Microwave Satellite Communication.

                                                 Satellite Microwave

A communication satellite can be seen as a microwave repeater in space. It is equipped with a number of devices called transponders, each of which listens to some portion of the electromagnetic spectrum, amplifies an incoming signal (the uplink), and re-broadcasts it at another frequency (the downlink). Geostationary satellites are placed in orbit above the equator at a height and speed that enables them to maintain a position above a specific location on the earth’s surface. The antenna used to receive signals from these satellites can thus be mounted in a fixed position.

The Astra 1H geostationary satellite

The downlink signal can be relatively narrowly focused (a spot-beam), or may cover a substantial fraction of the earth’s surface. The area covered by the signal is called its footprint. The size of the satellite dish required to receive a signal from a satellite depends on its location within the footprint (see below).

The footprint for the Astra 1G and 1H geostationary satellites

Although signals travel between earth stations and satellites travel at the speed of light (circa 3x10⁸ kilometres per second), the distances involved introduce substantial delays (typically 250-300 milliseconds). Satellites are also inherently broadcast media - very useful for some applications, but necessitating the use of encryption if security is an issue.

Low earth orbit (LEO) satellites are only visible for a short period. For this reason, large numbers of these satellites are required to implement a satellite communication system. When one satellite passes from view, another one replaces it. Satellite networks (or constellations) can provide worldwide telecommunication services using hand-held devices that communicate directly with the satellites.

Iridium is a constellation of 66 low earth orbit communication satellites

In the Iridium system, the satellites are positioned at a height of 750 kilometres in circular polar orbits, and are arranged in north-south necklaces, with one satellite every 32 degrees of latitude. Each satellite completes one orbit every 100 minutes, and six such necklaces are sufficient to provide coverage of the entire earth. The uplink and downlink frequencies used make it possible to communicate with a satellite using a small battery powered device equipped with an omni-directional antenna. Messages are received by one satellite and then relayed from one satellite to another around the world until the message can be transmitted to the destination mobile device (or via an Irridum gateway if the call is being routed via the public switched telephone network). The user simply requires line-of site between their antenna and one of the Iridium satellites. 

Modulation Microwave Communication

1, With the explosive growth of information flows, the current to the carrier's space satellite microwave communications technology gradually exposed the weaknesses of its own, that as the communication data rates have increased, as traditional means of microwave began to close to its maximum transfer rate bottleneck theory. In this context, it is natural to look to transfer to the laser signal with the optical communications, laser communications expect to rely on high data transfer rate to solve the problem.



Satellite optical communication is a new means of space communication. Use of artificial earth satellites as relay stations transmit laser signal can be achieved between multiple spacecraft and between spacecraft and earth station communications. The high transfer rate, high security and reliability, confidentiality and strong, terminal devices are small, light weight, low power consumption perseverance attracted national experts to explore the [1-4]. Space laser communication system in the structure should have an interface with the microwave communications. There is no one country has established space laser communication link, and therefore the satellite microwave communication and optical communications, few studies of mutual conversion process. In the field of optical communication has been a breakthrough, the successful realization of the satellite - terrestrial, satellite - satellite optical communication between tests in recent years is expected to enter the practical application [5-6]. Therefore, the space optical communications and microwave communications interconnection is a problem to be solved.



2, laser inter-satellite links Laser inter-satellite links include synchronous satellite communication link between, synchronous orbit and low orbit satellite communication link between China and LEO satellite communications links between satellites and ground stations and communication between link.



Based on consideration of the space environment, satellite and ground-based microwave link can only. Therefore, in order to meet the demanding star power, volume and complexity of the request, must study the satellite communications and microwave communications polish the mutual conversion technology. In addition, the existing satellite network using microwave technology, in order for satellite optical communication to optimize their effectiveness, the need to address the traditional satellite communications and satellite communications, optical networking technology, therefore, necessary to conduct in space optical communications and microwave communications interchangeable.



3, satellite communications and microwave communications optical conversion method

For most satellites, it is both microwave and optical communication link conversion node, is also a satellite routing optical network switching nodes. On the one hand, when the node up / down link, the satellite must complete the microwave / optical and optical / microwave interchangeable. On the other hand, routing is determined by the middle of the satellite onboard processors, according to the dynamic routing table lookup to complete, so that the packet needs to go through complex demultiplexing, demodulation process and routing the exchange of such treatment.



The relay satellite and the LEO satellite (GEO-LEO) relay link between the laser, the need to send back to the ground to LEO LEO high-speed data initially modulated laser communication terminals to the relay satellite complicated by the GEO laser communication terminal for receiving, processing obtained by the regeneration of the baseband demodulation signal, then microwave for QPSK modulation, the frequency to Ka-band, Ka-band satellite ground line via the link sent to ground stations. LEO sent to the first floor to the low-speed data, the spread spectrum, BPSK modulated, the ground line from the Ka-band satellite link sent to the relay satellite, GEO received baseband demodulation signal regeneration processing, enter the GEO optical laser communication terminals prepared and sent to LEO laser communication terminal. This microwave communication and optical communication method exists mutual conversion process complex, cumbersome equipment and network delay to increase the volume and other shortcomings, can not meet the requirements of the satellite payload.