GB2132451A

GB2132451A - Radio communications systems

Info

Publication number: GB2132451A
Application number: GB08234991A
Authority: GB
Inventors: Michael Naylor; David Brash; Robert Peile
Original assignee: Racal Research Ltd
Current assignee: Racal Research Ltd
Priority date: 1982-12-08
Filing date: 1982-12-08
Publication date: 1984-07-04
Also published as: SE8306640L; SE8306640D0

Abstract

A system for transmitting digital signals employs a continuously transmitted carrier frequency which is continuously changing in an analogue curve over a predetermined frequency band "B", such that the spectrum of the resultant transmission is spread over the frequency band and falls away sharply outside the band. <IMAGE>

Description

SPECIFICATION Radio systems The present invention relates to radio systems and methods of transmitting digital information such that the transmitted signal is spread over a relatively broad spectral band.

Various techniques have been proposed to enable a transmitted signal to be spread across the spectrum in order to minimize the effect of interference or jamming signals which may be present in regions of the spectrum over which the signal is spread. Such techniques are described in a book entitled "Spread Spectrum Systems" by R. C. Dixon published in 1976 by John Wiley.

One such technique known as 'direct sequence' requires the phase of the carrier signal to be changed at pseudo-random time intervals between two predetermined values such as 0 and 1800. The period between phase transitions is known as the chip period Tich. In such systems the chip period is normally less than the data bit period Tdb. In a normal transmission of digital data the bandwidth is inversely proportional to the data bit period. However, the direct sequence technique spreads the transmission over a bandwidth which is inversely proportional to the chip periods.The spectrum of the resultant transmission is in a form which has relatively large side lobes, the width of the central lobe being approximately 2/tich. Although the side lobes may be typically 13 dB down on the maximum amplitude at the centre of the spectral band, they can still cause considerable mutual interference if it is desired to use this technique for a number of adjacent channels. It has been found impracticable to use band pass filters to eliminate these side lobes as the performance requirements for such filters would be too high.

Frequency-hopping represents another method of spreading a transmitted signal over a broader spectral band. In this technique the carrier frequency of the transmission is periodically hopped in a pseudo random manner over a band of adjacent channels. Typically, the period between carrier frequency changes is significantly greater than the data bit period. At any one instant, the transmitted signal will be located in a relatively small portion of the available spectral band, although, over a longer period, it will be seen that the transmitted signal has traversed a relatively large bandwidth. If part of the total bandwidth is jammed or otherwise subject to interference during a frequency-hopping transmission, burst errors will be produced in the received signal when the carrier frequency is located in the portion of the spectrum subject to interference or jamming.However, when using a frequency-hopping technique, it is possible to have different radio systems operating in adjacent bands without causing mutual interference.

There is thus a problem to be solved if a radio system is to be provided which is both insensitive to narrow band interference or jamming and yet allows radio systems to be used in adjacent bands without mutual interference.

The present invention provides a radio system for transmitting digital signals comprising a radio transmitter and receiver, wherein the transmitter comprises means for synthesising a carrier signal and means for modulating the synthesised carrier signal with the digital signal to be transmitted, and the receiver comprises means for synthesising a signal corresponding to the carrier signal, and means for demodulating the received signal with said corresponding synthesised signal, wherein the frequency of the carrier signal is continuously changing within a preset range the sense and/or rate of change being periodically altered.

The present invention also provides a method of transmitting a digital signal by modulating a carrier signal with the digital signal to be transmitted, the carrier signal having a frequency which continuously varies over a predetermined frequency band in a gradual and continuous manner.

Preferably, the carrier signal is frequency modulated by the digital signal to be transmitted.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a block diagram of a radio transmitter; Figure 2 is a block diagram of a radio receiver; Figure 3 is a representation of a spectrum of a signal transmitted in accordance with the invention and signals transmitted using the direct sequence technique and the frequency-hopping technique; and Figure 4 is a diagram of frequency against time showing the output of a carrier signal synthesiser.

