KR100723634B1 - Method for generating Preamble Sequence using PN Sequence, and Method for Time Synchronization and Frequency Offset Estimation using PN Sequence in an OFDM communication system - Google Patents

Abstract

1. TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method of generating a preamble sequence using a PEN sequence in an OPMD system, and a method of time synchronization and frequency offset estimation.

2. The technical problem to be solved by the invention

According to the present invention, a preamble signal is generated in a frequency domain such that a PN sequence having good autocorrelation property is generated in a time domain at a transmitting side of an OFDM system, and a PN (Pseudo Noise) sequence used at the time of generating the preamble sequence is used at a receiving side. By performing symbol synchronization and subcarrier frequency offset estimation in the time domain, an object of the present invention is to provide a preamble sequence generation method using a PEN sequence in an OFDM system, and a time synchronization and frequency offset estimation method.

3. Summary of Solution to Invention

The present invention provides a method for estimating time synchronization and frequency offset in an OFDM receiving system, wherein a moving sum is maximized while taking a moving sum using a PN sequence on an OFDM received signal in a time domain. A time synchronization step of locating and synchronizing symbol with the preamble position; And a frequency offset estimating step of estimating a frequency offset of a subcarrier on a time domain by multiplying a received signal synchronized in the time synchronization step by a sequence of differentially encoding the PN sequence.

4. Important uses of the invention

The present invention is used for time synchronization and frequency offset estimation in an OFDM system.

Orthogonal Frequency Division Multiplexing, OFDM, Preamble Sequence, PN Sequence, Frequency Offset Estimation, Time Synchronization

Description

Method for generating preamble sequence using PN Sequence, and Method for Time Synchronization and Frequency Offset Estimation using PN Sequence in an OFDM communication system}

1 is a structural diagram of an OFDM frame in a time domain in an OFDM system;

2 is a structural diagram of an embodiment of a preamble in an OFDM frame according to the present invention;

3 is a flowchart illustrating a method of generating a preamble sequence using a PN sequence in an OFDM transmission system according to the present invention;

4 is a flowchart illustrating a method of estimating time synchronization and frequency offset using a preamble sequence in an OFDM reception system according to the present invention.

The present invention relates to a method of generating a preamble sequence using Pseudo Noise (PNN) sequence in an Orthogonal Frequency Division Multiplexing (OFDM) system, and a method of estimating time synchronization and frequency offset, and more particularly, to an OFDM system. The transmitter generates a preamble signal in the frequency domain such that the PN sequence has good autocorrelation in the time domain, and the receiver estimates symbol synchronization and subcarrier frequency offset in the time domain using the PN sequence used when generating the preamble sequence. The present invention relates to a preamble sequence generation method using a PEN sequence in an OFDM system, and a time synchronization and frequency offset estimation method.

Orthogonal Frequency Division Multiplexed Access (OFDM) is an efficient digital transmission method for band use. Digital transmission such as European digital audio broadcasting, digital video broadcasting, asymmetric digital transmission system (ADSL), and wireless local area network (WLAN) has recently been adopted. It is applied to the system. The orthogonal frequency division multiplexing scheme has the advantage of being robust to inter-symbol interference (ISI), which is a major problem in high-speed communication, and making frequency selective fading appear as frequency non-selective fading.

However, an OFDM system has a disadvantage in that it is more sensitive to carrier frequency errors due to Doppler frequency shift due to mismatches or movements of oscillators of a transmitter and a receiver, compared to a single carrier system. This carrier frequency error causes inter-carrier interference (ICI) because it breaks orthogonality between subcarriers in an OFDM system. 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.

And since there is no "eye opening" to find the optimal sampling time in an OFDM system, symbol synchronization in an OFDM system is different from that in a single frequency system. Time synchronization in OFDM means finding the starting estimate of one OFDM symbol. Since the cyclic prefix (CP) is usually used, the OFDM system is less sensitive to symbol synchronization error, but should be estimated so as not to deviate from the error within CP.

The current OFDM synchronization method can be divided into a blind method and a data dependent method. First, in the case of the data-dependent method, there is a method of using two pilot symbols to estimate both integer and fractional parts of time synchronization and frequency offset. However, this method has a problem in that the data rate is reduced because two pilot blocks are used, and there is also a problem in that there is an ambiguity in time synchronization estimation corresponding to CP length in time synchronization.

