JP2008199506A - Frequency offset correction device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a frequency offset correction device which reduces an estimation error according to the deviation of the specification timing of a pilot signal to improve the precision of a frequency offset even if the specification timing deviates from actual timing. <P>SOLUTION: An AFC part 230 of an OFDM receiver 200 is provided with a multiplication processing part 231 which multiplies a pilot signal included in a received OFDM signal by a pilot signal replica at multiplication timing relatively deviated on the basis of detection frame timing, and a frequency offset calculation processing part 233 which calculates a frequency offset amount based on a multiplication result. Thus, even if the precision of the detection frame timing is slightly deteriorated, an element based on the multiplication in exact frame timing is included in the multiplication result. The estimation error of the frequency offset resulting from the detection error of the frame timing is reduced by calculating the frequency offset based on the multiplication result. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to frequency offset calculation correction, and more particularly to a frequency offset correction apparatus that estimates a frequency offset using a replica signal.

  Currently, the adoption of orthogonal frequency division multiplexing (OFDM) is almost agreed in 3GPP as a next-generation mobile communication technology that realizes wireless broadband whose communication speed exceeds 100 Mbps.

  OFDM is a method for transmitting information signals in parallel by a plurality of orthogonal subcarriers. Since the bandwidth per subcarrier is narrow, OFDM is a high-speed transmission technique that can reduce the influence of frequency selective fading caused by multipath propagation paths.

  On the other hand, since OFDM transmits a plurality of narrowband signals, it is greatly affected by a slight frequency offset as compared with a method of transmitting a wideband signal such as code division multiple access (CDMA). Therefore, high frequency synchronization accuracy is required between the base station and the terminal.

  Realizing high frequency synchronization accuracy between the base station and the terminal can be realized by using rubidium with extremely high frequency accuracy for the original oscillation. However, since rubidium is very expensive, it is assumed that inexpensive commercial vibrations such as crystal will be used in future commercial machines. For this reason, implementation of automatic frequency control (AFC) is also essential in future commercial machines.

  As one of the AFC techniques, a technique has been proposed in which a frequency offset is estimated using a replica signal and frequency correction is performed based on the estimated amount, thereby improving resistance to noise (for example, Patent Document 1).

  Next, a conventional general AFC technique will be described. In the wireless receiver, first, a known signal is extracted from the received signal. At this time, the result of the frame timing detection process in the previous stage of AFC is used. A known signal cut out from the received signal is complex-multiplied with the replica signal. Thereafter, a sample averaging process is performed. In the sample averaging process, one symbol signal is divided into two parts in the first half / second half, and in-phase addition is performed with a half symbol length. The amount of phase rotation due to the frequency offset is equal to the phase difference between the calculated vectors.

In order to improve noise tolerance, branch synthesis and symbol / frame averaging are performed on the calculated phase difference between vectors. Thereafter, the phase of the averaged vector is calculated, and the frequency offset is calculated by the frequency offset calculation unit.
JP 2004-7439 A

  However, in the frequency offset estimation based on the conventional replica correlation, the characteristics are greatly deteriorated when the correlation timing is shifted. That is, when a known signal (pilot signal) is extracted from the received signal, there is a problem that the accuracy of estimation of the frequency offset is greatly deteriorated even if the time position to be extracted is shifted by about several samples.

  The present invention has been made in view of such points, and even when the specific timing of the pilot signal deviates from the timing at which the pilot signal is actually arranged, the estimation error corresponding to this deviation is reduced to reduce the frequency. An object of the present invention is to provide a frequency offset correction device that improves the offset accuracy.

  The frequency offset correction apparatus according to the present invention includes a multiplication processing unit that multiplies the known signal included in the received OFDM signal by the known signal replica signal at a multiplication timing relatively shifted with respect to the detected frame timing, And a frequency offset calculating means for calculating a frequency offset amount based on the result.

  According to the present invention, even when the specific timing of the pilot signal deviates from the timing at which the pilot signal is actually arranged, the frequency offset calculation correction that improves the frequency offset accuracy by reducing the estimation error corresponding to this deviation. Can be provided.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the embodiment, the same components are denoted by the same reference numerals, and the description thereof will be omitted because it is duplicated.