In the transmitter of Figure 1 input data in the form of a digital signal having a bit period, Tdb, is fed to a modulator 2, in which the digital signal frequency modulates a carrier signal fed from a synthesiser 4. The synthesiser 4 is under the control of a signal fed thereto along line 6. The signal fed to synthesiser 4 along line 6 is derived from a digital, serial code signal received along line 10 from a control generator (not shown). This serial code signal is fed to a D/A converter 12 which converts three bits at a time to an analogue voltage which is fed to a control input of the synthesiser 4 via a low pass filter 14. The data rate of the serial code signal is preferably the same as the signal data rate. The output of the modulator is fed along line 16 to an aerial 18 from which it is transmitted.

The transmitter may include a number of other elements for processing the digital signal to be transmitted either before or after the modulator 2.

Such processing steps are not essential to the subject of the present invention and accordingly will not be discussed herein.

The modulator 2 is a frequency shift keying modulator which shifts the frequency of the carrier signal by a fixed amount in a direction dependent on whether the data bit to be transmitted is a one or a zero. The synthesiser 4 is controlled to produce a frequency output which has a form such as that illustrated in Figure 4 to be described in more detail later. The serial code data is a pseudo random sequence of Os and 1 s.

The output of the D/A converter 12 changes at a rate equal to a third of the data rate of the code signal and, because it is derived from three bits, can have any one of eight possible output values.

If the output of the synthesiser were fed direct to the control input of the synthesiser 4, these eight possible outputs would cause the synthesiser to produce eight discrete frequencies within the operating frequency band, which may, for example, be 8MHz about the centre frequency.

However, because of the interposition of the low pass filter 14 the signal fed to the control input of the synthesiser gradually changes value, thus causing the output of the synthesiser to gradually and continuously change. The filter bandwidth and the code data rate are chosen such that the synthesiser output is substantially never steady.

Preferably the filter 14 has an analogue bandwidth which is 1/6 of the serial code data rate. The value of the D/A output determines the sense and rate of change of the synthesised frequency.

The receiver illustrated in Figure 2 includes an aerial 20 which supplies a received signal to a mixer 22 and then to a demodulator 24. The mixer 22 is supplied with a locally synthesised signal from synthesiser 26. The demodulator acts on the output of the mixer 22, which has a base frequency independent of the changing carrier frequency as a result of being mixed with the synthesised frequency which changes in synchronism with and in dependance on the changes in the carrier frequency as received at the receiver.

The output from demodulator 24 corresponds to the input digital signal and is fed along line 28.

Further processing stops (not shown) may be carried out on the received signal which are not essential to the disclosure of the present invention.

The synthesiser 26 operates under the control of a receiver control generator 30 which outputs a digital, serial code signal which corresponds to the code signal produced by the transmitter, but which is delayed by a time interval corresponding to the transmission path delay relative to the transmitter code signal. The serial code signal is fed along line 32 to a D/A converter 34, which converts the serial code, three bits at a time, into an analogue output which is passed, via a low pass filter 36, to a control input of synthesiser 26. The filter 36 operates similarly to filter 14 to cause the output of synthesiser 36 to gradually and continuously change.

Figure 4 illustrates a frequency against time graph of a portion of the output of synthesiser 4 or 26, when operating to produce a predetermined pseudorandom continously changing frequency output under the influence of a respective control generator. As shown, at equally spaced time instants T1 , T2, T3 etc. the output from the D/A converter 12 or 34 changes, thus causing the value of the voltage at the control input of the synthesiser to change gradually due to the presence of the low-pass filter. The synthesiser accordingly produces a frequency output which gradually and continuously changes in a smooth, analogue manner. The sense and rate of change of the frequency output depends on the previous and present values of the output of the D/A converter.Thus, before T1 the output of the D/A converter was such that, if fed directly to, the control input of the synthesiser would produce an output of F5. At T1 the D/A output changes to a value which would, if fed direct to the control input produce the lower frequency F,. Accordingly between T1 and T2 the synthesiser output gradually falls towards F,. It is possible that, if the frequency difference between F5 and F, is sufficiently great, the output of the synthesiser will never actually achieve F. This is immaterial provided that the transmitter and receiver synthesisers are sufficiently well matched for their outputs to correspond in response to the same changes of input.The output from modulator 2 is continuously transmitted unlike other frequencyhopping radio systems where there is a 'dead' period covering carrier frequency changes in which there is no transmission.

