JP2008283588A - Receiving apparatus and received signal amplification factor setting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a receiving apparatus capable of combining signals received by antennas while suppressing the quantity of noise therein in simple circuit configuration. <P>SOLUTION: When it is confirmed that differences of reception power in antennas 1a-1d become large, at an AGC control section 4, an amplification factor limit mode is set. During the amplification factor limit mode, an amplification factor for a digital broadcast signal received by an antenna of small receiving power is set to be smaller than a value in a case where the amplification factor is set based on a target value during a normal mode, thereby suppressing the effect of noise during combination due to a maximum ratio combining (MRC) section 5. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a receiving apparatus that receives a radio signal such as a broadcast signal, and more particularly to a receiving apparatus that receives a radio signal by a spatial diversity scheme configured by a plurality of antennas.

  In recent years, with the development of communication technology, digitalization in various broadcasts has progressed, and terrestrial television broadcasts are also being shifted from analog broadcasts to digital broadcasts. In this terrestrial digital television broadcasting, an OFDM (Orthogonal Frequency Division Multiplexing) system, which is a multi-carrier transmission system using a plurality of carriers simultaneously, is employed (see Patent Document 1 and Patent Document 2). In mobile communication, since frequency fading due to multipath fluctuates, a spatial diversity system in which a plurality of antennas are installed at different positions is employed.

  When this spatial diversity method is used, the received power at each antenna is different, so that the gain when the signal after channel selection is amplified is controlled according to the received power. AGC (Auto Gain Control) control Is used. However, the amount of noise superimposed on the signal is almost constant regardless of the magnitude of the reception power of the signal received by each antenna. Therefore, if the amplification factor of the signal with low reception power is increased by AGC control, the signal is superimposed. The noise that will be increased. For this reason, when the signals received by the respective antennas are combined, the influence received by the amplified noise is increased.

  First, in Patent Document 1, an orthogonal demodulation unit demodulates a broadcast signal received by a plurality of antennas using an OFDM method, and then controls the gain value after channel selection according to the signal level after demodulation. Gain Controller) is provided. Further, when the broadcast signals received by a plurality of antennas are combined by weighted addition of the signals after quadrature demodulation, the weighting coefficient is set in AGC based on the signal level after quadrature demodulation.

  Under the control by the AGC, setting is made so that the amplification factor after channel selection by the antenna with small received power becomes large. As described above, when the set amplification factor is increased, noise included in the received broadcast signal is increased. However, when the suppression amount is added to the weighting coefficient at the time of weighting addition in AGC, the noise is increased. It is possible to reduce the synthesis amount of the broadcast signal to be judged and prevent the influence of noise.

Further, in Patent Document 2, with respect to a plurality of tuner means or demodulation means provided in a receiving apparatus, different characteristics are given after giving different characteristics to a signal obtained by receiving and selecting or demodulating from a antenna. Are combined by the combining means. As a result, a combination ratio of a plurality of signals having different characteristics can be set, and a signal received by an antenna having a good reception state can be selected according to a reception situation, and reception characteristics by a spatial diversity system are improved. .
JP 2006-287899 A JP 2006-319608 A

  However, according to the configuration of Patent Document 1, it is necessary to provide a setting circuit for setting the weighting coefficient, which hinders downsizing of the receiving apparatus. Furthermore, when combining processing is performed for each of a plurality of subcarriers as in the OFDM method, it is necessary to set a weighting coefficient for each subcarrier, and the amount of signal processing increases, so the circuit scale is significantly increased. It will be something. Also, in Patent Document 2, the synthesis ratio at the time of synthesis by the synthesis unit is determined, but the amplification factor by the amplification unit by the tuner unit depends on the signal level of the signal input to the A / D conversion unit. Is set.

  Therefore, in both Patent Document 1 and Patent Document 2, the amplification factor of the selected signal is set according to the signal level of each signal, and is set according to the correlation of each signal. is not. Therefore, although Patent Document 1 has been described, the amount of signal processing in the arithmetic processing at the time of synthesizing increases, and the circuit scale for signal synthesizing increases and becomes complicated, thereby preventing downsizing of the apparatus.

  In view of such a problem, an object of the present invention is to provide a receiving apparatus that can synthesize a signal received by each antenna with a simple circuit configuration while suppressing the amount of noise.

  In order to achieve the above object, a receiving apparatus of the present invention selects a plurality of antennas that receive digital broadcast signals, selects a digital broadcast signal received by each of the plurality of antennas, obtains a baseband signal, and In a receiving apparatus comprising: tuner units equal to the number of antennas divided into carriers; and a combining unit that combines baseband signals acquired by the plurality of tuner units for each carrier. A signal level detection unit for detecting the signal level of each of the plurality of baseband signals, and an amplification factor after channel selection in the tuner unit based on the signal level of each of the plurality of baseband signals detected by the signal level detection unit The signal level of each of the baseband signals detected by the signal level detection unit and the amplification factor setting unit for setting the signal are compared. A mode switching control unit that switches the amplification factor setting operation in the amplification factor setting unit to the operation in the first mode and the second mode based on the comparison result, and the difference in signal level between the plurality of baseband signals Is smaller than a predetermined value, and when the amplification factor setting operation in the amplification factor setting unit is set to the first mode, the first target value for the signal level of the baseband signal when setting the amplification factor is constant. The amplification factor setting unit performs an amplification factor setting operation, and the difference in signal level between each of the plurality of baseband signals is a predetermined value or more, and the amplification factor setting operation in the amplification factor setting unit is set to the second mode. Then, the amplification factor in the first tuner unit that acquires the first baseband signal whose signal level is determined to be lower than that of the other baseband signals is expressed as the first mode. For the gain in the second tuner unit that obtains the baseband signal other than the first baseband signal, the gain is set to a value smaller than the gain when set in the The first target value is the same as that in the first mode.

  In such a receiving apparatus, when the digital broadcast signal is a signal modulated by an orthogonal modulation method such as the OFDM method, the tuner unit performs orthogonal demodulation on the baseband signal obtained by selecting a channel. A demodulation unit is provided, and the amplification factor in the tuner unit is set according to the signal level of the baseband signal demodulated by the orthogonal demodulation unit.

  Then, when the gain setting operation in the second mode is performed in the gain setting unit, the gain in the first tuner unit is set by the gain set for the second tuner unit. It does n’t matter.

  When the amplification factor setting operation in the second mode is performed in the amplification factor setting unit, the amplification factor in the first tuner unit is based on a second target value that is smaller than the first target value. It does not matter if it is set. Furthermore, when the amplification factor setting operation in the second mode is performed in the amplification factor setting unit, the amplification factor in the first tuner unit may be a predetermined constant value.

  Further, in the mode switching control unit, a difference in signal level between each of the plurality of baseband signals may be determined based on a difference between a minimum value and a maximum value of the signal level detected by the signal level detection unit. I do not care.

  An average value calculation unit that averages the signal level detected by the signal level detection unit and outputs the averaged signal level to the amplification factor setting unit, and a plurality of baseband signals acquired by the plurality of tuner units. A reception environment determination unit that determines a reception environment by the plurality of antennas by determining whether or not the influence of the multipath is greatly influenced, and the reception environment determination unit When it is determined that the first reception environment is greatly received, the average value calculation unit determines a period during which the signal level detected by the signal level detection unit is averaged in the second reception environment where the influence of multipath is small. It may be set shorter than the case where it is set.

