JP2000013357A - Ofdm receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the interference effect of a near analog broadcast signal with a simple and inexpensive configuration. SOLUTION: The output of a band extracting filter section 103 to extract a frequency band of an orthogonal frequency division multiplex OFDM signal converted to a 1st IF band is given to an adjacent interference elimination section 104 consisting of a notch filter such as a ceramic filter or an LC filter inexpensive in comparison with a SAW filter to eliminate an analog adjacent interference signal of the OFDM signal that cannot be eliminated only by the filter section 103, that is, an adjacent interference signal component of an audio carrier at a lower side band or a video carrier of an upper side band by means of the notch filter. Although the notch filter gives an effect on the phase of the signal in an OFDM band, since the phase change of the OFDM signal is compensated through equalization and group delay demodulation processing, the effect is negligible substantially.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to an OFDM receiver used for digital TV broadcasting by the (orthogonal frequency division multiplexing) method, and particularly to a digital TV.
When a broadcast and an analog TV broadcast are broadcast on adjacent channels, and when an OFDM signal is selectively demodulated from a signal in which a digital TV broadcast and an analog TV broadcast are mixed, interference caused by analog broadcast adjacent to the digital TV broadcast may occur. The present invention relates to an improved technique when demodulation performance deteriorates.

[0002]

2. Description of the Related Art In recent years, digital modulation systems have been actively developed for transmission of audio signals and video signals. In Europe and Japan, an OFDM (Orthogonal Frequency Division Multiplexing) system is to be adopted as an optimal modulation system for digitizing terrestrial TV broadcasting. In the OFDM system, modulation and demodulation are performed by allocating data to a plurality of orthogonal carriers. This is because IFFT (Inverse Fast Fourier Transform) is performed on a plurality of symbol data on the transmitting side.
This can be realized by performing processing and performing FFT (Fast Fourier Transform) processing on the received data on the receiving side. O
For details of the FDM method, refer to the document ITU-RS (TG11
/ 3) or The Institute of Television Engineers of Japan Vol.17.No.54.p
p7-12, BCS No. 93-33 (Sep. 1993), etc., and here, conventional techniques related to the present invention will be described.

[0003] When digital broadcasting based on the OFDM system is introduced, switching from analog TV broadcasting to digital TV broadcasting completely places an excessive burden on viewers, so that both broadcasting services are provided in parallel for a certain period of time. It has been proposed. In this case, a vacant channel of the current analog broadcast is considered as a frequency band assigned to the digital broadcast. However, since the bands are close to each other, measures against adjacent interference are an important issue.

In order to solve this problem, conventionally, OF
SAW to remove interference (lower band audio carrier and upper band video carrier) of analog broadcasting adjacent to DM signal
It has been proposed to construct an expensive band extraction filter such as a filter in two stages. That is, OFDM
If adjacent interference of analog TV broadcasting is present in the lower band of the signal, the frequency of the audio carrier is close to the OFDM signal band, so that the single-stage band extraction filter cannot remove the audio carrier. For this reason, if the fast Fourier transform is performed as it is, deterioration of the calculation accuracy of the OFDM signal itself or carrier interference due to the interference level of the analog TV broadcast occurs, and the demodulation performance deteriorates. Therefore, interference removal is performed by stacking expensive SAW filters or the like in two stages.

[0005]

As described above, O
In the digital TV broadcasting by the FDM system, for the time being, it coexists with the current analog TV broadcasting, and since the transmission band is close to each other, it is inevitable to take countermeasures such as using a multi-stage configuration of an expensive filter capable of obtaining steep characteristics. This greatly affects the cost of the receiving apparatus.

Accordingly, the present invention solves the above problems,
An object of the present invention is to provide an OFDM receiving apparatus that can reduce the influence of interference caused by an analog broadcast signal that is close with a simple and low-cost configuration.