Before the commencement of transmission using a radio system as above described, it is first necessary to bring the output of the synthesiser 26 at the receiver into synchronism with the carrier frequency being received. This may be achieved by a preliminary synchronising transmission of a known data word.

In order to maintain the output of synthesiser 26 in correspondence with the output of carrier signal synthesiser 4 both control generators are programmed with the same serial code signals. A re-synchronisation circuit 38 monitors the output of the demodulator 24 and detects and corrects any tendency of the output of the synthesiser 26 to get out of step with the received carrier signal by selectively adjusting the timing of the output of control generator 30.

A suitable re-synchronisation circuit 38 operates as follows: During transmission of a digital data signal, the carrier frequency is changed according to a predetermined continuous curve. If at a given point in this curve the frequency is increasing, and the receiver control generator 30 and receiver synthesiser 26 are operating slightly in advance of the transmitter control generator and synthesiser, then for a short period the frequency mixed with the incoming signal will be higher than the frequency on which the input data was moduiated and the demodulated output data will be at a lower than expected average voltage level.

However, in similar conditions, at a point on the curve when the frequency is decreasing, the average voltage level of the output data will be higher than expected. The relative levels will be reversed if the transmitter is ahead of the receiver.

Therefore, if the difference in the received and expected levels is inverted whenever the frequency curve is such that the frequency is decreasing, then by looking at the sense of the level deviation it can be determined whether the receiver control generator is operating either ahead of or behind the transmitter control generator. Thus, if an output from the resynchronisation circuit 38 is continuously indicative of a lower than erected average level the receiver control generator is ahead of the transmitter, whereas, if the indicated level appears high the receiver control generator is behind the transmitter.An appropriate correcting signal may therefore be applied to the receiver control generator 30 from the resynchronisation circuit 38 to bring the control generator 30 and therefore the output of synthesiser 26 back into synchronisation with the received carrier signal.

Figure 3 illustrates the different spectrum produced by a signal transmitted in accordance with the above described technique, line 40, the direct sequence technique, line 42, and a frequency-hopping technique, line 44. For each of these techniques a spectrum has been shown with the same bandwidth B. The spectra are not to relative scale as regards amplitude. The spectrum for the direct sequence technique 42 shows a considerable amount of energy in side lobes outside the desired bandwidth, whereas the spectrum for the present technique falls off uniformly outside the desired bandwidth without any undesired side lobes. The bandwidth B of the signal transmitted using the technique of the present invention is determined principally by the maximum possible frequency change produced by the synthesiser.Thus, in the illustrated example, the maximum frequency deviation, and hence the bandwidth is 8MHz. The frequency-hopping spectrum is shown as an envelope 44, however the spectrum at any given time will be as illustrated, for example, by the idealised narrow spectra 46 and 48.

It will be appreciated that if jamming or other interference is applied over a portion of the spectral bandwidth as indicated by shaded area 50, then in a frequency-hopping radio system operating with an instantaneous spectrum 46, there will be a burst of errors in the received signal. However, with either the direct sequence technique or the technique of the present invention this interference 40 will affect all data bits equally and there is a reasonable probability that the digital signal will be received with an error rate of an acceptable level.

It will be appreciated that the above-described technique combines the advantages of the frequency-hopping and direct sequence techniques without suffering from the disadvantages of either. This technique is thus able to provide a high degree of immunity to narrow band jamming or interference and also to be used for a number of radio systems operating in closely adjacent bands without causing undue mutual interference.

Claims (Filed on 14.11.83) 1. A radio system for transmitting digital signals comprising a radio transmitter and receiver, wherein the transmitter comprises means for synthesising a carrier signal and means for modulating the synthesised carrier signal with the digital signal to be transmitted, and the receiver comprises means for synthesising a signal corresponding to the carrier signal, and means for demodulating the received signal with said corresponding synthesised signal, wherein the frequency of the carrier signal is continuously changing within a preset range, the sense and/or rate of change being periodically altered.

2. A radio system as claimed in claim 1, wherein the sense and/or rate of change of the frequency of the carrier signal is altered after a regular fixed time period which corresponds to the time taken for a plurality of data bits of the digital signal to be transmitted.

3. A radio system as claimed in claim 2, wherein the fixed time period is substantially equal to the time for three data bits of the digital signal to be transmitted.

