GB2267198A - Interference reduction - Google Patents

Abstract

Interference on a prospective communication channel is preassessed and a filter arrangement 58 preset so as to suppress the interference when the system moves onto that channel. The channels may be those of a frequency hopped transmitter 30 which transmits on the upper and lower sidebands during the first and second halves respectively of each hop period. A receiver 32 monitors both sidebands continuously, passing the USB over switch 38 to the filter bank 58 through a one hop period delay unit 48. Switch 40 passes the LSB output (which will comprise merely the interference) to channel 50 which assesses the interference frequencies and stores this data at 54. During the next half hop period, switches 38 and 40 change over and the LSB output from the transmitter passes into the delay unit 48 while the USB output, carrying only interference, passes to the assessment circuit 52. The store 54 data sets the filter 58 to suppress the interference previously detected for the sideband output currently passing through the filter. <IMAGE>

Description

COMMUNICATIONS SYSTEM WITH INTERFERENCE SUPPRESSION The invention relates to communication systems and more specifically to interference suppression in radio communication systems.

Certain types of interference to radio communication can be reduced by filtering using a filter or filters set to reject a particular frequency or frequencies at which the interference is occurring. However, this necessitates measurement of the frequency of the interference and this cannot easily be done when the intended radio communication is itself taking place.

One particular example of a radio communication system to which the invention can be applied is a frequency hopping system in which a transmitter and a receiver in a communications system are arranged to change their operating frequency very rapidly in a pseudorandom manner and in synchronism with each other. In such a system, interference arises when the system hops onto a channel which is already occupied.

However, the invention is not restricted to frequency hopping systems but can be applied to fixed frequency systems.

According to the invention, there is provided a communications system in which transmission means transmits to receiving means during successive time periods but at respectively different frequencies during respective parts of each such period, and including assessment means operative during each time period to assess interference received at each said frequency by the receiving means during a respective time duration when the transmitting means is not transmitting at that frequency whereby to produce a respective control output corresponding to that assessed interference, interference suppression means connected to receive each transmission from the transmission means as received by the receiving means, and control means responsive to the said control outputs at such times as to adjust the interference suppression means so that the latter suppresses interference present in each transmission as received by the interference means.

According to the invention, there is further provided a frequency hopping radio communication system, comprising a radio transmitter controlled to operate at respectively different frequencies during successive time periods and to transmit one, only, of the sidebands at the respective frequency during the first half of each said time period and to transmit the other sideband, only, at the respective frequency during the second half of each time period, a radio receiver synchronised with the transmitter so as to be responsive at the respectively different frequencies during the successive time periods and operative to receive signals at both sidebands of each said frequency during both halves of each time period, means operative during each half of each time period to receive the sideband output as transmitted by the transmitter during that half period and as received by the receiver and to pass that sideband output to interference suppression means through time delay means having a predetermined time period, assessment means operative during each half of each time period to assess the interference received by the receiver in the band corresponding to the sideband not being transmitted by the transmitter during that half period and to produce corresponding control data, and control means operative to adjust the interference suppression means in response to each control data at such time that the interference suppression means acts to suppress the interference to which that control data corresponds when the sideband output corresponding to the band in which that interference has been assessed reaches the interference suppression means.

According to the invention, there is also provided a method of suppressing interference in a communications system in which transmission means transmits at respectively different frequencies during respective parts of successive time periods and in which receiving means is synchronised with the transmission means so as to be responsive at the respectively different frequencies, including the steps of assessing the interference received at each said frequency by the receiving means when the transmission means is not transmitting at that frequency and producing a respective control output corresponding to that interference, and subsequently carrying out, in response to each said control output, interference suppression on each received transmission, the interference suppression being carried out on each said transmission in response to the control output corresponding to the interference assessed at the frequency of that transmission.

Radio communications systems embodying the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a block diagram of one form of the system; Figure 2 is a block diagram of another form of the system; and Figure 3 is a timing diagram for the system.

The systems to be described with reference to the Figures are radio communications systems employing frequency hopping.

