FR2706231A1 - Radio telephony installation having mobile terminals, with a satellite link - Google Patents

Abstract

Device for correcting for the Doppler shift due to the relay satellite in a digital radio telephony link. The base station creates a FCCHD synchronisation burst which replaces the usual FCCH synchronisation burst which is of fixed frequency. The instantaneous frequency (fi) varies, for this FCCHd burst, according to a known law, for example a sum of two linear laws for the duration of the burst, with different coefficients (8, 9, Figure 3). The start time ( tau ) of this FCCHD burst is calculated by a rule of three. The Doppler shift (d) is calculated by elimination of the other terms from the received signal and spectral analysis. The correction for the Doppler shift is performed by correction of the frequency of the demodulation Local Oscillator.

Description

RADIO-TELEPHONY INSTALLATION WITH MOBILE TERMINALS

WITH SATELLITE LINK

  The present invention relates to an installation of digital radiotelephony with mobile terminals, called "car telephones", this installation being able to use a link by

  satellite traveling in low orbit.

  In the case of a Radio- installation

  digital mobile telephony with a purely conventional terrestrial link, radio coverage is currently organized as follows: The terrestrial coverage area is divided into a mosaic of hexagonal cells whose radius can, depending on the environment, be between one and thirty-five kilometers. In open countryside it is around thirty-five kilometers, while it is around one to three kilometers in built-up areas. A fixed base station, or "BSS" ("Base Station System"), is placed in the center of each hexagonal coverage cell and is responsible for the radio link with all mobile terminals

  which are located within the perimeter of this cell.

  It therefore brings together all the radio equipment necessary to cover a cell. A mobile radio installation of this type has at its disposal two different frequency bands: a first frequency band which is allocated to the transmission of information going from the mobile terminal to the base station, and a second frequency band which is assigned to the transmission of information from the base station to the mobile terminal. Each of these two frequency bands comprises a certain number of carrier frequencies spaced from one kilohertz from the other, and defining a corresponding number of frequency channels which are

  each with a width of 200 Kilohertz for example.

  The modulating signals are not emitted permanently on each carrier, but they are emitted in packets or "bursts". The duration of a burst is

  currently typically 0.577 milliseconds.

  The transmission is made on each carrier by successive periodic frames which are typically subdivided into eight time intervals which each comprise a packet or burst. Each frame has for example a duration of 4.615 milliseconds. Each burst therefore forms a physical channel on which it is possible to

  multiplex a number of logical channels.

  There are two types of information packets, or bursts, transmitted: "normal bursts" which are used to exchange information data (telephone conversations) with mobile terminals, and therefore correspond to traffic channels, the "access bursts" which are used to allow the initial access of the terminals to the network: they define the

signaling channels.

  In fact, each base station has a set of carriers, typically eight or sixteen carriers, which are used to carry signaling and traffic. In particular, there is a carrier called BCH ("Broadcasting CHannel") which carries signaling

  allowing the initial access of the terminals to the network.

  When it is powered up, a mobile telephone terminal performs the following operations: - it listens successively to all of the carriers received and, for each of them, measures the electrical power received or RSSI ("Received Signal Strength Intensity"), which allows it to select the carriers transmitted by the base station which, by its coverage area, is in fact geographically associated with it; - one of these carriers corresponds to the aforementioned BCH, and it periodically transmits a frame which includes a burst of fixed frequency, called FCCH ("Frequency CHannel") and a synchronization burst known as SCH ("Synchronization CHannel"): the terminal searches accordingly , among the carriers selected, the possible presence of an FFCH burst and it then proceeds to frequency synchronization; - it finally performs fine time synchronization using the SCH burst of the

  BCH carrier which corresponds to this FFCH burst.

  Access from this terminal to the network is then achieved and the user can then communicate with his correspondent via the base station on which he depends.

geographic point of view.

  The object of the invention is to provide an installation of mobile digital radiotelephony with satellite link between each terminal and its terrestrial base station, such an installation reusing as far as possible the systems already implemented in the context of purely terrestrial links. to minimize the number of additional modules to be developed and therefore the size and volume of the portable terminal. It is thus in particular possible to carry out a

  installation which uses dual-mode terminals,

  ie capable of being used both in an existing purely terrestrial network and in the mobile radio-satellite telephone network currently being planned. The principle of such an installation is to use the satellite, which travels in low orbit at an altitude for example of around 1390 kilometers and at a speed for example of the order of 7.2 kilometers per second, as a relay between the terminal

  mobile and its base earth station.

  The satellite therefore receives the radio wave from the base station and retransmits it towards the terminal. The role played by this satellite is a simple "mirror" role: it transmits the signal it receives from the base station, as is

  at most carrying out a frequency transposition.

  However, when it is received, the signal received by the telephone terminal is affected by a frequency displacement due to the Doppler effect, since the satellite moves relative to this terminal. This Doppler frequency displacement depends on the speed and the relative position of the satellite with respect to the telephone terminal, which itself can be considered to be fixed even if it is on board a motor vehicle. Three cases are possible: the satellite approaches the telephone terminal: in this case the Doppler movement is positive; its value is maximum when the satellite appears on the horizon, and is then typically of the order of 45 kilohertz, and it decreases as the satellite approaches; the satellite is at its zenith: in this case the value of the Doppler shift is zero; the satellite moves away from the telephone terminal: the Doppler movement then becomes negative; it increases in absolute value as the satellite moves away to reach a maximum value typically of the order of 45 kilohertz when the satellite

disappears on the horizon.

