Introduction
At the start of 2023, a survey collected the RF technology topics engineers most wanted to learn about in 2023. Millimeter-wave related topics received 21 nominations, ranking as the fourth most discussed topic. Millimeter-wave technology, once seen as specialized, has become a key focus for RF engineers.
Millimeter-wave technology has been used for many years in military and radar applications. In civil domains, it has gained wider attention with the adoption of 5G mobile communication, commercial satellite links, and automotive millimeter-wave radar.
China's Ministry of Industry and Information Technology recently issued a document in January 2023 listing 21.2–23.6 GHz and 71–76 GHz/81–86 GHz as millimeter-wave bands available for wireless communications in China [1]. According to estimates, 5G millimeter-wave phone shipments in 2023 are expected to exceed 100 million units, with a potential second rapid growth wave by 2025 [2].
Beyond phones, other millimeter-wave applications are growing rapidly. The charts below show automotive millimeter-wave radar market data and global satellite launches [3][4], both exhibiting fast growth in recent years.
This article discusses the commonly used system architecture in millimeter-wave systems: millimeter-wave phased-array structures (millimeter-wave phased array). It aims to clarify the fundamentals of these systems.
What Is Millimeter Wave?
Wireless communication is based on electromagnetic waves. Spectrum regulators allocate different frequency ranges to different applications to avoid mutual interference.
Millimeter wave generally refers to electromagnetic waves in the approximate 30 GHz to 300 GHz range. In vacuum their wavelengths are roughly 10 mm to 1 mm, hence the name millimeter wave.
Advances in semiconductor integrated-circuit technology and communications equipment have progressively extended usable spectrum upwards. For example, in civilian communications:
- In the early 20th century, broadcast TV and radio mainly used RF frequencies around 100 MHz.
- From the 1980s, microwave bands near 1–3 GHz enabled mobile phone networks.
- Since 2020, 5G has expanded beyond sub-6 GHz to include millimeter-wave ranges up to about 24–40 GHz.
Characteristics of Millimeter-Wave Communication
1. Large Bandwidth
The primary motivation to use higher-frequency spectrum is to access larger spectral resources for higher data rates. Compared with sub-6 GHz bands, the 30–300 GHz millimeter-wave range offers roughly 50 times more spectrum. This is analogous to adding many highway lanes beside a congested road, enabling much higher throughput. Therefore, a key characteristic of millimeter-wave communication is large bandwidth.
Large bandwidth enables higher data rates. According to Ookla Speedtest data [5], compared with 4G LTE, 5G Sub-6 GHz networks can provide about a 5x increase in speed, while 5G millimeter-wave networks can achieve around a 20x improvement.
Figure: Download speed comparison among 4G, 5G Sub-6 GHz, and 5G millimeter wave
2. High Resolution
Electromagnetic waves are also used for radar. By transmitting a signal and observing reflections from objects, radar can determine object size and distance. Diffraction effects make resolution proportional to wavelength: shorter wavelengths resolve finer details. Thus millimeter wave is useful for high-resolution radar.
Compared with ~1 GHz signals (wavelength ~0.3 m), millimeter-wave signals above 30 GHz provide much higher resolution. Automotive millimeter-wave radar commonly uses 24 GHz, 77 GHz, and 79 GHz bands to achieve centimeter-level precision.
Figure: Millimeter-wave radar applications in intelligent vehicles [6]
3. High Loss and Susceptibility to Blockage
Millimeter-wave communication has drawbacks: higher path loss and greater sensitivity to blockage. According to the Friis transmission equation, for a given distance, loss increases as wavelength decreases. Consequently, millimeter-wave coverage is much shorter than lower-frequency systems: where a 1 GHz base station may cover kilometers, millimeter-wave coverage may shrink to a few hundred meters. This increases deployment density requirements.
Short wavelengths also mean centimeter-scale objects can block and reflect millimeter waves. This is advantageous in radar detection but problematic for mobile communication because millimeter-wave links are largely line-of-sight and do not diffract well.
Figure: Millimeter-wave transmission is easily affected by physical obstructions
4. Small Circuit Size
In RF and microwave circuitry, component sizes generally scale with wavelength. Higher-frequency millimeter-wave circuits can be implemented in smaller physical sizes, which can reduce cost and enable dense arrays required for phased-array systems.
Reference [7] demonstrates a complete 24 GHz 4-channel millimeter-wave phased-array transmitter system including local oscillator, upconverter, and power amplifiers on a 2.1 mm x 6.8 mm chip—approximately the size of a grain of rice.
Figure: 4-channel 24 GHz millimeter-wave system
What Is a Phased Array?
Phased-array technology controls the phase and amplitude of each antenna element in an array to steer and shape the emitted or received beam.
The phased-array concept originated in the early 20th century and was first widely applied in military radar systems. As civilian electromagnetic frequencies have risen, phased arrays have also become important in commercial systems.
Two core concepts in phased arrays are the "array" and "phase control."
The "Array": Directional Transmission and Reception
Before antenna arrays, radiation was treated as nearly omnidirectional: emitted energy spreads spherically, and received energy per unit area decreases with distance, contributing to path loss. Increasing receiving aperture area collects more energy, but such solutions are large and require mechanical pointing.
