GB2092845A

GB2092845A - Radio communication system

Info

Publication number: GB2092845A
Application number: GB8202599A
Authority: GB
Original assignee: Redifon Telecommunications Ltd
Current assignee: Redifon Telecommunications Ltd
Priority date: 1981-01-30
Filing date: 1982-01-29
Publication date: 1982-08-18

Abstract

A radio communication system comprises a transmitter, Fig. 5, for producing a transmission 16 having no discernible carrier frequency component, an aerial 21 for radiating the transmission having a length less than that required for natural resonance at the transmission frequency, and a tuning circuit 19 for adjusting the aerial 21 to resonance at said transmission frequency, the resonant bandwidth of the aerial being such as barely to encompass the band occupied by the transmission. Two pilot waves having frequencies lying within or adjacent the band occupied by said transmission are produced at 22 and added to the transmission, one pilot wave having a frequency on the high frequency side of band centre and the other a frequency on the low frequency side of band centre. The tuning circuit is adjusted automatically 36 in response to the pilot waves detected 33 at the aerial 21 so that the resonant bandwidth of the aerial is maintained in a substantially symmetrical relationship to the band occupied by the transmission. The system also comprises a receiver, Fig. 6, in which the transmission is demodulated 47 using a carrier re-generated from the detected pilot waves 40-46. The pilot waves may themselves be modulated to convey synchronisation information, extracted at 54. The transmission may employ minimum shift FSK at VLF carrier frequency. <IMAGE>

Description

SPECIFICATION Radio communication system and apparatus The present invention relates to radio communications systems and apparatus in which the transmission is of a kind in which no discernible carrier frequency component is present and its bandwidth is such as substantially to occupy the bandwidth over which the aerial for radiating or receiving the transmission is resonant. Such systems may be operated in any part of the radio spectrum ranging from V.L.F.

to S.H.F. depending upon the absolute bandwidth of the transmission concerned in relation to the absolute bandwidth of the aerial employed.

An example of the kind of transmission referred to above is that known as Minimum Shift Keying (M.S.K.). This type of transmission is described in U.S. Patent No. 2977417 and is particularly suitable for use at carrier frequencies in the V.L.F. range (1030kHz) because of the small transmission bandwidth required for a given information transmission rate and system performance level.

In this frequency range a quarter-wave aerial, which is the minimum length for achieving natural resonance and a length widely used at higher frequencies, would be between 7 < and 2 < km long. Such aerials are impractical and any practical structures are electrically very short compared with a physical quarter-wave. This results in them exhibiting a very low radiation resistance in relation to a large radiator reactance, and this adverse ratio is the principal factor which determines that the bandwidth of practical aerials is low.

It follows that V. L. F. aerial systems are very susceptible to detuning as a result of environmental changes of a climatic nature, i.e. icing. It has therefore become an established practice in V.L.F.

engineering to provide automatic aerial tuning so as to eliminate the deleterious effects of climatic changes. While such automatic aerial tuning systems are relatively easy to implement where the transmission includes a radio frequency carrier component, a transmission employing M.S.K. has no such carrier component.

It is an object of the present invention to provide a radio communication system in which the transmission is of a kind in which no discernible carrier frequency component is present and in which means are provided for adjusting automatically the tuning of an associated aerial.

Accordingly, the invention provides a radio communication system comprising a transmitter for producing a transmission having no discernible carrierfrequency component, an aerial for radiating the transmission having a length less than that required for natural resonance at the transmission frequency, tuning means for adjusting said aerial to resonance at said transmission frequency, the resonant bandwidth of said aerial being such as barely to encompass the band occupied by said transmission, and a receiver for utilising the transmission provided by the transmitter, characterised by means for producing two pilot waves having frequencies lying within or adjacent the band occupied by said transmission, one pilot wave having a frequency on the high frequency side of band centre and the other a frequency on the low frequency side of band centre, and means responsive to said pilot waves for adjusting automatically the tuning means so that the resonant bandwidth of said aerial is maintained in a substantially symmetrical relationship to the band occupied by said transmission.

