GB2129655A - Improvements in and relating to radio communications - Google Patents

Abstract

A data telecommunication system is disclosed in which data to be transmitted is fed into an encoding unit 12 and activates a particular one of a plurality of tone generators 14 according to the value of each data unit. The tone generators may, for example, generate tones in the range 330 Hz to 640 Hz spaced at 10 Hz intervals, each tone being generated for a period of 100 milliseconds. The selected tone is fed to a modulator 16 where it is superimposed on an HF carrier and transmitted via an antenna 20. A keystream generator 24 controls a synthesizer 22 to produce a succession of randomly or pseudo-randomly arranged carrier frequencies on which the modulator 16 superimposes the tone frequencies. Each carrier frequency is present for a time period corresponding to the period (100 milliseconds) of each tone frequency. In this way, therefore, the transmitter applies frequency hopping to a multiple frequency shift keying or Piccolo system. The receiver has a demodulator driven by a keystream generator synchronised with the keystream generator 24 so as to reproduce the originally selected tone frequency. The tone frequencies are fed into a bank of tuned filters so as to detect the value of the tone frequency at any time and thus the value of the transmitted data. <IMAGE>

Description

SPECIFICATION Improvements in and relating to radio communications The invention relates to electrical circuit arrange ments and more specifically to circuit arrangements enabling radio communication using a frequency hopping technique, that is,wherethe operating frequencies ofthe transmitter and the receiver are rapidly changed, in synchronism, so as to reduce the possibility of interception or jamming by a third party.

Circuit arrangements to be described operate most beneficially in the HF band but cold operate in any of the radio bands.

Various novel features of the invention will be apparent from the following description, given by way of example only, of an electrical circuit arrangement for data communication and embodying the invention, reference being made to the accompanying drawings in which: Figure lisa block circuit diagram ofthetransmitter of one ofthe arrangements; Figure 2 is a block circuit diagram of a receiverfor use with the transmitter of Figure 1; Figure 3 shows waveforms occurring in the receiver of Figure 2; Figure 4 is a block circuit diagram of a transmitter for use with another of the circuit arrangements; and Figure 5 is a blockcircuitdiagram of a receiverfor use with transmitter of Figure 4.

More specificållyto be described below is a radio communications system operating with multiple frequency shift keying (MFSK) and with frequency hopping applied to the carrier frequency.

Thus, the invention provides a MFSK system with frequency hopping applied to its carrier.

Advantageously, the carrier frequency is main tained constantforthe duration of each tone frequency.

In a more specific sense, there will be disclosed below a data communications system comprising a data transmitter having means for generating a plurality of predetermined tone frequencies, encoding means responsive to each data unitto be transmitted to selectfor a predetermined period a predetermined tone frequency according to the value of the data unit, carrierfrequency generating means, means for superimposing the selected tone frequency on a carrier frequency produced by the carrier frequency generating means, means for transmitting the carrierfrequency and the superimposed tone frequency, and frequency hopping means operative to make step changes in the carrier frequency in a certain manner.

For example, the frequency hopping means advan tageously comprises means for generating a plurality of different and predetermined carrierfrequencies and means for selecting one of the carrierfrequencies as the transmitted carrier frequency for a length of time corresponding to the predetermined period. The values of the selected carrier frequencies may be random or pseudo-random.

The encoding means may comprise means for selecting a single predetermined one ofthetone frequencies for each possible value of a data unit.

Instead, however, it may comprise means for selecting a particular predetermined combination of tone frequencies, selected serially and each for a predetermined period, corresponding to each possible value of a data unit.

In such a system, there may be provided a receiver having receiving means for receiving each transmitted carrier frequency and the tone frequency superimposed thereon, demodulating means for removing the carrier frequency, a plurality oftuned filters each tuned to a particular one ofthe predetermined plurality of tone frequencies, and decoding means responsive to the outputs of the filters for deriving the value ofthe corresponding data unit.

Avantageously,the receiver includes carrierfrequency generating means and frequency hopping means controlling the carrierfrequency generating means to make step changes in the carrierfrequency and thereby to produce a predetermined plurality of carrier frequencies corresponding in value and order to those produced in the transmitter, means for synchronising the frequency hopping means in the transmitter, and means feeding the carrierfrequency produced by the carrierfrequency generating means in the receiver during any said predetermined period for controlling the demodulating means accordingly.

The foregoing are exemplary of and not exhaustive ofthe various novel features of the circuit arrangement now to be more specifically described.

