Airplanes are all equipped with 5G, so why is the signal on high-speed trains still so poor?

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This article is reprinted from the WeChat public account "BOO聊通讯", the author is BOO聊通讯. Please contact BOO聊通讯 public account to reprint this article.

I believe that students who have used the WiFi provided on airplanes (usually satellite communications) must have deeply felt the slow and unstable Internet speed. It can even be said that there is no difference between providing WiFi and not providing it.

However, the issue of surfing the Internet on airplanes seems to be taking a turn for the better. Recently, a Chinese operator announced that it will deploy an in-flight 5G network covering national routes, which is described in professional terms as an ATG (air to ground) network, to enable users to enjoy 5G network services on airplanes.

Is 5G network in the air meaningful? I believe everyone will give a positive answer. After all, it is really too painful for people to sit on the plane for several hours without playing with their mobile phones in this era, and the Internet speed of traditional satellite communication in the air has returned to the pre-liberation era overnight. If you can use 5G network on the plane, watch Douyin and TV series, you will not feel too bored even if the flight time is long.

How does ATG work?

The principle is very simple and crude, that is, directly use the same method as connecting to the 5G network on the ground: through a 5G base station. However, when surfing the Internet on the ground, the antenna connected to the base station emits radio electromagnetic waves to the ground, while when surfing the Internet with ATG, the antenna connected to the base station will be bent toward the sky and emit signals to the sky.

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Image source: Zhongguancun Online

In addition, for security reasons, the ATG 5G service will not allow users to directly connect to the 5G network using their mobile phones. After the dedicated equipment on the aircraft receives the 5G signal, it will eventually be converted into WiFi for users to access the Internet.

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Image source: ZTE

Therefore, ATG Internet access requires telecom operators to deploy dedicated base stations to serve airborne users in the ground areas corresponding to the routes they want to provide services.

Taking the site planning of China XX (the operator's name is hidden to avoid suspicion of advertising) as an example, its sites basically cover the entire Chinese territory. Therefore, in theory, after the ATG service is launched, the national routes can basically be fully covered.

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The coverage problem has been solved, but what about the network speed? China XX's ATG network uses a bandwidth of 100+10MHz, of which 100MHz is consistent with the current normal public 5G network bandwidth, and 10MHz is an aviation-specific frequency. Therefore, the frequency resources of the ATG 5G network are even a little more than those of terrestrial 5G.

According to the official statement of China XX, the ATG network it provides can achieve a peak rate of 1Gbps for a single aircraft. Due to the problems of ultra-long coverage, frequency interference, and high-speed mobile Doppler shift when the base station covers high altitude, this peak rate is somewhat lower than that of the ground 5G network, but it is already very good compared to the slow satellite Internet access.

However, please note a key word: single aircraft. This is very interesting. Because from the perspective of the operator's ATG site planning, the number of sites nationwide is only a few hundred, which is actually not as large as the number of 5G base stations in many larger counties.

For mobile communications, the bandwidth of a single base station is important, but what is more important is how many base stations can be provided in total. The accumulated bandwidth of these base stations is the overall capacity of the entire mobile communication system.

In 2019, the national civil aviation passenger volume was about 660 million people, which means about 1.8 million people were flying in the air every day for 365 days. Considering that each person usually flies for about 3 hours, we can use an analogy that the air coverage is for a city with a population of 1.8 million, where each person only uses the Internet for 3 hours a day.

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Data source: iiMedia Research

From this perspective, if the flight times and routes of flights across the country are evenly distributed, the scale of the planned sites is still acceptable and can basically meet the 5G network needs of users who do not have too high requirements.

However, based on common sense, you can guess that the flight schedules of flights across the country are busy and idle, and the flight frequencies of different routes vary greatly.

For example, the number of flights from Beijing to Shanghai at 10 a.m. every day must be much greater than the number of flights from Shaanxi to Liaoning at 23 p.m.

