METHOD AND DEVICE FOR MINIMAL PHASE GRADIENT MODULATION OF A SIGNAL REPRESENTATIVE OF DATA

Abstract

The data to be transmitted is converted into a bipolar modulating signal decomposable into alternating elements of even or odd rank. According to the invention, the bipolar elements are locked to the rate of a predetermined clock with a phase shift of 90 degrees depending on the parity of the rank of said elements. Then, amplitude modulation is performed by modulating the polarity of at least two main sequences of 10+ and Q90+ samples, alternating in polarity, from the count of the clock pulses modulo 2 and the locked elements. The two main sequences have respective envelope curves representing a symmetrical sinusoidal wave shifted by a predetermined width. Finally, the modulated samples thus combined are converted into a minimum phase gradient modulated signal.

Description

Method and device for minimum phase gradient modulation of a representative data signal The invention relates to data transmission. More particularly, it applies to minimum phase gradient modulation of a representative data signal. In French Patent Application FR-88 15855 (Main Patent) filed on December 2, 1988, the Applicants proposed a demodulation system for a signal representative of data transmitted under a minimum phase gradient modulation called MSK (from the Anglo-Saxon terminology "Minimum Shift"). Keying"). As is well known, MSK modulation consists, at the transmitter level, of modulating the amplitude of two carrier waves initially in quadrature according to successive bits, taken alternately and arranged in arpndi, of a digital signal representing the data to be transmitted. The extent of the modulation corresponds to a phase inversion, combined with amplitude modulation. In order to avoid the disturbances that may be caused by the high-level lines that a signal modulated by a series of short-period binary elements may exhibit, the person skilled in the art generally spreads the data in the form of consecutive segments of a pseudo-random sequence. MSK modulation has the advantage, on the one hand, of avoiding excessive variations in the envelope curve of the signal to be transmitted thanks to the respective offset of the two data trains modulating the two carrier waves, and, on the other hand, of reducing amplitude variations during phase jumps thanks to the rounding of bits by sinusoids. However, such a shift between the two data streams, shaped by the same sinusoids and rounded, is technologically difficult to achieve perfectly without resorting to complicated and expensive electronics. The invention provides a solution to this problem. One aim of the invention is to provide a modulation system MSK with minimum phase gradient of a representative data signal with spread of the spectrum of said data and which achieves a perfect offset between the two rounded data streams, using simple and cheap electronics. The invention thus aims to benefit from the good transmission qualities offered by MSK modulation, while simplifying and minimizing the costs of its implementation. The invention relates to a data transmission method, of the type known as minimum phase gradient modulation, comprising a) the conversion of the data to be transmitted into a bipolar modulating signal decomposable into alternating elements of even or odd rank b) the amplitude modulation, by rounding, of a first basic carrier signal as a function of said even-rank elements of the modulating signal, and of a second basic carrier signal, phase-shifted by 900 with respect to the first, as a function of said odd-rank elements of the modulating signal c) the emission of a wave derived from the two carrier signals thus modulated. According to a general definition of the invention, step b) comprises: b1) generating a basic clock signal and at least one submultiple thereof; b2) locking said bipolar elements to the rate of the submultiple of the clock with a phase shift of 900 according to the parity of the rank of said elements; b3) counting the clock pulses modulo 2; b4) modulating in polarity, from the clock pulse count and the locked elements, at least two periodic principal sequences of samples, alternating in polarity, the two principal sequences having a respective envelope curve representing a symmetric sinusoidal wave shifted respectively by a predetermined width; b5) combining all the samples thus modulated; and b6) converting the samples thus combined into a minimum phase gradient modulated signal. In the context of data spectrum spreading, step a) of the method according to the invention includes the preparation of the modulating signal, deduced from the elements of the data to be transmitted, in the form of consecutive alternating segments of at least one pseudo-random sequence. The invention also relates to a transmitting device for implementing the method according to the invention, of the type comprising: - a source suitable for delivering data to be transmitted; - means for converting the data to be transmitted into a bipolar modulating signal decomposable into alternating elements of even or odd rank; - means for amplitude modulation, in a rounded manner, of a base frequency as a function of said elements of the modulating signal, with a phase shift of 900° of the base frequency according to the parity of the rank of said elements; and - means for transmitting a signal derived from the signal thus modulated. According to an important feature of the device according to the invention, the modulation means comprise: - means for generating a basic clock signal and at least one submultiple thereof; - means for locking said bipolar elements to the rate of the submultiple of the clock with a phase shift of 900 according to the parity of the rank of said elements; - means for counting clock pulses modulo 2; - means for modulating in polarity from the counted clock pulses and the locked elements, at least two periodic main sequences of samples, alternating in polarity, the two main sequences having a respective envelope curve representing a symmetrical sinusoidal wave shifted respectively by a predetermined width; and - means for converting the samples thus modulated together into a signal with minimum phase gradient modulation. According to a preferred embodiment of the device according to the invention, the device further comprises a pseudo-random sequence generator, coupled to the source to produce a modulating signal, deduced from the elements of the data to be transmitted, in the form of consecutive alternating segments of at least one pseudo-random sequence. Other features and advantages of the