JP2007274726A - On-vehicle digital signal receiver and diversity system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform reception with the suitable distribution of an antenna sequence in accordance with the arrival state of radio waves. <P>SOLUTION: The present invention relates to an on-vehicle digital communication receiver which is mounted to a vehicle for receiving radio waves modulated by a digital signal, including: a plurality of antenna means each constituted of a plurality of antennas with different directivity for switching the antennas so as to select any antenna in accordance with a reception state; and a frequency composition diversity means for composing output signals from the antenna means with weighting based on the reception state for each frequency component. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention is a vehicle-mounted digital signal receiver for receiving a digital signal such as a radio wave mounted on a vehicle and modulated by an orthogonal frequency division multiplex (hereinafter sometimes abbreviated as “OFDM”) method. The present invention relates to an apparatus and an antenna.

  2. Description of the Related Art Conventionally, development of an OFDM system has been promoted as a transmission system for terrestrial digital television broadcasting. The OFDM scheme is a form of multi-carrier transmission scheme, and performs signal transmission by applying amplitude phase modulation (QAM) to 5300 subcarriers arranged at 1 kHz intervals, for example. The multicarrier scheme is generally known as a modulation scheme that has excellent resistance to frequency selective fading. Frequency selective fading has a problem of degrading channel quality in broadband wireless communication. In terrestrial digital television broadcasting, since it is necessary to transmit high-quality images, the OFDM system is adopted in a frequency band of about 500 MHz to 800 MHz. In the modulation signal of the OFDM system, the frequency band of the modulated wave overlaps between adjacent subcarriers, but the orthogonality that the correlation of the modulated wave band signal becomes zero is ensured by batch modulation / demodulation by fast Fourier transform (FFT). be able to. Furthermore, by adding a guard interval signal on the transmission side, it is possible to remove inter-symbol interference (ISI) due to multipath delay waves.

FIG. 29 shows the configuration of the transmitting side and the subcarrier frequency band arrangement according to the OFDM scheme. As shown in FIG. 29A, first, serial data to be transmitted is converted into parallel data composed of a plurality of, for example, N complex symbols in the baseband by the serial / parallel converter (S / P) 5. An IFFT (Inverse Fast Fourier Transform) calculation unit 6 performs inverse fast Fourier transform (IFFT) on the frequency data so that each symbol is transmitted by a subcarrier having a frequency having an interval shown in FIG. As a result, the frequency component of the subcarrier is converted into a multiplexed time signal. The real part Re and the imaginary part of the time signal are Im, LPF (Low
Pass Filter) 7a and 7b are input to the quadrature modulator 8 as I-ch and Q-ch, respectively. The quadrature modulator 8 performs quadrature modulation on the input data, and up-converts the data to a high-frequency signal by a subsequent transmission unit (not shown). On the receiving side, time-frequency conversion by FFT is performed as an inverse operation of IFFT on the transmitting side, and N symbols transmitted on N subcarriers are demodulated and output.

  FIG. 30 shows the guard interval. An OFDM signal constitutes one symbol by a guard period GI and an effective symbol period. In the guard period GI, the signal after the effective symbol period is cyclically copied. Insertion of the guard period GI is performed in order to prevent interference due to the delayed signal. The concept of frequency synchronization using signals in the guard period GI is described in, for example, pages 61 to 66 of the Television Society Technical Report (ITE Technical Report Vol.20 No. 53) published on October 17, 1996. It is reported as “Examination of frequency synchronization in ODFM demodulation”.

  When a traveling vehicle receives radio waves modulated by the OFDM method, a Doppler shift, which is a frequency shift based on the Doppler effect, becomes an obstacle. Doppler shift is particularly likely to cause deterioration in reception characteristics when a multipath through which radio waves propagate through a plurality of paths has frequency selectivity. This is because if frequency fluctuations occur independently for each subcarrier, synchronization between subcarriers on the receiving side becomes difficult, orthogonality is lost, and inter-carrier interference (ICI) occurs. Since the frequency shift caused by the Doppler shift is proportional to the speed of the vehicle, the inter-carrier interference becomes particularly noticeable during traveling at a high speed. In order to suppress inter-carrier interference in a multipath environment, it is necessary to estimate each path and equalize its fluctuation components. The present applicant proposes, as Japanese Patent Application No. 2001-279292, a method of improving characteristic deterioration due to inter-carrier interference in a multipath environment by providing a moving body with a plurality of directional antennas having different directivity directions and switching directivity. Yes.

  FIG. 31 shows a configuration basically equivalent to the OFDM receiving apparatus according to the first embodiment attached as FIG. 7 to Japanese Patent Application No. 2001-279292 as the basis of the present invention. 29 and 30 described above are basically the same as the drawings attached to Japanese Patent Application No. 2001-279292 as FIGS. 24 and 25, respectively. However, for convenience of explanation, there are portions where reference marks and the like are changed.

  In FIG. 31, the 360 ° range around the automobile as a moving body is divided into four sectors of 90 °, and directional antennas 11a, 11b, 11c, and 11d are provided in each sector. The antennas 11a and 11d have directivity in the forward direction and the backward direction of the vehicle, and the antennas 11b and 11c have directivity in the left and right direction of the vehicle. The radio reception units (RV) 12a, 12b, 12c, and 12d amplify radio signals received by the antennas 11a, 11b, 11c, and 11d, convert the frequencies to baseband signals, and input the baseband signals to the antenna switching unit 13. . The received power measuring unit 14 measures received power by measuring the received signal strength of each of the wireless receiving units 12a, 12b, 12c, and 12d, for example, RSSI (Received Signal Strength Indicator). The antenna selection unit 15 selects the antennas 11a to 11d having the maximum reception power based on the reception power. The received signal of the selected antenna is input to the orthogonal demodulator 16 that constitutes the demodulator DEM (DEModulator). The orthogonal demodulator 16 performs orthogonal demodulation processing on the received signal. The demodulated signal is timing-synchronized by the GI removal unit 17 and is output after the guard period (GI) is removed.

