JP4199349B2 - OFDM communication apparatus and OFDM communication system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to an OFDM communication apparatus used in a wireless communication systemAnd OFDM communication systemAbout.
[0002]
[Prior art]
A conventional OFDM communication apparatus will be described with reference to FIG. FIG. 15 is a block diagram showing a configuration of a conventional OFDM communication apparatus.
[0003]
In the OFDM communication apparatus shown in FIG. 15, first, an information signal for each subcarrier is digitally modulated by a modulation unit 1 using, for example, QPSK (Quadrature Phase Shift Keying) or QAM (Quadrature Amplitude Modulation). The synchronization symbol insertion unit 2 inserts a synchronization symbol.
[0004]
The signal in which the synchronization symbol is inserted is subjected to IFFT in an IFFT (Inverse Fast Fourier Transform) unit 3 to be an OFDM signal. A guard interval is inserted into the OFDM signal by the guard interval insertion unit 4. The frame of this signal is as shown in FIG. 16, and is composed of an AGC symbol 21, a synchronization symbol 22, a phase reference symbol or pilot symbol 23, a guard interval (guard interval) 24, and an effective symbol 25. Has been.
[0005]
The signal with the guard interval inserted is D / A converted by the D / A converter 5 to become a baseband signal. This baseband signal is subjected to normal wireless transmission processing by a wireless transmission unit (not shown) and transmitted as a transmission signal via an antenna.
[0006]
The signal received via the antenna is subjected to normal radio reception processing by a radio reception unit (not shown) to become a baseband signal. This baseband signal is subjected to quadrature detection processing by a quadrature detector, and unnecessary frequency components are removed by a low-pass filter (both the quadrature detector and the low-pass filter are not shown). This baseband signal is A / D converted by the A / D converter 6. Although the received signal is divided into an in-phase component and a quadrature component by the quadrature detection processing, it is shown as one signal path in the drawing.
[0007]
This baseband signal is subjected to an FFT calculation by an FFT (Fast Fourier Transform) unit 12 to obtain a signal assigned to each subcarrier. At this time, the baseband signal is delayed by the delay unit 7 and sent to the multiplier 8. In the multiplier 8, multiplication processing is performed between the baseband signal and the delayed signal, and the multiplication result is accumulated in the accumulation unit 9. Then, the integration result is sent to the subtracter 10, subtracted from the threshold value, and the threshold value is determined by the determination unit 11. Then, the determination result is sent to the FFT unit 12.
[0008]
The signal subjected to the FFT operation in the FFT unit 12 is sent to the demodulating unit 13 and subjected to delay detection processing. The determining unit 14 determines whether the signal is different from the signal one bit before and becomes a demodulated signal.
[0009]
When the symbol communication is performed in the OFDM communication apparatus having the above configuration, first, a baseband signal before the FFT operation and a signal obtained by delaying the signal before the FFT processing by one symbol by the delay unit 7 are sent to the multiplier 8. Therefore, complex multiplication processing is performed.
[0010]
Next, the output of the multiplier 8 is sent to the accumulator 9 and the complex multiplication results are accumulated. Here, since the phase reference symbol is the same signal as the synchronization symbol, the sum of the two results in a peak in the phase reference symbol of the signal delayed by one symbol, as shown in part A of FIG. Therefore, symbol synchronization can be established by detecting the timing at which the integration result exceeds the threshold value.
[0011]
[Problems to be solved by the invention]
However, in the OFDM communication apparatus, when establishing symbol synchronization, the timing at which the integration result exceeds the threshold is detected as described above. Therefore, when the propagation environment is bad, the integration result exceeds the threshold. There may not be. In this case, timing detection is not performed and symbol synchronization cannot be established. If symbol synchronization is not established in this way, the OFDM apparatus determines that a signal that has actually arrived has not been received.
[0012]
On the other hand, it is conceivable to lower the threshold value for threshold judgment performed on the integration result. In this case, however, the integration result may exceed the threshold at a timing earlier in time than the symbol establishment timing. Yes, there is a risk of synchronization loss.
[0013]
  The present invention has been made in view of such a point, and an OFDM communication apparatus capable of reliably establishing symbol synchronization even in a poor propagation environmentAnd OFDM communication systemThe purpose is to provide.
[0014]
[Means for Solving the Problems]
The gist of the present invention is that, when establishing symbol synchronization in an OFDM communication apparatus, a plurality of thresholds for establishing synchronization are prepared, and the correlation result exceeds the maximum threshold among the prepared thresholds. The timing is the synchronization establishment timing. As a result, symbol synchronization can be reliably established even in a situation where the propagation environment is poor.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
  Of the present inventionOne aspect of the OFDM communication apparatus is an OFDM transmission apparatus that transmits an automatic gain control signal and transmits a synchronization signal after the automatic gain control signal, immediately after the automatic gain control signal and the synchronization A configuration is adopted that includes null signal insertion means for generating an OFDM signal by inserting a null signal immediately before the signal for use, and transmission means for transmitting the generated OFDM signal.
[0016]
  One aspect of the OFDM communication apparatus of the present invention employs a configuration in which the length of the null signal section is set shorter than the length of the automatic gain control signal section.
[0017]
  One aspect of the OFDM communication apparatus of the present invention employs a configuration further comprising control means for controlling the length of the null signal section.
