# US 11,451,252 B2 · Digital predistortion for advanced antenna system

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- Number: US 11,451,252 B2
- Document ID: US11451252B2
- Status: Granted
- Kind: B2
- Application number: 17/220,541
- Filed: 2021-04-01
- Published: 2021-07-22
- Granted: 2022-09-20
- Earliest claimed: 2021-04-01
- Assignees: Telefonaktiebolaget LM Ericsson (publ)
- Applicants: Telefonaktiebolaget LM Ericsson (publ)
- Inventors: Raihan Rafique
- Web page: https://patentgrep.com/patent/US11451252B2

## CPC

- H04B 1/0475: {with means for limiting noise, interference or distortion (H04B1/0483 takes precedence)}
- H03F 1/3247: {using feedback acting on predistortion circuits (H03F1/3264 takes precedence)}
- H03F 1/3282: {Acting on the phase and the amplitude of the input signal}
- H03F 3/189: High-frequency amplifiers, e.g. radio frequency amplifiers
- H03F 3/245: {with semiconductor devices only}
- H03F 3/68: Combinations of amplifiers, e.g. multi-channel amplifiers for stereophonics {(power amplifiers using a combination of several semiconductor amplifiers H03F3/211; combinations of amplifiers using coupling networks with distributed constants H03F3/602)}
- H03F 2200/294: the amplifier being a low noise amplifier [LNA]
- H03F 2200/451: the amplifier being a radio frequency amplifier
- H03F 2201/3209: the amplifier comprising means for compensating memory effects
- H03F 2201/3215: To increase the output power or efficiency
- H03F 2201/3233: Adaptive predistortion using lookup table, e.g. memory, RAM, ROM, LUT, to generate the predistortion
- H03F 2203/21106: An input signal being distributed in parallel over the inputs of a plurality of power amplifiers
- H04B 2001/0425: {with linearisation using predistortion}
- H04B: TRANSMISSION
- H03F: AMPLIFIERS

## Abstract

Systems and methods are disclosed herein that provide low-complexity Digital Predistortion (DPD) for a transceiver system that uses an Advanced Antenna System (AAS) to provide analog or hybrid beamforming.

## Claims

### Claim 1

A system comprising:
a digital predistortion subsystem adapted to predistort a baseband transmit signal x(t) based on an adaptation model to thereby provide a predistorted baseband transmit signal x′(t); and
a beamforming front-end comprising:
a plurality of transmit branches coupled to a plurality of transmit antenna elements, respectively, wherein the plurality of transmit branches is adapted to process the predistorted baseband transmit signal x′(t) to transmit a plurality of radio frequency transmit signals xRF,1(t) through xRF,N(t) via the plurality of transmit antenna elements, respectively; and
a receive branch coupled to a receive antenna element, the receive branch configured to receive a combined receive signal y(t) comprising the plurality of radio frequency transmit signals xRF,1(t) through xRF,N(t) transmitted via the plurality of transmit antenna elements and received via the receive antenna element;
wherein the digital predistortion subsystem is further adapted to:
process the combined receive signal y(t) based on at least a plurality of known coupling factors (C1,j through CN,j) between the plurality of transmit antenna elements and the receive antenna element, respectively, to thereby provide a normalized receive signal ypp(t); and
update the adaptation model based on a comparison of the normalized receive signal ypp(t) and the predistorted baseband transmit signal x′(t).

### Claim 2 (depends on 1)

The system of claim 1 wherein the receive branch comprises:
a low-noise amplifier adapted to amplify the combined receive signal y(t); and
attenuating circuitry adapted to attenuate the combined receive signal y(t) prior to amplification by the low-noise amplifier such that the low-noise amplifier operates in a linear mode.

### Claim 3 (depends on 1)

The system of claim 1 wherein, in order to process the combined receive signal y(t) to thereby provide the normalized receive signal ypp(t), the digital predistortion subsystem is further adapted to:
provide the normalized receive signal ypp(t) in accordance with an equation expressed as:
y
pp
⁡
(
t
)
=
y
⁡
(
t
)
∑
i
=
1
N
⁢
g
i
⁢
e
j
⁢
⁢
φ
i
⁢
C
ij
where gi is one of a plurality of gain adjustments (g1 through gN) applied by the i-th transmit branch, φi is one of a plurality of phase adjustments (φ1 through φN) applied by the i-th transmit branch, and Cij is one of the plurality of know coupling factors between the i-th transmit antenna element and the receive antenna element.

### Claim 4 (depends on 3)

The system of claim 3 wherein the plurality of gain adjustments (g1 through gN) is equal.

