EP1946469A1

EP1946469A1 - Method for generating preamble sequence using pn sequence, and method for time synchronization and frequency offset estimation using pn sequence

Info

Publication number: EP1946469A1
Application number: EP06798731A
Authority: EP
Inventors: Hee-Wook Kim; Kun-Seok Kang; Do-Seob Ahn; Ho-Jin Lee
Original assignee: Electronics and Telecommunications Research Institute ETRI
Current assignee: Electronics and Telecommunications Research Institute ETRI
Priority date: 2005-11-08
Filing date: 2006-09-11
Publication date: 2008-07-23
Also published as: KR20070049420A; KR100723634B1; WO2007055469A1

Abstract

A method for generating a preamble sequence using PN sequence in an OFDM communication system and a method for time synchronization and frequency offset estimation are provided. The method for generating a preamble sequence using a PN sequence at an OFDM transmission system includes the steps of: generating pseudo noise (PN) sequence having high autocorrelation value; and generating a preamble sequence by multiplying the PN sequence by an inverse matrix of an inverse fast Fourier transform (IFFT) matrix in frequency domain so as to make the PN sequence become the preamble sequence in time domain.

Description

Description METHOD FOR GENERATING PREAMBLE SEQUENCE USING

PN SEQUENCE, AND METHOD FOR TIME SYNCHRONIZATION AND FREQUENCY OFFSET ESTIMATION

USING PN SEQUENCE

Technical Field

[1] The present invention relates to a method for generating a preamble sequence using a pseudo noise (PN) sequence, and a method for obtaining time synchronization and estimating frequency offset based on the PN sequence in an Orthogonal Frequency Division Multiplexing (OFDM) communication system. More particularly, the present invention concerns a method for generating a preamble sequence based on a PN sequence, and a method for achieving time synchronization and estimating frequency offset in an OFDM communication system. A transmitter part of the OFDM communication system generates a preamble signal in frequency domain so as to make the PN sequence have good autocorrelation properties in a time domain, and a receiver part of the OFDM communication system obtains a symbol synchronization and estimates a subcarrier frequency offset in the time domain by using the PN sequence to generate the preamble sequence. Background Art

[2] Orthogonal Frequency Division Multiplexing (OFDM) is a digital transmission scheme that is efficient in use of bandwidth. The OFDM is widely used in digital transmission systems, such as digital audio broadcasting or digital video broadcasting in Europe, Asymmetric Digital Subscriber Line (ADSL), Wireless Local Area Network (WLAN), and so on. The OFDM is robust against inter symbol interference (ISI) that is an important issue in the high-speed communication. Also, the OFDM can make a frequency selective fading appear as a frequency non-selective fading.

[3] However, compared with a single carrier system, OFDM communication system is sensitive to the mismatch of oscillators in the receiver and the transmitter, or carrier frequency error caused by Doppler frequency shift. The carrier frequency error destroys the orthogonality between the subcarriers in the OFDM communication system, causing intercarrier interference (ICI). Therefore, the frequency synchronization problem that may cause great performance degradation even at a small carrier frequency error is the most important issue in the implementation of the OFDM communication system.

[4] Because the OFDM communication system has no "eye opening"that can find an optimal sampling time, the symbol synchronization in the OFDM communication system is different from that in the single frequency system. The time synchronization in the OFDM communication system means finding a starting estimation point of an OFDM symbol. Because the OFDM communication system generally uses a cyclic prefix (CP), it is less sensitive to the symbol synchronization error, but the symbol synchronization has to be estimated within the CP without departing the error.

[5] OFDM synchronization methods may be classified into a blind method and a data dependent method. The data dependent method uses two pilot symbols for the time synchronization and the estimation of both integer and fraction parts of the frequency offset. However, this method has a problem in that data rate decreases because two pilot symbols are used. Also, in the time synchronization, there exists ambiguity of the time synchronization estimation corresponding to CP length.

[6] Meanwhile, the blind method uses the CP for the time synchronization and the estimation of the fraction part of the frequency offset. This method uses the correlation of a data symbol part and a CP part. However, this method cannot estimate the integer part of the frequency offset. Also, while this method can achieve the time synchronization, it cannot find the beginning of a frame. Disclosure of Invention Technical Problem

[7] It is, therefore, an object of the present invention to provide a method for generating a preamble sequence based on a PN sequence, and a method for performing time synchronization and estimating frequency offset in an OFDM communication system, in which a transmitter side of the OFDM communication system generates a preamble signal in frequency domain so as to make the PN sequence have good autocorrelation properties in time domain, and a receiver side of the OFDM communication system obtains a symbol synchronization and estimates a subcarrier frequency offset in time domain by using the PN sequence used to generate the preamble sequence.

