CA1276682C - Secure communication system for multiple remote units - Google Patents
Secure communication system for multiple remote unitsInfo
- Publication number
- CA1276682C CA1276682C CA000537960A CA537960A CA1276682C CA 1276682 C CA1276682 C CA 1276682C CA 000537960 A CA000537960 A CA 000537960A CA 537960 A CA537960 A CA 537960A CA 1276682 C CA1276682 C CA 1276682C
- Authority
- CA
- Canada
- Prior art keywords
- information signal
- frequency
- base station
- frequencies
- relay unit
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/713—Spread spectrum techniques using frequency hopping
- H04B1/7143—Arrangements for generation of hop patterns
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/713—Spread spectrum techniques using frequency hopping
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04K—SECRET COMMUNICATION; JAMMING OF COMMUNICATION
- H04K1/00—Secret communication
- H04K1/003—Secret communication by varying carrier frequency at or within predetermined or random intervals
Abstract
SECURE COMMUNICATION SYSTEM FOR
MULTIPLE REMOTE UNITS
Abstract of the Disclosure A frequency-hopping datalink for providing communication between a base station, relay unit and multiple remote units utilizes time-division mutiplexing and transmits information signals ccntaining information relevant to the next frequencies of transmission. Signals from the base station are transmitted on a first frequency to a relay unit which retransmits the signals on a second frequency to a group of remote units. The information signal from the base station contains information concerning the frequencies of the next transmission and the time slot allotted for the remote units to respond. The remote units respond back at the first frequency which is received by the relay unit and rebroadcast to the base station at the second frequency. The first and second frequencies are changed after each message in accordance with a pseudo-random sequence generated either at the base station (and transmitted in encrypted form) or by means of a pseudo-random sequence generated at each remote unit and the relay unit, whose initial state is reset by an internal or externally derived time reference and whose pattern is determined by an internal key.
MULTIPLE REMOTE UNITS
Abstract of the Disclosure A frequency-hopping datalink for providing communication between a base station, relay unit and multiple remote units utilizes time-division mutiplexing and transmits information signals ccntaining information relevant to the next frequencies of transmission. Signals from the base station are transmitted on a first frequency to a relay unit which retransmits the signals on a second frequency to a group of remote units. The information signal from the base station contains information concerning the frequencies of the next transmission and the time slot allotted for the remote units to respond. The remote units respond back at the first frequency which is received by the relay unit and rebroadcast to the base station at the second frequency. The first and second frequencies are changed after each message in accordance with a pseudo-random sequence generated either at the base station (and transmitted in encrypted form) or by means of a pseudo-random sequence generated at each remote unit and the relay unit, whose initial state is reset by an internal or externally derived time reference and whose pattern is determined by an internal key.
Description
12'~668Z
SECURE COMPIUNICATION SYSTEM FOR
MUI1TIPIE REMOTl~ UNITS
Technical Field The present invention relates to the field of secure data communication 3ystems and, more particularly, to such a system for use with multiple remote units.
Back~round Art As di6closed in copending Canadian patent application Ser. No. 486,105, filed J~ne 28, 1985 various types of communications techniques are employed in order to enhance the security of information being transmitted between a base or ground station and a remote station or vehicle, such as a pilotless reconnaissance aircraft, and to make such communications more resistant to intentional jamming or noise.
In the aforementioned copending application there i~ described a method and apparatus ror preventing intentional ~amming Or a tran~mitter and receiver in a secure communications ~ystem of the frequency-hopping type wherein an information ~ignal i8 generated at the transmitter, the information ~ignal including at least a portion repre~entative of the frequency of transmission of the next information signal. The information signal is th~n transmitted and received at a remote receiver at a fre~uency determined by the frequency representative port~on of an immediately preceding transmltted information signal.
~2~i'66~
While the system described in this copending a~plication is adequate where there is to be direct communication ~between a transmutter and a receiver, a need may arise for a base station to communicate with multiple remote units through a relay unit.
Disclosure of the Invention The present invention concerns a method and apparatus for communicating between a base station and remote unit through a relay unit in a frequency-hopping com~unications system. More particularly, the method of the present invention i3 for use in a communication system of the type having at least one base station including a transmitter and a receiver, at least one relay unit including a transmitter and receiver, and at least one remote unit including a transmitter and receiver, and comprises the steps of generating and transmitting at the base station an information signal at a first predetermined frequency, receiving the information signal at the relay unit and transmitting the information signal at a second predetermined frequency, and receiving the information signal at the remote unit at this second predetermined frequency.
