TWI513247B - Method for adjusting fft window of ofdm system - Google Patents

Description

Fast Fourier transform window adjustment method for orthogonal frequency division multiplexing system

The present invention relates to an Orthogonal Frequency Division Multiplexing (OFDM) system, and more particularly to an effective OFDM system that utilizes a guard interval in an orthogonal frequency division multiplexing (OFDM) system. FFT (Fast Fourier Transform) window adjustment method.

In general, an effective Fast Fourier Transform (FFT) window adjustment method using guard intervals in an orthogonal frequency division multiplexing (OFDM) system is performed before performing fast Fourier transform (FFT) on the received data. Use the Channel Impulse Response (CIR) signal to adjust the fast Fourier transform window to achieve inter-symbol interference (ISI, Inter Symbol Interference) and inter-channel interference (ISI). ICI, Inter Channel Interference) and noise power are minimized, thereby improving the performance of the entire system.

However, in the conventional OFDM system, only a square FFT window or only a portion that does not damage even with a guard interval (Guard Interval) is used, so that the longer the maximum delay length of the channel, the more limited the gain.

1 is a schematic conceptual diagram of modulation data in a transmission unit of a general OFDM system, and FIG. 2 is a conceptual diagram showing a process of pasting a protection interval in a transmission unit of a general OFDM system, in which a guard interval is inserted into a block to avoid a symbol. As shown in Fig. 2, only the inter-band interference and the inter-channel interference are copied and pasted to the front by the IFFT (Inverse Fast Fourier Transform).

3 is a conceptual diagram showing a pattern of symbols received from a channel having two paths with a maximum delay length shorter than a guard interval as a conventional method, and showing that the maximum delay length of the channel is shorter than the guard interval. Through the appearance of the signal of the channel, and for the sake of explanation, it is assumed that the path of the channel is two.

When a continuous OFDM symbol is transmitted as in FIG. 3, the A portion of the received symbol is added with the information of the previous symbol delayed by the path 2, and thus is the portion where the inter-symbol interference occurs. However, B and C only contain the same symbol information due to the guard interval. Therefore, if the synchronous receiving unit covers the square FFT window after setting the start position of the FFT to the B interval, and then performing FFT, no symbol will occur. Inter-channel interference and inter-channel interference.

In addition to the use of the guard interval for the purpose of not causing inter-symbol interference and inter-channel interference, there is also a method of using the guard interval for the purpose of estimating the maximum delay length of the channel and reducing noise. In the received symbols of FIG. 3, the A section and the B section are portions of the guard interval, that is, the rear portion of the copy symbol. Although the A section is damaged by the previous symbol, the B section is completely undamaged. If the fine timing synchronization and the maximum delay length of the channel are accurately inferred, it can be seen from where the B interval is. The guard interval that is completely undamaged as in the B interval has the same information in the original state in the posterior part of the symbol. Therefore, if the two are simply added and divided by 2, the signal component is generated as the original state, and the noise is generated. The effect of halving the power, so that the performance of the system is greatly improved.

The way to use the undamaged guard interval is that the longer the guard interval and the shorter the maximum delay length of the channel, the wider the interval in which the noise can be reduced, so good performance can be expected. However, the longer the maximum delay length of the channel, the lower the performance gain. In addition, due to the longest delay in the channel The degree of inter-symbol interference and inter-channel interference that occurs when the degree becomes longer than the guard interval causes severe performance degradation.

SUMMARY OF THE INVENTION An object of the present invention to solve the above problems is to provide an orthogonal frequency division multiplexing which can reduce performance degradation caused by inter-symbol interference and inter-channel interference when a delay length of a channel is long ( Fast Fourier Transform (FFT) window adjustment method for OFDM systems.

Another object of the present invention is to provide an orthogonality in which inter-symbol interference and inter-channel interference are minimized when the maximum delay length of the channel is longer than the guard interval, and not only the guard interval but also the external information of the symbol is used. Fast Fourier Transform (FFT) window adjustment method for frequency division multiplexing (OFDM) systems.

