# Technology Trends for Massive MIMO towards 6G
## 8 Massive MIMO for Non-Terrestrial and Deep-Space Communications
The use of either sub-7 GHz S-band or Ku/Ka/V-band could, to a certain extent, overlap with the spectrum of ongoing 5G and upcoming 6G systems,  
Figure 5: Illustrations of real-time orbital and location information of satellites in SpaceX Starlink constellation (as of 16 May 2023). along with other systems operating within these bands. This overlap could instigate interference and co-existence issues among various systems and networks. Both SpaceX and OneWeb have voiced concerns over potential interference encountered by non-geostationary orbit (NGSO) satellite internet if terrestrial 5G employs the 12 GHz band. The push to support more satellite direct links to user equipment (UE) using the sub-7 GHz S-band could further amplify these interference challenges. Thus, an in-depth exploration of spectral resources (e.g., higher frequency bands) and spectrum management for spaceborne massive MIMO is anticipated. * **Interference Management**: Moreover, a range of interferences can arise within a single space network and between different space networks [55]. For instance, in-band/out-band interference (or emission) can occur between user terminals (UTs) and ground stations within the same megacon constellation. In scenarios involving multiple space networks, satellite transmissions from one megaconstellation could interfere with the reception by UTs and ground stations of other megaconstellations. Similarly, transmissions from UTs/ground stations could cause interference with satellites in different constellations. Traditionally, co-existing space networks mitigate interference through the shared coordination of frequency allocations (both uplink and downlink). Nevertheless, the advent of increasingly complex scenarios necessitates the development of more sophisticated interference mitigation technologies. * **Channel modeling**: Additionally, various propagation conditions, as outlined by the ITU, can introduce interference to the satellite system. These conditions include line of sight, diffraction, tropospheric scatter, surface ducting, elevated layer reflection/refraction, and hydrometeor scatter. Consequently, the construction of precise channel models for massive MIMO satellite communications must take these physical conditions into account. The severe Doppler effect, induced by the high mobility of satellites and UTs, along with the longer propagation latency, complicates the acquisition of instantaneous channel state information (iCSI) at the transmitter [56]. As a result, spaceborne multi-user massive MIMO systems must strive to develop more cost-effective algorithms for computing precoding vectors. * **Hardware availability and readiness**: From a hardware standpoint, the RF performance of antennas and transceivers in multi-beam satellites is paramount as it influences key factors such as the EIRP (effective isotropic radiation power), frequency reuse, inter-beam interference, co-channel interference, adjacent channel interference, beam management, and more. At the satellite/ground station end, high-gain antennas like multi-beam reflector antennas and phased-array antennas are commonly utilized. For user terminals (UTs), the trend is towards adopting high-frequency (e.g., mmWave) phased-array antennas for high-throughput applications. The catalyst stems from the constant improvement of the antenna and integrated circuit designs, which makes high-efficiency RF front ends more available and affordable [57]. * **Artificial intelligence and cloud/edge computing**: In addition, certain emerging technologies have emerged as vital accelerators for the deployment of space broadband. In comparison to twenty years ago, AI and cloud/edge computing technologies have become instrumental in enabling high-performance satellite broadband connectivity and services. These services include beam-hopping, interference management, satellite traffic control, image processing, mapping, and computation offloading [58].