KR101449865B1

KR101449865B1 - Initial synchronization method for OFDM system and apparatus thereof

Info

Publication number: KR101449865B1
Application number: KR1020130089696A
Authority: KR
Inventors: 유영환; 신원재; 김병규
Original assignee: 세종대학교산학협력단
Priority date: 2013-07-29
Filing date: 2013-07-29
Publication date: 2014-10-13

Abstract

The present invention relates to an initial synchronization method in an OFDM system and a device thereof. According to the present invention, an initial synchronization method in an OFDM system which includes: a step of converting primary signal (PS) received by including a cell index from a plurality of cells from a time domain to a frequency domain; a step of classifying the PS signal into first and second subcarrier wave groups in the left and right with respect to the DC subcarrier wave and calculates a primary correlation value between adjacent subcarrier waves within the first and second subcarrier wave groups; a step of calculating a secondary correlation value between the first correlation value for the PS signal and a reference correlation value for the reference PS signal, comparing the second correlation value with the threshold value, and classifying the subcarrier waves into a plurality of sub-groups; a step of selecting integer frequency offset and cell index for initial synchronization from the sum made by multiplying signals which reflects integer frequency offset candidates to subcarrier waves within the sub-group and reference correlation value corresponding to the sub-group. According to an initial synchronization method in the OFDM system and a device thereof, an integer frequency offset which is one of the elements which can affect the performance of the OFDMA-based LTE downlink system and a sector cell index can be efficiently estimated.

Description

[0001] The present invention relates to an initial synchronization method for an OFDM system,

The present invention relates to an initial synchronization method and apparatus for an OFDM system, and more particularly, to an apparatus and method for effectively simultaneously estimating an integer frequency error and a sector cell index, which are factors that determine the performance of an OFDMA based LTE downlink system To an initial synchronization method in an OFDM system and apparatus therefor.

In the next generation wireless communication, an OFDMA (Orthogonal Frequency Division Multiple Access) method in which an entire channel is divided into a plurality of orthogonal subchannels and transmitted in parallel is used mainly for high-speed data transmission. In order to overcome the problem caused by the high peak-to-average power ratio (PAPR) of OFDMA in the uplink, the SC-FDMA (Single-Frequency Division Multiplexing) scheme adopts the OFDMA transmission scheme in the downlink of the LTE system standardized by the 3GPP Carrier Frequency Division Multiple Access) transmission method.

The LTE downlink system based on OFDMA is very sensitive to frequency error. The frequency error is caused by the Doppler effect or instability of the transmitter / receiver oscillator. The resulting frequency error is divided into an integer frequency error and a decimal frequency error. Particularly, the integer frequency error shifts the signal in the frequency domain after the Fast Fourier Transform (FFT), so that the position of the recovered data is shifted and demodulated into data completely different from the originally transmitted signal. Also, unlike other communication systems, the system does not have a preamble symbol, uses a limited PS (Primary Signal) signal for initial cell search and synchronization, and is very sensitive to cell index detection and detection time during initial cell search.

Conventionally, a method has been proposed in which integer frequency error estimation and sector cell index detection are simultaneously performed as a method for estimating an integer frequency error and performing a fast initial cell search. However, the conventional method is a method of estimating a cross-correlation value between a received PS signal and a previously known PS signal at a receiving end. Since a large number of complex multiplication and addition operations are performed at this time, high computational complexity is required, which is accompanied by high cost and hardware complexity in actual implementation.

The technique to be a background of the present invention is disclosed in Korean Patent Publication No. 2005-0066562 (published Jun. 30, 2005).

An object of the present invention is to provide an initial synchronization method and apparatus for an OFDM system capable of effectively simultaneously estimating an integer frequency error and a sector cell index, which are factors that influence performance of an OFDMA-based LTE downlink system. have.

