Industrial & Medical Technology

How lidar is clearing the way for 5G

29 January 2021

As 5G pushes the limits for wireless data transfer speeds, 5G antennas and repeaters must be placed much more carefully than older radio and cellular systems. The higher frequency signals of 5G are more easily absorbed by buildings and trees than existing 4G signals, meaning 5G repeaters must be able to be placed and moved more quickly as a city’s skyline changes. To accomplish all of this, the development of 5G infrastructure is using an unusual tool: Light Detection and Ranging, or LiDAR.

Device manufacturers boast of the high-speed data transfer with 5G-enabled devices. In order to achieve these speeds, 5G devices transmit at much higher frequencies than those of 4G service. While 4G devices use a standard 1.9 GHz, 5G devices transmit from 30 GHz to 300 GHz.

The higher frequencies used by 5G systems cause a few additional headaches for antenna design. Low frequencies require much larger antennas while higher frequencies can get away with much smaller antennas. At first glance this seems like an advantage, but it is a double-edged sword. Higher frequencies also mean that more pieces in the system act like antennas. This means that device components, feed lines, device power lines and so on are acting to collect signals -- including those signals that are unwanted.

When designing antennas, it is often useful to think in terms of wavelength, rather than in frequency. To convert frequency to wavelength, use the equation:

λ=c/f

Where λ is wavelength in m, c is the speed of light in m/s, and f is frequency in Hz. Many of the antenna calculations are based on wavelength, such as the “quarter-wave vertical antenna”, where optimal antenna height is one quarter the length of one complete wave. Table 1 shows a few existing radio systems, their frequencies, wavelength and quarter-wavelengths.

Table 1: A comparison of radio services and antenna heights

 Service Frequency Wavelength Vertical Height AM Broadcast 1 MHz 300 m 75 m FM Broadcast 100 MHz 3 m 0.75 m 4G 1.9 GHz 16 cm 4 cm 5G 30 GHz 1 cm 2.5 mm

This is just one type of antenna, and there are tricks to get the most performance out of a shorter antenna. Furthermore, the exact antenna dimensions are more important for transmitting performance than receiving, which is why a car can have a shorter radio antenna than 0.75 m.

In practice, all of this discussion of antenna design leads to one inescapable fact: 5G systems must be smaller than 4G systems. Their power supplies must be closer and their antennas cannot be fed by miles of coaxial cable.

Another technical problem arises with the higher frequency: it is attenuated more easily. Where an AM broadcast station can be received many miles from the station, 5G signals can only be received via line of sight. Because of this, even in flat areas, such as the Great Plains, there needs to be a 5G repeater every 300-700 ft, or every fourth or fifth utility pole.

1550 kHz AM Broadcast antennas in Belmont, California. Source: Wikicommons

Also, while an AM broadcast station can transmit music or talk radio, bouncing its signal off the ionosphere with a virtually undetectable time delay, the high-speed data transfer of 5G requires devices to be much closer together.

This creates a perfect storm for existing data infrastructure. Hardware for 5G systems cannot be simply placed on existing cellular phone towers and be expected to perform at high-speed. The unforgiving line-of-site requirements and high absorption of higher frequency signals require 5G hardware to be distributed smartly around the nation.

For existing cellular infrastructure, large towers were built at semi-regular intervals. Each tower required careful site planning. This level of site-planning is impractical for 5G systems, given that 5G systems need many more sites than the older 4G systems. Instead, there needed to be a way to evaluate sites quickly and deploy many, much smaller 5G repeaters.

Enter LiDAR. LiDAR uses pulsed light (1 million times per second) to make an accurate, 3D computer model of a geographic area, such as a town or shopping center. These models are created by measuring the time it takes for a light pulse to reflect back to a detector, in much the same way as radar or sonar do for radio and sound waves, respectively.

The resolution of this model, called a “digital twin”, can be accurate to 1 m. Data processing algorithms such as the Semi-Global Matching (SGM) can determine if an object is a tree, a park bench or a brick building.LiDAR image of San Francisco. Source: Wikicommons

The ability to detect and classify objects is incredibly useful for 5G planning. Some materials, such as a steel skyscraper, may reflect some of the signal whereas other materials, like the branches and leaves in a tree, can dampen the higher frequency signals.

The existing cellular network may have no change in performance if an empty lot is developed or a city-beautification project adds new trees in the median. The 5G signals, however, will be affected by these changes. The LiDAR data is updated frequently, meaning 5G site planning can be adjusted as a city evolves.

LiDAR can help fill in data holes from other data collection methods, such as aerial photography, site surveys, maps and building blueprints. LiDAR can be deployed from a number of platforms, including manned aircraft or with drones. In the near future, a technician will be able to fly a drone over a section of the city and map out where 5G data “shadows” exist, based on the buildings and vegetation, then plan and install additional 5G infrastructure in a few hours.

All of this mapping makes it possible to place 5G hardware, such as signal repeaters, in areas where they are needed, accounting for reflections, absorptions and shadows of nearby objects. This means a rapidly-developing tech-savvy city, such as the Austin Metro area, can quickly add or move 5G hardware as needed.

LiDAR creates the accurate maps needed to speed up planning and implementing 5G hardware infrastructure. For 5G placement, maps must be much more detailed than they were for previous technologies, as the data transmission speeds and higher frequencies used are more easily attenuated by buildings and vegetation.

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