KR101421305B1 - ractional frequency offset estimation method and receiver using the same - Google Patents

Abstract

The present invention provides a fractional frequency synchronization method. The method includes: performing time synchronization by estimating a correlation value for a cyclic prefix (CP) in at least one received symbol; Estimating a fractional frequency offset based on a position at which the correlation value becomes a maximum within each symbol during the time synchronization; Calculating at least one of an average and a variance of correlation values corresponding to the CP; Calculating an average of fractional frequency offsets based on the calculated mean and variance; And performing frequency-domain decimation based on an average of the calculated frequency-base-frequency offsets when the time-synchronization is completed.

Description

And a receiver using the same. [0001] The present invention relates to a frequency-

The present invention relates to fractional frequency offset estimation

As the mobile communication system evolves, the amount of data required by users and the processing speed thereof are also increasing. When data is transmitted on a wireless channel of a mobile communication system at a high speed, a bit error rate (BER) is obtained due to the influence of multipath fading and Doppler spread, There is a need for a radio access method suitable for the channel. A spread spectrum modulation scheme having advantages of relatively low output, i.e., relatively low transmit power and low detection probability, is widely used in the wireless access scheme.

On the other hand, an orthogonal frequency division multiplexing (OFDM) scheme has recently emerged as a radio access scheme suitable for high-speed data transmission. Recently, the OFDM scheme, which is used as a useful method for high-speed data transmission in a wired / wireless channel, is a scheme for transmitting data using a multi-carrier. The OFDM scheme converts parallel- Carrier modulation (MCM) scheme in which each subcarrier is modulated with a plurality of mutually orthogonal subcarriers.

A system employing such a multicarrier modulation scheme is not widely used because of its hardware complexity. Recently, a fast Fourier transform (FFT) and an inverse fast Fourier transform (Hereinafter referred to as " IFFT "), which is an inverse fast Fourier transform (IFFT) technique.

In general, in the above-described wireless communication system, time synchronization and frequency synchronization between a transmitter and a receiver must be essentially involved in order to satisfy the performance of a receiver.

In this case, in order to synchronize time in the digital wireless communication system, conventionally, after up-sampling a received signal received by a receiver, a correlation between an already known transmission signal and a received signal received by the receiver And the instant at which the correlation value has the maximum value is determined as the symbol timing.

Fractional frequency offset synchronization is performed after information on the time-synchronized sample index is obtained after the time synchronization is established after the time synchronization is established.

1 is a diagram illustrating an example of a time synchronization and frequency synchronization process according to the related art.

As can be seen with reference to FIG. 1, conventionally, the fractional frequency synchronization is performed by receiving information on a time index from a point in time synchronization is established.

First, correlation is performed on received data corresponding to one frame during the initial time synchronous operation state, and then the position of the sample having the maximum peak value is detected. At this time, the execution of the correlation value uses known repetition patterns. Using these pieces of information for several frames, an optimal sample position is detected and initial time synchronization is performed (S11).

When the initial time synchronization is completed (S12), the correlation is not performed for the entire one frame during the steady state time synchronization, but the correlation for the corresponding window period is performed for the specific position, and the steady state time synchronization is performed (S13).

When the steady-state time synchronization is completed (S14), a fractional frequency synchronization is performed on the time-synchronized received signal based on the detected time synchronization information (S15).

As described above, conventionally, when the time synchronization is completed, the information on the time index from the time after the time synchronization is established is acquired and the frequency multiples of the frequency are performed.

As described above, conventionally, when the time synchronization is completed, information on the time index from the time after the time synchronization is established is acquired, and the frequency multiplication of the frequency is performed.

Therefore, since the frequency synchronization is not possible until the time synchronization is established, there is a problem that the frequency synchronization acquisition time is delayed by the waiting time.

Accordingly, it is an object of the present invention to solve the above-mentioned problems.

Specifically, it is an object of the present invention to enable the frequency synchronization to be performed more quickly.

In order to achieve the above-mentioned object, the present invention provides a small number of frequency synchronization method. The method includes: performing time synchronization by estimating a correlation value for a cyclic prefix (CP) in at least one received symbol; Estimating a fractional frequency offset based on a position at which the correlation value becomes a maximum within each symbol during the time synchronization; Calculating at least one of an average and a variance of correlation values corresponding to the CP; Calculating an average of fractional frequency offsets based on the calculated mean and variance; And performing frequency-domain decimation based on an average of the calculated frequency-base-frequency offsets when the time-synchronization is completed.

