Down-converter device

Abstract

This down-converter device (21, 22, 23) comprises a band limiting filter (32, 43, 53) and a Farrow structure filter (33, 44, 54). The band limiting filter (32, 43, 53) suppresses the frequency bands of a portion of an input signal. The Farrow structure filter (33, 44, 54) converts the sample rate of a signal output from the band limiting filter (32, 43, 53). The band limiting filter (32, 43, 53) suppresses at least a portion of the frequency bands excluding a frequency band to be extracted.

Description

Downconverter Device

[0001] The present invention relates to a downconverter device.

[0002] In digital wireless communication systems, it is practically difficult to directly digitally process signals received in the radio frequency (RF) band. This is because real-time processing at a high sample rate is required. For this reason, signals received by an analog-to-digital converter (ADC) are downconverted by a downconverter and converted to baseband signals for processing. By downconverting the received signals in this manner, the processing speed required for digital signal processing is reduced, making real-time processing possible.

[0003] FIG. 10 is a block diagram showing a schematic functional configuration of a communication device according to the prior art. As shown in the figure, the communication device 1001 according to the prior art is configured to include the functions of an ADC unit 1011, a downconverter device 1002, and a demodulator 1015. The downconverter device 1002 is configured to include a frequency converter 1012, a decimator 103, and a band-limiting filter 1014. In this configuration, the ADC unit 1011 (A / D converter) quantizes a received signal and converts it into a digital signal. The frequency converter 1012 converts the frequency by applying phase rotation to the digital signal passed from the ADC unit 1011 in accordance with an externally provided frequency. The decimator 1013 performs decimation in accordance with an externally provided decimation ratio and converts it to the processing rate of the demodulator 1015. The band-limiting filter 1014 performs band-limiting within the band of the received carrier. Demodulation section 1015 demodulates the signal downconverted by downconverter device 1002. That is, in this configuration, downconverter device 1002 is arranged to extract a signal in a specific transmission band.

[0004] In the prior art, in order to keep the stopband of the digital filter of the band-limiting filter 1014 constant and fix the filter coefficients, the sample rate at the time of output from the decimation unit 1013 is set to a rate that is an integer multiple of the symbol rate. For example, when outputting at a double oversampling rate, if a 100 baud signal is to be processed, the signal is decimated to a sample rate of 200 sps (samples per second). Alternatively, for example, a 1000 baud signal is decimated to 2000 sps.

[0005] Non-Patent Document 1 describes a Farrow structure filter as one means for realizing the decimation unit 1013. The Farrow structure filter can change the sample rate of the received signal to an arbitrary sample rate. In other words, the Farrow structure filter can perform resampling at a real ratio. Furthermore, by using the Farrow structure filter, the circuit scale can be made smaller than that of a general decimation filter.

[0006] Non-Patent Document 2 describes a downconverter device that utilizes this property of the Farrow structure filter to achieve sample rate conversion of any rate.

[0007] That is, in a conventional digital wireless communication system, the ADC unit 1011 down-converts the received signal and converts it into a baseband signal for signal processing. The down-conversion process comprises resampling and frequency conversion. This conventional method allows for flexible resampling regardless of the sample rate of the ADC unit 1011, making it possible to achieve carrier extraction with a high degree of freedom.

[0008] CW Farrow, "A continuously variable digital delay element," 1988., IEEE International Symposium on Circuits and Systems, Espoo, Finland, pp. 2641-2645 vol.3, doi: 10.1109 / ISCAS.1988.15483.X. Liu, X. -X. Yan, Z. -K. Wang and Q. -X. Deng, "Design and FPGA Implementation of a Reconfigurable Digital Down Converter for Wideband Applications," in IEEE Transactions on Very Large Scale Integration (VLSI) Systems, vol. 25, no. 12, pp. 3548-3552, Dec. 2017, doi: 10.1109 / TVLSI.2017.2748603.

[0009] The Farrow structure filter according to the above-mentioned prior art is a linear time-varying system and has nonlinear characteristics. Therefore, the Farrow structure filter has a problem in that signals of a certain frequency are mixed with signals of other frequencies, causing out-of-band leakage. In an environment where strong signal components or noise components exist in the transmission band, the effect of this leakage becomes apparent and communication quality may deteriorate. An example of out-of-band leakage caused by a Farrow structure filter is described below.