The method includes converting a PS signal received from a plurality of cells, including a cell index, from a time domain to a frequency domain, and transmitting the PS signal to a first subcarrier and a second subcarrier based on a DC subcarrier, Calculating a first cross-correlation value between neighboring subcarriers in the first and second subcarrier groups, calculating a first cross-correlation value for the PS signal and a reference cross- Calculating a second cross-correlation value between a reference cross-correlation value and comparing the second cross-correlation value with a threshold value to classify each sub-carrier into a plurality of subsets, and for each PS signal corresponding to the cell index, Multiplying the sum of the signals reflecting the integer error candidate values to the subcarriers and the reference cross-correlation value corresponding to the subset, From the results, there is provided an initial synchronization method in an OFDM system including the steps of selecting a coarse error value and the cell indices for the initial synchronization.

The step of classifying each of the subcarriers into a plurality of subsets may further include a step of classifying the first cross-correlation value in a subcarrier having the smallest index among a plurality of subcarriers used in the calculation of the first cross- Calculating a second cross-correlation value between the reference cross-correlation values in a subcarrier, and calculating a second cross-correlation value larger than the threshold among a plurality of sub-carriers used in the calculation of the second cross- Classifying the derived subcarriers into a first subset together with subcarriers having the smallest index, calculating a second cross-correlation value from among the plurality of subcarriers excluding the first subset, And classifying the plurality of sub-carriers into a plurality of subsets by repeating the process.

The step of classifying each of the subcarriers into a plurality of subsets may compare a real part of the secondary cross-correlation value with the threshold value.

Also, the step of selecting an integer multiple error and a cell index for the initial synchronization may include calculating a sum of the signals of which the M integer error candidate values are respectively reflected in subcarriers in the subset with respect to each PS signal corresponding to N cell indices And multiplying the reference cross-correlation values of the sub-carriers having the smallest index among the sub-carriers in the sub-set by the subsets to derive L (= N x M) And selecting an integer-valued error value and a cell index to derive the maximum value among the plurality of indexes.

Here, a signal that reflects M integer error candidate values to subcarriers in the sub-set is a signal that reflects M integer error candidate values to the subcarriers, and a rounding value of the real part of the second-order cross- Can be used.

Also, the step of selecting the integer error and the cell index for the initial synchronization may use the following equation.

here,

And

D is an integer multiple frequency error candidate value determined by the D value, and d is an integer multiple of the frequency error candidate value determined by the D value. N _s _{, i} is the number of the subsets, k is a subcarrier index, S _i _{, j} is a subcarrier index belonging to the jth subset obtained from the ith cell,

Lt; RTI ID = 0.0 >

B _i (k) denotes a phase-transformed variable having a value of +1 or -1 as a rounding value of a real part of a second-order cross-correlation value in the corresponding subcarrier,

Denotes the reference cross-correlation value in a subcarrier having the smallest index among the subcarriers in the jth subset.

The present invention also provides a mobile station apparatus comprising: a Fourier transformer for converting a PS (Primary Signal) signal received from a plurality of cells, including a cell index, from a time domain to a frequency domain; A first correlator for calculating a first cross-correlation value between adjacent subcarriers in the first and second subcarrier groups and a second correlator for calculating a first cross-correlation value for the PS signal, A sub-set generator for calculating a second-order cross-correlation value between a reference cross-correlation value for a known reference PS signal and comparing the second cross-correlation value with a threshold value to classify each sub-carrier into a plurality of subsets, For a PS signal, a summation value of a signal that reflects an integer error candidate value to subcarriers in the subset, and a reference cross-correlation value And an error estimator for selecting an integer error value and a cell index for initial synchronization from a result of summing the multiplication results by the multipliers.

According to the initial synchronization method and apparatus of the OFDM system according to the present invention, it is possible to simultaneously and efficiently estimate the integer frequency error and the sector cell index, which are factors that determine the performance of the OFDMA-based LTE downlink system have.

