JP2005236666A - Ofdm demodulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an OFDM demodulator having a reception diversity system capable of preventing switching noise in an antenna from being mixed with a data symbol. <P>SOLUTION: An antenna switching circuit 16 is composed of a subcarrier RSSI detector 17 and a threshold determinator 18. The subcarrier RSSI detector 17 uses a propagation path estimation value H (f) by an equalization processing circuit 12 for calculating an amplitude of each subcarrier. The threshold determinator 18 determines whether the amplitude of each subcarrier is lower than a threshold. Then, the threshold determinator 18 uses a selection switch 2 for switching antennas 1A and 1B since desired signal quality cannot be obtained, when the number of low signal level subcarriers exceeds a fixed number, thus selecting the antennas 1A, 1B without changing the entire antennas 1A, 1B for each packet. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an OFDM demodulator used for a wireless local area network (LAN) or the like based on an orthogonal frequency division multiplex (OFDM) transmission system.

  In general, an OFDM demodulator is configured to perform a Fourier transform process for each symbol period of an OFDM signal received from an antenna and output a frequency domain signal composed of a plurality of subcarriers. Also, as an OFDM demodulator according to the prior art, there is known an apparatus that includes a plurality of antennas and adopts a reception diversity system that receives an OFDM signal by selecting an antenna having the best reception state among the plurality of antennas. (For example, refer to Patent Document 1).

JP 2000-188585 A

  In such an OFDM demodulator according to the prior art, switching to all antennas and detecting the subcarrier with the lowest reception level for each antenna, and selecting the antenna having the largest subcarrier with the lowest reception level among a plurality of antennas It was a composition.

  By the way, the conventional OFDM demodulator is configured to select the antenna in the best reception state after switching to all the antennas and detecting the reception state for each antenna. However, such an antenna selection operation is performed using a known symbol provided at the head of an OFDM signal frame (packet), whereas it switches to all antennas to detect the reception state of each antenna. There is a tendency for the antenna switching time to be insufficient at the beginning. As a result, since automatic gain control, automatic frequency control, etc. of the OFDM signal are also performed at the head of the frame, the time required for these controls is insufficient, and reception performance deteriorates, and antenna switching noise occurs in short frames. There is a problem that data symbols are mixed beyond the top of the frame (known symbols).

  Further, since the OFDM demodulator according to the prior art is configured to detect the lowest reception level among a plurality of subcarriers, it is necessary to compare the reception levels among all subcarriers, and the circuit scale is large. Therefore, there is a problem that the manufacturing cost tends to increase.

  The present invention has been made in view of the above-described problems of the prior art, and an object of the present invention is to provide an OFDM demodulator having a reception diversity system capable of preventing antenna switching noise from being mixed into data symbols. There is.

  In order to solve the above-described problem, an OFDM demodulator according to the invention of claim 1 is selected by a plurality of antennas, an antenna selection means for selecting one of the plurality of antennas, and the antenna selection means. A Fourier transform processing unit that performs a Fourier transform process for each symbol period in an OFDM signal packet received from an antenna; a propagation path is estimated for each OFDM signal packet; and the Fourier transform process is performed using the propagation path estimated value An equalization processing unit that performs equalization processing on the frequency domain signal output from the unit, and evaluates the reception state of the frequency domain signal for each subcarrier using the propagation path estimation value of the equalization processing unit. And antenna switching means for switching the antenna selected by the antenna selection means based on the number of subcarriers whose reception state has deteriorated. There.

  According to a second aspect of the present invention, the antenna switching means calculates a subcarrier amplitude calculation unit that calculates an amplitude for each subcarrier using a propagation path estimation value of the equalization processing unit, and a calculation by the subcarrier amplitude calculation unit. It is configured by a threshold determination unit that determines whether or not the value has decreased below a predetermined threshold and switches the antenna in accordance with the number of subcarriers that have decreased below the threshold.

  In the invention according to claim 3, the antenna switching means calculates a modulation accuracy for each subcarrier using a propagation path estimation value of the equalization processing unit, and a calculated value by the modulation accuracy calculation unit. And a threshold value determination unit that switches the antenna in accordance with the number of subcarriers that have decreased below the threshold value.