[0018]
  Of the present inventionOne aspect of the OFDM communication system is an OFDM transmitter that transmits an automatic gain control signal and transmits a synchronization signal after the automatic gain control signal, immediately after the automatic gain control signal and the synchronization An OFDM communication apparatus comprising: a null signal insertion means for generating an OFDM signal by inserting a null signal immediately before a signal for use; and a transmission means for transmitting the generated OFDM signal; And an OFDM communication apparatus that receives the OFDM signal.
[0019]
  According to these configurations, even when there is a correlation between the automatic gain control signal and the synchronization signal, it is possible to reduce synchronization acquisition error in a relatively short time after starting the symbol synchronization processing, Symbol synchronization can be reliably established even in a poor propagation environment.
[0035]
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
(Embodiment 1)
FIG. 1 is a block diagram showing a configuration of an OFDM communication apparatus according to Embodiment 1 of the present invention. The OFDM communication apparatus includes a transmission unit, but the configuration is omitted here.
[0036]
The signal received via the antenna is subjected to normal radio reception processing by a radio reception unit (not shown) to become a baseband signal. This baseband signal is subjected to quadrature detection processing by a quadrature detector, and unnecessary frequency components are removed by a low-pass filter (both the quadrature detector and the low-pass filter are not shown). This baseband signal is A / D converted by the A / D converter 101. Although the received signal is divided into an in-phase component and a quadrature component by the quadrature detection processing, it is shown as one signal path in the drawing.
[0037]
This baseband signal is subjected to an FFT calculation by an FFT (Fast Fourier Transform) unit 113, and a signal assigned to each subcarrier is obtained. At this time, the baseband signal is sent to the multiplier 103, delayed by the delay unit 102, and sent to the multiplier 103. The baseband signal and the delayed signal are subjected to correlation processing. That is, these signals are multiplied by the multiplier 103, and the multiplication result is accumulated by the accumulating unit 104. Then, the integration result is sent to the subtracters 105 and 108, and is subtracted between the threshold value 1 and the threshold value 2, respectively.
[0038]
These subtraction results are determined as positive or negative by the determination units 106 and 109, respectively, and it is determined whether or not the integration result exceeds a threshold value. The result of the threshold determination is sent to the counter 107. The threshold judgment result of the subtractor 105 is counted up by the counter 107. This count number is subtracted from the threshold value by the subtractor 110 and is determined to be positive or negative by the determination unit 111. Information whose count exceeds the threshold is sent to the selector 112, and the selector selects the timing at which the integration result exceeds the higher threshold when the count exceeds the threshold. This timing is sent to the FFT unit.
[0039]
The signal subjected to the FFT operation by the FFT unit 113 is sent to the demodulation unit 114, subjected to delay detection processing, and the determination unit 115 determines whether the signal is different from the signal one bit before and becomes a demodulated signal.
[0040]
On the other hand, on the transmission side, a signal in which an AGC symbol, a synchronization symbol, a pilot symbol, and a guard interval are inserted into effective symbols is D / A converted by a D / A conversion unit to become a baseband signal. The baseband signal is subjected to normal wireless transmission processing by the wireless transmission unit and transmitted as a transmission signal via the antenna. Note that the configuration on the transmission side is not shown.
[0041]
Next, the operation of the OFDM communication apparatus having the above configuration will be described.
The signal received from the antenna is subjected to normal radio reception processing. That is, the received signal is amplified and frequency-converted by a wireless receiver (not shown), and A / D converted by the A / D converter 101 to become a baseband signal. As described above, the received signal is processed by being divided into an in-phase component and a quadrature component by a quadrature detector (not shown), but is represented by one path in the figure. Thereafter, symbol synchronization is established for the baseband signal. Hereinafter, this symbol synchronization will be described.
[0042]
First, the baseband signal before the FFT operation and the signal before the FFT process are sent by the delay unit 102 to the multiplier 103, for example, by a signal delayed by one symbol, and the complex multiplication process is performed on both signals.
[0043]
Next, the output of the multiplier 103 is sent to the integrating unit 104, and the complex multiplication results are integrated. Here, since the phase reference symbol is the same signal as the synchronization symbol, the integration result of both is the phase reference symbol of the signal delayed by a unit symbol (here, one symbol) as shown in part A of FIG. Produces a peak. Therefore, symbol synchronization can be established by detecting the timing at which the integration result exceeds the threshold value.
[0044]
In the OFDM communication apparatus according to the present embodiment, the result of integration is determined using two types of threshold values. Although the case where two types of threshold values are used will be described here, the same processing is performed when the determination is performed using three or more types of threshold values.
[0045]
In this OFDM communication apparatus, it is assumed that threshold 2 is the above-described threshold and threshold 1 is smaller than threshold 2. The output from the integration unit 104, that is, the integration result is sent to the subtracters 105 and 108. The subtractor 105 performs a subtraction process between the input integration result and the threshold value 1, and the subtractor 108 performs a subtraction process between the input integration result and the threshold value 2.
[0046]
These subtraction results are sent to determination sections 106 and 109, respectively, where a positive / negative determination is made to determine whether or not a threshold value has been exceeded. Here, since the threshold value 1 is smaller than the threshold value 2, the integration result first exceeds the threshold value 1 at a certain time. At this time, a signal indicating that the integration result exceeds the threshold value 1 is sent from the determination unit 106 to the counter 107 and counted up by the counter 107.
