5G and satellite, what is the relationship?

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This article is reprinted from the WeChat public account "Xianzao Classroom", written by Zhou Lei. Please contact the WeChat public account "Xianzao Classroom" to reprint this article.

introduction:

In recent years, satellite communications have attracted widespread attention at home and abroad. People are generally optimistic about the long-term development of this technology, believing that it will bring disruptive changes to existing communication technologies and may even replace the most advanced mobile communication technology, 5G. However, some people believe that satellite communications have many technical bottlenecks and will not play a big role. What is the use of satellite communications? What is its relationship with 5G? In this article today, let's explore the answer...

At 11:31 a.m. Eastern Time on October 24, the U.S. Space Exploration Technologies Corporation (SpaceX) successfully completed the launch mission of the 15th batch of satellites in the "Starlink Project", sending 60 satellites into space using a "Falcon 9" rocket.

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Falcon 9 rocket and 60 Starlink satellites

As of now, SpaceX has launched a total of 893 Starlink satellites (895 if the two test satellites launched in February 2018 are included), which is an amazing progress.

There is no doubt that Musk is indeed a business genius and a technology fanatic. The disruptive innovation he led has greatly reduced the cost and threshold of launching satellites.

According to his vision, the "Starlink Project" will deploy about 12,000 satellites around the earth (which may increase to 42,000 later) to create a global network and provide users with high-speed Internet services.

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Many unscrupulous domestic self-media have hyped up the "Starlink Project", saying how powerful it is, that it is the United States' 6G, that it will replace 5G mobile communications, and that it poses a threat to China.

In fact, Musk himself has never said that the "Starlink Project" will replace 5G. As for 6G, Xiaozaojun's article about a "6G satellite" of a domestic university a few days ago also explained that many things are still unknown.

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The core threat of the "Starlink Project" lies in the occupation of orbital and spectrum resources.

The plan will apply for more than 1,000 satellite orbits each time. According to the current first-come, first-served principle (must be used within 7 years), the orbital resources below 1,000 kilometers are likely to be occupied by the "Starlink Project" in a few years.

The relationship between satellite communications and 5G

If satellite communication technology represented by the "Starlink Project" cannot replace 5G, then what is the relationship between satellite communication and 5G?

To answer this question, let us look at the work being done by international organizations.

At present, the representative organizations related to satellites and 5G at home and abroad are as follows:

• SaT5G (Satellite and Terrestrial Network for 5G)

This is a satellite and 5G integration project established in 2017 and funded by the European Union. Its members have made many contributions to ETSI and 3GPP standardization studies related to satellite integration into 5G.

• Non-terrestrial networks (NTN)

This is a project established by 3GPP. It is committed to integrating satellite communications with 5G and optimizing the 5G NR air interface and architecture so that it can provide more extensive and diversified communication services in the future.

• Space Communications Technical Working Committee (TC12)

This is an organization established by my country's CCSA (China Communications Standards Association) in 2019 with the aim of carrying out research on satellite-ground integration.

First, let’s take a look at SaT5G.

In their white paper, SaT5G gives some typical satellite communication use cases, focusing on the two major scenarios of 5G eMBB and mMTC (the propagation delay of the satellite system is an insurmountable obstacle for the uRLLC scenario):

Use case 1: 5G content distribution

Leveraging satellite’s broadcast/multicast capabilities, media entertainment content (or VNF software updates for mobile edge computing devices) can be efficiently distributed to the network edge.

Use case 2: 5G fixed backhaul

Promote the promotion of services in areas where terrestrial 5G networks cannot cover (such as maritime services, lakes, islands, mountainous areas, rural areas, isolated areas, etc.), and improve the performance of terrestrial networks in a cost-effective manner.

Use case 3: 5G to buildings

Complementing terrestrial network connectivity, such as combining with terrestrial wireless or wired to provide broadband connectivity to homes or offices in underserved areas.

Use Case 4: 5G Mobile Platform Backhaul

Broadband connectivity to mobile platforms, such as aircraft, ships, etc., provides continuity of service.

Source: SaT5G White Paper

It is worth mentioning that SaT5G members have conducted a series of live demonstrations of the integration of satellite and 5G network architecture at the European Network and Communications Conference (EuCNC) held in the past two years.

The following figure demonstrates 5G technology on an airplane. Satellite and ground-based 5G network equipment are combined to distribute content, provide entertainment services and connectivity solutions for passengers.

Source: SaT5G official document

Let’s take a look at 3GPP and see what progress they have made on integrating satellite into 5G.

The diagram below shows the overall work progress of relevant international organizations (including 3GPP).

Source: SaT5G official website

The relevant content and timeline of the 3GPP RAN working group are as follows:

R15 established the SI project for “NR support for non-terrestrial networks” and released the research report TR 38.811. The report defines the deployment scenarios and channel models of NTNs, including satellite networks, as well as the potential impact of NR[1].

The “NR Support for Non-terrestrial Networks” SI of R16 simulated and evaluated the performance of different deployment scenarios and analyzed the adaptability of NR. In December 2019, the SI was completed and TR 38.821[2] was released.

R17 converted R16 SI into work item WI, focusing on the study of NR NTN enhancement solutions[3].

To simplify the understanding, you can imagine moving the ground base station to the satellite platform in the air (in fact, this is indeed one of its deployment methods).

The difference between this situation and traditional terrestrial mobile communications is that in terrestrial mobile communications, the base station is stationary while the users are moving; whereas in satellite communications, the base stations in the air are moving at high speeds, and most users can be regarded as quasi-stationary when they are stationary or moving at low speeds.

