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

6G Enabled Advanced Transportation Systems

The 6th generation (6G) wireless communication network is envisaged to be able to change our lives drastically, including transportation. In this paper, two ways of interactions between 6G communication networks and transportation are introduced. With the new usage scenarios and capabilities 6G is going to support, passengers on all sorts of transportation systems will be able to get data more easily, even in the most remote areas on the planet. The quality of communication will also be improved significantly, thanks to the advanced capabilities of 6G. On top of providing seamless and ubiquitous connectivity to all forms of transportation, 6G will also transform the transportation systems to make them more intelligent, more efficient, and safer. Based on the latest research and standardization progresses, technical analysis on how 6G can empower advanced transportation systems are provided, as well as challenges and insights for a possible road ahead.Comment: Submitted to an open access journa

Joint Satellite-Transmitter and Ground-Receiver Digital Pre-Distortion for Active Phased Arrays in LEO Satellite Communications

A novel joint satellite-transmitter and ground-receiver (JSG) digital pre-distortion (DPD) (JSG-DPD) technique is proposed to improve the linearity and power efficiency of the space-borne active phased arrays (APAs) in low Earth orbit (LEO) satellite communications. Different from the conventional DPD technique that requires a complex RF feedback loop, the DPD coefficients based on a generalized memory polynomial (GMP) model are extracted at the ground-receiver and then transmitted to the digital baseband front-end of the LEO satellite-transmitter via a satellite–ground bi-directional transmission link. The issue of the additive white Gaussian noise (AWGN) of the satellite–ground channel affecting the extraction of DPD coefficients is tackled using a superimposing training sequences (STS) method. The proposed technique has been experimentally verified using a 28 GHz phased array. The performance improvements in terms of error vector amplitude (EVM) and adjacent channel power ratio (ACPR) are 7.5% and 3.6 dB, respectively. Requiring limited space-borne resources, this technique offers a promising solution to achieve APA DPD for LEO satellite communications

Recent Results from the Goldstone Orbital Debris Radar: 2016-2017

Since 1993, the NASA Orbital Debris Program Office has used the Goldstone Orbital Debris Radar (Goldstone) to sample statistically the orbital debris environment. Due to the sensitivity of this radar, which can detect an approximately 3 mm-diameter conducting sphere at 1,000 km, it has filled an important role in the characterization of the sub-centimeter-sized orbital debris population. Through the years, the capabilities of this system have increased recent updates include increased receiver bandwidth and a change in the bi-static observation geometry both of which enhance the radars ability to estimate orbital parameters. In 2016, dual polarization capability was added, making this the first year where both right- and left-hand circularly polarized information was available from this sensor. This additional polarization information may enable better characterization of sub-centimeter-sized particles in low Earth orbit, particularly since the receiver triggers on reflected energy from both left- and right-handed circular polarizations independently. In this paper, we present measurements and results derived from data taken during the calendar years (CY) 2016-2017 by Goldstone and compare this dataset to measurements taken by the Haystack Ultra-wideband Satellite Imaging Radar (HUSIR) during a similar timeframe

LEO Satellite-Enabled Grant-Free Random Access with MIMO-OTFS

This paper investigates joint channel estimation and device activity detection in the LEO satellite-enabled grant-free random access systems with large differential delay and Doppler shift. In addition, the multiple-input multiple-output (MIMO) with orthogonal time-frequency space modulation (OTFS) is utilized to combat the dynamics of the terrestrial-satellite link. To simplify the computation process, we estimate the channel tensor in parallel along the delay dimension. Then, the deep learning and expectation-maximization approach are integrated into the generalized approximate message passing with cross-correlation--based Gaussian prior to capture the channel sparsity in the delay-Doppler-angle domain and learn the hyperparameters. Finally, active devices are detected by computing energy of the estimated channel. Simulation results demonstrate that the proposed algorithms outperform conventional methods.Comment: This paper has been accepted for presentation at the IEEE GLOBECOM 2022. arXiv admin note: text overlap with arXiv:2202.1305

An Efficient Beam Steerable Antenna Array Concept for Airborne Applications

Deployment of a satellite borne, steerable antenna array with higher directivity and gain in Low Earth Orbit makes sense to reduce ground station complexity and cost, while still maintaining a reasonable link budget. The implementation comprises a digitally beam steerable phased array antenna integrated with a complete system, comprising the antenna, hosting platform, ground station, and aircraft based satellite emulator to facilitate convenient aircraft based testing of the antenna array and ground-space communication link. This paper describes the design, development and initial successful interim testing of the various subsystems. A two element prototype used in this increases the signal-to-noise ratio (SNR) by 3 dB which is corresponding to more than 10 times better bit error rate (BER)

