JP3839292B2 - OFDM signal receiver - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an OFDM (Orthogonal Frequency Division Multiplexing) signal receiver in a wireless communication system, and more particularly to symbol synchronization thereof.
[0002]
[Prior art]
First, in describing a conventional OFDM signal receiver, a method for generating an OFDM signal will be described.
FIG. 9 is a configuration block diagram of an OFDM signal transmitter that generates an OFDM signal.
In FIG. 9, 91 is a modulation unit, 92 is an IFFT unit, 93 is a guard interval adding unit, 94 is a preamble adding unit, and 95 is a digital-analog converter (hereinafter referred to as a D / A converter) that converts a digital signal into an analog signal. 96 is an analog part for transmitting an OFDM signal.
[0003]
The information data is subjected to coding modulation such as QSPK on the information data for each subcarrier in the modulation unit 91 so as to match the transmission format of the OFDM signal, and is output to the IFFT unit 92. In IFFT section 92, the output of modulation section 91 is subjected to IFFT (Inverse Fast Fourier Transform) and converted from a frequency domain signal to a time domain signal, and a digital modulated wave having a time axis waveform is generated. .
[0004]
The generated digital modulation wave has a guard interval at the head of the digital modulation wave for each subcarrier (that is, corresponding to an OFDM symbol described later) in order to make it difficult for the guard interval adding unit 93 to receive interference between symbols. An OFDM symbol to which a guard interval is added is generated.
[0005]
When transmitting a packet of the OFDM symbol to which this guard interval is added, the preamble added to the head of the packet by the preamble adding unit 94 is first converted into an analog signal by the D / A converter 95, and a radio wave output from the analog unit 96 Subsequently, the OFDM symbol to which the guard interval is added is similarly converted into an analog signal by the D / A converter 95 and is output from the analog unit 96 as a radio wave.
In this case, as long as the signal of the preamble part is known between transmission and reception, there is no problem with any shape, and usually a signal formed by an OFDM signal is often used.
[0006]
FIG. 10 is a diagram illustrating an example of a packet structure of an OFDM signal received by the OFDM signal receiver.
For example, as shown in FIG. 10, an OFDM signal packet is configured by adding a preamble part P composed of two preambles P1 and P2 to the head of an OFDM symbol packet Data to which the above-described guard interval is added. . In the OFDM signal receiver, the first preamble P1 is used for estimation of AGC (Auto Gain Control 1) or coarse frequency offset, and the next preamble P2 is used for OFDM symbol synchronization or high-accuracy frequency offset estimation. Is used for.
[0007]
FIG. 11 is a configuration block diagram of a conventional OFDM signal receiver.
11, 111 is an analog unit for receiving an OFDM signal, 112 is an AGC amplifier, 113 is an analog-to-digital converter (hereinafter referred to as an A / D converter) that converts an analog signal into a digital signal, and 114 is a quadrature demodulation. , 115 is a correlation unit, 116 is a guard interval removal unit, 117 is an FFT unit, 118 is a demodulation unit, 119 is a correlation value detection unit, and 120 is a symbol timing synchronization unit.
The OFDM signal is generally demodulated using a circuit configuration represented by these.
That is, the OFDM signal is received by the analog unit 111, the gain is controlled by the AGC amplifier 112 so that it is input to the A / D converter 113 with appropriate power, and is converted into a digital signal by the A / D converter 113. The
[0008]
In this example, the case where the signal is converted into a digital signal in the IF (Intermediate Frequency) region is shown, and the digital signal is converted into a baseband signal by the orthogonal demodulation unit 114. The converted baseband signal is correlated with a signal known in advance between transmission and reception by the correlator 115 in order to synchronize the OFDM symbols. The correlation value detection unit 119 detects the timing at which a correlation output having a level exceeding a preset threshold is output from the correlation unit 115.
[0009]
Here, the correlator 115 is configured by multiplying each sample by a discrete reception waveform and a function having the same resolution as that of the preamble P2 in FIG. Yes. This is the same configuration as a correlator used in a spread spectrum signal receiver or the like, and is a general one.
Therefore, in the correlation value detection unit 119 of the OFDM signal receiver, when there is no influence of noise or propagation path, the correlation value is detected at the timing when the last sample of the preamble P2 is input to the correlation unit 115.
