Shulou(Shulou.com)11/24 Report--

These days, the news about satellite mobile phones has become popular again.

Huawei's Mate 50 series of phones, to be launched on September 6, will have "satellite communication capability" to send emergency text messages through a satellite system where there is no network, according to media reports.

Coincidentally, there are rumors that Apple's iPhone 13 has hardware to connect to satellites. However, Apple did not discuss how to cooperate with the operators, so it was not made public. This feature is likely to be implemented on future iPhone 14.

The implication is that both Huawei and Apple are working on "satellite communications". It seems that a new "track" has appeared again.

So, is this really the case? Has the satellite age of mobile phones really begun?

In today's article, Xiao Zaojun will give you an in-depth interpretation.

The frequency of █ communication in recent years, the news about satellite communication has always been the focus of the media and the public.

In particular, after Musk engaged in it, there was news in two days, hyping the threat of satellite communications to ground cellular mobile communications and the threat to 5G.

In fact, many of these hype are lack of basic communication common sense, in addition to exposing some people's desire for quick success and instant benefits, mainly to serve the flow and capital operation.

Satellite communication, no matter now or in the future (at least 50 years), can not replace ground mobile communication and become the main means of communication for human beings.

Next, let me explain why--

First of all, we should understand and bear in mind that any wireless communication system is based on wireless electromagnetic waves.

Wireless electromagnetic wave, there is an important attribute parameter, that is frequency.

The characteristics of radio electromagnetic waves with different frequencies are different, as shown in the following figure:

From a macro point of view, it is mainly divided into light waves and radio waves.

At present, radio waves are mainly used in our human wireless communication. The low-frequency and intermediate-frequency electromagnetic waves used in the early days are now gradually developing to high-frequency and even terahertz (the THz in the middle of the picture above). There are also scientists who are studying visible light communication (towards light waves).

The radio waves of wireless electromagnetic waves seem to have a lot of frequency resources, but in fact, each frequency is defined by the relevant government departments for the corresponding use.

From the point of view of users, these frequency bands are divided into military, private network (public security, fire, railway, electric power, etc.), operators (public mobile communications), and so on.

In addition, there are ISM (Industrial Industry, Scientific Science, Medical Medicine) unlicensed band, which can be used without license, such as our Wi-Fi band.

No one can occupy the frequency casually. For example, it is illegal for me to invent a wireless communication device and then occupy the operator's 800MHz band to transmit wireless signals in this band.

When mobile phones carry out satellite communications, the first question to consider is: what frequency band does it use?

I don't know if you have noticed, but when we get the new phone, we will see a blue label on the back:

This tag is the license for the mobile phone to access the network. Before any mobile phone model is put on the market, the manufacturer should first take it to the Ministry of Industry and Information Technology to do the network test. The certificate will not be issued until it has passed the test.

This means that the state has strict requirements for every electronic product, especially those with wireless signal transceiver function: the radio working frequency must comply with the relevant national regulations and the legitimate use of the product.

Our phones are designed to use three kinds of frequency bands:

One is the operating frequency of the cellular communication system of the operator (that is, the working frequency band of the base station). If it is a more common all-Netcom mobile phone, it is necessary to support 2G / 3G / 4G / 5G band. Each frequency band is indicated on the official website.

The frequency bands supported by a certain model of Xiaomi

The second is Wi-Fi and Bluetooth. This is the unauthorized spectrum I mentioned earlier, the ISM band. As long as the transmission power meets the requirements (not too high), it can be used directly.

The third is the working frequency band of the satellite positioning system. Our mobile phones now support GNSS satellite positioning, which can send and receive satellite signals. There are many kinds of GNSS systems, GPS in the United States, BDS in China, Galileo in Europe, and so on.

Numerous satellite navigation and positioning systems

So, here's the problem. Many people shout that their mobile phones communicate by satellite every day. If they really want to communicate, what frequency band do they use?

Satellite communication belongs to space radio communication, which has exceeded the radio management capacity of a sovereign country. Therefore, the use of satellite communication working frequency resources should be applied to the International Telecommunication Union (ITU-R).

ITU-R has clear documents and strict requirements on declaration procedures and rules, which are often carried out in parallel with the application for satellite orbital resources.

Satellite communication works in the microwave band, and its frequency range is 1GHz~40GHz. According to the frequency band, it can be divided into L, S, C, X, Ku, K, Ka. Among them, the K-band is not suitable for satellite communication because it is in the frequency window with the greatest influence of atmospheric absorption loss. Therefore, the commonly used satellite communication frequency bands are: l, S, C, X, Ku, Ka:

What is worth mentioning is the Ka band.

As the C and Ku bands are nearly saturated, they can not carry more services, so Ka bands are becoming more and more popular. The Ka band is more vulnerable to the weather than the Ku band, but the working bandwidth is larger and the signal strength is higher.

