KR101055734B1

KR101055734B1 - Initial Sync Estimation Method and Apparatus in Code Division Multiple Access Mobile Communication System

Info

Publication number: KR101055734B1
Application number: KR20050103509A
Authority: KR
Inventors: 김남훈; 박용배
Original assignee: 엘지전자 주식회사
Priority date: 2005-10-31
Filing date: 2005-10-31
Publication date: 2011-08-11
Also published as: CN1960226B; KR20070046646A; CN1960226A

Abstract

The present invention relates to a method and apparatus for estimating initial synchronization with a base station in a terminal of a CDMA mobile communication system. According to the present invention, the terminal divides at least one or more sub-frames received from the base station into at least two areas during cell search and calculates correlations for each area, using the maximum correlation value for each area. Estimating the initial synchronization is a basic feature.

CDMA, TDD LCR, Initial Sync, Subframe, Sync Code, Timing Sync

Description

Method of estimating initial synchronization in CDMA mobile communications system and apparatus according to code division multiple access mobile communication system

1 is a flowchart illustrating a cell search process according to the prior art.

2 shows a format of a downlink subframe in a 3GPP TDD LCR system.

3 is a flowchart of an initial synchronization estimation method according to an embodiment of the present invention.

4 illustrates an example of autocorrelation for a SYNC-DL code and cross correlation with other codes.

FIG. 5 illustrates a process of obtaining a maximum value of correlation for each region, an index of a synchronization code having a maximum value, and a timing point according to an exemplary embodiment of the present invention.

FIG. 6 is a diagram for explaining an example of estimating timing synchronization point in one preferred embodiment of the present invention. FIG.

7 is a block diagram of an initial synchronization estimation device according to an embodiment of the present invention.

FIG. 8 is a graph showing simulation results for the case where various channels and frequency offsets as shown in Table 1 exist.

9A to 9D are initial synchronous histograms according to changes in SNR for various channels PA3, PB3, VA30, and VA120 as shown in Table 1;

The present invention relates to a code division multiple access (CDMA) mobile communication system. More specifically, the present invention relates to a method and apparatus for estimating initial synchronization with a base station in a terminal of a CDMA mobile communication system.

In the TD-SCDMA system, the UE searches for a downlink sync code (SYNC-DL), a basic midamble code, a scrambling code, a frame sync, broadcast channel (BCH) information, and a chip. As a process of acquiring chip timing information and the like, it is generally performed through the following four steps. 1 is a flowchart illustrating a cell search process according to the prior art.

1) Initial Sync Search Process (DwPTS)

The UE estimates initial synchronization in a downlink pilot time slot (DwPTS) using 32 possible SYNC-DL codes without knowing cell information to be searched. Through this process, the UE must find the SYNC-DL code and the timing synchronization point that are used in the cell among the 32 SYNC-DL codes.

2) Basic midamble and scrambling code identification

Since the DwPTS code found in step 1) corresponds to one code group having four basic midambles, the UE finds a basic midamble being used in a cell among four basic midambles. Since the basic midamble is associated with the scrambling code, the scrambling code can be found by looking for the basic midamble.

3) Control multi-frame sychronisation

The UE finds Master Information Blocks (MIBs) of BCH multi-frames from QPSK phase modulation of DwPTS for the P_CCPCH midamble.

4) Read the BCH

The terminal reads the BCH using the retrieved MIB to obtain BCH information.

In the above process, process 1) can affect the power consumption, performance of AFC (Automatic Frequency Control) and Sync Tracker, etc., and requires a large number of iterations, which requires a large amount of computation and memory. It is the most important part of the overall performance of the process.

In general, a downlink sync code (SYNC-DL) used in a current cell is estimated through correlation between a signal received from a base station and a reference signal. Techniques described below are used as a method of estimating initial synchronization from a signal received in a 3GPP TDD low chip rate (LCR) system.

The first method is to use the maximum value of the correlation. The method is easy to implement as a method of estimating the initial synchronization by finding the maximum value by obtaining the correlation in the received subframe. However, under low SNR and multipath fading channel environment, it is difficult to obtain a performance improvement of time correlation because it changes the level of the received signal quickly. Furthermore, when the sampling of the received signal is reduced, the timing error becomes larger.

The second method is to accumulate the correlation values over several subframes and use the average. The correlation between the signals received from the base station and the downlink sync code (SYNC-DL) is calculated, accumulated over several subframes, and then averaged. Take it. The initial value is estimated by finding the largest value among the average values.

