FR2707441A1 - Component for receiver or for differential direct spectrum spread spectrum signal transceiver and corresponding transceiver. - Google Patents

Abstract

The component comprises a single transducer adapted to the pseudorandom sequence and this is the input transducer. The component further comprises two output transducers (TRS1, TRS2) shifted by one period. Application to radiocommunications.

Description

COMPONENT FOR RECEIVER OR TME-RECEIVER
DIFFEEIENTIAL OF SPECTRUMALLY SPREADED SIGNALS BY
DIRECT SEQUENCE AND CORRECTIVE SPECTATOR TESTER-RECEIVER
DESCRIPTION
Technical area
The present invention relates to a direct sequence spread spectrum differential receiver component, a direct sequence spread spectrum differential transceiver component and a direct sequence spectrum spread transceiver.

State of the art
Spectrum spreading has been used for many years in military radiocommunications, mainly because it allows for discrete links, difficult interception and interference resistance. Two spreading techniques are conventionally used in terms of frequency hopping spectrum and direct sequence spectrum.

 Frequency hopping spread spectrum (also known as frequency evasion) is the technique used for military applications. It consists of changing the radio carrier frequency as often as possible (up to several hundred times per second) according to a law known only to the friendly receivers. This law, which is managed by one or more pseudo-random sequences, constitutes the access code to the link.

 This technique requires the use of agile frequency synthesizers. It is therefore expensive and there are few applications in the civil field.

The other technique, direct sequence spread spectrum (or, in short, ESSD or "Direct Sequence Spread Spectrum" (DDSS) in English terminology) is to transform the signal to be emitted, whose spectral band is Bs, in a signal having the properties of a white noise, whose spectral band is much larger, or BES. This spreading operation is performed by multiplying the message to be transmitted by a pseudo-random sequence formed of bits that can take the values +1 or -1. Yes
T is the period of the information elements to be transmitted, and TC the duration of a binary element of the pseudo-random sequence (also called "chip"), it is possible to define a fundamental quantity of the spread spectrum modulation, namely the processing gain N, by the relation N = (T / TC) = (BES / BS).

 This processing gain can vary according to the case from 10 to several tens of thousands, the conventional values in radiocommunications rarely exceeding 1000.

 In order to be able to retrieve the information, the receiver must perform a correlation operation between the message received and a pseudo-random sequence identical to that used to carry out the spreading operation. The receiver must know this sequence, which is the access key to the message sent. Two independent messages may be transmitted using the same frequency band and two orthogonal sequences, i.e. two sequences having low intercorrelation coefficients.

These transmission techniques are described in many books and articles. For example, the book entitled "Spread Spectrum Communicatuions" by Marvin K. SIMON et al., Computer Science
Press, 1983, vol. I and the following articles - "Design and experimental results for a direct-sequence spread-spectrum radio using differential phase shift keying modulation for indoor wireless communications", by M. KAVEHRAD et al., Published in TEEE
Journal on SAC, vol. SAC 5, No. 5, June 1987, pp.

815-823, - "Performance of differentially coherent digital communications over frequency-selective fading chan nelsnl by F.D. GARBER et al., Published in IEEE Trans on Communications, Vol 36, No. 1, January 1988, pp.

21-31, - "Direct-sequence spread spectrum and DPSK modulation and diversity for indoor wireless communications", by
MK KAVEHRAD et al., Published in IEEE Trans on communications, vol. COM-35, No. 2, February 1987, pp.

224-236.

We can also mention the American patent
US Patent 4,672,658.

 A direct sequence spread spectrum signal receiver must comprise a correlator, which outputs at its output a signal S1 resulting from the correlation between the received signal and the spreading sequence used at transmission and a delay line which delivers a signal 52 identical to S1 but offset by a duration equal to the period T of the information elements to be transmitted.

 The modulation used being generally a differential phase modulation, the information is carried by the phase difference between the signals S1 and S2. The extraction of this information is done using a multiplier. If the output signal of this circuit is positive, S1 and S2 are in phase. If it's negative, they're out of phase.