[0010] 11 and 12 are graphs showing examples of out-of-band leakage from a conventional Farrow structure filter, where the horizontal axis represents frequency and the vertical axis represents amplitude.

[0011] The graph in Fig. 11 shows the spectrum of a signal received at a sample rate of 400 ksps (immediately after reception by the ADC unit 1011 in Fig. 10). In this figure, signals exist in transmission band 1 and transmission band 2. This figure shows graphs of spectrum 1 and spectrum 2. However, in bands other than transmission band 1 and transmission band 2, there are many places where spectrum 1 and spectrum 2 overlap near -120 [dBV / 6.99 Hz].

[0012] The graph in Figure 12 shows the spectrum of a signal after conversion to 352 ksps using a Farrow structure filter. In other words, this graph shows the spectrum of a signal immediately after integer ratio oversampling rate conversion. In this graph, it can be seen that the signal in transmission band 2 leaks into transmission band 1. Furthermore, the level of spectrum 1 is high across the entire frequency band. In the situation shown in this graph, the signal in spectrum 1 is interference in transmission band 1, and the ratio of the interference signal to the desired signal (DU ratio, Desired to Undesired signal ratio) is degraded. The deterioration of the DU ratio degrades the demodulation performance of demodulation unit 1015 (Figure 10).

[0013] In other words, when a Farrow filter is used, the nonlinear characteristics of the Farrow filter cause some signal components to leak into other bands, resulting in a deterioration of the DUR characteristics. When other systems use other frequencies or when the system in question uses multiple frequency bands (FDD / FDMA system), the above-mentioned problem becomes apparent due to the shared use of multiple frequency bands.

[0014] The present invention has been made in consideration of the above circumstances, and aims to provide a downconverter device that can reduce the deterioration of the DU ratio caused by the nonlinear characteristics of a Farrow structure filter.

[0015] [1] In order to solve the above problem, a downconverter device according to one aspect of the present invention comprises a band-limiting filter that suppresses a part of the frequency band of an input signal, and a Farrow structure filter that converts the sample rate of the signal output from the band-limiting filter, wherein the band-limiting filter suppresses at least a part of the frequency band excluding the frequency band to be extracted.

[0016] [2] Furthermore, one aspect of the present invention is a downconverter device according to the above [1], further comprising a decimation unit that includes one or more linear time-invariant systems and converts the sample rate of an input signal and outputs the converted signal, the decimation unit being provided in either or both of the upstream and downstream stages of the band-limiting filter.

[0017] [3] Furthermore, one aspect of the present invention is a downconverter device according to the above [1] or [2], further comprising a frequency conversion unit that converts the frequency of a signal input to the downconverter device and passes the converted frequency signal to a subsequent stage.

[0018] [4] Furthermore, one aspect of the present invention is that in any of the downconverter devices [1] to [3] above, the band-limiting filter has a passband that is the frequency band of the carrier to be extracted, and a stopband that is the band of the nearest adjacent carrier on the high-frequency side and the nearest adjacent carrier on the low-frequency side of the carrier, and a band that is farther away from the frequency band of the carrier than the band of the adjacent carrier.

[0019] According to the present invention, a band-limiting filter suppresses components in a specific frequency band before applying a Farrow structure filter, which makes it possible to reduce the deterioration of the desired-to-undesired signal ratio (DU ratio) caused by the nonlinear characteristics of the Farrow structure filter.

[0020] 10 is a block diagram showing a schematic functional configuration of a communication device (including a downconverter device) according to a first embodiment of the present invention. FIG. 11 is a schematic diagram (graph) showing a spectrum immediately before passing through a band-limiting filter in the first embodiment. FIG. 12 is a schematic diagram (graph) showing a spectrum after passing through the band-limiting filter in the first embodiment. FIG. 13 is a schematic diagram (graph) showing the frequency characteristic (gain design) of the band-limiting filter 32 in the first embodiment. FIG. 14 is a block diagram showing a schematic functional configuration of a communication device (including a downconverter device) according to a second embodiment. FIG. 15 is a block diagram showing an example of the internal functional configuration of a pre-decimation unit in the second embodiment. FIG. 16 is a block diagram showing a schematic functional configuration of a communication device (including a downconverter device) according to a third embodiment. FIG. 17 is a block diagram showing an example of the internal functional configuration of a post-decimation unit in the third embodiment. FIG. 18 is a block diagram showing an example of the internal configuration of a computer that can be used to realize the first to third embodiments. FIG. 19 is a block diagram showing a schematic functional configuration of a communication device according to the prior art. FIG. 19 is a schematic diagram showing the spectrum of a signal received by a communication device according to the prior art (immediately after reception in the ADC unit of FIG. 10 ). 1 is a schematic diagram showing the spectrum of a signal after converting the signal to 352 ksps using a Farrow structure filter in the prior art, illustrating an example of out-of-band leakage due to the Farrow structure filter;