  According to a fourth aspect of the present invention, the threshold determination unit has two thresholds having different values, and when the calculated value of any subcarrier is lower than both thresholds, the antenna is switched, When the calculated value of the carrier rises above both thresholds, the antenna is held, the calculated values of all subcarriers rise above one threshold, and the calculated value of any subcarrier is two thresholds When the value is between the two, the antenna is switched according to the reception state of subsequent packets.

  According to the first aspect of the invention, the antenna switching means evaluates the reception state of the frequency domain signal for each subcarrier using the propagation path estimation value of the equalization processing unit, and the number of subcarriers whose reception state has deteriorated. Since the antenna selected by the antenna selection means is switched based on the reception allowable noise level for each subcarrier, the antenna can be switched without demodulating the OFDM signal in which a reception error occurs reliably. The reception error rate can be lowered.

  Also, since the antenna switching means evaluates only the reception state of the currently selected antenna, there is no need to switch to all antennas as in the prior art, and antenna switching noise is prevented from being mixed into the data symbols. Can do.

  Also, by comparing the amplitude and modulation accuracy of each subcarrier with a certain threshold value, it is possible to evaluate the quality of the reception state for each subcarrier. Compared with the comparison, the circuit scale can be reduced, and the manufacturing cost can be reduced.

  According to the invention of claim 2, the threshold value determination unit determines whether or not the value calculated by the subcarrier amplitude calculation unit is lower than a predetermined threshold value, and determines the antenna according to the number of subcarriers lower than the threshold value. When the subcarrier amplitude decreases and the number of subcarriers lower than the threshold value increases above a predetermined number, the predetermined signal quality (bit error rate) that enables demodulation of the data signal can be obtained. ) Cannot be obtained and the antenna can be switched.

  Further, since the subcarrier amplitude calculation unit calculates the amplitude for each subcarrier using the propagation path estimation value of the equalization processing unit, the subcarrier amplitude is calculated using the propagation path estimation value used for demodulation of the OFDM signal. Can be calculated, and the circuit scale of the subcarrier amplitude calculation unit can be reduced.

  According to the invention of claim 3, the threshold determination unit determines whether or not the value calculated by the modulation accuracy calculation unit is lower than a predetermined threshold, and switches the antenna according to the number of subcarriers lower than the threshold. Because of the configuration, when the subcarrier modulation accuracy decreases and the number of subcarriers lower than the threshold increases beyond a predetermined number, a predetermined signal quality (bit error rate) that enables demodulation of the data signal ) Cannot be obtained and the antenna can be switched.

  In addition, the modulation accuracy calculation unit uses the frequency domain signal after the equalization processing by the equalization processing unit to generate a signal having no distortion for each subcarrier and a signal after actually performing the equalization processing. Since the modulation accuracy as an error is calculated, the modulation accuracy of each subcarrier can be calculated using the output signal from the equalization processing unit used for demodulation of the OFDM signal, and the circuit scale of the modulation accuracy calculation unit is reduced. be able to.

  According to the invention of claim 4, since the threshold value determination unit has two threshold values of different values, whether or not the reception state of the signal in the frequency domain has deteriorated to the point where switching is required immediately. Can be determined using one threshold value, and it can be determined using the other threshold value whether the reception state of the signal in the frequency domain is a good state that can be reliably demodulated. When the value is between these two threshold values, the frequency domain signal can be demodulated according to the situation, so that the threshold value determination unit receives the current packet as it is and receives subsequent packets. The antenna can be switched according to the state.

  Hereinafter, an OFDM demodulator according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  1 to 4 show a first embodiment. In FIG. 1, reference numerals 1A and 1B denote diversity antennas. The antennas 1A and 1B are arranged at different positions, and a selection switch 2 described later. Is connected to the receiving circuit 3.

  Reference numeral 2 denotes a selection switch serving as an antenna selection unit that selects one of the two antennas 1A and 1B and connects to the reception circuit 3. The selection switch 2 includes the antenna 1A and the antenna 1A that are selected by a threshold determination unit 18 to be described later. 1B is configured to switch.

  Reference numeral 3 denotes a receiving circuit connected to the rear side of the selection switch 2. The receiving circuit 3 includes, for example, a low noise amplifier that amplifies a high frequency signal (RF signal) received from the antennas 1A and 1B, and a subsequent stage of the low noise amplifier. It is comprised by the mixer (all are not shown) etc. which were connected to the side. The mixer downconverts the high frequency signal to an intermediate frequency signal (IF signal) using a local transmission signal output from a local oscillator (not shown).

