GB2316560A - Improving the linearity of a transmitter amplifier using a pilot tone and feedback - Google Patents

Abstract

An amplifier circuit (10) includes an amplifier input for receiving a modulated input signal. A tone generator circuit is operably coupled to the amplifier input for incorporating a tone into the modulated input signal. An amplifier (18) receives the modulated input signal and tone and provides an amplified representation of the modulated input signal and tone. A tone receiver (22) is operably coupled to the amplifier (18) for extracting the tone from the amplified representation of the modulated output signal and tone. A processor is operably coupled to the tone receiver (22) for receiving and processing the extracted tone and adjusting the amplified representation of the modulated signal dependent upon the processed extracted tone.

Description

AMPLIFIER CIRCUIT, TRANSMITTER CIRCUIT AND METHOD OF OPERATION THEREFOR Field of the Invention This invention relates to amplifier technology. The invention is applicable to, but not limited to, amplifier technology for use in radio transmitters, and in particular in the linearisation of radio transmitters.

Background of the Invention Continuing pressure on the limited spectrum available for communication systems is forcing the development of spectrally-efficient linear modulation schemes. Since the envelopes of these linear modulation schemes fluctuate, intermodulation products can be generated in the non-linear power amplifier. Specifically in the private mobile radio (PMR) environment, restrictions on out-of-band emissions are severe (to the order of-60 to -70 dBc) and therefore linear modulation schemes require highly linear transmitters.

Linear modulation schemes require linear amplification of the modulated signal in order to minimise undesired out-of-band emissions.

The actual level of linearity needed to meet particular out-of-band emission limits is a function of many parameters, the most critical of which are modulation type and bit rate. Quantum processes within a typical RF amplifying device are non-linear by nature. Only when a small portion of the consumed DC power is transformed into RF power, can the transfer function of the amplifying device be approximated by a straight line, i.e. as in an ideal linear amplifier case. This mode of operation provides a low efficiency of DC to RF power conversion which is unacceptable for portable units.

The emphasis in portable PMR equipment is to increase battery life.

Hence, such operating efficiencies of the amplifiers used must be maximised. To achieve both linearity and efficiency, so called linearisation techniques are used to improve the linearity of the more efficient amplifier classes, for example class AB, B or C amplifiers. One such linearising technique often used in designing linear transmitters is Cartesian Feedback. This is a "closed loop" negative feedback technique which 'sums' the baseband feedback signal in it's digital "I" and "Q" formats with the "I" and "Q" input signals prior to amplifying and upconverting this signal to it's required output frequency and power level.

All linearisation techniques require a finite amount of time in which to linearise a given amplifying device. The "linearisation" of the amplifying device is often achieved by applying a training sequence to the amplifying device in order to determine the levels of phase and gain distortion introduced by the device. There are a number of benefits in using a training sequence, for example faster convergence of the linearisation algorithm, lower processing power requirement and a more stable and more efficient operation of the transmitter due to increased knowledge of the device characteristics.

Cartesian feedback linearisation is one technique that has been proposed for implementing a TETRA (Trans. European digital trunked radio). To linearise the transmitter prior to transmitting data or voice, phase and amplitude training of the transmitter is required to achieve linearity and stability. This is accommodated within a Common Linearisation Channel (CLCH) as is described in UK Patent Application No. 9222922.8 filed on 2 November 1992 (International Patent Application No. PCT/EP93/07243). The CLCH frame occurs both in the traffic channels and in the control channel to allow a radio to train prior to gaining access to the system. Hence, in order to continuously maximise the performance of the device, regarding stability, efficiency, linearity etc., an appropriate training sequence needs to be inserted over regular time intervals in a communication channel.

However, this requirement to use the amplifying device for training purposes impacts the time available for transmitting actual messages on the communication channel. Obviously, linearisation techniques requiring a training sequence are unsuitable for applications where no provision in a time frame structure has been made for this purpose. An undesirable alternative would be for linearised transmitters to transmit excessive levels of adjacent channel interference during linearisation periods or to operate with much lower smplifying efficiency.

This invention seeks to provide an improved transmitter design, and method of operation.