Calculating at least one of the average and the variance may include comparing the correlation values corresponding to the CP with preset threshold values; And calculating at least one of an average and a variance of correlation values corresponding to a predetermined number of moving averages among the correlation values equal to or greater than the threshold value.

The method comprising the steps of: comparing an average of the calculated fractional frequency offsets with a preset threshold; And calculating a frequency offset estimation value using a weight set according to the range of the threshold value.

At this time, the predetermined weight may vary according to the variance value for the correlation values.

In order to achieve the above object, the present invention provides a receiver. The receiver includes a CP remover for removing a CP (Cyclic Prefix) in a repeated symbol in a received frame, and a receiver for performing time synchronization by estimating a correlation value for the CP, And the average of the decimal multiple frequency offsets is calculated, and when the time synchronization is completed, the decimal multiple frequency offset is calculated based on the average of the calculated decimal multiple frequency offsets And may include time synchronization and fractional frequency synchronization portions.

The present invention makes it possible to perform frequency synchronization more quickly, thereby solving the problem of delaying the frequency synchronization acquisition time. In addition, the present invention can acquire frequency synchronization simultaneously with time synchronization as compared with the prior art.

In addition, the present invention utilizes the detection of a CP (Cyclic Prefix) to use the threshold value for the correlation value to obtain the frequency synchronization even before the time synchronization is completed. In addition, the present invention allows the target residual frequency offset amount to reach the target value even with a much smaller number of frames as compared with the prior art.

Hereinafter, the OFDM technology will be described while explaining the present invention, but the present invention is not limited thereto and can be applied to all communication methods to which the technical idea of the present invention can be applied.

It is noted that the technical terms used herein are used only to describe specific embodiments and are not intended to limit the invention. It is also to be understood that the technical terms used herein are to be interpreted in a sense generally understood by a person skilled in the art to which the present invention belongs, Should not be construed to mean, or be interpreted in an excessively reduced sense. Further, when a technical term used herein is an erroneous technical term that does not accurately express the spirit of the present invention, it should be understood that technical terms that can be understood by a person skilled in the art are replaced. In addition, the general terms used in the present invention should be interpreted according to a predefined or prior context, and should not be construed as being excessively reduced.

Furthermore, the singular forms "as used herein include plural referents unless the context clearly indicates otherwise. In the present application, the term "comprising" or "comprising" or the like should not be construed as necessarily including the various elements or steps described in the specification, Or may be further comprised of additional components or steps.

Furthermore, terms including ordinals such as first, second, etc. used in this specification can be used to describe various elements, but the elements should not be limited by the terms. The terms are used only for the purpose of distinguishing one component from another. For example, without departing from the scope of the present invention, the first component may be referred to as a second component, and similarly, the second component may also be referred to as a first component.

When an element is referred to as being "connected" or "connected" to another element, it may be directly connected or connected to the other element, but other elements may be present in between. On the other hand, when an element is referred to as being "directly connected" or "directly connected" to another element, it should be understood that there are no other elements in between.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings, wherein like reference numerals refer to like or similar elements throughout the several views, and redundant description thereof will be omitted. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. It is to be noted that the accompanying drawings are only for the understanding of the present invention and should not be construed as limiting the scope of the present invention. The spirit of the present invention should be construed as extending to all modifications, equivalents, and alternatives in addition to the appended drawings.

First, before describing the present invention, frequency synchronization will be briefly described. The frequency offset is caused by the difference between the local oscillator of the base station and the terminal, thereby deteriorating the performance of the system. Therefore, the synchronization of the frequency greatly influences the performance of the system.

The frequency offset is divided into a fractional offset and an integer multiple offset by a variable expression defined below, where frequency synchronization is limited to a fractional frequency offset.

Sampling factor: n

System Bandwidth: BW

Sampling frequency: F _s = Floor (n BW / 8000) x 8000

Subcarrier spacing :? F = F _s / N _FFT

Frequency offset: f _Δ

Integral multiple offsets and fractional _{offset: f Δ / Δf = (f} Δinteger, f Δfractional)

FIG. 2 is a block diagram of a receiver according to the present invention, and FIG. 3 is a view illustrating an example of a frequency synchronization process according to the present invention. 5 is an exemplary view showing the structure of a frame.

