Abstract:
A communications system comprising a signal transmission arrangement for outputting a first signal for transmission, a first controller operable to control transmission of the first signal such that the transmitted signal takes a pulsed form including a plurality of first periods in which the first signal is transmitted separated by second periods in which the first signal is not transmitted, a radio signal receiver arrangement including or forming a bandpass filter and arranged to receive a second signal, and a second controller operable such that during a plurality of third periods the second signal is processed by at least part of the radio receiver arrangement, the third periods being separated by fourth periods in which the radio receiver arrangement does not process the second signal, wherein the first and second controllers are synchronized such that the second periods are synchronized with the third periods, at the radio signal receiver arrangement, such that the pulsed signal received by the receiver arrangement is reconstituted in the bandpass filter into a continuous signal, free or substantially free of interference from the first signal.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority of British Patent Application No. 1306150.2, filed Apr. 5, 2013 and British Patent Application No. 1312534.9, filed Jul. 12, 2013, the contents of which are incorporated herein by reference in their entirety. 
     FIELD 
     This invention relates to a communications system, and in particular to a communications system whereby radio transmissions may be made from or received by a number of co-located, or closely located radio transmitters or receivers with reduced interference therebetween. 
     BACKGROUND 
     Where a number of radio systems are used in close proximity to one another, a relatively high power signal transmitted from one of the systems can swamp the signals received by other systems, even where the systems are operating at different transmission frequencies. The resulting desensitisation of a receiver impairs the ability of the receiver to receive and demodulate the incoming signal. Clearly, therefore, in these circumstances, communications difficulties can be faced, for example disrupting the reception of voice communications which may hamper organisation of personnel, or disrupting the reception of transmitted data signals. Depending upon the nature of the signals, the reception of which is disrupted, personnel may be put at unnecessary or avoidable risk. 
     SUMMARY 
     It is an object of the invention to provide a communications system whereby interference between closely located equipment is reduced or avoided. 
     According to the present invention there is provided a communications system comprising a signal transmission arrangement for outputting a first signal for transmission, a first controller operable to control transmission of the first signal such that the transmitted signal takes a pulsed form including a plurality of first periods in which the first signal is transmitted separated by second periods in which the first signal is not transmitted, a radio signal receiver arrangement including or forming a bandpass filter and arranged to receive a second signal, and a second controller operable such that during a plurality of third periods the second signal is processed by at least part of the radio receiver arrangement, the third periods being separated by fourth periods in which the radio receiver arrangement does not process the second signal, wherein the first and second controllers are synchronised such that the second periods are synchronised with the third periods, at the radio signal receiver arrangement. 
     By way of example, the second controller may be operable to control the second signal received by the receiver arrangement so that the signal takes a pulsed form including a plurality of third periods in which the second signal is received by the receiver arrangement separated by fourth periods in which the radio receiver does not receive the second signal. Alternatively, the signal received by the receiver arrangement may be modified, within the receiver arrangement under the control of the second controller, so that at least part of the receiver arrangement only processes those parts of the second signal received during the third periods. 
     It has been found that where a pulsed radio signal is passed through a bandpass filter, provided the repetition rate of the pulsed signal is greater than the bandwidth of the filter, any radio frequencies present in the pulsed signal that lie within the filter bandwidth will, in effect, be joined up and emerge from the filter as a continuous, albeit weaker, signal. Any typical radio receiver is, in effect, a tuneable bandpass filter. Accordingly, by transmitting a pulsed signal from the radio transmitter or transmission arrangement, and controlling the operation of the radio signal receiver arrangement in such a manner as to receive only the signal present between pulses of the transmitted signal, the pulsed signal received by the radio signal receiver arrangement can be reconstituted back into a continuous signal free from interference from the radio transmitter simply by the passage of the pulsed, received signal through the radio&#39;s bandpass filter. Little modification of a typical receiver is required to allow operation thereof in accordance with the invention. Furthermore, the transmitter used to transmit the received radio signal needs no modification. 
     The first periods may be equal in length, and the second periods equal in length, thereby defining a fixed pulse rate. Alternatively, the first and/or second periods may be of varying length, thereby defining a variable pulse rate. In either case, the pulse rate is conveniently greater than the bandwidth of the bandpass filter. It is preferably significantly greater than the bandwidth of the bandpass filter, for example it may be several times the bandwidth of the bandpass filter. 
     The first and second controllers may form part of a single control unit. This is convenient where the transmission arrangement and the radio signal receiver arrangement are adjacent one another. However, this need not always be the case. Provided adequate synchronisation can be maintained, for example by the use of a GPS based timing signal, the transmission arrangement and the radio signal receiver arrangement may be spaced apart. For example, the radio signal receiver arrangement could be a mobile, possibly hand held, unit, capable of use at locations remote from the transmission arrangement. 
