KR20110031620A - Method and apparatus for frame synchronizing - Google Patents

Abstract

PURPOSE: A method and apparatus for frame synchronization are provided to improve the accuracy in frame synchronization by using a synchronizing header, plural critical values and double confirmation criteria. CONSTITUTION: A correlation value calculation unit(210) calculates a standardized correlation value of a pre-stored synchronization header and a received sequence. A critical condition decision unit(220) judges whether or not the correlation value satisfies one of critical conditions. Based on the judgment result of the critical condition decision unit, a synchronization unit(230) regards as the starting point of a frame the corresponding point of the sequence having the correlation value which satisfies at least one of the critical conditions.

Description

Frame Synchronization Device and Method {METHOD AND APPARATUS FOR FRAME SYNCHRONIZING}

The present invention relates to an apparatus and method for frame synchronization, and more particularly, to an apparatus and method for high accuracy frame synchronization, which is suitable for a high noise channel.

In a digital communication system having one or more transmitters and one or more receivers, the receiver must capture the timing of the signal transmitted by the transmitter and synchronize it with it in order to extract information from the received signal. This synchronization method will be described with reference to a wireless communication system, which is an example of a digital communication system.

The timing of signals transmitted from fixed stations (eg, base stations) within a wireless communication system is generally referred to as system timing. In a cellular wireless communication system using Orthogonal Frequency Division Multiplexing (OFDM), the synchronization to the timing of a signal is a fast Fourier transform that is used by a receiver of the signal to extract information from the signal. FFT) allows for accurate positioning of the window.

In any cellular wireless communication system having multiple fixed stations and multiple mobile communication devices, a synchronization process must be frequently performed between the fixed station and the mobile communication devices for the system to operate. Hereinafter, mobile communication devices are simply referred to as user equipments (UEs).

In addition, as an example of a fixed station, each base station defines a geographical transmission area, commonly referred to as a cell, in which case a user terminal substantially close to a particular base station accesses the wireless communication system. Selecting a base station for a particular user terminal to access a cellular wireless communication system is called cell selection. To optimize the reception of the base station signal, the user terminal must identify the highest quality signal among the signals received from the different base stations and switch its receiver to tune to the best base station at a given time.

In addition, the user terminal may change from time to time due to mobility, and the synchronization process must be frequently performed because the user terminal must allow and support seamless handover from one base station to another.

In modern cellular wireless communication systems, high speed system access and cell selection are essential functions for proper mobile user terminal operation. The purpose of fast acquisition is to synchronize the user terminal to the desired base station. Cell selection and re-selection measures and synchronizes signal power (including interference) between adjacent base stations and selects and switches base stations with the highest signal quality, i.e., maximum carrier-to-interference (C / I) ratio. Although typically performed by the user terminal, some cellular wireless communication systems may perform cell selection, base station identification, and base station C / I ratio measurements.

On the other hand, this synchronization process is carried out by the frame synchronization device, which is the first functional module of the receiver, and on the premise that all other modules fulfill their respective functions, the frame synchronization device checks the exact start of the frame from the noise background, and pseudo noise A pseudo-noise sequence is usually used as a sync header in a frame because of its correlation index.

In the frame synchronization method according to the prior art, there is a method in which a receiver uses one pseudo noise sequence as a synchronization header and stores the pseudo noise sequence in the receiver in advance. According to this synchronization method, the receiver stores the sequence while calculating the correlation of the received data and regards it as the start of the frame when a peak is found.

However, this frame synchronization method differs significantly when the synchronization header is compared with the original value when the channel is severely interfered by noise, thus correlating the standard pseudonoise sequence even though it produces an apparent correlation peak. That is, it is not suitable for high noise channels.

Looking at another method of frame synchronization by the synchronization process according to the prior art, a method in which a receiver uses two identical pseudonoise sequences as synchronization headers and correlates received data without storing the pseudonoise sequences in the receiver is described. have. For example, assuming that two N-length pseudonoise sequences are used as synchronization headers, the received symbols X (n) to X (n + N-1) are X (n + N-1) to X (n + N− 1 + N-1). Here, "N" is a symbol currently received. Here, only "N" is the beginning of the header, and the correlation above is equal to the autocorrelation of the two pseudo-noise sequences to produce a peak value. This synchronization method can be used in high noise channels.