If the correlation is accumulated over several subframes and the average is found to find the maximum value, a highly reliable timing correlation can be obtained by reducing the level change of the received signal due to the high fading frequency effect. However, since all correlations must be stored for 32 codes for each time received over several subframes, the memory increases. In addition, when the over-sampled reception signal is implemented, a comparator that sequentially finds the maximum correlation has a burden of repeating and storing as much as [6400 × 32 × oversampling]. In addition, since the number of subframes cannot be infinitely limited, a limited number of subframes are accumulated, and an error value of a badly correlated error is accumulated and averaged by noise, which causes performance degradation.

Third, a method of tracking the synchronization code and the synchronization point with the highest frequency for several subframes, the frequency of the correlation is the highest in the original code when looking at the probability determined over the various subframes. This frequency of occurrence is used to remove error correlations and average with the remaining probable timing candidates.

If only the correlation value and the correlation value of the code having the largest correlation value are stored during one subframe, the required memory size per subframe is significantly reduced. However, in order to track high frequency codes and sync points during several subframes, many subframes must be observed, and when a small subframe is observed, the accuracy becomes low. In addition, when the channel environment is not good, more subframes must be observed because the correct downlink sync code cannot be found. Otherwise, the accuracy is significantly reduced.

SUMMARY OF THE INVENTION The present invention has been made to solve the problems of the prior art as described above, and an object of the present invention is to provide an initial method that can significantly reduce memory requirements when a UE estimates initial synchronization in a cell search process in a CDMA mobile communication system. A synchronization estimation method and apparatus are provided.

Another object of the present invention is to provide a method and apparatus for estimating initial synchronization with high reliability even with a small number of frames received at a terminal.

According to the present invention, the terminal divides at least one or more sub-frames received from the base station into at least two areas during cell search and calculates correlations for each area, using the maximum correlation value for each area. Estimating the initial synchronization is a basic feature.

In one aspect of the present invention, the initial synchronization estimation method in a CDMA mobile communication system according to the present invention, for each of the at least one sub-frame received from the base station by subdividing each sub-frame into at least two or more areas, each of the synchronization code Calculating a maximum correlation value with each other; and estimating initial synchronization by using the correlation maximum value for each subframe region.

In another aspect of the present invention, an initial synchronization estimation method in a CDMA mobile communication system according to the present invention divides one subframe received from a base station into at least two or more areas and maximizes correlation with a downlink sync code for each area. A first step of calculating a value, a second step of repeatedly performing the first step for at least two or more subframes, and combining correlation maximum values for each region and each downlink sync code; And a third step of estimating initial synchronization by using the third step.

In still another aspect of the present invention, an initial synchronization estimation apparatus in a CDMA mobile communication system according to the present invention divides at least one or more subframes received from a base station into at least two or more regions, and then correlates with downlink sync codes for each region. Means for calculating a relationship maximum value, means for combining correlation maximum values for each region and each downlink sync code calculated for the at least one or more subframes, and estimating initial synchronization using the combined values. It characterized in that it comprises a means for.

Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples in which the technical features of the present invention are applied to a 3GPP TDD LCR system (TD-SCDMA system). The following embodiments are merely exemplary, and it is obvious that the technical features of the present invention can be applied to other types of CDMA mobile communication systems.

2 shows a format of a downlink subframe in a 3GPP TDD LCR system. In the 3GPP TDD LCR system, there are 32 reference signal SYNC-DLs of downlink sync codes, and each chip has a length of 64 chips. The downlink sync code SYNC-DL is located after the first downlink time slot Ts0 of the subframe, as shown in FIG. The DwPTS consists of a guard period of 32 chips and a SYNC-DL code of 64 chips, and one of 32 SYNC-DL codes is selected and used for each cell.

3 is a flowchart of an initial synchronization estimation method according to an exemplary embodiment of the present invention.

The terminal divides one subframe received from the base station into several regions, calculates correlations with 32 downlink sync codes for each region, and calculates a correlation maximum for each region [S31]. This process is described in more detail as follows.

In the 3GPP TDD-LCR system, SYNC-DL codes are used as downlink sync codes, which use 32 sequences. In the initial cell search, the UE searches for the DwPTS in the subframe of FIG. 2 without information and timing information on the cell to be searched.

4 shows an example of auto-correlation for a SYNC-DL code and cross-correlation with another code. In other words, the maximum point becomes a portion having high correlation in the received signal.

Received signals {r} = (r ₀ , r ₁ , ....) are generally oversampled and received rather than chip rates. The l th SYNC-DL code is represented by {S _(l) = (s ₀ ^(l) , s ₁ ^(l) , ..., s ₆₃ ^(l) ), where s _i ^(l) is a QPSK symbol and 0 ≤l <32 is the index for the SYNC-DL codes. Correlation between the received signal and the l- th SYNC-DL code may be defined by Equation 1 below.