     The technique outline hereinbefore can be extended for use with two or more radio receivers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will further be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1   a  is a diagrammatic representation of a communication system in accordance with one embodiment of the invention; 
         FIG. 1   b  illustrates parts of the embodiment of  FIG. 1   a  in greater detail; 
         FIGS. 2   a  to  2   f  are diagrammatic representations of waveforms in various parts of the system; and 
         FIGS. 3   a  and  3   b  are diagrammatic representations of the effect of the use of a shaped waveform. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the accompanying drawings, a communications system is illustrated which comprises a first signal transmission system  10  arranged to output a first signal  14  (see  FIG. 2   a ) for subsequent transmission via an antenna  16 . The first signal  14  may take the form of a suitable modulated voice or data signal. A control unit or system  18  is provided which includes a first controller  20  operable to modify the first signal  14  to take a pulsed form. As illustrated, the first controller  20  may take the form of a fast electronic switch  22  operable by a control device  24  in response to a control waveform  26  (see  FIG. 2   b ). The switch  22  operates to modify the signal  14  to take a pulsed form which is passed to the antenna  16  for transmission. Between pulses, the switch  22  diverts the signal from the radio into a suitable load  22   b  where it is absorbed. An electronic attenuator  22   c  may also be provided to permit shaping of the transmitted radio pulses and minimise spectral spreading as described below.  FIG. 2   c  illustrates, diagrammatically, the transmitted signal  28 , and it will be apparent that the signal  28  is made up of a plurality of first periods  28   a  in which the first signal  14  is transmitted, separated by second periods  28   b  in which the first signal  14  is not transmitted. In the second periods  28   b , the first signal  14  is absorbed into the load  22   b  as mentioned above. 
     Whilst a simple switch  22  is illustrated in  FIG. 1   a , it will be appreciated that the function thereof may, in reality, be achieved in a number of ways. The switch function may also be integrated into the transmission system  10 , as described below. 
     The communications system further comprises a radio signal receiver arrangement  30  including a typical radio receiver device  32  operable to demodulate a received second signal in the usual manner. By way of example, the second signal  38   a  (see  FIG. 2   d ) may be transmitted from a remotely located transmitter  38 . The second signal  38   a  will typically be at a different frequency to the transmitted signal  28  mentioned hereinbefore. The control system  18  includes a second controller  34  operable to control a switch  36  to modify a received signal in accordance with a control waveform  29  (see  FIG. 2   e ) before the signal is supplied to the device  32 .  FIG. 2   f  is a diagrammatic representation of the modified received signal. It will be appreciated that the switch  36  of the second controller is controllable in a manner similar to the switch  22 , and that consequently the second signal received by the device  32  (illustrated diagrammatically in  FIG. 2   f  and denoted by numeral  40 ) is of pulsed form, including a plurality of third periods  40   a  in which the received radio signal is passed to the device  32  separated by fourth periods  40   b  in which the received radio signal is not passed to the device  32 . 
     The operation of the control system  18 , and the use of the control waveforms  26 ,  29  in the control thereof, is such that the third periods  40   a  in which the received signal is passed to the device  32  are synchronised with the second periods  28   b  in which the first signal is not transmitted. Accordingly, the signal received by the device  32  does not include any component of the first signal. 
     In addition to the switch  36 , the second controller  34  includes a low noise amplifier  44 , located upstream of the switch  36 , to boost the incoming signal level and so compensate, at least in part, for some of the losses which will be experienced during subsequent parts of the processing. Downstream of the switch  36  is located a fast electronic attenuator  46  to allow shaping of the signal envelope passed to the device  32  (described in further detail below). 
     The radio receiver device  32  is shown much simplified in  FIG. 1   a  but, in common with other radio receiver devices, it is essentially a bandpass filter  32   a  tuned to a desired signal frequency followed by a demodulator. It will be appreciated that the signal received by the device  32  is of pulsed form and, as mentioned hereinbefore, where pulsed signals are passed through a bandpass filter, the bandwidth of which is smaller than the pulse rate of the pulsed signal, the filter reconstructs or reconstitutes the signal as a continuous signal which can be demodulated by the device  32  in the usual manner to produce an output substantially the same as the output of a radio receiver receiving the unprocessed signal in the absence of the transmitted pulsed signal. 
     The switch  36  is ideally of fast acting form, thereby minimising the amount of signal loss whilst ensuring that the first signal is fully removed from the signal passed to the device  32 . To further ensure that the first signal is fully removed, the switch  36  should preferably be of high-isolation form. 