As can be seen with reference to FIG. 2, the fractional frequency synchronization according to the present invention can be performed at the front end of the fast Fourier transformer (FFT) 130 of the receiver 100.

The receiver 100 includes a cyclic prefix (CP) remover 110, a Fast Fourier Transform (FFT) 120, a Parallel to Serial 130, a time synchronization and decimation multiplier 140, .

Upon receiving the OFDM frame shown in FIG. 5 through the channel, the CP remover 110 removes the CP from repeated OFDM symbols in the frame.

The demultiplexer 130 demultiplexes and outputs the fast Fourier transformed signals.

In order to shorten the establishment time of the fractional frequency synchronization, the time synchronization and fractional frequency synchronization unit 140 may use a correlation value of the cyclic prefix (CP) for the correlation value of the initial state time synchronization or the decimal multiplication of the frequency , The acquisition time of the fractional frequency synchronization is reduced by utilizing the obtained fractional estimation frequency values before the time synchronization is completed.

3, correlation is performed on the received data corresponding to one frame during the initial time synchronous operation state, and then the position of the sample having the maximum peak value is detected, (S110).

In step S120, a phase difference generated between the symbols is estimated based on a position where the maximum peak value is calculated in the corresponding sample number. Specifically, an intra-symbol phase difference (a fractional frequency offset) is estimated at each symbol period and a calculation value corresponding to a CP (Cyllic Prefix).

If the initial time synchronization is completed (S130), the entire frame is not correlated during the steady state time synchronization, but the steady state time synchronization is performed by performing correlation with respect to the specific window at the specific position (S140).

When the steady-state time synchronization is completed (S150), a frequency-doubled frequency synchronization is performed on the time-synchronized received signal on the basis of the detected time synchronization information and the estimated decimal multiple frequency offset (S160).

4 is an exemplary diagram illustrating the operation of the time synchronous and fractional frequency synchronizer shown in FIG.

1) First, a correlation value in a cyclic prefix (CP) and a phase difference in symbol (fractional frequency offset) are estimated at every symbol period (S210).

Specifically, using the OFDM symbol shown in FIG. 5, a correlation value R for a corresponding interval is calculated for a cyclic prefix (CP) period, which is a repeated pattern in a symbol, according to the following equation (1). To give the maximum peak value R _k symbols in a unit period is detected, and subsequently in a number of samples corresponding to the length of a symbol and calculates a peak value corresponding to the value of the CP segment.

Next, the phase difference between symbols is estimated based on the position where the maximum peak value is calculated in the number of samples, and the result is converted into frequency as shown in the following equation (6).

-----------------------------------------------(1)

-----------------------------------------------(One)

------(2)At this time,

------(2)

--------------------(3)

-------------------- (3)

----------------------------------(4)

----------------------------------(4)

-------------------------(5)And,

------------------------- (5)

Where d is the input sample counter index initialized on a symbol basis, T _s is the input sample counter index initialized on a frame-by-frame basis, r _n is the nth received signal, D is the distance between the comparison sample locations, Sampling period. F is the generated frequency offset amount, S _n is the sample size, and n is the nth sample index.

------------------------------(6)

------------------------------ (6)

은 추정한 소수 배 주파수 옵셋 량이고, L은 CP 구간의 길이이고,

은 복소 값 z 의 angle이다.here,

Is an estimated fractional frequency offset amount, L is the length of the CP section,

Is the angle of the complex value z.

2) Next, an average and a variance of correlation values corresponding to the CP are obtained (S220)

Specifically, when the maximum peak value R _k in the symbol interval satisfies the condition TH _A < R _k , the average number M of the set moving average is calculated as an average R of the corresponding values as shown in the following equations (7) and (8) _ave and variance R _var .

At this time, TH _A : set normalized correlation threshold.

Here, R is the Correlation value corresponding to the CP interval, P is the corresponding normalized value of the interval, and R _{ave is} a moving average of M Corr values, as shown in the following equation (7).

----------------------------------------(7)

---------------------------------------- (7)

-----------------------(8)

-----------------------(8)

α : 0 <α≤ 1 Reflection weight factor value for the estimated frequency offset.

: M개 표본 값에 대한 분산 값과 반영 가중치 인자 α 와의 관계.

: Relation between the variance of M sample values and the reflection weight factor α .

