# Patent

## Title
Analog-to-digital conversion of a radiocommunication signal

## Abstract

A method for analog/digital conversion of a radiocommunication signal using an analog analog/digital converter preceded by variable gain amplifier, wherein the frequency band of the converted signal contains at least one useful channel, comprises steps for controlling the value of the gain of the amplifier in such a way that the power in the frequency band of the converted signal is less than a limit saturation value of the analog/digital converter. However, the power in the useful channel is substantially equal to a mean power level having a first predetermined value in the case of static propagation in the useful channel or a second predetermined value, different from said first predetermined value, in the case of dynamic propagation in the useful channel.

## Background
This application is a 371 of PCT/FRO1/02290 filed on Jul. 13, 2001.


FIELD OF THE INVENTION
The present invention relates to a method of analog/digital conversion of a radiocommunication signal, with the aid of an analog/digital converter preceded by a variable-gain amplifier. The converted signal has a determined frequency band containing at least one useful channel, in general from among several different channels. A useful channel is the frequency band of minimum width making it possible to recover a useful signal.
The invention finds applications in radiofrequency receivers, such as may be found for example in the fixed equipment (base stations) or the mobile equipment (portable terminals) of radiocommunication systems. In such an application, the signal considered is for example a radiofrequency signal such as a phase-modulated and/or amplitude-modulated carrier, or a signal resulting from the transposition of such a signal to an intermediate frequency or to baseband. The useful signal contains information coding voice and/or data and/or signaling information.
BACKGROUND OF THE INVENTION
In state of the art receivers, the frequency band of the converted signal corresponds to the useful channel. Accordingly, the radiofrequency signal received on a reception antenna is filtered, in general after transposition to an intermediate frequency or to baseband, by means of a filter disposed in the reception chain upstream of the analog/digital converter. In this way, the latter converts only the energy of the signal in the useful channel. When the receiver is a multichannel receiver, channel selection means, comprising a selective filter, make it possible to select the useful channel from among a plurality of different channels. In a manner known per se, a variable-gain amplifier can precede the analog/digital converter in the reception chain, so as to tailor the power level in the useful channel to the input dynamic swing in power of the converter. This is achieved by virtue of appropriate automatic gain control means.
Attempts are currently being made to dispose the analog/digital converter nearer to the reception antenna, and in particular upstream of the channel selection means. Hence, the signal converted by the analog/digital converter has a determined frequency band containing at least one useful channel, in general from among several different channels. The width of the frequency band of the converted signal, or converted band, is then greater than that of the useful channel. This converted band is determined by the (analog) filters disposed upstream of the converter. The channel selection means, disposed downstream of the analog/digital converter in the reception channel, then comprise a mixer and one or more digital filters for selecting the useful channel, before digital demodulation and decoding.
However, the power of the radiofrequency signal received on the reception antenna varies over time. These variations may be due to the appearance or the disappearance of obstacles between the transmitter and the receiver, to the appearance or the disappearance of other signals in the frequency band occupied by the signal, or to “fading” when there is a relative motion of the receiver with respect to one of the transmitters. One speaks of propagation of dynamic type in a channel when there is a relative motion of the receiver with respect to the corresponding transmitter, and of propagation of static type in the converse case. Fading is considerable when the Doppler frequency of f0×v/c is considerable, where fo is the central frequency of the channel, v is the relative speed of the receiver with respect to the transmitter and c is the speed of light. It is noted that when a signal is situated in a “fading hole” its power may become very small. The decrease in power of the signal in a “fading hole” is of short duration. In fact, the shorter the duration of the “fading hole”, the smaller the power of the signal in the “fading hole”.
Moreover, the input dynamic swing in power of the analog/digital converter is limited above by an upper limit value beyond which the converter is saturated, and below by a lower limit value beneath which the signal can no longer be distinguished from the noise introduced by the converter. One speaks of saturation value to designate said upper limit value, and of noise floor to designate said lower limit value. By convention, the desired power levels and the mean power values indicated subsequently in this document may be expressed in decibels (dB) with respect to the noise floor of the converter.
Furthermore, any radiocommunication system complies with specifications which determine the sensitivity and the rejection of the system with regard to the useful channel, as a function of the type of propagation in this channel. The sensitivity of the system corresponds to the minimum power level of the signal in the useful channel (signal-to-noise ratio), at which the system must still operate. The rejection of the system corresponds to the maximum power level which must be tolerated by the system inside the converted band, in the channels neighboring the useful channel. The sensitivity in the static case is less than the sensitivity in the dynamic case, and the rejection in the static case is greater than the rejection in the dynamic case. In one example, the dynamic sensitivity is equal to 15 dB above the noise floor of the converter, and the dynamic rejection is equal to 45 dB. Moreover, the static sensitivity is equal to 7 dB above the noise floor of the converter and the static rejection is equal to 70 dB. The input dynamic swing in power of the converter which is necessary is therefore equal to 60 dB in the dynamic case, to 77 dB in the static case, and hence to 85 dB to cover both the static case and the dynamic case if the useful signal level is fixed at the same level in both cases. At a rate of 6 dB per digit, a converter operating on 15 bits at output is therefore necessary.
SUMMARY OF THE INVENTION
An object of the present invention is to propose a method and a device which make it possible to use a converter operating on a smaller number of bits at output, and which is therefore less expensive.
Accordingly, the invention proposes a method of analog/digital conversion of a radiocommunication signal with the aid of an analog/digital converter preceded by a variable-gain amplifier, the frequency band of the converted signal containing at least one useful channel, consisting in controlling the value of the gain of the amplifier in such a way that the power in the frequency band of the converted signal is less than a limit saturation value of the analog/digital converter and that the power in the useful channel is substantially equal to a mean power level having a first predetermined value in the case of static propagation in the useful channel or a second predetermined value, different from said first predetermined value, in the case of dynamic propagation in the useful channel.
By distinguishing the static case from the dynamic case, it is possible to decrease the input dynamic swing in power of the converter. Thus, returning to the values of the example above, the values of the gain of the amplifier can be controlled in such a way that the power in the useful channel is substantially equal to 7 dB above the noise floor of the converter in the static case, and to 15 dB above the noise floor of the converter in the dynamic case. In this way, the input dynamic swing in power of the converter which is necessary to cover both cases is equal to 77 dB. A converter operating on 13 bits at output is therefore sufficient, and is much less expensive than a converter operating on 15 bits at output.
The invention also proposes a device for analog/digital conversion of a radiocommunication signal whose frequency band contains at least one useful channel, comprising an analog/digital converter preceded by a variable-gain amplifier, and means for controlling the value of the gain of the amplifier in such a way that the power in the frequency band of the converted signal is less than a limit saturation value of the analog/digital converter and that the power in the useful channel is substantially equal to a desired mean power level in the useful channel having a first predetermined value in the case of static propagation in the useful channel or a second predetermined value, different from said first predetermined value, in the case of dynamic propagation in the useful channel.
The invention further proposes a radiofrequency radiocommunication receiver incorporating such a device.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is the simplified diagram of a radiofrequency radiocommunication receiver incorporating a device according to the invention;
FIG. 2 is the diagram of a unit for measuring the power in the converted band or in the useful channel;
FIG. 3 is a curve illustrating the profile over time of the power of the signal in a useful channel;
FIG. 4 is a flow chart of the steps of the method according to the invention;
FIG. 5 and FIG. 6 are flow charts detailing substeps respectively of a step of determining the type of propagation in the useful channel and a step of determining a “fading hole” of the signal in the useful channel.



