GB2329560A

GB2329560A - Bias control for radio transmitter

Info

Publication number: GB2329560A
Application number: GB9815732A
Authority: GB
Inventors: Andrew David Vare
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 1997-07-28
Filing date: 1997-07-28
Publication date: 1999-03-24
Also published as: GB9815748D0; GB2329554B; GB9815732D0; ZA986684B; GB2329554A; GB9715842D0

Abstract

A broadband radio transmitter comprises a vector modulator 20 which generates a radio signal by modulating I and Q components of a baseband signal with a reference frequency signal having substantially the same frequency as the carrier frequency of the radio signal. An envelope detector 30 connected to the output of the vector modulator generates a signal representative of the power in the radio signal at the reference frequency. A comparator and loop controller 34 adjust the voltage biasing conditions of the I and Q components according to the power level signal so that a component of the radio signal at the reference frequency is minimised. The transmitter may be used in a CDMA system. An arrangement for providing increased signal separation between transmit and receive antennas of a transceiver is also disclosed.

Description

RADIO TRANSMITTER CONTROLLER The present invention relates to radio transceivers which operate to both transmit and receive radio signals. In particular, but not exclusively, the present invention relates to radio transceivers, which are provided with a means to isolate signals transmitted by a transmitter of the transceiver system and received by a receiver of the transceiver system.

In modern radio communications equipment it is often a requirement to both transmit and receive radio signals. Such transmitted and received radio signals may be on different frequencies separated by a guard band. As such, in order to ensure that signals transmitted from a transmit antenna of the transceiver do not corrupt and distort signals received on a receive antenna, it is necessary to provide isolation between the transmitter and receiver of the transceiver system.

In broadband Code Multiple Access systems (CDMA), the transmit and receive signals have a relatively wide bandwidth. As such, the bandwidth of the transmit and receive signals may be comparable to the guard band separating the transmit and receive frequency bands.

In known systems, a duplexing filter is provided which is coupled to the output of the transmitter and the input of the receiver in order to effect the desired frequency separation between transmitter and receiver.

However, a disadvantage with known duplexing filter based transceiver systems is that an inherent insertion loss associated with the connection of the duplexing filter to the receiver, causes a degradation in the receiver noise figure and furthermore reduces the power with which the transmitted signal may be transmitted. Another disadvantage with duplexing filters is that impairment is caused to the received and transmitted signals as a result of in-band amplitude and group delay ripple effects caused by the duplexing filter. This, in turn leads to loss of capacity in a radio system such as a CDMA system. The impairment is more significant in broadband systems such as broadband CDMA, in which the bandwidth of signals is comparable to the receiver and transmitter frequency separations.

The aforementioned disadvantages represent a technical problem which is addressed by the present invention.

According to the present invention there is provided a radio communications transceiver which operates to transmit radio signals via a transmit antenna and receive signals via a receive antenna, wherein said transmit and receive antennas are disposed to provide at least a substantial amount of radio frequency isolation therebetween, wherein said radio communications transceiver further including a cancellation device comprising a first coupler coupled to the transmit antenna and arranged to extract a sample of said transmit signal, a second coupler coupled to the receiver antenna and arranged to extract a sample of said received signal, a control unit fed on first and second inputs with said transmit and said received signal samples respectively, said control unit operating to generate an error signal representative of a correlation of said transmit and received radio signals, which error signal is fed to a modulator which operates to modulate said transmit signal sample, consequent upon said error signal which modulated transmit signal sample which is fed to said radio receiver via a further coupler, and which modulated transmit signal sample serves to substantially cancel transmit signals received by said receiver antenna, said cancellation device operating in combination with said disposal of said transmit and receive antennas to effect a desired frequency separation between said transmit and receive signals.

The transmit and receive antennas may be disposed so as to be substantially orthogonal. The transmit and receive antennas may be separated by a short distance, thereby providing said at least substantial frequency isolation.

The transceiver may further include a filter which is arranged to provide a substantial level of isolation between transmit and receiver antennas.

In modern radio communications equipment it is often a requirement to use wide band transmitters which are arranged to transmit radio frequency signals which occupy a substantial bandwidth, such as for example, broadband CDMA signals. Wide band transmitters suffer a problem in that generation of the wide band radio frequency signal within a predetermined frequency range using a conventional super heterodyne transmitter arrangement in which an intermediate frequency signal is formed, requires the use of transmit filters to reject spurious and mirror frequency signals. Such filters are expensive and furthennore, as a result of the wide band transmit signal, are difficult to implement. Therefore, providing an inexpensive transmitter arrangement wherein a broadband radio frequency signal is generated, in a cost effective manner, represents a technical problem which is addressed by the present invention.

