Adaptive High-Order Nonlinear Function Approximation Using Time-Domain Volterra Series to Provide Flexible High Performance Digital Pre-Distortion

• US 20150381216A1
• Filed: 06/27/2014
• Published: 12/31/2015
• Est. Priority Date: 06/27/2014
• Status: Active Grant

• Current Assignee: NXP USA, Inc. (NXP Semiconductors NV)
• Original Assignee: Freescale Semiconductor, Inc. (NXP Semiconductors NV)
• Inventors: Rashev, Peter Z., Shor, Roi M., Gal, Avraham D.
• Granted Patent: US 9,252,821 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: H03F 1/3241 using predistortion circuits H03F1/3211, H03F1/3217 take precedence
• 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
• CPC Class Codes: H04L 25/03853 Shaping by digital methods other than look up tables or up/down converters
• CPC Class Codes: H04L 25/10 Compensating for variations in line balance

Abstract

A method and apparatus are used to predistort input signal samples according to Volterra Series Approximation Model using one or more digital predistortion blocks (300) having a plurality of predistorter cells (301-303), each including an input multiplication stage (366-367) for combining absolute sample values received from an absolute sample delay line (362) into a first stage output, a lookup table (368) connected to be addressed by the first stage output for generating an LUT output, and a plurality of output multiplication stages (371-372, 373-374) for combining the LUT output with samples received from the amplitude sample delay line (362) and signal sample delay line (363) to generate an output signal sample y_Q from said predistorter cell, where the output signal samples y_Q from the predistorter cells are combined at an output adder circuit (375) to generate one or more Volterra terms of a combined signal (y_OUT[n]).

Claims

1. A method for predistorting an input signal to compensate for non-linearities of an electronic device that operates on the input signal to produce an output signal, comprising:

providing an input circuit for receiving a first input signal as a plurality of signal samples x[n] to be transmitted over a non-linear element;

providing one or more digital predistortion blocks coupled to the input circuit and generating an output signal y[n] by adaptively modifying the first input signal to compensate for distortion effects in the non-linear element, each digital predistortion block comprising;

a first delay line for storing a first plurality of signal samples x[n];

a second delay line for storing a first plurality of amplitude samples A[n] derived from the first plurality of signal samples x[n];

a plurality of Q predistorter cells, each comprising an input stage multiplier and multiplexer for combining amplitude samples received from the second delay line into a first stage output, a lookup table (LUT) connected to be addressed by the first stage output for generating an LUT output, and one or more output multiplication stages for combining the LUT output with signal samples and amplitude samples received from the first and second delay lines to generate an output signal sample y_Q from said predistorter cell; and

an output adder circuit connected to combine the output signal samples y_Q from the plurality of Q predistorter cells into a combined signal; and

processing the combined signal to generate the output signal y[n] for transmission to the non-linear element.

2. The method of claim 1, where providing the input circuit comprises connecting the input circuit to receive the first input signal as IQ baseband input signal samples from the baseband modem.

3. The method of claim 1, where the first delay line holds K complex signal samples.

4. The method of claim 1, where the second delay line holds K absolute values of the K complex signal samples.

5. The method of claim 1, where providing one or more digital predistortion blocks comprises initializing each LUT by programmably storing LUT entries with polynomial filtering coefficients.

6. The method of claim 1, where each LUT provides linear interpolation between LUT entries.

7. The method of claim 1, where each predistorter cell comprises:

first and second input multiplexers coupled to provide first and second amplitude samples from the second delay line;

a first multiplier circuit and a first input stage multiplexer forming the input stage multiplier and multiplexer, where the first multiplier circuit is connected to combine the first and second amplitude samples into a first product output which is supplied with the first amplitude sample value to the first input stage multiplexer which generates the first stage output;

a second multiplier circuit, second stage input multiplexer, and second stage multiplexer forming a first output multiplication stage, where the second multiplier circuit is connected to combine the LUT output with amplitude samples from the second delay line provided by the second stage input multiplexer into a second product output which is supplied with the LUT output to the second stage multiplexer which generates the second stage output; and

a third multiplier circuit, third stage input multiplexer, and third stage multiplexer forming a second output multiplication stage, where the third multiplier circuit is connected to combine the second product output with signal samples from the first delay line provided by the third stage input multiplexer into a third product output which is supplied with an empty term to the third stage multiplexer which generates the output signal sample y_Q.

8. The method of claim 7, where the second stage input multiplexer comprises an expansion input port for receiving an input signal Cin from another predistorter cell.

9. The method of claim 8, further comprising an expansion output port from the second stage multiplexer for providing an output signal Cout to another predistorter cell.

10. The method of claim 1, where the output adder circuit comprises an expansion input port for receiving a combined signal from another digital predistortion block.

11. The method of claim 1, where the first delay line comprises an expansion output port for providing a delayed first plurality of signal samples x[n] to another another digital predistortion block.