As shown in Figure 1, one form of transmitter has a data input line 10 feeding serial data characters, such as in 8-bit parallel form, through an input unit 11 to an encoder 12. The encoder 12 respondstoeachcharac- ter ofthe input data by activating a particular one of a plurality oftone generators 14, each tone generator being held activated for a fixed period of time, 100 milliseconds say. Atthe end of that period, the encoder 12 activates another ofthe tone generators (for 100 milliseconds) in accordance with the value of the nextcharacterto be encoded.

The tone generators produce predetermined frequencies which may, for example, be spaced at 10 Hz intervals between, say, 330 Hz and 640 Hz; the latter range therefore requires thirty two tone generators 14, permitting the encoding ofthirtytwo different characters.

Each activated tone generator 14 feeds its output toneto a modulator 16where it is added to a carrier and transmitted via transmission circuitry indicated generally at 18 and an antenna 20.

As so far described, the transmitter is similarto a standard multiple frequency shift keying (MSFK) transmitter, specifically a Piccolo transmitter.

However, in accordance with a feature ofthe invention, the carrierfrequencyto which the outputs ofthe tone generators 14 are applied is not constant but is arranged to change automatically (that is, to 'hop') so as to produce a frequency hopping signal transmitted by the antenna 20, in the HF band.

As shown (byway of example only) in Figure 1,the rapidly changing carrier frequency used by the modulator 16 is derived from a synthesizer 22 which is driven by data defining the successive carrierfrequen ciesandwhich is derived from a predetermined source such as a keystream generator 24. Thus, the keystream generator 24 stores a series of numbers (preferably generated randomlyorpseudo-randomly) representing the successive values ofthe carrier frequency. The keystream generator 24 may operate under control Of an accurate time of day clock 26 producing an accurate time-of-day, orTOD,signal on a line 28.

In this way, therefore, the carrierfrequency ofthe signal transmitted by the antenna 20 is rapidly changed in a random or pseudo-random manner, according to the nature of the keystream generator 24, thus making interception andjamming extremely difficult.

It is preferable that the keystream generator24and the encoder unit 12 are accurately synchronised, such as by means of clock signals derived from a clock 30, sothateach particularcarrierfrequency is maintained forexactlythe period (100 milliseconds mentioned above)forwhich each tone generator is activated.

The carrierfrequencies may be in the range 3 to 30 MHz, say (but could be in any of the radio bands).

Figure 2 shows one form which the receiver circuitry can take for receiving the signals transmitted bythe transmitter of Figure 1.

As shown in Figure 2, the receiver has an antenna 50 which feeds the received signals to normal RF receiver circuitry 52, the output ofwhich is fed to a demodulator unit 54. The latter is fed with a carrierfrequency derived from a synthesizer 56 which is in turn controlled by a keystream generator 58 ofthe same general form as the keystream generator 24 of Figure 1.

Assuming thatthe operations of the keystream generators 24and 58 are in synchronism (howthis may be achieved is described below), the carrier frequency which the synthesizer 56 applies to the demodulating circuit 54 will correspond to that which, forthe same 100 millisecond period, was applied by the synthesizer 22 (Fig. 1) to the modulator 16 in the transmitter. Therefore, the demodulator unit 54will output on a line 60 a particular one of the predetermined series of tones, this tone corresponding to the particular character.

Line 60 is connected to a bank of filters 62, each tuned to a particular one of the thirty two tones.

With thethirtytwo possibletones spaced 10 Hz apart and with each tone having a duration of 100 milliseconds, it will be seen thatthetones are orthogonally spaced. Figure 3A therefore shows the form which the output will take ofthe particular one of the filters 62 which is tuned to the tone currently being output on line 60 (Fig. 2). As shown, its output is a maximum at the end ofthe 100 millisecond period.

Figure 3B shows the output ofthe adjacent filter, that is,thefiltertuned 10 Hz more (or less) than the currenttone on line 60. lttherefore has a mximum at the centre ofthe 100 millisecond period and a null at the end.

Figure 3C shows the output of a filter spaced a further 10 Hz away. This has two peaks, and thus a null atthecentreofthe 100 millisecond period and a null again at the end of the 100 millisecond period.

Corresponding waveforms can be derived of course for the otherfilters.

ltthereforefollows that only one ofthe filters 62 will be producing a significant output atthe end ofthe 100 millisecond period. The particularfilter producing such output is determined by a decoder unit 64 which thus produces an output on a line 66 identifying the character transmitted.