However, the latter frame synchronization method as described above has a problem of low accuracy. Assuming that "N" is the correct starting point of the frame, the correlation value of symbols X (n + 1) to X (n + N) and X (n + N + 1) to X (n + N + N-1) Is smaller and lower than the peak, and the remaining symbols are also the same except for the first symbol.

The present invention has been proposed to solve such a problem of the prior art, and is suitable for high noise channels using a synchronization header and a plurality of thresholds and double acknowledgment structure in which the same pseudo noise sequence is repeated several times. This provides a high frame synchronization device and method.

A frame synchronization apparatus according to the first aspect of the present invention includes a correlation value calculator for calculating a normalized correlation value between a received sequence and a pre-stored synchronization header, and comparing the correlation values with a plurality of threshold conditions. A threshold condition determining unit that determines whether at least one of the plurality of threshold conditions is satisfied and the correlation value that satisfies the at least one threshold condition based on a determination result of the condition determining unit; It may include a synchronization unit for synchronizing by considering the corresponding position of the sequence as the start of the frame.

Here, the synchronization header may have a structure in which the same pseudonoise sequence is repeated several times.

The threshold condition determining unit compares the plurality of threshold conditions based on a preset threshold 1 and the threshold value 2 with the correlation value, and then the correlation value determines at least one threshold condition among the plurality of threshold conditions. It can be determined whether it is satisfied.

The threshold condition determination unit may set the plurality of threshold conditions when the correlation value exceeds the threshold value 1 (critical condition 1) and when the correlation value exceeds the threshold value 2 (critical condition 2). Can be.

According to a second aspect of the present invention, a frame synchronization method includes calculating a normalized correlation value of a received sequence and a pre-stored synchronization header, comparing a plurality of threshold conditions and the correlation value, and wherein the correlation value is the plurality of thresholds. Determining at least one threshold condition among the conditions, and starting a frame at a corresponding position of the sequence having the correlation value satisfying the at least one threshold condition based on a result of the determining step. It may include the step of synchronizing.

Here, the synchronization header may have a structure in which the same pseudonoise sequence is repeated several times.

The determining may include: comparing the plurality of threshold conditions based on the preset threshold value 1 and the threshold value 2 with the correlation value, and the correlation value selecting at least one threshold condition among the plurality of threshold conditions. It can be determined whether it is satisfied.

The determining may include making the plurality of threshold conditions when the correlation value exceeds the threshold value 1 (critical condition 1) and when the correlation value exceeds the threshold value 2 (critical condition 2). Can be.

According to an embodiment of the present invention, there is provided a new pattern for synchronization header and a new device and method for frame synchronization of a receiver. This solves the problem of low accuracy or inadequacy for high noise channels in synchronization according to the prior art.

The new pattern of the synchronization header according to the present invention is used by changing the pattern of the synchronization header according to the prior art, so that the conventional pseudo noise sequence generator can still be used. It does not require complex calculations or large storage space in the receiver.

Moreover, it is more beneficial in OFDM systems, and in particular, it is possible to estimate the approximate frequency deviation of the OFDM system, and if the length of the synchronization header is longer than the periodic prefix, then the approximate frequency deviation is more accurate than using the periodic prefix. There is an effect that can be estimated.

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but can be implemented in various different forms, and only the embodiments make the disclosure of the present invention complete, and the general knowledge in the art to which the present invention belongs. It is provided to fully inform the person having the scope of the invention, which is defined only by the scope of the claims.

In describing the embodiments of the present invention, if it is determined that a detailed description of a known function or configuration may unnecessarily obscure the gist of the present invention, the detailed description thereof will be omitted. In addition, terms to be described below are terms defined in consideration of functions in the embodiments of the present invention, which may vary according to intentions or customs of users and operators. Therefore, the definition should be based on the contents throughout this specification.

Combinations of each block of the accompanying block diagram and each step of the flowchart may be performed by computer program instructions. These computer program instructions may be embedded in the processor of a general purpose computer, special purpose computer, or other programmable data processing equipment such that the instructions executed by the processor of the computer or other programmable data processing equipment may be executed in each block or flowchart of the block diagram. It will create means for performing the functions described in each step of. These computer program instructions may be stored in a computer usable or computer readable memory that can be directed to a computer or other programmable data processing equipment to implement functionality in a particular manner, and thus the computer usable or computer readable memory. It is also possible for the instructions stored in to produce an article of manufacture containing instruction means for performing the functions described in each block or flow chart step of the block diagram. Computer program instructions may also be mounted on a computer or other programmable data processing equipment, such that a series of operating steps may be performed on the computer or other programmable data processing equipment to create a computer-implemented process to create a computer or other programmable data. Instructions that perform processing equipment may also provide steps for performing the functions described in each block of the block diagram and in each step of the flowchart.

