JP3437528B2 - Symbol / frequency synchronization method for OFDM signal using symmetric preamble - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a symbol / frequency synchronization method for orthogonal frequency division multiplexing (OFDM) signals, and more particularly, to a symbol structure of a symmetric structure for obtaining symbol synchronization of an OFDM signal by using a preamble having a symmetric structure. The present invention relates to a symbol / frequency synchronization method for an OFDM signal to which a preamble is applied.

[0002]

2. Description of the Related Art Orthogonal frequency division multiple access
l Frequency Division Multiplexing: Below, OFDM
(Referred to as) is effective in efficiently transmitting data through the channel. OFDM transmits a number of sub-carrier frequencies within the channel bandwidth for transmitting data.
requency). These sub-carriers are frequency division multiplexing (FD).
M) is arranged so as to optimize the bandwidth efficiency as compared with the conventional transmission method, so that inter-carrier interference (inter-c)
arrier interference (ICI) can be avoided. Also, O
Coding data on the FDM sub-carrier may be frequency diversity to mitigate the loss due to frequency-selective fading.
sity) can get the benefits.

In the case of an OFDM system, frequency error and symbol synchronization should be considered in order to accurately demodulate the transmitted signal. If the symbol starting point cannot be found correctly, inter-symbol interference:
ISI) occurs and the transmission signal cannot be correctly restored. Generally, the starting point of a symbol is searched for by using the correlation value between received signal sequences. The signal sequence of the specified preamble is used to obtain the correlation value.

A conventional symbol synchronization method for an OFDM signal will be described with reference to FIGS. 1 to 4. Figure 1 shows the conventional OFD
FIG. 2 is a block diagram of an M signal receiving device, and FIG.
4 is a diagram showing a preamble structure of an FDM signal, FIG. 3 is a diagram for explaining a detailed operation of a conventional autocorrelation unit, and FIG. 4 is a graph showing a correlation value according to a conventional preamble structure.

In order to recover the baseband data bits by receiving the OFDM signal, the OFDM receiver has an RF (radio)
frequency) Data r (n) is received via a receiver (not shown). The autocorrelation unit 20 delays the received data r (n) by a half (N / 2) of the symbol length N to delay the delayed signal r *.
Find the autocorrelation value of (kD) and the received signal r (n). The peak detector 40 calculates the average of the calculated N / 2 autocorrelation values, and detects the maximum value among the calculated average values for each sample. The time / frequency synchronization unit 50 obtains time domain synchronization between samples using the detected maximum value.
Frequency error after accurately searching for symbol synchronization in the time domain
(offset) The problem has to be solved. Frequency errors occur when the receiver oscillator is inaccurate. The frequency error distorts the amplitude and phase of the demodulated signal, causes inter-channel interference, and degrades the overall performance of the OFDM system. Generally, the frequency error is classified into an integral multiple frequency error and a fractional multiple frequency error. Time / frequency synchronization unit 5
When 0 is used, the guard interval is used in the time domain to estimate and compensate the fractional frequency error of the subcarrier. In order to compensate the integral multiple frequency, the signal compensated for the decimal multiple frequency error of the subcarrier and the received signal r (n) are IFFT (inverse fast fourie).
Inverse Fourier transform is performed by the r transform) unit 60. The cross-correlation value between the inversely transformed signal and the N differential signals v (k) is calculated, the average of the cross-correlation values is calculated, and the maximum cross-correlation value is detected for each sample. The integral multiple frequency synchronization unit 80 obtains integral multiple frequency synchronization from the detected maximum value.

Referring to FIG. 2, the conventional OFDM signal preamble is divided into two sections in the time domain so as to have exactly the same signal sequence. That is, the preamble is N /
It has a structure in which the same sample row is repeated with a distance of about two, and the distances between the same samples are all N / 2 uniform. FIG. 3 diagrammatically illustrates a method of obtaining the autocorrelation value by applying the received signal r (n) having the preamble of FIG. 2 to the system of FIG. The preamble of the received signal r (n) is composed of N samples in which the same sample sequence is repeated every N / 2 length. Therefore, the autocorrelation unit 20 obtains a signal delayed by N / 2 and an autocorrelation value for each sample, and transmits the autocorrelation value to the peak detection unit 40.

The vertical axis of the graph of FIG. 4 represents the autocorrelation value,
The horizontal axis is the sample number. The experimental condition of the graph shown is the result of experimenting at 5 dB in an additive white gaussian noise (AWGN) environment where the number of subcarriers is 64. It must be seen that the 64th sample must have a maximum value, but around the 64th sample a larger correlation value occurred. this is,
This means that the influence of noise and the channel increases the possibility that an error will occur in the symbol time synchronization around the sample that should have the maximum value in the first place. M in the correlation value calculation formula
Assuming 0 is the exact starting point of the symbol,
When the d samples are off, it can be expressed as follows.

