COVERT RADIO COMMUNICATIONS

This invention relates to apparatus for generating covert radio communications.

Many users of wireless systems have a need to send information to others such that the information is sent covertly, whereby potential opponents such for example as military adversaries or persons conducting criminal activities, cannot detect the information being sent. Known types of apparatus for generating covert radio communications may employ various approaches for creating low probability of intercept and detection. Most known apparatus focuses on using intermittent signals, special waveforms, low power, or similar techniques. Many of the known types of apparatus are vulnerable to detection techniques such as spectrogram and scalogram comparison.

It is an aim of the present invention to provide improved apparatus for generating covert radio communications.

Accordingly, in one non-limiting embodiment of the present invention there is provided apparatus for generating covert radio communications, which apparatus comprises:

(i) a radio frequency device for detecting a signal; and

(ii) transformation means for transforming the signal such that the signal appears to be the same as it was but actually contains added information through signal distortion, whereby the apparatus is operable to give a low probability of intercept and detection.

The apparatus may be one in which the transformation means operates to provide secure communications appropriate to a user’s requirements. For example, against opponents such as military opponents, the added information may provide parasitically via their signal distortion. Military personnel and other users of the apparatus may alternatively create their own signals to distort.

The apparatus may be one in which the radio frequency device for detecting a signal is configured to enable digital communication such that:

(a) a signal in a local environment is transformed to appear the same but contains the added information within the signal’s symbols by repeating a signal after the signal’s symbols have been manipulated via an amplitude modulation scheme;

(b) by manipulating a symbol on the repeated signal, additional data is able to be encoded such that the apparatus appears to other sources to be normal distortions warranting no attention; and

(c) a receiver is able to receive the encoded signal, decode it, and extract any data contained. The encoded signal may appear to be normal distortions to other sources such for example as other radios or to an opponent’s monitoring apparatus using monitoring signals.

The receiver may be a transceiver for transmitting back. Advantageously, the computational complexity of this transmitting back is low, which facilitates the use of real-time communications.

The apparatus may be one in which the radio frequency device for detecting a signal comprises:

(a) a discrete communication system that is part of a larger communications network;

(b) signal generating means which is able to manipulate emitted energy so that modulated signals contain additional information in a distorted symbol; and

(c) symbol providing means for providing a pulse or a burst of radio frequency radiation.

The discrete communication system may be a node. The node may be regarded as being a discrete communication system that is part of a larger network. Each node may be a radio comprising a circuit board, a receiver, and a transmitter or a transceiver. The circuit board may be used to provide management and control of the radio. With radios that are fitted with integrated circuits, then the radios can be programmed for advanced or complex radio frequency communication applications. The signal generating means may generate signals which are signals of radiated energy in the radio frequency range that are created by the application of time varied electrical currents and emitted by an antenna. The signal generating means may be a transceiver.

The symbol providing means may provide a symbol which is a pulse or a burst of radio frequency energy which may be determined by its spatial relationship and amplitude. The symbol may be a representation of a signal pulse.

The apparatus may be one in which there are two of the nodes, and in which the apparatus includes an integrated circuit which is configured to enable a receiver to quantize a received signal from a signal regardless of the received signal’s magnitude, a transceiver to then intercept a signal, and then manipulate a symbol of the signal by varying the amplitude of the symbol on the received background signal and then retransmit the signal.

The apparatus may be one in which the integrated circuit is configured to use the following high-level steps:

(i) after the primary data symbols are formed, set aside those data symbols that have the highest quadrature amplitude modulation values;

(ii) add covert data symbols to those highest quadrature amplitude modulation values; and (iii) transmit the modified symbols along with the unmodified symbols.

The apparatus may be one in which additional error correction is performed.

The apparatus may be one in which on the receiver equalised data symbols in l-Q form are assumed, and in which:

(i) one stream of l-Q data goes to a first quantizer;

(ii) a copy of the steam of l-Q data is split off before the first quantizer;

(iii) the output of the first quantizer is subtracted from the copy;

(iv) the resulting difference l-Q goes to a second quantizer;

(v) the output of the second quantizer goes to a symbol-to-bits mapper; and

(vi) the output of the symbol-to-bits mapper goes to a decoder that corrects for bit errors;

The apparatus may be one in which the decoder is a convolutional decoder that allows for a shift-register convolutional encoding of the bits at the transmitter.

