Satellite communication: How difficult is mobile phone + satellite?

 In the past few days, news about satellite phones has been hot again. According to media reports, the Huawei Mate 50 series of mobile phones, which will be released on September 6, will have "satellite communication capabilities", which can send emergency text messages through the satellite system in places where there is no network.

Coincidentally, another gossip claims that Apple's iPhone 13 has hardware to connect to satellites. However, Apple and operators have not negotiated cooperation methods, so it has not been disclosed. On the future iPhone 14, this feature is likely to be implemented.

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 era of mobile phones really begun? In today's article, Xiaozaojun will give you an in-depth interpretation.

Communication frequency problem

In recent years, news about satellite communications has always been the focus of media and public attention. Especially after Musk did it, there was news every three days and two days, hyping the threat of satellite communication to terrestrial cellular mobile communication and the threat to 5G. In fact, many of these hype lack basic communication common sense. In addition to exposing some people's quick success, it is mainly to serve traffic and capital operations.

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

Next, let me explain why?

First of all, we must understand and keep in mind that any wireless communication system is based on wireless electromagnetic waves for communication. Wireless electromagnetic waves have an important attribute parameter, that is, frequency. Wireless electromagnetic waves with different frequencies have different characteristics, as shown in the following figure:

From a macro perspective, it is mainly divided into light waves and radio waves. We humans communicate wirelessly, mainly using radio waves. The low-frequency and medium-frequency electromagnetic waves used in the early days are now gradually developing to high-frequency and even terahertz (that is, the THz in the middle of the picture above). There are also scientists who are studying visible light communication (towards light waves).

The radio wave part of the wireless electromagnetic wave seems to have a lot of frequency resources, but in fact, each frequency has its corresponding use stipulated by the relevant government departments.

From the perspective of users, these frequency bands are divided into military use, private network use (public security, fire protection, railway, electricity, etc.), operator use (public mobile communication), and so on. In addition, there are ISM (Industrial, Scientific, Medical) license-exempt frequency bands that can be used without applying for a license, such as our Wi-Fi frequency bands.

No one can arbitrarily occupy the frequency. For example, if I invented a wireless communication device myself, and then occupy the carrier's 800MHz frequency band and transmit wireless signals in this frequency band, it is illegal. Mobile phone for satellite communication, the first question to consider: what frequency band does it use?

I don’t know if you have noticed that when we buy a new mobile phone, we will see a blue label on the back of the phone:

This label is the mobile phone's network access license. Before any mobile phone model is listed, manufacturers must first obtain the Ministry of Industry and Information Technology for network access testing. If you pass the test, the certificate will be issued.

This means that the state has strict requirements on every electronic product, especially the electronic product with wireless signal sending and receiving function: the radio operating frequency must comply with the relevant national regulations and must comply with the legitimate use of the product. Our mobile phones are designed to use three frequency bands:

One is the operating frequency of the operator's cellular communication system (that is, the operating frequency band of the base station). If it is a more common full Netcom mobile phone, it must support 2G / 3G / 4G / 5G frequency bands. Each frequency band is indicated on the official website.

Bands supported by a certain model of Xiaomi. The second is Wi-Fi and Bluetooth. This is the unlicensed spectrum I mentioned earlier, the ISM band. As long as the transmit 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 and can send and receive satellite signals. There are many kinds of GNSS systems, GPS in the United States, Beidou (BDS) in China, Galileo in Europe, and so on.

Numerous satellite navigation and positioning systems. Well, here comes the question. Many people shout that their mobile phones communicate through satellites every day. If they really want to communicate, what frequency band should they use?

Satellite communication belongs to space radio communication, which has exceeded the radio management capability of a sovereign country. Therefore, an application to the International Telecommunication Union (ITU-R) is required for the use of satellite communication operating frequency resources.

ITU-R has clear documents and strict requirements on application procedures and rules, which are often carried out simultaneously with applications for satellite orbit resources. Satellite communication works in the microwave frequency 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 where atmospheric absorption loss has the greatest impact. Therefore, the commonly used satellite communication frequency bands are: L, S, C, X, Ku, Ka:

It is worth mentioning the Ka-band. Since the C and Ku frequency bands are nearly saturated and cannot carry more services, the Ka frequency band is now gradually becoming popular. The Ka-band is more susceptible to weather than the Ku-band, but operates with a larger bandwidth and higher signal strength.

