JP5187204B2 - Digital broadcast receiver - Google Patents

Description

  The present invention relates to a digital broadcast receiving apparatus, and more particularly to a technique for simultaneously achieving both improvement in reception sensitivity and distortion tolerance.

In a broadcast receiver, it is known that it is effective to reduce the NF (noise factor) of a broadcast receiver in order to improve reception sensitivity, which is reception performance with respect to weak radio waves. As a method of reducing the NF of the broadcast receiver, there is a method of connecting an LNA (low noise amplifier) to the first stage of a tuner built in the broadcast receiver.
By the way, in a small broadcast receiver such as a portable terminal having a function of receiving a broadcast signal of a television broadcast such as a so-called one-segment broadcast, there are restrictions on the scale of a circuit that can be mounted and the size of the entire broadcast receiver. Therefore, conventionally, an LNA having a fixed gain is generally used as an LNA used in such a broadcast receiver.

  However, in a broadcast receiver having an LNA with a fixed gain, it is relatively difficult to reduce NF and increase resistance to signal distortion. In order to reduce the NF of the broadcast receiver, for example, it is effective to increase the gain of the LNA. In such a case, when a strong signal is input to the LNA, the high gain of the latter stage of the LNA is increased. This is because a signal distortion occurs in the amplifying unit, and as a result, an error occurs and reception becomes impossible. On the other hand, if the gain of the LNA is lowered, the NF of the broadcast receiver increases and the reception sensitivity decreases.

  Therefore, a power supply switching circuit for switching the power supply voltage of the LNA, a means for detecting a bit error, and a determination means for determining the reception state based on the detected bit error (flag) are provided. When the reception state is not good, the LNA An invention has been proposed in which both the improvement of the receiving sensitivity and the improvement of the distortion tolerance can be realized by changing the LNA gain by switching the power supply voltage (Patent Document 1).

JP 2007-336463 A

  However, BER (bit error rate) degradation does not only occur due to signal distortion, but also due to the state of the transmission path. Therefore, as in the invention described in Patent Document 1, when the gain of the LNA is changed based only on the bit error, it is found that there is a problem that the BER may be deteriorated due to erroneous switching. .

  The present invention has been made paying attention to the above-described problems, and an object of the present invention is to provide a digital broadcast receiving apparatus that can achieve both improvement in reception sensitivity and improvement in distortion resistance.

  Another object of the present invention is to provide a digital broadcast receiver capable of stable reception even in an environment where the received power fluctuates greatly.

In order to achieve the above object, the invention described in claim 1 of the present application is
A low noise amplifier that amplifies the received signal, an RF processing unit that amplifies the received signal amplified by the low noise amplifier, converts the frequency to an intermediate frequency signal, and outputs the signal, and after converting the intermediate frequency received signal to a digital signal In a digital broadcast receiver comprising a demodulator for demodulating,
The demodulator
An amplitude detection circuit for detecting the amplitude level of the received signal;
An AD conversion circuit that converts a received signal of an intermediate frequency into a digital signal;
A data conversion circuit that converts time-axis data converted by the AD conversion circuit into frequency-axis data;
Based on the data on the frequency axis, frequency fluctuation detecting means for detecting the magnitude of the frequency fluctuation of the subcarrier due to fading;
A gain control circuit that generates a gain control signal for controlling the gain of the RF processing unit in accordance with the output of the amplitude detection circuit and outputs the gain control signal to the RF processing unit;
A gain switching control circuit for generating a gain switching control signal for switching the gain of the low noise amplifier according to the output of the frequency variation detecting means and the output of the amplitude detecting circuit and outputting the gain switching control signal to the low noise amplifier;
It is characterized by having.

The invention according to claim 2 is the digital broadcast receiving apparatus according to claim 1,
A reception power conversion circuit that converts received power based on the output of the amplitude detection circuit or the output of the gain control circuit; and the gain switching control circuit includes an output of the reception power conversion circuit and an output of the frequency fluctuation detection means The gain switching control signal is generated in response to the above.

The invention according to claim 3 is the digital broadcast receiving apparatus according to claim 1 or 2,
A pilot signal detection circuit for detecting a pilot signal included in the received signal is provided, and the frequency fluctuation detection means detects the magnitude of the frequency fluctuation based on the detection signal of the pilot signal detection circuit.