1 is a conceptual diagram illustrating a frame structure of a PS signal and an SS signal in an OFDMA-based downlink system according to an embodiment of the present invention.
2 is a configuration diagram of an initial synchronization apparatus in an OFDM system according to an embodiment of the present invention.
3 is a flowchart of an initial synchronization method using the apparatus of FIG.
FIG. 4 is a result of the subset configuration for each cell when the threshold value T = 0.99 in step S330 of the present invention.
5 shows an example of calculation for estimating an integer frequency error and a sector cell index for initial synchronization in a conventional technique.
Figure 6 shows the results of a comparison of the computational complexity for the method according to the embodiment of the present invention and the existing method.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily carry out the present invention.

Generally, in an OFDMA-based LTE downlink system, an initial cell search requires initial frame synchronization, frequency synchronization, sector cell index detection, and CP length detection for a UE to access a cell.

The present invention relates to an initial synchronization method and apparatus in an OFDM system, and corresponds to an initial cell search process that is performed for the first time in a mobile station. The initial cell search process is a process of searching for optimal cells for communication among neighboring cells (base stations), and can be roughly divided into PS (primary signal) signal detection and SS (secondary signal) signal detection.

Here, in the embodiment of the present invention, only a PS signal detection step is considered because a method of simultaneously estimating an integer frequency error and a sector cell index is proposed. According to the embodiment of the present invention, initial synchronization is performed by comparing a received PS signal with a known reference signal.

In the following embodiments of the present invention, N = 512, N _g = 128, f _c (Carrier frequency) = 800 MHz and D _f (Subcarrier spacing) = 15 kHz according to the standard of the 3GPP LTE downlink FDD mode OFDMA system, and considered the M.1225 channel model defined in ITU-R.

Prior to detailed description of the present invention, a frame structure of a PS signal and an SS signal in an OFDMA-based downlink system will be described below.

1 is a conceptual diagram illustrating a frame structure of a PS signal and an SS signal in an OFDMA-based downlink system according to an embodiment of the present invention. 1 corresponds to a signal frame structure in an OFDMA-based downlink system.

At this time, it is assumed that the transmitting terminal transmits an OFDM signal (transmission signal) having N subcarriers. The transmitting end transmits the transmission signal by inverse fast Fourier transform (IFFT). The transmission signal is transmitted in units of half a radio frame of 5 ms, and the transmission signal is used to distinguish frame synchronization from sector cell index information at the receiver. The PS signal, which is an OFDM signal transmitted from the transmitting end, is a signal to which a ZC (Zadoff Chu) sequence is assigned for detecting an integer multiple frequency synchronization error and a sector cell index based on cross-correlation.

In the frequency division duplex (FDD) mode of the OFDMA-based downlink system, one frame of 10 ms length is composed of 20 slots (0.5 ms). In Extended CP (Cyclic Prefix) mode, one slot consists of six OFDM symbols. The last symbols in the first and 11th slots of the 20 slots correspond to the positions of the PS signals, respectively (see the enlarged portion in FIG. 1).

In order to distinguish three types of sector cell index information, the PS signal is assigned a ZC sequence that is different from a root index (hereinafter referred to as a cell index) u. The ZC sequence is allocated to 63 subcarriers around a DC subcarrier, and a value of 0 is assigned to a DC subcarrier.

In the embodiment of the present invention, the cell index u is determined to be 25, 29, 34. Here, the type of the determined cell index is used for illustrative purposes, and the present invention is not necessarily limited thereto.

Here, the PS signal D _u (k) transmitted from the u-th cell is defined by Equation (1).

Here, D _u (k) is a PS signal in the frequency domain, and k represents a subcarrier index. The subcarrier index of D _u (k) is symmetric about the DC subcarrier. This is one of the important features in the embodiment of the present invention. Accordingly, the PS signal is divided into left and right first and second subcarrier groups on the basis of a DC subcarrier.

In the embodiment of the present invention, among the total of 72 subcarrier signals (corresponding to the number of subcarrier indices k) defined by Equation (1), five signals (kε [-36, -32] 62] (kε [-31, -1] ∪ [1,31]) except for the initial synchronization (32,36). Here, a signal corresponding to k? [-31, -1] is divided into a first subcarrier group and a signal corresponding to k? [1,31] is classified into a second subcarrier group.

