KR20120048362A - Integer frequency offset estimation method using correlations of adjacent pilots for ofdm systems - Google Patents

Abstract

PURPOSE: A method for estimating an integer frequency offset by using the correlation of adjacent pilots in an OFDM(Orthogonal Frequency Division Multiplexing) system is provided to estimate an integer frequency offset by using the correlation of adjacent pilots in a frequency domain. CONSTITUTION: Fast Fourier transform(201) changes a received signal into a frequency domain signal. A symbol acquisition unit(202) obtains one or more consecutive OFDM(Orthogonal Frequency Division Multiplexing) symbols through a signal passing the fast Fourier transform. A plurality of received signal moving units(203) obtains a second frequency band by transferring a first frequency band of the OFDM symbol which is obtained according to a plurality of integer frequency offset candidate values. A template generating unit generates a template to maximize the correlation value between samples. A plurality of correlation units(205) calculates the correlation value between a sample and the template in the most adjacent pilot index. An integer frequency offset estimating unit(206) finally determines an integer frequency offset.

Description

Integer frequency offset estimation method using correlations of adjacent pilots for OFDM systems

The present invention relates to a method for estimating integer frequency offset in an OFDM system. More specifically, the present invention relates to a method for estimating integer frequency offset using correlation of adjacent pilots in a frequency domain after a fast Fourier transform in a receiver.

Orthogonal frequency division multiplexing (OFDM) systems are robust to multipath fading environments and feature wireless local area networks (WLANs) and terrestrial digital video broadcasting due to features such as efficient spectrum usage. -terrestrial, hereinafter referred to as DVB-T. However, the performance of the OFDM-based system is greatly affected by the frequency offset caused by the Doppler frequency caused by the mismatch of the oscillator located at the transceiver and the relative speed of the transceiver. In general, the frequency offset is normalized and expressed by the size of the subcarrier, and divided into an integer part and a decimal part according to the size. The integer part causes the shift between subcarrier indices and the fractional part destroys orthogonality between subcarriers. Therefore, it can be said that estimating the frequency offset in the OFDM based system is very important.

The present invention provides a method for estimating integer frequency offset using correlation of adjacent pilots in a frequency domain after a fast Fourier transform.

In order to achieve the above object, the integer frequency offset estimation method according to an aspect of the present invention, the present invention relates to a method for estimating integer frequency offset, the step of performing a fast Fourier transform of the received OFDM signal, at least continuous Obtaining one OFDM symbol, moving a first frequency band of the obtained OFDM symbol according to a plurality of integer frequency offset candidate values, and obtaining a second frequency band in which pilot and data are expected to belong; For each of the samples in all pilot indices located within an OFDM symbol, generating a template using the values transmitted in the closest other pilot indices to maximize the correlation value between the samples in the closest other pilot indices; Samples at all pilot indices located in the same OFDM symbol in the second frequency band Calculating a correlation value with a template using a value transmitted from the other nearest pilot index for each, and finally integer frequency offset among the plurality of integer frequency offset candidate values according to the magnitude of the correlation value obtained through the operation Determining a step.

The calculating of the correlation value may include calculating a plurality of correlation values corresponding to each of the plurality of integer frequency offset candidate values.

According to the present invention, an excellent integer frequency offset estimation performance can be exhibited in an OFDM system for transmitting a pilot. Therefore, it can be used for integer frequency offset estimation algorithm when designing a receiver of an OFDM system that transmits pilot.

1 is a conceptual diagram illustrating arrangement in a frequency domain of continuous pilot and distributed pilot in an OFDM-based DVB-T system to which a method for estimating integer frequency offset according to an embodiment of the present invention can be applied.
2 is a configuration diagram for an integer frequency offset estimation method in an OFDM system according to an embodiment of the present invention.
3 is a conceptual diagram illustrating a correlation unit and an integer frequency offset estimation unit of FIG. 2.
4 is an experimental result graph showing the probability of success in integer frequency offset estimation of the conventional method and the estimation method according to an embodiment of the present invention in a Rayleigh multipath environment.

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. And is intended to enable a person skilled in the art to readily understand the scope of the invention, and the invention is defined by the claims. It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. In the present specification, the singular form includes plural forms unless otherwise specified in the specification. It is noted that " comprises, " or "comprising," as used herein, means the presence or absence of one or more other components, steps, operations, and / Do not exclude the addition.

