PCB News - The role of satellites in 5G networks

In order to support 5G construction, intensive deployment of cellular macro base stations and micro base stations is now underway. These base stations use complex radio technologies to support the data rate, capacity and coverage of 5G. The 16th version of 3GPP will be released in June this year, and the 17th version is expected to be released in the second half of 2021. At that time, detailed and specific instructions will be given on V2X, industrial Internet of Things, multi-SIM equipment, reliability and low-latency performance improvement, use of unlicensed spectrum within 71GHz, efficiency, and interference. In addition, as a supplement to the 24 major items discussed at the 3GPP meeting held in Spain at the end of last year, 5G New Radio (NR) support provided by non-terrestrial access PCB technologies such as satellites and high-altitude platforms will also be clarified. As a platform with inherent advantages, satellite technology can contribute to the global 5G architecture.
5G backhaul
Together with many 5G-enabled radio access technologies, the backhaul technology has undergone the necessary development-decomposing the baseband unit (BBU) and remote radio heads in the LTE network into a centralized unit (CU) and a distributed unit ( DU) and radio unit (RU) three separate functional modules. Carrier aggregation, downlink coordinated multipoint transmission/reception, MIMO and other radio technologies cooperate with each other to make full use of the limited spectrum below 6GHz, while massive MIMO (mMIMO) improves the network capacity of each cell site by improving spectrum efficiency And coverage. In addition, high-density deployment of millimeter-wave small base stations and other solutions move the frequency spectrum further to achieve greater access bandwidth. Various such technologies have contributed to the following 5G functions defined by the International Telecommunication Union (ITU) (Figure 1): 5G enhanced mobile broadband (eMBB); ultra-high reliability and low-latency communications (uRLLC); large-scale machine type Communication (mMTC).
As shown in Figure 2, the current strategy for 5G Radio Access Network (RAN) is the so-called gNodeB (gNB) base station. This type of base station uses the following two-layer architecture: Distributed Unit (DU), which provides low-latency performance for Factory automation and medical services; centralized unit (CU) for high power consumption processing. The separation of RU and DU exposes the Common Public Radio Interface (CPRI), which has been enhanced for 5G and is called an enhanced CPRI (eCPRI) interface. In some cases, DU and RU can be combined with each other, and the function is equivalent to a small base station.
The integration of 5G and satellite
At present, a number of studies are exploring the auxiliary use of the satellite-to-ground architecture for 5G radio access networks: The EU Horizon 2020 cooperation project involves a number of companies in the European continent, aiming to develop "satellite and ground networks for 5G"; European Space Agency funded Support the “Satellite-Ground Convergence Paradigm (SATis5G) in the Context of 5G” project; SpaceX, OneWeb, and Amazon are developing low-orbit (LEO) satellite networks that can provide connections to any location on the earth; geostationary orbit (GEO) runs high Flux Satellite (HTS) technology is another technology in the integration of satellite-to-earth networks and 5G, which can provide spot beam and multicast functions; the cellular communication standard organization 3GPP is also working on low orbit (LEO) and medium orbit (MEO), Research on non-terrestrial networks of geostationary orbit (GEO) satellites to clarify the functions of satellite communications in 5G1.
From the launch of Anik F2 with 4Gbps throughput in 2004 to the launch of EchoStar XIX with 200Gbps throughput in 2017, high-throughput satellite technology has achieved considerable development. In the near future, Ka-band transponders will provide Tbps-level speeds, and optimization techniques can also reduce the cost of propagation per bit. The "plug and play" capabilities of satellite networks are designed to support 5G through the following aspects: satellite network virtualization; allowing cellular networks to control satellite radio resources; development of link aggregation for small cell connections; and cellular access Key management and authentication between technology and satellite access technology to optimize security; fusion of the multicast advantages of satellite technology 2.

Fixed return

The fixed backhaul of satellites to base stations or independent small base stations can provide support for eMBB that cannot achieve cost-effective ground backhaul. This situation often occurs in underdeveloped areas and underserved areas on the planet where cellular network infrastructure and wireless access resources are minimal. In addition to eMBB, satellites can also provide support for mMTC in IoT applications such as smart agriculture.

HTS enabling technology

Satellite technology has evolved from the traditional fixed satellite service (FSS) to HTS technology, and continues to provide people with more and more functions and services.

Spot beam and frequency reuse

Figure 5: Spot beams and frequency separation improve the coverage and capacity of HTS.