곱하여, 소수배 주파수 옵셋 추정 값으로 반영한다(S240).3) Next, the reflection factor value ? Is selectively selected according to the range of the threshold value, which is the average and the variance value of the obtained correlation value (S230). In addition, the α

And reflects it as a fractional frequency offset estimation value (S240).

4) Next, when the time synchronization of the steady state is completed, the frequency synchronization of the frequency is performed in the steady state (S250).

FIG. 6 is a diagram illustrating a correlation value of a CP (Cyclic Prefix), and FIG. 7 is another example of a correlation value of a CP (Cyclic Prefix).

FIG. 6 shows a change in CP (Cyclic Prefix) when Eb / No is 5dB, and FIG. 7 shows a change in CP (Cyclic Prefix) when Eb / No is 20dB.

On the other hand, FIG. 8 shows the difference between the prior art and the present invention.

As can be seen with reference to FIG. 8, a red line indicates a residual frequency offset amount according to the prior art according to a frame, and a black line indicates a residual frequency offset amount according to the present invention according to a frame . That is, according to the present invention, the residual frequency offset amount is remarkably reduced.

In addition, as is within the dotted line (-100 to 100 Hz) of the lower end portion of FIG. 8, the present invention is advantageous in that the target residual frequency offset amount reaches the target value with a much smaller number of frames .

Although the OFDM frame shown in FIG. 5 has been described above, the present invention can be applied to any method of transmitting data using a multi-carrier. In addition, the present invention can be applied to a method of transmitting data using a single carrier.

In addition, the present invention can be applied to both CDMA, DS-CDMA, GSM, GPRS, WCDMA, IEEE 802.11, IEEE 802.16, and 3GPP Long Term Evolution (LTE).

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, May be modified, modified, or improved.

1 is a diagram illustrating an example of a time synchronization and frequency synchronization process according to the related art.

2 is a block diagram of a receiver according to the present invention.

3 is a diagram illustrating an example of a frequency synchronization process according to the present invention.

4 is an exemplary diagram illustrating the operation of the time synchronous and fractional frequency synchronizer shown in FIG.

5 is an exemplary view showing the structure of a frame.

6 is an exemplary diagram showing a correlation value of CP (Cyclic Prefix).

FIG. 7 is another example of a correlation value of CP (Cyclic Prefix).

8 is an exemplary diagram showing the difference in effect between the prior art and the present invention.

Claims (7)

Performing time synchronization by estimating a correlation value for a cyclic prefix (CP) in at least one received symbol; Estimating a fractional frequency offset based on a position at which the correlation value becomes a maximum within each symbol during the time synchronization; Calculating at least one of an average and a variance of correlation values corresponding to the CP; Calculating an average of fractional frequency offsets based on the calculated average or variance; And performing fractional frequency synchronization on the basis of an average of the calculated fractional frequency offsets when the time synchronization is completed. 2. The method of claim 1, wherein calculating at least one of the mean and variance comprises: Comparing the correlation values corresponding to the CP with preset threshold values; And calculating at least one of an average and a variance of correlation values corresponding to a predetermined number of moving averages among the correlation values equal to or greater than the threshold value. The method according to claim 1, Wherein the calculating of the average of the fractional frequency offsets comprises calculating an average of the fractional frequency offsets using a weight that varies depending on an average or variance value of the correlation values. A CP remover for removing a CP (Cyclic Prefix) in a repeated symbol within a received frame, Estimates a correlation value for the CP to perform time synchronization, estimates a fractional frequency offset based on a position where the correlation value becomes a maximum value within each symbol, calculates an average of the fractional frequency offsets And a time synchronization and decimal multiplication unit for performing a decimal multiplication when the time synchronization is completed, based on an average of the calculated decimal multiple frequency offsets. 5. The method of claim 4, wherein the average of the fractional frequency offsets Calculating at least one of an average and a variance of the correlation values corresponding to the CP, and calculating the average or variance based on the calculated average or variance. 5. The apparatus of claim 4, wherein the time synchronization and decimal multiplication unit Calculating a frequency offset estimation value by selecting a weight according to the calculated mean and variance value of the frequency-domain offset and a predetermined range of a predetermined threshold value, and then calculating a frequency offset estimation value using the frequency- And performs synchronization. 7. The method of claim 6, Wherein the correlation value is varied according to a variance value for the correlation values.

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