# Smart Beamforming for Direct LEO Satellite Access of Future IoT
## 6 Conclusions  
This paper presents a GF access scheme, which is referred to as RBSA. The proposed scheme is suitable for provisioning service to a large population of terminals that transmit simultaneously without requesting a grant. The RBSA, which was originally conceived for terrestrial communications, has been applied to a LEO satellite communications system with a regenerative satellite payload. It is considered that terminals are able to establish a direct link with the satellite at the Ka-band. In the scenario under study, the satellite is equipped with a uniform planar array that consists of a high number of elements. Hence, the system lies within the category of LEO satellite communications with massive MIMO. The merit of the RBSA is that it is able to exploit the spatial diversity to separate and distinguish users. The idea is to direct narrow digital beams towards the direction of users. The throughput analysis has revealed that the system can leverage on the angular discrimination to reduce the probability of collision, which allows us to increase the load of the network. To fulfil this objective we have designed a blind and open-loop beamformer. Hence, the satellite does not need to estimate the channel, request feedback from terminals, nor make a beam scan in azimuth and elevation. The work presented also takes into account practical implementation aspects, such as the determination of the number of users and the estimation of the covariance matrices. In addition, this work presents new findings about the beamforming design for terminals with the same repetition pattern. Namely, due to the use of a large scale array, these beamformers correspond with different eigenvectors; thus, no direction of arrival estimation is needed in this case to design separate beamformers. Numerical results show that the proposed beamformer design is able to achieve shapeable beams with angular resolution, which allows us to mitigate inter-user interference. As expected, the number of elements has an impact on the array gain and the interference mitigation capabilities. Another important result is that the overall performance depends on the accuracy of the sample-covariance matrices that are used to generate the beamformers. The better the accuracy, the higher the number of users that can access the media without experiencing significant SINR degradation. Therefore, high user density scenarios will require very accurate sample-covariance matrices to  
Figure 11: User positions and 4.3 dB contour of the beams generated by a fixed beamforming with a planar array of \(24\times 24\) antenna elements.  
decode overlapped signals. To understand the potential of the proposed beamforming technique, we carried out an assessment under ideal conditions, where covariance matrices are perfectly estimated. The most interesting finding is that the higher the number of repetition patterns \(P\), the lower the SINR degradation as the number of active users \(N_{\text{U}}\) increases. The numerical results also evidence that the digital beamforming clearly outperforms a beamforming network that points to a fixed grid.  
Conceptualization, M.C. and A.P.-N.; software, M.C.; validation, M.C. and A.P.-N.; investigation, M.C., A.P.-N. and E.M.; writing--original draft preparation, M.C., A.P.-N. and E.M.; writing--review and editing, M.C. and A.P.-N.; funding acquisition, A.P.-N. All authors have read and agreed to the published version of the manuscript.  
This work has received funding from the ministry of Science, Innovation, and Universities under project TERESATEC2017-90093-C3-1-R (AEI/FEDER,UE) and under grant RTI2018-099841-B-I00 and from the Catalan Government (2017-SGR-1479).  
Not applicable.  
The authors declare no conflict of interest.