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Convergence of satellite and terrestrial communications

For 5G, this may be just the icing on the cake in the second half; but for 6G, which is still in the imagination, the integrated communication of air, space, ground and sea is a sea of stars to be conquered.
So, here we go.

   Why is satellite communication needed? Mobile phones have connected all aspects of our lives in series. We take out our mobile phones anytime, anywhere, and can surf the Internet freely. All this is so taken for granted that we can’t even think of such an incredible question: Are mobile communication networks really ubiquitous? We all know that although the earth is called the “earth” ball, it is actually an out-and-out water polo, with only 29% of the land area and 71% of the ocean area. With such a small land area, the mobile communication network only covers 20%; in contrast, the network coverage of the ocean is even lower, only 5%. Overall, the mobile communication network only covers less than 10% of the world’s area! Global GSM network coverage

It is easy to understand that there are no people living in the ocean and there is no coverage. Why is the coverage on land so low? See the picture below to understand.
The global population density
turns out that there are not many places on land suitable for human habitation. Most of the places not covered by mobile communication networks are deserts, jungles, ice fields and other inaccessible places. Building a ground base station in this kind of place is purely a loss-making business, and it naturally becomes a blind spot for signals.
However, there are all kinds of ships on the ocean that only have access to the Internet, and inaccessible areas on land are not completely deserted. How can the communication needs of these forgotten fringes be met? In addition, in the face of nature, the power of human beings is small. Natural disasters such as floods, earthquakes, and tsunamis often lead to large-scale ground infrastructure damage, resulting in power outages, network outages, and circuit breaks, making rescue work difficult. In such a critical moment, how to get through the lifeline of rescue? The essence of the above problem is: how to build a communication network that covers the whole world and is not restricted by the ground environment? Therefore, people turned their attention to the sky, expecting that the communication base station, like the light of stars and moon, can spread the signal all over the land without prejudice, no matter whether the land below is prosperous or barren, whether it is a plain or a desert. Such a plan really exists, that is, “satellite communication”.


   How is satellite communication implemented? Under normal circumstances, the full name of what we call “satellite” is “artificial earth satellite”. Like the moon, the natural satellite of the earth, they also revolve around the earth at high speed in the sky.
So, what is the altitude of the satellite?
Although we often refer to the external world that we can see when we look up as the “sky”, in fact, the concepts of “sky” and “empty” are completely different.
“Empty” refers to the altitude range from the earth’s surface to the atmosphere, which can be reached by aircraft such as balloons, airplanes, and airships; the space outside the atmosphere can be called “sky”, which generally needs to be reached by rockets .
“Empty” and “sky” are generally bounded by an altitude of 100 kilometers, also known as the “Karmen line”. This was determined by the American engineer and physicist Theodore von Kármán by studying the altitude limit of aircraft. That is to say, the limit of the earth’s atmosphere is 100 kilometers, and above that is the vast universe outside the earth.
The communication carriers that can achieve “going to the sky” are all kinds of communication satellites. They cruise over our heads all the time, both familiar and strange. The height of the satellite orbit is generally divided into low orbit, medium orbit, geostationary orbit and high orbit. Low Earth Orbit (LEO): A satellite system with an altitude below 2,000 kilometers from the ground. Because low-orbit satellites are close to the earth, they have the characteristics of small path loss and low transmission delay (generally less than 10 milliseconds). As the launch cost decreases year by year, multiple LEO satellites can form a constellation to achieve true global coverage, and frequency reuse is more effective. Therefore, the LEO system is considered to be the most promising satellite Internet technology. Medium Earth orbit (MEO): The altitude from the ground is 2000km~35786Km, and the transmission delay is generally less than 50 milliseconds, which is greater than that of low-orbit satellites, but the coverage is also larger. When the orbital altitude is 10000Km, each satellite can cover 23.5% of the earth’s surface, so only a few satellites can cover the whole world. Geostationary Earth Orbit (GEO): 35786km above the ground, that is, geostationary geostationary orbit. That is to say, the angular velocity of GEO satellite motion is the same as the earth’s rotation, so these satellites are relatively stationary when viewed from the earth. Theoretically, global coverage can be achieved with three satellites in geostationary orbit. However, geostationary satellites have an unavoidable disadvantage, that is, the orbit is too far away from the earth, the link loss is serious, and the signal propagation delay is generally more than 250 milliseconds, much longer than LEO and MEO. High orbit (High Earth Orbits, HEO): the height above the ground is greater than 35786km. In addition, there are elliptical orbits, etc. Compared with the previous ones, these technologies have fewer applications, so I won’t repeat them here.
The distance scales of these different orbits are not obvious in digital form. As can be seen from the figure below, the geosynchronous geostationary orbit is the dividing line between the middle orbit and the high orbit, and their height range is very wide. In contrast, low-orbit is very close to the ground, and naturally there are much fewer satellites that can be accommodated.
According to the research report data of CCID Consulting, the Earth’s low-Earth orbit can accommodate about 60,000 satellites. It is predicted that by 2029, a total of 57,000 low-orbit satellites will be deployed in Earth’s low-Earth orbit. At present, 42,000 satellites have been planned for a single SpaceX star chain. If you don’t grab such a scarce resource, some people will grab it. Therefore, with the development of satellite Internet, the construction of low-orbit satellites has become hot.

To use satellites to realize long-distance wireless communication, spectrum resources are also crucial. With the demand for capacity, the frequency bands used by satellite communications range from intermediate frequency L and S bands to Ku and Ka, and then to millimeter waves all the way up, with higher frequencies and larger bandwidths.

Satellite communication frequency bands are “non-renewable resources” like space orbit resources, and the international principle is the use mode of “first come, first occupied”. At present, the main communication frequency bands (Ku and Ka) of LEO satellites have become saturated.   The architecture of satellite communication and the composition of terminal satellite communication system can be divided into three parts: space segment, ground segment and user segment.

The space segment mainly refers to the constellation composed of multiple communication satellites in the sky, and the communication link between satellites (ISL, Inter-satellite Link, also called inter-satellite link). The ground segment mainly includes earth stations (also called gateways), and auxiliary parts such as business control, monitoring management, and time injection. Network elements such as the transmission and core network of the ground network can also be regarded as a part of the ground segment. The user segment refers to the terminals that access satellites, mainly including antennas (we often say “pot”), signal processing and equipment that provides network access capabilities (such as routers, etc.), terminals that access the network (mobile phones, computers, etc.) ). As can be seen from the figure above, in places where neither wired network nor wireless network is covered, to achieve low-cost Internet access, you only need to install a satellite antenna on the roof and connect to the indoor router to realize computers, notebooks, mobile phones and other terminals. Internet together.
If you want to surf the Internet anytime, anywhere in the wild, Starlink’s solution is still to carry a small-sized electronic phased array antenna and router. A standard-sized antenna consumes 50-75 watts, and the router also needs power, so a vehicle power supply is essential. Looking at the picture above, one person enjoys the satellite high-speed network exclusively under the quiet starry sky, which is very pleasant.