Amplifier pre-distorter method and apparatus

1. A method of pre-distorting an RF input signal to an amplifier, said method comprising:

squaring said RF input signal to obtain a polynomial pre-distortion signal;

combining the polynomial pre-distortion signal with the input signal to form a combination; and

applying the combination to the amplifier; wherein

the polynomial pre-distortion signal is combined with the input signal by a step of multiplying the signals together to produce a combined output signal to be applied to the amplifier;

the input signal is split into inphase and quadrature signals and processed to produce inphase and quadrature polynomial pre-distortion signals which are combined to form the polynomial pre-distortion signal.

2. A method as claimed in claim 1 , wherein the squared input signal is used to generate even powers of a polynomial pre-distortion signal.

3. A method as claimed in claim 2 , wherein the input signal is squared by multiplying the signal by itself.

4. Apparatus for pre-distorting an RF input signal input to an amplifier, said apparatus comprising:

an RF input for receiving an RF signal to be pre-distorted;

a squarer for squaring the input signal to obtain a polynomial pre-distortion signal; and

a signal splitter for splitting the input signal into inphase and quadrature signals and inputting the inphase and quadrature signals into the squarer;

wherein the squarer comprises at least one multiplier for multiplying the input signal by itself, to create a squared signal.

5. A pre-distorter as claimed in claim 4 , wherein the squarer comprises a multiplier for each of the inphase and quadrature signals.

6. A pre-distorter as claimed in claim 5 , wherein each multiplier has an output operably coupled to a summer which sums the squared inphase and quadrature signals.

7. A pre-distorter as claimed in claim 6 , wherein the summed signal is applied to both inputs of a multiplier to generate a signal to the power four.

8. A pre-distorter as claimed in claim 7 , comprising a further multiplier to which the squared signal and the signal to the power four are applied to generate a signal to the power six.

9. A pre-distorter as claimed in claim 8 , comprising at least one further multiplier for further multiplying the signal from previous multipliers to generate a further signal.

10. A pre-distorter as claimed in claim 9 , wherein the further signal is one of an inphase signal and a quadrature signal.

11. A pre-distorter as claimed in claim 10 , comprising an inphase branch for multiplying the signal with inphase signals, and a quadrature signal branch for multiplying the signal with quadrature signals.

12. A pre-distorter as claimed in claim 11 , wherein the inphase branch and the quadrature branch both have a summer to combine the multiplied signals.

13. A pre-distorter as claimed in claim 12 , wherein the combined multiplied signals are input to a vector modulator.

14. A pre-distorter as claimed in claim 13 , wherein the vector modulator further comprises a multiplier for multiplying the combined multiplied signals with the input signal and a summer for combining the inphase and quadrature signals to provide a pre-distortion signal to be applied to the amplifier.

15. Radio communications apparatus comprising a signal predistorter as claimed in claim 4 , operably coupled to a power amplifier to provide a predistorted signal thereto.