Summarv of the Invention In a first aspect of the present invention, an amplifier circuit is provided. The amplifier circuit receives a modulated input signal. A tone generator circuit is operably coupled to an amplifier input for incorporating a tone into the modulated input signal. The amplifier receives the modulated input signal and tone and provides an amplified representation of the modulated input signal and tone. A tone receiver is operably coupled to the amplifier for extracting the tone from the amplified representation of the modulated input signal and tone. A processor is operably coupled to the tone receiver for receiving and processing the extracted tone and adjusting the amplified representation of the modulated signal dependent upon the processed extracted tone.

In this manner, the introduction of a tone-in-band signal fed into the amplifier enables the performance of the amplifier to be continuously optimised using the information obtained from the extracted tone.

In a second aspect of the present invention, a transmitter circuit is provided. The transmitter circuit includes the amplifier circuit described in the first aspect of the present invention circuit for receiving an input signal and for providing a modulated output signal. A tone generator circuit is operably coupled to the modulation circuit for incorporating a tone into the modulated output signal. An amplifier is provided for receiving the modulated output signal and tone and for providing an amplified representation of the modulated output signal and tone. A tone receiver is operably coupled to the amplifier for extracting the tone from the amplified representation of the modulated output signal and tone. A processor is operably coupled to the tone receiver for receiving and processing the extracted tone and adjusting the amplified representation of the modulated signal dependent upon the processed extracted tone.

In this manner, the performance of the transmitter is continuously optimised using the information obtained from the extracted tone. In particularly, and in a preferred embodiment of the invention, the linearity performance of a linearised transmitter can be continuously optimised to improve the operating efficiency of the transmitter.

In a third aspect of the present invention a method for optimising an amplifier circuit is provided. The amplifier circuit includes a tone generator circuit operably coupled to an amplifier input of an amplifier, whose output is connected to a tone receiver and a processor. The method includes the steps of combining a tone with a modulated input signal, amplifying the combined tone and modulated input to provide an amplified representation of the modulated input signal and tone and receiving the amplified representation of the modulated input signal and tone at the tone receiver. The tone is extracted from the amplified representation of the modulated input signal and tone and the extracted amplified tone processed. The amplified representation of the modulated signal is adjusted dependent upon the processed extracted tone.

In this manner, preferably using the described steps, the performance of the amplifier is continuously optimised using the information obtained from the extracted tone.

In a fourth aspect of the present invention the method for optimising an amplifier circuit is provided within a method of optimising a transmitter circuit. Preferably the transmitter circuit is a linearised transmitter circuit. The method includes the step of adjusting the amplified representation of the modulated signal to linearise the transmitter circuit output.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the drawings.

Brief Description of the Drawings FIG. 1 is a conceptual block diagram of an amplifier circuit according to a preferred embodiment of the invention.

FIG. 2 is a block diagram of a linearised transmitter circuit, according to a preferred embodiment of the invention.

FIG. 3 is a graph showing a baseband response of a tone frequency according to a preferred embodiment of the invention.

FIG. 4 is a graph showing a frequency response of a modulated signal with the tone frequency according to a preferred embodiment of the invention.

FIG. 5 is a graph showing a frequency response of a modulated signal with a non-optimised tone frequency.

FIG. 6 is a graph showing adjacent channel coupled power versus symbol rate for various third order intercept points of a power amplifier in a linearised transmitter according to a preferred embodiment of the invention.

FIG. 7 is a graph showing adjacent channel coupled power versus symbol rate for various third order intercept points of a power amplifier in a linearised transmitter with a non-optimised tone frequency.

FIG. 8 is a flowchart showing a method for optimising the performance of the amplifier circuit or transmitter circuit according to a preferred embodiment of the invention.

Detailed Description of the Drawings Referring first to FIG. 1, a conceptual block diagram of an amplifier circuit 10, according to a first aspect of the preferred embodiment of the invention, is shown. The amplifier circuit 10 includes a summing junction 14 having two inputs and an output where the output is connected to an amplifier 18. A feedback path 20 is provided to couple a portion of an output signal from the amplifier 18 to a tone-in-band receiver 22. The tonein-band receiver 22, for example a digital phase lock loop or digital filtering arrangement, is connected to a control logic function 24, which in turn is connected to the amplifier 18, to complete a feedback loop.

In operation, a digital data sequence 12 is input to the summing junction 14 where it is combined with a tone-in-band signal 16. The combined digital data sequence 12 and tone-in-band signal 16 is input to the amplifier 18 where it is amplified and frequency translated. A portion of the output from the amplifier 18 is coupled into the feedback path 20 and input to the tone-in-band receiver 22 where the tone-in-band signal 16 is extracted from the combined digital data sequence 12 and tone-in-band signal 16. The control logic function 24, processes the extracted tone-inband signal 16, to characterise the effects of the amplifier 18 on the digital data sequence 12 for transmission. The control logic function 24, then adjusts parameters in the amplifier 18, using the extracted tone information, to ensure the desired amplifier output, for example a linear output, from the amplifier 18.