  The variation average shift amount calculation unit 18 calculates a variation average shift amount Δθ per symbol of multipath fading by calculating a correlation between a known pilot signal and a received signal. The fading compensation unit 19 multiplies the received signal by exp (−jΔθ) to compensate for multipath fading. The FFT operation unit 20 converts the time domain signal into N carrier signals. The N carrier signals are subjected to error detection correction and decoding processing by an error detection correction decoding unit (not shown).

Prior arts related to receiving radio waves of digital signals such as OFDM by switching a plurality of antennas are disclosed in, for example, JP-A-11-205290, JP-A-2000-174726, JP-A-2000-278243, and JP-A-2001. -86091 and the like. In Japanese Patent Laid-Open No. 11-205290, in order to improve the degradation of reception characteristics caused in a multiple reflected radio wave propagation environment and a mobile reception environment, OFDM reception signals from a plurality of antennas are respectively delayed in time, and OFDM before and after the delay The prior art which selects the antenna with the maximum correlation value of the received signal and demodulates the OFDM signal from the selected antenna is disclosed. Japanese Patent Application Laid-Open No. 2000-174726 discloses a prior art that performs antenna switching by detecting a subcarrier level or the like so that a sufficient diversity effect can be obtained even in the case of a frequency selective fading propagation path. Japanese Patent Laid-Open No. 2000-278243 discloses a prior art for combining received signals from a plurality of antennas in order to improve multipath fading resistance. Japanese Patent Laid-Open No. 2001-86091 discloses a prior art that improves the characteristics of a received signal by combining signals received by a plurality of antennas before OFDM demodulation.
JP-A-11-205290 JP 2000-174726 A JP 2000-278243 A

  Each of the above-mentioned prior arts specifically relates to an antenna arrangement suitable for a diversity system for receiving a digital signal radio wave such as OFDM, such as installing an antenna having a traveling direction directivity and an antenna having a reverse directivity. No disclosure is found.

  In FIG. 31, using the directivity of the antennas 11a to 11d, the range for compensating for multipath fading is limited to the direction in which the received power is large. Since this direction changes as the vehicle moves, the direction is switched so that the received power becomes maximum. Since it is necessary to compare the received power from each of the antennas 11a to 11d to determine whether or not the received power is the maximum, radio receiving units 12a to 12d are provided for the respective antennas 11a to 11d. However, the radio receivers 12a to 12d that amplify the high-frequency signal and convert it to baseband handle the high-frequency signal of about 500 MHz as described above. Therefore, the radio receivers 12a to 12d are expensive and use the same number as the plurality of antennas 11a to 11d. This increases the manufacturing cost of the receiving device.

  An object of the present invention is to provide an in-vehicle digital signal receiving device that can reduce the number of radio receivers used compared to the number of antennas, and an antenna that can be easily mounted on a vehicle to obtain directivity. It is.

The present invention is an in-vehicle digital signal receiving device for receiving a radio wave mounted on a vehicle and modulated with a digital signal,
A plurality of antenna means comprising a plurality of antennas having different directivities, and switching to select one of the antennas according to the reception state;
An in-vehicle digital signal receiving apparatus comprising frequency combining diversity means for combining the output signals from the antenna means with weights based on reception states for each frequency component.

  According to the present invention, the antenna means has a plurality of directivities such that the gain is increased on a predetermined one-direction side with respect to the vehicle, and the directivity is increased such that the gain is increased in the opposite direction of the one-direction side. By switching so as to select any one of the directivities, it is possible to receive a radio wave of a digital signal modulated by an orthogonal frequency division multiplexing system or the like. Therefore, even if the radio wave of the received digital signal is affected by multipath fading, the direction of arrival of the received radio wave can be narrowed down by switching the directivity. In addition, the frequency synthesis diversity means receives the received signal from the antenna means as frequency synthesis diversity with weighting based on the superiority or inferiority of the reception state for each frequency component, so it is appropriate for the arrival state of radio waves. Receiving can be performed with a proper antenna sequence distribution.

  Hereinafter, an in-vehicle OFDM receiver as each embodiment of the present invention will be described with reference to FIGS. 16 to 24, the antenna according to each embodiment of the present invention will be described. 25 to 28, antenna switching according to still another embodiment of the present invention will be described. In each embodiment, the same reference numerals are given to the portions corresponding to the previously described embodiments, and redundant description will be omitted. Although the case of receiving an OFDM terrestrial digital television broadcast will be described, it is needless to say that the present invention is generally applicable to receiving a large-capacity digital signal.

  FIG. 1 shows a schematic configuration of an in-vehicle OFDM receiver 21 according to the first embodiment of the present invention. The in-vehicle OFDM receiver 21 is mounted on a vehicle and receives a radio wave modulated by an orthogonal frequency division multiplexing system, so that a forward beam antenna 22A, a backward beam antenna 22B, a changeover switch 23, an RF / IF unit 24, An OFDM demodulator 25, a level detector 26, an error corrector 27, an AGC unit 28, and an antenna selection circuit 29 are included.