  However, the base band having a width of 90 kilohertz, it is not possible to neglect the frequency shift of the spectrum of the signal due to this

Doppler effect.

  On the one hand indeed the frequency of the signal BCH, which should be fixed so that one can synchronize on it, varies constantly so that synchronization is impossible. On the other hand, the frequency of this signal, which is normally 67.708 kilohertz for example and is therefore included in the bandwidth of the terminal which is itself between 0 and 90 kilohertz, comes out of this bandwidth when the Doppler shift is positive, this

  BCH signal then being lost, and therefore unusable.

  In the end, the system cannot operate since the mobile terminal cannot

synchronize over the network.

  Note that the movement of the satellite, and therefore the Doppler effect, is without any noticeable drawback on the operation of the base station, which is fixed and which also functions permanently. The path of each satellite is in fact known with precision for each fixed earth station, and it is then possible to provide, for each base station, an algorithm

correction of the Doppler effect.

  The invention therefore also aims to remedy this impossibility of synchronization of the telephone terminal, due to Doppler frequency displacement

  generated by the movement of the satellite.

  Note that there are many systems, in particular used on ships, which use a

  geo-stationary satellite for which there is no-

  heard no Doppler frequency shift

  important enough to be taken into account.

  However, satellites traveling in low orbit were preferred in the context of this project for very obvious reasons of traffic capacity and propagation time of radio waves between

the earth and the satellite.

  The invention relates to these effects to a digital radiotelephony installation with mobile terminals, this installation being able to use a satellite link running in low orbit, and characterized in that, in order to allow access to a terminal telephone on the network despite the Doppler frequency shift (d) which is generated by the movement of the satellite: - Each base earth station is equipped with means for, when connected to a terminal via the satellite, thus transmit to this terminal, instead of the classic burst FCCH- of fixed frequency and normally intended, for a direct terrestrial link, for the frequency synchronization of the various mobile terminals, another burst FCCHd which replaces this burstFCCH and whose instantaneous frequency, instead of be fixed, varies during the duration of this burst FFCHd according to a known law, for example a sum of two linear laws with different coefficients famous; the terminal is equipped with means for: conventionally choosing the carriers to be scanned as a function of the field received and measured; among these selected carriers, choose the one which contains a burstFCCHd; then proceed, on the two quadrature channels (I) and (Q), to acquire the signal received with a number (N) of samples sufficient to perform the calculation which follows and to store samples sI (t ) and SQ (t), t being the variable "time" (continuous representation); then perform and record the various products if (kT) .sQ (kT), kT then being the variable "time" (discrete representation, T denotes the sampling period, k an integer index);

  then proceed to a digital pass filter

  low to eliminate the higher frequency component; then proceed to the spectral analysis of the filtered signal thus obtained in order to determine the difference (fr) of the instantaneous frequencies at the time of said acquisition from the sampling sequences on these two quadrature channels; knowing the duration (TB) of said FFCHd burst as well as the instantaneous frequencies (f I) and (fQ) on each of the two quadrature channels (I) and (Q) of a quadrature demodulator at the end of this burst, calculate l time interval (Z) which separates the real start of this FFCHd burst from the start of said acquisition by the formula: = TB. ft 2 (fI-fQ) o f. is determined by spectral analysis, and o fI is assumed to be greater than fQ; then synchronize the terminal with the start, now known, of this burst FCCHd; then determine the absolute value of the Doppler shift (d) by acquiring a complete FCCHd burst on one of the two quadrature channels (I) or (Q) of said receiver by means of sample storage, then by processing if necessary this signal thus stored, and moreover affected by said Doppler displacement (d), in order to keep only the term in cosine, or sine, of (2Wrdt), added with a phase term (Y) , and finally by carrying out the spectral analysis of the signal thus obtained in order to deduce therefrom the value of said Doppler displacement (d); determine the sign of this Doppler displacement (d) by proceeding in the same way to the calculation of two Doppler displacements (dl) and (d2) at sufficiently long successive intervals, of the order of at least one to several seconds for example, and by calculating the difference in absolute value Id21-ldlI of these two displacements in order to deduce therefrom the direction of variation of the Doppler displacement (d) and therefore its sign; and finally to compensate, in this terminal, for this Doppler frequency shift (d) by using, for the demodulation of the information signal received from the satellite, a Local Oscillator whose frequency is not the usual fixed frequency (fo) for such a demodulation, but is equal to this frequency (fo) affected by the value of this displacement Q

Doppler frequency (d).

  According to a first preferred embodiment, said determination of the absolute value of the Doppler displacement (d) is carried out: - by acquiring a complete FCCHd burst, by means of storage of samples, on the 'any of these two quadrature channels (I) or (Q) of said receiver; - then by processing, in a sampled manner, this burst FCCHd thus acquired in the following manner: the signal is separated into two channels and phase shifted by + 90 on one of these two channels; on the channel not phase shifted by + 90, the signal is multiplied, using a first multiplier, by another signal having a linear frequency variation as a function of time, recreated by the terminal, and identical, to a factor near proportionality, to the representative signal, on this channel (I) or (Q), of said receiver of the burst FCCHd as it was originally transmitted by the base earth station; on the other channel, the phase shifted signal by + 90 'is multiplied, in a second multiplier, by said other signal recreated by the terminal, after this other signal has also been phase shifted by + 90; the respective signals then obtained at the output of these two multipliers are added; and the signal then obtained is subjected to a spectral analysis, the result of this spectral analysis being the sought absolute value of said

Doppler displacement (d).

