JP2003143101A - Direct conversion receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a direct conversion receiver in which a circuit scale can be reduced by eliminating an analog IF filter. SOLUTION: The receiver is provided with a down converter capable of frequency-converting an analog RF signal modulated by an OFDM (orthogonal frequency division multiplexing) system so that the upper end frequency or the lower end frequency of the occupied frequency of a receiving object segment may be near a frequency of zero, an AD conversion means for converting a down conversion signal obtained by the down converter, an FFT (fast Fourier transformation) means for converting a time axis to a frequency axis by performing fast Fourier transformation to a signal obtained by the AD conversion means, and an unnecessary component removal means for removing unnecessary components except for the receiving object segment from the output signal of an FFT means.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a direct conversion receiver, and more particularly to a direct conversion receiver that can be used for partial reception of only one segment in terrestrial digital TV broadcasting.

[0002]

2. Description of the Related Art In recent years, in a system for transmitting a video signal or an audio signal, an OFDM (Orthogonal Frequency Division Multiplexing) system has been proposed as a system excellent in high quality transmission and improvement in frequency utilization efficiency. ing.

The OFDM system is a modulation system in which a large number of subcarriers are set up within the band of one channel. For example,
MPE after converting analog TV signal to digital signal
Data compression is performed by G (Moving Picture Experts Group). Byte interleaving and bit interleaving are performed to disperse the cause of error occurrence in the transmission line such as noise in this data signal, and QPSK (Quadrature
Phase Shift Keying), 16QAM (Quadrature Amplit)
ude Modulation) and other mapping methods.

The mapped data is subjected to IFFT (Inverse Fourier Transform) after quadrature modulation after time interleaving and frequency interleaving to disperse the cause of error occurrence in the transmission path such as fading.
It is converted into an RF frequency and transmitted.

FIG. 1 shows the configuration of a digital television receiver.

[0006] In the digital television receiver, the TV signal is demodulated by performing an operation which is completely opposite to that on the transmitting side.

The RF input input from the antenna is input to the mixer 21. The mixer 21 includes a local signal generator 22.
A signal according to the channel selection is also input from, and the desired frequency is BPF.
A signal whose frequency has been converted so as to fall within the band of the (bandpass filter) 23 is output. The BPF 23 extracts only the desired frequency component.

The output of the BPF 23 is input to the mixers 24 and 25, respectively. To the mixers 24 and 25, the cosine signal and the sine signal are input from the 90-degree phase shifter 27, which receives the signal from the local signal generator 26. The mixers 24 and 25 down-convert the output of the BPF 23, which is the IF frequency, to obtain the real axis (I axis) component and the imaginary axis (Q axis).
It is converted into a Low IF signal composed of (axis) components and output to the analog / digital converters 7 and 8.

The analog / digital converters 7 and 8 convert the analog signals (I-axis component and Q-axis component) into digital signals and output them to the FFT circuit 9. The FFT circuit 9 performs a fast Fourier transform on the input signal, converts the time axis data into frequency data, and outputs the frequency data to the frequency deinterleave circuit 12.

The frequency deinterleave circuit 12 restores the frequency interleave performed to compensate for the loss of the specific frequency signal due to the reflection of radio waves. The output of the frequency deinterleave circuit 12 is the time deinterleave circuit 13
Sent to. The time deinterleave circuit 13 restores the time interleave applied for anti-fading or the like.

The time-deinterleaved I-axis and Q-axis signals are sent to the demapping circuit 14 and converted into 2 bits (QPSK), 4 bits (16QAM) or 6 bits (64QAM). The demapped signal is sent to the bit deinterleave circuit 15. The bit deinterleave circuit 15 cancels the bit interleave performed for the purpose of increasing error resilience. The output of the bit deinterleave circuit 15 is sent to the Viterbi decoding circuit 16. The Viterbi decoding circuit 16 performs error correction using the convolutional code performed on the transmission side.

