JP4286476B2 - Orthogonal frequency division multiplexing modulation receiver - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is a transmission apparatus using an Orthogonal Frequency Division Multiplexing (hereinafter referred to as OFDM), which transmits an information code using a plurality of carriers orthogonal to each other as a transmission method. The present invention also relates to a method for reproducing a reference signal in a demodulator of a transmission apparatus that modulates a plurality of carriers by a modulation scheme using synchronous detection (synchronous modulation scheme).
[0002]
[Prior art]
In recent years, in the field of wireless devices, the OFDM method has attracted attention as a modulation method strong against multipath fading, and has many applications in the fields of next-generation television broadcasting, FPU, wireless LAN, etc. in countries such as Europe and Japan. Research is ongoing. Of these, the UHF band terrestrial digital broadcasting system is described in the Journal of the Institute of Image Information and Television Engineers 1998 Vol. 52, no. 11 is described in detail.
However, the carrier structure of the terrestrial digital broadcasting system in the UHF band is very complicated, and the difference between the conventional method and the method of the present invention may be difficult to understand. Accordingly, a conventional method for reproducing a reference signal will be described by taking a method having a simplified carrier structure as an example.
As shown in FIG. 11, the synchronous modulation OFDM system is provided with N (for example, approximately 1400) carriers orthogonal to each other within a certain transmission bandwidth, and a specified carrier is modulated by an information code such as 64QAM. This is a modulation scheme that modulates and transmits the data.
FIG. 12 is an enlarged view of a part of the carrier structure, and it can be considered that the same structure is repeated over the entire transmission band.
In FIG. 12, the horizontal direction represents frequency, the vertical direction represents the passage of time, and square marks “□” arranged in the horizontal and vertical directions each represent one carrier. Accordingly, one column of square marks “□” arranged in the horizontal direction represents one symbol constituting the OFDM signal.
A square mark “□” written as CP indicates the position of the pilot signal used for reproducing the reference signal at the time of demodulation. Also, “□” where nothing is written represents a signal position modulated by 64QAM.
[0003]
In the UHF band terrestrial digital broadcasting system in Japan, the pilot signal is dispersed in the frequency direction and the time direction, and is arranged at the position of the square mark “□” written as SP as shown in FIG. Therefore, this pilot signal is named SP (Scattered Pilot). On the other hand, in the carrier structure of FIG. 12, since pilot signals are continuously inserted in the time direction, the continuity is emphasized and changed to CP (Continual Pilot). Here, a carrier having pilot signal CP or SP in the time direction is hereinafter referred to as “pilot carrier”.
FIG. 14 is a circuit diagram showing a part related to the present invention extracted from the circuits constituting the OFDM transmission apparatus. The information code input to the transmission preprocessing circuit 1 is converted into an error correction code, modulated into a 64QAM signal, and subjected to preprocessing such as insertion of a pilot signal CP according to FIG. For example, it is converted into a signal sequence of a frequency distribution image of a 2048 sample clock, which represents a signal sequence of carriers arranged in a horizontal row in FIG.
The converted signal sequence is input to an IFFT circuit 2 that performs 2048-point inverse Fourier transform (IFFT), and is converted into a signal sequence representing a time waveform of the same 2048 sample clock. FIG. 15 schematically shows a time waveform transmitted from the transmission apparatus, and the IFFT circuit 2 outputs a time waveform during the Ts period of the OFDM signal. The guard interval insertion circuit 3 is a circuit that copies and inserts the portion b in the time waveform of the period Ts into the portion b ′. In this way, the signal with the guard interval b ′ inserted is further subjected to post-processing such as quadrature modulation, D / A conversion, and up-conversion in the post-transmission processing circuit 4 and then transmitted from the antenna 5.
[0004]
FIG. 16 is a block diagram showing a part related to the present invention extracted from circuits constituting an OFDM receiver. The signal received by the antenna 6 is subjected to pre-processing such as down-conversion, A / D conversion, orthogonal demodulation and the like in the reception pre-processing circuit 7 and then input to the signal clipping circuit 8 and corresponds to the Ts period in FIG. A signal sequence of 2048 sample clocks is cut out. The extracted signal sequence is input to an FFT circuit 9 that performs a 2048-point Fourier transform (FFT), and the signal in the horizontal row of FIG. 12 is a signal sequence of a frequency distribution image continuously arranged in the order of frequency magnitude. Return to column.
