Satellite Communications (SatCom) are systems and technologies that use satellites to relay information between points on Earth (and increasingly between satellites as well). The satellite acts as a repeater or node in a communication network, receiving signals on an uplink, processing or frequency-translating them, and transmitting them on a downlink—often to a different location or to many locations simultaneously. Basic principles • Links: o Uplink: Earth → satellite. o Downlink: satellite → Earth. o Inter-satellite link (ISL): satellite ↔ satellite (RF or optical). • Frequency bands: typically in the microwave and millimeter-wave range (L, S, C, X, Ku, Ka, Q/V bands), and increasingly optical (laser links). • Propagation: signals travel through the atmosphere (affected by rain, clouds, ionosphere) and vacuum; link budgets must account for free-space loss, atmospheric attenuation, antenna gains, noise, and interference. Orbits and architectures • GEO (Geostationary Earth Orbit): o Satellite appears fixed in the sky for a given ground location. o Ideal for broadcast (TV, radio), fixed services, and internet (e.g. HughesNet and Viasat). o High latency ( 2·height/c~250 ms). • MEO: o Intermediate altitude; used by some broadband and legacy systems. o Lower latency than GEO, fewer satellites than LEO. • LEO: o Low altitude, satellites move quickly across the sky. o Requires constellations of many satellites (thousands) and continuous handover (e.g. Starlink and in a future Kuiper) o Low latency and good link budgets, good for broadband and also for IoT. Architectures can be bent-pipe (transparent transponders) or regenerative: • Bent-pipe: satellite simply shifts frequency and amplifies; all routing and processing done on the ground (simpler payload). • Regenerative / processing satellites: demodulate, decode, switch/route, re-encode, and re-transmit; enables flexible resource allocation, onboard switching, and more advanced services. Multiple access and waveform aspects To share limited spectrum and satellite resources among many users, SatCom systems use: • Multiple access schemes: TDMA, FDMA, CDMA, OFDMA, NOMA, etc. • Advanced modulation and coding: QPSK, APSK, QAM, LDPC / Turbo / Polar codes, adaptive coding and modulation (ACM) to match link conditions. • Beamforming: fixed or electronically steered spot beams to reuse frequencies and increase capacity (high-throughput satellites, HTS). Applications • Broadcasting: o Direct-to-home (DTH) TV, radio, large-scale content distribution. • Fixed satellite services (FSS): o Corporate networks, government networks, backhaul for remote sites, trunk links for islands, etc. • Mobile satellite services (MSS): o Satellite phones, aero and maritime connectivity, land mobile terminals. o D2D direct to device (compatible with standard cellphones) • Broadband access: o Consumer and enterprise internet in underserved or remote regions. • Emergency and disaster communications: o Rapid deployment where terrestrial networks are damaged or absent. • IoT / M2M: o Low-data-rate connectivity for sensors, tracking devices, environmental monitoring, etc. Advantages and challenges Advantages: • Wide coverage: a single satellite (especially GEO or MEO) can cover huge areas, including oceans and remote regions. • Broadcast capability: inherently suited for point-to-multipoint services. • Infrastructure independence: no need to deploy extensive terrestrial infrastructure in the coverage area. • Resilience: can provide backup when terrestrial networks fail. • Broadband capabilities: especially for LEOs due to favorable link budget Challenges: • Latency: especially in GEO, affecting interactive applications and some protocols. • Doppler effect: depending on orbits and relative speeds • Capacity and spectrum scarcity: need efficient spectrum reuse and advanced techniques to increase capacity. That can be partly overcome by using optical links, specially between satellites and in future for feeder links • Propagation impairments: rain fade in Ku/Ka/Q/V bands, scintillation, RF interference. • Cost and complexity: satellite manufacturing/launch, regulatory coordination, gateway deployments. • Orchestration: complex and autonomous orchestration software (AI based) required for efficient operation From the [SA] Satellite Systems point of view, Satellite Communications is a specific class of satellite systems where: • The space segment is optimized for RF/optical relay (antennas, transponders, processors, ISLs). • The ground segment is essentially a telecommunication network infrastructure. • The user segment are all the terminals that provide connectivity to end-users or machines. Note: EO and NAV also make use of communication links (e.g. EO data downlinks, NAV signal broadcasting), but in [SC] Satellite Communications the communication itself is the primary mission, not just a support function.