[0047]
At this time, information indicating that the integration result has exceeded the threshold value 1 and timing information at that time are sent to the selector 112. Similarly, information indicating that the integration result has exceeded the threshold value 2 and at that time Is sent to the selector 112.
[0048]
The count number of the counter 107 (the number of times the integration result exceeds the threshold value 1) is subtracted from a predetermined threshold value by the subtractor 110, and the determination result is determined by the determination unit 111 as positive or negative. When the count number exceeds the threshold value, a signal indicating that is sent to the selector 112.
[0049]
The selector 112 refers to information from the determination unit 106 and the determination unit 109 when a signal indicating that the count number exceeds the threshold value is input. That is, it is referred to whether the integration result exceeds the threshold value 1 and the threshold value 2 or only the threshold value 1 is exceeded. This determines which timing is sent to the FFT unit 113. That is, if the integration result exceeds the threshold value 1 and the threshold value 2 after the lapse of a predetermined time, the timing at which the integration result exceeds the threshold value 2 is sent to the FFT unit 113, and the integration result is determined after the lapse of the predetermined time. If only the threshold value 1 is exceeded, the timing at which the integration result exceeds the threshold value 1 is sent to the FFT unit 113. Note that the count number of the counter 107 is reset when the integration result exceeds the higher threshold value 2.
[0050]
The FFT unit 113 performs an FFT process on the baseband signal using the sent timing as an FFT process start timing. In this way, timing is set so that FFT is started by synchronizing the symbol synchronization between the transmission side and the reception side. Further, the signal subjected to the FFT processing is demodulated by the demodulator 114, and the demodulated result is sent to the determiner 115, where quadrant determination is performed there to become a demodulated signal.
[0051]
As described above, according to the OFDM communication apparatus according to the present embodiment, a plurality of threshold values for use in establishing symbol synchronization are prepared, and the correlation result exceeds the maximum threshold value among the prepared threshold values. Since the timing is the symbol synchronization timing, it is possible to avoid the occurrence of symbol synchronization non-detection even when the propagation condition is bad, and to reduce the symbol synchronization error.
[0052]
(Embodiment 2)
In the present embodiment, a case will be described in which a threshold value used for establishing symbol synchronization is variable according to channel quality.
[0053]
FIG. 2 is a block diagram showing a configuration of an OFDM communication apparatus according to Embodiment 2 of the present invention. In the OFDM communication apparatus shown in FIG. 2, the same parts as those of the OFDM communication apparatus shown in FIG.
[0054]
In this OFDM communication apparatus, selectors 201 and 202 for selecting a threshold value to be input to the subtracters 105 and 108 are provided. The selectors 201 and 202 switch the threshold value for the integration result according to the line quality. That is, the selector 201 uses the threshold value 1 when the line quality is good and uses the threshold value 1 ′ when the line quality is bad. The selector 202 uses the threshold value 2 when the line quality is good and uses the threshold value 2 ′ when the line quality is bad. This channel quality is obtained by obtaining the difference between the demodulation result of the demodulation unit 114 and the determination result of the determination unit 115 by the subtractor 203. The difference is stored in the memory 204 as line quality information.
[0055]
The operation of the OFDM communication apparatus having the above configuration will be described.
For the integration result of the complex multiplication processing result between the baseband signal before the FFT operation and the signal before the FFT processing by the delay unit 102, for example, a signal delayed by one symbol, two threshold values 1 and 2 are used. Perform threshold judgment.
[0056]
The number of times that the integration result exceeds the threshold values 1 and 2 is counted by the counter 107 to determine the threshold value. Information indicating that the integration result exceeds the threshold value 1 and timing information at that time are sent to the selector 112, and the timing information is held for a predetermined time. Similarly, information indicating that the integration result exceeds the threshold value 2 and timing information at that time are sent to the selector 112. After a predetermined time elapses, at that time point, information from the determination unit 106 and the determination unit 109 is referred to determine which timing is to be sent to the FFT unit 113.
[0057]
That is, if the integration result exceeds the threshold value 1 and the threshold value 2 after the holding time elapses, the timing at which the integration result exceeds the threshold value 2 is sent to the FFT unit 113, and the integration result indicates that the holding time has elapsed. If only threshold value 1 is exceeded later, the timing at which the integration result exceeds threshold value 1 is sent to FFT section 113.
[0058]
The FFT unit 113 performs an FFT process on the baseband signal using the sent timing as an FFT process start timing. Further, the signal subjected to the FFT processing is demodulated by the demodulator 114, and the demodulated result is sent to the determiner 115, where quadrant determination is performed there to become a demodulated signal.
[0059]
Here, the channel quality is measured based on the received signal. For example, the difference (phase difference) between the demodulated signal and the signal subjected to quadrant determination by the determination unit 115 is obtained by the subtractor 203, and the phase difference information is stored in the memory 204. At this time, when the phase difference is large, it is determined that the propagation environment is bad, and when the phase difference is small, it is determined that the propagation environment is good.
