FR2745134A1 - Digital radiocommunication link establishment method for mobile terminals - Google Patents

Abstract

The method involves using a first mobile terminal which establishes communication with a second mobile terminal. the second terminal demodulates a call signal to provide a digital signal of m complex samples with a sampling period Tb. The difference signal, Sd(k), for the kth sample is equal to the term S(k).S(k)* and the difference signal Rd(t) is equal to the term Ref(t + Tb). Ref(t)*. These terms are used to define (35) a correlation function, C(p), where p lies between 0 and (q.n - 1) where q is an integer constant and the period of the sequence is n.Tb. The second terminal determines (36) the maximum modulus of the correlation function and estimates (39) the frequency difference, DELTA f, between the transmitted and demodulated signals, as the argument of the maximum modulus divided by 2 pi .Tb for comparison (38, 40) with thresholds.

Description

STORING PROCESS FOR COMMUNICATION BETWEEN DIS
MOBILE TERMINALS OF A RADIO COMMUNICATION NETWORK
Digital
The present invention relates to a method for establishing communication between mobile terminals of a digital radio network.

 The invention applies in particular to digital radiocommunication networks comprising one or more fixed or mobile relays able to communicate with the mobile terminals, the mobile terminals located under the cover of a relay being able to communicate with each other, or else with other mobile terminals of the network or with equipment of other networks according to the interfaces of which the relays are equipped.

 When a mobile terminal is outside the coverage of the relay (s) of the radio network, a procedure may be needed to establish direct communication between the mobile terminal and one or more other mobile terminals of the network, in order to enable the mobile terminal to terminal outside the coverage area to send emergency messages to one or more other users in the network.

 An object of the present invention is to provide a method for establishing such communication between two mobile terminals effectively and without disrupting the normal network traffic.

où q désigne une constante entière postive,
p désigne une variable entière comprise entre 0 et q.n-1, n étant un entier tel que la période de ladite séquence prédéterminée soit de n.Tb,
(.)* désigne le complexe conjugué,
Sd(k)=S(k).S(k)*, S(k) désignant le k-ième échantillon du signal utile,
Rd(t)=Ref(t+Tb) .Ref(t)*, Ref(t) désignant la valeur au temps t de ladite séquence périodique prédéterminée.According to the invention, a first mobile terminal transmits on a particular call channel a call radio signal representing a predetermined periodic sequence. At least one second mobile terminal regularly monitors the calling channel to detect the presence or absence of the call signal. Call channel monitoring by the second terminal includes demodulating the radio signal picked up by the second terminal on the calling channel to provide a useful digital signal of m complex samples with a sampling period of Tb, and detecting the presence or absence of the call signal from a maximization of the form correlation module

where q is a positive integer constant,
p denotes an integer variable between 0 and qn-1, n being an integer such that the period of said predetermined sequence is n.Tb,
(.) * denotes the conjugate complex,
Sd (k) = S (k) .S (k) *, S (k) designating the k-th sample of the useful signal,
Rd (t) = Ref (t + Tb) .Ref (t) *, Ref (t) designating the value at time t of said predetermined periodic sequence.

 This method of detecting a call signal between two terminals has the advantage of being robust to the ambient noise conditions on the calling channel, and to a possible frequency difference between the oscillators of the transmitter and the receiver. receiver.

 In a radio network, the relays usually have very precise oscillators (for example to within 0.5 ppm), whereas those of the mobile terminals are often less precise (a tolerance of 5 ppm for example). The mobile terminal that has escaped the coverage of the relay or relays and emits the call signal is no longer keyed to the local oscillator of a relay, so that the frequency offset between the mobile transmitter and receiver may exceed Allowed values for normal links between a mobile terminal and a relay. By employing the correlations C (p), the method according to the invention makes it possible to overcome the problems posed by such a frequency shift.

 The invention even makes it possible to estimate the value of this frequency shift. Thus, in a preferred version of the method, the monitoring of the calling channel by the second mobile terminal comprises a determination of a correlation having a maximum modulus among the correlations C (p) for 0spsq.n-1, and an estimate of a frequency difference between the frequency with which the first mobile terminal transmits the call signal and the frequency with which the second terminal demodulates the received signal, the estimated frequency deviation being the argument between -x and x of said maximum module correlation divided by 2X.Tb. The second terminal can decide that the call signal is absent if the frequency deviation thus estimated exceeds, in absolute value, a predetermined threshold.

où AF désigne l'écart de fréquence estimé et pmax désigne un entier compris entre 0 et q.n-1 pour lequel la corrélation
C(p) a un module maximal

The estimation of the frequency deviation can further be used to define a decision criterion for the presence or absence of the call signal, having a maximum robustness to the ambient noise. The monitoring of the calling channel by the second mobile terminal can thus include obtaining a corrected correlation S3 and two energies
Es, and according to

where AF designates the estimated frequency deviation and pmax denotes an integer between 0 and qn-1 for which the correlation
C (p) has a maximum modulus

 When the ratio S3I2 / (Es.Et) is lower than a predetermined threshold, the second mobile terminal detects an absence of the call signal.

