KR101284393B1 - Transmitter for modulating using cyclic delay diversity and receiver for estimating residual frequency offset of ofdm system - Google Patents

Abstract

The present invention relates to a transmitter and a receiver of an Orthogonal Frequency Division Multiplexing (OFDM) communication system to which a Cyclic Delay Diversity (CDD) scheme is applied. A fast Fourier transform unit for transforming (FFT); A pattern selector configured to estimate a pilot subcarrier pattern having the largest channel transfer function using the pilot subcarriers processed by the fast Fourier transform unit; And an estimator configured to calculate a phase value rotated from a correlation value calculated through autocorrelation based on an estimated pilot subcarrier pattern and a cyclic delay value of the received signal, and estimate a residual frequency offset. The cyclic delay value causes the channel transfer function (CTF) to have different sizes according to the cyclic delay values between different transmission antennas.
As described above, according to the present invention, when applying the CDD technique, when the CDD technique is applied, performance degradation of the residual frequency offset estimation due to the frequency selective characteristic of the generated channel can be improved.

Description

TRANSMITTER FOR MODULATING USING CYCLIC DELAY DIVERSITY AND RECEIVER FOR ESTIMATING RESIDUAL FREQUENCY OFFSET OF OFDM SYSTEM}

The present invention relates to a transmitter and a receiver of an orthogonal frequency division multiple modulation scheme communication system, and more particularly, orthogonal frequency division multiplexing (OFDM) to which a Cyclic Delay Diversity (CDD) technique is applied. It is a technique for estimating residual frequency offset (RFO) in a digital radio mondiale plus (DRM +) system.

Orthogonal Frequency Division Multiplexing (OFDM) is used in many radio broadcasting systems, including digital radio mondiale (DRM), in which the entire channel is divided into multiple orthogonal subchannels and transmitted in parallel for ultra-high speed data transmission. do.

In this case, the Cyclic Delay Diversity (CDD) technique has an advantage that it can be applied without changing the physical layer of the currently commercialized broadcast system because signal processing is performed in the OFDM symbol of the time axis.

However, the influence of the frequency selective characteristic of the channel due to the CDD technique causes a deterioration in the estimation performance of the residual frequency offset (RFO), a major weakness of OFDM.

In other words, the CDD technique causes severe performance degradation of the estimation of the residual frequency offset performed after the FFT because the cyclically delayed transmission data is transformed into the phase shift on the frequency axis due to a Fast Fourier Transform (FFT) operation. .

The background technology of the present invention is described in Korean Patent Publication No. 10-0890182.

The technical problem to be achieved by the present invention is the optimal repetition in the Digital Radio Mondiale plus (DRM +) system based on Orthogonal Frequency Division Multiplexing (OFDM) with Cyclic Delay Diversity (CDD) A transmitter and a receiver of an orthogonal frequency division multiple modulation scheme communication system for selecting a delay value and a pilot subcarrier pattern to estimate a residual frequency offset.

In the transmitter of an orthogonal frequency division multiple modulation scheme communication system according to an embodiment of the present invention, the cyclic delay such that channel transfer functions (CTFs) have different sizes according to cyclic delay values between different transmission antennas. A selection unit for selecting a value; And a cyclic delay unit configured to generate a continuous orthogonal frequency division multiplexed modulation signal cyclically delayed according to the cyclic delay value.

In this case, the selector may select the cyclic delay value such that the synthesized channel transfer function has the maximum size according to the cyclic delay value of the second transmit antenna.

According to another embodiment of the present invention, a receiver of an orthogonal frequency division multiple modulation scheme communication system includes: a fast Fourier transform unit for Fourier transforming a cyclically delayed received signal; A pattern selector configured to estimate a pilot subcarrier pattern having the largest channel transfer function using the pilot subcarriers processed by the fast Fourier transform unit; And an estimator configured to calculate a phase value rotated from a correlation value calculated through autocorrelation based on an estimated pilot subcarrier pattern and a cyclic delay value of the received signal, and estimate a residual frequency offset.

The cyclic delay value causes the channel transfer function (CTF) to have different sizes according to the cyclic delay values between different transmit antennas.