  According to another preferred embodiment, the aforementioned linearity coefficient for the variation of the instantaneous frequency is chosen to be equal to zero on the Q channel for example, where the frequency is therefore fixed and is moreover chosen to be equal to zero, throughout the duration of said FCCHd burst so that said determination of the absolute value of the Doppler displacement (d) is carried out simply: by acquiring a complete FCCHd- burst, by means of sample storage, on the quadrature channel of said receiver ,. then by proceeding directly to the spectral analysis of this signal thus sampled in order to

  deduce the value of said Doppler shift (d).

  In any case, the invention will be well understood, and its advantages and characteristics will emerge

  better, in the following description of an example not

  limiting of embodiment, with reference to the appended diagrammatic drawing in which: - Figure 1 schematizes a possible spatial arrangement for the basic earth station, two mobile terminals and a satellite; Figure 2 gives the two variation curves of the instantaneous frequency of the two signals of the synchronization burst which were emitted by this base earth station; - Figure 3 gives the two variation curves of the instantaneous frequency of these two same signals as they are received by a mobile terminal; - Figure 4 shows the temporal constitution of the signaling multiframe containing the FCCH and SCH bursts transmitted by the earth station on the "time slot" TN O of the carrier frequency BCH; - Figure 5 is a block diagram of the terminal synchronization device mobile; - Figure 6 is a block diagram of a general embodiment of the device for determining the absolute value of the Doppler displacement; - Figure 7 is a block diagram of a simplified form of this same determination of the absolute value of the Doppler displacement; Figure 8 is a block diagram of the device for determining the sign of this Doppler displacement; - Figure 9 is a block diagram of the device for correcting this Doppler effect; and - Figure 10 is a block diagram of the Doppler displacement tracking device which can equip the

  correction device according to Figure 9.

  In FIG. 1, a first mobile telephone terminal 1, or "radiotelephone", is assumed to be located, on the earth's surface 2, at a very short distance from a base earth station 3, this distance being included in the relatively small area of terrestrial coverage of this base station 3. The link between terminal 1 and base station 3 is then a direct microwave and two-way digital link. This is for example a digital link of the "GSM" type known per se, and the circuits of the mobile terminal 1 and the base station

3 are provided accordingly.

  Another mobile digital terminal 4, identical to terminal 1, is however positioned outside of

  the base station coverage area 3.

  The bi-directional radio link between the earth station 3 and the mobile terminal 4 is carried out via a relay satellite 5 which travels in low orbit at a speed of approximately 7.2 km / sec and an altitude of approximately 1390 km . This satellite is content to receive and retransmit the signal, with a possible frequency transposition. It does not perform any other processing on this signal, and therefore simply plays the role of a reflecting mirror. Its visibility time for a mobile station is about ten minutes, but another satellite is planned to relay it then in order to be able to continue any conversation

telephone in progress.

  As mentioned above, it is appropriate, to allow access to the terminal 4 on the telephone network via the satellite 5 and thanks to the base earth station 3, to take into account the Doppler frequency displacement d which is due to the

  scrolling of satellite 5, and which varies continuously.

  The earth station 3 and the terminal 4 are equipped with means which allow this terminal to access the network despite this Doppler movement. These

means will now be described.

  According to an advantageous characteristic of the invention, the installation is "dual-mode", that is to say it allows the same terminal to be used both for the existing terrestrial system (direct GSM link) and for the system using a satellite 5 to provide the link, this dual-value character being obtained without substantial modification of the "hardware" circuits

origin of this terminal.

  The basic general means which is the heart of the present invention consists in asking the fixed earth station 3 to substitute, in the case of a link via the satellite 5, for the conventional burst FCCH of fixed frequency and normally intended, for a link direct terrestrial, at the frequency synchronization of the various mobile terminals such as the aforementioned terminal 1, a replacement burst which will be called FCCHd and whose instantaneous frequency fi, instead of being fixed as in the traditional system, varies according to a known law, for example a sum of two linear laws throughout the duration of this burst,

with different coefficients.

  Concretely, this consists in replacing the pure sine wave of the traditional FCCH burst by the following signal: on channel I of a traditional transmitter: if (t) = cos (21fI t2) TB ot is the time variable which varies between time t = O which corresponds to the start of this burst FFCHd and the time t = TB which corresponds to the end of this burst, and o 2 fI is the instantaneous frequency at the end of this same burst and on this channel I On the channel Q of l above transmitter: SQ (t) = sin (2ffQ t_) TB o 2fQ is the instantaneous frequency at the end of the burst and on this channel Q. We then note that the instantaneous frequency, which is as we know proportional to the derivative of the phase term, that is to say of the term: 2Wf1f t2 for the channel I TB and 2iTrfQ t2 for the channel QQ t2 then varies linearly with time t having the same value for t = O (start of the burst) and different values (2fI and 2fQ respectively) at the end of the burst (t = TB). Graphically speaking, the variation over time, and during the duration of the burst FCCHd as it is transmitted by the base station 3, of the instantaneous frequency fi is represented, on the graph of FIG. 2 taken in the general case, by line segment 6 for channel I of the above-mentioned transmitter and by line segment 7 for channel Q. On the other hand, the instantaneous frequencies of the signals associated with channels I and Q of the receiver and obtained after demodulation of a burst FCCHd received at a given time, after reflection on satellite 5, are affected by the Doppler shift d, as shown

  respectively by curves 8 and 9 in Figure 3.

