JP3562994B2 - OFDM receiver - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an OFDM receiving apparatus that receives a transmission signal based on an orthogonal frequency division multiplexing (OFDM) modulation scheme.
[0002]
[Prior art]
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 particular, in digital terrestrial broadcasting, an OFDM modulation method having features such as resistance to multipath interference and high frequency use efficiency has been receiving attention.
[0003]
In digital terrestrial broadcasting, as shown in FIG. 2, it is considered that one frequency band (channel) is divided into a plurality of segments to transmit a plurality of layers or a plurality of services. For example, digital terrestrial TV broadcasting uses 13 segments, and digital audio broadcasting uses 3 segments or 1 segment. Details of these are described in Journal of the Institute of Image Information and Television Engineers, Vol. 52, no. 11, pp. 1562-1566.
[0004]
Therefore, an OFDM receiver for receiving digital terrestrial audio broadcasting needs to switch between three segments (wide band) or one segment (narrow band) to receive.
FIG. 4 is a block diagram showing a configuration of a conventional OFDM receiver.
[0005]
The OFDM modulated wave input to the input terminal 401 is selected by the tuner 402 based on channel selection information from the input terminal 412, converted into an IF signal, and supplied to the A / D conversion circuit 403 by a clock supplied from the timing reproduction circuit 410. It is converted to a digital signal. The output of the A / D conversion circuit 403 is converted into a baseband in-phase component (I signal) and a quadrature component (Q signal) in an IQ conversion circuit 405, and a carrier frequency error is canceled in a frequency conversion circuit 406. The output of the A / D conversion circuit 403 is also supplied to the AGC circuit 404, where the average power of the received signal is detected. From the result, an AGC control signal is obtained and fed back to the tuner 402. The output signal of the frequency conversion circuit 406 is converted into frequency axis data by an FFT (fast Fourier transform) circuit 407 by an FFT operation, and the received data is demodulated in a data demodulation circuit 408 and supplied to an output terminal 409.
[0006]
The timing reproduction circuit 410 receives the output signal from the frequency conversion circuit 406, reproduces a necessary timing signal in the FFT circuit 407, performs clock reproduction, and supplies a reproduced clock to the A / D conversion circuit 403.
[0007]
The frequency synchronization circuit 411 receives the output signal of the FFT circuit 407, detects a pilot signal inserted into a specific frequency slot and transmits the signal, detects a carrier frequency error, and outputs the frequency error signal to the frequency conversion circuit 406. Output, and the carrier frequency error is canceled.
[0008]
As described above, in the conventional OFDM receiver, the input received signal is selected by the tuner 402. At this time, in digital terrestrial audio broadcasting, it is adjusted to the broadcast using three segments (wide band) or one segment (narrow band). The filter characteristics of the tuner 402 are switched. In general, a tuner removes a signal outside a desired band by a band-pass filter in an IF stage. Therefore, there are two types of band-pass filters having a pass band of 3 segments (wide band) and a band-pass filter having a pass band of 1 segment (narrow band). It is possible.
[0009]
Alternatively, the pass band of the band-pass filter of the tuner 402 is set to only three segments (wide band), and when a one-segment (narrow band) transmission signal is received, the tuner 402 selects the station by regarding the three-segment (wide band) transmission signal. Then, reception can be performed by setting one center segment of the FFT operation result as valid demodulated data.
[0010]
[Problems to be solved by the invention]
In a conventional OFDM receiver, it is necessary to prepare two types of bandpass filters, one for receiving a three-segment (wideband) transmission signal and the other for receiving a one-segment (narrowband) transmission signal. This is a major problem in reducing the size and cost of the OFDM receiver.
[0011]
Even when a one-segment transmission signal is received using a band-pass filter of only three segments (broadband), adjacent channel interference of an analog TV signal exists on a transmission path on which digital terrestrial broadcasting and analog terrestrial broadcasting coexist. In this case, as shown in FIG. 5, when one segment at the end of the frequency band (channel) is received, the energy of the analog TV signal is concentrated near the video carrier and the audio carrier. The signal in the vicinity of the carrier cannot be suppressed, which causes a problem in receiving performance due to the influence of adjacent channel interference.
