Multi-Channel Communications Satellite Systems - Essay Example

Introduction Satellite communications has the ability of enabling a multi channel kind of communication. It is important to the fact that satellite communication use of space segmentation highly slices the operation costs. As such every effort in satellite communications should be focussed on the reduction of space segment and hence operating costs through ensuring spectral efficiency. Spectral efficiency can only be optimized through reduction of Channel Spacing (CS). However, reduction of Channel spacing is more likely to create structural overlapping of transmission and as such may end up in what is commonly called Adjacent Channel Interference (ACI) which greatly affects the performance of communication satellite systems (Roddy, 2006: pp123-127). This paper seeks to highlight the concept of multi-channel communications satellite systems, the concept of systems operation, the advantages and disadvantages of the system and how to improve performance and reliability of the system in the presence of man-made interference and atmospheric noise. Communications Satellite Satellite communication, heavily relies on the use of a spacecraft in orbit around the Earth. The spacecraft is able to receive and re-transmit signals mainly radio signals. The use of satellite systems for communication, have over time undergone evolution. They have been in use for some time now and many changes have been made enabling higher performance of these satellites. Communications satellites are not only able to amplify and route signals but they are also able to sort these signals. Earlier on they used to function like the ground microwave repeaters but as mentioned earlier, these systems have undergone great evolution and now they are quite different from ground microwave repeaters. Whereas ground microwave repeaters relay radio signals between two fixed points, the satellite communication systems are able to interconnect a multiple of locations both fixed and mobile. This is the superiority of these systems over the ground microwave repeaters. As far as evolution of these systems is concerned, the current functions of both switching and rerouting of signals with the switchboards being onboard and airborne. Communications Satellite Orbits The choice of orbit is very important to the performance of communications satellites. In fact, the height of the satellite which is in a circular orbit determines not only the time of orbit but also the coverage. For instance, a 35,860 km orbital satellite has a corresponding orbital period of about 23 hours 56 minutes and 4 seconds (roughly one day). The orbit of the satellite may coincide with the equatorial plane and this will therefore mean that the satellite will hover in one fixed point in relation to the rotating earth and as such it is said to be geostationary. A geostationary satellite has the capacity of supporting two fifths coverage of the earth's surface and this therefore implies that three geostationary satellites are able to support world coverage. Most of the satellites that support communications (fixed and mobile) are the satellites which are in geosynchronous equatorial orbit (Korhonen, 2003: pp1-7). The geostationary satellites have much capacity for coverage but are not able to cover the high latitude regions. These regions require that other types of satellites are used which are inclined at an angle with respect to the equatorial plane. For instance, the Russians launched a satellite with its orbit inclined at 63.50 in 1965. This satellite was meant for their domestic communications. The Molniya system is the type of this satellite system because it is found in the Molniya orbit and it was launched at 63.50 orbit inclination with respect the equatorial plane with the following specifications: Perigee - 500 km Apogee - 40,000 km Orbital period - 12 hours As far as the above inclination is concerned, there is no rotation of line of the aphides and as such there is reduced orbit correction sand manoeuvres (Takashi et al, 2003: pp168-172). The satellites need tracking and as such they require both the tracking and handover equipment which are more often than not found on earth stations. Besides the geostationary satellites that are high height and the highly inclined satellites, the low-altitude earth orbits (LEO) and medium-altitude earth orbits (MEO) satellites at less than 1000 km and 14,400 km respectively have got applications in the recent years. These have found excessive applications in mobile telephone services. LEOs as compared to geostationary satellites have shorter paths from the stations on earth that are linked to these satellites. Based on the short path, the power required for transmissions is very minimal. Similarly the path delays are very limited and this makes the LEOs advantageous for mobile services as compared to the other types of satellites. What are the other advantages of using LEOs in the mobile services The use of LEOs in the mobile services enables the use of low gain antennas which need not any tracking and if they do, it is little tracking. With the use of LEOs low power transmitters can be used and as such the system does not undergo any degradation as a result of delays or multiple hops (Bousquet & Maral, 2002: pp76-79). The period of interconnection is greatly dependent on the visibility of the satellite