  The curves according to Figure 2 and Figure 3 correspond to the most general case, which should be

of course to explain here.

  In reality, according to a practical and advantageous characteristic of the invention, one of the two guiding coefficients respectively associated with the two straight lines of variation of the instantaneous frequency can be, throughout the duration of the burst FCCHd,

chosen equal to zero.

  This arrangement simplifies installation, and will be detailed below, after

  description of a nonetheless advantageous form of

  treatment of the general case corresponding to Figures 2

  and 3, which will be done immediately.

  FIG. 4 diagrams the time structure of the signaling multiframe transmitted by the fixed base station 3 on the "time slot" TN 0 of the

carrier frequency BCH.

  Such a multiframe is typically composed of 51 frames of 8 bursts. The FCCHId and SCH logic channels are incorporated as follows: - the FCCHd burst always precedes the SCH burst and they are separated by 8 consecutive bursts, i.e. a frame, - the FCCHd burst is separated from the following FCCHd burst by bursts consecutive, i.e. 10 frames, - the SCH burst is also separated from the SCH burst

  following by 80 consecutive bursts, or 10 frames.

  Each of these two logical channels therefore uses 5

bursts in the multiframe.

  Typically, each burst has a duration of 0.577 milliseconds, so that a complete multiframe has

  a duration of 235.365 milliseconds.

  The first operation that the terminal must do to access the network via satellite 5, is to proceed, on the BCH carrier it has selected by measuring the electrical power received (called RSSI or "Received Signal Strength Intensity"), the acquisition of a burst -FCCHd. The search for such an FCCHd burst also enables it to ensure that it is indeed, on the selected carrier frequency, in the presence of a

BCH carrier.

  The first operation that the mobile terminal will perform after it has selected, by its power, a carrier which it thinks is a BCH carrier, is to test, among the eight bursts of the frame conveyed by this carrier, the presence of '' a burst

FCCHd, marked 10 in Figure 4.

  We will see below how can be detected the presence of such a burst among the others, but in any case each investigation is carried out by the acquisition, on the two channels in quadrature I and Q, of a sufficient number N of samples, for example thirty samples in a

  certain cases, to fix ideas.

  This acquisition of the N samples necessary for processing is carried out for a time tA (FIG. 4). In Figure 4, the acquisition of a burst, assumed here to be effectively the sought FCCHd burst 10, is

represented by hatching.

  However, even if this acquisition is carried out, as in FIG. 4, on the burst FCCHd 10, it is quite obvious that it is commanded at a time which is a priori arbitrary with respect to the real start O (FIG. 3) of the burst FCCHd . However, the terminal must be able to synchronize with precision at the start O of this

  burst FCCHd to be able to access the network.

  In order to be able to obtain this synchronization, it is therefore necessary to determine with precision the time interval which separates the start of the acquisition of N

  samples of the real start O of the FCCHd burst.

  According to the invention, this time interval Z is determined, by a simple calculation of similar triangles ending in a rule of three, in the following way (see Figure 3): If we consider Figure 3 of a point from a view of pure plane geometry, we observe the simple geometric relation:

OZ '= AB

OTB CD

  However, the OTB segment has a known value, which corresponds to the duration TB of the burst. It is the same for the CD segment, whose value corresponds to the difference of the instantaneous frequencies (known) at the end of the burst, respectively 2fú and 2fQ. The AB segment is the

1 Q

  difference in instantaneous frequencies 2fA and 2fB on channels I and Q of said receiver at the time of start of acquisition of N samples: this difference is

  easily determinable by spectral analysis.

  The value of the O7 segment that we are looking for is then calculated simply by the formula:

O E = OTB. AB

  CD or in other words by the rule of three: - = TB.2F-2F = TB f fB

BA-Q B -

  2fi-2 fQ fi fQ Finally, the synoptic and software architecture which makes it possible to carry out all this series of operations which corresponds to the acquisition of the BCHd 10 burst and to the synchronization of the terminal on this burst, is the architecture drawn in Figure 5, and which will be

  now explained in more detail.

  Referring to this FIG. 5, the analog signal respectively received, in baseband, on the two quadrature channels I and Q is, if it is a burst FCCHd, the following: on channel I: if (t) = cos (2l fI t2 + 2Wdt + Y) TB on channel Q: sQ (t) = sin (2'lfQ t2 + 2tdt + ô) TB od is the aforementioned value of the Doppler displacement and

o T is the phase shift term.

  These two signals sI (t) and SQ (t) are each applied to a respective analog low-pass filter, 13 and 14, whose cut-off frequency fc is equal to the operating bandwidth, i.e. 90 kilohertz

in this example.

  These filters 13 and 14 conventionally aim to get rid of parasitic intermodulation products. The signals filtered at outputs 15 and 16 of analog filters 13 and 14 are applied, to be digitized, to respective analog / digital converters 17 and 18, which are synchronized at the sampling frequency by a clock signal H coming from a block 19 which is integrated into the organ

central logic of the terminal.

  Two buses, for example of 8-bit format, respectively 20 and 21, connect these analog / digital converters 17 and 18 to two buffer memories

  22 and 23 respectively, which are synchronized

  also by the clock signal H. Each calculation sequence comprises, under control of block 19, the acquisition of N successive samples of each of the digitized signals sI (t) and sQ (t). This acquisition is shown schematically in Figure

  4, by the hatched part of the burst FCCHd 10.