Other features and advantages of the present invention will become apparent in the following description of nonlimiting exemplary embodiments, with reference to the appended drawings, in which:
FIG. 1 is a simplified diagram of a radiocommunication network
FIG. 2 is a general block diagram of a method for establishing a communication between mobile terminals according to the invention
FIGS. 3 and 4 are flowcharts illustrating the steps of a procedure for detecting an out-of-area call signal; and
FIG. 5 is a flowchart illustrating a variant of the procedure for detecting the out-of-area call signal.

 The invention will be described below, without being limiting, to a frequency division multiple access digital radio communication (FDMA) network. The network comprises one or more fixed or mobile radio relays. A single relay 10 is shown in Figure 1 to simplify the drawing. Each relay 10 is able to communicate with mobile terminals 11, 12, 13. For simultaneous access of several mobile terminals to the relay, a frequency sharing is provided for the available bandwidth, as is usual in FDMA techniques.

 As indicated above, the invention aims to provide a procedure allowing a mobile terminal 13 having escaped the coverage area 15 of the relay or relays 10 to communicate "out of area" with one or more mobile terminals of the network that could to be within reach. This allows the holder of the mobile terminal 13 to communicate urgent messages to other users of the network, who can, if it seems necessary to them and if they themselves are within range of a relay network, to share these messages to other users in the network.

 For this purpose, provision is made for the reservation of at least two frequency channels for this type of communication, namely an out-of-area call channel for the transmission of an out-of-area call signal by the mobile 13, and at least one out-of-area communication channel for carrying the corresponding speech signals. In general, it will be sufficient for out-of-area communications to be in alternate mode, so that only one out-of-area communication channel can be accommodated.

 The method of establishing an out-of-area call is illustrated in FIG. 2. The first step 20 consists, for the holder of the mobile terminal 13 who is unable to obtain normal communication with a relay 10, to command an out-of-area call. . This command can result from the manipulation of a button provided for this purpose on the mobile terminals.

 In response to this command, the terminal 13 transmits in step 21 an out-of-area call signal on the out-of-area call channel. This call signal represents a periodic reference sequence of period n.Tb, where n is an integer (for example n = 16) and Tb denotes the duration of a bit in the digital radiocommunication system, (for example Tb = 125 Rs for a sampling frequency of 8 kHz). The n-time bit pattern of the periodic sequence is repeated for a duration T1 which is for example of the order of 3 to 4 seconds. This pattern is composed of n samples of predefined values Ref (l), ..., Ref (n). These samples can be binary, real (represented for example on 8 bits) or complex. The call signal is modulated, for example according to a GMSK modulation, and transmitted around the central frequency of the out-of-area call channel. It can be sent at the maximum power allowed by the mobile to maximize the chances of detection by another mobile. After transmitting the call signal during the duration T1, the mobile terminal 13 is placed listening situation of the out-of-area communication channel (step 22).

The active mobile terminals of the network regularly listen to the out-of-area call channel. This listening is repeated at time intervals T2 <T1, for example T2 = 2 seconds. At each time interval T2, the mobile terminals are placed in reception on the out-of-area call channel for a duration T3 short enough not to disturb the normal operation of the network (for example
T3 = 20 ms). This duration T3 is divided into four periods, the first to perform frequency switching to the out-of-area call channel frequency, the second to perform an automatic gain control according to the power sensed on the out-of-area call channel, the third providing a useful signal window, and the fourth providing frequency switching to the channel on which the terminal was previously positioned. The useful signal window typically represents a duration of 4 ms, ie m = 32 complex samples at the same time. frequency 1 / Tb = 8 kHz.

Step 23 in FIG. 2 represents the demodulation of the signal picked up by a mobile terminal 12 on the out-of-area call channel, providing the m complex samples.
S (1), ..., S (m) of the useful signal. The mobile terminal 12 then detects, in step 24, the presence or absence of the out-of-area call signal, based on the samples S (1), ..., S (m) of the wanted signal and the values Ref (1), ..., Ref (n) of the pattern of the reference sequence represented by the out-of-area call signal. If the terminal 12 detects no out-of-area call signal on the channel, it is repositioned on its previous channel. If the presence of an out-of-area call signal is detected, the terminal 12 places itself in transmission condition on the out-of-area communication channel (step 25), and produces an alert signal (buzzer and / or light signal) to alert its holder of the out-of-area call. If he finds this alert signal, the holder of the terminal 12 can then enter into communication on the out-of-area communication channel with the user having requested the out-of-area call. The switching to the out-of-area communication channel of the terminal 12 having detected the call signal is preferably automatic as explained above, but it could also be controlled by its owner warned by the warning signal.