to the earth bases (stations) and as such there is need to put a group of satellites in orbit to guarantee a continuous communication. Therefore, it means that for continued connections and communications, there is need that a group of satellites are orbited such that by the time the current satellite being used goes out of the visibility of the earth bases, it is able to hand over to the already coming in satellite and this repeats itself for the next satellite until the cycle is complete by having a handing over to the satellite that was started with. The cycled go on and on and on. The handing over of coverage to the next incoming satellite is done in such an orderly manner so as to ensure continuous communication and wide coverage. Multi Channel Communications satellite Systems Having understood the concept of communications satellites and how they operate in the orbits it is important to look specifically at the multi channel communications satellites. First, these satellites use what is commonly called digital modulation. The old satellites use frequency modulation but the modern communications satellites use QPSK kind of modulation besides QAM. Besides the employment of frequency modulation in the communication satellites, the satellites have a feature called multiple-access. This term denotes the way the traffic carrying capability of a communication satellite can be enhanced. This feature of multi-access allows numerous earth stations to have access to the satellite simultaneously through the use of frequency-division or other techniques such as the time-division technique. The multi access may be fixed in that the number of telephone access channels availed to the earth stations is fixed. The number of telephone access channels may also be assigned depending on the traffic demand and as such this is called demand access. The frequency-division and the time-division techniques are addressed in detail in the following sections Frequency-division Method This method enables the communications satellites multiple access. The use of FDMA allows each transmission to and from the satellite assignment of specific bandwidth on a transponder and numerous FDMA systems can use a single transponder. Each ground station is able to send a carrier to the satellite in an FDM configuration. The satellite then receives the carrier and demodulates it on to the degree of de-multiplexing the channels addressed to the station in question. Ground stations that share a communication satellite receive their specific frequency spectrum allocated to them on the transponder. It is important to note that the FDMA system is greatly enhanced in use by both frequency modulation and frequency-division multiplexing. The breakdown in the communications satellite systems is interesting. The transponders can serve more than on earth station and the earth stations can serve a multiple blocks of telephone channels. These channels are assigned what is commonly called routes. As far as traffic studies are concerned, the number of circuits per route can be calculated. The routes that have little traffic can only require a few circuits and vice versa. Lower traffic telephone channels are more often unavailable and this explains why the transponder bandwidths allocated to these channels have to be broken into larger number carriers so as to enable allocation of a particular channel to each carrier in a system called single-channel per carrier (SCPC). This system can either have the carriers set or allocated as per the demand as earlier mentioned. Time-division Method This method is employed especially for digital systems which work differently from their counterparts. TDMA systems are able to access one transponder on the communications satellite at a time at different times. The earth station accessing the transponder at a given time will use all the power available from the transponder without sharing it. These time slots are allocated in a sequential manner and as such they are able to give an earth station exclusive use of the transponder during its access turn. Examples of such digital systems are the INTELSA T and EUTELSA T which are able to operate at a rate of about 120 Mb/s via a 72 MHz transponder. Each earth station takes its turn to access the transponder, transmit data using the transponder in the rate shown above for a fraction of the total time. It is imperative to note that the TDMA system does not regenerate signals in the satellite and as such the transponder acts as a down-converter. The multi udders of the transponder in this system transmit data to the satellite in bursts. The bursts reach the satellite in pre-assigned time sequence and occupy transponder bandwidth depending on the type of modulation used. As such, they may or may not occupy the entire transponder bandwidth. The ground stations are able to start and finish its burst without overlapping those bursts generated by other ground stations. A TDMA system has what is called a Frame (a TDMA frame). This is a cycle of frames that the ground station is able to generate. It is imperative to note that each there can only be one burst accessing the satellite at a time. The cycle can have three to hundred bursts accessing the transponder. How TDMA System Works Any time bursts are transmitted to the satellite, they have to have a preamble added before they are transmitted. The preamble is basically