  The successive samples which are acquired by the buffer memories, or "buffers", 22 and 23, are mixed one by one in a mixer, or multiplier,

digital 24.

  The digital signal resulting from this multiplication is then the following, except for the proportionality factors mentioned above: sI (t) .sQ (t) = cos (2-fI t2 + 2 dt +). Sin (2fYfQ t2 + 2? Wdt ± ) rE Ta be, by developing: sI (t) .sQ (t) = sin [4îÂdt + 2! î (fI + fQ) t 2 + 2f) .sin (2, r (ffQ) t2]

TB TB

  in the case where it is a burst FCCHd, what we

don't know yet at this point.

  This signal sI (t) .sQ (t) is then stored in a

other buffer 25.

  If now it is not a FCCHd burst, but another burst such as bursts 11 or 12, or a burst from a carrier which, although chosen because of its power, is not the BCH carrier sought, the analog signal received is, to within a proportionality factor: on channel I: if (t) = cos L4 (t) + 2lrdt + T] on channel Q: sQ (t) = sin [ t (t) + 2-Tdt + jo <'(t) is the information signal conveyed by this burst. The product si (t) .sQ (t), which is carried out by the digital multiplier 24 and which is stored, sample by sample, in the buffer memory 25, is then equal to: si (t). SQ (t) = sin (4kdt + 2 (t) + 2f) According to the invention, the various samples of the product sI (t) .sQ (t) which are stored in this buffer memory, are applied to a filter digital lowpass 26, whose cutoff frequency fc is equal to that of the system bandwidth

  cited above, that is 90 kilohertz in the example chosen.

  The resulting filtered signal is, sample by sample, stored in another buffer memory 27, to be finally subjected, in block 28, to a spectral analysis capable of determining, in this stored signal, the presence or absence of a frequency line corresponding to a pure sinusoid present in the frequency interval corresponding to the band

basic (here O at 90 KHz).

  In the case, which has just been mentioned above

  above, where in fact the selected burst is not an FCCHd burst, the aforementioned product obtained: sin [4 dt + 2 (t) + 2 E corresponds to a frequency component greater than the maximum frequency, ie 90 KHz, of the base band. This "high-frequency" component is eliminated by the low-pass filter 26, so that the spectral analysis 28 reveals no frequency line, but

only noise.

  As symbolically shown by the diamond 29, the answer to the question: "are we in the presence of an FCCHd burst?" is negative and a corresponding item of information 30 is applied to the calculation block 19 which consequently triggers the acquisition of the following sequence number burst. If after eighty successive investigations of this type, for which all the bursts of a multiframe have been processed, the response to 29 is still negative, this is because the carrier frequency selected is not the carrier frequency BCH of the base station, and the terminal selects another strong carrier accordingly,

  check again for the presence of an FCCHd burst.

  If on the other hand the burst selected is, in accordance with Figure 4, indeed a burst FCCHd, the higher frequency component of the formula, given above, which results from a calculation of the product if (t) .sQ (t), is eliminated by the low-pass filter 26, so that only the following term is stored in the buffer memory 27: if (t) .sQ (t) = - sin 20 (fi-fQ) t 2, TB

  always within a factor of proportionality.

  The instantaneous frequency fi of such a signal is represented by the derivative of the term: (f-fQ) t2 TB and is therefore equal to: fi = 2 (fi-fQ) t TB The spectral analysis performed by the analyzer 28 then necessarily reveals the presence of a frequency line corresponding to a pure sinusoid, and the answer to the question posed by the diamond 29 is then

  positive: it is indeed an FCCHd burst.

  It is then applied, via the link 31, to control information to a block 32 for calculating the time A above, which therefore separates the acquisition time of the first sample from the start of this burst FCCHd, this calculation being understood performed by the rule of

three above.

  This time Z being thus determined, the block 32 addresses, by a link 33, this information to the synchronization block 34 which then consequently performs the necessary synchronization of the terminal on the start of the burst FCCHd 10. The terminal is then, at this stage , synchronized in time, that is, it knows how to

  position at the start of each burst.

  What has so far been described with reference to Figures 4 and 5 is valid both in the general case which corresponds to Figure 3 and in the particular case, quite advantageous, where one of the two guiding coefficients of a of the two lines of variation of the instantaneous frequency of the transmitted signal is zero and the starting point of the two laws of variation is the origin of the axes of the diagram in FIG. 3. The second step of operating the terminal then consists in determining the value displacement

Doppler d.

  This second step will be explained with reference to Figure 6 in the above-mentioned general case of Figure 3, and with reference to Figure 7 in the above-mentioned particular case o one of the two directing coefficients respectively associated with the two lines of variation of the instantaneous frequency fi is chosen

equal to zero.

  The means used then consists in proceeding on a single quadrature channel, for example channel I according to Figure 6: To be carried out, after having previously filtered (low-pass filter 13 previously described) then digitized (analog / digital converter 17 previously describes) the analog signal: sI (t) = cos (2ifi t2 + 2'-dt + ô) IsB

  on the sampled acquisition of the full FCCHd burst.

  For example, we proceed with the acquisition of

  M = 156 samples of this complete burst.

  The storage of these M samples is carried out in the buffer memory 22 which has been previously described

(see Figure 5).

