KR20070060928A - Time and frequency offset estimation with antenna diversity in ofdm communication system - Google Patents

Abstract

A time and frequency offset estimating method and an apparatus using diversity in an OFDM(Orthogonal Frequency Division Multiplexing) communication system are provided to enhance performance in a low SNR(Signal to Noise Ratio) and multi-path environment by using weight. A time and frequency offset estimating apparatus using diversity in an OFDM communication system includes a delay device(101), a first adding device(104), a first summing device(105), a second adding device(107), a second summing device(108), a time offset detecting device(110), and a frequency offset outputting device(111). The delay device(101) delays a received signal by a random index. The first adding device(104) adds a square of the delayed signal and a square of the original received signal. The first summing device(105) adds the adding result of the first adding device(104) as much as the given random number, and calculates a first sum by multiplying the adding result by a value calculated by using weight set for the received signal, signal power, and noise power. The second adding device(107) adds the original received signal and a value obtained by changing a code of an imaginary number part of a complex number of the delayed received signal by the delay device(101). The second summing device(108) adds the adding result of the second adding device(107) as much as the given random number, and outputs a second sum. The time offset detecting device(110) subtracts an absolute value of the second summing device(108) and the result value of the first summing device(105), and detects a maximum value. The frequency offset outputting device(111) outputs an angle for the absolute value of the second summing device(108), and outputs a frequency offset by using a time offset detected by the time offset detecting device(110).

Description

Time and frequency offset estimation method using diversity in orthogonal frequency division multiplexing communication system and apparatus therefor {Time and Frequency Offset Estimation with Antenna Diversity in OFDM Communication System}

1 is a conceptual diagram illustrating a general OFDM system;

2 is a diagram illustrating a general OFDM symbol structure;

3 is a conceptual diagram illustrating an OFDM system using two antennas;

4 is a configuration diagram of an estimation apparatus for frequency offset and time synchronization in an OFDM communication system according to the present invention.

* Explanation of symbols for the main parts of the drawings

The present invention relates to a method and apparatus for estimating time and frequency offset in an Orthogonal Frequency Division Multiplexing (OFDM) communication system. More particularly, the present invention relates to a low signal-to-noise ratio by combining diversity of antennas in an OFDM communication system. The present invention relates to a method and apparatus for estimating time and frequency offset in a multipath channel environment.

In the present invention, an orthogonal frequency division multiplexing (OFDM) system includes an orthogonal frequency division multiple access (OFDMA) system, an orthogonal frequency and code division multiplexing (OFCDM) system, and the like.

Orthogonal Frequency Division Multiplexing (OFDM) access is an efficient digital transmission scheme in band usage, and was first proposed by Chang. Recently, the OFDM access method has been applied to digital transmission systems such as digital audio broadcasting, digital video broadcasting, Asymmetric Digital Subscriber Line (ADSL), and Wireless Local Area Network (WLAN) in Europe.

The orthogonal frequency division multiplexing scheme has the advantage of being robust against inter-symbol interference (ISI), which is a major problem in high frequency usage efficiency and high speed communication, and making frequency selective fading appear non-selective fading. However, the OFDM access scheme has a fatal disadvantage compared to a single carrier system in that oscillators of the transmitter and the receiver are inconsistent or sensitive to carrier frequency errors due to Doppler frequency shift in the receiver. This carrier frequency error breaks the orthogonality between subcarriers in an OFDM system, whereby each subcarrier is affected by intercarrier interference (ICI) from another carrier. Therefore, the frequency synchronization problem, which causes considerable performance degradation even with a small carrier frequency error, has been noted as one of the most important problems in implementing an OFDM system.

Many proposals have been made to solve the problems of the OFDM system, but it can be largely divided into a data dependent method and a blind method.

First, the data dependent method using two pilot symbols for estimating both integer and decimal number of time synchronization and frequency offset can be estimated faster and more reliably than the blind method, but since it uses two pilot blocks, The disadvantage is that the data rate is reduced and power efficiency is lowered.

In contrast, the blind method uses a cyclic prefix (CP) which is inserted to reduce the inter-symbol interference (ISI), so that the data rate is not reduced or the power efficiency is not reduced.

