A satellite is an artificial or man-made object that revolves around Earth. Satellites are relay stations in space for the transmission of voice, video and data communications. They are ideally suited to meet the global communications requirements of military, government and commercial organizations because they provide economical, scalable and highly reliable transmission services that easily reach multiple sites over vast geographic areas.
Satellite communication offers a number of advantages over traditional terrestrial point-to-point networks. Satellite networks can cover wide geographic areas and can interconnect remote terrestrial networks (“islands”). In case of damaged terrestrial networks, satellite links provide an alternative. Satellites have a natural broadcast capability and thus facilitate multicast communication. Finally, satellite links can provide bandwidth on demand by using Demand Assignment Multiple Access (DAMA) techniques.
Satellite communications involve four steps: An uplink Earth station or other ground equipment transmits the desired signal to the satellite; The satellite amplifies the incoming signal and changes the frequency; The satellite transmits the signal back to Earth, and The ground equipment receives the signal.
A satellite contains multiple channels, called transponders, that provide bandwidth and power over designated radio frequencies. The transponder’s bandwidth and power dictate how much information can be transmitted through the transponder and how big the ground equipment must be to receive the signal. In addition, the satellite’s antennas direct the signal over a specific geographic area.
Commercial satellite communications services are grouped into three general categories:
- Fixed Satellite Services (FSS), which use ground equipment at set locations to receive and transmit satellite signals. FSS satellites support the majority of our domestic and international services, from international internet connectivity to private business networks.
- Mobile Satellite Services (MSS), which use a variety of transportable receiver and transmitter equipment to provide communication services for land mobile, maritime and aeronautical customers
- Broadcast Satellite Services (BSS), which offer high transmission power for reception using very small ground equipment. BSS is best known for direct-to-consumer television and broadband applications such as DIRECTV.
Satellite Network Components
A satellite network (also called a satcom network) comprises a set of satellite terminals, one or more gateways and one NCC that is operated by one operator and uses a subset of the satellite resources (or capacity).
The ground segment is composed of a user segment and a control and management segment. In the user segment, one finds satellite terminals(ST) connected to the end-user customer premises equipment (CPE), directly or through a LAN and hub or gateway stations (GW)
— CPE are also called user terminals (UT) and they include equipment such as telephone sets, television sets and personal computers. User terminals are independent of network technology and can be used for terrestrial as well as satellite networks.
— Satellite terminals are earth stations connected to CPE, sending carriers to or receiving carriers from a satellite. They constitute the satellite access points of a network
— The gateway earth station (GW) provides internetworking functions between the satellite network and the Internet or a terrestrial network.
The control and management segment consists of:
— a mission and network management centre (MNMC) in charge of non-real-time, high-level management functions for all the satellite networks that are deployed in the coverage of a satellite.
— network management centres (NMC), also called interactive network management centres (INMC), for non-real-time management functions related to a single satellite network.
— network control centres (NCC) for real-time control of the connections and associated resources allocated to terminals that constitute one satellite network.
(ITU-R Rec. S.522) stipulates that the bit error rate (BER) must not exceed:
—one part in 10 exp(6) , 10-minute mean value for more than 20% of any month;
—one part in 10 exp (4) , 1-minute mean value for more than 0.3% of any month;
—one part in 10 exp (3), 1-second mean value for more than 0.05% of any month.
(ITU-R Rec. S.614) stipulates that the bit error rate for satellite transmission of data at 64 kbit/s at a frequency below 15 GHz on a link which is part of an integrated services digital network (ISDN) ust not exceed:
—10 exp (-7) during more than 10% of any month;
—10 exp (-6) during more than 2% of any month;
—10 exp (-3 ) during more than 0.03% of any month.
Availability is the fraction of time during which a service conforming to the specifications is provided. It is affected by both equipment breakdown and propagation phenomena.
(ITU-R Rec. S.579) stipulates that the unavailability for telephony must not exceed:
—0.2% of a year in the case of breakdown (interruption of the service must be less than 18 hours per year);
—0.2% of any month if the service interruption is due to propagation.
Delay builds up from sending user terminal to destination user terminal on account of:
—delay in terrestrial network, if any;
—propagation delay over satellite links;
—baseband-signal processing time;
Satellite networks are characterized by their topology (meshed, star or multi-star), the types of link they support and the connectivity they offer between the earth stations.
In a star network, each node can communicate only with a single central node, often called the hub. In a multi-star topology, several central nodes (hubs) are identified. The other nodes can communicate only with those central nodes. A meshed satellite network consists of a set of earth stations that can communicate one with another by means of satellite links consisting of radio-frequency carriers.
