Imported: 17 Feb '17 | Published: 10 Jan '12
USPTO - Utility Patents
The invention relates to a method of optimizing the resources used in a satellite or aircraft telecommunications system (onboard segment). The method includes expressing the difference in consumption E_{dB }between the consumption of onboard power and the consumption of onboard bandwidth as a function of a plurality of terms including a quality indicator, an informational term expressing onboard parameters and a corrective term. The quality indicator QaF′depends on the antenna system, onboard and on the ground, and on conditions of propagation and interference of the medium. The informational term ℑ_{dB }depends on modem parameters. The term K′ depends on onboard parameters such as the maximum incoming flux density and the bandwidth. A corrective term C_{dB }characterizing the consumption of onboard power attributable to the upward noise which is amplified and transposed on board. At least one of the parameters QaF′, K′, ℑ_{dB }is modified in order to tend toward the cancellation of the absolute value of the term E_{dB}.
The present application is based on, and claims priority from, French Application Number 0706361, filed Sep. 11, 2007, the disclosure of which is hereby incorporated by reference herein in its entirety.
The present invention relates to a method of optimizing the resources used in a satellite or aircraft telecommunications system. The invention can notably be used in a network planning phase, during the deployment of a communications system between stations of the terrestrial segment, the system using one or more onboard repeaters, that is to say present on a satellite or on an aircraft.
In a satellite telecommunications system, often called a “Satcom” system, earth stations communicate with each other via one or more repeaters in a transparent manner. The earth stations are provided with at least one modem, one frequency transposition module for each direction (for transmission, referenced Tx, and for reception, referenced Rx), one amplifier for each direction (Tx and Rx) and with an antenna, such that a transmitting station modulates the signals according to an appropriate waveform, sends signals which modulate a carrier wave ascending to the repeater, which amplifies them, transposes them and retransmits them to the ground on a descending carrier wave, a receiving station capturing the descending carrier wave in order to demodulate the signals. In order to obtain good performance at a minimal cost, the allocation of the resources used in the Satcom links must be optimized. Several criteria are involved in the performance of these links and their implementation cost. In order to increase the capacity of the network, it is possible, for example, either to increase the size of the antennas on the ground, or to increase the bandwidth (sometimes leased) of the repeater or to increase the Equivalent Isotropically Radiated Power (Puissance Isotrope Rayonée Equivalente-PIRE) at saturation of the repeater or its operational gain, each of these options also generating a corresponding additional financial cost.
The adjustment of certain elements of the communication system makes it possible to optimize the resources used. A first element to be minimized is the consumption of resources in the space segment, in other words the allocated bandwidth and the power used. A second element relates to the size of the equipment on the ground, notably the sizes of the antennas. A third element to be optimized is the setup of the modems, a good setup making it possible to increase the capacity of the system, that is to say the network spectral efficiency, referenced η. The spectral efficiency of the network is the capacity/allocated bandwidth ratio of the repeater, the capacity being the sum of the useful data rates of all the carriers which share that same allocated bandwidth of the repeater. Knowing that the modification of one of the aforesaid elements affects the connected elements, the problem of optimization of the resources used means the overall optimization of these three elements.
A system is said to be limited by power (or by bandwidth) when 100% of the power (or, respectively, of the bandwidth) available onboard is reached whereas the whole of the bandwidth (or of the power respectively) available on board is not consumed. It is known that in order to optimize the space resources, the Satcom system must neither be limited by bandwidth nor limited by power, which is equivalent to balancing the spectral consumption on board and the power consumption on board. In practice, this principle of balance naturally results in reducing the size of the antennas for a given network spectral efficiency or in increasing the network spectral efficiency for a given size of antennas. In the case of a heterogeneous set of antennas, optimization is carried out per antenna class, that is to say per group of connections sharing the same quality indicator (the “iso-QaF” class of connections will be described and the quality factor “QaF” is defined below). In order to increase the network spectral efficiency, either the capacity at constant bandwidth is increased, or the leased bandwidth on the satellite at constant capacity is reduced. It is also known that the modulation spectral efficiency is to be adapted according to the size of the antenna of the receiving stations. Notably, the following two articles published by the MILCOM can be mentioned:
More generally, it is known that there is a relationship between the modem parameters and the antenna parameters, but this relationship is not simply expressed and, at present, no method of the prior art makes it possible to plan the resources to be allocated in a Satcom network simply and in an optimal manner. In general, link budget experts use iterative algorithms in which several tens of parameters (or even about a hundred of them) are involved. The experience of these experts then guides their setup choices in order to refine the allocation of the resources for each equipment of the network. The methods used by these experts are multiple, suffer from a lack of transparency and often do not make it possible to obtain the result in just a few simple operations.