DETAILED DESCRIPTION
Represented in FIG. 1 is the diagram of a radiofrequency receiver incorporating a device according to the invention. The receiver comprises a reception antenna 10 linked to the input of a radiofrequency amplifier 11 which outputs a radiofrequency signal RF. This is for example a phase-modulated and/or amplitude-modulated signal. In one example, the spectrum of the RF signal lies in the 380-500 MHz band. It comprises several distinct channels which are, for example, all of the same width.
The RF signal is carried to a first input of a first mixer 12a. A second input of the mixer 12a receives a signal at a frequency fLO1 less than the frequency of the RF signal, delivered by a first local oscillator 13a. In one example, the frequency fLO1 is equal to 154 MHz. The mixer 12a outputs a signal IF1 which corresponds to the RF signal transposed to the intermediate frequency fLO1. The signal IF1 is filtered by means of a first bandpass filter 14a. The output of the filter 14a is linked to a first input of a second mixer 12b. A second input of the mixer 12b receives a signal at a frequency fLO2 less than the frequency fLO1 of the signal IF1, delivered by a second local oscillator 13b. In one example, the frequency fLO2 is equal to 500 KHz. The mixer 12a outputs a signal IF2 which corresponds to the RF signal transposed to the intermediate frequency fLO2. The signal IF2 is filtered by means of a second bandpass filter 14b. The signal IF2 thus filtered corresponds to the radiocommunication signal S according to the invention.
The device according to the invention comprises a variable-gain amplifier 15 whose input is linked to the output of the filter 14b so as to receive the signal S, and whose output delivers the amplified signal S. The device further comprises an analog/digital converter 16 whose input is linked to the output of the amplifier 15 so as to receive the amplified signal S and whose output delivers discrete instantaneous values or samples Sn of this signal. The sampling frequency fe of the converter 16 is for example equal to 2 MHz, complying with the Shannon condition.
Specifically, by reason of the passband of the bandpass filters 14a and 14b, the frequency band of the signal S, called the converted band, is around 150 KHz wide. Stated otherwise, it is assumed that the rejection, in terms of power, is big enough outside of this band for it to be possible to neglect the spectral components of the signal S outside of this band. Moreover it is centered on the frequency 500 KHz. In one example, a useful channel lying in the converted band is considered to be centered on the frequency 450 KHz. The converted band comprising several channels such as the useful channel, the bandwidth of this channel is less than that of the converted band.
The device comprises a unit 18 for measuring the power in the converted band, whose input receives the samples Sn, possibly but not necessarily via a digital filter 17 whose passband corresponds to the width of the converted band. The device also comprises a unit 20 for measuring the power in the useful channel, whose input receives the samples Sn via a channel selection module 19. The module 19 comprises means of digital transposition, for transposing the useful channel to baseband. In the example, these means comprise a digital mixer making it possible to transpose the converted band in such a way that it is centered on 50 KHz. The useful channel is then centered on 0 Hz. The module 19 also comprises digital means of low-pass filtering making it possible to isolate the components of the useful signal. The samples S′n output by the module 19 are also transmitted to the downstream part 23 of the radiofrequency receiver, which here is represented overall by a box. This downstream part 23 comprises in particular the means of demodulation and of decoding of the useful signal, which make it possible to extract the data transmitted in the useful signal.
The units 18 and 20 produce respectively values PB of the power in the converted band and values PC of the power in the useful channel, which are provided at the input of a management unit 21 of the device. Preferably, the device comprises, for example, in the power measuring units 18 and 20, means for compensating for the difference of delay in the transmission of the values PB and PC which is due to the difference of the paths taken. This makes it possible to deliver at the input of the unit 21 values of the power in the converted band and in the useful channel referring to identical samples Sn. The management unit 21 comprises means for implementing a method according to the invention, and outputs a control signal for an automatic gain control module 22. The module 22 produces an analog signal which is carried to a gain control input of the variable-gain amplifier 15, so as to control the gain of this amplifier in the manner which will now be described.
The gain of the amplifier is preferably controlled as a function of values of the mean power in the converted band and/or in the useful channel, in order for the device not to be too sensitive to the slight fluctuations in the power of the signal received. Specifically, taking account of values of the instantaneous power would give rise to changes in the gain of the amplifier which could prove to be inopportune in the sense that they might destabilize the reception chain. The values of the mean power are calculated over a determined time window. The larger this time window, the less sensitive is the device to the fluctuations in the power of the signal received. From another point of view, the measurement of the mean power of the signal is then available only after the expiry of this time window. This lag may be penalizing in certain cases, in particular on switching on the receiver. Specifically, it causes a delay in the adjusting of the reception chain which takes place in a receiver initialization phase.