[0008]

[Equation 4]

The received signal r (n + d)

[0010]

[Equation 5] Since the signals belong to different symbols, there should be no correlation, but in the above equation (4), r (n + d) and

[0011]

[Equation 6] Are still correlated with each other. Therefore, when the number of deviated samples is small, it is not possible to show a significant difference in correlation value with the accurate symbol start point. If AWGN is mixed in the sample that is out of order because the absolute value symbol is used,
It can have the largest correlation value near the actual starting point of the symbol.

As described above, according to the conventional OFDM signal symbol synchronization method, firstly, the preamble used to demodulate the conventional OFDM signal cannot be used in the synchronous modulation / demodulation method. Second, the number of uncorrelated signals each time it deviates from the exact starting point of the symbol matches the number of deviated samples. As a result, there is a high probability that an error will occur near the original symbol start point due to the effects of noise and channels. Third, since the same distance between samples is N / 2 in frequency error estimation, the fractional frequency error and the integer frequency error should be estimated respectively.

[0013]

SUMMARY OF THE INVENTION The present invention has been proposed to solve the above-mentioned problems, and an object of the present invention is to pass a multipath channel which causes noise and amplitude and phase distortion. An object of the present invention is to provide a symbol / frequency synchronization method for an OFDM signal with improved accuracy of timing synchronization and fine frequency synchronization of the OFDM reception signal. Another object of the present invention is to provide a symbol / frequency synchronization method for an OFDM signal with an improved error probability that can occur near the original symbol start point. Still another object of the present invention is to provide a symbol / frequency synchronization method for an OFDM signal, which can widen a frequency error estimation range.

[0014]

In order to achieve the above-mentioned object, a symbol / frequency synchronization method of an OFDM signal according to the present invention applies a symbol / frequency synchronization of an OFDM signal by applying a preamble having a symmetrical structure. To get The preamble is symmetrically configured to have a length of 1/2 or less than 1 / 2n of the OFDM symbol length N. An OFDM signal symbol / frequency synchronization method according to the present invention is (A)
Receiving an OFDM signal having a symmetric structure preamble, (B) converting the OFDM signal from an analog signal to a digital signal to obtain samples of the OFDM signal, (C) a symbol / frequency of the OFDM signal Separating a preamble from the received OFDM signal to obtain synchronization, (D) obtaining a correlation value between the same samples forming the symmetrical preamble, (E) obtaining symbol / frequency synchronization of the OFDM signal And stages.

The symbol synchronization acquisition step (E) is as follows:
(E1) obtaining an autocorrelation value between the same samples from N samples constituting the symmetric preamble,
(E2) obtaining the sum of the obtained autocorrelation values, and (E3) taking the absolute value of the sum of the correlation values and storing it in a buffer allocated to each sample,
(E4) detecting the sample n having the maximum data stored in the buffer allocated to each sample as the start point of the symbol.

The frequency synchronization acquisition stage of the (F) stage is (F)
1) The step of obtaining a phase value using the correlation value between the same samples forming the preamble, and the frequency error between the same samples using the distance between the phase value obtained in steps (F2) and (F1) and the sample. The step of calculating and the step of (F3) estimating the frequency error of the OFDM signal using the frequency error between the same samples. The frequency error can be estimated by applying different weighting values depending on the distances between the correlation samples forming the symbols at the time of (E) frequency synchronization acquisition. The weight value increases as the distance between the samples increases. The weight value increases exponentially as the distance between samples increases. The weight value increases as an integer multiple as the distance between the samples increases.

The preamble according to the present invention is composed of two symmetrical portions, and the maximum value is obtained as the starting point of the OFDM symbol by obtaining the correlation value between the same symbol sequences in the two symmetrical portions. Since the correlation values between the same signal sequences show a significant difference from each other, the starting point of the OFDM signal can be accurately estimated. Therefore, if you use a symmetric preampule,
It is possible to estimate not only the fractional frequency error but also the integer frequency error, and the performance of frequency synchronization can be improved by applying the weighted value according to the distance between samples.

DETAILED DESCRIPTION OF THE INVENTION Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. FIG. 5 is a block diagram of an OFDM signal receiving apparatus according to the present invention, FIG. 6 is a diagram showing a preamble structure according to the present invention, and FIG. 7 is for explaining a detailed operation of an autocorrelation unit according to the present invention. 8 is a graph showing a correlation value according to a preamble structure according to the present invention.