According to a fourth aspect of the present invention, in the digital broadcast receiving apparatus according to the third aspect,
The frequency fluctuation detecting means includes a comparison circuit that compares a fluctuation amount of the pilot signal detected by the pilot signal detection circuit with a predetermined level set in advance.

According to a fifth aspect of the present invention, in the digital broadcast receiving apparatus according to the fourth aspect,
The received signal is a signal modulated by a multicarrier digital modulation method,
The gain switching control circuit includes:
When the fluctuation amount of the pilot signal is less than or equal to the predetermined level, the gain of the low noise amplifier is switched to a high gain when the reception power is −70 dBm or less, while the gain of the low noise amplifier is changed when the reception power is −30 dBm or more. Switch to lower gain,
When the fluctuation amount of the pilot signal is larger than the predetermined level , the gain of the low noise amplifier is switched to a high gain when the reception power is −75 dBm or less, while the gain of the low noise amplifier is changed when the reception power is −20 dBm or more. It is characterized by switching to a low gain.

The invention described in claim 6 is the digital broadcast receiver according to any one of claims 1 to 5,
A through path that supplies a received signal to the RF processing unit without passing through the low noise amplifier, and a switching unit that switches whether the received signal is supplied to an input terminal of the low noise amplifier or the through path. The switching control circuit generates a signal for controlling the switching means.

  According to the present invention, since the gain in the LNA is switched according to two conditions of the magnitude of received power and the presence / absence of fading, a digital broadcast receiving apparatus that can achieve both improvement in reception sensitivity and improvement in distortion tolerance is realized. There is an effect that can be.

It is a block diagram which shows schematic structure of the receiver of the 1 seg digital broadcasting mounted in a mobile telephone etc. It is a block diagram which shows the structure of the low noise amplifier in the digital broadcast receiver of an Example. It is a block diagram which shows the structure of the RF process part in the digital broadcast receiver of an Example. It is a block diagram which shows the structure of the demodulation part in the digital broadcast receiver of an Example. It is a block diagram which shows the structure of the LNA control part in the digital broadcast receiver of an Example. It is a characteristic view which shows the relationship between the symbol position and magnitude | size when receiving the fading of the pilot signal contained in a received signal. It is a flowchart which shows an example of the switching control procedure of the LNA gain by the LNA control part in the digital broadcast receiver of an Example. FIGS. 8A and 8B show the relationship between applied noise converted to C / N and BER after Viterbi when the received power at the antenna end is −25 dBm and −30 dBm in the absence of fading. FIG. FIGS. 9C and 9D show the relationship between applied noise converted to C / N and BER after Viterbi when the received power at the antenna end is −70 dBm and −75 dBm in the absence of fading. FIG. FIGS. 10A and 10B are characteristics showing the relationship between applied noise converted to C / N and BER when the received power is −15 dBm and −20 dBm with fading corresponding to a fading frequency of 15 Hz. FIG. FIGS. 11C and 11D show characteristics indicating the relationship between applied noise converted to C / N and BER when the received power is −75 dBm and −80 dBm with fading corresponding to a fading frequency of 15 Hz. FIG.

  The present inventor considers that the deterioration of BER does not occur only due to signal distortion but also due to the state of the transmission path, and in the state where there is no fading and in the state where fading is applied, An experiment was conducted to measure the BER (bit error rate) of received data output from an error correction circuit by receiving radio waves while changing the strength (received power viewed from the receiver) and the noise applied to the signal. . The gain of the low noise amplifier (hereinafter referred to as LNA) is also considered to be related to BER, and measurement is performed when the received signal is amplified with a fixed gain LNA and when it is amplified without passing through the LNA (through). It was.

  8 to 11 show part of the experimental results. Of these, FIGS. 8 and 9 show the applied noise converted to C / N and the Viterbi (error) when the received power at the antenna end is −25 dBm, −30 dBm, −70 dBm, and −75 dBm without fading. The relationship with the bit error rate (hereinafter simply referred to as BER) after correction processing is shown.

