GB2171879A

GB2171879A - Radio communication systems

Info

Publication number: GB2171879A
Application number: GB08431207A
Authority: GB
Inventors: Christopher John Tarran; Gerald Raymond; Terrence Alvan Bristow
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1984-12-11
Filing date: 1984-12-11
Publication date: 1986-09-03
Also published as: GB2171879B

Abstract

In combination with radio transmitter 1 and radio receiver 2 located in relatively close proximity with one another and having different operating frequencies, an interference cancellation arrangement for cancelling cross-coupled interference between the respective antennae of the transmitter and receiver means, the cancellation arrangement comprising a vector modulator 6 or equivalent means which is arranged to be fed with a sample signal from the output of the transmitter and which is arranged to feed to the input of the receiver in dependence upon said sample signal and any correlated reference signal from the receiver an interference cancelling signal having an amplitude and phase which effectively nullifies an interfering signal at the receiver input. <IMAGE>

Description

SPECIFICATION Improvements relating to radio communication systems This invention relates to radio communication systems and relates more specifically to such systems in which the antennae offixed frequency orfrequency agile radio transmitters for transmitting radio signals at a fixed frequency or at stepped frequencies, as the case may be, are allocated in relatively close proximity with the antennae of radio receivers tuned for receiving radio signals at fixed or stepped frequencies different from those at which the radio transmitters are operating.Such collocated radio transmitters and radio receivers may, for example, form part of ships' radio equipment with the respective transmitter and receiver antennae being spaced apart by short distances (eg. 10 metres). Underthese conditions crosscoupled interference usually occurs between the co-l ocated transmitters and receivers whereby in terfering signals from the transmitters are picked up by the antennae ofthe radio receivers.It is already known to used multicouplers in fixed frequency collocated radio transmitters/receivers for the avoidance ofcross-coupled interference between the transmitter and receiver antennae. However, such multicouplers (eg. cavity filters) which are introduced in the transmitter and receiver antennae paths are bulky and expensive and, moreover, cannot be used in frequency agile or frequency hopping systems in which frequency switching is performed at medium or high speeds since the mechanical multicouplers cannot adjust quickly enough to the changing frequencies.

According to the present invention there is provided, in combination with a collocated radio transmitter and radio receiver having different operating frequencies, an interference cancellation arrange mentfor cancelling cross-coupled interference between the respective antennae ofthe transmitter and receiver, said cancellation arrangement comprising a vector modulator or other means which is arranged to be fed with signal sample or reference signal from the transmitter output and which is arranged to feed to the receiver input in dependence upon said signal sample and a control signal from a control loop associated with said vector modulator or other means and interference cancelling signal having an amplitude and phase which effectively nullifies an interfering signal at the receiver input.

The transmitter and receiver ofthe system may be ofthe fixed frequency or frequency agile type. In cases where the transmitter, or transmitter and receiver, are of a relatively high powerfrequency agile type the vector modulatorwill need to be of a fast dynamic range type and the circuitry ofthe control loop associated with the vector modulatorwill be required to adapt rapidly to the stepped changes in the transmitter frequency.

By way of example the present invention will now be described with reference to the accompanying drawings in which; Figure lisa schematic diagram showing a coilo- cated transmitter and receiver arrangement having an interference signal concellation arrangement in accordance with the present invention; Figure 2 is a schematic diagram showing a collo- cated frequency agile transmitter/receiver (transceiver) and a plurality of fixed frequency transmitters and associated receivers.

Figure 3 is a schematic diagram illustrating the switching arrangements forthe embodiment shown in Figure 2; and, Figure 4 is a circuit diagram of one exemplary interference cancellation arrangement according to the present invention.

Referring to Figure 1 ofthe drawings, this shows a radio transmitter 1 which may, for example, be the transmitter of a frequency agile UHF transceiver and a radio receiver 2 which may, for example, comprise a fixed frequency orfrequency agile receiver having an associated transmitter (not shown). The transmitter 1 and the receiver 2 mayform part of a ship's radio equipment in which case the respective antennae 3 and 4 will inevitably be located in relatively close proximity with oneotherwhich can give rise to -ross-coupled interference problems between the collocated transmitter and receiver antennae.

