Active Terminal Identification, Channel Estimation, and Signal Detection for Grant-Free NOMA-OTFS in LEO Satellite Internet-of-Things

This paper investigates the massive connectivity of low Earth orbit (LEO) satellite-based Internet-of-Things (IoT) for seamless global coverage. We propose to integrate the grant-free non-orthogonal multiple access (GF-NOMA) paradigm with the emerging orthogonal time frequency space (OTFS) modulation to accommodate the massive IoT access, and mitigate the long round-trip latency and severe Doppler effect of terrestrial-satellite links (TSLs). On this basis, we put forward a two-stage successive active terminal identification (ATI) and channel estimation (CE) scheme as well as a low-complexity multi-user signal detection (SD) method. Specifically, at the first stage, the proposed training sequence aided OTFS (TS-OTFS) data frame structure facilitates the joint ATI and coarse CE, whereby both the traffic sparsity of terrestrial IoT terminals and the sparse channel impulse response are leveraged for enhanced performance. Moreover, based on the single Doppler shift property for each TSL and sparsity of delay-Doppler domain channel, we develop a parametric approach to further refine the CE performance. Finally, a least square based parallel time domain SD method is developed to detect the OTFS signals with relatively low complexity. Simulation results demonstrate the superiority of the proposed methods over the state-of-the-art solutions in terms of ATI, CE, and SD performance confronted with the long round-trip latency and severe Doppler effect.Comment: 20 pages, 9 figures, accepted by IEEE Transactions on Wireless Communication

Smart Beamforming for Direct Access to 5G-NR User Equipment from LEO Satellite at Ka-Band

Study how spatial diversity can help in massive IoT and develp signal processing access for MIMO beamformingNon-Terrestrial Networks (NTN), in particular LEO Satellite Networks, 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 work proposes the implementation of a novel scheme called Resource Sharing Beamforming Access (RSBA), which seems a promising solution to deal with scenarios where Bit Error Rate (BER), probability of collision and/or achievable rate are important aspects of study. Given the system architecture presented in this work, RSBA will be proposed as solution in the 5G-NR Sat-IoT scenario. As it is expected, a huge amount of IoT devices will be transmitting in the uplink, and being the case of Non-Orthogonal-Multiple-Access (NOMA), the risk of collisions between devices will increase. The idea, after assessing the channel impairments of a direct link between a LEO Satellite and a NB-IoT device, it to study how spatial diversity via smart beamforming at the receiver will reduce the probability of collision between the devices, and thus increasing the number of users that can access to the media

Hummingbird: An Energy-Efficient GPS Receiver for Small Satellites

Global positioning system (GPS) is the most widely adopted localization technique for satellites in low earth orbits (LEOs). To enable many state-of-the-art applications on satellites, the exact position of the satellites is necessary. With the increasing demand for small satellites, the need for a low-power GPS for satellites is also increasing. However, building low-power GPS receivers for small satellites poses significant challenges, mainly due to the high speeds (similar to 7.8 km/s) of satellites and low available energy. While duty cycling the receiver is a possible solution, the high relative Doppler shift among the GPS satellites and the small satellite contributes to an increase in Time to First Fix (TTFF), which negatively impacts energy consumption. Further, if the satellite tumbles, the GPS receiver may not be able to receive signals properly from the GPS satellites, thus leading to an even longer TTFF. In the worst case, the situation may result in no GPS fix due to disorientation of the receiver antenna. In this work, we elucidate the design of a low-cost, low-power GPS receiver for small satellites. We also propose an energy optimization algorithm to improve the TTFF. With the extensive evaluation of our GPS receiver on an operational nanosatellite, we show that up to 96.16% of energy savings can be achieved using our algorithm without significantly compromising (similar to 10 m) the positioning accuracy

A Stochastic Geometry Approach to Doppler Characterization in a LEO Satellite Network

A Non-terrestrial Network (NTN) comprising Low Earth Orbit (LEO) satellites can enable connectivity to underserved areas, thus complementing existing telecom networks. The high-speed satellite motion poses several challenges at the physical layer such as large Doppler frequency shifts. In this paper, an analytical framework is developed for statistical characterization of Doppler shift in an NTN where LEO satellites provide communication services to terrestrial users. Using tools from stochastic geometry, the users within a cell are grouped into disjoint clusters to limit the differential Doppler across users. Under some simplifying assumptions, the cumulative distribution function (CDF) and the probability density function are derived for the Doppler shift magnitude at a random user within a cluster. The CDFs are also provided for the minimum and the maximum Doppler shift magnitude within a cluster. Leveraging the analytical results, the interplay between key system parameters such as the cluster size and satellite altitude is examined. Numerical results validate the insights obtained from the analysis.Comment: Accepted in IEEE International Conference on Communications (ICC) 202

NB-IoT via LEO satellites: An efficient resource allocation strategy for uplink data transmission

