FR2695772A1 - Component for differential signal receiver for spectra spread by direct sequence - includes first correlator having applied signal at entry and pseudo-random sequence with second correlator to retard data - Google Patents

Abstract

The component is located in a receiver of an intermediate frequency stage, and includes a filter (20), an amplifier (22) with gain control circuit (26), two amplifiers (34,36) for mixing (38) in the gain control circuit (40). The component is inserted between the amplifier (22) and the amplifiers (34,36) and has a coupler (62) of 3 dB. At one entry and two outputs (51,52) is a surface wave acoustic device (64), which has a first line with an entry transducer (TR1) and a first correlator (C1) delivering a signal (S1) and a second line with a transducer (TR2) and a second correlator (C2) delivering a signal (S2) . The second correlator is shifted w.r.t. the first to cause a retardation of the signal S2 to S1. The signal (E) of the intermediary frequency is split in two by the coupler (62). Two identical signals (E1,E2) are created with equal power. These two signals are filtered by surface acoustic wave filters. The exit signals (51,52) are identical but one is retarded by time T by the relationship S2 (t) = S1 (t-T). USE - Spread of spectra signal with retardation of signal for use in radio communications particularly military.

Description

DESCRIPTION
Technical area
The subject of the present invention is a component for a direct sequence spread spectrum differential receiver.

State of the art
The spread spectrum technique has been used for many years in military radiocommunications, mainly because it makes it possible to establish discrete links, of difficult interception and resistant to interference. Two spreading techniques are conventionally used: spread spectrum by frequency hopping and spread spectrum by direct sequence.

 Frequency hopping spread spectrum (also called "frequency evasion") is the technique used for military applications.

It consists in changing the radio carrier frequency as often as possible (up to several hundred times per second) according to a law which is known only to 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 has not yet found applications in the civil field.

The other technique, spread spectrum by direct sequence (or, for short, ESSD or "Direct
Sequence Spread Spectrum "(DSSS) in English terminology), consists in transforming the signal to be transmitted, whose spectral band is Bs, into a signal having the properties of white noise, whose spectral band is much larger BES This spreading operation is carried out by multiplying the message to be transmitted by a pseudo-random sequence formed of bits which can take the values +1 or -1. If 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"), one can 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 from 10 to several tens of thousands depending on the case, the conventional values in radiocommunications never exceeding 1000.

 To be able to find the information, the receiver must carry out a correlation operation between the received message and a pseudo-random sequence identical to that which was used to carry out the spreading operation. The receiver must therefore know this sequence, which constitutes the access key to the message sent. Two independent messages can be transmitted using the same frequency band and two orthogonal sequences, that is to say two sequences having low intercorrelation coefficients.

The most frequently highlighted advantages of the direct sequence spread spectrum technique are as follows:
- the same frequency band can be shared
by systems using modulation
narrow band and those who use the sta
spectrum; the first did not notice
will see a slight increase in noise
ambient radio since the sequences
spreading have spectral characteristics
the noise while the second operates
rejection of belt modulations
close thanks to the correlation operation - several spread spectrum links
can share the same frequency band
by using pseudo-random sequences
orthogonal res; this technique (which
bears the name of distributed multiple access
by codes or CDMA for short or CDMA
in English) is unfortunately difficult
to implement and impose management
of transmitted power - a spread spectrum transmission
is difficult to intercept since the re
receiver must know the sequence of spread
to be able to demodulate the data
correctly; in fact an acquaintance
partial sequence sufficient - spread spectrum transmission
is fairly resistant to fog
their and interference and this all the more
the greater the gain in treatment.

In fact, you have to temper this
advantage because compared to a bond
narrow band and in the case of an app
civil cation where one should not take into account
that interference and not consider
the case of ill-intentioned jammers
indeed, by multiplying the band by N,
we intercept a noise power due
to interference N times stronger in
average which we reject with a gain of
treatment equal to N hence resistance
vis-à-vis this interference no more
important than for standard techniques
sics.