A method of estimating integer frequency offset according to an embodiment of the present invention will be described with reference to FIGS. 1 to 3. 1 is a conceptual diagram illustrating arrangement in a frequency domain of a continuous pilot and a distributed pilot in an OFDM-based DVB-T system to which an integer frequency offset estimating method according to an embodiment of the present invention can be applied, and FIG. FIG. 3 is a schematic diagram illustrating a correlation unit and an integer frequency offset estimation unit of FIG. 2 in an OFDM system according to an embodiment of the present invention. FIG. 4 is a diagram illustrating a conventional technique in a Rayleigh multipath environment. An experimental result graph showing an integer frequency offset estimation success probability of the estimation method according to the embodiment of the present invention.

First, a DVB-T system for transmitting continuous pilots and distributed pilots will be described with reference to FIG. 1. The DVB-T system can be divided into 2K mode and 8K mode by the number of subcarriers. FIG. 1 exemplarily shows positions of continuous pilots and distributed pilots in a 2K mode DVB-T system using 1705 subcarriers out of a total of 2048 subcarriers for transmitting pilot and data.

The 2K mode DVB-T system transmits data and 45 consecutive pilots, 142 or 143 distributed pilots. The value of the pilot is +4/3 or -4/3, which is determined by a pseudo random binary sequence. Horizontal and vertical represent the subcarrier index and the symbol index of the OFDM system, respectively. Kmin (= 0) and Kmax (= 1704) respectively mean the smallest and largest index used to transmit data and pilot. The distributed pilot is inserted every 12 subcarriers on one OFDM symbol, and the position in one OFDM symbol is repeated every four OFDM symbols.

2 is a configuration diagram for an integer frequency offset estimation method in an OFDM system according to an embodiment of the present invention.

Referring to FIG. 2, a receiver for performing an integer frequency offset estimation method in an OFDM system according to an embodiment includes a fast Fourier transform unit 201, a symbol obtainer 202, a received signal mover 203, and a template generator. 204, a correlator 205, and an integer frequency offset estimator 206.

The fast Fourier transform unit 201 receives a received signal and converts it into a signal in a frequency domain.

The symbol obtainer 202 receives a signal passed through the fast Fourier transform unit 201 and obtains at least one consecutive OFDM symbol.

The plurality of received signal shifters 204 move the first frequency band of the acquired OFDM symbol according to the plurality of integer frequency offset candidate values to obtain a second frequency band in which the pilot and data belong. Here, the number of received signal shifters may be as many as the integer frequency offset candidate value.

The template generator generates a template in order to maximize the correlation value between samples in the adjacent pilot index located in the OFDM symbol.

The plurality of correlators 205 calculates a correlation value between the samples in the nearest pilot index and the template for the samples in all pilot indexes located in the same OFDM symbol in the second frequency band.

The integer frequency offset estimator 206 finally determines the integer frequency offset among the plurality of integer frequency offset candidate values according to the magnitude of the correlation value obtained through the calculation.

The above description will be described in more detail.

First, the OFDM signal in the time domain arrives at the receiving end after passing through the channel, and the n th sample y _ℓ (n) of the ℓ-th OFDM symbol arrived at the receiving end may be expressed by Equation 1 below.

인 덧셈꼴 백색 정규 잡음 (additive white Gaussian noise: AWGN) 샘플을 나타낸다. 신호대 잡음비는

으로 정의되며,

이다. x_ℓ(n)은 채널을 통과한 ℓ번째 OFDM 심볼의 n번째 샘플을 의미하며, 아래의 수학식 2와 같이 나타낼 수 있다.Here v denotes a frequency offset normalized to the OFDM subcarrier interval. N is the magnitude of the inverse fast Fourier transform, N _ℓ is the number of samples of one OFDM symbol including the guard interval, w _ℓ (n) is 0, and the variance is

Represents an additive white Gaussian noise (AWGN) sample. Signal-to-noise ratio

Is defined as

to be. x _l (n) refers to the n th sample of the l th OFDM symbol that has passed through the channel, and may be represented by Equation 2 below.

Where P _l (k) denotes data or pilot in the frequency domain transmitted by the k subcarrier of the l OFDM symbol, and H _l (k) denotes a channel response on the k subcarrier of the l OFDM symbol, respectively. . The frequency offset v can be divided into an integer part Δ and a fractional part ε, and it is assumed in the present invention that the fractional frequency offset is perfectly estimated and compensated (ε = 0). For example, assume that the fractional frequency offset is perfectly estimated and compensated before estimating the integer frequency offset at the receiver.

In this case, the k-th OFDM sample Y _l (k) of the l-th OFDM symbol in the frequency domain after the fast Fourier transform may be expressed as follows.