In orderforthe system to operate, it is clearly necessaryforthe synthesizers 22 and 56, in the transmitter and receiver respectively, to be operating in step so asto be producing the same carrier frequencies, and the keystream generator 58 is therefore programmed with the same series of numbers as the keystream generator 24.

A variety of techniques may be used to synchronise the two keystream generators when the receiver is first switched on. Figure 2 shows one technique.

In this arrangement, the keystream generator 24 in the transmitter is arranged so that particular ones of the frequencies represented by the numbers which it stores are termed syne frequencies and are not used fortransmitting data but are used to transmit predetermined sync characters. Forthis reason, and as shown in Fig. 1, the keystream generator 24 is connected to the input unit 11 by means of a line 68, and wheneverthe carrierfrequencyto which the keystream generator 24 is setting the transmitter is oneofthesyncfrequencies, line 68 causesthe unit 1 to apply the appropriate sync characterto the encoder 12 so that this is transmitted on the antenna 20.

As shown, the receiver has an accurate clock70 similarto the clock 26 ofthetransmitter as shown in Fig. 1. The clock 70 is controlled by a control unit 72 which responds to a START signal on a line 74, when the receiver is first switched on, by activating the clock 70 to produce a TOD signal on line 76 representing the currenttime of day (the clock70 is of course continuously running whether or notthe receiver is operating). The unit 72 also energises a line 77 to de-activate the keystream generator 58 temporarily and to activate a store 78.

TheTOD signal on line 60 is applied to the store 78 which holds pre-stored information relating each time of day to the known sequence ofsyncfrequencies and syne characters. In response to the TOD sig nal on line 76, therefore, the store 78 outputs a series of control signals on a line 80to the synthesizer 56so asto setthe synthesizer 56 to each ofthe syncfrequencies in turn and at such periods as should correspond with the periods for which the keystream generator 24 in the transmitter is setting the synthesizer 22 to the same respective frequencies (if the clock 70 of the receiver is exactly in step with the clock 26 ofthe transmitter).

Simultaneouslywith eachsyncfrequencyon line 80, the store 78 outputs the corresponding character on a line 82to a comparator 84 which is connected to the output line 66 of the decoder 64.

Therefore, itthetime of day clock 70 and 26 of the receiver and transmitter are exactly instep, the comparator 84 will receive from the decoder 64 the correct sync characters at the correct times, and the comparator84will produce a syne output on a line 86 which causes the control unit 72 to remove the signal on line 77 and to de-activate the store 78. The demodulator unit 54 is therefore now operating under direct control of the information from the keystream generator 58 which is synchronised with the keystream generator 24 in the transmitter.

If the comparator 84 does not detect the required sync characters on the line 66, it energises a line 90 which resets the time of day clock 70 by a predeter mined amountcorrespondingtothe length of half a character period (50 milliseconds in this example) and the store 78 re-outputs data representing the sync frequencies in turn, atthe times when they next occur according to the reset clock 70, together with the sync characters, and the process repeats. This continues until the comparator 84 determines thatthetime of day clock 70 is synchronised with the time of day clock 26 in the transmitter. When this is detected, line 86 is energised so as to switch the receiver into control by the keystream generator 58.During this process, the keystream generator 58 is reset in response to each resetting ofthe time of day clock 70, by means of a line 92, and in this way the two keystream generators are brought into synchronism.

Figure4 shows a transmitter corresponding to that of Figure 1 but embodying a different method for synchronising it with a receiver, the receiver being shown in Figure 5. Component parts of Figures 4 and 5 corresponding to those in Figures 1 and 3 respectively are similarly reference and will not be described again.

Thetransmitterof Figure4differsfromthatof Figure 1 in that the time of day clock 26 also feeds the TOD signal to the input gate 11 on a line 100. The keystream generator 24 is arranged so thatthe sequence offrequencies which its outputs represent contains a number of syne frequencies wh ich always occurattimes bearing afixed relationship to another frequency, the base frequency, in the sequence. The syne frequencies are used, in combination, to transmit characters representing the current time of day, that is, the currentTOD signal on line 28.

In addition, the keystream generator 24 is con nected to the input unit 11 by another line 102.

Each time the keystream generator 24 sets the synthesizer 22 to the above-mentioned base frequency, it energises line 102 which causes the input unit 11 to disconnectthe encoder 12temporarilyfrom the data input line 10 and to cause the encoder 12 to encode a sync character identifying the base frequency as such.

