Smart Beamforming for Direct LEO Satellite Access of Future IoT

Abstract

Non-terrestrial networks (NTN) are expected to play a key role in extending and complementing terrestrial 5G networks in order to provide services to air, sea, and un-served or under-served areas. This paper focuses the attention on the uplink, where terminals are able to establish a direct link with the NTN at Ka-band. To reduce the collision probability when a large population of terminals is transmitting simultaneously, we propose a grant-free access scheme called resource sharing beamforming access (RSBA). We study RBSA for low Earth orbit (LEO) satellite communications with massive multiple-input multiple-output (MIMO). The idea is to benefit from the spatial diversity to decode multiple overlapped signals. We have devised a blind and open-loop beamforming technique, where neither the receiver must carry out brute-force search in azimuth and elevation, nor are the terminals required to report channel state information. Upon deriving the theoretical throughput, we show that RBSA is appropriate for grant-free access to LEO satellite, it reduces the probability of collision, and thus it increases the number of terminals that can access the media. Practical implementation aspects have been tackled, such as the estimation of the required statistics, and the determination of the number of users.

Keywords: LEO; beamforming; grant-free; massive IoT; massive MIMO; non-orthogonal multiple access; orthogonal frequency division multiplexing.

Figure 1

Illustration of a satellite communication system based on narrow digital beams.

Figure 2

Sequence structure for the RSBA scheme.

Figure 3

Satellite geometry.

Figure 4

Scheme of the beamformer design at base band.

Figure 5

Beampattern displayed in the $uv$-plane. The beamformer points at a terminal that is located at $(\theta ,\varphi )=(24.91{}^{\circ },7.89{}^{\circ })$ or, equivalently $u=0.4172,v=0.0578$. There are nine interfering users and three of these select the same pattern as the user of interest.

Figure 6

Throughput of RBSA for ${\theta }_{\mathrm{MAX}}=$ 44.44${}^{\circ }$.

Figure 7

MAPE versus $N_{\mathrm{U}}$ for different values of $N_{\mathrm{S}}$ and $N_x,N_y$.

Figure 8

SINR versus $N_{\mathrm{U}}$ for different values of $N_{\mathrm{S}}$ and $N_x,N_y$.

Figure 9

SINR versus $N_{\mathrm{U}}$ in fixed and digital beamforming schemes for $N_x=N_y=24$.

Figure 10

User positions and 4.3 dB contour of the beams generated by the proposed beamforming with a planar array of $24\times 24$ antenna elements.

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.