Method of variable-bitrate communication with selection of a transmission interface and corresponding device

Abstract

A method for selecting a mode of transmission for a first telecommunication entity of a communication signal to a second telecommunication entity, each mode determining a physical bitrate. The method includes: determining for a given mode a first metric corrected by a second metric, the first metric measuring at a given distance d a relative degradation by the medium for transmitting the signal resulting from a relative degradation linked to a multipath effect at the link level with respect to a Gaussian channel and from a relative degradation linked to an effect of attenuation of the medium with respect to a model of attenuation in free space, the second metric determining a ratio between a mean bitrate and the physical bitrate for this mode of transmission; comparing for various modes of transmission, values of the first metric corrected to select at least one mode of transmission for distance d.

Claims

1. A method for selecting a transmission mode, for a first telecommunication entity comprising various modes (MODE.sup.i) of transmission, of a communication signal intended for a second telecommunication entity, each mode (MODE.sup.i) determining a physical bitrate (D.sup.i) in bits/s, wherein the method comprises the following acts performed by the first telecommunication entity: determining for a given transmission mode (MODE.sup.i), a value of a first metric (α.sup.i) corrected by a second metric (ν.sup.i), the first metric (α.sup.i) measuring, at a given distance d, a relative degradation introduced by the communication signal transmission medium that is the result of a relative degradation linked to a multipath effect at the link level relative to a Gaussian channel and that is the result of a relative degradation linked to an effect of attenuation of the transmission medium relative to a model of attenuation in free space, the second metric (ν.sup.i) determining a ratio between a mean bitrate (D.sub.moy.sup.i) and the physical bitrate (D.sup.i) for this same transmission mode; and comparing, for different modes (MODE.sup.i i=1, . . . N) of transmission, the values of the corrected first metric (α.sup.i) to select at least one transmission mode ({MODE.sup.j, . . . }) for a given distance d.

2. The method for selecting a transmission mode as claimed in claim 1, wherein the mean bitrate D.sub.moy is estimated by a polynomial relationship of the form D.sub.moy=ad.sup.2+bd+c with a, b and c determined coefficients.

3. The method for selecting a transmission mode as claimed in claim 1, wherein the first metric (α) is the result of a weighted sum of a relative degradation linked to the multipath effect and of a relative degradation linked to the effect of attenuation of the transmission medium.

4. The method for selecting a transmission mode as claimed in claim 1, whereby, for a distance d less than a distance threshold (d.sub.0.sup.i), the selection and comparison are made on the basis of the first metric (α) calculated for different transmission modes determining a same physical bitrate D in bits/s.

5. The method for selecting a transmission mode as claimed in claim 1, whereby the multipath effect is determined taking the difference between a multipath sensitivity threshold of the transmission mode and a sensitivity threshold of the transmission mode, the sensitivity threshold corresponding to a minimum power required to ensure the physical bitrate D with a target bit error ratio TEB representative of a quality of service QoS on a Gaussian transmission medium.

6. The method for selecting a transmission mode as claimed in claim 1, further comprising, for the selected transmission modes: determining a value of a third metric β corrected by the value of the second metric, this third metric β measuring excess power available at the distance d, which is the difference between the available power and a minimum power required to ensure the physical bitrate D with a target bit error ratio TEB representative of a quality of service QoS, choosing a transmission mode (MODE.sup.k) for which the corrected third metric β crosses a given threshold.

7. The method for selecting a transmission mode as claimed in claim 6, wherein the third metric β is calculated according to the following relationship: β=Gr+PIRE−α−S−PL.sub.FS(d), in which PIRE is a radiated power at an output of a transmission antenna of the first telecommunication entity, Gr is a gain of a reception antenna of the second telecommunication entity, S is the required minimum power to ensure the bitrate D with the given quality of service QoS for a Gaussian transmission medium, PL.sub.Fs(d) is attenuation of propagation in free space.

8. A communication entity comprising: at least two different transmission modes ensuring a physical bitrate (D) in bits/s; a processor; and a non-transitory computer-readable medium comprising instructions stored thereon, which when executed by the processor configure the communication entity to: determine a value of a first metric α which measures a relative degradation at a given distance d introduced by a communication signal transmission medium that is the result of a relative degradation linked to a multipath effect at a link level relative to a Gaussian channel and the result of a relative degradation linked to an effect of attenuation of the transmission medium relative to a model of attenuation in free space, for a given transmission mode, determine, at least beyond a distance threshold (d.sub.0.sup.i) the value of the first metric (α) corrected by a second metric (ν) determining a ratio between a mean bitrate (D.sub.moy.sup.i) and the physical bitrate (D) for a same given transmission mode, and compare values of the first metric α or of the corrected first metric α for different modes to select at least one transmission mode from among these modes.