[0060]
Information about the propagation environment is sent to the selectors 201 and 202 as channel quality information. Specifically, if the propagation environment is bad, the integration result may not exceed the threshold with the set threshold, so an instruction to lower the threshold is sent to the selector, and if the propagation environment is good, An instruction is sent to the selector to maintain or raise the threshold. In the selectors 201 and 202, the threshold value is changed based on an instruction related to raising or lowering the threshold value.
[0061]
That is, in response to an instruction to lower the threshold value, the selectors 201 and 202 change the threshold values 1 and 2 from the threshold values 1 and 2 to a smaller threshold value 1 ′ and 2 ′. Regarding the relationship between the phase difference and the change of the threshold values 1 and 2, for example, a threshold value for the phase difference is set, and when the phase difference exceeds the threshold value, the threshold values 1 and 2 are set. It is conceivable to make changes.
[0062]
In addition, as shown in FIG. 3, the threshold value for the count number of the counter 107 (the number of times the integration result exceeds the threshold value 1) may be switched to the threshold value 'by the selector 301 in accordance with the line quality. good. For example, if the propagation environment is bad, an instruction to lower the threshold value to the threshold value 'is sent to the selector 301 and the threshold value is switched to the threshold value'.
[0063]
As described above, in the present embodiment, the threshold value used for establishing symbol synchronization is made variable according to the line quality, so the threshold value can be optimized according to the line quality. Symbol synchronization timing can be detected with high accuracy.
[0064]
In this embodiment, the case where two types of threshold values are set according to the line quality is described. However, the threshold value to be changed according to the line quality may be set to three or more types. good. In this way, by increasing the number of thresholds to be changed according to the line quality, finer control can be performed according to the propagation environment.
[0065]
(Embodiment 3)
In the present embodiment, a case will be described in which signal level information is used for establishment of symbol synchronization to prevent establishment of symbol synchronization at a timing at which no signal is transmitted.
[0066]
FIG. 4 is a block diagram showing a configuration of an OFDM communication apparatus according to Embodiment 3 of the present invention. In the OFDM communication apparatus shown in FIG. 4, the same parts as those in the OFDM communication apparatus shown in FIG.
[0067]
This OFDM communication apparatus includes a memory 401 that stores a signal level of a received signal, a subtractor 402 that performs a subtraction process between the signal level and a predetermined threshold L, and makes a positive / negative determination on the subtraction result. The determination unit 403 and a selector 404 that switches whether to use the symbol synchronization timing based on the determination result.
[0068]
The operation of the OFDM communication apparatus having the above configuration will be described.
For the integration result of the complex multiplication processing result between the baseband signal before the FFT operation and the signal before the FFT processing by the delay unit 102, for example, a signal delayed by one symbol, two threshold values 1 and 2 are used. Perform threshold judgment.
[0069]
The number of times that the integration result exceeds the threshold values 1 and 2 is counted by the counter 107 to determine the threshold value. Information indicating that the integration result exceeds the threshold value 1 and timing information at that time are sent to the selector 112, and the timing information is held for a predetermined time. Similarly, information indicating that the integration result exceeds the threshold value 2 and timing information at that time are sent to the selector 112. After a predetermined time elapses, at that time point, it is determined which timing to send to the FFT unit 113 with reference to information from the determination unit 106 and the determination unit 109.
[0070]
The FFT unit 113 performs an FFT process on the baseband signal using the sent timing as an FFT process start timing. Further, the signal subjected to the FFT processing is demodulated by the demodulator 114, and the demodulated result is sent to the determiner 115, where quadrant determination is performed and becomes a demodulated signal.
[0071]
The reception level of the reception signal is measured from the demodulated signal, and the reception level is sent to the memory 401. A threshold is determined for the reception level stored in the memory 401. That is, the reception level and the threshold value L are subtracted by the subtractor 402, and the positive / negative determination is performed by the determination unit 403. This threshold value L is set to the lowest reception level at which a signal is transmitted, for example. The selector 404 selects whether to establish symbol synchronization depending on whether the reception level exceeds the threshold value L. Therefore, if the reception level is equal to or lower than threshold value L, the symbol synchronization establishment process is stopped. If the reception level exceeds threshold value L, the symbol synchronization establishment process is performed in the same manner as in the first embodiment.
[0072]
As described above, in this embodiment, since the signal level information is used for symbol synchronization establishment and the necessity of symbol synchronization establishment processing is selected, symbol synchronization is established at a timing when no signal is transmitted. It can be surely prevented.
[0073]
(Embodiment 4)
In the present embodiment, a case will be described in which it is determined that symbol synchronization has been established when the number of times that the correlation result exceeds the maximum threshold is equal to or greater than a certain number.
[0074]
FIG. 5 is a block diagram showing a configuration of an OFDM communication apparatus according to Embodiment 4 of the present invention. In the OFDM communication apparatus shown in FIG. 5, the same parts as those in the OFDM communication apparatus shown in FIG.
[0075]
This OFDM communication apparatus includes a counter 1 501 and a counter 2 502 that count the number of times the thresholds 1 and 2 are exceeded in the subtractors 105 and 108, the count number of the counter 501 and the counter 502, and a predetermined threshold 3. Subtracters 503 and 504 that perform subtraction processing between the subtractors 503 and 504.
[0076]
The operation of the OFDM communication apparatus having the above configuration will be described.