From Equation (1), it can be seen that the phase increase of D _u (k) is proportional to the square of the carrier index of the PS signal. The phase difference between adjacent subcarriers can be defined by Equation (2).

Hereinafter, a method and an apparatus for initial synchronization in an OFDM system according to an embodiment of the present invention will be described in detail with reference to the above description. 2 is a configuration diagram of an initial synchronization apparatus in an OFDM system according to an embodiment of the present invention.

3 is a flowchart of an initial synchronization method using the apparatus of FIG. 1 and 2, an initial synchronization apparatus 100 in an OFDM system according to an embodiment of the present invention includes a Fourier transformer 110, a first correlator 120, a second correlator 130, (140), and an error estimator (150).

First, the Fourier transformer 110 transforms a received primary signal (PS) signal including a cell index from a plurality of cells into a frequency domain from a time domain (S310). The step S310 will be described in detail as follows.

At the receiving end, each PS signal corresponding to cell indices 25, 29, and 34 is received from each cell. For example, a PS signal for cell index 25, a PS signal for cell index 29, and a PS signal for cell index 35 are received. In an embodiment of the present invention, one cell index may be determined as the cell search result for the initial synchronization using the analysis result of the received PS signal.

The PS signal transmitted from the cell is defined in Equation (1). The receiving end performs Fast Fourier Transform (FFT) on each received PS signal from the time domain to the frequency domain. Since the analysis procedure for each cell index is the same, the process of analyzing the PS signal for one cell index will be described as an example for convenience of explanation.

Assuming that the time and fractional frequency errors of the received signal in the OFDMA-based LTE downlink system are completely compensated, the PS signal converted into the frequency domain, i.e., Y (k), is expressed by Equation (3).

More specifically, Y (k) is the kth subcarrier signal for the PS signal in the received frequency domain,

H (k) denotes a channel response in the frequency domain, and W (k) denotes a complex additive white Gaussian noise (AWGN) with an average of zero. We have explained that u corresponds to three cell indexes.

After converting the received PS signal into the frequency domain, the first correlator 120 generates a first subcarrier group in the first subcarrier group (left subcarriers) composed of signals corresponding to k? [-31, -1] And calculates a first-order cross-correlation value between adjacent subcarriers. In the same manner, the second correlator 130 calculates a first-order cross-correlation value between adjacent subcarriers in a second subcarrier group (right subcarriers) composed of signals corresponding to k [1, 31] ( S320).

The first correlator 120 and the second correlator 130 are used to calculate the respective first-order cross-correlation values < RTI ID = 0.0 >

) Can be defined as shown in Equation (4).

Is a first order cross-correlation value between Y (k) and Y (k-1) subcarrier signals adjacent to each other in the received PS signal. Equation (4) uses the symmetric characteristic of the PS signal,

If the sub-carrier index k [-30, -1], and the cross-correlation of ^{Y (k) Y * (k} -1) in one case, the subcarrier index k is [2,31] has one Y (k) ^* Y ( k-1). Here, * corresponds to conjugation.

According to the operation of Equation (4), the first cross-correlation value between adjacent subcarriers for the received PS signal is derived for each of the first subcarrier group and the second subcarrier group.

In the case of the embodiment of the present invention, the first cross-correlation value (

) Is a reference cross-correlation value ("

&Lt; / RTI > and are then cross-correlated and compared. Here, the reference PS signal may have the same shape as the received PS signal (left-right symmetrical structure with respect to a DC subcarrier).

The reference cross-correlation value for the reference PS signal previously known at the receiving end (

) Can be defined as shown in Equation (5).

Is a reference cross-correlation value between the neighboring sub-carrier signals D _ui (k) and D _ui (k-1) in the known reference PS signal. The calculation method for the two carrier groups is the same as in Equation (4). In addition,

Is derived for each of the 30 carrier groups.

Based on the contents of Equations (4) and (5), the subset generator 140 generates a first-order cross-correlation value

) And the reference cross-correlation value for the known reference PS signal (

), Compares it with a threshold value, and classifies each subcarrier into a plurality of subsets (S330).