[0010]
FIG. 12 is a waveform diagram of the correlation output from the correlation unit 115 before and after receiving the preamble P2 shown in FIG.
The magnitude of the correlation output from the correlator 115 (correlation level) is actually affected by noise or a propagation path, and is as shown in FIG.
In this prior art, the correlation value detection unit 119 sets a threshold for the correlation output output from the correlation unit 115, and the timing at which the correlation output exceeding the correlation threshold is detected is the earliest detection timing. Hereinafter, it is assumed that the symbol data is used, and hereinafter, the information data is demodulated based on the symbol timing.
[0011]
In FIG. 12, the correlation output output from the correlation unit 115 exceeds the correlation threshold at the timing of t = a, and this time is adopted as the timing at which the last sample of the preamble P2 of the OFDM signal is received. Become. Thereafter, from the timing indicating the end of the preamble P2 obtained by the correlation value detection unit 119, the symbol timing synchronization unit 120 synchronizes with the subsequent OFDM symbol.
[0012]
The configuration of the symbol timing synchronization unit 120 can be realized by, for example, a simple counter because OFDM symbols are received in a known cycle in advance. Based on the timing generated by the symbol timing synchronization unit 120, the guard interval removal unit 116 removes the guard interval added to each OFDM symbol and is necessary for FFT (Fast Fourier Transform). Are extracted, and the data is transferred to the FFT unit 117. The FFT unit 117 performs FFT on the OFDM symbol (that is, digital modulation wave for each subcarrier) from which the guard interval is removed, demodulates the signal from the time domain to the signal in the frequency domain, and sends the data to the demodulation unit 118. Deliver. In the demodulator 118, general demodulation is performed on data that has been coded and modulated, such as QSPK.
[0013]
[Problems to be solved by the invention]
However, in the symbol synchronization method in the conventional OFDM signal receiver as described above, in a noisy environment, the correlation output of the correlation unit 115 exceeds a predetermined correlation threshold at a timing when there is no desired signal, thereby detecting a correlation value. Unit 119 erroneously detects a correlation value, and at a timing output from symbol timing synchronization unit 120 based on the erroneously detected correlation value, guard interval removal unit 116, FFT unit 117, and demodulation as an OFDM symbol demodulation unit There is a problem that the probability of malfunction is extremely high, such that the unit 118 enters the demodulation operation.
[0014]
Furthermore, there is a problem that the probability of losing a packet that is originally desired to be demodulated and losing the demodulation timing becomes high due to malfunction.
In addition, when other systems are communicating using the same frequency in a place close to the location, or when there is a strong radio wave adjacent to or adjacent to the frequency used by itself (neighboring adjacent), the desired signal There is also a problem that a phenomenon similar to the above problem occurs due to accidental correlation with a non-radio wave.
In view of the above-described problems, an object of the present invention is to provide an OFDM signal receiver with improved symbol synchronization performance.
[0015]
[Means for Solving the Problems]
In order to solve the above problems, an OFDM signal receiver according to the present invention includes a correlation unit that correlates an OFDM signal including a synchronization signal whose position and pattern in a packet are known, and a correlation output of the correlation unit. And a symbol timing synchronization unit for generating a timing signal for demodulating the OFDM signal based on the correlation value detection by the correlation value detection unit. A reception determination unit that detects reception of the OFDM signal, and the correlation in a predetermined time range having a lower limit time and an upper limit time on the time axis with the reception determination time of the reception determination unit as an origin A correlation detection control unit for detecting and operating the value detection unit, The correlation detection control unit does not include the reception determination time of the reception determination unit as a time within the predetermined time range, and until the correlation value detection of the synchronization signal is performed from the start of reception of the synchronization signal. Includes time “T” as time It is characterized by that.
[0016]
Thus, the OFDM signal receiver according to the present invention can observe the correlation value of the synchronization signal based on the correlation output output from the correlator only at a specific timing after the arrival of the signal radio wave is detected.
In the OFDM signal receiver according to the present invention, the reception determination unit detects a reception power measurement unit that measures the reception power of the signal radio wave, and a measurement value of the reception power exceeds a preset threshold value. A reception power determination unit that determines signal reception.
