JP2009253838A - Ofdm transmission apparatus and frequency conversion device therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an OFDM transmission apparatus and frequency conversion device therefor in which the configuration is simplified without deteriorating characteristics. <P>SOLUTION: By appropriately setting characteristics of a band pass filter 300, a desired band is positively extracted from a plurality of appearing image signals and is outputted as one carrier signal as it is. Namely, from the output of a digital/analog conversion circuit 200, the image signal which will be removed by a low-pass filter if it is usual, is conversely extracted using the band pass filter 300. Thus, a signal of a signal higher than the original Nyquist frequency of the digital/analog conversion circuit 200 can be obtained in simple configuration. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an OFDM transmission apparatus and a frequency conversion apparatus for retransmitting a video stream acquired from a network by performing OFDM (Orthogonal Frequency Division Multiplex) modulation.

  A so-called IPTV (IP Television) service is known that distributes a video stream to a viewer via a medium such as an IP (Internet Protocol) network. This type of service can be broadly divided into the following two. One is IP-VOD (Video on Demand), and the other is IP broadcasting including terrestrial digital-IP retransmission. Among various types of viewing services, in recent years, technological development relating to IP retransmission systems has been active.

  This type of system converts the video stream acquired from a high-speed network such as an optical fiber network into radio waves again and retransmits it within a local area (such as a private house or an apartment). By doing so, it is possible to obtain merits such as elimination of the troublesomeness of home wiring. As in the so-called terrestrial digital television broadcasting, the modulation method in the radio section is mainly the OFDM modulation method, and the set-top box (STB) connected to the ONU (Optical Network Unit) is a key device as in-home equipment.

The set-top box includes a digital / analog (D / A) conversion unit and a wideband frequency converter (upconverter) for OFDM modulation of the stream signal. Normally, the analog output from the D / A converter is mixed with the local signal, and is up-converted to a desired band. However, since the set top box is used for continuous operation for 24 hours, it is desired to reduce the number of parts as much as possible to reduce power consumption. Although there is a method of directly modulating a local signal using a quadrature modulator, this method requires two systems of D / A conversion circuits, and it is also necessary to precisely balance them.
Patent Documents 1 and 2 disclose techniques related to frequency conversion, but none of them is based on the OFDM method, and it is difficult to deteriorate characteristics in an analog high frequency region. It is difficult to apply these technologies to reduce the number of parts in the set top box.
JP 2001-339248 A JP 2002-26661 A

As described above, in the technical field of IP retransmission, there is a demand to reduce the number of components of a set-top box as an OFDM transmitter as much as possible without degrading characteristics, to reduce the size and to reduce power consumption. There is.
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an OFDM transmission apparatus and a frequency conversion apparatus thereof that can simplify the configuration without degrading characteristics.

  In order to achieve the above object, according to an aspect of the present invention, in an OFDM transmitter that OFDM-modulates a stream signal acquired from a backbone network and transmits the radio signal to a local area, the stream signal is based on ISDB-T. A processing circuit that outputs digital data that has undergone pre-modulation processing, a digital / analog conversion unit that digitally / analog converts the digital data and outputs an analog signal including an image signal, and is located in a carrier band of the image signal There is provided an OFDM transmitter comprising: a band-pass filter that passes an image signal to be transmitted; and a radio unit that synthesizes the image signal for each carrier and transmits the radio wave.

  That is, the inventor has focused on the fact that when a digital signal is converted from digital to analog, an image signal is output at an integer multiple of the sampling clock, and the present invention has been achieved. By actively using this property and using the image signal included in the analog signal output from the digital / analog converter as it is, a signal having a band higher than the sampling frequency can be obtained. Therefore, a circuit for up-converting the baseband signal to the radio band can be omitted, and the configuration can be simplified.

  According to the present invention, it is possible to provide an OFDM transmission apparatus and its frequency conversion apparatus that have a simplified configuration without degrading characteristics.

  FIG. 1 is a system diagram showing an example of use of an OFDM transmitter according to the present invention. The video distribution system shown in FIG. 1 is a system that distributes program content data by multicast using IP broadcast with the IP network 1000 as a core. Content data broadcast by radio waves from the broadcast station 10 is demodulated and reproduced by the tuner 30 and converted into IP data, and stream data in which video data for a plurality of channels is multiplexed is sent to the IP network 1000. As a result, an IP broadcast service including terrestrial digital-IP retransmission is realized.