 Aside from better management of the radio spectrum (since this technique makes it possible to share the same frequency bands as certain narrow band emissions), spectrum spreading by direct sequence does not seem to have many advantages. The primary interest of this technique appears when analyzing the behavior of this modulation in radio channels in the presence of multiple paths.

 The technique of spread spectrum by direct sequence finds applications in radio transmissions with motor vehicles, in wireless local computer networks, data transmissions in an industrial environment and, more generally in all radio transmissions. inside buildings.

 In this type of environment, the propagation of the radio wave takes place along multiple paths involving reflection (on walls and partitions), diffraction (on edges) and scattering phenomena. In addition, in the general case, there is no direct path between the transmitter and the receiver.

This propagation by multipath induces a certain number of parasitic effects which deteriorate the quality of the transmission until making it sometimes unusable. In addition to the significant attenuation of the radio signal during its propagation (since it must cross partitions), the other annoying effects are a) The widening of the impulse response
of the canal. This effect is linked to the fact that
all journeys have durations of
different propagation, which imposes a
information rate limit value in
the canal. The duration of the transmitted symbol
must be much higher than this
impulse response width for
obtain a reasonable error rate.

b) Fading, which has
originated by a vector summation
signal destructive on the antenna
reception. The level of the received signal is
therefore significant fluctuations during
the movement of the receiving antenna.

These fluctuations exist even with
fixed antennas. They are, in this case,
generated by vehicle movements
or personnel in the radio path. These
fainting spoils of course
the quality of the transmission. The
techniques conventionally used
to improve this quality are
so-called diversity techniques, which
use multiple antennas
reception. The increased cost
by adopting these techniques done
that they are used essentially
in military systems.

c) Doppler noise or parasitic modulation
frequency. This effect is all the more
annoying that the data rate is low.

I1 is not a frequency offset
but of a noise, since the directions
arrival of the different radio paths
are any. This phenomenon is all the more
louder than the radio frequency and the
vehicle speeds are high.

 In this type of radio channel, the spread spectrum technique by direct sequence with differential phase modulation makes it possible to ensure good quality of transmission, subject to respecting a few rules which fix the modulation parameters.

These transmission techniques are described in numerous books and articles. One can quote, for example, the work entitled "Spread Spectrum Communications" 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'1, by
M. KAVEHRAD et al., Published in IEEE Journal
on SAC, vol. SAC 5, n "5, June 1987,
pp. 815-823,
- "Performance of differentially coherent
digital communications over frequency-selec
tive fading channels ", by FDGARBER and
al., published in IEEE Trans on communica
tions, vol. 36, No. 1, January 1988,
pp. 21-31,
- "Direct-sequence spread spectrum with DPSK
modulation and diversity for indoor wireless
communications ", by MK KAVEHRAD et al.,
published in IEEE Trans on communications,
flight. COM-35, nO 2, February 1987, pp. 224-236.

 FIG. 1 appended illustrates a known receiver capable of processing the spread spectrum signals by direct sequence. This receiver firstly comprises a radio frequency stage, referenced E (RF), then an intermediate frequency stage E (FI) and finally a baseband stage E (BB).

The radio frequency stage E (RF) comprises a reception antenna 10, a filter 12 whose band is equal to or greater than BEYS, an amplifier 14, a local oscillator 16 providing a frequency
FOL and a mixer 18. The latter delivers an intermediate frequency signal FI resulting from the mixing of the incident frequency and the frequency of the local oscillator 16.

 The intermediate frequency signal processing device comprises a filter 20 whose band is always equal to or greater than BES and an amplifier 22 whose output 24 is connected to a gain control circuit 26, intended to automatically adjust the gains of the amplifiers 14 and 22 so that the power of the intermediate frequency signal at output 24 is constant.

 The signal delivered by the output 24 is then applied to a correlator 30, which delivers on its output a signal S1 result of the correlation between the received signal and the spreading sequence used on 11 transmission.

 This correlation signal is moreover applied to a delay line 32, which delivers a signal S2 identical to S1 but shifted by a duration equal to the period of the information elements to be transmitted. The two signals S1 and S2 are then applied to two amplifiers 34 and 36. A gain control circuit 40 adjusts the gain of the amplifiers 34 and 36.