[8] Other objects and advantages of the present invention can be understood more fully through the embodiments of the present invention. Also, the objects and advantages of the present invention can be easily implemented by means of the following claims and combination thereof.

Technical Solution

[9] In accordance with one aspect of the present invention, there is provided a method for generating a preamble sequence using a pseudo noise (PN) sequence at an OFDM transmission system, the method including the steps of: generating a PN sequence having high autocorrelation value; and generating a preamble sequence by multiplying the PN sequence in frequency domain by an inverse matrix of an inverse fast Fourier transform (IFFT) matrix so as to make the preamble sequence become the PN sequence in time domain.

[10] In accordance with another aspect of the present invention, there is provided a time synchronization method for an OFDM reception system, including the steps of: storing PN sequence that is used to generate a preamble sequence at a transmitter side; and performing a moving sum with respect to OFDM reception signals by using the PN sequence, finding a position where the moving sum becomes maximum, and achieving the synchronization to the found preamble position.

[11] In accordance with a further another aspect of the present invention, there is provided a method for obtaining a time synchronization and estimating a frequency offset using a PN sequence at an OFDM reception system, including the steps of: finding a preamble position of an OFDM reception signal in time domain and obtaining a symbol synchronization to the found preamble position; and estimating a frequency offset of a subcarrier in time domain by multiplying the synchronized reception signal by a sequence given by differentially encoding the PN sequence. Advantageous Effects

[12] In accordance with the present invention, the OFDM communication system, specifically the OFDMA uplink where many interferences exist, can estimate the time offset and wider range of the frequency offset effectively and simultaneously by designing the preamble sequence in frequency domain such that the preamble sequence becomes the PN sequence in time domain.

[13] Also, PAPR can be remarkably reduced by designing the preamble sequence to be the PN sequence in time domain. The time synchronization and the frequency offset estimation can be achieved rapidly and easily. Moreover, in the case of the uplink, interference from other users can be reduced.

[14] In addition, the integer part and the fraction part of the frequency offset can be estimated at the receiver side, so that the estimation range of the frequency offset is wide. Brief Description of the Drawings

[15] The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which:

[16] Fig. 1 illustrates an OFDM frame in time domain in an OFDM communication system;

[17] Fig. 2 illustrates a structure of a preamble in an OFDM frame in accordance with an embodiment of the present invention;

[18] Fig. 3 is a flowchart illustrating a method for generating a preamble sequence using PN sequence in an OFDM transmission system in accordance with an embodiment of the present invention; and

[19] Fig. 4 is a flowchart illustrating a method for obtaining time synchronization and estimating frequency offset using preamble sequence in an OFDM reception system in accordance with an embodiment of the present invention. Best Mode for Carrying Out the Invention

[20] Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, which is set forth hereinafter. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.

[21] Fig. 1 illustrates a structure of an OFDM frame in time domain in an OFDM communication system. Fig. 2 illustrates a structure of a preamble in an OFDM frame in accordance with an embodiment of the present invention. Fig. 3 is a flowchart illustrating a method for generating a preamble sequence using PN sequence in an OFDM transmission system in accordance with an embodiment of the present invention.

[22] Referring to Fig. 1, in the conventional frame structure, a preamble with a repetitive structure in time domain is transmitted and data is then transmitted. For this repetitive structure in time domain, PN sequence is transmitted only on odd subcarriers in frequency domain, but not on even subcarriers. The preamble sequence has peak to average power ratio (PAPR) which causes non-linearity of an amplifier in a transmitter.

[23] On the contrary, concerned with a preamble having a low PAPR, the present invention designs a preamble sequence in time domain, instead of frequency domain.

[24] In step 301, in order to generate a preamble sequence, a PN sequence having good autocorrelation property, i.e., high autocorrelation value, is generated. To reduce interference with other signals, other sequences having good autocorrelation properties may also be used as the preamble sequence.

[25] When the PN sequence is used as the preamble sequence in time domain, the PAPR that is the biggest problem in the OFDM becomes minimal, i.e., "1". Therefore, more margin can be provided with respect to the transmission power limitation due to PAPR and thus the preamble signal can be transmitted with larger power. Consequently, the synchronization can be achieved more exactly in high SNR. In the case of an OFDMA uplink system, since an amplifier of a mobile station has inferior performance to a base station, the OFDMA uplink system is more sensitive to PAPR. Therefore, much more advantages can be obtained if the preamble sequence having a minimal PAPR is generated.

[26] In step 302, a preamble sequence is generated by multiplying the PN sequence by an inverse matrix of an inverse fast Fourier transform (IFFT) matrix in frequency domain so that the PN sequence can become the preamble sequence in time domain.

[27] A process of calculating signals that must be transmitted in frequency domain so as to generate the PN sequence in time domain will be described below.

