JP5945256B2 - Residual frequency error estimation method and residual frequency error estimation device - Google Patents

Description

  The present invention relates to a residual frequency error estimation method and a residual frequency error estimation device.

  There is an IEEE802.11a standard as a high-speed wireless access system using a 5 GHz (gigahertz) band. This system uses an Orthogonal Frequency Division Multiplexing (OFDM) modulation method, which is a technique for stabilizing the characteristics in a multipath fading environment, and achieves a maximum throughput of 54 Mbps (megabits per second). (For example, refer nonpatent literature 1).

  Further, in IEEE 802.11n, a MIMO (Multiple Input Multiple Output) technique capable of realizing spatial multiplexing using a plurality of antennas using the same time and the same frequency channel, and the 20 MHz that has been used individually until now. It is possible to realize a high-speed communication with a technology that uses two frequency channels (megahertz) at the same time and uses a frequency channel of 40 MHz, and can realize a maximum transmission rate of 600 Mbps (for example, Non-Patent Document 1). .

  In addition, in the current wireless communication system, studies on OFDMA (Orthogonal Frequency Division Multiple Access) that realizes efficient communication by assigning data of a plurality of wireless stations to different subcarriers for transmission are also being conducted. (For example, refer nonpatent literature 2).

  One of the main drawbacks of the system using the OFDM modulation system as described above is that it is easily influenced by a frequency synchronization error. The frequency synchronization error is caused by a center frequency error and a clock frequency error of each radio station. When the above error exists, subcarrier orthogonality is lost, interference between subcarriers increases, and decoding cannot be performed correctly, resulting in degradation of throughput. In order to solve this problem, there is a method of estimating a frequency error between radio stations using known training information (a preamble or the like) embedded in a transmission signal (see, for example, Non-Patent Document 3).

  However, since noise between radio stations and analog attenuation components affect the estimation algorithm, the estimated frequency error does not match the actual frequency error. Therefore, there is a residual frequency error including a center frequency error and a clock frequency error defined as the difference between the estimated frequency error and the actual frequency error. The residual frequency error can be estimated by estimating the frequency error using the phase difference between successive symbols using pilot subcarriers (see, for example, Non-Patent Document 4).

Next, a specific residual frequency error estimation method according to the prior art will be described.
FIG. 9 is a block diagram of a conventional residual frequency error estimation device 9, which includes a pilot subcarrier extraction unit 900, a center frequency error estimation unit 901, a clock frequency error estimation unit 902, and a residual frequency error correction unit 903. Composed.

FIG. 10 is a diagram illustrating an input signal to the residual frequency error estimation device 9. As shown in the figure, the input signal is an OFDM modulation scheme signal, and is sequentially input from the OFDM symbol having the smallest number of OFDM symbols. In the OFDM signal, block B1 is a received data signal of one OFDM symbol, and block B2 is a received pilot subcarrier signal of one OFDM symbol. Received pilot sub-carrier signal in the figure is expressed as x (k _{m, n).} Subcarrier number _{k m is, k -M, ..., k -1} , k 1, ..., the sum the 2M _{k M,} while the absolute value of a character subscript m is similar, the subcarrier frequency The absolute value is the same. Further, n (n is an integer of 1 to N) is the number of OFDM symbols.

When an input signal is input, pilot subcarrier extraction section 900 extracts a pilot subcarrier signal arranged in a subcarrier (block B2) designated in advance from this input signal, and performs center frequency error estimation section 901. And output as a received pilot subcarrier signal to the clock frequency error estimator 902. Center frequency error estimation section 901 uses the received reception pilot subcarrier signal and the known transmission pilot subcarrier signal, and uses the following formulas (1) and (2) to calculate the center for OFDM symbol number n. The frequency error θ _n is estimated, and the estimation result is output to the residual frequency error correction unit 903.

_{Here, p (k m, n '} ) is, n' th (n 'is from n-a + 1 to n) is _{k m-th} transmission pilot sub-carrier signals of OFDM symbols, known in advance determined Signal. sgn () is the Sgn _{function, p (k m, n '} ) is 1, and when the negative when the positive is -1. a is the number of symbols used for time averaging.

The clock frequency error estimator 902 uses the input reception pilot subcarrier signal and transmission pilot subcarrier signal, and uses the following equations (3)-(6) to calculate the clock frequency error ω _n for the number of OFDM symbols _n. And the estimation result is output to the residual frequency error correction unit 903.

_{Here, x '(k m, n} ) is the signal when the temporary correction to the x _(k m, n). Note that ω ₋₁ is 0.
The residual frequency error correction unit 903 corrects the residual frequency error by the following equation (7) using the input center frequency error θ _n of the estimation result and the clock frequency error ω _n of the estimation result, and receives the corrected reception. Output data signals.

Here, r (k, n) is the received data signal of the kth subcarrier of the nth OFDM symbol, and r ′ (k, n) is the nth OFDM symbol after the residual frequency error correction. This is a received data signal of the kth subcarrier.
The above is the conventional residual frequency error estimation method.