  To separate into two channels 36 and 37 each of these M samples, taken successively in the

buffer 22.

  To be phase-shifted, in phase-shifter 35, by +90 degrees the

signal present on channel 37.

  To multiply, on channel 36 which is not phase-shifted by +90 degrees, in a first multiplier 38, the signal present on this channel by another signal 39, which is synthesized, digitized, and sampled by the central unit of the terminal and which is equal, except for a proportionality factor, to: cos (2 rfz t2) TB and which corresponds to a component of the signal FCCHd as it is transmitted by the base earth station 3. To be multiplied, over l other channel 37, and in a second multiplier 40, the signal 62 which is phase-shifted by +90 degrees by the phase shifter 35 by a signal 63 which is obtained after having phase-shifted, in another

  phase shifter 64, by +90 degrees the aforementioned signal 39.

  Concretely speaking, the signal present at the output 62 of the phase shifter 35 is of the form: - sin (2nf I t + 2 -7dt +4), TB while the signal present at the output 63 of the phase shifter 64 is of the form: - sin (2frt2 f), TB the proportionality factors still not being

  written here, as well as in everything that follows.

  It follows that after the respective multiplications in the multipliers 38 and 40, the signal present on the output of the multiplier 38 is of the form: cos (21 fI t2 + 21q'dt + Y). cos (2Yfi t 2

TB TB

  while the signal present on the output 66 of the multiplier 60 is of the form: sin (21rfI t2 + 2t'dt + '). sin (2rfI t2)

TB T

  The rest of the processing then consists in: Adding, in a digital summator 67, the two just-mentioned signals which are present on the

bus 65 and 66.

  After development and simplification, we note with happiness that the signal obtained on the output 68 of this summator 67 is quite simply of the form:

cos (2W dt +).

  Finally, in an analyzer 69 similar to the analyzer 38 of Figure 5, for example a Fast Fourier Transform or "FFT" analyzer, proceed to the spectral analysis of the signal thus obtained on the bus 68, this spectral analysis being performed finely enough to eliminate additive white Gaussian noise. We thus extract the absolute value sought from

Doppler displacement d.

  In the above, it is understood that the Doppler displacement d does not vary in practice

  so as to influence the results of the above treatment

  above for the duration of the FCCHd burst considered.

  If we now proceed according to the particular case, which is also interesting for its relative simplicity, o the burst FCCHd 10 is created, by the base station 3, so that the curve 7 in Figure 2 is a horizontal line segment and confused with the time axis Ot, the instantaneous frequency fi is constant and equal to zero throughout the duration of the burst FCCHd as it is synthesized by the base station 3, the determination of the Doppler displacement d is, according to Figure 7, fairly simplified. The signal received in baseband on the Q channel of said receiver is then of the form:

sQ (t) = sin (2îldt + ô).

  As in Figure 6, one proceeds, on channel Q, to the acquisition, in the buffer memory 23 already described in Figure 5, of M samples of a complete FCCHd burst. For this, the low-pass filter 14 and the analog / digital converter 18 which have already been used

  have been described with reference to Figure 5.

  The Doppler displacement d is then simply determined by the spectral analysis of the stored signal. For this we use for example the analyzer

69 of Figure 6.

  The next step will now be to determine the sign of this Doppler displacement D. This sign is obtained by determining, as explained above with reference to FIGS. 6 or 7, the absolute value of two successive Doppler displacements dl then d2, which are separated from one another by a time interval of one to a few seconds, sufficient therefore for the Doppler displacement to have had

  time to vary significantly.

  According to Figure 8, the absolute digitized values 1d21 and Idll of these two Doppler displacements are stored in a buffer memory 41, then compared

in a digital comparator 42.

  If the result of this comparison indicates that Id21 is greater than idll, it means that the satellite is moving away from terminal 4, and that consequently the sign

  of the Doppler shift is negative.

  If on the other hand this comparison indicates that 1d21 is less than Idl, it is because the satellite is approaching the terminal, and consequently that the sign

  of the Doppler shift is positive.

  If, which is inevitably extremely rare, this comparison indicates that 1d2I is practically equal to Idll, it is that the satellite has just passed at the zenith and will therefore start to move away from the terminal: the sign to take into account for move

  Doppler d is therefore in fact the negative sign.

  Finally, the last step will now consist in compensating, for the signal received by the terminal, the Doppler shift d, in sum allowing this

terminal to operate.

  Note that this Doppler displacement d must necessarily be compensated for the entire duration of use of the satellite for the telephone conversation in progress. Given the running time of such a satellite, this duration is

about ten minutes.

  Then the satellite having disappeared on the horizon, it is planned to relay it by another, and so on. It is therefore necessary to plan, in order to be able to correct it permanently and not only at a given instant, to follow this Doppler movement throughout the duration of passage and use

effective satellite.

  A first approach, which is in fact intellectually the simplest to design but which is in practice the heaviest to carry out, consists for the terminal in making measurements of the Doppler displacement at almost regular time intervals, by

  example every ten seconds or so.

  During each measurement, the terminal corrects on the one hand the frequency of the Local Oscillator of the Demodulator as a function of the Doppler displacement measured and, on the other hand, it adjusts the time interval separating two consecutive measurements relative to the importance of the frequency variation then so

  measured, between these two consecutive measurements.

  The very general scheme of compensation, or correction, for the Doppler shift d is given in

solid lines in Figure 9.

