The design of a satellite communication system is a complex process, involved compromises between many factors in order to obtain the maximum performance at an acceptable cost. Several factors dominate the design. These are
· The weight of the satellite
· The DC power that can be generated on board.
· The frequency bands allocated for satellite communication
· The maximum dimensions of satellite and ground station antennas,
· The multiple access technique used to share communications capacity between many earth stations
The 6/4 Ghz bands have been the most popular and heavily used for the first 15 years of satellite communication systems, because they offer the fewest propagation problems, lower sky noise, lower noise temperature and historically, RF components for these bands have been readily available.
The weight of the satellite is limited by the high cost of launching a spacecraft into geostationary orbit.
In the design of satellite link two main designs are considered. They are
· Uplink design
· Downlink design
Downlink Design:
The downlink of any satellite communication system must be designed with the following objectives. They are
To guarantee continuity of the link for a specified percentage of the time
To carry the maximum number of channels at a minimum capital and maintenance cost.
The first objective requires a minimum (C / N) ratio at the receiver input for 99.9 percentage of time and will almost certainly require a modulation scheme or signal processing technique that gives a (S / N) improvement over the receiver. Modulation is usually FM in analog systems and PSK in digital systems, but companded SSB is also used.
The second objective brings in series of compromises between antenna cost, receiver cost, tracking accuracy, station manning, modulation, and multiple access techniques. In the Intelsat system standard earth station specification is produced, which go some way to optimizing the earth segment of that system. In a maritime or a military system, a totally different earth station would be needed. An Intelsat standard a earth station costs several million dollars.
Uplink Design:
The uplink design is rather easier than the downlink in most cases, since an accurately specified carrier power density must be presented at the satellite transponder and it is feasible to use much higher power transmitters at earth stations than can be used on a satellite in most cases.
The cost of transmitters tends to be high compared to the cost of receiving equipment in satellite communication system. Generation of high stability, high power microwave carriers is invariably expensive, and considerable care is needed throughout the transmitter to control spurious emissions and group-delay effect. As a result, the major growth in satellite communications has been in point-to-multipoint transmission, as in cable TV distribution. One high cost transmit earth station provides service to many low-cost receive-only stations. The situation worsens as antenna diameter is reduced. Smaller antenna gain requires greater transmitter power for a given EIRP, and use of TDMA requires still more power if the satellite transponder is to be driven into saturation. For example, a TV receive only earth station for the 4 GHz band costs as a little as $1500 with a 10-ft dish (1984 prices). To add a 6 GHz transmitter to this station would increase the cost by factor of 10 or 15, given for a low output power.
At L-band (1000-2000 MHz) where the maritime system operate, transistor amplifiers can be used to generate 50 W of output power, costs are not so high as 14 or 30 GHz, where the traveling wave tube is the main HPA in use. The development of a low-cost, high-power, high-stability microwave source that could operate a millimeter wavelength would open up a vast range of new satellite communication services from nation wide mobile communication to wristwatch personal two-way radios.
The satellite transponder is quasilinear amplifier and the received carrier levels determine the output level. Where a traveling wave tube is used as the output high-power amplifier (HPA) in the transponder, as is often the case, and FDMA is employed, the HPA must be run with a predetermined back off to avoid inter modulation products appearing at he output. The output back off is typically 3 to 7 dB and is determined by the uplink carrier power level received at the spacecraft. Accurate control of the power transmitted by the earth station is therefore essential, and Intelsat specifies +/- 0.5 dB for standard A stations when operated in the FDMA mode.