Masahiro Morikura, Shuji Kubota, “Revised Third Edition 802.11 High-Speed Wireless LAN Textbook”, Impress R & D, March 27, 2008 Takeshi Hattori, “OFDM / OFDMA Textbook”, Impress R & D, September 21, 2008 Nobuaki Mochizuki, “A high performance frequency and timing synchronization technique for OFDM”, Global Telecommunications Conference, vol.6, pp.3443-3448, 1998. Young-Hwan you Sampling, “Pilot-assisted fine frequency synchronization for OFDM-based DVB receivers”, IEEE transaction on Broadcasting, pp.674-678, Sept. 2009.

  The residual frequency error estimation method described in the background art is an estimation method assuming the use of pilot subcarriers fixedly arranged so as to be paired with respect to the center frequency. If the system uses a frequency band that is symmetric from the center frequency, the received signal at the radio station includes all pairs of pilot subcarriers, so the residual frequency error is estimated by the conventional technique. be able to. However, in the case of an OFDMA system in which a plurality of radio stations are assigned subchannels that are aggregates of different subcarriers and transmit / receive simultaneously with a plurality of radio stations, only a part of pilot subcarriers can be received by each radio station, In addition, since there are cases where the pair does not exist, residual frequency error estimation using the conventional technique cannot be performed.

  In view of the above circumstances, the present invention is capable of estimating and correcting a residual frequency error using only some pilot subcarriers in a wireless communication system that performs multicarrier communication, and any subcarrier. An object of the present invention is to provide a residual frequency error estimation method and a residual frequency error estimation apparatus capable of arranging pilot subcarriers at positions.

  According to one aspect of the present invention, a pilot subcarrier extraction step of receiving a received signal composed of a plurality of subcarriers and extracting a received pilot subcarrier signal at a different subcarrier position from the input received signal; A phase is calculated based on a result obtained by correcting the received pilot subcarrier signal extracted in the pilot subcarrier extraction step by a clock frequency error and a center frequency error and a known transmission pilot subcarrier signal, and different subcarrier positions are calculated. Calculating a correction value of a clock frequency error from the calculated phase, and updating the clock frequency error with the calculated correction value; and calculating the clock frequency error calculated in the clock frequency error estimation step. Supplement A center frequency error estimating step for updating the center frequency error based on a value; the center frequency error updated in the center frequency error estimating step; and the clock frequency error updated in the clock frequency error estimating step. And a residual frequency error correction step for correcting the residual frequency error in the received signal using the residual frequency error estimation method.

  Another aspect of the present invention is the residual frequency error estimation method described above, wherein in the clock frequency error estimation step, the received pilot subcarrier signals extracted from different received signals are converted into a clock frequency error and a center frequency error. The phase averaged for each of the different subcarrier positions is calculated based on the result corrected in accordance with the known transmission pilot subcarrier signal, the correction value of the clock frequency error is calculated from the calculated phase, and the calculated correction The clock frequency error is updated with a value.

  Another aspect of the present invention is the residual frequency error estimation method described above, wherein in the center frequency error estimation step, the correction value of the clock frequency error calculated in the clock frequency error estimation step is extracted. Further, a correction value for the center frequency error is calculated using the received pilot subcarrier signal having the smallest absolute value of the number representing the subcarrier position among the received pilot subcarrier signals, and the calculated correction value for the center frequency error is calculated. The center frequency error is updated by the following.

  Another aspect of the present invention is the residual frequency error estimation method described above, wherein the signal power calculation step calculates a ratio between the power of the received signal and the power of the difference between the received signal and a known pilot signal. In the pilot subcarrier extraction step, a received pilot subcarrier signal whose ratio calculated in the signal power calculation step is larger than a threshold is extracted from the received signal.

  Further, according to one embodiment of the present invention, a pilot that extracts a pilot subcarrier that receives a received signal composed of a plurality of subcarriers and extracts received pilot subcarrier signals at different subcarrier positions from the input received signal. A phase is calculated based on a subcarrier extraction unit, a result obtained by correcting the received pilot subcarrier signal extracted by the pilot subcarrier extraction unit with a clock frequency error and a center frequency error, and a known transmission pilot subcarrier signal. Then, a clock frequency error correction value is calculated from the phase calculated for different subcarrier positions, and the clock frequency error estimator updates the clock frequency error with the calculated correction value, and is calculated by the clock frequency error estimator. Said clock frequency A center frequency error estimator that updates the center frequency error based on a difference correction value, the center frequency error updated by the center frequency error estimator, and the clock frequency updated by the clock frequency error estimator And a residual frequency error correction unit that corrects the residual frequency error of the received signal using an error.

  Another aspect of the present invention is the above-described residual frequency error estimation device, wherein the clock frequency error estimation unit determines the received pilot subcarrier signal extracted from different received signals based on a clock frequency error and a center frequency error. A phase averaged for each of the different subcarrier positions is calculated based on the corrected result and a known transmission pilot subcarrier signal, a correction value of a clock frequency error is calculated from the calculated phase, and the calculated correction value To update the clock frequency error.