9. The communication entity as claimed in claim 8, comprising several transmission interfaces, each of the transmission modes being associated with one of the transmission interfaces, such that the transmission interfaces belong to a list consisting of: an interface of power line transmission type PLT (CPL), an interface of radio type, an interface of optical type.

10. A telecommunication system comprising the communication entity as claimed in claim 8.

11. A non-transitory computer-readable information medium comprising program instructions stored thereon for implementing a method for selecting a transmission mode, when said program is loaded and run in a first telecommunication entity, the first telecommunication entity comprising various modes (MODE.sup.i) of transmission of a communication signal intended for a second telecommunication entity, each mode (MODE.sup.i) determining a physical bitrate (D.sup.i) in bits/s, and wherein the instructions configure the first telecommunication entity to: determine for a given transmission mode (MODE.sup.i), a value of a first metric (α.sup.i) corrected by a second metric (ν.sup.i), the first metric (α.sup.i) measuring, at a given distance d, a relative degradation introduced by the communication signal transmission medium that is the result of a relative degradation linked to a multipath effect at the link level relative to a Gaussian channel and that is the result of a relative degradation linked to an effect of attenuation of the transmission medium relative to a model of attenuation in free space, the second metric (ν.sup.i) determining a ratio between a mean bitrate (D.sub.moy.sup.i) and the physical bitrate (D.sup.i) for this same transmission mode; and compare, for different modes (MODE.sup.i i=1, . . . N) of transmission, the values of the corrected first metric (α.sup.i) to select at least one transmission mode ({MODE.sup.j, . . . }) for a given distance d.

12. The method for selecting a transmission mode as claimed in claim 1, comprising: the first telecommunication entity transmitting the communication signal through a respective transmission interface using the selected at least one transmission mode.

Description

LIST OF THE FIGURES

(1) Other features and advantages of the invention will become apparent from the following description of particular examples given in light of the attached figures given by way of nonlimiting examples.

(2) FIG. 1 is a schematic representation of the structure of a PPDU frame of the level one physical layer and of the corresponding MPDU frame of the level two MAC (media access control) layer, referring to the OSI model. The FCS field contains the bits, called cyclic redundancy check when an ARQ mechanism is put in place in the protocol for retransmission of the data at the MAC level.

(3) FIG. 2 is a flow diagram of a particular embodiment of a selection method according to the invention.

(4) FIG. 3 is a diagram of the simplified structure of a communication entity according to the invention.

(5) FIG. 4 represents curves of mean bitrate as a function of the distance obtained from simulations for different transmission modes of a communication entity.

(6) FIG. 5 represents the values of the corrected metric α′ obtained for the mean bitrate values corresponding to the curves of FIG. 3 for the 15-50 m distance range.

DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

(7) The selection method according to the invention exploits the metric α corrected by a metric ν to compare the performances obtained with the different transmission modes while taking account of a bitrate variation, each mode being associated by definition with a transmission interface which can be identical and/or different between the different modes.

(8) The metric α measures the relative degradation introduced by the transmission channel for a given telecommunication entity in a given environment relative to a reference model of the transmission medium, taking account, on the one hand, of the multipath effect and, on the other hand, of the attenuation of the propagation channel (PL(d)).

(9) The metric α for a distance d is typically the result in dB of a weighted sum of an MCM degradation linked to the multipath effect and an MCBE (narrowband channel margin) degradation linked to the effect of attenuation of the transmission medium:
α=η.sub.1MCM+η.sub.2MCBE  (9)

(10) The weighting coefficients η.sub.1, η.sub.2 have the default value of one.

(11) The multipath effect of the propagation channel impacts the performances at the physical layer level (radio performances) relative to a transmission over a so-called perfect channel. The multipath effect is reflected by a relative degradation which limits the reliability of the link for a given transmission mode. The measurement of this degradation is obtained by the difference in dB of the multipath sensitivity threshold S.sub.M of the given transmission mode and of the sensitivity threshold S of this same mode at the same bitrate. This measurement is denoted by the acronym MCM which means “multipath channel margin”. This MCM parameter makes it possible to compare the performance of different transmission modes at the link level.

