Satellite communication is a wireless communication method that uses satellites as a communication medium. When chatting with customers and friends, I often encounter customers and friends who are not very familiar with some terms in satellite communication. Later, we will introduce the relevant knowledge of satellite communication in very popular words in several times. We strive to make it possible for 99% of people to understand it even if they do not have a professional technical background in the communications and electronics industries.
First of all, this article will introduce the classification of communication satellites.
We often hear these terms: low-orbit satellite, Starlink, Lao Ma, high-throughput satellite, spot beam, single hop, Ku, Ka, C... Similar terms are basically related to the classification of communication satellites.
What is satellite communication
Satellite communication is a type of wireless communication, and its application scenarios are mainly divided into two categories:
1: Areas that cannot be covered by ground mobile communications
When we are in remote areas such as forests, deserts, oceans, mountains, and villages, where the ground mobile communication network is not effectively covered and mobile phones cannot communicate, satellite communications are needed, such as the shipboard satellite communication equipment on ocean-going cargo ships and the maritime satellite equipment used by hikers in the Gobi Desert.
Also, when natural disasters such as earthquakes and floods occur, ground mobile communications are interrupted, and satellite communications are also needed at this time.
2: Communication is blocked by line of sight
If I am in a mountainous area and there is no mobile phone signal, and I want to communicate with another person 10km away, what should I do?
If we draw a straight line between the two of us, and there is no other obstruction on the path of this line, the technical term is "no obstruction in line of sight". Line of sight literally means "at visible distance". If there is no obstruction in line of sight, then each of us can maintain communication with a communication radio in our hands, and satellite communication is not needed at this time.
On the contrary, if there is an obstruction in the line of sight, such as other mountains or trees, the two communication stations cannot maintain communication at this time, and satellite communication is required.
Summarize:
Scenarios where satellite communications are used:
1. The ground mobile communication network cannot provide effective coverage;
2. Line-of-sight communication is blocked.
The basic architecture of satellite communications
The basic architecture of satellite communication is that data is transmitted between two users through a satellite as an intermediary, as shown in the following figure:
The ultimate goal is: user A sends data to user B.
Divide this process into 2 steps:
Step 1: User A sends data to the satellite wirelessly;
Step 2: The satellite sends the data to user B wirelessly.
The whole process is very similar to mobile phone communication. Even if two mobile phones are placed next to each other, they cannot communicate directly with each other. They must communicate through the Wi-Fi of the local area network or through a mobile base station. Wi-Fi or base stations play the role of data transmission relay, and satellites in satellite communications also play the role of data transmission relay .
Classification of communication satellites
There are many ways to classify communication satellites. To sum up, there are mainly the following commonly used classification methods:
Classification by data flow
Classification by frequency band
Classification by capacity
Classification by beam coverage
Classification by charging method
Classification by data flow
According to the transmission path of data flow from user A to user B, there are three different types of satellite communications:
Point-to-point “single hop”
Point-to-point "two hops" or "multiple hops"
Single-hop access to ground network integration
***The basic data flow mode is point-to-point "single hop". As shown in the following figure:
User A's data is sent to the satellite, and the satellite forwards the data to user B. The entire data stream is transferred through the satellite only once, which is professionally called "once on satellite".
Another way is point-to-point "two-hop", as shown in the following figure:
User A's data is sent to the satellite. The satellite does not forward the data to user B, but first forwards the data to the satellite communication company's gateway station located on the ground. After the gateway station processes the data, it is sent from the gateway station to the satellite, and then the satellite forwards the data to user B. The entire data flow is transferred twice through the satellite, which is professionally called "twice on the satellite."
Since the data of user A has reached the gateway station in the first stage of "two satellite launches", and the gateway station is built on the ground, it can definitely be connected to the ground fixed broadband network, dedicated optical fiber network, etc. If user B can also connect to the ground network or mobile communication network, then the gateway station does not need to launch the satellite for the second time, and can directly forward the data to user B through the ground network. This is the third method. This method is shown in the following figure:
Classification by frequency band
Communication satellites mainly have three frequency bands: C band, Ku band and Ka band.
C-band: generally 3GHz~6GHz. The standard C-band satellite communication frequency range is 3.7 to 4.2GHz for downlink (from satellite to terminal), and 5.925 to 6.425GHz for uplink (from terminal to satellite).
