Mobile satellite communications are gaining more and more interest in recent years, with SOS-via-satellite, location updating and satellite mobile messaging being important applications for customers. An efficient satellite waveform is the key technology to enable the mobile satellite communication service that will be provided by satellite constellations in the 5G and 6G era. In this paper, we conduct an experimental evaluation of the satellite waveform transmitted by the first satellite-enabled smartphone, the iPhone 14. We forced the device to transmit location data via L-band using the Find My application, while it had no other connectivity than the Globalstar satellites. The results show the temporal and spectral properties of the satellite waveform, and we performed a first estimate of the modulation scheme and data rate.

To meet the target requirements of 5G and 6G systems in terms of throughput and global connectivity, the upcoming Low Earth Orbit (LEO) mega-constellations are set to seamlessly integrate into terrestrial communication networks. Although extensively utilized in terrestrial systems, multi-user multiple-input multiple-output (MU-MIMO) transmission has not yet been implemented in LEO mega-constellation communication systems. In this work, we compare the throughput performance of a MU-MIMO downlink transmission and full frequency reuse (FFR) with the throughput of a multibeam LEO system with conventional four-color frequency reuse (FR4). We employ a deterministic modeling approach of the spacecraft motion, the satellite channel and the pre-generation of the downlink channel state information (CSI). To mitigate co-channel interference in the MIMO case, zero-forcing precoding is applied. We study the effects of an outdated CSI on the MIMO downlink performance. Simulation results confirm that even under the effects of an outdated CSI at the precoder, the MIMO LEO satellite system with FFR substantially outperforms the multibeam scenario with the FR4.

We investigate the throughput of a multi-user multiple-input multiple-output aviation network where multiple aircraft are connected to a High Throughput Satellite (HTS). We assume full frequency reuse in the downlink and a multibeam antenna architecture with multiple reflectors on the HTS. To maximize the system throughput, we apply zero forcing precoding along with a scheduling algorithm to group multiple aircraft that are simultaneously served with data via space division multiple access. For a more realistic scenario, the flight paths of the airplanes are based on real airline routes, and interference from terrestrial fixed service stations is considered. Our simulation results show that the proposed concept enables a massive throughput increase while the required update rate of the precoder coefficients and the user groups remain moderate.

In satellite communication downlinks, physical layer security is challenging to achieve due to their broadcasting nature and Line-of-Sight channel characteristics. This paper provides a precoding algorithm to secure the downlinks of multiple users against multiple eavesdroppers with optimization of the minimum secrecy capacity. By the use of artificial noise, a positive secrecy capacity is achievable even if the number of eavesdroppers is higher than the number of beams. We demonstrate that a multiple-reflector antenna design provides a significantly higher secrecy and throughput performance when compared to a single-reflector design due to additional degrees of freedom, exhibited by the signal phases. The total vulnerability region is introduced as a new figure of merit with respect to unidentified eavesdroppers.

Precoding for multibeam satellite systems with full frequency reuse in a multi-user multiple-input multiple-output (MU-MIMO) downlink scenario is addressed. A testbed is developed to perform an over-the-air field trial of zero forcing precoding for the spatial multiplexing of two separate video streams over two co-located geostationary satellites. Commercialoff-the-shelf DVB-S2x receivers in two single-antenna user terminals (UTs) are operated to successfully decode two independent video streams. To this end, particular attention is paid to a comprehensive assessment of the practical synchronization tasks of a precoding based MU-MIMO system. In particular, carrier frequency recovery and carrier phase synchronization in the most challenging multi-satellite scenario with different oscillators in the payloads is performed. An estimation method for channel state information (CSI), i.e. the complex channel coefficients, is also proposed and implemented. The successful video transmission finally constitutes the first field trial of MUMIMO precoding and proves the feasibility of precoding concepts for multibeam satellite systems.

We apply the concept of spatial multiple-input-multiple-output (MIMO) to co-located satellites and discuss its potential to further increase the capacity of a single geostationary slot. We show that the achievable capacity in fixed satellite services applications strongly depends on the co-location method. To obtain a constantly high MIMO capacity gain, the geometry between the satellites must be maintained, which requires the implementation of a coordinated station-keeping strategy. The impact of satellite positioning errors on the capacity due to inaccuracies of the orbit determination or the thruster system is investigated. Monte Carlo simulations involving two satellites show that, even for a conservative assumption on the satellite positioning accuracy, a capacity increase of at least 1.7 for more than 90% of all observations can be realized compared to a conventional single-input-single-output (SISO) satellite system.

If multiple-input multiple-output (MIMO) satellite communications (SATCOM) systems use spatial multiplexing instead of polarization multiplexing, the channel capacity depends on the geometrical conditions of the antenna setup. This theoretical result is proven and confirmed for the first time by a true-MIMO measurement campaign. We utilize two Ku-band satellites and a ground station with two antennas as a 2 × 2 MIMO SATCOM probing system. The channel capacity is estimated and compared to its theoretical prediction. Moreover, an error analysis is provided for the capacity estimation.