[0017]
In the OFDM signal receiver according to the present invention, the reception determination unit detects a reception power measurement unit that measures the fluctuation of the reception power of the signal radio wave, and detects that the fluctuation value of the reception power exceeds a preset threshold value. And a reception power determination unit for determining signal reception.
Thus, the OFDM signal receiver according to the present invention can reliably determine the arrival of a signal radio wave even in an environment with a poor reception state.
[0018]
In addition, in the OFDM signal receiver according to the present invention, the symbol timing synchronization unit is configured such that the correlation value detection unit detects the correlation value after the time “T” within the predetermined time range, and the predetermined time range. A timing signal having the same contents as when the correlation value is detected by the correlation value detection unit at time “T” is generated.
[0019]
In the OFDM signal receiver according to the present invention, the reception determination unit is provided with a threshold control unit that changes the threshold stepwise in synchronization with a packet periodically transmitted at a specific time interval. It is characterized by.
As a result, even when the timing is shifted due to the influence of multipath, demodulation can be performed without intersymbol interference.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a demodulation block diagram of an OFDM signal receiver according to an embodiment of the present invention.
In FIG. 1, the same components as those of the prior art OFDM signal receiver already described in FIG. 11 are denoted by the same reference numerals as the numbers assigned to the components, and the description thereof is omitted. .
[0021]
In FIG. 1, the OFDM signal receiver according to the present embodiment converts a received power measuring unit 31 that measures the intensity of a radio wave of a received signal, and the power measured by the received power measuring unit 31 into a digital value. The A / D converter 32, the received power determination unit 33 that determines the magnitude of the received power of the A / D converted received signal, and the detection operation of the correlation value detection unit 19 based on the determination result of the received power determination unit 33. The correlation detection control part 34 to control is newly provided.
[0022]
The received power measuring unit 31 measures the power of the OFDM received signal, that is, the strength of the received radio wave, using the OFDM received signal output from the analog unit 11 before being input to the AGC amplifier 12. In general, the reception power measurement unit 31 includes an averaging circuit so that accurate reception power can be measured.
The power of the OFDM reception signal measured by the reception power measurement unit 31 is converted into a digital value by the A / D converter 32 and then output to the reception power determination unit 33 that constitutes the reception determination unit together with the reception power measurement unit 31. Is done.
[0023]
The received power determination unit 33 compares the received power level (magnitude) of the received signal with a predetermined threshold value to determine whether or not the received power level exceeds a predetermined threshold value. Determine.
When the received power level exceeds this threshold, the received power determination unit 33 determines that the OFDM signal radio wave originally intended to be received has arrived, and sends the OFDM signal to the correlation detection control unit 34. A reception detection signal for informing the reception of the signal radio wave is output.
[0024]
The correlation detection control unit 34 detects the reception of the OFDM signal by the input of the reception detection signal from the reception power determination unit 33 and then sets a preset “T” on the time axis with the reception detection time as the origin. −d1T ”(lower limit time) to“ T + d2T ”(upper limit time), a signal that becomes active is generated only in a predetermined time range, and the correlation value detection unit 19 outputs only the output period of the generated signal. Control is performed to detect the correlation value, that is, the symbol timing. Therefore, the signal that becomes active corresponds to the operation signal of the correlation value detection unit 19.
[0025]
Therefore, the correlation value detection unit 19 determines whether a correlation output exceeding a preset correlation threshold is output from the correlation unit 15 while the operation signal is input from the correlation detection control unit 34, When a correlation output exceeding the correlation threshold is detected, this correlation is detected and a correlation detection signal (symbol timing detection signal) is output to the symbol timing synchronization unit 20.
Based on the input of the symbol timing detection signal, the symbol timing synchronization unit 20 generates a symbol timing synchronization signal and outputs the symbol timing synchronization signal to the guard interval removal unit 16 to continuously perform the demodulation operation on the received OFDM signal.
[0026]
In contrast, if the correlation value detection unit 19 does not detect a correlation value that exceeds the correlation threshold, the process starts from the OFDM signal reception detection routine based on the magnitude of the power level of the reception signal described above.