invention will become apparent from an examination of the detailed description below and the accompanying drawings, in which Figure 1, identical to Figure 1 of the main patent, represents a schematic view of a modulated signal transmitter. MSK of known type - Figure 2, identical to Figure 2 of the Main Patent, represents timing diagrams illustrating the MSK modulated emission signals of known type - Figure 2 bis is a schematic view of the MSK modulator according to the invention - Figure 2 ter represents timing diagrams illustrating the bipolar signals of the pseudo-random sequence and the bipolar signals locked at the rate of the submultiple of the base clock according to the invention; and - Figure 2 quater represents timing diagrams illustrating the periodic sequences of samples stored in the read-only memory according to the invention. The attached drawings contain numerous elements of a definite nature. They can therefore not only serve to clarify the description below, but also contribute to the definition of the invention, if necessary. Figure 1, identical to Figure i of the Main Patent, shows the transmitting side of a known type data transmission system bearing the general reference 2 and comprising a digital source 4 producing the digital message to be transmitted and a transmitter 6 providing the signal carrying the message which must pass through a transmission channel (not shown). In a known manner, the transmitter 6 includes transmission means 12, preceded by a modulator 10 of the MSK minimum phase gradient modulation type in which the first and second carrier waves originally in quadrature are modulated by variation of amplitude as a function of successive bits, taken alternately, of a signal delivered by the digital source 4, whose bits are rounded and which represents the data to be transmitted. In a known manner, a pseudo-random sequence generator 8 is coupled to the digital source 4 to produce at least two bipolar signals consisting of consecutive segments s, taken alternately, of at least one pseudo-random sequence, said segments being rounded after polarity modulation by the successive bits of the data to be transmitted. The pseudo-random sequence generator 8, for example, consists of a 9-stage shift register designed to deliver a pseudo-random sequence with a maximum length of 511 elements. A binary element is removed to split the pseudo-random sequence into a non-prime integer number of consecutive pseudo-random segments, that is, into 510 segments. With a pseudo-random sequence of 510 elements, 3 bits of data can be covered, meaning that one bit of data will be covered by 170 pseudo-random segments. This data spectrum spreading operation helps to reduce the harmful effect of transmission jammers. In part A of Figure 2, identical to that of the main patent, the digital train delivered by the digital source 4 is shown. This train consists of bipolar di-elements with period Tn. For example, the transmission rate of the elements or bits is on the order of 6 x 10³ bits / second (T1 = 6 x 10³ Hz). In part B of figure 2, identical to that of the main Patent, a pseudo-random sequence split into 510 S segments or "CHIPS" of period Tsp has been represented. -For example, the frequency of the pseudo-random sequence has a value of the order of 106 Hz ( = 10.6 Hz). Tsp On parts C and D of figure 2, identical to those of the main Patent, the chronograms of two carrier waves originally in quadrature modulated by variation of amplitude, in rounded form, respectively following the first and second bipolar signals B1 and B2 made up of consecutive segments S', taken alternately, of the pseudo-random sequence S. In practice, the bipolar signal B1 consists of even-numbered segments S'0, S'2, S'4,..., while the bipolar signal B2 consists of the odd-rank segments S'1, S'3, S'5 Depending on the parity of the rank of said segments S', the two carrier waves have a phase shift of 900 (quadrature shift). After modulation, their amplitudes, shaped by rounding, have a respective shift of the duration of half a segment S'. It will be noted that the S' segments of the bipolar signals B1 and B2 have a duration Tb twice that of the S segments of the pseudo-random sequence Tsp in order to allow the superposition of the two bipolar signals Bl and B2 in order to obtain a signal with minimum phase gradient modulation. However, achieving such a precise offset between the two rounded bipolar signals is technologically challenging. Indeed, it requires sophisticated electronics designed to maintain this offset as precisely as possible while preserving the same rounded shape of the bipolar signals. The Applicants have raised the issue of implementing an MSK modulation system that addresses this drawback. Such an MSK modulation system according to the invention is shown in Figure 2 bis. It includes the digital source 4 producing the digital message to be transmitted and a transmitter 6' distinct from the transmitter 6 described in reference to figure 1 and which provides the signal carrying the message which must pass through a transmission channel (not shown). A pseudo-random sequence generator 8 identical to that described with reference to Figure 1 produces a modulating signal (part A of Figure 2 ter), deduced from the data to be transmitted in the form of consecutive segments S of at least one pseudo-random sequence, identical for example to that described with reference to part B of Figure 2. A voltage-controlled local oscillator OL1 with a frequency F0, for example, on the order of 16 MHz, drives the rate of the pseudo-random sequence at a value on the order of 10⁶ chips / second (the frequency F1 of the pseudo-random sequence is on the order of 1 MHz) via a divider D1 that divides the input frequency F0 by 16. The oscillator OL1 also drives the rate of the digital source 4 at a value, for example, on the order of 6 x 10³ bits / second (the frequency F2 of the digital source is on the order of 6 kHz) via the divider D1 and the dividers D3 and D4 respectively suitable for dividing the frequency F1 by 102 and 170. A phase-locked loop (PLL) inserted between the divider D3 and the local oscillator OL1 corrects the frequency and phase of the clock of said local oscillator OL1 as a function of the output signal of the divider D3, and a reference frequency F4 (here equal to 10 kHz, for example). The S segments delivered by the pseudo-random generator 8 are then locked in SP and SIP lock memories at the rate of the frequency F3 obtained at the output of a divider D2, with a phase shift of 90" according to the parity of the rank of said segments S. The divider D2 is suitable for