On the other hand, there is a method for estimating the baking part of the time synchronization and the frequency offset using the CP in a blind manner, which uses a correlation between the data symbol part and the CP part. However, this method has a problem in that it is impossible to estimate the integer part of the frequency offset, and there is a problem in that symbol synchronization can be achieved but the starting point of the frame cannot be found.

 The present invention has been proposed to solve the above problems, and the preamble signal is generated in the frequency domain so that the PN sequence of the autocorrelation property is good in the time domain at the transmitting side of the OFDM system, and the preamble sequence is used at the receiving side in generating the preamble sequence. By performing symbol synchronization and subcarrier frequency offset estimation in the time domain using the PN sequence, a preamble sequence generation method using a PEN sequence in an OFDM system and a time synchronization and frequency offset estimation method are provided.

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. It will also be appreciated that the objects and advantages of the present invention may be realized by the means and combinations thereof indicated in the claims.

 In order to achieve the above object, the present invention provides a preamble sequence generation method in an orthogonal frequency division multiplexing (OFDM) transmission system, comprising: generating a PN sequence (S) having a high autocorrelation value; ; And generating a preamble sequence p in the frequency domain using Equation 1 and Equation 2 such that the PN sequence s becomes a preamble sequence in the time domain. do.

On the other hand, the present invention provides a time synchronization method in an OFDM receiving system, comprising: a PN sequence storing step of storing a PN sequence used for generation of a preamble sequence at a transmitting side; And a time synchronization step of finding a position at which the mobile sum is maximized while synchronizing to a preamble position while taking a moving sum with respect to the OFDM received signal using the PN sequence.

Meanwhile, the present invention provides a method for estimating time synchronization and frequency offset in an OFDM receiving system, wherein a moving sum is maximized while taking a moving sum using a PN sequence on an OFDM received signal in the time domain. A time synchronization step of locating a symbol and synchronizing symbol with the preamble position; And a frequency offset estimating step of estimating a frequency offset of a subcarrier on a time domain by multiplying a received signal synchronized in the time synchronization step by a sequence of differentially encoding the PN sequence.

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.

1 shows a structural diagram of an OFDM frame in the time domain in an OFDM system, and FIG. 2 shows an exemplary structural diagram of a preamble in an OFDM frame. 3 is a flowchart illustrating a method of generating a preamble sequence using a PN sequence in an OFDM transmission system according to the present invention.

In the existing frame structure (see FIG. 1), a preamble having a repeating structure is transmitted, and then data is transmitted. In order to have such a repetition structure, conventionally, only the odd subcarriers are transmitted in the frequency domain and nothing is sent to the even subcarriers. Here, the preamble sequence transmitted to the odd subcarrier should be designed in a direction of reducing the peak to average power ratio (PAPR), thereby solving the nonlinearity of the amplifier in the transmitter.

On the other hand, in the present invention, the preamble sequence is not generated (designed) in the time domain but is generated (designed) in the frequency domain.

First, in order to generate a preamble sequence, a PN sequence having good autocorrelation property (high autocorrelation value) is generated (301). As described above, in order to reduce the interference of other signals in the present invention, PN sequences are used as preamble sequences, but other sequences having good autocorrelation may also be used.

When the PN sequence is used as a preamble sequence, the PAPR, which is the most problematic in OFDM, is minimized to "1", so that the preamble signal can be more afforded to the transmission power limitation problem due to the PAPR of the conventional method. The advantage is that it can transmit more power, allowing more accurate synchronization. In the case of the OFDMA uplink system, the amplifier of the terminal is more sensitive to the PAPR because the performance of the terminal is not as good as that of the base station. Thus, when the preamble sequence having the minimum PAPR is generated as in the present invention, there are more advantages.

Next, in operation 302, the preamble sequence is generated by multiplying the inverse matrix of the inverse Fourier transform (IFFT) matrix on the PN sequence so that the PN sequence becomes the preamble sequence in the time domain.

That is, to obtain the PN sequence in the time domain, a process of obtaining a signal to be transmitted in the frequency domain is as follows.

For an OFDM symbol having N data symbols, first, an Inverse Fourier Transform (IFFT) matrix M of N × N may be expressed as Equation 1 below.

와 같다.here,

Same as

In the above definition, for example, a gold sequence vector having a length of N having a good autocorrelation property among PN sequences is s , and the frequency domain signal (preamble signal) p to be transmitted is It can be obtained as in [Equation 2].