(12) The multipath sensitivity threshold S.sub.M of a transmission mode for a propagation scenario is the minimum power required to ensure a target physical transmission bitrate D in bits/s with a target bit error ratio TEB (QoS) when the propagation channel has multiple paths. The multipath sensitivity threshold S.sub.M depends: on the signal-to-noise ratio required for the transmission mode to reach the target physical bitrate D and a target bit error ratio TEB associated with the QoS (target bit error ratio typically 10.sup.−5), on the thermal noise P.sub.b.

(13) The thermal noise P.sub.b expressed in dBm describes the imperfections of the components of the RF (radio frequency) stages of a communication entity. This thermal noise P.sub.b exhibits very different variations from one transmission mode considered to another as a function of the transmission bandwidth, of the noise temperature T of the receiver and of the spectral efficiency. The thermal noise P.sub.b is often expressed as a function of a reference value P.sub.b.sub.0 equal to −114 dBm corresponding to a reference noise temperature T.sub.0 set at 290 K and to a transmission band of 1 MHz. The thermal noise contribution is given by:
P.sub.b=10 Log(kTB.sub.w)+L.sub.0=10 Log(kT.sub.0)+10 Log(T/T.sub.0)+10 Log(B.sub.w)+L.sub.0
P.sub.b=−114 dBm+10 log.sub.10(B.sub.w.sub.MHz)+NF+L.sub.0 (dBm)
P.sub.b=−114 dBm+10 log.sub.10(D)−10 log.sub.10(Eff)+NF+L.sub.0 (dBm)
Eff=D/B.sub.w.sub.MHz)  (1)

(14) with T the noise temperature of the communication entity, NF the noise factor (10 log.sub.10 (T/T.sub.0)), B.sub.w the effective bandwidth of the transmission mode, L.sub.0 the cable losses, k the Boltzmann constant and Eff the spectral efficiency of the transmission mode. The sensitivity threshold S does not depend on the transmitted power, or on antenna gains.

(15) The multipath sensitivity threshold S.sub.M depends on the transmission mode, on the desired quality (target bit error ratio), on the thermal noise contribution, on the signal-to-noise ratio deduced from the simulations at the link level in a multipath context associated with a propagation scenario; it can be expressed as follows:

(16) $\begin{array}{cc}{S}_M=SNR+P_b{S}_M=SNR+kT{B}_w+L_0=SNR+10\mathrm{Log}\left(kT\right)+10\mathrm{Log}\left(B_w\right)+L_0{S}_M=SNR+10\mathrm{Log}\left(k{T}_0\right)+10\mathrm{Log}\left(T/T_0\right)+10\mathrm{Log}\left(B_w\right)+L_0{S}_M=SNR-114\mathrm{dBm}+\mathrm{NF}+10{\log }_{10}\left(B_{w_{\mathrm{MHz}}}\right)+L_0\left(d\mathrm{Bm}\right){\mathrm{SNR}}_{TEBc=10^{-5}}={\left(\frac{Ebu}{N_0}\right)}_{TEBc=10^{-5}}\frac{D}{B_w}& \left(2\right)\end{array}$

(17) Considering the above equation of the SNR.sub.TEBc=10.sub.−5, the multipath sensitivity threshold S.sub.M can be expressed as a function of the bitrate D:

(18) $\begin{array}{cc}{\mathrm{SNR}}_{BER=10^{-5}}={\left(\frac{Ebu}{N_0}\right)}_{BER=10^{-5}}\frac{D}{B_w}{S}_M=SNR+kT{B}_w+L_0=\left(\frac{Ebu}{N_0}\right)+10\mathrm{Log}\left(kT\right)+10\mathrm{Log}\left(D_{Mbps}\right)+L_0{S}_M=\left(\frac{Ebu}{N_0}\right)+10\mathrm{Log}\left(k{T}_0\right)+10\mathrm{Log}\left(T/T_0\right)+10\mathrm{Log}\left(D_{Mbps}\right)+L_0{S}_M=\left(\frac{Ebu}{N_0}\right)-114\mathrm{dBm}+\mathrm{NF}+10{\log }_{10}\left(D_{Mbps}\right)+L_0\left(\mathrm{dBm}\right)& \left(3\right)\end{array}$

(19) The sensitivity threshold S of a transmission mode corresponds to the minimum power required to ensure the physical bitrate D, calculated over the data field, with a target bit error ratio (TEBc) representative of the QoS on a gaussian channel (perfect channel affected by a noise contribution AWGN (additive white gaussian noise)), that is to say without multiple path (typically a Dirac function). The expression of S is identical to that of S.sub.M using the notations:

