KR100340426B1 - Method for compensating frequency offset of fast wireless local area network - Google Patents

Abstract

The present invention relates to a method for accurately compensating for a frequency error of a high speed WLAN. The present invention relates to a frequency error compensation method of a high speed WLAN using orthogonal frequency division multiplexing (OFDM). step; A second step of taking a conjugate complex number for each of the short codes delayed in the first step; A third step of searching for and finding an effective size of the real part and the imaginary part of the conjugate complex number in the short code of the second step; A fourth step of obtaining a phase difference of the single code using the effective size of the third step; A fifth step of obtaining the total sum of the phase differences obtained in the fourth step and obtaining an average thereof; A sixth step of retrieving phase information corresponding to an average of the frequency errors obtained in the fifth step from a look-up table; Comprising a seventh step of compensating the phase of the subsequent short code by using the retrieved phase information; In accordance with the present invention, after searching the effective size in a high-speed wireless LAN, a reference table (look- By using the up table), the synchronization of the received signal can be performed quickly and accurately, thereby improving the performance of the fast WLAN.

Description

Compensation method for frequency error of high speed wireless LAN {METHOD FOR COMPENSATING FREQUENCY OFFSET OF FAST WIRELESS LOCAL AREA NETWORK}

The present invention relates to a method for accurately compensating the frequency error of a high-speed wireless LAN. In particular, the effective size is searched in the high-speed wireless LAN. The present invention relates to a method for precisely compensating for a frequency error of a high speed WLAN to improve the performance of the high speed WLAN by enabling accurate operation.

At present, IEEE 802.11 has almost finalized the 2M / 11M wireless LAN standard and drafted a high-speed wireless LAN supporting up to 54Mbps in 802.11a.

FIG. 1 is a block diagram of a general high speed wireless LAN. Referring to FIG. 1, a high speed wireless LAN using Orthogonal Frequency Division Multiplexing (OFDM) generally includes a received signal (5.7 GHz or 2.4 GHz) from an antenna ANT. A frequency converter 100 for down-converting the frequency and a baseband unit 200 for performing fast Fourier change, error correction, and demodulation after synchronizing the output signal of the frequency converter 100.

The baseband unit 200 may include a synchronizer 210 for performing coarse and fine synchronization by compensating for a positional shift with respect to a received signal, and an FFT unit 220 for performing a fast free transform. And an error correcting unit 230 for correcting an error with respect to the received signal using an error correction code, and finally, a demodulator 240 for demodulating the received signal and restoring the original data.

FIG. 2 is a format diagram of a received signal of a high speed WLAN. Referring to FIG. 2, the received signal of the fast WLAN includes 10 short codes for early synchronization with a preamble code. Two long codes for fine synchronization are placed and used to compensate for phase misalignment with respect to the received signal.

FIG. 3 is a conceptual diagram for explaining the fast Fourier transform (FFT) of FIG. 1. Referring to FIG. 3, the OFDM scheme transmits and receives a multi-carrier. In this case, the receiver converts an input serial signal into parallel and then converts each parallel signal. Subcarriers are removed by synthesizing a signal and an oscillation frequency having a predetermined frequency difference, respectively, in which the phases of each subcarrier of the parallel signal are exactly orthogonal to each other, that is, one subcarrier as shown in FIG. When is the maximum value, it is called orthogonal frequency division multiplexing (OFDM) because it is most important that the other neighboring subcarriers maintain the relationship with a value of "0", the phase is out of phase in a high-speed WLAN using this OFDM When it starts to lose, the LAN communication itself becomes impossible.

In a high-speed wireless LAN using orthogonal frequency division multiplexing (OFDM) configured as described above, how to compensate for a frequency offset corresponding to a phase shift is the most important technical content. Referring to FIG. As described above, ten short codes and two long codes are used.

Recently, a method for compensating for a frequency error in a high-speed wireless LAN using OFDM described above has been researched, and a technique for no method has been known. There is a need for a method for compensating synchronization, ie, frequency error, for a terminal using OFDM.

Referring to the conventional method for compensating for a frequency error in an OFDM high speed WLAN, which is made according to the existing needs as follows.

Referring to FIG. 6, in the first steps 41 and 42, the first short code is received and delayed. In the second step 43, a conjugate complex number is taken for each short code delayed in the first step 42. In the third step 44, the phase difference is obtained using the upper four bits of the real part and the imaginary part of the first short code of the second step 43. Then, in the fourth step (45, 46), the total sum of the phase differences obtained in the third step (44) is calculated and averaged. In the fifth step (47), the frequency error obtained in the fourth step (46) is obtained. Phase information corresponding to the average is retrieved from a look-up table. In the final sixth step (48, 49), the phase information of the subsequent short code is compensated for by using the phase information retrieved as described above.