In accordance with the present invention,forthe purpose of cancelling any cross-coupled interference between the transmitter 1 and receiver 2 a sample of the transmitter output signal derived through a coupler 5 and constituting a reference signal RT is fed to a vector modulator 6 connected in a cross-coupling path extending between the transmitter and the receiver antennae 3 and 4. This vector modulator 6 is arranged to produce an outputsignal Opwhich is applied overthe cross-coupling path to the input ofthe receiver2via a coupler7fornullifying any crosscoupled interfering signal received by the receiver antenna 4.Forthe purpose of controlling the amplitude and phase of the output signal OPto effect cancellation of an interfering signal in the receiver input the vector modulator 6 is controlled by means of an assoicated negative feedback loop which derives from the receiver input th rough a coupler 8 a sample signal SS which may include an interfering signal. A fast operating correlator 9 serves to correlate the sample signal SS derived from the receiver input and a reference signal RB derived from the transmitter through the coupler 5 and a further coupler 10. A vector modulator control signal CO from the correlator9 is fed through a low-passfilter 11 tothevector modulator6 in orderto control the amplitude and phase of the interference cancellation signal OP.The correlator 9 and low-pass filter 11 effectively carry out a multiplication ofthe reference and control signal inputs RB and SSto the correlator9 and the latterwill applyan inputto the vector modulator 6 in respect of any in put to the receiver 2which correlates with the transmitter sample or reference signals RTand RB.

The feedback loop indicated by the dashed line FB in the figure is arranged to provide negative feedback so that any signal at the receiver input which correlates with the transmitter reference or sample signal RTwill be reduced bya factorvery nearly equal to the loop gain to a level significantly below the so-called out-of-band desensitisation level ofthe receiver 2.

It may here be mentioned thatthe cross-coupled cancellation arrangement operates mainly on the carrierofthetransmitted signal and any associated noise of close frequency (up to 10MHz). Spurious signals and wide-band noise may not be cancelled and these may be suppressed at source, as by the addition ofsuitablefilters.

Referring to Figure 2 ofthe drawings, this shows howthe present invention may be applied to a frequency agile radio transceiver when collocated with a number offixed frequency radio transmitters and receivers In the present examplethefixed frequency collo- catedtransmitters 12,13, 14 and 15 and receivers16, 17,18,and 19 19 may comprise an existing shipborne radio installation having multicouplers (i.e. cavity filters) 20 and 21 forthe avoidance of cross-coupled interference between the collocated transmitter and receiver antennae 22 and 23.

As can be seen from Figure 2, the frequency agile transmitter/receiver (transceiver) 24 has a common antenna 25 as have the receivers 16,17,18 and 79 (ie antenna 23) and the transmitters 12,13 14 and 15 (ie antenna 22).

In orderto cancel cross-coupled interference between the transmitter ofthe transceiver 24 and the collocated fixed frequency receivers 16,17, 18 and 19 a negative feedback interference cancellation arrangement 26 is provided in a cross-coupiing path including couplers and extending between the antennae 25 and 23. The cancellation arrangement comprises a vector modulator and an associated negative feedback control loop circuit as described with reference to Figure 1.Forthe purpose of cancelling any crosscoupled interference between thefixed frequency transmitters 12,13,14 and 15 and the receiverofthe frequency agile transceiver 24 respective interference cancellation arrangements 27,28,29 and 30 similarto that indicated at 26 above are provided in respect of thetransmitters 12,13, 14and 15 and provide cross-coupling paths, including couplers, extending between these transmitters and the receiver input of the transceiver 24.

As will be appreciated, in operation ofthe system shown schematically in Figure 2 appropriate selective switching arrangements will need to be incorporated fortheselection of cross-coupling cancellation arrangements according totheparticularmodes and sequence of operations ofthe transceiver and fixed frequency transmitters and receivers ofthe system.

For example, b coupler module may be associated with the frequency agile transceiver 24 and arranged so thatthe cross-coupling cancellation arrangements 27,28,29 and 30 are by-passed when the frequency agile transmitter is selected andthe coupler module may also be arranged to supply a so-called delayed pressel signal (determining mode oftransceiver) to the frequency agile transmitter. This ensures that there is notransmission from thefrequency agile transmitterthroug h thecross-coupling paths to the fixedfrequencytransmitters 12,13, 14 and 15.

Duringtransmission by the frequency agile transmitter a sample of the transmitter output will be coupled to the cross-coupled cancellation arrange ment 26 which will be activated so as to protect the fixed frequency receivers 16, 17, 18 and 19 against interference overtheircommon receiver antenna 23.

When the fixed frequency radio equipment is operating the frequency agile receiver is protected againstinterferencefromthefourfixedfrequency transmitters 12,12,14 and 15 by the interference cancellation arrangements 27,28,29 and 30.The received signal from the frequency agile antenna 25 is passed to each of the loop controlled vector modulators of the cancellation arrangements in turn and then to each ofthe correlators of those cancellation arrangements afterwhich the signal is returned through the previously mentioned coupler module to the frequency agile transceiver 24. The fourfixed frequency transmitter sample signals are derived through couplers included in the transmitter paths.

Each of these transmitter sample signals is fed to an appertaining cancellation arrangement via a length of delay cable 31 which compensates for the delay of the cross-coupling path between transmitterand receiver antennae. Onespecificarrangementespeciallyenvis- aged for providing the sampling, coupling and switching arrangements described is shown schematically in Figure 3 ofthe accompanying drawings.