[0021] Hereinafter, several embodiments of the present invention will be described with reference to the drawings. In the embodiments described below, signal components in a band around a carrier to be extracted are removed by a band-limiting filter before applying a Farrow structure filter. As a result, the configuration of each embodiment reduces degradation of DUR characteristics due to the nonlinear characteristics of the Farrow structure filter.

[0022] 1 is a block diagram showing a schematic functional configuration of a communication device according to a first embodiment. As shown in the figure, the communication device 1 according to this embodiment includes an ADC unit 11, a downconverter device 21, and a demodulator 15. The downconverter device 21 also includes a frequency conversion unit 31, a band-limiting filter 32, and a Farrow structure filter 33. A feature of this embodiment is that the Farrow structure filter 33 is provided after the band-limiting filter 32.

[0023] The ADC unit 11 (Analog-to-Digital Converter) quantizes the received signal and converts it into a digital signal. The ADC unit 11 passes the digitally converted signal to the frequency conversion unit 31.

[0024] The frequency conversion unit 31 converts the frequency of the digital signal passed from the ADC unit 11 by applying a phase rotation to the digital signal according to an externally applied frequency. The frequency conversion unit 31 passes the frequency-converted signal to the band-limiting filter 32. In other words, the frequency conversion unit 31 converts the frequency of the signal input to the downconverter device 21 and passes the signal with the converted frequency to the subsequent stage.

[0025] The band-limiting filter 32 performs band-limiting based on a filter coefficient provided from the outside. That is, the band-limiting filter 32 limits the frequency bands other than the carrier of the signal passed from the frequency conversion unit 31 in the downconverter device 21 based on the filter coefficient. The band-limiting filter 32 passes the signal after band-limiting to the Farrow structure filter 33. That is, the band-limiting filter 32 suppresses a portion of the frequency band of the input signal and passes the result to the Farrow structure filter 33 in the subsequent stage. The band-limiting filter 32 suppresses at least a portion of the frequency band excluding the frequency band that is the target of extraction by the downconverter device 21.

[0026] The farrow structure filter 33 converts the sample rate in accordance with an externally applied decimation ratio. That is, the farrow structure filter 33 converts the sample rate of the signal passed from the band-limiting filter 32 to the signal processing rate of the demodulation unit 15. That is, the farrow structure filter 33 converts the sample rate of the signal output from the band-limiting filter and passes it to the demodulation unit 15 at the subsequent stage.

[0027] The demodulation unit 15 demodulates the signal down-converted by the down-converter device 21 .

[0028] That is, in the downconverter device 21 of this embodiment, the band-limiting filter 32 suppresses at least a part of the frequency band excluding the frequency band that is the extraction target of the downconverter device 21. Therefore, when the Farrow structure filter 33 converts the sample rate in accordance with a given decimation ratio, it is possible to reduce deterioration of the DU ratio due to the nonlinear characteristics of the Farrow structure filter 33.

[0029] The operation of the band limiting filter 32 will now be described in more detail with reference to FIGS.

[0030] 2 is a schematic diagram (graph) showing a spectrum immediately before passing through the band-limiting filter 32. In this diagram, the horizontal axis corresponds to frequency, and the vertical axis corresponds to signal strength. Note that the frequency on the horizontal axis is a normalized frequency, with +1 corresponding to 1 / 2 the sample rate at the time of input to the band-limiting filter 32 (i.e., a phase of π). The visible range is from -1 to +1 on the horizontal axis.

[0031] In the figure, carriers in three regions are shown by solid lines. The carrier in the center, whose normalized frequency is near 0, is the carrier extracted by the downconverter device 21. Carrier A shown on the positive side and carrier B shown on the negative side are adjacent carriers. Adjacent carriers A and B are, for example, signals transmitted by other systems. The frequency characteristics of the band-limiting filter 32 are shown by dashed lines. The bandwidth of this band-limiting filter 32 can be set. In the illustrated situation, the band-limiting filter 32 suppresses adjacent carriers A and B.