For the purpose solely of maintaining aerial resonance the pilot waves may have a very low, subliminal level. However, if their level is such as to permit reception at a distant receiver employed in the radio communication system they may be used to mitigate the problems involved in maintaining the frequency of a re-insertion carrier oscillator employed in conjunction with a synchronous demodulator in said receiver. Pilot waves at this level may also serve, in a system wherein the transmission is a data traffic stream, to synchronise data processing timing clocks as a receiver without requiring adaptation of the data stream to achieve synchronisation.

The invention also provides a transmitter for use in a radio communications system, the transmitter comprising means for producing a transmission having no discernible carrier frequency component,and in aerial tuning means for adjusting an associated aerial having a length less than that required for natural resonance at the transmission frequency concerned to a resonant condition, characterised by means for producing two pilot waves having frequencies within or adjacent the band occupied by said transmission, one pilot wave having a frequency on the high frequency side of band centre and the other a frequency side of band centre, and means responsive to said pilot waves for adjusting automatically the aerial tuning means so that the resonant bandwidth of the associated aerial is maintained in a substantially symmetrical relationship to the band occupied by said transmission.

The transmitter may also include, if desired, means for imparting to said pilot waves a modulatiori content, whereby additional information, for example, timing information in respect of a data traffic stream, may be conveyed to a receiver responsive to the transmission.

The invention also provides a receiver for use in a radio system in which a transmitter ofthe system is arranged to produce a transmission having no discernible carrier frequency component, comprising means for demodulating said transmission signal with the aid of a locally generated re-insertion carrier wave, characterised in that the transmission is produced with two pilot waves having frequencies lying within or adjacent the band occupied by said transmission, one pilot wave having a frequency on the high frequency side of band centre and the other a frequency on the low frequency side of band centre, and the receiver comprises means for deriving at least one of said pilot waves, and means responsive to the derived pilot wave or waves to produce a re insertion carrier wave to permit correct demodulation of the transmission signal.

If desired, a receiver of the kind referred to above may also include an auxiliary demodulator arranged to recover a modulation content from the pilot waves and means for making available at output terminals of said receiver the recovered pilot wave modulation content.

Embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawings, in which: Fig. lisa block schematic diagram of a radio communication system in accordance with the present invention; Fig. 2 is a representation of the RF spectrum of a M.S.K. data transmission in a system according to Fig. 1, overlaid with a smooth curve representing power spectral density; Fig. 3 is a representation of the power spectral density for frequencies further removed from band centre, in a system according to Fig. 1; Fig. 4 is a representation of the amplitude and phase responses of a V.L.F. aerial system; Fig. 5 is a block schematic diagram of transmitting apparatus for use in the system shown in Fig. 1; and Fig. 6 is a block schematic diagram of receiving apparatus for use in the system shown in Fig. 2.

In the system shown in Fig. 1 a transmitter 1 is arranged to produce a transmission in the V.L.F.

band, such as in M.S.K. transmission, having no discernible carrier frequency in respect of message signals applied to it over an input line 2. Associated with the transmitter 1 is a pilot wave generator 3 which produces two pilot waves having frequencies lying within or adjacent the band occupied by said transmission, one pilot wave having a frequency on the upper frequency side of band centre and the other a frequency on the low side of band centre.

The transmitter 1 is arranged to feed an aerial 4 which is short relative to a physical quarter-wave and is resonated by an aerial tuning device 5. The bandwidth ofthe aerial 4 is thus low and is substan tiallyoccupied by the transmission of the transmitter 1. The aerial 4 is maintained in a resonant condition by a pilot wave detector and processing means 6 which is coupled to a variable circuit element in the aerial tuning device 5 to maintain the resonant bandwidth of the aerial 4 in a substantial symmetrical relationship to the band occupied by said transmission.