  Another aspect of the present invention is the above-described residual frequency error estimation apparatus, wherein the center frequency error estimation unit is extracted with the correction value of the clock frequency error calculated by the clock frequency error estimation unit. A center frequency error correction value is calculated using the received pilot subcarrier signal having the smallest absolute value of the number representing the subcarrier position among the received pilot subcarrier signals, and the calculated center frequency error correction value is calculated according to the calculated center frequency error correction value. The center frequency error is updated.

  Another aspect of the present invention is the residual frequency error estimation apparatus described above, wherein the signal power calculation unit calculates a ratio between the power of the received signal and the power of the difference between the received signal and a known pilot signal. The pilot subcarrier extraction unit extracts a reception pilot subcarrier signal having a ratio calculated by the signal power calculation unit larger than a threshold value from the reception signal.

  According to the present invention, in a wireless communication system that performs multicarrier communication, it is possible to estimate and correct a residual frequency error using only some pilot subcarriers, and to add pilot subcarriers at arbitrary subcarrier positions. It becomes possible to arrange.

1 is an overall configuration diagram of a wireless communication system according to a first embodiment of the present invention. It is a figure which shows an example of the input signal to the residual frequency error estimation apparatus by the embodiment. It is a functional block diagram which shows the structure of the base station by the embodiment. It is a functional block diagram which shows the structure of the radio station by the embodiment. It is a functional block diagram which shows the structure of the residual frequency error estimation apparatus by the same embodiment. It is a functional block diagram which shows the structure of the residual frequency error estimation apparatus by 2nd Embodiment. It is a functional block diagram which shows the structure of the residual frequency error estimation apparatus by 3rd Embodiment. It is a functional block diagram which shows the structure of the residual frequency error estimation apparatus by 4th Embodiment. It is a functional block diagram which shows the structure of the residual frequency error estimation apparatus by a prior art. It is a figure which shows an example of the input signal to the residual frequency error estimation apparatus by a prior art.

  Hereinafter, a residual frequency error estimation method and a residual frequency error estimation apparatus according to an embodiment of the present invention will be described with reference to the drawings.

[A. First Embodiment]
FIG. 1 is an overall configuration diagram of a radio communication system according to the present embodiment. As shown in the figure, the wireless communication system of the present embodiment includes a base station 1 and a plurality of wireless stations 2. Each radio station 2 includes a residual frequency error estimation device 3. The plurality of wireless stations 2 are wireless stations 2-1, 2-2,..., 2-L (L is an integer of 2 or more).

  The base station 1 performs multicarrier communication by radio with each radio station 2. Here, a case where the OFDMA modulation method is used as multicarrier communication will be described. The base station 1 assigns subchannels, which are aggregates of different subcarriers, to the plurality of radio stations 2 and transmits OFDM signals to the plurality of radio stations 2 simultaneously.

  FIG. 2 shows an example of an input signal to the residual frequency error estimation device 3. The OFDM signal shown in the figure is transmitted from the base station 1 in order from the OFDM symbol having the smallest number of OFDM symbols, and is received by the radio station 2. In the OFDM signal, block B1 is a received data signal of one OFDM symbol, and block B2 is a received pilot subcarrier signal of one OFDM symbol. Different subchannels are assigned to the radio stations 2-1 to 2-L, and two or more pilot subcarriers are arranged at arbitrary subcarrier positions in each subchannel. In the radio station 2, the base station 1 receives the signal of the subchannel assigned to the radio station 2 among the OFDM signals, and uses the received signal as an input signal to the residual frequency error estimation device 3 provided in the radio station 2 input.

  FIG. 3 is a functional block diagram showing the configuration of the base station 1, and only functional blocks related to the present embodiment are extracted and shown. As shown in the figure, the base station 1 includes error correction coding units 100-1 to 100-N, interleave processing units 101-1 to 101-N, and subcarrier modulation units 102-1-1 to 102-1. S,..., 102-N-1 to 102-NS, pilot subcarrier generation unit 103, IFFT (Inverse Fast Fourier Transform) unit 104, GI (Guard Interval) addition unit 105, An RF (Radio Frequency) processing unit 106 and an antenna 107 are provided (N and S are integers of 2 or more).

  The error correction coding unit 100-i (i is an integer not smaller than 1 and not larger than N) performs convolutional coding of the input data, and outputs it to the interleave processing unit 101-i. Interleave processing section 101-i performs bit replacement so that transmission of separated bits after encoding is performed on subcarriers as far as possible, and divides the bits after replacement to generate subcarrier modulation section 102-i-1 To 102-i-S. The subcarrier modulation units 102-i-1 to 102-i-S convert the interleaved data into BPSK (Binary Phase Shift Keying) or QPSK (Quadrature Phase Shift) defined in a wireless local area network (LAN). Modulation is performed according to a modulation scheme such as (Keying), and the result is output to IFFT section 104.