The modulation used being a differential phase modulation, the information is carried by the phase difference between the signals S1 and
S2. This information is extracted using the multiplier 38. If the signal at the output of this circuit is positive, S1 and S2 are in phase. If it is negative, they are in phase opposition.

 As for the baseband stage, it includes a low-pass filter 42 whose role is to reject the harmonics of the mixture, and an integrator 44.

The role of this integrator is essential in the case of propagation by multipath. In this case, each path corresponds to a correlation peak in S1 and S2 and, therefore, each path provides information. The integrator 44 sums all the information conveyed by these paths. It therefore performs the diversity processing operation by paths.

 The signals at the output of the integrator 44 are then sent to a circuit 46 for recovering the clock and to two comparators 48 and 50.

A data recovery circuit 52 finally delivers the data D. Finally, as in most data transmission receivers, there can be found a module for detecting and correcting transmission errors 54.

 The circuit finally delivers on an output 56 the data (D) and on another output 58 the clock pulses (H).

 The correlator 30 and the delay line 32 can each be produced in the form of a surface acoustic wave device (SAW abbreviated for "Surface Acoustic Waves"). This is what is described for example in the article by Mohsen KAVEHRAD et al.

cited above, (Figure 4, page 817). The correlator is actually a suitable filter. It is produced as a surface wave device and is followed by an amplifier, the output of which is divided into two channels, one connected to a delay line produced as a surface acoustic wave device and the other, directly connected to a multiplier, which also receives the delayed signal.

 Many articles have described surface acoustic wave devices, and in particular the article entitled "Practical Surface Acoustic Wave Devices" published by Melvin G. HOLLAND et al., In Proceedings of the IEEE, vol. 62, n "5, May 1974, pp. 582-611.

The application to spread spectrum communications is more particularly 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 (central frequency in particular). 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; ideally, it should be equal to the time reverse of the signal to which the filter must be adapted. The transducers are of the "interdigitated combs" type, the electrodes (or fingers) of which may have constant or variable lengths depending on their position. They are produced by depositing aluminum (a few hundred to a few thousand Angstroms thick) on the substrate used. Such components are described and offered in the catalogs of several suppliers of surface wave components. It may be added that, in the case of filters adapted to phase-coded signals, as is the case for the receiver system described, the relative bandwidths required are generally too wide to allow the use of other types of transducers than those known conventionally and which are bidirectional. Surface acoustic waves other than those which propagate between the two transducers are attenuated by depositing on the areas located between each transducer and the edge of the substrate which is closest to it an appropriate material (generally an adhesive).

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

 Ideally, the delay line transfer function should be of constant amplitude and linear phase (i.e. constant delay) in a frequency band at least equal to that occupied by the signal system obtained at the output of the adapted filter so as not to deteriorate the characteristics of the latter. In addition, this delay must be achieved with great precision and must be stable in temperature for the phase comparison between two successive correlation peaks to be valid.

 The precision and the temperature stability of the delay generally require the use of quartz as a piezoelectric substrate. Unfortunately, the bandwidth of this delay line must be so large (130 MHz in this case for a central frequency of 261.12 MHz) that it is practically impossible to envisage the creation of a delay line. with sufficiently low phase distortions for this application.

Statement of the invention
The object of the present invention is precisely to remedy this drawback. To this end, it offers a component capable of fulfilling both the correlation function and the delay function, this delay being pure, that is to say unaffected by distortion.

Specifically, the subject of the invention is a component for a differential receiver of direct sequence spread spectrum signals, characterized in that it comprises
- a first correlator able to perform
the correlation between the applied signal
upon entry and a pseudo-random sequence
identical to that used for spreading
spectrum,
- a second differentiated correlator only
compared to the first by the fact that the
signal at the output of this correlator is re
delayed by a period T corresponding to the
duration of an information bit.

 According to one of the embodiments, the component comprises, in the same housing, two acoustic channels.

