Abstract:
The method of transmission between a transmitter and a receiver using a mode of adaptive modulation and coding, wherein the modulation and coding are selected based on the comparison of a characteristic variable of the signal to noise ratio measured by the receiver with a threshold value plus a margin, which margin is variable depending on the prior change in the signal to noise ratio. 
     The margin changes based on a statistical function of a higher order than 1 of the characteristic variable of the signal to noise ratio measured by the receiver over at least one time horizon.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to French Patent Application No. 1202672 filed Oct. 5, 2012. This application is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a method of transmission between a transmitter and a receiver using a mode of adaptive modulation and coding, wherein the modulation and coding are selected based on the comparison of a characteristic variable of the signal to noise ratio measured by the receiver with a threshold value plus a margin, which margin is variable depending on the prior change in the signal to noise ratio. 
     BACKGROUND OF THE INVENTION 
     The transmissions, in particular by satellite in the Ka (K-above) and EHF (Extremely High Frequency) bands are sensitive to various different phenomena that can degrade the budget of the link between a transmitter and a receiver. These phenomena can lead to very rapid variations, such as masking or interference. The connection of the link can then be reduced by several decibels per second. 
     Other phenomena, such as weather related variations, in particular rain fade or antenna pointing errors have rapid effects that lead to a reduction of the gain by a few tenths of decibels per second. 
     Finally, other phenomena, such as the geographic location or situation of the receiver when it is mobile may result in slower variations of the gain of the link of the order of a hundredth of a decibel per second. 
     In order to be better adapted to these variations, mechanisms for adapting the modes of modulation and coding have been implemented. The goal is to dynamically adapt the parameters of the waveform so as to be well adapted to the link budget. This mechanism is known by the acronym AMC in English, for “Adaptive Modulation and Coding”. 
     As it is known per se, the AMC mechanism makes it possible, by comparing the signal to noise ratio to the baseline reference values to define the mode of modulation and coding adapted to the conditions of the link. 
     The propagation of information between the entities of the chain of transmission for transmitting the information pertaining to the state of communication and orders of change in modulation and coding requires a substantial amount of time, so that when the signal to noise ratio decreases, it takes a certain amount of time for the transmitter to be able to react to this decrease. 
     In order to ensure that the signal to noise ratio of the link is never less than a baseline reference signal to noise ratio necessary for the receiver, it is a well known practice to provide for a margin, added to the baseline reference signal to noise ratio in order to anticipate the losses of the link budget and to be able to change the modulation and coding early enough before the conditions become far too degraded. 
     This margin is called AMC margin. 
     The AMC margin depends on the worst case scenario variation of link budget to which the transmission system must be resistant as well as the reaction time of the system. 
     In general, the AMC margin is static and is of the order of 2 to 3 decibels for transmissions in the Ka band and the AMC margin may be higher in the EHF band. 
     When the conditions for signal propagation are stable, typically with a clear sky, the margin is unnecessary since the signal to noise ratio does not vary. The transmission power is thus 2 to 3 decibels higher than necessary thereby causing a decrease of the speed or the bandwidth of the order of 50% to 100%. 
     It is a known technique to make the AMC margin vary based on the historical information related to the change in the signal to noise ratio. 
     These solutions have the drawback of sometimes impose unnecessarily high AMC margins. The variation in signal to noise ratio may be of the order of 20 decibels, leading to the possibility of retaining an AMC margin of around several decibels, without this improving the communication, the phenomena deemed to have caused the variation in signal to noise ratio having been very brief and thus not having needed to be compensated for by a change in modulation or coding. 
     The aim of the invention is to provide a method of transmission with adaptive modulation and coding in which the changing of the AMC margin:
         makes possible the optimisation of the transmission power when conditions for signal propagation are stable   does not lead to changes in the mode of coding or modulation considered unnecessary, in particular in the event of masking or interference.       