  The forward beam antenna 22A, the rear beam antenna 22B, and the changeover switch 23 have directivity that increases the gain in front of the vehicle, which is a predetermined one direction side, and the reverse direction of the one direction side. An antenna unit that can receive a radio wave modulated by the orthogonal frequency division multiplexing system is switched by the selector switch 23 so as to select any one of a plurality of directivities including a directivity whose gain increases in the direction. .

The RF / IF unit 24, the OFDM demodulation, the OFDM demodulation unit 25, the level detection unit 26, the error correction unit 27, and the AGC unit 28 input an electric signal corresponding to the received radio wave from the antenna unit by switching the directivity. It functions as an OFDM demodulating means for amplifying and demodulating. The RF / IF unit 24 amplifies the high frequency signal, converts it to an intermediate frequency, and further amplifies it. An OFDM demodulator 25 that demodulates an intermediate frequency signal includes a level detector 26 and an error corrector 27, and demodulates and outputs an effective symbol (TS). The level detection unit 26 outputs received power information and adjusts the gain of the RF / IF unit 24 via the AGC unit 28. The error correction unit 27 uses the error correction information included in the demodulated signal to perform error detection and correction within a possible range, and outputs error rate information (Bit Error Rate: BER). The received power information and the error rate information are input to the antenna selection circuit 29. The antenna selection circuit 29 functions as a directivity selection unit that detects information associated with travel of a predetermined vehicle and controls the antenna unit to switch and select directivity according to the detected information.

FIG. 2 shows a procedure of antenna directivity switching control performed by the antenna selection circuit 29 in the in-vehicle OFDM receiver 21 of FIG. In step a1, it waits for the reception error rate to become larger than a predetermined reference. That is, an increase in the reception error rate exceeding the reference is a starting condition for switching the antenna directivity. When the reception error rate is smaller than the reference, error correction is possible or the influence of errors is inconspicuous, and there is no need to switch the antenna directivity. When the error rate increases beyond the reference in step a1, the average reception level of the forward beam antenna 22A is detected over a time of, for example, several hundred milliseconds in step a2. In step a3, the selector switch 23 is switched to the backward beam antenna 22B side, and the average reception level of the backward beam antenna 22B is detected over a period of several hundred milliseconds, for example. In step a4, the forward beam antenna 22A and the backward beam antenna 22B are compared with the average reception level. In step a5, based on the comparison result in step a4, the average reception level is kept higher for a certain time, for example, several seconds to several tens of seconds, and the process returns to step a1.

  In step a2 to step a4, the antenna selection circuit 29 switches a plurality of directivities of the antenna means for a predetermined short period of time, and selects directivity with superiority or inferior reception state. Since directivity in multiple directions is switched for a predetermined short time, even if multiple directivities are not received at the same time, it is possible to easily compare the superiority or inferiority of the reception state between directivities to determine superiority or inferiority it can. Also, the superiority or inferiority of the reception state is determined based on the reception power information. Since the reception power information determines the superiority or inferiority of the reception state, the directivity of the antenna means can be switched in the direction in which the reception power is high. Such determination of superiority or inferiority of the reception state can also be made based on reception error rate information.

  FIG. 3 shows a schematic configuration of an in-vehicle OFDM receiver 31 according to the second embodiment of the present invention. The in-vehicle OFDM receiver 31 includes two systems of antenna means and OFDM demodulation means, and can perform reception by a frequency division diversity method. One system includes a forward beam antenna 22A1, a backward beam antenna 22B1, a changeover switch 23, an RF / IF unit 24, an OFDM demodulation unit 35, a level detection unit 36, and an AGC unit 28. The other system includes a forward beam antenna 22A2, a backward beam antenna 22B2, a changeover switch 43, an RF / IF unit 44, an OFDM demodulation unit 45, a level detection unit 46, and an AGC unit 48. The OFDM demodulation units 35 and 45 are basically the same as the OFDM demodulation unit 25 in the embodiment of FIG. 1 except that they include only the level detection units 36 and 46 and do not include the error correction unit 37. The forward beam antennas 22A1 and 22A2, the backward beam antennas 22B1 and 22B2, the changeover switches 23 and 43, the RF / IF units 24 and 44, the level detection units 36 and 46, and the AGC units 28 and 48 of the two systems are the same. It is.

  The outputs of the two OFDM demodulating units 35 and 45 are combined by the diversity combining unit 38, the combined output is input to the error correction unit 37, and the error rate information is input to the antenna selection circuit 39. Received power information from the level detectors 36 and 46 is also input to the antenna selection circuit 39 and used to control the changeover switches 23 and 43.

FIG. 4 shows a procedure of antenna directivity switching control performed by the antenna selection circuit 39 in the in-vehicle OFDM receiver 31 of FIG. In step b1, it waits for the reception error rate to become larger than a predetermined reference. When the error rate increases beyond the reference in step b1, the average reception level of the forward beam antenna 22A1 of one system is detected over a time of, for example, several hundred milliseconds in step b2. In step b3, the changeover switch 23 is switched to the backward beam antenna 22B1 side, and the average reception level of the backward beam antenna 22B1 is detected over a period of several hundred milliseconds, for example. In step b4, the forward beam antenna 22A1 and the backward beam antenna 22B1 are compared in terms of the average reception level and switched to the higher average reception level. In step b5, a certain time, for example, several tens
Wait for 0 ms. In step b6, based on the comparison result in step b4, the other system is switched to the one in which the average reception level is determined to be higher in one system. For example, if the average reception level of the forward beam antenna 22A1 is greater than the average reception level of the backward beam antenna 22B1 as a result of the comparison in step b4, the forward beam antenna 22A2 is selected. When the average reception level of the backward beam antenna 22B1> the average reception level of the forward beam antenna 22A1, the backward beam antenna 22B2 is selected. In step b7, a predetermined time, for example, several seconds to several tens of seconds is held, and the process returns to step b1.