  At this time, the reception environment determination unit may determine the magnitude of the influence of multipath by measuring the transition of the signal level in the frequency direction of the baseband signal after quadrature demodulation.

  Also, the received signal amplification factor setting method of the present invention detects the signal level of each of a plurality of baseband signals acquired by the same number of tuners as the antennas for selecting digital broadcast signals received by a plurality of antennas. A signal level detection step; an amplification factor step for setting an amplification factor after channel selection in the tuner unit according to the signal level of each of the plurality of baseband signals detected in the signal level step; and the signal level step. A mode switching control step for comparing the signal levels of the detected baseband signals and switching the amplification factor setting operation in the amplification factor setting step between the first mode and the second mode based on the comparison result; and The difference in signal level between each of the plurality of baseband signals is smaller than a predetermined value, When the amplification factor setting operation in the amplification factor setting step is set to the first mode, the first target value for the signal level of the baseband signal when setting the amplification factor is constant, and the amplification factor setting step When the gain setting operation at the gain setting operation in the gain setting step is set to the second mode, the difference between the signal levels of the plurality of baseband signals becomes a predetermined value or more, and the gain setting operation in the gain setting step is set to the second mode. In the rate setting step, set when the amplification factor in the first tuner unit that acquires the first baseband signal whose signal level is determined to be lower than other baseband signals is set in the first mode. The gain is set to a value smaller than the amplification factor in the case where the second tuner unit obtains the baseband signal other than the first baseband signal. Information, and setting by the first mode and the same of the first target value.

  An average value calculating step of averaging the signal level detected in the signal level detecting step and outputting the averaged signal level to the amplification factor setting unit; and a plurality of basebands acquired by the plurality of tuner units A reception environment determination step for determining a reception environment by the plurality of antennas by determining whether or not the signal is greatly affected by the multipath, wherein the reception environment determination step includes: When it is determined that the first reception environment is greatly affected, the period during which the signal level detected by the signal level detection unit is averaged in the average value calculating step is the second reception with less multipath influence. It may be set shorter than that set in the environment.

  According to the present invention, since the mode is switched when setting the amplification factor according to the magnitude of the difference between the signal levels of the plurality of baseband signals, the amplification factor for a signal with extremely small received power can be suppressed. . Thereby, when combining the signals received by the spatial diversity method, it is possible to suppress the influence of the noise superimposed on the signal with extremely low received power having a high noise content rate. Therefore, noise degradation in the combined signal can be prevented, and the occurrence of disturbance during signal reproduction can be reduced. In addition, since it is realized by changing the setting operation of the amplification factor, the amount of calculation does not increase and the circuit configuration can be simplified as compared with the case of switching the weighting coefficient at the time of synthesis. Therefore, it does not hinder downsizing of the receiving device.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an internal configuration of a receiving apparatus according to this embodiment.

  The receiving apparatus in FIG. 1 receives antennas 1a to 1d that receive digital broadcast signals, and RF signals in a high frequency band (RF band) that are digital broadcast signals input from the antennas 1a to 1d, respectively, as intermediate frequency bands (IF bands). Tuners 2a to 2d that perform frequency conversion to IF signals, and an orthogonal demodulator 3a that demodulates IF signals output from the tuners 2a to 2d based on an orthogonal frequency division multiplexing (OFDM) transmission scheme. AGC (Auto Gain Control) control unit 4 that adjusts the amplification factor in each of the tuner units 2a to 2d according to the signal level of the baseband signal obtained by the orthogonal demodulation units 3a to 3d, and the orthogonal demodulation units 3a to 3d The baseband signal obtained in 3d is synthesized by the MRC synthesis unit 5 for synthesizing the maximum ratio combining (MRC) and the MRC synthesis unit 5. The digital demodulator 6 that demodulates the baseband signal based on the digital modulation scheme for each subcarrier, and the MPEG (Moving Picture Experts Group) encoded signal obtained by demodulating the digital demodulator 15 into the MPEG compression scheme. An MPEG decoder 7 for decoding on the basis thereof.

(Broadcast signal reception operation)
For example, in the case of a vehicle-mounted receiving device, the receiving device configured as described above includes an antenna 1a, 1b installed on the windshield of the vehicle body and an antenna 1c, 1d on the rear glass of the vehicle body. A spatial diversity antenna composed of the antennas 1a to 1d is configured to be installed. In this way, digital broadcast signals received by the plurality of antennas 1a to 1d constituting the spatial diversity antenna are given to the tuner units 2a to 2d, respectively.

  The tuner units 2a to 2d perform a channel selection operation on the channel frequency corresponding to the desired channel. At this time, the channels selected by the tuner units 2a to 2d are the same channel, and digital broadcast signals having the same channel frequency are selected. When performing the channel selection operation, the tuner units 2a to 2d perform frequency conversion (down-conversion) on the digital broadcast signal, which is an RF signal at a channel frequency corresponding to a desired channel, so that an IF signal is obtained. A baseband signal is acquired.

  When the baseband signal that becomes the IF signal is given from each of the tuner units 2a to 2d to the orthogonal demodulation units 3a to 3d, a baseband signal that becomes a time-axis signal by fast Fourier transform (FFT) processing. To a baseband signal that is a frequency axis signal. Thereafter, the channel characteristics of each subcarrier for each time are estimated from the baseband signal that is the frequency axis signal, and equalization processing is performed based on the estimated channel characteristics.

  The quadrature demodulation units 3a to 3d in FIG. 1 perform FFT processing on the baseband signals acquired by the tuner unit 2 (corresponding to the tuner units 2a to 2d in FIG. 1) like the quadrature demodulation unit 3 shown in FIG. And an equalization processing unit 32 that performs equalization processing on the baseband signal that has been subjected to FFT processing in the FFT unit 31 and converted from a time-axis signal to a frequency-axis signal. In the orthogonal demodulator 3 configured as described above, the FFT unit 31 performs FFT processing in a state where symbol synchronization, carrier frequency synchronization, and sampling frequency synchronization are established.

  That is, the offset of the symbol length (time length of each symbol), the carrier frequency, and the sampling frequency (frequency of the subcarrier interval) generated by a transmission path such as multipath is corrected. At this time, symbol synchronization and carrier frequency synchronization are established by using a guard interval provided for each symbol and comprising the same signal as the latter half of the symbol. Also, sampling frequency synchronization is established by estimating the frequency offset using pilot symbols described later. The carrier frequency synchronization may be established by estimating a frequency offset using a pilot symbol.

  In this way, the FFT processing in the FFT unit 31 converts the baseband signal that is a time-axis signal into the baseband signal that is a frequency-axis signal. As shown in FIG. 4, the data symbols and pilot symbols arranged in the frequency direction and the time direction are collectively called an OFDM symbol sequence. In the ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) system, a scattered pilot symbol (hereinafter referred to as “SP symbol”) is used as a pilot symbol. The frequency direction and the time direction are also called a carrier direction and a symbol direction, respectively.

  In FIG. 3, a time number (symbol number) corresponding to the time direction is represented by t (integer of t ≧ 0), and a carrier number corresponding to the frequency direction is an integer of l (0 ≦ l ≦ (L−1)). , L: total number of subcarriers). Further, t represents time when the symbol length of the OFDM signal is used as a unit. Further, a position in the OFDM symbol string that is uniquely determined by uniquely defining t and l is called a carrier position, and this carrier position is represented by (t, l).