  4 is an automatic gain controller (hereinafter referred to as AGC 4) connected to the rear side of the receiving circuit 3, and the AGC 4 is configured so that the amplitude of the intermediate frequency signal output from the receiving circuit 3 becomes a substantially constant value. The gain is controlled.

  Reference numeral 5 denotes a synchronization circuit connected to the rear side of the AGC 4, and the synchronization circuit 5 is generally configured by, for example, an IQ signal detector 6, an A / D converter 7, an AFC 8, a variable frequency oscillator 9, and a synchronization processing unit 10 described later. Has been.

  Reference numeral 6 denotes an IQ signal detector connected to the receiving circuit 3 via the AGC 4. The IQ signal detector 6 performs quadrature detection of an intermediate frequency signal using a transmission signal So from a variable frequency oscillator 9 to be described later to obtain an I signal. A baseband OFDM signal composed of (in-phase signal) and Q signal (quadrature signal) is output.

  Reference numeral 7 denotes an A / D converter connected to the output side of the IQ signal detector 6. The A / D converter 7 digitally converts the baseband OFDM signal that forms an analog signal output from the IQ signal detector 6. It is converted into a signal.

  Reference numeral 8 denotes an automatic frequency control unit (hereinafter referred to as AFC8) for estimating a frequency error. The AFC8 is provided at the head of each packet (frame) in the OFDM signal output from the A / D converter 7. The leading known symbol S1 is extracted from the two known symbols S1 and S2, and a correlation (convolution) operation and convolution integration are performed between the extracted known symbol S1 and the previously stored comparison symbol S0 ( (See FIG. 3). At this time, the comparison symbol S0 is, for example, the known symbol S1 without distortion (frequency error or the like). Then, the AFC 8 calculates the phase Δθ and the frequency error Δf using this integral value, as shown in the following equation 1, and sends a control signal Sc corresponding to the frequency error Δf to the variable frequency oscillator 9 described later. Output toward.

  Reference numeral 9 denotes a variable frequency oscillator for correcting the frequency error Δf, and the variable frequency oscillator 9 outputs a transmission signal So whose frequency changes in accordance with the control signal Sc output from the AFC 8. Thereby, the IQ signal detector 6 can detect the OFDM signal in which the frequency error Δf calculated by the AFC 8 is corrected.

  Reference numeral 10 denotes a synchronization processing unit connected to the output side of the A / D converter 7. The synchronization processing unit 10 performs signal detection and signal synchronization for each packet with respect to the OFDM signal, and performs each packet of the OFDM signal. Is outputting a synchronization signal Ss. At this time, for example, the synchronization processing unit 10 detects each packet depending on whether or not the known symbol S1 (preamble period signal) provided at the beginning of each packet of the OFDM signal exceeds a signal detection threshold.

  Specifically, the packet is detected when the amplitude of the known symbol S1 exceeds the signal detection threshold at the beginning of the packet, and since the known symbol S1 is a known signal, the cross-correlation with the comparison symbol S0 or Synchronization is established by calculating delayed autocorrelation and performing peak detection.

  Reference numeral 11 denotes a fast Fourier transform circuit (hereinafter referred to as an FFT circuit 11) as a Fourier transform processing unit connected to the output side of the A / D converter 7. The FFT circuit 11 includes a plurality of symbols for each packet of an OFDM signal. In contrast to the period, a Fourier transform process is performed for each symbol period in the packet of the baseband OFDM signal using the synchronization signal Ss output from the synchronization processing unit 10. Specifically, after synchronization is established at the head of the packet by the synchronization processing unit 10, the timing of the FFT window is determined using a trigger signal generated at a constant interval, and a guard interval signal (GI signal) is obtained. In this state, Fourier transform processing is performed for each symbol period. Then, the FFT circuit 11 converts the baseband OFDM signal into a frequency domain signal (complex frequency signal Y (f)) composed of a plurality of subcarriers, and equalizes the complex frequency signal Y (f) described later. The output is directed to the device 14.

  Reference numeral 12 denotes an equalization processing circuit (equalization processing unit) that estimates a high-frequency signal propagation path (multipath) and performs an equalization process using the estimation result. The equalization processing circuit 12 is a propagation path described later. An estimation unit 13 and an equalizer 14 are included.