The signal subjected to the Viterbi decoding is sent to the byte deinterleave circuit 17. The byte deinterleave circuit 17 cancels the byte interleave performed for the purpose of increasing the error resilience like the bit interleave. The output of the byte deinterleave circuit 17 is sent to the RS decoding circuit 18. The RS decoding circuit 18 is an RS (Reed Solomon)
Decoding is performed and error correction is performed. The error-corrected signal is
It is sent to the MPEG decoding circuit 19. The MPEG decoding circuit 19 expands the error-corrected signal (compressed signal) and outputs it to the digital / analog converter 20. The digital / analog converter 20 converts the signal sent from the MPEG decoding circuit 19 into an analog video and analog audio signal and outputs it.

In the Japanese terrestrial digital broadcasting system,
A signal format divided into segments is adopted. In the case of television broadcasting, 13 segments are grouped together and transmitted within a band of 6 MHz. In addition, in the central one of the 13 segments, partial reception that allows data broadcasting and the like is possible with only one segment. In contrast to 13-segment reception, the bandwidth is only 1/13 in partial reception, and reception is possible with almost the same configuration.

FIG. 2 shows the spectrum of each part in FIG. 1 when partial reception is performed.

FIG. 2A shows terrestrial digital broadcasting (UH).
The RF signal spectrum for F) is shown. Figure 2
In (a), the spectra for UHF 14-16 channels are shown. 1 channel is 6 HMz
Thus, one channel is composed of 13 segments. The central segment of these is a signal for partial reception.

FIG. 2 (b) shows the spectrum of the RF signal of UHF15. Of 13 segments, 6 of the central part
The spectrum for one segment S15-0 to S15-6 is shown.

FIG. 2 (c) shows the spectrum of the IF signal for the UHF 15 output from the mixer 21.
Of the three segments, the six central segments S15
The spectrum for -0 to S15-6 is shown.

FIG. 2D shows the BP at the time of partial reception.
The spectrum of the IF signal output from F23 is shown. At the time of partial reception, U is set by the BPF 23.
Of the IF signal spectrum of HF15, 1 in the center
Only the spectrum for one partial reception target segment S15-0 is extracted.

FIG. 2E shows the spectrum of the LowIF signal output from the mixers 24 and 25 during partial reception.

As the reception system, the so-called superheterodyne system in which the IF frequency is once converted and then the IF frequency is converted into the LowIF has been described above. QPSK
In the single carrier transmission method such as modulation, a direct conversion method for directly converting an RF signal into a baseband signal is also used as another reception method. In the direct conversion method, the bandpass filter 23 or the like which is usually realized by the SAW filter is not necessary, so that the number of parts can be deleted.

[0021]

However, since the reception of OFDM uses the Low IF signal, the conventional direct conversion system cannot be applied. The reason for this is that, as shown in FIG. 4C described in the embodiment, due to direct conversion, the partial reception target segment S15-0 and the segment S15-1 adjacent thereto are received.
This is because the turn-around occurs between the adjacent segments and the adjacent segment is multiplexed to the partial reception target segment.

An object of the present invention is to provide a direct conversion receiver which can reduce the number of analog IF filters and can reduce the circuit scale.

[0023]

In a first direct conversion receiver according to the present invention, an analog RF signal modulated by the OFDM system has an upper end frequency or a lower end frequency of an occupied frequency of a segment to be received near zero frequency. As described above, a down converter for performing frequency conversion, an AD conversion unit for converting a down-converted signal obtained by the down converter into a digital signal, and a signal obtained by the AD conversion unit are subjected to fast Fourier transform,
FFT means for converting time axis to frequency axis, and FFT
It is characterized by comprising an unnecessary component removing means for removing an unnecessary component other than the reception target segment from the output signal of the means.

