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

Hello, everyone. I'm mayday.

In this issue, we talk about satellite communications, and the convergence of satellite and terrestrial communications: the story of non-terrestrial networks.

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

So, here we go.

Why do you need satellite communications? Mobile phones have connected all aspects of our lives. When we take out our mobile phones anytime and anywhere, we can surf the Internet freely.

All this is so taken for granted that we can't even think of such an incredible question:

Is mobile communication network really ubiquitous?

As we all know, although the earth is called a "earth" ball, it is actually a real water polo, with land area accounting for only 29% and ocean area accounting for 71%.

With such a small land area, mobile networks cover only 20% of the land area, while marine network coverage is even lower, at 5%.

Overall, the mobile communication network covers less than 10% of the world's area!

Global GSM network coverage can not live in the sea people do not have coverage is easy to understand, why is land coverage so low? Just look at the picture below.

The global population density turns out that there are not many suitable places for people to live on land. Most of the places not covered by the mobile communication network are deserts, jungles, ice sheets and other inaccessible places.

Building a ground base station in such a place is purely at a loss, and it naturally becomes a blind area for signals.

However, there are all kinds of ships on the sea to surf the Internet, and the inaccessible areas on land are not completely unattended. How can the communication needs of these forgotten fringes be met?

In addition, human power is small in front of nature. Natural disasters such as floods, earthquakes and tsunamis often lead to the destruction of a large area of ground infrastructure, resulting in power outage, network disconnection and road disconnection, which makes the rescue work difficult.

In such a critical moment, how to get through the lifeline of rescue?

The essence of the above question is: how to build a communication network that covers the whole world and is not restricted by the ground environment?

As a result, people look to the sky, hoping that the communication base station, like starlight and moonlight, can spread signals all over the earth without prejudice, no matter whether the earth is prosperous or barren, plain or Gobi.

There is indeed such a plan, that is, "satellite communications."

How to realize satellite communication? In general, the full name of what we call "satellite" is "man-made satellite". Like the moon, the natural satellite of the earth, they revolve around the earth at a high speed in the sky.

So, what is the altitude of the satellite?

Recall that when I introduced 5G ATG earlier, I used "5G, you can go to heaven!" Such a summary is not rigorous enough now.

Although we often verbally call the outside world we can see with our heads up as "sky", in fact, the concepts of "sky" and "emptiness" are completely different.

"sky" refers to the range of height from the earth's surface to the atmosphere, which can be reached by balloons, airplanes, airships and other aircraft, while the space outside the atmosphere can only be called "sky" and can only be reached by rockets.

The boundary between "sky" and "sky" is generally 100 kilometers above sea level, also known as the "Carmen line". This was obtained by Theodore von Kamen, an American engineer and physicist, by studying the extreme altitude of aircraft. In other words, the limit of the earth's atmosphere is 100 kilometers, and further up is the vast universe outside the earth.

The communication carriers that can realize "heaven" are all kinds of communication satellites. They swim over our heads all the time, familiar and unfamiliar.

The height of satellite orbit is generally divided into low orbit, medium orbit, geostationary orbit and high orbit.

Low Earth orbit (Low Earth Orbit,LEO): a satellite system that is less than 2000 kilometers above the ground. Because the low-orbit satellite is close to the earth, it has the characteristics of low path loss and low transmission delay (generally less than 10 milliseconds).

As the launch cost decreases year by year, multiple LEO satellites can form constellations to achieve real global coverage, and frequency reuse is more effective. Therefore, LEO system is considered to be the most promising satellite Internet technology.

Mid-orbit (Medium Earth orbit,MEO): at the height of 2000km~35786Km from the ground, the transmission delay is generally less than 50 milliseconds, which is larger 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 small number of satellites can cover the whole world.

Geostationary orbit (Geostationary Earth Orbit,GEO): 35786km above the ground, that is, the geostationary orbit. In other words, the angular velocity of GEO satellites is the same as the rotation of the earth, so these satellites are relatively stationary from the earth.

In theory, global coverage can be achieved with three geostationary satellites. However, the synchronous satellite has an inevitable disadvantage, that is, the orbit is too far from the earth, the link loss is serious, and the signal propagation delay is generally more than 250ms, which is much larger than that of LEO and MEO.

High Rail (High Earth Orbits,HEO): the height from the ground is greater than 35786km. In addition, there are elliptical orbits and so on. Compared with the previous ones, the applications of these technologies are less, so I will not repeat them here.

The distance scales of these different orbits are not obvious in digital form. As can be seen from the picture below, the middle and high orbits are separated by the geostationary orbit, and their height ranges are very wide. Low orbits, by contrast, are very close to the ground and can hold far fewer satellites.

