JP2003249907A - Transmitting device of ofdm system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce increase in a symbol error rate due to the distortion of a reproduction reference signal in mobile communication by reducing distortion that occurs in the vicinity of both ends of the band of a reference signal reproduced to be used in demodulation in a transmitting device of an OFDM (orthogonal frequency division multiplexing) system for modulating a plurality of carriers of an OFDM system with a modulation method using synchronous detection. <P>SOLUTION: A receiver calculates a reference signal of a carrier at the central part of a band by interpolating the reference signal with an LPF of three taps or more, calculates a reference signal of a carrier in the vicinity of both ends of the band with linear approximation between two pilot signals, or further forms a carrier structure, wherein a carrier interval of a pilot signal inserted in the vicinity of the both ends of the band is narrower than the a carrier interval of a pilot signal at the central part. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an orthogonal frequency division multiplex modulation system (Orthogonal Frequency Divisio) for modulating a plurality of carriers orthogonal to each other with information codes.
n Multiplexing: Hereinafter, it relates to a carrier structure of a transmission device using an OFDM method and reproduction of a reference signal performed by the demodulation device.

[0002]

2. Description of the Related Art In recent years, in the field of wireless devices, the OFDM system has been attracting attention as a modulation system that is resistant to multipath fading. In particular, the OFDM method in which a plurality of carriers are modulated by the synchronous modulation method is under extensive application research in the fields of next-generation television broadcasting, FPU, wireless LAN, etc. in countries such as Europe and Japan. The OFDM system, as shown in FIG. 2, has N orthogonal to each other within a certain transmission bandwidth.
Provide a carrier, for example, about 1400 carriers,
In this method, a designated carrier is modulated by an information code by a modulation method such as 64QAM and transmitted.

FIG. 3 is a view for explaining an example of the carrier structure in more detail, and is an enlarged view of a part thereof. It can be considered that a similar structure is repeated over the entire transmission band. In FIG. 3, “□” arranged in the horizontal direction each represents one carrier. The horizontal line "□" is OFDM
It represents one symbol of the signal, the vertical direction representing the passage of time. The “□” written as SP indicates the position of the pilot signal used to reproduce the reference signal required for demodulation. In addition, "□" with nothing written is 6
The signal position modulated by 4QAM is shown. In this arrangement, the pilot signals are arranged at positions scattered in the frequency direction and the time direction, so that SP (Scattered Pilo
t). The pilot signal P is shown in FIG.
It may be inserted continuously in the time direction such as “□” written as CP. In this case, CP (Conti
(nual Pilot).

FIG. 5 and FIG. 6 are circuit diagrams showing parts related to the present invention extracted from the circuits constituting the transmitter and the receiver of the OFDM system. Pre-transmission processing circuit 1
The information code input to is converted into an error correction code, 64
Pre-processing such as mapping to QAM and insertion of SP carriers according to FIG. 3 is converted into a signal sequence of a frequency distribution image of 2048 sample clocks, which represents the signals of the carriers arranged in one horizontal line in FIG. The converted signal train is input to the IFFT circuit 2 and converted into a signal train representing a time waveform by 2048-point inverse Fourier transform (IFFT). FIG. 7 schematically shows an OFDM signal transmitted from the transmission device. From the IFFT circuit 2,
The time waveform of the Ts period of the OFDM signal is output. The guard interval insertion circuit 3 is a circuit for copying and inserting the portion b of the time waveform of the period Ts into the portion b '. The signal with the guard interval inserted in this way 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.

