INTERFERENCE SIGNAL GENERATOR
The invention relates to an interference signal generator and is
applicable, notably, to the field of telecommunications.
5
The generation of interference signals, commonly known as
electronic jamming, is a method intended to prevent the use of part of the
electromagnetic spectrum by telecommunications networks belonging to third
parties, while allowing its use by networks considered to be "friendly". For this
10 purpose, devices commonly known as jammers are used. The principle of
these devices is that of transmitting an interference signal at the same time
as the transmitters to be jammed, in at least the same frequency band.
Some telecommunications networks are designed to make it
difficult to jam their transmitted signals. For this purpose, it is possible,
15 notably, to use transmission methods with frequency hopping. These
systems are known as frequency evasion systems and are commonly
designated by the acronym EVF.
In order to interfere with communications in EVF networks,
adapted jamming techniques must be used. Three types of prior art jammers
20 for achieving this objective are commonly used, and are known as follower
jammers, barrage jammers and repeater jammers.
A follower jammer includes means for synchronizing it with the
transmissions of the EVF system to be jammed, and it uses, notably, a
frequency analysis of the spectrum to be protected. A jamming signal can
25 thus be transmitted at the same time and across a frequency band matched
to the transmissions of the EVF system. This is an effective means of
counteracting fast EVF communications, that is to say communications with a
frequency hop rate of more than a hundred hops per second, by using a
jamming signal concentrated on one frequency channel, where the term
30 "frequency channel" denotes a frequency band centred on what is known as
a central frequency. The spread of the jamming power is thus limited.
Furthermore, the parallelization of receiving channels enables the
instantaneous jamming band to be increased. However, this type of jammer
can only jam one EVF system at a time.
A second type of jammer uses the barrage jamming method. In
this case, the jamming signal is a wideband signal and there is a spread of
jamming power over this band. This type of jamming can be used, notably, as
an effective method of counteracting fast EVF (frequency evasion)
5 communications. This jamming is usually limited to an instantaneous band
with a width of several tens of MHz. Since the interference signal is
wideband, it is difficult to protect friendly systems which use frequencies
included in the jammed frequency band. Furthermore, the use of this type of
jamming is not discreet.
10 A third type of jammer is called a repeater jammer. This type of
jammer records the signal received by the jamming device and retransmits
the recorded signal. This makes it possible to counteract fast EVF
communications effectively. The transmitted interference signal is
concentrated on one frequency channel. The spread of the jamming power is
15 thus limited. The drawback is that the networks to be jammed cannot be
distinguished from friendly networks. Therefore it is possible that friendly
networks will be jammed. This is because the temporal processing of the
signal received by the jammer, that is to say the recording and
retransmission of the signal in a given time slot, makes it impossible to
20 protect friendly systems.
Thus it appears that there is no jamming device which can
simultaneously jam the transmissions of a plurality of telecommunications
networks while protecting the transmissions of one or more friendly systems.
25 One object of the invention is, notably, to overcome the aforesaid
drawbacks. For this purpose, the invention proposes a device for jamming
the transmissions of at least one enemy telecommunications network, the
device including means for receiving and storing signals periodically in a
receiving band Br in a receiving time slot having a predetermined duration 60.
30 The device includes means for the frequency analysis of the stored signals
and for detecting the frequency channels used by the telecommunications
networks transmitting in the band Br, and means for generating a jamming
signal capable of jamming the signals present in the detected frequency
channels, the device additionally including means for ascertaining the
35 frequency channels used by at least one friendly network and implementing a
function of the follower type enabling the device to be synchronized with the
friendly frequency evasion network or networks, the jamming signal being
adapted so as to jam the frequency channels used by the enemy network
without jamming the frequency channels used by the friendly network.
5 In one embodiment, a receiving time slot is opened periodically
every A. seconds, & being known as the opening period, the jamming signal
being adapted periodically on the basis of the analysis and detection carried
out in these slots.
In another embodiment, the opening period A. is chosen as a
10 function of the frequency hop rate of the transmissions to be jammed.
By way of example, a fast Fourier transform applied to the stored
signal can be used for the frequency analysis.