  Pilot subcarrier generating section 103 places two or more pilot subcarrier signals for correcting residual frequency error at arbitrary subcarrier positions in each subchannel (for example, the subcarrier position of block B2 in FIG. 2). Deploy. The pilot subcarrier signal is composed of a known signal. The subcarrier position of the pilot subcarrier signal is set in advance in the radio station 2 or is previously notified from the base station 1 to the radio station 2 through communication.

  IFFT section 104 converts the input frequency series data into time series data by IFFT calculation and outputs the time series data to GI adding section 105. The input frequency series data includes a data signal generated by modulating data by subcarrier modulation sections 102-1-1 to 1022-1-S, ..., 102-N-1 to 102-N-S, and a pilot. It consists of pilot subcarrier signals arranged by subcarrier generation section 103. The GI adding unit 105 adds a GI by copying a fixed period at the rear end of the IFFT output signal input from the IFFT unit 104 and connecting it to the front end of the IFFT output signal, and outputs it to the RF processing unit 106. To do. The RF processing unit 106 converts the baseband signal to which the GI is added by the GI adding unit 105 into a wireless LAN signal using an analog RF device, and outputs the wireless LAN signal to the antenna 107. The antenna 107 radiates the wireless LAN signal output from the RF processing unit 106 into the air.

  FIG. 4 is a functional block diagram showing a configuration of the radio station 2 according to the present embodiment, and only functional blocks related to the present embodiment are extracted and shown. As shown in the figure, the radio station 2 includes an antenna 201, a frequency conversion unit 202, a GI removal unit 203, an FFT (Fast Fourier Transform) unit 204, a residual frequency error estimation device 3, and a subcarrier demodulation unit 205. -1 to 205-J (where J is an integer equal to or greater than 2), a deinterleave processing unit 206, and a decoding unit 207.

  The antenna 201 receives a wireless LAN signal radiated into the air and outputs it to the frequency conversion unit 202 as a received signal. The frequency conversion unit 202 converts the signal received by the antenna 201 into a digital signal having a baseband frequency, and outputs the digital signal to the GI removal unit 203. The GI removal unit 203 removes the GI from the frequency-converted digital received signal and outputs the GI to the FFT unit 204. The FFT 204 converts the received signal input from the GI removal unit 203 from frequency sequence data to time sequence data, and outputs the converted data to the residual frequency error estimation device 3. The residual frequency error estimation device 3 estimates the residual frequency error estimation, corrects the residual frequency error of the received signal input from the FFT unit 204 based on the estimation result, and outputs the subcarrier demodulation units 205-1 to 205-J as parallel signals. Output to. Reception subcarrier demodulation sections 205-1 to 205-J demodulate the received signal after residual frequency error correction input from FFT section 204 and output the result to deinterleave processing section 206. Deinterleave processing section 206 deinterleaves the reception signals input from reception subcarrier demodulation sections 205-1 to 205-J, and outputs the result to decoding section 207. The decoding unit 207 decodes the received signal input from the deinterleave processing unit 206.

  FIG. 5 is a functional block diagram showing a configuration of the residual frequency error estimating apparatus 3 according to the present embodiment, and only functional blocks related to the present embodiment are extracted and shown. As shown in the figure, the residual frequency error estimation apparatus 3 includes a pilot subcarrier extraction unit 300, a clock frequency error estimation unit 301, a center frequency error estimation unit 302, and a residual frequency error correction unit 303. Pilot subcarrier extraction section 300 includes pilot subcarrier signals (hereinafter referred to as “received pilot subcarrier signals”) included in subcarriers assigned to radio station 2 provided with residual frequency error estimating apparatus 3. Extract from the input signal. Clock frequency error estimator 301 calculates a clock frequency error based on the received pilot subcarrier signal extracted by pilot subcarrier extractor 300 and a known pilot subcarrier signal (hereinafter referred to as “transmitted pilot subcarrier signal”). presume. Specifically, the clock frequency error estimation unit 301 calculates the phase based on the result of correcting the received pilot subcarrier signal with the current clock frequency error and the center frequency error and the known transmission pilot subcarrier signal, The clock frequency error is updated with the correction value of the clock frequency error obtained from the phase calculated for different subcarrier positions. The center frequency error estimating unit 302 estimates the center frequency error based on the correction value of the clock frequency error calculated by the clock frequency error estimating unit 301. The residual frequency error correction unit 303 uses the clock frequency error estimated by the clock frequency error estimation unit 301 and the center frequency error estimated by the center frequency error estimation unit 302, to the received data signal included in the input signal. Perform residual frequency error correction.