In Figure 1, a transmitter 5 transmits radio signals at a succession of different transmitting frequencies each transmitting frequency being maintained for the same finite time, called the hop period. The different frequencies, that is, the channel frequencies respectively operative during the successive hop periods, may be pseudo-randomly arranged under control of a suitable coding system.

Figure 1 also shows a receiving station having two radio receivers, a receiver 6 and a receiver 8.

Receiver 6 is the primary receiver and is intended to produce an audio-frequency output on a line 10 corresponding to the information transmitted by the transmitter 5. It is therefore necessary for the receiver 6 to have its operating frequency continuously synchronised with that of the transmitter 5, that is, for its operating frequency to hop in synchronism with the frequency of the transmitter.

Various methods are known of maintaining a transmitter and a receiver in synchronism in a frequency hopping system. For example, not only may the transmitter have a coding unit producing a succession of control signals identifying pseudo-randomly arranged hop frequencies but so also may the receiver 6, the operating frequencies in the receiver's unit obviously being pseudo-randomly arranged in the same sequence as in the transmitter's unit. It is clearly necessary in such an arrangement to ensure that the two units are operating in synchronism, and this may be achieved by causing the transmitter 5 to transmit synchronisation information at one or more predetermined frequencies during predetermined hop periods. In Figure 1, the synchronisation process is indicated by the chain-dotted line 12, and it is assumed that receiver 6 is synchronised with transmitter 5.Forms of synchronisation systems for frequency hopping arrangements are disclosed, for example, in our co-pending U.K. patent application No.

8119492 (2100944).

Therefore, during any hop period, the transmitter 5 and the receiver 6 are operating at the same channel frequency and the receiver therefore produces an output on a line 14 corresponding to the signal received in that channel. Depending on reception conditions, this output will comprise the signals received from the transmitter 5 together with any other, interfering, signals present in that particular channel. Clearly, if the particular channel frequency which is operative during a particular hop period is in fact occupied by other transmissions, such other transmissions will produce interference on the line 14.

In order to reduce or remove such interference, the output on the line 14 is passed through an adjustable filter 16, an adjustable notch filter for example, so as to produce a relatively interference-free signal on line 10.

In order for the system to operate effectively, therefore, it is necessary for the filter 16 to be continually adjusted so as to filter out the particular frequency or frequencies at which the audio interference exists.

Adjustment of the filter 16 is carried out by the secondary receiver 8. Receiver 8 is similar to receiver 6 in that it incorporates a coding unit which is programmed with the same sequence of pseudorandom hop frequencies as the receiver 6 (and the transmitter 5). However, instead of being maintained in exact synchronism with the transmitter 5, it is arranged so as always to be tuned not to the channel frequency of the current hop period (to which the transmitter 5 and the receiver 6 are working) but to the channel frequency of the next following hop period. Therefore, the receiver 8 is monitoring any transmissions or other interference which exist at the channel frequency to which the transmitter 5 and the receiver 6 will next be tuned.The receiver 8 therefore demodulates its received signal and produces an audio output on a line 18 corresponding to the audio frequency or frequencies of the received signal.

Signal 18 is processed by a control unit 20 which produces a control signal on line 22 for adjusting the filter 16 in such a manner as to filter out or block signals at the particular audio frequency or frequencies on line 18.

Therefore, during the next hop period, when the transmitter 5 and the receiver 6 change to the next channel frequency corresponding to the frequency at which receiver 8 has just been operating, the filter 16 will be so set as to block the audio interference which has just been measured at that channel frequency. The audio output on line 10 will thus be relatively interference-free.

In order to control the operating frequency of the receiver 8, the latter also receives synchronisation signals from the transmitter 5 (as indicated by line 17) but of course its coding unit is set so that the synchronisation signals ensure that the receiver 8 is always operating at the next, rather than at the current, channel frequency. In practice, receiver 8 could receive synchronisation signals from receiver 6.

The filter 12 may be a filter which can be set to block a single interference frequency or a plurality of such frequencies. Such arrangements will be described in more detail later.