The conventional blind method is a method of estimating and then compensating for a frequency offset using a CP and using a maximum likelihood (ML), which is the best frequency synchronization estimation method. In the conventional blind method, when the time synchronization is correct, only the phase difference corresponding to the frequency offset between the portion corresponding to the CP and the portion copied to the CP uses the same property. However, such a blind method has good performance at high signal-to-noise ratio (SNR) in AWGN channel, but has low performance at low signal-to-noise ratio and multipath channel environment.

Accordingly, the present invention has been proposed to solve the above problems, and a method and apparatus capable of estimating a good performance time offset and frequency offset even in a low signal-to-noise ratio and a multipath channel environment by combining antenna diversity in an OFDM communication system The purpose is to provide.

Other objects and advantages of the present invention can be understood by the following description, and will be more clearly understood by the embodiments of the present invention. Also, it will be readily appreciated that the objects and advantages of the present invention may be realized by the means and combinations thereof indicated in the claims.

According to an aspect of the present invention, there is provided an apparatus for estimating a time and frequency offset of a signal received through at least two antennas, comprising: delay means for delaying the received signal by an arbitrary exponent; First adding means for adding a square value for the signal delayed by the delay means and a square value for the original received signal; After adding the given number to the addition result of the first adding means, the sum is multiplied by a value calculated by using the weight and signal power and noise power applied to the received signal, respectively, to add the first sum. First summing means for calculating a value; Second adding means for adding a value that changes the sign of an imaginary imaginary part of the received signal delayed by said delay means and an original received signal; Second adding means for adding a result of the second adding means by a given number to calculate a second sum value; Time offset detection means for detecting a maximum value by subtracting an absolute value of the second summation step and a result value of the first summation means; And frequency offset calculating means for calculating an angle with respect to the absolute value of the second summing means and then calculating a frequency offset using the time offset detected by the time offset detecting means.

In addition, the method according to the present invention for achieving the above object, in the method for estimating the time and frequency offset for a signal received through at least two antennas, for a signal delayed by any exponent for the received signal A first step of adding a square value and a square value of the original received signal; A second step of performing an addition by any given number to the addition result of the first step, and then multiplying the addition result by a value calculated by using the weight and signal power and noise power applied to the received signal, respectively; A third step of adding an original received signal with a value changed from a sign of a imaginary imaginary part of the received signal delayed by an arbitrary exponent; A fourth step of summing up any number given for the result of the third step; A fifth step of detecting a maximum time offset value by subtracting an absolute value of the total value of the fourth step and a result value of the second step; And a sixth step of calculating an angle with respect to the absolute value of the sum of the fourth step and then calculating a frequency offset using the time offset value detected in the fifth step.

The above objects, features and advantages will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, whereby those skilled in the art may easily implement the technical idea of the present invention. There will be. In addition, in describing the present invention, when it is determined that the detailed description of the known technology related to the present invention may unnecessarily obscure the gist of the present invention, the detailed description thereof will be omitted. Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Although an embodiment of the present invention describes a basic OFDM system using two antennas, the method and apparatus according to the present invention are applied to uplink and downlink links of all systems such as OFDMA based on OFDM using multiple antennas. The possibilities are natural.

1 is a diagram illustrating a general OFDM system showing a transmission process between one antenna.

As shown in FIG. 1, the transmission signal is transmitted to the receiver through an inverse discrete cosine converter (IDFT) 11 and a parallel / serial converter 12, and the receiver is inversely connected to the serial / parallel converter 13 and the discrete cosine converter. (DFT) 14 receives the signal transmitted.

1, S ₀ , S ₁ , S ₂ ,. , S _{N + L-1} corresponds to CP. 1 shows a system using a total of N carriers and having L CPs. Referring to FIG. 2, a process of estimating maximum likelihood (ML) in such a system is as follows.

First, 2N + L symbols are observed as shown in FIG. 2. Here, except for I and I ', each symbol is randomly inputted, so independence is guaranteed. Since I and I' are the same symbols except for frequency offset, there is correlation between the two symbols. Given the time offset θ and the frequency offset ε, the probability density function of the received signal r is obtained and then estimated using the log-likelihood function. The conventional technique for estimating the time offset and the frequency offset is a well-known technique and a detailed description thereof will be omitted.

