# Smart Beamforming for Direct LEO Satellite Access of Future IoT
## 1 Introduction
In the downlink, multiple non-orthogonal transmission schemes can be adopted to outperform orthogonal schemes [7]. Similarly, in the uplink it is possible to simultaneously serve several users on shared time and frequency resources. Note that scheduled transmission schemes, which involve close-loop signaling and multiple exchange of messages, are not suitable for low-duty IoT traffic. For the sporadic uplink transmission of multiple devices, the random access scheme is more efficient. To separate the messages in uplink NOMA, one can rely on the ideas highlighted in [8; 9], where a comprehensive review of cellular and satellite access schemes are provided. Essentially, the concept is based on exploiting a codebook of signatures to facilitate user separation and detection. To exhibit a distinguishing feature, different operations, such as repetition, spreading, multi-dimensional modulation, interleaving, and scrambling can be employed. The sparse code multiple access (SCMA) [10] and the contention resolution diversity slotted Aloha (CRDSA) [11] are advanced access schemes that require time slot synchronization. Prominent access schemes that are able to operate in asynchronous mode include the asynchronous contention resolution diversity Aloha (ACRDA) [12], the enhanced spread spectrum Aloha (E-SSA) [13; 14], and the asynchronous flipped SCMA (AF-SCMA) [15]. An alternative approach that is referred to as resource sharing beamforming access (RSBA) has been proposed in [16]. The innovative idea of the RBSA scheme, which is based on [17], stems from combining spatial signal processing and signature-based NOMA (S-NOMA) to achieve angular resolution. Then, it follows that terminals located in different spots can be separated and thus, the probability of collision is reduced. This scheme is blind as it does not require transmitting an specific training sequence to estimate the channel nor scanning in both elevation and azimuth. In this respect, the proposed RSBA is different from the ones that exist in the literature for terrestrial mMTC access with spatial diversity. In [18; 19], the authors simplify the user detection problem by resorting to the channel hardening and favorable propagation properties of terrestrial massive multiple-input multiple-output(MIMO). However, this channel behavior is not present in satellite communications due to the strong line-of-sight (LoS) component. Another approach is that in [20], where, first, the channel must be estimated, and next, for each detected pilot sequence, a maximum ratio combination is applied to the data symbols. This procedure relies on averaging the pilot collision events across the transmission slots and is suitable for delay-tolerant and low-rate applications. In contrast, our approach aims at detecting the device with only one slot transmission. It is noteworthy that RBSA can be regarded as a variant of the slotted Aloha access protocol. Indeed, RBSA does not exclude the possibility of benefiting from the features of other access schemes, such as spreading or repetition. This observation highlights that there is room for improvement in terms of efficiency and robustness to time and frequency offsets. In this work, we do not intend to conduct a thorough comparison with existing access, but to show that RBSA builds a strong foundation to harness spatial diversity gains in the random access channel. We see the access schemes that do not exploit the spatial dimension, as complementary solutions to assist RBSA in detecting packets. However, enabling RBSA to use more complex transmission and detection schemes may not be a trivial task. Since RBSA exhibits remarkable features that are not sufficiently studied, in this paper we study RBSA in its basic form. This means that we follow the assumption of time synchronization. Allowing RBSA to relax synchronization requirements will reduce the complexity of the access scheme at transmission. The required modifications to make RBSA asynchronous are deferred to future works.