Signals radiated from the aerial 4 are picked up by the aerial 7 of a distant receiving station at which a receiver 8 arranged to utilise the signals of the transmitter 1 is connected to said aerial 7 to provide message signals on an output line 9. The receiver 8 includes a demodulator requiring for its operation a re-insertion carrier. In orderto mitigate the problem involved in maintaining this re-insertion carrier at the correct frequency, an auxiliary signal processing device 10 is arranged to detect the pilot waves provided by the pilot wave generator 3 at the transmitter and to provide in response thereto a control signal which is applied to the re-insertion carrier oscillator in the receiver 8 over the line 11.Optionally, the pilot waves may be modulated in respect of a data sync hronising signal applied over the line 12 to the pilot wave generator 3 at the transmitter, the correspond ing demodulated synchronising signal being made available at the receiving station from the line 13.

In the spectrum representation shown in Fig. 2 it will be noted that there exists a series of minima disposed symmetrically about the band centre frequency where the power density drops to near zero.

These minima in the transmitted spectrum are used to accommodate pairs or multiple pairs of subcarriers. In-phase pairs of sidebands may be gener ated by amplitude modulation. Quadrature pairs may likewise be generated by low deviation index frequency modulation. To ease the filtering out of the subcarriers, pairs falling in a corresponding pair of main signal minima, which are relatively far removed from band centre where the adjacent signal power density is relatively low, may be employed.

This is illustrated forsubcarriers U and L in Fig. 3.

Fig. 4 shows the relationship between the upper and lower subcarriers SCu and SC, and the resonant frequency fo of the aerial system lies as shown, mid-way between Fscu and Fisc,. Then SCu and SC, are both subject to the same numerical phase shift e. It is also apparent that this situation | eJ =| e only pertains when fo, (band centre) is in the mid position. If fo drifts in frequency then analogue voltages representative of e, and Ou respectively differ in magnitude.

Fig. 5 shows a transmitting apparatus for use in the system shown in Fig. 1. This transmitter inc ludos an oscillator 14 controlling over the line 15 the nominal transmission frequency of a data transmitter 16 employing M.S.K. The data information for transmission is applied over the line 17. The RF output of the transmitter 16 is fed over a transmission line 18 to an aerial tuning unit 19 including a variable inductor 20 for adjusting the resonant frequency of an aerial 21. The carrier frequency of the transmitter 16 lies in the V.L.F. range 1 0-30kHz and the aerial 21 is electrically short at the carrier frequency concerned. the bandwidth of the aerial 21 is such as barely to encompass the transmission band of the signal generated by the transmitter 16 so that its adjustment for resonance is critical.

In order to maintain the aerial 21 at resonance two pilot waves are generated, one having a frequency withinn the upper frequency halr of the transmission band and the other within the lower frequency half.

The frequencies of the pilot waves are arranged to be equidistant from the nominal centre frequency of the transmission and to lie in minima which exist in the power spectrum ofthe transmitted signal.

The pilot waves are generated by a frequency and amplitude modulator device 22 which is arranged to receive a carrier wave input from the oscillator 14 by way of the line 23 and an audio frequency tone over a line 24. The frequency of the audio tone deter mines the frequency spacing of the pilot waves about the band centre or nominal carrier frequency of the transmission as determined by the oscillator 14. The pilot waves are fed over a line 25 to an amp lifier 26 and applied, after amplification to the trans mission line 18, together with the output of the transmitter 16.The amplifier 26 may be arranged to amplify the level of the pilot waves either to a very low, subliminal level at which they are avaiiable solely for the purpose of maintaining resonance of the aerial 21 or they may be amplified to a level such as to permit reception at a distant receiver, in which case they may be employed for additional purposes to be described more fully later.