The input transducers of these two channels are identical, so they can be supplied in parallel or via a 3 dB coupler. The first channel is the filter adapted to the signal to be received because of the spacing which naturally exists between the input and output transducers, the top of the correlation peak at the output of this filter has a delay L relative to the start of the input signal.

The second channel also performs the filtering adapted to the signal to be received, but the distance between the input and output transducers is greater than on the first channel and has been calculated so that the top of the correlation peak on this channel has a delay cry + T relative to the start of the input signal. Thus, there is simultaneously on the outputs of the device a correlation peak corresponding to the ith information bit and that corresponding to the (i-l) th. We can then perform the phase comparison to determine the information. The greater delay of the second channel results in signal distortion during propagation of the surface waves due to propagation losses and diffraction. These distortions are greater the greater the bandwidth and the delay. . However, CAD (computer-aided design) means for surface wave components allow these distortions to be taken into account and corrected by acting on the length of the electrodes of the transducers.

 According to other embodiments, the two channels can be placed in separate housings, which generally offers little interest (larger size, higher cost). They can also be produced on the same substrate.

 Another possibility is to take advantage of the bidirectionality of the surface wave transducers to have a common transducer, and to have the output transducers of channels 1 and 2 on either side of the latter.

 It is generally considerations on the dimensions of the substrate, on the usable boxes and on the manufacturing cost which determine the choice of one or the other of the solutions.

Brief description of the drawings
- Figure 1, already described, illustrates a re
spread spectrum receiver according to art
prior
- Figure 2 shows a component according to a
variant of the invention
- Figure 3 illustrates an embodiment
of this component,
- Figure 4 shows a component according to a
another variant.

Detailed description of an embodiment
The diagram in FIG. 2 shows the space occupied by the component of the invention in a receiver and more particularly in an intermediate frequency stage. Elements of this stage have already been shown in FIG. 1 and for this reason bear the same references. It is, upstream of the filter 20, of the amplifier 22 and of the gain control circuit 26 and, downstream, of the two amplifiers 34, 36, of the mixer 38 and of the gain control circuit 40. The component of the invention bears the reference 60 and is inserted between the amplifier 22 and the amplifiers 34, 36. I1 comprises a 3 dB coupler referenced 62, with one input e and two outputs sl, s2, and a wave device surface acoustic 64. As shown, this device comprises a first channel with an input transducer TR1 and a first correlator C1 delivering a signal S1 and a second channel with a transducer
TR2 and a second correlator C2 delivering a signal
S2. The second correlator is offset from the first, which leads to a delay from S2 to S1.

The operation of this component is as follows. The signal E at the intermediate frequency is separated into two using the coupler 62. Two identical signals El and E2 are therefore created with powers equal to half that of the induced signal. These two signals are then filtered by the surface acoustic wave filters of the two channels. The two outputs S1 and S2 of this filter are identical except for a delay, the value of this delay being chosen to be equal to the duration T of the binary symbol transmitted. So we have the following relation
S2 (t) = Sl (tT).

 This filter also performs the correlation operation between the input signal and the pseudo-random sequence used to spread the spectrum.

We have, for channel 1
Sl (t) = El (t) * I (t) where the sign * symbolizes the convolution operation and I (t) the impulse response of the first channel of the filter. This impulse response in baseband being the pseudo-random sequence reversed in time, S1 is the correlation product between El and the pseudo-random sequence.

The input transducer can be an interdigital comb with a reduced number of electrodes, so as to have a sufficiently wide passband. The surface acoustic wave (or wave of
RAYLEIGH) propagates from this transducer to the surface of the piezoelectric material with a speed of the order of 3150 m / s. It then passes under the electrode of the correlator C1, the length of which corresponds to the duration of a binary element of information. For a system with a data rate of 256 kb / s, the length of the output electrode is therefore worth
L = 3150 / 256.103, i.e. L = 12.3 mm.