According to the Sadie consultancy report, the Earth's low-Earth orbit can hold about 60,000 satellites. It is predicted that a total of 57000 Leo satellites will be deployed in low-Earth orbit by 2029.

At present, 42000 satellites have been planned in the SpaceX star chain alone. If you don't rob such scarce resources, there are plenty of people robbing them. Therefore, with the development of satellite Internet, the construction of low-orbit satellites has been hot.

In order to use satellite to achieve long-distance wireless communication, spectrum resources are also very important.

With the demand of capacity, satellite communications use frequency bands from intermediate frequency L, S band to Ku, Ka, and then to millimeter wave all the way up, the frequency is getting higher and higher, and the bandwidth is also getting larger and larger.

The frequency band of satellite communication, like space orbit resources, belongs to "non-renewable resources", and the international principle is the "pre-emptive" mode of use. At present, the main communication bands (Ku and Ka) of Leo satellites have become saturated.

The structure of satellite communication and the composition of terminal satellite communication system can be divided into three parts: space segment, ground segment and user segment.

Satellite system architecture (source: Starlink) the space segment mainly refers to the constellation composed of multiple communication satellites in the sky, as well as the communication link between satellites (ISL,Inter-satellite Link, also known as inter-satellite link).

The ground section mainly includes earth stations (also known as gateways), as well as auxiliary parts such as service control, monitoring and management, time injection and so on. The transmission of ground network, core network and other network elements can also be regarded as a part of the ground segment.

The user segment refers to the terminals connected to the satellite, which mainly include antennas (what we often call "pots"), devices that process signals and provide network access capabilities (such as routers, etc.), and terminals (mobile phones, computers, etc.) that access to the network.

Rural Digital connection Center (Source: Starlink) as can be seen from the above picture, in places where both wired and wireless networks are not covered, to achieve low-cost Internet access, you only need to install satellite antennas on the roof and connect indoor routers to achieve common Internet access by computers, notebooks, mobile phones and other terminals.

Satellite access to the Internet in the field (source: Starlink) if you want to surf the Internet anytime and anywhere in the field, the scheme of Starlink is still to carry small electronic phased array antennas and routers.

The power consumption of the standard size antenna is 50 to 75 watts, and the router also needs power supply, so the vehicle power supply is essential. Look at the picture above, a person in the quiet starry sky to enjoy the satellite high-speed network, very uncomfortable.

Starlink user terminal (source: Wikipedia) if you only need voice communications in an emergency and want to get rid of clunky antennas and travel light, you need a dedicated portable satellite phone.

What does a satellite phone look like?

Below are the three best-selling satellite phones in the United States in 2022, supporting two mainstream satellite communication systems, Inmarsat (International Maritime Satellite) or Iridium (Iridium Satellite).

At first glance, the shape of the first number, the 9-grid button, and the thick, large and long antenna have gone back to 20 years ago.

Looking at the price, boy, some functions that we think 2G phones have supported since its birth, such as calling, texting, GPS and emergency calls, have sold for nearly $1600, which is equivalent to more than 10, 000 yuan.

Let's take a look at what the domestic satellite communication system and satellite phone look like.

On January 10, 2020, Tiantong system, the first satellite mobile communication system independently built in China, officially provides services to the whole society and is operated by China Telecom.

Tiantong-01 satellite can provide voice service at a rate of 1.2 kbps and data communication service at a maximum of 384 kbps. Although the capacity is relatively low, it is possible to provide emergency communication services.

On May 17, 2022, China Telecom launched "Tiandi Wing Card", claiming that users can make satellite phones without changing SIM cards or mobile phone numbers, and never lose contact! If the signal from the ground base station can be received, the 4G and 5G networks can also be used normally.

Although you don't have to change your card to change your number, you have to change your cell phone. Mayfly searched on a shopping platform and found that there are still many mobile phone models that support Tiantong system, ranging from thousands to tens of thousands of prices, and flagship phones can indeed support 5G.

The following picture shows a mobile phone launched by Telecom in 2020, which can support 4G and Tiantong satellite. The appearance is a little higher than the traditional satellite phone, but the price is really expensive.

One of the reasons for this is that the positioning of satellite phones is not consumer goods for ordinary people, but industrial products specially for wild and ocean workers, which need to work stably in a harsh environment, and the cost is indeed relatively high; second, the satellite communications industry has relatively few users, and the system and terminal costs are difficult to dilute.

We can get a glimpse of it from the product introduction below.

As for the tariff, you can better see the location of the satellite phone: 100 yuan for 60-minute calls, 0.4 yuan for text messages, and 300 yuan for 20m traffic.