Next, the operation of the residual frequency error estimation process in the residual frequency error estimation device 3 will be described.
For example, when the base station 1 transmits the signal illustrated in FIG. 2, the wireless station 2 receives the signal transmitted from the base station 1 by the antenna 201. The frequency conversion unit 202 converts the received signal of the subchannel assigned to the own station in the RF band received by the antenna 201 into a digital signal of the baseband frequency, and the GI removal unit 203 converts the frequency-converted digital signal. GI is removed from the received signal. The FFT conversion unit 204 performs FFT on the received signal from which the GI has been removed, and inputs the received signal as an input signal to the pilot subcarrier extraction unit 300 of the residual frequency error estimation apparatus 3. Pilot subcarrier extraction section 300 extracts the received pilot subcarrier signal included in the subcarrier assigned to the own station from the input signal, and outputs it to clock frequency error estimation section 301. Here, for simplification of explanation, the subcarrier number k _i representing the subcarrier position of the extracted received pilot subcarrier signal is a total of I of k _−M ,..., K _{− (M−I + 1).} .

The clock frequency error estimator 301 estimates the clock frequency error ω _n using the input reception pilot subcarrier signal and the known transmission pilot subcarrier signal according to the following equations (8)-(12). Do. Specifically, the clock frequency error estimator 301 uses the equation (8) to receive the received pilot subcarrier signals of the maximum subcarrier number k _max and the minimum subcarrier number k _min among the subcarrier numbers k _i. Is corrected by the current clock frequency error and the center frequency error, and is set to a positive value using the sign of the transmission pilot subcarrier signal. Next, the clock frequency error estimator 301 uses the formulas (9) and (10) to calculate the calculation result of the formula (8) for the subcarrier numbers k _max and k _min in the nth OFDM symbol as the phase information ψ ( k _max , n) and ψ (k _min , n). Normally, since the pilot signal is 1, -1, the phase information is 0 when the frequency error is correctly corrected, but is a value other than 0 when the frequency error is not corrected correctly. The clock frequency error estimation unit 301 calculates the clock frequency error Δω per subcarrier using Equation (11) using the phase information calculated for the subcarrier numbers k _max and _kmin . The clock frequency error Δω is a difference (correction value) with respect to the current frequency error ω _n−1 . The reason why the subcarrier numbers k _max and _kmin are used is that the phase change increases with the subcarrier number. The clock frequency error estimator 301 updates the current clock frequency error ω _{n−1 according} to the equation (12) using the clock frequency error Δω per subcarrier calculated by the equation (11), and the clock frequency error ω _n is calculated.

In the above equation, x (k _i , n) is the k _i th received pilot subcarrier signal of the n th OFDM symbol, and p (k _i , n) is the k _i th of the n th OFDM symbol. Are transmitted pilot subcarrier signals. sgn () is the Sgn function, p _(k m, n) is 1, and when the negative when the positive is -1. ω _n-1 and ω _n are the clock frequency errors estimated for the number of OFDM symbols (n-1) and n, respectively, and θ _n-1 is the center frequency error estimated for the number of OFDM symbols (n-1). Yes, x ′ (k _i , n) is a signal when provisional correction is performed on x (k _i , n). Note that θ ₋₁ and ω ₋₁ are zero.
The clock frequency error estimation unit 301 outputs the clock frequency error Δω to the central frequency error estimation unit 302 and outputs the clock frequency error ω _n to the residual frequency error correction unit 303.

Using the clock frequency error Δω input from the clock frequency error estimation unit 301, the center frequency error estimation unit 302 estimates the center frequency error θ _n using the following formulas (13) and (14), and the residual frequency The estimation result is output to the error correction unit 303. Specifically, the center frequency error estimation unit 302 calculates the center frequency error Δθ per OFDM symbol using Equation (13). This center frequency error Δθ is a correction value for the current center frequency error θ _n−1 estimated for the number of OFDM symbols (n−1). Then, the center frequency error estimating unit 302 updates the center frequency error θ _n−1 using the center frequency error Δθ per OFDM symbol calculated by the equation (13) according to the equation (14), and the nth OFDM symbol. The center frequency error θ _{n of} is calculated.

Here, k _i is may be used which subcarrier numbers.
The residual frequency error correction unit 303 includes an estimation result ω _n of the estimation result input from the clock frequency error estimation unit 301 and a center frequency error θ _n of the estimation result input from the center frequency error estimation unit 302. Is used to correct the residual frequency error of the received data signal by the following equation (15), and the corrected received data signal is output to the subcarrier demodulating units 205-1 to 205-J.

  Here, r (k, n) is the received data signal of the kth subcarrier of the nth OFDM symbol, and r ′ (k, n) is the nth OFDM symbol after the residual frequency error correction. This is a received data signal of the kth subcarrier.

  Reception subcarrier demodulation sections 205-1 to 205-J demodulate the received signal whose residual frequency error has been corrected by residual frequency error estimation apparatus 3 and output the demodulated signal to deinterleave processing section 206. The deinterleave processing unit 206 deinterleaves the demodulated reception signal, and the decoding unit 207 decodes the deinterleaved reception signal to obtain data.

With the configuration of the residual frequency error estimation device 3 and the residual frequency error estimation method described above, residual frequency error estimation can be performed using some pilot subcarriers.
The above is the first embodiment.