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Data source: Lin Yaqing Civil Aviation Resource Network

This will result in a situation where there are a lot of passengers in certain time periods and on certain routes, and very few passengers in other periods. Although the operator's site planning shows a clear trend of more passengers in the eastern coastal areas and fewer in the west, the number of sites cannot offset the serious imbalance between busy and idle times.

Therefore, the reason why the operator mentioned the peak rate of a single aircraft is actually to give the best theoretical value of rate experience under ideal conditions (no other aircraft competing for resources). In reality, the user's Internet experience may be much worse. However, I think no matter how worse it is, it should be similar to the 4G network perception. What's the big deal?

Currently, the main inter-provincial transportation methods in China are airplanes and high-speed trains. Now that we have talked about those in the sky, let’s take a look at those on the ground.

I believe that many people are not very satisfied with the communication services of domestic high-speed rail lines. For example, a financial industry giant whom I have been following for a long time posted a Weibo post complaining about the high-speed rail signal a while ago:

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Why has the problem of signal coverage in the sky been solved, but the problem on the ground is still "stagnant"? Is it more difficult to cover high-speed rail lines than to cover air routes?

It’s true, and it’s much more difficult.

Because no matter where you want to cover, the base station and antenna are the ones that transmit and receive signals in the end. And whether it is a 245G network, the fundamental principle of mobile phone access to the Internet is that the radio electromagnetic waves of the base station antenna can be sent to the user's mobile phone, and the frequency resources of the electromagnetic waves are sufficient and will not cause congestion.

Let's first look at the aerial coverage environment: the unobstructed view of the sky allows a base station to easily cover the area of ​​a city; it doesn't matter where the base station is located, as the antennas are aimed into the sky; there are at most a few hundred people on a plane, sometimes even dozens; planes are far enough apart that they don't get crowded together.

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On the other hand, the high-speed rail lines have complex terrain that crosses mountains and ridges, resulting in wireless signals being blocked everywhere. The line length that a single base station and antenna can cover is only a few hundred meters or about 1 kilometer, and the selection of the location of each base station is very difficult; wireless signals cannot penetrate into large sections of tunnels at all, and network construction must be carried out in tunnels through special methods, but in many cases, construction in tunnels is not something that operators can decide; every high-speed train passing by, taking the CRH380AL EMU train used by the Beijing-Guangzhou High-Speed ​​Railway as an example, has 16 carriages and carries 1,000 passengers without standing tickets. In 2019, the number of passengers transported by my country's railways reached 3.6 billion, which is 6 times that of aviation.

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Base station construction covering a section of high-speed railway Source: People's Post and Telecommunications

Therefore, from the perspective of coverage, the high-speed rail coverage cannot be achieved seamlessly by building hundreds of stations across the country like ATG does. It is necessary to place a base station every few hundred meters to cover a section of the road based on the terrain, and even use a special method such as "leaky cables" to build coverage for each tunnel. From the perspective of capacity, every time a train passes, thousands of people will be online at the same time, and it is easy for people to watch videos and scroll through Douyin on the train, which makes it difficult for wireless network resources to support.

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Leaking cable source: Xinhuanet

What's worse is that the high-speed train is fast enough, so passengers in the carriage have to be frequently switched from one base station to another, and this handover of wireless signals further reduces the network experience. In terms of the coverage of a single ATG base station, from Beijing to Shanghai, there may only be a dozen or twenty base station switches, which is even less than the number of base stations that the high-speed train experiences in a small city.

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Image source: Zhihu Lin Lin Chuang

Many users always have an idea: that is, the Internet signal comes from the sky and should be everywhere, but in fact the signal is transmitted in the form of relay from a base station that is unobstructed and close enough to your location.

Therefore, the difficulty and complexity of high-speed rail coverage is far greater than covering the sky. It is easy to look up at the sky, but it is too damn difficult to get down to earth. If you want to achieve perfect coverage of high-speed rail lines, the resources invested, the funds spent, the manpower and material resources spent can be described as astronomical. Operators are also enterprises and they also have to consider costs. Don’t you think so? This is why the signal of high-speed rail lines is not ideal.