Here, “T” indicates the time from the reception detection of the OFDM signal until the correlation output exceeding the correlation threshold is detected, and is a specific value determined from the position of the synchronization preamble in the packet and the circuit configuration. That is, when it is assumed that the arrival of the signal radio wave can be detected ideally by the reception power determination unit 33, “T” is the correlation value of the synchronization preamble in the correlation unit 15 from the detection (reception detection) of the signal radio wave. Is the time until is output.
Further, “d1T” is determined by the time resolution of the received power measuring unit, and “d2T” is determined by the propagation path, as described later.
[0027]
FIG. 2 shows a simple timing diagram of the demodulation block of the OFDM signal receiver described above.
In FIG. 2, “POW” indicates a change in the power of the received signal measured by the received power measuring unit 31, that is, the received radio wave. However, a circuit that measures received power generally averages over time in order to reduce the influence of noise, so the rise of POW is smooth.
[0028]
“PDET” is a radio wave arrival detection signal generated by the reception power determination unit 33 and is activated when the reception power exceeds a predetermined determination threshold. In FIG. 2, it means that the arrival of radio waves (reception of radio waves) is detected at the timing of “t = a”.
“MAXEN” is an operation signal of the correlation detection unit 19 generated by the correlation detection control unit 34, and is a signal that becomes active only during the period “T−d1T” to “T + d2T” after PDET becomes active. It is. That is, the correlation value detection unit 19 performs the operation of detecting the correlation value of the synchronization preamble only during the period “T−d1T” to “T + d2T”.
[0029]
FIG. 3 shows a timing diagram of the correlation output waveform of the correlator 15 and MAXEN.
FIG. 3A shows the timing relationship between the correlation output waveform of the correlator 15 and MAXEN when the target signal radio wave is normally received.
Accordingly, the correlation value detection unit 19 exceeds the correlation threshold value at the timing “t = b” within the period “T−d1T” to “T + d2T”, and the correlation value of the synchronization preamble is Assuming that it has been detected, the normal demodulation can be performed by passing the timing to the symbol timing synchronization unit 20 hereinafter.
Further, in order to clarify the effect of defining the periods “T−d1T” to “T + d2T”, a description will be given with reference to FIGS. 3B and 3C.
[0030]
FIG. 3B shows the timing relationship between the correlation output waveform of the correlator 15 and MAXEN when noise is erroneously detected as a radio wave in the received power determination unit 33.
According to FIG. 3B, during the period from “T−d1T” to “T + d2T”, the correlation output exceeding the correlation threshold is not output from the correlation unit 15, and the correlation value is detected by the correlation value detection unit 19. However, since a correlation output exceeding the correlation threshold is accidentally output at a timing before “T-d1T” (“t = c” in the figure), the correlation value is detected. If "" to "T + d2T" is not specified, erroneous synchronization occurs. Thereafter, the circuit demodulates based on this incorrect timing, and error is detected by other means, for example, CRC (Cyclic Redundancy Check character) check of information data. Until the synchronization is recognized, the circuit must continue to operate in a false synchronization, which wastes time.
[0031]
FIG. 3C shows the timing relationship with the output waveform MAXEN of the correlator 15 when a signal other than the target OFDM packet is received.
According to FIG. 3C, a correlation output exceeding the correlation threshold is detected by chance at the timing after “T + d2T” (“t = d” in the figure) (the OFDM signal has a random waveform). Therefore, when there is an OFDM signal of another system, a correlation value may be detected by chance), and as in the case of FIG. 3B, a false synchronization occurs and the same problem occurs.
As a result, by defining the period “T−d1T” to “T + d2T” of the detection timing of the correlation value of the synchronization preamble with reference to the time of detection of the arrival of the signal radio wave, the probability that a wasteful demodulation operation due to erroneous synchronization occurs will occur. Can be very low.
[0032]
Next, a modified example of the reception power determination unit 33 of the OFDM receiver according to the present embodiment will be described.
In this example, the received power determination unit 33 as signal radio wave arrival detection means determines using the rate of change in power.
In the embodiment described above, the received power determination unit 33 compares the received power level (magnitude) with a predetermined threshold, and when the received power level exceeds the predetermined threshold, In contrast to the configuration in which it is determined that arrival (that is, reception) has been detected, in the present modification, the reception power determination unit 33 is configured to detect the signal radio wave when the rate of change in reception power exceeds a predetermined threshold. It is configured to determine that arrival has been detected.