(20) ${\left(\frac{Ebu}{N_0}\right)}_c^{AWGN}\mathrm{et}{\mathrm{SNR}}_c^{AWGN}.$

(21) The multipath channel margin MCM is a dimensionless datum which can be deduced from several variables, SNR, Ebu/N.sub.0 or minimum required power according to the following expressions:

(22) $\begin{array}{cc}\mathrm{MCM}={\left(\frac{Ebu}{N_0}\right)}_c-{\left(\frac{Ebu}{N_0}\right)}_c^{AWGN}MCM=SN{R}_c-SN{R}_c^{AWGN}\mathrm{MCM}=S_M-S& \left(4\right)\end{array}$
with (Ebu/N.sub.0).sub.c the average energy per useful bit divided by the noise spectral density which is required for a target bit error ratio TEBc, SNR.sub.c the corresponding signal-to-noise ratio and S.sub.M the minimum power required in reception for this same TEBc.

(23) The MCM parameter corresponds to the additional power (or the variation of the signal-to-noise ratio ΔSNR in dB or to the variation of the energy per useful bit divided by the noise spectral density ΔEbu/N.sub.0 in dB), in a multipath context, necessary to achieve a bit error ratio identical to the gaussian case, for a given transmission mode.

(24) For one and the same bit error ratio and one and the same mode, the multipath channel margin MCM is the SNR difference obtained between, respectively, a gaussian channel and a multipath channel.

(25) The effect of attenuation of the propagation channel in a multipath context and in a partially obstructive or obstructed link introduces an additional attenuation and leads to a reduction of the radio coverage (range) of a transmission interface, considering in succession an ideal point-to-point transmission without obstruction and a transmission in an environment comprising obstacles which obstruct the link and increase the attenuation due to the propagation channel. The attenuation due to the propagation channel modeled by an equation of the PL(d) type is a physical variable representative of the physical environment which is deduced from experimental measurements. The relative attenuation effect due to the propagation channel depends only on the environment and on the deployment scenario (range, antenna, etc.) and does not depend on the telecommunication entity apart from the impact of the transmission carrier frequency in the calculation of the attenuation. A signal transmitted with a power Pt is received at a distance d with a power Pr with Pr<Pt. The ratio between Pt and Pr represents the propagation attenuation for antenna gains equal to zero (the effect of the antennas (antenna gains GT and Gr) is not considered in order to provide the attenuation model for a given environment).

(26) The simplest attenuation model is the model in free space deduced from the Friis transmission equation known to the person skilled in the art. This model corresponds to the attenuation when no obstacle obstructs the link. The distance dependency of the attenuation varies in (d/d.sub.0).sup.2 in which d is the distance between the two measurement points and d.sub.0 a reference distance generally set at 1 m. The formula of the Friis transmission equation is as follows:
PL.sub.FS(d,fc).sub.dB=−27,55+20 log(fc.sub.MHz)+20 log(d.sub.m/d.sub.0=1 m)  (5)
with d.sub.m the distance expressed in m and fc.sub.MHz the carrier frequency expressed in MHz.

(27) When the link is obstructed or slightly obstructed, the transmission equation is modified and the attenuation as a function of the distance is proportional to (d/d.sub.0).sup.n with n>2. The modified formula has the following form:
PL.sub.FS(d,fc).sub.dB=PL.sub.FS(d.sub.0,fc).sub.dB+10×n×log.sub.10(d/d.sub.0)+σ  (6)
PL.sub.FS(d.sub.0,fc).sub.dB=PL.sub.FS(d.sub.0,fc.sub.0).sub.dB+20×Log(fc/fc.sub.0)  (7)
with fc.sub.0 the reference frequency and σ the standard deviation that are associated with the propagation model.

(28) The MCBE (narrowband channel margin) parameter corresponds to the additional attenuation between the two configurations: obstructed space and free space; it makes it possible to quantify the effect of the transmission medium on the selection of a transmission interface. For a same distance and a same mode, the narrowband channel margin MCBE is the attenuation difference obtained between, respectively, the free space and the obstructed space.