In accotdance with the invention, information for setting the first and second predetermined frequencies is transmitted as part of the information signal and the frequencies are periodically changed.
Preferably the frequencies are randomly changed in ordee to ensure communications security The information for setting the first and second predetermined frequencies may, for example, be a portion of the information signal representative of the next frequencies to be used for transmission and reception. Alternatively, the information for setting the first and second predetermined fre~uencie~ is a signal for changing the state of a pseudo-random frequency selector or a frequency mapping table forming part of the transmikters and receivers of the relay and remote units.
12~66~3Z
In order to further enhance the tran~mission security, the infor~ation for setting the first and second predetermined ~xequencies can be encrypted.
The present invention may further be used where a remote unit generates and tran mits a second infor-mation signal. This second information signal is transmitted at the first predetermined frequency and received at the relay unit. The relay unit then re-transmits the second information signal at the second predetermined frequency which i8 then received by the base station.
In order to prevent interference between the transmis~ion of the remote units the infor~ation signal transmitted by the ba~e station includes a portion indicative of separate and unique time delays for transmission of infor~ation signals generated by each of the remote units.
With the foregoing ~ystem, multiple remote units may communicate with a ba~e tation via a relay unit usinq frequency-hopping in order to prevent ~amminq of or interference with the signals between the base station and the remote unit~. Since the relay unit transmits at a frequency dif~erent than that at which the base station and remote units transmit, an eaves-dropper or potential ~ammer would only be able todetect one of the two rrequencies in use at a parti-cular time. Thus transmis~ion of a ~amming signal Oh the tran~mis~ion frequency of the relay unit would not affect the transmis~ion or reception of information signals from the base ~tation to the r~lay unit or from the remote units to the relay unit. Means may be provided at any of the base ~tation, relay unit or remote units for detecting the onset and frequency of a ~amming or other interfering signal ao that these units may be arranged to switch to a predetermined set of one or more fall-back transmi~sion frequencies or to avoid the frequencies of the detected interfering . -4-signal, such as set forth in the aforementioned copending application.
The foregoing arrangemen~ thus enables communication to take place between a base station, relay unit and remote units with a high probability of success in completing the transmis~ion even in the face of deliberate attempts to interfere with such communication.
Apparatus for performing the above method i5 also encompassed by the present invention.
Brie~ Description of Dra~inga These and other features and advantages of the present invention will be clear from the following detailed description of the preferred embodiments, when taken in conjunction with the drawing figure~
wherein:
Figure 1 i8 a block diagram of a data communication ~ystem arranged in accordance with a present invention;
Figure 2 shows diagrammatically a typical information signal as used in the present invention;
Figure 3 i8 a block diagra~ of a first embodiment of a frequency selector circuit for u~e in the pr¢sent invention; and Figure 4 is a block diagram of a second embodiment of a frequency ~elector circuit for use in the present invention.
Best Mod~e~ for Car~yir~lbut the Invention As shown in Figure 1, the pre~ent invention consists of a base station 1 comprised of a transmitter portion, To~ and a receiver portion, Ro~
constructed as shown in Figure 1 of the aforementioned copending application Ser. No. 48~,10S.
Transmitter To transmits an information signal on a so-called "up-link" frequency Fl to a relay unit 3 and receiver Ro receives a signal from relay unit 3 via "down-link" frequency F2.
~2~i682 Relay unit 3 comprises a similar transmitter portion Tr and receiver portion Rr for transmitting and receiving signals at frequencies F2 and F1, respectively.
Also shown in Figure 1 are one or more remote units 5a. 5b 5n which also include transmitter and receiver portions Tal Ra~ Tb,Rb,...Tn,Rn, respective-ly, similar to those of base station 1 and relay unit 3. Receivers Ra,Rb...~n receive signals from relay unit 3 at frequency F2 and transmitter~ Ta~ Tb...Tn optionally transmit a further information signal back to relay unit 3 via frequency Fl.
It is to be noted that transmission of ~ignals from relay unit 3 to both remote units 5a~5b 5n and base station 1 taXes place at frequency F2 while reception of signals fro~ remote units 5a~5b 5n and base station 1 by relay unit 3 takes place at frequency Fl.