By the way, in order to demodulate a signal modulated by 64QAM, as described in a general handbook, it is necessary to know the phase (direction) and magnitude of a reference signal corresponding to a rule in the signal space. The pilot signal CP in FIG. 12 is a signal inserted to enable reproduction of this reference signal.
In general, the phase and magnitude of a reference signal of a received signal are not constant, and are changed by each time or each carrier due to the influence of multipath generated in the transmission system. However, the method of change usually draws a smooth curve and has a strong correlation between the time direction and the carrier direction. Therefore, for example, the reference signal for the carrier 21 in FIG. 12 can be calculated by interpolating signals such as CP1 and CP2 which are a plurality of CPs in the same symbol arranged in the vicinity of the carrier in the carrier direction (frequency direction). Yes, 64QAM demodulation can be performed using the calculated reference signal.
[0005]
The carrier direction interpolation circuit 11 in FIG. 16 is a circuit that performs the interpolation calculation in the carrier direction. The signal sequence output from the FFT circuit 9 is input to the pilot signal selection circuit 10 and the delay circuit 12. The pilot signal selection circuit 10 takes out the CP inserted between the signal sequences output from the FFT circuit 9, converts the signal values of the other carriers to 0, and outputs them as symbol pilot signals.
The symbol pilot signal output from the pilot signal selection circuit 10 is interpolated in the carrier direction by the carrier direction interpolation circuit 11 and output as a regenerated reference signal. The delay circuit 12 is a circuit that applies time adjustment corresponding to the calculation time of the reference signal to the signal sequence output from the FFT circuit 9.
The reference signal output from the carrier direction interpolation circuit 11 and the signal sequence output from the FFT circuit 9 and time-adjusted by the delay circuit 12 are input to the demodulation & post-processing circuit 13, 64QAM demodulation, and demodulated code After post-processing such as error correction, it is output from the receiving device as a decoded information code.
[0006]
[Problems to be solved by the invention]
Incidentally, although not described in the above description, a complex filter having a large circuit scale and a complicated configuration is usually used for the carrier direction interpolation circuit 11 of FIG. Before describing the reason, the nature of the signal sequence input to the carrier direction interpolation circuit 11 will be briefly described.
Although a detailed description is omitted, when a delayed wave due to multipath is mixed in the received signal, the phase angle of each carrier of the direct wave component signal included in the signal sequence output from the FFT circuit 9 in FIG. Even if it is constant as shown in (a), the phase angle of each carrier of the delayed wave component signal mixed in the received signal becomes the carrier direction as the carrier number increases as shown in (b) of FIG. The phenomenon of rotating at a constant frequency occurs.
The rotational frequency changes with the delay time of the delayed wave. For example, if the number of FFT sample points is 2048, a rotation that rotates in the positive direction at a rate of n rotations per 2048 carriers occurs in the phase angle of each delayed wave carrier delayed by n sample clock periods with respect to the direct wave.
Accordingly, when the distribution of this rotational frequency is expressed with the carrier number as a time axis, there is no signal component received before the direct wave, so the rotational frequency of the phase angle is positive as shown by the hatched frame in FIG. The distribution appears only on the axis. In addition, when it is necessary to distinguish the frequency of rotation of the phase angle that rotates with the increase of the carrier number on the frequency axis of FIG. 17 from the frequency of vibration on the normal time axis, it is hereinafter referred to as “carrier direction frequency”. I write. The horizontal axis in FIG. 18 represents the carrier direction frequency as the number of rotations per 2048 sample clocks.
Here, the maximum carrier direction frequency to be supported is determined by the method of constructing the time direction signal of the received signal. The time waveform of one symbol of the OFDM signal in FIG. 15 is composed of a Ts ′ period obtained by adding the period of the guard interval b ′ to the signal of the Ts period output from the IFFT circuit 2. This guard interval b ′ is inserted in order to increase resistance to multipath fading. Although detailed description is omitted, this guard interval is set to N _GI If sample clocks are provided, in principle, the delay time of the mixed delay wave is this N _GI As long as it is within the sample clock time, it is possible to correctly demodulate the code. N _GI Is a positive integer.