In any case, the satellite communication band and the carrier cellular communication band are different. Ordinary mobile phones currently on the market do not support the wireless operating frequency of communication satellites at all, and naturally cannot communicate with satellites.

Only dedicated satellite phones provided by satellite communication service providers can establish connections with these communication satellites. These satellite phone calls are also recharged to the corresponding satellite phone service providers.

As mentioned earlier, mobile phones can communicate with GPS and Beidou, which are positioning and navigation satellites. Strictly speaking, not a communications satellite. However, our country's Beidou is very powerful. In addition to positioning, navigation and timing services, it has one more function than GPS, that is, the ability to communicate with short messages.

That is, in addition to location information, it can communicate with short messages (RDSS protocol). How short is it? Up to 78 hexadecimal numbers/English/Numbers, or 39 Chinese characters. The Huawei MATE 50 "satellite communication" function mentioned earlier refers to this. With the help of the short message communication channel of Beidou positioning satellite, in emergency situations, 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 very different from the satellite communication that everyone understands in the normal sense. The dedicated satellite phone mentioned above can make calls and use data services (Internet access).

At present, the mobile phones on the market do not support the frequency band of communication satellites. So, can it be added? not that simple.

The support of the mobile phone for the communication frequency band is not enough to add a root antenna. It involves the transformation of the SoC chip, baseband, radio frequency, and software, which is a very large and complex work. It's not that it can't be done, it's that it doesn't make sense.

Because in most cases, mobile phones have base station signals or Wi-Fi signals, there is no need to increase unnecessary costs for rare cases. (It is more feasible to build an external satellite communication kit.) This is the same as the principle of amphibious vehicles.

It is not impossible to add the function of a car to a ship. However, the price will double several times, and the utilization rate is extremely low, which is not conducive to the popularization of automobiles. So, if the mobile phone is not modified and does not support the operating frequency of the existing satellite communication, is there no way to carry out satellite communication? Neither.

We can change our minds. If the mobile phone does not support the frequency of satellite communication, then, let the satellite directly use the frequency of the ground cellular base station? In fact, the new project that the US operator T-Mobile and Musk worked on together did just that. They let the satellites use some of the 5G band, improve the antennas, and send wireless communication signals to the ground.

This way of working has an important premise, that is, low-orbit satellites must be used. Over the years, a very important development trend in satellite communications has been the rise of low-orbit satellites. In the past, the cost of launching satellites was high, so it was hoped that a satellite could cover a large area. If you want to cover a large area, you must put the satellite higher. As shown below:

High-orbit satellites, although covering a large area, have long distances and make communication 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 widespread application of one-rocket-N-satellite technology, the cost of launching satellites has dropped. Private companies represented by Musk's Space-X have adopted rocket recovery technology, which has further reduced the cost of satellite launch, making it possible to lay a large number of low-orbit communication satellites.

Low-orbit satellites are closer to the ground. If the weather factor is ignored, they are all line-of-sight coverage, so they can provide signal services to ground users.

The orbital altitude of SpaceX satellites was originally 1110 kilometers to 1325 kilometers, but was later lowered to 540 to 570 kilometers. Simply put, it is equivalent to hanging the base station in the sky, that is, it is a little farther away. So, what is the bandwidth rate that T-Mobile is tinkering with with this satellite communication technology? 2-4 Mbps. Yes, you read that right, it's the same level as ADSL dial-up Internet access back then. Reluctantly making phone calls, sending pictures, and videos is not enough.

Musk's SpaceX communication satellites mainly use dedicated frequency bands for satellite communication. Based on the advantage of the existing number of satellites, they can currently reach a service rate of about 300Mbps.

Notice! SpaceX communication satellites belong to conventional satellite communication systems. It is not the mobile phone that reaches this rate, but the satellite communication equipment it is equipped with. This device has a "pot" (butterfly antenna), and a router or something.

SpaceX Communications Satellite Facility Antenna Unfolded. So, why can't mobile phones achieve higher satellite communication rates? This has a lot to do with the phone hardware. Especially the type of antenna has a great influence.