In any case, the satellite communication band is different from the carrier cellular communication band. At present, ordinary mobile phones sold on the market do not support the wireless working frequency of communication satellites at all, so they are naturally unable to communicate with satellites.

Only dedicated satellite phones provided by satellite communications service providers can establish connections with these communications satellites. The telephone charges of these satellite phones also have to be recharged to the corresponding satellite phone service providers.

It was mentioned earlier that mobile phones can communicate with GPS and Beidou, which are positioning and navigation satellites. Strictly speaking, it's not a communications satellite. However, our country's Beidou is very powerful. In addition to positioning, navigation and time service, it has one more function than GPS, that is, short message communication capability.

That is, in addition to location information, it can communicate with short messages (RDSS protocol). How short is it? Up to 78 binary numbers / English / numbers, or 39 Chinese characters.

The "satellite communication" function of Huawei MATE 50 mentioned earlier refers to this. With the help of the short message communication channel of Beidou positioning satellite, in case of emergency, send and receive short messages and carry out emergency communication.

This is just a text message. There is no data service, no pictures, no phone calls or videos.

This is still very different from the satellite communications that you understand in the normal sense. The dedicated satellite phone mentioned above can make phone calls and use data services (surfing the Internet).

At present, all the mobile phones on the market do not support the frequency band of communication satellites. So, can we add it?

It's not that simple.

It is not necessary to add an antenna to support the communication band of the mobile phone. It involves the transformation of SoC chip, baseband, RF and software, and it is a very large and complex work.

It's not that it can't be done, but it's meaningless.

Because in most cases, mobile phones have base station signals, or Wi-Fi signals, there is no need to increase unnecessary costs in order to rarely occur. (it is more feasible to have an external satellite communications kit. )

This is the same as amphibious cars.

It is not impossible for a car to add the function of a ship. However, the price will increase several times, and the utilization rate is very low, which is not conducive to the popularity of cars.

So, if the mobile phone is not modified and does not support the working frequency of existing satellite communications, is it impossible to carry out satellite communications?

No, not really.

We can think differently. If the mobile phone does not support satellite communication frequency, why not let the satellite directly use the frequency of the ground cellular base station?

In fact, that's what the new project launched by American operators T-Mobile and Musk did. They let the satellite use part of the 5G band, improve the antenna, and then send a wireless communication signal to the ground.

This way of working, there is an important premise, that is, the use of low-orbit satellites.

In recent years, there is a very important development trend of satellite communications, that is, the rise of low-orbit satellites.

In the past, the cost of launching a satellite was very high, so it was hoped that a satellite could cover a large area. If you want to cover a wide area, you must put the satellite higher. As shown in the following figure:

High-orbit satellites, although the coverage area is large, but the distance is long, communication is more difficult. At that time, the communication technology was not very mature, so the bandwidth of the wireless channel was relatively low and the communication rate was relatively slow. This kind of low-speed communication can only barely meet the needs of positioning and navigation.

Now, with the maturity of technology and the wide application of one-arrow N-satellite technology, the cost of launching satellites has dropped. Private companies, represented by Musk Space-X, adopt rocket recovery technology, which further reduces the cost of satellite launch, which makes it possible to lay a large number of Leo communication satellites.

Dense phobia should be carefully entered.

Leo satellites are closer to the ground, and if you put aside the weather factors, they all have line-of-sight coverage, so they can provide signal services to ground users.

The altitude of the SpaceX satellite was originally 1110 km to 1325 km, but later reduced to 540 km to 570 km.

To put it simply, it is tantamount to hanging the base station in the sky, that is, a little further away.

So, what is the bandwidth rate that T-Mobile is messing with with this kind of satellite communication technology?

2-4 Mbps.

Yes, you read it correctly, which is the same level as ADSL dial-up Internet access. Reluctantly make phone calls, send pictures and videos, it's tough.

Musk's SpaceX communication satellite mainly uses the special frequency band of satellite communication. Based on the advantage of the existing number of satellites, it can reach the service rate of about 300Mbps.

Be careful! SpaceX communication satellite belongs to conventional satellite communication system. It is not the mobile phone that achieves this rate, but the satellite communication equipment it is equipped with. This device has a "pot" (butterfly antenna) and a router or something.

Open the box of SpaceX communication satellite equipment

Antenna deployment status of SpaceX communication satellite equipment

So why can't mobile phones achieve higher satellite communication rates?

This has a lot to do with mobile phone hardware. In particular, the type of antenna has a great impact.