In accordance with the present invention, frequencies F1 and F2 are periodically and randomly changed in order to prevent an ea~esdropper from receiving the trans~itted infor~ation signals and to prevent possible ~amming. To thi~ end, as shown in Figure 2, the information signal transmitted by base station 1 to relay unit 3 and for subsequent retrans-mis~ion to remote unit~ 5a~5b 5n contains a portion indicative of the next set of frequencies, Fl and F2~
which will be used by the ~y~tem for tran~mission and reception. This frequency indicative portion may be a signal representative of the next pair of trans~ission and reception frequencies, which signal may be further encrypted, as ~et forth in copending application Ser.
No. 486,105. Alternatively, the frequency indicative port~on may include information for setting the status of pseudo-rando~ number (PN) generators associated with the frequency selecting circuits included as part of the base station, relay unit and each of the remote ~Z';16~82 --s--units, as discussed more fully below. Thi~ frequency indicative portion may also be encrypted.
Since each of the remote units tran8mit8 it8 information signal back to relay unit 3 ~ia frequency Fl, it is necessary that each such remote unit be told to transmit only during predeter~ined time periods, so as to not interfere with each other. Accordingly, the information signal trans~itted by base station 1 includes an address portion unique for each remote unit and a portion indicative of the time period in which each remote uni~ i~ to respond to any ~essage from the base station relayed by relay unit 3. ~ore particularly, as shown in Figure 2, the infor~ation signal transmitted by base station 1 includes a portion indicatlve oP a unique time delay for each remote unit which specifies the amount of time the designated remote unit i~ to wait before respondi~g to the base station and/or trans~itting an information signal back to the base ~tation.
In operation, an initial information signal is transmitted by basa station 1 via frequency Fl to relay unit 3 which retran~mits the signal to remote units 5a~5~ ..5n at frequency F2. The relay unit and remote units have been initialized to receive the information signal at frequencies Fl and F2, re~pectively, in a manner a~ set forth in copending application Ser. No. 486,105. The portion of the information signal indicativ- of th~ next pair of different transmission frequencies F1 and F2 is decoded by the relay unit and remote unit~ in preparation for the next information signal to be broadcast by the base station.
Upon reception of the information ~ignal, each remote unit decodes the addre~s portion of the ~ignal and its a~sociated time delay unigue to Qach ramote unit. Each remote unit ~ay include a stable time reference or external time re~erence source (dascribed lZ76682 below) for this purpo6e. Each remote unit waits until its specified time delay has elapsed and then transmits its information signal bacX to the base station via the relay unit, as described earlier.
Upon reception of all information signals from the remote units by the base station, or after a predetermined maximu~ time delay (in the event one or more of the remote units is unable to communicate), the base station trans~its a new information signal at new frequencies Fl and F2 (a~ set during the previous trans~ission cycle). This process of transmission from the base station to the remote units and back again via the relay unit may continue for as long as necessary with the frequencies of transmission, Fl, and F2, being changed periodically and randomly 80 as to prevent eavesdropping or ~amming.
As mentioned earlier, the relay unit and remote unit may have their frequency selecting circuits retuned for the next frequencie~ of transmi~sion as set forth in copending application Ser. No. 486,105.
Alternatively, these frequency selQcting circuits may be set using internal frequency selecting apparatus whose state is initially ~et by a portion of the information signal transmitted by base station 1.
More particularly, a~ shown in Figure 3, the frequency selecting circuit include~ a PN generator 7 whose initial st~lte (e.g. the ~tarting point in the PN code sequence) i8 responsive to a ~ignal genQrated by key generator 9 and a stable time reference (e.g. a stable internal clock or a receiver tuned to receive a radio broadcasted time reference). The output of PN
generator 7 is applied to a mapping table 11 (e.g., a random access memory) which contains information reprQsentative of specific tran~mission and reception fr2quencies. Thus, the state of PN generator 7, as initialized by key generator 9 and the stable time reference, will cause mappinq table 11 to generate ~Z~66~3Z
-7a-different frequencies Fl and F2 for each tran-~mission cycle. To an eavesdropper, these frequencies will appear to periodically and randomly change or "hopn.
As shown in Figure 4, instead of using a stable time reference, the PN generator may be ~ade responsive to a portion of ~he infor~ation signal transmitted by the base station.
~Z76682 The frequency mapping table, block 11, in figures 3 and 4 may also be made responsive t~ a portion of the information signal transmitted by the base station. The sequenae of the-~first and second frequencie-~ can be controlled by either controlling the PN
5 generator state, by altering the frequency mapping table contents, or by any combination of these two technique~. m us the sequence or pattern of frequencies can be programmed init~ally, or changed dynamically by the transmitted information signal, to avoid certain frequency band~.