  According to this embodiment, a base station allocates subchannels, which are aggregates of different subcarriers, to a plurality of radio stations, and in a system using an OFDMA modulation scheme in which transmission and reception are performed simultaneously with a plurality of terminal stations, It is possible to estimate and correct the residual frequency error using some pilot subcarriers arranged at the carrier position.

[B. Second Embodiment]
In the second embodiment, when estimating the clock frequency error, the estimation accuracy of the residual frequency error can be improved by averaging the phase shift using the received pilot subcarrier signals extracted from other symbols. . Below, 2nd Embodiment is described centering on the difference with 1st Embodiment.

  The radio communication system according to the present embodiment has a configuration in which the radio station 2 shown in FIG. 1 includes a residual frequency error estimation device 3a shown in FIG. 6 below instead of the residual frequency error estimation device 3 shown in FIG. .

  FIG. 6 is a functional block diagram showing the configuration of the residual frequency error estimating apparatus 3a according to this embodiment. In FIG. 6, the same parts as those of the residual frequency error estimating apparatus 3 according to the first embodiment shown in FIG. The same reference numerals are given and the description thereof is omitted. As shown in the figure, the residual frequency error estimation device 3a includes a pilot subcarrier extraction unit 300, a clock frequency error estimation / average unit 305, a center frequency error estimation unit 302, and a residual frequency error correction unit 303. The As described above, the residual frequency error estimation device 3a according to the present embodiment includes the clock frequency error estimation / average unit 305 instead of the clock frequency error estimation unit 301 included in the residual frequency error estimation device 3 according to the first embodiment. It is a configuration. Thereby, pilot subcarrier extraction section 300 extracts the received pilot subcarrier signal included in the subcarrier assigned to the own station from the input signal, and outputs it to clock frequency error estimation / average section 305.

The clock frequency error estimation / average unit 305 uses the received pilot subcarrier signal input from the pilot subcarrier extraction unit 300 and the known transmission pilot subcarrier signal according to the following equations (16)-(20). The clock frequency error ω _n is estimated and averaged, and the estimation result is output to the center frequency error estimating unit 302 and the residual frequency error correcting unit 303.
Specifically, the clock frequency error estimation / average unit 305 calculates the received pilot subcarrier signal that has been temporarily corrected according to Equation (16). Then, the clock frequency error estimating / averaging unit 305 averages the received pilot subcarrier signals after provisional correction calculated for the subcarrier numbers k _max and _kmin for the past a symbols according to equations (17) and (18). The phase information ψ (k _max , n), ψ (k _min , n) is calculated. The clock frequency error estimator / averager 305 uses the equation (19) to calculate per phase per subcarrier from the phase information ψ (k _max , n) and ψ (k _min , n) calculated by the equations (17) and (18). The clock frequency error Δω is calculated. The clock frequency error estimator / average unit 305 updates the clock frequency error ω _n−1 using the clock frequency error Δω calculated by the equation (19) according to the equation (20), and calculates the clock frequency error ω _n .

Note that θ ₋₁ and ω ₋₁ are 0, and a is the number of symbols used for time averaging. The subsequent processing is the same as in the first embodiment.
The above is the second embodiment.

[C. Third Embodiment]
In the third embodiment, when estimating the center frequency error, the estimation accuracy of the residual frequency error can be improved by estimating the center frequency error with reference to the pilot subcarrier having the smallest absolute value of the subcarrier number. . In the following, the third embodiment will be described focusing on the differences from the first embodiment.

  The radio communication system according to the present embodiment has a configuration in which the radio station 2 shown in FIG. 1 includes a residual frequency error estimation device 3b shown in FIG. 7 below instead of the residual frequency error estimation device 3 shown in FIG. .

  FIG. 7 is a functional block diagram showing the configuration of the residual frequency error estimating apparatus 3b according to this embodiment. In FIG. 7, the same parts as those of the residual frequency error estimating apparatus 3 according to the first embodiment shown in FIG. The same reference numerals are given and the description thereof is omitted. As shown in the figure, the residual frequency error estimation device 3b includes a pilot subcarrier extraction unit 300, a clock frequency error estimation unit 301, a selective center frequency error estimation unit 306, and a residual frequency error correction unit 303. The As described above, the residual frequency error estimation device 3b according to the present embodiment includes the selective center frequency error estimation unit 306 instead of the center frequency error estimation unit 302 included in the residual frequency error estimation device 3 according to the first embodiment. It is a configuration. As a result, the clock frequency error estimating unit 301 outputs the clock frequency error Δω to the selective center frequency error estimating unit 306. The residual frequency error correction unit 303 uses the clock frequency error estimated by the clock frequency error estimation unit 301 and the center frequency error estimated by the selective center frequency error estimation unit 306 to generate a received data signal included in the input signal. On the other hand, the residual frequency error is corrected.

The selection-type center frequency error estimation unit 306 estimates the center frequency error θ _n using the clock frequency error Δω _n input from the clock frequency error estimation unit 301 using the formulas (21) and (22), The estimation result is output to the residual frequency error correction unit 303.