 Each output transducer can be constituted by a interdigitated comb with 255 elements. Each element made up of several complementary electrodes corresponds to one of the chips of the pseudo-random spreading sequence. These elements are polarized according to the value of the corresponding chip and, of course, the last chip in the sequence corresponds to the element closest to this input transducer. When the acoustic signal which is modulated by the pseudo-random sequence is in phase with all the elements, the output signal is maximum. One thus obtains, at the output of this filter, a pulse whose width at mid-height is worth 15 ns (duration of a "chip") every 3.9 jis (duration of a bit of information).

 The second path of the surface acoustic wave component differs from the first in the position of the output transducer. The input and output electrodes of the two channels are identical, but the output transducer of the second correlator is placed at such a distance from the input transducer that the signal S2 is delayed by a duration T corresponding to a bit of information with respect to S1 so as to realize the relation S2 (t) = Sl (tT). This delay must be achieved with extreme precision, but above all, this precision must be valid throughout the frequency band used by the spread spectrum modulation. Or, in the embodiment which has been considered, the output transducer of the second channel has been optimized to take into account the attenuations and distortions generated by the propagation of the acoustic waves on the substrate so that the relation S2 (t) = Sl (tT) is verified in the entire frequency band considered.

We can therefore, by this means, achieve a perfect delay.

 Furthermore, the precision on the delay must be very high. In fact, the two signals S1 and S2 have a central frequency equal to the intermediate frequency of the receiver which is, in the example considered, of 261.12 MHz or four times the frequency "chip". Now, it is a question of comparing the phase of the signals S1 and 52 since the information is conveyed by the phase difference between these two signals. For this comparison to be valid, the error on the delay must therefore not cause an error on the phase greater than ten degrees at the frequency of 261.12 MHz. This corresponds to an error on the maximum delay of a hundred picoseconds for a delay of 3.9 microseconds.

 Since the component has an operating temperature range which can vary within a range of more than 500 ° C., the delay must not be able to fluctuate by more than 100 ps throughout the range.

Figure 3 shows an embodiment of the surface acoustic wave device. This device comprises two similar paths, the elements of which are identified by references bearing the indices 1 and 2 respectively. It suffices to describe one. The first path includes a blade 701 serving as a piezoelectric substrate. This blade is preferably made of quartz, with an ST cut. The input transducer
TR1 is connected to a ground terminal 711 and to an input terminal 721. An acoustic absorbent 731 is placed on the left side of the transducer. Only the acoustic wave propagating to the right is therefore used. The transducer F1 has one of its electrodes connected to an output terminal 741 and the other to a ground pad 751 An acoustic absorbent 761 is disposed at the rear of the output transducer.

 The second channel is similar to the first, except that the second output transducer is offset downstream on its piezoelectric plate, as already explained and that it has been optimized to take into account propagation distortions.

 In FIG. 4, another variant of the component of the invention is seen with a single input transducer TR, connected to a single input e. Two surface waves leave this transducer, one on the right towards the output transducer F1, the other on the left towards the output transducer F2, the second being more distant from TR than is the first.

Claims (5)

 1. Component for differential receiver of signals with spread spectrum by direct sequence, characterized in that it comprises  - a first correlator able to perform  the correlation between the applied signal  upon entry and a pseudo-random sequence  identical to that used for spreading  spectrum,  - a second differentiated correlator only  compared to the first by the fact that the  signal at the output of this correlator is retar  dice of a period T corresponding to the duration  of a bit of information.  2. Component according to claim 1, characterized in that the first and the second correlators each consist of a surface acoustic wave device with an input transducer (TR1, TR2) and an output transducer (F1, F2), the distance separating TR2 from F2 being greater than that separating TR1 from F1 of a length such that the time taken to traverse it by surface waves is equal to a period T.  3. Component according to claim 2, characterized in that the input transducers are separate (TR1, TR2) or produced in the form of a single transducer (TR).  4. Component according to claim 3, characterized in that each output transducer is produced in the form of an interdigital comb having a length such that the time taken by the acoustic waves to pass through it is equal to the period (T ) a binary piece of information.  5. Component according to claim 4, characterized in that the two correlators can be produced on the same piezoelectric substrate or on two separate substrates.