10 and 11 show the relationship between the C / N conversion applied noise and the BER when the received power is -15 dBm, -20 dBm, -75 dBm, and -80 dBm with fading corresponding to a fading frequency of 15 Hz. Show. Here, the fading frequency means a fluctuation amount of the frequency of the subcarrier when fading is received.
8 to 11, the BER measurement result when the received signal is amplified with a fixed gain LNA (ON) is plotted with ■, and the BER measurement result when the LNA is passed (OFF) Plotted with marks.

  Comparing the graphs of FIG. 8 and FIG. 9, when there is no fading, the BER is better when the LNA is passed through when the reception power level is −25 dBm from FIG. 8A, and FIG. When the received power level is -75 dBm, the BER is better through the LNA. From FIGS. 8B and 9C, the BER does not change much when the received power level is -30 dBm and -70 dBm. I understand that. Although not shown, measurement results similar to those of −30 dBm and −70 dBm were obtained even between −30 dBm and −70 dBm.

  On the other hand, comparing the graphs of FIG. 10 and FIG. 11, when fading is applied, the BER is better when LNA is passed through when the received power level is −15 dBm from FIG. From (D), when the received power level is −80 dBm, the BER is better when the LNA is passed. From FIGS. 10 (B) and 11 (C), the BER is less at both the received power levels of −20 dBm and −75 dBm. You can see that there is no change. Although not shown, measurement results similar to −20 dBm and −75 dBm were obtained even between −20 dBm and −75 dBm.

  From the above measurement results, when there is no fading, when the received power level is −25 dBm or more, it is better to let the LNA pass through, and when the received power level is −75 dBm or less, the LNA is passed. It can be seen that the BER is better. In addition, when fading is present, if the received power level is -15 dBm or more, the BER is better through the LNA, and if the received power level is -80 dBm or less, the BER is passed through the LNA. Is found to be good.

  Based on the analysis of the measurement results as described above, the present inventor provided a circuit for detecting the presence or absence of fading in the digital broadcast receiving apparatus having the configuration shown in FIG. And a control method for switching the path of the LNA based on the output of the detection circuit and the signal for detecting the amplitude level of the received signal for AGC control to make the amplitude of the signal input to the demodulator constant. .

  Furthermore, with regard to the presence or absence of fading, a broadcast signal received by the one-segment digital broadcast receiving apparatus targeted by the present invention is subject to reception processing called a scattered pilot signal (hereinafter simply referred to as a pilot signal) in a subcarrier. A reference signal having a known amplitude and phase is included as a landmark. When this pilot signal undergoes fading, as shown in FIG. 6, the magnitude varies according to the symbol position in the frequency direction, so that the pilot signal is extracted and the magnitude of fading is detected from the variation amount. Can do.

  In FIG. 6, when certain fading is applied, 36 pilot signals included in one segment and one symbol are detected, their magnitudes are plotted, and lines connecting the points are indicated by solid lines. Yes. Further, in FIG. 6, a dashed line indicates a pilot signal when there is no fading. In addition, since the subcarrier and symbol position where the pilot signal enters are determined in advance, such as once every 12 carriers and once every 4 symbols in the one-segment broadcasting signal, it is easy to detect the pilot signal. Detection is performed for more distortion correction.

  One embodiment of a digital broadcast receiving apparatus invented by the present inventor from the above knowledge and idea will be described with reference to FIGS.

  FIG. 1 shows a schematic configuration of a digital broadcast receiving apparatus called one-segment installed in a mobile phone or the like.

  As shown in FIG. 1, the digital broadcast receiving apparatus includes a low noise amplifier (LNA) 2 that amplifies a broadcast reception signal received by an antenna 1 with low noise, and an intermediate frequency signal while further amplifying the amplified reception signal. RF processing unit 3 for converting (down-converting) into a signal, demodulating unit 4 for converting the converted intermediate frequency signal into a digital signal, demodulating it, and outputting it as a transport stream (TS), detected at the entrance of demodulating unit 4 An AGC control unit 5a that generates and feeds back the gain control signal Vgc of the RF processing unit 3 based on the received power level and an LNA control unit 5b that generates a control signal CS for switching the gain of the low noise amplifier 2 are provided. In general, a band-pass filter that allows a received signal in a desired frequency band among received signals to pass therethrough is provided between the antenna 1 and the LNA 2. However, the illustration is omitted in FIG.