The correlator and the delay line may each be embodied as surface acoustic wave devices (SAW abbreviated to "Surface
Acoustic Waves "), as described for example in the article by Mohsen K ~ VEHRAD et al, cited above (Figure 4, page 817), which is actually a suitable filter. in a surface acoustic wave device and is followed by an amplifier whose output is divided into two channels, one connected to a delay line made of a surface acoustic wave device and the other directly connected to a multiplier which also receives the delayed signal.

 Numerous articles have described surface acoustic wave devices, including the article entitled "Practical Surface Acoustic Wave Devices" published by Melvin G. HOLLAND et al., In Proceedings of the IEEE, Vol 62, No. 5, May 1974 , pp. 582-611.

The application to spread spectrum communications is more specifically described in the article entitled "Application of acoustic surface-wave technology to spread spectrum communications", published by D. T. BELL et al., In IEEE Trans on MTT, vol. MTT 21, No. 4, April 1973, pp. 263-271.

 The matched filter or correlator, which performs the correlation operation, is composed, like any surface acoustic wave filter, of two surface wave transducers deposited on a piezoelectric material which, in the case of phase-modulated signals, is generally quartz for reasons of temperature stability of the characteristics of the component (including central frequency). It is well known that the impulse response of such a device is the product of convolution of the impulse responses of the two transducers; it must be equal, ideally, to the reversed time of the signal to which the filter must be adapted. The transducers are of the "interdigital comb" type whose electrodes (or fingers) can have constant or variable lengths depending on their position. They are made by depositing aluminum (thickness of a few hundred to a few thousand Angstroms) on the substrate used. Such components are described and offered in the catalogs of several suppliers of surface wave components.

 It can be added that, in the case of filters adapted to phase-coded signals, as is the case for the described receiver system, the relative bandwidths required are generally too wide to allow the use of other types of transducers. than those classically known and which are bidirectional.

Surface acoustic waves other than those propagating between the two transducers are attenuated by depositing on the areas between each transducer and the edge of the substrate which is closest to it, a suitable material (usually an adhesive).

This prevents uncontrolled amplitude and phase signals due to reflections on the edges of the substrate.

 EP-A-0 409 538 describes a component which fulfills both the correlation function and the delay function. This component is illustrated in Figure 1 attached.

 It comprises an input transducer 10 and two output transducers 12 and 14, all deposited on the same substrate 16. The two output transducers are identical and arranged one behind the other. The component therefore has an input e and two outputs s1, s2.

The transfer function between the two outputs can be likened to a pure delay. If we call Ie (t), Is1 (t) and Is2 (t) the impulse responses respectively of the input transducer 10 and the two output transducers 12 and 14, in the embodiment described in this document, we have
Ie (t) = rectTc (t)
1s1 (t) = â (t)
1s2 (t) = to + (tT) = 1s1 (tT) where rectTc (t) is the rectangle function of width Tc (ie rectTc (t) = l if 0St <Tc and 0 everywhere else), where Tc is the duration of the chip, the binary element of the pseudo-random spreading sequence, (t) is the time inverted conjugated complex of the sequence a (t) used at the emission to spread the spectrum.

The structure of Figure 1 has at least two disadvantages
a) The two output transducers 12, 14 are
one behind the other so that the space-
between the end of the first and the beginning of
cond is small before Tc.v, where v is the speed
propagation of RAYLEIGH acoustic wave
on the piezoelectric substrate (v = 3150 m / s for
quartz with an ST cut). There is therefore, in
be these two transducers, a parasitic capacitance
important and, therefore, an electrical isolation
diocre. It is fundamental, in the case of a
use in spread spectrum, to have a
good isolation since the information is conveyed
by the phase difference between the two signals
Release.

b) Transducers 12 and 14 which are duplicated
are the most complicated transducers to rea
Liser. The manufacturing efficiency of such
component is reduced. If we wanted them
modify or program them, it would be necessary to duplicate
such modification or programming, which
would be long and complex.

 The present invention aims to overcome these disadvantages. To this end, it proposes a component that fulfills, by itself, the dual function of correlation and delay, but which contains only one transducer adapted to the pseudo-random spreading sequence instead of two and for which the two output transducers are naturally widely spaced from each other, which reduces the parasitic capacitance between them.