In addition, the wireless propagation environment and characteristics of the two are also very different.

So the question is, the terrestrial mobile communication network was not originally designed for such a scenario. These significant features brought by the NTN (non-terrestrial network project) will affect the architecture, protocols and implementation (especially the physical layer) of 5G to varying degrees.

Technical details of 5G NR supporting NTN

First, we need to understand the two typical modes of NTN beam coverage:

• Transparent forwarding

That is, the bent pipe solution, which can be considered as wireless signals being relayed via satellite.

• On-board processing

It can be considered that the satellite has all or part of the functions of a 5G base station.

Accordingly, four network architectures are proposed based on transparent forwarding, on-board processing, and with or without relays:

Source: 3GPP 38.811 V1.0.0

Secondly, if we look at the protocol stack perspective:

1. The user plane and control plane protocol stacks of the transparent forwarding architecture are as follows:

User plane protocol stack of transparent forwarding architecture

Control plane protocol stack of transparent forwarding architecture

2. The user plane and control plane protocol stacks of the onboard processing architecture are as follows:

User plane protocol stack of on-board processing architecture

Control plane protocol stack for onboard processing architecture

Finally, let’s look at the main impacts on the physical layer (and suggested solutions):

a) Physical layer control process

• Timing relationship

Compared with terrestrial networks, NTN has a larger two-way transmission delay RTT, which leads to a large offset in the uplink and downlink frame timing. It is necessary to enhance the physical layer timing relationship. The offset Koffset can be introduced and applied to modify the relevant timing relationship. The specific value of Koffset will also be different in different timing relationships. In addition, it is necessary to further discuss whether the Koffset value is obtained through broadcast or high-level parameter configuration.

Note: For the specific timing relationship of the impact, please refer to Section 6.2.1.2 of TR 38.821 V16.0.0

• Uplink power control

R16 discussed power control optimization solutions such as beam-specific and universal power control parameter configuration, prediction-based power control adjustment, and group-based power control parameter configuration, but no convergence conclusion has been reached. Therefore, the power control method of R15 will still be used.

• Adaptive Modulation Coding (AMC) and Delayed CSI Feedback

As we all know, AMC ensures the transmission quality of the link by adjusting the modulation mode and coding rate of wireless transmission. In order to solve the problem of outdated reporting of channel state information CSI, R16 discussed a variety of optimization solutions, but no convergence conclusion has been reached. According to the conclusion of SI, the CSI feedback mechanism defined in R15 can at least be used for NTN link adaptation in LOS scenarios.

b) Uplink timing advance and RACH enhancement

• TA Enhancement

Timing advance is used to instruct UE to send uplink data in advance according to the instruction. NR's TA mechanism cannot meet the transmission distance requirements of NTN of hundreds or even thousands of kilometers. The enhancement scheme considered by R16 is to use a combination of public TA and UE-specific TA: the first is to autonomously obtain the TA value based on the user's location and ephemeris information (i.e., the key orbital parameters of commercial satellites). The second is to adjust the TA based on the network-side instructions. There are still some enhancements to the above two methods that need to be further discussed in R17.

• RACH Enhancement

If the UE can accurately obtain the user location information and perform time-frequency offset pre-compensation, the R15 PRACH format and preamble sequence can be reused (the necessity of additional enhancement can be further discussed); otherwise, it is necessary to consider the enhanced PRACH format and preamble sequence design.

In addition, NTN may also consider adopting the two-step access in R16 to simplify the initial access process.

c) More delay-tolerant retransmission mechanisms

As we all know, the hybrid automatic repeat request HARQ mechanism can ensure information integrity and improve transmission reliability.

However, the RTT in NTN is larger, and the minimum number of HARQ processes required will be much larger than the 16 supported by NR.

The two main options currently under discussion are:

The first one is the HARQ off mechanism.

The second is to enhance the HARQ transmission mechanism. For example, increase the number of HARQ processes to match the longer satellite two-way transmission delay. Or disable UL HARQ feedback to avoid stopping and waiting in the HARQ process, and rely on RLC ARQ to improve reliability. These two enhancement mechanisms have not yet been finalized. R17 should further discuss the number of HARQ processes and consider HARQ feedback, buffer size, RLC feedback, and RLC ARQ buffer size.

d) There are many other topics that I will not list here due to space limitations...

Conclusion

Based on the research progress of major organizations, we can basically assume that satellite communications will be integrated into the entire 5G ecosystem as a beneficial supplement.

The integration of satellite communications and 5G will be a win-win result.

On the one hand, the scale effect of 5G has opened up new market opportunities for satellite communications. On the other hand, the "plug-and-play" satellite communication network will be an effective supplement to the terrestrial 5G network, making the 5G ecosystem more resilient and efficient.

From a standardization perspective, 3GPP is still in the process of formulating specifications for the integration of satellite and 5G networks. However, at present, the most important consideration is how to reuse terrestrial 5G key technologies and standards to the greatest extent possible.

I believe that by 2021, when R17 is released, we will see preliminary results. At that time, it will be the starting point for the deep integration of 6G satellite and ground in the future.

—— End of the article ——

Reference Documents:

[1] SaT5G Whitepaper.

[2]3GPP. 3GPP TR 38.811: Study on New Radio (NR) to support non terrestrial networks V15.0.0 (Release 15) [R]. 2018.06.

[3]3GPP. 3GPP TR 38.821: Solutions for NR to support non-terrestrial networks (NTN) V1.0.0 (Release 16 ) [R]. 2019.12.

[4]Thales. 3GPP RP-193234: New WID: Solutions for NR to support non-terrestrial networks (NTN) [R]. 2019.12.