[0007]
On the other hand, the carrier direction interpolation circuit 11 in FIG. 16 must be able to calculate the reference signal by correctly interpolating the delayed wave in the above delay time range. In other words, for example, when a guard interval of 64 sample clock periods is provided, it is necessary to be able to perform a correct interpolation operation for the rotational vibration in the hatched frame range of the carrier direction frequency 0 to 64 in FIG. .
Based on the above facts, the reason why a complex filter is used in the carrier direction interpolation circuit 11 of FIG. 16 will be described. In general, in order to perform an interpolation operation of a signal having a frequency distribution in the range of the hatched frame in FIG. 18, it is necessary to use a filter having the frequency range of the hatched frame in the pass region. Therefore, in the case of a normal digital LPF, it is necessary to use a digital LPF having the characteristics of the broken line 22 in FIG.
However, a filter with this characteristic allows a noise component in the negative range to pass through even if the carrier direction frequency is in the negative range, even though no signal is generated, so this noise is included in the reference signal obtained by interpolation. The S / N of the reference signal that is mixed and reproduced is greatly deteriorated. The S / N degradation amount reaches about 3 dB. In order to prevent the deterioration of the S / N, it is necessary to use a complex filter having only the frequency range of the hatched frame in FIG. 18 as the pass region.
By the way, in order to construct a complex filter, it is necessary to use four normal digital LPFs as shown in FIG. Moreover, in order to obtain the accuracy characteristics required for the interpolation operation, it is necessary to use a digital LPF having 63 to 127 taps or more as the number of taps of the digital LPF to be used.
In the current technology, one large IC of about 100 to 200 pins is required to configure one digital LPF having such a large number of taps. For this reason, four such ICs are required to construct a complex filter, which has the disadvantage of making the circuit of the receiving apparatus large and expensive.
The present invention eliminates these drawbacks, can reduce the circuit scale of the carrier direction interpolation circuit to almost half, and can obtain the same performance as that using a complex filter, that is, a circuit that can be configured in a small size and at low cost. Is to provide.
[0008]
[Means for Solving the Problems]
The first basic idea of the present invention is that the symbol pilot signal input to the carrier direction interpolation circuit is modulated in the carrier direction in advance, and the carrier direction frequency distribution signal in the range of the hatched frame in the upper stage of FIG. After shifting as in the lower row, the interpolation operation is performed.
The frequency distribution in the carrier direction in the lower stage of FIG. 2 is a symmetric distribution around the origin. Therefore, it is not necessary to use a complex filter for making the pass region asymmetrical, the number of expensive digital LPFs having a large circuit scale is reduced by half from four to two, the circuit scale is greatly reduced, and the cost is low. It becomes possible to.
The second basic idea of the present invention is to implement the method of shifting the symbol pilot signal as shown in the lower part of FIG. 2 by cyclically changing the order of signal trains input to the FFT circuit 9. .
Before explaining the principle of this method, the meaning of the operation performed by FFT will be briefly explained. FIG. 3A schematically shows a received direct wave and a range to be cut out for input to the FFT. The FFT circuit 9 of the receiving apparatus cuts out a signal sequence corresponding to 2048 sample clocks of (B + b) IFFTed by the transmitting apparatus and performs FFT. By the way, a detailed description will be given to a textbook related to Fourier transform. In FFT, discrete Fourier transform is performed on the assumption that an input signal sequence is repeated infinitely as shown in FIG. Therefore, as shown by the arrow 23 in FIG. 3, if the signal sequence for the first b ″ of B in FIG. 3B, for example, the signal sequence for the first 32 sample clocks is moved behind the part b and then FFT is performed. As shown in FIG. 3C, the same result as that obtained by performing FFT on the signal sequence in which the signal sequence of (B + b) of the direct wave in FIG. It is done.
[0009]
Based on the above facts, the principle of the second basic concept will be described. As described in FIG. 17B, the phase angle of each carrier of the delayed wave delayed by n sample clock periods with respect to the direct wave rotates in the positive direction at a rate of n rotations per 2048 carrier.