Generally, the hub is a large earth station (antenna size from a few meters to more than 10 m) with higher EIRP and G/T than the other earth stations in the network. A star network topology places fewer constraints on the EIRP and G/T of the earth stations than a meshed network topology relying on a transparent satellite, due to the fact that the earth stations communicate with a large earth station (the hub). This architecture is popular among networks populated with small earth stations (antenna size of about 1 m) called very small aperture terminals (VSAT). The link from any earth station to the hub is called an inbound link or return link. The link from the hub to the other earth stations is called the outbound link or forward link.
There are different kinds of satellite networks designed for various tasks. Mobile satellite telecommunications services use two types of satellite network – Low Earth Orbit (LEO) and Geostationary (GEO).
Geostationary (GEO) Satellite Networks
Geostationary satellite networks utilize a smaller number of satellites, and each satellite provides satellite coverage to a fixed area of the Earth. Geostationary satellites are generally located above the equator and follow the Earth’s orbit which means each Geostationary satellite stays in the same place relative to the Earth’s surface. Geostationary satellites are generally larger and require more power than LEO satellites as they need to cover a much larger area of the Earth’s surface.
Geostationary satellites generally have superior data bandwidth, but since the satellites are around 36,000 km above the Earth will experience greater latency or voice delay than LEO satellites. When using Geostationary satellite phones and terminals in a mobile environment it is preferable to have a high elevation angle, so these terminals work better closer to the center of the coverage area.
Low Earth Orbit (LEO) Satellite Networks
Low Earth Orbit (LEO) satellite networks are made up of a constellation of small satellites that orbit the Earth in a series of planes. In each plane, several satellites follow each other as they orbit the Earth, and the planes run parallel to each other to provide the radio coverage that is used for the services. The orbiting pattern of LEO satellite networks means that the individual satellites in the constellation are continually moving relative to the Earth’s surface.
From most locations, it is possible to see one or more LEO satellites at any time. Sometimes they may be blocked by physical barriers such as buildings or mountains that block the signal path to the satellite, however, because LEO satellites are always moving the satellites may move out from behind the physical barrier after a few seconds or minutes providing a clear radio path between the user terminal and the satellite.
LEO satellite networks are well suited to mobile applications, where you need to use the service while on the move. External antennas used on vehicles, vessels, and aircraft are generally smaller and relatively inexpensive on LEO satellite networks compared to Geostationary satellite antennas. LEO satellites are also much closer to the Earth (800-1,400 km) than Geostationary satellites (approximately 36,000 km), so latency or voice delay in the network can be considerably less. Pivotel products that use LEO satellite networks include all Iridium, inReach, SPOT and Globalstar satellite solutions.
Multibeam satellite Systems
When a communication link is established through a satellite network, two levels of connectivity need to be distinguished: the connectivity required at the service level and the connectivity required on board the satellite. Internet access service is characterised by a star or multi-star topology with multipoint-to-point connectivity: customer traffic goes necessarily through a point of presence (POP) and the CPE is connected to the nearest POP of the user’s ISP. A virtual private network (VPN) service is characterised by a meshed topology with point-to-point connectivity (multipoint to multipoint connectivity can also be requested for VPN multicast services). This service allows for interconnecting different LANs of a company to form a single LAN.
Satellite onboard connectivity defines how the satellite network resources are switched on board in order to meet the service-level connectivity requirements. It, therefore, depends on how the satellite resources (beams, channels, carriers, etc.) are organised on both satellite up- and downlinks and, primarily, on the type of coverage that the satellite system provides. In the case of global coverage, any user within the coverage can, in principle be connected to any other user.
In multi-beam coverage, interconnection of any user within different beams of the coverage requires on-board interconnection of beams and of the resources allocated to those beams. The multibeam satellite systems make it possible to reduce the size of earth stations and hence the cost of the earth segment. Frequency re-use from one beam to another permits an increase in capacity without increasing the bandwidth allocated to the system.
A satellite payload using multibeam coverage must be in a position to interconnect all network earth stations and consequently must provide interconnection of coverage areas. Depending on the on-board processing capability and the network layer, different techniques are considered for interconnection of coverage:
—transponder hopping (used when there is no on-board processing): Beam switching by transponder hopping is a solution when the number of beams is low. Because the number of transponders increases at least as the square of the number of beams, with a large number of beams the satellite payload becomes too complex and too heavy.
