JP5414484B2 - Fourier transform circuit, receiver and Fourier transform method - Google Patents

Description

  The present invention relates to a Fourier transform circuit, a receiving apparatus, and a Fourier transform method that do not limit the number of data points to be subjected to Fourier transform other than a power of 2.

  Conventionally, in a wireless communication system such as OFDM (Orthogonal Frequency Division Multiplexing) and SC-FDMA (Single Carrier-Frequency Division Multiple Access), a transmission apparatus converts a frequency domain signal into a time domain signal by inverse Fourier transform and transmits the signal. To do. In the receiving device, the received signal is converted into the frequency domain by Fourier transform. In the transmission apparatus and the reception apparatus, IFFT (Inverse Fast Fourier Transform) or FFT (Fast Fourier Transform) is generally used when performing inverse Fourier transform or Fourier transform in order to increase the processing speed.

  On the other hand, the number of data points to be processed is fixed in a circuit that implements IFFT or FTT (hereinafter referred to as FFT circuit). Therefore, in order to receive received signals having different numbers of subcarriers, the number of FFT circuits corresponding to the number of subcarriers is provided, which increases the device scale.

  As a technique for suppressing the device scale when receiving received signals having different numbers of carriers (number of subcarriers), for example, there is a technique described in Patent Document 1 below. The OFDM receiver described in Patent Document 1 below includes an FFT circuit having an input terminal having the maximum number of carriers (power of 2) of an assumed received signal, and receives a received signal having a number of carriers smaller than the number of input terminals. In addition, a reception signal having the number of data corresponding to the number of carriers is input to the input terminal, and 0 is inserted to the input terminal to which no reception signal is input. Then, the data output from the FFT circuit is thinned and output as the number of data corresponding to the number of carriers of the received signal. In this way, the OFDM receiver described in Patent Document 1 described below can cope with a single FFT circuit when receiving received signals having different numbers of carriers.

JP 2004-186852 A

  However, according to the technique described in Patent Document 1, reception signals with different numbers of carriers are handled by one FFT circuit, but the number of assumed reception signals is a power of 2, and the input terminal The number is an integral multiple of the number of carriers. At the time of FFT thinning, thinning is simply performed based on this integer ratio. Therefore, there is a problem that it is impossible to cope with cases where the number of carriers is other than a power of two.

  The present invention has been made in view of the above, and when processing a plurality of types of data having different numbers of input data points, the number of data points other than a power of 2 is not increased without increasing the circuit scale. An object of the present invention is to obtain a Fourier transform circuit, a receiving apparatus, and a Fourier transform method capable of performing Fourier transform or inverse Fourier transform at high speed on the above data.

In order to solve the above-described problems and achieve the object, the present invention is a Fourier transform circuit that performs an IDFT process, wherein N (N is a positive integer) point input data before and after the input data. 0 insertion means for inserting data having a value of 0 to generate an M-point data string that is a power of 2 greater than N, IFFT means for performing an M-point IFFT process on the data string, Conversion means for converting the M-point data string after the IFFT processing into an N-point output data string , wherein the conversion means passes through an area corresponding to the input data and is used as an aliasing component of the input data. A filter having a predetermined number of taps in which tap coefficients are set so as to perform filtering with a filter function that removes the corresponding region, and when the index of the output data sequence is n, Each time, m and p satisfying M × n / N = m + p (m is a positive integer, p is a number of 1 or less) are obtained, the filter shifts the input data by m points, and the filter function The filtering is performed by setting a tap coefficient corresponding to a delay filter function that is delayed by p .

  According to the present invention, when an input signal of N (N is a positive integer) point is input, 0 is inserted into the input signal to obtain a signal of M (M is a power of 2) point. The M-point FFT processing is performed and the signal after the FFT processing is converted into N-point data. Therefore, when processing multiple types of data with different numbers of input data points, the circuit scale is reduced. There is an effect that Fourier transform or inverse Fourier transform can be performed at high speed even for data having a number of data points other than a power of 2 without increasing.