Space MIMO: Direct Unmodified Handheld to Multi-Satellite Communication

This paper examines the uplink transmission of a single-antenna handsheld user to a cluster of satellites, with a focus on utilizing the inter-satellite links to enable cooperative signal detection. Two cases are studied: one with full CSI and the other with partial CSI between satellites. The two cases are compared in terms of capacity, overhead, and bit error rate. Additionally, the impact of channel estimation error is analyzed in both designs, and robust detection techniques are proposed to handle channel uncertainty up to a certain level. The performance of each case is demonstrated, and a comparison is made with conventional satellite communication schemes where only one satellite can connect to a user. The results of our study reveal that the proposed constellation with a total of 3168 satellites in orbit can enable a capacity of 800 Mbits/sec through cooperation of $12$ satellites with and occupied bandwidth of 500 MHz. In contrast, conventional satellite communication approaches with the same system parameters yield a significantly lower capacity of less than 150 Mbits/sec for the nearest satellite

Survey of Inter-satellite Communication for Small Satellite Systems: Physical Layer to Network Layer View

Small satellite systems enable whole new class of missions for navigation, communications, remote sensing and scientific research for both civilian and military purposes. As individual spacecraft are limited by the size, mass and power constraints, mass-produced small satellites in large constellations or clusters could be useful in many science missions such as gravity mapping, tracking of forest fires, finding water resources, etc. Constellation of satellites provide improved spatial and temporal resolution of the target. Small satellite constellations contribute innovative applications by replacing a single asset with several very capable spacecraft which opens the door to new applications. With increasing levels of autonomy, there will be a need for remote communication networks to enable communication between spacecraft. These space based networks will need to configure and maintain dynamic routes, manage intermediate nodes, and reconfigure themselves to achieve mission objectives. Hence, inter-satellite communication is a key aspect when satellites fly in formation. In this paper, we present the various researches being conducted in the small satellite community for implementing inter-satellite communications based on the Open System Interconnection (OSI) model. This paper also reviews the various design parameters applicable to the first three layers of the OSI model, i.e., physical, data link and network layer. Based on the survey, we also present a comprehensive list of design parameters useful for achieving inter-satellite communications for multiple small satellite missions. Specific topics include proposed solutions for some of the challenges faced by small satellite systems, enabling operations using a network of small satellites, and some examples of small satellite missions involving formation flying aspects.Comment: 51 pages, 21 Figures, 11 Tables, accepted in IEEE Communications Surveys and Tutorial

Opportunistic Navigation with Iridium Next LEO Satellites

The concept of opportunistic navigation arises from the future demands that autonomous vehicles will require in order to navigate in a reliable and accurate way in GNSS-challenged environments. Specifically, GNSS signals are not robust enough against intentional jamming attacks and they are unencrypted, making them accessible by hackers and completely spoofable. Some alternatives that have been identified as timely sources of positioning are signals of opportunity, which cluster a broad spectrum including broadband LEO (Low Earth Orbits) satellite signals, AM/FM radio signals, Wi-Fi signals and even cellular LTE/4G signals, and which can be exploited for navigation although they were not transmitted for this purpose. Particularly, LEO satellite signals have inherent attributes that make them even more desirable for opportunistic navigation. First, their received signal power is around 30dB higher than GNSS signals since they are located approximately twenty times closer to the Earth’s surface. Second, they will be abundant in the following years since private companies are planning to aggregately launch thousands of broadband Internet satellites into LEO. Third, they will be diverse in frequency and direction since each broadband provider will deploy its satellites into unique constellations. Unfortunately, there are several challenges with using LEO satellite signals for navigation as it is discussed throughout this document. For instance, there is a need of having specifically designed receivers that can extract navigation observables from LEO satellites and, furthermore, the internal clocks of LEO satellites are not as precisely synchronized as GNSS satellite clocks, requiring the receiver to account for extra timing shifts. In this way, the present thesis addresses the problem of navigating opportunistically with Iridium Next LEO satellite signals by proposing a complete receiver architecture that allows to make Doppler measurements to satellite signals in order to obtain a PNT (Positioning, Navigation and Timing) solution.Outgoin

Channel Estimation for LEO Satellite Massive MIMO OFDM Communications

In this paper, we investigate the massive multiple-input multiple-output orthogonal frequency division multiplexing channel estimation for low-earth-orbit satellite communication systems. First, we use the angle-delay domain channel to characterize the space-frequency domain channel. Then, we show that the asymptotic minimum mean square error (MMSE) of the channel estimation can be minimized if the array response vectors of the user terminals (UTs) that use the same pilot are orthogonal. Inspired by this, we design an efficient graph-based pilot allocation strategy to enhance the channel estimation performance. In addition, we devise a novel two-stage channel estimation (TSCE) approach, in which the received signals at the satellite are manipulated with per-subcarrier space domain processing followed by per-user frequency domain processing. Moreover, the space domain processing of each UT is shown to be identical for all the subcarriers, and an asymptotically optimal vector for the per-subcarrier space domain linear processing is derived. The frequency domain processing can be efficiently implemented by means of the fast Toeplitz system solver. Simulation results show that the proposed TSCE approach can achieve a near performance to the MMSE estimation with much lower complexity.Comment: accepted by IEEE Transactions on Wireless Communication