     SUMMARY OF THE INVENTION 
     To this end, the object of the invention relates to a method of transmission of the aforementioned type, characterized in that the margin changes based on a statistical function with an order greater than 1 of the characteristic variable of the signal to noise ratio measured by the receiver over at least one time horizon. 
     According to particular embodiments of implementation, the method comprises of one or more of the following characteristic features:
         the margin changes based on a linear combination of statistical functions with an order greater than 1 of the characteristic variable of the signal to noise ratio measured by the receiver over several time horizons of different lengths;   the number of time horizons considered is between 2 and 4;   the or each time horizon has a duration between 2 and 90 seconds;   the statistical function depends on the standard deviation of the characteristic variable of the signal to noise ratio measured by the receiver;   the statistical function depends on the standard deviation of the characteristic variable of the signal to noise ratio measured by the receiver or on a predetermined maximum value of the standard deviation if the standard deviation of the characteristic variable of the signal to noise ratio measured by the receiver is greater than the maximum value;   the said method includes the calculation of a predicted signal to noise ratio that corresponds to the difference between the measured signal to noise ratio minus at least one variable margin, and the modulation and coding are selected based on the comparison of the predicted signal to noise ratio with a threshold value plus a fixed margin.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood upon reading the description which follows, provided solely by way of example and with reference made to the drawings in which: 
         FIG. 1  is a schematic view of a transmission installation for the implementation of the method according to the invention; and 
         FIG. 2  is a flowchart of the algorithm for determination of the modes of modulation and coding in the transmission method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In  FIG. 1  a transmission installation  10  is represented that demonstrates the implementation of a transmitting station  11  a satellite  12  and a ground receiving station  14 . The transmission takes place for example by band or in EHF band from the transmitting station  11  to the ground station  14  via satellite  12 , by any known means considered suitable. In a similar fashion, the transmission also takes place in the reverse direction. 
     The transmission method implements a mechanism for adapting the modes of modulation and coding known by the acronym AMC in English for “Adaptive Modulation and Coding” that makes it possible to dynamically adapt the parameters of the waveform so as to be well adapted to the link budget. 
     The ground station  14  includes the means for transmitting to the station  11  via the satellite  12  the information concerning the measured characteristics of the transmission, and the requests made by the receiving station in order to satisfy its needs. 
     The station  11  comprises, as is known per se the means for determination of the mode of modulation and coding to be used for the transmission based on the information received from the station  14 , in particular depending on the signal to noise ratio required by the station  14 , this latter being denoted by C/N 0     —     predicted . 
     By design, the station  11  is capable of determining the mode of modulation and coding selected by comparison of the signal to noise ratio required by the ground station  14  C/N 0     —     predicted  with a baseline reference signal to noise ratio C/N 0     —     ref  plus a fixed AMC margin, denoted by Margin fixe . The fixed AMC margin Margin fixe  is for example equal to 0.5 decibels (dB). 
     This figure also provides an illustration of the clouds  16 , which can degrade the conditions of transmission, and thereby reduce the signal to noise measured by the ground station  14 , possibly requiring the modification of the mode of modulation and coding. 
     As is known per se, the transmission is carried out by frame, also called packet according to the mode of modulation and coding. 
     The algorithm described with reference to  FIG. 2  is set to run continuously during the transmission partially in the ground station  14  and in the station  11  by the means for computing that deploy the appropriate computer programmes. 
     As illustrated in  FIG. 2 , during the transmission and for each group of n of k frames a channel error denoted by ErrCanal(n) is calculated during the step  100  by the receiver, that is, the ground station  14  in the example considered. A group of k frames is called super frame SF (k is chosen in order for the duration of a super frame to be of the order of 100 ms so as to be statistically significant). This channel error is the difference between the signal to noise ratio measured by the ground station  14  over the super frame n denoted by ΔC/N 0     —     meas     —     ST     —     n  and the transmission power of the station  11  denoted by PIRE Consigne     —     ST     —     n . 
     Thus, ErrCanal(n)=ΔC/N 0     —     meas     —     ST     —     n −PIRE Consigne     —     ST     —     n . 
     During the step  102 , and for several different time horizons numbered i, the standard deviation of the channel denoted by DACMMA_σ i  is determined by the receiver over the N i  last seconds constituting the time horizon i considered. 
     For example, the time horizons constitute periods of 3, 10, 30 and 60 seconds such that N 1 =3; N 2 =10; N 3 =30; N 4 =60. 
     Thus, the standard deviation of the channel error for a determined time horizon i is given by 
               DACMMA_σ   i     =       [         1     nST   i       ⁢       ∑     n   =   1       nST   i       ⁢           ⁢       ChannelErr   ⁡     (   n   )       2         -       (       1     nST   i       ⁢       ∑     n   =   1       nST   i       ⁢     ChannelErr   ⁡     (   n   )           )     2       ]       1   /   2             
wherein
 
nST i  is the number of super frames in the time horizon i. During the step  104  a narrow (bounded) standard deviation is determined for each time horizon i by the receiver. This narrow standard deviation is denoted by Clip(i) and is given by Clip (i)=Min(DACMMA_σ i ; DACMMA MaxVariation) wherein DACMMA MaxVariation is a constant. Thus, the narrow standard deviation is equal to the standard deviation of the channel error if the latter is less than a predetermined maximum value of the standard deviation denoted by DACMMA MaxVariation or equal to the predetermined maximum value of the standard deviation if not, this being so in order to not take into account extremely large variations in the standard deviation.
 
     During step  106 , the receiver determines a time variable margin constituted by a linear combination of narrow standard deviations Clip(i) calculated over the four time horizons. Thus, the time variable margin is written as follows Margin time variable =α 1  Clip (1)+α 2  Clip (2)+α 3  Clip (3)+α 4  Clip (4) where α 1 , α 2 , α 3  and α 4  are non zero positive real numbers. By default, the coefficients á 1 , á 2 , á 3  and á 4  are all taken to be equal to 1. 
     During the step  112 , the receiver calculates a predicted signal to noise ratio denoted by C/N 0     —     predicted  which corresponds to the difference between the measured signal to noise ratio minus the variable margin calculated in step. 
     Thus C/N 0     —     predicted =C/N 0     —     meas −Margin time variable . 
     It is conceivable that with such a method, the AMC margin can be maintained at a highly reduced level during periods of low variation in the signal to noise ratio, in particular the periods with clear skies and that the AMC margin is shown to be increased in a rapid manner during significant but not abrupt changes in the signal to noise ratio, thereby making it possible to adequately anticipate the modifications in mode of modulation and coding in order for the signal to noise ratio to be maintained in all circumstances at a level higher than the signal to noise ratio required by the receiver, without the signal to noise ratio however being constantly much higher than the signal to noise ratio required at the receiver, in particular during periods of clear weather conditions.