 These results are obtained by a component in which it is the input transducer which is adapted to the spreading sequence and not the two output transducers, which can be made simply in the form of interdigitated combs. This type of embodiment is universally used in surface acoustic wave filters.

Both output transducers have a number of electrodes, alternately connected to two power supply lines. The output signal is taken from one of the lines, the other line being connected to ground. In the general case, if each output transducer has N electrodes spaced apart by d, the length of the transducers is
L = d (N-1)
In the case of the realization of the adapted filter, this length corresponds to the duration of a chip (binary element of the pseudo-random sequence of spreading) and one can write, if v is still the speed of propagation of the wave surface acoustic on the piezoelectric substrate L = v.Tc = d (Nl) for Tc = 20 ns this gives L = 63 Am.

 The input transducer is composed of two lines or electrodes connected, for example, signal input and ground respectively, and interdigitated electrode groups connected to the feed electrodes. The number of these groups of electrodes corresponds to the number of chips of the pseudo-random spreading sequence. The distance separating two consecutive groups is equal to L, the length that the acoustic wave travels during the duration Tc. These groups of electrodes are polarized according to the spread spectrum sequence considered. This filter is made by photolithography from a mask to the spreading sequence used.

 The output transducers are very far apart and intertransducer parasitic capacitance is very low. Naturally, in the device of the prior art, one could always shift the output transducers by placing them one above the other, but then the surface of the substrate would be increased and the cost would be also . In the invention, the parasitic capacitance between transducers is extremely low and the overall performance of the receiving system is improved, particularly with respect to resistance to radio interference and interference.

 In the invention, the spreading sequence, or code, determines the shape of a single transducer, namely the input transducer, instead of two (both output transducers). This is extremely interesting if one wants to make the component programmable, that is to say, adaptable to different spreading sequences. In the case of the invention, it will be necessary to program the elementary electrode groups of the input transducer whereas, in the prior art, it would be necessary to program the double.

 The distances separating the different components can be specified somewhat. If M is the number of chips of the pseudo-random sequence and S the distance separating the first group from the last, we can write S = (M-1) L = (M-1) vTc.

Knowing that the duration T of the symbol (or bit) transmitted by the spread spectrum system is
T = M.Tc it is also possible to describe T = M. (L / v)
The two output signals S1 (t) and S2 (t) must verify the relationship
S2 (t) = Sl (tT) the distance separating the two output electrodes must be such that the time taken by the acoustic wave to propagate from one to the other is equal to T. This distance is therefore S + L since S + L = (M-1) L + L = ML = M (Tv / M) = Tv

The advantages of the component of the invention compared to that of document ES-A-0 409 538 therefore appear clearly
the realization of the photo lithography mask is simpler since the device has only one transducer of complex shape instead of two. Manufacturing rejection will therefore be lower, yields better and unit cost reduced
the two output transducers are automatically separated from each other since the distance separating them is equal to S.

The component which has just been described has the additional advantage of being able to be very easily transformed into a transmitter component. For this, it is sufficient to add a third output transducer identical to the first two and coupled to the input transducer. Indeed, the impulse response in the baseband of the path constituted by the input transducer and the first output transducer is equal to the pseudo-random spreading sequence used for the time-reversed emission. If a (t) is this pseudo-random sequence, we can define, for the receiver-transmitter component, three impulse responses seen from the three outputs that it presents (two for the reception, ie S1, 2 one for the emission or
S3):
I1 (t) = a (rt)
12 (t) = 11 (tT) a (T + Tt)
I3 (t) = a (i + r ') where T, T t are times which depend on the relative positions of the output transducers with respect to the input transducer and T is the duration of the information binary symbol.

The third transducer thus makes it possible to directly generate, at the level of the intermediate frequency, the spread spectrum signal. To do this, it suffices to apply to the input transducer an electrical pulse signal of very short duration comparable to a DIRAC pulse. In this case, the output voltage
S3 of the third transducer will be worth
S3 (t) = I 3 (t) * 6 (t) = I 3 (t) = a (t + nT).

The subject of the present invention is therefore also a component for a differential transceiver of direct sequence spread spectrum signals, this component being characterized by the fact that it comprises
- a reception channel constituted by the
component that has just been defined,
- a transmission channel constituted by the trans
input driver of the receiving component
and, in addition, by a third transducer
similar output to the other two.