In either of the arrangements shown in Figures 3 or 4, the base station, relay unit, or remote units may include apparatus for detecting frequencies upon which jamming or interfering signal~ are present. ThLs information can then be used to avoid these frequencies which might otherw~se be seiected by the mapping table. -Advantageously, the foregoing arrangement enables multiple remote units to communicate with a base station via a relay unit using frequency hopping and time-division multiplexing principles to minimize interference and the effect3 of possible ~amming. Since the up-link and down-link frequencies are periodically and randomly changed it would be extremely d~fficult for an eavesdropper or ~ammer to discover any pattern to the frequency changes. Further, reception of one of the transmitted signals tells nothing about the frequency of transmis~ion of the other signal. Therefore a jammer would at best be able to ~am only half of the communications flow (i.e. the information being transmitted at the ~amming frequency) and then only until the next transmission cycle when the frequencies would be changed again. Even in this latter case, the communication system is designed to detect such jamming frequencies and take corrective action to avoid such frequencies, such as by dynamically altering the state of the frequency mapping table or PN generator in response to a portion of the information signal.
~ hile the present invention has been described in considerable detail, it is understood that various changes and modifications will lZ76~
g fall within the scope of the invention. For example, multiple relays, each assigned to a group of eemote units, could be used, with each relay bein~ assigned a different set of uplink/downlink frequencies by the base station. Alternatively, a relay unit could communicate with one or more additional relay units to extend the range of communication of the system. In this case one oe more addltional frequencies can be assigned for the intecmediate (relay-to-relay) link. It should therefore be understood that the foregoing detailed description of the preferred embodiments is merely illustrative, but not limitive, of the invention which is defined by the appended claims.
SECURE COMPIUNICATION SYSTEM FOR
MUI1TIPIE REMOTl~ UNITS
Technical Field The present invention relates to the field of secure data communication 3ystems and, more particularly, to such a system for use with multiple remote units.
Back~round Art As di6closed in copending Canadian patent application Ser. No. 486,105, filed J~ne 28, 1985 various types of communications techniques are employed in order to enhance the security of information being transmitted between a base or ground station and a remote station or vehicle, such as a pilotless reconnaissance aircraft, and to make such communications more resistant to intentional jamming or noise.
In the aforementioned copending application there i~ described a method and apparatus ror preventing intentional ~amming Or a tran~mitter and receiver in a secure communications ~ystem of the frequency-hopping type wherein an information ~ignal i8 generated at the transmitter, the information ~ignal including at least a portion repre~entative of the frequency of transmission of the next information signal. The information signal is th~n transmitted and received at a remote receiver at a fre~uency determined by the frequency representative port~on of an immediately preceding transmltted information signal.
~2~i'66~
While the system described in this copending a~plication is adequate where there is to be direct communication ~between a transmutter and a receiver, a need may arise for a base station to communicate with multiple remote units through a relay unit.
Disclosure of the Invention The present invention concerns a method and apparatus for communicating between a base station and remote unit through a relay unit in a frequency-hopping com~unications system. More particularly, the method of the present invention i3 for use in a communication system of the type having at least one base station including a transmitter and a receiver, at least one relay unit including a transmitter and receiver, and at least one remote unit including a transmitter and receiver, and comprises the steps of generating and transmitting at the base station an information signal at a first predetermined frequency, receiving the information signal at the relay unit and transmitting the information signal at a second predetermined frequency, and receiving the information signal at the remote unit at this second predetermined frequency.
In accotdance with the invention, information for setting the first and second predetermined frequencies is transmitted as part of the information signal and the frequencies are periodically changed.
Preferably the frequencies are randomly changed in ordee to ensure communications security The information for setting the first and second predetermined frequencies may, for example, be a portion of the information signal representative of the next frequencies to be used for transmission and reception. Alternatively, the information for setting the first and second predetermined fre~uencie~ is a signal for changing the state of a pseudo-random frequency selector or a frequency mapping table forming part of the transmikters and receivers of the relay and remote units.
12~66~3Z
In order to further enhance the tran~mission security, the infor~ation for setting the first and second predetermined ~xequencies can be encrypted.
The present invention may further be used where a remote unit generates and tran mits a second infor-mation signal. This second information signal is transmitted at the first predetermined frequency and received at the relay unit. The relay unit then re-transmits the second information signal at the second predetermined frequency which i8 then received by the base station.