The integrated circuit may govern the behaviour of the radio to receive signals and generate signals. This may include signal symbol selection, signal changes, encoding the signal, transforming the signal, and transmitting the signal. Conversely, when the integrated circuit receives the signals, the integrated circuit may also detect transformed signals, decode the signals, and process the signal data. This processing may be a two-way full duplex process. This enables both parties to be sending and receiving at the same time. Furthermore, an algorithm may enable initial negotiation of communication between nodes in order to commence with an initial handshake. This processing is able to take place at a high level in the integrated circuit and without the need for special receiver, transmitter or antenna design.

The apparatus of the present invention operates such that it uses signal distortion. There are potential other ways that this could be used for signals not using the signal distortion method.

The apparatus of the present invention makes covert or sensitive information difficult to detect. The information is sent over legitimate signals found in the environment and modifies a small part of a pulse (the symbol) that makes up a signal. However an opponent might somehow work out the devices that are transmitting and collect a large amount of the signals for analysis. By looking at a large amount of data, it could be possible for the opponent to detect that a symbol in the constellation seems to be abnormal. Reversing the signal and information could then point to hidden information. This could cause the opponent to look for such signals. In order to reduce the possibility of such detection, the apparatus of the present invention may be configured to increase the burden on potential opponents. This may be done by performing the same actions mentioned above, but by alternating between symbols in the constellation. The changed symbol may follow a semi-random switching of constellation positions. For a potential opponent who analyses the signal, this random changing of a distorted symbol with the hidden information makes it much more difficult to extract information from the signal and reduces the already small chance of detection. This method takes advantage of the fact that signals are normally distorted, which means that the minor change to symbols in the constellation is very difficult to detect.

In order to provide for the above, the apparatus of the present invention may be one in which the signal distortion provides a signal constellation, and in which the apparatus includes switching means for switching the signal constellation such as to alternate between symbols in the signal constellation.

The apparatus may be one in which the switching means comprises: a transmitting device which pairs with a receiving device, and both devices are configured to exchange “handshakes” and exchange encryption keys as well as information on managing hidden symbols within a normal signal’s constellations of symbols; the transmitting device emits a radio frequency signal which has been altered to hide information within a signal symbol; the transmitting device alters symbols according to pre-arranged patterns to reduce potential interception and discovery of the signal by altering different symbols in the constellation; and the receiving device detects a signal with hidden information, demodulates the signal and extracts information from symbols following a pre-arranged pattern.

Optionally, the apparatus may be one in which the receiving device is able to transmit back to the transmitting device. In this case, the receiving device is preferably configured to transmit back to the transmitting device in the same manner as the transmitting device transmits, for example with the above transmitting steps.

Optionally, the apparatus may be one in which the transmitting and receiving devices are also able to increase “bandwidth” by altering multiple symbols at the same time.

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which:

Figure 1 shows apparatus of the present invention for generating covert radio communications;

Figure 2 illustrates how the apparatus of the present invention operates in use;

Figure 3 shows a sequence of flow communications;

Figure 4 shows a process flow of the selecting and manipulation of a Long-Term Evolution (LTE) signal;

Figure 5 is a visual representation of a symbol map;

Figure 6 is a high level diagram illustrating the inventive concept of the invention;

Figure 7 is a first example of data mapped to a manipulated symbol; Figure 8 is a second example of data mapped to a manipulated symbol;

Figure 9 shows an l-Q constellation before carrier interferometer encoding;

Figure 10 shows how an l-Q constellation post carrier interferometer processing now looks like Gaussian noise; and

Figure 11 shows how overtime symbols are changed in a random order.