When the keystream generator 24then thereafter calls for each ofthe syne frequencies, the keystream generator 24 each time energises line 102 which again causes the input unit 11 to disconnectthe encoder 12 temporarilyfrom the data input line 10 and to connect the line 100 to the encoder 12, so thatthe encoder 12 encodes a respective part of the current time of day in correspondence with each of the syne frequencies. In this way, the final one ofthe syne freq uencies encodes the last digit of the time of day to the accuracy required.

In the receiver shown in Figure 5, the time of day clock 70 is connected to the keystream generator 58 via line 92 and a gate 110. The receiver has a control unit 72 which is activated buy a START signal on a line 74 and which thereupon energises a line 112 which temporarily prevents the keystream generator 58 from controlling the synthesizer 56. In addition, the signal on line 112 closes gate 110 so as to prevent the TOD signal on line 92 from affecting the keystream generator 58.

The control unit 72 activates a sone unit 1 14by means of a line 116and the sync unit 114thereupon sets the synthesizer 56 to the above-mentioned base frequency by means of a signal on a line 118. In addition, it produces a pre-stored signal representing the sync character and this is fed to a comparator 120 on a line 122. The receiver is thus held tuned to the base frequency and watches for reception ofthe sync character by comparing the character information received on line 122 with the output of the decoder 64 on line 66.

When the comparator 120 detects equality, it signals this to the sync unit 114 on a line 124 and the sync unit 114 thereupon sets the synthesizer 56 to each ofthe above-mentioned syne frequencies in turn and with thecorrecttimespacing, using timing information which may be derived from the time of day clock 70 on a line 126.

As the sync unit 114 sets the synthesizer 56 to each of the syncfrequencies in turn, it opens a gate 128 by means of a line 130 so thatthe decoded character received via each of the syncfrequencies in turn is fed into the time of day clock 70 and resets it, that is, synchronises it with the time of day clock 26 in the transmitter (Fig. 4).

When this process is complete, the syne unit 114 signals this to the control unit 72 on a line 132 and the control unit 72 de-activates line 112so as to open gate 110 and renderthe keystream generator 58 under control ofthe now-synchronised time of day clock 70.

Itwill be appreciated thatthe methods illustrated in the Figures for synchronising the transmitter and the receiver are merely examples of various different ways of carrying out the synchronisation process.

From the foregoing, it will be appreciated that the systems described applyfrequency hopping to the carrierfrequency of a basic MFSK system, specifically the Piccolo system. Piccolo systems have a number of advantages for data transmission over HF radio circuits. Such circuits are characterised by varying propagation conditions and multiple path effects giving rise to signal fading, distortion and consequent risk of data corruption. With a Piccolo system, the transmitted signal is rendered inherently far less susceptible to fading, interference and multiple path effects. This is mainly because Piccolo uses a very narrow information bandwidth (10 Hz in the example being considered) per character, so thatthe energy percharacter percycle of bandwidth ismaximised.

This gives very good protection against fading and atmospheric noise, enabling the system to workwith much lower received signal/noise ratios. Because each character is represented by a single tone instead of a group (five, say) of bits, the transmission time for each character can be longer with Piccolo,fivetimes longer, for example. Therefore, multi-path effects resulting in delays of a few milliseconds, which are common in other systems and have the effect of corrupting data step transitions, have virtually no effect in Piccolo systems when integrated over a full 100 millisecond period.

Otherforms of MFSK have similar advantages.

To all these advantages the systems described add the advantages offrequency hopping, that is, security against interception and jamming. The systems will have a very low probability of intercept. Furthermore, the advantage of frequency hopping, using a different carrier frequency for each transmitted tone, remove the characteristic sound which conventional Piccolo systems have (giving them the name "Piccolo"). This would make it much more difficult than with a bask Piccolo system for a third party to detect the transmission.

Thewiderthe bandspread over which hopping of the carriertakes place, the harder it will be for a potential interceptorto spotthe signal among all other occupants ofthe band. Advantageously, the hopping channels should be separated by morethan 3 KHz so that a 3 KHz bandwidth surveillance receiver would onlyencounterthesignal on a single frequency and at infrequent intervals.

If there are a large number of hopping channels, the total bandspread overwhich hopping takes place would therefore be substantial, and the limitations imposed bythe bandwidth of available antenna systems may limit the total possible hopping bandspread particularly at lowerfrequencies.

Advantageously, the hopping channels are spaced irregularly.