(Embodiment 1)
As shown in FIG. 1, an OFDM transmission apparatus 100 according to the present embodiment includes an encoding unit 110 that performs error correction encoding on transmission data, a modulation unit 120 that modulates a transmission signal after encoding, and a signal after modulation. A scrambler 130 for scrambling, an IFFT unit 140 for forming an OFDM signal by performing a fast inverse Fourier transform (IFFT) on the scrambled transmission signal, and predetermined radio processing (guard interval insertion, up-conversion) And the like. Modulator 120 maps the known signal so that the known signal (pilot signal) is arranged in a predetermined OFDM symbol. This known signal is used when frequency offset correction is performed on the receiving side.

  As shown in FIG. 2, OFDM receiving apparatus 200 according to the present embodiment uses radio reception section 210 that performs predetermined radio processing (down-conversion, guard interval removal, etc.) on the received OFDM signal, and a signal after radio processing. A frame detection unit 220 that detects frame timing, and an AFC (automatic frequency control) that calculates a frequency offset based on a correlation result between a known signal included in the received OFDM signal and a replica signal of the known signal based on the detected frame timing Unit 230, FFT unit 240 for forming a signal in the frequency domain by performing fast Fourier transform (FFT) on the received OFDM signal, and descrambling the signal in the frequency direction using a scrambling code specified by cell search Descrambling section 250 and descrambling A signal and the P / S conversion section 260 to parallel-serial convert, a demodulator 270 for demodulating the signal converted in series, and a decoder 280 for decoding the demodulated signal.

  An AFC (automatic frequency control) unit 230 calculates a frequency offset based on a multiplication result of a known signal included in the received OFDM signal and a replica signal of the known signal with reference to the detected frame timing, and wirelessly receives the frequency offset. Feedback to the department.

  Specifically, as shown in FIG. 3, the AFC unit 230 multiplies a known signal by a replica signal of the known signal, a replica processing unit 231, and a frequency offset based on the multiplication result. A frequency offset calculation processing unit 233 for calculating. The frequency offset calculation processing unit 233 includes a phase rotation vector calculation processing unit 234, a time direction synthesis unit 235, an averaging processing unit 236, and an offset calculation execution unit 237.

  Multiplication processing section 231 multiplies the pilot signal included in the received OFDM signal by the pilot signal replica formed by replica signal generation section 232. The multiplication processing unit 231 multiplies the pilot signal and the pilot signal replica at a plurality of multiplication timings by relatively shifting the pilot signal and the pilot signal replica with reference to the detection frame timing. That is, the multiplication processing unit 231 multiplies the known signal included in the received OFDM signal by the known signal replica signal at a multiplication timing relatively shifted with respect to the detected frame timing.

  Specifically, the multiplication processing unit 231 has a plurality of paths including a delay unit 2311 and a multiplier 2312. Different delay amounts are set in the delay units 2311-1 to 2311 -N of the respective paths. Therefore, in each of the multipliers 2312-1 to 231 -N, the pilot signal and the pilot signal replica are multiplied at different multiplication timings, and a plurality of multiplication results having different multiplication timings are output to the phase rotation vector calculation processing unit 234. . Note that a plurality of multiplication timings based on the frame timing can be set at time positions corresponding to the path positions of the delay profile calculated by the frame timing detection unit 220.

  The phase rotation vector calculation processing unit 234 calculates a phase rotation vector based on each of a plurality of multiplication results. Specifically, the phase rotation vector calculation processing unit 234 has N paths including a sample averaging processing unit 2341 and a phase rotation vector calculation unit 2342. The sample averaging processing units 2341-1 to 2341-1 receive multiplication results at different multiplication timings, and average the samples sampled at a predetermined timing within a predetermined period. The phase rotation vector calculation units 2342-1 to 2342 -N receive the average values obtained by the corresponding sample averaging processing units 2341-1 to 2341 -N, and calculate phase rotation vectors based on the average values.

  The time direction synthesis unit 235 obtains a time synthesis phase rotation vector by synthesizing the plurality of phase rotation vectors calculated by the phase rotation vector calculation processing unit 234 in the time direction. That is, the time direction synthesizing unit 235 sequentially holds the phase rotation vectors obtained at a plurality of time positions by the phase rotation vector calculation processing unit 234, and performs a time synthesis phase rotation by synthesizing the plurality of phase rotation vectors. Get a vector.

  The averaging processing unit 236 combines the time-combined phase rotation vectors between the antennas when receiving them with a plurality of antennas, and further averages them with respect to the symbol interval and frame interval, thereby finally obtaining the frequency. Improves noise immunity of offset. The averaging processing unit 236 includes a branch combining unit 2361, a symbol averaging processing unit 2362, and a frame averaging processing unit 2363.