  Reference numeral 13 denotes a propagation path estimator connected to the output side of the FFT circuit 11. The propagation path estimator 13 estimates and estimates a propagation path (multipath) for each packet of the OFDM signal using the known symbol S1. The reciprocal (1 / G (f)) of the transfer function G (f) consisting of the frequency response of the propagation path is output as the propagation path estimation value H (f).

  Specifically, for example, a known symbol S1 in which all subcarriers are 1 is provided at the beginning of each packet of the OFDM signal, and the propagation path estimation unit 13 includes the known signal among the OFDM signals output from the FFT circuit 11. A complex frequency signal Y1 (f) corresponding to the symbol S1 is extracted. At this time, the complex frequency signal Y1 (f) has a value that is shifted in amplitude and phase with respect to the known symbol S1 by the multipath, so the complex frequency signal Y1 (f) is a transfer function G (f) by the multipath. It is equal to. For this reason, the propagation path estimation unit 13 outputs the reciprocal (1 / Y1 (f)) of the complex frequency signal Y1 (f) as the propagation path estimation value H (f).

  Reference numeral 14 denotes an equalizer that performs adaptive equalization processing on the complex frequency signal Y (f) output from the FFT circuit 11 using the propagation path estimation value H (f) by the propagation path estimation unit 13. The unit 14 multiplies the propagation path estimation value H (f) and the complex frequency signal Y (f) by a complex number, and outputs the calculation result as an output signal X (f) (X (f) = H (f) Y (f)). The output signal X (f) that has been subjected to the adaptive equalization processing is used to modulate each subcarrier using a detection circuit (not shown) connected to the output side of the equalizer 14 using the demapping circuit 15. The signal is detected according to (BPSK, QPSK, 16QAM, 64QAM, etc.) and demodulated into a binary (1, 0) data signal by performing demapping after detection.

  16 is connected to the propagation path estimation unit 13 of the equalization processing circuit 12 and evaluates the reception state of the complex frequency signal Y (f) for each subcarrier using the propagation path estimation value H (f), and the reception state is deteriorated. An antenna switching circuit (antenna switching means) for switching the antennas 1A and 1B selected by the selection switch 2 based on the number of subcarriers. The antenna switching circuit 16 includes a subcarrier RSSI detection unit 17 and a threshold determination unit 18 described later. And is composed of.

  Reference numeral 17 denotes a subcarrier RSSI detection unit as a subcarrier amplitude calculation unit that calculates an amplitude for each subcarrier. The subcarrier RSSI detection unit 17 calculates the reciprocal of the propagation path estimation value H (f) of the equalization processing circuit 12. While calculating for every subcarrier, the amplitude of the reciprocal is calculated as a received electric field strength (RSSI: Receive Signal Strength Indicator) for every subcarrier.

  Reference numeral 18 denotes a threshold determination unit that determines whether or not the subcarrier amplitude (signal level) calculated by the subcarrier RSSI detection unit 17 is lower than a predetermined threshold A1. The threshold determination unit 18 is more than the threshold A1. The number of subcarriers that have decreased is counted, and the antennas 1A and 1B selected by the selection switch 2 are switched according to the number of subcarriers. At this time, the threshold value A1 is set to be changeable according to a modulation method such as BPSK or QPSK. In addition, the number of subcarriers for switching the selection switch 2 is a value at which a predetermined signal quality (bit error rate) cannot be obtained due to an increase in subcarriers having a low signal level. Is set to a value that makes it impossible to demodulate.

  Then, when the number of subcarriers having a low signal level exceeds a certain value, the threshold determination unit 18 uses the synchronization signal Ss from the synchronization processing unit 10 to set the antennas 1A and 1B at the guard interval time located between symbols, for example. Switch.

  The antennas 1A and 1B may be switched when the next frame (packet) is received. In addition, the number of packets having subcarriers lower than the threshold A1 is counted, and when a certain number of packets including subcarriers having a low signal level are continuously counted, the antennas 1A and 1B are counted. It is good also as a structure which switches.

  The OFDM demodulator according to the present embodiment is configured as described above. Next, the operation of the OFDM demodulator will be described with reference to FIGS.