  Similarly, the content data of the content holder 20 is converted to IP and then sent to the IP network 1000 to provide an IP-VOD format service. The IP packet that reaches the edge router 40 near the viewer is acquired by an IP television (IPTV) and the content is reproduced.

  By the way, an optical fiber F using PON (Passive Optical Network) technology is connected to the edge router 40, and this optical fiber F is terminated at the ONU 7 in the user home 60. The ONU 7 is further connected to a set-top box (STB) 8 in the house, and the IP video stream is retransmitted by radio waves from the antenna 6 after being processed by the STB 8. As a result, the television set (TV) 50 can be used to view the program anywhere in the user home 60. The STB 8 is positioned as an OFDM transmitter that retransmits stream data from the IP network 1000 by radio waves.

  At present, the standard system for terrestrial digital television broadcasting is ISDB-T (Integrated Services Digital Broadcasting for Terrestrial), and the orthogonal frequency division multiplexing (OFDM) system is adopted as a transmission system in Europe and Japan. The OFDM scheme has a feature that it is possible to improve delay interference resistance characteristics in a multipath propagation path by transmitting a wideband signal with a large number of carriers orthogonal to each other (hereinafter referred to as carriers).

  FIG. 2 is a functional block diagram showing the STB 8 of FIG. In FIG. 2, a video stream from the ONU 7 is subjected to digital signal processing by a CPU 1 including an arithmetic processing circuit such as a DSP (Digital Signal Processor) and is written in the memory 2. The written data is read from, for example, a plurality of processing circuits (hereinafter abbreviated as circuits) 31 to 3n corresponding to each channel, whereby the video data for each channel is separated. The separated data is subjected to premodulation processing and then input to the modulation unit 4. The modulation unit 4 performs OFDM modulation on the processed data, and the modulated signal is amplified at a high frequency by the wireless unit 5 and transmitted from the antenna 6 as a radio wave.

  FIG. 3 is a functional block diagram showing the main part of the STB 8 of FIG. FIG. 3 shows the configuration of one carrier among a number of carriers that are orthogonal frequency multiplexed. In FIG. 3, the circuits 31 to 3n of the STB 8 include an OFDM operation circuit 100, and the OFDM operation circuit 100 performs processing conforming to ISDB-T, such as data rearrangement, scrambling, and error processing. The data subjected to the modulation pre-processing is sampled at a constant rate in a D / A (digital / analog) conversion circuit 200 of the modulation unit 4 and converted into an analog signal, and a high-frequency component in a desired band by the band-pass filter 300 of the wireless unit 5. Are extracted, and then combined by a combining unit (not shown) and retransmitted from the antenna 6. Next, the operation of the above configuration will be described.

  FIG. 4 is a diagram for explaining the operation in STB 8 of FIG. As shown in FIG. 4 (normal signal), in a normal digital / analog conversion process, a band higher than the Nyquist frequency fN, which is a half frequency of the sampling frequency fs, is cut by a low-pass filter. The image signal is removed.

  In contrast, in this embodiment, as shown in FIG. 4 (signals to be used), by appropriately setting the characteristics of the band-pass filter 300, a desired band is actively extracted from a plurality of image signals that appear. This is output as a single carrier signal. That is, the image signal that is normally removed by the low-pass filter from the output of the digital / analog conversion circuit 200 is extracted using the band-pass filter 300. In this way, a signal having a frequency higher than the Nyquist frequency of the original digital / analog conversion circuit 200 can be obtained with a simple configuration.

  FIG. 5 is a block diagram showing a main configuration of an existing STB 8 for comparison. In FIG. 5, for example, in terrestrial digital broadcasting using the UHF band, it is necessary to handle an OFDM-modulated signal at a frequency of about 470 to 770 MHz. Therefore, first, a modulation signal in an intermediate frequency band of about 40 MHz is output by the digital / analog conversion circuit 200 'of about 100 MSPS. Further, a configuration in which an intermediate frequency signal is converted into a desired frequency by an up-converter including the frequency conversion circuit 400 and the local oscillation circuit 500 is generally used. If the signal is 500 MHz, the frequency is converted by multiplying it with the local signal of 460 MHz. At that time, the image signal is removed by the low-pass filter 600 provided on the output side of the digital / analog conversion circuit 200 '. In such a configuration, since the frequency conversion circuit 400 and the local oscillation circuit 500 are required, the configuration becomes complicated.

