GB2283882A - Secure communication system - Google Patents

Abstract

A communication system uses a frequency hopping technique in which the hopping rate is not less than the data bit rate of a transmitted message. The frequency transmitted at any instant is determined by the current output of one of a plurality of pseudo-random sequence generators 5, 6, the particular sequence used being selected in dependence on the current value of the data bit. (For example, if the current bit is "1" the output of generator 5 determines the frequency transmitted, whereas if the bit is "0" generator 6 determines the frequency). A cooperating receiver generates the same codes and frequencies in synchronism with the transmitter, and by comparing received frequency values with locally generated frequency values, the original message is reconstitued. The system is very resistant to jamming. <IMAGE>

Description

COMMUNICATION SYSTEM This invention relates to a communication system, and is particularly applicable to a secure system which is resistant to message jamming. By jamming is meant action which prevents the message being received by its intended recipient in a form from which the information content of the message can be correctly obtained. The invention is also useful for the transmission of communications via a "noisey" medium, that is to say in the case of the propogation of electromagnetic signals, a noisey medium would be one in which electromagnetic interference, possibly originating from atmopsheric conditions or electrical equipment, is present.In many communication systems, the presence of interfering noise having characteristics, such as frequency, similar to that of the communication signal, can lm- pair the quality of the message to the point where parts of it become unintelligible.

The present invention seeks to provide an improved communication system.

According to this invention a communication system includes a plurality of available sequences elements of which represent frequency values, each sequence representing a different data value; means for obtaining a message as a series of message segments, each having a data value; and means for allocating to successive message segments frequency values taken from that sequence which corresponds to its data value, nonallocated elements in the sequences being discarded; and means for transmitting the sequence of allocated frequency values.

Typically, the message is in the format of digital bits, in which each bit represents a message segment.

Thus in the case of a message originating as an analogue signal, sampled values are digitised to form a digital word representing each value, and there will thus be a plurality of message segments for each sampled value. Although there may be a number of elements in the sequence of frequency values which relate to a particular message segment, there cannot be more message segments than there are elements in the sequence of frequency values, and preferably each message segment is allocated just one frequency value for transmission.

Conveniently, the message consists of a stream of binary digital states, with one sequence of available frequency values corresponding to the logic '1' state, and another sequence of available frequency values corresponding to the logic '0' state.

Thus for binary data, two long codes are typically generated so that each element in the code corresponds to a predetermined frequency value.

These codes are available independently at a receiver in synchronism with those at the transmitter, and thus by comparing the received frequency values with the code sequences at the receiver1 the nature of the message can be fully determined.

It will be appreciated that such a transmission is not susceptible to conventional jamming techniques, since interference by other signals will not prevent a message being reconstituted. As a receiver will always expect the frequencies representing the available digital states, a jamming signal at the transmitted frequency will not be of avail, since the presence of that frequency will be detected at the receiver, as will the specific absence of those frequencies representing the alternative digital states. Clearly transmission of a very powerful wide band jamming signal is not practicable.In a sense the present invention is akin to what is known as a frequency hopping communication system, but whereas a jamming signal which follows the frequency hops will interfere adversely with a conventional system to the point where reception of the message may be impossible, this is not the case with the present invention as successful interference could be achieved only by a jamming signal occurring at a discarded, non-transmitted frequency of the sequence of available frequencies.The discarded nontransmitted frequencies are, of course, not known to a third party who might wish to jam the communication system.

The invention is further described by way of example with reference to the accompanying drawing in which: Figure 1 shows a transmitter forming part of a communication system, and Figure 2 shows a co-operating receiver.

Referring to Figure 1 a data source 1 is arranged to generate a message which is to be transmitted via a transmitting circuit 2 at a radiating antenna 3 in the form of a high frequency radio signal. The message is not transmitted at a constant frequency but instead is divided up into a series of small message segments each of which is transmitted at a different frequency which is determined by a frequency selection circuit 4. For this purpose the message generated by the data source 1 is converted into a digital format from an analogue format by repetitively and rapidly sampling analogue values, which are then converted into binary digital words, the number of bits of which is chosen with regard to the accuracy or fidelity required. Thus each digital word consists of a sequence of logic 'l's and 'O's which represent the information content of the message.In the context of this specific example each logic 1 or O in each digital word represents an individual message segment.

The frequency at which a particular message segment is transmitted is chosen in accordance with long pseudo-random code sequences which are generated by sequence generators 5 and 6, sequence generator 5 representing the logic 1 level whereas sequence generator 6 represents the logic 0 level. In practice, both code sequences are started from a known point in synchronism and thereafter re-cycled continuously. Each sequence may be very long indeed so that it does not complete a re-cycle during the length of a typical message. Although both code sequences may in fact, consist of streams of logic 'l's and 'O's, their information content will in general be multilevel, since a frequency generated by the generator 7 will depend on the effective value of an element of the code sequence.In practice, the frequency generator 7 is capable of generating any frequency within an extremely wide available frequency band at any given moment in time, the frequency which is actually generated at a specific instant being determined by the value of the code which is currently presented to it. Consequently, frequency generator 7 is capable of generating any one of possibly several hundred different spot frequencies in extremely rapid succession in a manner which is well known in connection with conventional frequency hopping communication systems. However, in the present instance, the frequency hopping rate is extremely rapid since it corresponds to the bit rate of the data source 1 with which the code sequence generators 5 and 6 are clocked by means not shown.