  FIG. 5 shows a schematic configuration of an in-vehicle OFDM receiver 51 according to the third embodiment of the present invention. The in-vehicle OFDM receiver 51 includes one system of antenna means and OFDM demodulating means as in the embodiment of FIG. 1, but the OFDM demodulator 55 includes a frequency synchronization circuit along with the level detector 26 and the error corrector 27. 56. The antenna selection circuit 59 also receives the error rate information from the error correction unit 27, the frequency shift information from the frequency synchronization circuit 56, and the vehicle speed pulse from the vehicle, detects the Doppler shift direction, and detects the changeover switch 23. Switch. A configuration equivalent to the frequency synchronization circuit is also included in the OFDM demodulation units 25, 35, and 45 in the embodiment of FIGS.

  As described above, in the present embodiment, the antenna selection circuit 49 serving as the directivity selection means converts the arrival direction of the received radio wave into information about the Doppler shift direction with respect to the frequency of the received radio wave and information about the traveling speed of the vehicle. Judgment based on. When the traveling direction of the vehicle approaches the arrival direction of the received radio wave, the Doppler shift shifts in a direction in which the frequency increases, and when the traveling direction moves away from the arrival direction, the Doppler shift shifts in a direction in which the frequency decreases. It can be considered that the frequency shift due to the Doppler shift is proportional to the traveling speed. Therefore, the arrival direction of the received radio wave can be known based on the information about the Doppler shift and the information about the traveling speed of the vehicle, and the directivity direction of the antenna means is selected in accordance with the arrival direction of the received radio wave. Can do.

  FIG. 6 shows a concept of performing Doppler shift direction detection in the embodiment of FIG. The traveling direction of the vehicle can be considered as a vector having a length of the traveling speed V. The angle formed by the direction of travel speed and the direction of arrival of radio waves is defined as θ. When the Doppler shift frequency fd is positive, the angle θ is in the range of −90 ° to + 90 °. When the Doppler shift frequency fd is negative, θ is in the range of + 90 ° to + 180 ° and −90 ° to −180 °. The Doppler shift frequency fd can be expressed by the following equation (1), where fc is the broadcast frequency and C is the radio wave propagation speed.

Therefore, the angle θ can be calculated by the following equation (2).

  Thus, if the angle θ is strictly calculated, the radio wave arrival direction can be calculated. However, the left and right direction cannot be distinguished from the traveling direction of the vehicle. The lateral direction can be detected. When the traveling speed V is low, the error becomes large. From equation (1), when fc = 470 MHz and V = 100 km / h, fd = 44 Hz.

  FIG. 7 shows a procedure of antenna directivity switching control performed by the antenna selection circuit 59 in the in-vehicle OFDM receiver 51 of FIG. In step c1, it waits for the reception error rate to become larger than a predetermined reference. When the error rate increases beyond the reference in step c1, the average value of the Doppler shift frequency fd is detected over a period of several seconds, for example, in step c2. In step c3, it is determined whether the Doppler shift frequency fd is higher than the threshold frequency fth and the traveling speed V is higher than the threshold speed Vth. When it is determined that both conditions are satisfied, in step c4, the antenna selection circuit 59 switches the changeover switch 23 so as to select the forward beam antenna 22A. If the condition is not satisfied in step c3, it is determined in step c5 whether the Doppler shift frequency fd is smaller than the threshold frequency −fth and the traveling speed V is larger than the threshold speed Vth. When it is determined that both conditions are satisfied, in step c6, the antenna selection circuit 59 switches the selector switch 23 so as to select the backward beam antenna 22B. If the condition is not satisfied in step c5, the selection of the currently selected antenna is continued in step c7. After step c4, step c6 and step c7, the process returns to step c1.

FIG. 8 shows a schematic configuration of an in-vehicle OFDM receiver 61 according to the fourth embodiment of the present invention. As in the embodiment of FIG. 3, the in-vehicle OFDM receiver 61 includes two systems of antenna means and OFDM demodulation means, and can perform reception by a frequency division diversity method. One system includes a forward beam antenna 22A1, a backward beam antenna 22B1, a changeover switch 23, an RF / IF unit 24, an OFDM demodulation unit 65, a level detection unit 36, and an AGC unit 28. The other system includes a forward beam antenna 22A2, a backward beam antenna 22B2, a changeover switch 43, an RF / IF unit 44, an OFDM demodulation unit 66, a level detection unit 46, and an AGC unit 48. OFDM demodulation sections 65 and 66 include only level detection sections 36 and 46, respectively, and further include a configuration equivalent to frequency synchronization circuit 56 of FIG. 5, and output frequency shift information.

  Similar to the embodiment of FIG. 3, the outputs of the two OFDM demodulation units 65 and 66 are combined by the diversity combining unit 38, the combined output is input to the error correction unit 37, and the error rate information is input to the antenna selection circuit 69. Is done. The antenna selection circuit 69 is also supplied with frequency shift information and vehicle speed pulses from the OFDM demodulation units 65 and 66 and is used to control the changeover switches 23 and 43. The antenna directivity switching in this embodiment can also be performed in the same manner as in FIG. However, in step c4 and step c6, both systems switch to directivity in the same direction.