  The transport stream generated by the demodulator 4 is subjected to processing such as extraction and decryption of a packet including an encryption key and data separation by a descrambler / DEMUX unit 6, and data is separated by a decoder / VIDEO encoder unit 7. The original video signal and audio signal are reproduced and output to the output unit 8 such as a display unit or a speaker.

  In the present embodiment, as shown in FIG. 2, a through path TPS for inputting the reception signal Rx to the RF processing unit 3 without passing through the low noise amplifier 2 in parallel with the low noise amplifier 2, and transmission of the reception signal A changeover switch SW for switching the path to the low noise amplifier 2 or the through path TPS is provided, and the switch SW is configured to be controlled by a control signal CS from the LNA control unit 5b. As a result, the apparent gain of the low noise amplifier 2 can be easily switched only by adding one through-pass TPS and one switch SW.

  As shown in FIG. 3, the RF processing unit 3 includes a first variable gain amplifier circuit 31 that amplifies the reception signal Rx, an oscillator 32 that generates an oscillation signal having a predetermined frequency, and the reception signal Rx and the oscillator 32 By combining the oscillation signal and generating a signal having a frequency corresponding to the sum of both frequencies and an intermediate frequency (IF) signal corresponding to the difference between the frequencies, the output from the mixer 33 corresponds to the sum of the frequencies. A low-pass filter 34 that passes the intermediate frequency signal by removing the frequency signal and a second variable gain amplifier circuit 35 that amplifies the passed intermediate frequency signal are provided. The gains of the first variable gain amplifier circuit 31 and the second variable gain amplifier circuit 35 are controlled by a gain control signal Vgc from an amplitude detection circuit 46 (see FIG. 4) provided in the demodulator 4 at the subsequent stage. In this way, a reception signal having a constant amplitude level is input to the demodulation unit 4.

As shown in FIG. 4, the demodulator 4 includes an AD conversion circuit 41 that converts the broadcast reception signal amplified to a predetermined amplitude level by the RF processing unit 3 into a digital signal, and fast Fourier transform of the AD-converted reception data. FFT 42 that converts time axis information to frequency axis information, waveform equalization circuit 43 that corrects distortion generated in the transmission path, and OFDM (orthogonal frequency division multiplexing) that is one of the multicarrier digital modulation systems A demodulation circuit 44 that demodulates the received data by a demodulation method corresponding to the modulation method, and an error correction circuit 45 that performs error correction of the received data based on an error correction code included in the received data.

  Further, the demodulator 4 includes an amplitude detection circuit 46 that detects the amplitude based on the reception data converted by the AD conversion circuit 41, and a pilot signal detection circuit 47 that detects a pilot signal from the data Fourier-transformed by the FFT 42. A transmission path estimation circuit 48 that estimates a transmission path based on the detected signal is provided. A signal indicating the amplitude level detected by the amplitude detection circuit 46 is supplied to the AGC control circuit 5a, and a signal (voltage) for controlling the gain of the variable gain amplification circuits 31 and 35 in the RF processing unit 3 based on this detection signal. Vgc is generated, and the waveform equalization circuit 43 is configured to correct the distortion of the received signal generated in the transmission path according to the output from the transmission path estimation circuit 48.

  In this embodiment, the detection signal of the pilot signal detection circuit 47 is supplied to the LNA control unit 5b, and the LNA control unit 5b switches the path of the low noise amplifier 2 in accordance with the pilot detection signal. Is generated and output.

  As shown in FIG. 5, the LNA control unit 5b determines whether or not the fluctuation amount of the pilot signal (for example, the difference between the maximum value and the minimum value) is greater than or equal to a predetermined magnitude based on the detection signal of the pilot signal detection circuit 47. A frequency fluctuation comparison circuit 51 that detects whether or not fading is large by determining, a received power conversion unit 52 that converts current received power based on a signal from the AGC control circuit 5a, and the received power conversion unit 52 And an LNA switching control unit 53 for switching the path of the low noise amplifier 2 based on the signal indicating the received power converted by the above and the signal from the frequency fluctuation comparison circuit 51. Since the reception power conversion unit 52 converts the reception power based on the signal from the AGC control circuit 5a, the amplitude detection circuit 46 can be shared with the AGC control circuit 5a, and an increase in circuit scale can be suppressed.

