Allocation of Downlink Carrier Power in LEO Communication Satellites

Abstract

A method is provided for simultaneously transmitting a plurality of signals from a LEO satellite towards a plurality of ground terminals located within a pre-defined range of distances from the LEO satellite, wherein the plurality of signals have a pre-defined overall capacity; at least two of the plurality of signals have each a power level that is different from a power level of the other of the at least two signals; and each signal transmitted to a respective ground terminal is selected so as to ensure that its power level is the lowest from among the signals that are simultaneously transmitted, yet the selected signal has a sufficient power to enable its proper reception at a distance which extends between the respective ground terminal and the LEO satellite.

Claims

1. A method for simultaneously transmitting a plurality of signals from a LEO satellite towards a plurality of ground terminals located within a pre-defined range of distances from the LEO satellite, wherein: a) said plurality of signals have a pre-defined overall capacity; b) at least two of the plurality of signals have each a power level that is different from a power level of the other of the at least two signals; and c) each signal transmitted to a respective ground terminal is selected so as to ensure that its power level is lowest from among the signals that are simultaneously transmitted from the LEO satellite to the ground terminals, yet the selected signal has a sufficient power to enable its proper reception at a distance which extends between said respective ground terminal and said LEO satellite.

2. The method of claim 1, wherein the LEO satellite's antenna is steered to illuminate a pre-defined, fixed area on the ground for a certain period of time, so that as the LEO satellite moves, the distances to different ground terminals located within the pre-defined range of distances from the LEO satellite, change in a non-uniform way while maintaining an optimal signals' power level.

3. The method of claim 2, wherein said optimal signal power is maintained by adopting a member of a group that consists of (i) modifying power of individual signals from among the plurality of signals; (ii) handing-over ground terminals by replacing a signal selected from among the plurality of ground terminals which is being transmitted to a specific ground terminal, with another signal selected from among that plurality of signals; (iii) any applicable combination of (i) and (ii).

4. The method of claim 1, wherein the LEO satellite's antenna is directed at a direction that is fixed relative to the nadir.

5. The method of claim 4, wherein a power of a signal being transmitted from a certain LEO satellite towards a specific ground terminal, is modified during transmission of communications from said certain LEO satellite to said specific ground terminal.

6. The method of claim 5, wherein said modification of the signal power is made by taking into consideration a change in distance between the LEO satellite and said specific ground terminal and/or a change in antenna gain that occurred with respect to said ground terminal.

7. The method of claim 1, wherein a LEO satellite's coverage area is divided into a plurality of cells and either: (i) each cell is covered by a separate beam used for transmission of a separate signal for communicating with ground terminals located within said respective cell; or (ii) each beam covers a number of cells by illuminating one cell at a time; or (iii) a combination of (i) and (ii).

8. The method of claim 7, further comprising a step of allocating power to signals based on which beam is used for their transmission, while taking into consideration a distance that extends between the LEO satellite and a cell covered by each respective beam.

9. The method of claim 7, further comprising using a beam-forming antenna array and combining the signals prior to their transmission by an array element.

10. The method of claim 9, further comprising a step of allocating power to signals in accordance with their beam association while taking into consideration that when said beam-forming antenna array's boresight is directed at the center of the coverage area of the LEO satellite, beams that cover cells close to an edge of the LEO satellite coverage area will be received at a lower gain by respective ground terminals located within said cells that are close to an edge of the LEO satellite coverage area.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The accompanying drawing, which is incorporated herein and constitutes a part of this specification, illustrates an embodiment of the disclosure and, together with the description, serves to explain the principles of the embodiments disclosed herein.

[0037] FIG. 1—demonstrates a flow chart exemplifying a certain example construed in accordance with an embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0038] Some of the specific details and values in the following detailed description refer to certain examples of the disclosure. However, this description is provided only by way of example and is not intended to limit the scope of the invention in any way. As will be appreciated by those skilled in the art, the claimed method and device may be implemented by using other methods that are known in the art per se. In addition, the described embodiments comprise different steps, not all of which are required in all embodiments of the invention. The scope of the invention can be summarized by referring to the appended claims.

[0039] The satellite communication network to which the present disclosure relates comprises a constellation of LEO satellites that circle the earth at a relatively low altitude. At these altitudes, the orbital period is in the order of 90 to 120 minutes and a satellite is only visible from any given location on the ground only for a small fraction of the time, and the satellite's field of view is limited to a few thousand km at the most.

[0040] Prior art satellite communication networks solutions, implement a method where signals are transmitted as single signals, each having a pre-defined capacity (measured, for example, in megabits per second) and which is strong enough to be properly received by all ground terminals located within the satellite coverage area.

[0041] In contradistinction to the prior art implementations of LEO satellites communication network referred to above, by the solution provided by the present disclosure, a plurality of signals replaces the single signal. This plurality of signals has the same overall pre-defined capacity as the single signal would have, and characterized in that each of the plurality of signals is transmitted to a respective ground terminal using the lowest power possible for such transmission to that specific ground terminal, yet it has a power level that is sufficient to be properly received by the respective ground terminal located within a certain range from the satellite (for example, up to 1200 km, up to 1400 km, etc.). In other words, the power of a signal being transmitted is the power of a signal of a shortest distance that may be transmitted to the specific ground terminal, which can still be properly received thereat, thereby ensuring that communications are transmitted to all terminals while using the lowest power available for a signal transmitted to a respective terminal. Thus, the satellite continuously transmits a plurality of signals (modulated carriers) at different power levels. A transmission to a specific terminal is modulated onto one of these signals based on the signal's power, i.e. the signals are continuously transmitted and the method provided enables to match which signal, from among the plurality of signals, will be used for transmission to each ground terminal.