  First, the antenna 1A is selected using the selection switch 2. At this time, when receiving a high frequency signal from the antenna 1A, the receiving circuit 3 converts the frequency of the high frequency signal into an intermediate frequency signal. The intermediate frequency signal is amplified to a constant amplitude by the AGC 4, and then demodulated into a baseband OFDM signal composed of the I signal and the Q signal by the IQ signal detector 6 of the synchronization circuit 5, and the A / D converter 7. Is converted into a digital signal. The digital signal output from the A / D converter 7 is input to the FFT circuit 11 and also input to the AFC 8 and the synchronization processing unit 10.

  At this time, the AFC 8 calculates the frequency error Δf using the known symbol S1 provided at the head of the OFDM signal packet, and outputs the control signal Sc. Thereby, since the variable frequency oscillator 9 outputs the transmission signal So corresponding to the frequency error Δf, the IQ signal detector 6 applies the frequency to the symbols S2, S3,... Inputted after the known symbol S1. The I signal and the Q signal can be detected with the error Δf corrected. The synchronization processing unit 10 detects the packet when the amplitude of the known symbol S1 exceeds the signal detection threshold at the beginning of the packet, and outputs the synchronization signal Ss.

  On the other hand, the FFT circuit 11 performs a Fourier transform process for each symbol period of the OFDM signal using the synchronization signal Ss from the synchronization processing unit 10, and outputs a complex frequency signal Y (f) to the equalization processing circuit 12.

  At this time, the propagation path estimation unit 13 of the equalization processing circuit 12 estimates the transfer function G (f) by multipath or the like using the known symbol S1, and the reciprocal (1 / G (1) of the transfer function G (f). f)) is output as the propagation path estimation value H (f). For this reason, the equalizer 14 of the equalization processing circuit 12 multiplies a complex number by the propagation path estimation value H (f) and the complex frequency signal Y (f), and uses this calculation result as the output signal X (f). Output. As a result, the output signal X (f) is equalized, amplitude and phase distortion due to multipath or the like is corrected, and the demapping circuit 15 is used for detection according to the modulation scheme of each subcarrier. By performing the demapping, the data signal is restored. Finally, the data signal is subjected to error code correction and the like and then input to an external device such as a computer (not shown).

  Here, the subcarrier RSSI detection unit 17 calculates the reciprocal of the propagation path estimation value H (f) of the equalization processing circuit 12 for each subcarrier, and calculates the reciprocal amplitude as the received electric field strength for each subcarrier. doing.

  Then, the threshold determination unit 18 counts the number of subcarriers lower than the threshold A1, and as shown in FIG. 4B, when this subcarrier exceeds a certain number, a predetermined signal quality is obtained. Since the data signal cannot be restored even if error correction is performed, the antenna 1A is switched to the antenna 1B using the selection switch 2 at the guard interval time or the like.

  On the other hand, when the next packet is received, the selection switch 2 selects the antenna 1B, so that the number of subcarriers lower than the threshold value A1 is equal to or less than a certain number as shown in FIG. At this time, since the data signal can be restored by performing detection and error correction by the demapping circuit 15, the selection switch 2 maintains the current state and maintains the selection of the antenna 1B.

  Thus, in the present embodiment, the antenna switching circuit 16 evaluates the reception state of the complex frequency signal Y (f) for each subcarrier using the propagation path estimation value H (f) of the equalization processing unit 12, and receives the received signal. Since the antennas 1A and 1B selected by the selection switch 2 are switched based on the number of subcarriers whose state has deteriorated, the reception allowable noise level can be compared for each subcarrier, and an OFDM signal in which a reception error is surely generated can be compared. Without demodulation, the antennas 1A and 1B can be switched, and the reception error rate can be reduced.

  Further, since the antenna switching circuit 16 evaluates only the reception state of the currently selected antenna (for example, the antenna 1A), it is not necessary to switch to all the antennas as in the prior art, and the antenna switching noise is generated by the data symbol S3, It is possible to prevent mixing into S4,.

  Furthermore, since the quality of the reception state for each subcarrier can be evaluated by comparing the amplitude (reception electric field strength) of each subcarrier with a certain threshold value A1, all subcarriers can be evaluated as in the prior art. Compared with the comparison of reception levels, the circuit scale can be reduced and the manufacturing cost can be reduced.

  In particular, the threshold determination unit 18 determines whether or not the value calculated by the subcarrier RSSI detection unit 17 (subcarrier amplitude) has decreased below a predetermined threshold A1, and depends on the number of subcarriers that has decreased below the threshold A1. Therefore, when the subcarrier amplitude decreases and the number of subcarriers lower than the threshold A1 increases beyond a predetermined number, a predetermined signal quality that enables demodulation of the data signal is achieved. It is possible to switch the antennas 1A and 1B by determining that (bit error rate) cannot be obtained.