  In a transmission state in which the fading signal causes fading, the magnitude of the pilot signal fluctuates depending on the symbol position in the frequency direction as shown in FIG. 6, but the magnitude of the pilot signal is known from the detection signal of the pilot signal detection circuit 47. The fluctuation comparison circuit 51 can be easily configured by using a circuit that detects the magnitude of fluctuation and a comparison circuit that compares a signal indicating the magnitude of fluctuation with a predetermined level (threshold).

  Further, the frequency fluctuation comparison circuit 51 outputs a signal with fading (for example, high level) when the fluctuation of the pilot signal is larger than a predetermined level, and no fading when the fluctuation of the detection signal is smaller than the predetermined level. The signal (for example, low level) is output. Thereby, the frequency fluctuation comparison circuit 51 can be configured easily. In addition, instead of supplying the signal from the AGC control circuit 5a, the received power conversion unit 52 that converts the received power is supplied with a signal indicating the amplitude level supplied from the amplitude detection circuit to the AGC control circuit 5a. It may be configured.

  Next, an example of a specific procedure for switching control in the LNA switching control unit 53 will be described with reference to the flowchart of FIG.

  The LNA switching control unit 53 first determines the presence or absence of fading based on the detection signal output from the frequency variation comparison circuit 51 (step S1). If it is determined that there is no fading, the process proceeds to step S2 to determine whether the received power is −70 dBm or less. If “Yes” (positive), the control signal CS for switching the switch SW so as to input the received signal to the LNA. Is output (step S5). If “No” (negative) in step S2, the process proceeds to step S3 to determine whether the received power is −30 dBm or more. If “No” (negative), the current control state of the LNA is maintained (step S4). If “Yes” (Yes), a control signal CS for switching the switch SW is output so that the received signal is passed through and not input to the LNA (step S8).

  On the other hand, if “Yes” (positive), that is, fading is determined in step S1, the process proceeds to step S6 to determine whether the received power is −75 dBm or less. If “Yes” (positive), the received signal is input to the LNA. A control signal CS for switching the switch SW is output (step S5). If “No” (No) in Step S6, the process proceeds to Step S7 to determine whether the received power is −20 dBm or more. If “No” (No), the control state of the LNA at that time is maintained (Step S4). If “Yes” (Yes), a control signal CS for switching the switch SW is output so that the received signal is passed through and not input to the LNA (step S8).

  By the control as described above, the gain in the LNA (which is regarded as a low gain when it is through) is determined according to the two conditions of the magnitude of the received power and the presence / absence of fading, and only the magnitude of the received power Alternatively, the BER can be improved as compared with the conventional control method in which the gain in the LNA is determined only by the quality of the BER, and the reception sensitivity can be increased to the limit within the allowable BER range. As a result, it is possible to obtain an effect that it is possible to achieve both improvement in reception sensitivity and improvement in distortion resistance. Further, when there is fading, the reception power range between the switching threshold levels can be set wide, so that stable reception is possible even in an environment where the reception power fluctuates greatly.

  Note that the switching control process according to the flowchart of FIG. 7 provides the highest accuracy if it is performed in a 4-symbol period (1.134 ms × 4 = 4.536 ms) in which pilot signals are arranged in a single line. Therefore, it is preferable to repeat the operation at a cycle of 100 ms, for example.

  Although the invention made by the present inventor has been specifically described based on the embodiment, the present invention is not limited to the above embodiment. For example, in the above embodiment, the presence / absence of fading is detected based on the pilot signal, but it is also possible to detect based on other signals.

  In the above-described embodiment, the example in which the switch SW for switching whether the received signal is input to the LNA to be amplified or through is provided has been described, but instead of providing the switch SW and the path TPS for passing through the LNA, Even if the LNA itself is configured so that the gain can be switched and the gain of the LNA is switched according to the magnitude of the received power and the presence / absence of fading, the BER is improved as in the above embodiment. can get. The description in parentheses in the boxes in steps S5 and S8 in FIG. 7 represents the control in such a case.

  Furthermore, the determination level (−70 dB or the like) at the time of determination of the received power level in steps S2, S3, S6, and S7 in the flowchart of FIG. 7 may be configured to have hysteresis. Thereby, malfunction due to noise or the like can be prevented.

  In the above embodiment, it has been described that the frequency fluctuation comparison circuit 51 includes a comparison circuit that compares the magnitude of pilot signal fluctuation with a predetermined level (threshold value). A circuit for changing the value may be provided, and the threshold value may be changed according to the condition.

