GB2267015A

GB2267015A - Interference cancelling a radio transceiver

Info

Publication number: GB2267015A
Application number: GB9210530A
Authority: GB
Inventors: Zalman Schwartzman; Tal Mor; Avraham Kopelman; Joseph Bercovich
Original assignee: Motorola Israel Ltd
Current assignee: Motorola Solutions Israel Ltd
Priority date: 1992-05-16
Filing date: 1992-05-16
Publication date: 1993-11-17
Anticipated expiration: 2012-05-16
Also published as: GB2267015B; GB9210530D0

Abstract

A radio transceiver has a receiver part (13, 14, 15, 16) using an injection frequency e.g. 806 MHz equal to the transmit frequency of a corresponding transmitter part, the latter including a modulator (10), synthesizer (11) and power amplifier (12), and modulates the injection frequency with an information signal when the power amplifier (12) is in an off state and the transceiver should not be transmitting, and if a signal is received from a radio infrastructure system, e.g. a base station, in response to the information signal, this indicates interference at a base site due to leakage from the injection frequency elements of the transceiver. Appropriate action is taken at the transceiver to stop the interference e.g. shutting off the radio for a certain period. The transceiver is used in radio trunking systems having a common control channel. <IMAGE>

Description

INTERFERENCE CANCELLING IN A RADIO TRANSCEIVER Background of the Invention This invention relates to a radio transceiver in which a receiver part uses an injection frequency equal to the transmit frequency of a corresponding transmitter part. The invention is particularly applicable to radio trunking systems.

Summary of the prior art Many radio trunking systems are based on the concept that a central control system uses a control channel common to all mobile radios. The control channel is used to communicate digital requests and voice channel assignments from mobile to system controller and vice versa. All idle mobile units are in receive mode on the control channel.

It has been found cost effective in full duplex mobile design to use a first IF centre frequency equal to the frequency spacing between the transmit and receive frequencies. This allows the use of a single frequency source (a synthesizer or local oscillator) for both transmitter driver and first mixer injection. A problem exists, however, in that when the radio is in idle mode, radiation from the first mixer injection may interfere on the control channel if the mobile unit is in close proximity to a system site (trunking base station). This can have the disastrous effect of blocking the entire site, because there is often only one control channel and all other units in the site need access to the control channel on which the interference is taking place.

Extensive shielding can be used to reduce the interference to a minimum, but this is bulky and expensive.

In existing trunking systems the system has no means to identify the interfering mobile. If the system attempts to switch to another frequency for the control channel, it commands all mobiles to switch to the same frequency and the interfering radio itself switches to the new frequency and interference continues, because the interfering radio simultaneously adopts a new injection frequency equal to the new control channel frequency.

Summarv of the Invention According to the present invention, a radio transceiver is provided comprising: a transmitter part having a synthesizer for providing a transmit signal and a power amplifier for amplifying the transmit signal and transmitting it through an antenna; and a receiver part comprising a mixer for injecting an injection frequency equal to the transmit frequency (thereby providing a receive intermediate frequency equal to the duplex separation frequency). Means are provided for modulating the injection frequency with an information signal while the power amplifier is in an off condition and detecting means are provided for detecting whether a signal is received from the infrastructure of the radio system in response to the information signal, thereby indicating injection frequency leakage and interference.

In trunking terminology, the information signal may be a "go to" channel signal and the injection frequency is modulated with this signal while the radio is in standby mode.

In response to detecting of whether a signal is received on the radio infrastructure system, the transceiver can be shut off or its injection frequency can be changed so as to cause it to be different to the control channel frequency.

This has the effect of switching the radio to another channel, for example a traffic channel so that it is no longer interfering on the control channel (and, of course, it will be no longer be able to receive control channel commands). The switching to another channel can be a temporary switch i.e the "interfering" radio returns to the control channel after a time-out period. During this period, the user of the interfering radio has the opportunity to move away from the base station being interfered with. In the meantime, it is not such a major problem that he may be interfering on a traffic channel, because there will be other traffic channels available for use.

The information signal used for modulating the injection frequency may be a request for channel grant command.

Accordingly, if the transceiver is in standby mode (with its power amplifier off) and it receives a channel grant command, this indicates that there must be interference at a nearby base station. The channel grant command contains the ID of the transceiver in question. From this, the transceiver knows" that the channel grant command has been made in response to an interfering signal from it's injecting signal.

Brief Description of the Drawings Figure 1 shows a full duplex radio with single synthesizer in accordance with the preferred embodiment of the present invention and Figure 2 shows a flow diagram showing the operation of the radio of figure 1.