The signal received by the antenna 6 of the receiver shown in FIG. 6 is down-converted in the reception preprocessing circuit 7.
After pre-processing such as A / D conversion and quadrature demodulation, it is input to the signal cutout circuit 8 and a 2048-sample signal sequence corresponding to the Ts period in FIG. 7 is cut out. The cut-out signal train is input to the FFT circuit 9 that performs 2048-point Fourier transform (FFT), and is returned to the signal train of the horizontal single row of FIG. 3 which is the signal train of the frequency distribution image. By the way, in order to demodulate a signal mapped by 64QAM, a reference signal corresponding to a rule on the signal space is required as described in general textbooks. The pilot signal SP of FIG. 3 is a signal inserted to enable reproduction of this reference signal. The signal sequence of the frequency distribution image output from the FFT circuit 9 of FIG. 6 is input to the reference signal reproduction circuit 10, and the reference signal is reproduced from the pilot signal SP inserted therein. The post-reception processing circuit 12 of FIG. 6 uses the regenerated reference signal for 64Q.
This is a circuit that demodulates AM and performs post-processing such as code error correction of demodulated code. The code output from the post-reception processing circuit 12 is output from the receiving device as a decoded information code. The delay circuit 11 delays the signal sequence of the frequency distribution image by the operation time for reproducing the reference signal,
It is a circuit for time adjustment for simultaneously inputting the reference signal and the signal sequence of the frequency distribution image to the post-reception processing circuit 12.

Next, a conventional reference signal reproducing method implemented in the reference signal reproducing circuit 10 will be described. Figure 8
The circuit portion for reproducing the reference signal vector of the OFDM receiver is extracted. Reference signal reproduction circuit 1
The signal sequence of the frequency distribution image input to 0 is first input to the time direction interpolation circuit 13. The time-direction interpolation circuit 13 passes the LPF having a predetermined number of taps for each carrier indicated by the diagonal lines in FIG. 9 that includes SP in the time direction and outputs the reference signal vector interpolated in the time direction. The carrier in which this SP is arranged is hereinafter referred to as a pilot carrier. FIG. 10 is a diagram schematically showing the above-mentioned interpolation method in the time direction by taking the carrier indicated by the alternate long and short dash line 14 in FIG. The horizontal axis is the time axis, and a memory is attached to each symbol. The circled circles represent the received SP signal vector. The reference signal vector of the symbol between SP1 and SP2 is interpolated by an LPF with a fixed number of taps using the signal vectors of a plurality of SPs before and after the symbol. By this interpolating operation in the time direction, the reference signal vector of the carrier at the six intervals indicated by the diagonal lines in FIG. 9 is calculated. In addition, the intermediate reference signal vector obtained by interpolating in the time direction,
Hereinafter, in order to distinguish from the finally obtained reference signal vector, it is referred to as a symbol pilot signal. On the other hand, the reference signal vector of the modulated signal A in FIG. 9 in the carrier in which the SP is not arranged is obtained by interpolating the symbol pilot signal of the pilot carrier in the carrier direction.

The carrier direction interpolation circuit 15 of FIG. 8 is a circuit for performing this interpolation calculation. FIG. 11A is a diagram schematically showing the above-described interpolating method in the carrier direction by taking the symbol of the alternate long and short dash line 16 of FIG. The horizontal axis is the frequency axis, and a memory is attached to each carrier position. The thick arrows indicate the symbol pilot signals W (1), W (6 + 1), W (2 ×) obtained for the shaded carriers in FIG.
6 + 1), ... Here, the numbers in parentheses are carrier numbers. The reference signal vector at the carrier position A without the thick arrow is calculated as follows. First, Fig. 1
The signal W (1) of FIG. 11 (b) obtained by setting the magnitude of the vector of the carrier without the thick arrow of 1 (a) to 0,
0, ..., 0, W (6 + 1), 0, ..., 0, W (2
.. are passed through a normal digital LPF having 23 taps, for example, to calculate a smooth interpolation signal represented by a broken line. The interpolated signal calculated in this way is output as the reference signal vector of the modulated signal A. The reference signal vector reproduced by the carrier direction interpolation circuit 15 of FIG. 8 is directly output from the reference signal reproduction circuit 10. The reference signal vector reproduced and output by the reference signal reproduction circuit 10 of FIG. 6 and the signal sequence of the frequency distribution image delayed by the delay circuit 11 by the calculation time for reproducing the reference signal are sent to the post-reception processing circuit 12. Entered, 64
Post-processing such as QAM demodulation and code error correction of the demodulated code is performed, and the demodulated information code is output from the receiving device.