(29) This MCBE deviation no longer depends on the frequency RF explicitly and consequently makes it possible to take account only of the relative degradation of the medium, independently of the explicit attenuation of the frequency and of the transmission powers (the calculation of MCBE is done for a given distance d between transmitter and receiver). The MCBE parameter is expressed in the following form:

(30) $\begin{array}{cc}\mathrm{MCBE}=P{L}_{MFS}\left(d\right)-P{L}_{FS}\left(d\right)=10\times n\times {\log }_{10}\left(\frac{d}{d_0}\right)-10\times {{\log }_{10}\left(\frac{d}{d_0}\right)}^2+\sigma \mathrm{MCBE}=10\times \mathrm{Log}\left({\left(\frac{d}{d_0}\right)}^{n-2}\right)+\sigma & \left(8\right)\end{array}$

(31) The MCBE parameter is calculated for a given environment with which there is associated an attenuation model.

(32) The MCM parameter is determined for a target QoS, typically a target bit error ratio TEBc (for example TEBc=10.sup.−5).

(33) The metric α gives a good measurement of a relative degradation introduced by the transmission medium of the communication signal for a given environment relative to a reference model of the transmission medium, since MCM corresponds to the additional power necessary for a multipath channel relative to a reference gaussian channel to achieve a same bit error ratio and MCBE corresponds to the additional attenuation obtained for an attenuation model in obstructed space relative to that obtained for a reference attenuation model in free space.

(34) Attenuation models are known to the person skilled in the art for each environment.

(35) The metric ν is a standardized submetric expressed in dB, dimensionless, which reflects the ratio between the mean bitrate D.sub.moy and the physical bitrate D for a given distance d:

(36) $\begin{array}{cc}{v\left(d\right)}_{dB}=10.Math.{\log }_{10}\left(\frac{D_{moy}\left(d\right)}{D}\right)& \left(10\right)\end{array}$

(37) It corrects the expression of the metric α of the ratio between the mean bitrate D.sub.moy and the physical bitrate D of the same transmission mode: α′=α−ν.

(38) When there is no retransmission nor any packet loss, the metric ν has a value close to zero which does not significantly alter the metric α. The selection then takes place simply on the basis of the metric α as long as the distance d is less than a distance threshold d.sub.0 beyond which the metric ν becomes meaningful. Beyond the distance threshold d.sub.0 the bitrate is degraded which increases the value of α′. Taking the metric ν into account can thus lead to the transmission mode previously selected for a distance less than the threshold d.sub.0 being eliminated and lead to the selection decision being modified. The distance threshold d.sub.0 from which a mode is degraded depends on this mode and on the propagation conditions which induce a degradation of the quality (BER) and, if appropriate, a reduction of the received power level RSSI (received signal strength indicator).

(39) FIG. 2 is a flow diagram of a particular embodiment of a selection method according to the invention.

(40) For a distance considered d and for each mode considered from among the different modes MODE.sup.i i=1, . . . N, the method 1 comprises the determination 2 of the value of the first metric α.sup.i corrected by the second metric ν.sup.i:α′.sup.i=α.sup.i−ν.sup.i, ν.sup.i determining the ratio between the mean bitrate D.sub.moy.sup.i and the physical bitrate D.sup.i for this same transmission mode.

(41) For the distance considered d, the method 1 further comprises the comparison 3 of the values of the corrected first metric α′.sup.i obtained for the different transmission modes considered MODE.sup.i i=1, . . . N to select at least one transmission mode {MODE.sup.i, . . . }, j∈[1, . . . , N].

(42) According to one embodiment of the comparison step 3, the selection method 1 performs a scheduling according to the increasing values α′.sup.q≤α′.sup.p . . . or decreasing values α′.sup.q≥α′.sup.p . . . , p and q∈[1, . . . , N] of the corrected first metric to select at least one transmission mode. The method can select the modes {MODE.sup.j, . . . } for which the value of the corrected first metric is minimal with a range of variation of 10%.

(43) The steps of determination 2 and of comparison 3 are repeated for each new value considered for the distance d.

(44) For each transmission mode MODE.sup.i there is a distance threshold d.sub.0.sup.i beyond which the mean bitrate D.sub.moy.sup.i, obtained with this mode deviates from the physical bitrate D.sup.i obtained with this same mode.

(45) When the distance d considered is less than the threshold d.sub.0.sup.i of each of the transmission modes MODE, the selection method can determine as many values of the corrected metric α′.sup.i as there are different transmission modes MODE.sup.1, MODE.sup.2 . . . , MODE.sup.N.

(46) When the distance d considered is greater than the threshold d of a transmission mode MODE.sup.i, this mode can be discarded in the comparison step based on the relative values between the modes considered of the corrected first metric α′.sup.i. At the very least, a mode is discarded when the mean bitrate that it obtains is close to zero.