Detailed Description of the Preferred Embodiment Referring to figure 1, there is shown a trunking radio transceiver comprising a transmitter part, comprising a modulator 10, a synthesizer 11 and a power amplifier 12 and a receiver part comprising a front end amplifier 13, a first mixer 14, a first IF stage 15 and a demodulator 16. The power amplifier 12 and front end amplifier 13 are coupled to an antenna 20 by means of a duplexer 21.

The synthesizer 11 is preferably a fractional-N synthesizer having a VCO locked to a reference source. The output frequency of the synthesizer is, by way of example, 806 MHz. This signal is coupled to the power amplifier and the mixer 14. During transmission mode, it is amplified by the power amplifier 12 and transmitted through the antenna 20 and at the same time it is injected into the mixer 14 and mixed with a 851 MHz receive signal from the front end amplifier 13, to provide a 45 MHz first IF frequency signal for the first IF stage 15. The demodulator 16 has a degree of transmit modulation cancellation, thereby cancelling to a degree the transmitted signal modulated on the injection path to the mixer 14.The modulation cancellation is only partial and in full duplex, a degree of side tone is quite acceptable, causing the user to hear through the ear-piece a degree of his own voice or other signal in the microphone.

Users find this a comforting feature during operation.

During standby mode, the power amplifier 12 is in an off condition and the synthesizer 11 is still providing an output signal of 806 MHz for injection into the mixer 14, thereby the transceiver is able to receive signals on 851 MHz. 851 MHz is a suitable frequency for a control channel and accordingly, in standby mode, the transceiver is able to receive the control channel.

Referring again to full duplex operation, when making a transmission, the user presses his push to talk button (PTT) and a channel grant request command is modulated by the modulator 10 and output at 806 MHz by the synthesizer 11 and amplified by the power amplifier 12. This signal is received by a base station and in return a channel grant command is transmitted at 851 Mflz on the control channel. This is received by the transceiver and causes the transceiver to switch to a traffic channel. The traffic channel may, for example, be 100 KHz separated from the control channel and in switching to the traffic channel, the synthesizer 11 moves its output through 100 KHz causing both the transmit and receive frequencies to change through 100 KHz simultaneously.

The first IF frequency always remains at 45 MHz.

When the transceiver enters standby mode, the steps shown in figure 2 are carried out. The power amplifier is switched off (step 101) and the synthesizer 11 is modulated with a request for channel grant command. This is a sort of "dummy transmission". If, in step 103, a "channel grant" command together with the ID of the unit in question is received, the transceiver shuts down in step 104 for a predetermined period of time. After a time-out (step 105), the process returns to step 102 and another request to channel grant command is transmitted. As an when no channel grant command is received, the radio waits for a delay period (106) and repeats the dummy transmission of step 102. If, at any time in the process of figure 2, a "go to channel" command is received, the process is interrupted and the radio responds to the go to channel command by changing its synthesizer frequency to the channel instructed and the radio is able to receive an incoming call.

In the radio transceiver described and claimed, the unit senses its proximity to the system site. If it is too close to a site, it shuts itself down, for example for a few minutes.

In this way, a very inexpensive synthesizer and IF design can be employed and a low degree of screening can be incorporated, so that if the unit is 100 to 200 metres from a site and causes interference from its injection frequency signal, it avoids blocking the control channel for the entire site by shutting down or moving to a different frequency.

Claims

1. A radio transceiver comprising: a transmitter part having a synthesizer for providing a transmit signal and a power amplifier for amplifying the transmit signal and transmitting it throught an antenna and a receiver part comprising a mixer for injecting an injection frequency equal to the transmit frequency characterised by means for modulating the injection frequency with an information signal while the power amplifier is in an off condition and detecting means for detecting whether a signal is received from a radio infrastructure system in response to the information signal.

2. A transceiver in accordance with claim 1 comprising means for shutting off the radio in response to the detecting means when a signal is received from the radio infrastructure system in response to the information signal.

3. A radio transceiver in accordance with claim 2 wherein the means for shutting off the radio off operates for a timeout period only.

4. A radio transceiver in accordance with claim 1 comprising means for changing the injection frequency in response to the detecting means when a signal is received from the radio infrastructure system in response to the information signal.

5. A radio transceiver according to claim 4 wherein the means for changing the injection frequency changes said frequency back to said transmit frequency after a time-out period.

6. A radio transceiver in accordance with any one of the preceding claims, wherein the means for modulating the injection frequency with an information signal comprises means for modulating it with a request for channel grant signal.

7. A radio transceiver in accordance with any one of claims 1 to 5, wherein the means for modulating the injection signal comprises means for modulating it with a unique command and wherein a radio system infrastructure transceiver is provided comprising means for responding to said unique command with a unique response indicating detection of interference.