For the integration result of the complex multiplication processing result between the baseband signal before the FFT operation and the signal before the FFT processing by the delay unit 102, for example, a signal delayed by one symbol, two threshold values 1 and 2 are used. Perform threshold judgment.
[0077]
The number of times that the integration result exceeds the threshold value 1 is counted by the counter 501, and the number of times that the integration result exceeds the threshold value 2 is counted by the counter 502. The number of times the integration result exceeds the threshold values 1 and 2 is subtracted from the threshold value 3 by the subtracters 503 and 504, respectively. With respect to the subtraction result, determination units 106 and 109 perform positive / negative determination, respectively. It is determined that the threshold values 1 and 2 are exceeded only when the determination units 106 and 109 determine that the threshold value 3 is exceeded.
[0078]
The number of times the integration result exceeds the threshold values 1 and 2 is counted by the counter 107, and the threshold value is determined. Information indicating that the integration result exceeds the threshold value 1 and timing information at that time are sent to the selector 112, and the timing information is held for a predetermined time. Similarly, information indicating that the integration result exceeds the threshold value 2 and timing information at that time are sent to the selector 112. After a predetermined time elapses, at that time point, it is determined which timing to send to the FFT unit 113 with reference to information from the determination unit 106 and the determination unit 109.
[0079]
The FFT unit 113 performs an FFT process on the baseband signal using the sent timing as an FFT process start timing. Further, the signal subjected to the FFT processing is demodulated by the demodulator 114, and the demodulated result is sent to the determiner 115, where quadrant determination is performed there to become a demodulated signal.
[0080]
In the present embodiment, the case where the threshold value 3 used for the threshold value determination of the count numbers of the counters 501 and 502 is the same is described, but in the present embodiment, as shown in FIG. The threshold value 3 used for the threshold value determination of the count number of the counter 501 by the subtractor 503 and the threshold value 4 used for the threshold value determination of the count number of the counter 502 by the subtractor 601 are set differently. Thus, symbol synchronization can be established with higher accuracy.
[0081]
In other words, the higher the threshold value for the integration result, the smaller the number of times that the threshold value is exceeded. It is preferable to set it smaller than 3. Thereby, the threshold value can be determined more accurately.
[0082]
In the above description, the case where the count number of the counters 501 and 502 is increased when the integration result exceeds the threshold values 1 and 2 is described. However, the integration result exceeds the threshold values 1 and 2. If not, the count number of the counters 501 and 502 may be decreased. When the propagation environment is bad and the signal-to-noise ratio is bad, the correlation value may suddenly increase in time. In such a case, the integration result thereafter does not exceed the threshold values 1 and 2. For this reason, by reducing the number of counts when the integration result does not exceed the threshold value, it is possible to cancel the sudden increase in the correlation value, and erroneously establish symbol synchronization. Can be prevented.
[0083]
As described above, in this embodiment, since it is determined that symbol synchronization has been established when the number of times the correlation result exceeds the maximum threshold value is greater than or equal to a certain number of times, the correlation result has suddenly exceeded the threshold value. Sometimes it is possible to avoid establishing symbol synchronization. Thereby, symbol synchronization can be established with higher accuracy.
[0084]
(Embodiment 5)
In this embodiment, a case will be described in which the threshold value of the number of times the correlation result exceeds the maximum threshold value is made variable according to the line quality.
[0085]
FIG. 7 is a block diagram showing a configuration of an OFDM communication apparatus according to Embodiment 5 of the present invention. In the OFDM communication apparatus shown in FIG. 7, the same parts as those in the OFDM communication apparatus shown in FIG.
[0086]
In this OFDM communication apparatus, selectors 701 and 702 that switch threshold values 3 and 4 used for threshold value determination of the number of counters 501 and 502 to threshold value 3 ′ and threshold value 4 ′ according to the line quality information. And a subtractor 203 for obtaining a phase difference between signals before and after quadrant determination with respect to the demodulated signal, and a memory 204 for storing the subtraction result.
[0087]
The operation of the OFDM communication apparatus having the above configuration will be described.
For the integration result of the complex multiplication processing result between the baseband signal before the FFT operation and the signal before the FFT processing by the delay unit 102, for example, a signal delayed by one symbol, two threshold values 1 and 2 are used. Perform threshold judgment.
[0088]
The number of times that the integration result exceeds the threshold value 1 is counted by the counter 501, and the number of times that the integration result exceeds the threshold value 2 is counted by the counter 502. The number of times the integration result exceeds the threshold values 1 and 2 is subtracted from the threshold value 3 by the subtracters 503 and 504, respectively. With respect to the subtraction result, determination units 106 and 109 perform positive / negative determination, respectively. It is determined that the threshold values 1 and 2 are exceeded only when the determination units 106 and 109 determine that the threshold value 3 is exceeded.
[0089]
The number of times the integration result exceeds the threshold values 1 and 2 is counted by the counter 107, and the threshold value is determined. Information indicating that the integration result exceeds the threshold value 1 and timing information at that time are sent to the selector 112, and the timing information is held for a predetermined time. Similarly, information indicating that the integration result exceeds the threshold value 2 and timing information at that time are sent to the selector 112. After a predetermined time elapses, at that time point, it is determined which timing to send to the FFT unit 113 with reference to information from the determination unit 106 and the determination unit 109.