This is why, according to an advantageous characteristic of the invention, the units 18 and 20 each produce N series of values, denoted P1n to PNn in what follows, of the mean power respectively in the converted band (aforesaid values PB) and in the useful channel (aforesaid values PC), where N is an integer, the values of these series being calculated over increasing time windows of respective width. In one example, N is equal to 5.
Represented in FIG. 2 is the simplified diagram of a power measurement unit such as the aforesaid unit 18 or the unit 20. In what follows, the terms “input signal” used with reference to the units 18 and 20 designate the signal corresponding respectively to the series of samples Sn and S′n provided as input to the units 18 and 20, that is the samples of the signal in the converted band and in the useful channel respectively. For the unit 18, the series of values P1n to PNn correspond to the values of the power in the converted band which are indicated overall by the reference PB in FIG. 1, while for the unit 20, the values P1n to PNn correspond to the values of the power in the useful channel which are indicated overall by the reference PC in FIG. 1. The description of a power measurement unit which follows with reference to FIG. 2 relates to the example of the unit 18 receiving the series of samples Sn as input signal. Given that the unit 20 is identical to the unit 18, this description also holds, with the necessary alterations to the notation, for the unit 20 receiving the series of samples S′n as input signal.
The power measurement unit of FIG. 2 optionally comprises a subsampling module 101 which carries out a subsampling of the samples of the input signal, at a subsampling frequency fse which is a submultiple of the sampling frequency fe. In one example, fse=fe/125, so that one sample Sn out of 125 is transmitted by the module 101. The values of the samples Sn are for example coded on p bits, where p is an integer.
The unit further comprises a module 102 for calculating instantaneous power receiving as input the series of values Sn. This module has the function of producing a series of values Pn of the instantaneous power of the input signal, from the series of values Sn. The values Sn being writable in the form of an imaginary number Sn=SIn+i. SQn, where SIn and SQn are real numbers and where i2=−1, the values Pn are obtained successively from the successive values Sn by performing for each the calculation Pn=S2In+S2Qn. The values Pn are therefore coded on 2p bits.
The unit further comprises, according to the invention, N mean power calculation modules arranged in cascade, where N is an integer. Each of these modules, referenced 1031 to 103N in FIG. 1, allows the continuous production of the series of values P1n to PNn respectively of the mean power of the input signal, calculated over increasing respective time windows directly or indirectly from the values Pn of the series of values of the instantaneous power of the input signal. The modules 1031 to 103N are in what follows called modules for calculating mean power of level 1 to N respectively. These are synchronous modules.
The module 1031 for calculating mean power of level 1 comprises a memory register 1041, as well as a counter C1 (not represented) counting up to N1, where N1 is an integer such that N1≧2, (not represented) and means for resetting to zero the register 1041 and the counter C1 (also not represented). It further comprises means of addition 1051, a first input of which is coupled to the output of the circuit 102 for calculating instantaneous power so as to receive the values Pn of the instantaneous power of the input signal, a second input of which is coupled to an output of the register 1041 so as to receive the current value stored in this register, and the output of which is coupled to the input of said register 1041. With each reception of a new value Pn, the means of addition 1051 produce a value equal to the sum of said value Pn and of said current value stored in the register 1041 this sum value being then stored in the register 1041 in place of said current value. Stated otherwise, the hereinabove means of the module 1031 form an accumulator register. Such a register is of very simple structure and requires little memory space, since the register 1041 must have a length enabling it to store the result of the addition of N1 words of 2p bits, that is equal to 2p+N1 only.
The module 1031 for calculating the mean power of level 1 outputs a series of values P1n which are obtained successively for example by averaging over N1 successive values Pn of the instantaneous power of the input signal. Preferably, this is an arithmetic mean, which is the simplest to implement since it requires just one complex step of division by N1. Accordingly, the counter is incremented by one unit with each reception of a new value Pn and corresponding updating of the value stored in the register 1041. When the counter reaches the value N1 the value stored in the register 1041 is divided by N1 to yield an arithmetic mean of the last N1 successive values Pn of the instantaneous power of the input signal which were received at the input of the module 1031. A value P1n of the mean power of level 1 of the input signal is thus produced. Moreover, the value of the counter C1 and the value stored in the register 1041 are reset to zero. Preferably, the integer N1 is an integer power of 2, that is there exists a nonzero integer k1 such that N1=2k1. This makes it possible to simplify the step of division by N1 since it is then sufficient to eliminate the N1 least significant bits of the value stored in the register 1041 to produce the value P1n.