[0012]
Therefore, in the present invention, when a signal of one segment (narrow band) at the end of a frequency band (channel) is received, the tuner selection frequency is offset to suppress the influence of adjacent channel interference, and the frequency offset is performed by a quadrature detection circuit. It is an object of the present invention to provide an OFDM receiving apparatus capable of suppressing the influence of adjacent channel interference by using a tuner having only a three-segment (wideband) bandpass filter by performing channel selection control to cancel.
[0013]
[Means for Solving the Problems]
In the OFDM receiving apparatus according to the present invention, one frequency band (channel) is divided into N divided frequency bands (segments), and M (1 ≦ M <N) divided frequency bands (segments) among the N divided frequency bands (segments). Is used, a first transmission mode in which a transmission signal is transmitted, and a division frequency band (segment) at the end of the frequency band (channel), L division (M <L ≦ N) of the N divisions An input end to which an orthogonal frequency division multiplexing (OFDM) modulated signal in which a frequency band (segment) is used and a second transmission mode in which the transmission signal is transmitted is input, and L divided frequency bands (segments) , A quadrature detecting means for quadrature detecting a signal from the tuner, and a signal of the M divided frequency bands (segments) at the end of the frequency band (channel) Upon reception, an offset means for offsetting a tuning frequency of the tuner in order to suppress the influence of adjacent channel interference, and receiving signals of the M divided frequency bands (segments) at the end of the frequency band (channel). A tuning control means for outputting a tuning control signal for canceling the offset by the quadrature detection means to the quadrature detection means.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail.
FIG. 1 is a block diagram showing a configuration of an OFDM receiving apparatus according to an embodiment of the present invention.
[0015]
The tuning information is input to a tuning control circuit 114 via an input terminal 113. The tuning control circuit 114 supplies a tuning frequency control signal corresponding to the input tuning information to the tuner 102 and also removes the offset. The signal is supplied to the adder 112. The OFDM modulated wave input to the input terminal 101 is tuned by the tuner 102 according to a tuning frequency control signal input from the tuning control circuit 114, is converted into an IF signal, and is supplied to the A / D conversion circuit 103. . In the A / D conversion circuit 103, the digital signal is converted by a reproduction clock supplied from the timing reproduction circuit 110.
[0016]
The output of the A / D conversion circuit 103 is subjected to quadrature detection by the frequency control signal from the adder 112 in the quadrature detection circuit 105, and is converted into a baseband in-phase component (I signal) and a quadrature component (Q signal). The output of the A / D conversion circuit 103 is also supplied to the AGC circuit 104, where the average power of the received signal is detected. From this result, the AGC control signal is obtained and fed back to the tuner 102.
[0017]
The output of the quadrature detection circuit 105 is supplied to a low-pass filter (hereinafter, referred to as LPF) 106, from which unnecessary high-frequency components are removed, and then to an FFT (fast Fourier transform) circuit 107 and an LPF 106. In the FFT circuit 107, the signal supplied from the LPF 106 is converted into frequency axis data by the FFT operation in the FFT circuit 107, and the signal converted into the frequency axis data is supplied to the data demodulation circuit 108 and the frequency synchronization circuit 111.
[0018]
The timing reproduction circuit 110 reproduces a necessary timing signal in each circuit by using a signal supplied from the LPF 106, supplies a timing signal required for FFT to the FFT 107, and converts the reproduced clock signal to an A / D conversion circuit. Output to 103.
[0019]
The frequency synchronization circuit 111 uses the signal supplied from the FFT circuit 107 to detect a pilot signal inserted into a specific frequency slot and transmitted to detect a carrier frequency error, and the detected frequency error signal is The signal is supplied to the adder 112.