In addition, each block or step may represent a portion of a module, segment or code that includes one or more executable instructions for executing a specified logical function (s). It should also be noted that in some alternative embodiments, the functions noted in the blocks or steps may occur out of order. For example, the two blocks or steps shown in succession may in fact be executed substantially concurrently or the blocks or steps may sometimes be performed in the reverse order, depending on the functionality involved.

1 is a structural diagram showing a pattern of a synchronization header used in a frame synchronization device according to an embodiment of the present invention.

As described above, the synchronization header 100 according to the embodiment of the present invention has a structure in which the same pseudonoise sequence 10 is repeated several times. For example, the synchronization header (100) X _H is made assuming the L- length pseudo noise sequence 10 is repeated a number of times T, the total length of the synchronization header (100) X _H is L × T, the same pseudo noise sequence (10) X _M is stored in the frame synchronization device or other functional module of the receiver.

2 is a block diagram of a frame synchronization device according to an embodiment of the present invention.

As described above, the frame synchronization apparatus according to the embodiment of the present invention may include a correlation value calculator 210, a threshold condition determiner 220, a synchronizer 230, and the like.

The correlation value calculator 210 reads pre-stored synchronization headers and calculates normalized correlation values of the received sequence and the synchronization header.

The threshold condition determination unit 220 compares the plurality of threshold conditions based on the threshold value 1 and the threshold value 2 previously set and the correlation values calculated by the correlation value calculation unit 210, and the correlation values are the plurality of threshold conditions. It is determined whether at least one of the threshold conditions is satisfied.

The synchronization unit 230 synchronizes the position of the sequence having a correlation value satisfying at least one threshold condition among the plurality of threshold conditions based on the determination result of the threshold condition determination unit 220 as the start of the frame. .

The frame synchronization process of the frame synchronization apparatus of FIG. 2 using the synchronization header of FIG. 1 will be described below with reference to the flowchart of FIG. 3.

When the receiver including the frame synchronization device according to an embodiment of the present invention is turned on to receive a sequence (S301), the correlation calculator 210 may store a synchronization header X _{M previously} stored in the frame synchronization device or other functional module of the receiver. Read (S303), and calculate the normalized correlation value C (N) of the received sequence X _F and the synchronization header X _M by the following equation (1) (S305).

The peak value T exists in the same order as C (N) of Equation 1 above, and each peak value is represented by the first symbol X _H (1) of the header and the first symbol X _H (L) of each repeated pseudo noise sequence. +1), X _H (2L + 1),... , X _H ((T-1) * L + 1), respectively.

Next, check the start of the synchronization header X _H.

First, the threshold condition determination unit 220 compares the C (N) value with the threshold value 1 set in advance (S307). Since C (N) has been normalized, if there is no noise, the peak value will be 1.0. Considering the interference of noise, threshold 1 is a number less than 1.0, and C (N) is found higher than threshold 1. In other words, it is determined whether the threshold condition that should exceed the threshold value 1 is satisfied (S309).

When the threshold condition determining unit 220 finds a C (N) higher than the threshold 1, that is, determines that the threshold condition that must exceed the threshold 1 is satisfied, the synchronization unit 230 determines the corresponding position N ₁ . The synchronization is performed by considering the start of the header and the data frame (S313).

In step S309, if the threshold condition determination unit 220 does not find a C (N) value larger than the threshold 1, it means that the header is severely interfered by noise. In this case, threshold 1 is changed to threshold 2 lower than threshold 1, and threshold 2 is again compared with C (N) (S311).

When the threshold condition determining unit 220 finds C (N) higher than the threshold 2, that is, determines that the threshold condition that should exceed the threshold 2 is satisfied, then C (N ₁ + L), C (N ₁ + 2L),... , Check the value of C (N ₁ + (T-1) * L). If the threshold value 2 exceeds all, the synchronization unit 230 regards the corresponding position N ₁ as the start of the header and the data frame and performs synchronization (S313).