Step S330 of generating a plurality of subsets for the sub-carriers constituting the PS signal may be performed according to the following Equation 6 (Step 1 to Step 5).

First, in step 1, i and j are initialized to 1.

Here, i denotes an i-th cell as a cell ID, and i = {1,2,3}. That is, it means that a subset generation process (S330) is performed for the PS signal received from the first cell (u = 25) among the three cells corresponding to u? [25,29,34] with i = 1. u = 25 if i = 1, u = 29 if i = 2, u = 34 if i = 3. When the subset generation is completed for i = 1, i is sequentially increased and the same process is repeated for the signals received from the remaining two cells.

J is the jth subset, and since there is no subset at the beginning, it is initialized to the first subset to be generated, j = 1. Hereinafter, for the sake of convenience of explanation, a subset generation process in the case of i = 1 (u = 25) will be described.

The procedure of Step 2 is as follows.

In step 2, first, a plurality of sub-carrier indexes

= [- 30,1] ∪ [2,31], the smallest subcarrier index

Select according to equation. Accordingly,

= -30. Note that,

Is a set of 60 integers made up of [-30,1] ∪ [2,31], and g _i _{, j} means 60 sets of [1,60].

Next, in Step 3, a first-order cross-correlation value (k) in subcarriers corresponding to k = -30, which is the smallest index among the plurality of subcarrier indexes used for the calculation of the first-

), A reference cross-correlation value in the plurality of subcarriers (

) Are mutually correlated with each other to obtain a second-order cross-correlation value. Then, from the second cross-correlation value,

) And compares it with a threshold value T to generate a first subset corresponding to j = 1.

More specifically, among the subcarriers used in the calculation of the second-order cross-correlation value, the subcarriers deriving the second-order cross-correlation value larger than the threshold value T are selected and compared with the subcarriers (k = -31) (S _{i, j} = S _1,1 ).

More specifically,

and

The cross-

and

Gt ;, < RTI ID = 0.0 >

and

As shown in FIG. The real part is then taken from the computed cross-correlation values.

The real number value taken in Step 3

Can be defined as shown in Equation (7). Equation (7) can be calculated using equations (4) and (5).

In Equation (7), Re {} corresponds to an operation that takes a real part for the {} component.

Step 4 will be described as follows. After the first subset (S ₁ _{, 1} ) is sorted for the first cell as described above,

And repeats the calculation of the second-order cross-correlation value and the comparison of the threshold values for the remaining subcarriers excluding the first subset (Step 2 to Step 4). Thereby dividing the plurality of subcarriers into a plurality of subsets.

In this process, as in Step 5,

Is empty, that is, when it becomes an empty set, the generation of the subset is stopped, and the j value at that time is finally defined as the number of total subsets (N _s _{, i} ). sure

If there is an element remaining in the element, the above procedure is repeated while j is incremented by 1 until the element is exhausted. A more specific calculation example of the subset generation process will be described later.

As described above, the subset generator 140 divides the PS signals received from the first cell into subcarriers constituting the corresponding PS signal into subcarriers (S ₁ _{, 1} ) in the first subset, subcarriers (S ₁ _, (S ₁ _{, 2} ), a third subset of subcarriers (S ₁ _{, 3} ), and the like. This also applies to the second cell and the third cell.

As described above, unlike the conventional method using correlation values of all PS signals in order to reduce computation complexity, the present invention uses an arbitrary subset exceeding predetermined threshold values of correlation values of PS signals to estimate errors .

After generating the subsets, the error estimator 150 calculates, for each PS signal corresponding to the cell index, the sub-

A signal (d) that reflects the integer error candidate value d

) And a reference cross-correlation value ("

) And multiplying them by the subsets, an integer multiple error value and a cell index for initial synchronization are selected (S340).

Hereinafter, step S340 will be described in detail with reference to Equation (8) below.

here,

And

Is the selected integer error and cell index, and u is the cell index corresponding to the plurality of cells.