However, in the conventional method of compensating for the frequency error in an OFDM high-speed WLAN, it is necessary to obtain a phase difference for all bits of the short code when obtaining the phase difference of the short code. , The upper 4 bits) is selected, which means that the phase difference is not accurate because it selects 4 bits from any size, for example, the most significant bit, regardless of the phase difference information in the short code. There was a problem that the compensation of the frequency was not made correctly.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems. Accordingly, an object of the present invention is to provide a fast synchronization for a received signal using a look-up table for an effective size in a high-speed wireless LAN. The present invention provides a method for precisely compensating for frequency error of a high speed wireless LAN to improve the performance of the high speed wireless wireless LAN.

1 is a block diagram of a typical high-speed wireless LAN.

2 is a format diagram of a received signal of a high speed WLAN.

FIG. 3 is a conceptual diagram for explaining a fast Fourier transform (FFT) of FIG. 1.

4 is a flowchart illustrating a frequency error compensation method of a conventional high speed WLAN.

5 is a block diagram of a synchronizer 210 for carrying out the present invention.

FIG. 6 is a diagram illustrating a phase error predicting unit 211a of the initial frequency error compensating unit 211 of FIG. 5.

7 is a flowchart illustrating a method for accurately compensating for frequency error of a fast WLAN according to the present invention.

Explanation of symbols on the main parts of the drawings

210: synchronization unit 211: frequency error initial compensation unit

212: frequency error fine compensation unit 211a: phase error prediction unit

211b: phase compensation unit 211a-1: delay unit

211a-2: conjugate complex portion 211a-3: synthesis portion

211a-4: Frequency error summing unit 211a-5: Frequency error averaging unit

211a-6: phase information predictor

As a technical means for achieving the above object of the present invention, the method of the present invention is a frequency error compensation method of a high-speed wireless LAN using Orthogonal Frequency Division Multiplexing (OFDM), the method for delaying receiving the first short code Stage 1; A second step of taking a conjugate complex number for each of the short codes delayed in the first step; A third step of searching for and finding an effective size of the real part and the imaginary part of the conjugate complex number in the short code of the second step; A fourth step of obtaining a phase difference of the single code using the effective size of the third step; A fifth step of obtaining the total sum of the phase differences obtained in the fourth step and obtaining an average thereof; A sixth step of retrieving phase information corresponding to an average of the frequency errors obtained in the fifth step from a look-up table; A seventh step of compensating for a phase of a subsequent short code using the retrieved phase information; Characterized in that made.

Hereinafter, a synchronizer for compensating for a frequency error of a fast WLAN according to the present invention will be described in detail with reference to the accompanying drawings. In the drawings referred to in the present invention, components having substantially the same configuration and function will use the same reference numerals.

FIG. 5 is a configuration diagram of a synchronization unit 210 for performing the present invention. As shown in FIG. 5, the synchronization unit 210 of a high-speed wireless LAN uses a short code to compensate for a frequency error. Compensation unit 211, and a frequency error fine compensation unit 212 for finely compensating the frequency error using a long code.

FIG. 6 is a configuration diagram of the phase error predicting unit 211a of the initial frequency error compensating unit 211 of FIG. 5. Referring to FIG. 6, the frequency error compensating unit 211 compensates for the frequency error using the short code. ) Is a delay unit 211a-1 for delaying the first single code, a conjugate complex unit 211a-2 which takes a conjugate complex number for the delayed short code, and searches the bits of the real and imaginary parts of the conjugate complex number. After retrieving the valid bits, a synthesis unit 211a-3 for dividing the phase difference of the short codes by dividing the phase difference of the single code by using the four bits from the effective bit, and the frequency error summation summing the phase differences obtained in the synthesis unit 211a-3 Phase 211a-4, frequency error averaging unit 211a-5 for obtaining an average of the phase differences summed by the adding unit 211a-4, and phase information corresponding to the average obtained in this averaging unit 211a-5. Example of phase information for finding effective size by using look-up table A phase compensator 212b which performs phase compensation from the next short code by using the phase error predictor 211a including the side 211a-6 and the phase information predicted by the phase error predictor 211a. ).

Operation according to the present invention configured as described above will be described in detail below based on the accompanying drawings.

Referring to FIG. 5 to FIG. 7, first, in the first steps 71 and 72, the first short code is received and delayed. The first short code includes 16 sample signals. The delay is a delay of 16 sample signals (16 bit status signals) included in one short code.

In the second step 73, a conjugate complex number is taken for one short code delayed in the first step 72, which means that each of the 16 sample signals included in the input single short code is " f (t) = I. + jQ1 "(I and Q include phase information), which is exp {jθ1} = e ^jθ1. Therefore, if a conjugate complex is taken, exp {-jθ1}.

Next, in the third step 74, the effective size for the real part and the imaginary part of the conjugate complex number is found in the short code of the second step 73, where the effective size is searched from the most significant bit of the short code. Search for the effective size by taking a predetermined bit from the bit with " 1 ".

In other words, the real part and the imaginary part of the conjugate complex number for the short code are simultaneously retrieved from the upper bits to the lower bits, which are reflected in the reference table from bits other than "0" (that is, bits that are "1"). Specific examples 1) and 2) are as follows.

