Multi-band digital compensator for a non-linear system

• US 10,644,657 B1
• Filed: 10/18/2019
• Issued: 05/05/2020
• Est. Priority Date: 05/11/2018
• Status: Active Grant

• Current Assignee: NanoSemi, Incorporated (Maxlinear Incorporated)
• Original Assignee: NanoSemi, Incorporated (Maxlinear Incorporated)
• Inventors: Megretski, Alexandre, Chuang, Kevin, Li, Yan, Mahmood, Zohaib, Kim, Helen H.
• Primary Examiner(s): Vo, Nguyen T
• Application Number:
  US16/656,686
• Publication Number:
  US 20200169227A1
• Time in Patent Office: 200 Days
• CPC Class Codes: H03F 1/0227 using supply converters
• CPC Class Codes: H03F 1/3247 using feedback acting on predistortion circuits H03F1/3264 takes precedence
• CPC Class Codes: H03F 1/3258 based on polynomial terms
• CPC Class Codes: H03F 2200/102 A non-specified detector of a signal envelope being used in an amplifying circuit
• CPC Class Codes: H03F 2200/451 the amplifier being a radio frequency amplifier
• CPC Class Codes: H03F 2201/3224 Predistortion being done for compensating memory effects
• CPC Class Codes: H03F 2201/3227 Adaptive predistortion based on amplitude, envelope or power level feedback from the output of the main amplifier
• CPC Class Codes: H03F 2201/3233 Adaptive predistortion using lookup table, e.g. memory, RAM, ROM, LUT, to generate the predistortion
• CPC Class Codes: H03F 3/189 High-frequency amplifiers, e.g. radio frequency amplifiers
• CPC Class Codes: H03F 3/193 with field-effect devices H03F3/195 takes precedence
• CPC Class Codes: H03F 3/21 with semiconductor devices only H03F3/245 takes precedence
• CPC Class Codes: H03F 3/24 of transmitter output stages
• CPC Class Codes: H04B 1/0475 with means for limiting noise, interference or distortion H04B1/0483 takes precedence
• CPC Class Codes: H04B 2001/0425 with linearisation using predistortion

Abstract

A pre-distorter that both accurately compensates for the non-linearities of a radio frequency transmit chain, and that imposes as few computation requirements in terms of arithmetic operations, uses a diverse set of real-valued signals that are derived from separate band signals that make up the input signal. The derived real signals are passed through configurable non-linear transformations, which may be adapted during operation, and which may be efficiently implemented using lookup tables. The outputs of the non-linear transformations serve as gain terms for a set of complex signals, which are functions of the input, and which are summed to compute the pre-distorted signal. A small set of the complex signals and derived real signals may be selected for a particular system to match the classes of non-linearities exhibited by the system, thereby providing further computational savings, and reducing complexity of adapting the pre-distortion through adapting of the non-linear transformations.

Claims

1. A method of signal predistortion for linearizing a non-linear circuit, the method comprising:

processing an input signal (u) comprising a plurality of separate band signals (u₁, . . . , u_Nb), each separate band signal having a separate frequency range within the input frequency range of the input signal, at least part of the input frequency range containing none of the separate frequency range, the processing producing a plurality of transformed signals (w), the transformed signals including at least one transformed signal equal to a combination of multiple separate band signals;

determining a plurality of phase-invariant derived signals (r) to be equal to respective non-linear functions of one or more of the transformed signals;

transforming the plurality of phase-invariant derived signals (r) according to a plurality of parametric non-linear transformations (Φ

) to produce a plurality of gain components (g);

forming a distortion term by accumulating a plurality of terms (k), each term being a combination of a transformed signal (w_ak) of the plurality of transformed signals and respective one or more time-varying gain components (g_i,i∈

Λ

_k) of the plurality of gain components; and

providing an output signal (v) determined from the distortion term for application to the non-linear circuit.

2. The method of claim 1, further comprising adapting the plurality of parametric non-linear transformations according to measured characteristics of the non-linear circuit.

3. The method of claim 1, wherein the at least one transformed signal comprise a degree-1 combination of the separate band signals.

4. The method of claim 3, where the at least one transformed signal further comprise at least one degree-2 or at least one degree-0 combination of the separate band signals.

5. The method of claim 3, further comprising transforming one or more of the derived signal (r_j) of the plurality of phase-invariant derived signals according to respective one or more parametric non-linear transformations (φ

_i,j) to produce a time-varying gain component (g_i) of a plurality of gain components (g).

