What is millimeter wave technology?

CW (continuous wave)

Principle: Radio waves of a certain frequency are continuously emitted and the reflected waves are received. The change in the frequency of the reflected waves can be used to determine how fast an object is moving.

Characteristics: Good for measuring speed, but not distance. Simple and cheap, it has specific uses.

2-frequency CW method

Principle: Two radio waves of different frequencies are sent out alternately and the reflected waves are received, allowing both speed and distance to be measured.

Characteristics: It can measure distance and speed simultaneously, but the distance accuracy is not as high as that of FMCW. It is a little more complicated than CW.

Pulse method

Principle: It emits a short radio wave pulse and measures the time it takes for the pulse to hit an object and bounce back, from which distance can be calculated.

Characteristics: Good for measuring distances, especially for detecting moving objects. The shorter the pulse, the more precise the distance measurement.

FMCW method

Principle: Radio waves whose frequency changes over time are emitted and the difference in frequency of the reflected waves is measured. From this difference, both distance and speed can be determined.

Characteristics: It can measure distance with extremely high accuracy and speed at the same time. Although the system is complex, it is widely used in car radars and other applications. Each method is chosen based on its respective characteristics. For example, the FMCW method is often used in car safety systems, while the pulse method is used in aircraft radars.

About Frequency Modulated Continuous Wave (FMCW)

The FMCW method is a method in which a continuous wave (CW) is transmitted at a varying frequency (FM) for a set period of time, and the distance and speed of the target are determined from the difference in frequency between the transmitted wave and the reflected wave. Furthermore, a signal whose frequency is modulated over time is called a chirp signal or chirp wave, and the FMCW method is characterized by its use of chirp signals.
The frequency of the transmitted wave is changed over time and compared with the frequency of the received wave. The distance is calculated from the difference in frequency. FMCW enables highly accurate distance measurement.
FMCW has the advantages of being able to measure distance and velocity simultaneously, having simple circuit configuration, and being able to obtain high distance resolution with relatively little signal processing.

The technical process of a typical FMCW method is explained below.

1. Frequency Modulation

Chirp signal generation:

FMCW radar generates a chirp signal whose frequency varies linearly with time.
Chirp signals are generated using a voltage controlled OSCILLATORS (VCO) or a direct digital synthesizer (DDS). The bandwidth and duration of the chirp affect the range resolution of the radar. A wider bandwidth provides better range resolution.

2. Sending and Receiving

send:

The generated chirp signal is transmitted through ANTENNAS towards the target.
The transmitted signal changes frequency continuously, resulting in a time delay when reflected from a target.

Receiving:

The signal reflected from the target is received by the receiving ANTENNAS.
The received signal has a time delay compared to the transmitted signal, and this delay corresponds to distance information.

3. Beat signal generation

mixing:

The received signal is mixed with the transmitted signal in a mixer.
This mixing produces a "beat signal" based on the frequency difference between the transmitted and received signals.

Beat Frequency:

The frequency of the beat signal is proportional to the time difference (delay) between the transmitted and received signals.
This beat frequency is the basis for calculating the distance to the target.

4. Distance Calculation

Beat frequency analysis:

The beat signal is digitized by an analog-to-digital converter (ADC).
The frequency components of the beat signal are analyzed using a Fast Fourier Transform (FFT).

Distance calculation:

Calculate the distance to the target ( R ) using the following formula:
R = (c * T_c * f_b)/(2 * B)
where:
c: speed of light
T_c: chirp duration
f_b : beat frequency
B: Chirp bandwidth

5. Calculating Velocity

Doppler shift analysis:

If the target is moving, the Doppler effect causes an additional frequency shift in the beat signal.
By analyzing this Doppler shift, the relative velocity of the target is calculated.

Calculating speed:

Calculate the target velocity (v) using the following formula:
v = f_d * λ /2
where:
f_d: Doppler shift
λ: wavelength of the transmitted signal

6. Calculating angles

Use of array ANTENNAS:

To detect the angle, an ANTENNAS ANTENNAS is used, with each ANTENNAS element receiving the reflected signal from the target independently.

Use of phase difference:

The reflected signal from the target travels different distances to reach each ANTENNAS element, resulting in a phase difference.
By analyzing this phase difference, the target's arrival direction (angle) can be estimated.