FIG. 1 is a diagram illustrating a functional configuration example of a receiving apparatus according to the present invention. FIG. 2 is a diagram illustrating a configuration example of the IDFT unit. FIG. 3 is a diagram illustrating an example of 0 insertion processing of the 0 insertion unit. FIG. 4 is a diagram illustrating an example of frequency characteristics of a filter included in the data thinning unit. FIG. 5 is a diagram illustrating error characteristics when the thinning process of Example 1 is performed. FIG. 6 is a diagram illustrating error characteristics when the thinning process of Example 2 is performed.

  Embodiments of a Fourier transform circuit, a receiving device, and a Fourier transform method according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

Embodiment.
FIG. 1 is a diagram illustrating a functional configuration example of an embodiment of a receiving device according to the present invention. As shown in FIG. 1, the receiving apparatus of this embodiment includes a receiving antenna 1, an FFT unit 2, an SC-FDMA demodulating unit 3, an IDFT (Inverse Discrete Fourier Transform) unit 4, an LLR (Log Likelihood Ratio). : Log likelihood ratio) calculating unit 5 and error correcting unit 6.

  In this embodiment, a case will be described in which the receiving apparatus performs LTE (Long Term Evolution) base station reception (uplink reception). The SC-FDMA scheme is applied in the LTE uplink. Therefore, the receiving apparatus of this embodiment receives a signal transmitted by the SC-FDMA method.

  First, the overall operation of the receiving apparatus will be described. The receiving antenna 1 receives a signal transmitted by the SC-FDMA method. The FFT unit 2 performs fast Fourier transform on the signal received by the receiving antenna 1 to convert the signal into a frequency domain signal (frequency domain signal). The SC-FDMA demodulator 3 performs a predetermined demodulation process based on the frequency domain signal. The IDFT unit 4 converts the demodulated signal into a time domain signal (time domain signal) by inverse discrete Fourier transform. The LLR calculation unit 5 calculates a log likelihood ratio (LLR) based on the time domain signal. The error correction unit 6 performs error correction decoding based on the LLR to obtain decoded data.

  In LTE, the number of data points input to the FFT unit 2 is a power of 2, but the number of subcarriers to which data is allocated is not necessarily a power of 2, so the number of data points input to the IDFT unit 4 is 2 It is not a power point. Therefore, an IFFT algorithm with a power-of-two point that is a high-speed IDFT algorithm cannot be applied as the IDFT unit 4.

The number of subcarriers to which data is assigned is 12 × 2 ^a × 3 ^b × 5 ^c (a, b, c are integers of 0 or more), and is 1200 at the maximum. The number of subcarriers to which data is allocated changes from moment to moment even during communication due to changes in the propagation environment. Therefore, the IDFT unit 4 needs to cope with a plurality of types of input data points.

  Therefore, in the present embodiment, the IDFT unit 4 is configured as follows. FIG. 2 is a diagram illustrating a configuration example of the IDFT unit 4. The IDFT unit 4 includes a 0 insertion unit 11, an IFFT unit 12, and a data thinning unit 13.

  The operation of the IDFT unit 4 of the present embodiment will be described with reference to FIG. As described above, the number N of data points of the signal after SC-FDMA demodulation processing input to the IDFT unit 4 (N is a positive integer) is equal to the number of subcarriers to which data is allocated on the transmission side. It is not always a power of. Therefore, first, the 0 insertion unit 11 adds 0 to the input N-point signal to obtain an M-point signal (M is a power of 2).

  FIG. 3 is a diagram illustrating an example of the 0 insertion process of the 0 insertion unit 11. As shown in FIG. 3, zero value signals are inserted on both sides (high frequency side) of an N-point input signal (input signal to the IDFT unit 4) in the frequency domain to obtain a total of M point signals. Here, in order to efficiently perform the subsequent IFFT processing, data processing is performed by setting the minimum frequency among the subcarriers to which data is allocated to 0 regardless of the actual frequency of the subcarrier. Therefore, in FIG. 3, the input signal to the IDFT unit 4 is shown as a signal in a frequency band centered on 0.

  The IFFT unit 12 performs IFFT processing similar to the conventional method on the M-point signal after 0 insertion, and outputs an M-point time domain signal. The data thinning unit 13 performs a thinning process on the M-point time domain signal output from the IFFT unit 12 to obtain an N-point time domain signal. That is, the data thinning unit 13 is a conversion unit that converts an M point signal into an N point signal.