[0090]
The FFT unit 113 performs an FFT process on the baseband signal using the sent timing as an FFT process start timing. Further, the signal subjected to the FFT processing is demodulated by the demodulator 114, and the demodulated result is sent to the determiner 115, where quadrant determination is performed there to become a demodulated signal.
[0091]
A difference (phase difference) between the demodulated signal and the signal subjected to quadrant determination by the determination unit 115 is obtained by the subtractor 203, and the phase difference information is stored in the memory 204. At this time, when the phase difference is large, it is determined that the propagation environment is bad, and when the phase difference is small, it is determined that the propagation environment is good.
[0092]
Information about the propagation environment is sent to the selectors 701 and 702 as channel quality information. Specifically, if the propagation environment is bad, the count value may not exceed the threshold value with the set threshold value, so an instruction to lower the threshold value is sent to the selector, and if the propagation environment is good, An instruction is sent to the selector to maintain or raise the threshold. The selectors 701 and 702 change the threshold value based on an instruction related to raising or lowering the threshold value.
[0093]
That is, in response to an instruction to lower the threshold value, the selectors 701 and 702 change the threshold value 3 or 4 to a smaller threshold value 3 ′ or 4 ′. As for the relationship between the phase difference and the change of the threshold values 3 and 4, for example, a threshold value for the phase difference is set, and when the phase difference exceeds the threshold value, the threshold values 3 and 4 are set. It is conceivable to make changes.
[0094]
In addition, as shown in FIG. 8, the channel quality information is not determined by one burst, but the channel quality information (threshold change instruction) of a plurality of bursts is averaged by the averaging unit 801 and the count number is adjusted. The threshold values 3 and 4 may be changed. Thereby, the threshold values 3 and 4 for the count number can be changed more optimally according to the line quality.
[0095]
As described above, in the present embodiment, the threshold value used for threshold determination with respect to the number of times the correlation result exceeds the threshold value is made variable according to the line quality. Since the value can be optimized, it is possible to reliably avoid the establishment of symbol synchronization when the correlation result suddenly exceeds the threshold value.
[0096]
(Embodiment 6)
In this embodiment, a case will be described in which 0 symbol is inserted before a synchronization symbol in a transmission signal on the transmission side, and the correlation value in the temporal direction is reduced on the reception side.
[0097]
FIG. 9 is a block diagram showing a configuration of an OFDM communication apparatus according to Embodiment 6 of the present invention. In FIG. 9, the configuration on the receiving side is omitted.
[0098]
First, an information signal for each subcarrier is digitally modulated by a modulation unit 901, for example, by QPSK (Quadrature Phase Shift Keying) or QAM (Quadrature Amplitude Modulation), and then synchronized by a synchronization symbol insertion unit 902. Symbols are added, and thereafter, 0 symbol insertion section 903 adds 0 symbols, which are correlation value suppression signals.
[0099]
The signal in which the predetermined symbol is inserted is subjected to an IFFT calculation by an IFFT (Inverse Fast Fourier Transform) unit 904 to become an OFDM signal. A guard interval is inserted into this OFDM signal by a guard interval insertion unit 905. The signal with the guard interval inserted is D / A converted by the D / A converter 906 to become a baseband signal. This baseband signal is subjected to normal wireless transmission processing by a wireless transmission unit (not shown) and transmitted as a transmission signal via an antenna.
[0100]
Next, the operation of the OFDM communication apparatus having the above configuration will be described using FIG. 9 and FIG. After the AGC symbol 1001 is inserted into the signal digitally modulated by the modulation unit 901, the synchronization symbol 1003 is inserted by the synchronization symbol insertion unit 902. The synchronization symbol insertion unit 902 is configured by a switch, and when the control signal 1 for inserting the synchronization symbol 1003 is input, the switch is switched and the synchronization symbol 1003 is inserted. Yes. Note that a phase reference symbol (pilot symbol) 1004 is inserted after the synchronization symbol 1003. This phase reference symbol 1004 is the same signal as the synchronization symbol 1003. Further, the insertion of the phase reference symbol 1004 is similarly performed by the synchronization symbol inserting unit 902.
[0101]
The 0 symbol 1002 is inserted by the 0 symbol insertion unit 903 before the synchronization symbol 1003 of the signal in which the synchronization symbol 901 is inserted. The 0 symbol insertion unit 903 is configured by a switch, and when a control signal 2 for inserting the 0 symbol 1002 is input, the switch is switched and the 0 symbol 1002 is inserted.
[0102]
The signal in which the synchronization symbol 1003 and the 0 symbol 1002 are inserted in this way is sent to the IFFT unit 904 and subjected to IFFT calculation. That is, IFFT section 904 obtains a signal waveform on the time axis by performing IFFT conversion on the time axis for each symbol period on complex number data including phase and amplitude information on the frequency axis.
[0103]
Next, a guard interval (guard interval) 1005 is inserted in the IFFT-converted signal waveform by the guard interval insertion unit 905. Specifically, a partial waveform at the rear end of the effective symbol 1006 is inserted as the guard interval 1005. Thus, by inserting the guard interval 1005 that allows the delay time, an increase in the bit error rate can be suppressed, and the multipath resistance can be improved.