Here, k ′ _min indicates the subcarrier number having the smallest absolute value among the subcarrier numbers k _i . The subsequent processing is the same as in the first embodiment.
The above is the third embodiment.

[D. Fourth Embodiment]
In the fourth embodiment, when extracting the received pilot signal, the ratio of the received signal power and the power obtained by subtracting the known transmitted pilot signal from the received signal is calculated for the received pilot subcarrier signal of each subcarrier number. Then, pilot subcarriers whose calculated ratio is larger than a certain threshold are selected and extracted. Accordingly, there is an adjacent communication cell that uses any one of the frequency channels assigned to the radio station, and the estimation accuracy of the residual frequency error can be improved even in an environment that causes interference. In the following, the fourth embodiment will be described focusing on differences from the first embodiment.

  The radio communication system according to the present embodiment has a configuration in which the radio station 2 shown in FIG. 1 includes a residual frequency error estimation device 3c shown in FIG. 8 below instead of the residual frequency error estimation device 3 shown in FIG. .

  FIG. 8 is a functional block diagram showing the configuration of the residual frequency error estimating apparatus 3c according to this embodiment. In FIG. 8, the same parts as those of the residual frequency error estimating apparatus 3 according to the first embodiment shown in FIG. The same reference numerals are given and the description thereof is omitted. As shown in the figure, the residual frequency error estimation device 3c includes a signal power calculation unit 307, a selective pilot subcarrier extraction unit 308, a clock frequency error estimation unit 301, a center frequency error estimation unit 302, and a residual frequency error correction unit. 303 is comprised. As described above, the residual frequency error estimation device 3c according to the present embodiment replaces the pilot subcarrier extraction unit 300 included in the residual frequency error estimation device 3 according to the first embodiment with the signal power calculation unit 307 and the selective pilot. The subcarrier extraction unit 308 is provided.

  The signal power calculation unit 307 receives an input signal and a replica signal. The replica signal is a known pilot signal. The signal power calculation unit 307, for each pilot subcarrier, the power of the input signal x (k, n), the input signal x (k, n), and the replica signal p (k, n) as shown in Equation (23). A ratio R (k, n) with the difference power from n) is calculated and output to the selective pilot subcarrier extraction section 308. The input signal x (k, n) is an input signal of the nth OFDM symbol, and the replica signal p (k, n) is a known pilot signal of the nth OFDM symbol.

The selective pilot subcarrier extraction unit 308 extracts pilot subcarriers having a power ratio R (k, n) input from the signal power calculation unit 307 of a predetermined threshold or more from the input signal, and a clock frequency error estimation unit 301. Output to. The subsequent processing is the same as in the first embodiment.
The above is the fourth embodiment.

  As described above, in the present embodiment, in a wireless communication system using the OFDMA modulation scheme, a base station arranges two or more known transmission pilot subcarriers at subcarrier positions in a subchannel and transmits a transmission signal. The residual frequency error estimator of the radio station extracts the pilot subcarrier signal arranged at the subcarrier position from the received signal as a received pilot subcarrier, and extracts the received received pilot subcarrier and the known transmitted pilot subcarrier. The clock frequency error and the center frequency error are estimated based on the carrier, and the residual frequency error is corrected using the estimated clock frequency error and center frequency error. As a result, the residual frequency error estimation device can estimate and correct the residual frequency error using a part of pilot subcarriers arranged at arbitrary subcarrier positions.

  You may make it implement | achieve a part of function of the base station 1 and the residual frequency error estimation apparatus 3, 3a, 3b, 3c in embodiment mentioned above with a computer. In that case, a program for realizing this function may be recorded on a computer-readable recording medium, and the program recorded on the recording medium may be read into a computer system and executed. Here, the “computer system” includes an OS and hardware such as peripheral devices. The “computer-readable recording medium” refers to a storage device such as a flexible medium, a magneto-optical disk, a portable medium such as a ROM and a CD-ROM, and a hard disk incorporated in a computer system. Furthermore, the “computer-readable recording medium” dynamically holds a program for a short time like a communication line when transmitting a program via a network such as the Internet or a communication line such as a telephone line. In this case, a volatile memory inside a computer system serving as a server or a client in that case may be included and a program that holds a program for a certain period of time. The program may be a program for realizing a part of the functions described above, and may be a program capable of realizing the functions described above in combination with a program already recorded in a computer system.

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and includes designs and the like that do not depart from the gist of the present invention.

  The present invention is applicable to a wireless communication system in which subchannels, which are aggregates of different subcarriers, are assigned to a plurality of wireless stations using the OFDMA modulation method, and transmission / reception is performed simultaneously with the plurality of wireless stations.