6. The method of claim 1, wherein each derived signal (r_j) of the plurality of derived signals is equal to a non-linear function of a respective subset of one or more of the transformed signals, at least some of the derived signals being equal to functions of different one or more of the transformed signals.

7. The method of claim 1, wherein each of the parametric non-linear transformations (Φ

) is decomposable into a combination of one or more parametric functions (ϕ

) of a corresponding single one of the derived signals (r_j).

8. The method of claim 1, further comprising filtering the input signal (u) to form the plurality of separated band signals (u₁, . . . , u_Nb).

9. The method of claim 8, wherein each of the separated band signals is represented at a same sampling rate as the input signal.

10. The method of claim 1 wherein the processing of the input signal (u) to produce a plurality of transformed signals (w) includes forming at least some of the transformed signals as combinations of subsets of the separate band signals or signals derived from said separate band signals.

11. The method of claim 10, wherein the combinations of subsets of the separate band signals or signals derived from said separate band signals make use of delay, multiplication, and complex conjugate operations on the separate band signals.

12. The method of claim 1, wherein the non-linear circuit comprises a radio-frequency section including a radio-frequency modulator configured to modulate the output signal to a carrier frequency to form a modulated signal and an amplifier for amplifying the modulated signal.

13. The method of claim 12, wherein the input signal (u) comprises quadrature components of a baseband signal for transmission via the radio-frequency section.

14. The method of claim 1, wherein the input signal (u) and the plurality of transformed signals (w) comprise complex-valued signals.

15. The method of claim 1, wherein processing the input signal (u) to produce the plurality of transformed signals (w) includes scaling a magnitude of a separate band signal according to an overall power of the input signal (r₀).

16. The method of claim 15, wherein forming at least one of the transformed signals as a linear combination includes forming a linear combination with at least one imaginary or complex multiple input signal or a delayed version of the input signal.

17. The method of claim 16, wherein forming at least one of the transformed signals, w_k, to be a multiple of D_α

w_a+j^dw_b, where w_a and w_b are other of the transformed signals each of which depend on only a single one of the separate band signals, and D_α

represents a delay by α

, and d is an integer between 0 and 3.

18. The method of claim 15, wherein forming the at least one of the transformed signals includes time filtering the input signal to form said transformed signal.

19. The method of claim 18, wherein time filtering the input signal includes applying a finite-impulse-response (FIR) filter to the input signal.

20. The method of claim 18, wherein time filtering the input signal includes applying an infinite-impulse-response (IIR) filter to the input signal.

21. The method of claim 1, wherein processing the input signal (u) to produce the plurality of transformed signals (w) comprises raising a magnitude of a separate band signal to a first exponent (α

) and rotating a phase of said band signal according to a second exponent (β

) not equal to the first exponent.

22. The method of claim 1, wherein processing the input signal (u) to produce the plurality of transformed signals (w) includes forming at least one of the transformed signals as a multiplicative combination of one of the separate band signals (u_a) and a delayed version of another of the separate band signals (u_b).

23. The method of claim 1, wherein the plurality of transformed signals (w) includes non-linear functions of the separate band signals (u_i).

24. The method of claim 23, wherein the non-linear functions of the separate signals (u_i) comprises at least one function of a form

u_i|u_j|², i≠

j, or

u_i|u_iu_j, i≠

j.

25. The method of claim 1, wherein determining a plurality of phase-invariant derived signals (r) comprises determining real-valued derived signals.

26. The method of claim 1, wherein determining a plurality of phase-invariant derived signals (r) comprises processing the transformed signals (w) to produce a plurality of phase-invariant derived signals (r).

27. The method of claim 26, wherein each of the derived signals is equal to a function of one of the transformed signals.

28. The method of claim 26, wherein processing the transformed signals (w) to produce a plurality of phase-invariant derived signals includes for at least one derived signal (r_p) computing said derived signal by first computing a phase-invariant non-linear function of one of the transformed signals (w_k) to produce a first derived signal, and then computing a linear combination of the first derived signal and delayed versions of the first derived signal to determine the at least one derived signal.

29. The method of claim 28, wherein computing a phase-invariant non-linear function of one of the transformed signals (w_k) comprises computing a power of a magnitude of the one of the transformed signals (|w_k|^p) for an integer power p≥

1.

30. The method of claim 29, wherein p=1 or p=2.