Here, it is assumed that there are no unused subcarriers. Even when both subcarriers are not used, the preamble sequence can be obtained by multiplying a matrix of signals of both carriers by "0" in this manner. 2 shows this frequency domain preamble structure. In other words, p in the frequency domain is transmitted in the PN sequence s because the Fourier transform matrix is multiplied and converted into a time domain signal before transmission.

When the preamble sequence is designed in the time domain as described above, it is easy to design an efficient preamble sequence for the synchronization algorithm because time synchronization is performed in the time domain. In addition, when the PN sequence is used in the time domain, not only can the time synchronization be more effectively synchronized by the good autocorrelation property of the PN sequence, but also because different users attempt to access the base station at the initial synchronization in the uplink, Although the user signal may interfere with the synchronized user signal, the synchronization performance may be degraded, but this may reduce the interference of other users by assigning different PN sequences to each user.

4 is a flowchart illustrating a method of estimating time synchronization and frequency offset using a preamble sequence in an OFDM reception system according to the present invention.

First, when receiving a signal in the OFDM receiving system, a time synchronization process for finding the location of the preamble in the time domain is performed (400 to 408).

Since the receiving end knows that the preamble sequence is generated (designed) and transmitted in the time domain as a PN sequence, the receiving end receives the PN sequence (PN sequence used when generating the preamble at the transmitter side) previously stored in the memory. The moving sum with the signal finds the maximum position and is time synchronized at that position.

Such a time synchronization method will be described with reference to FIG. 4 as follows.

When the k th received signal sample in the time domain is r _k , the received signal vector r _θ at the starting point of the moving sum is θ because the OFDM symbol consists of N sample values. Same as

Where T represents the transpose matrix.

therefore The value X (θ) obtained by moving the sum of the received signal vector r _θ and the PN sequence s is obtained as shown in Equation 4 below (402).

By finding θ at which X (θ) is maximum, time synchronization can be achieved.

That is, while increasing θ from θ = 0 (400) (406), as shown in Equation 4, the mobile summation is carried out using a PN sequence using a PN sequence (402). The position where the value is maximized is found and time synchronization is performed by using the position as the preamble position.

However, θ To reduce the search time, the threshold TH _θ is set (404) to synchronize time with θ when X (θ) exceeds the threshold, and then the time synchronization process is terminated (408).

When comparing the present invention as described above with the time synchronization method using a preamble of the existing repeating structure, as follows.

First, since the conventional method is sent through an iterative structure, the influence of the channel is small in a multipath environment, and it is also possible to estimate the fractional part of the frequency offset in the time domain through the iterative structure. On the other hand, the present invention cannot directly reduce the influence of the channel because the channel information is not known in the multipath environment. However, by using the good autocorrelation property of the PN sequence, interference by other channel delays can be significantly reduced.

The conventional method is N / 2 because the number of samples used for estimation has a repeating structure, but in the case of the present invention, it is N. Since the average estimation error increases approximately in inverse proportion to the number of samples, the present invention has an advantage of about 3 dB when viewed only as the number of samples.

In addition, in the conventional method, when a moving sum is performed, it does not matter if the synchronization is not correct over several sample intervals, but if the synchronization is not correct with one or two sample intervals, the synchronization is correct. It is not easy to find the maximum point because the difference from the moving sum of is not large. However, in the case of the present invention, because of the good autocorrelation property of the PN sequence, even if one or two samples are not synchronized, the moving sum is significantly smaller than that at the time of synchronization, so the threshold for finding the maximum value is small. The advantage is that it is much easier to set the value.

However, as mentioned earlier, because it does not have a repetitive structure, the frequency offset in the time domain has to be estimated in another way.

Hereinafter, the frequency offset estimation method 310 according to the present invention will be described.

First, the following sequence is defined as shown in [Equation 5].

Since s _k is a PN sequence, C _k differentially encoded It also becomes a PN sequence.

Assuming that time synchronization is correct, and assuming that the n th time domain received signal sample is r _n , Equation 6 below holds.

Here, the approximation is made on the assumption that the received SNR is high, with the assumption that the channel values between adjacent samples are nearly similar, and this assumption is generally valid in a typical system.

Therefore, the frequency offset ε can be estimated as shown in Equation 7 below.

That is, the frequency offset of the subcarriers is estimated on the time domain by multiplying the received signals synchronized in the time synchronization processes 400 to 408 by differentially encoding the PN sequence.