  On the other hand, in this embodiment, if a signal that appears at twice the sampling clock, for example, is used in the configuration shown in FIG. 3, a 50 MHz modulated wave is generated at 150 MSPS without using frequency conversion by an up converter. A radio band signal similar to that shown in FIG. 5 can be obtained.

  In addition, this embodiment assumes implementation in a transmitter of the OFDM modulation scheme. As is well known, in the OFDM modulation scheme, even if the interval between carriers is narrow, the transmission rate for each carrier can be kept low. Accordingly, the image signal generated by the digital / analog conversion is unlikely to deteriorate in analog characteristics even on the high frequency side. The inventor pays attention to this point, and conceives an OFDM transmission apparatus that has a simplified configuration without deterioration of characteristics by connecting a band pass filter 300 of an appropriate band to the output of the digital / analog conversion circuit 200. It came to.

  In other words, the inventor noticed that when digital / analog conversion of a digital signal is performed, an image signal is output at an integer multiple of the sampling clock, and this characteristic is actively used to output from the digital / analog conversion unit. It was conceived that a signal having a band higher than the sampling frequency can be obtained by using the image signal included in the analog signal as it is. In addition, an image signal with little degradation of analog characteristics can be obtained from the characteristics of OFDM modulation. Therefore, it is possible to simplify the configuration by omitting the circuit for up-converting the baseband to the radio band, and to suppress the deterioration of characteristics. From these facts, it is possible to provide an OFDM transmission apparatus and its frequency conversion apparatus that simplify the configuration without degrading characteristics.

The present invention is not limited to the above embodiment. For example, the STB 8 and the TV 50 may be connected by wire instead of radio waves by a coaxial cable or the like. If the modulation method in the meantime is OFDM, the same operation and effect can be obtained.
Although FIG. 2 shows an example in which the digital signal processing is performed by the CPU 1, the same processing can be performed by using only the hardware logic or both the CPU and the hardware logic instead.

  In addition, the frequency of the signal after frequency conversion and before frequency conversion is not limited to the numerical values described above. Furthermore, a means for varying the frequency of the signal processed and output by the OFDM arithmetic circuit 100 and the frequency of the band pass filter 300 may be provided. That is, the present invention can also be applied to a variable frequency type OFDM transmitter.

  Furthermore, the present invention is not limited to the above-described embodiments as they are, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment.

The system diagram which shows an example of utilization of the OFDM transmitter concerning this invention. The functional block diagram which shows STB8 of FIG. The functional block diagram which shows the principal part of STB8 of FIG. The figure for demonstrating the effect | action in STB8 of FIG. The block diagram which shows the principal part structure of the existing STB8 for the comparison.

Explanation of symbols

  1000 ... IP network, 10 ... broadcasting station, 20 ... content holder, 30 ... tuner, 40 ... edge router, F ... optical fiber, 6 ... antenna, 7 ... ONU, 8 ... set top box (STB), 50 ... TV set , 60 ... user home, 1 ... CPU, 2 ... memory, 31-3n ... processing circuit, 4 ... modulation unit, 5 ... radio unit, 100 ... OFDM arithmetic circuit, 200 ... D / A conversion circuit, 300 ... band pass filter

Claims (2)

In an OFDM transmission apparatus that modulates a stream signal acquired from a backbone network and performs radio wave transmission to a local area by OFDM (Orthogonal Frequency Division Multiplex) modulation,
A processing circuit for outputting digital data obtained by performing modulation preprocessing based on ISDB-T (Integrated Services Digital Broadcasting for Terrestrial) on the stream signal;
A digital / analog conversion section for digital / analog conversion of the digital data to output an analog signal including an image signal;
A band-pass filter for passing an image signal located in a carrier band among the image signals;
An OFDM transmitter comprising: a radio unit that synthesizes image signals for each carrier and transmits radio waves. In a frequency converter used in an OFDM transmitter that modulates a stream signal acquired from a backbone network with OFDM (Orthogonal Frequency Division Multiplex) and transmits the radio wave to a local area,
A digital / analog converter that digitally / analog-converts digital data subjected to modulation pre-processing based on ISDB-T (Integrated Services Digital Broadcasting for Terrestrial) to the stream signal and outputs an analog signal including an image signal; ,
A frequency converter comprising: a band-pass filter that passes an image signal located in a carrier band among the image signals.

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