  The offset calculation execution unit 237 calculates a frequency offset based on the time synthesized phase rotation vector after the averaging. Specifically, the offset calculation execution unit 237 includes a phase rotation amount calculation unit 2371 and a frequency offset calculation unit 2372.

  The phase rotation amount calculation unit 2371 calculates the phase rotation amount based on the time synthesis phase rotation vector. The frequency offset calculation unit 2372 calculates a frequency offset amount based on the calculated phase rotation amount. The frequency offset correction process is performed by feeding back the frequency offset amount.

  Next, processing in the AFC unit 230 will be described with reference to FIG.

  In the multiplication processing unit 231 of the AFC unit 230, the pilot signal included in the received OFDM signal and the pilot signal replica formed in the replica signal generation unit 232 are relatively shifted with reference to the detection frame timing, Multiplication is performed at a plurality of multiplication timings.

  Specifically, multiplication processing is performed at a plurality of different multiplication timings depending on the delay amount given to the pilot signal in each path. This is achieved by multiplying a pilot signal arranged at a predetermined position in a frame by a plurality of replica signals shifted in time with reference to the frame timing as shown in FIG. This is equivalent to performing multiplication processing at timing.

  By multiplying the pilot signal and the pilot signal replica by a plurality of multiplication timings based on the detected frame timing in this way, even if the detection accuracy of the detected frame timing is somewhat worse, What is obtained at the multiplication timing that matches the exact frame timing is included. By calculating the frequency offset based on such a multiplication result, it is possible to reduce the frequency offset estimation error caused by the frame timing detection error. Then, by performing the frequency correction process based on the frequency offset amount estimated with a small error, it is possible to improve the reception characteristics in the reception device.

  As described above, according to the present embodiment, the AFC unit 230 of the OFDM receiver 200 receives the pilot signal and the pilot signal replica included in the received OFDM signal at the multiplication timing relatively shifted with reference to the detected frame timing. And a frequency offset calculation processing unit 233 that calculates a frequency offset amount based on the result of the multiplication.

  In this way, since the pilot signal and the pilot signal replica can be multiplied by a plurality of multiplication timings based on the detected frame timing, even if the detection accuracy of the detection frame timing is somewhat poor, Are included at the multiplication timing that matches the exact frame timing. By calculating the frequency offset based on such a multiplication result, it is possible to reduce the frequency offset estimation error caused by the frame timing detection error. Then, by performing the frequency correction process based on the frequency offset amount estimated with a small error, it is possible to improve the reception characteristics in the reception device.

  The multiplication processing unit 231 gives a plurality of different delay amounts to the pilot signal, thereby forming delay units 2311-1 to 2311 -N that form a plurality of pilot signals shifted in timing from each other, the plurality of pilot signals, and a pilot signal replica And multipliers 2312-1 to 2312 -N.

  The frequency offset calculation processing unit 233 includes a time direction synthesis unit 235 that forms a time direction synthesized signal from a plurality of multiplication results obtained by the multiplication processing unit 231, and calculates a frequency offset amount based on the time direction synthesized signal. calculate.

(Embodiment 2)
In the first embodiment, a time direction synthesized signal is formed from a plurality of multiplication results having different multiplication timings, and a frequency offset amount is calculated based on the time direction synthesized signal. On the other hand, in the second embodiment, the AFC unit detects the maximum multiplication result among a plurality of multiplication results obtained by the multiplier, and calculates the frequency offset amount based on the detected maximum multiplication result. To do.

  As shown in FIG. 5, the frequency offset calculation processing unit 233A of the AFC unit 230A in the OFDM receiving apparatus of Embodiment 2 includes an averaging processing unit 236A and a maximum value detection unit 238.

  The averaging processing unit 236A combines each of the phase rotation vectors related to each multiplication timing obtained by the phase rotation vector calculation processing unit 234 between the antennas when receiving by a plurality of antennas, and further performs symbol interval. By averaging the frame intervals, the noise tolerance of the finally obtained frequency offset is improved. The averaging processing unit 236A has N paths including a branch combining unit 2361, a symbol averaging processing unit 2362, and a frame averaging processing unit 2363.

  Maximum value detector 238 detects the maximum phase rotation vector among the phase rotation vectors corresponding to each path. Here, the phase rotation vector obtained by multiplying the pilot signal replica at the wrong detection position in the N path, that is, the timing other than the accurate frame timing, is added to the in-phase component by the processing in the averaging processing unit in the previous stage. Therefore, the vector size is small. Therefore, the maximum phase rotation vector corresponds to an accurate frame timing.

