JP2005184385A - Ofdm receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an OFDM receiver capable of reducing the effect of Doppler shift attended with movement with a simple configuration. <P>SOLUTION: OFDM demodulation (24) is applied to a signal received by a directivity antenna 10 having directivity forward in a traveling direction and a signal received by a directivity antenna 14 having directivity backward in the traveling direction and a weighting composite section (26) composites the demodulated signals with maximum ratio composition. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an OFDM (Orthogonal Frequency Division Multiplexing) receiver, and more particularly to an OFDM receiver suitable for in-vehicle use affected by Doppler shift.

  On the transmission side of the OFDM system, complex data converted to the time domain is obtained by performing I-FFT (Inverse Fast Fourier Transform) on complex parallel data arranged on the frequency axis, and this is orthogonally modulated and transmitted. Conversely, on the receiving side, parallel data is obtained on the frequency axis by performing FFT after orthogonal demodulation.

  The OFDM modulation scheme is originally a modulation scheme that is resistant to multipath fading, but it is conceivable to employ a diversity scheme in order to further enhance multipath tolerance and enable transmission of more information. For example, Japanese Patent Laid-Open No. 2002-271291 discloses that weighting synthesis is performed for each carrier according to transmission path characteristics after receiving by a plurality of antennas on the receiving side and performing OFDM demodulation on each of them.

  However, in the case of a receiver mounted on a moving body such as a vehicle, a Doppler shift proportional to the moving speed occurs with movement. When this is combined with multipath, the frequency depends on the direction of arrival of radio waves. The shift amount and direction are different, and diversity combining is not effective for this.

  Japanese Patent Laid-Open No. 2003-87213 solves the problem of Doppler shift by providing a plurality of directional antennas having directivity in different directions in OFDM reception, and selecting one of them according to the reception state. Doing (switched beam method) has been proposed.

  When the switched beam disclosed in Japanese Patent Laid-Open No. 2003-87213 and the diversity combining disclosed in Japanese Patent Laid-Open No. 2002-271291 are combined, for example, a configuration of an OFDM receiver as shown in FIG. 1 can be considered. In FIG. 1, two directional antennas 10 and 12 having directivity in the front of the traveling direction and two directional antennas 14 and 16 having directivity in the rear are provided, and the switch 18 is controlled according to the control signal. One of the front directional antennas 10 and 12 and the rear directional antennas 14 and 16 is selected. Received signals at the selected two antennas having forward or backward directivity are converted into IF signals by the frequency converters 20 and 22, OFDM demodulated by the OFDM demodulator having two branches, and weighted and combined by the carrier unit 26. Weighted and synthesized. The controller 28 switches the switch 18 based on the reception information such as the reception level.

JP 2002-271291 A JP 2003-87213 A

  The configuration of FIG. 1 requires the switch 18 and the controller 28, and the configuration is complicated.

  Accordingly, an object of the present invention is to provide an OFDM receiver that realizes reduction of the influence due to Doppler shift that occurs with movement with a simpler configuration than that described above.

  According to the present invention, a plurality of directional antennas having directivity in different directions, and OFDM (Orthogonal Frequency Multiplex) demodulation of received signals respectively received by the plurality of directional antennas, each of which includes a plurality of directional antennas. An OFDM demodulator having a plurality of branches each generating a data signal on a carrier, and a data signal output from the plurality of branches of the OFDM demodulator according to a weight corresponding to each carrier and a transmission path response characteristic estimation value of each branch An OFDM receiver comprising a weighting / combining unit that performs weighting / combining for each carrier is provided.

  FIG. 2 shows the configuration of the in-vehicle OFDM receiver according to the first embodiment of the present invention. In FIG. 2, signals received by the antenna 10 having directivity in the forward direction of the vehicle and the antenna 14 having directivity in the rear are converted into IF signals by the frequency converters 20 and 22, respectively, and the OFDM demodulator 24. OFDM is demodulated in each of the two branches, and weighted and synthesized by the weighting / synthesizer 26 for each carrier.

