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# Basics of Millimeter Wave Technology

This article provides an introduction to millimeter waves (mmWaves) including their frequencies, propagation characteristics, and advantages and disadvantages for common applications.

### What Is a Millimeter Wave?

As implied by the name, millimeter waves are electromagnetic waves with a wavelength (λ) that is approximately 1 mm (1 to 10 mm, to be more precise). Converting that wavelength into frequency using the equation f = c/λ, where c is the speed of light (3 x 108 m/s), gives a frequency range of 30-300 GHz. The millimeter wave band is designated the “extremely high frequency” (EHF) band by the International Telecommunication Union (ITU). The term “millimeter wave” is also often shortened to “mmWave”.

Figure 1 includes examples of applications that utilize the mmWave spectrum and also demonstrates the location of the mmWave spectrum in relation to other electromagnetic frequency bands.

##### Figure 1. Millimeter wave spectrum overview. Image courtesy of Analog Devices

Now that we have basic definitions out of the way, let’s talk about how millimeter wave signals propagate.

### Millimeter Wave Propagation

Millimeter wave signal propagation is characterized by:

• High free space path loss
• Significant atmospheric attenuation
• Diffuse reflections
• Limited penetration depth

The following subsections will examine in more detail each of these four propagation characteristics.

#### Free Space Path Loss

One limitation of millimeter wave radio frequency (RF) communication is the free space path loss (FSPL) for direct line-of-sight communication between two antennas. The FSPL is inversely proportional to the square of the wavelength and is given by the following equation:

\$\$FSPL = left( frac{4πd}{λ} right) ^2\$\$

where:

• d is the distance between the two antennas in m
• λ is the wavelength in m.

As can be seen from this equation, a 10X decrease in the wavelength results in a 100X increase in the free space path loss. Thus the attenuation at millimeter wavelengths is many orders of magnitude higher than the attenuation of more traditional communication frequencies like FM radio or Wi-Fi.

In RF communication calculations, this loss equation is often converted to provide a result in dB, with the frequency measured in GHz and the distance measured in km. After this conversion, the equation becomes:

\$\$FSPL (dB) = 20 * log_{10}(d) + 20 * log_{10}(f) + 92.45\$\$

A free calculator for evaluating the free space path loss is available here.

#### Atmospheric Attenuation

Another drawback of millimeter wave transmission is the atmospheric attenuation. In this range of wavelengths there is additional attenuation caused by the presence of atmospheric gases – primarily oxygen (O2) and water vapor (H2O) molecules.

As can be seen in Figure 2, the atmospheric attenuation can be very severe in certain bands.

##### Figure 2. Atmospheric attenuation by frequency and elevation. Image courtesy of 5G Americas

For example the oxygen peak at 5 mm (60 GHz). Rain increases the attenuation across the full spectrum.

#### Diffuse Reflection

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.

##### Figure 3. Diffuse and specular reflection. Image courtesy of Hermary

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.

#### Limited Penetration

Because of their shorter wavelengths, millimeter waves do not penetrate deeply into or through most materials. For example, a study of common building materials found that attenuation ranged from approximately 1 to 6 dB/cm and the penetration losses through a brick wall at 70 GHz may be five times higher than at 1 GHz. Outdoors, foliage will also block most millimeter wavers. Therefore, most millimeter wave communication is limited to line-of-sight operation.

### Advantages of mmWave Frequencies

For many applications, the free space path loss, atmospheric attenuation, diffuse reflection, and limited penetration of millimeter wave signals are detrimental. However, it turns out that these characteristics can also be exploited as benefits in certain applications. The advantages of millimeter waves include:

• Wide bandwidths
• High data rates
• Low latency
• Small antennas
• Limited range
• Limited reflection
• Limited penetration
• Increased resolution

Each of these advantages and how they are exploited in some applications will be explained in the following subsections.

#### Wide Bandwidths and High Data Rates

For communication applications, wide bandwidths mean higher peak data rates. This can mean the ability to either handle more simultaneous communication channels for a given data rate, or send more data in a single communication. The lower frequency spectrums are heavily used and, therefore, do not provide these desirable wide bandwidths.

For example, 3GPP’s 5G New Radio (NR) specification allocates a maximum channel bandwidth of only 100 MHz below 6 GHz, but up to 400 MHz in bands above 24 GHz. As these 5G specifications continue to evolve, some parties are lobbying for even wider bandwidth allocations in the mmWave spectrum.

It is because of these wide bandwidths and high data rates that millimeter waves have long been used in satellite communication at 27.5 GHz and 31 GHz. Advances in high-frequency circuit technology including silicon carbide (SiC) and gallium nitride (GaN) and associated lower manufacturing costs are bringing millimeter wave communications to terrestrial, mask-market consumer applications like 5G NR.

#### Low Latency

Latency in communication networks can have multiple meanings. With regards to one-way communication, latency is the time from the source sending a data packet to the destination receiving the same data packet. The higher frequencies of millimeter waves mean more data can be transmitted in a shorter amount of time. Therefore, for a fixed data packet size, a high-frequency system will have lower latency than a low-frequency system.