[0008]

By the way, in the method of reproducing the reference signal vector using the correlation in the carrier direction as described above, the following problems occur. That is, the digital LPF used for interpolation in the carrier direction is, for example, a filter having 23 taps, and in order to obtain the interpolated value of an arbitrary carrier number, as shown in (b) of FIG. Signals for 11 carriers before and after are required. When calculating the interpolated value for the carrier inside the band, there is always the required number of pilot carriers before and after that, so there is no problem. However, in the vicinity of the boundary of the band, as shown in FIG. 12, one of the carriers does not exist. Since there is no symbol pilot signal indicated by the thick dashed arrow for performing the correct interpolation calculation, the correct interpolation value cannot be obtained. The magnitude of the distortion vector, which is the difference between the original reference signal vector and the interpolated value, is illustrated in FIG. In FIG. 13, a memory is attached to the carrier position on the frequency axis. In addition, carriers marked with "○" on the frequency axis indicate that they are pilot carriers. The vertical axis represents the ratio of the size of the reproduced reference signal and the size of the distortion vector, and is expressed in dB. That is, it is a value corresponding to the S / N of the reproduced reference signal.

The distortion of the interpolated value in the carrier direction from the correct reference signal vector approaches the end carrier within the range of the number of carriers which is approximately 1/2 of the number of taps of the digital LPF used for interpolation in the carrier direction. Increase rapidly according to. In the carrier between the end pilot carrier 17 and the second pilot carrier 18, distortion exceeding 1/7 of the correct magnitude of the reference signal vector occurs depending on the value of the carrier spacing Mp of the pilot carriers. Since the code of the received signal is discriminated using the reproduced reference signal, if the reference signal is distorted, the error rate of the code demodulated increases and the performance of the transmission device deteriorates. Therefore, how to reproduce a reference signal with less distortion is a decisive factor in realizing a high-performance transmission apparatus with a low code error rate.

A method of reproducing the reference signal with less distortion is described in Japanese Patent Laid-Open No. 2002-9726. This reference signal reproducing method extrapolates and interpolates an assumed complex vector value of the symbol pilot signal even for a carrier portion outside the band where the pilot signal is not transmitted. That is, for example, the value of the symbol pilot signal at the carrier position at the end of FIG. 12 is copied as a symbol pilot signal at the position of the thick broken arrow outside the band and extrapolated and interpolated. Then, an interpolation operation is performed using the extrapolated symbol pilot signal including all symbol pilot signals including the interpolated complex vector to reproduce the reference signal vector. Therefore, also in the calculation of the reference signal vector for the carrier near the band boundary, the necessary number of symbol pilot signals exists, and the effect of reducing the amount of distortion of the obtained reference signal vector can be obtained.

In this method, if the extrapolation is performed correctly, the region in which large distortion occurs in FIG. 13 moves out of the band, so that a large effect can be obtained. However, the actual symbol pilot signal waveform changes in a complicated manner under the influence of multipath and the like. For this reason, there is a drawback that correct extrapolation cannot be performed unless the favorable conditions such as fixed radio, and the error rate of the demodulation code cannot be reduced. In addition, since a certain amount of vibration component is generated even under favorable conditions, there is a drawback that a modulation circuit having a large circuit scale is necessary to correct this vibration waveform. An object of the present invention is to add a comparatively small circuit to the conventional circuit, to reproduce a reference signal with comparatively little distortion even when the symbol pilot signal waveform changes in a complicated manner, and to obtain a code error rate of a demodulation code. It is an object of the present invention to provide a good OFDM-type transmission device capable of reducing noise.