3 illustrates a model of a system using two receive antennas for technical understanding before explaining the method for estimating time and frequency offset according to the present invention.

In general, an OFDM diversity receiver includes two antennas, two demodulators that convert a signal received through each antenna into a digital signal, and then demodulate the original signal through a fast Fourier transform, and a demodulated signal through each demodulator. And two equalizers for compensating for distortion occurring in the transmission with respect to the < RTI ID = 0.0 > and < / RTI >

In FIG. 3, it is assumed that the first antenna receives the r1 signal and the second antenna receives the r2 signal. The two antennas are separated by λ / 2 or more so that the two received signals are independent. The received signals are P ₁ and P ₂ , respectively. As the weight is added. In the present invention, the ML estimator is derived using the weighted signal.

Assuming that the AWGN channel, the received signal r can be expressed as Equation (1).

If we consider the received signal r as a symbol belonging to I and a symbol not belonging to I, a symbol belonging to I has a correlation at I 'and I' apart from N because it has CP copied the symbol to I '. Since the remaining signals are transmitted randomly, independence is established between signals separated by N. Therefore, the log-likelihood function using the probability density function of the received signal for ML (Maximum Likelihood) estimation is calculated as in Equation 2.

Using this to define γ, Φ as shown in equation (3),

The time and frequency offset for maximizing the log-likelihood function can be estimated by the method shown in Equation 4.

Therefore, according to Equation 4, the time and frequency offset estimator is equal to Equation 5.

In Equation 5 ρ ₂ has the same value as Equation 7, because the correlation between the signals corresponding to I is equal to Equation 6.

은 신호 전력을,

은 잡음 전력을 각각 의미한다.In Equations 6 and 7

Signal power,

Means noise power respectively.

FIG. 4 is a diagram illustrating an embodiment of a time and frequency offset estimating apparatus implementing the time and frequency offset estimating process described above.

The time offset is first estimated using the signals received through the two antennas, and then the frequency offset is estimated. Where P ₁ and P ₂ depend on the antenna coupling method. In the case of equal gain combining method, P ₁ = P ₂ = 1, and in the selective combining method, the signal part having the largest signal-to-noise ratio is 1, the rest is 0, and in the case of MRC (Maximum Ratio Combining), two signals are added. P ₁ and P ₂ are determined to maximize the signal-to-noise ratio.

Referring to FIG. 4, the apparatus for estimating time and frequency offset according to the present invention is as follows.

The apparatus for estimating time and frequency offset according to the present invention includes a delay 101 for delaying a signal received through two antennas by an arbitrary Z index, and a square value for a signal delayed by the delay 101. A first square value calculator 102, a second square value calculator 103 for calculating a square value of a signal received through an antenna, and a result of the first square value calculator 102 A first adder 104 for adding a value and a resultant value of the second square value calculator 103 and a sum of an arbitrary number given to the output of the adder 104, and then two received A first summer 105 that multiplies a weight applied to the signal by an arbitrary value given by the signal power and the noise power, and a conjugator for changing the sign of the imaginary imaginary part of the signal delayed by the delayer 101. (conjugator) 106 and the scene received via the antenna And a second adder 107 for adding the output of the conjugator 106, a second adder 108 for performing any number of additions to the output of the second adder 107, and A subtractor 109 which subtracts the output of the first summer 105 from an absolute value of the result value of the second summer 108 and a time offset that is the maximum among the subtraction results of the subtractor 109 is detected. A frequency in radians for the absolute value of the detector 110 and the result of the second summer 108, and a frequency offset calculated using the time offset detected by the detector 110. The offset calculator 111 is included.

The biggest feature of the time and frequency offset estimator according to the present invention is to add the square value of the delayed received signal and the square value of the received signal, and then add the sum as many as the given number, and to the result of performing the sum, The time offset is estimated by reflecting a weight applied to the received signal and an arbitrary value given by the signal power and the noise power. A method of estimating time and frequency offset according to the present invention will be described with reference to FIG. 4 as follows.

First, a received signal weighted to a signal received through two antennas is input and delayed by an arbitrary index. The first square value, which is the square value of the delayed received signal, is calculated, and the second square value, which is the square value of the non-delayed received signal, is calculated to add the first square value and the second square value.