A preferred example of the use of the tone-in-band technique is described below. The tone-in-band signal 16, for example a sinusoid, together with the digital data sequence 12 are input to an amplifier 18.

Both signals are amplified and distorted by the amplifier due to the non ideal characteristic. The sinusoid tones are placed on both sides of the carrier (centre) frequency, so that the output signal contains crossproducts which are a measure of the amplifier linearity. The output signal is coupled to the processing unit where the level of cross products is monitored and the amplifier parameters are adapted accordingly. In addition to monitoring the cross product levels it is possible to measure phase changes of the sinusoid tones. Hence, using a tone-in-band technique, both amplitude modulated to amplitude modulated (AM-AM) effects and amplitude modulated to phase modulated (AM-PM) effects of the linear amplifier are monitored and compensated for. Advantageously, this characteristic enables the tone-in-band technique, proposed herein, to be used for any digital or analogue modulation scheme.

In this manner, the undesirable effects of the non-ideal amplifier 18 are counteracted for by adjustment of at least one operational parameter of the amplifier, for example amplifier bias voltage level. In addition, the introduction of a tone-in-band signal 16 does not affect the composition of the digital data sequence 12 for transmission. As such the tone-in-band signal 16 can be introduced whenever required to optimise the linearity performance of the amplifier output. This enables the amplifier output to continuously operate at maximum efficiency, which is not possible with all current amplifiers, in particular when the amplifier is used as part of a linearised transmitter architecture, as described in FIG. 2.

Referring now to FIG. 2, a block diagram of a Cartesian feedback linearised transmitter circuit 29 of a digital radio, according to a second aspect of the preferred embodiment of the invention, is shown. The invention is described in detail with regard to the Cartesian feedback linearised transmitter circuit 29, but it is within the contemplation of the invention, that the particular inventive concept described herein, is applicable to any amplifier tuning method and in particular to all linearised transmitter architectures. FIG. 2 shows a lineariser and tone combiner function 30 having a modulation circuit 32, a baseband amplifier circuit 34, a baseband combining circuit 36 and a tone combining circuit 38. The lineariser and tone combiner function 30 receives an input from a tone generator circuit 52 and a microprocessor 56. An output of the lineariser and tone combiner function 30 is connected to an up-converter 40, which in turn is connected to a power amplifier 42. The power amplifier 42 is connected to an antenna 44 for transmitting a linear output signal. A feedback path 46 is connected to the output of the power amplifier 42 for coupling a portion of the linear output signal to a downconverter 48. The down-converter 48 is connected to the microprocessor 56 and to the lineariser and tone combiner function 30 to complete a real-time feedback loop. Furthermore, the down-converter 48 receives an input from the lineariser and tone combiner function 30 via a phase adjustment function 50. The microprocessor 56 receives an extracted tone signal from a tone receiver 54 which receives a second portion of the linear output signal via the feedback path 46.

In operation, the lineariser and tone combiner function 30 receives digital baseband 'I' and 'Q' signals in quadrature to each other. The digital baseband signals are input to the modulation circuit 32 where they are modulated, amplified in the baseband amplifier circuit 34, combined in the baseband combining circuit 36 with a feedback signal and combined with a tone in the tone combining circuit 38 to produce a compound baseband modulation signal. In the preferred embodiment of the invention, the tone is a digital tone generated in the tone generator circuit 52, but it is within the contemplation of the invention that it is equally applicable to other forms of tone or tones such as analogue generated tones. It is also within the contemplation of the invention that the functions of modulation, amplification and combining are performed in any order, as known, in general principles, to those skilled in such art.

The compound baseband modulation signal is input to the up-converter 40 where the signal is frequency translated to a desired output frequency and applied to the power amplifier 42 in order to generate the desired output signal from the antenna 44. A portion of the desired output signal is coupled from the output of the power amplifier via the feedback path 46 to the down-converter 48. The down-converter 48 receives a phase adjustment value from the phase adjust function 50 to adjust the phase of the feedback signal to ensure negative feedback is achieved when the down-converted signal is passed to the lineariser and tone combiner function 30. A tone receiver 54 is included in path 'A' to extract the tone generated by the tone generator 52, subsequently distorted by the power amplifier 42.