[0104]
Next, the signal with the guard interval inserted in this manner is D / A converted by the D / A conversion unit 906. Thereafter, the D / A converted signal is transmitted after being subjected to normal radio transmission processing. That is, the signal is frequency-converted and amplified by a radio transmission unit (not shown) and transmitted from the antenna.
[0105]
In this embodiment, a 0 symbol 1002 is inserted before a synchronization symbol 1003 in a signal frame. Accordingly, when a correlation is obtained between the received signal and the signal delayed by one symbol, a correlation process is performed between the 0 symbol of the received signal and the synchronization symbol. In this case, since correlation processing is performed with 0 symbols, the correlation result is very small. For this reason, the threshold value is not exceeded in a relatively previous time zone, and the FFT processing start timing can be accurately detected.
[0106]
As described above, in this embodiment, since 0 symbols are inserted before the synchronization symbols, the correlation output before the time can be reduced. Therefore, it is possible to suppress exceeding the threshold value in time. As a result, it is possible to prevent the occurrence of a synchronization error in front of time and accurately detect the FFT processing start timing.
[0107]
Further, in the present embodiment, the same effect can be obtained even when a 0 signal in a section shorter than the synchronization symbol period is inserted before the synchronization symbol.
[0108]
In the OFDM communication apparatus shown in FIG. 11, the transmitter performs IFFT processing on the signal in which the synchronization symbol is inserted, inserts the guard interval, and then inserts the 0 signal. For this reason, the 0 symbol insertion unit 902 is deleted from the OFDM communication apparatus shown in FIG. 9, and a 0 signal insertion unit 1101 is provided after the guard interval insertion unit 905.
[0109]
Next, the operation of the OFDM communication apparatus having the above configuration will be described using FIG. 11 and FIG. In the same manner as described above, a synchronization symbol insertion unit 902 inserts a synchronization symbol 1003 into the signal digitally modulated by the modulation unit 901. Note that a phase reference symbol 1004 is inserted after the synchronization symbol 1003. This phase reference symbol 1004 is the same signal as the synchronization symbol 1003. Further, the insertion of the phase reference symbol 1004 is similarly performed by the synchronization symbol inserting unit 902.
[0110]
The signal with the synchronization symbol 1003 inserted is sent to the IFFT unit 904 for IFFT calculation. Next, a guard interval (guard interval) 1005 is inserted in the IFFT-converted signal waveform by the guard interval insertion unit 905.
[0111]
Next, a 0 signal, which is a correlation value suppression signal, is inserted into the signal into which the guard interval is inserted by the 0 signal insertion unit 1101. The 0 signal insertion unit 1101 is configured by a switch. When the control signal 3 for inserting the 0 signal 1201 is input, the switch is switched and the 0 signal 1201 is inserted. The interval of the 0 signal 1201 is set shorter than the period of the synchronization symbol 1003. For example, it is preferable to set to about 1/4 symbol. As a result, it is possible to shorten the section during which a signal without power is sent as much as possible.
[0112]
Next, the signal in which the 0 signal is inserted in this manner is D / A converted by the D / A converter 906. Thereafter, the D / A converted signal is transmitted after being subjected to normal radio transmission processing. That is, the signal is frequency-converted and amplified by a radio transmission unit (not shown) and transmitted from the antenna.
[0113]
According to such a configuration, it is possible to reduce the correlation output ahead in time, prevent the occurrence of synchronization error in time ahead, and suppress the decrease in transmission efficiency.
[0114]
Further, in the present embodiment, as shown in FIG. 13, the control signal 3 input to the 0 signal insertion unit 1101 may be switched according to the line quality. That is, the line quality information is input to the switch 1301 for switching the control signals 3 and 3 ′, and the section in which the 0 signal is inserted is controlled. For example, when the channel quality is poor, the interval for inserting the 0 signal is lengthened, and when the channel quality is good, the interval for inserting the 0 signal is shortened.
[0115]
According to such a configuration, since the 0 signal insertion interval is made variable according to the channel quality, the 0 signal insertion interval can be adaptively optimized according to the channel quality. As a result, it is possible to prevent the occurrence of a synchronization error ahead in time with higher accuracy and to suppress a decrease in transmission efficiency.
[0116]
(Embodiment 7)
In the present embodiment, a case will be described in which the result of integration when a smaller threshold is exceeded among the plurality of prepared thresholds is reflected in the next higher threshold.
[0117]
FIG. 14 is a block diagram showing a configuration of an OFDM communication apparatus according to Embodiment 7 of the present invention. In the OFDM communication apparatus shown in FIG. 14, the same parts as those in the OFDM communication apparatus shown in FIG.
[0118]
This OFDM communication apparatus includes a switch 1401 that stores an accumulation result at that time in a memory 1402 based on a threshold determination result determined by the determination units 106 and 109.
[0119]
The operation of the OFDM communication apparatus having the above configuration will be described.
For the integration result of the complex multiplication processing result between the baseband signal before the FFT operation and the signal before the FFT processing by the delay unit 102, for example, a signal delayed by one symbol, two threshold values 1 and 2 are used. Perform threshold judgment.
