Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)

13.2 ARCHITECTURE OF A SATELLITE COMMUNICATION SYSTEM

As discussed in Chapter 3, "Transmission Media", satellite communication systems operate in two configurations: (a) mesh; and (b) star. In mesh configuration, two satellite terminals communicate directly with each other. In star configuration, there will be a central station (called a hub), and remote stations communicate via the hub. The star configuration is the most widely used configuration because of its cost-effectiveness, and we will study the details of satellite communication systems based on star configuration in this chapter.

Communication satellites operate in two configurations: (a) mesh; and (b) star. In mesh configuration, a remote station can communicate directly with another remote station. In star configuration, two remote stations communicate via a central station or hub.

The architecture of a satellite communication system is shown in Figure 13.1. The system consists of two segments:

• Space segment

• Ground segment

Figure 13.1: Architecture of a satellite communication system.

The space segment consists of the satellite, which has three systems: fuel system, telemetry control system, and transponders.

Space segment: The space segment consists of the satellite, which has three main systems: (a) fuel system; (b) satellite and telemetry control system; and (c) transponders. The fuel system is responsible for making the satellite run for years. It has solar panels, which generate the necessary energy for the operation of the satellite. The satellite and telemetry control system is used for sending commands to the satellite as well as for sending the status of onboard systems to the ground stations. The transponder is the communication system, which acts as a relay in the sky. The transponder receives the signals from the ground stations, amplifies them, and then sends them back to the ground stations. The reception and transmission are done at two different frequencies. The transponder needs to do the necessary frequency translation.

Ground segment: The ground segment consists of a number of Earth stations. In a star configuration network, there will be a central station called the hub and a number of remote stations. Each remote station will have a very small aperture terminal (VSAT), an antenna of about 0.5 meter to 1.5 meters. Along with the antenna there will an outdoor unit (ODU), which contains the radio hardware to receive the signal and amplify it. The radio signal is sent to an indoor unit (IDU), which demodulates the signal and carries out the necessary baseband processing. IDU is connected to an end systems, such as a PC, LAN, or PBX.

The central station consists of a large antenna (4.5 meters to 11 meters) along with all associated electronics to handle a large number of VSATs. The central station also will have a Network Control Center (NCC) that does all the management functions, such as configuring the remote stations, keeping a database of the remote stations, monitoring the health of the remotes, traffic analysis, etc. The NCC's main responsibility is to assign the necessary channels to various remotes based on the requirement.

The communication path from a ground station to the satellite is called the uplink. The communication link from the satellite to the ground station is called the downlink. Separate frequencies are used for uplink and downlink. When a remote transmits data using an uplink frequency, the satellite transponder receives the signal, amplifies it, converts the signal to the downlink frequency, and retransmits it. Because the signal has to travel nearly 36,000 km in each direction, the signal received by the satellite as well as the remote is very weak. As soon as the signal is received, it has to be amplified before further processing.

Communication satellites are stationed at 36,000 km above the surface of the earth, in geostationary orbit. Two separate frequencies are used for uplink and downlink.

13.2.1 Frequencies of Operation

The three widely used frequency bands in satellite communication systems are C band, Ku band, and Ka band. The higher the frequency, the smaller will be the antenna size. However, the effect of rain is greater at higher frequencies.

The various bands of operation are:

C band: Uplink frequency band 6GHz (5.925 to 6.425 GHz)

Downlink frequency band: 4GHz (3.7 to 4.2 GHz)

Ku band: Uplink frequency band: 14GHz (13.95 to 14.5 GHz)

Downlink frequency band: 11/12GHz (10.7-11.25 GHz, 12.2-12.75 GHz)

Ka band, with uplink frequency band of 30GHz and downlink frequency band of 20GHz, is used for broadcasting applications. Direct broadcast satellites, which broadcast video programs directly to homes (without the need for distribution through cable TV networks) operate in the frequency band 17/ 12GHz, with uplink frequency band being 17.3 to 18.1 GHz and downlink frequency band being 11.7 to 12.2 GHz.

Because the frequency of operation is higher in the Ku band, the antenna size will be much smaller as compared to C band antennas. However, the effect of rain is greater in Ku band than in C band. For many years, only C band was used for satellite communication. With advances in radio components such as amplifiers, filters, modems, and so on, the effect of rain on Ku band can be nullified by necessary amplification. Presently, Ku band is used extensively for communication.

Communication satellites operate in different frequency bands: C band (6/4GHz), Ku band (14/ 12GHz), and Ka band (30/20GHz) band. The higher the frequency, the smaller the antenna.