dividing by 2 the output of the divider D1. The phase shift is ensured, for example, by an inverter. IV. I1 is translated here by a respective shift of the locked bipolar segments S" by the duration of half a segment Ta / 2. The frequency -F1 is twice as fast as the frequency F3, it follows that the bipolar segments S" locked in the lock memories SP and SIP respectively according to the parity of the rank of the segments S have a duration Ta twice that of the segments Tsp (parts B and C of figure 2 ter). From the modulo 2 counting of clock pulses of frequency F0 as well as the counting of locked segments S" at the rate of frequency F3, we modulate in polarity at least two periodic main sequences of samples I0+ and Q90+, alternating in polarity, the two main sequences having a respective envelope curve representing a symmetric sinusoidal wave, shifted respectively by a predetermined width. In practice, the main sequences I0+ and Q90+ include a respective complementary sequence I0- and Q90- consisting of samples with an algebraic value opposite to that of the samples of its respective main sequence. These periodic sequences I0+ and Q90+ and their complement I0 and Q90- are stored in memory pages of a read-only memory The samples can be obtained by tabulating a suitable envelope waveform. Advantageously, this envelope corresponds to the envelope that would result from passing the modulated carrier wave through a linear-phase band-limiting filter. We now refer to figure 2 quater which represents chronograms illustrating the periodic sequences of samples. In part A of figure 2 quater, the envelope curve of the periodic sequence of samples I0+, contained in the memory page PI0+, is represented. I1 is an envelope curve of a symmetric sinusoidal wave of period Ta containing for example 16 samples e alternating in polarity, spaced from each other by a width of one sample. Advantageously, these are half-width samples or pulses, which do not alter the spectral power distribution of the signal. In part B of figure 2 quater, the envelope curve of the periodic sequence of samples I0- contained in the memory page P I0- is represented. I1 is an envelope curve of a symmetric sinusoidal wave, identical to that of I0+ but made up of samples with an algebraic value opposite to that of I0+. In part C of figure 2 quater, the envelope curve of the periodic sequence of samples Q90+ contained in the memory page PQ90+ is represented. I1 is an envelope curve of a symmetric sinusoidal wave identical to that of I0+, but with a shift with 10+ of a predetermined width. In part D of Figure 2 quater, the envelope curve of the periodic sequence of samples, Q90- contained in the memory page PQ90-, is shown. It is an envelope curve of a symmetrical sinusoidal wave, identical to that of Q90+ but made up of samples with an algebraic value opposite to that of Q90+. In practice, the samples of the I0+ and I0- sequences are shifted relative to the samples of the Q90+ and Q90- sequences by a width of one sample so as to allow their nesting, which is possible with half-width samples without distortion of the spectrum. These periodic sample sequences are intended to be modulated by the S" segments locked in the SP and SIP lock memories at a rate of F1 with a phase shift of 900 depending on the parity of the rank of the S segments delivered by the pseudo-random generator 8. Consequently, the periodic sample sequences I0+ and I0- are shifted relative to the sequences Q90+ and Q90- by half a segment, respectively. S" / 2 so as to be in phase shift of 900 (in quadrature). The periodic sequences of samples are modulated in polarity by counting the clock pulses modulo 2 and by counting the locked elements S''. More precisely, segment S''-1 modulates the polarity of the periodic sequence of samples contained in memory page P1O-; segment S''1 modulates the polarity of the periodic sequence of samples contained in memory page P1O+; and segments S''0 and S''2 modulate the polarity of the periodic sequence of samples contained in memory page PQ9O+. The counting can be performed by conventional counters (not shown). Next, all the samples thus modulated by the consecutive segments S" are combined. In fact, they appear successively at the output of the MM memory, according to its addressing. A digital-to-analog converter (DAC) then processes the samples thus combined in order to deliver an MSK signal with a frequency of approximately 4.08 MHz, amplitude modulated according to the segments locked in the SP and SIP lock memories. This signal is obtained through an FL1 filter. We saw earlier that a data bit covers 170 chips or S segments. Consequently, at the output of the MM memory, depending on the polarity of the bit, it is necessary to take the signal thus obtained or its algebraic opposite (either by addressing the memory appropriately, or by playing on the digital / analog converter DAC). The FL1 filter should be a linear-phase band-limiting filter if the tabulated envelope waveform were sinusoidal. As mentioned above, a suitable choice of the tabulated envelope allows for a simple, much less expensive harmonic-eliminating filter in FL1. I1 is followed by an amplifier A1. The MSK signal with a frequency of 4.08 MHz can be transmitted at a frequency of approximately 70 MHz via a transmission channel comprising a local oscillator OL2 suitable for delivering signals with a frequency of approximately 65.92 MHz, an amplifier A2, a mixer RU for producing the product of the signals from amplifiers A1 and A2 respectively, filtering means FL2, an amplifier A3 and transmission means 12 identical to those described with reference to Figure 1. The invention thus makes it possible to produce a phase gradient modulated signal using inexpensive digital electronics which are simple to implement. The person skilled in the art will understand that the described digital process makes it possible to synthesize an envelope of "rounded" conformation which is not a sinusoid, but which would result from the passage, through a spectral band limiting filter, discarding the unuseful spectrum, of a signal having an envelope shaped according to a desired basic curve. Passing through the filter in question alters the shape of the envelope curve, which can be accounted for by the values ​​of the tabulated samples. This alteration is accompanied by a temporal "spreading" of the envelopes on either side of the theoretical limits (corresponding to an infinite spectrum), resulting in inevitable interference between symbols. We can then consider all possible cases of signal succession to calculate the set of possible samples to be tabulated.