D is an attempted value of the integer frequency error candidate, and d is the M number of integer frequency error candidate values determined by the D value. Since D = 2 in the present embodiment, a total of five frequency error candidate values are determined as d = {- 2, -1, 0, 1, 2}. In addition, N _s _{, i} denotes the number of the subsets, k denotes a subcarrier index, S _i _{, j} denotes a subcarrier index belonging to the jth subset obtained from the ith cell _{, and} S _i _{, j} denotes a subcarrier index.

Also,

Lt; RTI ID = 0.0 >

Referring to Equation (8), in operation S340, for each PS signal corresponding to N cell indices, subcarriers in the subset

(M) of the error value (d)

) And a reference cross-correlation value at a sub-carrier having the smallest index among the sub-carriers in the sub-set

) Are multiplied together, and the result is summed by the subset to derive L (= NxM) resultant values. Note that,

Is expressed by Equation (8)

Means the summation of the signal multiplied by B _i (k). B _i (k)

Is used because there is a case where a negative value is outputted.

In this embodiment, since the number of used cell indexes is 3 and the integer error candidate value is 5, a total of 15 result values are obtained with respect to Equation (8). Thereafter, an integer-valued error value and a cell index that derive the maximum value among these 15 result values are selected. This selected information corresponds to the estimated information soon. For example, in step S340,

Is 0 out of -2, -1, 0, 1, 2, and the estimated cell index

May be 25 out of 25, 29, or 34.

Hereinafter, calculation examples of steps S330 through S340 will be described. Here, the case where u = 25 (i = 1) will be exemplified.

First, the calculation example of the step S330 is as follows. The formula for comparing the cross-correlation value (primary cross-correlation value) of the PS signal received at the receiving end with the cross-correlation value (reference cross-correlation value) known at the receiving end has been described with reference to Equation (7).

In Equation (7)

The

and

(Second-order cross-correlation value). n and m are composed of [-30, -1] ∪ [2,31] as described above. The configuration of the subsets uses Equation (6) using the real part of the values correlated with each other.

In constructing the original subset (j = 1)

M = -30,

Using n = [- 30,1] ∪ [2,31]

Respectively.

Is calculated by the following equation (9). This corresponds to some of the calculation examples out of 60 total.

In this embodiment, the threshold value is T = 0.99. Selects subcarrier indices that derive a value greater than 0.99 from Equation (9), and associates subcarriers corresponding thereto with the first subset group. As a result of calculation, sub-carriers corresponding to sub-carrier indexes -30, -25, -1, 2, 26, and 31 belong to the first subset corresponding to j = 1.

After that

= [- 30,1] U [2, 31] is subtracted from the first subset to generate a second subset corresponding to j = 2. Here, when j = 2

= -29. That is, in constructing the second subset with j = 2,

Of m = -29,

N of 54 is used except for {-30, -25, -1, 2, 26, 31} in [-30,1] ∪ [2,31]. Accordingly, a total of 54

Respectively. The threshold value T used is equal to 0.99.

As a result, sub-carriers corresponding to the sub-carrier indexes -29, -24, 25, and 30 belong to the second subset corresponding to j = 2. A total of 15 subsets can be generated by repeating this process. In this way, the subsets at u = 29 and 34 can be obtained.

FIG. 4 is a result of the subset configuration for each cell when the threshold value T = 0.99 in step S330 of the present invention. For u = 25, a total of 15 subsets were constructed. For u = 29 and 34, a total of 13 subsets were constructed. In the present embodiment, when u = 29, 34, the result of the construction of the subset is the same, and the result is displayed in the same area. Of course, this subset configuration example is merely an example, and the present invention is not necessarily limited thereto.

Next, a calculation example of step S340 will be described. In the case of u = 25, it is confirmed that there are 15 subsets (j = 1 to 15). The operation of Equation (8) may be performed by using the sum of the 15 results derived from Equation (10) below. Equation (10) is an example in which d = 0 in Equation (8).