An OFDM receiving system (not shown) receives an OFDM signal R (n) via an RF (radio frequency) receiver (not shown) to receive the OFDM signal and recover the baseband data bits. (Step A). OFDM
Let s (k) be the signal to be transmitted by a transmitter (not shown). If the input signal goes through the IFFT (not shown) of the transmitter, it is expressed as follows.

[0020]

[Equation 7] The transmitted signal goes through a multipath channel, AWGN is added, and is expressed as follows.

[0021]

[Equation 8] Here, w (n) represents AWGN and h (n-τ) represents the time domain response function of the channel. Considering the frequency error generated from the receiver, the received signal is expressed as follows.

[0022]

[Equation 9] Here, δ is a form obtained by multiplying 1 / NT by a constant.
This is to model the frequency error generated by the receiver. N is the size of FFT, and T represents the sampling period. H (k) represents the channel response function in the frequency domain.

The received OFDM signal contains a symmetrical preamble used to obtain symbol / frequency synchronization. The structure of the preamble is shown in FIG. This is to get the exact starting point of the symbol,
If the symbol length is N, the length of the preamble is N /
This is a structure in which two sample rows are arranged symmetrically. That is, the first sample value and the Nth sample value are the same, and the N / 2nd sample value and the (N / 2 + 1) th sample value are the same. Therefore, the distance between the same samples is different for each sample. The preamble sample sequence may be symmetrically configured to have a length of 1 / 2n or less. At this time, n is an integer.

The OFDM signal is converted from an analog to a digital signal to obtain samples of the received OFDM signal (step B). The preamble is separated from the OFDM signal to obtain symbol / frequency synchronization of the OFDM signal (step C). Since the correlator 220 calculates the frequency error using the symmetric preambles separated in step (C), it obtains the correlation value between the same samples forming the symmetric preamble (step D). Referring to FIG. 7, as the correlation value, the correlation value between the same samples having different distances is obtained and transferred to the symbol timing / frequency synchronization unit 260.

Symbol timing / frequency synchronization section 260
Performs step E to obtain symbol / frequency synchronization of the OFDM signal. That is, the symbol / frequency synchronization acquisition step E of the OFDM signal includes the following detailed steps. An autocorrelation value between the same samples is obtained from N samples forming the symmetric preamble (step E1). Stage E1
Then, the total of the autocorrelation values obtained in (3) is obtained (step E2).
The absolute value of the sum obtained in step E2 is taken and stored in the buffer allocated to each sample (step E3). Stage E
In step 3, the sample n having the maximum data stored in the buffer assigned to the sample is detected as the start point of the symbol (step E4). Starting point S of OEDM symbol
_start is _calculated by the following formula.

[0026]

[Equation 10] The starting point of the symbol is the most accurate reference for finding the fractional frequency error. The position of the starting point of the detected symbol is verified by the following formula.

[0027]

[Equation 11]

[0028]

[Equation 12]

Here, d is an integer other than 0 and represents the number of deviated samples. When even one sample is off, the correlation value calculation result becomes 0 on average. This is because there is no correlation between the signal components applied to each other. Even if AWGN is added to the received signal, there is no correlation between AWGN and the transmitted signal, so that it has an extremely small value.

Symbol timing / frequency synchronizing section 260
In step E, which obtains the symbol / frequency synchronization of the OFDM signal, estimates the frequency error to obtain the frequency synchronization of the OFDM signal. Frequency The frequency synchronization acquisition step is performed by the following detailed steps. First, step F1 of obtaining a phase value using the correlation value between the same samples forming the preamble is performed. Then, the frequency error between the same samples is calculated using the phase value and the distance between the samples obtained in the step F1 (step F2). The frequency error of the OFDM signal is estimated using the frequency error between the same samples (step F
3).

The frequency error estimation is obtained by the following equation.

[Equation 13] here,

[0032]

[Equation 14] Is the error estimate and N represents the length of the symbol. That is, the error estimated value is calculated by dividing each correlation value calculation result by the distance between samples and adding N / 2 samples to obtain N /
It is divided by two.

The frequency error estimation is characterized in that the range of frequency error that can be estimated differs depending on the distance between two samples. Generally, only a fractional frequency error can be estimated with a correlation value between two samples by GIB that does not divide each correlation value calculation result into the distance between samples. However, if the distance between the two samples is reduced, the range becomes wider, and even a frequency error that is an integral multiple can be estimated. When calculating the correlation value, the distance between the samples is the shortest when there is one sample difference. The symmetric preamble structure satisfies such a condition. In this way, not only the decimal multiple frequency error but also the integer multiple frequency error can be estimated by using the correlation value having the short sample distance.
That is, the estimation range of the frequency error is widened.