It is within the contemplation of the invention that this tone extraction may be performed as a function in the microprocessor 56, the lineariser and tone combiner function 30, as shown in path 'B' or indeed any other appropriate receiver arrangement, for example a receiver of a transceiver wherein the transmitter of the present invention is the corresponding transmitter part of the transceiver.

In the preferred embodiment of the invention, the extracted tone is processed in the microprocessor 56, to determine the effects of the power amplifier on the signal to be transmitted. These effects include, amongst others, phase and amplitude distortion introduced into the signal by the power amplifier 42. The microprocessor 56 uses the information on the extracted tone to control the characteristics of the transmit output signal, and, preferably, to make a linearisation adjustment of the transmitter output signal. In the preferred embodiment of the invention, this is achieved by controlling the amplification parameter in the baseband amplifier circuit 34 and the phase adjustment value provided by the phase adjust function 50. However, it is within the intention of the present invention that such control of the transmit output signal is performed by adjustment of at least one operational parameter of the transmitter and can be administered in other ways, for example bias control of the amplifier bias voltage level of the power amplifier thereby adjusting the output power level from the power amplifier or control of gain factors associated with the up-converter 40 and down-converter 48. Such adjustment is performed in a real-time manner, so as to simultaneously facilitate the continuous feedback operation of the Cartesian feedback transmitter.

Referring now to FIG. 3, a graph 58 showing a baseband response of a tone frequency is provided, according to the preferred embodiment of the invention. The graph 58 shows a response of a raised cosine shaped pulse 60 and orthogonal tone 62, presented as I sin(x) I. The raised cosine filter used to generate the response was of 20 symbols duration, with the roll-off set to 0.35. The frequency of the tone was chosen to minimise inter-symbol interference (ISI) when introduced with the data stream (or in this case the raised cosine shaped pulse).

In this manner, a sinusoid crosses the zero crossing point whenever a digital signal passes through its maximum, as shown in FIG. 3.

Previously, tone-in-band techniques have been used for the purpose of synchronisation and/or channel sounding, but until now their application and associated benefits have not been applied to optimising amplifier performance and in particular for applying to linearising transmitter technology as described.

Referring now to FIG. 4, a graph 70 showing a frequency response of a modulated signal with a tone frequency is provided, according to the preferred embodiment of the invention The graph 70 shows the output spectrum of a modulated signal 74, in this case a X1/4 differential quadrature phase-shift keyed (DQPSK) signal. A tone 76 has been introduced into the modulated signal 74, as shown, within the desired frequency band, in this case a 25 KHz frequency band with the adjacent channel of the upper side being positioned at 12.5 KHz from the centre frequency 72, as shown. The tone-in-band amplitude level has been set so as not to affect the adjacent channel coupled power (ACCP) performance of the transmitter, which is a critical performance parameter in such transmitters. The ACCP in the graph 70 is shown to be approximately - 60 dBc (relative to the desired output signal power). The cross-product 78 at (fo+2df) is barely noticeable, with the raised cosine filter again being set to 20 symbols duration, with the roll-off set to 0.35.

Referring now to FIG. 5, a graph 80 showing a frequency response of a modulated signal with a non-optimised tone frequency, is provided. The graph 80 shows the output spectrum of a modulated signal 84, again a 7t/4 differential quadrature phase-shift keyed (DQPSK) signal. A tone 86 has of higher amplitude level than the tone 76 of FIG. 4 has been introduced into the modulated signal 84, as shown. The desired frequency band is again a 25 KHz frequency band with the adjacent channel of the upper side being positioned at 12.5 KHz from the centre frequency 82, as shown. The tonein-band amplitude level has been set too high and affects the adjacent channel coupled power (ACCP) performance of the transmitter. The ACCP in the graph 80 is shown to be approximately - 50 dBc (relative to the desired output signal power). The cross-product 88 at (fo+2df) is much more noticeable than the corresponding cross-product 78 of FIG. 4, due to the amplitude level of the tone. The raised cosine filter is again set to 20 symbols duration, with the roll-off set to 0.35.

Hence, the graphs of FIG. 4 and FIG. 5 highlight the fact that the introduction of a tone of an appropriate amplitude level to the transmitted signal has a negligible detrimental affect to the output spectrum and thereby transmitter performance, whereas a tone set at too high an amplitude level will affect the output spectrum.

