# US 9,538,537 B1 · Blind carrier synchronization method for OFDM wireless communication systems

> Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt

- Number: US 9,538,537 B1
- Document ID: US9538537B1
- Status: Granted
- Kind: B1
- Application number: 15/235,052
- Filed: 2016-08-11
- Granted: 2017-01-03
- Earliest claimed: 2015-08-11
- Assignees: PhasorLab, Inc.
- Applicants: PhasorLab, Inc.
- Inventors: Jian Cui; Joshua C. Park
- Web page: https://patentgrep.com/patent/US9538537B1

## CPC

- H04L 27/26524: {Fast Fourier transform [FFT] or discrete Fourier transform [DFT] demodulators in combination with other circuits for demodulation}
- H04J 11/0036: {at the receiver}
- H04L 5/0007: {the frequencies being orthogonal, e.g. OFDM(A) or DMT}
- H04L 27/2657: {Carrier synchronisation}
- H04L 27/2659: {Coarse or integer frequency offset determination and synchronisation}
- H04L 27/2676: {Blind, i.e. without using known symbols}
- H04L 2025/0342: {QAM}
- H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M)
- H04J: MULTIPLEX COMMUNICATION (transmission in general H04B; peculiar to transmission of digital information H04L5/00; systems for the simultaneous or sequential transmission of more than one television signal H04N7/08; in exchanges H04Q11/00; stereophonic systems H04S)

## Abstract

Systems and methods are disclosed herein for blind frequency synchronization. In one embodiment, a method is disclosed, comprising: downconverting a received orthogonal frequency division multiplexed (OFDM) signal to baseband; identifying, from the downconverted received signal, a series of OFDM symbols in the time domain; performing a fast Fourier transform (FFT) on a block of several time domain samples to turn the time domain OFDM symbols into frequency domain OFDM symbols, one sample per subcarrier in the received OFDM signal; computing a cross-correlation between in-phase and quadrature samples in each subcarrier and for each frequency domain OFDM symbol, wherein the cross-correlation may be computed as a sum of products of either squares or absolute values of the in-phase and quadrature samples; and summing the computed cross-correlation across the series of symbols and across all subcarriers to determine a frequency offset for the received OFDM signal.

## Claims

### Claim 1

A method, comprising:
downconverting a received orthogonal frequency division multiplexed (OFDM) signal to baseband;
identifying, from the downconverted received signal, a series of OFDM symbols in the time domain;
performing a fast Fourier transform (FFT) on a block of several time domain samples to turn the time domain OFDM symbols into frequency domain OFDM symbols, one sample per subcarrier in the received OFDM signal;
computing a cross-correlation between in-phase and quadrature samples in each subcarrier and for each frequency domain OFDM symbol, wherein the cross-correlation is computed as a sum of products of either squares or absolute values of the in-phase and quadrature samples; and
summing the computed cross-correlation across the series of symbols and across all subcarriers to determine a frequency offset for the received OFDM signal.

### Claim 2 (depends on 1)

The method of claim 1, wherein the method is performed at a radio receiver.

### Claim 3 (depends on 1)

The method of claim 1, wherein a quantity of the series of symbols is based on an arbitrarily-configured number sufficient to cause a synchronization algorithm to converge to within a desired error range.

### Claim 4 (depends on 1)

The method of claim 1, wherein the summed cross-correlation is a cumulative phase measurement.

### Claim 5 (depends on 1)

The method of claim 1, wherein a subset of the received symbols that are repeated symbols that have been inserted for carrier synchronization are discarded.

### Claim 6 (depends on 1)

The method of claim 1, wherein the method is initiated at device power on, upon signal acquisition, at scheduled intervals, or upon detecting a loss of synchronization, with a same number of input samples being used each time the method is performed.

### Claim 7 (depends on 1)

The method of claim 1, wherein the received OFDM signal is quadrature amplitude modulation (QAM) modulated.

### Claim 8 (depends on 1)

The method of claim 1, wherein the received OFDM signal is a Wi-Fi, WiMAX, WiGig, or Long Term Evolution (LTE) signal.

### Claim 9 (depends on 1)

The method of claim 1, wherein the series of OFDM symbols in the time domain are a subset of the received OFDM symbols.

### Claim 10 (depends on 1)

The method of claim 1, further comprising processing the received OFDM signal with the frequency offset.

### Claim 11 (depends on 1)

The method of claim 1, further comprising using the frequency offset to achieve time synchronization with a transmitter of the received OFDM signal.

### Claim 12

A blind frequency synchronization method comprising:
obtaining orthogonal frequency domain multiplexed (OFDM) symbols from a received information signal that are in the time domain;
performing a Fourier transform on the time domain OFDM symbols to obtain OFDM symbols in the frequency domain; and
determining a frequency offset based on an estimated correlation between the in-phase signal samples and the quadrature signal samples summed over each subcarrier and summed over each frequency domain OFDM symbol.

### Claim 13 (depends on 12)

The method of claim 12, wherein the time domain OFDM symbols are baseband OFDM symbols, and further comprising obtaining the OFDM symbols from the information signal by downconverting a received modulated carrier signal with a local oscillator (LO) signal to produce baseband OFDM symbols.

### Claim 14 (depends on 12)

The method of claim 12, further comprising processing the received information signal using the frequency offset to correct for frequency offset in the received information signal.

### Claim 15 (depends on 12)

The method of claim 12, further comprising using the frequency offset to achieve time synchronization with a transmitter of the received information signal.

### Claim 16 (depends on 12)

The method of claim 12, wherein the estimated correlation between the in-phase signal samples and the quadrature signal samples is based on squared in-phase samples and squared quadrature samples, or is based on absolute values of in-phase samples and absolute values of quadrature samples.

### Claim 17 (depends on 12)

The method of claim 12, wherein determining the frequency offset further comprises time averaging and integrating a product of either squares of or absolute values of the in-phase signal and the quadrature signal.

### Claim 18 (depends on 12)

The method of claim 12, wherein determining the frequency offset further comprises calculating:
∑
m
⁢
∑
l
⁢
{

I
R
⁡
(
l
,
m
)

2
⁢

Q
R
⁡
(
l
,
m
)

2
}
where m refers to each subcarrier, l refers to each OFDM symbol, IR(l,m) refers to an in-phase part of an mth subcarrier in an lth OFDM symbol, and QR(l,m) refers to a quadrature part of the mth subcarrier in the lth OFDM symbol.

### Claim 19 (depends on 12)

The method of claim 12, wherein determining the frequency offset further comprises calculating:
∑
m
⁢
∑
l
⁢
{

I
R
⁡
(
l
,
m
)

·

Q
R
⁡
(
l
,
m
)

}
where m refers to each subcarrier, l refers to each OFDM symbol, IR(l,m) refers to an in-phase part of an mth subcarrier in an lth OFDM symbol, and QR(l,m) refers to a quadrature part of the mth subcarrier in the lth OFDM symbol.

### Claim 20

A system, comprising:
a radio receive chain for receiving an input orthogonal frequency domain multiplexed (OFDM) signal;
a baseband processor coupled to the radio receive chain and capable of computing a Fourier transform; and
a non-transitory computer-readable medium comprising instructions that, when executed by the baseband processor, cause the system to perform steps comprising:
obtaining orthogonal frequency division multiplexed (OFDM) symbols from the input OFDM signal that are in the time domain;
performing a Fourier transform on the time domain OFDM symbols to obtain OFDM symbols in the frequency domain; and
determining a frequency offset based on an estimated correlation between the in-phase signal samples and the quadrature signal samples summed over each subcarrier and summed over each frequency domain OFDM symbol.