[0033]
In this modification, as a means for obtaining the rate of change in received power, a method of obtaining from the difference in power level at successive measurement times is adopted, but the specific configuration for obtaining the rate of change in received power is as follows: It is not limited to this configuration.
This modification is effective when the received power to be measured is fixedly high although it is not the desired radio wave.
[0034]
There are several possible causes for such a case, but there are cases where it cannot be avoided simply by increasing the accuracy of the circuit.For example, the frequency used for transmission / reception is used in multiple communication systems, and your communication This is the case where signal radio waves are transmitted for many hours outside the area.
In this case, since the signal radio wave is constantly transmitted at a distance, the determination of only the power level causes the signal radio wave to be captured constantly, and the desired signal radio wave is transmitted at a level that can be received. Nevertheless, it cannot be captured and communication is impossible.
[0035]
Such a problem also occurs when similar communication is performed near the frequency being used.
In general, signal radio waves transmitted adjacent or adjacent to each other can be removed by a filter or the like in the receiver, but the transmitter transmitting using the adjacent etc. is in the vicinity (locally) of the terminal. In some cases, it is difficult to remove the signal to a problem-free level, and the problem that the signal cannot be synchronized with the radio wave of the terminal itself is caused.
[0036]
FIG. 4 is a diagram showing a timing diagram of correlation value detection when such interference waves are present.
4A shows a case where a determination unit based on the level of the power level is used as the reception power determination unit 33. FIG. 4B shows a determination unit based on the rate of change in the power level as the reception power determination unit 33. It is a case where it is used for.
[0037]
“POW” is a reception level, “DIFFPOW” is a rate of change in received power, “PDET” is a signal indicating that a radio wave generated by the received power determination unit 33 in FIG. 1 has been received, “MAXEN” and “correlation output” 3 is the same as “MAXEN” and “correlation output” in FIG. 3 described above, and “SYNCMISS” is during the period “T−d1T” to “T + d2T” while MAXEN is active, that is, after PDET becomes active. When a correlation value exceeding the threshold is not detected, this is a control signal for redoing the radio wave determination.
[0038]
In the case where the power value is used for the determination of the arrival of the radio wave shown in FIG. 4A, “PDET” is repeatedly detected by the interference wave, and therefore the timing “t = a” at which reception of the actual desired wave is started. However, there is a problem that “PDET” does not become active and the correlation value generated at the timing “t = b” cannot be detected normally. This means that “MAXEN” is not active at the timing of detecting the correlation value of the synchronization preamble of the desired wave indicated by “t = b” in FIG. 4A.
[0039]
However, as shown in FIG. 4B, when the rate of change in power is used to determine the arrival of radio waves, “MAXEN” is erroneously activated at the start of interference wave reception. After detecting SYNCMISS, “PDET” does not become erroneously activated, and the reception operation can be normally started at the desired wave reception start timing “t = a”. The correlation value of the synchronization preamble of the desired wave output at “t = b” can be detected.
This is because “MAXEN” is active at “t = b” as shown in FIG.
Next, the configuration of the reception power determination unit 33 described above will be described.
[0040]
FIG. 5 is a configuration circuit diagram of the reception power determination unit 33 that performs determination based on reception power.
In FIG. 5A, the power obtained from the received power measuring unit 31 is input after A / D conversion, compared with a predetermined threshold value by the comparator 51, and if the power value is larger than the threshold value, the comparator 51 is High. Is configured to output.
The output of the comparator 51 is input to two-stage D-type flip-flops (DFF) 52 and 53 for generating a detection pulse waveform.
[0041]
The DFFs 52 and 53 include an input terminal, an output terminal (Q), an inverted output terminal (Q with an overline), an enable (En), a reset terminal (RS), and a clock terminal (CLK).
A logical product of the output signal of the DFF 52 and the inverted output signal of the DFF 53 is obtained by a logical product circuit 54 to generate a PDET signal.