Referring to Figure 1 , there is shown apparatus 2 for generating covert radio communications. The apparatus 2 comprises a first radio 4 and a second radio 6. The apparatus 2 may include more radios if required. The radios 4, 6 and any other radios employed have the ability to send and receive signals 8. Figure 1 shows schematically how the signals 8 are received by the radio 4. The radio 4 then transmits signals 10 to the radio 6. The signals 10 have the same form as the signals 8 and thus the signals 10 appear to be the same as the signals 8. However, the signals 10 actually contain added information which provides parasitic signal distortion. This added information is shown by the illustrated numbers underneath the signals 10. The apparatus 2 is able to able to operate to give a low probability of communications intercept and detection. The radios 4, 6 are designed to allow for the covert communication using the parasitic communications by leveraging original signal data symbols to store information by manipulating the signals.

Referring now to Figure 2, there is shown how apparatus 12 of the invention is able to operate. More specifically, Figure 2 shows a first node 14 and a second node 16. A transmitter 18 transmits a background signal 20 to the first node 14. The first node 14 samples the background signal 20. The first node 14 transforms the background signal 20 to add parasitic signal distortion. The first node 14 then transmits the transformed signal 22 to the second node 16. The second node 16 receives the transformed signal 22 and decodes the transformed signal 22. The second node 16 may communicate back with the first node 14. The components 12, 14, 16, 18, 20, 22 may be regarded as transformation means for transforming the signal such that the signal appears to be the same as it was but actually contains added information through distortion. As shown in Figure 2, an opponent with monitoring apparatus 24 may or may not find the background signal 20 and the transformed signal 22. If the opponent does find the background signal 20 and the transformed signal 22, then the opponent will believe that the background signal 20 and the transformed signal 22 are the same. Thus the apparatus 12 is operable to give a low probability of detection.

The apparatus 12 shown in Figure 2 may operate as follows. Each of the first and second nodes 14, 16 may be in the form of a radio having a receiver, and a transmiter or a transceiver. The first and second radios need to communicate with each other in a covert manner due to surveillance. The first node 14 samples the background for legitimate signals such as those being permited by a nearby cell. The sampled signal is then processed to embed information by manipulating a symbol in the radio frequency signal, for example in the LTE signals constellation. The signals constellation is a collection of symbols that contain data. The transformed signal 22, which to all intents and purposes is the same as the original sampled background signal 20, is then transmitted to the second node 16. At the second node 16, the transformed signal 22 is processed to extract relevant information. The transmission of signals can be a one-way process or a two-way process. The communication may be full duplex.

The opponent with the monitoring apparatus 2 has several challenges. Firstly the opponent will be trying to find abnormal signals, which will be difficult since the communications between the first and the second nodes 14, 16 are legitimate signals which can be processed by the original sender’s recipient. Secondly the opponent, assuming that the opponent has found the signals to look at, must find the encoded symbol and decode it. This is exceptionally difficult.

Figure 3 shows a flow of communications as occurs in the apparatus of the present invention;

Figure 4 shows an example of a process flow in the selecting and manipulation of a LTE signal.

Reference will now be made to Figures 5 and 6 to describe in more detail the present invention.

Each radio can be provided with nodes which enable transmission and/or reception of signals. The symbols may be regarded as pulses of energy from a radio that are mapped to specific data bits. Thus, for example, a pulse picked up by one radio may be mapped to a bit of binary data. The radio transceiver uses some form of digital modulation which gathers the type and number of signals in each transmission. Basically, this takes the form of a burst of information that can be described as symbol constellation in a specific slice of time. One way to visualise these transmissions in time is given below with reference to Figure 5, which shows a constellation diagram.

The apparatus of the invention may be regarded as focussing on a digital modulation scheme in that the invention may use quadrature amplitude modulation. Other digital modulation techniques may be employed.

Referring to Figure 5, there is shown a visual representation of a symbol map. The digital modulation technique picks a specific symbol such as 1001. For example, as shown in Figure 5, each symbol is mapped to a word which is just a specific set of binary data of a specific length. The symbols are then translated to binary bits which are translated into forms of data to make up the information passed by the radio according to a specific radio protocol or scheme.

Referring now to Figure 6, there is shown apparatus 26 of the present invention. The apparatus 26 comprises radio frequency means comprising a first transceiver 28 and a second transceiver 30. The carrier signal is shown as signal 32. The radio frequency means comprising the first transceiver 28 and the second transceiver 30 may be regarded as a radio frequency device.