Referring now to FIG. 6, a graph 90 showing adjacent channel coupled power versus symbol rate for various third order intercept points of a power amplifier in a linearised transmitter is provided, according to the preferred embodiment of the invention. The graph 90 of FIG. 6 provides a more detailed ACCP analysis of the performance shown in FIG.

4. A particular point of interest 92 is the ACCP value of -60 dBc at a symbol rate of 18 ksymbols/sec, where a third order intercept point of 50 dB for the power amplifier is used to achieve this transmitter output performance; the tone being set to an appropriate amplitude level.

Referring now to FIG. 7, a graph 96 showing adjacent channel coupled power versus symbol rate for various third order intercept points of power amplifier in a linearised transmitter with a non-optimised tone frequency is provided. The corresponding point of interest 98 indicates that it is not possible to use any power amplifier to meet the transmitter output performance shown in FIG. 6.

Referring now to FIG. 8 a flowchart showing a method for optimising the performance of an amplifier circuit, for example as shown in FIG. 1, or a transmitter circuit, for example as shown in FIG. 2, according to the preferred embodiment of the invention, is provided. The method includes the steps of receiving an input signal at the modulation circuit, as shown in step 100, and modulating the input signal to provide a modulated output signal, as in step 102. The tone is combined with a modulated output signal to form a combined signal, as shown in step 104.

The combined signal is input to an amplifier, where it is amplified, as in step 106, to provide an amplified representation of the modulated output signal and tone. A portion of the amplified representation of the modulated output signal and tone is coupled off to a tone-receiver, as shown in step 108, where the tone is extracted. The extracted tone is then processed, as in step 110. At least one operational parameter of the transmitter circuit is then adjusted according to the processed extracted tone, as shown in step 112, thereby adjusting the amplified representation of the modulated output from the transmitter circuit.

Advantageously, the present invention removes the requirement for a special time slot to be assigned for transmitter training, or even intermittent re-training. Currently in the European Telecommunications Standard Institute (ETSI) Trans. European Trunked Radio (TETRA) standard, transmitter training occurs once per second to allow the transmitter to adapt its performance, and in particular its linearity performance, to changing conditions, for example desired output power, frequency, bias voltage, device temperature etc. The present invention preserves the benefits previously described whilst retaining the full signalling capacity of the communication channel.

The preferred embodiment uses a novel tone-in-band technique that also provides the capability of continuous re-training during normal transmissions. A tone is selected at a certain frequency within the communications channel, so as to be transparent to the receiver, and the tone is overlaid over the digital message to be transmitted. Hence, the digital message is transmitted normally, the decoding performance is not affected, but more importantly an assessment of the performance and current characteristics of the amplifying devicel transmitter is obtained continuously from the overlaid tone.

A further advantage of the preferred embodiment of the invention, is the applicability to linear base station transmitter technology. The introduction of a tone, say a linearisation tone, in the transmission signal from a base station transmitter, where there is a requirement to transmit a number of linear signals on a number of different frequencies, possibly, in alternative time slots in a time domain communication system, is particularly attractive. Typically, the linearised signals are each different and the linearised signals may well be transmitted on a plurality of frequencies, for example in either a Time-Division Multiple Access (TDMA) or Code Division Multiple Access (CDMA) communication system. Such communication systems provide time-divided periods for transmission on a plurality of frequencies, and the linear base station incorporates a tone into a number of the modulated signals in respective time-divided periods. Such a tone-extrapolation implementation can be used without impacting the communications on other channels, for example in the case of a multi-channel linear base station. Using such a technique provides an in-depth assessment of the transmitter characteristics prior to, and during, transmissions. Hence, there is no need to initially train each transmitter and the performance of each transmitter is continuously optimised. Furthermore, the technique allows that intermodulation products are assessed on a continuous basis and adequate measures are taken to keep them down to the minimum.

Moreover, not just passive monitoring is possible but also active characterisation of the amplifying device can be performed. By changing the input level of the tone-in-band and measuring cross-products and tone phase at the transmitter output, AM-AM and AM-PM characteristics are established. In addition, all of these benefits are provided for with a tone which uses a minimal portion of the signal energy conveyed by communication channel The level of energy that should be used for the tone depends upon the type of modulation used in the communication device, the tolerable level of splattering (interference) that is allowed to be transmitted into adjacent channels and also the characteristics of the power amplifier device. If the associated communication system operates a power control facility then the tone energy would be a function of the output power as well. Typically these levels are pre-defined. In such a situation, the amplitude level of the tone can be adapted during each transmission.