Here, the processing in the weighting synthesis unit 26 is the same as the processing in the weighting synthesis unit 26 in FIG. That is, for example, as described in Patent Document 1, data D _l (k) obtained for each carrier k in each branch l based on the following equation is weighted and synthesized with a weight W _l (k) for each carrier. A diversity output Dout (k) is obtained.

The weight W _l (k) is a channel response characteristic estimation value H calculated for each branch and each carrier based on the reception state of the pilot signal inserted at predetermined intervals in the frequency direction and time direction of the OFDM demodulation result. _l (k), preferably for maximum ratio synthesis

Calculated by

  Conventionally, the weighted synthesis is to combine signals received by a plurality of antennas having no directivity or having directivity in the same direction. However, in the present invention, multiple weights having directivity in different directions are used. By weighting and synthesizing the signals received by the antennas, it is possible to reduce the influence of Doppler shift due to movement as a result of selecting a directional antenna in an appropriate direction.

  FIG. 3 shows an OFDM receiving apparatus according to the second embodiment of the present invention. In addition to the configuration of FIG. 2, the controller 30 includes a weighting synthesis unit based on information from the level detection units 32 and 33, the frame synchronization units 34 and 35, and the C / N measurement units 36 and 37 existing in the OFDM demodulation unit 24. 26 is controlled. More specifically, the frame lock completion flag from the frame synchronization units 34 and 35 is monitored, and control is performed so that only the frame synchronization completion branch is synthesized. The C / N measuring units 36 and 37 calculate the C / N (carrier / noise ratio) for each carrier in each branch, but the carriers in which the C / N of some branches is equal to or less than a predetermined threshold value. In the weighted composition for, a branch whose C / N is not more than a threshold is excluded from composition. Also, based on the level information from the level detection unit, branches whose level is equal to or lower than a predetermined threshold are excluded from synthesis.

  These three types of conditions may be used alone or in combination of two or more. When combining a plurality of conditions, a branch that does not satisfy at least one of the plurality of conditions is excluded from synthesis.

  Further, when speed information is input from the vehicle or the navigation system and the vehicle speed, that is, the moving speed of the OFDM receiver is below a predetermined threshold, the transmission path response characteristic estimated value, the reception level, the frame lock state, and the C / N value Regardless, the level shortage in the case of a weak electric field may be improved by performing a simple addition of the outputs of all branches and combining them.

  The controller 30 can be realized by a CPU and software for controlling its operation.

It is a block diagram of the conventional OFDM receiver which combined the switched beam and diversity combining. It is a block diagram of the OFDM receiver which concerns on the 1st Embodiment of this invention. It is a block diagram of the OFDM receiver which concerns on the 2nd Embodiment of this invention.

Explanation of symbols

10, 12... Directional antennas 14 and 16... Directional antennas 14 and 16.

Claims (5)

A plurality of directional antennas having directivity in different directions;
An OFDM demodulator having a plurality of branches each performing OFDM (Orthogonal Frequency Multiplexing) demodulation of received signals respectively received by the plurality of directional antennas, each of which generates a data signal on a plurality of carriers;
An OFDM receiving apparatus comprising: a weighting synthesis unit that weights and synthesizes data signals output from a plurality of branches of the OFDM demodulating unit for each carrier with a weight according to a transmission channel response characteristic estimation value of each carrier and each branch.   2. The OFDM receiver according to claim 1, wherein the weighting / combining unit targets only a branch for which frame synchronization has been completed.   3. The OFDM receiving apparatus according to claim 1, wherein the weighting / combining unit excludes, for each carrier, a branch having a carrier / noise ratio equal to or less than a predetermined threshold.   The OFDM weight receiving apparatus according to any one of claims 1 to 3, wherein the weighting / combining unit excludes a branch whose reception level is equal to or less than a predetermined threshold.   When the moving speed of the OFDM receiver is equal to or less than a predetermined threshold, the weighting combiner combines the outputs of all branches regardless of the channel response characteristic estimation value, the reception level, the frame lock state, and the carrier / noise ratio. The OFDM receiver according to any one of claims 1 to 4.

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