In practice of course, there would be a plurality of receiving stations, each having two receivers. Of necessity, therefore, the system shown in Figure 1 requires duplication of receivers. The system now to be described with reference to Figure 2 does not have this disadvantage.

In the system of Figure 2, there is a transmitter 30 which corresponds generally to the transmitter 5 of the Figure 1 system, and a single receiver 32 at each station, instead of the two receivers of Figure 1. In a manner to be explained in more detail below, the transmitter 30 is a single sideband transmitter but transmits alternately on the upper and lower sidebands. During each hop period, it operates at a different channel frequency (in the same manner as transmitter 5) but during the first half of each hop period it transmits on the upper sideband only and during the second half of each hop period it transmits on the lower sideband only.

The receiver 32 is synchronised with the transmitter in the same way as for receiver 6 in the Figure 1 system in that it changes its channel frequency for each hop period so as always to be operating in the same channel as the transmitter, this being achieved by the same general type of synchronisation as is described above in relation to Figure 1 and being indicated in Figure 2 by the chain-dotted line 12.

However, the receiver 32 monitors both the upper and the lower sidebands continuously, at the relevant channel frequency.

In a manner to be described in more detail below, during the first half of each hop period (while the transmitter is transmitting on the upper sideband), the receiver is receiving the transmission on the upper sideband from the transmitter 30 (together with any interference) while, simultaneously, it is merely receiving any other interfering transmissions (not from the transmitter 30) which may be present on the lower sideband. The received signal at the upper sideband, consisting of the transmission received from the transmitter 5 together with any interference in that sideband, is not passed directly to an output but is delayed for one complete hop period and then passed to the output through an adjustable filter arrangement similar to the filter arrangement 16 of Figure 1.Any interference picked up on the lower sideband is used, in a manner to be described, to produce data for setting up the filter arrangement.

During the second half of the hop period, the operation is reversed: that is, the receiver is now receiving the transmission from the transmitter 30 on the lower sideband (together with any interference) while, simultaneously, it is receiving any interference in the upper sideband from any other transmissions in that sideband. Again, any such interference in the upper sideband is used to produce data for the filter arrangement. The audio output from the lower sideband, consisting of the transmission from the transmitter 30 and any interference in that sideband, is passed to the system output through the one hop period delay and then the adjustable filter arrangement.

During the first half of the next hop period, the audio output produced at the upper sideband by the receiver during the first half of the preceding hop period is passed through the adjustable filter arrangement which is simultaneously set up by the data produced by the receiver when assessing the interference on the upper sideband (this data was of course produced during the second half of the preceding hop period). The filter output thus is relatively interference free.

During the second half of the second hop period, the operation is similar except that the audio output passed to the filter arrangement is the audio output produced during the second half of the preceding hop period, that is, the audio output at the lower side band, and during this half hop period, the filter arrangement is set up by the data produced during the first half of the first hop period, this data corresponding to the interference at the lower side band.

By splitting each hop period into two halves in this way, therefore, interference can be assessed, and the filter arrangement can be adjusted accordingly, without the need for the two receivers of the Figure 1 system.

The system of Figure 2 will now be described in more detail.

Because the receiver 32 has to be capable of receiving both the upper and the lower sidebands continuously and simultaneously, it has to be provided with channel filters for both sidebands and with two single sideband demodulators. It therefore produces an audio output from the upper sideband on a line 34 and an audio output from the lower sideband on a line 36, and these outputs are both fed to two change-over switches 38 and 40 which would in practice be implemented electronically. The switches 38 and 40 are controlled by a switch control unit 42 which in turn is operated by a timing unit 44. Timing unit 44 operates at twice the frequency of the hop periods and produces timing signals on a line 46 each synchronised with the start of a respective half-hop-period, each such signal causing the unit 42 to change over the switches 38 and 40.During the first half of each hop period, switches 38 and 40 are held in the positions opposite to those illustrated, so that the upper sideband output on line 34 passes into an output channel 47 including a delay unit 48 having a delay exactly equal to one hop period, while the lower sideband output on line 36 passes into an assessment channel 50. During each second half-hop-period the switches 38 and 40 are held in the positions illustrated, and the lower sideband output on line 36 passes into the output channel 47 and the upper sideband output into the assessment channel 50.