This new component makes it possible to synthesize more efficient modulations than simple modulation
DPSK. It is indeed possible to consider producing (and therefore demodulating) modulations of the MSK or GMSK type, spectrally more efficient than the DSSK modulation and this without the slightest increase in complexity.

 The present invention also relates to a differential spread spectrum transceiver.

This transceiver is characterized in that it comprises the transceiver component that has just been defined, this component being used in its transmission path to produce a spread spectrum intermediate frequency signal, this component being Moreover, it is used in its reception channel as an intermediate frequency signal processing device capable of correlating with the pseudo-random sequence used on transmission. This transceiver is further characterized in that the means of the transmission channel for transposing the intermediate frequency signal into a radio frequency signal are the same as the means of the reception channel for transposing the radio frequency signal into intermediate frequency signal.

Brief description of the drawings
- Figure I, already described, illustrates a
component according to the prior art,
FIG. 2 shows an exemplary embodiment
a receiver component according to trin
vention,
FIG. 3 shows an exemplary embodiment
of a transceiver component
according to the invention,
FIG. 4 represents a part of an emitter
spread spectrum receiver-receiver
reading the component of the invention.

Detailed presentation of an embodiment
The component shown in FIG. 2 comprises, on the same substrate t8, an input transducer TRE, and two output transducers, respectively TRS1,
TRS2.

 The input transducer comprises two lines or electrodes 20, 21, the first connected to a signal input E, the second connected to ground.

 The output transducers TRS1, TRS2 also include two lines or electrodes, one connected to a signal output, respectively S1 and S2, the other to ground.

In the illustrated variant, the transducer Denter is made in the form of an interdigitated comb comprising as many (M) groups of electrodes G1, ...,
GM that there are elements in the pseudo-random sequence. In the figure, we see 5 groups but we must understand that, in practice, M is between a few tens and a few hundred. Each group of electrodes has a length (L) corresponding to the duration of a binary element of the pseudo-random sequence. The length (S + L) of the input transducer corresponds to the duration (T) of a binary information symbol.

 As for the output transducers TRS1 and TRS2, they are made in the form of an interdigitated comb with a length L corresponding to the duration (Tc) of a binary element of the pseudo-random sequence.

 The difference between the two output transducers is equal to S + L.

 The component illustrated in Figure 3 differs from that of Figure 2 by the presence of a third output transducer, TRS3, placed substantially symmetrically of TRS1 relative to TRE. This third output transducer has an electrode connected to an output S3 and another electrode connected to ground.

It is identical in structure to the other two output transducers TRS1 and TRS2. All
TRE-TRS3 is a transmission channel, while the TRE-TRS1-TRS2 is a reception channel.

i
Figure 4 illustrates a part of a transceiver according to the invention. This circuit comprises a radiofrequency stage E (RF), an intermediate frequency stage E (FI), and, in between, frequency transposition means TF.

 This device comprises six switches, referenced from 11 to 16, at two positions E or R. The position E corresponds to a transmission operation and puts into service means constituting a "transmission channel".

The position R corresponds to reception operation and activates means constituting a "reception channel".

The transceiver of FIG. 4 comprises, generally speaking
A) in the emission channel
means 130 with a clock input 132
and a data entry 134; these means
are able to code the data D in symbo
binaries with a certain duration T
these symbols can be pulses
positive or negative,
a filter having an impulse response
which is a pseudo-random sequence of
equal to the duration T of a symbol of
binary training, this filter being
killed by the emission channel of component 80,
means for transposing the signal to a frequency
intermediate quence into a radio signal
frequency, these means comprising a bone
local oscillator 110 and a mixer 112,
a radiofrequency amplifier 1022, a
bandpass filter 104 and an antenna
100,
B) in the receiving lane
- a reception antenna which is confused
with the emission antenna 100 and an amp
radiofrequency factor 1021,
means for transposing the signal of
frequency in a frequency signal in
intermediate, these means being precisely
the means 110, 112 of the transmission channel
to transpose the signal at frequency in
intermediate in radio frequency signal,
a device for processing the signals of
intermediate frequency comprising
means capable of correlating
between the intermediate frequency signal
and the pseudo-random sequence used at
emissions these means being constituted by
the receiving channel of the component 80.