### Claim 5 (depends on 1)

The system of claim 1 wherein the digital predistortion subsystem is further adapted to:
determine whether a measured Adjacent Channel Leakage-power Ratio, ACLR, is acceptable when using the adaptation model;
update the adaptation model based on a comparison of the normalized receive signal ypp(t) and the predistorted baseband transmit signal x′(t) if the measured ACLR is not acceptable; and
refrain from updating the adaptation model based on the comparison of the normalized receive signal ypp(t) and the predistorted baseband transmit signal x′(t) if the measured ACLR is acceptable.

### Claim 6 (depends on 1)

The system of claim 1 wherein the beamforming front-end is adapted to form a single transmit beam with all of the plurality of transmit antenna elements and the plurality of transmit branches coupled to the plurality of transmit antenna elements.

### Claim 7

A method for providing digital predistortion in a radio node utilizing an Advanced Antenna System, AAS, and including a plurality of antenna elements for beamforming, comprising:
predistorting a baseband input signal x(t) based on an adaptation model to thereby provide a predistorted baseband transmit signal x′(t);
processing the predistorted baseband transmit signal x′(t) to transmit a plurality of radio frequency transmit signals xRF,1(t) through xRF,N(t) via a plurality of transmit antenna elements, respectively;
receiving a combined receive signal y(t) comprising the plurality of radio frequency transmit signals xRF,1(t) through xRF,N(t) transmitted via the plurality of transmit antenna elements and received via a receive antenna element;
processing the combined receive signal y(t) based on at least a plurality of known coupling factors (C1,j through CN,j) between the plurality of transmit antenna elements and the receive antenna element, respectively, to thereby provide a normalized receive signal ypp(t); and
updating the adaptation model based on a comparison of the normalized receive signal ypp(t) and the predistorted baseband transmit signal x′(t).

### Claim 8 (depends on 7)

The method of claim 7 wherein receiving the combined receive signal y(t) comprises attenuating the combined receive signal y(t) prior to amplification by a low-noise amplifier such that the low-noise amplifier operates in a linear mode.

### Claim 9 (depends on 7)

The method of claim 7 wherein processing the combined receive signal y(t) to thereby provide the normalized receive signal ypp(t) comprises providing the normalized receive signal ypp(t) in accordance with an equation expressed as:
y
pp
⁡
(
t
)
=
y
⁡
(
t
)
∑
i
=
1
N
⁢
g
i
⁢
e
j
⁢
⁢
φ
i
⁢
C
ij
where gi is one of a plurality of gain adjustments (g1 through gN) applied by the i-th transmit branch, φi is one of a plurality of phase adjustments (φ1 through φN) applied by the i-th transmit branch, and Cij is one of the plurality of know coupling factors between the i-th transmit antenna element and the receive antenna element.

### Claim 10 (depends on 9)

The method of claim 9 wherein the plurality of gain adjustments (g1 through gN) is equal.

### Claim 11 (depends on 7)

The method of claim 7 further comprising:
determining whether a measured Adjacent Channel Leakage-power Ratio, ACLR, is acceptable when using the adaptation model;
wherein:
updating the adaptation model comprises updating the adaptation model based on a comparison of the normalized receive signal ypp(t) and the predistorted baseband transmit signal x′(t) if the measured ACLR is not acceptable; and
the method further comprises refraining from updating the adaptation model based on the comparison of the normalized receive signal ypp(t) and the predistorted baseband transmit signal x′(t) if the measured ACLR is acceptable.

### Claim 12 (depends on 7)

The method of claim 7 wherein a single transmit beam is formed with all of the plurality of transmit antenna elements and the plurality of transmit branches coupled to the plurality of transmit antenna elements.

### Claim 13

A system for providing digital predistortion in a radio node utilizing an Advanced Antenna System, AAS, comprising a plurality of antenna elements to provide analog or hybrid beamforming, the system adapted to:
predistort a baseband input signal x(t) based on an adaptation model to thereby provide a predistorted baseband transmit signal x′(t);
process the predistorted baseband transmit signal x′(t) to transmit a plurality of radio frequency transmit signals xRF,1(t) through xRF,N(t) via a plurality of transmit antenna elements, respectively
receive a combined receive signal y(t) comprising the plurality of radio frequency transmit signals xRF,1(t) through xRF,N(t) transmitted via the plurality of transmit antenna elements and received via the receive antenna element;
receive a combined receive signal y(t) comprising the plurality of radio frequency transmit signals xRF,1(t) through xRF,N(t) transmitted via the plurality of transmit antenna elements and received via a receive antenna element;
process the combined receive signal y(t) based on at least a plurality of known coupling factors (C1,j through CN,j) between the plurality of transmit antenna elements and the receive antenna element, respectively, to thereby provide a normalized receive signal ypp(t); and
update the adaptation model based on a comparison of the normalized receive signal ypp(t) and the predistorted baseband transmit signal x′(t).