Conversely, the phase angle of each carrier of the preceding wave that precedes the direct wave by n sample clock periods rotates in the negative direction at a rate of n rotations per 2048 carrier. In other words, if the signal component of the direct wave indicated by the thick arrow in the upper part of FIG. 2 can be preceded by some method for 32 sample clock periods, it is moved to the position of the carrier direction frequency indicated by the thick arrow in the lower part of FIG. Can do. The same is true for the delayed wave indicated by the hatched frame in the upper part of FIG. Therefore, the signal in the upper hatched frame in FIG. 2 can be shifted as shown in the lower row in FIG. The second basic idea of the present invention is that this carrier direction frequency shift is carried out by performing FFT after a part of the signal sequence inputted to the FFT circuit 9 is cyclically replaced as indicated by an arrow 23 in FIG. It is realized.
[0010]
The present invention realizes the first basic concept and the second basic concept and achieves the above-mentioned object by using a plurality of carrier waves (hereinafter referred to as carriers) orthogonal to each other to encode information codes. In a transmission device using an orthogonal frequency division multiplexing modulation scheme (OFDM scheme) for transmission, an FFT circuit that sequentially performs a discrete Fourier transform on a signal sequence cut out from a received signal for each predetermined signal length to a reception device of the transmission device; The signal order in the extracted signal sequence is cyclically changed, and an FFT input signal cyclic circuit that sequentially outputs to the FFT circuit, and a predetermined pilot signal is extracted from the FFT circuit output and interpolated to obtain a reference signal vector. The reproduction circuit and a demodulation circuit for demodulating the FFT circuit output based on the reproduced reference signal vector are provided.
The FFT input signal cyclic circuit is a circuit that cyclically replaces the time waveform signal of a predetermined period from the head of each of the extracted signal sequences to the end of the signal sequence.
Further, the pilot signal is a pilot signal (CP) having a carrier structure inserted continuously at a predetermined carrier interval in the carrier direction in the time direction, and the receiver of the transmission apparatus includes the FFT circuit and the FFT input. A signal cyclic circuit; a pilot signal selection circuit that converts a signal value of a carrier other than the carrier having the CP in the signal sequence output from the FFT circuit to 0 and outputs the signal as a symbol pilot signal; and the symbol pilot signal The first digital LPF that performs the interpolation calculation of the real number component and the second digital LPF that performs the interpolation calculation of the imaginary number component have a carrier direction interpolation circuit.
The pilot signal is a pilot signal (SP) having a carrier structure that is inserted intermittently in the time direction and at a predetermined carrier interval in the carrier direction. The receiving apparatus of the transmission apparatus includes the FFT circuit and the FFT input. Obtained by selecting the SP from the signal cycle circuit and the signal sequence output from the FFT circuit and performing interpolation in the time direction, and converting the signal value of carriers other than the carrier having the SP to 0 A first digital LPF for interpolating a real component of a symbol pilot signal output from the time direction interpolation circuit and an imaginary component thereof are provided. It is configured to have a carrier direction interpolation circuit configured by a second digital LPF that performs an operation.
Further, instead of the pilot signal selection circuit, the CP of the signal sequence output from the FFT circuit is band-limited in the time direction, and the signal value of carriers other than the carrier having the CP is converted to 0. A time-direction LPF for outputting the symbol pilot signal obtained in this way is provided, and the symbol pilot signal output from the time-direction LPF is input to the carrier direction interpolation circuit.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a configuration example of the receiving apparatus according to the first embodiment of the present invention will be described with reference to FIG. The most significant difference from the conventional circuit configuration of FIG. 16 is that an FFT input signal cyclic circuit 30 is newly provided, and a part of the signal sequence cut out by the signal cutout circuit 8 for input to the FFT circuit 9 is shown in FIG. This is the point that the signal is input to the FFT circuit 9 after being cyclically switched as indicated by the arrow 23. The second difference is that the carrier direction interpolation circuit 11 constituted by four digital LPFs is replaced with a carrier direction interpolation circuit 31 constituted by only two digital LPFs.