Each circuit 103j for calculating the mean power of level j, where j is an index such that 2≦j≦N, produces a j-th series of values Pjn of the mean power of level j of the input signal from Nj values of the j−1-th series of values Pj−1n of the mean power of level j−1 of the input signal, where Nj is an integer such that Nj≧2. It is necessary to distinguish between the last module 103N (for which j=N) and the other modules 103j (for which 2≦j<N).
For the values of j such that 2≦j<N, the values of the j-th series of values Pjn of the mean power of the input signal are obtained successively by averaging over successive Nj-tuples of successive values of the j−1-th series of values Pj−1n of the mean power of level j−1 (immediately lower level) of the input signal. Preferably, this is an arithmetic mean, which is the simplest to implement since it requires few complex calculations.
Accordingly, each module 103j for calculating the mean power of level j can have the same structure as the module 1031 for calculating the mean power of level 1 described hereinabove, with a counter Cj counting up to Nj.
Nevertheless, in a preferred exemplary embodiment, each module 103j for calculating the mean power of level j comprises, in place of the memory register 1041 of the module 1031, a shift register 104j of length Nj, that is comprising Nj elementary registers in series, as well as a counter Cj (not represented) counting up to Nj and means (also not represented) for resetting to zero the counter Cj and optionally the register 104j. It further comprises means of addition 105j with Nj inputs which are linked respectively to the outputs of the Nj elementary registers 104j so as to receive the Nj values stored in the shift register 104j. The input of each module 103j is coupled to the output of the module 103j−1 so as to receive the values Pj−1n and its output is coupled to the input of the module 103j+1 so as to send it the values Pjn. Each time a value Pj−1n is input into the shift register 104j, the counter Cj is incremented by one unit. When Nj values Pj−1n of the mean power of level j−1 (immediately lower level) have been input into the shift register 104j, that is when Cj=Nj, these Nj values are added together in the adder 105j. The sum obtained is then divided by Nj to produce a value Pjn of the mean power of level j of the input signal. Moreover, the shift register 104j can be emptied of the values which it contains, by virtue of the abovementioned means for resetting to zero. Preferably, each integer Nj is an integer power of 2, that is there exists an integer kj such that Nj=2kj. This simplifies the step of division by Nj, as has been set forth previously.
The structure of the modules 103j for calculating mean power of level j for 2≦j<N thus enable them to keep in memory, in the shift register 104j, the previous values of the mean power of level j−1. This history of the values of the mean power can thus be used at any instant, as will be described hereinbelow.
Let us now see the particular case of the last module 103N. The values of the last series of values PNn of the mean power of level N of the input signal are obtained successively by taking a sliding average over the successive KN-tuples of the last NN values of the N−1-th series of values PN−1n of the mean power of level N−1 (immediately lower level) of the input signal.
Accordingly, the module 103N for calculating the mean power of level N can have the same structure as the module 1031 for calculating the mean power of level 1 described earlier, with a counter CN counting up to NN, but which is not reset to zero after the calculation of each value PNn.
Nevertheless, in a preferred exemplary embodiment, the module 103N for calculating the mean power of level N of the input signal comprises a shift register 104N of length NN, that is comprising NN elementary registers in series, and an adder 105N with NN inputs for receiving respectively the NN values stored in the shift register 104N, where NN is an integer. The input of the module 103N is coupled to the output of the module 103N−1 for calculating the mean power of level N−1 (immediately lower level). With each input of a new value PN−1n of the mean power of level N−1 of the input signal into the shift register 104N, the values which are stored therein are shifted so that the oldest value PN−1n stored in the shift register 104N is lost. A counter CN (not represented) able to count up to NN is incremented by one unit with each input of a new value PN−1n into the shift register 104N. Moreover, a sum of the NN values newly stored in this register is calculated by virtue of the means of addition 105N. As soon as the counter has reached the value NN (CN≧NN), the sum thus obtained is divided by NN to produce the value PNn of the mean power of level N of the input signal, according to (preferably) an arithmetic mean. Preferably, the integer NN is an integer power of 2, that is there exists an integer kN such that NN=2kN, this simplifying the step of division by NN as was set forth previously. This calculation produces a value PNn of the mean power of level N of the input signal. The counter CN is not reset to zero after the calculation of each value PNn.
The modules 1041 to 104N for calculating the mean power of level 1 to N respectively of the input signal are for example embodied in the form of hardware and/or software modules, for example in a microcontroller, an ASIC circuit, a DSP circuit, an FPGA circuit, or the like.
As will have been understood, the successive values Pn of the instantaneous power of the input signal which are delivered by the circuit 102 bring about the cascaded generation of the series of values P1n to PNn of the mean power of level 1 to N respectively of the input signal. Thus, a value P1n of the mean power of level 1 is a value of the input signal mean power calculated over a time window of width equal to N1 times an elementary duration separating two successive values Pn of the instantaneous power of the input signal. This elementary duration is equal to 