1) When the value of the valid bits of the real and imaginary part is large and the difference between the two values is small,

Real part = 0 101 0000 1011 1111B = 20671, upper 4 bits = 1010B = 10, valid 4 bits = 1010B = 10

Imaginary part = 0010 1001 0000 1010B = 10506, upper 4 bits = 0101B = 5, valid 4 bits = 0101B = 5

In this case, arctan (10506/20671) = 26.94 ° with all bits used

Example using only upper 4 bits (conventional) arctan (5/10) = 26.56 °, error = 0.38 °

Example using only 4 significant bits (invention) arctan (5/10) = 26.56 °, error = 0.38 °

In this case, there is no performance difference between the conventional method and the present invention,

2) If the difference between the two values is small while the magnitude of the valid bits of the real and imaginary parts is small,

Real part = 0000 0010 1011 1111B = 703, upper 4 bits = 0000B = 0, valid 4 bits = 0010B = 2

Imaginary part = 0000 1001 0000 1010B = 2314, upper 4 bits = 0001B = 1, valid 4 bits = 1001B = 9

In this case, arctan (2314/703) = 73.1 ° with all bits used

Example using only upper 4 bits (conventional) arctan (1/0) = 90 °, error = 16.95 °

Example using only 4 significant bits (invention) arctan (9/2) = 77.47 °, error = 4.37 °

In this case, there is a big difference in performance between the conventional method and the present invention. In this case, according to the method of the present invention, a precise error can be calculated with almost no error.

Next, in the fourth step 75, the phase difference of the short codes is obtained by dividing by using the effective size of the short codes retrieved in the third step 74. In theory, a single code is explained. Exp {-jθ1} and another short code, that is, exp {jθ2}, are synthesized by the synthesis unit 211a-3 of FIG. 5, where exp {-jθ1} * exp {jθ2} = exp {j ( θ2-θ1)}, the phase difference can be finally obtained by such calculation.

Since the calculation related to the sine, cosine, and tangent of the present invention takes a lot of time in the digital circuit, the calculation as described above is not suitable for the high-speed WLAN, and the present invention is performed after coarse synchronization is performed. Since the phase synchronization θ is smaller than the predetermined value k, the phase can be obtained by simple division without calculation using the cosine, sine and tangent. have.

For example, if the first input signal is I, Q and the 16th input I ', Q', the division θ = Q '/ I'-Q / I is θ = arctan {Q' / I. Since it is almost the same as '} -arctan {Q / I}', we can find the phase difference by simple division.

If the first reference table is prepared in advance by matching the phase difference and the sample signal (I, Q) obtained by such a calculation formula, I, Q of the first sample signal and I ', Q' of the 16th sample signal The corresponding phase difference can be found directly from the first reference table, so that the phase difference can be obtained at high speed.

Next, in the fifth step 76, 77, the sum of the phase differences (θ2-θ1 = θd) obtained in the sample signal order in the fourth step 75, that is, all 16 phase differences are summed, and then the average (eg, Sum / 16).

Next, in the sixth step 78, phase information corresponding to the average of the frequency errors obtained in the fifth step 76, 77 is retrieved from a look-up table, which is not shown. This table is set in advance by mapping each phase difference and phase information (I, Q) to each other. Using this reference table takes a long time to calculate sine, cosine and tangent values in a digital circuit. If you use the reference table, you can execute at high speed.

Finally, in the seventh step 79, the phase compensation unit 212b shown in FIG. 5 compensates the phase for the next short code by using the retrieved phase information I.Q.

According to the present invention as described above, by using a look-up table to target the effective size in a high-speed wireless LAN to perform a fast and accurate synchronization of the received signal, thereby improving the performance of the high-speed wireless LAN There is a special effect to improve.

The above description is only a description of one embodiment of the present invention, the present invention is capable of various changes and modifications within the scope of the configuration.

Claims (2)

In the frequency error compensation method of a fast WLAN using Orthogonal Frequency Division Multiplexing (OFDM), A first step of receiving and delaying a first short code; A second step of taking a conjugate complex number for each of the short codes delayed in the first step; A third step of searching for and finding an effective size of the real part and the imaginary part of the conjugate complex number in the short code of the second step; A fourth step of obtaining a phase difference of the single code using the effective size of the third step; A fifth step of obtaining the total sum of the phase differences obtained in the fourth step and obtaining an average thereof; A sixth step of retrieving phase information corresponding to an average of the frequency errors obtained in the fifth step from a look-up table; A seventh step of compensating for a phase of a subsequent short code using the retrieved phase information; Precision compensation method of the frequency error of the high-speed wireless LAN, characterized in that consisting of. The method of claim 1, wherein the third step is A method for precisely correcting a frequency error of a high-speed WLAN, characterized by searching from the most significant bit of a single code and taking a predetermined bit from a bit where a "1" appears and processing it into an effective size.