Then, the sum is performed as many as the given number for the addition result, and then the weight calculated by the weight and the signal power and the noise power applied to the signals received through the two antennas, respectively, ρ ₂ ) is multiplied by 2 to calculate the first sum.

At the same time, the sign of the complex imaginary part of the delayed received signal is changed, and then added with the original received signal, the sum result is added by an arbitrary number to calculate a second sum value, and then the second sum value. Calculate the absolute value of.

Then, the first sum value is subtracted from the absolute value of the second sum value to detect a time offset value that maximizes.

Thereafter, the frequency offset is estimated using the detected time offset. An angle in radians is obtained with respect to an absolute value of the second sum value, and the frequency offset is calculated using the detected time offset.

The present invention described above is capable of various substitutions, modifications, and changes without departing from the technical spirit of the present invention for those skilled in the art to which the present invention pertains. It is not limited by the drawings.

As described above, in the diversity OFDM communication system, time and frequency offsets are estimated using weights, and thus time and frequency offset estimation can be performed with good performance even in a low signal-to-noise ratio and a multipath channel environment.

Claims (7)

An apparatus for estimating time and frequency offset for signals received via at least two antennas, the apparatus comprising: Delay means for delaying the received signal by any exponent; First adding means for adding a square value for the signal delayed by the delay means and a square value for the original received signal; After adding the given number to the addition result of the first adding means, the sum is multiplied by a value calculated by using the weight and signal power and noise power applied to the received signal, respectively, to add the first sum. First summing means for calculating a value; Second adding means for adding a value that changes the sign of an imaginary imaginary part of the received signal delayed by said delay means and an original received signal; Second adding means for adding a result of the second adding means by a given number to calculate a second sum value; Time offset detecting means for detecting a maximum value by subtracting an absolute value of the second adding means and a resultant value of the first adding means; And A frequency offset calculating means for calculating an angle with respect to an absolute value of the second summing means and then calculating a frequency offset using the time offset detected by the time offset detecting means; Time and frequency offset estimation apparatus in an orthogonal frequency division multiplexing communication system comprising a. The method of claim 1, The first summing means, A sum of any given number is added to the addition result of the first adding means, and then the weights P ₁ and P ₂ applied to the received signal and the signal power (

) And noise power (

) The value (ρ ₂₎ of OFDM time and frequency offset estimation apparatus in a communication system, characterized in that multiplying the integrated value calculated by the equation (8) used. [Equation 8]

The method of claim 2, And the weights P ₁ and P ₂ have a value of 1 when the antenna coupling method is equal gain coupling. The method of claim 2, The weights P ₁ and P ₂ indicate that the signal portion having the largest signal-to-noise ratio is 1 and the rest is 0 when the antenna coupling method is selective coupling. Time and frequency in the orthogonal frequency division multiplexing communication system, characterized in that Offset estimation device. The method of claim 2, The weights P ₁ and P ₂ are time and frequency offset in an orthogonal frequency division multiplexing communication system, when the two antennas are combined when the antenna combining method is MRC (Maximum Ratio Combining). Estimation device. A method of estimating time and frequency offset for a signal received via at least two antennas, the method comprising: A first step of adding a squared value for the signal delayed by any exponent for the received signal and a squared value for the original received signal; A second step of performing an addition by any given number to the addition result of the first step, and then multiplying the addition result by a value calculated by using the weight and signal power and noise power applied to the received signal, respectively; A third step of adding an original received signal with a value changed from a sign of a imaginary imaginary part of the received signal delayed by an arbitrary exponent; A fourth step of summing up any number given for the result of the third step; A fifth step of detecting a maximum time offset value by subtracting an absolute value of the total value of the fourth step and a result value of the second step; And A sixth step of calculating a frequency offset using the time offset value detected in the fifth step after calculating an angle with respect to an absolute value of the sum of the fourth step; Time and frequency offset estimation method in an orthogonal frequency division multiplexing communication system comprising a. The method of claim 6, The second step, After adding up any number given to the addition result of the first step, the weights P ₁ and P ₂ applied to the received signal and the signal power (

) And noise power (

) The value calculated by the equation (8) with (ρ _2), orthogonal frequency division multiplexing the estimated time and frequency offset in a communication system, characterized in that for multiplying said integrated value.

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