Claims

Demands 1. Data transmission method, of the type known as minimum phase gradient modulation, according to one of the claims of the main Patent, comprising a) the conversion of the data to be transmitted into a bipolar modulating signal decomposable into alternating elements of even or odd rank b) the amplitude modulation, by rounding, of a first base carrier signal as a function of said even rank elements.of the modulating signal, and of a second basic carrier signal, phase-shifted by 900 with respect to the first, according to said odd-rank elements of the modulating signal c) the emission of a wave drawn from the two carrier signals thus modulated, characterized in that step b) comprises 1) the generation of a basic clock signal and at least one submultiple thereof b2) the locking of said bipolar elements to the rate of the submultiple of the clock with a phase shift of 90" according to the parity of the rank of said elements b3) the counting of the clock pulses modulo 2 b4) the modulation in 0.5.arality from counting, clock pulses and locked elements, of at least two periodic principal sequences of samples, alternating in polarity, the two principal sequences having a respective envelope curve representing a symmetric sinusoidal wave shifted respectively by a predetermined width; b5) the joining of all the samples thus modulated; and b6) the conversion of the samples thus joined into a minimum phase gradient modulated signal.

2. Method according to claim 1, characterized in that step a) comprises a) the preparation of the modulating signal, deduced from the elements of the data to be transmitted, in the form of consecutive alternating segments of at least one pseudo-random sequence.