A second direct conversion receiver according to the present invention is an analog R modulated by the OFDM system.
Down converter for converting the frequency of the F signal so that the upper frequency or the lower frequency of the occupied frequency of the segment to be received is near frequency zero, and a filter means and a filter means for removing high frequency components from the down converted signal obtained by the down converter. From the output signal of the FFT means for converting the time axis to the frequency axis by performing a fast Fourier transform on the signal obtained by the AD conversion means , And an unnecessary component removing means for removing an unnecessary component other than the reception target segment.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to FIGS. 3 and 4.

FIG. 3 shows the structure of a digital television receiver for performing partial reception. FIG. 4 shows the spectrum of each part of FIG.

In FIG. 3, the same parts as those in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted. The receiver of FIG. 3 and FIG.
The difference from the conventional example is as follows.

(1) A circuit (mixer 2) for converting an RF signal into an IF signal and further into a Low IF signal
1, the local signal generator 22, the BPF 23, the local signal generator 26, the mixers 24 and 25, and the 90-degree phase shifter 27) have been deleted. (2) A down converter for directly converting the RF signal into a Low IF signal, that is, a down converter for frequency-converting the RF signal so that the lower end frequency (or upper end frequency) of the occupied frequency of the partial reception target segment becomes near zero frequency. Converter (local signal generator 1, mixers 2, 3, 90
Degree phaser 4) and LPF 5, 6 added. (3) The unnecessary component removing circuits 10 and 11 are added.

In terrestrial digital broadcasting, the spectrum is transmitted as shown in FIG. In FIG. 4A, U
The spectra for the HF 14-16 channels are shown. One channel is 6 HMz and one channel is composed of 13 segments. Of these, the central segment is a signal for partial reception, and only the partial reception unit can receive it. A case where the UHF 15 is partially received will be described.

FIG. 4B is an enlarged view of the UHF 15, in which the segment which is the partial receiving section is S15-0 and the preceding and following segments are S15-1 to S15-12, of which S1 is
5-0 to S15-6 are shown.

The RF signal of the UHF 15 is frequency-converted to Low IF by the down converter (local signal generator 1, mixers 2, 3, 90-degree phase shifter 4) of FIG. From the local signal generator 1, the segment S15 of the partial receiving unit
A signal having a frequency such that −0 becomes Low IF (for frequency conversion of the RF signal so that the lower end frequency of the occupied frequency of the partial reception target segment S15-0 becomes near frequency zero) is output, and the real axis, To output the imaginary axis component,
The 90-degree phase shifter 4 outputs a cosine signal and a sine signal to the mixers 2 and 3.

The spectrum after frequency conversion by the down converter is as shown in FIG. 4 (c). Since the down converter frequency-converts the RF signal so that the lower end frequency of the occupied frequency of the partial reception target segment S15-0 becomes near the frequency zero, the partial reception target segment S15-0
The signal in which the component of the segment S15-1 is folded is multiplexed in the −0 frequency band.

Similarly, segments S15-2 and S15-
3, S15-4 and S15-5 overlap. LPFs 5 and 6 remove harmonic components from this signal. LPF
The output spectrum after removing the harmonic components by 5 and 6 is
It becomes as shown in FIG.

With respect to this signal, the FFT circuit 9 converts the time axis into the frequency axis. The number of points of the FFT circuit 9 is required to be two or more segments in order to separate the signals of two segments. Further, since the segments in the UHF 15 have orthogonality, the partial reception target segment S1
The orthogonality is maintained in both 5-0 and the segment S15-1. Therefore, in the FFT circuit 9, the segment S15-
1 negative frequency component and partial reception target segment S15-
Positive frequency components of 0 are completely separable. The output spectrum of the FFT circuit 9 is as shown in FIG.

By the unnecessary component removing circuits 10 and 11, FF
Segment 1 which is a negative frequency component from the output of the T circuit 9
5-1 is removed and output to the frequency deinterleave circuit 12. Therefore, the spectrum of the output signals of the unnecessary component removing circuits 10 and 11 is only the partial reception target segment S15-0 as shown in FIG. 4 (f). The signal processing after the frequency deinterleave circuit 12 is the same as that of the receiver of FIG.