The fast Fourier transform window adjustment method of the orthogonal frequency division multiplexing system of the present invention for achieving the above-mentioned purposes is characterized in that it comprises the following steps: Step A, which is that the maximum delay length of the channel is shorter than the guard interval. In the case, the guard interval (A ₁ ) impaired by the path attenuation (Fading) channel is multiplied by the weighting value (α); step B, which is not damaged by the path delay (Fading) channel delay The interval (A ₂ ) is multiplied by the weighting value (1-α); and the C step is to add the A step and the B step and input (A ₃ ) to the FFT.

In addition, the fast Fourier transform window adjustment method of the orthogonal frequency division multiplexing system of the present invention for achieving the further object as described above is characterized in that it comprises the following steps: Step A, which is that the maximum delay length in the channel is longer than In the case of the guard interval, the FFT start position (A ₁ ) is multiplied by the weight value (α); Step B, which is the multiplication of the position (A ₂ ) deviated from the FFT length by the weight value (1-α) And a C step of adding the A step and the B step and inputting (A ₃ ) to the FFT.

In the case of using the fast Fourier transform window adjustment method of the orthogonal frequency division multiplexing system of the invention, there is an effect of minimizing the inter-symbol interference, the inter-channel interference, and the noise of the damaged guard interval.

In addition, the present invention is that the longer the maximum delay length of the channel, the greater the performance gain can be obtained, and the performance gain can be expected even if the maximum delay length of the channel is longer than the guard interval.

1 is a schematic conceptual diagram of modulation data in a transmission unit of a general orthogonal frequency division multiplexing (OFDM) system.

2 is a conceptual diagram showing a process of attaching a guard interval to a transmitting unit of a general orthogonal frequency division multiplexing (OFDM) system.

3 is a conceptual diagram showing a pattern of symbols received from a channel having a maximum delay length shorter than a guard interval of a normal channel and having two paths.

4 is a conceptual diagram showing a damaged guard interval received from a channel having two paths having a maximum delay length shorter than a guard interval from the channel of the present invention.

Fig. 5 is a block diagram showing a method of minimizing inter-symbol interference, inter-channel interference, and noise by using the damaged guard interval of the present invention.

Fig. 6 is a conceptual diagram showing a pattern of symbols received from a channel having a maximum delay length longer than a guard interval of the channel of the present invention.

Figure 7 is a diagram showing that the maximum delay length of the channel of the present invention is longer than the guard interval, and the path is two, starting with Fast Fourier Transform (FFT) When the start position is set to the position of the first path, the concept map of the inter-symbol interference of the fast Fourier transform (FFT) input is entered.

Fig. 8 is a block diagram showing a method for minimizing inter-symbol interference, inter-channel interference, and noise in a channel having a maximum delay length longer than a guard interval in the channel of the present invention.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings and mathematical formulas as follows.

4 is a conceptual diagram showing a damaged guard interval received from a channel having two paths having a maximum delay length shorter than a guard interval from the channel of the present invention. The following is known, and therefore, it is not explained that the portion of the received symbol that is not damaged in the guard interval has effectively removed noise in the prior art.

Referring to FIG. 4, the A ₁ interval of the received symbol is a guard interval that is damaged by the previous symbol, that is, the multipath fading channel delay, and is expressed by Mathematical Formula 1 as described below. The guard interval, that is, the A ₂ interval in which the inter-symbol interference occurs in the A ₁ interval.

[Math 1] A ₁ = S ₁ + I + N ₁ A ₂ = S ₁ + S ₂ + N ₂

Here, I is an interference component due to the previous symbol, and N ₁ and N ₂ respectively mean noise applied to the A ₁ interval and the A ₂ interval.

In the present invention, the A ₁ section which is not used in the prior art due to the impairment of the inter-symbol interference is used in the case where the inter-symbol interference and the inter-channel interference are used by the channel pulse signal noise ratio (SNR). Minimize, reduce noise power and improve overall performance.

FIG 5 is a diagram showing a guard interval impaired utilization of the present invention will be inter-symbol interference, inter-channel interference, a block diagram of a method for minimizing noise, and A ₁ represents a section method.