Here, W _l (k) means a complex AWGN sample in the frequency domain. It can be seen from Equation 3 that the integer frequency offset rotates the phase of the OFDM signal and results in cyclical movement of the transmission data and the pilot.

In order to calculate a correlation value between adjacent pilots in one OFDM symbol, a template according to Equation 4 below is considered. This template consists of sample values transmitted in the actual pilot index at the transmitter and is used to maximize the correlation between pilots having different values.

Here, n ∈ p _ℓ , p _ℓ means a set of pilot subcarrier indices in the l-th OFDM symbol. I means the pilot index nearest to the pilot with index n.

 By using the correlation between each pilot in one OFDM symbol and the nearest pilot, a correlation function as shown in Equation 5 below can be obtained.

Where f is a candidate value of integer frequency offset, L is the number of consecutive OFDM symbols used to estimate the integer frequency offset,

를 각각 의미하며,

Means each,

에서 i는 색인 n을 갖는 파일럿과 가장 가까운 곳에 위치한 파일럿의 색인을 의미한다.

Where i is the index of the nearest pilot to the pilot with index n.

Referring to the above process using the simplified OFDM symbol shown in Figure 3 as follows. If the actual integer frequency offset value and the candidate value are the same, pilot 1 and pilot 2, pilot 2 and pilot 1, pilot 3 and pilot 4, pilot 4 and pilot 5 located in the OFDM symbol of FIG. The correlation values are calculated using the sample values in the pilot 5 and pilot 4 indexes, respectively. If two OFDM symbols are used to estimate integer frequency offset, pilot 6 and pilot 7, pilot 7 and pilot 8, pilot 8 and pilot 9, pilot 9 and pilot 10, Correlation values are calculated using sample values at the indexes of pilot 10 and pilot 9.

The equation represented by Equation 5 may be an integer frequency offset through Equation 6 below to find the largest value.

는 추정된 정수 주파수 옵셋값을 의미한다.From here

Denotes an estimated integer frequency offset value.

4 shows the probability of success of integer frequency offset estimation in the conventional method and the estimation method according to an embodiment of the present invention in a Rayleigh multipath environment.

 Through simulations, we evaluate the performance of the proposed technique. The 2K mode DVB-T system is considered and data is generated by quadrature phase shift keying (QPSK).

In the simulation, the guard interval length was set to 128 samples, which is 1/16 of the total number of samples in one OFDM symbol, and one OFDM symbol used for integer frequency offset estimation. The multipath channel has 10 paths and the time difference of each path is set to 10 samples. The path sizes follow a Rayleigh distribution that is independent of each other. The magnitude of each path follows an exponential distribution, with 20 dB difference in power between the first and tenth paths. The phase of each path has a uniform distribution at (-π, π) and the Doppler frequency is set to 100 Hz. Can be.

Specifically, a plurality of correlation values corresponding to each of a plurality of integer frequency offset candidate values are calculated, and then a real component is calculated for each of the plurality of correlation values, and the integer of which the real component is the largest among the plurality of correlation values. The frequency offset candidate value may be finally determined as the integer frequency offset.

The present invention relates to a method for estimating integer frequency offset in an OFDM system transmitting a pilot, and uses correlation between nearest pilots for all pilots located in an OFDM symbol to estimate the integer frequency offset. In other words, the technique proposed in the prior art uses only the correlation between the distributed pilots located closest to the center of the continuous pilot, but the present invention uses not only this relationship but also the correlation between the pilots located closest to the center of the distributed pilots. It was.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be defined not only by the scope of the following claims, but also by the equivalents of the claims.

Claims (2)

A method for estimating integer frequency offset by receiving an OFDM symbol using a pilot,
Fast Fourier transforming the transmitted OFDM signal;
Obtaining at least one contiguous OFDM symbol;
Moving a first frequency band of the obtained OFDM symbol according to a plurality of integer frequency offset candidate values to obtain a second frequency band in which pilot and data are expected to belong;
Generating a template using samples of pilot indexes transmitted from the closest pilot index for each sample of all pilot indexes located within the OFDM symbol to maximize the correlation value between samples in the other nearest pilot index step;
Calculating a correlation with each template for each sample at all pilot indices located in the same OFDM symbol in the second frequency band; And
Finally determining an integer frequency offset among the plurality of integer frequency offset candidate values according to the magnitude of the correlation value obtained through the operation
Method for estimating an integer frequency offset comprising a. The method of claim 1,
The template includes, for each of all the pilots, information about the ratio with the sample of the nearest pilot index;
Estimating an integer frequency offset.

Images