In order to prevent interference between the transmis~ion of the remote units the infor~ation signal transmitted by the ba~e station includes a portion indicative of separate and unique time delays for transmission of infor~ation signals generated by each of the remote units.
With the foregoing ~ystem, multiple remote units may communicate with a ba~e tation via a relay unit usinq frequency-hopping in order to prevent ~amminq of or interference with the signals between the base station and the remote unit~. Since the relay unit transmits at a frequency dif~erent than that at which the base station and remote units transmit, an eaves-dropper or potential ~ammer would only be able todetect one of the two rrequencies in use at a parti-cular time. Thus transmis~ion of a ~amming signal Oh the tran~mis~ion frequency of the relay unit would not affect the transmis~ion or reception of information signals from the base ~tation to the r~lay unit or from the remote units to the relay unit. Means may be provided at any of the base ~tation, relay unit or remote units for detecting the onset and frequency of a ~amming or other interfering signal ao that these units may be arranged to switch to a predetermined set of one or more fall-back transmi~sion frequencies or to avoid the frequencies of the detected interfering . -4-signal, such as set forth in the aforementioned copending application.
The foregoing arrangemen~ thus enables communication to take place between a base station, relay unit and remote units with a high probability of success in completing the transmis~ion even in the face of deliberate attempts to interfere with such communication.
Apparatus for performing the above method i5 also encompassed by the present invention.
Brie~ Description of Dra~inga These and other features and advantages of the present invention will be clear from the following detailed description of the preferred embodiments, when taken in conjunction with the drawing figure~
wherein:
Figure 1 i8 a block diagram of a data communication ~ystem arranged in accordance with a present invention;
Figure 2 shows diagrammatically a typical information signal as used in the present invention;
Figure 3 i8 a block diagra~ of a first embodiment of a frequency selector circuit for u~e in the pr¢sent invention; and Figure 4 is a block diagram of a second embodiment of a frequency ~elector circuit for use in the present invention.
Best Mod~e~ for Car~yir~lbut the Invention As shown in Figure 1, the pre~ent invention consists of a base station 1 comprised of a transmitter portion, To~ and a receiver portion, Ro~
constructed as shown in Figure 1 of the aforementioned copending application Ser. No. 48~,10S.
Transmitter To transmits an information signal on a so-called "up-link" frequency Fl to a relay unit 3 and receiver Ro receives a signal from relay unit 3 via "down-link" frequency F2.
~2~i682 Relay unit 3 comprises a similar transmitter portion Tr and receiver portion Rr for transmitting and receiving signals at frequencies F2 and F1, respectively.
Also shown in Figure 1 are one or more remote units 5a. 5b 5n which also include transmitter and receiver portions Tal Ra~ Tb,Rb,...Tn,Rn, respective-ly, similar to those of base station 1 and relay unit 3. Receivers Ra,Rb...~n receive signals from relay unit 3 at frequency F2 and transmitter~ Ta~ Tb...Tn optionally transmit a further information signal back to relay unit 3 via frequency Fl.
It is to be noted that transmission of ~ignals from relay unit 3 to both remote units 5a~5b 5n and base station 1 taXes place at frequency F2 while reception of signals fro~ remote units 5a~5b 5n and base station 1 by relay unit 3 takes place at frequency Fl.
In accordance with the present invention, frequencies F1 and F2 are periodically and randomly changed in order to prevent an ea~esdropper from receiving the trans~itted infor~ation signals and to prevent possible ~amming. To thi~ end, as shown in Figure 2, the information signal transmitted by base station 1 to relay unit 3 and for subsequent retrans-mis~ion to remote unit~ 5a~5b 5n contains a portion indicative of the next set of frequencies, Fl and F2~
which will be used by the ~y~tem for tran~mission and reception. This frequency indicative portion may be a signal representative of the next pair of trans~ission and reception frequencies, which signal may be further encrypted, as ~et forth in copending application Ser.
No. 486,105. Alternatively, the frequency indicative port~on may include information for setting the status of pseudo-rando~ number (PN) generators associated with the frequency selecting circuits included as part of the base station, relay unit and each of the remote ~Z';16~82 --s--units, as discussed more fully below. Thi~ frequency indicative portion may also be encrypted.