The low level pilot waves on the line 25 are also fed to respective band pass filters 27, 28 which serve to separate the individual pilot waves for amplification over lines 29, 30 to respective phase detectors 31,32. The phase detectors 31,32 have applied to them a sample of the aerial current which is derived from a pick-off device 33 which may be a current transformer. The outputs of the phase detectors 31, 32 are applied to a summing device 34 the output from which is applied to an amplifier 35 which in turn is arranged to adjust the aerial tuning coil 20 by means of a motor 36. The current sampling device, phase detectors, summing device, amplifier 35, motor 36 and coil 20 constitute a servo system for maintaining aerial resonance.The error signal of this servo system is the summed outputs of two detectors 31,32 which is zero when e, = Ou, i.e. when the resonance point of the aerial is mid-way between the subcarriers.

A receiver capable of utilising pilot waves produced at the transmitter when they are at a level sufficient to permit distant reception for mitigating the problems involved in maintaining the frequency of a re-insertion carrier oscillator is shown in Fig. 6.

Signals to be received are picked up by an aerial 37 which is arranged to supply signals to two parallel signals over the lines 38, 39. The line 38 passes signals to a pair of band pass filters 40,41 which serve to separate the individual pilot waves which are then applied to a summing device 42. The output of the summing device 42 feeds a phase reversing switch 43 which is arranged to operate at each zero crossing of the input signal. The switch 43 is driven by a cusp detector 44 which is arranged to be responsive to a half-wave rectified version of the signal from the summing device 42 after it has been detected by an envelope detector 45. The phase switched output from the switch 43 is then passed through a band pass filter 46 before application to a demodulator 47 as the re-insertion carrier wave for demodulating the main transmission.This main transmission reaches the demodulator 47 from the line 38 after passing through a delay device 48, which has a delay such as to compensate for the delay suffered by the pilot waves during their transmission between the input to the filters 40, 41 and the output of the filter 46, a signal frequency amplifier 49, mixer 50 and intermediate frequency amplifier 51. The mixer 50 is also arranged to receive, in known manner, a frequency conversion wave from a conversion oscillator 52.

The frequency of the oscillator 52 may be controlled to maintain a desired relationship with that of the re-insertion carrier wave by means of a frequency control device 53.

In a system wherein the transmission is a data traffic stream the pilot waves may be utilised to synchronise a data processing timing clock at the receiver installation, so avoiding the requirement to adapt the data stream to achieve synchronisation. In certain circumstances, the data may be encrypted and any periodic adaption which occurs in it may compromise its security. The signalling rate required to maintain sychronisation is usually very low in relation to the data rate so that the pilot waves may themselves be modulated to convey the synchronisation information and their bandwidth still remain within the power minima in the spectrum of the transmitted signal. The receiver shown in Fig. 6 may therefore also be provided with a synchronisation signal generator 54 arranged to receive signals from the detector 45 and to provide, in response thereto, synchronising signals on its output line 55 for controlling a data decoder.

If desired, the filter 46 may comprise a phaselocked loop rather than an UC or active filter arrangement.

Claims

1. A radio communication system comprising a transmitter for producing a transmission having no discernible carrier frequency component, an aerial for radiating the transmission having less than that required for natural resonance at the transmission frequency, tuning means for adjusting said aerial to resonance at said transmission frequency, the resonant bandwidth of said aerial such as barely to encompass the band occupied by said transmission, and a receiver for utilising the transmission provided by the transmitter, characterised by means for procuding two pilot waves having frequencies lying within or adjacent the band occupied by said transmission, one pilot wave having a frequency on the high frequency side of band centre and the other a frequency on the low frequency side of band centre, and means responsive to said pilot waves for adjusting automaticallythetuning means so that the resonant bandwidth of said aerial is maintained in a substantially symmetrical relationship to the band occupied by said transmission.

2. A radio communication system according to claim 1, in which the pilot waves are transmitted at a level sufficient to enable their reception at the receiver, wherein the receiver comprises means for deriving at least one of said pilot waves and means responsive to the derived pilot wave or waves to produce a re-insertion carrier wave to permit correct demodulation of the transmission signal.