As a preamble, a list of the notations used subsequently for indicating physical values is given below:
One purpose of the invention is to propose a simple method making it possible to optimize the allocation of the resources used in a satellite or aircraft communications system. For this purpose, the subject of the invention is a method of optimizing the resources used in the telecommunications carried out via an onboard segment, formed by a satellite or by an aircraft, between stations of the terrestrial segment comprising at least one modem, characterized in that it comprises at least the following steps:
According to one implementation of the optimizing method, the difference E_{dB }(in a logarithmic scale) between the onboard bandwidth consumption and the onboard power consumption is equal to:
QaF′+K′−ℑ_{dB}−C_{dB} [E0]
According to one implementation of the optimizing method, the satellite or the aircraft comprises at least one repeater, and the parameter K′, expressed in dBW/Hz, is equal to
K′=DF_{max}+10 log_{10}(4πd^{2})−10 log_{10}(B)+margin [E0bis]
According to one implementation of the optimizing method, the parameter ℑ_{dB }is equal to
According to one implementation of the optimizing method, the parameter QaF, expressed in dBHz/W, is equal to the quantity that is taken from the noise density (C/No)_{requis }in dBHz (with a system margin here), in order to find the value of PIRE_{sol }(dBW).
In fact, there is the equation QaF=(C/No)_{requis}−PIRE_{sol }(see below)
Where, by notational convention,
and where, by purely notational convention, the following two prime values are used in the text:
QaF′=QaF−M_{sys}etK′=−K+M_{sys }
The Satcom stations of the terrestrial segment can be placed on the ground or on a naval or air carrier such as a helicopter for example.
Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious aspects, all without departing from the invention. Accordingly, the drawings and description thereof are to be regarded as illustrative in nature, and not as restrictive.
The method according to the invention is based on the use of the following equation (equivalent to the equation [E0]) which will be explained below:
E_{dB}=QaF−K−ℑ_{dB}−C_{dB} [E16]
where:
Depending on the technical constraints, the equation [E16] can be used in different ways for optimizing the resources of a Satcom system.
In a first case, when the parameters on the ground and the onboard parameters are fixed, that is to say for example when antennas and the repeater are already installed, the use of the equation [E16] makes it possible to adjust the modem parameters in order to maximize the capacity of the Satcom network. In order to do this, it is appropriate to modify the pair of modem parameters (e, E_{b}/N_{0}), until the difference E_{dB }becomes substantially zero in absolute value.
An example of the method corresponding to this first case is given in FIG. 1. It is an iterative method executed, for example, by a calculating processor used during the network planning phase, the method making it possible to tend towards a desired value of E_{dB }by means of successive modifications of the modem parameters. A tolerance Tol can, for example, be fixed with respect to the desired value so that the difference E_{dB }obtained at the end of the complete execution of the method is distanced at most by the value Tol from this desired value. Advantageously, the desired value of the difference E_{dB }is zero, which results in stopping the iterations when the absolute value of the difference E_{dB }is less than or equal to the tolerance Tol. It is also possible to search for the minimum of the absolute value of the difference E_{dB }among all of the configurations of pairs (e, E_{b}/N_{0}) available for the modem used.
The method according to the example comprises the following steps:
In a second case, when the modem parameters and the onboard parameters are already fixed, the equation [E16] can be used for minimizing the size of the antennas on the ground. An iterative method acting on the QaF indicator can be used for obtaining this result. An example of this method is given below:
A similar method could be used for optimizing the characteristics of the repeater under the constraint of a fixed antenna system and fixed modem parameters, in which case it would be necessary to act on the parameter K.
More generally, it is possible to seek an optimal solution by fixing only one or two of the parameters QaF, K and ℑ_{dB}, the criterion corresponding to the optimum choice of the parameters remaining the elimination of the difference E_{dB}.