In FIG. 1, a signal received by an antenna 6 is input to a reception preprocessing circuit 7 and processed in the same manner as a conventional receiving apparatus. Then, the signal cutout circuit 8 cuts out a signal sequence for 2048 sample clocks input to the FFT circuit 9 and inputs it to the newly provided FFT input signal circuit 30.
A configuration example of the internal circuit of the FFT input signal circuit 30 will be described with reference to FIG. The signal sequence of the 2048 sample clock that is cut out and output by the signal cutout circuit 8 is input to the FIFO 301 and the switch 302. 3B is stored in the FIFO 301 under the control of the enable pulse ENin. The portion (B + b) following the portion b ″ passes through the switch 302. , Output directly from the FFT input signal circuit 30.
The portion b ″ accumulated in the FIFO 301 is read out under the control of the enable pulse ENout immediately after the portion b is output through the switch 302, and is output from the FFT input signal circuit 30 through the switch 302.
By this processing, the signal extraction circuit 8 extracts the signal sequence (B + b) in FIG. 3B as the signal sequence input to the FFT, like the signal sequence input to the FFT in FIG. 3C. Converted.
[0012]
The signal sequence output from the FFT input signal circulation circuit 30 is FFTed by the FFT circuit 9, but the signal sequence input to the FFT circuit 9 is a diagram cut out from the received signal as shown in FIG. 3C. This is a signal sequence equivalent to a signal sequence cut out from a signal that precedes the signal sequence of (b) of 3 by 32 sample clock periods. Then, from the FFT circuit 9, the carrier direction frequency distribution is output as a signal sequence having a symmetrical distribution around the origin as shown in the lower diagram of FIG. Therefore, it is not necessary to use a complex filter for making the pass region asymmetric as the carrier direction interpolation circuit 31, and only two digital LPFs that individually band limit real components and imaginary components as illustrated in FIG. Can be configured.
The reference signal output from the carrier direction interpolation circuit 31 and the signal sequence output from the FFT circuit 9 and time-adjusted by the delay circuit 12 are input to the demodulation & post-processing circuit 13 as in the conventional receiver. , 64QAM demodulation, post-processing such as demodulated code error correction, etc., and then output as a decoded information code from the receiver.
As described above, in this embodiment, the conventional receiving apparatus has a large circuit scale as shown in FIG. 19 and requires four expensive digital LPFs, as shown in FIG. It can be configured only with a digital LPF. Therefore, the circuit scale of the carrier direction interpolation circuit 31 can be greatly reduced to about half, and the circuit price can be greatly reduced. Also, the shift in the carrier direction frequency distribution for enabling the carrier direction interpolation circuit to be configured by only two digital LPFs can be performed only from the FIFO without using a modulation circuit that requires a slightly large complex multiplication. Therefore, the circuit scale can be further reduced and the circuit can be configured at a low price. Therefore, it is possible to obtain an OFDM transmission device that is low in cost, small in size, and easy to use.
[0013]
Next, FIG. 6 shows an example of the circuit configuration of the receiving apparatus according to the second embodiment of the present invention. In this embodiment, the present invention is applied to the case of a carrier structure similar to the UHF band terrestrial digital broadcasting system in Japan. That is, the present invention is applied to a carrier structure in which pilot signals are arranged in a frequency direction and a time direction as indicated by a square mark “□” written as SP in FIG.
In the case of the carrier structure of FIG. 13, demodulation of the received signal is performed by first interpolating SP in the time direction and calculating a pilot carrier reference signal (pilot carrier signal) indicated by hatching. The arrangement of pilot carriers indicated by hatching in FIG. 13 is the same as the arrangement of carriers having CP in FIG. 12 except that the period in which pilot carriers are inserted is changed every three carriers. Therefore, by using a pilot carrier signal obtained by interpolation in the time direction instead of the pilot signal CP in the first embodiment, the information code can be demodulated in the same manner as in the first embodiment.
The circuit in FIG. 6 has a configuration in which the pilot signal selection circuit 10 in FIG. 1 is replaced with a time direction interpolation circuit 32 that calculates a pilot carrier signal. In the same manner as in the first embodiment, the signal sequence cut out by the signal cutout circuit 8 is changed in order by the FFT input signal circuit 30 and then input to the FFT circuit 9.