Antenna type problem

Our current mobile phones have built-in antennas, and the antennas are all inside the mobile phone. A dedicated satellite phone will have a larger and more prominent antenna, as shown in the following figure:

According to the antenna principle, when the length of the antenna is 1/4 of the wavelength of the radio signal, the transmission and reception conversion efficiency of the antenna is high. The antenna of the satellite phone is so large, indicating that the working wavelength of this communication system is longer. It also means that it operates at a shorter frequency (frequency = speed of light ÷ wavelength).

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

The signal transmission and reception capability of the whip antenna is relatively weak after all. Stronger than it is the dish antenna, also known as the "pot". Dish antenna, as long as the direction is aligned, can achieve better communication effect.

So, I would like to ask, if the mobile phone uses satellite communication, would you like to carry another pot on the mobile phone? Then this pot, you still try to find the correct direction. Even if you are willing to take the blame, you still face a problem - the problem of transmitting signals.

The satellite transmits signals to the mobile phone, and the transmit power can be adjusted up. Cell phones transmit signals to satellites, which is difficult. For handheld communication terminals, the transmit power is strictly limited. You don't want to have a microwave in your hand to communicate, do you?

With the mobile phone's milliwatt-level wireless transmission power, how difficult is it to make the satellite receive this kind of signal? You can imagine for yourself. What's more, thousands or even hundreds of thousands of mobile phones send signals to satellites in space. Do you think satellites can capture all the signals?

Musk's SpaceX communication satellite also has the problem of insufficient uplink speed. This is a dead end.

Capacity issues

Well, we continue to analyze down. Next, let's talk about capacity issues. Musk's SpaceX communications satellite, known as 12,000 satellites, is used to cover the world. A simple average, China's 9.6 million square kilometers, the earth's area of ​​510 million square kilometers, accounting for about 1.88%. For satellites, there are 226.

So, what about the number of ground base stations? The total number of base stations in China is about 10.35 million. For China Mobile alone, there are about 5.5 million seats. 226 vs 5500000, even a fool can understand, there is no substitution relationship. Without terrestrial cellular base stations, how many satellites would we need to get everyone online at high speed?

Taking a ten thousand steps back, even if the satellite can be point-to-multipoint and undertake a massive demand for mobile broadband connections, what should we do with the signal backhaul (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.

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

Previously, my country conducted a test mission on a satellite, using a high-speed high-order coherent laser communication terminal to achieve a transmission rate of 10Gbps. It seems that it is not much different from 25Gbps, but this is wireless communication, and its stability and anti-interference are far inferior to optical fibers. Moreover, the number of satellites is far less than that of base stations. If it is comparable to land base stations, its quantity demand is huge.

Indoor coverage problem

In short, if satellite communications replace terrestrial cellular base station communications, what about indoor signal coverage? Basements, tunnels, what to do? What if there is bad weather?

So much has been said, everyone should understand it. Satellite communication is completely unable to replace the terrestrial cellular communication system. It's either stupid or bad if someone tells you that communications satellites will replace base stations. As a special communication technology, the role of satellite communication is still a supplement to the terrestrial cellular communication system.

There are three types of applications for satellite communications:

  1. In sparsely populated areas, in areas where optical fibers and base stations are inconvenient to deploy, satellites will play a great role. In foreign countries, in the Midwest of China, there are still many such areas. For some outdoor enthusiasts, as well as special working groups such as mineral exploration and nature protection, satellite phones are very useful.
  2. Ocean and sky scenes, such as sailing ships and civil aviation aircraft, have a strong demand for satellite communications. Especially in civil aviation, high-value groups such as business people have a great demand for air communication in recent years, and such applications have developed rapidly.
  3. Emergency rescue scenarios. When natural disasters such as earthquakes and floods occur and the ground communication system fails, satellites are needed for emergency communication.

When you look at satellite communications, you must be rational and objective. Its market size is not at 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, TC12, are all studying air-ground integrated communication.

I think the market potential is there, but it's not as big as imagined. In particular, NTN mainly focuses on the integration of satellite communication and 5G, which is the supplement of satellite to 5G. It's not about breaking away from cellular communication and starting anew, everyone should have a basic understanding.

For satellite communications, in addition to technical bottlenecks, it also involves a series of factors such as law, environment, and politics. Due to space limitations, we will not introduce them one by one.

Post a Comment