1 Base Stations 100-1 to 100-N Error Correction Encoding Units 101-1 to 101-N Interleave Processing Units 102-1-1 to 102-1-S, 102-N-1 to 102-N-S Subcarriers Modulation section 103 Pilot subcarrier generation section 104 IFFT section 105 GI addition section 106 RF processing section 107 Antenna 2 Radio station 201 Antenna 202 Frequency conversion section 203 GI removal section 204 FFT sections 205-1 to 205-J Subcarrier demodulation section 206 Interleave processing unit 207 Decoding unit 3, 3a, 3b, 3c Residual frequency error estimation device 300 Pilot subcarrier extraction unit 301 Clock frequency error estimation unit 302 Center frequency error estimation unit 303 Residual frequency error correction unit 305 Clock frequency error estimation / average unit (Clock frequency error estimator)
306 Selective center frequency error estimator (center frequency error estimator)
307 Signal power calculation unit 308 Selective pilot subcarrier extraction unit (pilot subcarrier extraction unit)
9 Residual frequency error estimation device 900 Pilot subcarrier extraction unit 901 Center frequency error estimation unit 902 Clock frequency error estimation unit 903 Residual frequency error correction unit

Claims (8)

A pilot subcarrier extraction step of receiving a received signal composed of a plurality of subcarriers and extracting a received pilot subcarrier signal at a different subcarrier position from the input received signal;
A phase is calculated based on a result of correcting the received pilot subcarrier signal extracted in the pilot subcarrier extraction step with a clock frequency error and a center frequency error, and a known transmission pilot subcarrier signal, and different subcarrier positions are calculated. A clock frequency error estimating step of calculating a correction value of a clock frequency error from the phase calculated with respect to and updating the clock frequency error with the calculated correction value;
A center frequency error estimating step for updating the center frequency error based on the correction value of the clock frequency error calculated in the clock frequency error estimating step;
A residual frequency error correction step for correcting a residual frequency error in the received signal using the center frequency error updated in the center frequency error estimation step and the clock frequency error updated in the clock frequency error estimation step. When,
A residual frequency error estimation method comprising: In the clock frequency error estimation step, the different subcarriers are based on a result of correcting the received pilot subcarrier signals extracted from different received signals with a clock frequency error and a center frequency error and a known transmission pilot subcarrier signal. Calculating an averaged phase for each position, calculating a correction value of a clock frequency error from the calculated phase, and updating the clock frequency error with the calculated correction value;
The residual frequency error estimation method according to claim 1, wherein: In the center frequency error estimating step, the correction value of the clock frequency error calculated in the clock frequency error estimating step and the absolute value of the number representing the subcarrier position among the extracted received pilot subcarrier signals are the largest. A center frequency error correction value is calculated using the small received pilot subcarrier signal, and the center frequency error is updated with the calculated center frequency error correction value.
The residual frequency error estimation method according to claim 1 or 2, wherein the residual frequency error is estimated. A signal power calculating step of calculating a ratio between the power of the received signal and the power of the difference between the received signal and a known pilot signal;
In the pilot subcarrier extraction step, a received pilot subcarrier signal whose ratio calculated in the signal power calculation step is larger than a threshold is extracted from the received signal;
The residual frequency error estimation method according to any one of claims 1 to 3, wherein the residual frequency error is estimated. A pilot subcarrier extracting unit that receives a received signal composed of a plurality of subcarriers and extracts a pilot subcarrier that extracts received pilot subcarrier signals at different subcarrier positions from the received received signal;
A phase is calculated based on a result of correcting the received pilot subcarrier signal extracted by the pilot subcarrier extraction unit using a clock frequency error and a center frequency error, and a known transmission pilot subcarrier signal, and different subcarrier positions are calculated. A clock frequency error estimator that calculates a correction value of a clock frequency error from the phase calculated for the above, and updates the clock frequency error with the calculated correction value;
A center frequency error estimator that updates the center frequency error based on the correction value of the clock frequency error calculated by the clock frequency error estimator;
A residual frequency error correction unit that corrects a residual frequency error in the received signal using the center frequency error updated by the center frequency error estimation unit and the clock frequency error updated by the clock frequency error estimation unit When,
A residual frequency error estimation device comprising: The clock frequency error estimator is configured to correct the received pilot subcarrier signals extracted from different received signals with a clock frequency error and a center frequency error and the different subcarrier positions based on a known transmitted pilot subcarrier signal. Calculating an averaged phase every time, calculating a correction value of a clock frequency error from the calculated phase, and updating the clock frequency error with the calculated correction value;
The residual frequency error estimation apparatus according to claim 5, wherein The center frequency error estimator has the smallest absolute value of the correction value of the clock frequency error calculated by the clock frequency error estimator and the number representing the subcarrier position among the extracted received pilot subcarrier signals. A center frequency error correction value is calculated using the received pilot subcarrier signal, and the center frequency error is updated with the calculated center frequency error correction value.
The residual frequency error estimation apparatus according to claim 5 or 6, characterized by: A signal power calculation unit that calculates a ratio between the power of the received signal and the power of the difference between the received signal and a known pilot signal;
The pilot subcarrier extraction unit extracts, from the received signal, a received pilot subcarrier signal in which the ratio calculated by the signal power calculation unit is larger than a threshold;
The residual frequency error estimation apparatus according to any one of claims 5 to 7, wherein

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