The assessment channel 50 comprises a channel assessment circuit 52 which monitors the sideband output received from switch 40 and (in a manner to be described in more detail below) assesses the frequency or frequencies at which the interference exists. Data corresponding to the assessed frequency or frequencies is passed to and stored in a store 54 in suitable form. The store is connected to a control unit 56 which controls the setting of a notch filter arrangement 58 connected to the output of the delay unit 48. The store 54 is controlled by the timing unit 44 so that it feeds data out to the control unit 56 at such time that the latter sets the filter arrangement 58 to block or minimise interference when the appropriate sideband output is received from the delay unit 48. The filtered output is passed to an output terminal 60.

The operation will now be described in more detail with reference to the timing diagram of Figure 3.

Figure 3 shows four successive hop periods I,II,III and IV, each divided into first and second half hop periods labelled with letters A and B respectively.

Diagram 3A shows the signal fed into the output channel 47 by the switch 38 during each half hop period while waveform 3B shows the signal fed into the assessment channel 50 by the switch 40 during each half hop period. The upper sideband output from the receiver 32 during the successive hop periods is labelled USBI, USBII, USBIII and USBIV respectively while the corresponding lower sideband outputs are labeled LSBI,LSBII,LSBIII and LSBIV. As shown by diagram 3A, the output presented to the input of channel 47 during half hop period IA is USBI and the corresponding output at the input of channel 47 during half hop period IB is LSBI, and this pattern is repeated for the following hop periods.Diagram 3B shows that the output presented to the assessment channel 50 is LSBI during half hop period IA and is USBI during half hop period IB, and again this pattern is repeated for following hop periods.

Diagram 3C shows the signals presented to the filter 58 during each half hop period. Diagram 3C is therefore the same as diagram 3A but delayed by one whole hop period - because of the delay unit 48, Fig.

2. Diagram 3D shows the interference data assessed by the assessment unit 52 during each half hop period, this being indicated by LSBI-DATA etc. Dependent on the form of the assessment circuit 52, the data would in practice be accumulated over the whole of each half hop period, and might thus not be completed until the end of that half hop period. The data is of course stored in the store 54.

Finally, diagram 3E shows the data which the store 54 presents to the control unit 56 during each half hop period, under the control of the timing unit 44, the correct data always being presented to the control unit 56 so as to set up the filter bank 58 to the correct frequency or frequencies to suppress the interference previously assessed for that particular sideband output.

It will be apparent from Figure 3 that the store 54 has to be controlled so that the data assessed for the upper sideband output during the second half of each hop period has to be read out of the store to the control unit 56 during the immediately following half hop period, that is, the first half of the next hop period. On the other hand, the data assessed for each lower sideband output during the first half of each hop period does not have to be read out until two further half hop periods have passed, that is, it does not have to be read out until the second half of the next following hop period.

It will be apparent that the data used to set up the filter bank 58 during each first half hop period is derived from the upper sideband output existing during the corresponding second half hop period, while the data used to set up the filter bank during each second half hop period is derived from the lower sideband output existing during the corresponding half hop period. The operation therefore assumes that the interference does not change significantly during each hop period (even though the wanted signal will of course change during each hop period).

The channel assessment circuit 52 may comprise any suitable means for assessing the frequency or frequencies at which interference occurs on the particular sideband output. For example, it could comprise a phase-locked loop incorporating a variable frequency oscillator and a phase detector which compares the phase of the frequency of the oscillator with the phase of the strongest interference frequency and produces a control signal for adjusting the frequency of the oscillator accordingly, so as to achieve phase lock. The oscillator therefore now has a frequency corresponding to that of the interfering signal and this frequency can be converted into digital data in any suitable way for storage in the store 54.