[0032] 3 is a schematic diagram (graph) showing the spectrum after passing through the band-limiting filter 32. In this diagram, as in FIG. 2, the horizontal axis corresponds to frequency and the vertical axis corresponds to signal strength. In the spectrum shown in this diagram, adjacent carriers A and B are suppressed by the action of the band-limiting filter 32. In other words, the energy of adjacent carriers A and B is sufficiently lower than the energy of the carrier to be extracted by the downconverter device 21.

[0033] FIG. 4 is a schematic diagram (graph) showing the frequency characteristics (gain design) of the band-limiting filter 32. In this figure, as in FIGS. 2 and 3, the horizontal axis corresponds to frequency and the vertical axis corresponds to signal strength. In this figure, a broken line 301 represents the frequency characteristics of the band-limiting filter 32. This broken line 301 has a polygonal line shape in the normalized frequency range from -1 to +1. In other words, the frequency characteristics of the band-limiting filter 32 are expressed as a function of frequency -(1 / R １ ) -α to +(1 / R １ The signal passband is between frequencies -1 and f + α. The carrier to be extracted by the downconverter device 21 is present within this passband. ｒ２ and frequency f ｒ１ The range from f to +1 is the stop band of the band limiting filter 32. The adjacent carrier A shown in the figure is a signal that exists in this stop band and is suppressed by the band limiting filter 32. The range between the pass band and the stop band is the transition band. That is, the frequency f ｒ２ From -(1 / R １ ) -α and frequency + (1 / R １ ) + α to f ｒ１ The frequency characteristics change linearly between

[0034] That is, the band-limiting filter 32 sets the frequency band of the carrier to be extracted (the carrier shown as "carrier to be extracted by the down converter" in FIG. 4) as the passband. The band-limiting filter 32 also sets at least the bands of the nearest adjacent carriers (if such adjacent carriers exist) on the high-frequency side and the low-frequency side of the carrier, and the bands farther away from the frequency band of the carrier than the adjacent carriers. That is, in the case of FIG. 4, the frequency f ｒ１ 4 does not show the nearest adjacent carrier on the low frequency side of the carrier itself, but even if an adjacent carrier exists on the low frequency side of the carrier itself, the band farther from the frequency band of the adjacent carrier from the frequency band of the carrier itself may be set as the stop band.

[0035] 2, 3, and 4, in this embodiment, the band-limiting filter 32 is provided in the upstream stage of the farrow structure filter 33. That is, before input to the farrow structure filter 33, the band-limiting filter 32 suppresses adjacent carriers (adjacent carrier A and adjacent carrier B shown in FIG. 3). This makes it possible to reduce the influence of band leakage due to nonlinear distortion from adjacent carriers. In other words, the farrow structure filter 33 also reduces adjacent carrier interference. This improves the communication quality in communications performed using the frequency of the adjacent carrier (corresponding to transmission band 1 in FIG. 11).

[0036] Second Embodiment Next, a second embodiment of the present invention will be described. Note that the description of the matters already described in the previous embodiment may be omitted. Here, the description will focus on matters unique to this embodiment.

[0037] The second embodiment realizes more flexible sample rate conversion than the first embodiment.

[0038] 5 is a block diagram showing a schematic functional configuration of a communication device according to the second embodiment. As shown in the figure, the communication device 2 according to this embodiment includes an ADC unit 11, a downconverter device 22, and a demodulator 15. The downconverter device 22 also includes a frequency converter 41, a pre-decimator 42, a band-limiting filter 43, and a Farrow-structure filter 44.

[0039] The ADC unit 11, frequency conversion unit 41, band-limiting filter 43, Farrow structure filter 44, and demodulation unit 15 in the second embodiment have the same functions as the ADC unit 11, frequency conversion unit 31, band-limiting filter 32, Farrow structure filter 33, and demodulation unit 15 in the first embodiment ( FIG. 1 ), respectively. A feature of this embodiment is that a pre-decimation unit 42 is provided after the frequency conversion unit 41 and before the band-limiting filter 43.

[0040] The pre-decimation unit 42 is a linear time-invariant system. The pre-decimation unit 42 performs decimation in accordance with an externally provided decimation ratio A. In other words, the pre-decimation unit 42 includes one or more linear time-invariant systems, and converts the sample rate of an input signal and outputs the converted signal. Note that the pre-decimation unit 42 in this embodiment is provided in the stage preceding the band-limiting filter 43. Note that the pre-decimation unit 42 may also be simply referred to as a "decimation unit."