  The SP symbol is arranged at a carrier position satisfying l = 3 × (t mod 4) + 12p. Note that mod represents a remainder operation, and p is an integer. That is, as shown in FIG. 3, when attention is paid to a signal at a certain time t, the SP symbol is arranged every 12 subcarriers on the frequency axis. Each time the time t advances by one symbol, the SP signal is shifted in the frequency direction by three subcarriers. In other words, when attention is paid to a signal of a certain subcarrier l, SP symbols are arranged every four symbols on the time axis. Thus, for example, at time t = 0, SP symbols are arranged at carrier positions (0, 0), (0, 12), (0, 24), (0, 36),. Then, SP symbols are arranged at carrier positions (1, 3), (1, 15), (1, 27), (1, 39),. Further, data symbols are arranged at carrier positions other than the carrier position where the SP symbols are arranged.

  When the baseband signal from the FFT unit 31 is supplied to the equalization processing unit 32, the SP symbols arranged in the frequency axis direction and the time axis direction as shown in FIG. Transmission path characteristics for each position are estimated, and equalization processing is performed based on the estimated transmission path characteristics. At this time, first, transmission path characteristics with respect to the carrier position of each SP symbol are estimated by each SP symbol. That is, the SP symbol generated from the received signal is complex-divided by the SP symbol generated in the equalization processing unit 32, thereby estimating the transmission path characteristic with respect to the carrier position of the SP symbol.

  Then, for each subcarrier, interpolation in the time axis direction is performed using transmission path characteristics with respect to the carrier position of each SP symbol arranged every four symbols on the same subcarrier. As a result, the channel characteristics are estimated for the carrier positions of all the subcarriers having the SP symbol every four symbols. That is, by performing interpolation processing on the data symbols arranged between the SP symbols arranged in the time axis direction, the transmission path characteristics in each data symbol are estimated. When the transmission path characteristics in the time axis direction are interpolated, for example, an averaging process of the transmission path characteristics estimated from the SP symbols is performed by an IIR (Infinite Impulse Response) type low-pass filter.

  Further, interpolation in the frequency axis direction is performed for each symbol based on the channel characteristics estimated for the carrier position of the SP symbol and the channel characteristics estimated by performing interpolation in the time axis direction. As a result, the transmission path characteristics of all the subcarriers where no SP symbol is arranged are determined by the transmission path estimated from the SP symbol directly or every three subcarriers by time axis interpolation. Is estimated. That is, for subcarriers arranged between subcarriers having SP symbols in the frequency axis direction, transmission path characteristics in the data symbols are estimated. When performing the interpolation of the channel characteristics in the frequency axis direction, for example, the channel characteristics estimated for the subcarrier having the SP symbol are input to a FIR (Finite Impulse Response) type low-pass filter. Thus, the transmission path characteristics of subcarriers where no SP symbol is arranged are estimated.

  In this way, when the transmission path characteristics of all data symbols are estimated based on the SP symbols, the estimated transmission path characteristics are obtained for each data symbol obtained from the signal on the frequency axis from the FFT unit 31. By performing complex division, equalization processing is performed, and distortions in amplitude and phase due to multipath effects and the like are removed. As this equalization processing, for example, a zero focusing equalization method that directly divides the estimated transmission path characteristic is used. Since this zero-focusing equalization process has a problem of noise enhancement, there is a Minimum Mean Square Error (MMSE) equalization method for reducing the noise enhancement problem. In the case of this MMSE equalization method, it is also necessary to estimate the average power value of noise (additional noise) added on the transmission line.

  As described above, in the quadrature demodulation units 3a to 3d, the FFT unit 31 performs FFT processing to convert the baseband signal on the frequency axis, and then the transmission path and the power estimated by the equalization processing unit 32 are equal. By performing the conversion processing, a baseband signal from which distortion in amplitude and phase due to the influence of multipath and the like is removed can be obtained. When the baseband signals obtained by the orthogonal demodulation units 3a to 3d are supplied to the MRC combining unit 5, the MRC combining unit 5 performs weighted addition of the four baseband signals with an optimum gain for each subcarrier. Combining (carrier diversity method) to form one baseband signal.

  When one baseband signal obtained by synthesizing by the MRC synthesizing unit 5 is supplied to the digital demodulating unit 6, it is demodulated by a digital modulation method set for each subcarrier. Examples of the digital modulation method include QAM (Quadrature Amplitude Modulation), QPSK (Quadrature Phase Shift Keying), and DQPSK (Differential Quadrature Phase Shift Keying). Then, when the MPEG encoded signal obtained by demodulating by the digital demodulating unit 6 is given to the MPEG decoder 7, it is decoded based on the MPEG compression method to obtain a video signal. This video signal is further applied to a display device (not shown) such as a display, so that the video is reproduced.

(AGC control operation)
In this way, when the spatial diversity reception is realized by synthesizing the digital broadcast signals received by the antennas 1a to 1d and performing the reproduction output of the digital broadcast, the AGC control unit 4 uses the orthogonal demodulation units 3a to 3d. Based on the magnitude of the received power of each symbol of the baseband signal obtained in step 1, the amplification factors in the tuner units 2a to 2d are set. The configuration and operation of the AGC control unit 4 will be described below.

1. Configuration of AGC Control Unit As shown in FIG. 4, the AGC control unit 4 detects the signal level of the baseband signal that has been equalized by the equalization processing unit 32 of each of the orthogonal demodulation units 3a to 3d. Detection unit 41a-41d, average value calculation unit 42a-42d for calculating the average value of the signal level detected by each of signal level detection unit 41a-41d, and variation in the signal level from each of average value calculation unit 42a-42d A reception environment determination unit 43 that sets a period for calculating an average value by the average value calculation units 42a to 42d based on the state, and tuner units 2a to 2d based on signal levels calculated by the average value calculation units 42a to 42d, respectively. The amplification factor setting unit 44 for setting the amplification factor of the signal and the amplification factor setting unit 44 based on the signal level calculated by each of the average value calculation units 42a to 42d. And a mode switching control unit 45 for switching the width ratio setting operation.

  In the AGC control unit 4 configured as described above, the signal levels of the baseband signals output from the quadrature demodulation units 3a to 3d are detected by the signal level detection units 41a to 41d, so that the antennas 1a to 1d are detected. The received power at each is detected. The average value calculation units 42a to 42d store the signal levels detected by the signal level detection units 41a to 41d for each predetermined period set by the reception environment determination unit 43, and calculate the average value. The obtained average value is output to the amplification factor setting unit 44 and the mode switching control unit 45.

  The mode switching control unit 45 switches the setting operation of the amplification factor in the tuner units 2a to 2d in the amplification factor setting unit 44 by confirming the relationship between the signal levels detected by the average value calculation units 42a to 42d. Do. That is, by detecting the signal level detected by the average value calculation units 42a to 42d, and confirming a baseband signal having a poor CNR (Carrier to Noise Ratio), which is the ratio between the signal level and noise, all the tuners are detected. The normal mode in which the amplification factors of the units 2a to 2d are set with the same target value is switched to the amplification factor limiting mode in which the amplification factor of the baseband signal having a poor CNR is suppressed.

2. Outline of AGC Control Operation An outline of the AGC control operation by the AGC control unit 4 configured as described above will be described with reference to the drawings.