(47) According to one embodiment, for the selected transmission mode or the different selected transmission modes {MODE.sup.j, . . . }, the selection method 1 determines 4, for a selected transmission mode, the value of a third metric β.sup.j corrected by the second metric ν.sup.j:β′.sup.j=β.sup.j+ν.sup.j.

(48) The third metric β measures the excess power available at the distance d, that is to say the difference between the available power and the required minimum power: β=Pa(d)−S.sub.M. The minimum power S.sub.M required to ensure a transmission bitrate D for a given transmission mode corresponds to the multipath sensitivity threshold. The available power Pa(d) depends on the environment considered, on the radiated power PIRE at the output of the transmission antenna and on the antenna gain in reception.

(49) The value of the third metric β varies notably with the noise contribution in a given transmission band which, when it increases, requires a stronger transmission power. The available power Pa(d) should be at least equal to the minimum power S.sub.M required to establish the communication according to a transmission mode selected on the basis of the first metric α.

(50) For a multipath propagation model, Pa(d) is given by:
Pa(d)=PIRE−PL.sub.MFS(d)+Gr in dBm  (10)

(51) with Gr the gain of the reception antenna, PIRE the radiated power at the output of the transmission antenna of the transmitting entity given by the expression:
PIRE=Pt+Gt in dBm  (11)

(52) with Pt the power of the transmission antenna input, Gt the gain of the transmission antenna.
Also: α=MCM+MCBE=(S.sub.M−S)+(PL.sub.MFS(d)−PL.sub.FS(d)),
i.e.: −PL.sub.MFS(d)−S.sub.M=−PL.sub.FS(d)−α−S,

(53) with S the minimum power required to ensure the physical bitrate D for a gaussian channel (perfect channel affected by an AWGN noise contribution).
Therefore β=Pa(d)−S.sub.M=PIRE−PL.sub.MFS(d)−S.sub.M+Gr

(54) The third metric β can therefore be expressed according to the following relationship:
β=PIRE−PL.sub.FS(d)−α−S+Gr  (12)

(55) Given that the mean bitrate can vary as a function of the distance, the third metric β is corrected by the value of the second metric: β′=Pa(d)−S.sub.M+ν

(56) This corrected third metric β′ can therefore be expressed according to the following relationship:
β′=PIRE−PL.sub.FS(d)+Gr−α−S+ν
β′=PIRE−PL.sub.FS(d)+Gr−(α−ν)−S
β′=PIRE−PL.sub.FS(d)+Gr−α′−S

(57) Thus, the available power Pa(d) must be at least equal to the required minimum power S.sub.M to which there is added the power loss due to the reduction of the bitrate, a loss taken into account by the second metric to establish the communication according to a transmission mode selected on the basis of the corrected first metric α′:
Pa(d)>S.sub.M−ν i.e.: β′>0.

(58) The corrected first metric α′ assures the achievement of a certain quality, target QoS, typically a target bit error ratio TEBc=10.sup.−5, to deliver a bitrate at the distance d and the corrected second metric β′ makes it possible to check that the power available at the distance d for the selected mode is indeed sufficient.

(59) The transmission modes {MODE.sup.j, . . . }, j∈[1, . . . , N], selected on the basis of the corrected first metric α′ for which the corrected second metric β ′ is less than zero are discarded because they do not ensure a sufficient power at the distance d.

(60) If several modes {MODE.sup.k, . . . }, k∈[1, . . . , N], of transmission lead to a corrected second metric β′ greater than zero then the selection method chooses 6 a mode MODE.sup.m, m∈[1, . . . , N], from among these modes {MODE.sup.j, . . . }. The chosen mode MODE.sup.m is that for which the corrected second metric is maximal between two values β′.sub.min and β′.sub.max. β′.sub.min is equal to zero plus, possibly, a margin of ⅔ dB and β′.sub.max is of the order of 35 dB. Taking β′.sub.max into account is optional, and its aim is to limit the transmission powers and improve the coexistence between communication entities present in the same coverage zone.

(61) The method for selecting a transmission mode is implemented by a communication entity (access point, base station, terminal, etc.). The simplified structure of such an entity is described hereinbelow and illustrated by FIG. 3.