[0041] 6 is a block diagram showing an example of the internal functional configuration of the pre-decimation unit 42. As shown in the figure, the pre-decimation unit 42 is composed of, for example, an FIR filter 421, a CIC filter 422, a polyphase filter 423, a downsampling unit 424, and an upsampling unit 425.

[0042] The pre-decimation unit 42 may be configured with one or more FIR filters, a CIC filter, a polyphase filter, a downsampling unit, and an upsampling unit, as in the example shown in Fig. 6. Fig. 6 shows an example in which the processing order is the FIR filter 421, the CIC filter 422, the polyphase filter 423, the downsampling unit 424, and the upsampling unit 425. However, the processing order within the pre-decimation unit 42 does not necessarily have to be this order.

[0043] The pre-decimation unit 42 performs decimation using a pre-arranged linear time-invariant system. The decimation ratio in the processing of the pre-decimation unit 42 is the decimation ratio A given from the outside.

[0044] The band limiting filter 43 performs band limiting in accordance with an externally provided filter coefficient. This band limiting prevents interference from adjacent carriers from occurring in the downstream Farrow structure filter 44.

[0045] The Farrow structure filter 44 performs sample rate conversion in accordance with a decimation ratio B given from the outside.

[0046] The product of the decimation ratio A in the pre-decimation unit 42 and the decimation ratio B in the Farrow structure filter 44 is the total decimation ratio of the downconverter device 22. This value of decimation ratio A x decimation ratio B is adjusted to a sample rate that can be processed by the demodulation unit 15.

[0047] [Third Embodiment] Next, a third embodiment of the present invention will be described. Note that the following description may omit the matters already described in the previous embodiments. Here, the description will focus on matters unique to this embodiment.

[0048] The third embodiment realizes more flexible sample rate conversion than the second embodiment.

[0049] 7 is a block diagram showing a schematic functional configuration of a communication device according to a third embodiment. As shown in the figure, the communication device 3 according to this embodiment includes an ADC unit 11, a downconverter device 23, and a demodulator 15. The downconverter device 23 also includes a frequency converter 51, a pre-decimator 52, a band-limiting filter 53, a Farrow structure filter 54, and a post-decimator 55.

[0050] The ADC unit 11, frequency conversion unit 51, pre-decimation unit 52, band-limiting filter 53, Farrow structure filter 54, and demodulation unit 15 in the third embodiment have the same functions as the ADC unit 11, frequency conversion unit 41, pre-decimation unit 42, band-limiting filter 43, Farrow structure filter 44, and demodulation unit 15 in the second embodiment ( FIG. 5 ), respectively. A feature of this embodiment is that the downconverter device 23 is provided with a post-decimation unit 55 in the stage subsequent to the Farrow structure filter 54.

[0051] The post-decimation unit 55 is a linear time-invariant system that performs decimation according to a decimation ratio C given from the outside.

[0052] That is, like the pre-decimation unit 52, the post-decimation unit 55 includes one or more linear time-invariant systems, and converts the sample rate of the input signal and outputs the converted signal. In this embodiment, the pre-decimation unit 52 is provided before the band-limiting filter 53. The post-decimation unit 55 is provided after the band-limiting filter 53. Each of the pre-decimation unit 52 and the post-decimation unit 55 may be simply referred to as a "decimation unit."

[0053] 8 is a block diagram showing an example of the internal functional configuration of the post-decimation unit 55. As shown in the figure, the post-decimation unit 55 is composed of, for example, an FIR filter 551, a CIC filter 552, a polyphase filter 553, a downsampling unit 554, and an upsampling unit 555.

[0054] The post-decimation unit 55 may be configured with one or more FIR filters, a CIC filter, a polyphase filter, a downsampling unit, and an upsampling unit, as in the example shown in Fig. 8. Fig. 8 shows an example in which the processing order is the FIR filter 551, the CIC filter 552, the polyphase filter 553, the downsampling unit 554, and the upsampling unit 555. However, the processing order within the post-decimation unit 55 does not necessarily have to be this order.

[0055] Alternatively, post-decimation unit 55 may be a matched filter.

[0056] The post-decimation unit 55 performs decimation using a pre-arranged linear time-invariant system. The decimation ratio in the processing of the post-decimation unit 55 is the decimation ratio C given from the outside.