Therefore, the downlink sync code having the maximum correlation value in each region and its location information

Can be obtained by a maximum likelihood detection method as shown in Equation 2 below.

here

and

Denotes an SYNC-DL code having the maximum correlation value and an estimated value for the one having the maximum correlation value in the region. R is the size of the region. This process is repeated for one subframe to obtain the maximum value of the correlation for each region and the index of the synchronization code and timing point having the maximum value. 5 shows the above process.

Preferably, the step S31 is repeated for at least two subframes. After obtaining the maximum correlation value for each subframe and for each sync code for the second and subsequent subframes, the correlation is performed for each subframe to the corresponding subframe and for each sync code. The relationship maximum is added together and combined [S32].

After summing up the maximum correlation value for each region and sync code up to the corresponding subframe, it is checked whether a predetermined condition is satisfied [S33]. The predetermined condition is satisfied when the difference between the largest value and the second largest value among the combined values of correlations for each region and synchronization code is greater than or equal to a preset threshold. In other words, if the largest value and the second largest value among the combined values of the correlation maximum value are more than the predetermined threshold, it is unlikely that the wrong information will be tracked, and no more frames are observed at that time. Initial synchronization is estimated [S36]. The predetermined condition may be applied only to the first subframe. In addition, the S33 process may be omitted.

If the predetermined condition is not satisfied, the UE indexes the combined value of the correlation maximum value for each subframe up to the corresponding subframe and the synchronization code and timing point having the maximum correlation value for each subframe and area. To store it [S34].

Next, check the number of subframes for which the maximum correlation value for each area is H [S35]. If the number is smaller than H, the process from S31 is repeated for the next subframe. The initial synchronization is estimated using the value [S36]. H may be selected within a range capable of ensuring the accuracy of the initial synchronization estimation.

When receiving H subframes, the equation for correlation described above may be redefined as in Equation 3 below.

here

It is assumed that the received signal is oversampled n times.

The sum of correlation maximum values in one region during H subframes may be defined as in Equation 4 below.

If the predetermined condition is satisfied in step S33 or the number of subframes becomes H in step S35, the initial synchronization is estimated using a combined value of correlation maximum values for each region and each sync code up to that point. [S36]. Here, estimating initial synchronization means estimating an index of a downlink synchronization code and a timing synchronization point in use in a cell in which a terminal is located.

As the downlink sync code index, the index of the sync code corresponding to the largest value among the combined values of the correlation maximum values for each region and each sync code is selected. The index of the timing synchronization point may be estimated by various methods described below in an area corresponding to the largest value among the combined values of the correlation maximum value. It may be possible to estimate the timing synchronization point by a method other than the methods described below.

The first method selects the index of the timing point where the correlation maximum value of the selected synchronization code is most frequently found in the region corresponding to the largest value among the combined values of the correlation maximum value as the index of the timing synchronization point. That's how. FIG. 6 is a diagram for explaining the first method, since point i is the point where the correlation maximum value occurs most frequently in a region corresponding to the largest value among the combined values of the correlation maximum value. Denotes the timing synchronization point.

The second method obtains an average value of the indexes of the timing points of the correlation maximum values of the selected synchronization code in the region corresponding to the largest value among the combined values of the correlation maximum values, and selects the index of the timing synchronization points. It is a way.

The third method weights by weighting the timing point index of the correlation maximum values of the selected synchronization code according to the frequency of the correlation maximum value in the region corresponding to the largest value among the combined values of the correlation maximum values. A method of obtaining a weighted average and selecting the index as the index of the timing synchronization point.

7 is a block diagram of an initial synchronization estimation device according to an embodiment of the present invention. Since the present invention relates to the initial synchronization estimation in the terminal of the mobile communication system, an initial synchronization estimation apparatus as shown in FIG. 7 will actually be implemented inside the terminal.

In FIG. 7, the initial synchronization estimating apparatus divides at least one or more subframes received from a base station into at least two or more regions, and calculates a correlation maximum value calculating module 71 for calculating a correlation maximum with a downlink sync code for each region. And a correlation maximum value combining module 73 for combining the correlation maximum values for each region and each downlink sync code calculated for the at least one subframe, and the combined values of the correlation maximum values. An initial synchronization estimation module 75 for estimating initial synchronization using the combination value of the correlation maximum value of each region and each downlink synchronization code, and the correlation of each subframe, region, and each downlink synchronization code. And a memory module 77 for storing the index of the point having the maximum value. Since the functions of each module are as described above, a detailed description thereof will be omitted. Each component shown in FIG. 7 may be easily implemented in software, hardware, or a combination of both in the terminal with reference to the present invention.