3. A radio communications system according to claim 2, wherein the transmission is a data traffic stream and the pilot waves are modulated to convey synchronisation information, wherein the receiver comprises means for deriving a synchronising signal from the derived pilot wave or waves.

4. Atransmitterfor use in a radio communications system, the transmitter comprising means for producing a transmission having no discernible car rierfrequency component, and an aerial tuning means for adjusting an associated aerial having a length less than that required for natural resonance at the transmission frequency concerned to a resonant condition, characterised by means for producing two pilot waves having frequencies within of adjacent the band occupied by said transmission, one pilot wave having a frequency on the high frequency side of band centre and the other a frequency on the low frequency side of band centre, and means responsive to said pilot waves for adjusting automatically the aerial tuning means so that the resonant bandwidth of the associated aerial is maintained in a substantially symmetrical relationship to the band occupied by said transmission.

5. Atransmitteraccording to claim 4, comprising a data transmitter circuit receiving the output of an oscillator operating at the nominal transmission frequency of the transmitter and a data signal, an aerial tuning unit receiving the output of the data transmitter circuit, a modulator receiving the output of the oscillator and an audio frequency tone and providing at its output the two pilot waves, and means for combining the outputs of the data transmitter circuit and the modulator.

6. A transmitter according to claim 5, comprising a pick-off device for deriving a sample of the aerial current, two phase detectors each receiving an aerial current sample signal from the pick-off device and a respective pilot wave from the modulator, a summing circuit for summing the outputs of the phase detectors, and means for controlling the aerial unit to minimise the output of the summing circuit.

7. A receiver for use in a radio system in which a transmitter of the system is arranged to produce a transmission having no discernible carrier frequency component, comprising means for demodulating said transmission signal with the aid of a locally generated re-insertion carrier wave, characterised in that the transmission is produced with two pilot waves having frequencies lying within or adjacent the band occupied by said transmission, one pilot wave having a frequency on the high frequency of band centre and the other a frequency on the low frequency side of band centre, and the receiver comprises means for deriving at last one of said pilot waves, and means responsive to the derived pilot wave or waves to produce a re-insertion carrier wave to permit correct demodulation of the transmission signal.

8. A receiver according to claim 7, for use in a radio system in which the two pilot waves are arranged to be equidistant in frequency from the nominal centre of the transmission, comprising means for separating the two pilot waves from the received signal, a summing circuit for summing the two pilot waves, and a phase reversing switch con nected to the output of the summing circuit and arranged to operate at each zero crossing of the input signal, the output of the phase reversing switch providing the re-insertion carrier.

9. A receiver according to claim 8, wherein the phase reversing switch is controlled by the output of a cusp detector, the cusp detector being connected to an envelope detector receiving the output of the summing circuit.

10. A receiver according to claim 8 or 9, compris ing a mixer connected between a transmission signal frequency amplifier and in intermediate fre quency amplifier, and a conversion oscillator the output of which is supplied to the mixer, wherein the frequency of the conversion is controlled by a frequency control device connected to receive the said reinsertion carrier.

11. A receiver according to any one of claims 7 to 11, in which the transmission is a data traffic stream and the pilot waves are modulated to convey synchronisation information, wherein the receiver comprises means for deriving a synchronising signal from the derived pilot wave or waves.

12. A receiver according to claim 11, as depen- dent upon claim 9 or 10, comprising a synchronising signal generator connected to the envelope detector, and a data decoder connected to the output of the synchronising signal generator.

13. A radio communication system substantially - as hereinbefore defined with reference to the accompanying drawings.

14. Atransmitterfor use in a radio communications system, the transmitter being substantially as hereinbefore described with reference to Fig. 5.

15. A receiver for use in a radio communications system, the receiver being sustantially as hereinbefore defined with reference to Fig. 6.