Referring to FIG. 5, each of the latter is impaired guard interval i.e. the interval A ₁ and A ₂ are not damaged by the weighting section value addition, the method made FFT input (A ₃₎ of the. Fig. 5 can be expressed by a mathematical expression as follows.

[Math 2] A ₃ = αA ₁ +(1 - α ) A ₂

Here, in order to obtain the optimum weighting value, that is, the alpha value, the mathematical expression 2 is substituted into the mathematical expression 1 and expanded, as in the mathematical expression 3.

[Math 3] A ₃ = αA ₁ + (1 - α ) A ₂ = α( S ₁ + I + N ₁ ) + (1 - α )( S ₁ + S ₂ + N ₂ ) = α S ₁ + α I +α N ₁ + S ₁ + S ₂ + N ₂ -α S ₁ -α S ₂ -α N ₂ )= S ₁ + S ₂ -α S ₂ +α I +α N ₁ +(1- α ) N ₂

Here, if A ₃ = S ₁ + S ₂ , the noise becomes 0, and the condition of inter-symbol interference and inter-channel interference disappears. Therefore, it is only necessary to obtain the α value of the remaining term other than the S ₁ + S ₂ term from the mathematical expression 3 and the α value closest to 0 as in the mathematical formula 4.

[Math 4] -α S ₂ +α I +α N ₁ +(1 - α ) N ₂

Here, since the value of Mathematical Formula 4 is an instantaneous value, in order to obtain a value whose value is closest to 0 on the average, the average power is obtained, and it is sufficient to find the value of α which is closest to 0. All the terms of Mathematical Formula 4 are independent, so the average power is simply expressed by Math.

Here, as can be seen from FIG. 4, both Ps ₂ and P _I are the average power of the symbol and the interference made by the second path of the channel, and since the average power between the OFDM symbols is the same, it can be Ps ₂ =P _I . Thus, when Equation 5 is developed into a quadratic equation form with respect to α, Mathematical Formula 6 can be obtained.

Here, since there is no real solution that satisfies α of Mathematical Formula 6, if α is differentiated in order to find the α value closest to 0 on the left side, the α value can be obtained as follows. The value is as follows: When applied in Figure 5, inter-symbol interference, inter-channel interference, and noise are minimized.

The above description has been made on the case where the maximum delay length of the channel is smaller than the guard interval, and the case where the maximum delay length of the channel is longer than the guard interval will be described below.

Fig. 6 is a conceptual diagram of a pattern of symbols received from a channel having a maximum delay length longer than a guard interval of the channel of the present invention.

Referring to Fig. 6, the A section of the received symbol is an interval received in the following manner: the current symbol generated by the first path and the information of the previous symbol generated by the second path are added; the B interval is only present. The interval of the information of the current symbol; the C interval is an interval in which the information of the next symbol generated by the first path and the information of the current symbol generated by the second path are added.

The interval of the uninter-symbol interference is only the B interval, but since the length of the B interval is shorter than the length of the FFT, the conventional OFDM system has the following problem: in this case, it is difficult to determine the FFT start position. The reason is that when the power of the path 1 is greater than the power of the path 2, when P _{1 is} set to the FFT start position, the inter-symbol interference and the inter-channel interference become the smallest, but conversely in the path 2 In the case of higher power, only P _{2 is} set to the FFT start position to minimize inter-symbol interference and inter-channel interference.

In the embodiment of Fig. 6, in order to simplify the description, only two paths are provided, but if the number of paths becomes larger, finding the optimum FFT start position becomes complicated.

In the present invention, the FFT input is adjusted in a manner that aligns the FFT start position to the first of all paths, minimizing the resulting inter-symbol interference and inter-channel interference, in order to find the best FFT. The method of performing complicated calculations at the starting position is as follows.

7 is a diagram showing the concept of inter-symbol interference entering the FFT input when the maximum delay length of the channel of the present invention is longer than the guard interval and the path is two, and the FFT start position is set to the position of the first path. FIG. 8 is a diagram showing a method in which the maximum delay length of the channel of the present invention is longer than the guard interval and the path is two, and the inter-symbol interference, the inter-channel interference, and the noise are minimized.