Since each of the remote units tran8mit8 it8 information signal back to relay unit 3 ~ia frequency Fl, it is necessary that each such remote unit be told to transmit only during predeter~ined time periods, so as to not interfere with each other. Accordingly, the information signal trans~itted by base station 1 includes an address portion unique for each remote unit and a portion indicative of the time period in which each remote uni~ i~ to respond to any ~essage from the base station relayed by relay unit 3. ~ore particularly, as shown in Figure 2, the infor~ation signal transmitted by base station 1 includes a portion indicatlve oP a unique time delay for each remote unit which specifies the amount of time the designated remote unit i~ to wait before respondi~g to the base station and/or trans~itting an information signal back to the base ~tation.
In operation, an initial information signal is transmitted by basa station 1 via frequency Fl to relay unit 3 which retran~mits the signal to remote units 5a~5~ ..5n at frequency F2. The relay unit and remote units have been initialized to receive the information signal at frequencies Fl and F2, re~pectively, in a manner a~ set forth in copending application Ser. No. 486,105. The portion of the information signal indicativ- of th~ next pair of different transmission frequencies F1 and F2 is decoded by the relay unit and remote unit~ in preparation for the next information signal to be broadcast by the base station.
Upon reception of the information ~ignal, each remote unit decodes the addre~s portion of the ~ignal and its a~sociated time delay unigue to Qach ramote unit. Each remote unit ~ay include a stable time reference or external time re~erence source (dascribed lZ76682 below) for this purpo6e. Each remote unit waits until its specified time delay has elapsed and then transmits its information signal bacX to the base station via the relay unit, as described earlier.
Upon reception of all information signals from the remote units by the base station, or after a predetermined maximu~ time delay (in the event one or more of the remote units is unable to communicate), the base station trans~its a new information signal at new frequencies Fl and F2 (a~ set during the previous trans~ission cycle). This process of transmission from the base station to the remote units and back again via the relay unit may continue for as long as necessary with the frequencies of transmission, Fl, and F2, being changed periodically and randomly 80 as to prevent eavesdropping or ~amming.
As mentioned earlier, the relay unit and remote unit may have their frequency selecting circuits retuned for the next frequencie~ of transmi~sion as set forth in copending application Ser. No. 486,105.
Alternatively, these frequency selQcting circuits may be set using internal frequency selecting apparatus whose state is initially ~et by a portion of the information signal transmitted by base station 1.
More particularly, a~ shown in Figure 3, the frequency selecting circuit include~ a PN generator 7 whose initial st~lte (e.g. the ~tarting point in the PN code sequence) i8 responsive to a ~ignal genQrated by key generator 9 and a stable time reference (e.g. a stable internal clock or a receiver tuned to receive a radio broadcasted time reference). The output of PN
generator 7 is applied to a mapping table 11 (e.g., a random access memory) which contains information reprQsentative of specific tran~mission and reception fr2quencies. Thus, the state of PN generator 7, as initialized by key generator 9 and the stable time reference, will cause mappinq table 11 to generate ~Z~66~3Z
-7a-different frequencies Fl and F2 for each tran-~mission cycle. To an eavesdropper, these frequencies will appear to periodically and randomly change or "hopn.
As shown in Figure 4, instead of using a stable time reference, the PN generator may be ~ade responsive to a portion of ~he infor~ation signal transmitted by the base station.
~Z76682 The frequency mapping table, block 11, in figures 3 and 4 may also be made responsive t~ a portion of the information signal transmitted by the base station. The sequenae of the-~first and second frequencie-~ can be controlled by either controlling the PN
5 generator state, by altering the frequency mapping table contents, or by any combination of these two technique~. m us the sequence or pattern of frequencies can be programmed init~ally, or changed dynamically by the transmitted information signal, to avoid certain frequency band~.
In either of the arrangements shown in Figures 3 or 4, the base station, relay unit, or remote units may include apparatus for detecting frequencies upon which jamming or interfering signal~ are present. ThLs information can then be used to avoid these frequencies which might otherw~se be seiected by the mapping table. -Advantageously, the foregoing arrangement enables multiple remote units to communicate with a base station via a relay unit using frequency hopping and time-division multiplexing principles to minimize interference and the effect3 of possible ~amming. Since the up-link and down-link frequencies are periodically and randomly changed it would be extremely d~fficult for an eavesdropper or ~ammer to discover any pattern to the frequency changes. Further, reception of one of the transmitted signals tells nothing about the frequency of transmis~ion of the other signal. Therefore a jammer would at best be able to ~am only half of the communications flow (i.e. the information being transmitted at the ~amming frequency) and then only until the next transmission cycle when the frequencies would be changed again. Even in this latter case, the communication system is designed to detect such jamming frequencies and take corrective action to avoid such frequencies, such as by dynamically altering the state of the frequency mapping table or PN generator in response to a portion of the information signal.
