# Technology Trends for Massive MIMO towards 6G
## 8 Massive MIMO for Non-Terrestrial and Deep-Space Communications  
With the completion of 5G standardization phases 1 and 2, as defined by 3 GPP release 15 and 16, the first half of 2022 brought about the third evolution of the global 5G standard, culminating in the system design completion through 3 GPP release 17. This release is recognized as a significant expansion of 5G, accommodating a host of new devices and applications. Notably, 3 GPP release 17 has brought 5G NR support to satellite communications, a key constituent of non-terrestrial networks. Essentially, the NTN concept includes any network incorporating aerial or space-based elements, with satellite communication networks, high-altitude platform systems (HAPS)--including aircraft, balloons, and airships--and air-to-ground networks forming the NTN family [52]. The primary objective of 3 GPP NTN is to enable advanced features like ubiquitous connectivity and coverage for remote and rural regions through satellite communication networks. These networks serve two distinct functions: facilitating satellite backhaul communications for application scenarios such as customer-premises equipment (CPE) and direct low data rate services for handhelds, and adapting eMTC (enhanced Machine Type Communication) and NB-IoT (Narrowband Internet-of-Things) operation for satellite communications. Recent years have seen a surge of interest and development in Low Earth Orbit (LEO) satellites, which enable broadband access and services [54]. Among the significant players in this field, including Starlink (SpaceX), Kuiper (Amazon), OneWeb, Boeing, and Telestat, Starlink is the frontrunner, owing to the scale and dimension of its satellite megaconstellation and its service subscriber base. As illustrated in Figure 5, a total of 4288 Starlink satellites have been launched, with 3526 of them currently in active service as of 16 May 2023 [53]. Figure 4: Massive MIMO for high-speed applications, a design example of hybrid demodulation scheme. Several key factors have spurred the rapid growth of spaceborne broadband access [54], such as a decrease in launch costs, private investment, the widespread implementation of AI and cloud/edge computing, and advances in satellite wireless and networking technologies. From the perspective of wireless communications and particularly massive MIMO, an overview of trends, challenges and possible solutions is provided below. * **Competition and coexistence**: The deployment and operation of expanding satellite constellations below 2000 km present challenges in managing competition and facilitating coexistence. Many LEO megaconstellation developers aim to deploy satellites as low as possible to minimize propagation delay, while the ITU follows a \"First Come, First Served\" approach in accessing orbit/spectrum resources through the ITU cooperative system. With increasing congestion and reported collision incidents in LEO [54], regulatory measures, strategies, and technologies are needed to manage growing space traffic and ensure safe disposal of satellites/spacecraft. With the escape velocity being at 7.8 km/s, tracking, localizing the satellites/spacecraft and further enabling collision-avoidance can be difficult even with contemporary AI-assisted sensing and detecting technologies. * **Spectrum management**: Furthermore, spectrum management is a pivotal factor as the scarcity of spectrum resources introduces significant hurdles. To enable 5G NR for NTN, 3 GPP release 17 has embarked on investigations to support satellite backhaul communication for CPEs and facilitate direct links to handheld devices for low data rate services using the sub-7 GHz S-band. Frequencies beyond 10 GHz will be the subject of research in 3 GPP release 18. Concurrently, Starlink's first-generation system, Gen1, has predominantly utilized Ku-band and Ka-band for diverse link types and transmission directions, with the second-generation (Gen2) intending to incorporate the V-band.