  Further, since the subcarrier RSSI detection unit 17 calculates the amplitude for each subcarrier using the propagation path estimation value H (f) of the equalization processing unit 12, the propagation path estimation value H (used for demodulation of the OFDM signal) The amplitude of each subcarrier can be calculated using f), and the circuit scale of the subcarrier RSSI detector 17 can be reduced.

  Next, FIGS. 5 and 6 show a second embodiment of the present invention. The feature of this embodiment is that an antenna switching circuit is connected to a frequency domain signal (output) after equalization processing by an equalization processing unit. A modulation accuracy calculation unit that calculates the modulation accuracy for each subcarrier using a signal), and determines whether or not a value calculated by the modulation accuracy calculation unit is lower than a predetermined threshold. A threshold value determination unit that switches the antenna according to the number of carriers. In the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted.

  21 is connected to the equalization processing circuit 12 and uses the complex frequency signal Y (f) (output signal X (f)) subjected to the equalization process, and there is no distortion with the output signal X (f) for each subcarrier. Switching circuit for evaluating the modulation accuracy N (f) that makes an error with the complex frequency signal Y0 (f) of the first and second antennas 1A and 1B selected by the selection switch 2 based on the number of subcarriers whose reception state has deteriorated Thus, the antenna switching circuit 21 includes a modulation accuracy calculation unit 22 and a threshold value determination unit 23 which will be described later.

  A modulation accuracy calculation unit 22 calculates the modulation accuracy N (f) for each subcarrier. The modulation accuracy calculation unit 22 is connected to the output side of an equalizer (not shown) constituting the equalization processing circuit 12. The modulation accuracy N (f) that makes an error between the output signal X (f) and the complex frequency signal Y0 (f) without distortion is calculated for each subcarrier using the output signal X (f). .

  Specifically, the modulation accuracy calculation unit 22 waits until an output signal X2 (f) obtained by performing equalization processing on the second known symbol S2 is output from the equalizer. At this time, the propagation path estimation unit (not shown) estimates the propagation path estimation value H (f) using the first known symbol S1 provided at the head of the packet, and the equalizer uses the propagation path estimation value H ( Using f), an output signal X2 (f) in which the amplitude and phase are corrected with respect to the complex frequency signal Y2 (f) of the second known symbol S2 is output. When the output signal X2 (f) is input to the modulation accuracy calculation unit 22, the modulation accuracy calculation unit 22 receives the output signal X2 (f) and the complex frequency signal Y0 (f) without distortion. Is calculated as modulation accuracy N (f) (see FIG. 6).

  Reference numeral 23 denotes a threshold determination unit that determines whether or not the subcarrier modulation accuracy N (f) calculated by the modulation accuracy calculation unit 22 is lower than a predetermined threshold A2. The threshold determination unit 23 is more than the threshold A2. The number of subcarriers that have decreased is counted, and the antennas 1A and 1B selected by the selection switch 2 are switched according to the number of subcarriers. At this time, the threshold A2 is set to be changeable according to a modulation method such as BPSK or QPSK. Further, the number of subcarriers for switching the selection switch 2 is a value at which a predetermined signal quality (bit error rate) cannot be obtained due to an increase in subcarriers having a low signal level. Is set to a value that makes it impossible to demodulate. Further, since the modulation accuracy N (f) increases as the output signal X2 (f) corrected by the propagation path estimation value H (f) moves away from the complex frequency signal Y0 (f) without distortion, the threshold determination unit 23 Determines that the modulation accuracy N (f) is lower than the threshold A2 when the modulation accuracy N (f) is greater than the threshold A2 (N (f)> A2).

  Then, when the number of subcarriers with low modulation accuracy N (f) exceeds a certain value, the threshold determination unit 23 uses, for example, the synchronization signal Ss from the synchronization processing unit 10 to perform the antenna at the guard interval time located between the symbols. Switch between 1A and 1B.

  Note that the antennas 1A and 1B may be switched when receiving the next frame (packet) in substantially the same manner as in the first embodiment. In addition, the number of packets having subcarriers lower than the threshold A2 is counted, and when a certain number of packets including subcarriers with low modulation accuracy N (f) are continuously counted, It is good also as a structure which switches antenna 1A, 1B.