  Next, a thinning process performed by the data thinning unit 13 will be described. In the present embodiment, since M is not necessarily an integer multiple of N, a method of simply thinning out data every predetermined number cannot be applied. Here, as an example, a thinning process using a filter will be described.

  FIG. 4 is a diagram illustrating an example of a frequency characteristic (transmission characteristic) of a filter included in the data thinning unit 13. As shown in FIG. 4, the filter included in the data thinning unit 13 is an input signal to the IDFT unit 4 (N-point signal having a frequency corresponding to a subcarrier to which data is allocated, and a portion filled with a diagonal line in FIG. ) Has a flat frequency characteristic, and signals in this frequency band are allowed to pass through.

  On the other hand, the signal output from the IFFT unit 12 generates a high frequency component due to aliasing in the vicinity of an integer multiple of the sampling frequency fs by IFFT processing. In FIG. 4, an unfilled rectangle centered at fs is shown. The filter included in the data thinning unit 13 has a frequency characteristic that removes the high-frequency component. The frequency characteristic of the filter shown in FIG. 4 is an example, and a frequency that passes a frequency band corresponding to an input signal (N-point data) to the IDFT unit 4 and removes a high-frequency component near an integral multiple of fs. Any filter may be used as long as it has characteristics.

  In the present embodiment, the data thinning unit 13 performs a thinning process using the above filter. As an example of the thinning process using a filter, first, the number of data allocation subcarriers (= number of input data points to the IDFT unit 4) N is set to 1200, and the number of points after 0 insertion (= number of IFFT points) M is set to 2048. The case where it does is demonstrated. In this example, thinning may be performed at a sample interval of 2048/1200 = 128/75. Therefore, if the index of each output signal of the data thinning unit 13 is n (n = 0, 1,..., 1199), a signal corresponding to the 128 × n / 75th of the input signals of the data thinning unit 13 is obtained. Sampling.

128 × n / 75 can be transformed into the following equation (1) using positive integers m and i.
128 × n / 75 = m + i / 75 (m is a positive integer, i is a positive integer of 75 or less) (1)

  That is, a point that is further advanced by i / 75 samples of the m-th sample of the M-sample input signal may be sampled. Therefore, the data thinning unit 13 obtains m and i / 75 (= p, p ≦ 1) based on N and M, and inputs the shift amount of the filter shift register as m according to the value of m. The tap coefficient is set so that the signal is shifted, and the filter function of the filter (filter as illustrated in FIG. 4) is delayed by p samples. For example, by storing the tap coefficient as a table for each i, the processing amount required for calculating the tap coefficient can be reduced.

  Next, an example different from the above example of obtaining the filter tap coefficient (hereinafter referred to as Example 1) will be described as Example 2. In Example 2, the filter function for the thinning process is approximated by a polynomial, and the coefficients of the polynomial are held as a table.

  Assuming that the tap length is K, the output signal y (n) of the data thinning unit 13 can be expressed as in the following equation (2).

  However, cΔ (k) is a tap coefficient corresponding to the tap k, and d (m) is an mth input signal of the thinning filter unit 13. Further, Δ is the delay amount of the filter function. Δ is a delay amount corresponding to p (= i / 75) in the above formula (1).

If cΔ (k) is expressed by an L-th order polynomial of L (L is a positive integer), and an l-th order polynomial coefficient is b _k (l), cΔ (k) is expressed by the following equation (3). be able to.

  By substituting Equation (3) into the right side of Equation (2), Equation (2) can be modified as follows.

Accordingly, the polynomial coefficient b _k (l) is calculated in advance according to the frequency characteristics of the filter to be applied, and b _k (l) is stored as a table, so that the tap coefficient is based on b _k (l). Is set, the data decimation process (that is, the calculation of y (n)) can be performed using the above equation (4).

  Next, an example of calculating an error in the case where the process of the IDFT unit 4 of the present embodiment is performed with respect to the case where the N-point IDFT process is performed according to the definition formula shown in the following formula (5) will be described. Note that the two data thinning methods of Example 1 and Example 2 are applied to the thinning process of the IDFT unit 4 of the present embodiment.

  Note that t is an index in the time domain, f is an index in the frequency domain, X (f) indicates a signal in the frequency domain, and x (t) is X (f) obtained by IDFT processing in the time domain. Shows the converted signal.