4. A radio system as claimed in any one of claims 1 to 3, wherein the means for synthesising a carrier signal comprises a control generator for outputting a serial digital code signal, a digital to analogue converter for receiving said digital code signal and converting a predetermined number of bits to an analogue output signal, a voltage controlled frequency synthesiser having a control input to which the output of the digital to analogue converter is fed via a low pass filter.

5. A radio system as claimed in any one of the preceding claims, wherein the modulating means comprises means for frequency modulating the carrier signal with the digital signal to be transmitted.

6. A radio system as claimed in claim 4, wherein the means for synthesising a signal corresponding to the carrier signal in the receiver comprises the same elements as the means for synthesising a carrier signal in the transmitter, the receiver further comprising a re-synchronisation means for controlling the timing of the receiver control generator in dependence on the output of the demodulating means.

7. A radio system as claimed in claim 6, wherein the re-synchronisation means comprise means for comparing the level of the received signal with an expected level of the received signal, inverting the sense of said difference whenever the frequency of the carrier signal is decreasing, and means for adjusting the timing of the control generator in dependence on the sense of said adjusted difference.

8. A radio system substantially as herein described with reference to the accompanying drawings.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. reversed if the transmitter is ahead of the receiver. Therefore, if the difference in the received and expected levels is inverted whenever the frequency curve is such that the frequency is decreasing, then by looking at the sense of the level deviation it can be determined whether the receiver control generator is operating either ahead of or behind the transmitter control generator. Thus, if an output from the resynchronisation circuit 38 is continuously indicative of a lower than erected average level the receiver control generator is ahead of the transmitter, whereas, if the indicated level appears high the receiver control generator is behind the transmitter.An appropriate correcting signal may therefore be applied to the receiver control generator 30 from the resynchronisation circuit 38 to bring the control generator 30 and therefore the output of synthesiser 26 back into synchronisation with the received carrier signal. Figure 3 illustrates the different spectrum produced by a signal transmitted in accordance with the above described technique, line 40, the direct sequence technique, line 42, and a frequency-hopping technique, line 44. For each of these techniques a spectrum has been shown with the same bandwidth B. The spectra are not to relative scale as regards amplitude. The spectrum for the direct sequence technique 42 shows a considerable amount of energy in side lobes outside the desired bandwidth, whereas the spectrum for the present technique falls off uniformly outside the desired bandwidth without any undesired side lobes. The bandwidth B of the signal transmitted using the technique of the present invention is determined principally by the maximum possible frequency change produced by the synthesiser.Thus, in the illustrated example, the maximum frequency deviation, and hence the bandwidth is 8MHz. The frequency-hopping spectrum is shown as an envelope 44, however the spectrum at any given time will be as illustrated, for example, by the idealised narrow spectra 46 and 48. It will be appreciated that if jamming or other interference is applied over a portion of the spectral bandwidth as indicated by shaded area 50, then in a frequency-hopping radio system operating with an instantaneous spectrum 46, there will be a burst of errors in the received signal. However, with either the direct sequence technique or the technique of the present invention this interference 40 will affect all data bits equally and there is a reasonable probability that the digital signal will be received with an error rate of an acceptable level. It will be appreciated that the above-described technique combines the advantages of the frequency-hopping and direct sequence techniques without suffering from the disadvantages of either. This technique is thus able to provide a high degree of immunity to narrow band jamming or interference and also to be used for a number of radio systems operating in closely adjacent bands without causing undue mutual interference. Claims (Filed on 14.11.83)

1. A radio system for transmitting digital signals comprising a radio transmitter and receiver, wherein the transmitter comprises means for synthesising a carrier signal and means for modulating the synthesised carrier signal with the digital signal to be transmitted, and the receiver comprises means for synthesising a signal corresponding to the carrier signal, and means for demodulating the received signal with said corresponding synthesised signal, wherein the frequency of the carrier signal is continuously changing within a preset range, the sense and/or rate of change being periodically altered.

9. A method of transmitting a digital signal by

modulating a carrier signal with the digital signal to be transmitted, the carrier signal having a frequency which continuously varies over a predetermined frequency band in a gradual and continuous manner.

10. A method of transmitting a digital signal substantially as herein described with reference to the accompanying drawings.