If it is desired to suppress two interfering frequencies in this way, then the assessment circuitry may comprise two cascaded phase-locked loops with a filter between them for filtering out one of the two frequencies so that the second loop may be set to the other frequency. The settings of the phase-locked loops are stored and used to set up the filter bank 58.

The filter bank 58 advantageously comprises a bank of switched capacitor notch filters set up by frequency control of the switching frequency.

Instead, the channel assessment circuit could comprise a Fourier transform circuit which analyses the incoming sideband signal into its component frequencies and assesses the respective magnitudes of the signals at the different frequencies. In this way, such a circuit could produce a plurality of outputs corresponding to the frequencies at which the most significant interference is occurring, these outputs being converted into data in suitable form for storing in the store 54. The stored outputs could then be used to set up inverse Fourier transform circuits in the filter bank 58. During the actual assessment process, the Fourier transform circuit could produce outputs corresponding to those L frequencies at which strongest interference is detected.Instead, it could produce outputs corresponding to all those frequencies where interference above a predetermined threshold was detected.

The arrangement described with reference to Figures 2 and 3 may be used with advantage in a system operating at a fixed frequency (that is, not a frequency hopping system). For example, in such a fixed frequency system, the transmitter would transmit at the same frequency during each of successive time periods but could transmit on the upper sideband, only, during the first half of each period and on the lower sideband, only, during the second half of each period. During each half period, the assessment circuitry would, as described above, monitor any interference received on the sideband on which the transmitter was not transmitting and use the information to set up the filter bank 58 in the manner explained. As before, the received sideband would be delayed by one whole period before being passed to the filter.

Claims (17)

1. A communications system in which transmission means transmits to receiving means during successive time periods but at respectively different frequencies during respective parts of each such period, and including assessment means operative during each time period to assess interference received at each said frequency by the receiving means during a respective time duration when the transmitting means is not transmitting at that frequency whereby to produce a respective control output corresponding to that assessed interference, interference suppression means connected to receive each transmission from the transmission means as received by the receiving means, and control means responsive to the said control outputs at such times as to adjust the interference suppression means so that the latter suppresses interference present in each transmission as received by the interference means.

2. A system according to claim 1, in which there are two respective parts of each said period, each making up one half thereof, and in which the receiving means comprises first and second receivers, the first of which is arranged to be set during each of the successive half periods to the same said frequency as the transmission means is set during that half period and the second of which is arranged to be set during each said half period to the particular frequency to which the transmission means will be set during the next following half period, and in which the assessment means comprises means operative to assess the interference received by the second receiver during each said half period whereby to produce a respective said control output in response to the interference assessed during that half period, the interference suppression means comprising means connected to receive each transmission from the transmission means as received by the first receiver, and the control means comprising means responsive to each said control output to adjust the interference suppression means during the respective said next following half period.

3. A system according to claim 2, in which the said frequencies are arranged pseudo-randomly.

4. A system according to claim 1, in which there are two respective parts of each said period, each making up one half thereof, and in which during respective halves of each said time period the transmission means transmits the upper and lower sidebands of its transmission frequency which is constant for that period, which sidebands constitute the said different frequencies, and in which the receiving means comprises means operative to receive each particular transmitted sideband during the respective half of each said time period and simultaneously to receive any interference in the band corresponding to the sideband which is not being transmitted during that half of the time period, and in which the assessment means comprises means operative to assess the interference received in the latter band and to produce a corresponding said control output, and including time delay means arranged so that the control means controls the interference suppression means in response to each said control output when the interference suppression means is receiving from the receiving means the transmission at the particular sideband equal to the band to whose interference that control output corresponds.

5. A system according to claim 4, in which the said transmission frequency during each said period is pseudo-randomly selected from a plurality of predetermined frequencies.

6. A system according to any preceding claim, in which the transmission means comprises radio transmission means and the receiving means comprises radio receiving means.