When there are very few FSS beams that span a large range (as large as across the entire continent), HTS satellites can use multiple spot beams balanced by frequency reuse to increase throughput by 20 times under the same frequency allocation scheme (Figure 5). Among them, compared with the wide beam of the FSS satellite, each spot beam can provide more power to the target area. In this way, no matter which band the satellite transponder is working on (C-band, K-band or Ka-band), the spectrum can be optimally utilized. In order to reduce the risk of interference and signal loss, the spot beam layout is such that the frequencies of adjacent beams are not close to each other. There is the following trade-off between spot beam frequency separation and satellite flux: the closer the frequencies between spot beams are, the higher the frequency reuse rate, which makes the satellite capacity larger. This concept is similar to the relationship between the data rate and capacity increase of mMIMO in which hundreds of active antenna elements and beamforming units provide multiple beams for users in different locations. However, this concept is significantly different from spatial diversity: when the terrestrial mMIMO system reduces co-channel interference by increasing the number of beams, the environment where the satellite is located is not rich in scattering, so co-channel interference becomes a concern. problem. This problem can be alleviated by the "four-color" frequency multiplexing (FR4): adjacent beams achieve orthogonality through non-intersecting frequencies with different polarization directions. In general, this orthogonality is maintained to the user terminal.

multicast

HTS technology inherently has a multicast function: a message to be sent to one thousand users only needs to be sent once, without sending one thousand times, so that spectrum and data resources can be efficiently used. Compared with terrestrial wireless services, HTS technology has the following characteristics: satellite beam coverage area is large; long channel code can overcome noise; transmission signal contains information of multiple users. In addition, the corresponding frames of this technology can be encoded by the DVB-S2X framing protocol, and can be decoded by groups of users, thereby realizing multicast transmission6. In this way, the more devices that receive broadcast content, the more bandwidth can be saved. An example of a multicast service is a video conference: each participant forms a multicast source for all other participants (ie, multipoint-to-multipoint). Although multicast services tend to be the source of high bandwidth consumption for terrestrial systems, it is relatively not a problem for HTS.

Spectrum shift up

The recently launched HTS uses Ka-band transponders. The purpose of frequency shifting is to obtain a larger bandwidth, thereby achieving more spot beams. The next few generations of satellites will provide Tb/s-level capacity, so it may be necessary to use Q-band and V-band to achieve agglomeration of greater user traffic and use thousands of spot beams in the coverage area.

LEO's low latency

The LEO satellite network can provide functions that a single GEO satellite cannot achieve. The main advantages of LEO are: LEO satellites can reduce delay; and LEO satellite networks can achieve greater coverage. The altitude of the GEO satellite is about 36000km, and the end-to-end propagation delay is 280ms; the altitude of the MEO satellite is 10000km, and the delay is 90ms; the altitude of the LEO satellite is 350-1200km, and the delay is 6-30ms. Although the low latency of LEO satellites can only support limited low-latency 5G services, the synchronization chains of most low-latency 5G services require extremely small round-trip delays and corresponding timing errors (Table 1).

With ubiquitous global coverage, LEO satellite network becomes the best choice for mMTC applications. Although high-throughput GEO satellites can provide services to predetermined areas through a spot beam architecture with frequency reuse function, as long as they have sufficient ground infrastructure, LEO satellite networks can also achieve global coverage. The world’s first LEO satellite network, Iridium, declared bankruptcy shortly after its launch in 1998. However, the satellite network has been providing low data rate services for more than a decade, and has been upgraded through a new generation of satellites8.

The operation of the LEO satellite network is facilitated by a variety of technologies including digital payload, advanced modulation, frequency reuse, high power density GaN power amplifier (PA), and beam agile active phased array.

LEO communication

The LEO satellite network involves ground-to-satellite, ground-to-ground station (G2G), satellite-to-satellite (S2S), and satellite-to-ground round robin communication. These physical links are divided into ground-to-satellite links and inter-satellite links. The communication between satellite and satellite and between ground station and ground station is another difference between LEO and HTS PCB. The LEO satellite communication network can achieve strict control of data transmission between users, control terminals and telemetry terminals (such as status, diagnosis, configuration).

Unlike GEO, which maintains a fixed position in space, LEO satellites pass the ground section at a very fast speed, so multiple satellites are needed to achieve consistent coverage of a certain area. Among them, the ground station needs to perform complex switching through a mechanical scanning reflector antenna or an active phased array antenna with high gain and high directivity. When the status is updated, with the support of the G2G link, the beam hopping between the satellite and the user can reach remote areas without corresponding infrastructure. In addition, satellites equipped with cameras and sensors can track space junk through close coordination with each other5.

Onboard processing

For high-throughput GEO and LEO satellites, in order to increase the satellite throughput, the satellite architecture needs to be adjusted. The main architectural adjustment is to transform the previous forwarding topology into a regenerative topology.