Here, the estimation range of the frequency offset ε is only a fractional part of the frequency offset of -1/2 <ε <1/2, whereas the conventional method estimates -N / 2 <ε <N / 2 in the present invention. There is an advantage that the offset estimation range is much wider. In addition, in the case of the present invention, the interference can be averaged over N-1 more samples, whereas the conventional method averages N / 2 samples. And since C _k has a PN sequence, there is also an interference cancellation effect.

Compensation is made by using a more accurate time offset (θ _correct ) and a frequency offset (ε _correct ) obtained through the time synchronization process and the frequency offset estimation process according to the present invention as described above (412). Since the compensation method used at this time uses an existing method, the description thereof will be omitted.

Although the embodiments according to the present invention as described above have been described in the basic OFDM system, the present invention is applicable to the uplink and downlink of all systems such as OFDMA based on OFDM.

As described above, the method of the present invention may be implemented as a program and stored in a recording medium (CD-ROM, RAM, ROM, floppy disk, hard disk, magneto-optical disk, etc.) in a computer-readable form. Since this process can be easily implemented by those skilled in the art will not be described in more detail.

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 present invention, in a frequency-divided orthogonal frequency division multiplexing (OFDM) system, particularly in a system with much interference such as OFDMA uplink, the preamble sequence in the frequency domain is designed to be a PN sequence in the time domain. There is an effect that can effectively estimate a wide range of frequency offsets at the same time.

In addition, the present invention, by designing the preamble sequence to be a PN sequence in the time domain, it is possible to significantly lower the PAPR, and to quickly / easily estimate the time synchronization and frequency offset, as well as other users in the case of uplink There is an effect to reduce the interference of.

Further, the present invention has the effect that the estimation range of the frequency offset is wide by estimating the integer part and the fractional part of the frequency offset together at the receiving side.

Claims (8)

delete A method for generating a preamble sequence in an orthogonal frequency division multiplexing (OFDM) transmission system, A PN sequence generating step of generating a PN sequence s having a high autocorrelation value; And A preamble generation step of generating a preamble sequence p in the frequency domain using Equations 1 and 2 so that the PN sequence s becomes a preamble sequence in the time domain. Method of generating a preamble sequence using a PN sequence in an OFDM transmission system comprising a. [Equation 1]

here,

[Equation 2]

Where s is PN sequence of length N, M represents an N × N inverse Fourier transform (IFFT) matrix. In the time synchronization method in an OFDM receiving system, A PN sequence storing step of storing a PN sequence used for generating a preamble sequence at a transmitting side; And Step of synchronizing with the preamble position by finding a position where the mobile sum is maximum while taking a moving sum using the PN sequence for the OFDM received signal. Time synchronization method using a PN sequence in an OFDM receiving system comprising a. The method of claim 3, wherein The time synchronization step, The time synchronization method using a PN sequence in the OFDM receiving system, characterized in that the time synchronization by comparing the moving sum value with a predetermined threshold value, the maximum value of the moving sum value exceeding the threshold value. A time synchronization and frequency offset estimation method in an OFDM reception system, A time synchronization step of finding a position at which the mobile sum is maximized while performing a moving sum using a PN sequence on the OFDM received signal in the time domain, and synchronizing the symbol with the preamble position; And A frequency offset estimating step of estimating a frequency offset of a subcarrier on a time domain by multiplying a received signal synchronized in the time synchronization step by a sequence of differentially encoding the PN sequence Time synchronization and frequency offset estimation method using a PN sequence in an OFDM receiving system comprising a. The method of claim 5, The PN sequence is A time synchronization and frequency offset estimation method using a PN sequence in an OFDM receiving system, characterized in that the PN sequence used when generating a preamble on a transmitting side. The method according to claim 5 or 6, The frequency offset estimating step, A method of estimating time synchronization and frequency offset using a PN sequence in an OFDM receiving system, characterized by estimating a frequency offset ε on a time domain using Equations 3 and 4 below. [Equation 3]

[Equation 4]

here, S denotes the PN sequence used when generating the preamble sequence at the transmitting side, C _k denotes a sequence of differentially encoding the PN sequence, N denotes the number of samples included in one OFDM symbol, and r _n denotes the n th received signal sample. The method of claim 7, wherein The time synchronization step, A time synchronization and frequency offset estimation method using a PN sequence in an OFDM receiving system, characterized in that time synchronization is performed by comparing the mobile sum value with a predetermined threshold value to maximize the mobile sum value exceeding the threshold value. .

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