How do base stations go to the sky?

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This article is reprinted from the WeChat public account "Network Optimization Mercenary", the author is in correspondence. To reprint this article, please contact the WeChat public account "Network Optimization Mercenary".

The base station is installed on a flying platform about 14 km above the ground. It uses a large-scale antenna array to transmit 120 beams to the ground. Each beam corresponds to a cellular cell, thus forming a continuous coverage area consisting of 120 cells on the ground.

The base station just went up to the sky!

It can solve all kinds of problems encountered in building ground cellular networks, such as difficult planning, difficult site selection, difficult site construction, difficult coverage, high costs, etc.!

Recently, Deutsche Telekom demonstrated the latest results of its High Altitude Platform System (HAPS) project and ambitiously announced the above plan.

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For a time, the topic of "base stations going to space" once again became a hot topic of discussion in the industry.

So how did the base station get to the sky?

1

In the HAPS project successfully demonstrated by Deutsche Telekom, the antenna is installed on a remote-controlled flying platform located in the stratosphere about 14km above the ground, which can form a coverage cell with a diameter of about 10km on the ground, and mobile phones can directly connect to the antenna on the aircraft within the cell. The flying platform is connected to the ground core network using high-frequency, large-bandwidth wireless backhaul.

The project uses 5G Ready LTE technology with an operating frequency band of 2.1GHz and a channel bandwidth of 10MHz, which can provide VoLTE voice and data communications.

The test results show that the smartphone can connect to Deutsche Telekom's ground mobile network through the antenna on the aircraft, with a download speed of 70Mbps and an upload speed of 20Mbps. At the same time, the phone can also switch back and forth between the cells provided by the flying platform and the cells of the ground base station.

Deutsche Telekom said this was the first time in the world that HAPS was successfully integrated with a commercial terrestrial mobile network.

HAPS has a wide coverage area and can especially solve the coverage blind spots caused by the shadow effect of ground base station signals blocked by buildings, hills, etc., so it is a good supplement to the ground mobile network.

Deutsche Telekom said: "This demonstration shows that we can bring fast Internet connections to anywhere in the future, especially in places where traditional ground base stations are difficult to locate and build. 'Air Base Stations' will become a cost-effective supplement to our mobile communication network."

It is reported that the project was jointly developed by Deutsche Telekom and SPL (Stratosphere Platforms Limited), with SPL providing the flight platform. The project started in 2016 and has been invested for 5 years.

Both parties are very excited about this successful demonstration and believe that it is a model of innovation for Deutsche Telekom and even the whole of Europe.

Looking to the future, Deutsche Telekom has also announced an ambitious plan:

A large-scale antenna array consisting of 1,028 antenna units with an installation area of ​​about 9 square meters is installed on the flying platform. It can transmit 200 beams, each beam corresponding to a cell. The 200 cells can simultaneously form a continuous coverage area with a diameter of about 140 km on the ground.

Deutsche Telekom said that one flying platform can deploy hundreds of cells. In theory, only 30 aircraft are needed to cover the whole of Germany, 50 aircraft are needed to cover the whole of Kenya, and only 670 aircraft are needed to cover the whole of the United States.

In addition, the project is also developing antenna versions that support sub-6GHz and millimeter wave frequency bands. In the future, HAPS will not only be used to solve outdoor wide coverage, but also to solve indoor coverage, deploy fixed wireless services to replace fixed home broadband, and cover aircraft, etc. More advanced inter-cell coordination technology will be used to avoid frequency interference between ground cells and HAPS cells.

2

Similar to Deutsche Telekom, Japanese operator SoftBank announced the establishment of a joint venture company HAPSMobile in April 2019 and also launched the HAPS program to build an aerial communication system for global business.

According to SoftBank, HAPS refers to a flying platform in the stratosphere that can directly connect to mobile phones and IoT terminals like a base station, and interconnect with the ground telecommunications network, thus realizing a three-dimensional network integrating the sky and the earth.

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HAPS is located in the stratosphere about 10 to 50 km above the earth's surface, which is higher than the flight altitude of civil aircraft and lower than the satellite orbit altitude. The air flow in the stratosphere is relatively stable and smooth. Therefore, HAPS has the advantages of both satellite and terrestrial communication systems, with the characteristics of wide coverage, low cost, small transmission delay, and easy configuration.

SoftBank said that one HAPS can cover an area of ​​about 200km in diameter on the ground, and about 40 HAPS can cover the entire Japanese archipelago.

The HAPS flight platform is solar-powered and can hover continuously for six months at any coordinate.

From the perspective of SoftBank's HAPS networking architecture, the flight platform includes a Service link and a Feeder link. The former is a link that directly communicates with mobile phones or IoT terminals; the latter is a backhaul link, which is connected to the ground gateway via high-frequency wireless and then connected to the mobile core network via the ground transmission network.

For Service link, SoftBank plans to use the 450MHz to 2.6GHz frequency band; for Feeder link, it will use high frequency bands such as 21GHz, 26GHz, 28GHz, 31GHz, 38GHz, and 47GHz.

3

While operators are keen on innovative network architecture and service models such as "base stations in the sky", the standards organization 3GPP is also studying NTN (Non-Terrestrial Networks).

The dream of 5G and 6G is full connectivity and full coverage. But it is too difficult to realize this dream. Operators have to spend a lot of money to build a lot of base stations, especially in remote mountainous areas. The cost of building a station is horrifyingly high, and there is almost no income. Even if there is no shortage of money, how can you cover ships sailing on the sea and planes flying in the sky?

The best way is to make the ground cellular network "connected to the sky", that is, to integrate it with the non-terrestrial network (NTN), such as the satellite network or UAS (unmanned aerial system), to create a three-dimensional wide coverage.

As can be seen from the above figure, 3GPP's NTN is similar to Deutsche Telekom and SoftBank's HAPS networking architecture.

Specifically, it is divided into the following three categories:

1) Based on satellite transparent transmission of 5G NR wireless signals

5G base stations (gNB) are still deployed on the ground, but NR wireless signals are transmitted through satellites and sent to gNBs via the ground-based NTN gateway. Simply put, satellites are used as repeaters.

2) Satellite-mounted 5G base stations (gNB)

3) Satellite-mounted gNB-DU

The 5G base station (gNB) is divided into CU and DU parts, and the DU part is carried on the satellite.

In order to identify business needs to help operators increase business revenue, 3GPP has also identified 12 major use cases for NTN networks, including broadcast and multicast services based on satellite coverage, wide-area Internet of Things (NB-IoT and mMTC) based on satellite coverage, satellite backhaul, 5G ubiquitous connectivity, etc.

In the past, ground mobile communication networks and the satellite industry have been developing in parallel and rarely intersecting, but now with the development of commercial space and the demand for 5G and 6G, the two are merging. There is no doubt that this will strongly promote the realization of a world where everything is connected.

But in the end, NTN or "base stations in space" also faces some challenges, such as:

1) The satellite industry lacks unified standards. The mobile communications industry has always been based on global unified standards, and each G can quickly achieve ecological scale, but the satellite industry is relatively lacking in this regard.

2) Integrating satellite technology into mobile phones and IoT terminals will bring challenges in cost and power consumption, and will require support from the global industrial chain.

3) Satellites provide limited network capacity, especially in supporting wireless backhaul.

4) Satellites may not be of any help for urban 5G or 6G deployments, where terrestrial deployments are cheaper.

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