Scanned Arrays (AESA) with simple electronically switchable elements for 2D scanning . Dielectric lenses offer the advantages of simpler mechanical steering by adjusting the feed around the lens without rotating the whole antenna. These lenses are spherically symmetric so a single lens can be used with several feeds to look over a wide angle, ideal for the seamless detection of multiple targets. Less complex than phased antenna arrays, the switched beam arrays are often implemented in car radar and can be steered through connecting the parasitic elements to pin-diode switches. This way, RF energy is not wasted on antenna elements that are not facing in a useful direction while still being able to leverage digital beamforming (DBF) methods.
MM-Wave Avionics Navigation Systems
Along with the many recent advances in military and civil avionics, Enhanced Flight Vision Systems (EFVS) and instrument landing systems (ILS) can operate at mm-wave frequencies to obtain a higher positional accuracy than their RF avionics counterparts. ILS systems have been in use for many decades at major airports to guide a landing aircraft to a runway without hitting obstacles or buildings in any weather condition. The ILS uses two major components, the localizer and glidescope; the localizer operates laterally and the glidescope vertically. These systems work in tandem and in cooperation with the transmitting airport runway to properly navigate an aircraft into a safe landing. Typically, the localizer sends out two beams at the same carrier frequency (between 108.10 MHz and 111.95 MHz) from two different antennas with differing modulations. The glidescope signal (between 329.15-335.0 MHz) is radiated from an antenna array with a radiation pattern with two vertically overlapping beams to provide the pilot with vertical guidance.
The ILS system is critical in landing commercial aircraft but needs enhancement in scenarios with tight tolerances and no prebuilt ground infrastructure such as the navigation of Unmanned Aerial Vehicles (UAVs) or even for military aircraft that perform unconventional landings. Similar to LRR in automobiles, both autonomous and manual landings in physically and electromagnetically cluttered environments such as urban landscapes or indoors necessitate mm-wave technology. For instance, BAE Systems’ Autonomous Approach Landing Capability (AALC) system leverages the 94 GHz frequency (W-Band) with an optical sensor for multi-spectral visibility . It has been found that at 94 GHz there is an atmospheric window which permits radar to image through fog, so many mm-wave avionics navigation systems operate at this frequency.
EFVS (Figure 2) are installed on aircraft to provide a real-time display of the forward external scene topography through the use of imaging sensors, including but not limited to forward-looking infrared, millimeter wave radiometry, millimeter wave radar, or low-light level image intensification . Similar to other mm-wave radar, an EFVS can leverage reconfigurable aperture antennas with a switched grid of metallic patches. Each patch in the grid is electrically connected to neighboring patches through a switched link. These individual switches can be opened or closed in predetermined arrangements to reconfigure the resulting radiation pattern) . These antenna arrays allow for easier integration by effectively eliminating a mechanical gimbal. Similar to most aircraft radar systems, the antennas are protected by radomes to shield the system from heat, wind, and rain.
MM-Wave Imaging for Security Applications
Security applications leverage imaging to detect concealed threats with high volumetric resolution and small antennas. In portal-based applications such as airport mm-wave scanners as shown in Figure 3, cylindrically scanned synthetic aperture radar (SAR) algorithms are often leveraged as the moving platform (or moving target) can reconstruct images with higher spatial resolution than the diffraction-limited imagery possible with a stationary aperture and target . In a mm-wave body scanner, the mm-waves are beamed over the target with two rotating antennas and the energy reflected back is processed to create images to reveal potentially objectionable equipment. Some portal systems also use stacks of slotted waveguide antennas with lens covers to vertically focus the antenna beam while other portal technology leverage transmitting and receiving planar antenna arrays with complex beamforming algorithms . Scanning can occur at the 60 GHz unlicensed spectrum or well into the W-Band. At the 60 GHz range, the background illumination from the sky is similar to the background radiation from the ground, creating a much more uniform image background compared to other mm-wave frequencies to allow for reliable automated threat detection routines .
MM-Wave Potential in Medical Applications
Mm-wave technology allows for continuous-contactless vital sign monitoring in medical applications in order to maintain better health or for early detection of health issues. Traditionally, heart health is measured through ergospirometry (stress test) or through bioinstrumentation such as an electrocardiogram (ECG/EKG) (HF-Band). Typically, only individuals with potential heart issues undergo these tests through a healthcare provider in order to obtain critical health information. Heart rate monitors (2.4 GHz ISM band) require wearable equipment for continuous monitoring. MM-wave radar also opens doors for remote sensing multiple humans simultaneously. While there are currently no commercially available systems, there is potential as 60 GHz-based systems become more ubiquitous .
While there is a great deal of opportunity for mm-wave radar in commercial, medical, and military applications, there are challenges towards its ubiquity. Components and equipment with mm-wave functionality tend to be much more expensive than their RF counterparts. For instance, precision connectors such as 3.5 mm, 2.92 mm, 2.4 mm, and 1.85 mm can cost more than ten times the price of a SMA, BNC, or N-type connector. Development kits are few and far between, thereby limiting research and development to those who have the overhead and expertise to develop a highly customized system. From the antenna to the radar and system simulator, to the post-processing stage, mm-wave radar test beds require a great deal of experimentation.
While phased array antennas or reconfigurable aperture antennas offer electronically controlled beamforming with high directivity and gain, many test set ups rely on horn antennas and programmable rotators for their moderate directivity and cost-effectiveness. The 60 GHz unlicensed spectrum allows the benefit of testing at mm-wave without the need to invest in broadcast licensing, making it an ideal range for highly experimental test and measurement for mm-wave radar and wireless setups. Figure 4 shows a very basic test setup for mm-wave radar measurements with a signal generator, spectrum analyzer, and a 60 GHz development module. These systems can be the backbone of a more complex test and measurement setup but provide an accessible platform to start with.
With automotive LRR, advanced avionics navigations systems such as ILS and EFVS, and portal-based scanners, mm-wave has already proliferated into many radar markets. The small size of the antenna and generally good performance in harsh weather conditions allow for mm-wave to be a desirable alternative to RF-based radar. The development of AESAs allow for complex beamforming techniques and not only have a longer life expectancy, but also have extremely high directivity and flexibility of motion. These types of antenna arrays are not only being implemented in a vast array of applications in RF due to their utility, but are also a natural fit in mm-wave applications as there is normally not much real estate for a gimbal. With talk of leveraging the mm-wave spectrum for wireless applications, there is much R&D also going into communications applications. This gives rise to the advancement of mm-wave based technology and can naturally feed into the advancement of mm-wave radar applications.
A recent Market and Markets report predicts the global mm-wave technology market is estimated to reach USD 8.69 billion by 2025 . Even with the upward trend of mm-wave radar, there is a general inaccessibility to components and development kits. Antennas, test equipment such vector network analyzers (VNA), and development kits with mm-wave capability are significantly more expensive than RF-based equipment. MM-wave transmit/receive modules can greatly simplify the prototyping phase and cut down cost and development time.