# Smart Beamforming for Direct LEO Satellite Access of Future IoT
## 2 Massive MIMO in LEO Satellites for Massive IoT  
To deal with a high number of communications, satellite systems typically tessellate the coverage area in multiple spotbeams by means of array antenna systems. This paves the way to handling users' traffic needs, since each beam is processed separately, which scales down the problem. In addition, in order to increase the spectral efficiency, high frequency reuse can be applied. In this case, MIMO precoding techniques must be implemented to handle the interbeam interference. In general, these techniques have been studied in a single feed per beam payload architecture [25]. However, the recent needs of flexible satellite systems, which are capable of accommodating varying capacity demand distributions, raised the interest of the industry in active antenna systems, where many feeds can be combined to synthesize arbitrary beam shapes. This interest is growing in parallel with the development of massive MIMO [26] technology at terrestrial wireless base stations in 5G.  
Motivated by the latest developments in 5G technologies, some works have proposed approaches to massive MIMO for satellite communications [27], where the key challenges to adopt massive MIMO in satellites are studied. The work in [28] further works in this direction and exploits massive MIMO when a large-scale active phased array antenna system is equipped at the LEO satellite side. We propose to further work in this direction and design adaptive beamformers for direct radiating arrays. This solution seems to adapt very well to the necessity of pointing specific areas by employing narrow beams, and thus, increase the directivity of the antennas and compensate the higher propagation losses in satellite mm-wave communications at Ka-band. Otherwise, note that, for satellites in LEO, the orbital dynamics make the adoption of conventional satellite precoding techniques (i.e., with fixed beams) very challenging.  
The present work studies the practicality of a smart beamforming technique to enable IoT access to a LEO satellite that is equipped with a digital beamforming phased array. The proposed setup departs from the hybrid beamforming architecture, where there is a fixed beamforming matrix. The proposed spatial processing provides the needed flexibility to steer the LEO reception pattern to any designed location. The idea is illustrated in Figure 1. The main feature of the proposed beamformer is its blind nature; thus, it does not need the acquiring channel state information at the receiver (CSIR) nor does it need to make a beam scan in the spatial domain. Hence, it is also an open-loop beamforming technique. In this way it overcomes the cumbersome implementation of precoding schemes requiring user feedback due to the adopted frequency division duplexing (FDD) in satellite communications. Furthermore, the proposed beamformer does not require any previous channel estimation, making it very suitable for IoT grant-free (GF) access, and bypassingthe impossibility to use time division duplexing (TDD) schemes in satellite systems [2]. In addition, the satellite line of sight channel simplifies the problem of direction of arrival user identification, which is intricate in terrestrial communications due to the ambiguity that multipath introduces. Finally, the implementation of the proposed smart beamforming, as directly weighting the output of each active antenna element, will allow the system to reduce the collision probability in massive IoT access.