 The process of detecting the out-of-area call signal is detailed on the flow charts of Figures 3 and 4. Figure 3 shows preliminary processes requiring a small amount of computation and in most cases allowing the call signal to be determined. is absent. These preliminary treatments make it possible to minimize the average computing load and therefore the electrical power consumed by the mobile terminals for the reception processing on the out-of-area call channel.

The first test 31 relates to the energy Es of the useful signal which is compared with a threshold X0. The energy of the wanted signal is calculated in step 30 according to

 If the test 31 shows that ES <Xo, the mobile terminal detects the absence of the out-of-area call signal. It is then considered that the useful signal only represents background noise.

If the test 31 shows a sufficient energy of the useful signal, then the periodicity of this signal is tested. The reference sequence having a periodicity n.Tb, this periodicity must be found in the useful signal received if the call signal is present. In step 32, the mobile terminal calculates an autocorrelation S1 and two energies E1,
E2 according to

 The ratio S1t2 / (El.E2) is between 0 and 1.

A decision threshold X1 between 0 and 1 is set to decide whether the useful signal has the periodicity of the reference sequence. The test 33 compares the ratio ls, 12 / (E1.E2) to the threshold X1. If /S112/(E1.E2) <X1, the mobile terminal detects the absence of the out-of-area call signal. Otherwise, the mobile terminal proceeds to the processes illustrated in Figure 4.

The treatments shown in FIG. 4 begin with obtaining differential signals Sd (k) and Rd (t), according to FIG.
Sd (k) = S (k + 1) .S (k) *
Rd (t) = Ref (t + Tb) .Ref (t) * for k = 0,1, ..., ml and t = (i + p / q) Tb with i = 0,1, ... , m-1 and p = 0,1, ..., qn-1, q denoting a positive integer, with the periodicity property Ref (t + n.Tb) = Ref (t). The integer q is preferably equal to 2. The differential signal Sd (k) is calculated by the mobile terminal as a function of the wanted signal, while the differential signal Rd (t) can be stored once and for all since the reference is fixed.

In step 35, the terminal then calculates, for p = 0.1, ..., qn-1, the correlations C (p) given by

 Maximization 36 is performed to determine the integer pmax between 0 and qn-1 for which the correlation modulus C (p) is maximal. C max = C (pmax) is the value of the maximum module correlation. The quantity 2x.pmax / (n.q) represents the phase difference between the useful signal and the periodic pattern of the reference sequence.

In the exemplary embodiment illustrated in FIG. 4, the maximization step 36 is followed by the calculation 37 of two energies Ed and Er of the differential signals, given by:

 The ratio Cmax2 / (Ed.Er) is between 0 and 1.

A decision threshold X2 between 0 and 1 is set to decide whether the useful signal has sufficient quality compared to the reference sequence. The ratio Cmaxi2 / (Ed.Er) is compared with this threshold X2 during the test 38.

If Cmaxi2 / (Ed.Er) <X2, the mobile terminal detects the absence of the out-of-area call signal. Otherwise, the mobile terminal proceeds to step 39 of estimating the frequency deviation AF between the frequency with which the mobile terminal 13 has emitted the out-of-area call signal and the frequency with which the terminal 12 has demodulated the signal picked up on the out-of-area call channel. The frequency deviation AF is estimated according to the argument of the complex number Cmax: AF = arg (Cmax) / (2x.Tb), the argument arg (Cmax) being taken between -s and K.

 If the estimated frequency deviation AF exceeds, in absolute value, an AFmaX frequency threshold (test 40), the mobile terminal decides that the out-of-area call signal is absent. If it does not, it decides that the out-of-area call signal is present. For computing convenience, the test 40, expressed by [| AF | > AFMAX?] In Figure 4, can be performed in the form: [ïRe (Cmax) <a. Im (Cmax) I? ], denoting the constant cotg (2x.Tb.AFmax).