7. A frequency hopping radio communication system, comprising a radio transmitter controlled to operate at respectively different frequencies during successive time periods and to transmit one, only, of the sidebands at the respective frequency during the first half of each said time period and to transmit the other sideband, only, at the respective frequency during the second half of each time period, a radio receiver synchronised with the transmitter so as to be responsive at the respectively different frequencies during the successive time periods and operative to receive signals at both sidebands of each said frequency during both halves of each time period, means operative during each half of each time period to receive the sideband output as transmitted by the transmitter during that half period and as received by the receiver and to pass that sideband output to interference suppression means through time delay means having a predetermined time period, assessment means operative during each half of each time period to assess the interference received by the receiver in the band corresponding to the sideband not being transmitted by the transmitter during that half period and to produce corresponding control data, and control means operative to adjust the interference suppression means in response to each control data at such time that the interference suppression means acts to suppress the interference to which that control data corresponds when the sideband output corresponding to the band in which that interference has been assessed reaches the interference suppression means.

8. A system according to claim 7, in which the predetermined time delay of the time delay means corresponds to the length of one of the time periods.

9. A system according to claim 7 or 8, including storage means for storing the control data and timing means synchronised with the timing of the said time periods for reading the data out of the storage means.

10. A system according to claim 8 or 9 in which the control data produced by the assessment means and corresponding to the interference received during the second half of each time period adjusts the interference suppression means during the first half of the next following time period, and the control data produced during the first half of each time period by the assessment means and corresponding to the interference received during the first half of each time period adjusts the interference suppression means during the second half of the next following time period.

11. A system according to any preceding claim, in which the interference suppression means comprises one or more tunable notch filters.

12. A system according to any preceding claim, in which the assessment means comprises phase-lock means operative to phase-lock oscillator means to a frequency depending on a frequency of the said interference.

13. A method of suppressing interference in a communications system in which transmission means transmits at respectively different frequencies during respective parts of successive time periods and in which receiving means is synchronised with the transmission means so as to be responsive at the respectively different frequencies, including the steps of assessing the interference received at each said frequency by the receiving means when the transmission means is not transmitting at that frequency and producing a respective control output corresponding to that interference, and subsequently carrying out, in response to each said control output, interference suppression on each received transmission, the interference suppression being carried out on each said transmission in response to the control output corresponding to the interference assessed at the frequency of that transmission.

14. A method according to claim 13, in which there are two respective parts of each said period each making up one half thereof, and in which, simultaneously with the step of receiving during each said half period the transmission from the transmission means at the respective frequency for that half period, any interference is also received at the frequency respective to the next following half period, and in which the assessment step comprises the step of assessing any such interference.

15. A method according to claim 14, in which the said frequencies are selected pseudo-randomly from a predetermined plurality of frequencies.

16. A method according to claim 13, in which there are two respective parts of each said period, each making up one half thereof, and in which during respective halves of each said time period the transmission means transmits one of the two sidebands, only, of its transmission frequency which is constant for that period, and in which during each said half of each time period the transmitted sideband is received and simultaneously any interference in the band corresponding to the sideband not being transmitted is received, the assessing step comprising the step of assessing any such interference.

17. A method of radio communication, substantially as described with reference to Figures 2 and 3 of the accompanying drawings.

17. A method according to claim 16, including the step of time-delaying the passage of each received transmission at a particular sideband so as to enable interference suppression to take place when the interference in the band corresponding to that sideband has been assessed.

18. A method according to claim 16 or 17, in which the said transmission frequency during each said period is pseudo-randomly selected from a plurality of predetermined frequencies.

19. A radio communications system, substantially as described with reference to Figure 1 of the accompanying drawings.

20. A radio communications system, substantially as described with reference to Figures 2 and 3 of the accompanying drawings.

21. A method of radio communication, substantially as described with reference to Figure 1 of the accompanying drawings.

22. A method of radio communication, substantially as described with reference to Figures 2 and 3 of the accompanying drawings.