~ hile the present invention has been described in considerable detail, it is understood that various changes and modifications will lZ76~
g fall within the scope of the invention. For example, multiple relays, each assigned to a group of eemote units, could be used, with each relay bein~ assigned a different set of uplink/downlink frequencies by the base station. Alternatively, a relay unit could communicate with one or more additional relay units to extend the range of communication of the system. In this case one oe more addltional frequencies can be assigned for the intecmediate (relay-to-relay) link. It should therefore be understood that the foregoing detailed description of the preferred embodiments is merely illustrative, but not limitive, of the invention which is defined by the appended claims.
Claims (18)
1. In a secure communication system of the frequency-hopping type having at least one base station including a transmitter, at least one relay unit including a transmitter and a receiver, and at least one remote unit including a receiver, a method of preventing intentional jamming of the system comprising the steps of:
(a) generating and transmitting at the base station an information signal at a first predetermined frequency:
(b) receiving the information signal at the relay unit:
(c) retransmitting the information signal by the relay unit at a second predetermined frequency; and (d) receiving the information signal at the remote unit, wherein information for setting the first and second predetermined frequencies is transmitted as part of the information signal and said predetermined frequencies are periodically changed in accordance with a random pattern.
(a) generating and transmitting at the base station an information signal at a first predetermined frequency:
(b) receiving the information signal at the relay unit:
(c) retransmitting the information signal by the relay unit at a second predetermined frequency; and (d) receiving the information signal at the remote unit, wherein information for setting the first and second predetermined frequencies is transmitted as part of the information signal and said predetermined frequencies are periodically changed in accordance with a random pattern.
2. The method of claim 1, wherein the base station includes a receiver and the remote unit includes a transmitter, further including the steps of:
(e) generating a second information signal at the remote unit;
(f) transmitting the second information signal at the first predetermined frequency;
(g) receiving the second information signal at the relay unit;
(h) retransmitting the second information signal by the relay unit at the second predetermined frequency; and (i) receiving the second information signal at the base station.
(e) generating a second information signal at the remote unit;
(f) transmitting the second information signal at the first predetermined frequency;
(g) receiving the second information signal at the relay unit;
(h) retransmitting the second information signal by the relay unit at the second predetermined frequency; and (i) receiving the second information signal at the base station.
3. The method of claim 1, wherein the information signal includes a portion representative of the first and second frequencies of transmission of the next information signal and wherein the relay unit retransmits the information signal at said second frequency which is determined by the frequency representative portion of an immediately preceding transmitted information signal.
4. The method of claim 2, wherein a first information signal transmitted by the base station includes a portion representative of the first and second frequencies of transmission of the next first and second information signals, and wherein the relay unit receives said first and second information signals at said first frequency and retransmits both the first and second information signals at said second frequency, said first and second frequencies being determined by the frequency representative portion of an immediately preceding information signal transmitted by the base station.
5. The method of claim 2, wherein in step (h), the second information signal is transmitted by said remote unit at a predetermined time after the relay unit retransmits a first information signal transmitted by the base station.
6. The method of claim 5, wherein there are a plurality of remote units and wherein said first information signal includes a portion indicative of separate and unique time delays for transmission of information signals generated by each of said remote units.
7. The method of claim 3, wherein the portion of the information signal representative of the first and second frequencies of transmission is an encrypted version of said first and second frequencies.
8. The method of claim 3, wherein the portion of the information signal representative of the first and second frequencies of transmission is a signal for changing the state of a pseudo-random frequency selector forming part of the transmitters and receivers of said relay unit and said remote unit.
9. The method of claim 3, wherein the portion of the information signal representative of the first and second frequencies of transmission is a signal for changing the state of a frequency mapping table containing data indicative of a plurality of predetermined frequencies, said frequency mapping table forming part of the transmitters and receivers of said relay unit and said remote unit.