[0057] In the third embodiment, the band-limiting filter 53 performs band-limiting in accordance with an externally provided filter coefficient. This band-limiting prevents interference from adjacent carriers from occurring in the downstream Farrow structure filter 54.

[0058] The Farrow structure filter 54 performs sample rate conversion in accordance with a decimation ratio B given from the outside.

[0059] The product of the decimation ratio A in the pre-decimation unit 52, the decimation ratio B in the Farrow structure filter 54, and the decimation ratio C in the post-decimation unit 55 is the total decimation ratio of the downconverter device 23. This value of decimation ratio A × decimation ratio B × decimation ratio C is adjusted to a sample rate that can be processed by the demodulation unit 15.

[0060] [Method of Setting the Coefficients of the Band-Limiting Filter] Here, a method of setting the coefficients of the band-limiting filter, which is common to the first to third embodiments, will be described. The band-limiting filters (32, 43, 53) of each embodiment perform band-limiting based on a band containing a carrier extracted by the downconverter device (21, 22, 23). The oversampling rate immediately before passing through the band-limiting filter (32, 43, 53) is set to R １ Let the rate R １ is the value obtained by dividing the sample rate immediately before passing through the band-limiting filter by the symbol rate of the carrier to be extracted.

[0061] As shown in FIG. 4, in this case, the carrier to be extracted is the normalized frequency −(1 / R １ ) - α [×π radian / sample] to + (1 / R １ ) + α [×π radian / sample], where α is the value obtained by converting the roll-off rate and maximum Doppler frequency into normalized frequencies and adding them together.

[0062] The nearest adjacent carrier is +f ｒ１ [×π radian / sample], the normalized frequency −(1 / R １ ) -α to +(1 / R １ ) + α is the pass band of the band limiting filters (32, 43, 53). １ ) + α to + f ｒ１ The transition band with gain attenuation is up to the normalized frequency + f ｒ１ This allows the suppression of adjacent carriers without attenuating the carrier itself.

[0063] In the example shown in FIG. 4, the frequency f ｒ２ The frequency characteristic of the band-limiting filter is set based on the above. ｒ２ = -f ｒ１ Alternatively, f ｒ２ may be the frequency at which the nearest adjacent carrier on the lower frequency side resides.

[0064] [Modification: Configuration without Frequency Conversion Unit] As a modification common to the first to third embodiments, the downconverter devices (21, 22, 23) may be configured without the frequency conversion units (31, 41, 51). This is the case, for example, in a low-frequency acoustic communication system, where the frequency of the original transmission band is low and frequency conversion is not required. In other words, when applied to a low-frequency acoustic communication system, the frequency conversion units (31, 41, 51) may be omitted.

[0065] [Variation: Arrangement of Decimation Unit] In the second embodiment, the pre-decimation unit 42 is provided in the stage preceding the band-limiting filter 43, and no decimation unit is provided in the stage following the band-limiting filter 43. In the third embodiment, the pre-decimation unit 52 is provided in the stage preceding the band-limiting filter 53, and the post-decimation unit 55 is provided in the stage following the band-limiting filter 53. As a variation of these, a decimation unit may be provided only in the stage following the band-limiting filter. In other words, the decimation unit may be provided in either or both of the stage preceding and the stage following the band-limiting filter.

[0066] [Possible Configurations of the Downconverter Device] A number of embodiments and their modified examples have been described above. That is, as an embodiment, the downconverter device includes a band-limiting filter and a Farrow structure filter provided downstream of the band-limiting filter. The decimation unit may be provided upstream or downstream of the band-limiting filter, or both. Furthermore, a frequency conversion unit may be provided at the beginning of processing by the downconverter device, which converts the frequency of a signal input to the downconverter device and passes the converted frequency signal to the downstream side.

[0067] [Realization by Computer and Program] The communication devices 1, 2, and 3 according to the first to third embodiments (including modified examples) can also be realized using a computer and a program. The downconverter devices 21, 22, and 23 included in each of the communication devices 1, 2, and 3 can also be realized using a computer and a program. The program can be recorded on a recording medium or provided via a communication network.