  We recognize in this Figure 9 the classic diagram of the quadrature phase demodulation of

  analog signal received by the terminal.

  The non-demodulated analog signal which is present on the receiving line is conventionally divided into two channels: channel I on line 44, and channel Q

on line 45.

  The signal from channel I is applied to a first analog mixer 46 which receives on its other input 47 the fixed frequency signal supplied to it by

Local Oscillator 48.

  However, this Local Oscillator is not a conventional oscillator of immutable natural frequency fo, but is a V.C.O (voltage controlled oscillator) which receives on its control input a voltage representative of the Doppler displacement d, determined by

  the terminal as described above.

  The frequency of Local Oscillator 48 is therefore not its natural frequency fo that it would have in the absence of Doppler displacement correction, but the algebraic sum (fo + d) of this frequency fo and the

Doppler displacement d.

  The demodulated signal on channel I, at output 50 of the mixer 46 is therefore indeed a demodulated signal and

  for which Doppler displacement is compensated.

  Likewise, and in itself very conventional, this frequency signal (fo + d) produced by the Local Oscillator 48 is applied, via a conventional 90-degree phase shifter referenced 51, to another mixer 52 which also receives the signal from channel Q and whose output 53 then carries the demodulated signal on channel Q. According to the first approach mentioned above, the signal present on the control input 49 of Local Oscillator 48 is therefore produced directly from the value of the Doppler shift d measured during correction. This solution is nevertheless rather heavy, because it obliges to carry out the calculation of the Doppler displacement

  quite a number of times.

  We therefore generally prefer a second solution which consists (see in dashed lines in Figure 9) in providing, for the development of the Doppler correction d to be applied to the control input 49 of the Local Oscillator 48, a correction in real time or by means of a logic device 54 for automatically monitoring the Doppler displacement d as a function of the known displacement of the satellite considered, and of a limited number of Doppler displacement measurements carried out as described above and entered into the device logic 54 on an entry door 55 of the latter. The constitution and operation of the logic block 54 for automatic tracking of the Doppler movement

  will now be explained with reference to Figure 10.

  For a given mobile terminal, the temporal variation of the value of the Doppler displacement depends on its relative position relative to the trajectory followed by the satellite. The latter, seen from the terminal, is then characterized by a parameter called maximum elevation and designating the maximum opening of the angle formed over time by the

  terminal-horizon and terminal-satellite lines.

  Concretely, the maximum elevation varies between 20 and 90 degrees. There is provided in a "ROM" memory 56 (non-volatile memory) of the terminal, a table 57 containing the known temporal variations of the Doppler displacement d for different maximum elevations of each satellite. In other words, the terminal has, in this non-volatile memory 56 and for each particular trajectory of each satellite (therefore for a given maximum elevation), the variation curve

associated Doppler displacement.

  To fix the ideas by a numerical example, curves are thus memorized, the resolution for

  the maximum elevation being taken equal to one degree.

  When powering up, the terminal (block 58 comprising a RAM memory, or volatile memory, 59) performs and stores a series of N measurements of the Doppler displacement: d (1), d (2), ..., d (NOT). The number

  N is for example of the order of a few tens.

  These measurements are compared, in a logic block, with the data prerecorded in the ROM memory 56. It is thus determined, if the measurements carried out allow it, what is the variation curve of the Doppler displacement which must presently

be taken into account.

  Another calculation block 61 is therefore provided, which has a role of predicting and supplying the voltage for correcting the Doppler displacement d at the aforementioned Local Oscillator.

  (see link 49, compare with Figure 9).

  In the case where the aforementioned initial measurements d (1) to d (N) do not allow the logic block 60 to determine which is the prerecorded curve which should be taken into account, the terminal then performs, by the logic block 58 , a series of complementary measures. It goes without saying that the invention is not limited to the embodiment which has just been described. On the contrary, it is capable of being implemented in multiple other equivalent forms.

Claims (4)