The overall optimizing of the network is obtained by proceeding with the optimizing of each iso-QaF class. In this case “iso-QaF class” refers to all of the links which share the same QaF (the same merit factor of the receiving antenna, substantially the same conditions of propagation—and therefore the same link availability—, of interference, of intermodulation and of upward and downward geographic advantages).
In order to understand the equation [E16] better, an explanation is given below.
As explained in the introduction, the balance between the consumption of onboard power and the onboard spectral consumption corresponds to the optimum in terms of the use of resources. By denoting the percentage used (by a carrier) of the onboard power by the term conso^{PW}, and the percentage used (by a carrier) of the onboard spectral bandwidth by the term conso^{BW}, the optimum can be conditioned to the achieving of the following criterion (for each iso-QaF carrier class):
conso^{BW}=conso^{PW} [E1]
which criterion, transposed on a logarithmic scale (in decibels in this case), is equivalent to the elimination of a difference E_{dB }between two consumption values conso^{BW}_{dB }and conso^{PW}_{dB}. A Satcom system is therefore optimized when
E_{dB}=conso_{dB}^{BW}−conso_{dB}^{PW}=0 [E2]
On the one hand, the spectral consumption conso^{BW }is defined as the ratio of the spectral bandwidth ΔF of the communication channel to the spectral bandwidth of the repeater B:
Moreover, the modulation spectral efficiency e is equal to the quotient of the data rate D of data transmitted on the communication channel over the spectral bandwidth ΔF of the said channel:
which makes it possible to express the onboard spectral consumption as a function of the data rate D, of the spectral bandwidth B of the repeater and of the spectral efficiency e as follows:
this latter expression, transposed into decibels, is equivalent to the following formula:
conso_{dB}^{BW}=10 log_{10}(D)−10 log_{10}(B)−10 log_{10}(e) [E6]
On the other hand, the power consumption conso^{PW}_{dB }in decibels can be expressed as a function of the power transmitted on the ground PIREsol_{dB }and of onboard parameters as follows:
conso_{dB}^{PW}=PIREsol_{dB}+K_{1} [E7]
where K_{1}=IBO−SFD−162.1−A_{up}^{nfs}−ΔA_{up}^{fs }
In fact, a geostationary satellite is placed at a distance d_{o}=35,786 km from the earth's surface at the sub-satellite point, in order words at the point of zero latitude and of the same longitude as that of the satellite. Now, the attenuation of the power of a signal transmitted by a station separated by d_{o }from the satellite is proportional to the surface of a sphere of radius d_{o}, which, on a logarithmic scale, translates as 10·log(4πd_{o}^{2})=162.1 dBm^{2}. Taking the difference between d_{0 }and the real distance d_{up }separating the transmitting station from the satellite into account, the additional free space attenuation is determined as ΔA_{up}^{fs}=20 log(d_{up}/d_{o}).
By expressing this additional free space attenuation, on a linear scale, as follows: ΔA_{up}^{fs}=(d_{up}/d_{0})^{2}, the flux Φ received by the satellite corresponding to the carrier transmitted at PIREsol_{dB }can be written on a linear scale as:
and therefore the onboard power consumption is written as:
Moreover, a patent application published under Number FR2877785 mentions the use of a quality indicator QaF to facilitate the establishment of a Satcom link budget, the name QaF being chosen with reference to the expression “Quality Aggregate Factor”, for “total quality figure”. As a reminder, this quality indicator QaF is a coefficient of proportionality between the PIRE_{sol }and the power ratio of the power of the signal to the total noise density C/N_{O }on reception of the said signal. Thus, on a logarithmic scale, the indicator QaF is expressed as the difference between the PIRE_{sol }and the required (C/N_{0})_{req }(in which a system margin is incorporated here) in order to comply with a binary error rate making it possible to carry out a demodulation:
The higher the value of this indicator QaF, the less is the required PIRE_{sol }in order to obtain a C/N_{0 }ratio sufficient to be able to demodulate the signal on reception. The indicator QaF depends on interference having an effect on the link and on the area of the antennas used on the ground and on the satellite or aircraft. The quality indicator equal to the difference between the required (C/N_{0})_{req }and the PIRE_{sol}, without integrating a system margin, is referenced QaF′.
