Basics of Millimeter Wave(mmWave)
After entering the 21st century, the development of millimeter wave technology has been unstoppable, and it has found numerous applications in fields such as communication, radar, remote sensing, and radio astronomy. What exactly are millimeter waves, and what makes them so special? This article will introduce you to the basics of millimeter waves, including their definition, propagation principles, limitations, and advantages.
1. What is mmWave?
Millimeter wave is not a new concept, and its first discovery can be traced back to the 1890s when Indian physicist Jagadish Chandra Bose successfully transmitted and received electromagnetic waves with a frequency of 60 GHz over a distance of 23 meters. Millimeter waves are electromagnetic waves with wavelengths of 1-10 millimeters and frequencies ranging from 30 to 300 GHz. In practical applications, frequencies above 24 GHz are considered millimeter waves, which lie in the wavelength range where microwaves and far-infrared waves overlap, thus exhibiting characteristics of both spectra. The International Telecommunication Union (ITU) designates this radio frequency band as "extremely high frequency" (EHF).
2. Propagation Characteristics of mmWave
Free space path loss
The frequency and distance dependence of the loss between two isotropic antennas is expressed in absolute numbers by the following equation: LFSL = (4πR/λ) 2 Free Space Loss where R: distance between transmit and receive antennas; λ: operating wavelength. After converting to units of frequency and putting in dB form, the equation becomes: LFSL dB = 92.4 + 20 log f + 20 log R where f: frequency in GHz; R: Line-of-Sight range between antennas in km.
The figure above shows the free-space path loss at different frequencies. A change of eight octaves in distance results in a 6 dB difference in attenuation. For example, when the distance increases from 2 kilometers to 4 kilometers, the attenuation increases by 6 dB. It is worth noting that even at short distances, free-space path loss can be very high. This poses a great challenge for the design and deployment of millimeter-wave communication systems.
"Atmospheric Windows" & "Attenuation Peaks"
Millimeter wave transmission is characterized by atmospheric attenuation. Water vapor and oxygen in the atmosphere can absorb electromagnetic waves, so millimeter wave application research mainly focuses on several "atmospheric window" frequencies and three "attenuation peak" frequencies. The so-called "atmospheric window" refers to the frequency band with high transmission rate and less reflection, absorption, and scattering of electromagnetic waves passing through the atmosphere. It mainly concentrates around the frequencies of 35GHz, 45GHz, 94GHz, 140GHz, and 220GHz. However, in the vicinity of frequencies of 60GHz, 120GHz, and 180GHz, there is a significant attenuation peak. Generally, the "atmospheric window" frequency band is suitable for point-to-point communication, while the "attenuation peak" frequency band is suitable for multi-branch diversity hidden networks and systems that meet the requirements of network security.
Longer wavelengths often rely on direct (specular) reflected power to assist in transmission around obstacles (think of mirror-like reflection). However, many surfaces appear “rough” to millimeter waves, which results in diffuse reflections that send the energy in many different directions. This can be seen in Figure 3.
Thus, less reflected energy is likely to reach a receiving antenna. Millimeter wave transmissions are therefore very susceptible to shadowing by obstacles and are typically limited to line-of-sight transmission.
Due to their short wavelengths, millimeter waves cannot penetrate or penetrate most materials deeply. For example, a study of common building materials found an attenuation range of about 1 to 6 dB/cm, and the penetration loss through a brick wall at 70 GHz may be five times greater than at 1 GHz. Outdoors, foliage can also block most millimeter waves. As a result, most millimeter wave communications are limited to line-of-sight operations.
3. Advantages of mmWave technology
For many applications, the free-space path loss, atmospheric attenuation, and diffuse reflection of millimeter-wave signals are detrimental. However, it has been proven that these characteristics can also be utilized as advantages in certain applications. The advantages of millimeter waves include: 1.Small component size Compared with microwave components, millimeter wave components are much smaller in size. Therefore, millimeter wave systems are easier to miniaturize. 2.Strong detection capability The wideband spectrum can be used to suppress multipath effects and clutter echoes. There are a large number of frequencies available, effectively eliminating mutual interference. A large Doppler frequency shift can be obtained under target radial velocity, thereby improving the detection and recognition capability of low-speed moving objects or vibrating objects. 3.High transmission quality Due to the high frequency band, millimeter wave communication has almost no interference sources, and the electromagnetic spectrum is extremely clean. Therefore, the millimeter wave channel is very stable and reliable, and its error rate can be maintained at the level of 10^-12 for a long time, comparable to the transmission quality of optical fibers. 4.Extremely wide bandwidth The millimeter wave frequency range is usually considered to be 26.5 to 300 GHz, with a bandwidth of up to 273.5 GHz, which is 10 times higher than the bandwidth from DC to microwave. Even considering atmospheric absorption, only four major windows can be used for propagation in the atmosphere, but the total bandwidth of these four windows can also reach 135 GHz, which is five times the sum of the bandwidths below the microwave band. This is undoubtedly very attractive in situations where frequency resources are scarce. 5.Narrow beamwidth Under the same antenna size, the beamwidth of millimeter waves is much narrower than that of microwaves. For example, a 12cm antenna has a beamwidth of 18 degrees at 9.4GHz, while the beamwidth is only 1.8 degrees at 94GHz. Therefore, it can distinguish smaller targets that are closer together or observe target details more clearly. 6.Limited range, reflection, and penetration depth Limited range, diffuse reflection, and limited penetration depth can actually benefit telecommunications. These features are being used to allow many small cells to be very close to each other without interference. This provides spatial reuse of the spectrum, allowing more high-bandwidth consumers to be supported in an area.