[0042] From the point of view of their coverage area, LEO satellites are not stationary (unlike geosynchronous earth orbit—GEO—satellites). As a LEO satellite is moving in its orbit, its distance to any given ground terminal is changing. In other words, the satellite's nadir moves over the ground along with the satellite movement, and together with this movement, the equal-distance concentric circles around it, move too. To maintain an optimal power allocation, the above scheme may be implemented in either one of following two embodiments.

[0043] According to one embodiment, the method provided comprises a step of steering the satellite antenna to illuminate a pre-defined, fixed area on the ground for a certain period of time. As the satellite is moving, the distances to different ground terminals are changing in a non-uniform way and an optimal signal power is maintained.

[0044] In accordance with another embodiment, the optimal signal power is maintained by adopting one of the following options: (i) modifying the power of individual signals; (ii) “handing-over” ground terminals from one of the signals to another. In other words, at a certain point in time, the signal that is used to communicate with a certain ground terminal, is replaced with another signal; and (iii) any applicable combination of the above options (i) and (ii).

[0045] By one of the options that are possible to implement the method provided, the satellite antenna is directed at a direction that is fixed relative to the nadir. In this case, the antenna beam sweeps across the ground and—from the point of view of the satellite—any given terminal that enters the antenna beam, moves within its footprint relative to its center (or boresight) and eventually leaves it. According to this option, as the gain of the antenna towards a specific terminal changes with its relative position, the power of the signal received by the ground terminal, would be changed accordingly. When modifying the signal power and/or when handing over ground terminals from one signal to another, the variations due to distance and/or antenna gain may be taken into consideration when determining what should be the appropriate modification for signals transmitted to that ground terminal.

[0046] According to another option for the implementation of the solution provided herein, a LEO satellite's coverage area is divided into a plurality of cells, in order to enable increasing capacity through frequency re-use, as well as improving link conditions through higher antenna gain. Each of these cells may be covered by a different (separate) beam, or a “hopping beam” technique may be used, to enable covering a number of cells by cyclically directing the beam to each cell for a certain period of time.

[0047] Each beam may be used to transmit a separate signal for communicating with ground terminals located within the respective cell. In this case, the power of each beam's signal may be changed according to the average distance from the satellite to a respective cell, and this change of power may be implemented according to a pre-defined policy, as determined by the LEO satellite system operator. A possible policy may be one whose target is to reduce the overall satellite energy consumption, while another target may be set to increase the total transmission capacity, whereas another target may be provisioning of services to users in accordance with pre-defined individual service level agreements. Yet another target may be a combination of the above.

[0048] As in the single antenna case, the way that distances that extend from the satellite to the cells, and consequently the optimal power allocation, change along with the satellite movement, depends on whether the antennas creating the beams are fixed in orientation or are steered to illuminate a fixed area.

[0049] In accordance with another option of implementing the disclosure, a beam-forming antenna array is used rather than using separate antennas for generating a plurality of beams. When such an array is used instead of transmitting each signal by its own individual antenna, the signals are combined in any one of a number of ways that are known in the art per se, and each combination is transmitted by an array element. By controlling the way how signals are combined, a plurality of beams having desired directions and gains, may be generated by the array in a flexible and programmable way. When a beam-forming antenna array is used to generate a multi-beam coverage of a LEO satellite's service area, signals' combining can be modified to transmit, over each beam, at a signals' power level that is optimized for the distance to a cell which is illuminated by a respective beam from among the plurality of beams.

[0050] In certain implementations of beam-forming antenna arrays, the maximum beam gain that the antenna can generate falls off with the angle between the beam center and the array's boresight. If the beam-forming antenna array's boresight is directed at the center of the satellite's coverage area, beams that cover cells close to the edge of coverage will have a lower gain. Therefore, according to another embodiment of the present disclosure, the method provided further comprises a step of allocating power among beams' signals while taking into account the above factor, and wherein the allocation of power among the beams' signals is carried out according to a pre-determined power allocation policy.

[0051] The flow chart depicted in FIG. 1, describes a certain example of implementing the solution provided by the present invention. Obviously, this is only one example from among the different options to implement the solution, some of which were described above.

[0052] In step 100a plurality of signals is provided, wherein the aggregated transmission capacity of all these signals has a pre-defined value. Next, at step 110, for any given ground terminal located within a pre-defined distance range from the LEO satellite, a signal is selected from among the plurality of signals. The selected signal has the lowest power level, yet its power level allows it to be properly received at the given ground terminal.

[0053] The communication that should be transmitted to the given ground terminal is modulated onto the selected signal (step 120).

[0054] As the LEO satellite is moving, the distance to the ground terminal changes and the satellite antenna is steered to illuminate a pre-defined, fixed area on the ground for a pre-determined period of time (step 130).

[0055] As the gain of the antenna toward the given ground terminal changes, the power level of the signal being transmitted is modified to ensure that the pre-defined overall transmission capacity is not exceeded, and that the modified signal still has the lowest power level from among the signals that have sufficient power level to be properly received at the ground terminal in its location relative to the new location of the satellite (step 140).

[0056] Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.