  The antenna selection circuit 69 can also obtain information on the traveling speed of the vehicle by detecting a change in the course direction by the gyro. Information on the traveling speed of the vehicle is detected based on the change in the direction of the course by the gyro and the vehicle speed pulse. Therefore, the influence of the Doppler shift is evaluated based on the speed, and the directivity is appropriately selected according to the direction of the course. be able to.

  FIG. 9 shows a configuration in the case where lateral directionality control is performed in the first to fourth embodiments of the present invention. An antenna having directivity of the front beam 71, the rear beam 72, and the lateral beams 73 and 74 is mounted on the vehicle body 70. If the lateral beams 73 and 74 are combined, lateral directivity can be realized. When performing two-line reception as shown in FIGS. 3 and 8, two pairs of the configurations shown in FIG. 9 may be provided.

  FIG. 10 shows a control procedure in the case of performing the Doppler shift direction detection and also performing the lateral directionality control with the configuration shown in FIG. Steps d1 to d6 are the same as steps c1 to c6 in FIG. In step d7, the state of the horizontal directivity by synthesis as shown in FIG. 9 is selected and switched. After step d4, step d6, and step d7, the process returns to step d1.

  Note that although one of the determination conditions is that the traveling speed V is greater than the threshold speed Vth in Step c3, Step c5, Step d3, and Step d5 of FIGS. 7 and 10, this determination is not essential. When the traveling speed of the vehicle increases, the influence of Doppler shift increases, and the necessity of matching the arrival direction of the received radio wave with the directivity of the antenna means increases. If the time point when the traveling speed rises above a predetermined reference is determined as the time when the switching start condition is satisfied, radio wave reception can be performed under appropriate conditions by matching the directivity with the arrival direction of the received radio wave.

  In the procedures of FIGS. 2, 4, 7 and 10, the control procedure start condition is that the reception error rate is large in step a1, step b1, step c1 and step d1, but it is unconditional and always A control procedure can also be executed. Further, instead of the reception error rate, it is possible to use the start condition that the received power is smaller than the reference, or to use both conditions of the reception error rate and the reception power.

  FIG. 11 shows a schematic configuration of an in-vehicle OFDM receiver 80 that is the fifth embodiment of the present invention. The in-vehicle OFDM receiver 80 includes a DTV receiver 81 that receives a digital television broadcast, an antenna selection circuit 82, a navigation device 83, and an antenna directivity control device 84. The antenna selection circuit 82 obtains the broadcast station direction from the broadcast station position table, current position information, and vehicle direction information, and selects the antenna directivity. The current position information is obtained based on a GPS (Global Positioning System) function provided in the navigation device 83 or the like.

  FIG. 12 shows an outline of a test section for confirming the effect of frequency division diversity. The length of section A is about 2 km, and the entire circumference is about 8 km. FIG. 13 shows a case where diversity is performed using a bidirectional beam antenna such as a standard dipole antenna, a case where diversity is performed using a directional antenna, and a case where diversity is not performed. The result compared with the case is shown. In section A of FIG. 12, the rear of the traveling direction is the radio wave arrival direction. From FIG. 13, it can be seen that the diversity method in which directivity is provided before and after the vehicle has the highest reception rate and is excellent. This confirms the effectiveness of making the vehicle have both directivity in the front and rear directions even in a test using an actual vehicle.

  FIG. 14 and FIG. 15 show the mounting position of the directional antenna in consideration of the mountability of the passenger car on the vehicle body 70. As shown in FIG. 14, a windshield-attached film antenna can be considered in front of the vehicle body 70. A rear bumper-attached film antenna can be considered behind the vehicle body 70. As shown in FIG. 15, a front beam 71 is obtained in front of the vehicle body 70, and a rear beam 72 is obtained in the rear of the vehicle body 70. Assuming such an arrangement, it is conceivable to use the metal body of the vehicle body 70 as a reflector to provide directivity.

  FIG. 16 shows a basic configuration of a directional antenna 90 that uses a metal body of a vehicle as a reflector as an embodiment of the antenna according to the present invention. The directional antenna 90 connects one end of the first power supply line 91 and the second power supply line 92 to the central power supply part of the pair of dipole elements 90a and 90b. The first feed line 91 and the second feed line 92 are parallel and have a length of ¼ of the wavelength λ in consideration of the shortening rate. The other end of the first feeder 91 is connected to a ground 93 to a metal body. The other end of the second feeder 92 is connected to the core wire at the tip of the coaxial cable 94. The outer sheath at the end of the coaxial cable 94 is connected to the ground 93 to the body.

  The directional antenna 90 connects the metal body of the vehicle and the feed points of the dipole elements 90a and 90b with the first and second feed lines 91 and 92 having a length of 1/4 wavelength. It can be operated as a reflector. With a dipole antenna mounted on a vehicle, a metal body can be operated as a reflector to provide directivity. Since the end of the coaxial cable 94 only needs to reach the connection to the metal vehicle body, it can be hidden from view.

  FIG. 17 shows a state where the directional antenna 90 of FIG. 16 is attached to the windshield 95 in (a), and shows the directivity in the horizontal plane obtained in (b). As shown in FIG. 17A, the lengths of the dipole elements 90a and 90b when pasted on the windshield 95 are shortened by the dielectric constant of the glass, and the element length 90 mm × 2 is the element length 140 mm × in free space. It corresponds to 2. FIG. 18 shows the result of examining the frequency characteristics by using the roof 96 as a metal vehicle body to be grounded and varying the distance Y from the roof 96 at the mounting position. 18A shows the forward gain at three frequencies, and FIG. 18B shows the F / B ratio that is the ratio of the forward gain and the backward gain. It is desirable that the F / B ratio is 10 dB or more.