The initial setting up of the linear transmitter requires accurate knowledge on how the energy contained in a cross-product relates to the adjacent channel coupled power. This is determined by initially calibrating the linear transmitter design. The calibration information is stored in form of a look-up table and control algorithm can be similar to that described in the UK Patent Application No: 9401703.5 "Power amplifier for radio transmitter and dual mode remote radio".

A significant disadvantage associated with most amplifier and transmitter designs is the limitation of not being able to always transmit at optimum efficiency. This is primarily due to the characteristics of a power amplifier being dependent upon a number of factors including frequency of operation, operating temperature, bias voltage, input power level etc.

Using the tone-in-band technique, proposed herein, with continuous updated knowledge on how the power amplifier is performing, it is possible to always operate at maximum efficiency within other constraints imposed on the transmitter such as adjacent channel transmissions.

It is also within the contemplation of the invention, that the tone-inband technique can be applied to any linear transmitter method.

Linearised transmitter techniques such as LInear amplification using Non-linear Components (LINC) or one of the feedforward techniques are utilised when a wideband transmitter performance is required. Such linearised transmitter technologies still require a significant amount of fine tuning to ensure a linear output. Typically, a feedback path for the transmitted signal is not used in such techniques for fine-tuning purposes as this would severely restrict the bandwidth of operation. However, with the opportunity of using a narrowband tone, as proposed in the present invention, a feedback path can be used to characterise the performance of the power amplifier as well as the transmitter as the tone bandwidth is minimal and does not impact the operational bandwidth associated with such techniques, namely the av

Claims (26)

Claims

1. An amplifier circuit comprising: an amplifier input for receiving a modulated input signal; a tone generator circuit operably coupled to the amplifier input for incorporating a tone into the modulated input signal; an amplifier for receiving the modulated input signal and tone and for providing an amplified representation of the modulated input signal and tone; a tone receiver operably coupled to the amplifier for extracting the tone from the amplified representation of the modulated input signal and tone; and a processor operably coupled to the tone receiver for receiving and processing the extracted tone and adjusting the amplified representation of the modulated signal dependent upon the processed extracted tone.

2. The amplifier circuit of claim 1, wherein the processor is operably coupled to the amplifier for adjusting an operational parameter in response to the processed extracted tone.

3. The amplifier circuit according to claim 2, wherein the operational parameter includes any of the following: an amplifier bias voltage level, an input power level.

4. A transmitter circuit comprising: an amplifier input for receiving a modulated input signal; a tone generator circuit operably coupled to the amplifier input for incorporating a tone into the modulated input signal; an amplifier for receiving the modulated input signal and tone and for providing an amplified representation of the modulated input signal and tone; a tone receiver operably coupled to the amplifier for extracting the tone from the amplified representation of the modulated input signal and tone; and a processor operably coupled to the tone receiver for receiving and processing the extracted tone and adjusting the amplified representation of the modulated input signal dependent upon the processed extracted tone.

5. The transmitter circuit according to claim 4, further comprising: a modulation circuit for receiving an input signal and providing the modulated input signal to the amplifier input, wherein the tone generator circuit is operably coupled to the modulation circuit for incorporating a tone into the modulated input signal.

6. A transmitter circuit according to claim 4 or 5, wherein the processor is operably coupled to the modulation circuit for adjusting the input signal dependent upon the processed extracted tone.

7. The transmitter circuit according to claim 4, wherein the processor is operably coupled to the amplifier circuit for adjusting an amplification parameter of the amplifier circuit, dependent upon the processed extracted tone, to adjust the amplified representation of the modulated input signal.

8. The transmitter circuit according to any one of the preceding claims 4 to 7, further comprising a lineariser circuit operably coupled to the modulation circuit for linearising the amplified representation of the modulated input signal, wherein adjustment of the amplified representation of the modulated input signal dependent upon the extracted tone is a linearisation adjustment.

9. The transmitter circuit according to claim 8, further comprising a feedback path between the amplifier and both the tone receiver and the lineariser circuit for feeding back the amplified representation of the modulated input signal to the lineariser circuit and for feeding back the amplified representation of the tone to the tone receiver, to facilitate the linearisation of the amplified representation of the modulated input signal dependent upon the extracted tone.