4. Mmwave Wave Radar
As the name suggests, millimeter wave radar is a radar that operates in the millimeter wave frequency band. Millimeter waves (MMW) refer to electromagnetic waves with a length of 1-10mm and a frequency range of 30-300GHz. Radar comes from the acronym for Radio Detection and Ranging, meaning "radio detection and ranging." It uses radio waves to discover targets and determine their spatial position, which reveals that the most important task of radar is to detect the distance, velocity, and direction of target objects.
The ranging principle of millimeter wave radar is very simple. It emits radio waves (millimeter waves) and then receives the echoes, measuring the position data and relative distance of the target based on the time difference between transmission and reception. According to the propagation speed of electromagnetic waves, the distance formula of the target can be determined as: s=ct/2, where s is the target distance, t is the time for the electromagnetic wave to be transmitted from the radar and received as an echo from the target, and c is the speed of light. The principle of measuring speed using millimeter-wave radar is to send a beam of millimeter-wave signals and then receive the reflected signal to measure the target's speed. When the signal is sent to a moving target, the reflected signal undergoes a Doppler frequency shift, the size of which is directly proportional to the target's speed. By analyzing the Doppler frequency shift in the received signal, the target's speed can be calculated. Compared with traditional radar, millimeter-wave radar has higher accuracy and resolution.
Mmwave radar data is shown as point clouds, with X, Y (and possibly Z) coordinates, RCS and Doppler info. Converting reflected signals to point clouds enables high-precision 3D modeling and recognition of the environment. This technology has been widely used in autonomous driving, intelligent security, robot navigation, and other fields.
For many years, aerospace radar has been the main application of millimeter wave technology. Its wide bandwidth is very suitable for determining the distance to objects, resolving two distant objects at a close distance, and measuring the relative speed with the target. For example, assuming two objects are moving towards or away from each other, in the most basic form, the Doppler frequency shift (Δf) is given by the following formula: Δf= (2*Vrel)/λ
- Vrel is the relative velocity (m/s)
- λ is the wavelength (m)
Due to the shorter wavelength (such as millimeter waves), the frequency shift is greater, making it easier to measure the resulting frequency shift. The ability to use smaller multi-element antennas and adaptive beamforming also makes millimeter waves an ideal choice for radar applications. For the same reasons, millimeter wave radar is suitable for aerospace applications and is widely used in autonomous vehicle applications, including emergency braking, adaptive cruise control (ACC), and blind spot detection (as shown in Figure 5).
The ability to rapidly and accurately measure distance and relative velocity is obviously important for the operation of autonomous vehicles. Millimeter waves have long been used in radar applications and play a very important role in security, intelligent transportation, industry, and military fields in addition to aerospace and automotive ADAS applications.
5. In Summary
In summary, millimeter wave technology is one of the fastest-growing technologies of the 21st century and has been widely used in communication, radar, remote sensing, and radio astronomy. Although millimeter waves have limitations such as transmission distance, atmospheric attenuation, and blockage reflection, these limitations can also be turned into advantages in some applications. Millimeter wave technology has the advantages of small component size, strong detection ability, high transmission quality, wide bandwidth, narrow beam, etc., and therefore has great potential in communication and radar applications. Millimeter wave radar refers to radar that operates in the millimeter wave frequency band, which can be used for ranging, velocity measurement, and acquiring 3D data of objects, suitable for autonomous driving, aerospace, and other fields. In addition to radar applications, millimeter wave technology also plays an important role in security, intelligent transportation, industry, and military fields. The application prospects of millimeter wave technology are broad, and its unique performance and advantages make it an indispensable part of many fields.