A signal is detected, for example, if the values obtained by
application of the fast Fourier transform exceed a first predetermined
15 threshold value andlor do not exceed a second predetermined threshold
value.
In one embodiment, the transmission of the jamming signal is
interrupted when the receiving time slot is opened.
The jamming signal preceding the last receiving time slot is
20 retransmitted during the processing time required for the analysis and
detection (302) of the stored signal.
The analysis, detection and jamming signal generation functions
can be implemented in an ASIC or FPGA circuit.
In one embodiment, the device interacts with monitoring means
25 which can be used to ascertain the frequencies used by friendly networks.
The device includes, for example, means for communicating by a
wire link with the monitoring means.
The monitoring means are, for example, implemented by an
apparatus belonging to the friendly network, this apparatus being installed in
30 such a way that it interacts with the device.
The device includes, for example, means for communicating by a
wireless link with the monitoring means (401).
The invention also proposes a system for jamming the
transmissions of at least one enemy telecommunications network, this
35 system being composed of a first and a second device.
The first device includes:
means for periodically receiving and storing (300, 301) signals
in a receiving band Br in a receiving time slot having a
predetermined duration 60,
means for the frequency analysis (301) of the stored signals
and for the detection (302) of the frequency channels used by
the telecommunications networks transmitting in the band Br,
and means for generating (304) a jamming signal adapted so
as to jam the signals present in the detected frequency
channels,
means for interacting with the second device in order to acquire
the frequency channels used by at least one friendly network
(303,307)
the jamming signal being adapted so as to jam the frequency
channels used by the enemy network without jamming the
frequency channels used by the friendly network.
The second device includes:
means for ascertaining the frequency channels used by at least
one friendly network (303, 307) and implementing a function of
the follower type enabling the device to be synchronized with
the friendly frequency evasion network or networks.
According to one aspect of the invention, the first and the second
device communicate by an optical, wire or radio link.
25 The invention has, notably, the advantage of enabling a plurality of
EVF networks to be jammed without requiring synchronization with any of
these networks. Furthermore, the jamming energy is concentrated solely on
the frequencies used by the networks to be jammed; in other words, the
jamming is of the narrow band type.
30
Other characteristics and advantages of the invention will be made
clear by the following description, provided for illustrative purposes and
without restrictive intent, which refers to the attached drawings, in which:
- Figure 1 is a simplified illustration of the jamming principle
according to the invention;
- Figures 2A and 2B show examples illustrating the effect of the
jamming rates;
- Figure 3 provides a simplified example of a jamming device
according to the invention;
- Figure 4 provides an example of a scenario in which a
jamming device according to the invention can be used.
10 As explained above, a repeater jammer periodically records the
signal that it receives and then retransmits it a number of times. Thus the
recorded signal extracts are used to jam the transmissions of the
telecommunications networks present, since the retransmitted signals
correspond to the frequency bands used by these networks. However, the
15 use of this type of jammer is limited, because all the signals in its receiving
band are jammed. This means that friendly networks are also jammed. The
jammer according to the invention can be used to jam a plurality of networks
while still allowing the passage of signals transmitted by friendly networks.
For this purpose, periodic and systematic jamming is applied while using
20 frequency analysis, the aim of this processing being to protect certain stages,
that is to say certain frequency channels used during a given period by one
or more friendly networks.
Figure 1 is a simplified illustration of the principle of the jammer
25 according to the invention. In this example, two EVF networks to be jammed
transmit signals at different instants. These transmissions are shown on a
two-dimensional graph. The horizontal axis is the frequency axis and the
vertical axis is the time axis. The transmissions associated with two EVF
networks to be jammed are represented. Figure 1 shows two successive
30 transmissions 100, 101 at the instants tl, t2. These transmissions have
different durations and are sent from a first EVF network. Four other
successive transmissions 102, 103, 104, 105 from a second EVF network are
also shown, these transmissions having different durations and being sent,
respectively, at the instants t'l, t'2, tB3t,h .
The principle of the invention is that a receiving slot is regularly
opened, this slot being called the open slot. During this slot, frequency
detection is carried out on the received signal. If the presence of one or more
signals is detected, the jamming is activated; in other words, a jamming
signal adapted to the results of the detection is transmitted. This adaptation
implies the generation of a jamming signal covering only those portions of the
frequency band that are used by the detected networks.