[28] For OFDM symbol having N number of data symbols, NxN IFFT matrix M can be expressed as Eq. (1) below

[29] [30]

Eq. (1)

[31] [32] where

W = _ ^-J 2π/N

[33] For example, if "s" is a gold sequence vector having N length and good autocorrelation property as a PN sequence, frequency domain signal (preamble signal) vector p that must be transmitted can be given as Eq. (2) below

[34]

Eq. (2)

[35] It was assumed that there are no unused subcarriers. Even if the subcarriers are not used, the preamble sequence can be calculated by multiplying a matrix that makes signals on the carriers zero. Fig. 2 illustrates a structure of the frequency domain preamble. That is, "p" in the frequency domain is multiplied by a Fourier transform matrix prior to transmission and converted into time domain signal, and it is transmitted as the PN sequence "s".

[36] When the preamble sequence is designed in time domain by the above method, the time synchronization is achieved in time domain. Therefore, it is easy to design the preamble sequence that is effective to the synchronization algorithm. Also, when the PN sequence is used in time domain, the time synchronization can be much effectively achieved due to the good autocorrelation property of the PN sequence. In the case of the uplink, several users try to access the base station at the same time in an initial operation, other user signals interfere with the user signal being synchronized, thus degrading the synchronization performance. However, this can reduce other user interferences by allocating different PN sequences to the users.

[37] Fig. 4 is a flowchart illustrating a method for obtaining time synchronization and estimating frequency offset using the preamble sequence in the OFDM reception system in accordance with an embodiment of the present invention.

[38] In steps 400 to 408, when the OFDM reception system receives a signal, a time synchronization process of finding a preamble position in time domain is carried out.

[39] Because the receiver side knows that the preamble sequence is generated, in other words, designed, in a PN sequence in time domain and then transmitted, the receiver side performs a moving sum of the PN sequence previously stored in a memory and the reception signal, and finds a maximum value position. Herein, the PN sequence is used to generate the preamble at the transmitter side. Then, the receiver side adjusts the time synchronization to the maximum value position.

[40] This time synchronization method will be described below with reference to Fig. 4.

[41] Assuming that r is a k reception signal sample in time domain, an observation k window includes N sample values and thus an observed reception signal vector rθ having θas a starting point of a moving sum is expressed as Eq. (3).

[42]

[43]

Vθ⁼ Y fθ fθ+λ ^{' ' '} fθ+N-2 Fθ-N^{^}Λ ] ^T

Eq. (3) [44]

[45] where T represents a transposed matrix.

[46] Therefore, a timing metric X(O) given by the moving sum of the reception signal vector r and the PN sequence s is calculated by Eq. (4). θ

[47] [48] X(θ)= s^τr_θ

Eq. (4)

[49]

[50] The time synchronization can be obtained by finding θ that maximizes X(θ).

[51] That is, in steps 400, 402 and 406, while increasing θfrom zero, the position where the moving sum becomes maximum is found while performing a moving sum with respect to the OFDM reception signal of Eq. (4) using the PN sequence. Then, the time synchronization is achieved using the found position as the starting position of the preamble.

[52] In steps 404 and 408, in order to reduce the time necessary to find θ, a threshold value TH θ is set, and the time synchronization is achieved using θ that is given when

X(O) first exceeds the threshold value. Then, the time synchronization process is terminated.

[53] The above-described method of the present invention will now be compared with the conventional time synchronization method using the preamble having the repetitive structure.

[54] In the conventional method, because the preamble is transmitted through the repetitive structure, the influence of channel is small in multi-path environment. Also, the fraction part of the frequency offset can be estimated through the repetitive structure in time domain. On the contrary, in the method of the present invention, the influence of channel cannot be directly reduced because the channel information is not known in multi-path environment. However, using the good autocorrelation properties of the PN sequence, the interference caused by other channel delays can be remarkably reduced.

[55] In the conventional method, the number of samples used for estimation is N/2 because of the repetitive structure. On the contrary, in the present invention, the number of samples is N. Because the mean estimation error is increased in inverse proportion to the number of the samples, the present invention has a merit of about 3 dB in view of the number of the samples.

[56] In the conventional method, there is no problem when the found position is not matched with the actual starting point at intervals of several samples. However, when the synchronization is not matched at intervals of one or two samples, it is difficult to find the maximum point where a timing metric has a maximum value because the timing metric reaches a plateau which has a length equal to the length of cyclic prefix (CP) in the conventional method. On the contrary, in the present invention, even if one or two samples are not matched, the moving sum is very small compared with the value when correctly synchronized, because of the good autocorrelation properties of the PN sequence. Therefore, it is much easier to set the threshold value for finding the maximum value. [57] However, as described above, the present invention does not have the repetitive structure, the frequency offset has to be measured in time domain using other methods. [58] Hereinafter, the method for estimating the frequency offset in accordance with the present invention will be described. [59] First, a sequence of Eq. (5) below is defined.