The specific form of the cross-coupling interference cancellation arrangements provided in accordance with the invention may vary but one specificform which is envisaged is shown in Figure 4 ofthe accompanying drawings. In the arrangement iliustrated the vector modulator comprises two PIN diode bridges 32 and 33 serving as weighting elements together with a quadrature phase splitter34and combiner 35. The correlator consists of circuits 36,37, 38 and 39 and the filtering ofthe correlator output is provided by video amplifiers 40 and 41 which provide the vector modulatorcontrol voltages.

A coupler 42 applies the transmitter sample signal tothe vector modulator andto one side ofthe correlatorthrougha limiter43and44. Undertransmit conditions the limiter 43 goes into limit and the output of 44constitutes the correlator reference signal. When the interference cancellation arrangement is active the vector modulator output is coupled into the receiver antenna path by the coupler 45 and the coupler 46 couples the resulting radio frequency outputto an amplifier chain 47,48 and 49. Limiters 50 and 51 operate to avoid amplifier saturation when an uncan celled signal is still in the antenna path. The output from the amplifierchain referred to provides the second input to the correlator 36,37,38 and 39 and completes the negative feedback control loop forthe vector modulator.

The control loop delay compensation cable 52 is adjustable to equalise the delays along the two paths from thetransmittersample input at terminal 53 to the correlator.

From the foregoing brief description of the specific arrangements shown in Figures 3 and4 of the accompanying drawingsitwilf bereadily understood by those skilled in the art howthe broader concept of the present invention and the benefits thereof can be realised by specific circuit configu rations.

Claims

1. In combination with radio transmitter means and radio receiver means located in relatively close proximity with one another and having different operating frequencies, an interference cancellation arrangement for cancelling cross-coupled interference between the respective antennae ofthetransmit- ter and receiver means, the cancellation arrangement comprising avectormodulatororequivalentmeans which is arranged to be fed with a sample signal from the output ofthe transmitter means which is arranged to feed to the input receiver means in dependence upon said sample signal and any correlated reference signal from the receiver means an interference cancelling signal having an amplitude and phase which effectively nullifies an interfering signal at the receiver means input.

2. An interference cancellation arrangement as claimed in claim 1, in which the sample signal from the transmitter means and the correlated reference signal from the receiver means input are fed to correlator means which provides a control signal for controlling the output from the vector modulator.

3. An interferencecancellation arrangement as claimed in claim 1 or claim 2, in which the sample and reference signals are derived from the outputofthe transmitter means and the inputto the receiver means, respectively, through coupler means and the outputfrom the vector modulator is applied to the receiver means inputthrough further coupler means.

4. In combination with a plurality of radio transmitters and radio receivers located in relatively close proximity with one another and having different operating frequencies, an interference cancellation arrangementfor ca ncel ling cross-coupled interference between antennae oftransmitters and receivers, in which the cancellation arrangement comprises a plurality of vector modulators arranged to be fed with sample signals from the transmitter outputs and arranged to feed to receiver inputs in dependence upon thetransmitteroutputs sample signals and any correlated reference signals from the receiver inputs interference cancelling signals having amplitude and phases which effectively nullify any interfering signals atthe receiver inputs.

5. An interference cancellation arrangement as claimed in claim 4, in which the sample signalsfrom the transmitters and the correlated reference signals from the receiver inputs are fed to respective correlators associated with individual vector modulators to provide control signals for controlling the outputs from the vector modulators.

6. An interference cancellation arrangement as claimed in claim 4 or claim 5, in which one of the transmitters and one ofthe receivers is constituted by afrequency-agiletransceiverwhilstothertransmitters and receivers are fixed frequency transmitters and receivers.

7. An interference cancellation arrangement as claimed in claim 6, in which cross-coupled interference between the antennae of the fixed frequency transmitters and receivers is provided by cavity filters whereas cancellation of cross-coupled interference between theantennaeofthefrequency agile transceiver and the antennae ofthefixed frequency transmitters and receivers is provided by the said cancellation arrangementincluding the vector modulators.

8. An interferencecancellationarrangementas claimed in claim 7, in which selective switching means are provided for the selection of appropriate crosscoupling cancellation paths according to the particular modes and sequence of operations ofthefrequency agile transceiver and the fixed frequency transmitters and receivers.

9. An interference cancellation arrangement as claimed in any one of claims 4to 8, in which each of the transmitter output sample signals is fed to an appertaining vector modulatorthrough a delay cable which compensatesforthedelayofthe cross- coupling path between transmitter and receiver antennae.

10. An interference cancellation arrangement sub stantially as hereinbefore described with reference to the accompanying drawings.