 A complete circuit further comprises demodulation and demodulated signal processing means which are not shown in FIG. 4 since they are well known to those skilled in the art. These means restore the information D used in the program.

The differential transceiver of FIG. 4 is characterized in particular by the fact that it comprises the transceiver component which has been described above with reference to FIG. 3. This component, referenced 80, is used in its transmission channel. transmission TRE-TRS3 to produce the spread spectrum intermediate frequency signal and, in its reception channel TRE,
TRS2, TRS2, as an intermediate frequency signal processing device capable of correlating with the pseudo-random sequence used at 1 transmission.

 A first switch I1 is provided for connecting the antenna 100 either to the reception amplifier 1021 or to the transmission amplifier 1022. A second switch I2 is also provided for connecting, in synchronism with the first, either the receiving amplifier 1021, which is the transmission amplifier 1022 to the common signal transposition means 110, 112.

 Furthermore, a bandpass filter 104 may be provided just after the amplifier 1022.

In the illustrated embodiment, the transceiver further comprises
an intermediate frequency amplifier 114,
an intermediate frequency bandpass filter
116,
a third switch I3 connecting the input
said amplifier 114 is to the means
common frequency transposition 110,
112, the transmission channel TR3, F3 of the
component 80,
a fourth switch I4 for connecting in
synchronism with the third switch
I3, the output of the bandpass filter 116,
at the reception channel TR1, Fl, TR2,
F2, the common means of transposition
frequency arrangement 110, 112.

 An automatic gain control circuit 118 is also provided to adjust the gain of the amplifier 114 from the received signal.

 The structure shown in FIG. 4 clearly shows the use of several subassemblies, both on transmission and on reception. The transition from the transmission mode to the reception mode is done through switches II to I5. These switches may be diode switches, so that the transition from one mode to another is done quickly.

In the diagram of Figure 4, all switches are in the receiving position (R).

 In transmission mode, the gain of the amplifier 114 is fixed and preset by means of a potentiometer 120.

It is conceivable to use this gain control input of the amplifier 114 to adjust the transmission power either with calibrated voltages or with an automatic gain control loop whose setpoint input would be the power of the signal transmitted by the antenna.

 In receive mode, the signal provided by the antenna is amplified by 1021 and filtered by 106 whose band is equal to or greater than Bes. In transmission mode, the signal output from the mixer 112 is filtered by 106, amplified by 1022 and then filtered again by 104 before being emitted by the antenna 100. The filter 104 is necessary to avoid any secondary lobe that could be produced by the nonlinearities in the amplifier 1022.

 The mixer 112 operates in down-converter mode, that is, the signal present at the input is mixed with the local oscillator signal and the frequency of the signal provided The intermediate frequency is the difference in frequencies between the frequency of the local oscillator and the radio frequency, the intermediate frequency being lower than the frequency of the local oscillator and the radio frequency, the frequency IF being lower than the frequencies OL. and RF.

 It operates in up-converter mode and the signal present at the input is mixed with the local oscillator signal to produce the radio frequency signal whose frequency is higher than the frequency intermediate.

 The intermediate frequency stage operates as follows in reception mode. The intermediate frequency signal at the output of 112 is amplified by 114 then filtered by 116 whose band is greater than Bes, before being injected at the input of the component 80 whose detailed operation has been described above. The gain control circuit 118 automatically adjusts the gain 114 so that the power of the intermediate frequency signal at the input input of the filtered component is constant. The signal is applied to the input E of the transducer TRE of the component 80. The two outputs S1 and S2 of the component are identical to a delay, the value of this delay being T, the duration of the transmitted bihaire symbol, as already indicated.

 In the transmission mode, the switches rl to 15 are in position E. To produce the transmission signal, the transmission channel of the component, that is to say the transducer TRE and the third output transducer TRS3, is used.

