The band of spectrum ranging from 30GHz to 300GHz is called the millimeter-wave region or millimeter wave spectrum.
mmWave signals propagate in the air at the same speed as light, i.e. 3 x 108 m/s. Therefore, the wavelength proportional to the designated frequency range is from 10 mm to 1 mm.
In practical applications, frequencies above 24 GHz are considered millimeter waves, which lie in the wavelength range where microwaves and far-infrared waves overlap. Millimeter waves thus exhibit characteristics of both spectra. The International Telecommunication Union (ITU) designates this radio frequency band as “extremely high frequency” (EHF).
Propagation Characteristics of mmWave:
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:
where:
- d is the distance between the two antennas in m
- λ is the wavelength in m.
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 ∗ log10(d) + 20 ∗ log10(f) + 92.45
The figure above shows the free-space path loss at different frequencies. A change of one 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 Attenuation: 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.
Atmospheric attenuation trend map of millimeter wave in different frequency bands
The “atmospheric window” refers to the frequency band with high transmission rate and less reflection, absorption, and scattering of electromagnetic waves passing through the atmosphere.
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.
Diffuse Reflections: 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.
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 waves. Therefore, most millimeter wave communication is limited to line-of-sight operation.
Advantages of mmWave technology:
1. Wide bandwidths and high data rates: 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.
As well, due to its high frequency range mmWave bands can deliver higher data rates than the lower frequency spectrums.
2. Reduced Antenna size: Since mmWaves have very short wavelengths, the antennas used at these frequencies can be very small. This allows a significantly larger number of antenna elements to be integrated and used within a smaller area, enabling the use of phase array antennas, electronically steered antennas, and various other antenna technology.
3.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, mmWave can distinguish smaller targets that are closer together or observe target details more clearly.
4.Strong detection capability: mmWave’s 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.
5. 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.
Applications of Millimeter Wave Technology:
1. Radar
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= (2*Vrel)/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
Applications of millimeter wave radar for autonomous vehicles.
The ability to quickly and accurately measure distance and relative velocity are clearly important for autonomous vehicle operation.
2. 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.
Adaptive beamforming to support both stationary and mobile users.
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.
3. 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,
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 proposed applications include:
- Radio astronomy
- Soil moisture evaluation
- Snow cover measurements
- Iceberg location
- Supplementing optical detection in adverse weather
- Weather mapping
- Measuring wind speeds
- Medical treatments
Conclusion:
Millimeter wave technology is one of the fastest-growing technologies in this decade. Higher demand for high-speed data, ultra-high definition multimedia, HD gaming, security and surveillance, etc will drive millimeter wave technology to the next level. It will continuously develop and offer wide spectrum of applications in the future.