In the foregoing description, for clarity of explanation, the process of comparing the threshold 1 and the C (N) and the process of comparing the threshold 2 and the C (N) were sequentially performed by a serial structure. In actual implementation, it can be performed simultaneously by a parallel structure. Moreover, parallel processing is more desirable because it is not only impossible and unreasonable for the receiver to store all C (N) values.

For example, the receiver compares C (N) with both threshold 1 and threshold 2 (only when it does not exceed threshold 1). If C (N) exceeds either, N is recorded and subsequent actions are executed.

When C (N) does not exceed either threshold, the signal-to-noise ratio (SNR) is very low and the signal is almost hidden by noise. In practice, however, frame synchronization at such a low signal-to-noise ratio is meaningless.

In order to prove the effect of the frame synchronization method according to the embodiment of the present invention as follows, the realization paradigm and Matlab simulation results thereof are provided.

For example, L = 31 and T = 2. The 31-length pseudonoise sequence refers to the M-sequence (maximum length sequence) passing through the polynomial x ⁵ + x ³ +1, and as shown in FIG. 4, an exclusive or operator with a shift resist 401. And a 31-length M-sequence generator, including 403). For example, by the 31-length M-sequence generator of FIG. 4, {0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 1, 1, 1, 0, 0, 0, 1, 1 , 0, 1, 1, 1, 0, 1, 0, 1, 0, 0, 0, 0, 1} is generated with the sequence X _M.

Use the same X _M at the receiver to correlate the received data, and record each C (N) value.

5 is a graph showing an example of the correlation value C (N) of the synchronization header when the noise level is not too high. In FIG. 5, the signal-to-noise ratio is 28 dB, and the up-sampling rate is 4.

In Fig. 5, two peak values close to 0.8 can be seen, and when tested in a good signal-to-noise ratio environment, correlation peaks are confirmed by comparison with threshold 1, which is a number smaller than 1.0 and lower. At the same time, it can be seen that the distance between the two peak values is 124 symbols, which is equal to 31 (sequence length) x 4 (upsampling ratio).

6 is a graph showing an example of the correlation value C (N) of the synchronization header when the noise level is high. A 10dB signal-to-noise ratio is a requirement for Binary Phase Shift Key (BPSK), with an upsampling ratio of 4. A signal-to-noise ratio below 10dB means you no longer synchronize to the frame.

In Figure 6, although there is an apparent peak value, it can be seen that it decreased to 0.3. In this case threshold 1 cannot identify the peak. At this time, compare the threshold 2 whose correlation is lower than 0.3.

In a situation where threshold 2 is very low, it is possible for another symbol to achieve its value after interference by strong noise rather than the starting frame. Therefore, it is necessary to check the value 124 symbols away from the first peak. If the first peak is not the start of the M-sequence, the probability of a point exceeding threshold 2 is very low.

However, you can notice the problem from the numbers above. The correlation value near the peak points is also very high. In the present invention, the value is filtered by an absolute threshold, and a synchronization error may be indicated by identification of one to three symbols before the exact start as the beginning of the frame. In low modulation instructions, a small phase error can be ignored after demodulation, but in high modulation instructions, the Bit Error Rate (BER) is increased.

An algorithm is added to solve this problem. The peak (for both threshold 1 and threshold 2) is identified, and the receiver also calculates a correlation of 10 symbols after the peak. As can be seen from Figures 5 and 6, the 10 symbols behind the peak cover all of the relatively high values.

The above mentioned synchronization header may be deployed at the physical layer of all digital communication systems. However, it is more effective and beneficial in OFDM systems. Since OFDM is very sensitive to the frequency deviation between the transmitter and the receiver, all OFDM systems need estimation and compensation of the frequency deviation.

In order to estimate the correct frequency deviation, the estimation process is divided into two parts. First, the coarse frequency offset is estimated and compensated, and then the correct frequency deviation is estimated. The coarse frequency deviation estimates a frequency deviation greater than half of the subcarrier width, which is the basis for estimating the exact frequency deviation.

The approximate frequency deviation can be estimated through the periodic prefix of the OFDM symbol, and the long period prefix is more accurate but the physical efficiency is lower. In addition, the length of the periodic prefix is determined by the delay parameter of the channel and is not related to the accuracy for the coarse frequency deviation.

Here, the present invention uses the above-described synchronization header designed to estimate the coarse frequency deviation.

In an OFDM system, a parameter called "relative frequency deviation" is used to find the frequency deviation. The definition is as shown in Equation 2 below.