12. The method of claim 1, where providing one or more digital predistortion blocks comprises:

providing a first digital predistortion block configured to generate one or more static Volterra polynomial terms;

providing a second digital predistortion block configured to generate one or more dynamic Volterra memory polynomial terms;

providing a third digital predistortion block configured to generate one or more type 1 Volterra cross-terms; and

providing one or more fourth digital predistortion blocks configured to generate one or more type 2 or type 3 Volterra cross-terms.

13. The method of claim 12, where providing one or more fourth digital predistortion blocks comprises providing a plurality of cascaded digital predistortion blocks which are connected together and configured to generate second order type 2 or type 3 Volterra cross-terms.

14. The method of claim 1, where providing the input circuit comprises providing a multiplexer for selecting between the first input signal and a second input signal of a plurality of delayed signal samples x_in[n].

15. A circuit for predistorting a signal according to Volterra Series Approximation Model for transmission over a non-linear element, comprising:

a first digital predistortion block comprising a first sample delay line for storing a plurality of first input signal samples x₁[n];

a first vector magnitude computation unit for computing a plurality of first amplitude samples A₁[n] derived from the plurality of first signal samples x₁[n];

a first amplitude sample delay line for storing the plurality of first amplitude samples A₁[n];

a first plurality of predistorter cells, each comprising an a first input stage for combining amplitude samples received from the first amplitude sample delay line into a first stage output, a first lookup table (LUT) connected to be addressed by the first stage output for generating a first LUT output, and one or more first output multiplication stages for combining the first LUT output with signal samples and amplitude samples received from the first sample delay line and first amplitude sample delay line to generate a first output signal sample y_Q from said predistorter cell; and

a first output adder circuit connected to combine the first output signal samples y_Q1 from the first plurality of predistorter cells into a first combined signal; and

a second digital predistortion block comprising a second sample delay line for storing a plurality of second input signal samples x₂[n];

a second vector magnitude computation unit for computing a plurality of second amplitude samples A₂[n] derived from the plurality of second signal samples x₂[n];

a second amplitude sample delay line for storing the plurality of second amplitude samples A₂[n];

a second plurality of predistorter cells, each comprising an second input stage for combining amplitude samples received from the second amplitude sample delay line into a second stage output, a second lookup table (LUT) connected to be addressed by the second stage output for generating a second LUT output, and one or more second output multiplication stages for combining the second LUT output with signal samples and amplitude samples received from the second sample delay line and second amplitude sample delay line to generate a second output signal sample y_Q2 from said predistorter cell; and

a second output adder circuit connected to combine the second output signal samples y_Q2 from the second plurality of predistorter cells into a second combined signal;

where the first and second digital predistortion blocks are connected in a cascaded arrangement to produce a type 2 or type 3 Volterra series cross term.

16. The circuit of claim 15, wherein the second digital predistortion block is connected to receive the plurality of first input signal samples x₁[n] as the plurality of second input signal samples x₂[n], and wherein the second output adder circuit comprises an additional input port connected to receive the first combined signal from the first digital predistortion block.

17. The circuit of claim 15, wherein the first sample delay line comprises an output connection port for providing delayed versions of the plurality of first input signal samples x₁[n] to the second digital predistortion block as the plurality of second input signal samples x₂[n], and wherein the second output adder circuit comprises an additional input port connected to receive the first combined signal from the first digital predistortion block.

18. The circuit of claim 17, wherein each of the first plurality of predistorter cells comprises an output connection port for providing an output signal from the one or more first output multiplication stages to a corresponding one of the second plurality of predistorter cells as an input signal.

19. The circuit of claim 15, wherein the second digital predistortion block is connected to receive the plurality of first input signal samples x₁[n] as the plurality of second input signal samples x₂[n], wherein the second output adder circuit comprises an additional input port connected to receive the first combined signal from the first digital predistortion block, and wherein each of the first plurality of predistorter cells comprises an output connection port for providing an output signal from the one or more first output multiplication stages to a corresponding one of the second plurality of predistorter cells as an input signal.

20. The circuit of claim 15, further comprising a third digital predistortion block comprising a third sample delay line for storing a plurality of third input signal samples x₃[n];

a third vector magnitude computation unit for computing a plurality of third amplitude samples A₃[n] derived from the plurality of third signal samples x₃[n];

a third amplitude sample delay line for storing the plurality of third amplitude samples A₃[n];

a third plurality of predistorter cells, each comprising a third input stage for combining amplitude samples received from the third amplitude sample delay line into a third stage output, a third lookup table (LUT) connected to be addressed by the third stage output for generating a third LUT output, and one or more third output multiplication stages for combining the third LUT output with signal samples and amplitude samples received from the third sample delay line and third amplitude sample delay line to generate a third output signal sample y_Q3 from said predistorter cell; and

a third output adder circuit connected to combine the third output signal samples y_Q3 from the third plurality of predistorter cells into a third combined signal;

where the first, second, and third digital predistortion blocks are connected in a cascaded arrangement to produce a type 2 or type 3 Volterra series cross term.