By combining the expressions [E9] and [E10], the difference E_{dB }between the consumption values can be expressed as a function of the quality indicator QaF:
Now, the power C of the signal is equal to the product of the data rate D in bits and the mean energy per user bit E_{b}, which, transposed on a logarithmic scale, results in expressing this power C as follows:
(C)_{dB}=(E_{b})_{dB}+10 log_{10}(D) [E12]
By dividing this power by the noise density N_{0 }received at the demodulator of the receiving station and by integrating the system margin M_{sys }in it, the following is derived:
By combining the expressions [E11] and [E13], the difference E_{dB }between consumptions is expressed as follows:
The terms E_{b}/N_{o }and 10 log_{10}(e) can be gathered into a single term ℑ_{dB}, the term ℑ_{dB }thus grouping the informational parameters related to the modem. Also, the difference E_{dB }between the consumption of onboard power and the onboard spectral consumption can be expressed as follows:
and where K=M_{sys}+10 log_{10}(B)−SFD+IBO−10 log_{10}(4πd^{2})+−ΔA_{up}^{fs }
Knowing the two parameters of the first order high-pass Butterworth filter, defined in the abovementioned patent application numbered FR2877785, which are the gain of the filter (referenced R1) and the −3 dB cut-off diameter of the said filter, and knowing the antenna diameter of the receiving station of the link, the quality indicator QaF can be calculated in two operations. As the calculation of the other parameters of the equation [E15] is fast, it is thus demonstrated that the method according to the invention necessitates few calculation operations.
The term “K′” (in dBW/Hz), such that K′=−K+M_{sys}, can be interpreted as the spectral density of maximum PIRE which would be transmitted from the ground in band B, that is to say that which corresponds to an incoming flux at the satellite of value DF_{max}=SFD−IBO. The equations [E0] and [E0bis] are thus obtained again from the demonstrated expression [E15].
If the upward noise (thermal noise and possible interference and/or possible onboard intermodulations) is not negligible in terms of onboard power consumption, the equation [E15] is refined by adding a corrective term C_{dB}:
where (C/No)_{up }is the signal to noise density ratio obtained by all of the upward noises (at least the thermal noise) and possibly the noises generated on board, notably by intermodulations.
FIG. 2 gives an example showing the variation of the spectral efficiency η of a Satcom network as a function of the spectral modulation efficiency e for different values of the indicator QaF.
Each value of the indicator QaF corresponds to a different curve 11, 12, 13 in increasing order of QaF. The higher the QaF indicator, the greater is the capacity of the Satcom network, since the indicator QaF increases with an improvement in the performance of the antenna system used.
Moreover, each of the curves 11, 12, 13 passes through a maximum spectral efficiency value of the network η_{11}, η_{12}, η_{13}. Each maximal value η_{11}, η_{12}, η_{13 }is obtained by choosing an optimal modulation spectral efficiency e, e_{opt11}, e_{opt12}, e_{opt13}, corresponding to the balance between the spectral consumption and the power consumption. A spectral modulation efficiency value e which is distant with respect to the optimal value e_{opt11}, e_{opt12}, e_{opt13 }indicates a bad configuration of the modem. In fact, when the modulation spectral efficiency e is less than the optimal value, the Satcom system is limited by bandwidth (E_{dB}>0), in other words a portion of the available power is not used. That is why it is necessary to increase the “e” parameter and therefore the quantity ℑ_{dB}, which reduces the difference E_{dB}. Inversely, when the modulation spectral efficiency e is higher than the optimal value, the Satcom system is limited by power (E_{dB}<0), which means that a portion of the available bandwidth is not used. That is why it is necessary to reduce the parameter “e” and therefore the quantity ℑ_{dB}, which increases the difference E_{dB}. The method according to the invention makes it possible to reach these optimal values e_{opt11}, e_{opt12}, and e_{opt13 }quickly.
It will be readily seen by one of ordinary skill in the art that the present invention fulfils all of the objects set forth above. After reading the foregoing specification, one of ordinary skill in the art will be able to affect various changes, substitutions of equivalents and various aspects of the invention as broadly disclosed herein. It is therefore intended that the protection granted hereon be limited only by definition contained in the appended claims and equivalents thereof.