  
    1
    
      fe
      ×
      fse
    
  
  .




Likewise, a value P2n of the mean power of level 2 is a value of the input signal mean power calculated over a time window of width equal to N1×N2 times this elementary duration. Expressed in a general manner, this signifies that a value Pjn of the mean power of level j of the input signal is a value of the signal S mean power calculated over a time window of width equal to N1×N2× . . . ×Nj−1×Nj times the duration separating two consecutive values Pn of the instantaneous power of the input signal. These time windows are therefore of respective increasing widths.
Thus, the more the level j of the mean power of the input signal increases, the more the small variations in the values of the input signal are masked in the value Pjn of this mean power of level j. Nevertheless, the smaller the level of this mean power, the more quickly are the values Pjn of the mean power available after the unit has been switched on. In one example, N is equal to five, N1, N4 and N5 are equal to eight, and N2 and N3 are equal to two. Moreover, the values of the power in the converted band and/or the values of the power in the useful channel which are taken into account in an initialization phase are the values P1n of mean power of level 1 which are calculated over a time window having a first determined width, while the values of the power in the converted band and/or the values of the power in the useful channel which are taken into account in a holding phase are the values P5n of mean power of level 5 which are calculated over a time window having a second determined width, greater than said first determined width. Of course, values of the mean power of different levels may be taken into account for the power in the converted band and for the power in the useful channel. In one example, the initialization phase is considered to begin when the device is brought into service and to terminate as soon as a value P5n of the mean power of level 5 in the useful channel is available. However, the device is brought back to the initialization phase and the memory registers and the counters Ci for i lying between 1 and N of the units 18 and 20 are reinitialized with each modification of the value of the gain of the amplifier 15. Moreover, and preferably, the values of the power in the frequency band of the converted signal and/or the values of the power in the useful channel which are taken into account are calculated on the basis of measurements of the instantaneous power after a first determined lag has elapsed after bringing into service or modifying a parameter of an analog part upstream of the analog/digital converter.
The values PB of the power in the converted band and the values PC of the power in the useful channel which are produced respectively in the units 18 and 20, are by nature decimal values on a linear scale. They are for example expressed in watts (W) or in milliwatts (mW). Moreover, the mean power values are calculated from values in watts or in milliwatts. Nevertheless, it is advantageous to express them in decibel milliwatts (dBm), that is on a logarithmic scale. Specifically, the values of the gain of the amplifier 15 which can be controlled by the gain control signal delivered by the management unit 21 are generally expressed in dB. Likewise, the saturation value Psat and the noise floor Pmin of the converter 16 are generally expressed in dBm in the specifications. Also, the mean power level PCo desired in the useful channel and the mean power level PBo desired in the converted band are expressed in dBm. As was stated earlier, all these values can be expressed through a deviation in dB with respect to the value Pmin of the noise floor of the converter expressed in dBm. Likewise, various margins used in the comparison steps can be expressed in dB. It is indeed advantageous to deal with values expressed in dB, since operations of multiplication or of division on values expressed linearly are then performed by means of simpler operations of addition and of subtraction respectively.
This is why the values PB of the power in the converted band and the values PC of the power in the useful channel are converted into values in decibels by means of a predetermined conversion table stored for this purpose in the units 18 and 20 respectively. Such a table can take the form given by table I below. In the unit 20, there exists such a table for each type of power measurement of a channel (sampling frequency, channel filter used) lying in the frequency band of the radiocommunication signal S. In each of these tables, each column corresponds to one of the predetermined values which can be taken by the gain of the amplifier 15. These gain values go from a minimum value Gmin to a maximum value Gmax with a stepsize of for example 1 dB. Advantageously, the power values converted into decibels by means of the table are then independent of the current value of the gain G of the amplifier 15.
Likewise each row of the table corresponds to a measured power value going from a minimum value Pmin, which corresponds to the value of the noise floor of the converter possibly increased by a margin, to a maximum value PBmax (for the power in the converted band) or PCmax (for the power in the useful channel), with a stepsize ΔP of for example 0.5 dB. Each row of the table therefore corresponds to an index j such that the power value indicated in this row corresponds to Pmin+j.ΔP for j going from 0 to Np, where Np is an integer.









TABLE I









Gain















Index
PB
Gmin
. . .
. . .
. . .
. . .
. . .
Gmax





0
Pmin
X
X
X
X
X
X
X


1
Pmin + ΔP
X
X
X
X
X
X
X


2
Pmin + 2.ΔP
X
X
X
X
X
X
X


.
.
X
X
X
X
X
X
X


.
.


.
.


j
Pmin + j.ΔP
X
X
X
X
X
X
X


.
.
X
X
X
X
X
X
X


.
.


.
.