In other words, the jammer has open slots 106, 107, 108, 109,
11 0, 11 1, 112, 1 13, 114, 11 5, each having a duration of 60. This duration 60 is
referred to as the open time in the remainder of the description. These open
slots are available, for example, periodically, every A. seconds. The duration
of the open period & can be chosen as a function of the hop rate of the
transmissions to be jammed.
The transmission of the jamming signal is interrupted when the
slot is opened. The signal received by the jammer during the period 60 is
stored. The purpose of the frequency detection is to determine the
frequencies on which signals corresponding to radio transmissions are
present. In order to do this, the stored portion of signal with a duration of 60
must be analysed. This analysis requires a certain amount of computation,
and therefore a certain processing time tic following the open slot is needed
for the analysis.
Thus, two transmissions 100, 102 are present during the first open
slot 106 shown in Figure 1. By analysing the stored signal portion it is
possible to detect the two frequency bands used for these transmissions,
these bands being referred to as frequency channels in the remainder of the
description.
Thus, at the end of the open slot, the signal portion to be analysed
for the purpose of detection is acquired and analysed. The characteristics of
the jamming signal are available after the analysis. The jammer then
transmits a jamming signal to create interference on the two detected
channels.
In order to reduce the time intervals in which no jamming signal is
sent, it is possible to retransmit the jamming signal used before the last open
slot during the processing time &. This makes the operation independent of
the processing time, if a signal is present at a frequency identical to the
frequency before the open slot. If this is not the case, the excess frequency
11 6 of the jamming signal has no impact on the effectiveness of the jamming.
It is clear from this example that the jammer according to the
5 invention can be used for the simultaneous jamming of a plurality of
transmissions originating from different telecommunications networks.
It is also possible to protect a friendly network by using a function
of the follower type for synchronization with the friendly frequency evasion
network or networks. For this purpose, a protection mask corresponding to
10 the set of frequencies which must not be jammed is generated, and can be
used to prevent the jamming of frequencies used by the friendly network or
networks.
Figures 2A and 2B show examples illustrating the effect of the
15 jamming rate.
The jamming rate denoted as Rb is the proportion between the
jamming time and the non-jamming time, the jamming time being the time in
which a jamming signal is transmitted.
Figure 2A shows an example of a scenario in which two frequency
20 channels are used by an EVF network for transmission. The jammer
according to the invention includes means for determining the frequencies
present in the spectrum, using portions of signals received by the jammer
during the open slots 200, each of which has a duration of &. In this
example, in order to simplify the description, the processing time & is
25 considered to be negligible. In the example shown in Figure 2A, for an
opening period with a duration of Ao, the jamming rate Rb is 0.5 and it can be
described by the following expression:
In other words, the transmissions of the EVF network are jammed
for half the time. Most EVF networks designed to resist jamming are
implemented in such a way that digital data are transmitted with a high level
of redundancy. For this purpose; low efficiency channel codes are used. In
this case, redundancy enables the transmissions to be made robust against
jammers providing low jamming rates, of about 0.5 for example.
Figure 2B shows an example of a scenario in which two
5 frequencies are used by an EVF network for its transmissions, with a jammer
operating with a jamming rate of more than 0.7.
In this example, the processing time ac is allowed for. Thus, when
the signal received by the jammer has been acquired in an open slot 201, a
frequency detection process is executed for the period &. If no jamming
10 signal is transmitted during this period, the jamming rate can be described by
the following expression:
15 The transmission of the signal is therefore jammed over a period of & - (a0 +
6c)-
As explained above, during the processing time 6, it is possible
either not to transmit the jamming signal 204 or to retransmit the jamming
20 signal 202, 203 on the same frequency channels as those used before the
last open slot. In this second example, the transmissions of the EVF network
may have been subjected to a frequency hop. Consequently, the jamming
signal 202 retransmitted during the processing time tic is not transmitted on
the frequency band used by the EVF network during this time. However, if
25 there is no frequency hop between the start of the open period and the end of
the processing time, the jamming signal 203 is transmitted on the frequency
channel which is actually used by the EVF network. By retransmitting the
jamming signal during the processing period 6, in the last frequency band
used, the jamming rate Rb can be improved. This improvement is shown in
30 the following inequality:
Figure 3 provides a simplified example of a jamming device
35 according to the invention. The jammer has a number of processing stages.