█ antenna type problem our current mobile phones are built-in antennas, antennas are in the phone. Dedicated satellite phones will have larger and more eye-catching antennas, as shown in the following figure:

Satellite telephone

According to the principle of the antenna, when the length of the antenna is 1 / 4 of the wavelength of the radio signal, the transmitting and receiving conversion efficiency of the antenna is higher. The large antenna of the satellite phone means that the working wavelength of the communication system is longer. It also means that the shorter the operating frequency (frequency = speed of light / wavelength).

Based on the knowledge of middle school physics, low-frequency (large-wavelength) wireless signals have stronger diffraction ability and longer propagation distance. This matches the scene of satellite communications.

After all, the signal sending and receiving ability of whip antenna is relatively weak. Stronger than it is the dish antenna, which is commonly known as the "pot".

The dish antenna can achieve better communication effect as long as it is oriented correctly.

So, I would like to ask, if the mobile phone for satellite communication, would you like to carry another pot on the phone? And then this pot, you still try to find the right direction.

Even if you are willing to carry the pot, you still have a problem-the problem of transmitting signals.

When a satellite sends a signal to a mobile phone, it can increase the transmission power. It is difficult for a mobile phone to send a signal to a satellite.

The transmission power of the handheld communication terminal is strictly limited. You don't want to communicate with a microwave oven, do you?

With the milliwatt wireless transmission power of a mobile phone, how difficult is it for a satellite to receive such a signal? You can imagine for yourself.

What's more, thousands or even hundreds of millions of mobile phones send signals to satellites in space. Do you think satellites can capture all the signals?

Musk's SpaceX communications satellite also has the problem of insufficient uplink speed. This is a loophole.

The problem of █ capacity is all right, let's move on.

Next, let's talk about capacity.

Musk's SpaceX communications satellite, known as 12000 satellites, is used to cover the world. On average, China covers an area of 9.6 million square kilometers and the earth covers an area of 510 million square kilometers, accounting for about 1.88%. If you have a satellite, it's only 226.

What about the number of ground base stations?

The total number of base stations in China is about 1035. For China Mobile alone, it is about 5.5 million seats.

226 vs 5500000, even a fool can see that there is no substitution relationship. If there were no cellular base stations on the ground, how many satellites would we need to allow everyone to have high-speed Internet access?

To say the least, even if the satellite can be one-to-many and take on a huge demand for mobile broadband connections, what about the return of the signal (that is, the satellite sends the signal to the ground station)? Data centers are all on the ground, and cloud services are also on the ground, so we have to send them back.

The current 5G base station covers an area of a few square kilometers to dozens of square kilometers, hundreds of users, and the forward bandwidth has to reach the 25Gbps level. Can satellite communication achieve the 25Gbps communication rate with the ground station?

Before this, our country did an experimental mission on the satellite, using high-speed high-order coherent laser communication terminal to realize the transmission of rate 10Gbps.

It seems that it is not much different from 25Gbps, but this is wireless communication, and its stability and anti-interference are far less than that of optical fiber. Moreover, the number of satellites is far less than that of base stations, if you want to compare with land base stations, its quantity demand is enormous.

█ indoor coverage problem in short, if satellite communications replace ground cellular base station communications, then, indoor signal coverage, what should we do? Basement, tunnel, what should I do? What should I do if I encounter bad weather?

█ 's conclusion has been said so much that everyone should understand it. Satellite communication can not replace the ground cellular communication system at all. If someone tells you that communication satellites will replace base stations, it's either stupid or bad.

As a special communication technology, the role positioning of satellite communication is still a supplement to the ground cellular communication system.

There are three types of applications in satellite communications:

1. In sparsely populated areas, satellites will play an important role in areas where optical fiber and base stations are not convenient to deploy. There are still many such areas abroad and in the central and western regions of the country. For some outdoor enthusiasts, as well as special workers such as mineral exploration and nature conservation, satellite phones are very useful.

2. Ocean and sky scenes, such as maritime ships and civil aircraft, have a strong demand for satellite communications. Especially civil aviation, in recent years, high-value people, such as business people, have a great demand for air communications, and this kind of applications are developing rapidly.

3. Emergency and disaster relief scenarios. When earthquake, flood and other natural disasters occur and the ground communication system fails, it is necessary to carry out emergency communication with the help of satellites.

When we look at satellite communications, we must be rational and objective. Its market volume is not at all the same level as cellular mobile communications. However, the market segment it faces does have a lot of room for development.

In recent years, 3GPP's NTN (Non-terrestrial networks, non-terrestrial network project), as well as SaT5G and TC12, have been studying air-to-ground integrated communication.

I think there is market potential, but it is not as big as we thought. In particular, NTN mainly focuses on the integration of satellite communication and 5G, which is the supplement of satellite to 5G. It is not to break away from cellular communication and start a new stove, we should have a basic understanding.

For satellite communication, in addition to the technical bottleneck, it also involves a series of factors, such as law, environment, politics and so on. Because of the limited space, we will not introduce them one by one.

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.