FIG. 8 is a graph illustrating simulation results when various channels and frequency offsets are present as shown in Table 1, and the downlink sync code detection error rate according to SNR changes for various channels as shown in Table 1 is shown as SYNC-. DL Code Detection Error Rate). The best performance was found in the PA3 channel and the worst performance in the VA120 channel.

ITU Walk (Pedestrian) A
Speed 3km / h
(PA3) ITU Walk B
Speed 3km / h
(PB3) ITU vehicle (vehicular) A
Speed 30km / h
(VA30) ITU Vehicle A
Speed 120km / h
(VA120) Relative
Delay [ns] Relative mean
Power [dB] Relative
Delay
[ns] Relative mean
Power
[dB] Relative
Delay [ns] Relative mean
Power [dB] Relative
Delay
[ns] Relative mean
Power
[dB] 0 0 0 0 0 0 0 0 110 -9.7 200 -0.9 310 -1.0 310 -1.0 190 -19.2 800 -4.9 710 -9.0 710 -9.0 410 -22.8 1200 -8.0 1090 -10.0 1090 -10.0 2300 -7.8 1730 -15.0 1730 -15.0 3700 -23.9 2510 -20 2510 -20

9A to 9D are initial synchronization histograms (Initial Synchronization Histogram) according to changes in SNR for various channels (PA3, PB3, VA30, and VA120) as shown in Table 1. In FIG. 9A to FIG. 9D, 0 is a case where an accurate sync point is found, and it is determined that an error within ± 4 chips is acceptable. Values within one chip interval are in over-sampling units. <-4, 4 <is a case where a sync point is found outside the ± 4 chip, and determined as an initial sync error. 9A to 9D, the lower the SNR, the worse the performance.

It will be apparent to those skilled in the art that the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the above detailed description should not be construed as limiting in all aspects and should be considered as illustrative. The scope of the invention should be determined by reasonable interpretation of the appended claims, and all changes within the equivalent scope of the invention are included in the scope of the invention.

The estimated initial sync is used as a reference sync point not only for frame sync but also for time tracker, automatic frequency control (AGC) and automatic gain control (AGC), so it is important to find the correct initial sync. Do. According to the present invention, since the correlation maximum values for each region are summed over several subframes, only the correlation value of the number of regions needs to be stored, compared to the method in which all correlation values must be included in one subframe, thereby preventing an increase in memory. In addition, there is an effect that an initial synchronization with high reliability can be obtained with only a few frames, rather than a method having only one maximum correlation value for one subframe.

Claims

An initial synchronization estimation method in a terminal of a code division multiple access (CDMA) mobile communication system,

Calculating a maximum correlation value with a sync code for each region by dividing each subframe into at least two regions for at least one subframe received from the base station; And

Estimating initial synchronization by using correlation maximum values for respective sub-frames.

The method of claim 1,

In the initial synchronization estimation step,

If the difference between the largest value and the second largest value among the combined values of the correlation maximum values of each region and each sync code for the at least one subframe is greater than or equal to a preset threshold, the region corresponding to the largest value is selected. And selecting to obtain a synchronization code index and a timing synchronization point.

The method of claim 1,

The initial synchronization estimation step,

Combining correlation maximum values calculated for at least two subframes for each region and for each sync code;

And selecting a region having the largest value among the combined values of the correlation maximum values to obtain a synchronization code index and a timing synchronization.

The method of claim 2 or 3,

And the selected sync code index is an index of a sync code corresponding to the largest value among the combined values of the correlation maximum values for each region and each sync code.

The method of claim 4, wherein

And the timing synchronization point is estimated at the highest frequency of the correlation maximum value with the selected synchronization code in the selected area.

A first step of dividing one subframe received from a base station into at least two areas to calculate a maximum correlation value with a downlink sync code for each area;

A second step of repeating the first step for at least two subframes and combining correlation maximum values for each region and each downlink sync code; And

And a third step of estimating initial synchronization using the combined values.

The method of claim 6, wherein in the third step,

If the difference between the largest value and the second largest value among the combined values of the correlation maximum values for each region and each sync code for the at least two subframes is greater than or equal to a preset threshold, the region corresponding to the largest value is determined. And selecting to obtain a sync code index and a timing sync point.

The method of claim 6, wherein in the third step,

The method according to claim 7 or 8,

An apparatus for initial synchronization estimation in a code division multiple access (CDMA) mobile communication system,

Means for dividing at least one or more subframes received from the base station into at least two regions to calculate a correlation maximum with the downlink sync code for each region;

Means for combining a correlation maximum value for each region and each downlink sync code calculated for the at least one subframe; And

Means for estimating initial synchronization using the combined values of the correlation maximum values.

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