It is true that this price can only meet the most critical communication needs in times of emergency.

From this point of view, the gap between satellite communications and our daily life is undoubtedly huge: there are few scenarios, mobile phones are not easy to use, and the price is still very expensive.

The integration of 5G and satellite communications, so is there any hope for satellite mobile phones to enter the field of consumption?

This year, Huawei launched its flagship Mate 50, which supports Beidou satellite messages, and Apple has partnered with Globalstar to launch iPhone14, which supports satellite calls for help. These two products are the prelude to the exploration and consumption of satellite communications.

It is reported that Huawei Mate 50 can send text, location, trajectory map and other information to individuals via Beidou satellite, but the content will be reviewed. Only information related to rescue can be sent, and no reply can be received.

The content that iPhone14 can post is also a preset distress signal, and it has its own location coordinates, but it cannot be directed to individuals. Messages will be sent to public or paid rescue organizations, but can receive responses from rescue agencies.

Thus it can be seen that at present, in the field of consumption, satellite communications on mobile phones and 4G and 5G networks on the ground are still two independent systems, which are not technologically integrated, and the satellites are only located in emergency communications. this is not essentially different from the traditional satellite phone.

Even, due to technical limitations, Huawei and Apple can't even make satellite calls.

So, can we go a step further and integrate the two systems and send 5G signals directly from the satellite? In this way, we may not even have to change our cell phones, and we will be able to connect to 5G directly by satellite in the wilderness.

The answer is yes. And Starlink has taken the first step in this respect.

On August 25 this year, SpaceX announced that it would reach a spectrum sharing agreement with T-Mobile, and the new generation of Starlink V2 satellite will provide services to mobile phones on the current network through 1.9GHz.

The mobile phone in the current network does not have a large antenna like a dedicated satellite phone, and the transmission power is set at a very low level (usually 0.2 watts). So in order to achieve the goal, we can only work on the satellite.

The Starlink V2 satellite has enhanced its signal reception capability, extending the satellite antenna to 7 meters and the panel to 25 square meters, making the communication performance 10 times that of the previous generation V1.

As a result, existing mobile phones on the network can finally be directly connected to 5G satellites, and the throughput is expected to reach 2~4Mbps.

Although this rate can not be compared with the usual speed of hundreds of megabytes of 5G, it is easy to support phone calls and smooth Internet access is also guaranteed.

Compared with the 5G network on the ground, this floating 5G network is naturally called "non-terrestrial network (Non-Terrestrial Network)", or NTN for short.

The broad sense of NTN includes a wide range of unmanned aerial vehicles, ground-to-air communications, high-altitude base stations, satellite networks and so on. Satellite-based NTN is naturally the focus of attention, so in general, what we call NTN is based on satellite.

The following figure shows 3GPP's progress and plans for NTN technology standardization. It can be seen that the 5G NTN is still in the research stage in R15 and R16. Starting from R17, the access technology of 5G through NTN has begun to be standardized and will continue to move forward in subsequent versions.

The idea of NTN is: the satellite launches 5G signals, connects directly with the user's mobile phone, and then sets up a gateway station on the ground as a gateway, and finally connects to the 5G core network.

Among them, the link between the satellite and the user is called the service link (Service Link); the link between the satellite and the gateway station is called the feed link (Feeder Link); the link between the satellite is called the intersatellite link (Inter-Satellite Link,ISL).

In the current NTN related protocols, two implementation architectures are defined, namely "transparent payload" and "renewable payload".

The so-called "transparent payload", also known as transparent forwarding, actually treats the satellite only as a link for signal relay. The 5G base station is deployed behind the gateway station as part of the ground network. The satellite does not pay attention to what is sent by the base station and does not do any processing to the signal, as long as the mobile phone is smoothly connected to the gateway station.

Transparent payload architecture can make use of existing satellites, which is technically easy to implement and low cost, but the path between satellite and base station is long, the time delay is large, and does not support inter-satellite cooperation, so a large number of gateway stations need to be deployed.

Renewable payloads, also known as base stations on satellites, are equivalent to deploying 5G base stations on satellites. The inter-satellite link between the satellite and the satellite is like the Xn interface between the ground base station; the feed link between the satellite and the gateway station is actually a part of the backhaul network between the base station and the core network.

The architecture of renewable load must be modified and new satellite launch, which is complex in technology and high in cost. The advantage is that the time delay between the mobile phone and the satellite base station is short, and because of the existence of inter-satellite link, the gateway station can be deployed less.

On the basis of these two architectures, the realization of 5G NTN is essentially the integration of satellite communications and terrestrial cellular communications, which are originally distinct systems.