The hopping rate could however, be as slow or fast as the desired rate of transmission.

Thus the code values, which vary from moment to moment, are applied to a frequency selection circuit 4 which acts to allocate a value from one or other of the code sequences in dependence on whether it is currently receiving a logic 1 or a logic 0 from the date source 1. Thus the output of the frequency selection circuit 4 consists of those code values which are allocated to message segments depending on the nature of the message from the data source 1. This sequence of code values is converted to frequency values by the generator 7 and the frequency hops are passed to the transmitting circuit 2 for transmission at the antenna 3. The frequencies represented by the discarded code values are therefore not actually generated.

The transmitted signal therefore appears as a sequence of different frequency values and superficially appears to be a conventional frequency hopping system.

With reference to Figure 2, a co-operating receiver is provided with an antenna 10 at which the radiated signal is received and is routed to two similar receiving circuits 11 and 12. The receiving circuits 11 and 12 incorporate a frequency selective gate so that only frequencies lying within a very narrow specified band are output to a respective comparator 13 or 14, the value of the frequency gate being determined by a respective frequency generator 15 or 16 which in principle corresponds to the frequency generators 7 and 8 of the transmitter. Frequencies produced by the generators 15 and 16 are determined by respective code sequence generators 17 and 18 which generate exactly the same code sequences as code generators 5 and 6 and in exact synchronism therewith.

In practice, synchronism can be achieved by any conventional known means used typically in conjunction with standard frequency hopping communication systems.

The frequency received by the comparator 13 from the receiving circuit 11 is compared with that of the frequency generator 15 and an output provided to a data extraction circuit 19 in dependence thereon. The comparator 14 is operative in a similar manner.

It will already be appreciated that the transmitter radiates a signal which has a frequency which is indicative of whether the message instantaneously represents a logic 1 or a logic 0 level. Once the frequency is known at the receiver, its binary representation can be deduced only by reference to the encryption process represented by the code sequences produced by the generators 5, 6, 17, 18.

Thus at the receiver shown in Figure 2, the receiving circuits 11 and 12 both receive frequencies representative of the message. It is, however, necessary to identify whether the message segment being received represents a logic 1 or a logic 0. Thus the frequency generator 15 is operative to allow only those signals representing logic 1 levels to pass from the receiving circuit 11 to the comparator 13. Similarly, comparator 14 receives from the receiving circuit 12 only those frequency components known to represent logic 0 levels at that particular instant in time. If the frequency from the receiving circuit 11 is in fact equal to that generated by the frequency generator 15 or 16 as the case may be, a logic 1 or a logic 0 is positively identified. Clearly, it is not possible for both to be present at the same time.The outputs of the comparators 13 and 14 are fed to a data extraction circuit 19 which is operative to reconstitute the original message as a positive input from comparator 13 represents a logic 0 and that from comparator 14 represents a logic 1.

Reception of an interfering jamming signal at the frequency of transmission would not significantly impair the ability of the receiver to detect the correct signal. This is because the transmitted signal will still be present in the receiver and will be capable of giving a successful comparison at the comparator 13.

The presence of a jamming signal at the same frequency will simply raise the level of the output signal from the receiving circuit 11. Conversely, a jamming signal occurring accidentally at the same frequency as that generated by the frequency generator 15 in the absence of such a frequency being generated by the transmitter, will not give rise to an error which cannot be detected since the receiver 12 will be receiving the frequency component corresponding to a logic 0 level. This is because at any given moment in time the receiver is expecting to receive a frequency component coresponding to one or other of the two available binary logic levels. Any degradation of message segments in this way will be of an isolated nature since the nontransmitted frequencies of the discarded code values are not known to a third party, and any errors could easily be overcome by the use of simple error correction coding of the transmitted message.

With reference to Figure 1, a number of data sources 1 could be provided, with their content being obtained in a multiplexed manner from a common source.

In this case the transmission and reception circuits could simply be duplicated.

Claims (6)

1. A communication system including a plurality of available sequences, elements of which represent frequency values, each sequence representing a different data value; means for obtaining a message as a series of message segments, each having a data value; and means for allocating to successive message segments frequency values taken from that sequence which corresponds to its data value, non-allocated elements in the sequences being discarded; and means for transmitting the sequence of allocated frequency values.

2. A system as claimed in claim 1 and wherein each data value represents a separate bit of a digital word.

3. A system as claimed in claim 2 and wherein each digital word is binary in nature.

4. A system as claimed in claim 1, 2 or 3 and wherein for each data value, a respective pseudorandom code is used to determine the allocation of a frequency value to each message segment.

5. A system as claimed in any of the preceding claims and wherein a co-operating receiver is provided at which the same sequencies of frequency values are available in synchronism with those at the transmitter; and wherein said same sequencies are utilised to determine the data value represented by a received frequency value so as to permit reconstitution of said message.

6. A communication system substantially as illustrated in and described with reference to Figures 1 and 2 of the accompanying drawing.