Method and apparatus for crest factor reduction in telecommunications systems

1. A method for reducing a Peak-to-Average Power Ratio, PAPR, of a transmitted signal in a wireless telecommunication system utilizing Crest Factor Reduction, CFR, technique, the method comprising:

receiving at each clipping stage in a plurality of cascaded clipping stages at least one complex sample associated with that clipping stage and based on the transmitted signal;

computing at each clipping stage a Look Up Table (LUT) input based on a difference between a squared absolute value of said at least one complex sample and a squared maximum allowable amplitude individually configured for that clipping stage;

providing at each clipping stage the LUT input computed for that clipping stage to a LUT in that clipping stage;

filtering the output of said LUT to suppress out-of-band emissions after at least one of said clipping stages; and

receiving a clipped signal based on an output of the LUT associated with a last clipping stage in said plurality of cascaded clipping stages.

2. The method according to claim 1 , wherein each LUT of each clipping stage comprises a plurality of pre-computed values of a clipping function.

3. The method according to claim 1 , wherein said computing said LUT input comprises:

performing a complex multiplication of said complex sample with a complex conjugate of said complex sample to compute said squared absolute value of said at least one complex sample.

4. The method according to claim 1 , further comprising:

individually configuring one or both of said maximum allowed amplitude or a windowing function tap for each clipping stage prior to computing the LUT input at that clipping stage.

5. The method according to claim 4 , wherein said filtering includes out-of band filtering and said method further comprises:

individually configuring a tap of an out-of-band filter prior to said filtering.

6. The method according to claim 1 , wherein each said clipping stage is individually configured to perform a different level of clipping.

7. The method according to claim 1 , wherein said computing said LUT input comprises:

computing a squared in-phase component and a squared quadrature component of said at least one complex signal with at least one real multiplier; and

adding said squared in-phase component and said squared quadrature component with an adder to obtain said squared absolute value of said at least one complex sample.

8. The method of claim 1 , wherein said filtering comprises filtering said output of said LUT with one or more of: a digital filter comprises a Finite Impulse Response (FIR) or an Infinite Impulse Response (IIR) filter.

9. A device for reducing Peak-to-Average Power Ratio (PAPR) of a signal transmitted over a wireless telecommunication system utilizing Crest Factor Reduction (CFR) technique, the device comprising:

a clipping module comprising at least two cascaded CFR clipping stages for performing peak-windowed clipping of a transmitted signal, the clipping module configured to:

receive at each CFR clipping stage at least one complex sample associated with that CFR clipping stage and based on the transmitted signal;

compute at each CFR clipping stage a Look Up Table (LUT) input based on a difference between a squared absolute value of said at least one complex sample and a squared maximum allowable amplitude individually configured for that clipping stage;

provide at each CFR clipping stage the LUT input computed for that CFR clipping stage to a LUT in that CFR clipping stage; and

provide a clipped signal based on an output of the LUT associated with a last of the CFR clipping stages; and

a digital filter coupled after at least one of said CFR clipping stages.

10. The device according to claim 9 , wherein each said LUT comprises a plurality of pre-computed values of a clipping function for said peak-windowed clipping.

11. The device according to claim 9 , wherein said digital filter comprises one or both of a Finite Impulse Response (FIR) or an Infinite Impulse Response (IIR) filter.

12. The device according to claim 9 , wherein said clipping module is further configured to:

perform a complex multiplication of said complex sample with a complex conjugate of said complex sample to compute said squared absolute value of said at least one complex sample.

13. The device according to claim 9 , wherein one or both of said maximum allowed amplitude or a windowing function tap is individually configured for each clipping stage prior to computing the LUT input at that clipping stage.

14. The device according to claim 13 , wherein said digital filter comprises an out-of band filtering and a tap of said out-of-band filter is individually configured for a particular stage prior to filtering.

15. The device according to claim 9 , wherein said each said clipping stage is individually configured to perform a different level of clipping.

16. The device according to claim 9 , wherein said clipping module is further configured to:

compute a squared in-phase component and a squared quadrature component of said at least one complex signal with at least one real multiplier; and

add said squared in-phase component and said squared quadrature component with an adder to obtain said squared absolute value of said at least one complex sample.

17. An apparatus for reducing a Peak-to-Average Power Ratio (PAPR) of a transmitted signal in a wireless communication system utilizing Crest Factor Reduction, CFR, the apparatus comprising:

means for receiving at each clipping stage in a plurality of cascaded clipping stages at least one complex sample associated with that clipping stage and based on the transmitted signal;

means for computing at each clipping stage a Look Up Table (LUT) input based on a difference between a squared absolute value of said at least one complex sample and a squared maximum allowable amplitude individually configured for that clipping stage;

means for providing at each clipping stage the LUT input computed for that clipping stage to a LUT in that clipping stage;

means for filtering the output of said LUT to suppress out-of-band emissions after at least one of the clipping stages; and

means for receiving a clipped signal based on an output of the LUT associated with a last clipping stage in the set of cascaded clipping stages.

18. The apparatus according to claim 17 , wherein each LUT of each clipping stage comprises a plurality of pre-computed values of a clipping function.

19. The apparatus according to claim 17 , wherein said means for computing said LUT input comprises:

means for performing a complex multiplication of said complex sample with a complex conjugate of said complex sample to compute said squared absolute value of said at least one complex sample.

20. The apparatus according to claim 17 , further comprising:

means for individually configuring one or both of said maximum allowed amplitude or a windowing function tap for each clipping stage prior to computing the LUT input at that clipping stage.

21. The apparatus according to claim 20 , wherein said means for filtering comprises an out-of-band filter, said apparatus further comprising:

means for individually configuring a tap of said out-of-band filter prior to said filtering.

22. The apparatus according to claim 17 , wherein each said clipping stage is individually configured to perform a different level of clipping.

23. The apparatus according to claim 17 , wherein said means for computing said LUT input comprises:

means for computing a squared in-phase component and a squared quadrature component of said at least one complex signal with at least one real multiplier; and

means for adding said squared in-phase component and said squared quadrature component with an adder to obtain said squared absolute value of said at least one complex sample.

24. The apparatus of claim 17 , wherein said filtering comprises filtering said output of said LUT with one or more of: a digital filter comprises a Finite Impulse Response (FIR) or an Infinite Impulse Response (IIR) filter.