Therefore, the signal sequence output from the FFT circuit 9 is a signal sequence having a carrier direction frequency distribution in the lower stage of FIG. 2 as in the first embodiment.
In the time direction interpolation circuit 32, the SP inserted between the signal sequences output from the FFT circuit 9 is selected, and the time direction interpolation operation is performed. At the same time, other than the carrier having the SP (pilot carrier) The symbol pilot signal obtained by converting the signal value of the carrier of 0 to 0 is calculated and output.
[0014]
The symbol pilot signal output from the time direction interpolation circuit 32 is a signal having the same structure as the symbol pilot signal output from the pilot signal selection circuit 10 of FIG. Accordingly, as in the first embodiment, the symbol pilot signal is input to the carrier direction interpolation circuit 31 composed of only two digital LPFs to reproduce the reference signal, and the information code is reproduced using the reproduced reference signal. Can be demodulated.
In this way, in this embodiment as well, as in the first embodiment, the carrier direction interpolation circuit can be configured with only two digital LPFs, so that the circuit scale of the carrier direction interpolation circuit 31 is greatly reduced to about half. In addition, the cost of the circuit can be greatly reduced. Also, the shift in the carrier direction frequency distribution for enabling the carrier direction interpolation circuit to be configured by only two digital LPFs can be performed only from the FIFO without using a modulation circuit that requires a slightly large complex multiplication. Therefore, the circuit scale can be greatly reduced and the circuit can be configured at a low price. Therefore, it is possible to obtain an OFDM transmission device that is low in cost, small in size, and easy to use.
[0015]
Next, FIG. 7 shows a configuration example of a circuit of the receiving apparatus according to the third embodiment of the present invention. This embodiment is applied to a carrier structure in which pilot signals CP are continuously inserted in the time direction as shown in FIG. 12, and the pilot signal selection circuit 10 in FIG. 1 is replaced with the LPF 33 in the time direction. This is different from the first embodiment.
In the case of the carrier structure of FIG. 12, since pilot signals are continuously inserted in the time direction in any pilot carrier, no interpolation operation in the time direction is necessary. However, if the band is limited in the time direction, the S / N of the reference signal to be reproduced can be improved. Therefore, in the LPF 33 in the time direction, the band of the CP continuously inserted in the time direction is band-limited, and at the same time, the symbol pilot signal obtained by converting the signal value of the carrier in which the CP is not inserted into 0 Is output. Since other signal processing is the same as that of the first embodiment, description thereof is omitted.
As described above, in this embodiment, the same effect as that of the first embodiment can be obtained, and a low-cost, small-sized and easy-to-use OFDM transmission apparatus can be obtained.
In order to avoid confusion in the explanation, in each of the above embodiments, the position of the signal sequence cut out for input to the FFT circuit is set as Ts output from the IFFT circuit of the transmission apparatus as shown in FIG. The description has been made assuming that the signal sequence is set to the 2048 sample clock signal in the period. However, in actual receivers, taking into account synchronization acquisition errors, as shown in FIG. 8, a signal sequence of 2048 sample clocks in a range moved to the b ′ side for several sample clock times, for example, 4 sample clock periods, is cut out. FFT.
In such a case, it should be noted that it is necessary to change the amount by which the order is cyclically changed in the FFT input signal cyclic circuit 30 from 32 to 28 (= 32-4) or the like.
[0016]
Further, in order to avoid confusion in the description, the above embodiments have been described on the assumption that the acquisition is performed so that the direct wave is at the timing as shown in FIG. However, in practice, the synchronization acquisition is usually performed so that the signal having the maximum power at that time has the timing shown in FIG. In this case, since the main wave is not always a direct wave, if there is a preceding wave received prior to the main wave, the preceding wave is received. For this reason, in the frequency distribution in the carrier direction, as shown in the upper part of FIG. 9, in addition to the delayed wave component indicated by the hatched frame, the preceding wave component indicated by the dot frame 24 appears. Therefore, it should be noted that in such a case, it is necessary to change the amount by which the order is cyclically changed in the FFT input signal cyclic circuit 30 so that the distribution in the lower stage of FIG.
