Reduction of digital-to-analog converter distortion using constellation rotation

1. A method for signal conversion, comprising:

generating a complex digital signal, which comprises digital In-phase (I) and Quadrature (Q) components, for conversion into respective analog I and Q components by first and second Digital-to-Analog Converters (DACs);

reducing a distortion generated by the DACS in the analog I and Q components by applying a phase rotation to the complex digital signal; and

after applying the phase rotation, converting the digital I and Q components of the complex digital signal into the respective analog I and Q components using the first and second DACs,

wherein the analog I and Q components have respective first and second Peak-to-Average Ratios (PARs), and wherein the applied phase rotation reduces at least one of the PARs.

2. The method according to claim 1 , wherein reducing the distortion comprises identifying the phase rotation that reduces the distortion generated by the DACS, and applying the identified phase to the complex digital signal prior to conversion by the DACs.

3. The method according to claim 1 , and comprising measuring the first and second PARs, and computing the phase rotation based on the measured PARs.

4. The method according to claim 1 , wherein the applied phase rotation reduces a probability that at least one of the analog I and Q components exceeds a given amplitude.

5. The method according to claim 1 , and comprising applying to the complex digital signal a pre-distortion operation prior to conversion by the DACs, wherein the applied phase rotation depends on a characteristic of the pre-distortion operation.

6. The method according to claim 5 , wherein applying the pre-distortion operation comprises filtering the complex digital signal with one or more coefficients, and wherein the applied phase rotation depends on the coefficients.

7. The method according to claim 5 , wherein the pre-distortion operation compensates for a response of a Power Amplifier (PA) that amplifies a Radio Frequency (RF) signal derived from the analog I and Q components, and wherein the applied phase rotation is selected so as to match the response.

8. The method according to claim 7 , wherein the applied phase rotation depends on a temperature of the PA.

9. The method according to claim 7 , wherein the PA belongs to a set of PAs, and wherein selecting the phase rotation comprises choosing a common phase rotation for the set.

10. The method according to claim 1 , wherein applying the phase rotation comprises modifying the phase rotation over time.

11. The method according to claim 1 , wherein the applied phase rotation reduces the distortion and further meets an additional criterion defined with respect to the analog I and Q components.

12. The method according to claim 1 , and comprising generating a Radio Frequency (RF) signal from the analog I and Q components, and transmitting the RF signal to a receiver.

13. Apparatus for signal conversion, comprising:

first and second Digital-to-Analog Converters (DACs), which are configured to convert digital In-phase (I) and Quadrature (Q) components of a complex digital signal into analog I and Q components, respectively; and

a phase rotator, which is configured to reduce a distortion generated by the DACS in the analog I and Q components by applying a phase rotation to the complex digital signal prior to conversion by the DACs,

wherein the analog I and Q components have respective first and second Peak-to-Average Ratios (PARs), and wherein the applied phase rotation reduces at least one of the PARs.

14. The apparatus according to claim 13 , and comprising a control unit, which is configured to identify the phase rotation which, when applied by the phase rotator, reduces the distortion generated by the DACs, and to cause the phase rotator to apply the identified phase rotation to the complex digital signal.

15. The apparatus according to claim 13 , and comprising a control unit, which is configured to measure the first and second PARs, to compute the phase rotation based on the measured PARs, and to cause the phase rotator to apply the computed phase rotation to the complex digital signal.

16. The apparatus according to claim 13 , wherein the applied phase rotation reduces a probability that at least one of the analog I and Q components exceeds a given amplitude.

17. The apparatus according to claim 13 , and comprising a pre-distortion unit, which is configured to apply to the complex digital signal a pre-distortion operation prior to conversion by the DACs, wherein the applied phase rotation depends on a characteristic of the pre-distortion operation.

18. The apparatus according to claim 17 , wherein the pre-distortion unit is configured to filter the complex digital signal with one or more coefficients, and wherein the applied phase rotation depends on the coefficients.

19. The apparatus according to claim 17 , wherein the pre-distortion operation compensates for a response of a Power Amplifier (PA) that amplifies a Radio Frequency (RF) signal derived from the analog I and Q components, and wherein the applied phase rotation is selected so as to match the response.

20. The apparatus according to claim 19 , wherein the applied phase rotation depends on a temperature of the PA.

21. The apparatus according to claim 19 , wherein the PA belongs to a set of PAs, and wherein the applied phase rotation is selected in common for the set.

22. The apparatus according to claim 13 , and comprising a control unit, which is configured to modify the applied phase rotation over time.

23. The apparatus according to claim 13 , wherein the applied phase rotation reduces the distortion and further meets an additional criterion defined with respect to the analog I and Q components.

24. The apparatus according to claim 13 , and comprising transmission circuitry, which is configured to generate a Radio Frequency (RF) signal from the analog I and Q components, and to transmit the RF signal to a receiver.

25. The apparatus according to claim 13 , and comprising a mapper, which generates the complex digital signal and comprises the phase rotator.

26. A method for signal conversion, comprising:

generating a complex digital signal, which comprises digital In-phase (I) and Quadrature (Q) components, for conversion into respective analog I and Q components by first and second Digital-to-Analog Converters (DACs);

reducing a distortion generated by the DACS in the analog I and Q components by applying a phase rotation to the complex digital signal; and

after applying the phase rotation, converting the digital I and Q components of the complex digital signal into the respective analog I and Q components using the first and second DACs,

wherein reducing the distortion comprises identifying the phase rotation that reduces the distortion generated by the DACS, and applying the identified phase to the complex digital signal prior to conversion by the DACs.

27. Apparatus for signal conversion, comprising:

first and second Digital-to-Analog Converters (DACs), which are configured to convert digital In-phase (I) and Quadrature (Q) components of a complex digital signal into analog I and Q components, respectively;

a phase rotator, which is configured to reduce a distortion generated by the DACS in the analog I and Q components by applying a phase rotation to the complex digital signal prior to conversion by the DACs; and

a control unit, which is configured to identify the phase rotation which, when applied by the phase rotator, reduces the distortion generated by the DACs, and to cause the phase rotator to apply the identified phase rotation to the complex digital signal.