In operation of the apparatus 26, the transceiver 28 samples the carrier signal 32 or generates the carrier signal 32. The transceiver 38 then manipulates the signal’s amplitude according to a shared key. The transceiver 28 transmits the carrier signal with manipulated symbols as a transformed signal 34 to the transceiver 30. The transceiver 30 receives the signals 34 looking for a specific symbol manipulation. The transceiver 30 then decodes data from the symbol, thereby recovering hidden data. By way of example, the apparatus 26 may operate with the following steps.

1. The nodes in the radio communicating perform a traditional handshake or key pair exchange.

2. The transmitting node detects a background signal it wants to “piggy back” data onto, or creates a dummy signal to use as a carrier for covert data.

3. The transmitting node takes the carrier signal and selects a specific symbol or pulse and modulates its amplitude based on the key exchange and pairing in step 1 . This is basically amplitude modulation for a specific signal.

4. The original signal is then manipulated or transformed to add the changed symbol. That system is then broadcast.

5. The receiving node then looks for a signal with its amplitude modulation scheme being used by a specific symbol and then decodes it.

In the apparatus of the present invention, the signal processing may be, for example, as follows:

1. After the primary data signals are formed, set aside those data symbols that have their highest quadrature amplitude modulation values.

2. Add covert data symbols to those highest quadrature amplitude modulation values. 3. Transmit the modified symbols along with the unmodified symbols.

For the receiver, assume that there are equalised data symbols in l-Q form. The steps may then be as follows.

1. One stream of l-Q data goes to a first quantiser.

2 A copy of this stream is split off before the first quantiser.

3. The output of the quantiser is subtracted from the copy.

4. The resulting difference l-Q goes to a second quantiser.

5. The output of the second quantiser goes to a symbol-to-bits mapper.

6. The output of the symbol-to-bits mapper goes to a decoder that corrects for bit errors. A convolutional decoder may be preferred, whereby a shift register convolution encoding of the bits at the transmitter is able to be performed.

Referring to Figure 7, there is shown a selected symbol for manipulation. More specifically, the receiver can do successive interference cancellation. For example, as shown in Figure 7, the receiver receives a signal near point 1101. A, so it decides that the primary signal is the quadrature amplitude modulation symbol 1101. A and subtracts this estimate from the actual signal that was received. Now, only the secondary signal remains and some noise. Next the receiver determines which constellation point in 1101.B, the secondary signal corresponds to. Figure 8 shows an example of data mapped to the manipulated signal. Figure 8 shows remaining covert data after receiving it.

To make the covert data look like noise, Figures 9 and 10 show how carrier interferometer (Cl) coding applied to the quadrature amplitude modulation signals transforms the quadrature amplitude modulation constellation to a purely Gaussian distribution. Figure 10 shows how the l-Q constellation post Cl processing now looks like Gaussian noise. In Figure 10, it can be seen that Cl coding transforms the quadrature amplitude modulation symbols into a Gaussian distribution. This might seem harder to decode but surprisingly it is easier because each Cl symbol is a randomised linear combination of the original data symbols, so there is spreading gain. Since the quantization error has zero mean, the Cl spreading increases the signal-to- noise.

Figure 11 shows how over time the illustrated symbols are changed in a random order. This may be, for example, random signal constellation switching. If the symbols are charged in a random order, it increases the burden on potential opponents to work out the devices that are transmitting. For a potential opponent who analyses a signal, the random changing of distorted symbols with hidden information makes it much more difficult to extract information from the signals, and thereby reduces the already small chance of detection.

The apparatus of the present invention is able to use amplitude modulation and carrier interferometry to add additional information to an existing signal to make the changes look like noise. Once the signal is manipulated, it is able to be rebroadcast and only receives that share of the same algorithm that can extract the data. The approach uses carrier interferometry applied to the modulation type to create a Gaussian distribution that looks like noise in the signal.

It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications may be effected. Individual components shown in the drawings are not limited to use in their drawings and they may be used in other drawings and in all aspects of the invention. The invention also extends to the individual components mentioned and/or shown above, taken singly or in any combination.