[0007]

In order to achieve the above object, the present invention provides an OFDM receiving apparatus used in an environment where digital broadcasting using the OFDM system is broadcast on a channel adjacent to analog broadcasting. A channel selection channel is extracted from the received signal of the digital broadcast by the frequency conversion unit, frequency-converted to a first intermediate frequency band, and channel selection converted to the first intermediate frequency band by the frequency conversion unit by the first intermediate frequency band filter unit. A signal in the OFDM transmission band is extracted from the digital broadcast reception signal of the channel, and the first intermediate frequency band signal obtained by the first intermediate frequency band filter means is converted into a second intermediate frequency band signal by the second frequency conversion unit. The band of the second intermediate frequency band signal obtained by the second frequency converter is limited to only low-frequency components by the low-pass filter, and the A / D conversion is performed. The analog signal obtained in the first intermediate frequency band filter means into a digital signal by the parts,
A quadrature detection unit performs quadrature detection on the signal converted to a digital signal, a fast Fourier transform unit performs a fast Fourier transform on the quadrature detection output, converts it from the time domain to the frequency domain, and a demodulation unit performs synchronous detection or Equalization processing and demodulation are performed by delay detection, and an error in the demodulation result is corrected by an error correction unit. The first intermediate frequency band filter means converts a digital broadcast reception signal of the selected channel into a signal of an OFDM transmission band. And a band removal filter unit for locally removing adjacent interference components due to the analog broadcast generated at a known frequency from the digital broadcast reception signal of the selected channel. .

[0008] In the above configuration, the first intermediate frequency band filter means, together with a band extracting filter section for extracting the band of the OFDM signal, locally removes the adjacent interference component caused by the analog broadcast occurring at a known frequency. By providing a band rejection filter unit and performing multi-stage connection in any order, analog adjacent interference of an OFDM signal that cannot be removed only by a band extraction filter unit, that is, adjacent interference components of a lower band audio carrier and an upper band video carrier are removed. Like that.

[0009]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows an OFD according to the present invention.
FIG. 3 shows the configuration of an M receiving apparatus, in which an RF signal received by a receiving antenna 101 is converted into a first IF band corresponding to a selected channel by a frequency conversion unit 102. This tuning,
This is performed by changing the frequency of the local oscillator 112 according to the tuning information signal input to the tuning information input terminal 111. The output of the frequency conversion unit 102 is input to a band extraction filter unit 103, where the OFDM signal transmission band is extracted, and further, the adjacent interference removal unit 10 using a notch filter.
At 4, the interference component is removed. This adjacent interference removal unit 10
4 is mixed with a local oscillation signal from an oscillator 114b by a mixer 114a constituting a second frequency conversion unit 114 and converted into a second IF band, and then is converted to a low-pass filter (LPF) 115 by a low-pass filter 115. The band is limited only to the component, and the signal is further converted into a digital signal by an A / D (analog / digital) converter 105.

[0010] Thereafter, quadrature detection is performed by quadrature detection section 106, conversion from time domain to frequency domain is performed by FFT section 107, demodulation section 108 performs demodulation processing by delay detection or synchronous detection, and performs error correction. Correction processing of an error occurring during transmission is performed by the unit 109 and output from the output terminal 110. The A / D conversion clock and clocks and timing signals used in other digital circuits are reproduced by the synchronous reproduction unit 113 from the received signals themselves.

The features of the above-described configuration that are different from the conventional configuration will be described below. As described above, a digital TV broadcast based on the OFDM scheme will be transmitted adjacent to an analog TV broadcast. in this case,
If the analog TV broadcast is adjacent to the lower band of the OFDM signal, the frequency of the audio carrier is close to the OFDM signal band, so that one stage of the band extraction filter cannot remove the audio carrier. If the fast Fourier transform is performed as it is, the calculation accuracy of the OFDM signal itself deteriorates due to the interference level of the analog TV broadcast, the carrier interference occurs, and the demodulation performance deteriorates.
It has been considered to adopt a technique of removing interference by configuring a two-stage SAW filter or the like that is more expensive than a notch filter.

On the other hand, according to the present invention, the output of the band extracting filter section 103 for extracting the band of the OFDM signal converted into the first IF band is constituted by a notch filter such as an inexpensive ceramic or LC as compared with a SAW filter. Input to the adjacent interference removal unit 104
The analog adjacent interference of the OFDM signal, which cannot be removed by only 3, that is, the adjacent interference component of the lower band audio carrier and the upper band video carrier is removed by the notch filter.

Here, the notch filter affects the phase in the OFDM band. In the case of the OFDM, however, the phase change is compensated for by the demodulation processing of equalization and differential detection, so that the effect is substantially eliminated. .

Referring to FIG. 2, as an example of the analog TV broadcast interference included in the OFDM signal, an operation of removing the interference of the OFDM signal received by the lower analog voice carrier by the adjacent interference remover 104 will be described. .