According to a first aspect of the present invention there is provided a transmitter for transmitting broadband radio frequency signals, comprising a vector modulator which operates to generate said broadband radio frequency signal from I and Q components of a baseband signal by modulating a reference frequency signal with said I and Q components of said baseband signal which reference frequency signal is substantially at a carrier frequency of the broadband radio frequency signal, an envelope detector connected to an output of the vector modulator and arranged to generate a signal representative of the power in the broadband radio frequency signal at said reference frequency, and a comparator loop controller which operates to adjust biasing conditions of said I and Q components in dependence upon said signal representative of the power in said broadband radio frequency signal at said reference frequency detected by said envelope detector so that a component at the reference frequency signal is substantially minimised.

By providing a transmitter with an arrangement such that the broadband radio frequency signal is generated in a single stage by modulating an appropriate reference frequency signal with the I and Q components of the baseband signal, a requirement for expensive filters and other components associated with generating the radio frequency signal from intermediate frequency signals is obviated. However, where the transmitted signal is a broadband radio frequency signal, and which radio frequency signal must vary within a predetermined bandwidth, a technical problem exists in suppressing a component of the transmitted signal at the reference or carrier frequency. By arranging for an envelope detector to detect the power at the reference frequency, and feeding back the detected power to a comparator and loop controller, which operates to adjust the voltage bias of the conductors communicating to respective modulators I and Q components, the carrier component or reference component can be reduced by correspondingly adjusting the voltage bias at which the I and Q components are communicated to the modulator.

The voltage bias of the I and Q components operates to change the power component of the transmitted signal at the reference or carrier frequency.

The transmitter may further include a low pass filter connected to an output of the envelope detector and an input to the comparator and loop controller, which low pass filter serves to average out amplitude fluctuations of the signal representative of the envelope of the broadband radio frequency signal.

The transmitter may further include a power amplifier connected between the output of the vector modulator and an input to the envelope detector.

One embodiment of the present invention will now be described by way of example only, with reference to: FIGURE 1 which is a schematic block diagram of a transceiver for radio frequency communications, FIGURE 5 which is a schematic block diagram of a transmitter circuit, wherein there is provided a controller for controlling a carrier component of the broadband radio frequency signal.

CDMA systems are fully duplexed system which means that they transmit and receive signal contemporaneously. In order to avoid the blocking of the received by the transmitter, an isolation of 40-50 dB is required between the transmitter output and the receiver input.

In known systems, the RX/IX isolation is provided by a duplexing filter which connects the transmitter output and the receiver input to a common antenna The duplexing filter also provides selectively between the received and transmit frequencies which in turn isolates the transmitter output from the receiver input and therefore prevents blocking of the receiver.

The disadvantage of the duplexing filter is that due to the inherent insertion loss of the filter degrades the receiver noise figure and reduces the transmitted power available at the antenna.

Furthermore, since the duplexing filter exhibits amplitude and group delay ripple, it impairs both the received and transmitted signals which in turn leads to loss of capacity in CDMA systems. The impairment is more significant in broadband systems such as B-CDMA in which the signal bandwidth is comparable to the receive/transmit frequency separation.

An illustrative embodiment of the present invention substantially eliminates the need for a duplexing filter and therefore all the disadvantages associated with the duplexing filters. Furthermore, since the receive and transmit separation is now not based on frequency selectivity, the receive and transmit frequencies can be allocated more closely, which lead to more flexible network planning.

A method of transmitting and receiving signals contemporaneously using two antennas separated by a short distance or two co-located but orthogonal antennas, comprises the steps of measuring the inherent coupling between two antennas, using a cancellation device connected between the two antennas to form a cancellation path, and a control device which operates to control the cancellation device in accordance with the measured coupling so that the cancellation path is equal in magnitude and opposite in phase of the coupling between the two antennas.

An example embodiment of the present invention is provided in figure 1. In figure 1, a CDMA transmitter 1 is shown to feed a transmitter 8 with I and Q components of the CDMA signal.