For example, a first stage 308 comprises a first analogue to digital converter
(ADC) 300 which serves to convert the received signal obtained from a
radiofrequency receiving circuit. This stage 308 also comprises a second
converter of the digital to analogue type (DAC) 305, which serves to convert
5 the digital version of the jamming signal to be transmitted into an analogue
version which is then transmitted to a radiofrequency circuit for transmission.
The output of this ADC converter 300 is, for example, connected
to an FPGA or ASIC processing circuit 309, which includes a number of
functional units. A first analytical functional unit 301 receives the converted
10 digital signal at its input and serves to record the signal for a period Go and
then to carry out a frequency analysis on the recorded signal. For this
purpose, a fast Fourier transform (FFT) can be used. Advantageously, the
duration of EVF channel interception is reduced because only one FFT
needs to be applied.
15 The result of the FFT is used by a functional detection unit 302
which serves to compare these results with a threshold value in order to
determine the presence of transmissions originating from one or more
networks, as well as the frequency channels used for these transmissions.
The detected channels to be jammed are then used by a multiple
20 waveform generator 304, using a multiple jamming waveform ("Multi-JWF")
algorithm. This functional unit 304 serves to generate a digital version of the
jamming signal, this signal being used to jam the frequency channels
indicated by the logical detection unit 302. By way of example, the Multi-JWF
unit 304 is chosen in such a way that it can generate a jamming signal
25 covering up to 6 frequency channels at once.
The digital jamming signal is then converted to an analogue signal
by the DAC converter 305 for transmission by the jammer.
Additionally, a functional unit 303 for protecting friendly networks
provides information to the logical detection units 302 and/or the unit
30 implementing the Multi-JWF algorithm 304 to identify the frequency channels
which are used by these networks. The purpose of this information is to
ensure that the jamming signal does not cover the frequencies used by the
friendly networks.
By way of example, the information on the channels used by the
35 friendly network protection unit 303 can allow the detection unit 302 to
disregard certain frequency channels for which a signal has been detected,
on the basis that a predetermined threshold value has been exceeded. Thus
the information sent by the detection unit to the Multi-JWF unit 304 only
concerns the frequencies used by networks other than friendly networks.
5 The processing circuit 309 can be controlled by a host apparatus
310 such as a computer. The latter includes a human-machine interface 306
which, notably, allows the result of the frequency analysis 301 in the form of
spectral rays to be displayed graphically on a screen. Additionally, the host
apparatus 310 includes monitoring means 307 which, for example, transmit a
10 frequency mask, that is to say data identifying the frequencies used by the
friendly networks, to the friendly network protection functional unit 303 of the
processing circuit 309. These monitoring means 307 can be located either
inside the host apparatus as shown in the drawing, or outside it, as for
example in the case of a follower jammer configured in interception-detection
15 mode.
A device of this kind has, for example, the capacity to receive
signals for their analysis and for frequency detection over a band with a width
of several tens of MHz, owing to the analytical algorithms 301 which are
20 used, even if the radiofrequency circuits used have capacities which are
usually greater. If a wider band is to be covered, a plurality of ADC converter
units 308 and analysis units 309 covering separate bands can be used in
parallel.
25 Figure 4 provides an example of a scenario in which a jamming
device according to the invention can be used.
The aim is to interfere with communications 402, 403, 404 on
three enemy EVF networks, while avoiding interference with communications
405 on a friendly EVF network.
30 One solution is to use a jamming device 400 according to the
invention. This transmits a jamming signal 406 for interfering with
communications on three enemy networks on the basis of the analysis of the
received signals 407 originating from these networks. Additionally, because
of the monitoring capacity enabling the device to know the transmission times
35 and frequencies 408 used by the friendly network, the jamming signal is
generated in such a way that the friendly network can operate normally,
because the jamming signal does not interfere with its transmissions. The
monitoring means 401 can be external to the jamming device, as explained
above. In this case, an interactive procedure between two separate devices
5 400, 401 is followed. These two devices are, for example, linked by a wire or
wireless link.