FIG. 2A shows an output spectrum of the frequency conversion unit 102 when adjacent interference exists in the upper band and the lower band of the OFDM signal. Since the frequency spectrum shown here is in the IF band, and the frequency relationship in the RF band is interchanged, the lower adjacent interference has a higher frequency than the OFDM signal, and the upper adjacent interference has a lower frequency than the OFDM signal. ing.

FIG. 2B shows the filter characteristics of the OFDM signal extraction filter unit 103. FIG.
(A) and FIG.
Since the voice carrier of the lower adjacent interference of the M signal has a frequency very close to the OFDM signal band, it cannot be removed by one stage of the OFDM signal extraction filter having the characteristics shown in FIG.

FIG. 2C shows the band elimination filter characteristics of the notch filter used in the adjacent interference elimination unit 104. This filter has a characteristic that a notch is formed in the voice carrier portion of the lower adjacent interference of the OFDM signal. This can be easily achieved because the frequency of the voice carrier is known. By using this notch filter, the band-pass filter unit 1 having the characteristics shown in FIG.
03, it is possible to reduce the voice carrier component of the lower band adjacent interference that cannot be removed in the OFDM signal.

FIG. 2D shows a spectrum of the OFDM signal output from the adjacent interference removal unit 104 to the A / D conversion unit 105. In this way, the signal to be A / D converted is an OFDM signal from which the component of the voice carrier that is the adjacent interference in the lower band is removed.

Although the above embodiment has been described on the assumption that adjacent interference always exists, there may be cases where adjacent interference does not occur. Therefore, in the adjacent interference removal unit 104,
A case in which interference removal is adaptively performed depending on whether there is adjacent interference will be described with reference to FIG.

FIG. 3 shows a configuration of the adjacent interference removing unit 104.
This shows a configuration in the case of performing interference removal adaptively depending on whether there is adjacent interference. Here, the adjacent interference can be detected, for example, by monitoring the signal subjected to the fast Fourier transform of the FFT unit 107 and determining whether there is an abnormal peak at a frequency at which the adjacent interference may occur.

First, the signal from the band extraction filter unit 103 is input to the interference removal filter unit 104a using the notch filter and the switching unit 104b. On the other hand, the signal from the FFT unit 107 is
c to detect whether there is adjacent interference, and output the result to the switching unit 104b.

The switching unit 104b selects the output of the interference removal filter unit 104a when there is interference and outputs the signal from the band extraction filter unit 103 when there is no interference according to the detection result of the adjacent interference detection unit 104. And outputs it to the A / D converter 105. According to this configuration, O
Since the interference component is removed only when there is adjacent interference in the FDM signal band, the influence on the OFDM signal band can be reduced when there is no adjacent interference.

In the above embodiment, the adjacent interference removal unit 1
Although 04 is arranged after the band extraction filter unit 103, the operation is the same even if it is arranged before.

In the above embodiment, the case where the audio carrier component of the lower band analog broadcast of the digital broadcast is removed has been described. However, the video carrier component of the upper band analog broadcast of the digital broadcast can also be removed by the same method. . The removal of both carrier components can be realized by configuring a band elimination filter such as a notch filter in two stages.

Further, in the above embodiment, the first IF
Adjacent interference removal is performed in the band, the signal is converted to the second IF band in a stage preceding the A / D converter 105, and the band is limited through the LPF 115. Such a configuration can widen the dynamic range after the second frequency converter 114 located at the subsequent stage, and can be said to be more advantageous than directly converting to the second IF band.

[0026]

As described above, according to the present invention, it is possible to provide an OFDM receiving apparatus capable of reducing the influence of interference caused by an analog broadcast signal with a simple configuration at a low cost.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of an OFDM receiving apparatus according to the present invention.

FIG. 2 is a waveform chart for explaining how to remove a voice carrier that becomes lower adjacent interference in the embodiment.

FIG. 3 is a block diagram showing a configuration for operating the adjacent interference removal only when there is interference in the embodiment.

