GB2270444A

GB2270444A - Improved cross-coupled interference cancellation in a transmitter/receiver

Info

Publication number: GB2270444A
Application number: GB9218857A
Authority: GB
Inventors: Ian Michael Saunders
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 1992-09-05
Filing date: 1992-09-05
Publication date: 1994-03-09
Anticipated expiration: 2012-09-05
Also published as: GB2270444B; GB9218857D0

Abstract

The cancellation system including a weighting network 6 via which a sample of a transmitted signal is fed from the radio transmitter to the co-located radio receiver weighting coefficients of the network being controlled via a feedback loop in dependence upon levels of interference from the transmitter such that the interference received by the receiver tends to be nullified. The loop includes a filter (11), whose time constant is arranged to be set in accordance with the level of interference. This arrangement reduces transients at the beginning of a fast rise-time interfering signal. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO RADIO COMMUNICATIONS SYSTEMS This invention relates to radio communications systems, and more especially it relates to co-located transmitters and receivers.

When radio receivers are located in relatively close proximity to radio transmitters, the receivers can experience severe reception problems caused by transmission signals which are coupled into the receive antenna. This can occur even when systems are operating at different frequencies. The problems are however, exacerbated when the output waveform of a local transmitter has a fast rate of change, such as that from a frequency agile transmitter which may have a rise time of about 10 ills, for example.

As explained in GB patent specification 2171879, one solution is to employ an interference cancellation system in the receive antenna path which adds an anti-phase sample of the interference to the receiver antenna to effect interference cancellation. However, when used against a fast rise time interferring signal, such systems exhibit a transient of uncancelled signal at the output due to their limited speed of response. This occurs because cancellation does not take place until the transmit sample approaches its maximum value thereby providing sufficient loop gain for the cancellation process. If the cancellation loop is given a higher gain, such that cancellation occurs with low levels of transmit sample, the system tends to become unstable when a higher level is reached.

It is an object of the present invention to provide an interference cancellation system for co-located transmitters and receivers wherein efficient interference cancellation is effected even when a local transmitter has a fast rate of change such as might be found with a frequency agile transmitter.

According to the present invention a co-located radio transmitter and receiver comprising a cancellation system, including a weighting network via which a sample of a transmitted signal is fed from the radio transmitter to the co-located radio receiver, weighting coefficients of the said network being controlled via a feedback loop in dependence upon levels of interference from the transmitter such that the interference received by the receiver tends to be nullified, characterised in that the said loop includes a filter, the time constant which is arranged to be set in accordance with the level of interference.

By providing a cancellation system with a loop filter having a time constant the value of which is controlled in dependence upon the level of the transmit sample, the bandwidth of the cancellation system can be reduced at higher levels of transmit sample, thereby maintaining loop stability. This is in effect equivalent to increasing the loop time constant. By using a cancellation system with an adaptive time constant, the cancellation system can be arranged to have a faster speed of response even at low levels of transmit sample, such that the interferring signal is continually cancelled on its rising edge, thereby reducing any output cancellation transient to a minimum.

One embodiment of the invention will now be described by way of example only, with reference to the accompanying drawing, which is a block schematic diagram of a co-located transmitter and receiver having an interferrence signal cancellation arrangement.

Referring now to the drawing, the arrangement comprises, 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 or frequency agile receiver having an associated transmitter (not shown). The transmitter 1 and the receiver 2 may form 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 one another which can give rise to cross-coupled interference problems between the co-located transmitter and receiver.

For the purpose of cancelling any cross-coupled interference between the transmitter 1 and the receiver 2, a sample of the transmitter output signal derived through a coupler 5 and constituting a reference signal 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 output signal which is applied over the cross-coupling path to the input of the receiver 2 via a coupler 7 for nullifying any cross-coupled interferring signal received by the receiver antenna 4 .For the purpose of controlling the amplitude and phase of the output signal to effect cancellation of the interferring signal in the receiver input, the vector modulator 6 is controlled by means of an associated negative feedback loop which derives from the receiver input through a coupler 8 and an amplifier A2 a sample signal which may include an interferring signal. A fast operating correlator 9 serves to correlate the signal sample derived from the receiver input and a reference signal derived from the transmitter through the coupler 5 and a further coupler 10. A vector modulator control signal from the correlator 9 is fed through a low pass filter 11 via an amplifier Al to the vector modulator 6 in order to control the amplitude and phase of the interference cancellation signal.It will be appreciated that the correlator 9 and low pass filter 11 effectively carry out a multiplication of the reference and control signal inputs to the correlator 9 and the latter will apply an input to the vector modulator 6 in respect of any input to the receiver 2 which correlates with the transmitter sample or reference signal.

This feedback loop is thus arranged to provide negative feedback so that any signal at the receiver input which correlates with the transmitter reference or sample signal will be reduced by a factor, very nearly equal to the loop gain, to a level significantly below the so-called out-of-band desensitisation level of the receiver 2.

It may be mentioned that the cross-coupled cancellation arrangement operates mainly on the carrier of the transmitted signal and any associated noise of close frequency. Spurious signals and wideband noise may not be cancelled and these may be suppressed at source as by the addition of suitable filters.

One of the problems with the arrangement thus far described is that with interferring signals having a fast rise time cancellation can be less than perfect due to the limited speed of response of the cancellation loop. This limitation occurs because cancellation does not take place until the transmit sample approaches its maximum value, thereby providing sufficient loop gain for the cancellation process. If the cancellation loop is arranged to have a higher gain such that cancellation occurs with low levels of transmit sample the system becomes unstable when higher levels are reached. In order to overcome this problem the time constant of the filter 11 is controlled by means of a signal on a line 12 which is derived via an amplifier A3 and represents a sample of the interferring signal. Thus the loop time constant is arranged to be set adaptively in dependence 1 upon the level of the input reference signal, which enables the cancellation system to produce low levels of cancellation transient when operating with interferring signals having a fast rise time. Various modifications may be made to the arrangements shown without departing from the scope of the invention and for example the invention may be applied to systems as described in detail in GB patent 2171879.

Claims

1. A co-located radio transmitter and receiver comprising a cancellation system, including a weighting network via which a sample of a transmitted signal is fed from the radio transmitter to the co-located radio receiver, weighting coefficients of the said network being controlled via a feedback loop in dependence upon levels of interference froni the transmitter such that the interference received by the receiver tends to be nullified, characterised in that the said loop includes a filter, the time constant which is arranged to be set in accordance with the level of interference.

2. A co-located transmitter and receiver as claimed in Claim 1 wherein a sample of the transmitted signal is derived via a correlator and fed back to control the weighting coefficients via the filter.

3. A co-located transmitter and received as claimed in Claim 2 wherein a signal derived from the sample is used to control the time constant of the filter.

4. A co-located transmitter and receiver as claimed in Claim 3 wherein the correlator is arranged to correlate the sample against a signal derived via the receiver.

5. A co-located transmitter and receiver arrangement substantially as herein before described with reference to the accompanying drawing.