Ku band: generally 10Ghz~14GHz. The standard Ku band satellite communication frequency range is 14GHz~14.5GHz for uplink (terminal to satellite) and 12.25~12.75GHz for downlink (satellite to terminal)
Ka band: generally 20GHz~30GHz. For example, the Ka band of China Satellite Communications' ChinaSat-16 satellite is: the user transmission frequency range is 29.46GHz~30GHz, and the user receiving frequency is 18.7Ghz~20.2GHz.
Many communication satellites carry multiple transponder payloads that can support one or more of the C, Ku, and Ka frequency bands at the same time. For example, the ChinaSat-10 satellite has both a Ku-band transponder payload and a C-band transponder payload.
For end customers, choosing which frequency band to use as a means of satellite communication requires comprehensive consideration of multiple factors, including terminal installation form, power supply, data communication rate requirements, cost, satellite coverage, and convenience of using satellite resources.
usually
1. The higher the frequency, the smaller the terminal antenna size, the easier it is to miniaturize the terminal, and the lighter the terminal weight.
2. The higher the frequency band, the greater the spatial transmission loss from the satellite to the terminal. On rainy days, the signal energy loss is greater. (The satellite communication industry calls the loss of signal energy due to rainfall "rain attenuation")
3. The frequency band is high, and the manufacturing cost of terminal equipment increases.
Classification by satellite orbit
Because Ma's Starlink is so popular, satellites are divided into high-orbit satellites, medium-orbit satellites and low-orbit satellites according to their orbits. I'm sure everyone has already known these terms. Commonly used communication satellites are divided into two major categories: high-orbit satellites and low-orbit satellites.
High-orbit satellites are geosynchronous satellites. They are located above the equator, about 36,000 km from the earth's surface, and their orbital period is synchronized with the earth. If the earth is used as the coordinate system, the position of geosynchronous satellites is unchanged. For example, the position of Zhongxing 10 is: longitude 110.5°E, latitude is 0 (because it is above the equator). This position will not change, and the longitude is always 110.5°E. The advantage of this is that:
1. The satellite’s coverage is fixed, and users within the satellite’s coverage can communicate with the satellite uninterruptedly 24 hours a day.
2. The satellite has a very wide coverage area, which can reach thousands of kilometers
3. Because the satellite is relatively stationary with respect to the ground, it is relatively easy for the terminal antenna to track the satellite
The disadvantages of high-orbit satellites are: because the satellites are too far from the ground, the signal attenuation during the entire transmission process is too large, affecting the communication rate of the entire system, and the signal transmission delay is large. The space transmission delay of the signal from the terminal to the satellite is 120ms (360000/speed of light). When user A transmits data to user B via satellite, the space transmission delay is 240ms. Add the processing delay of the terminal itself and the processing delay of the satellite, and the actual transmission delay from user A to user B is greater than 300ms.
The distance between a low-orbit satellite and the earth's surface is about 200km~2000km. Due to the low orbit altitude of the satellite, the orbit period of the satellite is also short. It usually takes a few hours to circle the earth.
Let's take a low-orbit satellite with an orbital altitude of 500 kilometers as an example. It takes about 90 minutes, or 1.5 hours, to orbit the earth. In fact, the time from when the satellite rises above the horizon to when it disappears across the horizon is the theoretical time for the terminal to communicate with the satellite, which is much less than 90 minutes. In addition, considering the satellite's orbital inclination, the location of the terminal and other factors, the actual communication time with the satellite (the professional term is "overhead time") is only about ten minutes. Therefore, there is a continuous switching process between the terminal and the low-orbit satellite.
Satellite A rises from the horizon and can communicate with the terminal. After more than ten minutes, satellite A falls from the horizon and can no longer communicate with the terminal. Before that, another satellite B must also rise from the horizon, and the terminal switches from communicating with satellite A to communicating with satellite B to ensure uninterrupted communication.
Benefits of low-orbit satellites:
1. Small space loss, which helps to improve data transmission rate
2. Small delay in signal transmission and strong real-time performance
3. Small space loss and strong ground signal power can reduce the size of the terminal antenna, thereby reducing costs;
The disadvantages of low-orbit satellites are: the time they pass over the top is short, and satellites need to be switched constantly. Due to the fast movement of satellites, it is difficult for terminal antennas to track them.
Classification by capacity
We often hear the term "high-throughput satellite", which is actually a classification based on the capacity of the satellite. The communication capacity of each satellite is limited, and the satellite communication industry divides communication satellites into traditional satellites and high-throughput satellites based on capacity.
High-throughput satellite, as the name suggests, means a satellite with large capacity. So how large a capacity is considered high-throughput?