[0020]
In the data demodulation circuit 108, the output of the FFT circuit 107 is demodulated, and the demodulated received data is supplied to the output terminal 109 and output. The LPF 106 has a filter characteristic having a pass band characteristic of one segment (narrow band) at the time of initial synchronization pull-in. After synchronization, the LPF 106 decodes a transmission parameter multiplexed and transmitted on a transmission signal, and decodes the decoded transmission parameter. Is changed to a one-segment (narrow band) or three-segment (wide band) passband characteristic in accordance with.
[0021]
The adder 112 adds the offset removal signal from the tuning control circuit 114 and the frequency error signal from the frequency synchronization circuit 111 and supplies the result to the quadrature detection circuit 105 as a frequency control signal. Thereby, the quadrature detection circuit 105 cancels the carrier frequency error.
[0022]
Tuning information is input to the tuning control circuit 114, and when the segment to be tuned is one segment (narrow band) at the end of the frequency band (channel) as shown by oblique lines in FIG. The tuner 102 is supplied with a tuning frequency control signal for offsetting the tuning frequency of the tuner 102 so that adjacent channel interference can be effectively removed, and further, the offset removal for canceling the frequency offset by the quadrature detection circuit 105. The signal is output to the adder 112.
[0023]
In the OFDM receiver having the above configuration, even when one segment (narrow band) at the end of the frequency band (channel) is received, the tuner 102 can remove adjacent channel interference and perform good reception.
[0024]
FIG. 3 is a block diagram illustrating a configuration example of the tuner 102.
The OFDM modulated wave from the input terminal 101 in FIG. 1 is supplied to the input terminal 601. The tuning frequency control signal from the tuning control circuit 114 in FIG. 1 is supplied to the input terminal 609. In the local oscillator 607, a local oscillation signal whose frequency is controlled by the tuning frequency control signal supplied via the input terminal 609 is supplied to the frequency conversion circuit 602. In the frequency conversion circuit 602, the OFDM modulated wave from the input terminal 601 is frequency-converted into a first IF signal by a local oscillation signal from the local oscillator 607, and is supplied to the band-pass filter 603. From the first IF signal, only a desired OFDM signal is extracted by the band-pass filter 603 and supplied to the frequency conversion circuit 604.
[0025]
In the frequency conversion circuit 604, the signal from the band-pass filter 603 is frequency-converted into a second IF signal by a local oscillation signal from the local oscillator 607, and is supplied to the low-pass filter 605. From this second IF signal, only a low-frequency signal is extracted by a low-pass filter 605 and supplied to an output terminal 606 as an IF signal. Note that the filter characteristic of the low-pass filter 605 is such that the aliasing component in the subsequent A / D conversion circuit 103 is removed, and the band-pass filter characteristic of the tuner 102 is a three-segment (wide band) reception. It is for.
[0026]
The frequency offset amount can be arbitrarily determined within a range where adjacent channel interference is removed by a bandpass filter of the tuner 102 and a received OFDM signal is not removed by the bandpass filter. Thus, the tuner 102 can cope without reducing the tuning frequency control interval, and the phase noise characteristic of the local oscillator 607 in the tuner 102 does not deteriorate due to the reduction of the tuning frequency control interval of the tuner. .
[0027]
In the above embodiment, the three-segment receiver has been described as the wideband OFDM receiver, and the one-segment receiver has been described as the narrowband OFDM receiver. However, the present invention is not limited to this. The present invention can be similarly applied to a 13-segment receiving apparatus by applying the present invention even when the narrow-band OFDM receiving apparatus is a 3-segment receiving apparatus.
[0028]
【The invention's effect】
As described above, according to the present invention, when a wideband OFDM receiving apparatus that receives an OFDM signal divided into a plurality of segments and transmitted receives a narrowband OFDM signal at a frequency band (channel) end, By offsetting the frequency of the tuner and performing channel selection control so as to cancel the frequency offset by the quadrature detection circuit, adjacent channel interference can be removed, and an OFDM receiver with improved reception performance can be obtained.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of an OFDM receiver according to the present invention.