Digital Beamforming (DBF):

It uses digital beamforming technology to enhance signals coming from certain directions and suppress signals coming from other directions.
Various algorithms are used to estimate the direction of arrival with high accuracy.

Reflective properties

Millimeter waves have the property of easily bouncing off certain objects, especially metal and hard surfaces.
Taking advantage of this property, millimeter wave sensors can measure the distance to an object and the speed at which that object is moving. Specifically, by analyzing the time it takes for millimeter waves to hit an object and bounce back, and changes in the reflected waves, the position and movement of an object can be tracked with great accuracy.
Simply put, millimeter waves are radio waves that are good at collecting information about objects using reflection, and can be used to learn more about the position and movement of objects.

Transmission and absorption characteristics

The penetration and absorption of millimeter waves are properties that relate to whether they can pass through or be stopped by an object. Millimeter waves can pass through non-metallic materials such as cloth and plastic to a certain extent, which is called "penetration."
This property allows mmWave sensors to see inside objects, for example to detect objects hidden under clothing.
On the other hand, materials that contain water or are very dense tend to absorb millimeter waves, a phenomenon known as "absorption." When millimeter waves are absorbed, they cannot travel any further, resulting in a shorter distance they can travel. In other words, this affects the reach of millimeter waves.
Simply put, millimeter waves have the property that they can pass through cloth and plastic, but are weak against water and dense materials.

Radio wave reach is short

Millimeter waves are significantly attenuated by the atmosphere and walls, and have a shorter reach than microwaves and other radio waves with longer wavelengths. They are easily blocked by objects and absorbed by air and water, making it difficult to transmit them over long distances.
However, the fact that it does not reach far means that it is less susceptible to interference from long-distance radio waves.
Therefore, millimeter waves are more suitable than other electromagnetic waves for short-distance communication.

Directivity (beamforming and directionality)

"Beamforming and directionality" refers to the way mmWave sensors send signals.
"Beamforming" is a technology that allows mmWave sensors to focus their radio waves in a specific direction, allowing the sensor to examine a specific area in detail, for example, by focusing on a specific location or object and scanning that area thoroughly.
"Highly directional" refers to the property of radio waves being sent strongly in a specific direction, which reduces unwanted reflections from the surrounding area and allows mmWave sensors to detect objects more accurately.
Simply put, beamforming and directionality enable mmWave sensors to "see what you want to see and nothing you don't," improving detection accuracy.

Source: Structural Planning Institute [Beamforming analysis of phased array ANTENNAS for 5G]

Miniaturization and integration possible

The short wavelength of millimeter waves allows for miniaturization of sensor devices.
"Miniaturization and integration" refers to how small a millimeter-wave sensor can be made and how many functions can be integrated into one. Because millimeter-waves have short wavelengths, sensors can also be miniaturized. This makes it possible to incorporate many ANTENNAS into a single sensor. Arranging multiple ANTENNAS in this way is called an "array."
Using an array allows the sensor to gather more information, improving its sensitivity (ability to sense small details) and precision (ability to measure accurately).
Simply put, the shorter wavelength of millimeter waves allows sensors to be "smaller but have more functionality" and therefore perform better.

Source: Texas Instruments [Why Automotive Radar Systems are Moving from 24GHz to 77GHz]

High resolution can be obtained

Millimeter waves have short wavelengths and therefore offer high spatial resolution. "High resolution" refers to the ability to see things in detail.
Millimeter waves are a type of radio wave characterized by their very short wavelength. A short wavelength makes it easier for radio waves to reach the fine details of an object. This is what provides "high spatial resolution."
Specifically, mmWave sensors can detect small objects and their minute movements, even the slightest movement, for example.
This is because millimeter waves have short wavelengths that allow them to capture the fine details of objects.
Simply put, millimeter wave sensors have "eyes that can see even the smallest details."

Source: Texas Instruments [mmWave Radar for Automotive and Industrial Applications]

Excellent environmental resistance

Millimeter waves are easily attenuated by air and water, making it difficult to transmit them over long distances, but they have a higher degree of directionality than infrared or ultrasonic waves, making them suitable for use in relatively poor environments and for short-distance operation.In addition, because they do not use light, they can be detected at night or against backlight.