10. A secure communication system of the frequency-hopping type comprising:
at least one base station including a transmitter;
at least one relay unit including a transmitter and a receiver; and at least one remote unit including a transmitter and receiver, said base station transmitter including means for generating and transmitting an information signal at a first predetermined frequency;
said relay unit transmitter and receiver including means for receiving the information signal and for retransmitting the information signal at a second predetermined frequency and said remote unit receiver including means for receiving the information signal, wherein information for setting the first and second predetermined frequencies is transmitted by the base station as part of the information signal, said base station further including means for periodically changing said predetermined frequencies in a random pattern and for transmitting information representative thereof to the relay unit and remote unit.
at least one base station including a transmitter;
at least one relay unit including a transmitter and a receiver; and at least one remote unit including a transmitter and receiver, said base station transmitter including means for generating and transmitting an information signal at a first predetermined frequency;
said relay unit transmitter and receiver including means for receiving the information signal and for retransmitting the information signal at a second predetermined frequency and said remote unit receiver including means for receiving the information signal, wherein information for setting the first and second predetermined frequencies is transmitted by the base station as part of the information signal, said base station further including means for periodically changing said predetermined frequencies in a random pattern and for transmitting information representative thereof to the relay unit and remote unit.
11. The system according to claim 10, including:
means for generating a second information signal at the remote unit:
means located at the remote unit for transmitting the second information signal at the first predetermined frequency, the second information signal being received by the receiver of the relay unit and retransmitted by the transmitter of the relay unit at the second predetermined frequency; and receiver means located at the base station for receiving the second information signal.
means for generating a second information signal at the remote unit:
means located at the remote unit for transmitting the second information signal at the first predetermined frequency, the second information signal being received by the receiver of the relay unit and retransmitted by the transmitter of the relay unit at the second predetermined frequency; and receiver means located at the base station for receiving the second information signal.
12. The system according to claim 10, wherein the information signal generated by the base station includes a portion representative of the first and second frequencies of transmission of the next information signal and wherein the relay unit retransmitting means retransmits the information signal at said second frequency which is determined by the frequency representative portion of an immediately preceding transmitted information signal.
13. The system according to claim 11, wherein the base station generating and transmitting means generates and trans-mits a first information signal at said first frequency in-cluding a portion representative of the first and second fre-quencies of transmission of the next first and second inform-ation signals, and wherein the remote unit receives said first and second information signals at said first frequency and the relay unit retransmitting means retransmits both the first and second information signals at said second frequency, said first and second frequencies being determined by the frequency representative portion of an immediately preceding information signal transmitted by the base station.
14. The system according to claims 11, wherein there are a plurality of remote units-and wherein said first information signal includes a portion indicative of separate and unique time delays for transmission of information signals generated by each of said remote units.
15. The system according to claim 12, wherein the base station includes means for encrypting the portion of the information signal representative of the first and second frequencies of transmission.
16. The system according to claim 12, wherein the por-tion of the information signal representative of the first and second frequencies of transmission is a signal for changing the state of a pseudo-random frequency selector forming part of the transmitters and receivers of said relay unit and said remote unit.
17. The system according to claim 12, wherein the portion of the information signal representative of the first and second frequencies of transmission is a signal for changing the state of a frequency mapping table containing data indicative of a plurality of predetermined frequencies, said frequency mapping table forming part of the transmitters and receivers of said relay unit and said remote unit.
18. The system according to claim 10, wherein any of said base station, relay unit, or remote unit includes means for randomly changing said first and second predetermined frequencies.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US86713786A | 1986-05-27 | 1986-05-27 | |
| US867,137 | 1986-05-27 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1276682C true CA1276682C (en) | 1990-11-20 |
Family
ID=25349179
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000537960A Expired - Lifetime CA1276682C (en) | 1986-05-27 | 1987-05-26 | Secure communication system for multiple remote units |
Country Status (5)
| Country | Link |
|---|---|
| US (1) | US4829540A (en) |
| EP (1) | EP0247790A3 (en) |
| JP (1) | JPS6372233A (en) |
| CA (1) | CA1276682C (en) |
| IL (1) | IL82561A (en) |
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-
1987
- 1987-05-18 IL IL82561A patent/IL82561A/en unknown
- 1987-05-21 EP EP87304508A patent/EP0247790A3/en not_active Withdrawn
- 1987-05-26 CA CA000537960A patent/CA1276682C/en not_active Expired - Lifetime
- 1987-05-27 JP JP62131054A patent/JPS6372233A/en active Pending
- 1987-10-29 US US07/115,601 patent/US4829540A/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| IL82561A0 (en) | 1987-11-30 |
| IL82561A (en) | 1991-12-15 |
| US4829540A (en) | 1989-05-09 |
| EP0247790A2 (en) | 1987-12-02 |
| JPS6372233A (en) | 1988-04-01 |
| EP0247790A3 (en) | 1989-02-08 |
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| MKLA | Lapsed | ||
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