[60] [61]

Eq. (5)

[62]

[63] Because s is the kth sample of the gold sequence vector as a PN sequence, a differentially encoded C is also a sample of the PN sequence.

[64] Assuming that the accurate time synchronization is achieved and r n is the n* time domain reception signal sample, Eq. (6) below is satisfied. [65] [66]

Eq. (6)

[67]

[68] The approximation is performed on the assumption that the channel values of the neighboring samples are almost similar to one another and the reception signal- to-noise ratio (SNR) is high. This assumption is generally proper in the general system.

[69] Therefore, the frequency offset (ε) can be estimated as Eq. (7) below.

[70] That is, the frequency offset of the subcarrier is estimated in time domain by multiplying the reception signal synchronized through the time synchronization procedures (steps 400 to 480) by the sequence given by differentially encoding the PN sequence.

[71]

[72]

s = -(N/ (r_n /r_n+ι )C:) Eq. (7)

[73]

[74] While the existing method estimates only the fraction part of the frequency offset of

-l/2<ε<l/2, the method of the present invention has the estimation range of the frequency offset (ε) of -N/2<ε<N/2. That is, compared with the related art, the estimation range of the frequency offset is widened. Also, while the number of the samples averaged in the related art is N/2, the preset invention can average the interference using (N-I) number of the samples. In addition, because C has the PN sequence, the interference removal effect can be obtained.

[75] In step 412, a compensation operation is performed using the time offset (θ ) correct and frequency offset (ε ) obtained through the time synchronization and frequency correct offset estimation procedures. Since the compensation method is well known, its detailed description will be omitted.

[76] Although the embodiments of the present invention have been described based on the basic OFDM communication system, the present invention can also be applied to uplink and downlink of all systems using OFDMA based on the OFDM.

[77] The above-described methods in accordance with the present invention can be stored in computer-readable recording media. The computer-readable recording media may include CD ROM, RAM, ROM, floppy disk, hard disk, optical magnetic disk, and so on. Since these procedures can be easily carried out by those skilled in the art, a detailed description thereof will be omitted.

[78] The present application contains subject matter related to Korean patent application

No. 2005-0106554, filed with the Korean Intellectual Property Office on November 8, 2005, the entire contents of which is incorporated herein by reference.

[79] While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.

Claims

[1] A method for generating a preamble sequence using a pseudo noise (PN)sequence in an OFDM transmission system, the method comprising the steps of: a) generating a PN sequence having a high autocorrelation value; and b) generating a preamble sequence by multiplying the PN sequence by an inverse matrix of an inverse fast Fourier transform (IFFT) matrix in frequency domain to make the PN sequence become the preamble sequence in time domain.

[2] The method as recited in claim 1, wherein when a length of the PN sequence (S) is N, the preamble sequence (P) is generated based on equations 1 and 2 which are expressed as:

Eq. (1) where W =e^"j2/N

N p = JVI-¹S

Eq. (2) where S represents PN sequence having length of N, and M represents an NxN

IFFT matrix.

[3] A time synchronization method in an OFDM reception system, comprising the steps of: a) storing a PN sequence that is used to generate a preamble sequence at a transmitter side; and b) performing a moving sum onto OFDM reception signals by using the PN sequence, finding a position where the moving sum becomes maximal, and setting the position as a preamble position.

[4] The time synchronization method as recited in claim 3, wherein the step b) includes the steps of: bl) comparing the moving sum with a threshold value; and b2) achieving the time synchronization by setting the moving sum value that exceeds the threshold value for the first time as a maximum value.

[5] A method for obtaining time synchronization and estimating frequency offset based on a PN sequence in an OFDM reception system, comprising the steps of: a) finding a preamble position of an OFDM reception signal in time domain and synchronizes symbols according to the preamble position; and b) estimating frequency offset of a subcarrier in time domain by multiplying a synchronized reception signal by a sequence obtained by differentially encoding the PN sequence.

[6] The method as recited in claim 5, wherein the step a) includes the steps of: performing a moving sum onto the OFDM reception signals based on the PN sequence; finding a position where the moving sum becomes maximal; and setting the position as the preamble position. [7] The method as recited in claim 5, wherein the frequency offset (ε) is estimated in frequency domain based on equations (3) and (4) which are expressed as:

Eq. (3)

Eq. (4) where s denotes the PN sequence used to generate the preamble sequence at a transmitter side; C denotes the kth sample of the sequence given by differentially encoding the PN sequence; N denotes the number of samples having a single OFDM symbol; and r denotes an n* reception signal sample.