[0012]

In order to achieve the above object, the present invention provides an orthogonal frequency division multiplex modulation (OF) method for transmitting an information code by a plurality of carriers which are orthogonal to each other.
(DM system), the OFDM system signal has a structure in which a carrier having a pilot signal in the time direction (pilot carrier) is inserted between carriers in a certain band, , Input the received plurality of carrier signals and select the pilot signal inserted between the carrier signals to be a symbol pilot signal, or calculate a signal by interpolating the pilot signals in the time direction A symbol pilot signal, and a reference signal reproducing circuit for reproducing and outputting a reference signal from the calculated symbol pilot signal, selecting a symbol pilot signal with a carrier interval Mpc from the calculated symbol pilot signal, A signal obtained by filtering the selected symbol pilot signal with three or more taps A filter circuit that outputs a first interpolated signal; a linear approximation circuit that outputs a signal obtained by performing a linear approximation between the calculated symbol pilot signals as a second interpolated signal; A signal and the second interpolated signal are input, the second interpolated signal is selected as an interpolated signal in a certain range from both ends within a certain band, and the first interpolated signal is used as an interpolated signal in the other range. Is a receiving device having a reference signal reproduction circuit having a selection circuit that selects the interpolated signal of and outputs the selected interpolated signal as a reproduction reference signal. The interval for inserting the pilot carrier is
The interval of pilot carriers inserted between carriers near both ends in the band is narrower than the interval Mpc of pilot carriers inserted between carriers near the center in the band. At least the carrier spacing Mp
The pilot carrier is inserted for each c, and in the vicinity of both ends in the band, one or more pilot carriers are further provided between the pilot carriers inserted at the carrier interval Mpc.

According to the present invention, the reference signal in the inner region of the band is calculated by the interpolation operation in the carrier direction by the filter circuit as in the conventional case. On the other hand, the reference signal in the region near both ends of the band where large distortion occurs when using the filter circuit is calculated by linear approximation in which symbol pilot signals of pilot carriers are connected by a straight line. In the demodulation of 64QAM or the like, the reference signal obtained by these two types of calculations is switched between the internal region of the band and the region near both ends to be used for demodulation. As for the interpolation operation, the linear approximation is inferior in performance to the filter circuit. That is, inside the band, the distortion of the reference signal calculated using the filter circuit is smaller than that of the reference signal calculated by linear approximation. However, the distortion of the reference signal calculated using the filter circuit rapidly increases near the boundary of the band, and its magnitude is opposite to the magnitude of the distortion of the reference signal calculated by the linear approximation. For example, in the case of a signal in which the pilot carrier interval is 6 carrier intervals and the symbol pilot signal is modulated by one rotation of 128 carriers, the distortion amount of the reference signal calculated by linear approximation is as shown by the solid line in FIG. Therefore, the distortion amount is smaller than that of the reference signal indicated by the broken line calculated by the filter circuit. Therefore, in the transmission apparatus of the present invention which demodulates by switching the reference signal obtained by these two types of operations in the internal region of the band and the region near both ends, the code error rate of the demodulated code increases due to the distortion of the reference signal. It is possible to obtain a good transmission apparatus of the OFDM system with less number of channels. Further, this effect can be realized only by adding a simple linear approximation circuit and a switch circuit to the conventional circuit, so that an effect of suppressing an increase in circuit scale can be obtained. Further, if the pilot carriers are arranged more densely in the regions near both ends of the band than in the inner region of the band, the accuracy of the linear approximation can be further increased, and the effect of further reducing the distortion amount can be obtained.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION A first embodiment according to the present invention will be described. The configuration of the transmitter may be the same as that of the conventional circuit shown in FIG. The circuit configuration of the receiver shown in FIG. 6 is also the same as the conventional circuit except the internal circuit configuration of the reference signal reproducing circuit 10, and the information code demodulation procedure is also the same as the conventional one. Therefore,
Only the internal circuit configuration of the reference signal reproducing circuit 10 different from the conventional one and its effect will be described. FIG. 1 shows a configuration example of the internal circuit of the reference signal reproducing circuit 10. The internal circuit configuration of the carrier direction interpolation circuit 20 is significantly different from the conventional circuit configuration of FIG. The symbol pilot signal calculated by the time direction interpolation circuit 13 is input to the carrier direction interpolation circuit 20 as in the conventional case. The input symbol pilot signal is branched into two, one of which is input to a filter circuit 19 having the same configuration as the filter circuit of the conventional circuit of FIG.
A first interpolated signal is calculated. The other branched one is input to the newly provided linear approximation circuit 21 to calculate the second interpolation signal calculated by the linear approximation. Filter circuit 1
9 and the first interpolation signal and the second interpolation signal calculated by the linear approximation circuit 21 are input to the switch circuit 22. Then, under the control of the control signal, the second interpolation signal is selected as the reference signal of the carrier in the region a near the boundary of the band in FIG. 14, and the first interpolation signal is selected for the region b inside the band. A signal is selected and output from the carrier direction interpolation circuit 20.