the number of bits added to the traffic burst so as to enable the earth station receives the correct signal. The preamble is generated and added to the traffic burst by the TDMA equipment and the burst assembler respectively. QPSK modulator receives the assembled burst and produces a burst at the IF which is then taken to the transmitting equipment. The transmitting equipment allocates the frequency to the burst before radiating it through the aerial. Once the IF signal has been radiated or transmitted from the satellite, the receiving earth station needs to demodulate the IF signal, recover synchronization pulses to be able to identify the start and end of each frame. The original telephony signal is then assembled before being relayed to the destination. Because of this, the TDMA system needs to be synchronized to ensure that the traffic burst from various ground stations do not overlap Advantages and Disadvantages of Communications Satellite Systems Advantages Noise reduction and adjacent channel interference can be minimized by increasing the use of intelligent interference mitigation schemes The space segment costs can be minimized by increasing the spectral efficiency which can be achieved through channel spacing. Multi channel communications satellites are able to amplify sort and route signals Switching and rerouting of signals has been enhanced greatly with the advent of on-board switchboards which contain the transponders traffic carrying capability of a communication satellite systems can greatly be enhanced using the frequency-division and the time-division techniques Wide coverage especially the remote areas where fibre optic cables have not reached On-board switchboards help in maintaining functions such as the station-keeping, attitude, power systems and telemetry among others. Multi functionality of the multi channel communications satellite systems which are able to support applications such as the satellite television, fixed service satellite, direct broadcast satellite, mobile satellite technologies, satellite radio, amateur radio, satellite internet and some military uses (Green, 2000: pp408-420). Geostationary orbit hence the geostationary satellites is helpful for communications because the ground antennas which have to be directed to the satellites can easily function without the need for very expensive equipment for tracking the satellites These systems can handle both the digital and analog signals. Disadvantages The space segment costs can be minimized by increasing the spectral efficiency which can be achieved by through channel spacing but this brings about spectrally overlapping transmissions which lead to what is commonly termed as adjacent channel interference (ACI) which has been found to greatly hamper both the performance and efficiency of satellite communication systems Sometimes the satellites have to be tracked using very expensive tracking equipment. Interconnection duration is limited to the interval of satellite visibility to the mobile earth terminals and base stations Channels with less traffic are likely to be unavailable if the single-channel per carrier (SCPC) is not applied Use of TWT as the final power amplifier leads to inter-modulation products which lead to Recommendations In presence of manmade noise, noise reduction in the communications satellite systems is possible through the increased use of intelligent interference mitigation schemes especially in the LEOs A TWT power amplifier is most efficient when operated at, or near saturation; this is where it is supposed to be used and not as a final power amplifier. The IDR system should be embraced as opposed to the TDMA. The IDR system uses the QPSK modulation which reduces or effectively manages the intermodulation products which cause distortion of the signal. Conclusion In conclusion, the use of the multi functional multi channel communications satellite systems is gaining popularity as opposed to the fibre optic cables because of its wide coverage especially the remote areas where fibre optic cables have not reached. The TDMA system is specifically efficient but for its noise as a result of the intermodulation products, it is losing popularity and the digital system known as Intermediate Data Rate (IDR) is slowly taking over. The IDR system uses the QPSK modulation which reduces or effectively manages the intermodulation products which cause distortion of the signal. The use of the multi channel communication satellite systems have proved to be highly efficient because of their ability to be accessed by a number of ground stations thus not only reducing the extra costs of having a satellite per ground station. With, the continued evolution in communication technology and the advent of systems such as the IRM, much efficiency is expected for the multi channel communications satellite systems. References Roddy D, (2006): Satellite communications, McGraw-Hill Professional, pp123-127 Bousquet M & Maral G, (2002): Satellite Communications Systems: Systems, Techniques and Technology, John Wiley and Sons, 2002 Korhonen J, (2003): Introduction to 3G mobile communications, Artech House Publishers, pp1-7 Takashi et al, (2003): Satellite Communications in the 21st Century: Trends and Technologies, American Institute of Aeronautics and Astronautics, pp168-172 Green D C, (2000): Radio Communication, Longman Publishers, pp408-420 Read More