In this way, if we obtain the results for all u (total 3) and d (total 5) values, Equation 8 can be derived as a total of 15 (= 3 x 5) branches, And finds the integer frequency error and the sector cell index corresponding thereto.

In the embodiment of the present invention as described above, the computational complexity can be reduced while the performance is similar to that of the conventional technique. The reason for this is described below in comparison with the conventional technique.

The conventional technique estimates an integer frequency error using Equation (11) below.

In Equation (11), the same factor as Equation (8) means the same element. In the conventional technique, C (k) obtained by cross-correlation of received Y (k) and Y (k-1) and D _u (k) and D _u After correlating the obtained Q _u (k) with each other, a point at which the value is maximized is estimated to simultaneously detect an integer frequency error and a sector cell index. Then, u and d are selected from among the index u for searching the sector cell index and the frequency error d of integer frequency.

5 shows an example of calculation for estimating an integer frequency error and a sector cell index for initial synchronization in a conventional technique. In the conventional technique, C (k + d) reflecting d to C (k) is correlated with Q _u (k), and is performed for all k values. This process is also performed for all d values. 5 is an example of calculation when d = -1, 0 is applied. That is, 60 cross-correlation operations are required for each d value. However, according to the present invention, there is a difference from the prior art in that a process of generating a subset includes only a number of subsets for each d value.

Figure 6 shows the results of a comparison of the computational complexity for the method according to the embodiment of the present invention and the existing method.

The complex multiplication calculation complexity of the method of simultaneously estimating the integer frequency error and the sector cell index according to the embodiment of the present invention is

And, which is affected by the number N _{i, j} of sub-carriers belonging to a number of N _{s, i,} and a sub-set of the sub-set.

On the other hand, the complex multiplication calculation complexity according to the conventional method is

Lt; / RTI > It can be seen from this that the embodiment of the present invention reduces the computational complexity by about 38.6% compared with the conventional method.

Further, the complex addition computation complexity according to the conventional method is

, Which is also influenced by N _s _{, i} and N _i _{, j} . On the other hand, the complex addition computational complexity according to the conventional method is

Lt; / RTI > From this, it can be seen that the complexity of the entire system is lowered because the method according to the embodiment of the present invention reduces computational complexity in addition and multiplication operations compared to the conventional method.

As described above, according to the initial synchronization method and apparatus of the OFDM system according to the present invention, it is possible to simultaneously and efficiently estimate the integer frequency error and the sector cell index, which are factors that determine the performance of the OFDMA based LTE downlink system There is an advantage.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. Accordingly, the true scope of the present invention should be determined by the technical idea of the appended claims.

100: initial synchronization device in OFDM system
110: Fourier transformer 120: First correlator
130: second correlator 140: subset generator
150: error estimator

Claims

Converting a primary signal (PS) signal including a cell index from a plurality of cells into a frequency domain from a time domain;
Dividing the PS signal into left and right first and second subcarrier groups on the basis of a DC subcarrier, and calculating a first-order cross-correlation value between adjacent subcarriers in the first and second subcarrier groups;
Calculating a second cross-correlation value between a first cross-correlation value for the PS signal and a reference cross-correlation value for a known reference PS signal, comparing the second cross-correlation value with a threshold value, and classifying each sub-carrier into a plurality of subsets ; And
For each of the PS signals corresponding to the cell index, a sum of a sum of signals obtained by reflecting the integer error candidate values to subcarriers in the subset and a reference cross-correlation value corresponding to the subset are multiplied by each other, And selecting from the result an integer error value and a cell index for initial synchronization.

The method according to claim 1,
Wherein the step of classifying each subcarrier into a plurality of subsets comprises:
The second cross-correlation value between the first cross-correlation value in the subcarrier having the smallest index among the plurality of subcarriers used for the calculation of the first cross-correlation value and the reference cross-correlation value in the plurality of sub- Computing;
Classifying the subcarriers that derive the second cross-correlation value larger than the threshold among a plurality of subcarriers used for the calculation of the second cross-correlation value into a first subset together with subcarriers having the smallest index; And
And a step of classifying the plurality of subcarriers into a plurality of subsets by repeating the calculation of the second cross-correlation value and the comparison of the threshold values with respect to the plurality of remaining subcarriers excluding the first subset, In the initial synchronization method.