In contrast to present space communication systems in the Ultra-High-Frequency (UHF) band, like military communications or communication to space crafts, present and upcoming animal tracking systems introduce a novel UHF propagation channel with a larger bandwidth, low-gain antennas and new challenging propagation scenarios like for example the forest. The ground reflection, as a phenomenon of wave propagation, is relevant in this scenario. Unlike other multipath components with larger excess delays or lower power, the ground reflection cannot be equalized; even not at large system bandwidths. In this paper, the relevance of the ground reflection for animal tracking systems is proven by measurement and simulation. Further results from a measurement campaign are presented, where the forest as a novel scenario is included. A statistical model is provided to predict the received signal power, and to estimate the required additional fading margin to close the link of UHF animal tracking systems in the presence of a reflected signal from the ground.

Due to the steady increase of satellite networks in the geostationary arc, operators and governments must take measures to ensure that each network can operate without being interfered by adjacent satellite networks. Usually this is achieved by strict regulation of the allowed power spectral density within the shared operating spectrum. Unfortunately, limiting the allowed transmit power ceteris paribus manifests in limited data rates, especially if terminals with small apertures are used. In this paper we will show that the recently proposed Multiple Input-Multiple Output (MIMO) technology for communication satellite systems is a convenient concept to mitigate data rate limitations induced by adjacent satellite interference (ASI). Given a fixed data rate required by the end user, MIMO systems are superior over conventional Single Input - Single Output (SISO) systems because they work at less satellite transmit power and, therefore, enable more narrow satellite spacing or smaller ground terminal dish sizes without violating negotiated ASI limits.

A multiple-input multiple-output (MIMO) satellite communications system for fixed-satellite services (FSS) is considered. The MIMO Line-of-Sight (LOS) satellite channel is optimized to achieve maximum spectral efficiency. This requires an orthogonal MIMO channel matrix, ending up with large antenna spacing of the ground terminal antennas. As the main drawback, considerable propagation delays between the MIMO paths are observed, and the MIMO signal sub-streams interfere asynchronously in the order of several tens or hundreds of a symbol duration at the receiver. To cope with these delays, we propose a system architecture that uses a sufficiently large guard-interval or cyclic prefix between successive data blocks. The delay equalization at the receiver is then performed by a zero-forcing frequency domain equalizer for single-carrier transmission (SC-FDE). Thanks to the orthogonality of the considered MIMO channel, the SC-FDE architecture achieves even perfect spatial equalization of the multiplexed signal streams at considerably low implementation effort. The capacity reduction caused by the guard-interval is, moreover, mitigated by long data blocks, which can be used because of the long channel coherence times in FSS applications. The system concept reveals superior BER and transinformation performance, which is verified through fair comparison with current single-input single-output (SISO) systems. If the channel orthogonality is violated through narrow antenna spacing, the system performance degrades dramatically.

We investigate the impact of atmospheric impairments on the theoretical bandwidth efficiency of Multiple-Input Multiple-Output (MIMO) geostationary satellite links which are shaped to optimize the channel bandwidth efficiency. We analyze the impairments caused by precipitation, since this is the most severe atmospheric effect causing capacity degradations. By theory, the MIMO channel capacity is strongly affected by signal attenuation as well as signal phase shifts that might reduce the number and strength of spatial subchannels (eigenmodes). We will show, however, that the characteristics of the phase disturbances prevent a loss of capacity. Regarding the additional attenuation, which the signals may encounter passing through the troposphere, we will quantify outage values for several levels of link capacity degradation. Although a loss of capacity cannot be avoided in total, it still turns out that MIMO systems outperform conventional Single-Input Single-Output (SISO) designs in terms of reliability. Even in the presence of atmospheric perturbations, MIMO systems still provide enormous capacity gains and vast reliability improvements. Thus, the MIMO satellite systems presented are perfectly suited to establish the backbone network of future broadband wireless standards (e.g. DVB-SH), supporting high data rates for a variety of worldwide services.

MIMO satellite links have recently attracted a high interest with respect to possible link capacity enhancements. In it has been shown that especially in Line-of-Sight (LOS) satellite channels maximum multiplexing gain can be achieved via the construction of orthogonal channels by means of the geometrical arrangement of the ground terminal antennae in relation to the antennae in orbit. Based on these theoretical results, we present practically relevant design and configuration examples for satellite communication systems, involving transparent pay loads for the first time. Thus, we significantly extend the results, that have been limited to regenerative payloads. The examples cover multiple-satellite and single-satellite MIMO scenarios. The assets and drawbacks of the applications are investigated, especially highlighting system-inherent design uncertainties.

Different multiple input - multiple output (MIMO) channel models are analyzed with regard to the indoor line-of-sight (LOS) MIMO channel. In the literature this channel is proven to achieve high capacities while being highly correlated. The modeling results from physical as well as analytical models are compared with measured channels, showing that none of the established models is exactly suitable for the considered indoor LOS MIMO channel. However, there is a promising approach that applies an extended Saleh model in conjunction with the physically correct spherical wave propagation. Although in its current state this basic approach cannot be regarded as a complete MIMO channel model, at least it provides an adequate origin for the correct prediction of MIMO capacities in different types of LOS channels. This is due to the fact, that the crucial prerequisites for an appropriate LOS MIMO channel model, which are by name the application of the physically correct spherical wave propagation, the correct modeling of the high correlation in LOS channels and the inclusion of the geometrical antenna setup, are regarded correctly. Contrarily, these prerequisites are included in none of the current models that are widely accepted in MIMO radio communications nowadays. The proof of this statement using measured data is a major objective of this paper.