In this case, the pattern selector estimates a gain reference cell pattern having the largest magnitude of the channel transfer function using the cyclic delay value, and the estimator is configured to estimate the gain of the channel transfer function among the gain reference cell patterns repeated every four symbols. The residual frequency offset is estimated only at the symbol with the largest magnitude.

The estimator may further include a size of the channel transfer function determined by a cyclic delay value of a second antenna, a number of gain reference cell patterns of a pilot subcarrier, a period of a gain reference cell pattern in a time domain, and a number of pilot subcarriers. The largest gain reference cell pattern can be estimated.

As described above, according to an embodiment of the present invention, the residual frequency offset is selected by selecting an optimal cyclic delay value and a pilot subcarrier pattern to estimate the residual frequency offset, and the residual due to the frequency selective characteristic of the channel generated when the CDD scheme is applied in the conventional OFDM system. The performance degradation of the frequency offset estimation can be improved.

In particular, when there is no cyclic delay in the first transmit antenna and there is a cyclic delay satisfying the condition of the difference in cyclic delay between adjacent antennas, an optimal cyclic delay value for having a different channel transfer function for each pilot subcarrier pattern The estimated residual frequency offset can be estimated more accurately than the prior art by finding a, and estimating the residual frequency offset by selecting a pilot subcarrier pattern having the largest magnitude of the channel transfer function.

1 is a block diagram showing a configuration of an orthogonal frequency division multiple modulation scheme communication system according to an embodiment of the present invention;
2 is a flowchart illustrating a method of estimating a residual frequency offset (RFO) of an orthogonal frequency division multiple modulation scheme communication system according to an embodiment of the present invention;
3 shows MSE performance of the conventional method and the residual frequency offset estimation method according to an embodiment of the present invention in an urban channel environment with mobile velocity and SNR in an orthogonal frequency division multiple modulation communication system according to an embodiment of the present invention,
4 illustrates MSE performance of a conventional method according to various mobile velocities in an urban channel environment and a residual frequency offset estimation method according to an embodiment of the present invention in an orthogonal frequency division multiple modulation scheme communication system according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The terms used are terms selected in consideration of the functions in the embodiments, and the meaning of the terms may vary depending on the user, the intention or the precedent of the operator, and the like. Therefore, the meaning of the terms used in the following embodiments is defined according to the definition when specifically defined in this specification, and unless otherwise defined, it should be interpreted in a sense generally recognized by those skilled in the art.

1 is a block diagram showing a configuration of an orthogonal frequency division multiplexing (hereinafter, referred to as "OFDM") communication system according to an embodiment of the present invention, Figure 2 is a view according to an embodiment of the present invention A flowchart illustrating a method of estimating residual frequency offset (RFO) of an orthogonal frequency division multiple modulation scheme communication system.

Here, the OFDM communication system is a DRM + (Digital Radio Mondiale plus) system in which a Cyclic Delay Diversity (CDD) scheme having Nt transmit antennas and a single receive antenna is applied.

Referring to FIG. 1, an OFDM communication system includes a transmitter 100 and a receiver 200.

Here, the transmitter 100 includes a selector 110 and a cyclic delay unit 130, each component is as follows.

The selector 110 selects an optimal cyclic delay value such that a channel transfer function (CTF) has a different magnitude value according to a cyclic delay value between a plurality of transmit antennas.

In this case, an optimal cyclic delay value is calculated such that the combined channel transfer function (CTF) has the maximum size according to the selection of the cyclic delay value of the second transmit antenna among the plurality of transmit antennas.

The cyclic delay unit 130 generates a continuous OFDM signal by performing a cyclic delay according to the cyclic delay value selected by the selector 110.

In addition, the receiver 200 includes a fast Fourier transform unit 210, a pattern selector 230, an autocorrelation unit 250, and an estimator 270, and each component is as follows.

The fast Fourier transform unit 210 demodulates the fast delayed Fourier transform (FFT) from the time axis to the frequency axis by the cyclically delayed received signal transmitted from the transmitter 100.

The pattern selector 230 estimates the position of a gain reference cell (GRC) having the largest channel transfer function (CTF) using the pilot subcarriers processed by the fast Fourier transform unit 210. That is, the gain reference cell (GRC) pattern having the largest magnitude of the channel transfer function (CTF) is selected using the cyclic delay value selected by the selector 110 for the signal converted into the frequency axis by the fast Fourier transform unit 210. do.