Although omitted in the above description, when the symbol pilot signal is interpolated in the carrier direction in the carrier direction interpolation circuit, a large distortion occurs in the reference signal in the vicinity of the band boundary among the reproduced reference signals. . In order to reduce this distortion, it is necessary to interpolate after smoothly extrapolating the symbol pilot signal calculated from the received CP or SP outside the band. In this case, extrapolation is clearly easier when the carrier phase angle is not twisted as shown in FIG. 17B than when it is twisted as shown in FIG. Therefore, as shown in FIG. 8, when a signal sequence at a shifted position is extracted from the signal sequence in the Ts period output from the IFFT circuit of the transmission apparatus and input to the FFT circuit, the signal sequence corresponding to the shifted sample clock. Are input to the FFT circuit. That is, the signal value of the first sample clock of the time waveform of the effective symbol converted by the IFFT circuit of the transmission device is cyclically switched so that it becomes the signal value of the first sample clock of the time waveform input to the FFT circuit. To the FFT circuit. Then, since the phase angle of the signal sequence output from the FFT circuit becomes flat as shown in FIG. 17A, an effect of facilitating smooth extrapolation of the symbol pilot signal can be obtained. For reference, an insertion position of the extrapolation circuit 34 required at this time is illustrated in FIG.
[0017]
【The invention's effect】
As described above, when the means according to the present invention is used, the carrier direction interpolation circuit, which requires four expensive digital LPFs with a large circuit scale in the conventional receiving apparatus, can be configured with only two digital LPFs. Therefore, the circuit scale of the carrier direction interpolation circuit can be greatly reduced to about half, and the circuit price can be greatly reduced. In addition, the carrier direction frequency distribution shift for enabling the carrier direction interpolation circuit to be configured by only two digital LPFs is composed of only a FIFO without using a modulation circuit that requires a slightly large complex multiplication. Since it can be realized with a simple circuit, the circuit scale can be greatly reduced and the circuit can be configured at a low price. Therefore, it is possible to obtain an OFDM transmission apparatus that is low in cost, small in size, and easy to use.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a circuit configuration of a receiving apparatus according to a first embodiment of the present invention.
FIG. 2 is a schematic diagram for explaining a first basic concept according to the present invention.
FIG. 3 is a schematic diagram for explaining a second basic concept according to the present invention.
FIG. 4 is a block diagram showing an example of a circuit configuration of an FFT input signal cyclic circuit according to the present invention.
FIG. 5 is a block diagram showing an example of a circuit configuration of a carrier direction interpolation circuit according to the present invention.
FIG. 6 is a block diagram showing a circuit configuration of a receiving apparatus according to a second embodiment of the present invention.
FIG. 7 is a block diagram showing a circuit configuration of a receiving apparatus according to a third embodiment of the present invention.
FIG. 8 is a schematic diagram for explaining the position of a signal string input to an FFT circuit in an actual receiving apparatus.
FIG. 9 is a schematic diagram for explaining a carrier frequency distribution and a processing method in an actual receiving apparatus.
FIG. 10 is a block diagram showing a circuit configuration when extrapolating the symbol pilot signal of the present invention
FIG. 11 is a schematic diagram for explaining the carrier structure of the OFDM system;
FIG. 12 is a schematic diagram for explaining a carrier structure in which a CP is arranged as a pilot signal.
FIG. 13 is a schematic diagram for explaining a carrier structure in which SPs are arranged as pilot signals.
FIG. 14 is a block diagram illustrating an example of a circuit configuration of a transmission apparatus.
FIG. 15 is a schematic diagram for explaining a time waveform of an OFDM signal;
FIG. 16 is a block diagram showing an example of a circuit configuration of a conventional receiving apparatus.
FIG. 17 is a schematic diagram for explaining the rotation of the phase angle of the FFT output signal.
FIG. 18 is a schematic diagram for explaining the influence of carrier direction frequency distribution and delay wave.
FIG. 19 is a block diagram showing a circuit configuration of a conventional complex filter.