FIG. 2 is a diagram illustrating filter characteristics of a tuner 102;
FIG. 3 is a block diagram showing a configuration example of a tuner 102.
FIG. 4 is a block diagram showing a configuration of a conventional OFDM receiver.
FIG. 5 is a view showing filter characteristics of a tuner 402.
[Explanation of symbols]
102: tuner, 103: A / D conversion circuit, 104: AGC circuit, 105: quadrature detection circuit, 106: low-pass filter, 107: FFT circuit, 108: data demodulation circuit, 110: timing reproduction circuit, 111: frequency synchronization circuit .., 112... An adder, 114.

Claims (4)

A first transmission mode in which one frequency band is divided into N divided frequency bands, and a transmission signal is transmitted using M divided frequency bands (1 ≦ M <N) among the N divided frequency bands; When the frequency band includes the divided frequency band at the end of the frequency band, an orthogonal frequency in which L (M <L ≦ N) of the N divided frequency bands are used and the second transmission mode in which the transmission signal is transmitted exists. An input end to which the division multiplex modulation signal is input,
A tuner having bandpass filters of L divided frequency bands,
Orthogonal detection means for orthogonally detecting the signal from the tuner,
When receiving the signals of the M divided frequency bands at the frequency band edge, offset means for offsetting the tuning frequency of the tuner to suppress the influence of adjacent channel interference,
Tuning control means for outputting a tuning control signal for canceling the offset by the quadrature detection means to the quadrature detection means when receiving the signals of the M divided frequency bands at the frequency band end. An OFDM receiver, comprising: A first transmission mode in which one frequency band is divided into N divided frequency bands, and a transmission signal is transmitted using M divided frequency bands (1 ≦ M <N) among the N divided frequency bands; When the frequency band includes the divided frequency band at the end of the frequency band, an orthogonal frequency in which L (M <L ≦ N) of the N divided frequency bands are used and the second transmission mode in which the transmission signal is transmitted exists. An input end to which the division multiplex modulation signal is input,
According to the input tuning information, a tuning frequency control signal generating means for outputting a tuning frequency control signal for tuning at least a part of the transmission signal,
Quadrature detection carrier generation means for outputting a quadrature detection carrier for detecting at least a part of the transmission signal according to the tuning information,
First frequency conversion means for frequency-converting the orthogonal frequency division multiplex modulation signal from the input terminal into a first intermediate frequency band according to the tuning frequency control signal;
First filter means for extracting the L divided frequency band components from the output of the first frequency conversion means;
Quadrature detection means for performing quadrature detection on the output of the first filter means with the quadrature detection carrier frequency-controlled by an input detection carrier control signal;
Second filter means for extracting a band component in which the transmission signal is transmitted from an output of the quadrature detection means;
Discrete Fourier transform means for transforming the output of the second filter means from time domain to frequency domain by discrete Fourier transform;
Data demodulating means for demodulating symbol data transmitted to each subcarrier of the orthogonal frequency division multiplex modulation signal from the output of the discrete Fourier transform means;
The output of the discrete Fourier transform means and a detection carrier control signal generation means for outputting the detection carrier control signal from the quadrature detection carrier,
When the tuned OFDM signal includes the divided frequency band at the end of the frequency band, the tuned frequency control signal generating means outputs a tuned frequency control signal frequency-offset to the center of the frequency band, and An OFDM receiver, wherein the control signal generating means outputs the detection carrier control signal for canceling the frequency offset. 3. The OFDM receiver according to claim 2, wherein the tuning frequency control signal generating means outputs the tuning frequency control signal for offsetting the frequency at intervals of an integral multiple of the division frequency band. The second filter means has a filter characteristic of passing the OFDM signals of the M segments at the time of initial synchronization pull-in. 3. The OFDM receiver according to claim 2, wherein the filter characteristic is changed according to the transmission parameter.

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