  19 and 20 show a state in which the front beam 71 is formed by pasting the directional antenna 90 on the windshield of the vehicle. FIG. 19 is a plan view, and FIG. 20 is a perspective view. In such a directional antenna 90, the dipole elements 90a and 90b, the first feed line 91, and the second feed line 92 can be formed as a conductor pattern on an electrically insulating synthetic resin film. As a result, the first and second feeders 91 and 92 can be handled in a state where they are connected to the feeders at the center of the dipole elements 90a and 90b.

  FIG. 21 shows a schematic configuration of the directional antenna 100 with improved lateral directivity in (a), and shows a feeding method to the directional antenna 100 in (b). The directional antenna 100 is realized in a form in which the dipole elements 90a and 90b of FIG. 16 are installed in a state where the directivity is directed before or after the vehicle, and the additional element 100a in the lateral direction is added. Sex 101 is obtained. As shown in FIG. 20B, the dipole elements 90a and 90b are fed through the coupling circuit 102, and the additional element 100a is fed through the 90 ° phase shifter 103.

  FIG. 22 shows a configuration of a directional antenna 104 as another embodiment of the antenna according to the present invention. FIG. 22A shows a dipole antenna using dipole elements 105 and 106 having a length of ¼ of the wavelength λ used in combination with a reflector, similarly to the dipole elements 100a and 100b of FIG. Is shown in a state where it is bent 90 ° in the middle. FIG. 22B shows a state in which the total length of the dipole elements 105 and 106 is 100 mm, and the tip portion of 30 mm is bent. The directionality in the lateral direction can also be improved by bending the tip in this way.

  FIG. 23 shows a configuration of a directional antenna as still another embodiment of the antenna according to the present invention. FIG. 23A shows a state where a single pole antenna 107 is erected on the trunk 108 of the vehicle. Even if the pole antenna 107 is stood behind the rear glass, the directivity 110 exhibits omnidirectionality in the horizontal plane. As shown in FIG. 23 (b), if one pole antenna 107b is added and operated as a reflector, it can be used as a directional antenna 111 that provides directivity in the direction of the reflector 107a. Such directivity can also be obtained with a two-element Yagi antenna.

  FIG. 24 shows an example of combinations of antenna directivities and switching methods that can be employed in the on-vehicle OFDM receiver described above. An implementation method of the antenna element is also shown. Furthermore, the installation location is also shown.

  In each of the embodiments described above, it is possible to receive radio waves modulated by the orthogonal frequency division multiplexing method by switching so as to select one of a plurality of directivities. Therefore, even if the received radio wave modulated by the orthogonal frequency division multiplexing system is affected by multipath fading, the direction of arrival of the received radio wave can be narrowed down by switching the directivity. Since a plurality of directivities can be switched and input, the number of parts used for high-frequency amplification and frequency conversion to baseband can be reduced.

  FIG. 25 shows a partial configuration of the in-vehicle OFDM receiver 121 as the sixth embodiment of the present invention in (a), and shows a transition state of switching in (b). In the present embodiment, this is performed via the selector 123 of the antennas 22A and 22B. The selector 123 is supplied with a pair of digital / analog conversion outputs (hereinafter abbreviated as “D / A outputs”) from the antenna directivity control device 124 that change so that the voltage level rises and falls. The output switched by the selector 123 is input to the tuner 125, where processing such as demodulation is performed.

  The selector 123 forms a DC circuit with diodes 130A and 130B, NPN transistors 131A and 131B, coils 132A and 132B, coils 133A and 133B, and resistors 134A and 134B, respectively. The direct current values flowing through the diodes 130A and 130B change according to the D / A output given from the antenna directivity control device 124 to the bases of the transistors 131A and 131B, and the impedances of the diodes 130A and 130B correspond to the current values. Change. Signals from the antennas 22A and 22B are input to the diodes 130A and 130B via the capacitors 135A and 135B, and are output to the tuner 125 via the capacitors 136A and 136B.

  By changing one of the pair of D / As output from the antenna directivity control device 124 to be lowered and the other to be raised, as shown in FIG. 25B, the impedance of one diode 130A is increased. Thus, the attenuation rate is increased, the reception level is decreased and faded out, the impedance of the other diode 130B is decreased, the attenuation rate is decreased, and the reception level is increased and fade-in is performed. it can.

  When receiving an OFDM radio wave, the frequency component of the scattered pilot signal changes every about 1 ms, but a periodicity that makes a round every about 4 ms is given. Using this periodicity, the received signal level in the time axis direction can be predicted and corrected. However, if the antennas 22A and 22B are switched, the prediction is lost and the reception performance is degraded. In this embodiment, a time constant of about 200 milliseconds is provided, and the switching is performed slowly so that the correction in the time axis direction by the tuner 125 can follow.

  Note that the concept of the present embodiment can be similarly applied to the above-described embodiments. Further, it is preferable to connect the capacitors 134A and 134B via the buffer amplifier instead of directly connecting to the antennas 22A and 22B because the influence of impedance change can be avoided.