  Thus, the present embodiment can obtain the same operational effects as those of the first embodiment. However, in this embodiment, the threshold value determination unit 23 calculates the modulation accuracy N (f) calculated by the modulation accuracy calculation unit 22. Is determined to be lower than a predetermined threshold A2, and the antennas 1A and 1B are switched according to the number of subcarriers lower than the threshold A2, so that the subcarrier modulation accuracy N (f) is When the number of subcarriers lower than the threshold value A2 increases and exceeds a predetermined number, it is determined that a predetermined signal quality (bit error rate) that enables demodulation of the data signal cannot be obtained. The antenna can be switched.

  Since the modulation accuracy calculation unit 22 calculates the modulation accuracy N (f) using the output signal X2 (f) after the equalization processing by the equalization processing circuit 12, the equalization processing circuit used for demodulation of the OFDM signal 12, the modulation accuracy N (f) of each subcarrier can be calculated using the output signal X2 (f) from 12, and the circuit scale of the modulation accuracy calculation unit 22 can be reduced.

  Next, FIG. 7 shows a third embodiment of the present invention. The feature of the present embodiment is that the threshold value determination unit has two threshold values having different values, and the calculated value of any subcarrier ( If (amplitude) drops below both thresholds, the antenna is switched. If the calculated values of all subcarriers rise above both thresholds, the antenna is held, and the calculated values of all subcarriers are When the calculated value of any subcarrier becomes a value between the two threshold values, the antenna is switched according to the subsequent packet reception state. In the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted.

  Reference numeral 31 denotes a threshold determination unit according to the present embodiment. The threshold determination unit 31 has two thresholds A3 and A4 (for example, A3) having different values of subcarrier amplitudes (signal levels) calculated by the subcarrier RSSI detection unit 17. It is determined whether or not it is lower than <A4).

  At this time, the thresholds A3 and A4 are set to be changeable according to the modulation method such as BPSK and QPSK, and for example, the threshold A3 is set to the extent that the reception state of the complex frequency signal Y (f) needs to be switched immediately. It is determined whether or not the state is deteriorated, and it is determined whether or not the reception state of the complex frequency signal Y (f) is a good state in which the data signal can be reliably demodulated.

  For this reason, when the signal level of one of the subcarriers is lower than both thresholds A3 and A4, the reception state of the complex frequency signal Y (f) has deteriorated to the point where switching is immediately required. The threshold determination unit 31 switches the antennas 1A and 1B at a guard interval time located between symbols in the current packet, for example. The antennas 1A and 1B may be switched when the next frame (packet) is received.

  In addition, when the signal levels of all subcarriers rise above both threshold values A3 and A4, the reception state of the complex frequency signal Y (f) is in a good state where the data signal can be reliably demodulated. Therefore, the threshold determination unit 31 holds the current antennas 1A and 1B.

  On the other hand, when the signal levels of all the subcarriers are higher than the threshold value A3 and the signal level of any of the subcarriers is a value between the two threshold values A3 and A4, the complex frequency signal Y (f) Therefore, the threshold determination unit 31 receives the current packet with the current antenna (for example, the antenna 1A), and switches the antennas 1A and 1B according to the subsequent packet reception state.

  Specifically, for example, in the subsequent packets, when the signal level of any subcarrier falls below both thresholds A3 and A4, the antennas 1A and 1B are switched, and the signal levels of all subcarriers are both When the thresholds A3 and A4 are raised, the current antennas 1A and 1B are held. Also, when the signal level of all subcarriers rises above the threshold value A3 several times in subsequent packets, and the signal level of any subcarrier becomes a value between the two threshold values A3 and A4. Switches the antennas 1A and 1B.

  Thus, the present embodiment can obtain the same effects as those of the first embodiment. However, in the present embodiment, the threshold value determination unit 31 has two threshold values A3 and A4 having different values. Therefore, it is possible to determine whether or not the reception state of the complex frequency signal Y (f) has deteriorated to the point where it is necessary to immediately switch, using one threshold value A3, and the complex frequency signal Y (f). It is possible to determine using the other threshold A4 whether or not the reception state is a good state in which demodulation is possible. When the value is between these two threshold values A3 and A4, the complex frequency signal Y (f) can be demodulated according to the situation, so that the threshold value determination unit 31 receives the current packet as it is. Then, the antennas 1A and 1B can be switched according to the reception state of subsequent packets.