Basic MFSK systems, that is, without frequency hopping, require very accu rate frequency standards because ofthe need to control and measure the tones very accurately (e.g. to plus or minus 5 parts in 108).

The availability in basic MFSK systems of such a high frequency standard can be used with advantage to provide the necessary timing control to maintain the keystream generators in synchronism.

Advantageously, some suitable form oferrorpro- tection is provided in orderto reduce the output error rate ofthe system to an acceptable level.

Ifthetransmitter or receiver is movable at a significant speed (e.g. in an aircraft), the doppler shift of the tone frequencies may cause error and this needs to be allowed for.

Forthe avoidance of doubt, it is repeated that the referencesaboveto Piccolo systems are purely byway of example; itwill be appreciated thatthe system described may be applied to modifications and developments of Piccolo and to multiple frequency shift keying systems in general.

For example, instead of transmitting each character by means of a single tone, each character may use a sequence of two tones, each of 50 millisecond duration say. In this case, the tones would be spaced by 20 Hz so asto give orthogonal spacing. Again, preferably the time for each 'hop' (i.e. the duration of each hop frequency) would bethe same asthe duration of each tone (50 milliseconds in this ex ample).

Multiple channel transmission may be used by combining, in time series, two more input data streams.

CLAIMS (Filed on 22/7/83) 1. A radio communications system having multi plefrc ,uency shift keying (MFSK) and with frequency hopping applied to the carrierfrequency.

2. A system according to claim 1, in which the carrierfrequency is maintained constant for the duration ofeach tone frequency.

3. A data communications system comprising a data transmitter having means for generating a plurality of predetermined tone frequencies, encoding means responsive to each data unitto be transmitted to select for a predetermined period a predetermined tone frequency accordingtothevalue of the data unit, carrierfrequency generating means, meansforsuperimposing the selected tone frequency on a carrier frequency produced bythe carrierfrequency generating means, meansfortransmitting the carrierfrequen- cy and the superimposed tone frequency, and frequency hopping means operative to make step changes in the carrierfrequency in a certain manner.

4. A system according to claim 3, in which the frequency hopping means comprises means generating a plurality of different and predetermined carrier frequencies and meansforselecting one ofthe carrier frequencies as a transmitted carrierfrequencyfor a length of time corresponding to the predetermined period.

5. Asystem according to claim 4, in which the values of the selected carrier frequencies are random or pseudo-random.

6. A system according to any one of claims 3 to 5, in which the encoding means comprises meansfor selecting a single predetermined one ofthe tone frequencies for each possible value of a data unit.

7. A system according to any one of claims 3 to 5, in which the encoding means comprises meansfor selecting a particular predetermined combination of tone frequencies, selected serially and each for a predetermined period, corresponding to each possi blevalueofa data unit.

8. Asystem according to any one of claims 3 to 7, including a receiver having receiving meansfor receiving each transmitted carrierfrequency and the tone frequency superimposed thereon, demodulating means for removing the carrier frequency, a plurality oftuned filters each tuned to-a particular one ofthe predetermined plurality oftonefrequencies, and decoding means responsivetothaoutputs of the filters for deriving the value of the corresponding data unit 9.Asystem accqrding to claim 8, in which the receiver includes carrier frequency generating means and frequency hopping means controlling the carrier frequency generating means make step changes in the carrierfrequency and thereby to produce a predetermined. plurality of carrier frequencies corresponding in value and orderto those produced in the transmitter, means for synch ronising the frequency hoppingmeansinthetransmitter,andmeansfeeding the carrier frequency produced by the carrier frequency generating means in the receiver during anysaid predetermined periodforcontrolling the demodulat- ing means accordingly.

10. A communications system, substantially as described with referenceto Figure 1 oftheaccom panying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (13)