Referring to Fig. 7, only the front portion of the FFT input (the maximum delay length of the channel - the length of the guard interval) is interfered by the inter-symbol, and the A ₁ interval corresponds to the former portion, and can be represented by A _{1 of} Math.

In addition, when the FFT input is adjusted in such a manner that all inter-symbol interference, inter-channel interference, and noise are removed, the optimum value (A _opt ) is as follows.

[Math 8] A ₁ = S ₁ + I ₂ + N ₁ A _opt = S ₁ + S ₂

In the FFT input of FIG. 8, if the A ₃ section is used to approximate the optimum value A _{opt of} only the signal component, and the A ₁ section and the A ₂ section are used as in the mathematical expression 3, the expression can be expressed as in the mathematical expression 9. .

[Math 9] A ₃ = αA ₁ + (1 - α ) A ₂ = α( S ₁ + I ₂ + N ₁ ) + (1 - α ) ( I ₁ + S ₂ + N ₂ ) = α S ₁ +α I ₂ +α N ₁ + I ₁ + S ₂ + N ₂ -α I ₁ -α S ₂ -α N ₂ )= S ₁ + S ₂ -(1-α) S ₁ +α I ₂ +α N ₁ + I ₁ + N ₂ -α S ₂ -α I ₁ -α N ₂ = S ₁ + S ₂ -(1-α) S ₁ +α I ₂ +α N ₁ +(1-α) I ₁ +(1-α) N ₂ -α S ₂

In the same manner, the average power of the remaining items other than the term of A _opt can be expressed by Math.

Here, since Ps ₁ and P _I1 are the average power of the symbols passing through the first path, the same is true, and Ps ₂ and P _I2 are the same as the average power of the symbols passing through the second path. Therefore, Mathematical Formula 10 can sort α and be represented again by Math.

Since the α value of the mathematical expression 11 closest to 0 is required, if it is placed in the form of a quadratic equation about α, there is no α value which makes the mathematical expression 11 a real number, so the differential can be utilized, for example, the mathematical expression 12 The alpha value closest to zero is obtained as usual.

Using the channel impulse response signal to noise ratio information, the inter-symbol interference and inter-channel interference are minimized as described above, and the noise power is minimized instead of fixing the FFT start position to the first path. In the case where the maximum delay length of the channel is longer than the guard interval, performance degradation can be minimized. In addition, the method of utilizing the damaged guard interval can be simultaneously applied to further improve the performance.

As described above, the preferred embodiments of the present invention have been described in the detailed description of the present invention, but those who have ordinary knowledge in the technical field to which the present invention pertains can realize various kinds without departing from the scope of the present invention. Deformation. Therefore, the scope of the invention should be determined by the claims and the equivalents thereof

Claims (4)

A fast Fourier transform window adjustment method for an orthogonal frequency division multiplexing system includes the following steps: Step A, when the maximum delay length of the channel is shorter than the guard interval, receiving the protection interval damaged by the path attenuation channel delay The sign of the multiplication by the weighting value (α); step B, multiplying the symbol received by the section not damaged by the path attenuation channel delay by the weighting value (1-α); and step C, the step A Add to step B for fast Fourier transform. The fast Fourier transform window adjustment method of the orthogonal frequency division multiplexing system according to claim 1, wherein the weighting value (α) is obtained by using the following mathematical formula: Ps ₂ is the average power of the symbols passing through the second path, and P _N is the average power of the noise. A fast Fourier transform window adjustment method for an orthogonal frequency division multiplexing system includes the following steps: Step A, in the case where the maximum delay length of the channel is longer than the guard interval, the fast Fourier transform start position in the received symbol Multiplying the weighted value (α); step B, multiplying the position of the received symbol from the length of the fast Fourier transform by the weighted value (1-α); and step C, the step A and B The steps are added for fast Fourier transform. The fast Fourier transform window adjustment method of the orthogonal frequency division multiplexing system according to claim 3, wherein the weighting value (α) is obtained by using the following mathematical formula: Ps ₁ and Ps ₂ are the average power of the symbols passing through the first and second paths, respectively, and P _N is the average power of the noise.