The present invention provides a linear radio frequency (RF) power amplifier which is particularly suited for amplifying a linearly modulated signal.
More particularly, the present invention provides a linear RF power amplifier and transmitter in which the polar components of the input signal are manipulated at baseband in order to correct for any non-linearities introduced within the amplifier and hence provide for increased power amplifier (PA) efficiency.
PA efficiency is a vital parameter in portable radio equipment as it directly affects talk time. Linear modulation schemes, such as QPSK or DQPSK are characterized by having a non-constant amplitude envelope, as opposed to FSK or GMSK schemes which are constant envelope. Although linear modulation schemes are more bandwidth efficient than constant envelope schemes, thus allowing more channels to be allocated within a given band, they are also more demanding of the linearity of the PA. In contrast, with constant envelope modulation schemes, the power amplifier linearity is not relevant, as the amplitude envelope of the input signal does not vary. This allows the PA to be operated at the point of maximum efficiency, although at such a point the PA response is characterised by being highly non-linear. If a non-constant modulation envelope signal were to be applied as an input signal to a PA biased to such a point, the resulting output signal would be highly distorted due to the non-linear characteristic. This distortion would lead to spreading of the PA output spectrum into the adjacent channels, therefore effectively negating any gains from the bandwidth efficiency of the linear modulation schemes.
Various solutions to fulfill the requirement of a linear output PA when using non-constant envelope modulation schemes have been previously proposed, and are well known in the art. One of the most commonly used prior art solutions is to bias the power amplifier as if it is handling a much larger signal than the transmitted signal, so that the transmitted signal is amplified linearly. This is known by those skilled in the art as “backing off”, and the backoff ratio is essentially paid for in wasted battery power. As a result of this the talk time for a given output power is reduced in proportion to the backoff ratio when compared with a portable radio equipment that employs a constant envelope modulation scheme. Another resulting problem of “backing off” the output PA is the increased power dissipation in each of the output devices which increases both the cost and the difficulty of integrating transmitter circuitry into an integrated circuit.
A further known prior art scheme is the use of a technique called feedforward amplification. This subtracts a scaled down version of the power amplifier output from the power amplifier input in such a manner that the wanted signal is cancelled and the distortion generating error signal remains. The error signal is then amplified and combined with the PA output in such a manner that the PA distortion is cancelled. This has the effect of raising efficiency but requires the use of power hybrid combiner and delay lines which cannot be integrated onto silicon and also contribute to power loss. Furthermore, such a system relies on cancellation and therefore is prone to drift, and must be corrected adaptively. One method of achieving this is the use of pilot tones, as previously described in U.S. Pat. No. 3,922,617 “Adaptive feedforward system”, W. R. Denniston R. F. Hertz, NY 1975. However, care has to be taken to avoid spurious problems in an integrated transmitter, extra complication is introduced by adding more local oscillators, and the presence of discrete narrowband tones may create an interference problems in wideband communications channels. Another drawback is the increased alignment and test time which will increase cost over an equivalent constant envelope transmitter.
A third prior art method is envelope feedback. In this method, the input signal is resolved into its amplitude and phase components. FIG. 1 shows a block diagram of a prior art linear transmitter employing envelope feedback, as originally disclosed in U.S. Pat. No. 5,430,416 “Power amplifier having nested amplitude modulation controller and phase modulation controller” Black et al. 1994. In this case the amplitude component (r) of the signal is applied to the PA (1001) to amplitude modulate (AM) the PA output, by means of a closed loop around an envelope detector (1003). For the modulation of the phase component (8), a phase locked loop (1005) is used to provide the phase modulation (PM) means, and this also corrects for any incidental PM introduced by the AM modulator. Correction of incidental PM introduced in the PA is most commonly achieved by means of modulation of the divide ratios in a fractional N synthesizer.
An envelope feedback linear transmitter as in the described prior art has one advantage that almost all of the functional blocks can be incorporated into an integrated circuit (IC). there are, however, a number of disadvantages of such an arrangement, particularly when high modulation rates are required, wherein the arrangement becomes particularly unsuitable. Furthermore, the system requirements for an envelope feedback linear transmitter which could work with high modulation rates are harsh. A number of the disadvantages together with explanations of the requirements for operation at a high modulation rate are given below:                1) In order to operate effectively with high modulation rates, a closed loop amplitude control around the envelope detector would be required with a very fast response time.        2) The fractional-N phase locked loop (PLL) would have to cope with very high speed phase reversals in the modulation.        3) There is no facility for controlling time slippage between the AM and PM modulation means.        4) The use of a large divider in the PLL would result in adjacent channel interference due to synthesizer phase noise and hence will increase the likelihood of spurious signal generation.        
