Question

You are designing a satellite communications system with the following parameters:

Satellite parabolic antenna has a diameter of 1 meter

Earth Station parabolic antenna has a diameter of 2.5 meters

Operating frequency of the system is 4 GHz

Transmission system bandwidth of 106 Hz

Satellite transmitter power of 10 watts

Assume earth receiver operating at room temperature (294 kelvin)

a. What is the maximum distance the satellite can be from earth, in kilometers, so that the SNRdb at the earth station is no less that 0db?

b. The ideal (non noise) maximum data rate for this channel is?

c. The maximum data rate with consideration for noise on this channel is?

Answer 1

Communications satellites comprise a number of elements. Typically they incorporate the following main elements:

• Communication Payload: This could be considered to be the main element of the communications satellite. It consists of transponders, antenna, and switching systems
• Engines: These are used to bring the satellite to its required
• Station Keeping Tracking: This subsystem incorporates a stabilisation system and small propulsion system to maintain the orientation and correct orbit during the operational life of the satellite.
• Power subsystem: The power subsystem of the communications satellite is used to power the satellite systems. It normally contains two elements:
  
  • Solar cells: These are used to provide power when the satellite is in sunlight. These normally consist of large arrays of solar cells often on extended arms. Some satellites may just be covered in solar cells to reduce the overall footprint of the satellite.
  • Batteries: Batteries are required to power the satellite when it is in an area of darkness such as when it passes the dark side of the earth, or during solar eclipses.
• Command & Control: This sub-system maintains communications with ground control stations. The ground earth stations continually monitor the satellite state and health and control its functionality. This will vary over different phases of its life-cycle. At end of life it may be necessary to place the satellite into a different orbit, or even send it into outer space where it will not clutter the orbit.

Satellite communications basics

When used for communications, a satellite acts as a repeater. Its height above the Earth means that signals can be transmitted over distances that are very much greater than the line of sight. An earth station transmits the signal up to the satellite. This is called the up-link and is transmitted on one frequency. The satellite receives the signal and retransmits it on what is termed the down link which is on another frequency.

Using a satellite for long distance communications

The circuitry in the satellite that acts as the receiver, frequency changer, and transmitter is called a transponder. This basically consists of a low noise amplifier, a frequency changer consisting a mixer and local oscillator, and then a high power amplifier. The filter on the input is used to make sure that any out of band signals such as the transponder output are reduced to acceptable levels so that the amplifier is not overloaded. Similarly the output from the amplifiers is filtered to make sure that spurious signals are reduced to acceptable levels. Figures used in here are the same as those mentioned earlier, and are only given as an example. The signal is received and amplified to a suitable level. It is then applied to the mixer to change the frequency in the same way that occurs in a superheterodyne radio receiver. As a result the communications satellite receives in one band of frequencies and transmits in another.

In view of the fact that the receiver and transmitter are operating at the same time and in close proximity, care has to be taken in the design of the satellite that the transmitter does not interfere with the receiver. This might result from spurious signals arising from the transmitter, or the receiver may become de-sensitised by the strong signal being received from the transmitter. The filters already mentioned are used to reduce these effects.

Block diagram of a basic satellite transponder

Signals transmitted to satellites usually consist of a large number of signals multiplexed onto a main transmission. In this way one transmission from the ground can carry a large number of telephone circuits or even a number of television signals. This approach is operationally far more effective than having a large number of individual transmitters.

Obviously one satellite will be unable to carry all the traffic across the Atlantic. Further capacity can be achieved using several satellites on different bands, or by physically separating them apart from one another. In this way the beamwidth of the antenna can be used to distinguish between different satellites. Normally antennas with very high gains are used, and these have very narrow beamwidths, allowing satellites to be separated by just a few degrees.

Separating satellites by position

Satellite communications channel characteristics

Satellite communications links need to be designed to enable the inherent link characteristics to be accommodated:

• Propagation delay - latency: In view of the altitude of many satellites - those in geostationary orbit - there are significant propagation delays. This can affect signalling and extended timeout windows may be required to accommodate the latency of the system.
• Limited bandwidth: Bandwidth is an issue for all users of the radio spectrum. Some satellites are affected more than others. Accordingly many systems will require to use the available bandwidth very effectively. Data compression schemes are normally used.
• Noise: The path length and the fact that power levels are limited, especially on the satellite means that signals do not operate with a large margins. To overcome this, directive antennas are normally employed. However in addition to this robust error correction techniques are normally required for data transmission.