A transmitter coupler 12, connected to the transmit antenna 2, removes a sample of the transmit signal. The sample of the transmit signal represents the transmitted CDMA signal but with a small part of the total power. The transmit signal sample is fed to a vector modulator 20, and a correlator 22.

The line 14, in Figure 1 is representative of a coupling between the transmit and received antennas. This coupling has an effect of causing the transmit signal to be present at a front end of the receiver, as a result of coupling between the transmit and receive antenna. As such a component of transmit signal detected by receiver 6. A representative sample of the receiver signal may be thereby taken from the receiver and fed to a second input of the correlator 22. The correlator 22, serves to generate error signals Vl, V2, representative of I and Q components of a correlation between the transmit signal sample fed on a first input to the correlator 22, and the received signal fed on a second input to the correlator. The error signals Vl, V2, are fed to first and second inputs of a vector modulator 20, with the transmit signal sample fed to a third input of the vector modulator 20. The vector modulator 20, serves to modulate the transmit signal sample, with the complex error signal Vl, V2. The modulated transmit signal sample is thereafter fed to the receiver 6, via a coupler 28. By arranging for this correlation apparatus, in combination with a separation of the transmitter and received antennas, an isolation of the transmitter and receiver can be effected without a requirement of a duplexing filter.

Figure 5 provides an example embodiment of a transmitter for broadband CDMA signals where the radio frequency signal is generated in a single stage. In Figure 5, a vector modulator 20, is fed with I and Q components of a baseband CDMA signal on conductors 22, 24. The resulting radio signal is fed to an amplifier 26 and then to a transmit antenna 28. Also connected to an output of the power amplifier 26 is an envelope detector 30, which in this example embodiment is a diode operating as a rectifier. The envelope detector 30 is connected to a low pass filter 32. The output of low pass filter 32 feeds a comparator and loop controller 34. The comparator and loop controller 34 generates first and second output signals on conductors 36, 38, which are connected to the conductors 22, 24, respectively.

The envelope detector 30 rectifies the RF envelope and the low pass filter 32 averages out the amplitude varying components. Broadband CDMA used in CDMA link and also the IS95 system waveforms passes a high proportion of amplitude modulation, this means that through correct choice of the low pass filter 32 cut off, the dc voltage fed to the comparator and loop controller 34 contains a component due to the modulation and a component due to any non time varying signal such as carrier leakage. Thus, for a given output power, the input to the comparator and loop controller 34 is a voltage proportional to the carrier leakage plus and offset due to the modulation.

The error signal is processed by the comparator and loop controller 34 and it produces two dc levels that are used to dc offset the I and Q inputs of the I and Q vector modulator 20.

DC levels at the inputs to the I and Q vector modulator 20 offset the carrier impression, thus if the name of the feedback is correct, the comparator and loop controller will adjust the I and Q input level to reduce the error signal to a minimum, thus reducing the carrier level at the output of the modulator.

As will be appreciated by those skilled in the art, the present invention finds application with other radio systems and with a variety of modulation schemes besides CDMA.

Claims

1) A radio transmitter control system comprising a transmitter for transmitting broadband radio frequency signals, a vector modulator which operates to generate said broadband radio frequency signal from I and Q components of a baseband signal by modulating a reference frequency signal with said I and Q components of said baseband signal which reference frequency signal is substantially at a carrier frequency of the broadband radio frequency signal, an envelope detector connected to an output of the vector modulator and arranged to generate a signal representative of the power in the broadband radio frequency signal at said reference frequency, and a comparator loop controller which operates to adjust voltage biasing conditions of said I and Q components in dependence upon said signal representative of the power in said broadband radio frequency signal at said reference frequency detected by said envelope detector so that a component of the reference frequency signal is substantially minimised.

2) A radio transmitter control system as claimed in Claim 1, wherein said broadband radio frequency signal is generated in a single stage.

3) A radio transmitter control system as claimed in Claims 1 and 2, wherein said transmitter includes a low pass filter connected to an output of said envelope detector and an input to said comparator and loop controller, and serves to average out amplitude fluctuations of said signal representative of the envelope of said broadband radio frequency signal.

4) A radio transmitter control system as claimed in any preceding Claim, wherein said transmitter includes a power amplifier connected between an output of said vector modulator and an input to said envelope detector.

5) A radio transmitter control system as hereinafter described with reference to the accompanying drawings.