Let’s take the Asia-Pacific 6D satellite, ChinaSat 16 satellite and ChinaSat 26 satellite as examples. The satellite capacity of the Asia-Pacific 6D satellite is 50Gbps, the satellite capacity of the ChinaSat 16 satellite is 20Gbps and the single satellite capacity of the ChinaSat 26 satellite is 100Gbps.
Classification by coverage
The special terms corresponding to satellite coverage are "large beam satellite" and "spot beam satellite".
Large beam satellite
Large beams generally refer to geosynchronous satellites, where the coverage of one antenna beam is several thousand kilometers. For example, the following is the coverage of the Ku-band of ChinaSat-10, where one beam covers almost the entire China.
Spot beam satellite
The spot beam satellite has multiple antennas, and each antenna beam points to a small area on the ground. The range of this area is much smaller than that of the large beam satellite, usually a few hundred kilometers. The following figure is the satellite coverage map of APSTAR-6D. We can see from the figure that the APSTAR-6D satellite has 90 spot beams, each of which can only cover a small area. These 90 spot beams together can basically cover the entire Asia-Pacific region.
For large-beam satellites, the satellite's transmission power covers a range of several thousand kilometers, so the average satellite signal landing power per unit area is low. For spot beams, the satellite transmission power covers a small range, so within the spot beam coverage, the satellite signal landing power per unit area is much stronger. The benefit is that ground terminals of the same size within the spot beam coverage can transmit at a higher data rate, or a smaller ground terminal can be used for the same data rate.
The disadvantage of spot beams is that the coverage of each beam is limited. If a user flies from one spot beam to another, there is a problem of beam switching, and network management of satellite communications is required.
Classification by tariff
There are two types of satellite communication charges:
1. Charges are based on the bandwidth and duration of the radio frequency signal used.
2. Charge according to data flow
The first type charges according to the occupied RF signal bandwidth and duration, which corresponds to the point-to-point "single-hop" mode in the data flow method.
For example: ChinaSat 10 satellite, Ku band, the frequency range of terminal A transmission is 14.05GHz~14.055MHz, occupying a total of 5MHz signal bandwidth. As shown below:
Then, 5MHz of radio frequency bandwidth needs to be reserved on the satellite for use by terminal A. This occupation is exclusive, that is, once terminal A occupies this 5MHz of radio frequency bandwidth, other users can no longer occupy this 5MHz of radio frequency bandwidth. The satellite communication company charges terminal A based on the radio frequency bandwidth occupied by terminal A and the amount of time it occupies the bandwidth.
The charging standard is MHz.hour
The second type is charged according to data traffic, which corresponds to the point-to-point "two-hop" or "multi-hop" mode in the data flow method, and the single-hop and access to the ground network mode.
The user's data flow enters the satellite communication company's gateway station, which has a user management system, traffic detection system, and billing system that can charge according to the data flow. This is the same as the traffic billing of mobile phones.
As shown below:
The charging standard is MBytes
In short, if the satellite communication user data passes through the ground gateway, it will be charged according to the flow. If the satellite communication user data is forwarded directly by the satellite without passing through the gateway, it will be charged according to the bandwidth.
Communication satellite example
Taking several commonly used communication satellites as examples, the following classifications of communication satellites are explained:
ChinaSat 10: point-to-point single hop, C/Ku band, high-orbit geosynchronous satellite, traditional satellite, large beam, charged by bandwidth and duration
ChinaSat 12: point-to-point single hop, C/Ku band, high-orbit geosynchronous satellite, traditional satellite, large beam, charged by bandwidth and duration
ChinaSat-16: point-to-point two-hop, single-hop access to ground network, Ka-band, high-orbit geosynchronous satellite, high-throughput satellite, spot beam, charging according to traffic;
ChinaSat-26: point-to-point two-hop, single-hop access to ground network, Ka-band, high-orbit geosynchronous satellite, high-throughput satellite, spot beam, charging according to traffic;
Asia-Pacific 6D: point-to-point two-hop, single-hop access to ground network, Ku band, high-orbit geosynchronous satellite, high-throughput satellite, spot beam, charging according to traffic;
From the above example we can see that:
The large beam corresponds to the traditional capacity satellite, which is a point-to-point single hop and charges according to bandwidth and duration.
Spot beams correspond to high-throughput satellites, point-to-point two-hop, single-hop access to ground networks, and charges based on traffic volume
Some parts of this article are not very rigorous, and some classification methods have special cases that are not covered. This article is only used for popular science purposes.