Chapter 2
Satellite Connectivity
Satellite communications is emerging as an important enabling technology for ubiquitous connectivity. There are currently more than 2,000 communication satellites in orbit around the Earth serving applications such as TV and radio broadcasting, navigation, telemetry, imaging, and remote sensing. Communication satellites usually fall under one of three orbital classifications: geostationary (GEO), middle Earth orbit (MEO), and low Earth orbit (LEO).
Use of LEO satellite constellations for wireless connectivity is an emerging trend. With an orbital altitude ranging from 160 to 1000 km above the Earth’s surface, these systems are slated to provide high-speed internet connectivity no matter where you are on Earth.
Several challenges are inherent to the designs of high-speed satellite communication links.
Scenario Modeling
An important requirement is the design, analysis, and visualization of satellite orbits. Unlike GEO satellites, MEO and LEO satellites do not remain visible in the sky to a fixed point on the Earth continually, so continuous communication requires a large constellation of fast-moving satellites. This approach necessitates handoffs of ground station links from one satellite to the next as they become visible on the horizon. Often multiple hops between multiple satellites are required to provide connectivity between two ground stations.
Orbit Propagation and Visualization
In order to study motion of satellites, it is also important to have accurate orbit propagators. The simplest orbit propagator is the Two-Body-Keplerian propagator, which is based on the relative two-body model that assumes a spherical gravity field for the Earth. For more accurate orbit propagation, there are several propagators, including the Simplified General Perturbations (SGP4) propagator, that model various effects such as the variations in the orbit due to Earth oblateness, solar and lunar gravitational effects, and orbital decay using a simple drag model.
Link Budget Analysis
Transmissions over long distances require calculation of the transmit and receive powers and the necessary margins while accounting for the various losses suffered by the link. These calculations often require complex propagation loss models and channel models. One such propagation loss model is the International Telecommunications Union (ITU) P.618 model, which is targeted toward the design of Earth-space telecommunication systems.
Mobility and Doppler Analysis
Since LEO satellites are fast-moving objects at substantial distances from the Earth, it is also important to study the effects of latency and Doppler distortion on the transmitted signals.
Waveform Generation and Link-Level Simulations
It is essential to design waveforms, transmitters, and receivers that can compensate for the large latency and Doppler shifts suffered by satellite links. Satellite communications standards include DVB-S, DVB-S2X, CCSDS, and GPS. The latest standard for satellite links is developed by the 3GPP standards body as the specifications for 5G NR non-terrestrial networks (NTN). In order to study the performance of these systems, it is essential to have easy access to the waveforms, satellite-specific channel models (including land mobile channel models), and reference receiver designs.