The autocorrelation unit 250 calculates an autocorrelation value using its signal based on the selected cyclic delay and the gain reference cell (GRC) pattern.

The estimator 270 estimates a residual frequency offset (RFO) by calculating phase values in which the autocorrelation value calculated by the autocorrelation unit 250 is rotated.

The operation of the transmitter 100 and the receiver 200 of the OFDM communication system described so far are as follows.

Referring to FIG. 2, the selector 110 of the transmitter 100 designs an optimal cyclic delay between transmit antennas to which a cyclic delay diversity (CDD) scheme is applied, and transmits channels according to cyclic delay values between a plurality of transmit antennas. The optimal cyclic delay value is selected so that the function (CTF, Channel Transfer Function) has different size values (S101).

Next, the cyclic delay unit 130 of the transmitter 100 generates a continuous OFDM signal by performing a cyclic delay based on the cyclic delay value selected in step S101 (S103).

Next, the fast Fourier transform unit 210 of the receiver 200 performs FFT demodulation of the signal received from the transmitter 100 from the time axis to the frequency axis (S105).

Next, the pattern selector 230 of the receiver 200 uses the cyclic delay value selected in step S101 to convert the signal converted to the frequency axis into a gain reference cell GFC having the largest value of the channel transfer function CTF. ) To estimate the pattern (S107).

The GRC pilot subcarriers inserted in the received signal are generated through a continuous OFDM signal cyclically delayed at the transmitter 100.

Next, the estimator 250 of the receiver 200 has a residual frequency only at a symbol having the largest channel transfer function CTF among the gain reference cell (GFC) patterns repeated every four symbols selected by the pattern selector 230. The offset is estimated (S109).

The process of estimating the residual frequency offset will be described in more detail by using Equation.

First, the transmitter 100 is applied to an OFDM-based DRM + system to which a CDD scheme having N _T transmit antennas and a single receive antenna is applied. The transmitter 100 generates an OFDM signal having N subcarriers and a guard interval size of N _{g by} an Inverse Fast Fourier Transform (IFFT).

In this case, the OFDM symbol is transmitted by performing a cyclic delay of δ _t at each transmit antenna after the Inverse Fast Fourier Transform (IFFT), and the OFDM symbol has a length of N _s = N + N _g . .

In the OFDM-based DRM + system to which the CDD scheme is applied, the l-th OFDM symbol of the t-th transmit antenna can be expressed as follows.

Where N is the size of the FFT, ( _N ) modulo N operation, N _T is the number of transmit antennas, and δ _t is the cyclic delay value at the t-th antenna.

In this case, the maximum cyclic delay between adjacent transmit antennas may be represented by Equation 2 below.

Assuming that the time error of the received signal R _l (k) is perfectly estimated and compensated, the k-th subcarrier signal R _l (k) of the l-th symbol considering the minute residual frequency error after the FFT may be expressed as follows.

인 복소 가산성 백색 가우시안 잡음(AWGN, Additive White Gaussian Noise)이다. 그러므로 동일한 채널 전달 함수

이고, H^l _t(k)는 t 번째 전송 안테나로부터 평균이 0이고 분산이

인 주파수 채널 응답이다.Where N _g is the length of the guard zone and N _s = N + N _g , ㅿ _c is the residual frequency offset normalized by the subcarrier spacing, W _l (k) is the mean 0 and the variance

Additive White Gaussian Noise (AWGN). Therefore the same channel transfer function

H ^l _t (k) is 0 from the t th transmit antenna and the variance is

Is the frequency channel response.

In addition, a frequency reference cell (FRC) does not exist in the DRM + system. Thus, the autocorrelation unit 250 of the receiver 200 performs a correlation calculation using a gain reference cell GRC, which can be represented by Equation 4 below.

는 N_p개의 파일럿을 가지는 GRC들의 집합을 나타낸다. 이 때,

라고 가정하면, 수학식 4의 상관식을 근사화 할 수 있고,

는 위의 수학식 4와 같이 나타낼 수 있다. 이러한 수학식 4를 사용하여 수신기(200)의 추정부(270)는 잔여 주파수 오프셋을 추정하며, 다음과 같이 표현할 수 있다.Where D _t is the period of the GRC pattern in the time domain,

Denotes a set of GRCs having N _p pilots. At this time,

If we assume that, the correlation of Equation 4 can be approximated,

May be expressed as in Equation 4 above. Using Equation 4, the estimator 270 of the receiver 200 estimates a residual frequency offset and may be expressed as follows.