  Here, as a specific example, a Zadoff-Chu sequence having a sequence length of N is performed by performing IDFT according to Equation (5) and by processing (0 insertion processing + IFFT processing + data thinning processing) of the IDFT unit 4 of the present embodiment. The result of calculating the square error with the case is shown. The filter used for data thinning is a cosine roll-off filter with a roll-off rate of 0.414.

  FIG. 5 is a diagram illustrating error characteristics when the thinning process of Example 1 is performed. FIG. 5 shows errors with respect to eight types of tap lengths from 3 to 10 with respect to the case where IDFT processing is performed according to the equation (5). The horizontal axis represents the IDFT point number N (number of input signal points). Here, the number of IFFT points (M) is a minimum power of 2 greater than N. As shown in FIG. 5, by setting the tap length to 6 or more, the error can be suppressed to −40 dBc or less.

  FIG. 6 is a diagram illustrating error characteristics when the thinning process of Example 2 is performed. FIG. 6 shows errors with respect to the case where the tap length is fixed to 6 and the degree L of the polynomial is 5 types from 1 to 5, respectively, when the IDFT processing is performed according to the equation (5). As shown in FIG. 6, the error can be suppressed to -40 dBc or less by setting the degree of the polynomial to 3 or more.

  Increasing the tap length and the polynomial order reduces the error, but increases the amount of processing. For this reason, the minimum tap length and the order of the polynomial may be selected so as to be less than the error allowed by the system.

  Note that the thinning-out processing method (conversion method from M points to N points) described in this embodiment is merely an example, and any method may be used without being limited to the above method.

  Further, when the ratio of the number of input points N of the IDFT unit 4 and the number of input points M of the IFFT unit 12 (hereinafter referred to as conversion ratio) is the same (for example, N = 1200, M = 2048 and N = 30, M = Since the same tap coefficient can be used in the 512 case), the size of the tap coefficient table can be reduced by holding the tap coefficient for each conversion ratio.

  Even when the conversion ratios are different, the conversion ratio denominator (M is the denominator and N is the numerator) is the same (for example, N = 1200, M = 2048 (2048/1200 = 384/225). In the case of N = 900 and M = 2048 (2048/900 = 512/225), the same tap coefficient can be used. Therefore, the tap coefficient table can be reduced in size by sharing the tap coefficient table for those having the same conversion ratio denominator.

  Further, in the present embodiment, the IFFT point number M is the minimum power of 2 that is larger than the point number N of the input signal to the IDFT unit 4. As a result, the number of points of IFFT processing can be minimized and the amount of processing can be reduced. However, the number of IFFT points M is not limited to this, and any number that is a power of 2 greater than N can be used. It is good also as such a value. For example, the IFFT point number M may be a minimum power of 2 greater than the number of points of the input signal to the maximum IDFT unit 4 assumed in the system (for example, the number of subcarriers to which the maximum data is allocated). In this case, even when a plurality of N values are assumed, only one IFFT circuit is required, and the circuit scale can be reduced.

  In the above example, the case of performing IDFT for converting from the frequency domain to the time domain has been described. However, for the DFT for converting from the time domain to the frequency domain, the IFFT unit 12 is replaced with the FFT unit, and the data thinning process is performed. By setting the filter as a filter that performs filtering in the time domain, it is possible to process input signals other than powers of 2 by similarly performing zero insertion and thinning.

  In this embodiment, the LTE uplink has been described as an example. However, the present invention is not limited to this, and for all systems that perform DFT or IDFT that have a signal having a number of points that is not limited to a power-of-two point. Processing similar to that in this embodiment can be applied.

  In this way, when an N-point input signal is input to the IDFT unit 4, the 0 insertion unit 11 inserts 0 into the input signal to generate an M-point signal, and the IFFT unit 12 receives M points for the signal. The data thinning unit 13 converts the IFFT-processed signal into N-point data. Therefore, inverse Fourier transform can be performed at high speed without increasing the circuit scale for data of any number of data points that are not necessarily power-of-two points.