In this paper, we focus on the use of Low-Eart Orbit (LEO) satellites providing the Narrowband Internet of Things (NB-IoT) connectivity to the on-ground user equipment (UEs). Conventional resource allocation algorithms for the NBIoT systems are particularly designed for terrestrial infrastructures, where devices are under the coverage of a specific base station and the whole system varies very slowly in time. The existing methods in the literature cannot be applied over LEO satellite-based NB-IoT systems for several reasons. First, with the movement of the LEO satellite, the corresponding channel parameters for each user will quickly change over time. Delaying the scheduling of a certain user would result in a resource allocation based on outdated parameters. Second, the differential Doppler shift, which is a typical impairment in communications over LEO, directly depends on the relative distance among users. Scheduling at the same radio frame users that overcome a certain distance would violate the differential Doppler limit supported by the NB-IoT standard. Third, the propagation delay over a LEO satellite channel is around 4-16 times higher compared to a terrestrial system, imposing the need for message exchange minimization between the users and the base station. In this work, we propose a novel uplink resource allocation strategy that jointly incorporates the new design considerations previously mentioned together with the distinct channel conditions, satellite coverage times and data demands of various users on Earth. The novel methodology proposed in this paper can act as a framework for future works in the field.Comment: Tis work has been submitted to the IEEE IoT Journal for possible publication. Copyright may be transferred without notice, after which this version may no longer be accessibl

Energy-Efficient Design of Satellite-Terrestrial Computing in 6G Wireless Networks

In this paper, we investigate the issue of satellite-terrestrial computing in the sixth generation (6G) wireless networks, where multiple terrestrial base stations (BSs) and low earth orbit (LEO) satellites collaboratively provide edge computing services to ground user equipments (GUEs) and space user equipments (SUEs) over the world. In particular, we design a complete process of satellite-terrestrial computing in terms of communication and computing according to the characteristics of 6G wireless networks. In order to minimize the weighted total energy consumption while ensuring delay requirements of computing tasks, an energy-efficient satellite-terrestrial computing algorithm is put forward by jointly optimizing offloading selection, beamforming design and resource allocation. Finally, both theoretical analysis and simulation results confirm fast convergence and superior performance of the proposed algorithm for satellite-terrestrial computing in 6G wireless networks

Doppler shift compensation strategies for LEO satellite communication systems

Development of signal processing for communication in NGEO systems in order to counteract the high doppler component.Broadband connectivity only covers one third of the earth's surface. LEO satellite communication systems can be a solution to extend and complement terrestrial networks. Nevertheless, mobile ground-terminals receive high Doppler shift over the satellite channel, due the relative motion between the satellite and the mobile terminal. In this project is analysed the Doppler shift over the satellite channel observed by a mobile terminal. The analysis is done implementing a Matlab Orbital simulator transmitting an OFDM signal. Also is proposed a Doppler shift compensation strategy for LEO communications systems with the implementation of an ML Doppler estimator in the OFDM receiver. Several results are presented to evaluate the Doppler observed by the terminals in different positions as well as the BER for different SNR and Doppler shift.La conectividad de banda ancha actualmente sólo cubre un tercio de la superficie terrestre. Los sistemas de comunicación por satélite LEO pueden ser una solución a tener en cuenta para ampliar y complementar las redes terrestres. Sin embargo, los terminales terrestres móviles reciben un cambio de la frecuencia elevado (efecto Doppler) sobre el canal satélite debido a las velocidades de movimiento relativas entre el satélite y el terminal móvil. En este proyecto se estudia el efecto Doppler sobre el canal satélite observado por un terminal móvil. El análisis se hace implementando un simulador Orbital Matlab que transmite una señal OFDM. También se propone una estrategia de compensación Doppler para los sistemas de comunicaciones LEO con la implementación de un estimador Doppler ML en el receptor OFDM. Se presentan varios resultados en términos de Doppler observado por los terminales en diferentes posiciones, así como la BER para diferentes SNR y Doppler recibido por los terminales.La connectivitat de banda ampla actualment només cobreix un terç de la superfície terrestre. Els sistemes de comunicació per satèl·lit LEO poden ser una solució a tenir en compte per ampliar i complementar les xarxes terrestres. No obstant això, els terminals terrestres mòbils reben un canvi de la freqüència elevat sobre el canal satèl·lit a causa de les velocitats de moviment relatives entre el satèl·lit i el terminal mòbil. En aquest projecte s'analitza el efecte Doppler sobre el canal satèl·lit observat per un terminal mòbil. L'anàlisi es fa implementant un simulador Orbital Matlab que transmet un senyal OFDM. També es proposa una estratègia de compensació Doppler per als sistemes de comunicacions LEO amb la implementació d'un estimador Doppler ML en el receptor OFDM. Es presenten diversos resultats en termes de Doppler observat per els terminals en diferents posicions, així com la BER per a diferents SNR i Doppler rebut per els terminals

Data Communication With A Nano-satellite Using Satellite Personal Communication Networks (s-pcns)

Satellites typically communicate with locations on the ground to receive commands and send data back. Establishing reliable communications generally requires dedicated ground stations, which in turn require hardware and expertise. Developers of nano-satellites, however, may not have the expertise or resources necessary for establishing a dedicated ground station. Therefore, the use of an existing communication system, such as the Satellite Personal Communication Networks (S-PCNs), is attractive. Another shortcoming of the fixed ground stations, already available, is that they are normally only able to communicate with Low Earth Orbit (LEO) nano-satellites four times per day (two10-minute windows separated by 90 minutes, followed 12 hours later by two more such 10-minute windows). This drawback is also overcome by the use of S-PCNs which provide increased access times, smaller gaps in contact between the satellites and ground stations, and easier tracking of satellite health. In this thesis, the capabilities of S-PCNs for communications with a nano-satellite are explored. Software simulation and analysis have been performed to assess system performance. Ground testing of the hardware is done to understand the use of such systems for small satellites