Claims (10)

 1. Component for differential receiver of spread spectrum signals by direct sequence, these signals comprising a series of binary symbols drink formation having a certain duration (T), each symbol being constituted by a pseudo-random sequence of a certain number (M ) of bits each having a duration (Tc), this component comprising a surface acoustic wave device with an input transducer (TRE) and two output transducers (TRS1, TRS2), the distance separating one the output transducer (TRS2) of the input transducer (TRE) being larger than the distance between the other output transducer (TRS1) of the input transducer (TRE) by an amount corresponding to the duration (T) a binary information symbol, this component being characterized in that only the input transducer (TRE) is adapted to the pseudo-random sequence.  2. Component according to claim 1, characterized in that the input transducer (TRE) is produced in the form of an interdigitated comb comprising as many (M) electrode groups (G1, ..., GM) as there are of binary elements (M) in the pseudo-random sequence, each electrode group (G1, ..., GM) having a length (L) corresponding to the duration (Tc) of a binary element of the pseudo-random sequence, the length ( S + L) of the input transducer (TRE) corresponding to the duration (T) of a binary information symbol.  3. Component according to claim 1, characterized in that each output transducer (TRS1, TRS2) is produced as an interdigitated comb having a length corresponding to the duration (Tc) of a binary element of the pseudo-random sequence.  4. Component for a differential direct spectrum spread spectrum signal transceiver, characterized in that it comprises  - a reception channel constituted by the  component (TRE, TRS1, TRS2) according to one  any of claims 1 to 3,  - a transmission channel constituted by the trans  input duct (TRE) of said component, and  in addition, by a third transducer  output (TRS3).  5. Differential transceiver component according to claim 4, characterized in that the third output transducer (TRS3) of the transmission path is in the form of an interdigitated comb having a length corresponding to the duration (Tc) of a binary element of the pseudo-random sequence to be transmitted.  6. Differential transceiver component according to claim 5, characterized in that the output transducer (TRS3) of the transmission channel and the first output transducer (TRS1) of the reception channel are arranged substantially symmetrically. the other with respect to the input transducer (TRE).  7. Direct sequence spread spectrum differential transceiver, comprising A) a transmission channel comprising  means (130) for coding information  (D) into binary symbols having some  duration (T),  a filter having an impulse response  which is a pseudo-random sequence of  equal to the duration (T) of a symbol of  binary training,  means (110, 112) for transposing the  intermediate frequency signal into a signal  radio frequency,  an emission antenna (100), B) a reception channel comprising  - a receiving antenna (100) and an amp  radiofrequency indicator (1022),  means (110, 112) for transposing the  radiofrequency signal into a freq signal  intermediate  a device for processing the signals of  intermediate frequency comprising  means capable of correlating  between the intermediate frequency signal  and the pseudo-random sequence used in transmission, this differential transceiver being characterized by comprising the transceiver component (80) according to any one of claims 4 to 6, which component (80) is used in its transmission path for producing the spread spectrum intermediate frequency signal, and in its reception channel as an intermediate frequency signal processing device adapted to correlate with the pseudo-random sequence used on transmission.  8. Transceiver according to claim 7, characterized in that the means (110, 112) of the transmission path for transposing the intermediate frequency signal into a radio frequency signal are the same as the means (110, 112). ) of the reception channel for transposing the radio frequency signal into an intermediate frequency signal.  9. Differential transceiver according to claim 8, characterized in that the antenna (100) of the transmission channel is common with the antenna (100) of the reception channel, this common antenna (100) being connected to the common frequency transposition means (110, 112) by a receiving amplifier (1021) and a transmitting amplifier (1022) in parallel, a first switch (11) being provided to connect the common antenna (100) to either the receiving amplifier (1021) or the transmitting amplifier (1022), a second switch (12) being provided for connecting in synchronism with the first (11) or the receiving amplifier (1021). or the transmit amplifier (1022) to the common frequency transposition means (110, 112).  10. Transceiver according to claim 8, characterized in that it further comprises  an intermediate frequency amplifier (there),  an intermediate frequency bandpass filter  diary (116),  a third switch (I3) connecting the  said amplifier (114) either to  common ways of transposing fre  quence (110, 112), either to the path of emis  (TR3, F3) of the component (80), - a fourth switch (t4) for connecting in  synchronism with the third switch  (13), the output of the bandpass filter  (116), that is to say the reception channel (TR1,  F1) (TR2, F2), the common means of  frequency transposition (110, 112).