Patent Application
20130009809
This invention relates to a multi-spot satellite system with efficient space resource allocation and is particularly, but not exclusively, applicable to telecommunications satellites.
Satellite based systems can be used to provide multimedia digital services over large geographical areas. Those are particularly useful to cover large areas with low population density and distant from urban centres. If satellites are not used, the access to digital services will require a ground infrastructure implying high investment costs for low profitability.
In order to cover a given service area, most of broadband satellites systems make use of a multi-beam architecture that provides means to reuse frequencies.
Existing solutions are mostly based on one or several geostastionary satellites. For example, a service area can be covered by one satellite that uses a plurality of beams, a beam being associated to a spot. A spot is an area covered thanks to one of the system's beam. The service area is usually covered by a plurality of spots.
For a given frequency bandwidth allocated to such a satellite system, an efficient frequency use can be achieved by applying the well known four-color theorem. In that case, the bandwidth allocated to the system is splitted into four portions called colors, each spot being associated to one color. This allows the reuse of the same frequencies in different spots.
Most of the existing systems are based on a regular grid mesh where beam sizes and allocated bands per beam are even over the service area. An example is illustrated on
The user geographical distribution is the key driver to estimate the capacity required over a given service area. As illustrated on
The consequences of increasing the number of beams by using smaller spot size is that more ground stations transmitting towards the satellites are required, and the interference level increases significantly. The said stations are called gateways in the sequel. Another consequence is that the cost and complexity of the system are increased as well as the satellite's payload size and complexity.
According to the invention, there is provided a satellite system comprising at least one satellite that receives and transmits signals from/to user terminals located in a service area covered by a plurality of beams, frequencies used in each beam being allocated in order to allow frequency reuse. The beam sizes, the bandwidth used in each beam and the power density in each beam are chosen as a function of the user density in the service area.
According to a complementary aspect, a plurality of on board power amplifiers are used by the satellite to generate a different density of power distribution for each beam or for different subsets of beams.
The satellite system may use adaptative modulation and coding, the use of a given adaptative modulation and coding scheme being promoted in a beam by choosing an appropriate power density value.
A system according to any one of the preceding claims, wherein the system frequency plan is based on a frequency reuse scheme at least four colours.
In a preferred embodiment, a plurality of beam sizes are used in a ratio of 5 between the smallest and the biggest.
According to an other aspect of the invention, the chosen power densities are chosen on a 6 dB range.
A better understanding of the embodiments of the present invention can be obtained from the following detailed description in conjunction with the following drawings, in which:
Advantageously, the second satellite can be added later to the system in order to double the capacity. The two satellites will have the same architecture, which will reduce the engineering costs. Additionally, if one of the satellites breaks down, the system remains operational.
In this specification, the expression “space resources” embraces frequency, transmission power and beam/spot size.
Three beam sizes can be chosen, for example 0.4°, 0.7° and 1°. Those values are given only as an example. Other values as well as a different number of spot sizes can be used, for example in a ratio of 5 between the smallest and the bigger spots.
Additionally, the system uses a frequency bandwidth allocation in which contiguous beams are using different frequencies, even those that have the same size. To do so, an efficient frequency plan needs to be set up in order to fit the frontier between large beams and small beams and to limit the internal interferences in the system. The frequency plan is based for example on a four colour basic scheme. Further, additional sub-colors may be used inside an existing color.
According to a complementary aspect of the invention, different power densities can be used for different beams and/or sub-sets of beams. For that purpose, several Travelling Wave Tube Amplifiers TWTA can be used for different portions of the system bandwidth in order to generate various densities of power distribution in every spots or in different sub-set of spots of the system. For example, if three distinct 500 MHz bandwidth are used for three contiguous spots, one of the combinations below can be chosen:
During the system conception, gateways 501, 502, 503, 504, 505, 506, 507 should be located carefully so that they do not interfere with each other. Additionally, the boresight of the antennas gateways 406, 407, 408, 409 can be located so that they can transmit a beacon to the satellite for its Antenna Pointing System. This enables to make use of small size beams.
The gateways can be located close to existing infrastructures such as networks, facilities and power supplies in order to optimize deployment and operational costs.
As an example, the above schemes can be used:
The use of those schemes leads to efficiencies varying from 1 to 6.6, that is to say from 0.5 bits per symbol for M=QPSK, R=1/4 to a 3.3 bits per symbol for M=16-APSK and R=5/6.
Considering that the system allows the use of different spot sizes, different bandwidth sizes in a spot and different power densities in one or several spots, the solution proposed by the invention enables to address throughput ratio variations going from 1 to more than 300 depending on the user densities in the service area.
A first step 700 consists into analyzing system requirements such as the service area size and the user density over the said area. Then, the number of beams and the beams sizes are chosen 701 in order to cover the entire service area. A portion of the available bandwidth is allocated 702 for each beam taking into account frequency reuse and the throughput required in each beam. For example, a bandwidth of 62.5 MHz, 125 MHz, 250 MHz, 500 MHz or 1 GHz can be allocated for each beam. It is then possible to determine how many gateways are required 703 and to locate them. Then, different power densities can be chosen for each beam 704. High power densities can be allocated to areas where the user density is high. In a preferred embodiment, a power density varying on_a 6 dB range can be used. Once the beams sizes, the bandwidths and the power densities have been chosen, it is possible to estimate the link quality 705 over the service area. For that purpose, the carrier to noise and interference ratio CNIR can be estimated. It is possible to assess the link efficiency 706 by taking into account the ACM schemes that are used over the service area. Finally, the system capacity 707 is deduced from the estimated link efficiency and from the allocated bandwidth. It is possible to verify 708 if the system requirements are fulfilled. If not, the power densities 709 and/or the allocated bandwidth 710 can be adapted and the system capacity re-estimated 705, 706, 707. The number of beams and the beams sizes can be adapted too 712. A system-planning tool can implement this method.
This repeater is used by the satellite to retransmit signals received from gateways towards the correct spots. For that purpose, the repeater comprises for example g RF receivers 800, where g is the number of gateways used in the system. The g RF signals are then amplified by using g Low Noise Amplifiers LNA 801. A set of p local oscillators LO can be used for frequency conversion purpose. Signals resulting from this frequency conversion can be filtered by using bandpass filters 803. In order to amplify the filtered signals with the aim to introduce different power densities in the spots, m High Power Amplifier HPA 804 can be used. For that purpose, a TWTA and can be used for one or several spots. The gain of each TWTA can be controlled independently. If a TWTA is used for more than one spot, the said spots may have different frequencies allocated to each of them. After the amplification 804, a plurality of bandpass filters 805 as well as u RF transmitters 806 are used to transmit the signal in the spots of the system, u being the number of spots that are covering the service area.
As noted above, the system and method described are merely exemplary and the skilled person would appreciate that a number of alternatives exist to implement aspects of the invention. Embodiments of the invention may be also used in a wide variety of applications and contexts, wherever multi-spots satellite communications may be required. It will also be apparent to the skilled person that various sequences and permutations on the system and method described are possible within the scope of this invention as disclosed.