  Furthermore, in the above description, the case where the present invention is applied to a one-segment broadcast signal receiver has been described as an example. However, the present invention can be widely used for a digital broadcast signal receiver.

DESCRIPTION OF SYMBOLS 1 ... Antenna, 2 ... Low noise amplifier (LNA), 3 ... RF processing part, 4 ... Demodulation part, 5a ... AGC control part, 5b ... LNA control part, 6. ..Descrambler / DEMUX unit, 7... Decoder / VIDEO encoder unit, 8... Output unit, 31... First variable gain amplifier circuit, 32. ... Low pass filter, 35 ... Second variable gain amplifier circuit, 41 ... AD converter circuit, 42 ... FFT (Fast Fourier Transform circuit), 43 ... Waveform equalization circuit, 44 ..Demodulation circuit 45 ... Error correction circuit 46 ... Amplitude detection circuit 47 ... Pilot signal detection circuit 48 ... Transmission path estimation circuit 51 ... Frequency fluctuation comparison circuit (Frequency fluctuation) Detection means), 52... Received power conversion unit, 53 ·· LNA switching control unit

Claims (6)

A low noise amplifier that amplifies the received signal, an RF processing unit that amplifies the received signal amplified by the low noise amplifier, converts the frequency to an intermediate frequency signal, and outputs the signal, and after converting the intermediate frequency received signal to a digital signal In a digital broadcast receiver comprising a demodulator for demodulating,
The demodulator
An amplitude detection circuit for detecting the amplitude level of the received signal;
An AD conversion circuit that converts a received signal of an intermediate frequency into a digital signal;
A data conversion circuit that converts time-axis data converted by the AD conversion circuit into frequency-axis data;
Based on the data on the frequency axis, frequency fluctuation detecting means for detecting the magnitude of the frequency fluctuation of the subcarrier due to fading;
A gain control circuit that generates a gain control signal for controlling the gain of the RF processing unit in accordance with the output of the amplitude detection circuit and outputs the gain control signal to the RF processing unit;
A gain switching control circuit that generates a gain switching control signal for switching the gain of the low noise amplifier in accordance with the output of the frequency fluctuation detecting means and the output of the amplitude detection circuit and outputs the gain switching control signal to the low noise amplifier. A digital broadcast receiver characterized by the above.   A reception power conversion circuit that converts received power based on the output of the amplitude detection circuit or the output of the gain control circuit; and the gain switching control circuit includes an output of the reception power conversion circuit and an output of the frequency fluctuation detection means The digital broadcast receiving apparatus according to claim 1, wherein the digital broadcast receiving apparatus is configured to generate the gain switching control signal according to the frequency.   The pilot signal detection circuit for detecting a pilot signal included in the received signal is provided, and the frequency fluctuation detecting means detects the magnitude of the frequency fluctuation based on a detection signal of the pilot signal detection circuit. 3. The digital broadcast receiver according to 1 or 2.   4. The digital broadcasting according to claim 3, wherein the frequency fluctuation detecting means includes a comparison circuit that compares the fluctuation amount of the pilot signal detected by the pilot signal detection circuit with a predetermined level set in advance. Receiver device. The received signal is a signal modulated by a multicarrier digital modulation method,
The gain switching control circuit includes:
When the fluctuation amount of the pilot signal is less than or equal to the predetermined level, the gain of the low noise amplifier is switched to a high gain when the reception power is −70 dBm or less, while the gain of the low noise amplifier is changed when the reception power is −30 dBm or more. Switch to lower gain,
When the fluctuation amount of the pilot signal is larger than the predetermined level , the gain of the low noise amplifier is switched to a high gain when the reception power is −75 dBm or less, while the gain of the low noise amplifier is changed when the reception power is −20 dBm or more. The digital broadcast receiving apparatus according to claim 4, wherein the digital broadcast receiving apparatus is switched to a low gain.   A through path that supplies a received signal to the RF processing unit without passing through the low noise amplifier, and a switching unit that switches whether the received signal is supplied to an input terminal of the low noise amplifier or the through path. 6. The digital broadcast receiving apparatus according to claim 1, wherein the switching control circuit generates a signal for controlling the switching means.

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