Np
Pmin + NpΔP
X
X
X
X
X
X
X









The conversion of any power value is carried out as follows. The value to be converted, expressed in watts or in milliwatts, is compared with the values from the column of the table corresponding to the current value of the gain G of the amplifier 15, which values are expressed in the same unit (W or mW). It is perhaps equal to one of these values or lies between two of these values contained in two adjacent rows of the table. If it is less than Pmin or greater than Pmin+NpΔp, it is set to Pmin or to Pmin+NpΔp, respectively. From this is then deduced the value of the index j corresponding to the row of the table whose value, for the relevant column, is closest to the value to be converted. The value of this index is saved in memory and is used to compare the power value with other power values converted in the same way. It was seen that Pmin corresponds to the zero value of the index j. In one example, Psat corresponds to the value Np of the index j. Stated otherwise, Psat=Pmin+Np.Δp. Index values (integers) are thus compared, instead of comparing values in watts or in milliwatts (decimal numbers). This is simpler. Moreover, the index values can be saved in place of the corresponding values in watts or in milliwatts. This occupies less room in memory.
In FIG. 3, the curve 50 represents an example of the profile versus time of the power PC in a determined useful channel. Horizontal lines represent an interval around a desired predetermined mean power level PCo which is regarded as satisfactory for the channel taking into account the input dynamic swing in power of the analog/digital converter 16. This interval is delimited by an acceptable maximum value PCmax and by an acceptable minimum value PCmin.
In case of static propagation in this channel, that is when the corresponding transmitter is fixed with respect to the receiver, the power PC hardly varies over time. Its slight fluctuations are due only to parasitic glitches in the channel. In case of dynamic propagation in the channel, that is when the corresponding transmitter is moving with respect to the receiver, the power PC varies somewhat more, and it may pass below the level PCmin as indicated for example by the reference 52 in FIG. 3, or above the level PCmax.
According to the invention, the desired mean power level PCo, and possibly also the deviation between the acceptable minimum level PCmin and/or acceptable maximum level PCmax on the one hand and PCo on the other hand, depend on the type of propagation in the useful channel. In one example, the values of PCo, PCmin and PCmax are respectively equal to 4 dB, 7 dB and 10 dB above the noise floor Pmin of the analog/digital converter 16 (PCmin=Pmin+4 dB; PCo=Pmin+7 dB; PCmax=Pmin+10 dB) in the static case, and respectively to 12 dB, 15 dB and 18 dB above Pmin (PCmin=Pmin+12 dB; PCo=Pmin+15 dB; PCmax=Pmin+18 dB) in the dynamic case.
The type of propagation, static or dynamic, can be determined as a function of the state of the receiver (when the latter comprises means for detecting that it is moving), or of data received from the transmitter (when the latter comprises means for detecting and signaling that it is moving). Nevertheless, the device according to the invention preferably comprises means for determining the type of propagation, static or dynamic, in the useful channel as a function of the history of the values of the power in the useful channel which are obtained in the absence of saturation of the analog/digital converter as will be explained in greater detail hereinbelow in conjunction with FIG. 5.
When the signal in the channel is in a “fading hole”, the power PC in this channel may suddenly become less than the minimum level PCmin, as indicated for example by the references 51 and 53 in FIG. 3. However, this sudden decrease in the power in the useful channel is of short duration. Consequently, it may be preferable not to modify the gain of the variable-gain amplifier 15 (FIG. 1) on account of the power measurements in such a “fading hole”.
This is why the device comprises means for determining whether the signal in the useful channel is in a “fading hole”, and for modifying the gain of the amplifier 15, as appropriate, only if the signal in the useful channel is not in a “fading hole”. A “fading hole” is detected by the abrupt variation of the successive values of the power PC in the channel, as will be explained in greater detail hereinbelow in conjunction with FIG. 6.
The manner of operation of the device, according to the method of the invention, is described hereinbelow with reference to the flow chart of FIG. 4.
When the receiver is switched on, the device operates according to an initialization phase. Subsequently, it operates according to a holding phase. The method of analog/digital conversion whose steps are represented on the flow chart of FIG. 4 and implemented both during the initialization phase and during the holding phase. It will be now be described in the case of the initialization phase. This description is, making the necessary alterations which will be pointed out, also valid for the implementation in the holding phase. It will therefore not be repeated for the latter, so as to avoid a redundancy.
The method begins with a step 31 consisting in assigning a predetermined initial value to the gain G of the amplifier 15, which normally makes it possible to avoid the saturation of the analog/digital converter 16. A step 32 then consists in comparing a value PB of the power in the converted band with the limit saturation value Psat. If PB is not greater than Psat minus a predetermined margin, we then go to a step 34. If on the contrary PB is greater than Psat minus said margin, then, in a step 33, the gain G of the amplifier 15 is decreased in such a way that the power PB in the converted band is substantially equal to a desired value PBo less than or equal to Psat minus said margin. For this purpose, two cases are distinguished. If PB is less than or equal to PBmax, then the current value G of the gain of the amplifier 15 is replaced by G+PBo−PB. If on the contrary PB is strictly greater than PBmax, then the current value G of the gain of the amplifier 15 is replaced by G−ΔG, where ΔG constitutes a relatively high variation in the gain relative to the gain stepsize of the conversion table (which is 0.5 dB). For example ΔG equals 2 dB. After step 33 we return to the comparison step 32. In this way, possibly after several iterations of step 33, the gain G of the amplifier 15 is such that the power PB in the converted band is at most substantially equal to the value PBo of the mean power level desired in the converted band.