  As described above, when the signal level detection units 41a to 41d detect the signal levels of the baseband signals output from the quadrature demodulation units 3a to 3d, the detected signal levels are converted into the average value calculation unit 42a. To 42d. As shown in FIG. 5, in the average value calculation units 42a to 42d, when the moving body is moving in an urban area or the like and multipath is likely to occur, the signal level detection units 41a to 41d detect it in a short time τ1. The average value of the measured signal level is calculated, and when the mobile object is in a situation where multipath is unlikely to occur, such as when moving in a sparsely populated area or the like, the signal is output for a long time τ2 (τ2> τ1). An average value of the signal levels detected by the level detectors 41a to 41d is calculated.

  It is assumed that the signal level detectors 41a to 41d and the average value calculators 42a to 42d that operate in this way detect the signal level and calculate the average value for each subcarrier of each baseband signal. That is, a temporal average value (hereinafter referred to as “time average signal level”) of the signal level for each subcarrier of each baseband signal obtained by the average value calculation units 42 a to 42 d is given to the reception environment determination unit 43. As a result, the reception environment determination operation described later is performed.

  The signal level detectors 41a to 41d multiply the signal levels of the baseband signals output from the quadrature demodulators 3a to 3d by the reciprocals of the amplification factors set by the tuner units 2a to 2d to attenuate them. . Thereby, the signal level of the baseband signal corresponding to the received power in the antennas 1a to 1d can be detected. That is, the signal level detection unit 41a will be described as an example. When the amplification factor of the tuner unit 2a is set to A, the signal level detection unit 41a is a signal of a baseband signal output from the quadrature demodulation unit 3a. When the level Ls is detected, the signal level Ls is multiplied by the inverse 1 / A of the amplification factor A. A value Ls / A obtained by multiplying the signal level of the baseband signal by the reciprocal 1 / A of the amplification factor A is output to the average value calculator 42a as the detected signal level.

  Further, for the amplification factor setting unit 44 and the mode switching control unit 45, an average value obtained by averaging the average value of the signal level for each subcarrier of each baseband signal in the frequency direction from the average value calculation units 42a to 42d. Is given. That is, for the amplification factor setting unit 44 and the mode switching control unit 45, average values (hereinafter referred to as average values) averaged in both the time direction and the frequency direction of each baseband signal from the average value calculation units 42a to 42d. , “Time and frequency average signal level”). Further, the signal level detectors 41a to 41d may detect only the signal level of the subcarrier serving as the reference symbol SP symbol without detecting the signal level of the fluctuating data symbol.

  When the average value of the signal level of each baseband signal calculated by the average value calculation units 42a to 42d is given to the amplification factor setting unit 44, the time obtained from the average value calculation units 42a to 42d in the normal mode And the amplification factor in each of the tuner units 2a to 2d is set so that the frequency average signal levels L1 to L4 become the target value L set in advance. That is, the time and frequency average signal level L1 from the average value calculation unit 42a makes the amplification factor of the tuner unit 2a L / L1 times the current amplification factor, and the time and frequency average signal level from the average value calculation unit 42b. By L2, the amplification factor of the tuner unit 2b is set to L / L2 times the current amplification factor, and the amplification factor of the tuner unit 2c becomes the current amplification factor by the time and frequency average signal level L3 from the average value calculation unit 42c. The amplification factor of the tuner unit 2d is set to L / L4 times the current amplification factor based on the time and frequency average signal level L4 from the average value computation unit 42d.

  Further, when in the amplification factor limiting mode, the maximum value Lx is detected in the time and frequency average signal levels L1 to L4 obtained from the average value calculation units 42a to 42d, and the time and frequency average signal at which the maximum value is obtained. Compare the level Lx with other time and frequency average signal levels. When calculating the amplification factor of the tuner unit 2 (corresponding to the tuner units 2a to 2d) corresponding to the time and frequency average signal level at which the difference between the maximum time and the frequency average signal level Lx is equal to or greater than a predetermined value, The target value is set to be smaller than the target value L in the normal mode. As a result, a digital broadcast signal (digital broadcast signal with low received power) obtained by a baseband signal having a small time and frequency average signal level obtained from the average value calculation unit 42 (corresponding to the average value calculation units 42a to 42d) is acquired. The amplification factor of the tuner unit 2 can be limited.

  The mode switching control unit 45 switches between the amplification factor limiting mode and the normal mode. The mode switching control unit 45 calculates the difference ΔL (= Lx−Ly) between the maximum value Lx and the minimum value Ly when given the time and frequency average signal level calculated by the average value calculation units 42a to 42d. To do. Then, the difference ΔL between the maximum value and the minimum value in the time and frequency average signal level calculated by the average value calculation units 42a to 42d is compared with a predetermined threshold value LTH. At this time, when the difference ΔL between the maximum value and the minimum value is larger than the predetermined threshold value LTH, the gain limiting mode is set, and the difference ΔL between the maximum value and the minimum value is equal to or less than the predetermined threshold value LTH. If so, the normal mode is set.

  Further, the time average signal level of each baseband signal calculated by the average value calculation units 42a to 42d is also given to the reception environment determination unit 43, and the reception environment determination unit 43 determines the signal level of each baseband signal. By detecting the fluctuation amount, it is confirmed whether the current reception environment is likely to cause multipath. At this time, for example, the signal level of the SP symbol of each baseband signal may be detected, and it may be determined whether or not the reception environment is likely to cause multipath based on the amount of variation in the frequency direction of the SP symbol. .

3. Reception Environment Determination Operation Hereinafter, an example of reception environment determination by the reception environment determination unit 43 based on the variation amount of the SP symbol in the frequency direction will be described with reference to FIG. FIG. 6 shows changes in signal levels of SP symbols and data symbols in the frequency direction in the presence or absence of multipath interference. FIG. 6 shows the transition of the signal level in a state where the contents of the data symbols are assumed to be the same.

  That is, when multipath does not occur, as shown in FIG. 6A, the signal level of each symbol different in the frequency direction has a constant value (this state is called “flat fading”). When a multipath occurs, a plurality of signals delayed in time are overlapped and received, thereby causing attenuation and amplification depending on the frequency. Therefore, as shown in FIG. The signal levels of the symbols are different (this state is called “frequency selective fading”).

  Actually, the data symbols of the subcarriers have different signal levels due to different data contents. Therefore, by comparing the signal level of the reference SP symbol for each subcarrier having a different frequency, it is determined whether the state is close to “flat fading” or greatly influenced by “frequency selective fading”. By checking, the magnitude of the influence of multipath interference is determined. By determining the magnitude of the influence of this multipath interference, it is determined whether or not the reception environment is likely to cause multipath, and the average values are calculated by the average value calculators 42a to 42d. Is set.

(3-1) First example of determination example of reception environment As described above, whether or not the reception environment is likely to cause multipath is determined by the amount of variation in the frequency direction of the signal level of the SP symbol. As an example of the determination method, for example, the signal levels of SP symbols of different subcarriers existing in the frequency direction are confirmed, and the maximum signal level Lmax and the minimum signal level Lmin are detected. Then, a difference Δ (Lmax−Lmin) between the maximum value Lmax and the minimum value Lmin in the signal level of the detected SP symbol is calculated, and the difference Δ (Lmax−Lmin) is compared with the threshold value Lth, thereby generating multipath. It is determined whether the reception environment is easy.