[0042]
Once the radio wave is detected by the comparator 51, the received power determination unit 33 must not operate. Therefore, until the En of the DFF 52 is controlled by the inverted output signal of the output of the DFF 52 and the RS is reset, It is necessary to control so that it does not operate.
If “SYNCMISS” described with reference to FIG. 4 is input to this reset signal, if no correlation value is detected, the initial state can be quickly returned to and a signal radio wave detection routine can be started.
[0043]
FIG. 5B is a block diagram of the reception power determination unit 33 that performs determination based on the rate of change in reception power.
Since the basic configuration of the circuit is the same as in FIG. 5A, only the differences will be described.
The input power value is delayed by one sample by the delay unit 55. The subtractor 56 takes a difference between the power values before and after the delay unit 55. This difference is input to the comparator 51 and the arrival of the desired radio wave is detected by comparing a predetermined threshold value. That is, if the value obtained by subtracting the power value of the current sample from the power value of the previous sample exceeds the threshold value, the output of the comparator 51 is controlled to be active.
[0044]
Next, a method for setting an optimum value of “d1T” will be described.
It has been shown above that the shorter the observation times “T−d1T” to “T + d2T”, the lower the probability of performing a wasteful operation due to false synchronization.
“D1T” depends on the accuracy of the received power measurement unit 31.
For the sake of simplicity, considering the state where the propagation path is static (the state where there is no multipath), if the received power measurement unit 31 works completely in time, the correlation value obtained from the correlation value detection unit 19 is time “ It will be obtained at T ″.
[0045]
However, considering that noise characteristics are strengthened, there are many cases of erroneously responding to noise unless the power is averaged on the time axis. As described above, the shorter the observation times “T−dT” to “T + d2T”, the lower the probability of erroneous detection.
From the above, assuming that the time resolution of the received power measuring unit 31 is “N”, for example, “d1T = N” is the optimum value for the synchronization of the OFDM symbol packet.
[0046]
Next, a method for setting an optimum value of “d2T” will be described.
Naturally, “d2T” is preferably shorter, but in reality, the correlation output exceeding the threshold may be shifted behind the actual prediction due to the influence of multipath.
Correspondingly, “d2T” needs to be lengthened.
Therefore, the optimum value of “d2T” varies depending on the propagation path environment to be used, and it is optimal to match the expected delayed wave with the slowest arrival. Normally, when the system uses OFDM, a guard interval is provided in order to reduce the influence of intersymbol interference caused by delayed waves. Therefore, if “d2T” has the same time as the guard interval length “L”, it takes into account the delayed wave that arrives the slowest in the system.
On the other hand, when demodulating, when a delayed wave gives a correlation output exceeding the correlation threshold, there is the following problem if symbol synchronization is taken accordingly.
[0047]
FIG. 6 is a diagram illustrating the state of symbol synchronization when the correlation is shifted backward due to the delay wave.
In FIG. 6, Effective Symbol indicates an effective OFDM symbol, and GI indicates a guard interval that is a replica of the effective OFDM symbol.
Then, in FIG. 6, Effective Symbol # n is the received nth OFDM symbol, GI # n is the guard interval attached to the received nth OFDM symbol Effective Symbol # n, GI # (n + 1). Denotes a guard interval attached to the “n + 1” th OFDM symbol [Effective Symbol # (n + 1)].
[0048]
In FIG. 6, since it is assumed that the correlation value of the delayed wave 3 in which the correlation output of the correlation unit 15 exceeds the correlation threshold is detected by the correlation detection unit 19, the symbol synchronization is performed at that timing, so that the “n” th It can be seen that the “n + 1” th direct wave and the delayed waves 1 and 2 have an influence on the “n” th OFDM symbol that is currently demodulated.
This means that intersymbol interference occurs and the characteristics of the OFDM signal are impaired. Therefore, when the correlation value is detected by the correlation detection unit 19 after time T, it is more likely that the effect of intersymbol interference is reduced if the window synchronization (FFT Window) is taken and demodulated at time T. I understand.
[0049]
Next, a case of synchronizing with a frame transmitted at a constant period will be described.
When synchronizing with frames transmitted at a fixed period, unlike when synchronizing with randomly generated packets, there is no system problem even if synchronization is performed using multiple frames, and it is necessary to ensure frame synchronization. There is.