In the present embodiment, as shown by the solid line in FIG. 15, the interpolated signal with a small distortion amount in FIG. 14 is selected and used as the reference signal, so that it is output from the reference signal reproduction circuit 10 in FIG. The distortion of the reproduction reference signal is reduced, and the effect that a good information code having a lower code error rate than that of the conventional receiving apparatus can be demodulated is obtained. Further, in order to realize the present embodiment, it suffices to add a simple linear approximation circuit and a switch circuit,
The effect that the increase in the circuit scale can be suppressed small is obtained. As described above, when the reference signal regenerating circuit according to the present embodiment is used, the distortion of the reference signal vector generated near the boundary of the band is reduced, so that a good OFDM transmission apparatus with a small code error rate is obtained. be able to.

A second embodiment according to the present invention will be described. The circuit configuration used in this embodiment is basically the first
The embodiment is the same as that of Embodiment 1 except for the structure of the carrier of the OFDM signal to be transmitted. FIG. 16 shows an example of a carrier structure used in this embodiment. This figure is an enlarged view of the carrier structure in the leftmost region of the band.
The spacing between pilot carriers within the band is 6 as in the conventional case. However, at the edge of the band, one pilot carrier is added between the pilot carriers at 6 intervals. That is, the interval of pilot carriers inserted between carriers near both ends of the band is narrower than the interval Mpc of pilot carriers inserted between carriers near the center of the band. Then, when the first interpolation signal is calculated by the filter circuit 19 of FIG. 1, the interpolation calculation is performed using the symbol pilot signals calculated by the pilot carriers inserted at the same six intervals as in the band. carry out. On the other hand, when the second interpolation signal is calculated by the linear approximation circuit 21, linear approximation is performed between the symbol pilot signals calculated by the most adjacent pilot carriers. Therefore, in the vicinity of both ends of the band, a linear approximation value is calculated between the symbol pilot signals calculated by the pilot carriers at three intervals, and the reference signal with less distortion than in the first embodiment is reproduced. You can

As described above, according to the present embodiment, the first
In addition to the effect obtained in the embodiment, it is possible to obtain the effect of further reducing the distortion of the reproduced reference signal. Even when pilot carriers CP are continuously inserted in the time direction as shown in FIG. 4, the same holds if the carrier with CP is used as the pilot carrier and the CP carrier signal itself is used as the symbol pilot signal. Is clear. In this case, it is obvious that the time direction interpolation circuit 13 of FIG. 1 is unnecessary. Further, in the above description, the case where the spacing between the pilot carriers is set to 6 is used, but it goes without saying that the same holds for any carrier spacing. In this case, the carrier interval M
If pc is set to a carrier interval of a power of 2, such as an interval of eight lines, a linear approximation operation can be realized by a bit shift and a simple addition / subtraction circuit, and the effect of reducing the circuit scale can be obtained.