  That is, when it is confirmed that the difference Δ (Lmax−Lmin) between the maximum value Lmax and the minimum value Lmin in the signal level of the detected SP symbol is larger than the threshold value Lth, it is determined that the reception environment is likely to cause multipath, and the average A period during which the average value is calculated by the value calculators 42a to 42d is set as the period τ1. When it is confirmed that the difference Δ (Lmax−Lmin) between the maximum value Lmax and the minimum value Lmin in the signal level of the detected SP symbol is equal to or less than the threshold value Lth, it is determined that the reception environment is unlikely to generate multipath, and the average A period during which the average value is calculated by the value calculators 42a to 42d is set as the period τ2.

(3-2) Second example of determination example of reception environment As another example of the determination example, the fluctuation amount of the signal level of SP symbols of different subcarriers existing in the frequency direction is confirmed, and the SP symbol in the frequency direction is confirmed. The maximum value of the signal level is detected. Then, in the detected maximum value, the number of maximum values that are equal to or greater than the threshold value Lth1 is counted, and whether or not the reception environment is likely to cause multipath is determined based on the number of maximum values. That is, in the transition in the frequency direction of the SP symbol, when the number of local maximum values that are equal to or greater than the threshold Lth1 is large, it is determined that the signal level varies greatly in the frequency direction of the SP symbol, and the reception environment is likely to cause multipath. It is determined.

  Therefore, when the number of local maximum values exceeding the threshold Lth1 is equal to or greater than a predetermined number N (N ≧ 1 natural number), it is determined that the reception environment is likely to cause multipath, and the average values are calculated by the average value calculation units 42a to 42d. The period to be set is set to the period τ1. When the number of local maximum values exceeding the threshold Lth1 is less than the predetermined number N, it is determined that the reception environment is unlikely to cause multipath, and the period for calculating the average value by the average value calculation units 42a to 42d is set as the period τ2. .

(3-3) Third example of determination example of reception environment As still another example of the determination example, the fluctuation amount of the signal level of SP symbols of different subcarriers existing in the frequency direction is confirmed, and the SP symbol in the frequency direction is confirmed. The minimum value of the signal level is detected. Then, in the detected minimum value, the number of minimum values that are equal to or less than the threshold value Lth2 is counted, and it is determined whether the reception environment is likely to cause multipath based on the number of the minimum values. That is, in the transition in the frequency direction of the SP symbol, when there are a large number of local minimum values that are less than or equal to the threshold value Lth2, it is determined that the signal level varies greatly in the frequency direction of the SP symbol, and a reception environment in which multipath is likely to occur. It is determined.

  Therefore, when the number of minimum values that are equal to or less than the threshold Lth2 is equal to or greater than a predetermined number N (N ≧ 1 natural number), it is determined that the reception environment is likely to cause multipath, and the average values are calculated by the average value calculation units 42a to 42d. The period to be calculated is set to the period τ1. When the number of minimum values that are equal to or less than the threshold value Lth2 is smaller than the predetermined number N, it is determined that the reception environment is unlikely to cause multipath, and the period for calculating the average value by the average value calculation units 42a to 42d is set as the period τ2. To do.

  When determining the reception environment in this way, the average values (time average signal levels) La to Ld of the same subcarriers of the baseband signals calculated by the average value calculators 42a to 42d are averaged. The reception environment may be determined based on the signal level (La + Lb + Lc + Ld) / 4. The reception environment may be determined based on the signal level that is the maximum value among the time average signal levels La to Ld of the subcarriers calculated by the average value calculation units 42a to 42d.

  Further, an average value Lav based on the signal levels of the SP signals of different subcarriers obtained from the same baseband signal is calculated for the above-described threshold values Lth, Lth1, and Lth2 that serve as a reference for determining the reception environment. It may be set based on the value Lav. At this time, for each baseband signal, the average value Lav of the signal levels of the SP signals of different subcarriers is calculated, and the average value or the maximum value is obtained, and the above-described threshold values Lth, Lth1, Lth2 are set. It doesn't matter. For example, when setting with the average value Lav, the values of the threshold values Lth, Lth1, and Lth2 may be changed in proportion to the average value Lav. That is, the above-described thresholds Lth, Lth1, and Lth2 for determining the reception environment may be linearly changed with respect to the reception power of the signal level of the baseband signal.

  The reception environment determination unit 43 determines the reception environment based on the average value (time average signal level) of each baseband signal calculated by the average value calculation units 42a to 42d. The reception environment may be determined by performing the same operation as described above based on the signal level of each baseband signal detected by the level detectors 41a to 41d.

4). Mode Switching Operation and Gain Setting Operation The mode switching operation by the mode switching control unit 45 when the AGC control unit 4 operates as described above will be described with reference to FIG. FIG. 7 is a diagram illustrating the relationship of the received power of the antennas 1a to 1d. In FIG. 7, the received power of the antennas 1a to 1d is represented by the time and frequency average signal level obtained from the average value calculation units 42a to 42d.

  As described above, when the time and frequency average signal levels L1 to L4 obtained from the average value calculation units 42a to 42d are given to the mode switching control unit 45, the maximum value Lx and the minimum value Ly are detected. At this time, the maximum value Lx of the time and frequency average signal level corresponds to the maximum value of the received power of the antennas 1a to 1d (hereinafter referred to as “maximum received power”), and the time and frequency average signal The minimum level Ly corresponds to the minimum value of received power of the antennas 1a to 1d (hereinafter referred to as “minimum received power”). Then, by calculating the difference ΔL between the maximum value Lx and the minimum value Ly, the difference between the maximum received power and the minimum received power in the antennas 1a to 1d is detected.

  In this way, the difference ΔL between the maximum value Lx and the minimum value Ly representing the difference between the maximum received power and the minimum received power in the antennas 1a to 1d is compared with the predetermined threshold value LTH. For example, the predetermined threshold LTH is a value when the difference between the maximum received power and the minimum received power in the antennas 1a to 1d is 10 dB.

  As shown in FIG. 7A, when the difference ΔL between the maximum value Lx and the minimum value Ly is larger than the threshold value LTH, the difference between the maximum reception power and the minimum reception power at the antennas 1a to 1d is large. It is determined that the digital broadcast signal obtained by receiving from the antenna 1 with low power (corresponding to any of the antennas 1a to 1d) is a signal with CNR deteriorated. Therefore, the gain setting unit 44 switches to the gain limiting mode for reducing the gain of the digital broadcast signal obtained by receiving from the antenna 1 with low received power (corresponding to one of the antennas 1a to 1d). Performs gain setting operation.

  Further, as shown in FIG. 7B, when the difference ΔL between the maximum value Lx and the minimum value Ly is equal to or smaller than the threshold value LTH, the difference between the maximum reception power and the minimum reception power at the antennas 1a to 1d is small. It is determined that the CNR degradation is small even in a digital broadcast signal obtained by receiving from the antenna 1 with low power (corresponding to any of the antennas 1a to 1d). Therefore, the gain setting operation of the gain setting unit 44 is performed by switching the gain of the digital broadcast signal received from the antennas 1a to 1d to the normal mode set by the same target value.