FIG. 26 shows a schematic configuration of an in-vehicle OFDM receiver 141 as a seventh embodiment of the present invention. In this embodiment, the selector 123A and the tuner 125A are provided as the branch A, the selector 123B and the tuner 125B are provided as the branch B, and the frequency synthesis is performed by the diversity frequency synthesizer 148 between the branch A and the branch B. The selectors 123A and 123B and the tuners 125A and 125B are the same as the selector 123 and the tuner 125 of FIG. The diversity frequency synthesizer 148 includes OFDM demodulating units 25A and 25B, a weighting circuit 150, a weight changing circuit 151, and a combining circuit 152. The OFDM demodulation units 25A and 25B are equivalent to the OFDM demodulation unit 25 in FIG. The weighting circuit 150 sets weighting according to the received signal level to the outputs of the branches A and B from the OFDM demodulating units 25A and 25B for each carrier constituting the OFDM signal, and the combining circuit 152 synthesizes them. However, when the antenna directivity control device 144 performs switching of the antenna on one of the branch A or the branch B, the weight on the branch side where switching is performed is halved, for example, and the weight on the branch side where switching is not performed The weighting change circuit 151 changes the weighting to increase the appendage accordingly.

  When using frequency synthesis diversity by the diversity frequency synthesizer 148, weighting synthesis of two branches is performed, and when switching the directivity of the antenna, the weight of the branch that is switched is lowered, thereby reducing the reception performance at the time of switching. Reduction can be reduced. For example, when switching the antenna of branch A, the weight of branch A is reduced by about 50%, and the weight of branch B is increased accordingly. When switching the antenna of branch B, the weight of branch B is lowered by about 50%, and the weight of branch A is increased accordingly. For example, when switching the branch A, if the weight of the original branch A is 60%, the weight is lowered to about 30% and the weight of the branch B is raised to about 70%. It is also possible to set the weight of the switching person to 0.

FIG. 27 shows a procedure performed as an eighth embodiment of the present invention by switching between control of switching antennas according to the reception level and control of combining front and rear antennas in each of the above-described embodiments. The procedure starts from step e0, and the reception level is acquired at step e1. In step e2, it is determined whether or not the electric field is weak based on the acquired reception level. When the reception level is lower than the reference and it is determined that the electric field is weak, in step e3, the front and rear antennas are combined. That is, both the diodes 130A and 130B are set in a low impedance state by the selector 123 of FIG. 25, and the outputs of the two antennas 22A and 22B are combined. When it is determined in step e2 that the electric field is not weak, antenna switching control of each embodiment is performed in step e4. After step e3 or step e4, the process returns to step e1 at a constant cycle and the procedure is repeated.

  Whether or not the electric field is weak can be determined by directly measuring the reception level or converting the AGC voltage level. It is also possible to make a determination based on the BER by using an error correction function included in the digital signal. When a directional antenna is used, reception power is reduced by reducing directivity, and reception performance deterioration due to reduction in reception power becomes dominant in weak electric field areas. In the present embodiment, in a weak electric field area, reception signals from antennas directed in a plurality of directions such as front and back of a directional beam can be combined to increase reception power and improve reception performance.

FIG. 28 shows, as a ninth embodiment of the present invention, a control procedure in which before and after combining is performed without performing antenna switching at the time of search or channel selection in each of the above-described embodiments. The procedure is started from step f0. In step f1, it is determined whether or not a search for a signal to be received or a selection of a broadcast station to be received is being performed. When it is determined that a search or channel selection is in progress, in step f2, as in step e3 in FIG. 27, the reception signals from the antennas that are directed in a plurality of directions, such as front and rear, are synthesized. Receive. When it is determined in step f1 that searching or tuning is not in progress, antenna switching control in each embodiment is performed in step f3. After step f2 or step f3, the process returns to step f1 at a constant cycle and the procedure is repeated. Since signals to be received are not determined during searching or tuning, various synchronizations are not achieved, and switching antennas facilitates fluctuations in the reception state and makes it difficult to receive stably turn into.

It is a block diagram which shows the structure of the vehicle-mounted OFDM receiver 21 which is the 1st Embodiment of this invention. It is a flowchart which shows the directivity switching procedure in the vehicle-mounted OFDM receiver 21 of FIG. It is a block diagram which shows the structure of the vehicle-mounted OFDM receiver 31 which is the 2nd Embodiment of this invention. It is a flowchart which shows the directivity switching procedure in the vehicle-mounted OFDM receiver 31 of FIG. It is a block diagram which shows the structure of the vehicle-mounted OFDM receiver 51 which is the 3rd Embodiment of this invention. It is a figure which shows the relationship between the traveling direction of a vehicle, and a radio wave arrival direction. It is a flowchart which shows the directivity switching procedure in the vehicle-mounted OFDM receiver 51 of FIG. It is a block diagram which shows the structure of the vehicle-mounted OFDM receiver 61 which is the 4th Embodiment of this invention. It is a top view which simplifies and shows the structure in the case of performing horizontal directionality control in each embodiment of this invention. It is a flowchart which shows the control procedure in the case of performing Doppler shift direction detection and lateral directionality control in each embodiment of this invention. It is a block diagram which shows the structure of the vehicle-mounted OFDM receiver 80 which is the 5th Embodiment of this invention. It is a figure which shows the example of the area which tests each embodiment of this invention. It is a chart which shows the test result in the test section of FIG. It is a perspective view which shows the position which mounts a directional antenna in the vehicle body 70 of a vehicle. 4 is a simplified plan view showing a state where directional antennas are mounted on the front and rear of a vehicle body 70. FIG. It is a figure which shows the structure of the directional antenna 90 which is one Embodiment of this invention. It is a figure which shows the state which sticks the directional antenna 90 of FIG. 16 on the windshield 95 of a vehicle, and the directivity obtained. It is a graph which shows the change of the characteristic of a forward gain and F / B ratio when changing the attachment position Y of FIG. FIG. 17 is a simplified plan view showing a state where the directional antenna 90 of FIG. 16 is attached to a windshield 95 on a vehicle. It is the simplified perspective view which shows the state which has affixed the directional antenna 90 of FIG. 16 on the windshield 95 to a vehicle. It is a figure which shows the structure and the electric power feeding method of the directional antenna 100 which are other embodiments of this invention. It is a figure which shows the structure of the directional antenna 104 which is further another form of implementation of this invention. It is a figure which shows the structure of the directional antenna 111 which is other form of implementation of this invention. It is a table | surface shown about the combination of directivity obtained in each embodiment of this invention, the switching method, the structure of an element, and an attachment location. It is a block diagram which shows the partial structure of the vehicle-mounted OFDM receiver 121 which is the 6th Embodiment of this invention. It is a block diagram which shows schematic structure of the vehicle-mounted OFDM receiver 141 which is the 7th Embodiment of this invention. It is a flowchart which shows the schematic control procedure as 8th Embodiment of this invention. It is a flowchart which shows the schematic control procedure as 9th Embodiment of this invention. It is the block diagram which shows the structure of the transmission side of an OFDM system, and the graph which shows a part of subcarrier. It is a figure which shows the insertion state of the guard period GI of the signal of an OFDM system. It is a block diagram which shows the structure of the vehicle-mounted OFDM receiver used as the foundation of this invention.