10. The transmitter circuit according to claim 8 or 9, wherein the lineariser circuit combines the fedback amplified representation of the modulated input signal with the input signal to linearise the amplified representation of the modulated signal in a real-time manner.

11. The transmitter circuit according to any one of the preceding claims 4 to 10, wherein the transmitter circuit operates in a digital mobile radio and the tone is a digital tone.

12. The transmitter circuit according to any one of preceding claims 4 to 10, wherein the transmitter circuit operates in a linear base station for transmission of a plurality of linearised signals on a plurality of frequencies.

13. The transmitter circuit according to claim 12, wherein the linear base station operates in a time-domain communications system providing time-divided periods for transmission on the plurality of frequencies, and the linear base station incorporates a tone into respective modulated signals in respective time-divided periods.

14. A method for optimising at least one parameter of a transmitter circuit, the method comprising the steps of: combining a tone with a modulated input signal; inputting the combined tone and modulated input signal to an amplifier; amplifying the combined tone and modulated input signal to provide an amplified representation of the modulated input signal and tone; receiving the amplified representation of the modulated input signal and tone at a tone receiver; extracting the tone from the amplified representation of the modulated input signal and tone; processing the extracted amplified tone; and adjusting the amplified representation of the modulated input signal dependent upon the processed extracted tone.

15. The method for optimising at least one parameter of an transmitter circuit in accordance with claim 14, wherein the transmitter circuit includes a modulation circuit, a tone receiver, a processor and an amplifier circuit and the step of adjusting the amplified representation of the modulated input signal is performed via adjustment of an operational parameter of the transmitter circuit.

16. The method for optimising in accordance with claim 15, the method further comprising the step of receiving an input signal at the modulation circuit for providing a modulated input signal, and wherein the step of adjusting the amplified representation of the modulated input signal includes adjusting the input signal dependent upon the processed extracted tone.

17. The method for optimising a transmitter circuit in accordance with claim 14 or 15, wherein the step of adjusting the amplified representation of the modulated input signal includes adjusting an operational parameter of the amplifier circuit, dependent upon the processed extracted tone.

18. The method for optimising a transmitter circuit in accordance with any one of preceding claims 14 to 17, wherein the transmitter circuit further comprises a lineariser circuit operably coupled to the modulation circuit for linearising the amplified representation of the modulated input signal, wherein adjustment of the amplified representation of the modulated input signal dependent upon the extracted tone is a linearisation adjustment.

19. The method for optimising a transmitter circuit in accordance with claim 18, wherein the transmitter circuit further comprises a feedback path between the amplifier and both the tone receiver and the lineariser circuit, the method further comprising the steps of: feeding back the amplified representation of the modulated input signal to the lineariser circuit; and feeding back the amplified representation of the tone to the tone receiver, thereby facilitating linearisation of the amplified representation of the modulated input signal dependent upon the extracted tone.

20. The method for optimising a transmitter circuit in accordance with claim 18 or 19, wherein the lineariser circuit combines the fedback amplified representation of the modulated signal transmitter circuit with the input signal to linearise the amplified representation of the modulated input signal in a real-time manner.

21. A method for optimising an amplifier circuit having an amplifier input, a tone generator circuit operably coupled to the amplifier input, an amplifier, a tone receiver and a processor, the method comprising the steps of: generating a tone at the tone generator circuit; combining the tone with a modulated input signal at the amplifier input; amplifying the combined tone and modulated input signal to provide an amplified representation of the modulated input signal and tone; receiving the amplified representation of the modulated input signal and tone at the tone receiver; extracting the tone from the amplified representation of the modulated input signal and tone; processing the extracted amplified tone; and.

adjusting the amplified representation of the modulated signal dependent upon the processed extracted tone.

22. The method for optimising an amplifier circuit according to claim 21, wherein the step of adjusting the amplified representation of the modulated signal is performed by adjusting a power level of the modulated input signal.

23. The method for optimising an amplifier circuit according to claim 21, wherein the step of adjusting the amplified representation of the modulated input signal is performed by adjusting a bias voltage level of the amplifier circuit.

24. An amplifier circuit substantially as described herein with respect to FIG. 1 of the drawings.

25. A transmitter circuit substantially as described herein with respect to FIG. 2 of the drawings.

26. A method of optimising a transmitter circuit substantially as described herein with respect to FIG. 8 of the drawings.