 The AFmaX frequency threshold is chosen based on the nominal frequency of the out-of-area call channel and the accuracy tolerances of the mobile terminals and relays. If, for example, the frequency of the out-of-area call channel is of the order of 450 MHz with tolerances of 5 ppm for the mobile and 0.5 ppm for the relays, it is possible to take AFmaX ~ 450.106. -6 + 5.10 7) = 2475 Hz.

max
The maximized and normalized intercorrelation, compared with the threshold X2 in step 38, is independent of the frequency difference, so that the detection procedure described above is not very sensitive to the frequency difference that may exist between the oscillators of the mobile transmitter and receiver, the test 40 to ensure that this frequency difference is not excessive.

The embodiment variant shown in FIG. 5 has the advantage of being less sensitive to the background noise present on the out-of-area call channel. In this variant, the maximization step 36 is directly followed by the calculation 39 of the frequency deviation AF and the corresponding test 40. If the test 40 shows that the frequency deviation AF AF | remains below the threshold AF max 'the mobile terminal calculates in step 41 a corrected cross correlation S3, according to

soit en recalculant cette énergie Et, soit en la lisant dans une mémoire puisque sa valeur est une constante.In step 41, the mobile terminal also obtains the energy Et given by

either by recalculating this energy And either by reading it in a memory since its value is a constant.

 The quality test 38 of FIG. 4 is replaced by another quality test 42, where the ratio 53 2 / (Es.Et) is compared with a threshold X3 between 0 and 1. In this report, the energy Es is that of the useful signal that was calculated in step 30. If the IS312 / (ES.Et) ratio is lower than the threshold X3, the mobile terminal detects an absence of the out-of-area call signal. If not, it detects the presence of the out-of-area call signal.

 Typical orders of magnitude for the thresholds used in the periodicity test 33 and the quality test 38 or 42 are 0.2sol0.4 and 0.3sX2, X3s0.5. The precise values of these thresholds are adjustable according to the particular characteristics of the network and the probabilities of non-detection and false alarm that can be tolerated.

Claims (6)

 A method for establishing a communication between mobile terminals of a digital radiocommunication network, characterized in that a first mobile terminal transmits on a particular call channel a radio call signal representing a predetermined periodic sequence in that at least one second mobile terminal regularly monitors the calling channel to detect the presence or absence of the call signal, and in that the call channel monitoring by the second terminal includes the demodulation of the radio signal picked up by the second terminal on the calling channel to provide a useful digital signal of m complex samples with a sampling period of Tb, and detecting the presence or absence of the call signal to from a maximization of the correlation module of the form

 where q denotes a positive integer constant,  p denotes an integer variable between 0 and q.n-1, n being an integer such that the period of said predetermined sequence is n.Tb,  (.) * denotes the conjugate complex,  Sd (k) = S (k + 1) .S (k) *, S (k) designating the k-th sample of the useful signal,  Rd (t) = Ref (t + Tb) .Ref (t) *, Ref (t) designating the value at time t of said predetermined periodic sequence.  2. Method according to claim 1, characterized in that the monitoring of the calling channel by the second mobile terminal comprises a determination of a correlation (Cmax) having a maximum modulus among the correlations C (p) for Ospsq.nl, and estimating a frequency deviation (AF) between the frequency at which the first mobile terminal transmits the call signal and the frequency at which the second terminal demodulates the received signal, the estimated frequency deviation being the argument between -s and x of said maximum module correlation (Cmax) divided by 27t.Tb.  3. Method according to claim 2, characterized in that when the estimated frequency difference (AF) is greater in absolute value than a predetermined threshold (AFmax), the second mobile terminal detects an absence of the call signal.  4. Method according to claim 2 or 3, characterized in that the monitoring of the calling channel by the second mobile terminal comprises obtaining a corrected correlation S3 and two energies Es, and according to

 where AF designates the estimated frequency deviation and pmax denotes an integer between 0 and q.n-1 for which the correlation C (p) has a maximum modulus

 and in that when the ratio S3! 2 / (Es.Et) is less than a predetermined threshold, the second mobile terminal detects an absence of the call signal.  5. Method according to any one of claims 1 to 3, characterized in that the monitoring of the call channel by the second terminal comprises the calculation of two energies Ed, Er according

  and in that when the ratio ïCmaxi / (Ed.Er) is less than a predetermined threshold, the second mobile terminal detects an absence of the call signal, Cmax denoting a maximum module correlation among the correlations C (p) for Ospsq .nl.  6. Method according to any one of claims 1 to 5, characterized in that the number m of samples of the wanted signal is greater than n, and in that the monitoring of the calling channel by the second terminal comprises, before calculating the correlations C (p), calculating an autocorrelation S1 and two energies E1, E2 according to

 and in that when the ratio 1s112 / (E1.E2) is less than a predetermined threshold, the second mobile terminal detects an absence of the call signal and does not calculate the correlations C (p).