  Then, the maximum value detection unit 238 outputs the detected maximum rotation phase vector to the offset calculation execution unit 237. The offset calculation execution unit 237 calculates a frequency offset based on this maximum rotation phase vector.

  As described above, in the present embodiment, the frequency offset calculation processing unit 233A of the AFC unit is provided with the maximum value detection unit 238 that detects the maximum multiplication result among the plurality of multiplication results obtained by the multiplication processing unit 231, The frequency offset calculation processing unit 233A can calculate the frequency offset amount from the multiplication result corresponding to the accurate frame timing by calculating the frequency offset amount based on the detected maximum multiplication result. As a result, it is possible to reduce the frequency offset estimation error caused by the frame timing detection error. Then, by performing the frequency correction process based on the frequency offset amount estimated with a small error, it is possible to improve the reception characteristics in the reception device.

(Embodiment 3)
In Embodiments 1 and 2, each of a plurality of pilot signals with different timings obtained by giving a plurality of delay amounts to a pilot signal included in a received OFDM signal is multiplied by a pilot signal replica. Thus, a plurality of multiplication results of pilot signals and pilot signal replicas having different multiplication timings were obtained. On the other hand, in Embodiment 3, a superimposed pilot signal replica is formed by superimposing a plurality of pilot signal replicas (identical to each other) at different timings, and the superimposed pilot signal replica and the received OFDM signal are combined. A frequency offset is calculated based on a multiplication result based on a detection frame timing with the included pilot signal.

  As shown in FIG. 6, OFDM receiving apparatus 300 according to the present embodiment includes AFC section 330. As shown in FIG. 7, the AFC unit 330 includes a replica signal generation unit 331, a multiplier 332, and a frequency offset calculation processing unit 333.

  The replica signal generation unit 331 forms a superimposed replica signal by superimposing the pilot signal replica while relatively shifting the superposition timing. Specifically, the replica signal generation unit 331 includes a plurality of delay units 3311-1 to 2 t arranged in parallel as shown in FIG. 8 and an adder 3312 that adds the outputs of the delay units 3311-1 to 2 t. It comprises. Different delay amounts are set in the respective delay units 3311-1 to 2 t. Therefore, a superimposed replica signal can be formed by adding the outputs of the delay units 3311-1 to 2 t.

  Multiplier 332 multiplies the pilot signal included in the received OFDM signal by the superimposed replica signal from replica signal generation section 331, and inputs the multiplication result to frequency offset calculation processing section 333.

  The frequency offset calculation processing unit 333 calculates a frequency offset based on the multiplication result from the multiplier 332.

  Specifically, as illustrated in FIG. 7, the frequency offset calculation processing unit 333 includes a phase rotation vector calculation processing unit 334, an averaging processing unit 336, and an offset calculation execution unit 337.

  The phase rotation vector calculation processing unit 334 calculates a phase rotation vector based on the input multiplication result. Specifically, the sample averaging processing unit 2341 averages samples obtained by sampling the multiplication results at a predetermined timing within a predetermined period. The phase rotation vector calculation unit 2342 receives the average value obtained by the sample averaging processing unit 2341 and calculates a phase rotation vector based on the average value.

  Averaging processing unit 336 combines the phase rotation vectors between the antennas when they are received by a plurality of antennas, and further averages them with respect to the symbol interval and the frame interval to obtain the finally obtained frequency offset. Improve noise immunity. The averaging processing unit 336 includes a branch combining unit 2361, a symbol averaging processing unit 2362, and a frame averaging processing unit 2363.

  The offset calculation execution unit 337 calculates a frequency offset based on the averaged phase rotation vector. Specifically, the offset calculation execution unit 337 includes a phase rotation amount calculation unit 2371 and a frequency offset calculation unit 2372.

  Next, processing in the AFC unit 330 will be described with reference to FIG.

  In the replica signal generation unit 331 of the AFC unit 330, as shown in FIG. 9, a superimposed replica signal is formed by superimposing pilot signal replicas with a relatively shifted superposition timing.

  Multiplier 332 multiplies the superimposed replica signal and the received pilot signal. This is a process equivalent to the process of multiplying at multiple multiplication timings by shifting the pilot signal and the pilot signal replica relative to each other with reference to the detected frame timing in the first and second embodiments. .

  The detection accuracy of the detected frame timing is obtained by multiplying the superimposed replica signal obtained by superimposing the pilot signal replica with the superimposed timing shifted in this way and the received pilot signal based on the detected frame timing. The result of multiplication includes an element based on the multiplication of the pilot signal replica and the received pilot signal at the correct frame timing even if the signal is slightly worse. By calculating the frequency offset based on such a multiplication result, it is possible to reduce the frequency offset estimation error caused by the frame timing detection error. Then, by performing the frequency correction process based on the frequency offset amount estimated with a small error, it is possible to improve the reception characteristics in the reception device.