[0050]
However, too much time is also convenient, and it is important to synchronize with the frame quickly and reliably. In the case of packet communication that does not constitute a frame, it is necessary to set the threshold value of the reception power determination unit 33 in FIG. 1 so that it can be received even with the lowest reception power conceivable in the system. As a result, erroneous synchronization is often caused.
[0051]
As shown in the previous embodiment, it is possible to reduce the probability by limiting the operation of the correlation value detection unit 19 by time, but the probability of false synchronization still remains.
Therefore, when synchronizing with a frame transmitted at a fixed period, by making the threshold of the reception power determination unit 33 higher than the reception sensitivity, there is no false detection due to noise, and the reception power level is sufficient, It becomes possible to perform frame synchronization at high speed.
On the other hand, if the received power level is low, it cannot be detected and frame synchronization cannot be established, but if a certain amount of time power cannot be detected, the threshold is lowered, and if the same thing is done, power detection becomes possible.
[0052]
FIG. 7 is a demodulation block diagram of an OFDM receiver according to another embodiment of the present invention.
Since the basic configuration of FIG. 7 is the same as the demodulation block of the OFDM receiver of FIG. 1, only the operation of different blocks will be described, and the description of the operation of the same block will be omitted.
In FIG. 7, reference numeral 41 denotes a threshold control unit. The threshold control unit 41 has a timer, and is a circuit that controls the threshold to be low when power exceeding the threshold for a certain time is not detected.
[0053]
FIG. 8 is a diagram illustrating how signal radio waves are detected when the threshold value is changed by the threshold value control unit 41.
In FIG. 8, “POW” indicates the power value measured by the received power measuring unit 31, “t = a” is a point where radio wave search is started, and “t = b” is the head of a frame that is repeatedly transmitted. It is assumed that the timing of receiving is indicated.
The relationship between the threshold values is threshold 0> threshold 1> threshold 2. Among these thresholds, threshold 2 is the lowest reception sensitivity required for the system.
[0054]
In the configuration of the OFDM signal receiver according to the embodiment described so far, the threshold value of the reception power determination unit 33 is set to only the threshold value 2 in this example. Therefore, “t = c, d” illustrated in FIG. When a signal radio wave is erroneously detected at the timing, and the timing d and the timing b are close in time, that is, the timing at which the search for the radio wave is started and the timing at which the beginning of the frame that is repeatedly transmitted is close May miss the detection of radio waves at the original reception timing b.
If the threshold is set to the minimum receiving sensitivity of the system, such a case may occur frequently, and there is a problem that synchronization takes too much time.
[0055]
Here, as an example of changing the threshold value in two stages, if threshold value 1 and threshold value 2 are used, the threshold value is set to threshold value 1 at the beginning, so that such false detection is completely eliminated and synchronization is performed at high speed. Is possible. Further, when the relationship between the received power and the threshold value is such that the threshold value 0 and the threshold value 2 are used, since the radio wave is not detected at the threshold value 0, the threshold value is changed to the threshold value 2 by the action of the timer.
In this description, the case where the power value is used as the power determination unit 33 has been described. However, the present invention is not limited to this, and it is obvious that the same effect can be obtained even if the rate of change in the power value is used.
[0056]
Even when the rate of change is used, even in a packet communication that occurs randomly, it is necessary to detect even a very low power change due to system noise or the like.
However, this simultaneously causes a malfunction due to noise, and is not suitable as a means for synchronizing with a frame transmitted at a constant period.
Therefore, in the same manner as described above, the same effect can be obtained by switching the threshold value for determining the rate of change from higher to lower.
The present invention is not limited to the embodiment described above.
[0057]
【The invention's effect】
According to the present invention, even when there is a lot of noise in the receiver, the probability of malfunction is reduced, and at the same time, the probability of missing a packet to be received due to malfunction is also reduced.
The same effect can be obtained also when other systems are communicating at the same frequency in the vicinity, or when strong interference waves are adjacent to adjacent and adjacent neighbors in the frequency.
Further, even if there is an influence of multipath, it is possible to demodulate without intersymbol interference.
Furthermore, when packets are transmitted at regular intervals, synchronization can be performed efficiently.