CLAIMS 1. A communicrions system, comprising transmission means operative :1ring respective halves of each of a plurality of successive time periods to transmit the upper and lower sidebands of its transmission frequency which is constant for that period, receiving means comprising means operative to receive each particular transmitted sideband during the respective half of each said time period and simultaneously to receive any interference in the band corresponding to the sideband which is not being transmitted during that half of the time period, assessment means comprising means operative to assess the interference received in the latter band and to produce a control output corresponding to the assessed interference, interference suppression means connected to receive each sideband as received by the receiving means, and control means responsive to each said control signal and operative to control the interference suppression means accordingly when the interference suppression means is receiving from the receiving means the transmission at the particular sideband equal to the band to whose interference that control output corresponds.

2. A system according to claim 1, in which the control means includes time delay means for appropriately delaying each said control outpllt, 3. A system according o clair 1 or 2, in which the said transmission frequency during each said period is pseudo-randomly selected from a plurality of predetermined frequencies.

4. A system according to any preceding claim, in which the transmission means comprises radio transmission means and the receiving means comprises radio receiving means.

5. A frequency hopping radio communication system, comprising a radio transmitter controlled to operate at respectively different frequencies during successive time periods and to transmit one, only, of the sidebands at the respective frequency during the first half of each said time period and to transmit the other sideband, only, at the respective frequency during the second half of each time period, a radio receiver synchronised with the transmitter so as to be responsive at the respectively different frequencies during the successive time periods and operative to receive signals at both sidebands of each said frequency during both halves of each time period, means operati.ve during each half of each time period to receive the sideband output as transmitted by the transmitter during that half period and as received by the receiver and to pass that sideband outpI to inter~rence suppression means through time delay means having a predetermined time period, assessment means operative during each half of each time period to assess the interference received by the receiver in the band corresponding to the sideband not being transmitted by the transmitter during that half period and to produce corresponding control data, and control means operative to adjust the interference suppression means in ' response to each control data at such time so that the interference suppression means acts to suppress the interference to which that control data corresponds when the sideband output corresponding to the band in which that interference has been assessed reaches the interference suppression means.

6. A system according to claim 5, in which the predetermined time delay of the time delay means corresponds to the length of one of the time periods.

7. A system according to claim 5 or 6, including storage means for storing the control data and timing means synchronised with the timing of the said time periods for reading the data out of the storage means.

8. A system according to claim 6 or 7, in which the control data produced by the assessment means and corresponding to the interferenre received during the second half of each tim period adjusts the interference suppression means during the first half of the next following time period, and the control data produced during the first half of each time period by the assessment means and corresponding to the interference received during the first half of each time period adjusts the interference suppression means during the second half of the next following time period.

9. A system according to any preceding claim, in which the interference suppression means comprises one or more tunable notch filters.

10. A system according to any preceding claim, in which the assessment means comprises phase-lock means operative to phase-lock oscillator means to a frequency depending on a frequency of the said interference.

11. A method of suppressing interference in a communications system in which, during respective halves of each of a plurality of time periods, transmission means transmits the upper and lower sidebands of its transtission frequency which is constant for that period and in which a receiving means is operative to receive each particular transmitted sideband during the respective hr f of each said time period and simultaneously o receive any interference in the band corresponding to the sideband which is not being transmitted during that half of the time period, and including the steps of assessing the interference received in the latter band and producing a respective control output corresponding to that interference, and subsequently carrying out, in response to each said control output, interference suppression on each transmission received at a respective one of the sidebands, the interference suppression being carried out on each said transmission in response to the control output which corresponds to the interference corresponding to that sideband and which is assessed during the respective half ofthe time period when that sideband is not being transmitted.

12. A method according to claim 11, including the step of time-delaying the passage of each received transmission at a particular sideband so as to enable interference suppression to take place when the interference in the band corresponding to that sideband has been assessed.

Amendments to the claims have been filed as follows 13. A method according to claim 11 or 12, in which the said transmission frequency during each said period is pseudo-randomly selected from a plurality of predetermined frequencies.

14. A radio communications system, substantially as described with reference to Figure 1 of the accompanying drawings.

15. A radio communications system, substantially as described with reference to Figures 2 and 3 of the accompanying drawings.

16. A method of radio communication, substantially as described with reference to Figure 1 of the accompanying drawings.