For reference, the principle that the FFT circuit 9 can detect a negative frequency component will be described.

For simplification, there is one carrier in each of the segment S15-0 and the segment S15-1 which are the partial receivers, and the signals of the respective carriers are represented by S0 and S of the following equations (1) and (2).
It is assumed to be 1.

[0038] S0 = A * cos ((wc + α) t) + B * sin ((wc + α) t)) (1) S1 = C * cos ((wc-α) t) + D * sin ((wc-α) t)) (2)

Here, A to D are signal amplitudes, and wc is a segment S15-0 and a segment S15- which are partial receiving sections.
The frequency at the boundary with 1. Therefore, the RF signal is
It becomes as shown in the following formula (3).

[0040]   RF = A * cos ((wc + α) t) + B * sin ((wc + α) t)) + C * cos ((wc-α) t) + D * sin ((w c-α) t))… (3)

This RF signal is used as Lo for each of the real axis and the imaginary axis.
This RF signal is converted to cos (wc
t), sin (wct), and then remove the harmonic components (coefficient 1 /
2 is omitted), the real axis component I is as shown by the following equation (4), and the imaginary axis component Q is as shown by the following equation (5).

[0042]   I = A * cos (αt) + B * sin (αt) + C * cos (-αt) + D * sin (-αt) (4)   Q = A * sin (-αt) + B * cos (αt) + C * sin (αt) + D * cos (αt)… (5)

Since this signal is FFTed,
os (-αt) -jsin (-αt) is multiplied and integrated for one cycle, A +
When jB is obtained, and cos (αt) -jsin (αt) is multiplied and integrated for one period, C + jD is obtained, so that the positive frequency component and the negative frequency component can be completely separated.

In order to simplify the explanation, the carriers of the segment S15-0 and the segment S15-1 which are the partial receivers are defined as in the equations (1) and (2). For this reason,
In the FFT conversion, the positive and negative of the frequency are reversed, but this is not an essential problem.

[0045]

According to the present invention, it is possible to reduce the number of analog IF filters that are usually realized by SAW filters, and it is possible to reduce the cost of the receiver.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a conventional digital television receiver.

FIG. 2 is a schematic diagram showing a spectrum of each part of FIG.

FIG. 3 is a block diagram showing a configuration of a digital television receiver according to the embodiment of the present invention.

FIG. 4 is a schematic diagram showing a spectrum of each part of FIG.

[Explanation of symbols]

1 Local signal generator A few mixers 4 90 degree phaser 5, 6 LPF 9 FFT circuit 10, 11 Unnecessary component removal circuit

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kiyoo Hanafusa             2-5-3 Keihan Hondori, Moriguchi City, Osaka Prefecture             Within Yo Denki Co., Ltd. F-term (reference) 5K022 DD01 DD33

Claims (2)

[Claims] 1. An analog RF modulated by the OFDM system
Down converter for converting the frequency of the signal so that the upper or lower frequency of the occupied frequency of the reception target segment is near zero frequency, AD conversion means for converting the down-converted signal obtained by the down converter into a digital signal, AD conversion Fast Fourier transform is performed on the signal obtained by the means to transform the time axis into the frequency axis.
Means for removing unnecessary components other than the reception target segment from the output signal of the FFT means, and a direct conversion receiver characterized by the above-mentioned. 2. An analog RF modulated by the OFDM system
A down converter for converting the frequency of the signal so that the upper or lower frequency of the occupied frequency of the reception target segment is near zero frequency; a filter means for removing high frequency components from the down converted signal obtained by the down converter; AD that converts the output signal into a digital signal
FFT for performing fast Fourier transform on the signal obtained by the converting means and the AD converting means to convert the time axis into the frequency axis
Means for removing unnecessary components other than the reception target segment from the output signal of the FFT means, and a direct conversion receiver characterized by the above-mentioned.