However, with the evolution of cellular communication protocols from 2G, 3G, 4G to 5G, the challenge of integration with satellite communication is huge, and a lot of updates have to be made in the protocol.

1. High transmission delay. The transmission delay of GEO satellite is more than 250ms (for transparent forwarding satellite). Such a high delay will greatly affect the timeliness of the interaction between base station and mobile phone, especially in the process of multiple signaling interaction, such as access and handoff.

Under such a high delay, it is very likely that the timer of the system has been restarted over time, and the signaling has not been delivered yet. Therefore, the relevant protocol process needs to be improved or redesigned.

2. Doppler shift. Because the satellite in non-geosynchronous orbit moves at a high speed relative to the earth, this will lead to serious Doppler shift.

The frequency offset of the ground 5G system in general scenarios is very small, even in special scenarios such as high-speed rail, only the frequency offset compensation of several thousand hertz needs to be considered.

However, for Leo satellite systems, it is necessary to deal with not only tens of kilohertz or even megahertz Doppler offset, but also tens of microseconds of timing drift. These are a great challenge to the design of 5G NTN system.

3. Super-large community radius. The ground cellular network cell is generally several hundred meters to thousands of meters, and the ultra-long-range coverage is more than 100 kilometers, while the coverage of NTN cells is much larger, the LEO beam can reach 1000 kilometers, and the GEO beam can reach 3500 kilometers.

Therefore, the delay difference between the center and the edge of the satellite cell will be more obvious, which will also have a certain impact on the timing synchronization of the system. 5G is a synchronous communication system, so it is necessary to enhance the synchronization mechanism to avoid inter-user interference.

4. Mobility management. Because the non-geosynchronous satellite moves at high speed relative to the user, this will lead to frequent mobility problems such as cell handoff and reselection.

On the one hand, in the decision-making of mobility management, the mobile state information of the cell needs to be taken into account to avoid unnecessary handover or reselection; on the other hand, the mobile state information of the cell can be further used to carry out pre-cell or beam handover to reduce signaling interaction overhead.

These technical challenges are tricky, but it doesn't matter, as long as there is demand, as long as the market is there, with the unremitting efforts of various experts, technology will always find a way out.

The expectation of the future is in 2022, and we have seen the solid steps of the industry one by one.

On June 21, Ziguang Zhanrui announced that it had joined hands with Beijing Pengyi to complete the world's first satellite measurement of the R17 IoT-NTN-based 5G satellite Internet of things.

On July 11, Ericsson, Qualcomm and French aerospace company Therese jointly completed the research on "5G mobile phone directly connected LEO satellite" and will deploy 5G on the LEO orbiting satellite network for testing.

On July 28, Nokia and AST SpaceMobile reached a five-year agreement to build a space-based mobile broadband network accessible to 4G and 5G mobile phones. Transparent payload architecture is expected, and Nokia mainly provides ground base stations.

On August 17, MediaTek announced that it had partnered with Rhode Schwartz to complete the world's first direct test of a 5G NTN satellite and mobile phone based on the 3GPP R17 standard in the laboratory. The laboratory simulated LEO satellite has an altitude of 600km and a moving speed of up to 27000 km per hour.

On August 26, China Mobile, ZTE and Transportation Communications Information Group jointly released the outfield verification results of the world's first 3GPP R17-based operator 5G NTN technology. This test is based on GEO satellite, the delay of ping 64 bytes is about 4s, and realizes text short message, voice intercom and other services.

Source: as can be seen from ZTE's official website, these plans and tests are still in a relatively early stage, but they truly verify that the NTN technical architecture is feasible, that is, a breakthrough from "0" to "1".

Reviewing the development history of satellite communications, it is generally believed that it can be divided into three stages:

The first stage is that more than 20 years ago, satellite communications and terrestrial communications were finally defeated through head-to-head confrontation and competition.

The second stage is about more than a decade from 2000 to 2014, when satellite communications dormant, only as a supplement and backup of ground communications, to survive in the gap.

The third phase since 2014, with the operation of O3b as the starting point and the rise of Starklink as the most exciting part, the broadband satellite Internet based on LEO has developed rapidly and accelerated its integration with terrestrial communications.

5G NTN, which is the product of the integration of satellite communication and ground communication, is a seed of hope.

It is hoped that in the near future, each of our mobile phones will be able to receive full signals from satellites in the uninhabited desert wasteland, in the rough sea and in the ruined homes ravaged by natural disasters.

-END-

This article comes from the official account of Wechat: wireless Deep Sea (ID:wuxian_shenhai), author: mayfly acquisition

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.