  The mode switching control unit 45 obtains the threshold value LTH for the difference ΔL between the maximum value Lx and the minimum value Ly representing the difference between the maximum received power and the minimum received power at the antennas 1a to 1d from the average value calculating units 42a to 42d. The magnitude may be changed based on the maximum value Lx of the time and frequency average signal level. That is, when the maximum value Lx of the time and frequency average signal level is increased, the threshold value LTH for the difference ΔL between the maximum value Lx and the minimum value Ly is increased, and when the maximum value Lx of the time and frequency average signal level is decreased, The threshold value LTH for the difference ΔL between the maximum value Lx and the minimum value Ly is reduced.

  In addition, when the antennas 1a and 1b are installed on the windshield of the vehicle body and the antennas 1c and 1d are installed on the rear glass of the vehicle body as in this embodiment, the antennas 1c and 1d installed on the rear glass are The antenna is affected by the influence. Therefore, the antennas 1c and 1d often have poor reception sensitivity compared to the antennas 1a and 1b installed on the windshield. Therefore, the average value (L1 + L2) / 2 of the time and frequency average signal level indicating the reception sensitivity of the antennas 1a and 1b, and the average value (L3 + L4) / 2 of the time and frequency average signal level indicating the reception sensitivity of the antennas 1c and 1d. It is also possible to switch between the normal mode and the amplification factor limiting mode by obtaining the difference between and the threshold LTH.

5. Gain Setting Operation As described above, when the mode for performing the gain setting operation is determined by the mode switching control unit 45, the gain setting unit 44 uses the mode determined by the mode switching control unit 45 according to the mode determined by the mode switching control unit 45. An amplification factor is set in each of the tuner units 2a to 2d. That is, the amplification factor setting unit 44 performs the amplification factor setting operation described below in each of the normal mode and the amplification factor limiting mode specified by the mode switching control unit 45.

  The amplification factor setting operation of the amplification factor setting unit 44 in each of the normal mode and the amplification factor limiting mode switched by the mode switching control unit 45 will be described below with reference to the drawings. FIG. 8 is a diagram illustrating a change in received power before and after amplification when the amplification factor is set in the normal mode. FIG. 9 is a diagram illustrating before and after amplification when the amplification factor is set in the amplification factor limiting mode. It is a figure which shows the change of the received power after.

(5-1) Normal Mode As shown in FIG. 7B, the difference ΔL between the maximum value Lx and the minimum value Ly representing the difference between the maximum received power and the minimum received power in the antennas 1a to 1d is equal to or less than the threshold value LTH. When the mode switching control unit 45 designates the normal mode, the amplification factor setting unit 44 adjusts the tuner units 2a to 2 by the time obtained from the average value calculation units 42a to 42d and the frequency average signal levels L1 to L4, respectively. Set the amplification factor in 2d.

  That is, the time and frequency average signal level L1 from the average value calculation unit 42a makes the amplification factor of the tuner unit 2a L / L1 times the current amplification factor, and the time and frequency average signal level from the average value calculation unit 42b. By L2, the amplification factor of the tuner unit 2b is set to L / L2 times the current amplification factor, and the amplification factor of the tuner unit 2c becomes the current amplification factor by the time and frequency average signal level L3 from the average value calculation unit 42c. The amplification factor of the tuner unit 2d is set to L / L4 times the current amplification factor based on the time and frequency average signal level L4 from the average value computation unit 42d.

  As shown in FIG. 8A, it is assumed that the time indicating the received power of the antennas 1a to 1d before amplification and the frequency average signal levels L1 to L4 have a relationship of L1> L2> L3> L4. And since the noise level superimposed on the digital broadcast signal received by the antennas 1a to 1d is considered to be the same in any of the antennas 1a to 1d, the noise levels appearing in the time and frequency average signal levels L1 to L4 are The value Nz is substantially constant. Then, the amplification factor for each of the tuner units 2a to 2d is set so as to be the target value L.

  That is, by setting the amplification factors L / L1 to L / L4 to the tuner units 2a to 2d so that the time and frequency average signal level indicating the received power of the antennas 1a to 1d become the target value L, As shown in FIG. 8B, the time and frequency average signal level indicating the received power of the antennas 1a to 1d are L. At this time, noise superimposed on the digital broadcast signal received by the antennas 1a to 1d is also amplified. Therefore, the noise levels appearing in the time and frequency average signal levels are L / L1 × Nz, L / L2 × Nz, L / L3 × Nz, and L / L4 × Nz for the antennas 1a, 1b, 1c, and 1d, respectively. It becomes.

  In this way, the noise level is also amplified, but the difference in magnitude between the amplification factors L / L1 to L / L4 set in each of the tuner units 2a to 2d is small. Therefore, it is possible to suppress variations in the size of the noise level L / L1 × Nz to L / L4 × Nz. Thereby, when the baseband signals from the orthogonal demodulation units 3a to 3d are combined by the MRC combining unit 5, the influence of noise appearing in the baseband signals is suppressed.

(5-2) First Example of Amplification Limit Mode As shown in FIG. 7A, the difference ΔL between the maximum value Lx and the minimum value Ly representing the difference between the maximum received power and the minimum received power in the antennas 1a to 1d is A first example of the operation of the amplification factor setting unit 44 when the mode switching control unit 45 designates the amplification factor limiting mode when it is larger than the threshold value LTH will be described below. In this example, the amplification factor of the tuner unit 2 (corresponding to any of the tuner units 2a to 2d) with respect to the antenna 1 (corresponding to any one of the antennas 1a to 1d) with small received power is the amplification with respect to the maximum received power. Set to rate.

  That is, as shown in FIG. 9A, the time indicating the received power of the antennas 1a to 1d before amplification and the frequency average signal levels L1 to L4 are in a relationship of L1> L2> L3> L4. It is detected that the received power of the antenna 1a is the maximum received power. For the time and frequency average signal levels L2 to L4 representing the received power of the other antennas 1b to 1d, the difference between the time and frequency average signal level L1 indicating the maximum received power is greater than or equal to ΔLa (ΔLa ≦ LTH). It is confirmed whether or not.

  At this time, when the difference between each of the time and frequency average signal levels L3 and L4 and the time and frequency average signal level L1 indicating the maximum received power is equal to or larger than ΔLa as shown in FIG. It is determined that the received power of 1d is small and the amplification factor needs to be limited. Therefore, the amplification factor setting unit 44 sets the amplification factors of the tuner units 2c and 2d to the same amplification factor as that of the tuner unit 2a connected to the antenna 1a having the maximum received power.

  Thereby, the amplification factors L / L1 and L / L2 for the tuner units 2a and 2b are set so that the time and frequency average signal level indicating the received power of the antennas 1a and 1b become the target value L. As shown in FIG. 9B, the time and frequency average signal level indicating the received power of the amplified antennas 1a and 1b are L. Further, since the amplification factors of the tuner units 2c and 2d are set to the same amplification factor as that of the tuner unit 2a, the respective amplification factors are set to L / L1. As a result, as shown in FIG. 9B, the time and frequency average signal levels indicating the received power of the amplified antennas 1c and 1d are L3 × L / L1 and L4 × L / L1.

  At this time, noise superimposed on the digital broadcast signal received by the antennas 1a to 1d is also amplified. Therefore, the noise levels appearing in the time and frequency average signal levels after amplification in each of the tuners 2a to 2d are L / L1 as shown in FIG. 9B with respect to the antennas 1a, 1b, 1c, and 1d. × Nz, L / L2 × Nz, L / L1 × Nz, and L / L1 × Nz.