  As described above, according to the present embodiment, the replica signal that forms the superimposed replica signal by superimposing the pilot signal replica on the AFC unit 330 of the OFDM receiving apparatus 300 as the multiplication processing unit with the superimposed timing shifted relatively. A generation unit 331 and a multiplier 332 that multiplies the superimposed replica signal and the pilot signal are provided.

  By so doing, it is possible to multiply the superposed replica signal obtained by superimposing the pilot signal replica with the superposition timing relatively shifted, and the received pilot signal on the basis of the detected frame timing. Even if the timing detection accuracy is somewhat poor, the multiplication result includes an element based on the multiplication of the pilot signal replica and the received pilot signal at the correct frame timing. By calculating the frequency offset based on such a multiplication result, it is possible to reduce the frequency offset estimation error caused by the frame timing detection error. Then, by performing the frequency correction process based on the frequency offset amount estimated with a small error, it is possible to improve the reception characteristics in the reception device.

  The frequency offset correction apparatus according to the present invention improves the frequency offset accuracy by reducing the estimation error corresponding to the deviation even when the specific timing of the pilot signal deviates from the timing at which the pilot signal is actually arranged. Useful as.

FIG. 1 is a block diagram showing a configuration of an OFDM transmission apparatus according to Embodiment 1 of the present invention. FIG. 1 is a block diagram showing a configuration of an OFDM receiving apparatus according to Embodiment 1 The block diagram which shows the structure of the AFC part of FIG. The figure which serves for explanation of processing of the AFC section of FIG. FIG. 3 is a block diagram showing a configuration of an AFC unit of an OFDM receiving apparatus according to Embodiment 2 FIG. 3 is a block diagram showing a configuration of an OFDM receiving apparatus according to Embodiment 2 The block diagram which shows the structure of the AFC part of FIG. The block diagram which shows the structure of the replica signal generation part of FIG. FIG. 7 is a diagram for explaining the processing of the AFC unit in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 OFDM transmitter 110 Encoding part 120 Modulation part 130 Scrambling part 140 IFFT part 150 Wireless transmission part 200,300 OFDM receiver 210 Radio reception part 220 Frame detection part 230,330 AFC part 231 Multiplication process part 232 Replica signal generation part 233, 333 Frequency offset calculation processing unit 234, 334 Phase rotation vector calculation processing unit 235 Time direction synthesis unit 236, 336 Averaging processing unit 237, 337 Offset calculation execution unit 238 Maximum value detection unit 240 FFT unit 250 Descrambler 260 P / S conversion unit 270 demodulation unit 280 decoding unit 331 replica signal generation unit 332, 2312 multiplier 2361 branch synthesis unit 2311 delay unit 2341 sample averaging processing unit 2342 phase rotation vector calculation Part 2362 symbol averaging section 2363 frame averaging unit 2371 phase rotation amount calculating unit 2372 frequency offset calculating unit 3312 adder

Claims (5)

Multiplication processing means for multiplying the known signal and the known signal replica signal included in the received OFDM signal at a multiplication timing relatively shifted with respect to the detection frame timing,
Frequency offset calculating means for calculating a frequency offset amount based on the result of the multiplication;
A frequency offset correction apparatus comprising: The multiplication processing means includes:
Known signal forming means for forming a plurality of known signals with different timings by giving different delay amounts to the known signal;
A multiplier for multiplying the plurality of known signals by the known signal replica signal;
The frequency offset correction apparatus according to claim 1, comprising:   The frequency offset calculation means includes time direction synthesis means for forming a time direction synthesized signal from a plurality of multiplication results obtained by the multiplier, and calculates a frequency offset amount based on the time direction synthesized signal. The frequency offset correction apparatus according to 1.   The frequency offset calculating means includes maximum value detecting means for detecting a maximum multiplication result among a plurality of multiplication results obtained by the multiplier, and calculates a frequency offset amount based on the detected maximum multiplication result. The frequency offset correction apparatus according to claim 1. The multiplication processing means includes:
Superimposing replica signal forming means for forming a superimposed replica signal by superimposing the known signal replica signal by relatively shifting the superimposition timing;
A multiplier for multiplying the superimposed replica signal by the known signal;
The frequency offset correction apparatus according to claim 1, comprising:

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