3. Method according to claim 1, characterized in that the two symmetric sinusoidal envelope curves representing the two main sequences of samples are shifted respectively by the duration of a locked half-element while inside said envelope curves the samples are shifted respectively by the width of a sample.

4. Method according to claim 1, characterized in that the generation of a basic clock signal is adjusted to a frequency (F0) of the order of 16 MHz.

5. Method according to claim 1, characterized in that the data rate (F2) to be transmitted is adjusted to a value of approximately 6 kbits / second.

6. Method according to claim 1, characterized in that the frequency (Fl) of the pseudo-random sequence is adjusted to a value on the order of 1 MHz.

7. Method according to claim 6, characterized in that the pseudo-random sequence is split into 510 segments, a binary data element being covered by 170 segments.

8. Method according to claim 1, characterized in that each locked element modulates a periodic sequence of 16 samples.

9. Method according to claim 1, characterized in that the samples of the same sequence are spaced from each other by a width of one sample.

10. Method according to claim 1, characterized in that step b4) comprises b4-l) the initial storage in a read-only memory (ROM) of at least the two periodic main sequences of samples.

11. A method according to claim 1, characterized in that the main sequences comprise a respective complementary sequence consisting of samples with an algebraic value opposite to that of the samples of its respective main sequence.

12. Method according to claim 1, characterized in that step c) comprises c1) the generation of an transmit clock signal; and c2) the transmission of the minimum phase gradient modulation signal obtained in step b6) at the rate of the transmit clock signal plus that of the minimum phase gradient modulation signal.

13. Transmitter device for implementing the method according to any one of claims 1 to 12, of the type comprising: - a source (4) suitable for delivering data to be transmitted; - means for converting the data to be transmitted into a bipolar modulating signal decomposable into alternating elements of even or odd rank; - means for amplitude modulation, by rounding, of a first basic carrier signal as a function of said even-rank elements of the modulating signal, and of a second basic carrier signal, phase-shifted by 900 with respect to the first, as a function of said odd-rank elements of the modulating signal; - means for transmitting (12) a wave derived from the two carrier signals thus modulated.characterized in that the modulation means comprise: - means for generating (OL1) a basic clock signal and at least one submultiple thereof; - means for locking (SP and SIP) said bipolar elements at the rate of the submultiple of the clock with a phase shift of 900 according to the parity of the rank of said elements; - means for counting clock pulses modulo 2; - means for modulating in polarity from the counting of clock pulses and the locked elements, of at least two periodic main sequences of samples. I0+ and Q90+, alternating in polarity, the two main sequences having a respective envelope curve representing a symmetric sinusoidal wave shifted respectively by a predetermined width; and - means of conversion (DAC) of the samples thus modulated, combined, into a signal with minimum phase gradient modulation.

14. Device according to claim -13, characterized in that it further comprises a pseudo-random sequence generator (8), coupled to the source (4) to produce a modulating signal, deduced from the elements of the data to be transmitted, in the form of consecutive alternating segments of at least one pseudo-random sequence.

15. Device according to claim 13, characterized in that the two symmetric sinusoidal envelope curves representing the two main sequences of samples are shifted respectively by the duration of a locked half-element while inside said envelope curves the samples are shifted respectively by the width of a sample.

16. Device according to claim 13, characterized in that the means for generating a clock signal (OL1) are suitable for delivering a clock signal with a frequency of the order of 16 MHz.

17. Device according to claim 13, characterized in that the data rate of the source (4) is on the order of 6 kbits / second.

18. Device according to claim 15, characterized in that the throughput of the pseudo-random sequence generator (8) is on the order of 106 elements / second.

19. Device according to claim 18, characterized in that the pseudo-random sequence is split into 510 segments, a binary element being covered by 170 segments.

20. Device according to claim 13; characterized in that each locked segment S' modulates in polarity a periodic sequence of 16 samples.

21. Device according to claim 13, characterized in that the samples of the same sequence are spaced from each other by a width of one sample.

22. Device according to claim 13, characterized in that it further comprises a read-only memory (ROM) in which at least the two main periodic sample sequences (I0+ and Q90+) are stored.

23. Device according to claim 13, characterized in that the main sequences (10+ and Q90+) comprise a respective complementary sequence (10- and Q90-) consisting of samples with an algebraic value opposite to that of the samples of its respective main sequence.

24. Device according to claim 13, characterized in that it further comprises means for generating an emission clock signal (OL2) for the purpose of emitting the phase gradient modulation signal at the frequency of said emission clock signal plus that of the minimum phase gradient modulation signal.