  In this way, the noise level is also amplified, but for the digital broadcast signals received by the antennas 2c and 2d having a large CNR, which is the ratio of noise to the received power, the amplification factor in each of the tuner units 2c and 2d is amplified. The amplification factor is the same as that of the tuner unit 2a having the smallest rate. Thereby, compared with the case where the amplification factor is set in the normal mode, the absolute amount of noise given from each baseband signal when synthesized by the MRC synthesis unit 5 can be reduced. Therefore, the influence of noise on the baseband signal obtained after being synthesized by the MRC synthesis unit 5 can be suppressed.

  In this example, for the antenna 1x (corresponding to the antennas 1a to 1d), the difference between the time indicating the maximum received power and the frequency average signal level is a received power corresponding to the time and frequency average signal level equal to or greater than ΔLa. The tuner 2x (corresponding to the tuners 2a to 2d) to which the antenna 1x is connected is used as the amplification factor of the tuner 2y (tuner corresponding to the antennas 1a to 1d), which is the maximum received power. (Corresponding to the parts 2a to 2d).

  However, the tuner to which the antenna 1z (corresponding to the antennas 1a to 1d) having the reception power corresponding to the time and the frequency average signal level in which the difference between the time indicating the maximum reception power and the frequency average signal level is smaller than ΔLa is connected. It may be set to the amplification factor of the unit 2z (corresponding to the tuner units 2a to 2d), or may be an average value of the amplification factors set to the tuner units 2y and 2z. That is, in the case of the example shown in FIG. 9A, in this example, the amplification factors of the tuner units 2c and 2d are the amplification factors L / L1 of the tuner unit 2a connected to the antenna 1a that is the maximum received power. However, the amplification factor L / L2 of the tuner unit 2b may be used, or the average value ½ × (L / L1 + L / L2) of the amplification factors of the tuner units 2a and 2b may be used.

(5-3) Second Example of Gain Limiting Mode Next, a second example of operation of the gain setting unit 44 when the mode switching control unit 45 is designated to be in the gain limiting mode will be described below. explain. In this example, the amplification factor of the tuner unit 2 (corresponding to any of the tuner units 2a to 2d) with respect to the antenna 1 (corresponding to any of the antennas 1a to 1d) having low received power is set to the preset target value Lk ( Lk <L).

  Also in this example, when the relationship between the time indicating the received power of the antennas 1a to 1d before amplification and the frequency average signal levels L1 to L4 is the relationship shown in FIG. The antennas 1c and 1d in which the difference between the time indicating the maximum received power and the frequency average signal level L1 is equal to or greater than ΔLa are detected. Thereby, the amplification factor setting unit 44 sets the target value for setting the amplification factors of the tuner units 2a and 2b to L, and sets the target value for setting the amplification factors of the tuner units 2c and 2d to Lk. Set. At this time, for example, the target value Lk may be a value smaller than the target value L by ΔLa.

  Thereby, the amplification factors L / L1 and L / L2 for the tuner units 2a and 2b are set so that the time and frequency average signal level indicating the received power of the antennas 1a and 1b become the target value L. As shown in FIG. 9C, the time and frequency average signal level indicating the received power of the amplified antennas 1a and 1b are L. Further, by setting the amplification factors Lk / L3 and Lk / L4 to the tuner units 2c and 2d so that the time and frequency average signal level indicating the received power of the antennas 1c and 1d become the target value Lk, As shown in FIG. 9C, the time and frequency average signal level indicating the received power of the amplified antennas 1c and 1d are Lk.

  At this time, noise superimposed on the digital broadcast signal received by the antennas 1a to 1d is also amplified. Therefore, the noise levels appearing in the time average and frequency average signal levels after amplification in each of the tuners 2a to 2d are L / L1 as shown in FIG. 9 (c) for the antennas 1a, 1b, 1c, and 1d. × Nz, L / L2 × Nz, Lk / L3 × Nz, and Lk / L4 × Nz.

  In this way, the noise level is also amplified, but for the digital broadcast signals received by the antennas 2c and 2d having a large CNR, which is the ratio of noise to the received power, the amplification factor in each of the tuner units 2c and 2d is set as the target. The target value Lk is sufficiently smaller than the value L. Thereby, compared with the case where the amplification factor is set in the normal mode, the absolute amount of noise given from each baseband signal when synthesized by the MRC synthesis unit 5 can be reduced. Therefore, the influence of noise on the baseband signal obtained after being synthesized by the MRC synthesis unit 5 can be suppressed.

  Further, in this example, when it is determined that the received power is small and a plurality of tuner units 2 (corresponding to any one of the tuner units 2a to 2d in FIG. 1) for setting the amplification factor by the target value Lk are confirmed, It may be set uniformly with the amplification factor set for the highest received power. That is, in the case of FIG. 9A, it is assumed that only the tuner units 2a and 2b are set with the target value L as described above. And about the tuner parts 2c and 2d, the time and frequency average signal level corresponding to each become the relationship of L3> L4. Thus, the amplification factor of the tuner unit 2c is set to Lk / L3 by the target value Lk, and the amplification factor of the tuner unit 2d is set to Lk / L3 that is the same as the amplification factor of the tuner unit 2c.

(5-4) Third Example of Gain Limiting Mode Next, a third example of operation of the gain setting unit 44 when the mode switching control unit 45 is designated to be in the gain limiting mode will be described below. explain. In this example, the amplification factor of the tuner unit 2 (corresponding to any of the tuner units 2a to 2d) with respect to the antenna 1 having low received power (corresponding to any of the antennas 1a to 1d) is determined by the preset amplification factor A1. Is set. The amount of amplification by the amplification factor A1 is sufficiently small, and the time and frequency average signal level obtained by amplification by the tuner unit 2 (corresponding to any one of the tuner units 2a to 2d) set to the amplification factor A1 are the target. The value is smaller than the value L.

  Also in this example, when the relationship between the time indicating the received power of the antennas 1a to 1d before amplification and the frequency average signal levels L1 to L4 is the relationship shown in FIG. The antennas 1c and 1d in which the difference between the time indicating the maximum received power and the frequency average signal level L1 is equal to or greater than ΔLa are detected. As a result, the amplification factor setting unit 44 sets the target value for setting the amplification factors of the tuner units 2a and 2b to L, and sets the amplification factors of the tuner units 2c and 2d to A1. At this time, for example, the amplification factor A1 may be set to a value smaller than Lk / (L−ΔLa) so that the time after amplification and the frequency average signal level are close to the target value Lk of the second example. I do not care.

  Thereby, the amplification factors L / L1 and L / L2 for the tuner units 2a and 2b are set so that the time and frequency average signal level indicating the received power of the antennas 1a and 1b become the target value L. As shown in FIG. 9D, the time and frequency average signal level indicating the received power of the amplified antennas 1a and 1b are L. Further, by setting the amplification factor A1 to each of the tuner units 2c and 2d, as shown in FIG. 9D, the time and frequency average signal level indicating the received power of the amplified antennas 1c and 1d are L3. × A1, L4 × A1.

  At this time, noise superimposed on the digital broadcast signal received by the antennas 1a to 1d is also amplified. Therefore, the noise levels appearing in the time and frequency average signal levels after amplification in each of the tuners 2a to 2d are L / L1 as shown in FIG. 9D with respect to the antennas 1a, 1b, 1c, and 1d. × Nz, L / L2 × Nz, A1 × Nz, A1 × Nz.