[Explanation of symbols]
1: pre-transmission processing circuit, 2: IFFT circuit, 3: guard interval insertion circuit, 4: post-transmission processing circuit, 5, 6: antenna, 7: pre-reception processing circuit, 8: signal extraction circuit, 9: FFT circuit, 10: Pilot signal selection circuit, 11: Carrier direction interpolation circuit, 12: Delay circuit, 13: Demodulation & post-processing circuit, 30: FFT input signal cyclic circuit, 31: Carrier direction interpolation circuit, 302: Switch, 301: FIFO, 32: time direction interpolation circuit, 33: time direction LPF, 34: extrapolation circuit.

Claims (5)

  In a receiving apparatus using an orthogonal frequency division multiplex modulation scheme (OFDM scheme) that transmits information codes using a plurality of orthogonal carriers (hereinafter referred to as carriers), a predetermined carrier in the carrier direction is continuous in the time direction. An FFT circuit that sequentially performs a discrete Fourier transform on a signal sequence cut out at predetermined signal lengths from a received signal including a pilot signal (CP) having a carrier structure inserted at intervals, and cycles the signal sequence in the cut out signal sequence An FFT input signal cyclic circuit that sequentially outputs to the FFT circuit, a circuit that extracts a predetermined pilot signal from the FFT circuit output, interpolates, and reproduces a reference signal vector, and the reproduced reference signal vector A demodulating circuit for demodulating the output of the FFT circuit based on the signal, and the CP of the signal sequence output from the FFT circuit in the time direction The LPF in the time direction for outputting the symbol pilot signal obtained by converting the signal value of the carrier other than the carrier having the CP to 0 and the interpolation operation of the real component of the symbol pilot signal is performed. An orthogonal frequency division multiplexing modulation receiver comprising: a digital LPF of 1 and a carrier direction interpolation circuit configured of a second digital LPF that performs an interpolation operation of its imaginary component. In the receiving apparatus using orthogonal frequency division multiplexing modulation scheme for transmitting information symbols (OFDM system) using a plurality of carriers (hereinafter referred to as carriers) orthogonal to each other, the receiving communication apparatus, the predetermined signal length from the received signal An FFT circuit that sequentially performs discrete Fourier transform on the signal sequence cut out every time, an FFT input signal cyclic circuit that cyclically changes the signal order in the cut out signal sequence and sequentially outputs the signal to the FFT circuit, and the output of the FFT circuit Based on the reproduced reference signal vector, a carrier direction interpolation circuit that extracts a predetermined pilot signal from the signal and limits the bandwidth of the real component and the imaginary component separately to interpolate in the carrier direction and reproduce the reference signal vector. A demodulation circuit for demodulating the FFT circuit output,
The FFT input signal cyclic circuit is a circuit that cyclically replaces a time waveform signal of a predetermined period from the beginning of each of the extracted signal sequences to the end of the signal sequence, and the predetermined period includes the output of the FFT circuit A receiving apparatus, wherein a carrier direction frequency distribution with respect to rotation of a phase of a pilot signal is set to be a symmetric distribution around an origin. The receiving device according to claim 2,
The extracted signal sequence is extracted at a position that is a Ts period output from the IFFT circuit that is the transmission source of the received signal and that is advanced from the Ts period for the main wave by a predetermined clock time considering a synchronization supplement error. If the signal sequence is
The receiving apparatus according to claim 1, wherein the predetermined period is set to a period obtained by subtracting the predetermined clock time from a half period of the guard interval period.   3. The receiving apparatus according to claim 2, wherein the pilot signal is a pilot signal (SP) having a carrier structure inserted intermittently in a time direction at a predetermined carrier interval in a carrier direction, and is output from the FFT circuit. A time direction interpolation circuit for selecting the SP from the signal sequence and performing a time direction interpolation operation and outputting a symbol pilot signal obtained by converting the signal value of a carrier other than the carrier having the SP to 0 And a circuit configuration for inputting the symbol pilot signal output from the time direction interpolation circuit to the carrier direction interpolation circuit.   The receiving apparatus according to claim 2, wherein the CP of the signal sequence output from the FFT circuit is band-limited in the time direction, and a signal value of a carrier other than the carrier having the CP is converted to 0. A receiving apparatus comprising: a time-direction LPF that outputs an obtained symbol pilot signal; and a circuit configuration that inputs the symbol pilot signal output from the time-direction LPF to the carrier direction interpolation circuit.

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