  Further, even when a plurality of types of data having different numbers of input data points are processed, if the minimum power of 2 exceeding N is equal, the IFFT unit 12 is connected to one IFFT circuit within the same range. The circuit scale can be reduced. Further, if M is the minimum power of 2 greater than the number of points of the input signal to the maximum IDFT unit 4 assumed in the system, the IFFT unit 12 even when the minimum power of 2 exceeding N is not equal. Can be realized with one IFFT circuit, and the circuit scale can be reduced.

  As described above, the Fourier transform circuit, the receiving apparatus, and the Fourier transform method according to the present invention are useful for a system that performs a Fourier transform that does not limit the number of data points to be subjected to Fourier transform other than a power of two. This is suitable for a system in which a plurality of data points are input to the conversion process.

DESCRIPTION OF SYMBOLS 1 Reception antenna 2 FFT part 3 SC-FDMA demodulation part 4 IDFT part 5 LLR calculation part 6 Error correction part 11 0 insertion part 12 IFFT part 13 Data thinning part

Claims (8)

A Fourier transform circuit for performing IDFT processing,
Insertion of data with a value of 0 before and after the input data for N (N is a positive integer) point input data to generate an M-point data string that is a power of 2 greater than N Means,
IFFT means for performing M-point IFFT processing on the data string;
Conversion means for converting the M-point data string after the IFFT processing into an N-point output data string;
Equipped with a,
The converting means includes
A filter with a predetermined number of taps in which tap coefficients are set so as to perform filtering with a filter function that passes the region corresponding to the input data and removes the region corresponding to the aliasing component of the input data;
With
When the index of the output data string is n, m and p satisfying M × n / N = m + p (m is a positive integer and p is a number of 1 or less) are obtained for each n.
The Fourier transform circuit , wherein the filter performs the filtering by setting a tap coefficient corresponding to a delay filter function obtained by shifting the input data by m points and delaying the filter function by p . A Fourier transform circuit for performing DFT processing,
Insertion of data with a value of 0 before and after the input data for N (N is a positive integer) point input data to generate an M-point data string that is a power of 2 greater than N Means,
FFT processing means for performing M point FFT processing on the data string;
Conversion means for converting the M-point data string after the FFT processing into an N-point output data string;
Equipped with a,
The converting means includes
A filter with a predetermined number of taps in which tap coefficients are set so as to perform filtering with a filter function that passes the region corresponding to the input data and removes the region corresponding to the aliasing component of the input data;
With
When the index of the output data string is n, m and p satisfying M × n / N = m + p (m is a positive integer and p is a number of 1 or less) are obtained for each n.
The Fourier transform circuit , wherein the filter performs the filtering by setting a tap coefficient corresponding to a delay filter function obtained by shifting the input data by m points and delaying the filter function by p . The converting means includes
Holding the tap coefficient for each assumed p, and setting the held tap coefficient according to p,
The Fourier transform circuit according to claim 1 or 2 . The converting means includes
Holding a coefficient of the polynomial when the tap coefficient for generating the filter function is approximated by a polynomial related to p, and setting the tap coefficient according to p using the held coefficient.
The Fourier transform circuit according to claim 1 or 2 . Let M be set every N, and let M be the smallest power of 2 greater than N.
The Fourier transform circuit according to any one of claims 1 to 4 . Let M be the smallest power of 2 greater than the assumed maximum value of N.
The Fourier transform circuit according to any one of claims 1 to 4 . A receiving device for receiving a signal using a plurality of subcarriers,
The Fourier transform circuit according to claim 1,
With
N is the number of subcarriers to which data is assigned,
A receiving apparatus. A Fourier transform method in a Fourier transform circuit that performs IDFT processing,
Insertion of data with a value of 0 before and after the input data for N (N is a positive integer) point input data to generate an M-point data string that is a power of 2 greater than N Steps,
IFFT step for performing M-point IFFT processing on the data string;
A conversion step of converting the M-point data string after the IFFT processing into an N-point output data string;
Only including,
The converting step includes
A filtering step of performing filtering with a filter function that passes a region corresponding to the input data and removes a region corresponding to an aliasing component of the input data;
With
In the conversion step, when n is an index of the output data string, m and p satisfying M × n / N = m + p (m is a positive integer and p is a number of 1 or less) for each n. In the filtering step, the filtering is performed by setting the tap coefficient corresponding to a delay filter function obtained by shifting the input data by m points and delaying the filter function by p. .

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