The influence of the frequency deviation in the time domain signal may be expressed as Equation 3 below using the "relative frequency deviation" Δf.

Where N.itrp is the upsampling rate and N.sub is the total number of OFDM subcarriers.

According to this, the first pseudo noise sequence of the synchronization header is represented by Equation 4 below due to the presence of the frequency deviation.

In Equation 4, noise indicates interference in the same channel as Gaussian noise. L is the length of the pseudonoise sequence, for example L is 31 when using the 31-length M-sequence generator of FIG.

The second pseudonoise sequence is represented by Equation 5 below (assuming T = 2).

By applying X ₁ (n + L) = X ₁ (n) to Equations 4 and 5 above, Equation 6 below can be obtained.

Equation 6 above has only one unknown value Δf and is not related to n. As such, Δf can be obtained by separating the second pseudonoise sequence with respect to the first received sequence. And, if the average is obtained according to the L value, the influence from noise can be reduced and the accuracy can be increased.

The present invention can be applied to a digital communication system for providing all network environments regardless of a wireless network environment and a wired network environment. In particular, wireless communication systems using OFDM such as CDMA networks, WCDMA networks, HSDPA networks, GSM networks, etc., as well as fixed stations (e.g., base stations) and mobile communication devices in all future mobile communication systems including the fourth generation. Can be used for synchronization.

1 is a structural diagram showing a pattern of a synchronization header used in a frame synchronization device according to an embodiment of the present invention;

2 is a block diagram of a frame synchronization device according to an embodiment of the present invention;

3 is a flowchart illustrating a frame synchronization method according to an embodiment of the present invention;

4 is an exemplary diagram of a sequence generator capable of generating a pseudo noise sequence constituting a synchronization header used in the present invention;

5 is an exemplary diagram showing a correlation value of a synchronization header when the noise level is not too high to prove the effect of frame synchronization according to an embodiment of the present invention.

6 is an exemplary diagram illustrating a correlation value of a synchronization header when a noise level is high to prove the effect of frame synchronization according to an embodiment of the present invention.

<Explanation of symbols for the main parts of the drawings>

10: pseudonoise sequence

100: sync header

210: correlation value calculation unit

220: threshold condition determination unit

230: synchronization unit

401: shift resist

403: exclusive OR operator

Claims (8)

A correlation value calculator for calculating a normalized correlation value between a received sequence and a pre-stored synchronization header; A threshold condition determination unit which compares a plurality of threshold conditions and the correlation value and determines whether the correlation value satisfies at least one of the plurality of threshold conditions; A synchronization unit for synchronizing a corresponding position of the sequence having the correlation value that satisfies the at least one threshold condition based on a determination result of the condition determination unit as a start of a frame and synchronizing Frame synchronization device comprising a. The method of claim 1, The synchronization header has a structure in which the same pseudonoise sequence is repeated several times. Frame synchronizer. The method of claim 1, The threshold condition determining unit compares the plurality of threshold conditions based on a preset threshold 1 and the threshold value 2 with the correlation value, and then the correlation value determines at least one threshold condition among the plurality of threshold conditions. To determine whether we are satisfied Frame synchronizer. The method of claim 3, wherein The threshold condition determining unit sets the plurality of threshold conditions when the correlation value exceeds the threshold value 1 (critical condition 1) and when the correlation value exceeds the threshold value 2 (critical condition 2). Frame synchronizer. Calculating a normalized correlation value between the received sequence and a pre-stored synchronization header; Comparing the correlation value with a plurality of threshold conditions to determine whether the correlation value satisfies at least one of the plurality of threshold conditions; Synchronizing the position of the sequence having the correlation value that satisfies the at least one threshold condition as the start of a frame based on a result of the determining step Frame synchronization method comprising a. The method of claim 5, The synchronization header has a structure in which the same pseudonoise sequence is repeated several times. Frame sync method. The method of claim 5, The determining may include: comparing the plurality of threshold conditions based on the preset threshold value 1 and the threshold value 2 with the correlation value, and the correlation value selecting at least one threshold condition among the plurality of threshold conditions. To determine whether we are satisfied Frame sync method. The method of claim 7, wherein The determining may include setting the plurality of threshold conditions when the correlation value exceeds the threshold value 1 (critical condition 1) and when the correlation value exceeds the threshold value 2 (critical condition 2). Frame sync method.

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