The method of claim 2,
Wherein the step of classifying each subcarrier into a plurality of subsets comprises:
And comparing the real part of the second cross-correlation value with the threshold value.

The method of claim 2,
Wherein the step of selecting an integer error and the cell index for the initial synchronization comprises:
For each PS signal corresponding to N cell indices, the sum of the sum of the signals of which M integer error candidates are reflected on the subcarriers in the subset, Multiplying the correlation values by each other, and summing the resultant values on the subsets to derive L (= NxM) resultant values; And
And selecting an integer-valued error value and a cell index to derive a maximum value among the L result values.

The method of claim 4,
The signal that reflects M integer candidate error values to subcarriers in the sub-
Wherein a value obtained by multiplying the subcarriers by the M round error values is multiplied by a rounding value of a real part of a second cross correlation value in the corresponding subcarrier.

The method of claim 5,
The step of selecting an integer-time error and a cell index for the initial synchronization may include: an initial synchronization method in an OFDM system using the following equation:

here,

And

D is the trial value of the integer error candidate, d is the M number of integer error candidate values determined by the D value, and N _s (n) is a cell index corresponding to the selected integer error and cell index, u is a cell index corresponding to the plurality of cells, _{, i} is the number of the subsets, k is a subcarrier index, S _{i, j} is a subcarrier index belonging to the jth subset obtained from the ith cell,

Lt; RTI ID = 0.0 >

A Fourier transformer for converting a primary signal (PS) signal including a cell index from a plurality of cells into a time domain to a frequency domain;
The first and second subcarrier groups are divided into first and second subcarrier groups on the basis of a DC subcarrier based on the PS signal and first and second subcarrier groups, A correlator;
A second cross-correlation value between a first cross-correlation value for the PS signal and a reference cross-correlation value for a known reference PS signal, compares the second cross-correlation value with a threshold value, and classifies each sub- Set generator; And
For each of the PS signals corresponding to the cell index, a sum of a sum of signals obtained by reflecting the integer error candidate values to subcarriers in the subset and a reference cross-correlation value corresponding to the subset are multiplied by each other, From the result, an error estimator for selecting an integer error value and a cell index for initial synchronization.

The method of claim 7,
Wherein the subset generator comprises:
The second cross-correlation value between the first cross-correlation value in the subcarrier having the smallest index among the plurality of subcarriers used for the calculation of the first cross-correlation value and the reference cross-correlation value in the plurality of sub- After calculation,
Among the plurality of subcarriers used for the calculation of the second cross-correlation value, subcarriers deriving the second cross-correlation value larger than the threshold value are classified into a first subset together with subcarriers having the smallest index,
An initial synchronization in an OFDM system for dividing the plurality of sub-carriers into a plurality of subsets by repeating the calculation of the second-order cross-correlation values and the comparison of the threshold values for a plurality of sub-carriers excluding the first sub- Device.

The method of claim 8,
Wherein the subset generator comprises:
And comparing the real part of the second cross-correlation value with the threshold value.

The method of claim 8,
The error estimator includes:
For each PS signal corresponding to N cell indices, the sum of the sum of the signals of which M integer error candidates are reflected on the subcarriers in the subset, L (= N x M) result values are derived by multiplying the correlation values by the sum of the correlation values,
And an integer-valued error value and a cell index that derive the maximum value among the L result values are selected.

The method of claim 10,
The signal that reflects M integer candidate error values to subcarriers in the sub-
And a value obtained by multiplying the subcarriers by the rounded values of the real part of the second-order cross-correlation value of the subcarrier with respect to the signal that reflects the M error candidates.

The method of claim 11,
The error estimator includes:
An initial synchronization unit in an OFDM system using the following equation when selecting an integer error and a cell index for the initial synchronization:

here,

And

Lt; RTI ID = 0.0 >