If a wire link is used, the friendly network sends data for the
synchronization of the device 401. For this purpose, an apparatus of the
friendly network is installed so as to interact with the jammer.
10 If a wireless link is used, the monitoring means 401 are such that
the device can be synchronized with the friendly station according to the
interception-detectionlfollower process. In this case, it is not necessary to
have an apparatus in the friendly network installed with the jammer.

CLAIMS
1- Device for jamming the transmissions of at least one enemy
telecommunications network, the device including means for
periodically receiving and storing (300, 301) signals in a receiving
band Br in a receiving time slot having a predetermined duration 60,
characterized in that it includes means (301) for the frequency
analysis of the stored signals and for the detection (302) of the
frequency channels used by the telecommunications networks
transmitting in the band Brand means (304) for generating a jamming
signal capable of jamming the signals present in the detected
frequency channels, the device additionally including means for
ascertaining the frequency channels used by at least one friendly
network (303, 307) and using a function of the follower type enabling
the device to be synchronized with the friendly frequency evasion
network or networks, the jamming signal being adapted so as to jam
the frequency channels used by the enemy network without jamming
the frequency channels used by the friendly network.
2- Device according to Claim 1, characterized in that a receiving time slot
(106, 107, 108, 109, 110, 111, 112, 113, 114, 115) is opened
periodically every A. seconds, & being known as the opening period,
the jamming signal being adapted periodically on the basis of the
analysis and detection carried out in these slots.
3- Device according to Claim 2, characterized in that the opening period
A. is chosen as a function of the frequency hop rate of the
transmissions to be jammed.
4- Device according to any one of the preceding claims, characterized in
that a fast Fourier transform applied to the stored signal is used for the
frequency analysis (301 ).
5- Device according to Claim 4, characterized in that a signal is detected
(302) if the values obtained by the application of the fast Fourier
transform exceed a first predetermined threshold value and/or do not
exceed a second predetermined threshold value.
6- Device according to any one of the preceding claims, characterized in
that the transmission of the jamming signal is interrupted when the
receiving time slot is opened.
7- Device according to any one of the preceding claims, characterized in
that the jamming signal preceding the last receiving time slot is
retransmitted during the processing time 6c for the analysis (301) and
detection (302) of the stored signal.
8- Device according to any one of the preceding claims, characterized in
that the functions of analysis (301), detection (302) and jamming
signal generation (304) are implemented in an ASIC or FPGA circuit.
9- Device according to any one of the preceding claims, characterized in
that it interacts with monitoring means (401), these means making it
possible to know the frequencies used by the friendly networks.
10- Device according to Claim 9, characterized in that it includes means
for communicating by a wire link with the monitoring means (401).
I I- Device according to Claim 10, characterized in that the monitoring
means (401) are implemented by an apparatus belonging to the
friendly network, this apparatus being installed in such a way that it
interacts with the device.
12- Device according to Claim 9, characterized in that it includes means
for communicating by a wireless link with the monitoring means (401).
13- System for jamming the transmissions of at least one enemy
telecommunications network, the system being composed of a first
and a second device, the first device including:
means for periodically receiving and storing (300, 301) signals
in a receiving band Br in a receiving time slot having a
predetermined duration 60,
means for the frequency analysis (301) of the stored signals
and for the detection (302) of the frequency channels used by
the telecommunications networks transmitting in the band Br,
and means for generating (304) a jamming signal capable of
jamming the signals present in the detected frequency
channels,
means for interacting with the second device in order to acquire
the frequency channels used by at least one friendly network
(303, 307)
the jamming signal being adapted so as to jam the frequency
channels used by the enemy network without jamming the
frequency channels used by the friendly network,
and the second device including:
means for ascertaining the frequency channels used by at least
one friendly network (303, 307) and implementing a function of
. the follower type enabling the device to be synchronized with
the friendly frequency evasion network or networks.
14-System according to Claim 13, in which the first and the second
device communicate by an optical, wire or radio'link.
Dated this 17.06.2013
OF REMFRY & SAGAR
ATTORNEY FOR THE APPLICANT[S]