Antennas in arrays, invented by Nobel laureate Karl Ferdinand Braun in 1905, enable directional transmission without mechanical steering. Braun's array used three vertical monopoles placed at the vertices of an equilateral triangle with 1/4 wavelength spacing. By controlling input phase, the three antennas produced constructive and destructive interference in selected directions, enabling directional radiation.
Figure: Braun's 1905 antenna array and its far-field pattern
Array antennas attracted military interest for their directional properties, long range, and lack of mechanical steering. By the 1920s, research in the U.S. and Germany explored arrays for radar; by 1941 the SCR-270 radar deployed in Pearl Harbor used a 32-element array, confirming the feasibility of array radar systems. Phased arrays are now widely used in modern military systems.
Phase Control: Steering the Beam
The array provides the structure for directional transmission and reception, while phase control aligns signals from different elements so they sum coherently from a desired direction. In receive mode, signals arriving at different elements have different path delays; phase shifting aligns them before summation. This process, phase control, allows the array to receive signals from specific directions.
In transmit mode, setting element phases controls the direction of constructive interference, steering the beam without physically moving the antenna. For a two-element array, when both elements are in phase, their signals add at the center and cancel elsewhere, producing a beam perpendicular to the array. Introducing a phase difference shifts the main lobe to one side. Controlling element phase differences is commonly called beamforming.
Figure: Beam steering by phase difference between two antennas
Implementing Phase Shifters
Phase shifters change the phase of signals and are essential components in phased-array systems. They introduce controlled delays or combine signals to produce the required phase shift.
Phase shifters are often categorized as passive or active. Common circuit implementations and characteristics vary between these types.
Phased-Array System Types
Phased-array systems are broadly classified as passive phased arrays and active phased arrays.
Both system types enable directional transmission and reception. In passive phased arrays, the array and phase shifters are passive components, while a central transmitter and receiver handle the signals. In active phased arrays, each radiating element is paired with its own active transmit/receive module.
Active phased arrays place power amplifiers close to array elements, improving system stability and robustness: the failure of a few transmit/receive modules has limited impact on overall performance. Independent channel operation enables grouping of elements for multi-target tracking, among other capabilities.
Passive systems are simpler and lower cost due to a single transmit/receive unit, but active phased arrays are more flexible and reliable, and are widely used in radar and wireless communications.
Active Phased-Array Architectures
In active phased-array implementations, phase shifting can be placed at different points in the signal chain. Common architectures include:
- RF phase-shift architecture
- LO phase-shift architecture
- Digital phase-shift architecture
Why Combine Millimeter Wave and Phased Arrays?
Millimeter wave and phased-array technologies complement each other:
- Millimeter wave offers large bandwidth but suffers higher path loss and short range. Phased arrays focus energy into narrow beams to extend effective range through directional transmission.
- Phased arrays require many elements, increasing circuit area. Millimeter-wave circuits are physically smaller, enabling large-scale arrays.
The combination, millimeter-wave phased arrays, leverages the strengths of both technologies and is effective in many applications.
Applications of Millimeter-Wave Phased Arrays
5G Smartphones
Millimeter-wave phased-array technology is already present in many 5G smartphones. In October 2020, Apple introduced millimeter-wave support in the U.S. versions of the iPhone 12. The iPhone 12 used Qualcomm millimeter-wave solutions with four-element arrays placed at the top and sides of the phone to handle millimeter-wave transmit/receive functions [11].
Apple reported peak downlink rates up to 4 Gbps for devices equipped with millimeter-wave technology.
Automotive Millimeter-Wave Radar
Automotive millimeter-wave radar transmits millimeter-wave signals and receives reflections from targets. By measuring time delay between transmit and receive, it detects object range and velocity.
Figure: Typical automotive radar operating principle [12]
Automotive radars often use millimeter-wave phased arrays to shape beams precisely and perform accurate detection. An example 24 GHz automotive radar design uses a 4-channel phased-array receiver architecture [12].
Satellite Communications
Satellite communications, especially low-Earth-orbit (LEO) systems, are a major focus due to their low latency and wide coverage potential. However, achieving ground-to-satellite links is challenging: even LEO satellites are on the order of 1,000 km away, requiring high transmit power or highly directional antennas. Rapid satellite motion also demands fast beam steering from ground terminals.
Millimeter-wave phased arrays provide narrow beams and rapid electronic scanning suitable for LEO links. SpaceX's Starlink uses millimeter-wave phased arrays. The Starlink user terminal (Starlink Dish) is about 58.9 cm in diameter and contains a dense arrangement of 1,280 antenna array elements [13]. Phase shifters and RF transceiver circuits enable high directionality and fast beam scanning to track satellites moving at about 30,000 km/h at distances beyond 550 km.
Figure: Components of a Starlink ground transceiver
Conclusion
Since the discovery of electromagnetic waves in the late 19th century, wireless technology has advanced rapidly. Over more than a century, wireless systems have evolved from simple transmit/receive setups to sophisticated systems combining multiple frequency bands and advanced antenna techniques.
Millimeter-wave phased arrays represent a significant technical advance by using phase control across large antenna arrays to enable directional transmission of wideband millimeter-wave signals, mitigating high path-loss issues.
Prior to 2020, research on millimeter-wave phased arrays was concentrated in military and academic domains. Since 2020, the growth of 5G, automotive millimeter-wave radar, and commercial satellite communications has driven wider civilian adoption of millimeter-wave phased-array systems.