—on-board switching (used when there is transparent and regenerative processing):The payload in analogue transparent switching includes a programmable switching matrix having a number of inputs and outputs equal to the number of beams. This matrix connects each uplink beam to each downlink beam by way of a receiver and a transmitter. When the period of time separating two connection states is a frame, since interconnection between two beams is cyclic, stations must store traffic from users and transmit it in the form of bursts when the required interconnection between beams is realised. This technique, called satellite switched time division multiple access (SS-TDMA) can thus be used in practice only with digital transmission and access of the TDMA type. The
—beam scanning. Each coverage area is illuminated cyclically by an antenna beam whose orientation is controlled by a beam-forming network which is part of the antenna subsystem on board the satellite. In the absence of on-board storage, at least two beams are necessary at a given instant—one to establish the uplink and one to establish the downlink An inherent advantage of this type of system is the disappearance of fixed simultaneous beams and hence of co-channel interference (CCI).
A protocol is the rules and conventions used in conversation by agreement between communicating parties. Basic protocol functions include segmentation and reassembly, encapsulation, connection control, ordered delivery, flow control, error control, routing and multiplexing. Protocols are needed to enable parties to understand each other and make sense of received information. A protocol stack is a list of protocols (one protocol per layer). A network protocol architecture is a set of layers and protocols.
The layering principle is an important concept for network protocols and reference models. In the 1980s, the International Organisation for Standardisation (ISO) derived the seven layer reference model. One major trend in any telecommunications network is to move towards IP network technologies. Satellite networks are following the same trend. As with all other communications protocols, TCP/IP is composed of different layers.
The IP network layer is based on a datagram approach, providing only best-effort service, i.e. without any guarantee of quality of service (QoS). IP is responsible for moving packets of data from router to router according to a four-byte destination IP address (in the IPv4 mode) until the packets reach their destination. Management and assignment of IP addresses is the responsibility of the Internet authorities.
The transmission control protocol (TCP) and user datagram protocol (UDP) are transport layer protocols of the Internet protocol reference model. They originate at the end-points of bidirectional communication flows, allowing for end-user terminal services and applications to send and receive data across the Internet.
TCP is responsible for verifying the correct delivery of data between client and server. Data can be lost in the intermediate network. TCP adds support to detect errors or lost data and retransmit them until the data is correctly and completely received. Therefore TCP provides a reliable service though the network underneath may be unreliable, i.e., operation of Internet protocols does not require reliable transmission of packets, but reliable transmission can reduce the number of retransmissions and thus improve performance.
UDP provides a best-effort service as it does not attempt to recover any error or packet loss. Therefore, it is a protocol providing unreliable transport of user data. But this can be very useful for real-time applications, as re-transmission of any packet may cause more problems than losing packets.
The main issue affecting the performance of TCP/IP over satellite links is very large feedback delay compared to terrestrial links. The inherent congestion control mechanism of TCP causes source data rate to reduce rapidly to very low levels with even a few packet loss in a window of data. The increase in data rate is controlled by ACKs received by the
source. Large feedback delay implies a proportional delay in using the satellite link efficiently again. Consequently, a number of TCP enhancements (NewReno, SACK) have been proposed that avoid multiple reductions in source data rate when only a few packets are lost.The enhancements in end-to-end TCP protocol are called End System Policies.
The application layer protocols are designed as functions of user terminals or servers. The classic Internet application layer protocols include HTTP for the Web, FTP for file transfer, SMTP for email, Telnet for remote login, DNS for domain name services
Satellite network trends
The satellite communication industry is going through an enormous transformation. Next generation satellite technology is evolving to multi-orbit constellations that include Non-Geostationary Orbit (NGSO) constellations, Very High Throughput Satellites (VHTS), as well as the traditional Geosynchronous Earth Orbit (GEO) satellites. This next phase will enable ubiquitous connectivity for fixed and mobility sites, 5G and IoT, and will require innovation to provide the needed higher throughput, higher flexibility and network orchestration between ground and space.
The growing congestion of the C and Ku bands have increased the interest of satellite system developers in the Ka-band. Several factors influence the development of broadband satellite networks at Ka-band frequencies:
Adaptive Power Control and Adaptive Coding:Adaptive power control and adaptive coding technologies have been developed for improved performance, mitigating propagation error impacts on system performance at Ka-band.
High Data Rata:A large bandwidth allocation to geosynchronous fixed satellite services (GSO FSS) and non-geosynchronous fixed satellite services (NGSO FSS) makes high data rate services feasible over Ka-band systems.
Advanced Technology:Development of low noise transistors operating in the 20 GHz band and high power transistors operating in the 30 GHz band have influenced the development of low cost earth terminals. Space qualified higher efficiency traveling-wave tubes (TWTAs) and ASICs development have improved the processing power. Improved satellite bus designs with efficient solar arrays and higher efficiency electric propulsion methods resulted in cost effective launch vehicles.