     The use of switch  36  alone to divide an incoming radio signal into rectangular pulses will cause spectral spreading, creating numerous weaker images or copies of the radio signal spaced in frequency at multiples of the pulse rate. Thus, as shown in  FIG. 3   a , a second unwanted radio signal  50  at a frequency other than the frequency of a desired signal  52  entering the switch  36  will give rise to numerous images  50 ′, one of which could fall within the filter bandwidth of the radio receiver  32  and thus cause interference with the desired signal  62 . This spectral spreading effect can be minimised, as shown in  FIG. 3   b , by using the attenuator  46  to smoothly vary the envelope of the pulses supplied to the device  32  so that they are not of sharp, square or rectangular form but rather are of more smoothly curved form. A number of standard mathematical functions, known as window functions, exist to compute the shape of such envelopes to achieve an optimal effect. Similarly, spectral spreading of the pulsed transmission due to the operation of the switch  22  can be minimised by using attenuator  22   c  to shape the envelope of the transmitted pulses. 
     Whilst in the description hereinbefore the first signal transmission system  10  is described only as being capable of transmitting signals, and the signal receiver arrangement  30  is described only as receiving remotely transmitted signals, in practise it is likely that both will take the form of radio transceivers in which case the first and second controllers  20 ,  34  may take the form shown in  FIG. 1   b , including a transmission leg including the switch  22  and a reception leg including the switch  36 . 
     Relays  50 ,  52  may be provided to control which leg is active at any given time. When it is required to transmit a signal the relays  50 ,  52  are controlled in such a manner as to take the switch  36  and associated components off-line, and vice versa. 
     It will be appreciated that the use of the invention allows the reception of radio signals, for example relatively weak signals transmitted from remote locations, despite the use of a transmitter transmitting relatively high power signals in the vicinity of the receiver. The swamping of the received signal from the remote transmitter is thus avoided as desensitisation of the receiver does not occur. 
     The transmitted signal transmitted by the transmission system  10  can be received and demodulated by a remote receiver  10   a  in the usual manner, the signal being reformulated into a substantially continuous signal in the manner described hereinbefore, and so can be received and demodulated without requiring modification of the remote receiver  10   a.    
     It will be appreciated that the communications system may be used in the transmission of voice signals or data signals, so long as the radio filter bandwidth is less than the signal pulse rate. It requires no modification to the transmitter used to transmit the subsequently received signal, and requires only limited modification of a typical radio receiver device. 
     Many of the components in the second controller  34  can be integrated directly into the receive path within radio  32 , typically located immediately preceding filter  32   a . This arrangement can allow some simplification of the design. For example, relays  50 ,  52  and bypass path  48  would not be required as the components are no longer in the transmit signal path, and low-noise amplifier  44  is likely to already be present at the front end of any typical radio receiver design. Similarly, many of the components in the first controller  20  can be integrated into the transmit circuitry of the radio  12 . 
     Furthermore, in a software-based radio many of the components can be conveniently implemented in the radio&#39;s digital signal processing. Such a radio uses an analogue-to-digital converter to translate the incoming radio signal into a stream of data values that are then filtered and demodulated by a digital processing system. Switch  36  and attenuator  46  could be implemented as a digital multiplier immediately after the analogue-to-digital converter, multiplying each incoming data value by a value X that varies between 0 and 1. During each fourth period X is set to 0 to suppress incoming signal data, and during each third period X is set to 1 to allow signal data through unaltered. X may also be cycled smoothly between 0 and 1 using a mathematical function as described earlier to minimise any co-site interference effects. The resulting bursts of data are joined up by the subsequent digital filters in exactly the same manner as described earlier. 
     In the description hereinbefore, the controller  18  operates to control the second signal supplied to the radio receiver arrangement  30 , removing or attenuating those parts of the received signal received outside of the third periods. In an alternative arrangement, the radio receiver arrangement  30  may be arranged to receive the complete received signal, including the parts received outside of the third periods, avoiding the need to provide the switch  36  and attenuator  46 . The radio receiver arrangement  30  may instead be controlled in such a manner that the parts of the second signal received outside of the third periods are ignored by, for example, the final IF filter and/or demodulator of the radio receiver arrangement  30 , recovering and demodulating the received signal only during the third periods. Any suitable filtering technique may be used to recover the signal carrier and compensate for the periods during which the second signal is being ignored. A suitable timing signal may be used to control the operation of the controller  18  in such an arrangement. 
     Such an arrangement would most likely be achieved by appropriate programming of a software based radio device. 