Step 34 consists in determining the type of propagation, static or dynamic, in the useful channel as a function of the history of the values PC of the power in the useful channel which are obtained in the absence of saturation of the converter 16. It will be detailed hereinbelow in conjunction with the flow chart of FIG. 5. It is followed by a step 35 of determining a possible “fading hole” in the useful channel. This step 35 will be detailed hereinbelow in conjunction with the flow chart of FIG. 6.
Next, the method comprises a step 36 consisting in comparing a value PC of the power in the useful channel with the mean power level PCo desired in this channel.
More exactly, a check verifies whether the value PC lies in the interval around Pco defined by the values PCmin and PCmax. If PC is not outside this interval, then the end 38 of the method has been reached. Conversely, if PC is outside this interval, then, in a step 37, the gain G of the amplifier 15 is modified so that the value PC is inside said interval. However, this modification of the value of the gain G must not entail any risk of the saturation of the converter 16. This is why a test for validating change of the gain is performed so as to verify that the envisaged new value of the gain does not entail any risk of saturation of the converter 16. The new value of the gain which is envisaged to replace the current value G is for example G+PCo−PC. The test for validating the change of gain consists in comparing the value PB of the power in the converted band with the limit saturation value Psat of the converter 16. More exactly, if (PCo−PC)+PB≦Psat−margin, then the envisaged new value of gain can be adopted since there is no risk of it causing the saturation of the converter 16. Otherwise, the new value of the gain must be limited to Psat−margin−PB. In this way, priority is given to the avoiding of the saturation of the converter 16 over the obtaining of the best possible sensitivity in the useful channel. Stated otherwise the gain of the amplifier is controlled in such a way that the power in the useful channel is substantially equal to the predetermined mean power level PCo. Thus, the gain of the amplifier is increased at most by only a value such that the power in the converted band remains less than the limit saturation value Psat of the converter 16 minus said determined margin.
After step 37 we are at the end 38 of the method. Nevertheless, steps 32 to 37 may be repeated cyclically both during the initialization phase and during the holding phase.
Represented in FIG. 5 is a flow chart showing substeps of step 34 of determining the type of propagation in the useful channel.
During substep 341, the values PCi of the instantaneous power of the useful channel are produced and kept over a duration T. The value of T depends on a speed v of the mobile above which the mobile is regarded as being in the dynamic regime and below which the mobile is regarded as being in the static regime. T must then be greater than the period of the fadings, that is greater than 1/(2*fd), where fd represents the Doppler frequency, given by the expression fd=f*v/c; f represents the carrier frequency of the signal and c the speed of light. To dimension T, the variations of f from one channel to another of one and the same system are disregarded. By way of example, the value of v is fixed at 10 km/h.
During step 342, the maximum and the minimum of the power values PCi over the time interval T are calculated. These two values can also be calculated using respectively the N largest values (in place of the maximum) or the N smallest values (in place of the minimum). The deviation between these two extreme values will make it possible to update the regime to be considered.
If the deviation is less than a threshold S1, this signifies that the power PCi has varied little over the time interval T and that the propagation is of the static type in the useful channel. A substep 343 is then performed which consists in assigning a value corresponding to this type of propagation to the predetermined mean power level PCo. If conversely the difference is greater than the threshold S1, then this signifies that the power PCi has varied significantly over the time interval T and hence that the propagation is of the dynamic type. A step 344 is then performed which consists in assigning a value corresponding to this type of propagation to the predetermined power level PCo.
In one example, in step 343 the value Pmin+7 dB is given to Po, that is the predetermined mean power level is 7 dB above the value Pmin of the noise floor of the converter 16. Correspondingly, the value Pmin+4 db is given to the value PCmin, this signifying that the acceptable minimum level for the power for the useful channel is situated 4 dB above the value Pmin of the noise floor of the converter 16. Again likewise, the value Pmin+10 dB is given to the value PCmax, this signifying that the acceptable maximum value of the power in the useful channel is situated 10 dB above the noise floor Pmin of the converter 16. In step 344, the values Pmin+15 dB, Pmin+12 dB, and Pmin+18 db are given to the values PO, PCmin and PCmax respectively.
Represented in FIG. 6 is a flow chart showing substeps of step 35 of determining a “fading hole” of the signal in the useful channel.
During steps 351 and 352, partly merged with step 341 of FIG. 5, two consecutive values PCi and PCi+1 of the power in the useful channel are produced. These two values are instantaneous values of the power in the useful channel. In a step 353, the difference PCi+1−PCi between these two consecutive values is compared with a second determined threshold S2 which is greater than the threshold S1 mentioned hereinabove in conjunction with FIG. 5. If this difference is less than the threshold S2 then this signifies that the signal in the useful channel is not in a “fading hole”. This is why in a step 354, the second value PCi+1 is taken into account in the calculation of the mean values P1n to P5n of the power in the useful channel. In the converse case, represented symbolically by substep 355, this signifies that the signal in the useful channel is in a “fading hole”. This is why the second value PCi+1 is not taken into account in the calculation. It is quite simply ignored.