[0120]
The number of times that the integration result exceeds the threshold values 1 and 2 is counted by the counter 107 and the threshold value is determined using the threshold values 3 and 4. Information indicating that the integration result exceeds the threshold value 1 and timing information at that time are sent to the selector 112, and the timing information is held for a predetermined time. Similarly, information indicating that the integration result exceeds the threshold value 2 and timing information at that time are sent to the selector 112. After a predetermined time elapses, at that time point, it is determined which timing to send to the FFT unit 113 with reference to information from the determination unit 106 and the determination unit 109.
[0121]
The FFT unit 113 performs an FFT process on the baseband signal using the sent timing as an FFT process start timing. Further, the signal subjected to the FFT processing is demodulated by the demodulator 114, and the demodulated result is sent to the determiner 115, where quadrant determination is performed there to become a demodulated signal.
[0122]
Information determined that the integration result exceeds the threshold values 1 and 2 is sent to the switch 1401. At this time, the integration result at that time is stored in the memory 1402, the integration result is input to the subtractor 108, and the threshold value 2 is updated to the integration result.
[0123]
That is, when the threshold value 1 is exceeded but the higher threshold value 2 is not exceeded, if the timing exceeding the threshold value 1 is the symbol synchronization timing, it is still ahead of the accurate timing position in time. Synchronization may be established. Therefore, the integration result when the maximum threshold value at that time is exceeded is updated as a higher threshold value. As a result, the threshold value can be determined more accurately, and symbol synchronization can be established with higher accuracy.
[0124]
The OFDM communication apparatuses according to Embodiments 1 to 7 can be applied to communication terminal apparatuses and base station apparatuses such as mobile station apparatuses in wireless communication systems.
[0125]
In addition, this invention is not limited to the said Embodiment 1-7, It can change and implement variously. Moreover, Embodiments 1 to 7 can be implemented in combination as appropriate.
[0126]
【The invention's effect】
As described above, the OFDM communication apparatus according to the present invention prepares a plurality of threshold values for use in establishing symbol synchronization, and the timing at which the correlation result exceeds the maximum threshold value among the prepared threshold values is symbolized. Since the synchronization timing is used, it is possible to avoid the occurrence of symbol synchronization non-detection even when the propagation condition is bad, and to reduce the symbol synchronization error.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of an OFDM communication apparatus according to Embodiment 1 of the present invention.
FIG. 2 is a block diagram showing a configuration of an OFDM communication apparatus according to Embodiment 2 of the present invention.
FIG. 3 is a block diagram showing a configuration of an OFDM communication apparatus according to Embodiment 2 of the present invention.
FIG. 4 is a block diagram showing a configuration of an OFDM communication apparatus according to Embodiment 3 of the present invention.
FIG. 5 is a block diagram showing a configuration of an OFDM communication apparatus according to Embodiment 4 of the present invention.
FIG. 6 is a block diagram showing a configuration of an OFDM communication apparatus according to Embodiment 4 of the present invention.
FIG. 7 is a block diagram showing a configuration of an OFDM communication apparatus according to Embodiment 5 of the present invention.
FIG. 8 is a block diagram showing a configuration of an OFDM communication apparatus according to Embodiment 5 of the present invention.
FIG. 9 is a block diagram showing a configuration of an OFDM communication apparatus according to Embodiment 6 of the present invention.
FIG. 10 is a signal frame diagram used in the OFDM communication apparatus according to the embodiment.
FIG. 11 is a block diagram showing a configuration of an OFDM communication apparatus according to the above embodiment
FIG. 12 is a signal frame diagram used in the OFDM communication apparatus according to the embodiment.
FIG. 13 is a block diagram showing a configuration of an OFDM communication apparatus according to the above embodiment
FIG. 14 is a block diagram showing a configuration of an OFDM communication apparatus according to Embodiment 7 of the present invention.
FIG. 15 is a block diagram showing a configuration of a conventional OFDM communication apparatus
FIG. 16 is a signal frame diagram used in a conventional OFDM communication apparatus;
FIG. 17 is a diagram showing timing of a received signal and a correlation result
[Explanation of symbols]
101 A / D converter
102 Delay part
103 multiplier
104 Integration unit
105, 108, 110 Subtractor
106, 109, 111, 115 determination unit
107 counter
112 selector
113 FFT section
114 Demodulator

Claims (4)

  An OFDM transmitter that transmits an automatic gain control signal and transmits a synchronization signal after the automatic gain control signal,
  Null signal insertion means for generating an OFDM signal by inserting a null signal immediately after the automatic gain control signal and immediately before the synchronization signal;
  Transmitting means for transmitting the generated OFDM signal;
  An OFDM communication apparatus comprising:   2. The OFDM communication apparatus according to claim 1, wherein a length of the null signal section is set shorter than a length of the automatic gain control signal section.   The OFDM communication apparatus according to claim 2, further comprising control means for controlling a length of a section of the null signal. An OFDM transmitter for transmitting an automatic gain control signal and transmitting a synchronization signal after the automatic gain control signal, and inserting a null signal immediately after the automatic gain control signal and immediately before the synchronization signal An OFDM communication apparatus comprising: null signal insertion means for generating an OFDM signal, and transmission means for transmitting the generated OFDM signal;
  An OFDM communication apparatus for receiving an OFDM signal transmitted from the OFDM communication apparatus;
  An OFDM communication system comprising:

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