[0018]

As described above, when the means according to the present invention is used, a reference signal having relatively little distortion can be provided even if the symbol pilot signal waveform changes in a complicated manner by simply adding a relatively small circuit to the conventional circuit. It is possible to obtain a good OFDM transmission device that can be reproduced and can reduce the code error rate of the demodulation code.

[Brief description of drawings]

FIG. 1 is a block diagram showing a reference signal reproducing circuit configuration according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram of a carrier structure of the OFDM system.

FIG. 3 is a detailed explanatory diagram of a carrier structure of an OFDM system.

FIG. 4 is a detailed explanatory diagram of a second carrier structure of the OFDM system.

FIG. 5 is a block diagram showing a circuit configuration of a conventional transmitter.

FIG. 6 is a block diagram showing a circuit configuration of a conventional receiving device.

FIG. 7 is an explanatory diagram of the structure of an OFDM signal.

FIG. 8 is a block diagram showing a configuration of a conventional reference signal reproducing circuit.

FIG. 9 is a first explanatory diagram of an interpolation method in the time direction according to the present invention.

FIG. 10 is a second explanatory diagram of an interpolation method in the time direction according to the present invention.

FIG. 11 is an explanatory diagram of an interpolation method in the carrier direction.

FIG. 12 is an explanatory diagram of problems in the interpolation method in the carrier direction.

FIG. 13 is an explanatory diagram of an example of a distortion amount due to interpolation in the carrier direction.

FIG. 14 is a schematic diagram showing an example of a distortion amount by linear approximation of carrier direction interpolation according to the present invention.

FIG. 15 is a schematic diagram showing the amount of interpolation distortion in the carrier direction according to the first embodiment of the present invention.

FIG. 16 is a schematic diagram showing a carrier structure according to a second embodiment of the present invention.

[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: reception pre-processing circuit, 8: signal clipping circuit,
9: FFT circuit, 10: reference signal reproduction circuit, 11: delay circuit, 12: post-reception processing circuit, 13: time direction interpolation circuit, 15, 20: carrier direction interpolation circuit, 19: filter circuit, 21: straight line Approximation circuit, 22: switch circuit.

Claims (3)

[Claims] 1. An orthogonal frequency division multiplexing modulation method for transmitting an information code by a plurality of carriers which are orthogonal to each other.
(OFDM system) receiving apparatus, wherein the OFDM system signal has a structure in which a carrier having a pilot signal in the time direction (pilot carrier) is inserted between carriers in a certain band. Input the received plurality of carrier signals and select the pilot signal inserted between the carrier signals as a symbol pilot signal, or a signal obtained by interpolating the pilot signals in the time direction. A reference signal reproducing circuit for reproducing a reference signal from the calculated symbol pilot signal and outputting the symbol pilot signal, the carrier interval Mpc from the calculated symbol pilot signal.
Of the symbol pilot signal, and a filter circuit for outputting a signal obtained by performing a filtering process on the selected symbol pilot signal with a number of taps of 3 taps or more, and the calculated symbol pilot signal. A linear approximation circuit that outputs a signal obtained by linearly approximating between signals as a second interpolation signal, the first interpolation signal and the second interpolation signal are input, and both ends within a fixed band are input. To the second interpolated signal as an interpolated signal in a certain range, the first interpolated signal as an interpolated signal in the other range, and the selected interpolated signal as a reproduction reference signal. A receiving apparatus having a reference signal reproducing circuit having a selecting circuit for outputting. 2. The space for inserting the pilot carriers according to claim 1, wherein the space between the carriers near both ends of the band is more than the space Mpc of the pilot carriers inserted between the carriers near the center of the band. A transmission device characterized in that a space between pilot carriers to be inserted is narrowed. 3. The pilot carrier according to claim 1, wherein the pilot carrier is inserted at least every carrier interval Mpc, and in the vicinity of both ends in the band, the pilot carrier is further inserted between the pilot carriers inserted at the carrier interval Mpc.
A transmission device having at least two pilot carriers.