[Brief description of the drawings]
FIG. 1 is a demodulation block diagram of an OFDM signal receiver according to an embodiment of the present invention.
FIG. 2 shows a simple timing diagram of a demodulation block of an OFDM signal receiver.
FIG. 3 is a timing diagram of the correlation output waveform of the correlator 15 and MAXEN.
FIG. 4 is a diagram showing a timing diagram of correlation value detection when an interference wave exists.
FIG. 5 is a configuration circuit diagram of a reception power determination unit 33 that performs determination based on reception power.
FIG. 6 is a diagram showing a state of symbol synchronization when the correlation is shifted backward due to the influence of a delayed wave.
FIG. 7 is a demodulation block diagram of an OFDM signal receiver according to another embodiment of the present invention.
FIG. 8 is a diagram showing how signal radio waves are detected when the threshold value is changed by the threshold value control unit 41;
FIG. 9 is a configuration block diagram of an OFDM signal transmitter that generates an OFDM signal.
FIG. 10 is a diagram illustrating an example of a packet structure of an OFDM signal received by an OFDM signal receiver.
FIG. 11 is a configuration block diagram of an OFDM signal receiver that demodulates a conventional OFDM signal.
FIG. 12 is an output waveform diagram of the correlation unit 115 before and after receiving the preamble P2.
[Explanation of symbols]
11 Analog part
12 AGC
13 A / D converter
14 Quadrature demodulator
15 Correlator
16 Guard interval remover
17 FFT section
18 Demodulator
19 Correlation value detector
20 Symbol timing synchronization unit
31 Received power measurement unit
32 A / D converter
33 Received power judgment unit
34 Correlation detection control unit
41 Threshold control unit

Claims (9)

A correlation unit that correlates an OFDM signal including a synchronization signal whose position and pattern in the packet are known; a correlation value detection unit that detects a correlation value of the synchronization signal based on a correlation output of the correlation unit; In an OFDM signal receiver comprising a symbol timing synchronization unit that generates a timing signal for demodulating an OFDM signal based on correlation value detection by a correlation value detection unit,
A reception determination unit that detects reception of an OFDM signal;
A correlation detection control unit that detects and operates the correlation value detection unit in a predetermined time range having a lower limit time and an upper limit time on the time axis with the reception determination time of the reception determination unit as an origin,
The correlation detection control unit does not include the reception determination time of the reception determination unit as a time within the predetermined time range, and until the correlation value detection of the synchronization signal is performed from the start of reception of the synchronization signal. An OFDM signal receiver comprising time “T” as time .   2. The OFDM signal receiver according to claim 1, wherein the synchronization signal is a preamble located at a packet head of the OFDM signal.   3. The OFDM signal receiver according to claim 1, wherein the correlation value detection is performed by detecting a timing at which a correlation output of the correlation unit exceeds a preset threshold value.   The reception determination unit includes a reception power measurement unit that measures the reception power of a signal radio wave, and a reception power determination unit that detects signal reception by detecting that the measurement value of the reception power exceeds a preset threshold value. The OFDM signal receiver according to claim 1, further comprising: an OFDM signal receiver according to claim 1.   The reception determination unit includes a reception power measurement unit that measures fluctuations in received power of signal radio waves, and a reception power determination unit that detects signal reception by detecting that a fluctuation value of reception power exceeds a preset threshold value. The OFDM signal receiver according to any one of claims 1 to 3, further comprising: When "T-d1" the lower limit time of the predetermined time range, "d1" the OFDM signal receiver according to claim 1, characterized in that the time resolution "N" of the reception determination unit. When "T + d2" the upper limit time of the predetermined time range, "d2" the OFDM signal receiver according to claim 1, characterized in that said guard interval length contained in the OFDM signal "L". When the correlation value detection is performed by the correlation value detection unit after the time “T” within the predetermined time range, the symbol timing synchronization unit performs the correlation at the time “T” within the predetermined time range. OFDM signal receiver according to claim 1, wherein the generating a timing signal having the same contents as when it is detected correlation value by the value detection unit.   The threshold value control part which changes a threshold value stepwise in synchronization with the packet transmitted regularly at a specific time interval is attached to the reception determination part. OFDM signal receiver.

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