은 추정된 잔여 주파수 오프셋이며,

는 복소수에서 각을 구하는 연산자이다. 채널이 항상 일정하거나 거의 변화가 없다는 가정 하에 수학식 4의 MSE (Mean Square Error)를 구하면 다음 수학식 6과 같이 나타낼 수 있다.here,

Is the estimated residual frequency offset,

Is the operator to find the angle in complex numbers. Assuming that the channel is always constant or almost unchanged, MSE (Mean Square Error) of Equation 4 can be obtained as Equation 6 below.

와

은 평균 SNR이다.here,

Wow

Is the average SNR.

을 결정한다.In this case, to improve the estimation performance of the residual frequency offset when the CDD scheme is applied in the DRM + system, it is assumed that there are no cyclic delays in the first transmission antenna when there are two transmission antennas, and the selector 110 of the transmitter 100 Cyclic Delay at Second Antenna

.

은 주파수 비 선택적이라 가정하면서, 수학식 4에 나타난 채널 전달 함수(CTF)는 다음 수학식 7과 같이 나타낼 수 있다.channel

Assuming that is frequency non-selective, the channel transfer function (CTF) shown in Equation 4 may be expressed as Equation 7 below.

,

,

,

,

,

는 각각 x의 실수부와 허수부를 나타낸다.here,

,

,

,

,

,

Denotes the real and imaginary parts of x, respectively.

의 지연 값, 그리고 시스템 파라미터 N_p, D_f, N에 의해 결정된다. As such, the channel size of the received signal is the second antenna

The delay value of, and the system parameters N _p , D _f , N are determined by.

If δ ₀ = 0 and δ ₁ = N / 2, the channel transfer function CTF in Equation 7 has the same channel size for all gain reference cell GRC patterns.

At this time, the selector 110 of the transmitter 100 determines the cyclic delay δ ₁ in the second antenna in order to have a different channel size for each gain reference cell (GRC) pattern, which is calculated through Equation (8). Can be.

)를 하기 수학식 9를 통해 도출한다.In addition, the pattern selector 230 of the receiver 200 estimates a gain reference cell (GRC) pattern for causing the channel transfer function (CTF) to have a maximum size.

) Is derived through Equation 9 below.

)을 산출한다.After the pattern selector 230 of the receiver 200 selects a gain reference cell (GRC) pattern in Equation 9, the estimator 270 of the receiver 200 uses a residual frequency offset (Equation 10).

).

이고, 순환 지연 값이 수학식 8에서 구한 정수 값

일 경우,

이다. 이때, 수학식 8의 최대값을 갖는 채널 전달 함수(CTF)는 수학식 11과 같이 나타낼 수 있다.Where, in equation (7)

The cyclic delay value is an integer value obtained from Equation 8

If it is,

to be. In this case, the channel transfer function CTF having the maximum value of Equation 8 may be represented by Equation 11.

)이 수학식 12를 통해 도출된다.Then, based on Equations 4 and 10, the final residual frequency offset (

) Is derived from Equation 12.

Meanwhile, the results of simulating the contents described so far are the same as those of FIGS. 3 and 4.

3 is a proposed method for estimating the residual frequency offset of the conventional method and the present invention in an urban channel environment with mobile velocity v = 0 km / h and SNR = 40 dB in an orthogonal frequency division multiple modulation scheme communication system according to an embodiment of the present invention. 4 shows the MSE performance of the scheme, and FIG. 4 shows the conventional method according to various mobile velocities in the Urban channel environment and the proposed residual frequency offset estimation scheme of the present invention in an orthogonal frequency division multiple modulation scheme communication system according to an embodiment of the present invention. Indicates MSE performance.

For actual implementation, it is assumed that the transmission antenna N _T = 2, and channels of each antenna are irrelevant.

OFDM with N = 214, N _g = 24, N _p = 14, D _f = 16, D _t = 4, ㅿ _c = 0.01, BW (Bandwidth) = 95kHz conforming to DRM mode E standard The system was considered and the Urban channel model was used.

