# Smart Beamforming for Direct LEO Satellite Access of Future IoT
## 3 System Model  
Current wireless systems are not designed to support massive connectivity with a large number of devices. The conventional random access consists of four steps: (i) Random access preamble transmission, (ii) random access response, (iii) connection request message, and (iv) contention resolution phase. Then, resources are allocated to users so that subsequent communications occur on scheduled channels. However, in the context of massive connectivity, this procedure is inefficient. The handshake required in the access procedure to allocate resources to users involves a close-loop signaling and multiple exchange of messages. To improve the spectrum efficiency, the use of bulky procedures to establish the link should be avoided. Hence, typical assumptions are not applicable to satellite massive IoT scenarios. To overcome the issues related to the traditional 4-step procedure, we propose the adoption of GF access schemes. GF access is characterized to allow devices to transmit data without waiting for a grant. Therefore, the overhead and latency are reduced. However, the complexity is placed at the receive side, as the detector must be able to decode the data when users transmit on shared time and frequency resources. At the user side, the idea is to combine a reference sequence with the data into a single message. With this configuration, the format of the reference sequence will dictate the strategy followed by the receiver to avoid possible collisions. The proposed RSBA is based on GF access and its goal is to reduce the probability of collision by combining S-NOMA and spatial signal processing. For the scenario under study, the S-NOMA makes use of the repetition division multiple access (RDMA) to ease the beamforming design. This signature-based scheme employs different repetition patterns at the sample level to design device-specific signatures, providing time diversity. This feature is exploited by the detector that we are proposing so as to create blind beamformers pointing at the user of interest, thanks to the redundancy that RDMA presents. It is important to remark that RBSA is a GF access scheme that lies within the category of Aloha access protocols. Therefore, the proposed scheme is well suited for IoT scenarios where the  
Figure 1: Illustration of a satellite communication system based on narrow digital beams. terminals transmit without requesting permission from the base station. To formulate the system model, let \(x_{k}[n]\) be the baseband signal transmitted by the \(k\)-th terminal, namely,  
\[x_{k}[n]=\sum_{m=0}^{N_{\text{S}}-1}x_{k}^{m}[n-Nm], \tag{1}\]  
for \(k=1,\cdots,N_{\text{U}}\). Note that the transmitted signal relies on a packet format to transmit \(N_{\text{S}}\) symbols consecutively in a time division multiplexing fashion and with a symbol spacing of \(N\) samples. We assume that the symbols span \(M\) samples, i.e., \(x_{k}^{m}[n]\
eq 0\), for \(n\in\{0,\cdots,M-1\}\) and \(0\) otherwise. This is a sufficiently general model to represent different multicarrier schemes. We favor multicarrier in lieu of single-carrier to achieve a high degree of commonality with 5G. It must be emphasized that users are not frequency multiplexed, but they occupy the whole bandwidth. Interestingly, for \(N=M\), the notation can be used to model orthogonal frequency division multiplexing (OFDM) and single carrier frequency division multiplexing (SC-FDM) waveforms. For \(M>N\), overlapping is allowed. This case lies within the category of filter bank multicarrier (FBMC). To achieve a high user terminal power efficiency, which is of paramount importance to achieve long transmission ranges, it is desirable to avoid signals that exhibit peaks or sudden drops. In conclusion, low peak to average power ratio (PAPR) is a desirable feature. Special attention must be paid to discrete Fourier transform (DFT) spreading techniques to create single-carrier like signals [29; 30], which are natively more robust to non-linear effects.