Explanation of symbols

21, 31, 51, 61, 80, 121, 141 In-vehicle OFDM receiver 22A, 22A1, 22A2 Forward beam antenna 22B, 22B1, 22B2 Backward beam antenna 23, 43 Changeover switch 24, 44 RF / IF unit 25, 25A , 25B, 35, 45, 55, 65, 66 OFDM demodulation unit 26, 36, 46 Level detection unit 27, 37 Error correction unit 29, 39, 59, 69, 82 Antenna selection circuit 38 Diversity combining unit 56 Frequency synchronization circuit 70 Car body 83 Navigation device 84, 124, 144 Antenna directivity control device 90, 100, 104, 111 Directional antenna 90a, 90b, 105, 106 Dipole element 91 First feed line 92 Second feed line 93 Installation on body 94 Coaxial cable 95 Freon Glass 96 Roof 100a additional elements 107a, 107b pole antenna 123,123A, 123B selector 125, 125a, 125B tuner 148 diversity frequency synthesizer 150 weighting circuit 151 weights changing circuit 152 synthesizing circuit

Claims (14)

An in-vehicle digital signal receiving device for receiving a radio wave mounted on a vehicle and modulated with a digital signal,
A plurality of antenna means comprising a plurality of antennas having different directivities, and switching to select one of the antennas according to the reception state;
A vehicle-mounted digital signal receiving apparatus comprising frequency combining diversity means for combining the output signals from the antenna means with weights based on reception states for each frequency component. The in-vehicle digital signal receiving apparatus according to claim 1, wherein the antenna means switches the antenna when the reception error rate deteriorates beyond a reference. The antenna means is provided with two systems capable of receiving radio waves independently of each other, and at the same time, the antenna is selected except for at least one antenna means. The in-vehicle digital signal receiver according to 2. The antenna means is provided with a plurality of systems capable of receiving radio waves independently of each other, and the antenna is selected except for at least one system of antenna means at the same time. The in-vehicle digital signal receiver according to 2. Each of the antenna means includes a directional antenna whose gain is increased in one predetermined direction with respect to the vehicle, and a directional antenna whose gain is increased in the opposite direction of the one direction. The in-vehicle digital signal receiving device according to claim 1, wherein the on-vehicle digital signal receiving device is provided. 6. The in-vehicle digital signal receiving apparatus according to claim 5, wherein the one direction side is a front direction of the vehicle, and a reverse direction of the one direction side is a rear direction of the vehicle. 7. The in-vehicle digital signal according to claim 5 or 6, wherein each antenna means selects an antenna having the same directivity as the antenna selected by each antenna means of one system, and switches the antenna means with a time difference. Receiver device. A diversity system for an in-vehicle digital signal receiver for receiving radio waves that are mounted on a vehicle and modulated with a digital signal,
A plurality of antenna means comprising a plurality of antennas having different directivities, and switching to select one of the antennas according to the reception state;
Diversity system comprising frequency combining diversity means for combining the output signals from the antenna means with weighting based on the reception state of each frequency component. 9. The diversity system according to claim 8, wherein the antenna means switches the antenna when the reception error rate deteriorates beyond a reference. 9. The antenna means is provided with two systems capable of receiving radio waves independently of each other, and the antenna is selected except for at least one system of antenna means at the same time. 9. The diversity system according to 9. 9. The antenna means is provided with a plurality of systems capable of receiving radio waves independently of each other, and the antenna is selected except for at least one antenna means at the same time. 9. The diversity system according to 9. Each of the antenna means includes a directional antenna whose gain is increased in one predetermined direction with respect to the vehicle, and a directional antenna whose gain is increased in the opposite direction of the one direction. The diversity system according to claim 8, wherein the diversity system is provided. 13. The diversity system according to claim 12, wherein the one direction side is a front direction of the vehicle, and the opposite direction of the one direction side is a rear direction of the vehicle. 14. The diversity system according to claim 12 or 13, wherein each antenna means selects an antenna having the same directivity as that of the antenna selected by each antenna means, and switches the antenna means with a time difference.

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