  1 - Installation of digital radio telephony with mobile terminals (4) and at least one fixed base earth station (3) associated with each mobile terminal (4), this installation being able to use a satellite link (5) scrolling in low orbit, characterized in that, in order to allow access to a telephone terminal (4) on the network despite the Doppler frequency movement (d) which is generated by the movement of the satellite (5): - said station base (3) is equipped with means for, when in connection with said terminal (4) via said satellite (5), thus transmit to this terminal, instead of the classic burst FCCH of fixed frequency and normally intended, for a another direct ground link (3-1), at the frequency synchronization of the various mobile terminals (1), another burst FCCHd which replaces this classic burst FCCH, and whose instantaneous frequency (fi), instead of being fixed, varies according to a known law, for example a so even of two linear laws; - the terminal (4) is equipped with means (..., 19 to 69, ...) for: conventionally choosing the carriers to be scanned as a function of the field received and measured; among these selected carriers, choose the one which contains an FCCHd burst (10); proceed then, on the two channels (I) and (Q) of a quadrature receiver, to the acquisition (17,18) of a number (N) of samples sufficient to carry out the calculation which will follow, and storage (22,23) of these samples if (t) and SQ (t), t being the variable "time" then perform (24) and record (25) the various products sI (kT) .sQ (kT), kT designating the variable "time" (discrete representation);   then proceed to a digital pass filter   bottom (26) in order to eliminate, from each of these products thus obtained, the higher frequency component; then proceed to the spectral analysis (28) of the filtered signal thus obtained (27) in order to determine the difference (f) of the instantaneous frequencies on these two quadrature channels (I, Q) at the time of said acquisition; knowing the duration (TB) of said burst FCCHd (10) as well as the instantaneous frequencies (fI) and (fQ) at the end of the burst of the transmitted signal, calculate (32) the time interval (t) which separates the actual start of this burst FCCHd of the start of said acquisition by the formula: Z = TB. fC 2 (fT-fQ) o fz is determined by spectral analysis, and o fI is assumed to be greater than fQ; then synchronize (34) the terminal with the start, now known, of this burst FCCHd; then determine the absolute value of the Doppler shift (d) by acquiring (22 Figure 6, 23 Figure 7) a complete FCCHd burst on one of the two channels of the receiver in quadrature by means of sample storage , then by processing, if necessary (35 to 39, 63 to 68), this signal, thus stored and otherwise affected by said Doppler displacement (d), in order to keep only the term in cosine, or sine, of ( 21Y dt), added with a phase term (Y), and finally by carrying out the spectral analysis (69) of the signal thus obtained to deduce therefrom the value of said Doppler displacement (d); determine the sign of this Doppler displacement (d) by proceeding in the same way to the calculation of two Doppler displacements (dl) and (d2) at sufficiently long successive intervals, of the order of at least one to several seconds for example, and by calculating (41,42) the difference in absolute value ld21-ldlI of these two displacements in order to deduce therefrom the direction of variation of the Doppler displacement (d) and therefore its sign; and finally to compensate, in this terminal, this Doppler frequency displacement (d) by using, for the demodulation (46,51,52) of the received information signal (43,44, 45) coming from the satellite, a Local Oscillator (48) whose frequency is not the usual fixed frequency (fo) for such demodulation, but is equal to this frequency (Fo) affected by the value of this displacement Doppler frequency (d).   2 - Installation according to claim 1, characterized in that said determination of the absolute value of the Doppler movement (d) is carried out: - by acquiring (17,22) a complete FCCHd burst, by means of d 'storage of samples, on any one of these two channels of the quadrature receiver (I) or (Q); - then by processing, in a sampled manner, this burst FCCHd thus acquired (22) as follows: the signal is separated into two channels (36,37) and phase shifted (35) by +90, on one (37) of these two ways; on the channel (36) not phase shifted by +90, the signal is multiplied, using a first multiplier (38), by another signal having a linear frequency variation as a function of time, recreated by the terminal, and identical, except for a proportionality factor, to the representative signal, on this channel (I) or (Q), of said receiver of the FCCHd burst as it is originally transmitted by the base earth station; on the other channel (37) the signal (67) phase shifted by +90 is multiplied, using a second multiplier (40), by the aforementioned signal (39) phase shifted (64,63) by +90; the respective signals (65,66) then obtained at the output of these two multipliers (38,40) are added (67); and the signal (68) then obtained is subjected to a spectral analysis (69), the result of this spectral analysis (69) being the absolute value sought for said Doppler displacement (d). 3 - Installation according to claim 1, characterized in that the aforementioned linearity coefficient, identical to one of the two guiding coefficients associated respectively with the two straight lines of variation of the instantaneous frequency, can be during all the duration of said burst FCCHd (10 ) chosen equal to zero, so that said determination of the absolute value of the Doppler displacement (d) is carried out simply: by acquiring (18,23) a complete FCCHd burst, by means of storage of samples, on the quadrature channel (Q) where the instantaneous frequency was chosen to be constant and zero, then then proceeding directly to the spectral analysis (69) of this signal thus sampled for   deduce therefrom the value of said Doppler displacement (d).   4 - Installation according to any one of   claims 1 to 3, characterized in that each   mobile terminal (1,4) is designed to be used both for a direct terrestrial link (1-3) with said base station (3) and for a link (4-5-3) by satellite-relay (5) with this same station base (3).   - Installation according to any one of   Claims 1 to 4, characterized in that, in order to   perform a Doppler displacement correction (d) in real time for the entire duration of the movement of the satellite (5), the terminal (4): performs Doppler displacement measurements   (d) at almost time intervals   regular, corrects, during each of these measurements, the frequency (fo) of the Local Oscillator (48) as a function of the Doppler displacement measured, and adjusts the time interval separating two consecutive measurements relative to the importance of the variation frequency, then thus measured, between these two consecutive measurements. 6 - Installation according to any one of   Claims 1 to 4, characterized in that, in order to   perform a Doppler displacement correction (d) in real time during the entire period of travel of the satellite (5), provision is made in the terminal (4) for developing the Doppler correction (d) to be applied to the control input (49) of the local demodulation oscillator (48), a correction in real time by means of a logic device (54) for automatic monitoring of the Doppler movement (d) as a function of the known movement of the satellite considered (5), and a limited number of   Doppler displacement measurements (d).   7 - Installation according to claim 6, characterized: in that it is provided, in this logic block (54) for real-time monitoring of the Doppler movement (d), a memory (56) which contains a table (57) of known temporal variations of the Doppler displacement for different maximum elevations of each satellite; in that the terminal performs and stores, in a volatile memory (58, 59) a limited series of measurements of the Doppler displacement, and compares these measurements, in a logic block (60), with the prerecorded data of said table (57), in order to thus determine which is the variation curve of the Doppler displacement which must presently be taken into account; and in that there is consequently provided another calculation block (61) which has a role of prediction and supply (49) of the voltage for correcting the Doppler displacement   to the Local Demodulation Oscillator (48).