## Claims

1. A method of analog/digital conversion of a radiocommunication signal whose frequency band contains at least one useful channel, using an analog/digital converter preceded by a variable-gain amplifier which receives said signal and which has a gain of a given value, the method comprising steps for controlling the value of the gain of the amplifier in such a way that power in the frequency band of the signal is less than a limit saturation value of the analog/digital converter and that power in the useful channel is substantially equal to a desired mean power level in the useful channel having a first predetermined value in the case of static propagation in the useful channel or a second predetermined value, different from said first predetermined value, in the case of dynamic propagation in the useful channel.

2. The method of claim 1 comprising the steps of:
comparing values of the power in the frequency band of the converted signal with the limit saturation value of the analog/digital converter; and 
if a value of the power in the frequency band of the signal is greater than said limit saturation value of the analog/digital converter minus a predetermined margin, decreasing the gain of the amplifier in such a way that the power in the frequency band of the signal is at most substantially equal to a desired mean power level in the band of the signal.

3. The method of claim 2, wherein the values of the power in the frequency band of the converted signal and/or the values in the useful channel which are taken into account in an initialization phase are values of mean power calculated over a time window having a first determined width, while the values of the power in the frequency band of the converted signal and/or the values of the power in the useful channel which are taken into account in a holding phase are values of mean power calculated over a time window having a second determined width, greater than said first determined width.

4. The method of claim 2, wherein the values of the power in the frequency band of the converted signal and/or the values of the power in the useful channel which are taken into account are calculated on the basis of measurements of an instantaneous power after a first determined lag has elapsed after bringing into service or modifying a parameter of an analog part upstream of the analog/digital converter.

5. The method of claim 2, wherein the values of the power in the frequency band of the converted signal and/or the values of the power in the useful channel which are taken into account are calculated on the basis of measurements of an instantaneous power after a second determined lag has elapsed after the modification gain of the amplifier.

6. The method of claim 2, wherein the values of the power in the frequency band of the converted signal and/or the values of the power in the useful channel which are taken into account are converted into values in decibels independent of the gain of the amplifier by means of a predetermined conversion table.

7. The method of claim 1 comprising a step of determining the type of propagation, static or dynamic, in the useful channel.

8. The method of claim 7 wherein the type of propagation in the useful channel is determined as a function of the history of the values of the power in the useful channel which are obtained in the absence of saturation of the analog/digital converter.

9. The method of claim 1 comprising the steps of:
comparing values of the power in the useful channel with said determined mean power level; 
if a value of the power in the useful channel is outside a determined interval around said determined mean power level, modifying the gain of the amplifier so that the value of the power in the useful channel is inside said determined interval.

10. The method of claim 9, wherein the gain of the amplifier is controlled in such a way that the power in the useful channel is substantially equal to said determined mean power level.

11. The method of claim 9, wherein the gain of the amplifier is increased, at most, only by a value such that the power in the frequency band of the converted signal remains less than the limit saturation value of the analog/digital converter minus said determined margin.

12. The method of claim 9, further comprising a step of determining whether the signal in the useful channel is situated in a fading hole, the gain of the amplifier being modified, if appropriate, only if the signal in the useful channel is not situated in a fading hole.

13. A device for analog/digital conversion of a radiocommunication signal whose frequency band contains at least one useful channel, comprising an analog/digital converter preceded by a variable-gain amplifier which receives said signal and which has a gain of a given value, and means for controlling the value of the gain of the amplifier in such a way that power in the frequency band of the converted signal is less than a limit saturation value of the analog/digital converter and that power in the useful channel is substantially equal to a desired mean power level having a first predetermined value in the case of static propagation in the useful channel or a second predetermine value, different from said first predetermined value, in the case of dynamic propagation in the useful channel.

14. The device of claim 13 comprising:
means for comparing values of the power in the frequency band of the converted signal with the limit saturation value of the analog/digital converter; and 
means for decreasing the gain of the amplifier in such a way that the power in the frequency band of the converted signal is at most substantially equal to said limit value minus a determined margin, if a value of the power in the frequency band of the converted signal is greater than said limit saturation value of the analog/digital converter minus said determined margin.

15. The device of claim 13 further comprising means for determining the type of propagation, static or dynamic, in the useful channel as a function of the history of the values of the power in the useful channel which are obtained in the absence of saturation of the analog/digital converter.

16. The device of claim 13, further comprising:
means for comparing values of the power in the useful channel with said predetermined mean power level; and 
means for modifying the gain of the amplifier so that the value of the power in the useful channel is inside a determined interval around, said determined mean power level, if a value of the power in the useful channel is outside said determined interval.

17. The device of claim 16, further comprising means for determining whether the signal in the useful channel is situated in a fading hole, the gain of the amplifier being modified, if appropriate, only if the signal in the useful channel is not situated in a fading hole.

18. The device of claim 13, further comprising a first unit for measuring power delivering first values of the power in the frequency band of the converted signal which are values of mean power calculated over a time window having a first determined width, and second values of the power in the frequency band of the converted signal which are values of mean power calculated over a time window having a second determined width, greater than said first determined width.

19. The device of claim 18, wherein the first and/or the second unit for measuring power comprise:
a) means for producing a series of successive samples of the signal; 
b) means for producing a series of successive instantaneous power values of the signal, each of these values being obtained from the value of a respective sample of the series of successive samples of the signal; 
c) means for producing N series of successive values of the mean power of the signal over respectively N time windows of respective increasing widths, where N is an integer such that N≧2, from the values of the series of successive values of the instantaneous power of the signal.

20. The device of claim 18 further comprising a management unit connected to the output of the first and/or of the second unit for measuring power so as to receive respectively the values of the power in the frequency band of the converted signal and/or respectively the values of the power in the useful channel, and means for compensating for any difference of delay in the transmission of these respective values which is due to a difference of the paths taken, so as to deliver at the input of said management unit values of the power in the converted band and values of the power in the useful channel referring to identical samples of the converted signal of the converted signal.

21. The device of claim 13, further comprising a second unit for measuring power delivering first values of the power in the useful channel which are values of mean power calculated over a time window having a first determined width, and second values of the power in the useful channel which are values of mean power calculated over a time window having a second determined width, greater than said first determined width.