###### Abstract  
This paper provides an overview of recent results of a design, development and performance evaluation study of satellite gateways to receive and manage the traffic from a large population of uncoordinated user terminals. In particular, direct satellite access scenarios for machine-to-machine communications and the Internet of Things have been targeted. Tests were carried out in a representative laboratory environment emulating realistic system scenarios. Performance results, as presented in this paper indicate that the proposed gateway architecture, based on an efficient access protocol, is capable of managing a very high number of uncoordinated terminals transmitting short messages with a low duty cycle. The applicability of the proposed solution to both geostationary and non-geostationary satellite systems has also been examined. The key concept of the gateway is based on a novel receiver architecture that implements the linear minimum mean square error (MMSE) spread spectrum signal detection and successive interference cancellation techniques. The receiver uses features such as a multi-stage detector together with a robust preamble detection. The end-to-end solution includes also the use of a new waveform with a quasi-constant envelope at the terminal to modulate and transmit data packets to be received and detected by the gateway via a satellite link. CPU/GPU architectures; E-SSA protocol; ME-SSA; MMSE; random access; spread spectrum 202222256517 November 2021 8 January 202  
lower energy consumption and lower user terminal complexity, high flexibility in terms of required bandwidths and supported data rates, and asynchronous transmission of short messages with a low duty cycle. Other important design factors are minimum signaling on the forward link for terminals' population coordination to connect remote terminals to a central entity, e.g., smart grid, sensor networks or connected cars. The protocol known as Enhanced-Spread Spectrum ALOHA (E-SSA) [4] turns out to meet these requirements while providing a high spectral efficiency in terms of normalized aggregated throughput as highlighted in [3]. The E-SSA protocol has been adopted as an open standard, known as the S-band Mobile Interactive Multimedia (S-MIM) [5]. Some variations of the original E-SSA protocol have also been deployed by the satellite industry in other operating frequency bands (e.g., VHF, Ku and Ka bands). An example of such evolution is known as the Fixed Satellite Interactive Multimedia (F-SIM) protocol [6] used for uncoordinated return access of broadcast interactive systems [6]. The E-SSA protocol combines the Spread Spectrum ALOHA (SSA) protocol, used by terminals at the transmitter, with an enhanced receiver based on successive interference cancellation (SIC). In the baseline E-SSA approach [4], terminals transmit Binary Phase Shift Keying (BPSK) spread spectrum bursts sharing the same time/frequency resources in a random and uncoordinated manner. The overlapping bursts are subject to multiple access interference (MAI) at the receiver. In the case of a conventional SSA receiver, the impact of partially overlapping bursts is handled to some extent thanks to the properties of the spread spectrum signals. At a low traffic load (i.e., a low probability of burst collisions), the despreading operation allows for correctly demodulating the interfering received bursts. The conventional SSA receiver is highly sensitive to power imbalances across received bursts leading to performance degradation and the loss of throughput [4]. The SIC mechanism, as implemented as part of the E-SSA receiver, allows the system to decode multiple overlapping bursts even at high MAC load conditions, increasing the overall aggregated throughput. On the contrary to conventional SSA receiver, the power imbalance among the received bursts improves the performance of the SIC operation leading to a higher aggregated throughput. Essentially, the E-SSA receiver is based on a sliding window memory whose size is typically around five times the length of a burst.