     Although the description hereinbefore relates to the case where one remotely transmitted signal is to be received in the vicinity of a relatively high power transmission, the invention is also applicable to more complex arrangements or scenarios in which, for example, two or more remotely transmitted signals, transmitted at respective frequencies, require reception and demodulation. This may be achieved simply by controlling each receiver in substantially the manner described hereinbefore, respective controllers being used to modify the received signals to take pulsed form, using the same pulse pattern, to remove the locally transmitted pulsed signal  28  from the received signals such that subsequent demodulation results in the production of the desired substantially continuous waveforms. 
     For simplicity, in much of the description hereinbefore the transmission system  10  and receiver arrangement  30  are described as having different functions. In reality, as mentioned hereinbefore, each device will typically serve both as a transmitter and a receiver, the control unit controlling the pulsing of the transmitted and received signals in accordance with which of the devices is actually transmitting at any given time. If neither device is transmitting, then the pulsing operation will be redundant and both devices can receive continuously until such time as one or other device needs to commence transmission. If both devices are transmitting simultaneously, then the control unit applies the pulses such that they are out of phase with one another. If desired, three or more transmitters and receivers may be located within the vicinity of one another and, by appropriate adjustment of the phasing of the control waveforms, all may be used simultaneously without the aforementioned signal swamping or deafening issues. 
     Where individual units are capable of both transmitting and receiving signals, the switches, etc, of the controllers thereof will need to be able to withstand the relatively high transmission power and so may be of relatively complex form. As shown in  FIG. 1   b , separate transmission and reception legs may be provided to accomplish this. However, it may be possible to provide a single, bidirectional leg. 
     Whilst in the description hereinbefore the controller  18  is described and illustrated as a separate component, this need not be the case and its functionality could be incorporated into the signal transmission arrangement  10  and/or the radio signal receiver arrangement  30 . Moreover the controller  18  could be divided into two parts, one controlling the arrangement  10  and one controlling the radio receiver arrangement  30 , and will operate correctly so long as the two parts remain synchronised. 
     The precise timing of the various control pulses described above may be individually adjusted and optimised to compensate for component response times and/or propagation delays in interconnecting cables. Moreover, where the signal transmission arrangement  10  and radio signal receiver arrangement  30  are spaced apart from one another by a significant distance then signal propagation times could result in misalignment of the various timing periods and undesirable inclusion of part of the first signal into the pulsed signal passed to the device  32 . Rather than have the control signals exactly synchronised at the point of transmission, it may be preferred to have them slightly displaced from one another to compensate for such propagation delays, so that they are synchronised at the receiver location. If the elements of the system are in fixed locations, then the delays may be fixed and built into the system. Alternatively, particularly if one or other of the elements of the system is mobile, then the GPS system or similar used to maintain synchronisation may also be used to provide position information which can then be used in the calculation of a timing offset by which the pulses are displaced relative to one another to compensate for the spacing of the elements. Alternatively it may be possible to manually adjust the displacement until a clear demodulated signal is received, or to employ an automated adaptive algorithm to monitor the received signal quality and adjust the displacement to maintain optimum performance. 
     Whilst in the arrangements described hereinbefore the transmitted and received signals use different radio frequencies, the purpose of the invention being to avoid swamping and desensitisation and thereby allowing relatively weak signals to be received and demodulated in the presence of a relatively high power transmission, the invention is also applicable to arrangements in which it is desired to transmit several signals at the same frequency whilst avoiding interference between those signals. In such an arrangement, all of the transmitters and receivers used in the communications system will need to incorporate controllers to allow the conversion of signals, whether for transmission or demodulation, into pulsed forms, correctly synchronised to one another, to extract the required signals from the other signals, such that subsequent filtering and demodulation results in the pulsed signals being converted back into substantially continuous copies of the original signals. As described hereinbefore, the synchronisation should preferably take into account propagation time variations and the like. 
     The use of this technique would allow a single transmitter to transmit different signals to a series of different receivers using the same transmission frequency. 
     If the controller  18  is able to accommodate sufficiently fast switching, then the device  10  could be rapidly switched between transmit and receive modes, thereby enabling the transmission of a pulsed signal and, simultaneously, the reception of a pulsed signal. Accordingly, a radio capable of pseudo simultaneous transmission and reception may be possible. 
     It will be appreciated that a wide range of modifications and alterations may be made to the arrangements described hereinbefore without departing from the scope of the invention as defined by the appended claims. For example, whilst the description hereinbefore relates primarily to the transmission of signals between just two locations, it will be appreciated that the system may be used between more locations than this, provided the second controllers at each location are synchronised, appropriate compensation being made for propagation times, etc.