[0068] FIG. 9 is a block diagram showing an example of the internal configuration of a computer that can be used to realize the above-described embodiments. In each of the first to third embodiments (including modified examples), at least a portion of the communication device or at least a portion of the downconverter device can be realized using a computer. As shown in the figure, the computer includes a central processing unit 901, a RAM 902, an input / output port 903, input / output devices 904 and 905, and a bus 906. The computer itself can be realized using existing technology. The central processing unit 901 executes instructions contained in a program read from the RAM 902 or the like. In accordance with each instruction, the central processing unit 901 writes data to the RAM 902, reads data from the RAM 902, and performs arithmetic and logical operations. The RAM 902 stores data and programs. Each element included in the RAM 902 has an address and can be accessed using the address. RAM stands for "random access memory." The input / output port 903 is a port through which the central processing unit 901 exchanges data with external input / output devices, etc. Input / output devices 904 and 905 exchange data with the central processing unit 901 via an input / output port 903. A bus 906 is a common communication path used within the computer. For example, the central processing unit 901 reads and writes data from / to the RAM 902 via the bus 906. Also, for example, the central processing unit 901 accesses the input / output port 903 via the bus 906.

[0069] At least some of the functions of the communication devices 1, 2, and 3, or at least some of the functions of the downconverter devices 21, 22, and 23, in the above-described embodiments, can be implemented by a computer and a program. In this case, the program for implementing this function may be recorded on a computer-readable recording medium, and the program recorded on the recording medium may be loaded into a computer system and executed. Note that the term "computer system" as used herein includes hardware such as an OS and peripheral devices. Furthermore, the term "computer-readable recording medium" refers to portable media such as flexible disks, optical magnetic disks, ROMs, CD-ROMs, DVD-ROMs, and USB memory, as well as storage devices such as hard disks built into computer systems. In other words, a "computer-readable recording medium" may be a non-transitory computer-readable recording medium. Furthermore, the term "computer-readable recording medium" may also include media that temporarily and dynamically store programs, such as communication lines used when transmitting programs via networks such as the Internet or communication lines such as telephone lines, or media that store programs for a certain period of time, such as volatile memory within the computer systems that serve as the server or client in such cases. Furthermore, the above program may be one that realizes part of the functions described above, or may be one that can realize the functions described above in combination with a program already recorded in the computer system.

[0070] As described above, according to the embodiment (including the modified example), the band-limiting filter suppresses components of a specific frequency band before the application of the Farrow structure filter. This process makes it possible to reduce the deterioration of the desired-to-undesired signal ratio (DU ratio) due to the nonlinear characteristics of the Farrow structure filter. In other words, it is possible to prevent interference due to leakage into other bands.

[0071] The present invention can be used in, for example, a communication device, but the scope of use of the present invention is not limited to the example given here.

[0072] 1, 2, 3 Communication device 11 ADC unit 15 Demodulation unit 21, 22, 23 Downconverter device 31 Frequency conversion unit 32 Band-limiting filter 33 Farrow structure filter 41 Frequency conversion unit 42 Pre-decimation unit (decimation unit) 43 Band-limiting filter 44 Farrow structure filter 51 Frequency conversion unit 52 Pre-decimation unit (decimation unit) 53 Band-limiting filter 54 Farrow structure filter 55 Post-decimation unit (decimation unit) 421 FIR filter 422 CIC filter 423 Polyphase filter 424 Downsampling unit 425 Upsampling unit 551 FIR filter 552 CIC filter 553 Polyphase filter 554 Downsampling unit 555 Upsampling unit 901 Central processing unit 902 RAM 903 Input / output port 904, 905 Input / output device 906 Bus

Claims

1. A downconverter device comprising: a band-limiting filter that suppresses a portion of the frequency band of an input signal; and a Farrow structure filter that converts the sample rate of the signal output from the band-limiting filter, wherein the band-limiting filter suppresses at least a portion of the frequency band excluding the frequency band to be extracted.

2. The downconverter device according to claim 1, further comprising a decimation unit that includes one or more linear time-invariant systems and converts the sample rate of an input signal and outputs the converted signal, wherein the decimation unit is provided either before or after the band-limiting filter, or both.

3. The downconverter device according to claim 1, further comprising a frequency conversion section that converts the frequency of a signal input to the downconverter device and passes the converted signal to a subsequent stage.

4. A downconverter device as described in claim 1, wherein the band-limiting filter has a passband that is the frequency band of the carrier to be extracted, and a stopband that is the band of the nearest adjacent carrier on the high-frequency side and the nearest adjacent carrier on the low-frequency side of the carrier, and a band that is farther away from the frequency band of the carrier than the band of the adjacent carrier.

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