[Explanation of symbols]

101: receiving antenna, 102: first frequency converter,
103: band extraction filter, 104: adjacent interference removal unit, 104a: interference removal filter unit, 104b: switching unit, 104c: adjacent interference detection unit, 105: A / D conversion unit, 106: quadrature detection unit, 107: FFT unit , 108 ...
Demodulation unit, 109: error correction unit, 110: output terminal, 11
1 ... channel selection information input terminal, 112 ... local oscillator, 113 ...
Synchronous reproduction unit, 114 ... second frequency conversion unit, 114a ...
Mixer, 114b: oscillator, 115: low-pass filter.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tatsuya Ishikawa 8 Shinsugita-cho, Isogo-ku, Yokohama-shi, Kanagawa Prefecture Inside Toshiba Multimedia Technology Research Laboratories (72) Inventor Noboru Taga 3-3-9 Shimbashi, Minato-ku, Tokyo No. Toshiba Abu E Co., Ltd. (72) Inventor Takashi Seki 8 Shinsugita-cho, Isogo-ku, Yokohama-shi, Kanagawa Prefecture Inside Toshiba Multimedia Technology Research Laboratories (72) Inventor Yuji Ohashi 3 Shimbashi, Minato-ku, Tokyo 3-9, F-term in Toshiba AV EE Co., Ltd. 5K022 DD01 DD31 DD33 5K052 AA01 BB03 BB21 CC06 DD04 EE12 EE17 EE30 EE40 FF04 GG02 GG14 GG33 GG48

Claims (6)

[Claims] An OFDM receiving apparatus used in an environment in which digital broadcasting using an orthogonal frequency division multiplexing (hereinafter, OFDM) system is broadcast on a channel adjacent to analog broadcasting, wherein a channel is selected from a received signal of the digital broadcasting. A first frequency conversion unit that extracts a channel and converts the frequency to a first intermediate frequency band; and converts an OFDM transmission band signal from a digital broadcast reception signal of a selected channel converted to the first intermediate frequency band by the frequency conversion unit. A first intermediate frequency band filter that is extracted; and a second intermediate frequency band signal that is converted by the first intermediate frequency band filter into a second intermediate frequency band signal.
A low-pass filter unit for band-limiting the second intermediate frequency band signal obtained by the second frequency conversion unit to only low-frequency components; and an analog signal obtained by the low-pass filter unit. An A / D conversion unit that converts the signal into a digital signal; a quadrature detection unit that performs quadrature detection on the signal converted into a digital signal by the A / D conversion unit; and a detection output of the quadrature detection unit from the time domain by fast Fourier transform. A fast Fourier transform unit for converting to the frequency domain; a demodulation unit for equalizing and demodulating the fast Fourier transform result of the fast Fourier transform unit by synchronous detection or delay detection; and an error in the demodulation result obtained by the demodulation unit. An error correction unit that corrects the OFDM transmission band signal from the digital broadcast reception signal of the selected channel. A band extracting filter section for extracting, and a band removing filter section for locally removing adjacent interference components due to the analog broadcast generated at a known frequency from the digital broadcast reception signal of the selected channel. OFDM receiver. 2. The band elimination filter section of the first intermediate frequency band filter means has a characteristic of removing at least one of an audio carrier component of a lower band analog broadcast and a video carrier component of an upper band analog broadcast of a digital broadcast. 2. The OFDM receiver according to claim 1, further comprising: 3. A band rejection filter section of the first intermediate frequency band filter means, comprising: a first band rejection filter having a characteristic of removing an audio carrier component of a lower band analog broadcast of a digital broadcast; and an upper band of the digital broadcast. 2. A multi-stage second band rejection filter having a characteristic of removing a video carrier component of an analog broadcast.
The OFDM receiver according to any one of the preceding claims. 4. The first intermediate frequency band filter means further comprises: an interference detection unit for detecting whether there is a known adjacent interference from the digital broadcast reception signal of the selected channel,
A switching unit that selectively outputs the output of the band elimination filter unit when the interference is detected by the interference detection unit, and passes and outputs the band elimination filter unit when no interference is detected. The O according to claim 1, wherein
FDM receiver. 5. The interference detection unit monitors the conversion output of the fast Fourier transform unit and detects the presence or absence of the adjacent interference based on whether there is an abnormal peak at a frequency at which adjacent interference may occur. The OFDM receiving apparatus according to claim 4, wherein: 6. The apparatus according to claim 5, wherein said interference detection unit monitors a frequency corresponding to at least one of an audio carrier of a lower band analog broadcast and a video carrier of an upper band analog broadcast. OFDM
Receiver.