**WARNING** start of CLMS field may overlap end of DESC **. step transitions, have virtually no effect in Piccolo systems when integrated over a full 100 millisecond period. Otherforms of MFSK have similar advantages. To all these advantages the systems described add the advantages offrequency hopping, that is, security against interception and jamming. The systems will have a very low probability of intercept. Furthermore, the advantage of frequency hopping, using a different carrier frequency for each transmitted tone, remove the characteristic sound which conventional Piccolo systems have (giving them the name "Piccolo"). This would make it much more difficult than with a bask Piccolo system for a third party to detect the transmission. Thewiderthe bandspread over which hopping of the carriertakes place, the harder it will be for a potential interceptorto spotthe signal among all other occupants ofthe band. Advantageously, the hopping channels should be separated by morethan 3 KHz so that a 3 KHz bandwidth surveillance receiver would onlyencounterthesignal on a single frequency and at infrequent intervals. If there are a large number of hopping channels, the total bandspread overwhich hopping takes place would therefore be substantial, and the limitations imposed bythe bandwidth of available antenna systems may limit the total possible hopping bandspread particularly at lowerfrequencies. Advantageously, the hopping channels are spaced irregularly. Basic MFSK systems, that is, without frequency hopping, require very accu rate frequency standards because ofthe need to control and measure the tones very accurately (e.g. to plus or minus 5 parts in 108). The availability in basic MFSK systems of such a high frequency standard can be used with advantage to provide the necessary timing control to maintain the keystream generators in synchronism. Advantageously, some suitable form oferrorpro- tection is provided in orderto reduce the output error rate ofthe system to an acceptable level. Ifthetransmitter or receiver is movable at a significant speed (e.g. in an aircraft), the doppler shift of the tone frequencies may cause error and this needs to be allowed for. Forthe avoidance of doubt, it is repeated that the referencesaboveto Piccolo systems are purely byway of example; itwill be appreciated thatthe system described may be applied to modifications and developments of Piccolo and to multiple frequency shift keying systems in general. For example, instead of transmitting each character by means of a single tone, each character may use a sequence of two tones, each of 50 millisecond duration say. In this case, the tones would be spaced by 20 Hz so asto give orthogonal spacing. Again, preferably the time for each 'hop' (i.e. the duration of each hop frequency) would bethe same asthe duration of each tone (50 milliseconds in this ex ample). Multiple channel transmission may be used by combining, in time series, two more input data streams. CLAIMS (Filed on 22/7/83)

1. A radio communications system having multi plefrc ,uency shift keying (MFSK) and with frequency hopping applied to the carrierfrequency.

2. A system according to claim 1, in which the carrierfrequency is maintained constant for the duration ofeach tone frequency.

3. A data communications system comprising a data transmitter having means for generating a plurality of predetermined tone frequencies, encoding means responsive to each data unitto be transmitted to select for a predetermined period a predetermined tone frequency accordingtothevalue of the data unit, carrierfrequency generating means, meansforsuperimposing the selected tone frequency on a carrier frequency produced bythe carrierfrequency generating means, meansfortransmitting the carrierfrequen- cy and the superimposed tone frequency, and frequency hopping means operative to make step changes in the carrierfrequency in a certain manner.

4. A system according to claim 3, in which the frequency hopping means comprises means generating a plurality of different and predetermined carrier frequencies and meansforselecting one ofthe carrier frequencies as a transmitted carrierfrequencyfor a length of time corresponding to the predetermined period.

5. Asystem according to claim 4, in which the values of the selected carrier frequencies are random or pseudo-random.

6. A system according to any one of claims 3 to 5, in which the encoding means comprises meansfor selecting a single predetermined one ofthe tone frequencies for each possible value of a data unit.

7. A system according to any one of claims 3 to 5, in which the encoding means comprises meansfor selecting a particular predetermined combination of tone frequencies, selected serially and each for a predetermined period, corresponding to each possi blevalueofa data unit.

8. Asystem according to any one of claims 3 to 7, including a receiver having receiving meansfor receiving each transmitted carrierfrequency and the tone frequency superimposed thereon, demodulating means for removing the carrier frequency, a plurality oftuned filters each tuned to-a particular one ofthe predetermined plurality oftonefrequencies, and decoding means responsivetothaoutputs of the filters for deriving the value of the corresponding data unit

9.Asystem accqrding to claim 8, in which the receiver includes carrier frequency generating means and frequency hopping means controlling the carrier frequency generating means make step changes in the carrierfrequency and thereby to produce a predetermined. plurality of carrier frequencies corresponding in value and orderto those produced in the transmitter, means for synch ronising the frequency hoppingmeansinthetransmitter,andmeansfeeding the carrier frequency produced by the carrier frequency generating means in the receiver during anysaid predetermined periodforcontrolling the demodulat- ing means accordingly.

10. A communications system, substantially as described with referenceto Figure 1 oftheaccom panying drawings.

11. A communications system, substantially as

described with reference to Figures 1,2 and 3 ofthe accompanying drawings.

12. Acommunications system, substantially as described with reference to Figure 4 ofthe accompanying drawings.

13. A communications system, substantially as described with reference to Figures 4 and 5 ofthe accompanying drawings.