(62) This entity STA comprises several transmission modes MODE.sup.1, MODE.sup.2, . . . , MODE.sup.N that make it possible to achieve a certain bitrate. Each transmission mode is associated with a transmission interface. The entity comprises one or more different interfaces. When a communication is established with another communication entity (access point, base station, terminal, etc.), the choice of a common transmission mode MODE.sup.m must be made by the entities. This choice MODE.sup.m is made by the transmitting entity by implementing in particular a selection method according to the invention.

(63) The entity STA comprises a memory MEM comprising a buffer memory and a processing unit μP equipped for example with a microprocessor and driven by a computer program Pg to implement a selection method according to the invention.

(64) On initialization the code instructions of the computer program Pg are for example loaded into a fast memory before being executed by the processor of the processing unit μP.

(65) The microprocessor of the processing unit μP implements a selection method according to the invention described previously, according to the instructions of the computer program Pg.

(66) The implementation of the method is illustrated by FIGS. 4 and 5 that are derived from simulations.

(67) The simulated system considered is a single-technology system. It comprises a single transmission interface of IEEE 802.11ad type which operates at 60 GHz with a bandwidth of 2160 MHz. For such a system, a communication entity (access point, base station) and a terminal are considered that are separated by a distance d. Different transmission modes can be selected to deliver a target bitrate D.

(68) According to the simulations, the access point AP sends, according to a communication protocol of CSMA/CA (carrier sense multiple access with collision avoidance) type, data packets to the terminal during a simulation time T.sub.sim with a given transmission mode making it possible to deliver a bitrate at a given distance. The distance d considered between the two devices is variable to obtain bitrate values at different distances.

(69) On reception, the received power is calculated for the given distance d and as a function of the parameters of the propagation model. This received power makes it possible to determine a signal-to-noise ratio SNR. The probability of error per bit BER and consequently the probability of packet error PER are determined knowing the SNR and by using the quality tables predefined for each mode TM (transmission mode). In the case where the probability of packet error is zero, PER=0, all the packets transmitted are correctly received and the bitrate provided is the target physical bitrate. When the probability of packet error is greater than zero, PER>0, i.e. some packets are erroneous or absent at reception, retransmissions are requested which introduces latencies and therefore a loss of bitrate.

(70) Thus, for each distance value d, the bitrate is calculated as a function of the number of packets correctly received which makes it possible to plot the curves which give the variation of the bitrate as a function of the distance d.

(71) Parameterizable configurations make it possible to distinguish different transmission modes. A first type of configuration relies on a single-antenna technique SISO (single input single output), and a second type of configuration relies on an MISO technique (2,1) (multiple input single output). This second type of configuration is associated with a space-time block coding (STBC) using the Alamouti code. A single spatial flow is considered for this second type of configuration: the coded symbols are sent on two transmission antennas and received by a single reception antenna. Thus, the bitrate PPDU without retransmission or acknowledgment procedure (maximum bitrate D.sub.M on the PPDU layer) does not vary between these two types of configuration. According to the example, these two types of configuration are each associated with three different modulation and coding schemes (MCS):

(72) TABLE-US-00001 MCS Modulation Coding rate D (Mbps) D.sub.M (Mbps) 15 QPSK ½ 1386 1237 17 QPSK ¾ 2079 1822 18 16QAM ½ 2772 2374
to distinguish, according to the example, six different modes, MODE.sup.1, MODE.sup.2, . . . , MODE.sup.6, N=6.

(73) The propagation model used is called OLOS (obstruct line of sight). This model represents the case in which an obstacle obstructs the main path. The parameters for this model are as follows: d.sub.0=reference distance generally taken to be equal to 1 m, ƒ.sub.c=carrier frequency, σ=standard deviation of a gaussian random variable representing the mask effects and the variations of losses due to the movements of the obstacles in the propagation channel. The parameters of the losses due to the propagation model are recalled in the table below.

(74) TABLE-US-00002 OLOS PL.sub.FS (d.sub.0, f c).sub.dB 59.83 N 2.56 σ 5.04

(75) FIG. 4 represents the values of the mean bit rate D.sub.moy as a function of the distance d obtained at the end of the simulations. Each of the two types of configuration is combined with the three MCSs 15, 16 and 17 to define the six different transmission modes:

(76) MODE.sup.i={SISO QPSK ½, SISO QPSK ¾, SISO 16 QAM ½, MISO QPSK ½, MISO QPSK ¾, MISO 16 QAM ½ }.