  In this way, the noise level is also amplified, but for the digital broadcast signals received by the antennas 2c and 2d having a large CNR, which is the ratio of noise to the received power, the amplification factor in each of the tuner units 2c and 2d is The amplification amount is set to a sufficiently small value. Thereby, compared with the case where the amplification factor is set in the normal mode, the absolute amount of noise given from each baseband signal when synthesized by the MRC synthesis unit 5 can be reduced. Therefore, the influence of noise on the baseband signal obtained after being synthesized by the MRC synthesis unit 5 can be suppressed.

  In the receiving apparatus of the present embodiment, ON / OFF switching of the AGC control unit 4 may be set. That is, when the antennas 1a to 1d are always equal in received power level difference, such as a rod antenna, it is not necessary to change the amplification factors of the tuner units 2a to 2d. Therefore, by setting the AGC control unit 4 to OFF, the amplification factors of the tuner units 2a to 2d are fixed at predetermined values.

  The present invention can be applied to a receiving apparatus that performs mobile communication, and can also be applied to a receiving apparatus that receives a digital broadcast signal by the OFDM method. In addition, the receiving device can be applied to a vehicle-mounted receiving device or a portable terminal device having a broadcast receiving function such as a mobile phone or a PDA.

These are block diagrams which show the internal structure of the receiver in embodiment of this invention. FIG. 2 is a block diagram showing a configuration of an orthogonal demodulation unit in the receiving apparatus of FIG. 1. FIG. 4 is a diagram showing a relationship between pilot symbols and data symbols in an OFDM symbol sequence. FIG. 2 is a block diagram showing a configuration of an AGC control unit in the receiving apparatus of FIG. 1. These are figures which show the period for the average value calculation changed according to the presence or absence of multipath. These are figures which show transition of the reception power by the movement condition of a receiver. These are the figures which show the relationship of the received power of each antenna in the receiver of FIG. These are the figures which show the relationship before and behind amplification in the normal mode of each received power of the antenna in the receiver of FIG. These are figures which show the relationship before and behind amplification in the amplification factor restriction | limiting mode of each received power of the antenna in the receiver of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1a-1d Antenna 2a-2d Tuner part 3a-3d Orthogonal demodulation part 4 AGC control part 5 MRC synthetic | combination part 6 Digital demodulation part 7 MPEG decoder

Claims (9)

A plurality of antennas for receiving digital broadcast signals; a tuner unit having the same number as the antennas for selecting a digital broadcast signal received by each of the plurality of antennas to obtain a baseband signal and dividing it into carriers; and In a receiving apparatus comprising: a combining unit that combines the baseband signal acquired by the tuner unit for each carrier;
A signal level detection unit for detecting a signal level of each of a plurality of baseband signals acquired by the plurality of tuner units;
An amplification factor setting unit that sets an amplification factor after channel selection in the tuner unit according to the signal level of each of the plurality of baseband signals detected by the signal level detection unit;
The signal levels of the baseband signals detected by the signal level detection unit are compared, and based on the comparison result, the amplification factor setting operation in the amplification factor setting unit is changed to the operation in the first mode and the second mode. A mode switching control unit for switching,
With
When the difference in signal level between the plurality of baseband signals is smaller than a predetermined value, and the amplification factor setting operation in the amplification factor setting unit is set to the first mode,
With the first target value for the signal level of the baseband signal at the time of setting the amplification factor being constant, the amplification factor setting unit performs an amplification factor setting operation,
When the difference between the signal levels of the plurality of baseband signals is a predetermined value or more, and the amplification factor setting operation in the amplification factor setting unit is set to the second mode,
Amplification in the case where the amplification factor in the first tuner unit that acquires the first baseband signal whose signal level is determined to be lower than other baseband signals is set in the first mode. The gain in the second tuner unit that sets the value smaller than the rate and acquires the baseband signal other than the first baseband signal is set by the same first target value as in the first mode. And a receiving device.   When the amplification factor setting operation in the second mode is performed in the amplification factor setting unit, the amplification factor in the first tuner unit is set by the amplification factor set for the second tuner unit. The receiving device according to claim 1.   When the amplification factor setting operation in the second mode is performed in the amplification factor setting unit, the amplification factor at the first tuner unit is the minimum amplification factor among the amplification factors set for the second tuner unit. The receiving apparatus according to claim 2, wherein the receiving apparatus is a rate.   When the amplification factor setting operation in the second mode is performed in the amplification factor setting unit, the amplification factor in the first tuner unit is set based on a second target value that is smaller than the first target value. The receiving apparatus according to claim 1.   The amplification factor in the first tuner unit is set to a predetermined constant value when the amplification factor setting operation in the second mode is performed in the amplification factor setting unit. The receiving device described. An average value calculation unit that averages the signal level detected by the signal level detection unit and outputs the averaged signal level to the amplification factor setting unit;
A reception environment determination unit that determines a reception environment by the plurality of antennas by determining whether or not a plurality of baseband signals acquired by the plurality of tuner units are greatly affected by multipath;
Further comprising
When the reception environment determination unit determines that the first reception environment is greatly affected by multipath, the average value calculation unit determines a period for averaging the signal level detected by the signal level detection unit. The receiving apparatus according to claim 1, wherein the receiving apparatus is set to be shorter than that set in the second receiving environment in which the influence of multipath is small.   7. The reception according to claim 6, wherein the reception environment determination unit determines the magnitude of the influence of multipath by measuring a transition of a signal level in the frequency direction of the baseband signal after quadrature demodulation. apparatus. A signal level detecting step for detecting a signal level of each of a plurality of baseband signals acquired by the same number of tuners as the antennas for selecting digital broadcast signals received by each of the plurality of antennas;
An amplification factor step for setting an amplification factor after channel selection in the tuner unit according to the signal level of each of the plurality of baseband signals detected in the signal level step;
The signal levels of the baseband signals detected in the signal level step are compared, and based on the comparison result, the gain setting operation in the gain setting step is switched to the operation in the first mode and the second mode. A mode switching control step;
With
When the difference in signal level between each of the plurality of baseband signals is smaller than a predetermined value, and the gain setting operation in the gain setting step is set to the first mode,
With the first target value for the signal level of the baseband signal when setting the amplification factor being constant, performing the amplification factor setting operation in the amplification factor setting step,
When the difference in signal level between the plurality of baseband signals is a predetermined value or more, and the amplification factor setting operation in the amplification factor setting step is set to the second mode,
In the amplification factor setting step, the amplification factor in the first tuner unit that acquires the first baseband signal whose signal level is determined to be lower than other baseband signals is set in the first mode. The gain in the second tuner unit that acquires the baseband signal other than the first baseband signal is the same as that in the first mode. A received signal gain setting method, wherein the received signal gain is set according to the first target value. An average value calculating step of averaging the signal level detected in the signal level detecting step and outputting the averaged signal level to the amplification factor setting unit,
A reception environment determination step of determining a reception environment by the plurality of antennas by determining whether or not a plurality of baseband signals acquired by the plurality of tuner units are greatly affected by multipath;
Further comprising
If it is determined in the reception environment determination step that the first reception environment is greatly affected by multipath, a period for averaging the signal level detected by the signal level detection unit in the average value calculation step is determined. The reception signal amplification factor setting method according to claim 8, wherein the reception signal amplification factor is set to be shorter than that in the second reception environment where the influence of multipath is small.

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