  In the third embodiment, the threshold value determination unit 31 having two threshold values A3 and A4 is applied to the antenna switching circuit 16 according to the first embodiment. However, the present invention is not limited to this. For example, a threshold determination unit having two thresholds may be applied to the antenna switching circuit 21 according to the second embodiment.

  In each of the embodiments described above, the present invention is applied to an OFDM demodulator having two antennas 1A and 1B, but may be applied to an OFDM demodulator having three or more antennas.

  Further, in each of the embodiments, the propagation path estimation unit 13 calculates the propagation path estimation value H (f) using the frequency domain signal (complex frequency signal Y (f)) on the rear stage side of the FFT circuit 11. However, a configuration may be adopted in which the propagation path estimated value is calculated using the OFDM signal on the front stage side of the FFT circuit 11.

  Further, in each of the above embodiments, a receiving circuit that down-converts a high frequency signal into an intermediate frequency signal is provided. However, the present invention is not limited to this. For example, the receiving circuit may be omitted, and a direct conversion configuration in which a signal received by the antenna is directly quadrature detected may be employed.

1 is an overall configuration diagram showing an OFDM demodulator according to a first embodiment. FIG. 2 is an overall configuration diagram showing the OFDM demodulator in FIG. 1 with a receiving circuit omitted. It is explanatory drawing which shows the structure of the OFDM signal used for the OFDM demodulation apparatus in FIG. It is explanatory drawing which shows the state which demodulated the complex frequency signal which consists of a some subcarrier for every antenna. It is a whole block diagram which shows the OFDM demodulation apparatus by 2nd Embodiment. FIG. 6 is an explanatory diagram showing an arrangement of signal points when a known symbol S2 is demodulated using the OFDM demodulator in FIG. It is a whole block diagram which shows the OFDM demodulation apparatus by 3rd Embodiment.

Explanation of symbols

1A, 1B Antenna 2 selection switch (antenna selection means)
11 FFT circuit (Fourier transform processing unit)
12 Equalization processing circuit (equalization processing unit)
16, 21 Antenna switching circuit (antenna switching means)
17 Subcarrier RSSI detector (subcarrier amplitude calculator)
18, 23, 31 Threshold determination unit 22 Modulation accuracy calculation unit

Claims (4)

  A plurality of antennas, an antenna selecting means for selecting one of the plurality of antennas, and a Fourier transform process for each symbol period in the packet of the OFDM signal received from the antenna selected by the antenna selecting means A Fourier transform processing unit and an equalization process for estimating a propagation path for each OFDM signal packet and performing equalization processing on the frequency domain signal output from the Fourier transform processing unit using the propagation path estimation value And an antenna which is connected to the equalization processing unit and evaluates the reception state of the signal in the frequency domain for each subcarrier, and switches the antenna selected by the antenna selection means based on the number of subcarriers whose reception state has deteriorated An OFDM demodulator comprising switching means.   The antenna switching means includes a subcarrier amplitude calculation unit that calculates an amplitude for each subcarrier using a propagation path estimation value of the equalization processing unit, and a value calculated by the subcarrier amplitude calculation unit is less than a predetermined threshold value. 2. The OFDM demodulator according to claim 1, further comprising: a threshold determination unit that determines whether or not the antenna has decreased and switches the antenna according to the number of subcarriers that has decreased below the threshold.   The antenna switching means includes a modulation accuracy calculation unit that calculates a modulation accuracy for each subcarrier using a signal in a frequency domain after equalization processing by the equalization processing unit, and a value calculated by the modulation accuracy calculation unit is predetermined. 2. The OFDM demodulator according to claim 1, further comprising: a threshold determination unit configured to determine whether or not the threshold is lower than the threshold and switch the antenna according to the number of subcarriers lower than the threshold.   The threshold determination unit has two thresholds of different values, and when the calculated value of any subcarrier falls below both thresholds, the antenna is switched, and the calculated values of all subcarriers are both If the antenna is held above the threshold, the calculated value of all subcarriers rises above one threshold, and the calculated value of any subcarrier is between the two thresholds The OFDM demodulator according to claim 2 or 3, wherein the antenna is switched according to a reception state of subsequent packets.

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