(77) Below a distance threshold d which depends on the mode MODE.sup.i, the mean bitrate is substantially constant and substantially equal to the bitrate D.sub.M corresponding to the PPDU bitrate without HARQ or SAW and SR mechanisms. The bitrate PHY is the bitrate of the data payload field in the PSDU field (physical bitrate D). Above the threshold d the mean bitrate D.sub.moy varies.

(78) For the SISO 16 QAM ½ and SISO QPSK ¾ modes, the threshold has a value lying between 20 and 25 m. For the MISO 16 QAM ½ mode, the threshold has a value lying between 30 and 35 m. For the SISO QPSK ½ and MISO QPSK ¾ modes, the threshold is equal to approximately 40 m. For the MISO QPSK ½ mode, the threshold has a value close to 50 m.

(79) At least approximately from d=30 m, the SISO QPSK ¾ and SISO 16 QAM ½ modes have zero mean bitrates and therefore lead to invalid values of α′. It follows therefrom that, for this distance and the greater distances, the selection of the transmission mode excludes these two modes that have become out of radio range and compares the modes that are still valid. At least approximately from d=40 m, the MISO 16 QAM ½ mode has a zero mean bitrate and therefore leads to an invalid value of α′. It follows therefrom that, for at least this distance and the greater distances, the selection of the transmission mode excludes this mode which has become out of radio range and compares the modes that are still valid.

(80) FIG. 5 illustrates the change of selection of a mode when the distance exceeds a threshold. This figure gives the value of the corrected metric α−ν, called α′, as a function of the distance. The values of α′ are obtained for the same simulations as those having led to the bitrate values represented in FIG. 4.

(81) Given the bitrate values represented in FIG. 4, the selection method leads to selecting between the SISO 16 QAM ½ and MISO 16 QAM ½ modes for the distances less than approximately 25 m on the basis of the first metric α. Beyond 25 m, the SISO 16 QAM ½ mode is invalidated on the basis of the corrected metric α given that the distance threshold is crossed for this mode. The selection method according to the invention based on the corrected metric α therefore leads to the selection of the MISO 16 QAM ½ mode for d≥25 m. The threshold for this MISO 16 QAM ½ mode has a value close to 33 m according to the illustration of FIG. 5. Thus, beyond this threshold, the selection method according to the invention based on the corrected metric α leads to this MISO 16 QAM ½ mode being eliminated in favor of a mode with a value less than α′ compared to that obtained for the MISO 16 QAM ½ mode. According to the illustration, beyond approximately 36 m, the SISO QPSK ½, MISO QPSK and MISO QPSK modes lead to values of α′ less than that obtained for the MISO 16 QAM ½ mode. Beyond approximately 36 m, the selected mode therefore forms part of one of these three modes. The MISO QPSK mode which leads to the metric α′ with the lowest value is selected on the basis of this metric.

(82) According to an embodiment relying on effective simulations and measurements of the PPDU bitrate of the simulation tool ns−3, after retransmission of packets in an HARQ mechanism, the mean bitrate D.sub.moy is approximated by a polynomial relationship of the form D.sub.moy=ad.sup.2+bd+c with a, b and c coefficients determined during simulations by multiple linear regression. The aim is to approximate the function y=ƒ(x) representative of a measurement by a second order polynomial function described by g(x)=ax.sup.2+bx+c. In the context of the invention, this is the measurement of the bitrate D.sub.moy of the transmission mode considered as a function of each point x representative of a transmitter-receiver distance d. For this, the approximation method relies on the least squares method in which the minimization of the function Rx=(Y−G(X)).sup.2 makes it possible to determine the coefficients a, b and c of the polynomial function. The minimization consists in calculating the partial derivative of the function Rx for each of the coefficients a, b and c to be determined and in considering the cancelation of these three equations with the partial derivatives. Three equations with three unknowns (a, b and c) (Kramer system) are then obtained that can be solved by conventional methods.

(83) ${\left(Y-G\left(X\right)\right)}^2=\underset{i=1}{\overset{N}{.Math.}}{\left(y_i-a{x}_i^2-bx-c\right)}^2$$\frac{\partial }{\partial a}{\left(Y-G\left(X\right)\right)}^2=0$$\frac{\partial }{\partial b}{\left(Y-G\left(X\right)\right)}^2=0$$\frac{\partial }{\partial c}{\left(Y-G\left(X\right)\right)}^2=0$

(84) Although the present disclosure has been described with reference to one or more examples, workers skilled in the art will recognize that changes may be made in form and detail without departing from the scope of the disclosure and/or the appended claims.