Referring to FIG. 3, the MSE performance of the conventional method and the proposed residual frequency offset estimating technique of the present invention in an urban channel environment with moving vehicle speed v = 0 km / h and SNR = 40 dB are shown.

That is, a low MSE is shown in the cyclic delay value δ ₁ = {13, 27, 40, 67, 80, 94, 120, 134, 147, 174, 187, 201} obtained using Equation 8.

However, when δ ₁ = {53, 107, 161}, it can be seen that each time the GRC patterns have the same channel transfer function size, no large performance difference is seen. This simulation confirmed that there is some tradeoff between the performance of the residual frequency offset estimator and the setting of the cyclic delay value.

Referring to FIG. 4, the MSE performance of the conventional method and the proposed residual frequency offset estimating method according to various mobile velocities in an urban channel environment are shown.

In this simulation, it can be seen that the performance of the proposed residual frequency offset estimator shows better performance than the conventional method when v = 2, 20 and 60 km / h.

However, it can be seen that as the speed of the moving object increases, the overall performance of the residual frequency offset estimator occurs. This is due to the increased time-selective characteristics of the channel.

Meanwhile, embodiments of the present invention can be implemented by computer readable codes on a computer readable recording medium. A computer-readable recording medium includes all kinds of recording apparatuses in which data that can be read by a computer system is stored.

Examples of the computer-readable recording medium include a ROM, a RAM, a CD-ROM, a magnetic tape, a floppy disk, an optical data storage device and the like, and also a carrier wave (for example, transmission via the Internet) . The computer readable recording medium can also be distributed over network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. And functional programs, codes and code segments for implementing the present invention can be easily inferred by programmers in the art to which the present invention belongs.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, Therefore, the present invention should be construed as a description of the claims which are intended to cover obvious variations that can be derived from the described embodiments.

100: Transmitter
110: selection unit
130: circular delay unit
200 receiver
210: fast Fourier transform unit
230: pattern selection unit
250: autocorrelation
270: estimator

Claims (8)

A transmitter of an orthogonal frequency division multiple modulation scheme communication system,
The channel transfer function (CTF) has a different size according to a cyclic delay value between at least one different transmit antenna, and is synthesized according to the cyclic delay value of the transmit antenna among the at least one different transmit antenna. A selection unit for selecting a cyclic delay value such that the channel transfer function has a maximum magnitude; And
And a cyclic delay unit configured to generate an orthogonal frequency division multiplexed modulation signal circulated and continuous according to the cyclic delay value.
The selector may include a cyclic delay value (

A transmitter using a cyclic delay diversity scheme that selects

Here, N is the FFT size and D _f means the periodicity of the gain reference cell pattern on the frequency axis. delete delete In the receiver of an orthogonal frequency division multiple modulation scheme communication system,
A fast Fourier transform unit for performing Fourier transform (FFT) on the cyclically delayed received signal;
A pattern selector configured to estimate a pilot subcarrier pattern having a largest channel transfer function and a gain reference cell pattern using the pilot subcarriers processed by the fast Fourier transform unit; And
Based on the estimated pilot subcarrier pattern and the cyclic delay value of the received signal, a rotated phase value is calculated from a correlation value calculated through autocorrelation, and the magnitude of the channel transfer function among the gain reference cell patterns repeated every four symbols. Includes an estimator for estimating the residual frequency offset only in the largest symbol,
The cyclic delay value causes a channel transfer function (CTF) to have different sizes according to the cyclic delay values between different transmit antennas,
The gain reference cell pattern may include a cyclic delay value of a second antenna among at least one different antenna of the transmitter, a number of gain reference cell patterns of a pilot subcarrier, a period of a gain reference cell pattern in a time domain, and a number of pilot subcarriers And estimate a residual frequency error based on the magnitude of the maximum channel transfer function determined by the maximum. delete delete 5. The method of claim 4,
The pattern selection unit,
By using the following equation, the gain reference cell pattern (

Receiver to estimate the residual frequency error

Here, H _{(l) 4} (k) represents a channel transfer function of the k-th subcarrier of the l-th symbol. 5. The method of claim 4,
Wherein the estimating unit comprises:
The residual frequency offset (

A receiver estimating the residual frequency error

here,

Where N _g is the length of the guard interval, N is the number of subcarriers, and D _t represents the period of the GRC pattern in the time domain.

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