Inadequacies in any of the above would be manifested as unwanted adjacent channel sidebands. U.S. Pat. No. 5,732,333 “Linear transmitter using predistortion” Cox et al, 1998 discloses a technique for overcoming these problems in which an IQ coherent downconversion to baseband using the receiver A/Ds is undertaken. The demodulated signal is then compared with the transmitted signal and the appropriate corrections are made. The problems with this technique are that extra processing is required to keep the two signals coherent, and extra local oscillators are required. Furthermore, there is also a risk of introducing unwanted signals at the transmitter output by the downconverter, and extra A/Ds are required for a full duplex system.
The present invention improves upon all of the previously described prior art by making use of one of the properties of an amplifier that is operating at the point of maximum efficiency. As already described, a conventional RF power amplifier must be “backed off” from the point of maximum efficiency else there will be no linear relationship of the output signal with the input signal. However, an amplifier which is operating at maximum efficiency (i.e. a heavily compressed amplifier), whilst having nonlinear input/output characteristics, will also have a nearly linear and predictable amplitude response in relation to its power supply. If an input linear signal is therefore split into its amplitude and phase components, a faithful recreation of the signal's amplitude response can be achieved by means of the power supply voltage. Although, phase errors will remain with such a direct modulation method, these can be corrected by means of small signal circuits. The whole essence of this means of modulation is to force the power amplifier to operate in its mode of maximum efficiency, with the linearity been determined by small signal components. In this way the linearity of the transmitter is determined by small signal circuits which have more readily determined properties, and which can achieve linearity with much less power.
In order to implement the above modulation scheme, according to the present invention there is provided a linear RF transmitter for the transmission of non-constant envelope modulated signals, comprising:
baseband processing means arranged to resolve an input signal into phase components, and to further resolve the phase components into Inphase (I) and quadrature (Q) components;
conversion means arranged to generate analogue representations of the signal components;
phase modulation means arranged to receive the analogue representations of the In-phase and quadrature components and to upconvert and phase modulate the I and Q components into an RF signal;
output power amplifier means arranged to receive the phase modulated RF signal and amplify the signal for transmission;
direct amplitude modulation means arranged to receive an amplitude component of the input signal and to control the output power amplifier means in accordance with the amplitude component whereby to amplitude modulate the RF signal; and
synchronising means arranged to monitor the RF signal and control the conversion means in response to the RF signal.
From one aspect, the present invention can be arranged to resolve the input signal into both phase and amplitude components at baseband, wherein the amplitude component is then converted to analogue in the conversion means and then fed directly to the amplitude modulation means.
From another aspect, the present invention is arranged to resolve only the phase component from the input signal at baseband. The phase components are then used to phase modulate an intermediate frequency signal and this modulated intermediate frequency signal is applied to an envelope detector and limiter from which an analogue amplitude component of the signal is extracted. The analogue amplitude component is then fed to the amplitude modulation means.
Further, the present invention is particularly adapted to ensure synchronisation between the amplitude modulation and phase modulation, by correcting any time slippage therebetween, and various synchronising means are provided to achieve this. This correction has the effect of removing any errors in the modulation synchronisation introduced by various factors in the system components such as limited bandwidth, and hence contributes to the linearity of the transmitter as a whole.
The present invention has an advantage in that it allows for a maximum level of transmitter integration onto an IC.
Furthermore, the present invention provides a linear PA with a sufficiently linear characteristic so that a non-constant envelope modulation scheme may be used with comparable power efficiency to a constant envelope transmitter circuits.
There is also the further feature and advantage in that the arrangement provided by the present invention does not require any extra local oscillators or analogue to digital converters. This contributes to the previous advantage of maximum integration onto silicon.
As an additional advantage, the arrangement provided by the present invention does not generate any extra signals in the output stage of the transmitter. As a result of this, screening requirements are reduced and spurious problems are minimised. Furthermore, spectral spreading is also reduced and bandwidth efficiency may be increased.
Finally, the present invention allows a for a degree of self-alignment. This reduces the time required to test the circuit and hence reduces the cost over equivalent non-constant envelope transmitters.