However, since the accuracy becomes low as the estimation range widens, the frequency error can be estimated by applying different weighting values depending on the distance between the correlation samples forming the symbols in the F stage. Referring to FIG. 6, assuming that the distance α between the N / 2nd sample and the (N / 2 + 1) th sample is 1, the distance β between the second sample and the (N−1) th sample is (N− 3), and the distance γ between the first sample and the Nth sample becomes (N-1). That is, the distance between the same samples is different. At this time, the weight value can be applied by increasing the weight value as the distance between the samples increases. For example, the weight value increases exponentially as the distance between samples increases. In other methods, the weight value is increased to an integral multiple as the distance between samples increases.

FIG. 8 shows the correlation value when the proposed preamble having a symmetrical structure is used. This is the result of an experiment in which the number of subcarriers is 64 and 5 dB in the AWGN environment. The synchronization method using the symmetric structure preamble has no correlation even if only one sample is deviated at the original symbol start point, so that all other than the 64th sample have a small correlation value.

[0036]

The effects of the OFDM signal symbol / frequency synchronization method of the present invention are as follows. First,
It is possible to perform symbol synchronization of an OFDM signal with improved accuracy of timing synchronization and fine frequency synchronization of an OFDM reception signal that has passed through a multipath channel that causes noise, amplitude, and phase distortion. Secondly, it can be applied to both asynchronous and synchronous modulation / demodulation schemes by adopting a symmetric preamble. Third, the error probability that can occur near the original symbol start is improved. Fourth, the frequency error estimation range is wide.

Although the preferred embodiments of the present invention have been shown and described above, the present invention is not limited to the above-described embodiments, but is not limited to the above-described embodiments, and does not deviate from the gist of the present invention claimed in the scope of the invention. It goes without saying that any person having knowledge can make various modifications, and such modifications are within the scope of the claims.

[Brief description of drawings]

FIG. 1 is a block diagram of a conventional OFDM signal receiving apparatus.

FIG. 2 is a diagram showing a preamble structure of a conventional OFDM signal.

FIG. 3 is a diagram illustrating a detailed operation of a conventional autocorrelation unit.

FIG. 4 is a graph showing a correlation value according to a conventional preamble structure.

FIG. 5 is a block diagram of an OFDM signal receiving apparatus according to the present invention.

FIG. 6 is a diagram showing a preamble structure according to the present invention.

FIG. 7 is a diagram illustrating a detailed operation of an autocorrelation unit according to the present invention.

FIG. 8 is a graph showing a correlation value according to a preamble structure according to the present invention.

[Explanation of symbols]

220 Correlation part 240 Peak detector 260 symbol timing / frequency synchronization unit

Front page continuation (72) Inventor Zhao Nanshin 134 Shinchon-dong, Seodaemun-gu, Seoul, Republic of Korea (72) Inventor Zhao Xi-hee 134 Shinchon-dong, Seodaemun-gu, Seoul, Republic of Korea Inventor Masahiko Kang, Seoul, Republic of Korea 134, 72, Shinchon-dong, Seodaemun-gu, Hong Kong Inventor Hong Daeuk, 134, 56, Shinchon-dong, Seodaemun-ku, Seoul, Republic of Korea References JP 2001-102972 (JP, A) JP 2000-244371 (JP, A) Table 10-512733 (JP, A) Timothy M.M. Schmid l, Donald C .; Cox, LOW-Overhead, Low-Complexity [Burst] Synchnization for OFM, Proc. of ICC'96, p. 1301-1306 (58) Fields investigated (Int.Cl. ⁷ , DB name) H04J 11/00

Claims (4)

(57) [Claims] 1. A step of: (A) receiving an OFDM signal having a symmetric structure preamble; and (B) the O signal to obtain a sample of the OFDM signal.
Converting an FDM signal from an analog signal to a digital signal; (C) separating a preamble from the received OFDM signal to obtain symbol synchronization of the OFDM signal; and (D) configuring the symmetrical preamble. And (E) Correlation between the same samples forming the preamble.
Of the phase value obtained by using the value and the phase value obtained in steps (F) and (E) and the sample
Stage to calculate frequency error between same samples using distance
Floor, and (G) a symbol using the frequency error between the same samples
Weights that differ depending on the distance between the correlation samples that make up
Applying a value to estimate the frequency error of the OFDM signal
Symbol / frequency synchronization method of OFDM signals, characterized in that it is composed of a. 2. The symbol / frequency synchronization method for an OFDM signal according to claim 1 , wherein the weight value increases as the distance between samples increases. 3. The method of synchronizing symbols / frequency of an OFDM signal according to claim 1 , wherein the weight increases exponentially as the distance between samples increases. 4. The symbol / frequency synchronization method for an OFDM signal according to claim 1 , wherein the weight value increases as an integer multiple as the distance between the samples increases.