What are mmWaves or Millimeter Waves?

The band of spectrum ranging from 30 GHz to 300 GHz is called as millimeter-wave region or millimeter wave spectrum. Any signal that operates in this frequency band is said to be operating in the mmWave region and are hence known as millimeter waves. 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.

The mmWave band falls under the Extremely High Frequency (EHF) band and is standardized by the International Telecommunication Union (ITU).

5G deployment has been accelerating around the world in the last couple of years and 3GPP decided to use the mmWave band for 5G networks. As a result of this the frequency band has seen an increased amount of utilization in the years to come as more telecom operators have started to adopt the standalone (SA) model.

Another advantage of the mmWave band is that as we go up in the frequency, the amount of unused spectrum increases so the available bandwidth increases. Therefore, any technology operating in this frequency band can serve a relatively larger number of users worldwide compared to 3G UMTS, 4G LTE, LTE-Advanced infrastructure with higher data rates. However, there are several factors in addition to bandwidth that determine the feasibility of using the mmWave band.

We have listed some advantages and disadvantages of using the mmWaves frequency band.

Advantages of mmWaves

1. Wide bandwidths and high data rates: mmWave bands can deliver higher data rates than the lower frequency spectrum due to its high frequency range and the fct that large amounts of frequency spectrum in this frequency band are not used. 

2. Latency: The higher frequency and consequently higher bandwidth of mmWaves allows it to reduce latency compared to LTE services. Latency is the time taken for a packet sent by a transmitter to reach the receiver. Higher bandwidth means that a relatively larger amount of data can be transmitted over a given period of time.

3. Reduced Antenna Size: Since mmWaves have very short wavelength which means 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. 

4. Interference Mitigation: mmWaves experience a relatively larger amount of free space path loss compared to lower frequency bands, which limits their propagation distance. As a result, the frequency spectrum can be reused by a group of users in a different cellular system since the mmWaves will not propagate and interfere with another neighboring cellular system. Therefore, the interference can be mitigated due to the limited range is a key advantage of operating in the mmWave band.

5. Increased Resolution: In mmWave radars, the higher frequency and bandwidth (a narrow pulse width in time domain) allows for more accurate distance and velocity measurements. This is because the propagation time can be more accurately measured using mmWaves owing to its narrow pulse width. It also helps to resolve the ambiguity between two closely spaced targets which are often detected in radars operating at low frequencies.

Disadvantages of mmWaves

1. Line-of-Sight (LOS) operation: The free space path loss (FSPL) is relatively high for millimeter waves than for other low frequency bands based on the relationship given below:

For example, if the wavelength is reduced by 10 times, the FSPL increases by a factor of 100. The resulting attenuation is orders of magnitude higher than traditional FM radios or Wi-Fi devices that are operating in the relatively lower frequency bands.

2. Atmospheric Attenuation: In addition to attenuation caused by propagation, obstacles and the surrounding environment, this mmWave band of frequencies will face an additional attenuation due to the presence of atmospheric gases that are primarily oxygen and water vapor molecules. The below figure indicates the attenuation of mmWaves due to atmospheric gases and molecules.  Image Credit: Gases Tutorial (mike-willis.com)

3. Diffuse Reflection: Wavelengths longer than mmWaves often undergo direct (specular) reflected power for transmission around obstacles i.e. a mirror-like reflection as shown in the figure below.

However, for a mmWave, the same surface will appear rougher, thereby resulting in diffuse reflections. This means that the incident wave energy will scatter in different directions. The below figure illustrates the point.

Therefore, less energy is reflected and is likely to reach the receiving antenna. As a result, mmWaves are very susceptible to shadowing effects caused by obstacles and are limited to only line-of-sight (LOS) communication links.

4. Limited Penetration: mmWaves are shorter in wavelengths and hence, cannot penetrate deeply into most materials, unlike low-frequency signals. For example, research shows that the penetration loss through a brick wall for a signal at 70 GHz is roughly five-fold compared to a signal at 1 GHz. In outdoor environments, foliage will mostly block mmWaves and hence, signals in this band are limited to LOS operation.

5. Increased Hardware Cost: Additionally, as the size of antenna and other mmWave components reduce, the costs associated with manufacturing and fabricating components into a smaller area with greater precision is also high.