Low latency is important for many time-sensitive applications including industrial automation, wireless augmented or virtual reality and automated driving systems. The wide bandwidth of millimeter waves enables shorter transmission time intervals and lower radio-interface latency to facilitate the introduction of and support for low-latency-sensitive applications.

#### Small Antennas

One of the most important advantages of millimeter waves is smaller antennas and the ability to use a large number of these smaller antenna elements in arrays to enable beamforming. For example, automotive radars are transitioning from 24 to 77 GHz. The wavelength is more than three times smaller so the antenna array area can be over nine times smaller, as illustrated in Figure 4.

##### Figure 4. Relative antenna array sizes for 24 GHz and 77 GHz. Image courtesy of Texas Instruments

Large arrays of very small antenna elements are also going to be used in millimeter wave communication systems like 5G. Beamforming can focus the radiated power toward individual users for higher quality signals and longer range communication. With adaptive beamforming, the beams can even be changed dynamically as a function of the number of users and their location with respect to the transmit antenna.

#### Limited Range, Reflection, and Penetration

The limited range, diffuse reflections, and limited penetration depths can actually be a benefit for telecommunications. These characteristics are being exploited to allow many small cells to be placed very near each other without interference. This provides spatial reuse of the frequency spectrum and, therefore, allows more high bandwidth consumers to be supported in an area.

#### Increased Resolution

In radar applications, the higher frequency and increased bandwidth of millimeter wave signals support more accurate distance measurements, more accurate velocity measurements, and the ability to resolve between two closely spaced objects.

### Applications of Millimeter Wave Technology

For many years, aerospace radar applications were the primary application of millimeter wave technology. The wide bandwidths are ideal for determining the distance to an object, for resolving between two distant objects that are close together and measuring the relative velocity to the target.

For example, in its most basic form assuming two objects moving either directly toward or away from each other, the Doppler frequency shift (Δf) is given by the equation:

\$\$Δf = frac{(2 * V_{rel})}{λ}\$\$

where

• Vrel is the relative velocity (m/s)
• λ is the wavelength (m)

Because the frequency shift is larger with shorter wavelengths (like millimeter waves), it is easier to measure the resulting frequency shift. The ability to use smaller multi-element antennas and adaptive beamforming also make millimeter waves ideal for radar applications.

For the same reasons that millimeter wave radar is desirable for aerospace applications, it is widely being adopted for automated vehicle applications including emergency braking, adaptive cruise control (ACC), and blind-spot detection (as illustrated in Figure 5).

##### Figure 5. Applications of millimeter wave radar for autonomous vehicles. Image courtesy of Rohde & Schwarz

The ability to quickly and accurately measure distance and relative velocity are clearly important for autonomous vehicle operation.

#### Telecommunications

Satellite systems have long used millimeter waves for their communications due to the wide bandwidths, low latency, small antennas, and multi-antenna array beamforming. These same features are driving many terrestrial telecommunication networks to employ millimeter waves.

For example, because of the increased bandwidth, millimeter waves can support the wireless transmission of ultra high definition (UHD) video. In addition, the smaller antennas support integration into devices like smartphones, digital set top boxes, game stations, and more. Emerging industry standards that will employ millimeter waves include 5G and IEEE 802.11ad WiGig for Gb/s data rates.

Particularly in indoor and urban environments, spatial reuse and adaptive beamforming of millimeter waves will enable the delivery of high bandwidth communications to a large number of users, as can be seen in Figure 6.

##### Figure 6. Adaptive beamforming to support both stationary and mobile users. Image courtesy of Fujitsu via Phys.org

Massive MIMO (Multiple Input Multiple Output) systems will enable spatial diversity, spatial multiplexing, and beamforming to provide better functionality to more users while using lower power.

#### Security Scanners

Millimeter waves are also employed for human body security scanners. Thousands of transmit and receive antennas work together to scan with high precision as illustrated in Figure 7.

##### Figure 7. Millimeter-wave body scanner system. Image courtesy of Rohde & Schwarz

These systems transmit at a frequency range between 70 GHz to 80 GHz and emit only about 1 mW of power. ​​The millimeter waves pass through most clothing and reflect off the skin and other surfaces back to the receiving antennas. The received signal can be used to create a detailed image of the individual and reveal articles hidden under the clothing. The low power and limited penetration depth of millimeter waves provide improved safety.

#### Other Applications of Millimeter Waves

These are just a few of the many applications for millimeter-wave technology. Other applications that have been proposed or implemented include, but are certainly not limited to:

• Soil moisture evaluation
• Snow cover measurements
• Iceberg location
• Supplementing optical detection in adverse weather
• Weather mapping
• Measure wind speeds
• Medical treatments

### Summary

Millimeter waves have long been used in radar applications and are increasingly being applied to new applications with the most prominent being high data rate telecommunications. The short wavelengths and unique propagation characteristics provide both challenges and opportunities to design engineers working in these fields.

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