# US 8,787,438 B1 · Reconditioning equalizer filter for non-constant envelope signals

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- Number: US 8,787,438 B1
- Document ID: US8787438B1
- Status: Granted
- Kind: B1
- Application number: 14/138,959
- Filed: 2013-12-23
- Granted: 2014-07-22
- Earliest claimed: 2006-11-24
- Assignees: Altera Corporation
- Applicants: Altera Corporation
- Inventors: Kiomars Anvari
- Web page: https://patentgrep.com/patent/US8787438B1

## CPC

- H04L 25/03885: {adaptive}
- H04L 27/2624: {by soft clipping}
- H04L 27/366: {Arrangements for compensating undesirable properties of the transmission path between the modulator and the demodulator}
- H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M)

## Abstract

A technique for a reconditioning equalizer filter for non-constant envelope signals is described. The input to a transmitter chain is modified by a reconditioning equalizer filter, prior to being applied to the transmitter. The reconditioning equalizer filter modifies and smoothens the amplitude of the signal. The modified and smoothened signal has its peaks reduced which results in lower Crest Factor. The input to the reconditioning equalizer filter could be a baseband, intermediate frequency (IF) or radio frequency (RF) signal. When the signal is an IF or RF signal, it needs to be down-converted to baseband before being applied to the reconditioning equalizer filter. The reconditioning equalizer filter could be performed in a digital or analog domain.

## Claims

### Claim 1

A reconditioning equalizer method for use with non-constant envelope signals to enhance the performance of a transmitter, the reconditioning equalizer method comprising:
delay-and-gain adjusting a main input baseband signal to produce a primary delay-and-gain-adjusted main baseband signal;
peak-reduction filtering the main input baseband signal to produce a peak-reduced main baseband signal;
injecting an in-band-and-out-of-band-adjusted baseband signal into the main baseband signal in a feedforward loop, wherein the injecting comprises:
delaying the main baseband signal,
gain-adjusting the delayed main baseband signal to produce a second delay-and-gain-adjusted main baseband signal,
subtracting the second delay-and-gain-adjusted main baseband signal from the peak-reduced main baseband signal,
correlating the main baseband signal and the result of the subtraction of the second delay-and-gain-adjusted main baseband signal from the peak-reduced main baseband signal to calculate the amount of delay adjustment in the delaying and the amount of gain adjustment in the gain-adjusting,
low-pass filtering the result of the subtraction of the second delay-and gain-adjusted main baseband signal from the peak-reduced main baseband signal to adjust in-band and out-of-band components in the baseband signal,
adjusting the gain of the filtered subtraction of the second delay-and-gain-adjusted main baseband signal from the peak-reduced main baseband signal for injection into the primary delay-and-gain-adjusted main baseband signal, and
summing the primary delay-and-gain-adjusted main baseband signal and the low-pass-filtered and gain-adjusted subtraction of the second delay-and-gain-adjusted main baseband signal from the peak-reduced main baseband signal to produce a modified main baseband signal;
adjusting the gain of the modified main baseband signal to produce an output; and
using the main baseband signal, the output of the low pass filter, the output of the reconditioning equalizer filter, and external input to control the peak-reduction filtering, the correlating, the delay-and-gain adjusting, and the adjusting the gain of the filtered subtraction.

### Claim 2 (depends on 1)

The method of claim 1 wherein said low-pass filtering is performed using a digital FIR filter.

### Claim 3 (depends on 2)

The method of claim 2 wherein said low-pass filtering is performed using a configurable digital FIR filter.

### Claim 4 (depends on 1)

The method of claim 1, wherein the peak-reduction filtering uses a phase rotation lookup table to rotate the in-phase “I” component and quadrature phase “Q” component of the complex main baseband signal and uses the magnitude of the main baseband signal to point to the required rotation angle, said peak-reduction filtering comprising:
using a complex-to-real conversion to produce the in-phase “I” component and quadrature phase “Q” component from the main baseband signal;
using the in-phase “I” component and quadrature phase “Q” component of the main baseband signal to calculate the magnitude of the main baseband signal;
using the magnitude of the main baseband signal, and an external control parameter, to produce a pointer to the phase-rotation lookup table, thereby providing from the lookup table, based on the pointer, phase-rotation cosine and sine values for the in-phase “I” component and the quadrature phase “Q” component;
multiplying the in-phase “I” component of the main baseband signal with the in-phase “I” phase-rotation cosine value to produce a modified in-phase “I” component;
multiplying the quadrature phase “Q” component of the main baseband signal with the quadrature phase “Q” phase-rotation sine value modified quadrature phase “Q” component;
using a real-to-complex conversion to produce the peak-reduced main baseband signal from the modified in-phase “I” component and modified quadrature phase “Q” component.

### Claim 5 (depends on 4)

The method according to claim 4, wherein at least one of the calculating the magnitude of the main baseband signal, multiplying the in-phase “I” component, or the multiplying the quadrature phase “Q” component, is performed using one of programmable logic, a Field Programmable Gate Array (FPGA), a Gate Array, an Application Specific Integrated Circuit (ASIC), or a DSP processor.